Illumination

(Lighting Engineering)

Lecture notes

Power Distribution & Utilization

By Abid Mushtaq

http://en.wikipedia.org/wiki/File:Night_yamagata_city_2.jpg

Illumination Light is

emitted and absorbed in tiny "packets" called photons, exhibits properties of both waves and particles

simply a very small part of the electromagnetic spectrum,

sandwiched between ultraviolet and infrared radiation.

IlluminationVisible Spectrum

700 nm 400 nm

Illumination

Primary properties of light are Intensity, Propagation direction, Frequency or wavelength spectrum, and polarization, while its speed, about 300,000,000 meters

per second (300,000 kilometers per second) in a vacuum, is one of the fundamental constants of nature.

Illumination

Illumination: To provide light to a surface is called

illumination. The required level of light may vary from

surface to surface and purpose to purpose. For example , in a drawing room, we need more

illumination than in verandah. A watch repairer needs high illumination (in the form of direct light) as compared to the illumination required in a reading room.

Illumination

Some Important Term: Luminous flux:

It is the energy radiated (in the form of light waves) by a body per second.

Luminous or Radiant Efficiency:

The ratio between the energy radiated in the form of light to the total energy radiated (in all forms i.e, heat etc) is known as Radiant energy(ηrad.)

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Solid angle:

The angle subtended by an area at a point is called solid angle.

Illumination

Lumen: It is the unit of luminous flux and may be defined as luminous flux, emitted per unit

solid angle, from a light source of 1 candle power (C.P).

Lumen = Candle Power x Solid Angle = C.P x ω

Total Luminous flux emitted by the source of 1 C.P = 4π lumens

Illumination

Candle Power:

We have seen that

Lumen = C.P x ω

C.P = Lumen/ω The candle power may be defined as:

the number of lumens emitted by a source per unit solid angle in a given direction.

It represents the light emitting capacity of a source.

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Foot Candle: It is the unit of illumination. It is the illumination of the inside of a sphere of

radius 1ft with a source of 1 C.P at its centre.

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Some Important Concepts: Transmission

When light passes through an object, it is called transmission.

Absorption, reflection, refraction, and diffusion all affect light transmission.

Illumination Absorption

Instead of completely transmitting light, an object can absorb part or all of the incident light, usually by converting it into heat.

Many materials absorb some wavelengths while transmitting others, which is called selective absorption.

Sky, snow etc? Glass filters: Sunglasses

Illumination

Filtering A transmissive filter is a material that absorbs

some wavelengths and transmits others, while a reflective filter absorbs some

wavelengths and reflects others.

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Diffusion (Scattering) When light strikes a

perfectly smooth surface, the reflection is laminar,

When light strikes a rough surface, the light is reflected or transmitted in many different directions at once, which is called diffusion or scattering.

Illumination

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Laws of Illumination: The Inverse Square Law:

The illumination at a point on a surface when the surface is perpendicular to the direction of the source varies directly with the luminous intensity of the source and inversely with the square of the distance between the source and the point:

where: E = illumination in footcandles (or lux)I = luminous intensity in candlepower (or candela)d = distance in feet (or meters)

Illumination

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Lambert’s Cosine Law

The illumination of any surface varies as the cosine of the angle of incidence, θ, where the angle of incidence is the angle between the normal to the surface and the direction of the incident light.

Combined with the equation just given, the formula becomes:

Illumination

Processes/phenomena which are responsible for generation of light Incandescence Electric Discharge Arc formation Electro-luminance Photo-luminance

Illumination Light is emitted from a body

due to any of the following phenomenon.

Incandescence Solids and liquids emit visible

radiation when they are heated to temperatures about 1000K.

As the temperature increases, the intensity increases and the appearance becomes whiter.

Illumination Electric Discharge

When an electric current is passed through a gas the atoms and molecules emit radiation whose spectrum is characteristic of the elements present.

Electrons fall from High to low energy levels resulting in infrared, visible light, or ultraviolet radiation

Illumination

Electro luminescence Light is generated when

electric current is passed through certain solids such as semiconductor or phosphor materials.

Used for automobile dashboard lighting,

speedometers, etc LCD displays back lights Advertisement boards

Illumination Photoluminescence

Radiation at one wavelength is absorbed, usually by a solid, and re-emitted at a different wavelength.

When the re-emitted radiation is visible the phenomenon may be termed either fluorescence or phosphorescence.

Thank You