INTRODUCTION

As the name implies, it is a system in which the circuit is closed and all the elements are directly connected. This is unlike broadcast television where any receiver that is correctly tuned can pick up the signal from the airwaves. Directly connected in this context includes systems linked by microwave, infrared beams, etc. This article introduces the main components that can go to make up CCTV systems of varying complexity.

THE APPLICATIONS FOR CCTV

Probably the most widely known use of CCTV is in security systems and such applications as retail shops, banks, government establishments, etc. The true scope for applications is almost unlimited. Some examples are listed below.

Monitoring traffic on a bridge.

Recording the inside of a baking oven to find the cause of problems.

A temporary system to carry out a traffic survey in a town centre.

Time lapse recording for the animation of plasticine puppets.

Used by the stage manager of a show to see obscured parts of a set.

The well-publicised use at football stadiums.

Hidden in buses to control vandalism.

Recording the birth of a gorilla at a zoo.

Making a wildlife program using a large model helicopter.

Reproducing the infrared vision of a goldfish!

Aerial photography from a hot air balloon.

Production control in a factory.

The list is almost endless and only limited by the imagination.

THE CAMERA

The starting point for any CCTV system must be the camera. The camera creates the picture that will be transmitted to the control position. Apart from special designs CCTV cameras are not fitted with a lens. The lens must be provided separately and screwed onto the front of the camera.

There is a standard screw thread for CCTV cameras, although there are different types of lens mounts.

Diagram 1 Camera And Lens

Not all lenses have focus and iris adjustment. Most have iris adjustment. Some very wide angle lenses do not have a focus ring. The 'BNC' plug is for connecting the coaxial video cable. Line powered cameras do not have the mains cable. Power is provided via the coaxial cable.

THE MONITOR

The picture created by the camera needs to be reproduced at the control position. A CCTV monitor is virtually the same as a television receiver except that it does not have the tuning circuits.

 

Diagram 2 CCTV Monitor

SIMPLE CCTV SYSTEMS.

The simplest system is a camera connected directly to a monitor by a coaxial cable with the power for the camera being provided from the monitor. This is known as a line powered camera. Diagram 3 shows such a system. Probably the earliest well-known version of this was the Pye Observation System that popularised the concept of CCTV, mainly in retail establishments. It was an affordable, do-it-yourself, self-contained system.

Diagram 3 A Basic Lines Powered CCTV System

The next development was to incorporate the outputs from four cameras into the monitor. These could be set to sequence automatically through the cameras or any camera could be held selectively. Diagram 4 shows a typical arrangement of such a system. There was even a microphone built into the camera to carry sound and a speaker in the monitor.

The speaker, of course, only put out the sound of the selected camera. There were however a few disadvantages with the system, although this is not to disparage it. The microphone, being in the camera, tended to pick up sound close to it and not at the area at which it was aimed. There was a noticeable, and sometimes annoying, pause between pictures when switching. This was because the camera was powered down when not selected and it took time for the tube to heat up again.

The system was, though, cheap to buy and simple to install. It came complete in a box with camera, 16mm lens, bracket, switching monitor and 12 metres of coaxial cable with fitted plugs. An outlet socket for a video recorder was provided, although reviewing could be a little tedious when the cameras had been set to sequence.

There are now many systems of line powered cameras on the market that are more sophisticated than this basic system. Most of the drawbacks mentioned have been overcome.

Cameras had been around for a long time of course, before this development. The example is given to show the simplest, practical application. The use of some line powered cameras can impose limitations on system design. They do though, offer the advantage of ease of installation.

 

Diagram 4 A Four-Camera Line Powered CCTV System

MAINS POWERED CCTV SYSTEMS.

The basic CCTV installation is shown in diagram 5 where the camera is mains powered as is the monitor. A coaxial cable carries the video signal from the camera to the monitor. Although simple to install it should be born in mind that the installation must comply with the relevant regulations such as the Institute of Electrical Engineers latest edition. (Now incorporated into British Standard BS7671). Failure to do so could be dangerous and create problems with the validity of insurance.

This arrangement allows for a great deal more flexibility in designing complex systems. When more than one camera is required, then a video switcher must be included as shown in diagram 6. Using this switcher any camera may be selected to be held on the screen or it can be set to sequence in turn through all the cameras. Usually the time that each camera is shown may be adjusted by a control knob or by a screwdriver.

Diagram 5 A Basic Mains Powered CCTV System

Diagram.6 A Four-Camera System With Video Switcher

 

SYSTEMS WITH VIDEO RECORDING

The next development of a basic system is to add a video recorder, the arrangement would be as shown in diagram 7.

Diagram 7A Multi Camera System With Video Recorder

With this arrangement the pictures shown during play back will be according to the way in which the switcher was set up when recording. That is, if it was set to sequence then the same views will be displayed on the monitor. There is no control over what can be displayed.

MOVABLE CAMERAS

So far all the cameras shown have been fixed with fixed focal length lenses. In many applications the area to be covered would need many fixed cameras. The solution to this is to use cameras fixed to a movable platform. This platform can then be controlled from a remote location. The platform may simply rotate in a horizontal plane and is generally known as a

scanner. Alternatively the platform may be controllable in both horizontal and vertical planes and is generally known as a pan, tilt unit. A basic system is illustrated in diagram 8.

This chapter does not deal with how cameras are controlled or wired; it is just showing the facilities that may be incorporated into a CCTV system. Therefore the diagrams that follow are simply descriptive block diagrams and not connection drawings.

Diagram 8 Basic Movable Camera Systems

Cameras may be used indoors or outdoors. When used outdoors they will always require a protective housing. For indoor use the environment or aesthetic constraints will dictate whether a housing is needed. Systems may contain a combination of both fixed and movable cameras.

Diagram 9 Multiple Camera System

OTHER CONSIDERATIONS

This has been an introduction to some of the fundamentals of CCTV. Recent developments have made some very sophisticated systems possible. These include concepts such as multiple recording of many cameras; almost real time pictures over telephone lines; true real time colour pictures over the ISDN telephone lines; switching of hundreds, even thousands, of cameras from many separate control positions to dozens of monitors; reliable detection of movement by electronic evaluation of the video signal; immediate full colour prints in seconds from a camera or recording; the replacement of manual controls by simply touching a screen;

The Other Side of Using CCTV

Nearly all the articles and examples published about CCTV relate to security applications. However, there many far more interesting uses where imagination and flare can bring immediate and tangible benefits. So, in this issue we are looking at non-security applications for CCTV. Many of these applications were one off requirements, which for a purchased installation would have been impossibly expensive. The solution was the short term hire of a system with temporary cabling, this made it affordable for the customer and very profitable to the installer.

Time lapse recording for animating plasticine puppets

The original experimental animation of plasticine puppets was made using a time lapse video recorder and replayed in real time. The rest is history.

Carrying out a traffic survey for a town centre regeneration

A town was planning a major reconstruction of the town centre, one problem though, was that it was also an intersection of two main trunk roads and three local roads. An analysis of traffic flow was obviously a necessity and the usual method was for observers to sit by each junction and count the vehicles passing. It was considered that this would be expensive and not very accurate to cover a complete weeks traffic twenty four hours a day. Another problem was that although traffic flow could be counted at junctions, it would be difficult to determine where it went off.

The solution proposed was a CCTV system to monitor the traffic flow. There was a multi-storey car park right in the centre of the intersection where cameras could be mounted covering every junction. To purchase such a system would have been very expensive (although not prohibitively so) for just a couple of weeks. The answer was to hire the complete system on a short term contract.

The system consisted of eight fixed monochrome cameras, connected to a multiplexer and 12 hour time lapse video recorder. The installation would be temporary with cables simply laid along the roof, therefore, 24 volt cameras were used to eliminate the need for the expense of complying with wiring regulations. This made a significant reduction in the potential installation costs.

Two tapes was used each day to provide a continuous record of traffic for the seven days. The tapes could then be analysed in significant detail including the types of vehicles and the routes taken in and out of the intersection. This proved to be not a cost effective solution but provided far more useful data than a manual survey could achieve.

 

Monitoring traffic on a bridge

This was similar to the first example, in that a very old bridge could no longer take two way traffic and a bypass was needed. Once a time lapse VCR and two cameras provided far more information on traffic flow than manual logs could have provided.

Recording inside a baking oven

A large bakery was producing thousands of danish type pastries every night for distribution to a chain of high street shops. The pastries were conveyed on a wire mesh chain conveyor through the oven and discharged onto a flat belt conveyor to cool and pass to packing.

On rare occasions, maybe four or five times a year, instead of discharging, the pastries jammed at the transfer with the following pastries piling up behind. It only needed a couple of minutes for the entire oven and feed line to be a complete mess of uncooked, overcooked pastry filling the space. Not only was a complete nights production lost, but it needed several days to clean up the mess and start production again. The repercussions could be that it was not just the danish pastry sales that were lost but confidence in the company would be diminished. On every occasion, the conveyors and drives were stripped down, inspected and reassembled, but nothing untoward was found.

Again the solution was simple and temporary. A small camera was fitted in a water-cooled housing and mounted inside the oven, viewing the discharge area. This was connected to an 8-hour video recorder and the entire production shift recorded. The tape was simply overwritten each night there was no incident. It was over two months before disaster struck again. However, this time there was a video from inside the oven.

Detailed analysis of the tape produced an answer that could never have been found by any other means. There was a small kink on one side of the conveyor chain, and a small flaw in one of the driving sprockets. Due to the gear ratios and a fluid coupling the possibility of the mesh kink meeting the sprocket flaw was thousands to one. To make the incidence even more remote, this coincidence had to occur just as the first batch of pastry emerged from the oven, once it was flowing there was no problem. However, it was determined that this was in fact what had occurred. It was decided to leave the camera in place permanently, but once cured the problem, never presented itself again and they all lived happily ever after.

Recording the birth of a gorilla in a zoo at night

Apparently, gorillas are very private animals, particularly when expecting a birth. The BBC wildlife programme wanted to record the events leading up to and after

the birth, but the problem was during the night when any illumination would be unacceptable. The solution was to use an infrared illuminator with an 850 nanometer filter which would be totally unobtrusive with an infrared sensitive camera and time lapse VCR. The result was the first recorded birth of a gorilla in captivity.

Making a wildlife program in an isolated area using a model helicopter

Many exotic locations for wildlife filming are too remote or inaccessible to reach on foot or from conventional helicopters. One solution was to fit a miniature camera and radio transmitter to a small model helicopter. This was radio controlled and comparatively unobtrusive to the local wildlife, creating unique footage of film.

Reproducing the infrared vision of a goldfish

A university was studying the ability of fish to apparently ‘see’ and navigate through murky water. The theory was that goldfish had vision that was sensitive to infra light. It would seem that where the visible part of the spectrum was largely reflected by water, infra light penetrates further. To simulate this, a camera was fitted with a filter that restricted its sensitivity to only the infrared part of the spectrum. An infrared illuminator was directed from above and the views from the camera noted. I never saw the results of this and don’t know what, if anything, was proven. It was interesting to set up and different from run-of-the-mill CCTV.

Safety at Grand Prix racing

After the tragic accident of Nicki Lauder at The Nurburg Ring in Germany, Grand Prix racing drivers banned the track for major events. In May 1994 a new Nurburg Ring was opened with a computer-designed track and many new safety measures. The particular item of interest is a Geutebruck system of cameras connected back to a video motion detection system in the control room. Each camera monitors an area of the track, with zones defined alongside the track. If a car leaves the track it is detected and a view of the area instantly displayed at the control room and the appropriate action can be set into motion. If it is an accident, emergency teams can be directed to the scene immediately, even saving seconds can make the difference between life and death. The system would also detect a spectator straying onto the trackside. If a car leaves the track and rejoins the race, the system is automatically reset.

There are many other examples of the innovative use of CCTV, other than security, such as:

Production control in factories.

In a stage show to see obscured parts of a set.

Use at football stadia.

Arial photography from a hot air balloon.

Many of these applications require some lateral thinking and flexibility on the part of installation companies, maybe this is what is lacking today. Many of these systems have provided excellent value for money for the end user and can be very profitable for the installer.

Understanding Cameras

Specification of the right CCTV camera for a project is not always the easiest of processes. There are many factors that have to be taken into account: technical specifications, the application and its requirements, as well as any physical constraints the site may impose. With ever increasing product ranges available in the marketplace, and technology constantly evolving to optimise performance, reliability and functionality, it is quite a challenge to make an informed decision to meet the requirements for the job whilst remaining within projected budget. Understanding the many variables within CCTV camera technology today can only be an advantage in helping you make the right choices.

At the heart of the CCTV camera technology is a CCD sensor (Charge Coupled Device) that converts light into an electrical signal. This electrical signal is then processed by the camera electronics and converted to a video signal output that can then be either recorded or displayed on to a monitor.

However, the treatment of the video signal is then dependant on the type of camera. CCD chip cameras can be divided into two principal types: analogue or the more recently introduced digital versions.

These can be sub-divided further into the following: medium resolution monochrome

Medium resolution color High resolution monochrome High resolution color Day /night cameras that provide color in the day and monochrome at night.

To complicate matters even further, each of the above is generally available with different levels of performance -like a car model varying from 'base features' to 'top of the range'.

Monochrome or Colour?

The human eye remembers and recalls things better if they appear in colour - it's easier to track down a brown-haired person wearing a red sweater and blue jeans than a dark, grey-clad figure that would be produced in monochrome.

Color cameras carry an additional premium in price compared with monochrome cameras. But they are also less sensitive making night usage an impractical option unless good lighting is available.

Monochrome cameras can offer Infra Red (IR) sensitivity allowing their use with covert IR illumination possible. This can be particularly useful where planning permission makes extra lighting impractical or the security requirement is such that intruders should not be alerted to the existence of CCTV surveillance.

Analogue or Digital?

Until recently most cameras have been of the analogue type, producing good quality images at an affordable price. However, the introduction of Digital Signal Processing (DSP) has increased both the flexibility of using security cameras whilst enhancing the quality of the colour images produced.

At the heart of DSP lies computer microchips, or 'chip sets' which have replaced the conventional integrated circuits in the camera head. This enables DSP camera manufacturers to offer installer friendly, feature-rich products.

The market for DSP technology falls into two broad categories: 'standard' and 'premium' DSP. Standard DSP cameras generally offer more consistent picture quality than their analogue counterparts, operating over a wider range of lighting conditions. Premium DSP cameras, however, have much richer functionality. This includes programmable intelligent backlight compensation (BLC), Video Motion Detection, remote set-up and control using a serial data link; built-in character generator and on-screen menus. These features make Premium DSP cameras the ideal choice for complex surveillance conditions such as those encountered in town centres.

Some situations may require a standard DSP camera, but with a specific premium feature. A good example of this is the Vista NCL634 colour camera. Using digital signal processing, the NCL634 splits the screen into 64 zones. The DSP function calculates the average brightness within each of these zones and then compares it with those in all 64 zones. The camera can then adjust the picture detail for areas that are in silhouette.

This innovative feature is ideal in awkward lighting situations, e.g. a camera looking towards a shop window. In the morning, the sun may be in the top left corner of the window, but then moves across the field of view during the day, causing poor picture quality in most cameras. Intelligent backlight compensation is a function that will ensure crisp detailed pictures automatically throughout the day.

CCD Chip Size

CCTV cameras generally use COD chips that are designed for the consumer camcorder market. Originally, the chips used were half-inch image diagonal, but the drive for reduced size led to the development of third-inch and more recently quarter-inch chips.

The half-inch chips are capable of producing the highest sensitivity and resolutions owing to the simple fact that they are able to gather more light. Third-inch chips now form an increasing part of the market and as product development continues their performance is approaching that of their larger brothers.

Quarter-inch chip sets are a relatively recent development and are being widely used in consumer camcorders. Currently their use in CCTV is still somewhat limited because of the lack of availability and range of quarter-inch format lenses.

As a general rule, quarter-inch cameras provide the lowest cost and performance while half-inch cameras provide premium performance and are more expensive.

Mid-priced third-inch cameras make up the bulk of cameras used in the market today.

Cameras and lenses made simple

1. CAMERAS

The subject of specifying cameras is a jungle of jargon and misinformation, this brief article attempts to shed a little light on some of the mysteries surrounding it. Only CCD cameras will be considered because they are now the most commonly used type for CCTV.

The imaging device:

CCD means a Charged Coupled Device and consists of a flat array of tiny, light sensitive photodiodes. Each diode produces a voltage that is directly proportional to the amount of light falling on it. No light would produce no voltage and therefore a black level. Maximum light would produce a maximum voltage and therefore a white level. In between these would be shades of grey, and is the luminance information of a video signal. In the case of a colour camera, a chrominance signal is superimposed onto the luminance signal to carry the colour information. (If a colour camera is connected to a monochrome monitor, then a monochrome picture would be produced from the luminance information and the chrominance would not be processed). See also colour cameras with separate Y/C outputs under resolution.

