Page 327 - Electrician - TT (Volume 1)

ELECTRICIAN - CITS

Law of illumination
The illumination on a surface depends upon the luminous intensity, distance between the source and surface and
the direction of rays of light. It is governed by following laws:

1 Inverse square law
2 Lambert’s cosine law
Inverse Square Law
This law states that the Illumination (E) is directly proportional to the luminous intensity (I) of the source.
E α I

Illumination (E) is inversely proportional the square of the distance between the source and plane.
E α 1/d²
Illumination lumen/ sq.m.
Where,
I- luminous intensity in a given direction.

d- the distance.
Lambert’s cosine law
The law states that Illumination varies directly as the cosine of the angle between the direction of the incident light
and the normal to the plane.
E α cosθ
combining two laws,
E= I cosθ/d²

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CITS : Power - Electrician & Wireman - Lesson 54-59