Q.2 What is the principle of photometry?

Ans : Principle of Photometry.: If two light sources produce equal illumination in a screen, their illuminating powers are proportional to the squares of their distances from the screen. This is the basic principle of photometry. In photometry, the illuminating powers of light sources are compared. This principle is applicable only for those two light sources that emit lights of same colour. Illustration: Let S1 and S2 be the two sources of light kept at distances r1 and r2 respectively from a screen M as shown in figure. Let L1 and L2, be the illuminating powers of the sources. If they produce same illumination on the screen, the principle of photometry may be established in the following way. The illumination produces on the screen by

This indicates principle of photometry.