The range of light levels that a CCD can cope with is very limited, therefore means have to be introduced to restrict the light range within certain limits.

The video signal:

A field of video is created by the CCD being scanned across and down exactly 312 1/2 times and this reproduced on the monitor. A second scan of 312 1/2 lines is exactly 1/2 a line down and interlaced with the first scan to form a picture with 625 lines. This is known as a 2:1 interlaced picture. The combined 625 line is known as a frame of video and made up from two interlaced fields. The total voltage produced is one volt from the bottom of the sync pulse to the top of the white level, hence one volt peak to peak(p/p). The luminance element of the signal is from 0.3 volts to one volt, therefore is 0.7 volts maximum. This is known as a composite video signal because the synchronising and video information are combined into a single signal.

 

Note that the imaging device is scanned 625 times but the actual resolution is defined by the number of pixels making up the device.

There are several factors that make up a complete camera specification and are all be inter-related. These are:

Sensitivity

Signal to noise ratio.

Automatic gain control.

Resolution.

Sensitivity:

The most common factor people look for in a camera specification is the sensitivity, although it is not always the most important. Sensitivity is the amount of light, in lux, necessary to produce a video signal of some, usually unspecified, level. This factor seems to be the marketing battleground upon which all manufacturers fight to show their cameras as being better than the competition!

 

Signal to noise ratio. (S/n).

As seems obvious this is the ratio of the level of the video signal to the amount of noise present. Noise in a video is seen as snow or graininess, resulting in a poorly defined image on the monitor or video recording. The unit for expressing s/n ratio is decibels (dB), but do not be too worried because it can be expressed as a ratio. The following table shows the equivalent ratio for values given in dB.

dB Ratio

100 100,000:1

60 1,000:1

50 316:1

40 100:1

30 32:1

20 10:1

10 3:1

It can be seen that a s/n ratio of 40Db is equivalent to a ratio of 100:1, that is the signal is 100 times the noise level. Conversely the noise is one hundredth of the signal. Note that at a s/n ratio of 20Db, the noise is 10% of the signal and would produce an unacceptable picture. The following table provides a guide as what quality to expect from various s/n ratios.

 

 

S/N ratio dB S/N ratio:1 Picture quality

60 dB 1,000 Excellent, no noise apparent

50 dB 316 Good, a small amount of noise but picture quality good.

40dB 100 Reasonable, fine grain or snow in the picture, fine detail lost.

30 dB 32 Poor picture with a great deal of noise.

20 dB 10 Unusable picture.

 

Automatic gain control (AGC).

When the light falling on to an imaging device reduces to a certain level, there is insufficient to create a full level video signal. AGC acts to increase the amount of amplification in these conditions to bring the signal up to the required level. As well as amplifying the video signal, additional noise can be introduced, and the signal to noise ratio reduced. The result is frequently a very much degraded signal and poor picture on the monitor.

Resolution.

The value referred to here is the horizontal resolution in TV lines, that is the number of black to white transitions that can be resolved across the image. This is a function of the number of pixels that make up the CCD imaging area and the bandwidth of the camera circuitry. Typical camera resolution is 350 TV lines, with high resolution cameras producing better than 450 lines. Note that resolution costs money!

There are now colour cameras that instead of superimposing the chrominance onto the luminance signal, provide the chrominance as a separate signal. This is known as Y/C separation and requires two coaxial cables from the camera to carry each signal separately. The effect of this technique is to increase the bandwidth and therefore the resolution, typically to better than 500 TV lines.

912 words.

Cameras and lenses made simple.

2. LENSES

Introduction

The human eye is an incredibly adaptable device that can focus on distant objects and immediately re-focus on something close by. It can look into the distance or at a wide angle nearby. It can see in bright light or at dusk adjusting automatically as it does so. It also has a long 'depth of field' therefore scenes over a long distance can be in focus at the same time. It sees colour when there is sufficient light but switches to monochrome vision when there is not. It is also connected to a brain that has a faster updating and retentive memory than any computer. Therefore the eyes can swivel from side to side and up and down, retaining a clear picture of what was scanned. The brain accepts all the data and makes an immediate decision to move to a particular image of interest. It can then select the appropriate angle of view and re-focus. The eye has another clever trick in that it can view a scene of great contrast and adjust only to the part of it that is of interest.

By contrast the basic lens of a CCTV camera is an exceptionally crude device. It can only be focused on a single plane, everything before and after this becomes progressively out of focus. The angle of view is fixed at any one time it can only view a specific area that must be predetermined. The iris opening is fixed for a particular scene and is only responsive to global changes in light levels. Even an automatic iris lens can only be set for the overall light level although there are compensations for different contrasts within a scene. Another problem is that a lens may be set to see into specific areas of interest when there is a lot of contrast between these and the surrounding areas. However as the sun and seasons change so do light areas become dark and dark areas become light so the important scene can be 'whited out' or too dark to be of any use.

One of the most controversial but important aspects of designing a successful CCTV system is the correct selection of lens. The problem is that the customer may have a totally different perspective of what a lens can see compared to the reality. This is because most people perceive what they want to view as they see through their own eyes. Topics such as identification of miscreants or number plates must be subjects debated frequently between installing companies and customers.

The selection of the most appropriate lens for each camera must frequently be a compromise between the absolute requirements of the user and the practical use of the system. It is just not

possible to see the whole of a large loading bay and read all the vehicle number plates. The solution may be more cameras or viewing just a restricted area of particular interest. The company putting forward the system proposal should have no hesitation of pointing out the restrictions that may be incurred according to the combination of lens versus the number of cameras. Better this than an unhappy customer who is reluctant to pay the invoice.

 

Fixed Focal Length

These are sometimes referred to as monofocal lens. As the name implies this type of lens is specified when the precise field of view is fixed and will not need to be varied when using the system. The angle of view can be obtained from the supplier's specification or charts provided. They are generally available in focal lengths from 3.7mm to 75mm. Longer focal lengths may be produced by adding a 2x adapter between the lens and the camera. It should be noted that this will increase the f number by a factor of two (reducing the amount of light reaching the camera). If focal lengths longer than these are required then it will be necessary to use a zoom lens and set it accordingly.

Except for very wide angle lenses all other lenses have a ring for adjusting the focus. In addition cameras include a focusing adjustment that moves the imaging device mechanically relative to the lens position. This is to allow for minor variations in the back focal length of lens and manufacturing tolerances in assembling the device in the camera. Correct focusing requires setting of both these adjustments. The procedure is to decide the plane of the scene on which the best focus is required and then set the lens focusing ring to the mid position. Then set the camera mechanical adjustment for maximum clarity. Final fine focusing can be carried out using the lens ring.

The mechanical focusing on cameras is often referred to as the back focus. This was because a screw at the back of the camera moved the tube on a rack mechanism. Modern cameras now have many forms of mechanical adjustment. Some have screws on the side or the top, some still at the back. There are cameras that have a combined C/CS-mount on the front that also has the mechanical adjustment and can accept either type of lens format. The longer the focal length of the lens the more critical is the focusing. This is a function of depth of field.

Variable Focal Length

This is a design of lens that has a limited range of manual focal length adjustment. It is strictly not a zoom lens because it has quite a short focal length. They are usually used in internal situations where a more precise adjustment of the scene in view is required which may fall between two standard lenses. They are also useful where for a small extra cost one lens may be specified for all the cameras in a system. This saves a lot of installation time and the cost of return visits to change lenses if the views are not quite right. For companies involved in many small to medium sized internal installations such as retail shops and offices this can save on stockholding. It makes the standardisation of systems and costing much easier.

Manual Zoom Lens

A zoom lens is one in which the focal length can be varied manually over a range by means of a knurled ring on the lens body. It has the connotation of 'zooming in' and therefore infers a lens with a longer than normal focal length. The zoom ratio is stated as being for instance 6:1 this means that the longest focal length is six times that of the shortest. The usual way of describing a zoom lens is by the format size, zoom ratio and the shortest and longest focal lengths, i.e. 2/3," 6:1, 12.5mm to 75mm. Again, great care must be taken in establishing both the camera and the lens format. The lens just described would have those focal lengths on a 2/3" camera but a range of 8mm to 48mm on a 1/2" camera. Similarly a lens giving the same performance on a 1/2" camera would be a 1/2," 6:1, 8mm to 48mm.

Motorized Zoom Lens

Manual zoom lenses are not widely used in CCTV systems because the angle of tilt of the camera often needs to be changed as the lens is zoomed in and out. The most common need for a zoom lens is when used with a pan tilt unit. The lens zoom ring is driven by tiny DC motors and controlled from a remote source. With a correctly set up camera lens combination the focus should not change from one limit of zoom to the other.

With the development of ever smaller cameras and longer focal length lenses the method of mounting the camera/lens combination must be taken into account. There are many cases where the lens is considerably larger than the camera and it may be necessary to mount the lens rigidly with the camera supported by it. In other cases it may be necessary to provide rigid supports for both camera and the lens. Always check the relationship between the camera and lens sizes and weights when selecting a housing or mounting. Most manufacturers of housings can provide lens supports as an accessory.

The most frequent reason for the focus changing when zooming is that the mechanical focus of the camera has not been set correctly.

Motorized Zoom Lens With Pre-sets

There are many situations where it is required to pan tilt and zoom to a predetermined position within the area being covered. It is possible to obtain motorized lenses with potentiometers fitted to the zoom and focusing mechanisms. These cause the lens to zoom automatically and focus to the setting by measuring the voltage across the potentiometer and comparing it with the signals in the control system. All other functions are as for motorized zoom lenses. Pre-set controls are only possible with telemetry controlled systems. The specification of the telemetry controls should be checked to see whether the pre-set positions are set from the central controller or locally from the telemetry receiver.

Understanding Covert Cameras

Covert Cameras, in essence, are a means of offering surveillance of an undetected or more discreet nature. Suitable for use in a broad range of internal applications, these miniature Cameras have been designed in developed to provide monitoring tools that are disguised in the form of everyday commercial and domestic objects.

This ensures that they are able to blend inconspicuously into any background and consequently do not catch people's attention. As a result, there are a number of state-of-the-art products which have been introduced into the market to meet security demands, varying from office clocks to Passive InfraRed (PIR) sensors, containing a minute camera within. These products are available in monochrome or colour versions and with optional audio.

Covert cameras tend to be used where there is a requirement to achieve particular objectives. These tend to fall into the following categories:

A) Covert surveillance - where there is a requirement to monitor activities in a particular location, completely undetected, e.g. in areas of high security like jewellers and banks. They are also useful for back-up surveillance in installations where the primary CCTV equipment is of a more traditional nature, i.e. standard cameras. In this case Covert

Cameras can operate as a back-up where primary cameras are disabled by an intruder.

B) Discreet/Unobtrusive surveillance - often there is a need for a surveillance system that is less conspicuous, not necessarily as an attempt to hide the fact that monitoring is taking place, but more from marketing or style considerations.

When introducing a covert system, it is important to recognise that access to recorded material must be kept to a minimum to ensure the privacy of individuals who may appear. A responsible policy should be introduced to ensure that footage from covert cameras is used for the purposes it was intended.

Analogue and Digital - What's the Difference?

The issue of digital images as evidence is in focus as this new technology takes off in the security world. Demand for digital is rising rapidly as the cost of commercial applications falls (particularly for storage and maintenance). The quality of digital technology is clear to see quite literally with superior images that are more flexible to store and transfer.

So what is the difference between traditional analogue video images and images obtained from digital surveillance technology - and why all the fuss?

Traditional analogue Images are recorded in some physical form, such as frequency, amplitude or in the case of a photograph, the activation of photo-chemical emulsion.

A digital Image is recorded as a series of binary digits (called bits) - either ones or zeroes. The image is then focused onto an electronic sensor comprising individual light-sensitive elements known as pixels (picture elements). These act as switches to modify an electrical current on or off and the information is processed by a computer. It can then be displayed on a screen, stored in a variety of media or printed out.

Traceability

The Select Committee Report, 'Digital Images and Evidence', seeks to clarify the difference (see panel above) and makes recommendations to the Government on the way forward with digital CCTV images. For a court, the key word is 'traceability' - having a cast-iron audit trail that takes you right back to the original recording. This means that whatever happens to an image if it is enlarged, printed out, even tampered with - the original remains for a court to examine. Because digital technology is so new, people are having to get to grips with the fact that a digital image consists of a series of ones and noughts that are converted by a computer into an electronic image. But that doesn't mean they should be any less valid than a traditional analogue image.

Far from saying digital images cannot be used as evidence the Report lays out guidelines about ensuring their authenticity. Like analogue images, suitable procedures should be followed in collecting and monitoring what is captured on camera. Indeed, the Select Committee established that digital images have already been used as evidence in court. For example, images from a system installed in the car parks at Heathrow Airport have been successfully used as evidence.

It seems certain that the increasing popularity of digital technology coupled with the fact that images can be replayed countless times with no diminution in quality means its widespread use and acceptance as evidence is inevitable. Analogue or digital images are unlikely to be the only evidence presented in a court case. In fact, they are far more likely to be used before a trial to make a person admit their involvement in a situation.

From our understanding of the Report, the Government is saying that methods of storage and authentication of surveillance images should continue as before. Many of the issues created by new digital technology will be governed by the new Data Protection Act.

The Data Protection Act is significant because unlike analogue images, digital images are covered by the Act. This seeks to protect individuals from the use of personal information without their consent, such as their names and addresses. It is very detailed about the way data must be handled and stored. By falling within the remit of the Act, digital recordings are therefore governed by very stringent guidelines and controls.

What It All Means for Installers

What is important is that end users of digital surveillance equipment know what is expected of them in terms of the way they record, store and use digital images. It's not so much installers but the impact on their customers that need to be considered. Installers should make sure their customers know what is expected of them.

We're helped here by a number of specific recommendations made by the Select Committee and endorsed in the Government's official response. Digital technology has the capacity for encryption and security coding so some kind of electronic audit trail involving file coding of digital images is suggested. A permanent physical record of the data that cannot be amended is one idea - this could be some form of write-once read many times' (WORMS) disk. Creating an audit trail would reduce the chances of undetected tampering of images.

These are some of the main Report recommendations which the Government has said it hopes will help to form 'best practice' in the security industry and elsewhere:

Responsibility for proving the reliability and authenticity of data is with the body that captures, processes and modifies it. A suitable audit trail is essential

Where digital images are considered as evidence, courts should place greater weight on evidence that can be shown to be derived from an authenticated original. Juries should be informed of anything to doubt the authenticity of digital images

As with analogue images, proper records must be maintained showing who was in control of the equipment at the time of an incident and subsequently in charge of any images created, and who is responsible for the storage and retrieval of those images

The Data Protection Act 1998 should provide the regulatory framework to cover CCTV-derived images, including digital data. The Government supports the idea of devising some kind of incentive such as endorsing codes of good practice that are based on the quality, integrity and authenticity of data. Factors here might include:

The way in which systems are tested, including on-site, by installers or users The way systems are set up, calibrated and maintained Environmental conditions Operating procedures Training of users Automatic quality warnings.

What installers need to make their customers aware of is not just the fact that digital and analogue images differ but to ensure that the same careful approach is taken to the way any image is captured, stored and maintained. They need to make sure their customers understand the importance of ensuring traceability of surveillance images.

Digital Images as Evidence

At last it is out, the long awaited report from the House of Lords Select Committee on Science and Technology.

Most people in the industry and many end users have been waiting with baited breath for what was expected to be a series of draconian regulations imposing severe restrictions on the use of digital recordings. Everyone I spoke to on the subject had their own views of impending doom and enormous increases in costs to comply with these, as then, unpublished papers.

The result? I found the report to be an extremely down-to earth, pragmatic document that deals with the subject realistically. It recognises the inevitability of progress along the digital route and

the problems of creating legislation based on technological criteria. The recommendations do come down very heavily on the need for secure audit trails from initial recording to copies produced as evidence.

Although the report does not require forms of encryption, watermarking or other anti-tamper measures it should be remembered that these techniques may well be necessary for other security requirements. The report deals with digital images as evidence only.

There is one great difference between analogue and digital recording. If a copy is made from a tape recording, the copy will be of a lessor quality than the original, if further copies were made from copies the results may well be unusable. A digital recording however, consists of a series of binary digits which can be copied an unlimited number of times with no degradation of the images compared to the original. If an 'original' set of images was on a CD for instance and then copied several times, it would be impossible to determine which was the original and which were copies.

The report draws on a great deal of experience in relation to the digital storage and copying of documents and applies this to digital images. Following is a selection of extracts from the report.

