A proposed uni ® ed system of photometry

A uni ® ed system of photometry is proposed that is based on human vision and allows the speci ® cation of visual stimuli at all light levels. To be useful, however, photometry can never be entirely synonymous with vision. Additivity is an essen-tial characteristic of photometry, yet many visual responses, such as brightness matching, incorporate visual channels that are inherently non-additive in respond-ing to light. Following a review of models of mesopic photometry based on a num-ber of experimental techniques, a system of photometry based on reaction times is proposed to bridge conventional photopic and scotopic luminous ef ® ciency functions through the mesopic region. The basis for the uni ® ed system is a para-meter, X, which describes the proportion of photopic luminous ef ® cacy at any luminance, and luminance can be calculated from a simple closed-form equation. The utility of the system is described, including instrumentation for the measure-ment of uni ® ed luminance. 1