Short notes on Illumination and Units of Illumination


Illumination is that which produces no strain on the eye.

Light – Radiant energy which produce a sensation upon the human eye.

(A) Luminous Flux (Flux of Light):


Light energy radiated/send from a luminous body.

Unit – Luman

(B) Luman:

Flux emitted/unit solid angle from uniform source of one candle power.


(C) Luminous Intensity Unit Candle Power/Luminosity of a source in any particular direction is given by the Luminous flux radiated/ unit solid angle in that direction

Point source –

Luminous flux crossing any section of narrow cone of solid angle.

(D) Solid Angle:


An area of the part of the surface of a sphere of radius solid angle

Laws of Illumination:

(i) Cosine law:

Illumination Varies as the cosine of the angle between the normal to the surface & the direction of the incident light. Design of lighting scheme.



Transmission factor, coefficient of transmission, is the ratio of the total transmitted light to the total incident light.


Reflectance factor, reflectance coefficient. The ratio of reflected to incident light.



(Photometric brightness) Reproducible state of object. Problem A 40 W, 430 m.a, 122 cm fluorescent tube produces 3200. What is the luminance on the floor of a 3m x 3m room assuring 40% overall efficiency & uniform illumination. Solution-useful lumens

Calculate the luminance of a standard inside frosted 100 W light bulbs (an A-19 bulb) with a maintained output of 1700 lm.

Assume that the optical glass globe of 20 cm diameter and a transmittance of 35% surround the above bulb. Calculate the luminance of the globe. One candle produces 4 n lumens, distributed spherically.

Lighting Schemes/Systems: These are –

(i) Direct lighting is thrown towards the ceiling where it is diverted to the room through diffuse reflection. This lighting suitable for drawing offices, workshops.

Illumination will be depressive to the eye.

(ii) Semi direct lighting 50% of the light is sent from the source direct the reading plane and 30% is send upward.

Different globes are used scheme provides uniform distributed light

(iii) Semi indirect lighting 40% light is sent upward and 40% is sent directly on the surface.

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