Scene brightness is a sensation that is not well predicted by light quantities measured in photopic illuminance or luminance. Experiments with light sources of different spectral compositions indicate that scenes illuminated by sources with greater short-wavelength (<500 nm) output are perceived as being brighter than those illuminated by sources with less short-wavelength content at the same light level. This has been shown to be true not only under mesopic, but also under photopic conditions. Furthermore, the relative impact of short-wavelength contributions to scene brightness perception appears to increase with light level. Experimental results are consistent with short-wavelength (S) cone as well as melanopsin contributions to scene brightness, in addition to contributions from V(λ). An empirical model that incorporates increasing S-cone and melanopsin contributions with increasing light levels gave better predictions than a model that only incorporates an S-cone increase. This was confirmed in a series of follow-up experiments.
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