The color rendering index “CRI” and correlated color temperature (CCT) describe the quality of the light. As lighting technology evolves into various types and colors, simply measuring the lumens provides an inadequate basis for predicting how well people can see. In bright lighting, our pupils contract, allowing more detail to be perceived, while depth of field and perceived brightness also increases. In low light, our eyes dilate to allow in more light.
Sam Berman, formerly with the Lighting Systems Research Group at Lawrence Berkeley Laboratory and a major supporter of the importance of the P/S ratio in lighting selection, developed a conversion factor that applies the P/S ratio to lumen output of various light sources. The ratio expresses the effective lumens the eye will perceive for vision based on the size of the pupil and the effect on vision (see Table 1 below). Some lighting, such as a low-pressure sodium lamp, loses most of its output using this method. A high-quality Induction lighting, on the other hand, maintains its output throughout nearly its entire lifetime.