Measurement of Luminous Flux, Quantity of Light, and Luminous Emittance

Towards the end of the last century it was gradually realised that the correct quantity characterising a light source is not the luminous intensity, but the total luminous flux. Until then no other method of determining luminous flux was known than that of computing it from the light distribution. This method is not straight forward, however, and efforts have quite naturally been made to devise an instrument by means of which luminous flux could be measured in a simple manner. Various suggestions were put forward, but none of these gave satisfactory results until in 1900 Ulbricht 1 ) solved the problem by introducing his sphere photometer , or integrating photometer .