What is an original? The report prefers; "The original is the data first recorded in memory. Thus any printed or displayed image created from these is a copy. Consequently digital recording technology provides no original that could be produced in evidence. All that is available for evidence is a copy of the first, probably temporary, recording in memory, and this will be admissible as evidence. Its weight as evidence will depend on proper authentication and other matters".

In the case of a digital camera, it is probable that the original would be the digital file representing the image. This would be stored on a memory chip or series of chips and immediately transferred to some other form of storage (hard disc, etc.) and the memory chip being overwritten with the next image. "This does not represent a problem under the Law of England and Wales because if the original of a document no longer exists, copies or even copies of copies are admissible as evidence and it is irrelevant that the original was destroyed by the person seeking to produce the copy as evidence. Nor is it a problem in Scotland because although the general rule that copies of documents are admissible whether or not the originals still exist does not apply to visual images, copies of a document which no longer exists are admissible under the best evidence rule. The fact that a document is a copy goes to its weight as evidence, not its admissibility. It will therefore be necessary for the user to be able to give evidence of the procedures used for generating, processing and storing digital images. So as to be able to prove that the image produced to the court is an accurate copy of the original". ....In general the court is likely to admit the evidence, the judge will direct the jury on the weight they should consider attaching to it.

There is a section of the report dealing with the possibilities of modifying a digital image. There are a variety of inexpensive software packages that can perform sophisticated alterations to digital images. These techniques can be used for several reasons ranging from simple enhancement to making significant and possibly malicious alterations to an image. "There is no

clear distinction between acceptable 'enhancement' and unacceptable 'manipulation'. Any changes have to be considered on their merits.

The need for caution. With modern processing and tele-communications techniques even an image that purports to be analogue may have had a 'digital past' For example, it is possible that an image stored digitally may have been generated with a standard analogue camera, the signal or picture may be converted into digital format for transmission and then converted back to analogue again to be displayed. Thus in many circumstances, it can be difficult to make a distinction between what is and what is not a digital image. The ease with which images, when in digital format, can be copied and modified suggests that caution must be exercised when any image is used as evidence: all images, both analogue and digital might be suspect.

But are these concerns real? ......We concluded that before the advent of digital technology it might have been a time consuming and costly exercise to produce a modified image in which it was difficult to detect tampering, but with the present widespread availability of digital technology it could now be a low cost operation to produce an image in which the modifications would be undetectable. The existence of a technology that can be used to modify images in this way need in itself be of no great concern; even the widespread availability of the technology at low cost might not cause concern. .........This means that when presented with an image the observer should have no more, and no less, faith in it than if the information had been text on a sheet of paper of questionable provenance".

The report continues to state that witnesses regarded the differences between digital images and other evidence as being one of 'degree rather than of fundamental kind'.

There is a distinction between the admissibility of evidence between civil and criminal cases. "......This means that in criminal cases, any use of a digital image as evidence must be accompanied by the certificate required under section 69 (of the Police and Criminal Evidence Act 1984) This certificate, given by a person responsible for the computer system in question, must state that either the computer system was at all times operating properly, or that any defect in its operation was not such as to affect the accuracy of the record".

The Law Commission has recently recommended the repeal of Section 69 of PACE because 'it serves no useful purpose'. With the repeal of this section, a presumption of proper functioning would be applied to computers.

The report emphasised many times the need for an accurate recorded audit trail from the initial image to the copy produced in court. "A prosecutor or party to litigation will always need to be prepared to offer further evidence about the source of a digital image and the processing and storage it has undergone since it was first recorded. It has been held that the person adducing a recording as evidence must describe its provenance and history, so as to satisfy the judge that there is a prima facie case that the evidence is authentic".

The next chapter concentrates on the use of digital images and continues the theme of authentication. "The fact that a document is a copy may reduce its weight as evidence,

unless there is sufficient authentication evidence to convince the court that is an accurate copy. This authentication evidence would normally be in the form of an audit trail connecting the original image with the computer record which is to be adduced in evidence and recording what has occurred to that record in the interim.

Here the report refers to a British Standard Code of Practice for the Legal Admissibility of Information on Electronic Document Management Systems. (DISC PD 0008, February 1996). This sets out procedures and documentation required for the audit of systems producing documents or other images that may be used as evidence in a court of law. In the absence of any Code of Practice more relevant to CCTV images, it may be that we all need to obtain a copy of this BS.

I was very surprised to note the following comment. "Of considerable interest to us was that no defence teams in the United Kingdom had, as yet, ever requested an audit trail be produced in any case where video images were being used by the prosecution This may change as defendants and their lawyers become more familiar with the technology".

I have a feeling that this situation is going to change rapidly, and, of course will not only apply to digital recordings.

The report goes into some length about watermarking and encryption technologies and the pros and cons of their usefulness in authenticating digital recordings. It continues:

"The arguments against specifying new criteria which must be met before evidence can be admitted are:-

It would be very difficult to specify the nature of the authentication technology in such a way that it would not quickly become outdated as the technology advances.

It would take an appreciable time for manufacturers of digital image technology to incorporate such measures, and even longer for such technology to become widely used;

When technology advances, the courts will be faced with the position that images over which there is no dispute as to their reliability cannot be received as evidence because they were not captured by technology which met the specification; and

The clear trend in the development of the law is to remove prior requirements for all forms of documentary evidence, leaving it to the courts to determine whether the evidence is reliable.

For these reasons we are not convinced that some sort of criteria must be met before evidence can be admitted. Rather we agree with the witnesses who said that there should not be different rules about admissibility based on the technology used to capture the evidence".......

"We recommend that evidence should not necessarily be inadmissible because it does not conform with some form of technological requirement".

However, it continues, that although there should not be technological requirements which digital images must meet, it does not mean that the report is against authentication technologies. Quite the reverse, "We support the application of any technology which can help with the verification of an image and provide assistance to the court in assessing its worth".

"We recommend that the Government encourage the use of authentication techniques. Members of the legal profession should be aware of the benefits of these techniques, their value in adding weight to evidence and the possible significance of their omission.".

Technical procedures can only be part of the authentication process, provenance of evidence can be greatly enhanced by the correct procedural measures. "We recommend that the Government produce guidance on the benefits of conformance with procedural measures to establish the reliability of evidence, with particular reference to existing standards. When this guidance is available, we recommend that the trade associations of those organisations likely to be concerned with it produce training material on its use.

The report continues with a chapter on Civil Liberty implications and a final summary and recommendations.

So, digital images are admissible as evidence in court whether or not they have been manipulated. Their weight as evidence will be decided by authentication methods such as encryption or watermarking and particularly a secure audit trail from initial image to copy produced in court.

I found the document quite readable and interesting, bringing out many aspects of digital documents and images when used in evidence. I would commend everyone who has an interest in digital recording, whether end user, manufacturer, installer or consultant to obtain a copy. The full report is 37 pages and at £7.80 it's a steal.

Digital storage – More facts and Hype

Digital storage is likely to be flavour of the month for a long time yet, as is the thriving hype and misinformation industry. I originally produced an article on digital recording in the July 1996 issue, when it really was very much in its infancy. Incidentally the article was sub-titled ‘The hype and the facts’, so in one respect nothing has changed since then except that manufacturers now have bigger numbers to confuse us with.

I expected that digital recording in CCTV would develop at a rate comparable with the PC industry but this has not happened. For instance in 1996, a CD-ROM drive was many hundreds of pounds and a CD writer was several thousand pounds. Now you can buy a CD-ROM for as little as £35.00 and a CD-RW for not much over £100.00. Similarly PC central processors have increased tenfold in power and speed at significantly lower prices. One area of great interest to digital storage of video images is the capacity of hard disc drives (HDD). This has increased from about 1/2Gb in 1996 to common 18Gb today with 36Gb available, still not the 100Gb I hoped for in the original article.

There is no question as to the benefits of digital recording for event recording, ATMs, etc. This article is looking to the future of continuous recording as we currently enjoy with the VCR.

A lot of progress has been behind the scene with developments and availability of various compression techniques to, create more efficient storage of data with smaller file sizes. It may be worth revising some of the techniques involved in digital recording. The following is a brief extract from The Principles and Practice of CCTV 2nd edition.

Principles of Digital Video Recording.

In digital recording each field is divided in to an array of individual points or pixels. At each one of these points, analogue to digital converters convert voltages representing the colour and brightness at that point to a binary digital number. This array of binary digital numbers can then be stored digitally in a file with a name cross referenced against time and date. A single frame of monochrome video needs about 450Kb (Kilobytes) of space for storage and single frame of colour needs about 650Kb. This is the uncompressed size that would be needed for storage on hard disc or other storage medium.

Consequently to store the same number of images as a videotape, a total storage capacity of about 280Gb (Gigabytes) would be needed for one camera. This is considerably larger than hard discs and other media generally available and would also be tremendously expensive. Consequently some means is required of reducing the amount of space required without adversely affecting picture quality. The technique of reducing the amount of space required is generally referred to as compression.

The video frame contains a large amount of redundant information that can be eliminated without a great loss in perceived picture quality. Consequently, common types of compression used are known as "lossy compression" because the redundant information is discarded. Most compression methods are effective up to a certain point, or "Knee", beyond which the image quality quickly degrades.

To assist in reducing the amount of size required for storage the video signal can be represented in a form known as YUV. The YUV format consists of the Y (luminance) and UV (colour difference) signals (for further descriptions of luminance and video signal components see section 2). The advantage of using YUV format is that fewer bytes are needed to digitise the video. Normally, recording all of the colour components; red, green, blue (RGB recording) would need three bytes, one byte for each colour. By using YUV format the luminance can be digitised as one byte and the colour difference signal as one byte. Consequently only two bytes are needed rather than three, a saving of one third of the storage space required. This technique can be used together with compression to minimise the amount of space required for storage

Types of Compression.

The technology for compressing video pictures originated in the storage of still photographs on computers. The most commonly used standard, JPEG, takes its name from the Joint Photographic Expert Group by whom it was developed. Using JPEG compression, the knee

occurs at about 8:1 compression. The most commonly used standard is Motion JPEG for which the knee occurs at about 15:1 compression. Consequently, M-JPEG reduces a 450Kb file to only 30Kb. While this is still too large to fit the same number of images as a video tape on to a hard disk it is small enough to permit, say, 2 frames per second to be recorded for 24 hours on to a 6Gb hard disk, which is a size generally available, costing a few hundred pounds.

Another more recent compression standard was devised by the Motion Picture Expert Group specifically for the digitisation of moving images. This standard is given the name MPEG. This standard makes use of the redundancy between adjacent frames.

MPEG-1 contains three types of encoded frames. Intracoded frames (I-frames) contain all of the video information required to make a complete picture. Predicted frames (P-frames) are generated by previous I-frames or P-frames and are used to generate future P-frames. Bi-directional Predicted frames (B-frames) are generated using both previous and future frames. A complete sequence of frames is made up of a series of these different frame types with more than one I-frame for every 10 P- or B-frames. This process is known as inter-frame correlation and allows compression ratios of 100:1 to be achieved.

MPEG-2 is the format used in the latest Digital Video Disk (DVD) technology, which can store about 90 minutes of VHS quality video and audio on to only 650Mb of storage space, such as a CD-ROM. However there are a number of disadvantages to MPEG compression. Firstly, in order for MPEG to achieve high compression it needs the video signal not to change abruptly from frame to frame. Since many video recording applications require multiplexing because more than one camera must be recorded, the rapid change from frame to frame as cameras are switched defeats the inter-frame correlation technique used in MPEG.

FORMAT KNEE WITH INTERFRAME

JPEG 4 –8:1 NOT AVAILABLE

M-JPEG 10 -15:1 NOT AVAILABLE

MPEG 10 –15:1 100:1

FRACTAL 20 –30:1 >100:1

WAVELET 30:1 >100:1

Comparison of compression formats

 

Recording devices

There is now a far greater range of recording devices available at easily affordable prices than before. This is a brief review of the main characteristics for each.

ANALOGUE DEVICES

Video Cassette Recorder (VCR)

This is not intended to praise or condemn the humble VCR, simply to include it in the list of available devices. To record 16 cameras over 24 hours will provide a picture update time of 5.12 seconds. The tape can then be removed and stored for as long as the Code of Practice requires. This is frequently 31 days but sometimes 90 days. With S-VHS resolution can be up to 500 lines, depending on multiplexers, cameras, lenses, transmission, etc. Rewind time for a E180 tape is in the order of three minutes, this would be the time to locate a scene at the opposite end of the tape. The stored signal is an analogue video signal and can be replayed on any other make of VCR. The information stored on tape is permanent and can normally only be deliberately wiped off. VCRs require regular, relatively expensive maintenance and frequent replacement of tapes.

Other analogue devices

There are other types of higher quality analogue video recorders such as U-Matic, but these are rarely used in CCTV systems, they are mainly the province of broadcast television.

DIGITAL DEVICES

As discussed earlier, there are many different formats of compression and analogue to digital conversion and different compressed file sizes. There are also many varying claims for the resolution produced for these combinations. A future article will attempt to compare these to a common base. For simplicity of comparisons, this article will be based on a final file size of 20Kb which is a compression ratio of about 30:1 for a colour picture. There will be 16 cameras with a picture update time of 5.12 seconds to compare with conventional recording.

Hard Disc Drive

HDDs are found in every computer and have evolved to be extremely reliable devices requiring virtually no maintenance. There are no touching components in a HDD although there are mechanical parts to rotate the disc and move the read/write head. Seek time is virtually instantaneous to retrieve a scene from any part of the disc by many search parameters. It is possible, although unlikely, to accidentally delete all the data from a hard disc. Current disc capacity is up to 36Gb, with 18Gb being readily available, this is likely to increase dramatically over the next few years.

The example would require 5.4Gb per 24 hours, a 36Gb disc would provide 6.67 days of continuous recording, (26M images). The options therefore would to accept an archive period of just less than 7 days or transfer the full disc to another removable medium for longer archiving. The medium could be another HDD or a DAT.

HDDs can be removable slot-in devices, therefore it would be practicable to remove a full disc and replace it with a blank pre-formatted disc to continue recording, just as is done with VCR tapes. One example of this is digital recording in trains where it is not practical to review incidents on the train. The hard disc is replaced with a blank disc and the original taken back to a central control for reviewing.

A major advantage of hard disc recording is that any part of the disc can be reviewed without interrupting the continuous recording.

Digital Audio Tape (DAT)

DAT drives are miniature audiocassettes incorporating magnetic tape similar to a VCR and can have capacities up to 50GB. (A 50GB tape costs in the order of £40.00). One common use of DAT drives would be to download from a HDD when it is full for archiving. As many tapes as necessary could be used to provide the total storage time required. Rewind time would be about 3 minutes for a full tape. Although search parameters may be similar to a HDD, the seek time could be comparable to a VCR. If involved searches are required, the DAT could be downloaded to a HDD for faster output. It should be noted that transfer rates of data can be quite slow, from 1 to 12 Mb/sec. At the best rate, transferring 50Mb could take over one hour or up to four hours at the slower rates. Similar comments apply to a DAT as to a VCR cassette; there is a thin magnetic tape being drawn across read/write heads. Again similar to VCRs, because the cassette is a fixed size, greater capacity is achieved by using thinner tape.

Digital Versatile Disc (DVD)

This used to be known as Digital Video Disc, but is now used for all types of data storage. It utilises the same principle as a CD in that indentations are burned on to the disc by a laser. The same laser reads these indentations. These drives are now readily available as read/write devices at modest prices and can be used exactly the same as a HDD. Capacities of discs are quoted as being 2.6 or 5.2Gb, the latter uses both sides. A 5.2Gb single sided disc will be available shortly. DVD drives can read a range of devises such as, CD-ROM, CD-RW, PD format, DVD-RAM and DVD-ROM. Beware though of quoted capacities because data protocols use quite a lot of space. A DVD formatted as FAT 16, which is the international standard for CD-ROMs, reduces the capacity to 2GB. Another format is used for long continuous files of video is known as UDF, in this case capacity is limited to 2.32GB. At the previously noted files sizes and number of cameras this would equate to nearly 9 hours of continuous recording, (11M images). This medium could be useful for downloading excerpts from a HDD drive for evidence or distribution. If formatted to FAT 16 it could be replayed on any PC. Most DVD drives include MPEG1 compression software so that recordings could be made directly from a composite or S VHS input and replayed on a PC with MPEG1 decoding. (Most have this). DVD discs cost about £13.00 for 2.6Gb or £19.00 for 5.2GB.

CD writable and re-writable discs (CD-R, CD-RW)

CD writer drives are now available for under £200.00, with CD-R discs less than £1.00 each. The capacity though is limited to 640MB with several caveats. This is nearly 3 hours (.3M images) of recording on the previous basis. CD-RW discs are written to ISO9660 standards so any CD device may read them. Now for the caveats, and these apply to your CD writer that you use for every day applications. The header information requires 27Mb, so this leaves only 627Mb for data. If you write several separate sessions, then 5 sessions needs 79Mb for headers leaving 561Mb for data, and 10 sessions leaves only 490Mb for data. Writing speed is up to 900Kb/sec, so 600Kb of data would be read in about 11 seconds. As with the DVD, this would be a inexpensive medium for transferring data.

