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Illumination Photometer

Instruments calibrated to give direct reading of illumination are called illumination photometers.

1. Benjamin food-candle metre: A translucent screen is illuminated from below by a lamp run at a standard voltage. The illumination at different parts of a screen is marked on the scale. The scale is calibrated and a Bunsen grease spot over the screen and when the spot disappears the illumination on its upper surface is equal to that of the screen beneath. Thus, the illumination can be read from the scale.

2. Weber illumination meter: A is a ground glass screen receiving the unknown illumination and B is another screen whose illumination can be varied by enhancing its distance from a standard lamp. The scale at B is calibrated to give the illumination of B directly. When A and B, viewed through the Lummer-Brodhum cube C appear to be equally bright, they are equally illuminated. Thus, illumination from A is directly measured from the reading of the scale at B.