Digital camera

A Digital Camera is a device that records video or still photographs by capturing images on an an electronic image sensor. A digital camera is similar to a 35mm film camera in that it has a body, lens and light sensitive recording ability. The major difference is that instead of silver halide film there is an electronic semiconductor sensor.

WORKING:

If you are looking at an image the light bouncing off the object you are photographing, passes into the camera, through a set of lenses and onto a mirror. From there, the light bounces up and into a piece of glass called a pentaprism (a five sided prism). Once light enters the pentaprism, it travels around in a complicated way until it passes through the eyepiece and enters your eye. If you are looking at a photo, when you press the button on the camera, the mirror flips up, in front of the pentaprism. Instead of the light from the object bouncing on the pentaprism, the light passes directly to the back of the camera. There, it either hits photographic film or starts a chemical reaction (single use camera), or the light it impacts on a ray of light-sensitive cells that release a tiny electric charge in each activated cell (digital camera).

Image resolution

The resolution of a digital camera is often limited by the image sensor (typically a CCD or CMOS chip)

that turns light into discrete signals. The sensor is made up of millions of "buckets" that essentially

count the number of photons that strike the sensor. The brighter the image at a given point on the

sensor, the larger the value that is read for that pixel. The number of pixels in the image determines

its "Pixel Count". For example, a 640x480 image would have 307,200 pixels, or approximately 307

kilo pixels; a 3872x2592 image would have 10,036,224 pixels, or approximately 10 megapixels.

Methods of image capture

There are three main methods of capturing the image, each based on the hardware configuration of the sensor and colour filters.

The first method is often called single-shot, pertaining to the number of times the camera's sensor is exposed to the light passing through the camera lens. Single-shot capture systems use either one CCD with a Bayer filter, or three separate image sensors (one each for the colours red, green, and blue) which are exposed to the same image.

The second method is referred to as multi-shot because the sensor is exposed to the image in a sequence of three or more openings of the lens aperture. There are several methods of application of the multi-shot technique. The most common originally was to use a single image sensor with three filters (once again red, green and blue) passed in front of the sensor in sequence to obtain the additive colour information. Another multiple shot method is called Microscanning. This technique utilizes a single CCD with a Bayer filter but actually moved the physical location of the sensor chip on

the focus plane of the lens to "stitch" together a higher resolution image than the CCD would allow otherwise. A third version combined the two methods without a Bayer filter on the chip.

The third method is called scanning because the sensor moves across the focal plane much like the

sensor of a desktop scanner. Their linear or tri-linear sensors utilize only a single line of photo

sensors, or three lines for the three colours. In some cases, scanning is accomplished by moving the

sensor e.g. when using colour co-site scanning or rotate the whole camera; a digital rotation line

offers images of very high total resolution.

Most current consumer digital cameras use a Bayer filter mosaic in combination with an optical anti-aliasing filter to reduce the aliasing due to the reduced sampling of the different primary-colour images. A demosaicing algorithm is used to interpolate (interpolation is a method of constructing new data points within the range of a discrete set of known data points) colour information to create a full array of RGB image data.

Image Sensors:

An image sensor is a device that converts an optical image into an electrical signal. Early analog sensors were video camera tubes; most currently used are digital charge-coupled device (CCD) or complementary metal–oxide–semiconductor (CMOS) active pixel sensors.

Both CCD (charge-coupled device) and CMOS (complementary metal-oxide semiconductor) image sensors start at the same point -- they have to convert light into electrons. One simplified way to think about the sensor used in a digital camera (or camcorder) is to think of it as having a 2-D array of thousands or millions of tiny solar cells, each of which transforms the light from one small portion of the image into electrons. Both CCD and CMOS devices perform this task using a variety of technologies.

The next step is to read the value (accumulated charge) of each cell in the image. In a CCD device, the charge is actually transported across the chip and read at one corner of the array. An analog-to-digital converter turns each pixel's value into a digital value. In most CMOS devices, there are several transistors at each pixel that amplify and move the charge using more traditional wires. The CMOS approach is more flexible because each pixel can be read individually.