Standards

As with most systems, there are no common standards for video data storage in the CCTV industry. The various systems on the market incorporate most of the types of compression mentioned earlier. Add to this many methods of encryption and watermarking and there are the makings of a massive problem of the use of digitally recorded video. Life was simple when we had VHS, there were even problems when S VHS was introduced and that was only two standards, although they are internationally agreed.

Conclusion

There is no doubt that digital recording is now a potent force in the CCTV armoury and will prove to be the most effective and efficient method of video recording and archiving. It is still a case of ‘caveat emptor’, be suspicious of the specification that states 8 video inputs and offers continuous 24 hour recording with an 8Gb hard disc. You will probably find that this is only for one camera with a 15Kb file size and 5 frames per second.

Domes: In a Spin?

Dome cameras offer great flexibility and user-appeal during the day, but don't work quite as well at night. Right? Wrong, says Shaun Cutler of Derwent Systems.He argues that with the right lighting - which is too often overlooked - domes can offer improved performance at night.

Fully functional dome cameras are increasingly popular, bringing customers significant benefits - they're more aesthetic, are less intimidating, they move faster and through more angles than pan/tilt/zoom (PTZ) cameras, and they are good for discreet security surveillance. Colour and Monochrome dome units are now common-place, giving the user full colour pictures during daylight hours, and a low light sensitive monochrome image at night. A potential disadvantage, however, is that domes don't come with an in-built and obvious lighting solution offering totally effective performance at night.

Infra-Red (IR) lamps cannot be fitted into the dome because of their size, and problems with overheating and internal reflection from the dome bubble. In contrast, the traditional PTZ camera

set up can have powerful IR lamps mounted on the sides to sweep round and light up whichever area the camera is focused on.

There are other factors which further reduce night time performance of some domes:

tinted dome covers reduce light pick-up by blocking out some available light combined cameras/lens combinations may not be as sensitive or effective as models

specifically designed for performance in low light many domes are based on smaller CCDs with integral lenses, which do not provide the

most sensitive low light performance

The result is that users have a system with great visual appeal and flexibility during the day, but provides reduced performance during the vulnerable hours of darkness.

However, the performance of many dome cameras at night can be significantly improved through the correct use of IR lighting.

Dome cameras can be divided into two categories: fixed domes and fully functional domes.

Fixed DomesExternal fixed domes are often vandal resistant and used for short-range surveillance purposes. Because the unit is fixed, the low light issues involved are similar to those of other standard cameras. In low light or zero lux conditions, fixed domes will require additional lighting. Typically, IR lighting provides the best, most practical and cost-effective solution.

 

Many dome systems are sold with smoked domes that can reduce the IR that reaches the camera by up to 70%. Bear this in mind when considering the camera's performance in low light. Increased IR illumination levels may be required to compensate for the dome's IR attenuation. To maximise low light performance it is best to use clear domes.

To ensure full coverage of the scene, the IR illumination must be matched to the field of view of the camera. Narrow beam illumination should be used to match narrow field of camera view, and wide beam illumination for a wide field of view. Failure to match camera field of view and IR beam can dramatically reduce system performance.

  Make sure that you match the camera's field of view with that of the Infra-Red.

Fully Functional DomesThe challenge with fully functional dome cameras lies in the inability to mount Infra-Red illuminators on the moving part of the dome.

360° wide area illumination360° IR coverage would ensure that wherever the camera moves there is sufficient IR illumination to enable the camera to view the scene effectively. But full 360° coverage would require location of approximately eight 45° spread Uniflood IR lamps. Domes mounted on the

corner of a building may only require 270° IR coverage, and domes mounted on walls may require only 180° IR coverage.

Specific target illuminationAn alternative solution can be achieved by using 'specific target' illumination. This is a method of providing illumination to specific areas of risk rather than the whole area being viewed by the dome camera. The technique IR illuminators are positioned strategically to illuminate targeted locations and vulnerable areas such as gates, doors or pathways. During the full 360° rotation of the dome camera, there may be only two or three specific targets that need to be viewed. IR units may be mounted on and around the camera pole to illuminate these targets, increasing the effectiveness of the camera's monitoring.

Local Area illuminationThe IR illumination may be located above or near a specific target. Good matching of the camera angle of view and that of the IR illumination is essential for maximum performance.

  Angle of illumination and camera field of view are critical to success.

For both specific target illumination and local area illumination the IR lamp may be triggered on/off via a pre-set on the dome. A PIR detector may also be used to activate the IR lamp, which is especially useful for discreet applications or to save energy and Bulb life, especially for high voltage IR lamps.

  Using dome pre-sets or PIRs to activate specific target or local area illumination will reduce running costs of the system.

Frame IntegrationSome dome cameras include frame integration techniques to attempt to overcome the problems of obtaining clear images in poor light. A slow shutter speed is used to capture enough light in dark areas of the scene. This may be acceptable only in a limited number of applications because of the inability of these systems to work with moving objects. If an intruder moves through these areas during the dome's tour the person will only be recorded as a blur and vital information and detail will be missed. The net result will be a large and potentially serious gap in the surveillance system's total coverage.

Infra-Red and Video Motion Detection (VMD)Speed domes are often integrated with computer-driven digital video recorders using a 'video follow' or Video Motion Detection (VMD) systems. The VMD works by actively analysing pixel changes occurring in the video picture. But a person walking through a dark scene is unlikely to cause any pixel changes if there is insufficient illumination to detect pixel changes, thereby defeating the system. Infra-Red illumination can dramatically increase the effectiveness of VMD systems & intelligent video analysis systems.

Summary Designers of CCTV systems need to consider some fundamental issues involved in achieving effective 24/7 pictures using dome cameras, particularly in low light conditions. Illuminating the field of view of the camera with sufficient IR lighting is essential. Three

successful ways of achieving this are 360° wide area illumination, Specific Target illumination and Local area illumination. IR is also a crucial element in ensuring the success of Video Motion Detection, Intelligent Video Analysis & other sophisticated software functions.

 

Software functions at risk of failureThe scenario goes something like this: an intruder enters your premises and is picked up by your dome camera CCTV system; the VMD software works successfully and triggers an alarm condition; a response is generated. A week later the same thing happens - except this time the VMD software doesn't activate the alarm.Why? The second event took place during the night.

Uneven, poor quality and inadequate lighting means the dome has failed to capture sufficient quantity and quality of video to enable the VMD to work.

Following tests conducted on dome cameras in Derwent's workshops we found that subjects moving through the field of view, under low and zero light conditions, could get past without triggering an alarm. In order to ensure a successful alarm trigger, the system requires good quality, consistent illumination across the scene.

A scene that may appear at first sight to the human eye to be reasonably illuminated will often cause dome cameras to fail under low light conditions, because the illumination may have been designed for human use(pedestrians, drivers etc) and not the CCTV camera. There will often be poorly lit, shadowy parts of the scene.

Any organisation using such a system must consider the night-time performance of the system as well as during the day.

It's not just VMD software that can be 'fooled' - the problem may well become more widespread as tools such as Intelligent Scene Analysis are adopted. Designers and end users must recognise the need for well designed illumination and specifically the benefits of Infra-Red.

With the dark nights looming, the question every security manager should be asking is 'will my system fail me during the night?'

Derwent has produced a Night-time Handbook that provides a good starting point to help understand some of the key generic issues.

Common misconceptions about the performance of individual parts of the system

Cameras

Light levels: A salesperson was heard to say recently, "that's a great camera it's got a good lux level and it's cheap." This is not an indictment of the salesperson it is a criticism of the level of training provided. It is also not untypical of the jargon used by people desperate to create an

impression of knowledge. The camera in question was in a distributors' catalogue and described as 'sensitivity .6 lux.

Resolution: 580 lines; 750(H) x 580(V); 435,000 pixels. What do they all mean? The most useful method of specifying resolution is by the number of lines in this case 580 horizontal. With the increasing number of CCD cameras it is becoming common to state the number of horizontal and vertical pixels, in this example 750 x 580. If this is the only value given it can easily be mistaken for resolution in lines and appears to be better than it is. The last value is the total number of pixels in the chip, impressive but no practical use. Apart that is from the salesperson who needs to impress,-- "their camera only has 580 lines, mine has 435,000 elements!"

An approximation to convert horizontal pixels to equivalent lines is TV LINES = PIXELS X 0.7. I.e. 750 pixels is approximately 525 lines.

Colour cameras generally have lower resolution than monochrome. Be careful not to be taken by the specification for some colour cameras that specify resolution as 450 lines Y/C. These can only be used in certain situations and the resolution can only be obtained when using a compatible monitor and associated control equipment. (See similar note under monitors).

Auto exposure control: Sometimes called electronic iris. This is a development in CCD cameras and electronically controls the amount of light reaching the CCD sensor. Several manufacturers claim that this eliminates the need for an automatic iris lens. Using manual iris lenses instead can make a significant saving on a system. If this type of camera is to be used from full daylight to dusk then use caution. Call the manufacturers' technical department, describe the application and go by their advice.

Sensor sizes: Early cameras had a circular tube as the sensor therefore the size was decided by the diameter of the tube, which is the diagonal measurement of the picture. This is still the case today so although CCD sensors are flat rectangular chips the nominal size is the diagonal measurement.

The sensor sizes shown in the diagram must be considered in relation to the lens selected. This is because lenses are also designed for a particular size of sensor. See further notes under lenses.

Note: It is a little known fact that the video output from a colour camera should be 1.2 volts peak-to-peak compared to 1.0 volts for a monochrome signal.

Lenses

Lens functions: The CCTV lens performs two main functions. First it determines the scene that will be shown on the monitor, this is a function of the focal length. Second it controls the amount of light reaching the sensor, this is a function of the iris. The focal length may be fixed-- or variable as on a zoom lens. The iris may be manual-- or automatic and controlled by the camera.

Lens mountings: All CCTV lenses are based on what is known as the 'C' mount. This is a photographic standard that specifies the thread type and dimensions. There is now a generation of lens mounts for CCTV known as the 'CS' mount. The first point to make is that the thread type and its dimensions are identical for both types of lenses. Therefore, either type of lens may be screwed to cameras having either type of mount without causing any damage. The wrong combination will be impossible to focus but there is no apparent mechanical indication to an installer that the wrong combination has been used.

The difference between the two types is the optical distance from the back of the mounting flange to the face of the sensor. This is known as the 'flange back length.' In the case of the 'CS' lens this distance is shorter. This allows the use of smaller glass elements in the make up of the lens and fewer elements to be needed. The result is a lens that is more compact and cheaper to manufacture. The differences are shown in the diagram.

The lenses are not totally interchangeable. A 'CS' lens may only be used on a camera with a 'CS' format mounting. A 'C' mount lens may be used on a 'CS' mount camera by adding a 5mm-adapter ring.

Lens sizes: It can be very confusing to establish the actual field of view that will be obtained from a combination of sensor size and lens specification. Lenses are specified as being designed

for a particular sensor size. A lens designed for one sensor size may be used on a smaller size but not the reverse. The reason is that the extremities of the scene will be outside the area of the sensor. Many people in the CCTV industry have grown up with the 2/3" camera as being the most popular and are familiar with the fields of view produced. However the 1/2" and 1/3" cameras are now being extensively used and therefore there are important factors that must be taken account.

The diagram shows the effect of using one lens on two different sizes of sensor. The result of using a larger lens format on a smaller camera format is to create the effect of a longer focal length, that is, a narrower angle of view.

To summarise then:

1. A lens designed for one format may be used on a smaller format camera but will produce a narrower angle of view.

2. A lens designed for one format may not be used on a larger format camera. 3. Assuming a focal length has been assessed based on a particular format of camera and

lens. It is then decided to use a smaller format camera. The same field of view will only be obtained if a shorter focal length lens is used.

4. Always check the angle of view for the particular lens and camera combination it is intended to use.

Monitors

Size: As with camera sensors the size of monitors is the diagonal measurement of the screen. The distance at which it is to be viewed generally decides the size of monitor. Typical figures in metres are:

9"-----0.3--0.6 12"----0.6--1.0 15"----1.0--1.3 17"----1.3--1.6

Another consideration is for viewing multiscreen displays. A 15" monitor is normally the minimum for viewing a quad display. For multiplex displays that can show up to sixteen pictures then a 17" is the minimum with a 21" preferred.

Resolution: With monochrome monitors resolution is not generally the limiting factor. As always specifications need interpretation for instance most monitor resolution figures are given as number lines at the centre. A figure of 600 lines at the centre may only be 400 lines at the edges. The difference is likely to be greater the larger the monitor for two reasons. First the problem of maintaining accuracy of the scanning magnets over a larger area. Second, it is more difficult to produce larger monitors with as fine a coating as smaller monitors. This is why the picture on a 9" monitor always looks sharper than when seen on a 17" monitor.

The resolution of colour monitors is less than can be obtained with monochrome. This is because three spots are needed to make each point-- red, green and blue. Typical resolutions for colour monitors are from 280 lines to 350 lines. There is the same fall off towards the edges of the screen as with monochrome monitors. Some colour monitors are specified as 450 TVL Y/C. Take care because these monitors only produce this resolution when using cameras and control equipment that produce separate chrominance and luminance signals.

Switchers

A simple video switcher is designed to direct the signal from one of a series of cameras to a monitor. Most switchers have a control to enable the monitor to sequence automatically through each camera in turn. The time between each sequence is generally adjustable. A switcher should be selected that incorporates what is known as 'vertical interval' switching. This delays the actual moment of switching until the blanking period of the sync pulse in the composite video signal. The result of this technique is to prevent picture 'bounce' between successive pictures. One picture simply replaces the previous one without any rolling caused by waiting for the next sync pulse. Some switchers can provide output to two monitors. One monitor can be locked on to a specific camera while the other sequences.

Matrix switchers are now becoming common place in the market due to the development of microprocessor technology. This type of switcher can process the signals from a large number of cameras to many monitors. There can also be many control positions, each of which can call up any of the cameras. In a railway system for instance it is possible to have two hundred stations each with twenty cameras. Each station would have individual control of its own cameras to sequence or select. All the stations would be connected back to a central location that could control all four thousand cameras. The central control could then be divided into say ten regions each with a control and bank of monitors for its own group of cameras.

Recording

Recording of CCTV cameras has had a fairly mixed press over the last few years. Obviously the failures to identify culprits and lousy pictures seen on programs like Crimewatch have not

helped. There are though hundreds of systems incorporating video recording that have paid for themselves time and time over. The most successful systems are obviously in conditions of good consistent lighting using good quality colour cameras and a well-maintained video recorder. This then is the key to successful video recording, the right conditions, the correct equipment and proper maintenance of the system.

Until recently, one problem has been the limitation of a video recorder to provide only 240 lines resolution. This is a function of the maximum bandwidth that can be used on the standard width VHS tape. It does not really matter what quality of camera and monitor is used. The limiting resolution is that of the recorder. That is why the superb pictures seen on the screen during commissioning and handover of a system are not reproduced when an incident occurs. Reusing the same tape repeatedly frequently aggravates this. Also, by lack of maintenance on the recorder. The problems of poor video recordings could be dramatically reduced if customers insisted that the manufacturers' recommendations for maintenance and limits of tape use were strictly followed.

S-VHS: (Super VHS) There is a new generation of video recorders using the S-VHS system. This provides greater resolution of up to 500 lines but only in colour. In a composite video signal there are two elements that make up the colour information. They are known as the chrominance (C) and the luminance (Y). The chrominance is specific to colour signals and determines the colour content of the picture. The luminance is used in both monochrome and colour signals and determines the brightness. In S-VHS colour signals these two components are separated at the camera and transmitted as separate signals. Therefore point one- the camera has to provide separate Y/C components. This requires two coaxial cables to be run from each camera. The video recorder must be able to accept the separate Y/C signals as also does the monitor. The use of this improved quality is limited because at present there are no switchers or multiplexers that can pass the Y/C components. Note that there is no improvement with a monochrome signal.

Digital Recording

There has been much written about digital recording recently and is not within the scope of this article to reiterate all the advantages and loopholes in specifying this type of recording. However, there are still tens of thousands of analogue recorders still in use and thousands still being installed. Analogue recorders are still the main recording medium for many small installations.

Weatherproof Housings

Weatherproof housings must be about the most mundane aspect of a CCTV installation. Or so it seems because many engineers simply consider the housing as a protection against the elements. However there are many aspects to consider and many suppliers of housings. It is about the cheapest element of an external system yet price seems to be the main factor in selecting which to use. Important considerations should be:

Ease of access for pre-assembly in workshop.

