PURPOSE: To minimize the loss of the quantity of light, and to obtain lighting with a uniform illuminance distribution through simple structure, by arranging a treated photosensitive medium, obtaind in a prescribed way, between a light emitting body and a photodetection surface.

CONSTITUTION: A shutter 6 is opened for a prescribed time to irradiate a photosensitive medium 4 with light from a light emitting body 1 through a condenser lens 2. The medium 4 is detached and development and fixation are performed to obtain a pattern blackened in proportion to the quantity of exposure on the medium 4. The photosensitive medium 4 is fitted to a photosensitive medium fixture 5 again in the same state. Luminous flux with a spatially uneven distribution from the light emitting body 1 is passed through the medium 4 to obtain lighting with nearly uniform illuminance on a photodetection surface 3. The loss of the quantity of light due to the deviation of the photosensitive characteristics of the medium 4 from linearity is relatively small as compared with that when an ideal photosensitive medium is used.