Photometry has traditionally been conceptualized primarily in terms of measuring light sources. However, all units of photometry can also refer to measurements of light as it is received. A description of how to measure incident light is developed from the basis that light forming an image is just the reverse of light emitted from a source. The incident approach to photometry reveals how to derive luminance from a measure of illuminance and how to correctly calculate the illumination of an image. It is further recommended that photometric specification of incident light be accompanied by a description of its effectiveness relative to the receiver threshold.
© 1983 Optical Society of America
Thorny H. Nilsson, "Incident photometry: specifying stimuli for vision and light detectors," Appl. Opt. 22, 3457-3464 (1983)