Ease of access during installation. Ease of access for future service needs. Is the camera mounting plate insulated from the case? Can the mechanical focusing screws on the camera be accessed? Some are at the back,

some at the side and some on top. Can the lens be focused and the peak/average settings adjusted on site? Can one man remove the cover and work on the inside? If there is a telemetry board fitted can it be accessed without removing the camera?

The most common type of housing is that where the camera is mounted on a flat bed. A rectangular box shaped cover drops down over the complete assembly and is held in place by four spring clips. This is great for assembly in the workshop because everything is nicely accessible. The problem comes when a service engineer at the top of a ladder needs to work on it. Many engineers know that it needs four hands to hold the clips clear and two to remove the cover. Especially if it has been on for some time and the cover is welded to the rubber seal. Once the cover is off everything is exposed to the elements and it is no quick job to replace it.

Another type that was in vogue a few years ago and still around is the extruded aluminium design. The housing is a complete box and the camera plate slides out of the back in guides. The cable glands are usually in the rear sealing plate therefore sufficient slack must be allowed for the length of the plate to be slid all the way out. At this point the engineer is faced with the whole assembly and cable in his hands. It now needs two hands to hold it and two more to work on it. With the camera and lens on a loose platform and the cables hanging down it is really fun to focus the camera and set up the lens adjustments.

There is a variation on the first type mentioned but instead of using clips there is a pivot at the front. There are clips at the rear and when released the cover swings right up and forward. This exposes the complete interior and a stay rod holds up the cover. It's just like opening a rear-pivoting bonnet of a car. In another design there is a simple gas strut to hold the cover open. This needs one hand to open the cover and leaves two hands to work on the inside.

Another design is a box like housing with two latches on each side. When the latches on one side are released, the cover pivots open on the opposite side latches. The cover may be opened in either direction. If all four latches are released, the cover can be completely removed.

There are other designs around that may be just as engineer friendly. It is worth spending a little time on this often overlooked item to make future servicing easier and cheaper.

Finally there is the ubiquitous dome. There has been a proliferation of dome variants introduced recently probably more than any other development. In addition they are becoming ever smaller and faster. But they are not necessarily the panacea for all PTZs. The main fallibility of dome housings is in the material and manufacturer of the actual dome itself. Poorly moulded domes can lead to disastrous loss of focus, particularly at long focal lengths. It is always advisable to arrange a test and see the results for the longest focal lengths and distances. If you intend to purchase a dome and fit your own camera/lens make sure that there is sufficient clearance between the lens and the inside of the dome to allow focussing at long focal lengths.

Most domes now allow for quick release fixing of the pan tilt mechanism and plug/socket for the telemetry and video. This can be especially important when fitted at the top of a pole in a high street.

Understanding Fixed Dome Cameras

Fixed domes provide a popular means of monitoring a specific area in a more discreet manner than a traditional camera. Housed inside a plastic casing, usually with a smoked, gold or silver finish, they offer protection from tampering whilst preventing observers from detecting what area of surveillance the dome is covering.

Complimentary to any CCTV installation, these cameras can be supplied in a range of formats and specifications dependant on the requirement, including monochrome and Vandal Resistant.

Due to the increase in demand for ruggedised products in particular applications, some fixed domes have been developed to offer vandal resistance. These vary in strength, from protection against the impact of heavy hammers to bullet proofing.

Resolving the Problem of Focus Shift

Focus Shift is the condition that occurs when images that are sharp and in focus under artificial lighting (such as external cameras illuminated with IR lighting at night) are out of focus (appear soft or blurred) in daylight conditions and vice versa. The problem is caused by the nature of light. Different types of light have different wavelengths which means that an image viewed in different light conditions will appear slightly differently. Unless the lens is adjusted for different light conditions, it is impossible for a standard lens to produce a sharply focused image in all types of light.

To overcome this lenses either have to be adjusted manually for day and night time performance (impractical) or, if the camera is fitted with a remote controlled motor zoom lens, the picture may easily be brought back into focus (not possible with static lenses, of course, or on an unmanned site). A third option is to use the more expensive IR corrected lenses.

Ernitec A/S is a leading manufacturer of IR corrected lenses. Niels-Christian Andreasen, Lens Product Manager, is understandably keen on IR corrected lenses but believes they should be adopted more widely than just for day/night colour/monochrome environments:

"IR-corrected lenses should always be used and not only when using IR-illumination at night time. Many light sources include a part of IR-light, small or light. In connection with monochrome- or day/night cameras IR-corrected lenses will provide a sharper picture because all the light is focused, resulting in a far crisper picture compared to ordinary lenses. Sunlight contains much IR-light, but also many artificial light sources, especially halogen. Also, ordinary incandescent light bulbs include a considerable amount of IR-light. When having illumination with a mixture of visible and infrared light, it is possible to obtain a considerable improvement of the picture reproduction without having to replace the camera - just by replacing the lens. So there are excellent reasons for using IR corrected lenses in all environments.

"It is not possible to modify standard lenses to make them IR-corrected, e.g. just by coating the lens elements - at least not with optimum results. From the early stages of the design phase IR-correction must be considered and included in the design; special glass must be used for the lens elements and special coating of the lens element surfaces is required. Previously, this caused IR-corrected to be fairly expensive, but new efficient production technologies have reduced costs dramatically."

IR illumination manufacturer Derwent is pleased to see the increased use and market awareness of IR corrected lenses. Shaun Cutler, Derwent's Marketing Director, highlighted that 'the recent increased growth in day and night cameras has contributed towards the demand for IR corrected lenses and naturally, IR illumination. The industry is accepting that colour cameras can not perform as effectively at night as monochrome camera supported by IR illumination. Lens IR sensisitivity and Focus Shift are a real issue and it is pleasing to see that many of the leading lens manufactures such as Computar, Pentax, Fujion and Ernitec are responding to this.'

The Manufacturer Extreme CCTV believes that whilst single ccd day and night cameras can provide a good 24hour solution, their performance can not be compared to a twin camera solution. Extreme argue that a single camera solution can only provide a compromise between a perfect daytime picture and a perfect night-time picture (this is more evident with fixed filter models).

Extreme's engineers claim that the perfect, no compromise picture day and night picture can only truly be achieved with two separate cameras and two separate lenses integrated with IR illumination - in other words, two cameras in a single package.

Mark Vernon of Extreme CCTV commented: "This is not just a focus shift issue; it's also about accurate colour rendition, achieving the best picture in day and night conditions. During the day cameras must accommodate extremes of bright light yet at night gather as much light as possible. As a result it will always be a compromise trying to get the best picture 24 hrs a day from a single camera. With the dual camera each lens is optimised to get the best day and night image with the result that there is no focus shift, no colour rendition and very high IR sensitivity thereby matching the IR illumination to the camera field of view. When light levels drop the unit shifts to a monochrome camera which is then maximised for low light sensitivity with a high f-stop lens to gather as much light as possible and is already pre-focused for when the IR illumination switches on."

Guidelines for identification

Some time ago the Home Office issued guidelines for the identification of persons and vehicles. This is fine, but many system engineers stumble when trying to find what camera and lens combination will satisfy these guidelines. And what about end users who know even less about camera and lens formats, how can they assess the merits of competing specifications? This month all will be revealed for both groups.

Charts showing the horizontal and vertical fields of view for many lenses and four formats are given in ‘The Principles and Practice of CCTV’ and were published in the first issue of CCTV

Today (Jan ‘94). They also show the % of the screen height of a 1.7M person. The Home Office guidelines had not been published for general use when I produced the first draft of the book and so the 1.7M was my guess at the average height. The current guidelines use 1.6M as the height.

The values for various degrees of identification are given as the percentage the 1.6M figure would occupy of the monitor screen. I call this the ‘screen height ratio’. The complete guidelines are provided in several Home Office publications and so only the basic ratios are given in this article. The publications are available free from the Home Office and provide a lot more information as well.

These criteria are now becoming increasingly used as part of the specification for many CCTV systems, particularly in Town Centre schemes. Sometimes the specification will state the distance from the camera for each criterion, sometimes the specification will ask the question, ‘at what distances from the camera will the criteria apply’? In either case it involves calculations that are not too difficult but can be tedious to keep repeating for each lens and camera location.

Another problem that many people find difficulty in resolving are the different fields of view obtained from various camera and lenses formats, i.e. what is the result of fitting a 2/3" lens onto a 1/2" camera, and how does this affect the screen height ratio at certain distances?

A word of caution, just about all lens manufacturers brochures give the HORIZONTAL angle of view, whereas these calculations require the VERTICAL angles of view. The vertical angle of view is the horizontal angle times 3/4.

Field of view

The field of view is the ratio of the sensor size to the focal length and the distance to the subject. This is shown in diagram1. The 'width to height' ratio of the sensor is 4:3. The horizontal and vertical angles and therefore fields of view are different and must be considered separately.

Diagram 1 Field Of View

Note when using these ratios all the units must be the same, i.e. millimetres or Metres.

Sensor Sizes

Diagram 2 shows the sensor sizes to be used when calculating fields of view and angles of view.

Diagram 2 Sensor Dimensions

Example

Supposing it required to recognise a known person at 50M, using a 2/3" lens, the following is the calculation.

 

The scene height at 50M needs to be twice the standard height, 2 x 1.6=3.2M. Therefore:

.

The nearest standard would be a 10.5:105mm zoom lens to satisfy this requirement.

The formula can be worked backwards to find the scene height for a given lens. It is a simple matter to put all these criteria into a spreadsheet program and find the result for any combination. However, this may not be very convenient for the many salespersons on the road.

 

Do it the easy way

The following table summarises the main three criteria for the Home Office guidelines, 10% to ‘see’ a person, 50% to recognise a known person and 120% to identify an unknown person.

 

The focal lengths of lenses for four camera formats is listed in the first four columns. The next column lists the vertical angle of view for each. The remaining columns list the percentage of the screen a 1.6M target would be at various distances.

Using the table

1. Select a lens focal length under the camera format column and look along that line to find the % of the screen height for different distances.

 

For instance, a 50mm lens on a 2/3" camera would produce a ratio of 9.7% at 125M, 60.6% at 20M and 121.1% at 10M.. The scale across is linear so intermediate distances can be interpolated. In this example the 50% criteria would be met at 25M. (between 20 & 30).

 

On the other hand a 50mm lens on a 1/2" camera would produce ratios of 11.2% at 150M, 56% at 30M and 120% at about 16M (interpolated).

 

2. For convenience the table has been shaded into yellow, blue and grey areas. Any lens/distance within yellow area complies with the 10% rule, in the blue band to the 50% rule and in the grey area to the 120% rule.

 

Note that it is the FOCAL LENGTH of the lens that is significant not the lens format. The lens format is significant in that a larger format lens can be used on a smaller format camera but not the reverse

 

3. The table can also be used to solve the knotty problem of what is the equivalent of a lens on one format to that on another. Simple, look across the focal lengths under the sensor sizes and the equivalent focal lengths is shown for each format. I.e. an 8mm lens on a 2/3" format will have the same angle of view as a 5.8mm on a 1/2" or a 4.4mm on a 1/3" camera.

4.

Note that the angles of view are calculated from the sensor sizes, The actual angle may vary between manufacturers but will only have a marginal effect on the results from the table. If a precise value is required then the vertical angle of view must be taken from manufacturers data and not calculated from the sensor size.

Understanding Hard Disk Recorders

The late 1980's saw the introduction of video multiplexers offering users the facility to record pictures on to a single video tape, eliminating the need for VCRs dedicated to recording a single camera output.

The use of time-lapse VCRs as a storage medium for those images is well known, as are their inevitable drawbacks - introduction of noise, wear and tear and the simple requirement that the tape needs to be rewound to access information. In a practical situation the reviewing of tapes to secure the important face shot" or "scene of crime" can involve long and tedious work.

The late 1990's have seen the emergence of Hard Disk Recorders (HDRs) that are essentially multiplexers with a computer hard disk memory to store images. HDRs are excellent at reproducing high quality images with little noise or picture degradation and are extremely useful in calling up an alarmed picture.

A problem that HDRs faced, however, was that computer memory was still relatively expensive compared with a storage medium such as videotape.

The struggle for many HDR manufacturers was to produce a machine that provided the features and performance required with sufficient memory to make it a practical machine at a realistic price. Now, with computer memory being available in hundreds of Gb at a relatively low cost, Hard Disc Recorders have finally come of age. The advantages over VCR's are many. HDRs are able to record in VHS mode (the same quality as a standard VCR), SVHS mode (the standard used by the highest quality VCRs and giving over 60% better resolution than standard VHS) and SVHS+ (not available with VCRs).

Further to the above a VCR and it's tapes begin to wear and deteriorate from the moment they begins recording while Hard Disk recording should remain at the same high standard throughout it's working life. This means that even in VCR mode the quality will, in most cases, be superior to what would be achieved on a standard Video Recorder.

In addition to the above HDRs offer a number of additional features not available with Multiplexers.

1. The ability to view and control the system from computers around the world. 2. Interconnectivity to Computer networks. 3. Built in Motion Detection for setting alarm events and immediate retrieval. 4. The ability to go direct to a time or incident without the need to search through hours of

videotape.

Next-Generation Digital Video and Audio Recording

Also available now is the next-generation digital video and audio recording solution, this is designed for casinos and other high-security, real-time recording applications.

Highlights

Up to 30 frames per second NTSC/25 fps PAL, per camera - high frame rates enable users to extract the most information from their digital images

Up to 96 camera inputs per unit - saves space, requires fewer components, and increases reliability

Improved picture quality - high recording resolution provides superior definition and clarity allowing for optimum event monitoring and analysis

Enhanced integration capabilities - 32bit API enables seamless integration with existing security systems thus positively impacting the profitability of the business.

Understanding Housings

Housings, in essence are casings used to protect Cameras from a variety of conditions, dependent on the environment in which they are mounted.

At first sight, most Camera Housings may seem similar. In practise to ensure the optimum appearance and performance of appropriate for a Camera installation a number of factors have to be taken into consideration:

1. Location. 2. Risk of vandalism. 3. The total load weight of the housing and constituent

elements (including Camera, Lens and any other equipment encased within, the hanging bracket and fixing surface)

4. The housing chosen has sufficient physical space for the Camera, Lens (which may have to be changed at a later stage), electrical wiring and enough room to make the connections and allow for future maintenance.

5. Try to aluminium or rust proof products. Steel is more vulnerable to the elements and will rust in time!

6. Housings should only be mounted onto load bearing points. 7. It is recommended that the top five or six levels of brick work on buildings it is avoided

when mounting a Camera housing.

Environmental conditions are also a primary consideration in selecting an appropriate housing for a camera system, but often one which is not given the due attention. As a result, a housing may not give the level of protection required in its specific application - wasting time, money and effort. If a camera is to be mounted externally in a coastal location, for example, the housing will require a marine finish to protect against the damaging effects of salt which can induce premature corrosion.

Climatic effects also need to be considered. Rising and falling temperatures can dramatically effect the workings of electrical equipment and as a result requires pro-active consideration. In hot conditions, the severity of the sun may require the use of air blowers and sun shields to maintain the temperature of the camera at an optimum level and ensure clear viewing. Conversely, in cold conditions, it may be that a camera requires a heater and thermostat built into

the housing. In rainy conditions, wipers may be required to keep the housing glass clear to maintain the cameras viewing quality.

The standard for the degree of protection that housing afford to its contents is defined by the IP system (see following charts)

1st IP Digit Degree of Protection Definition

0 Zero protection No special protection

1 Protection against solids greater than 50mm

Large surface of human body such as a hand, none against deliberate access

2 Protection against solids greater than 12mm Fingers or similar objects not exceeding 80mm in length

3 Protection against solids greater than 2.5mm

Tools/wires etc of thickness or diameter greater than 2.5mm

4 Protection against solids greater than 1.0mm Wires or strips greater than 1.0mm

5 Dust protected Total protection against dust is not provided but sufficient protection to allow satisfactory operation

6 Dust tight No ingress of dust 2nd IP Digit Degree of Protection Definition

0 Zero protection No special protection

1 Protection against dripping water Vertically dripping water shall have no effect

2 Protection against dripping water when tilted up to 15°

Vertically dripping water shall have no harmful effect, when enclosure is titled at any angle up to 15°

3 Protection against water spray Water falling as a spray at an angle up to 60° from vertical shall have no harmful effect

4 Protection against splashing water

Water splashed against the enclosure from any angle shall have no harmful effect

5 Protection against water lets Water protected by a nozzle against the enclosure from any angle shall have no harmful effect

6 Protection against heavy seas Water from heavy seas or water projected by powerful jets shall not enter the enclosures in harmful quantities

Understanding Infra Red Lamps

The range that your camera will see in the dark will depend on the sensitivity and spectral response of the camera and lens combination. Some cameras have a better IR performance than others. For maximum performance choose IR sensitive cameras.