CCDs use a special manufacturing process to create the ability to transport charge across the chip without distortion. This process leads to very high-quality sensors in terms of fidelity and light sensitivity. CMOS chips, on the other hand, use traditional manufacturing processes to create the chip -- the same processes used to make most microprocessors. Because of the manufacturing differences, there have been some noticeable differences between CCD and CMOS sensors.

CCD sensors, as mentioned above, create high-quality, low-noise images. CMOS sensors, traditionally, are more susceptible to noise.

Because each pixel on a CMOS sensor has several transistors located next to it, the light sensitivity of a CMOS chip tends to be lower. Many of the photons hitting the chip hit the transistors instead of the photodiode.

CMOS traditionally consumes little power. Implementing a sensor in CMOS yields a low-power sensor.

CCDs use a process that consumes lots of power. CCDs consume as much as 100 times more power than an equivalent CMOS sensor.

CMOS chips can be fabricated on just about any standard silicon production line, so they tend to be extremely inexpensive compared to CCD sensors.

CCD sensors have been mass produced for a longer period of time, so they are more mature. They tend to have higher quality and more pixels.

Based on these differences, you can see that CCDs tend to be used in cameras that focus on high-quality images with lots of pixels and excellent light sensitivity. CMOS sensors traditionally have lower quality, lower resolution and lower sensitivity. CMOS sensors are just now improving to the point where they reach near parity with CCD devices in some applications. CMOS cameras are usually less expensive and have great battery life.

Sensor size

(Relative sizes of sensors used in most current digital cameras)

OTHER IMPORTANT COMPONENTS:

SHUTTER: a shutter is a device that allows light to pass for a determined period of time, for the purpose of exposing photographic film or a light-sensitive electronic sensor to light to capture a permanent image of a scene. A shutter can also be used to allow pulses of light to pass outwards, as in a movie projector.

APERTURE: It is referred to the lens diaphragm opening inside a photographic lens. The size of the diaphragm opening in a camera lens REGULATES amount of light passes through onto the film inside the camera the moment when the shutter curtain in camera opens during an exposure process. The size of an aperture in a lens can either be a fixed or the most popular form in an adjustable type (like an SLR camera).

LENS: An optical lens or assembly of lenses used to regulate the manner in which the light enters into the camera. It could comprise of a single lens or even an complex assembly of number of lenses.

The amount of light falling on the sensor is controlled by various factors like:

Shutter speed: The speed at which the camera shutter opens and closes back while taking a picture.

Aperture size: The aperture is an opening provided to control the amount of light entering into the camera. The operator can always control the light by varying the size of the aperture.

Lens Assembly: Certain filters are fitted into the camera to filter some wavelengths of light from entering it.

MOSAICING AND DE-MOSAICING:

A conventional CCD or CMOS sensor can see only in shades of grey. In other words it senses the brightness of the light falling on it. Colour is given by putting a filter in front of the sensor, with a particular pattern of red, green and blue filters, one for each sensor pixel. This is known as a Bayer Filter and is represented diagrammatically in the image below.

There are more green filters because the human eye is most sensitive to changes in green. The filter and sensor produces a mosaic of red, green and blue light levels.

Powerful computer routines (called de-mosaicing algorithms) in the camera circuitry then convert this into an image that can be interpreted by the human eye.

Before the image is stored further in-camera processing deals with white balance, contrast and edge sharpness. The final computer readable file is then placed on the camera storage card.

Types of digital cameras

Digital cameras are made in a wide range of sizes, prices and capabilities. The majority are camera phones, operated as a mobile application through the cellphone menu. Professional photographers and many amateurs use larger, more expensive digital single-lens reflex cameras (DSLR) for their greater versatility. Between these extremes lie digital compact cameras and bridge digital cameras that "bridge" the gap between amateur and professional cameras. Specialized cameras including multispectral imaging equipment and astrographs continue to serve the scientific, military, medical and other special purposes for which digital photography was invented. Some hyperspectral cameras include HySpex VNIR-640, VNIR-1600,SWIR-320i ETC..