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