The human eye cannot see infra-red light, however most mono CCTV cameras can. As such the invisible light can be used to illuminate a scene, this allows night time surveillance without the

need for additional artificial lighting. Infra-red also provides many other benefits above conventional lighting. The infra-red beam shape can be designed to optimise CCTV camera performance, as such it is important to remember to design illumination for the CCTV camera and scheme.

Infra-red lamps cannot work with colour cameras. Normal artificial light e.g. sodium light, causes problems to the quality of the picture, not producing accurate colour quality. There are two options: It is possible to use a mirrored shift filter lamp that produces good colour rendition with a good quality low-light colour camera or to use a dual technology camera (colour by day, monochrome at night) together with IR lamps.

Bulb life is dependant on filament ruggedness, design and power management and control. Standard IR lamps claim average life between 2,000 to 5,000 hours. Choose lamps with long average bulb life to reduce maintenance costs. For short-range, low power applications consider LED products with a greater than 5-year life.

Infra-red lamps come in varying wavelengths from approx. 730nm to 950nm. 730nm filters are overt and give a red glow - like a traffic light. 830nm filters are semi-discreet and produce a dull red glow. 950nm filters are effectively totally covert - giving off no visible illumination. Viewing distances are reduced with 830nm and 950nm lamps. A 950nm will require a highly sensitive night time camera.

Infrared Illumination, HOW FAR DOES YOUR LIGHT SHINE?

I wanted to know a lot more about the black art (or should it be red art?) of infrared illumination and how to know what results can be expected at different ranges and angles of lenses. I wanted practical answers as well as the theory, so I approached Trevor Duffy, Marketing Director of Derwent Systems to see what he could tell me. The reason being that Derwent are the acknowledged leaders in the field of infrared illumination and have probably carried out more research on this subject than any other company. The result was that he and Brent Midgley, their Technical Director, gave me a full day seminar on the subject which of course included their unique approach to the subject of infrared illumination. Combining this with data gathered for The Principles and Practice of CCTV has produced the most comprehensive analysis of the subject ever published.

The main problem in designing a CCTV system incorporating infrared illumination is that infrared light cannot be measured in lux. So how do you know how to specify camera sensitivity and how do you know what will be seen at various distances? Other questions to be answered are how sensitive are different cameras to infrared light, how are spectral diagrams to be interpreted and the effect of the lens.

To understand the answers to these and many more questions requires a basic understanding of three main aspects of physics. One is the wavelengths of light, another is the inverse square law of illumination and finally the transmission of light through a lens.

Infrared illumination is used to provide light over scenes that would otherwise be too dark for a camera to create an image. It is a compromise because the best results can only be obtained by providing sufficient white light but of course this is not always possible. In many cases using powerful floodlights would cause a considerable nuisance and could be dangerous where there is road traffic moving towards the lights. It is also difficult to cover a large area when a pan tilt camera is being used, in this case the illumination is only required where the camera is directed and infrared lights provide the answer.

principles of light.

Electromagnetic Radiation

Light is energy in the form of electromagnetic radiation. The different forms of electromagnetic radiation all share the same properties of transmission although they behave quite differently when they interact with matter. Electromagnetic radiation is measured in nanometres which is the wavelength. One Metre is 1,000,000,000 Nanometres (nm). The frequency is the speed of light divided by the wavelength, i.e. a wavelength of 830 nm is 3.6 x 1014 Hz.

Diagram 1

Light is that part of the electromagnetic spectrum that can be detected by the human eye. This is a very narrow band within the total spectrum as shown in diagram 1.

Electromagnetic Waves

The Transmission of light energy can be conveniently described as a wave motion and having the following properties:

Electromagnetic waves require no medium and therefore can travel in a vacuum.

Different types of electromagnetic radiation have different wavelengths or frequencies. From radio waves through visible light to gamma rays.

All electromagnetic waves travel at the same velocity, which is approximately 300,000,000 Metres per second in a vacuum, the speed of light..

The waves travel in a straight line but can be affected by:

Reflectance. Which is the reversal of direction that occurs at the surface of an object i.e. a mirror.

Refraction. A change of the angle that occurs at the boundaries of different surfaces. Different wavelengths have different angles of refraction i.e. a stick apparently bending in the water.

Diffraction. Which is a deflection that occurs at apertures or edges of objects.

Visible Radiation

These are the wavelengths of light that are visible to the human eye and are from approximately 380 nm to 760 nm. When all these wavelengths are seen simultaneously the eye cannot distinguish the individual wavelengths and the result is seen as white light. Therefore, white light is not one wavelength but a combination of them all. This effect can be demonstrated in reverse by passing white light through a prism. As stated previously, different wavelengths have different angles of refraction, therefore when the light is passed through a prism it is dispersed into its constituent spectra because each wavelength is refracted differently. The result is that if a white screen is placed to show the light passing out of the other side of the prism it will show all the individual colours. This effect is shown in diagram 2. The result is to show the spectrum of light and the seven significant colours of the rainbow. In reality there is a continuous range of hues but the eye sees mainly the main colours. A real rainbow is created in the same way by the light being reflected and refracted by droplets of moisture in the atmosphere. (Remember from school days "Richard Of York Gained Battles In Vain").

 

Diagram 2

 

Infrared light is considered to be wavelengths Longer than 715 nm. The range of wavelengths that the human eye can see is compared with the wavelengths of two main types of infrared light used in CCTV is illustrated in the diagram 3.

 

Diagram 3

Measurement of light energy

As stated light is a form of electromagnetic radiation, its power is measured in Watts and its intensity measured in Watts per square Metre (W/M2). This goes for all wavelengths. The visible spectrum is, however, normally measured in lumens for power and intensity in lux. The lumen is related to perceived power or brightness and because of this, the relationship between lumens and Watts is dependant on wavelength. Lumen values diminish virtually to zero at infrared wavelengths,. This is why it is not possible to express infrared radiation in terms of lux values.

In order to measure light radiation in terms of Watts it is necessary to use a radiometer which normally only exists in the sacred domain of laboratories and needless to say tend to be very expensive and beyond the means of normal installation companies.

Camera sensitivity to infrared light.

The sensor in a CCD camera is composed of thousands of tiny photo-sensitive diodes, a camera with a resolution of 570 lines incorporates over 400,000 such diodes. There are two main materials used in the production of sensors, germanium and silicon. These have very different responses to wavelengths of light as shown in diagram 4.

 

Diagram 4

Therefore for CCTV applications the material used in the chip has a great bearing on the response to either visible of infrared light. The development of sensors for CCTV cameras is driven by the vast Camcorder market and Camcorders are only used in the visible part of the spectrum. Therefore any response outside the visible is wasted and reduces the overall sensitivity of the sensor. An enormous amount of money has been expended by sensor manufacturers to develop a sensor that accurately follows the spectral range of the human eye, this is known as a photopic curve. Such a sensor has by definition a low response in the infrared part of the and therefore cannot be classified as infrared cameras. This is an important point for CCTV designers to consider because with new cameras arriving on the market every month it is essential to keep checking on their suitability for infrared illumination. Some manufacturers are not very helpful in this respect and often optimistic. Beware of such comments as ‘suitable for I/R’, always ask for a spectral response diagram.

Incidentally, all camera sensors are monochrome, colour is obtained by inserting red, green and blue filters in front. This is why colour cameras have less resolution than monochrome cameras. Also due to the filters, colour cameras are not sensitive to infrared light. Therefore all the discussion on camera sensitivity and suitability for infrared illumination is confined to monochrome cameras. This is except for the Dual Mode cameras now becoming available which potentially offer the best of both worlds. This however is evidence of the fact that colour cameras cannot be as sensitive as monochrome and the industry is finally recognising the fact.

Camera spectral response

Cameras have different responses to the spectrum of light. This is usually shown in diagrammatic form and is known as the relative spectral sensitivity. Diagram 5 shows the relative response to each part of the spectrum and is usually in a range from 0 to 1.0. Curves from two cameras are shown in the following diagram. It can be seen that camera A covers the visible part of the spectrum very effectively whereas camera B is sensitive far into the infrared. It could be thought that camera B would suit all requirements because of the wide range of wavelengths covered, but not so.. A Further point is that using an infrared sensitive camera in daylight can produce different ranges of grey tones because they see a higher content of infrared that the eye cannot see. Also the infrared sensitive camera can cause the automatic iris to close due to the amount of infrared light instead of visible light. This is particularly noticeable if there

is foliage in the scene, chlorophyll reflects at about 715 nm and often appears bright white instead a shade of grey. In practice the sensors sensitive to extended infrared light are more sensitive than photopic sensors.

 

Diagram 5

A high power infrared lamp generally uses a tungsten halogen bulb which due to its operating temperature emits a high proportion if light in the infrared part of the spectrum as well as covering the visible spectrum. (Some longer life bulbs now use quartz halogen). In front of the light source is a filter that blocks wavelengths up to the required infrared part of the spectrum. There are various filters that allow different wavelengths to pass, the most common passes light at 715 nm and above. This wavelength is just at the threshold of visible light and shows as a dull red glow. At 780 nm the light is considerably reduced and at 830 it is almost invisible to the human eye.

If optimum performance is to be achieved it is necessary to match the camera to the light source. Reference to diagrams 6 and 7 it can be seen that the important part of the graph is where the camera response overlaps the infrared filter response. The camera sensor effectively integrates all the wavelengths of light falling on it within the response curve. Therefore the measure of the cameras’ sensitivity to infrared radiation is the AREA UNDER THE OVERLAPPING CURVES, not just at say 715 or 830 nm. This is why it is essential to know the shape of both the camera and the infrared filter response curves. It is not the height or amplitude of the curve but the area enclosed by the camera curve and the light source curve that determines sensitivity.

Diagrams 6 and 7 superimpose typical infrared light performance curves over the camera curves in the previous diagram. The resulting areas under the curves indicate the reduced sensitivity even with the camera that does have response up to 900 nm.

Diagram 8 shows the area under the curve for an infrared sensitive camera used only in daylight.

Diagram 6 Diagram 7

Diagram 8

The normally undetected radiation 2x the area under the curve, therefore all other things being equal the sensor will be twice as sensitivity.

Light and illumination

Only natural light provides absolutely even illumination, although it is of course affected by clouds and shadows. All forms of artificial light suffer from the fact that as the distance increases from the light source so the illuminance reduces. This is due to the inverse square law of illumination where the illuminance falls to a quarter of its value if the distance is doubled..

Inverse Square Law Of Illumination

As the luminous flux travels away from the light source the area over which it spreads increases, therefore the illuminance (lux) must decrease. The relationship is expressed by the inverse square law and illustrated in diagram 9

Diagram 9 Inverse Square Law Of Illumination

The relationship between illuminance and it's effect at a distance is given by:

This factor is particularly important in considering the light available for a camera. For instance a light source providing a level of 30 lux at 20 Metres will provide 7.5 lux at 40 Metres and only 3.3 lux at 60 Metres. The other effect of this is that the wide range of light levels can cause problems with automatic iris lenses. Unless set up correctly, the foreground light will cause the iris to close and lose definition in the distance. The reverse is if the iris is set to the distant light level in which case there will be a lot of flare in the foreground.

Diagram 10

Diagram 10 illustrates the actual light levels over 100 Metres, the ratio from 100 M to 10M is 100:1. This illustration is for a white light because infrared light cannot be measured in lux. The effect of this law though, affects infrared light in exactly the same way. The effects of this when using infrared illumination on pan tilt zoom units can be catastrophic. If it is required to zoom in to distant subjects the power must be sufficient for the distance. The problem is that when focused on near subjects the picture is frequently flared out.

Cosine Law Of Illumination

Another factor that affects the light level of an area is if the light is striking the surface at an angle. This is often the case in CCTV systems when an infrared lamp is located well above ground level. Without going into too much theory the principle is shown in diagram 12.7.

Diagram11 Cosine Law Of Illumination

Diagram 11 shows the effect of light striking a diffuse surface at an angle. The effective area of the surface is reduced proportionally to the cosine of the incident light. Avoiding too much mathematical complexity, this means that the reflected light from a diffuse target will be approximately proportional to the cosine of the striking angle and the specific reflectance of target or scene. This is only true for a diffuse reflector. If a very highly reflective surface (specula) is encountered, virtually no energy will be reflected back along the incident light path to the camera. This is why a mirror may appears black when viewed at an angle.

Diagram 12 Adjustment For Angle Of Beam And Lamp

The maximum illumination is at the centre of the beam of light. This falls off towards the extremities. As can be seen from diagram 12 there is a greater reduction of light at the farthest distance. The cosine law formula can be modified to approximate the light levels at the outside ranges of a lamp. It is necessary to take account of both the angle of the beam and the angle at which the lamp is directed. The total angle at the farthest point will be the angle of the lamp from the vertical plus half the included beam angle.

For example a 300-watt lamp gives 20 lux at a distance of 10 Metres, has a beam angle of 60o and is mounted at 30o. The light level at the farthest point will then be approximately 10 lux.

Uniform light distribution

The Derwent lamp utilises design principles borrowed from radar antennas to achieve even levels of illumination for targets at short and long range and high and low altitude.

As has been seen from the previous text, for targets at say, 100 Metres it is necessary to radiate 100 times the amount of energy needed to radiate a target at 10 Metres. This is necessary to cancel out the effects of the inverse square law.

The Derwent system is designed to provide equal illumination for short and long range targets. Supposing, for instance, a lamp will radiate targets at 100 Metres and 10 Metres with an illumination level of 60 mW/M2. This is illustrated in diagram 13 and is more than sufficient illumination to provide 0.7 volts of video from an average photopic CCD camera using an average f1.4 lens.

Diagram 13

The design concept is that a single lamp will provide the equivalent of five lux evenly distributed over a scene from 10 to 100 Metres. If twin lamps are used the equivalent light distribution will be ten lux. The inverse square law may be applied to obtain the illuminance at other distances. Therefore at two hundred Metres a single lamp will provide 1.25 lux equivalent, i.e. a quarter of the light for twice the distance.

The beam power transmitted by the lamp is actually 1,000 milliwatts to provide 10 milliwatts per square Metre at 100 Metres and 10 milliwatts to provide 10 milliwatts per square Metre at 10 Metres. The structure of the vertical beam is such that a subject at 10 Metres will be radiated by the same intensity of energy as one at 100 Metres.

Cameras

Camera sensitivity

For research purposes a comparison was made of five commonly available cameras to compare the energy required on the sensor to achieve certain levels of performance. The tests were carried out using light sources that could put out a calibrated level of light energy measured in microwatts per square Metre. Two monochromatic light sources were used, one at 595 nm, the other at 880 nm. These simulate the wavelengths in the middle of the visible spectrum and well into the infrared spectrum. The tests were also measured at two output levels of the sensor. One

to provide a signal to noise ratio of 12 dB, the other to provide a full 0.7 volts video signal. The results are listed in table 1.

Four of the cameras had similar specifications for sensitivity, around the 0.1 lux level, one had a sensitivity of 0.05 lux. Note the greatly different performance at 880 nm! Although as always they are all qualified by various factors such as; usable picture; acceptable picture; 50 IRE, etc. In other words they are all the same but different!

With the 595 nm light source which is in the middle part of the visible spectrum there is almost no difference in the energy required at the 12 dB level. For the full video there is more variation with factor of 4.7:1 between the greatest and the least.

With the 880 nm light source the variation is 5.7:1 for the 12 dB level but for full video the range stretches to 10:1.

It is beyond the scope of this article to analyse these results further but they are presented to illustrate the variations in light energy required by different sensors at different wavelengths.

Light source 595 nm Light source 880 nm

Camera

ref.For S/N 12dB For 0.7 v video For S/N 12dB For 0.7 v

video

A 10.2 32 32 80

B 10.7 32 14 32

C 9.0 80 80 300

D 9.2 80 54 320

E 8.9 17 31.6 74

Table 1, W/M2 required on sensor.

 

Lenses

The first component that light from a scene has to pass through is the lens. If this is wrong then everything after gets progressively worse. Apart from selecting the focal length for a particular scene, there is generally very little thought given to this prime element in the chain to get a picture from a scene to a monitor. There are several factors in lens selection that will affect the effectiveness of a system under infrared light. Often they will determine whether anything at all will be seen.

Spectral response

As has been previously discussed, cameras have varying response to wavelengths of light but the same also applies to lenses. Just as CCTV camera sensors are led by the Camcorder market, so CCTV lenses are led by the photographic market, as well as lenses on Camcorders. They are all based on the need to create images in visible light. Even in the dark a flash is used to simulate visible light. Therefore lenses have a spectral response which is biased towards the visible part of the spectrum.

An example of a lens spectral response diagram is shown in diagram 14. In this example the response at 715 nm is only 60% of that at 550 nm. This can vary greatly between different makes of lens and can have a significant affect on the infrared performance of a system. So, get on to your lens manufacturer and obtain diagrams, beware of those that cannot provide them.

Diagram 14

Aperture

The f number of a lens is the ratio of the focal length to the effective object lens diameter. It is a mechanical ratio and does not infer the efficiency of a lens. It does affect the amount of light energy passed to the sensor and will play a significant part in the resulting picture. Traditionally camera manufacturers have specified sensitivity with a lens having an aperture of f 1.4. This would be fine if they all did it the same but they don’t. Some say with 75% reflectance some say 89% and so on. Then again some will state the sensitivity with AGC on but not what the AGC gain is. Camera specmanship is too vast a subject to expand on in this article but suffice to say

the f number of the lens is a most important consideration. In simple terms the smaller the f number the more light is passed to the sensor, therefore f1.2 is better than f1.8. The percentage of light passed by different apertures is listed in table 2. This shows the percentage of light falling on the lens that is passed to the sensor. The f stops in bold face are full stops and each number in the scale halves or doubles the light passed. There are two intermediate stops shown because they are common stops found in CCTV lens.

F number

f1.0 f1.2 f1.4 f1.8 f2.0 f2.8 f4.0 f5.6

% passed 20% 15% 10% 7.5% 5% 2.5% 1.25% 0.625%

Table 2 light passed by f stops

Yes, it is true that with an aperture of f1.4 only 10% of the light on the lens is passed to the sensor. Some manufacturers specify camera sensitivity as that on the faceplate or sensor. In these cases use these ratios to convert to the light required on the lens. i.e. 1 lux faceplate sensitivity requires 10 lux with an f1.4 lens or 20 lux with an f2.0 lens.

It may seem relatively unimportant to quibble about the difference between an f1.2 lens and an f1.4 lens, especially when the latter is much cheaper than the first. It is significant though because the f1.4 lens needs 50% more light for the same energy on the sensor. An example taken from a distributors’ catalogue shows two 12mm auto iris lenses, one f1.4 and one f1.2. The f1.4 is £75.00 the f1.2 is £152.00! Which lens would a comparatively inexperienced estimator use in a competitive tender? In a ten camera system it could mean a saving of £750.00 or about £1,000.00 on the tender price- a considerable temptation.

The effect of sensor size

As stated earlier, light is energy measured in Watts per square Metre. Therefore if the area of a sensor is known then the resultant power in watts can be easily calculated. The nominal areas of the sensors in common use are listed in table 3.

Sensor size 2/3" 1/2" 1/3"

Nominal area, mm2 68 36 17

Table 3, areas of sensors

The power produced by each individual pixel in the sensor is directly proportional to its area. If three cameras are considered each with the same resolution of say 500 lines then the number of pixels on each sensor must be the same. The result of this is that the pixels on each smaller size of sensor must also be smaller. Therefore the power produced will be less for the same aperture setting, i.e. the same amount of light energy.

If for the sake of an example a light source of 1,000 milliwatts per square Metre is passed to the sensors via an f1.8 aperture lens. The amount of light passed by the f1.8 lens will be 7.5% = 75 mW/M2 . From this the power output can be calculated for each sensor. This will be the power multiplied by the area of the sensor. The result is shown in table 4.

 

Sensor size 2/3" 1/2" 1/3"

Sensor area 68mm2 36 mm2 17 mm2

Aperture f1.8 f1.8 f1.8

Power output 5.1mW 2.7mW 1.275 mW

Table 4 power output of sensors

Therefore the 1/2" and 2/3" sensors will be producing insufficient power for a full video signal. The answer is to use a lens with a larger aperture for these sensors so that more energy is passed to maintain the output power. This is summarised in table 5.

 

Sensor size 2/3" 1/2" 1/3"

Sensor area 68mm2 36 mm2 17 mm2

Aperture f1.8 f1.2 f0.85

% light passed 7.5% 15% 30%

Power output 5.1mW 5.1mW 5.1mW

Table 5 power output corrected by lens f stop

This is the reason that many 1/3" cameras have the sensitivity specified with an f1.0 or sometimes an f 0.9 aperture. Beware though, there are only a limited number of lenses made to the 1/3" format. If the longer focal length lenses must be used they usually have smaller apertures (higher f numbers) and pass less light energy.

The contra to this argument is that if a sensor of one size has the same size pixels as a larger one, then the light required will be the same. However the total number of pixels will be fewer and the resulting resolution will be proportionally less. There is no such thing as a free lunch!

Sensor and lens format

The range of lenses available for the 1/2" and particularly the 1/3" cameras is limited. The largest range of lenses are still 2/3" and 1" format. It is alright to use a larger format lens on a smaller format camera but there is another penalty to pay. A 2/3" lens will focus the scene on the equivalent area around a 1/2" sensor but only the light energy relative to the area of the sensor will be converted to power. See diagram 15.

Diagram 15, 2/3" lens with a 1/2" sensor.

The area of a 1/2" sensor is 53% that of a 2/3" sensor, therefore only this proportion of the light energy will be converted to output power and thus a video signal. This would make an f1.4 lens equivalent to about f2.0. The area of a 1/3" sensor is only 25% that of a 2/3" which is equivalent to an f2.8 lens! So this is yet another correction factor that needs to be applied.

Transmission ratio

As stated, the f number is simply a ratio and although it is a measure of the amount of light that a lens will pass it is not a measure of the efficiency of the lens. The efficiency of a lens is measured by its transmission ratio referred to as the 'T' number. A lens, as stated earlier, is

several different glass elements arranged to provide the correct projection of the image being viewed on to the sensor of the camera. Every time light is transmitted through a standard glass to air surface some is lost. Lens designers and manufacturers use a range of techniques to reduce this factor. The amount of light that is additionally lost depends on the glass materials, the thickness and curves of the glass and the coating of each lens element. Light may be lost by absorption and reflectance in passing through a lens. It can also be lost by internal reflections.

The mathematics are quite complicated to calculate. The Transmission Ratio is calculated from a factor known as the Transparency Ratio. Suffice to say that the effect is to adjust the effective aperture to a value depending on the quality of the lens. For example a high quality f1.4 lens with a Transparency Ratio of 0.85 would have an effective aperture of f1.58. A lower quality lens may have a Transparency Ratio of 0.6 and would produce an effective aperture of f1.8. This represents a significant loss of available light to the sensor.

Other lens factors

There are several other factors in lens design that will affect the final picture seen on the monitor.

Flare, caused by some light being lost due to internal reflections.

Astigmatism, caused by the curvature of the lens being different in the vertical and horizontal planes.

Spherical Aberration, Where the surface of a convex lens in not perfectly spherical and all the rays of light do not focus at the same point.

Barrel Distortion, Where the rays from the outer areas of the lens do not focus at the same point as those from the centre, usually exaggerated in very wide angle lenses.

Chromatic Aberration, where different frequencies of light refract at different angles causing blurred images.

A high quality lens will minimise these effects, a low quality lens will exaggerate them. The difference in cost between a high quality lens and a cheap one may have be a small percentage of a tender price but it could have a dramatic affect on performance.

Focusing with infrared light

The distance to an object in focus will be different under infrared light compared to natural light. This is due to the different angle of refraction and the fact that a lens compensates for refraction mainly in the visible part of the spectrum. Therefore focusing of cameras illuminated by infrared light requires that they must be set up and back focused at night under the infrared light. The focusing is also more critical under infrared light because the aperture will be at its maximum with resulting decrease in depth of field. This will generally be compensated for in daylight with smaller apertures. Another factor is that the angle of the lamp must be accurately adjusted to the

angle of the camera, just a couple of degrees difference can lose a large amount of the available light energy.

There are lenses available that will focus visible light and infrared light at the same focal point, however the price penalty (or benefit!) is a factor of about four times the cost.

Reflectance

The camera only sees the light energy reflected from the subject, so this factor must also be considered. Some typical values for reflectance are given as follows.

Matt white test card 89%

Snowy scene 85%

Glass windows and walls 70%

White matt paint on concrete 60%

Unpainted concrete, car park 40%

Red bricks 35%

Open country, trees, grass 20% ( This can be 60%-70% for I/R)

Empty asphalt area 5%

It is interesting to note that photographic cameras with built-in light meters use a reflectance factor of 18% in the calibration of the meter.

What about the glass in a camera housing? Try a simple experiment, Go to the office window and measure the light from the inside, then measure it from the outside. The loss can be in the order of 25%-35%, so add this into the equation and see what happens!!

 

 

 

In conclusion a typical example may be as follows using the real world in which we all live.

 

Supposing a system is designed to provide 60mW/M2 of infrared power, what on earth does that equate to in plain English?

In very approximate terms it can be assumed that 270 Lux equates to 1 Watt/M2 light power over a wide spectrum.

Therefore 60 mW equates to a nominal 16 Lux if it were over the visible part of the spectrum.

This sounds like a lot of light power for a camera with the following specification:

Sensitivity: 0.1 lux for usable video at f1.0 (90% reflectance)

However if the camera specification is examined more closely some surprises transpire.

A. 0.1 Lux is at photopic wavelength.

 B. This refers to a target reflectance of 90%.

 C. This usually relates to 50% video or ‘usable video’

 D. Using a non-standard lens with an aperture of f 1.0.

 Now come back to planet earth and see how the camera performs at 730 nm!!!

 A. The camera has about 23% sensitivity at 730 nm.

B. It is required to see targets with a reflectance of 10%.

C. A full video signal of 0.7 volts is required, not 50%.

D. The lens actually fitted will be f1.4.

The infrared power required will actually be the equivalent of 16 Lux ! visible light . What a coincidence

Add just one more factor, supposing that the area from 750 nm under the lens spectrum curve is 50% of the total. The infrared power now required will be:

The equivalent Of 32 Lux! of visible light

Is the camera in a housing? Oh dear this could lose another 25%. So now the light power needed is::

The equivalent of 40 LUX! of visible light

It is probably best to forget about a zoom lens where the effective aperture may reduce at maximum zoom, or that an f1.4 lens may not be available for the focal length required!

 Summary

When a system is required to operate under infrared light all the components are being pushed to their limit of functionality. With all the factors to be considered it is amazing that any systems function at all, and of course many do not. More attention given to the many factors discussed in this article will improve the chances of achieving a system that will provide optimum performance. These comments equally apply to any system operating under low light conditions without infrared light where components are operating towards the limit of their specification.

The emphasis in system design for low light conditions must be to assess the products carefully, ask questions and do not shy away from quality products for the sake of a few percent on the price.

 

So, how far can your light shine? The answer is, of course FOREVER.

But, how far can your CAMERA see?

Understanding Lenses

Selecting the most appropriate Lens from Installation can be a complex task - the choices constantly expanding in response to new Camera and Lens technology. Lenses have a number of characteristics that must be considered to match a particular requirement with the best Lens for the job.

Fixed Focus Length Lens

Fixed focus lenses are the simplest type of Lens, and therefore the cheapest. The presets focal length means a precise calculation is required to select the Lens most suitable for the location, based on the desired size of viewing area and its distance from the Camera. Typical Lens sizes offer either 30 degree view - narrow to allow more detail at distance - or 60 degree, which offers a much wider angle of view.

Varifocal Lens

Varifocal lenses offer more flexibility, allowing the field of view to be adjusted manually. Although more expensive these lenses of popular because the use it is able to get the view required rather than the limited by the constraints of the fixed Lens. Finally, Zoom Lens are the most complex type,

offering the greatest functionality once installed - unsurprisingly, Zoom lenses offer the widest choice of associated features and technologies.

Zoom lenses can be remotely adjusted to allow variation of the focal length. This means that a single Lens can be used to view a wide area until an intruder is detected whereupon do it can be zoomed into capture facial details. Generally Zoom lenses incorporate an Auto Iris mechanism to permit 24-hour usage.

Formats

Lenses are also categorised according to size format. As Camera technology has advanced, sensor chips have reduced in size, requiring lenses to produce smaller images at the focal point. This has made smaller lenses possible (less glass resulting in less physical size and weight) although the requirements of precision manufacturing doesn't permit a proportional price reduction - the component materials of a Lens being a very small proportion of the overall manufacturing cost. The quoted format of the Lens (1", 1/2", 1/3" and now even 1/4") is derived from the ratio of diameter to the viewing image produced. Whilst it is often most cost effective to match the lens format to the camera sensor size, it is possible to use a larger lens on a smaller size camera since the image only needs to be at least as large as the sensor.

Using a larger lens can often be advantageous, since it offers greater depth of field (the range of distances from the lens before objects are too close or too far away to be in focus). Larger lenses also mean that the area of the image that is used is taken entirely from the central, flatter part of the lens causing much less corner distortion and better focus.

Aspherical

Lenses have traditionally been shaped to the arc of a sphere, which has the effect of causing some distortion of image at the very edges of the lens, as well as reducing its light gathering capability.

A recent innovation in lens manufacturing, aspherical technology, allows the edges of a lens to be less curved, producing a larger area of accurate image and allowing transmission of a greater amount of light. Aspherical lenses can therefore reduce distortion and give a lower effective f-stop permitting camera to operate at lower light levels.

Iris

To provide optimum performance neither too much nor too little light should fall on the camera sensor. This can be adjusted by means of the lens iris.

A smaller iris opening offers greater depth of field and better focus, but the reduced amount of light admitted into the camera results in poor quality images in low lighting levels. A fixed iris lens offers no adjustment to different lighting conditions, so is therefore limited in use and not suitable for applications where fine detail is consistently required. A manual iris can be adjusted at the time of installation, allowing an optimum picture to be obtained for a fixed lighting level.

These lenses are best suited to indoor applications, where the lighting level is controllable and consistent. Both manual and fixed iris lenses can be used with cameras which offer a feature known as 'electronic iris' - an on-board technology to effectively reduce the sensor exposure to compensate for the lack of iris control. This can be cost effective, but does not provide the increased depth of field offered by a correctly sized iris.

For external use (where conditions generally vary the most), an automatic iris lens offers the best performance, as the iris aperture automatically adjusts to create the optimum image by monitoring the output signal from the camera. There are a number of different lens types offering this method of iris control. The original design for automatic iris (Al) lenses was wholly self-contained, with the image analysing technology built into the lens and an iris that was adjusted by servomotors.

Market demand to produce smaller, lower cost lenses led to the introduction of direct drive technology which requires circuitry within the camera, replacing that previously located in the lens. This technique used a different iris control - galvanic drive. Subsequently this technology has been introduced into the original style auto iris lens where onboard camera circuitry is not required.

Today these are the choices for auto-iris control - traditional servo drive, galvanic iris and direct drive.

The final lens characteristic to take account of is the light-gathering speed of the lens-expressed as an f-stop number. This literally measures the amount of light captured by the lens in a given period of time; the lower the f-stop range, the more light that can be transmitted.

Using a 35mm Camera to Calculate CCTV Lenses

There are many methods of calculating the field of view needed for a particular application and the appropriate lens selection. Previous articles in CCTV Today have covered this topic in depth. Probably the most useful method is to make a video recording using a camera and zoom lens from a hydraulic hoist at the actual camera positions being proposed. Many companies now offer a service using a custom designed vehicle with several types of camera mounted on hydraulic masts. Inside the vehicle are various switching and recording devices. However, they can only cover a limited number of surveys compared to the total number of opportunities. This article looks at other ways of determining fields of view and lens specifications.

There are several types of CCTV viewfinders which are like a small telescope with a zoom lens. You look through the viewfinder and adjust the lens until the required field of view is in the frame. The lens focal length for various camera formats is then shown on scales on the barrel. One problem is that some of these useful devices only adjust to comparatively wide angles and are mainly suitable for internal systems. Also they do not provide a permanent record of the field of view ,which could lead to disputes in the future.

If you have a 35mm camera with a zoom lens, you can accomplish two benefits at one go. First, find the correct lens angle for any camera by reading the zoom setting when the required field of

view is composed through the camera viewfinder. Second, make a permanent record of this by taking a shot, keeping a note of the shot and the zoom setting.

The accompanying table lists the lens focal lengths and angles for various zoom settings on a 35mm camera and the equivalent focal lengths for camera formats.

35mm 1" 2/3" 1/2" 1/3" Angle

11 4.0 2.8 2.0 1.5 115.6

15 5.3 3.7 2.7 2.0 100.5

16 5.7 3.9 2.9 2.2 96.2

17 6.0 4.2 3.0 2.3 93.2

18 6.5 4.5 3.3 2.5 88.7

23 8.0 5.5 4.0 3.0 76.9

24 8.5 5.9 4.3 3.2 73.5

27 9.4 6.5 4.7 3.6 68.1

28 10.0 6.9 5.0 3.8 64.8

30 10.5 7.3 5.3 4.0 62.3

33 11.6 8.0 5.8 4.4 57.4

35 12.5 8.7 6.3 4.7 53.9

43 15.0 10.4 7.6 5.7 45.9

45 16.0 11.1 8.1 6.0 43.3

51 18.1 12.5 9.1 6.8 38.7

57 20.0 13.9 10.1 7.6 35.2

65 23.1 16.0 11.6 8.7 30.7

68 24.0 16.6 12.1 9.1 29.6

71 25.0 17.3 12.6 9.4 28.5

85 30.0 20.8 15.1 11.3 23.9

136 48.0 33.3 24.2 18.1 15.1

142 50.0 34.6 25.2 18.9 14.5

213 75.0 52.0 37.8 28.3 9.7

255 90.0 62.4 45.4 34.0 8.1

283 100 69 50 38 7.3

298 105 73 53 40 6.9

Today, Monitors are available in a vast array of sizes, resolutions and aesthetic designs; with options in monochrome and colour, to enable a wide range of applications and budgets to be met. When considering the

Understanding Monitors

most appropriate Monitor for a particular security application, a number of factors may determine the selection.

The Physical Distance Available from the Placement of the Monitor to the Viewer

Often, in manned surveillance operations, a viewer is expected to concentrate on one or more Monitors for long periods of time. To protect both the health and safety interests of the operator and to ensure that they remain alert to potential incidents, guidelines have been established to determine the optimum viewing distances.

Optimum Viewing Distances

A simple model is available to help calculate the optimum viewing distance of the viewer to the Monitor.

Monitor Size (inches) Optimum Viewing Distance (feet)9 1-312 3-614 6-917 7-1120 9-15

The Amount of Physical Space Available

Due to the wide range of Monitor sizes in the marketplace today, it is important to consider the available space the your installation. There is no point buying a 21in Monitor when there is only space or a 14in model. It is also worth taking a long-term view and considering the other factors such as heat and room installation. As a security installation develops, there may be the requirements to incorporate additional Monitors.

The Level of Viewing Detail Required

For high-quality manned surveillance systems, where there is a requirements to view high resolution pictures as well as record images, then a high resolution monitor with Y/C inputs is advisable.

The Need to Show Colour or Monochrome Pictures

Colour monitors are advantageous in applications where identification is important, e.g. someone wearing a red jumper and

blue jeans can be identified more effectively on a colour monitor than someone who appears dressed in dark grey on a monochrome monitor.

However, if budget is a concern or usage means that low light camera viewing is required, a monochrome monitor may be more appropriate.

Whether Audio is Required

Not all monitors have audio capabilities. Consequently, it is a feature that needs to be specified in advance, especially in applications in staff protection for example.

The Available Budget

Price variances in monitors arise for different reasons dependant on whether a monochrome or colour monitor is required. The majority of monochrome monitors today are supplied with high resolution CRTs. Consequently price differences in these models simply tend to reflect the screen size. Colour monitor pricing, on the other hand, is determined by both resolution and screen size.

However, as a result of the vast quantity of 14" monitors that are currently produced for the PC market, the Security Industry has been able to offer 14"Is high-resolution colour displays at extremely affordable prices.

Understanding Pan & Tilts

Broadly categorised into internal and external usage, pan/tilts are normally selected on the basis of the maximum load they can take. This of course will dramatically vary from application to application. Careful consideration is required to ensure the total weight of the housing, camera, lens, Infra Red lamps and any other equipment featured.

This article was supplied by Norbain and used with permission. Norbain are manufacturers of the Vista Protos range of state-of-the-art products designed for leading edge and performance critical applications - where extensive features and functionality are the key.

Another way of looking at monitors

The aspect ratio of a standard CCTV picture is 4 units wide by 3 units high, typically complicated by being based on the diagonal measurement of the tube or sensor. A 12" monitor would have dimensions of about 220mm wide by 165mm high. Camera lenses have the vertical and horizontal angles of view in the same proportions.

It is standard practice to set up cameras and monitors to view in this normal plane, but is this always the best way to look at a scene?

Many systems protecting a perimeter are looking along a long narrow field of view as illustrated in the diagram 1. This shows the view on a monitor in its normal orientation and with the camera mounted conventionally. The field of view will be determined by the vertical angle of the lens. It can be seen that there is a great deal of the screen showing areas not necessarily important in relation to the scene being monitored.

 

If the camera is turned through 90 and also the monitor, as in diagram 2, the part of the scene being monitored is now represented in greater detail. This is because the orientation of the required scene is in a better relationship with that of the monitor.

 

(Believe it or not, the two screens shown are the same size and the corect ratio)

It is not just a question of rotating the camera and monitor, because the field of view will now be determined by the horizontal angle instead of the vertical angle of the lens. As previously stated this is in the ratio of 4:3, therefore a lens with a longer focal length will be required. This is quite straightforward. Having calculated the required lens using the vertical angle, simply find a lens

that has the same horizontal angle of view. For instance a 12.5mm lens has horizontal and vertical angles of view of28.4 and 21.3 respectively. A 16mm lens has horizontal and vertical angles of 22.3 and 16.8 respectively. Therefore the horizontal angle of the 16mm lens is nearly equal to the vertical angle of the 12.5mm lens.

The longer focal length lens, though, produces a larger image on the screen for the same scene content. This is illustrated in diagram 3. The angle of view is the same in both diagrams.

 

Diagram 3

It can be seen therefore that more of the important part of the scene is displayed when the camera and monitor are rotated through 90 .

This is obviously a somewhat controversial point of view and in reality must take into consideration factors of other cameras to be viewed which may require the conventional arrangement. However there may be occasions when some lateral thinking may pay dividends.

Understanding Video Compression

As the CCTV industry continues to move towards digital devices, such as Digital Video Recorders (DVRs) and IP devices, technicians need to be familiar with the subject of Compression – the methods such as MPEG, Wavelet™, and similar. In this article we spell out some of the basics of compression technology.

So first – why is data compression necessary? Because without it, the volumes of data produced by digitising CCTV image streams would swamp the available storage and communications systems.

To overcome this, the process of compression is applied to the image stream, reducing the amount of information that needs to be transmitted and stored. In fact compression of the camera

signal is not new - many people do not realise that all ‘analogue video’ has always been compressed.

Similarly, there has long been a need for data compression in the computer industry. Specialist mathematicians have worked for many years on solving the basic problem of how to reduce the image size to produce the best compromise between image clarity, the data size of the image, and the amount of processing power it takes to run the compression method.

Different applications have different priorities regarding clarity of the image, data volumes, and processing power – for example identification evidence has a different picture quality requirement compared to monitoring the length of a queue. So if you are selecting digital equipment, you’ll need to select the compression format that suits the network or the application you are installing.

Different sorts of compression are described as lossless or lossy. In general, the less compression the better the playback and recorded image, so naturally in that sense lossless is always better than lossy; however, less compression means more data to be transmitted and stored, and thus incurs higher system costs.

Compression reduces the signal in three ways. The first is by various mathematical tricks that are lossless to the image, and can be reversed at the time of display so that the full image is viewed. The second is to remove parts of the signal that are redundant to human viewing of the image. The third method is to start to visibly reduce image quality – definition, frames per second, and colour range – and it is this type of compression that is called lossy.

The compression formats used in CCTV vary by manufacturer and by product. But the four most commonly used compression formats are:

H261 Motion JPEG, also written M-JPEG or M-JPG Wavelet™ MPEG, also written mpg

H261

H261 is a digitisation and compression scheme for analogue video. It is widely used in video conferencing and is aimed at providing digitised video at a bit rate of 64Kbps-1Mbps, which is the bandwidth range of public data networks.

Compression rates as high as 2500:1 are achieved, but of course at the cost of quality. The format is good for high frame rates, showing movement, but the resolution of those frames is not high. This is not good if, say, person identification images are required. But if the application is a non-security application such as video-conferencing, the quality is likely to be adequate.

Uniquely among the compression formats discussed here, H261 encoded signals can also be decoded or decompressed by reversing the process(es) from a valid reference or I-Frame. That means you can get back to the original high quality if you ever need to.

Motion JPEG (M-JPEG)

Motion JPEG (JPEG stands for Joint Photographic Experts Group) is an adaptation of the popular JPEG image compression for still digital photos. JPEG is a lossless compression technique, losing very little data in the image. Motion JPEG creates a video stream from a succession of JPEG-compressed still photos. Because it is based on these high quality lossless stills, it delivers a much higher quality image than H261. But at a cost – it requires a considerably greater transmission bandwidth and storage capacity compared to its H261 counterparts.

An advantage of Motion JPEG is that, because it is based on still images, it can produce any of its frames as a single image for identification purposes. As we will explain, some compression techniques cannot provide such images.

MPEG

MPEG (named after the Moving Pictures Experts Group) is purpose designed for moving pictures, rather than being based on still image compression. This means that each frame is defined as the previous frame plus changes, rather than a full frame. The advantage of this is that compression is more efficient – the same quality can be displayed from less data. However, the method has problems when there is extensive motion between one frame and the next – there is a danger that the image gets ‘blocky’ and vague, losing some definition in the areas of the frame where the movement occurs.

There is not one MPEG standard but several , changing over time, of which only the first two are relevant at present.. MPEG -1 was designed to output 15 frames per second video from limited bandwidth sources, such as CD-ROMs. MPEG-2, designed for high bandwidth applications such as High Definition TV. (HDTV), delivers 30 frames per second video at full CCIR 601 resolution but requires special high speed hardware for compression and playback – PCs cannot handle this.

Wavelet™ Compression

Like Motion-JPEG, Wavelet™ compression delivers high-quality moving images by starting with still images, applying a compression method to them, and putting them together to form moving pictures. It compresses images by removing all obvious redundancy and using only the areas that can be perceived by the human eye. Wavelet™ is up to four times more effective in reducing the volume of data than JPEG and M-JPEG.

Wavelet™ is also seen as offering superior development potential to current MPEG compression, giving a greater amount of compression with equivalent quality. It transforms the whole image and not just blocks of the image, so as the compression rates increase, the image degrades gracefully, rather than into the 'blocky' artefacts seen with some other compression methods. Wavelet™ applications can have their preferred level of compression selected by the user – higher or lower.

Thus, although Wavelet™ is not as established as some other compression techniques, it is growing in popularity.

It all depends on the application - Image Resolution

There is no ‘good’ or ‘bad’ in compression methods. The idea of ‘horses for courses’ applies, and the table below summarises when each method is best.

Method Compression Ratio Bandwidth and Storage Required

Frames per second Quality

M-JPEG Low High 25 HighH261 Very high Low 25 Low

MPEG Low Very high 25 Very highWavelet™ High Low 25 High

Video Recording in Security Surveillance Systems

Security monitoring of premises and the movement of people by the use of CCTV systems can be beneficial in the need to protect premises and ensure the safety of staff and visitors. In combination with access control, fire and intruder alarm systems it can prove to be a formidable tool in the fight against crime. But no matter how good the system is, its effectiveness will be diminished unless cameras are monitored, pictures recorded and a means by which its use can be documented and its integrity proved, is established from the outset.

The following are suggestions for ways of ensuring best practice in relation to the gathering and presentation of video evidence. If these general guidelines concerning the handling of evidential video tapes and equipment are followed it will greatly assist prosecution cases. This may lead to an increase in 'Guilty' pleas at court and a decrease in the amount of staff time wasted in attending court to give evidence.

The Police and Criminal Evidence Act 1984 requires that the gathering of Police evidence be both procedurally correct and as far as possible, technically verifiable. It therefore puts the emphasis on improving the standard of evidence required, to catch suspects 'in the act' of committing a crime.

The use of video recordings have already proved their worth in court proceedings, enabling the viewing of events as they took place and as an added bonus, the publicity surrounding such events can act as a form of Crime Prevention. This preventative effect will only last as long as prosecutions using video evidence succeeds. It is vital therefore that total integrity of any system is maintained from beginning to end.

Quality

It should be established at the outset whether the purpose of the system is intended to identify an incident or to provide identification evidence of suspects suitable for presentation to the courts.

The quality of any recording depends on the standard and condition of both the video tape and the system used to make the recording. Equipment used must be in good order and regularly, professionally maintained and serviced, details of which should be recorded from the date of purchase and commissioning of the system.

Unless the camera is set to record a fixed point, eg a particular door or piece of equipment, a preference should tend towards pan, tilt and zoom cameras. This will facilitate the provision of an identifiable picture of the subject but successful system operation is dependent on an operator being available to manually control the system.

Adequate lighting (colour cameras) or infra red assisted (monochrome) recording should be employed for night-time operation of a CCTV system.

A member of staff should be in a position to explain to any court, procedures relating to the systems installation and use.

  Video Tape Purchase, Usage and Storage

Much criticism is levelled by the police and others that the standard of video reproduction is of poor quality. There may be many reasons for this but the most common is over-use of the recording tape. In an ideal world tapes would be used only once. It is appreciated that cost effectiveness is a necessity, therefore it is suggested that a library of 31 tapes is established, one for each day of the month. With the complete library being changed at the end of the twelve month period, this gives a maximum usage of twelve times per tape at which point it would tend to show signs of wear and deterioration.

Obviously, the more recording machines used, the greater number of 'libraries' will be required. It should be borne in mind that although this may appear to be excessive, the cost of the tapes is only a small percentage of the overall cost of the complete video based security system.

Tapes should be stored in a secure cupboard or cabinet so that their integrity can be maintained also avoiding the possibility of accidental damage or use. Once purchased it is important that the life of the tapes are fully recorded, a guide to this is shown at the end of this document.

Cataloguing

It is of the utmost importance, when presenting a video recording as evidence, that the tapes have not been interfered with and that their integrity can be proved. The best manner to prove this is to establish strict procedures for usage, these being fully documented in a prepared register which can be subsequently produced at court if required. Each tape should be given a unique reference number and labelled accordingly. The principle aim of the register is to be able to prove the life of the tape, its movements and usage.

The register will also prove to be a useful management tool in evaluating the system as it will contain information relating to the number of tapes used and the number of cases where video evidence was presented. In conjunction with this, where manually monitored, a daily 'incident log' should be kept on which the person monitoring the system can record details of occurrences that have been recorded on the tape. The log should include the date, time, a brief description of the incident and the tape counter reading at the start and finish of the incident. It should also bear details of the person who monitored the incident. It follows therefore that not only will provision have to be made for the secure storage of video cassettes but also for the tape register and daily incident logs.

Making Recordings Before recording check that the equipment is in good working order See that the tape counter is set to zero Check that the time and date generator is correctly set and is being recorded from a single

source only

All recordings should be made without interruption unless it is absolutely necessary, any interruptions should be recorded so that allegations of malpractice can be disproved.

Tape Re-usage

Before re-using a tape it should be erased by the use of a bulk eraser, which uses a magnetic field to erase previously recorded material. It should be erased just prior to its re-use, thereby giving you one month in which to decide if any recordings on the tape are required for possible future use. By starting with a freshly erased tape that has been documented to that effect, its integrity is enhanced.

The Video Tape As Evidence - Conclusions There must be evidence of continuity of handling of the videotape from the time it is first

taken into use, up to its production as an exhibit in court. The videotape evidence must be the original recording and there must be no evidence of

editing, either by physically cutting and splicing or mechanically recording from separate sources.

The tape used should either be new or evidence should be produced to show that it was erased prior to its use.

Time and date must be encapsulated to the tape. At no time during an incident, nor in the period following, should the recording be

touched until the Police Officer investigating the incident arrives. Under no circumstances should a member of staff be allowed to remove the tape from the recorder or playback the video recording of the incident. This is against Police procedure and will effect both the quality of the recording and the usefulness of the tape for evidential purposes.

It should be appreciated that the identification of a defendant must not exclusively rely on evidence from the videotape, the proof of the crime must be supported by other testimony.

Action to be taken by Police

When Police are called to a venue where a CCTV system is installed and it is apparent that what has taken place may have been recorded the officer will request permission to remove the recording cassette from the recorder. It is of the utmost importance that the tape is not rewound or reviewed.

Statements will be required from the following: A member of staff who is capable of proving the system and who can provide details of

the equipment used. The member of staff who last placed the tape into the video library store following

normal use procedures. The member of staff who erased the video tape (if applicable) and put it into the recorder. The member of staff who monitored the incident (if applicable and if different from

above).

The officer will then treat the tape as he would any other exhibit in the case except that the plastic exhibit bag should be of the perforated type in order to avoid the build up of condensation which could harm the tape.

The video tape recording should then be transported to the laboratory or other designated place, where a working copy will be made. From this working copy all viewing or further copies will be made, the original retained as the exhibit for production at court.

Further statements will be required from the person who transported the tape for copying, the person who copied the tape and the person who retrieved the original.

The fact that there is video tape evidence in a case must be declared to the Crown Prosecution Service at the earliest opportunity in order that its existence can be declared to the defence.