Higher Color Temperature Lamps May Not Appear Brighter

Abstract

This study examines the belief that rooms illuminated with higher correlated color temperature (CCT) lamps will appear brighter than rooms illuminated with lower CCT lamps at the same illuminance. This belief is held by many illuminating engineers and lighting designers, despite the fact that it is not supported by theory, and has received mixed support in experimental studies. The linear brightness models of Guth and Howett, and the nonlinear color appearance models of Nayatani and Hunt predict that higher CCT sources will appear dimmer than lower CCT sources, though the predicted brightness differences are small. We report results from two experiments to test the predictions of these models. Both experiments involved the visual comparison of two identical side-by-side full-scale rooms. One room was illuminated with lamps having a nominal CCT of 3500K; the other room was illuminated with lamps having a nominal CCT of 6500K. All other photometric characteristics of the two rooms were held constant and the rooms were identically furnished as mirror images with typical office furniture. In the first experiment forty-one subjects used a forced choice survey instrument to identify which room appeared brighter. In the second experiment twenty-eight subjects used a dimming adjustment task to match the brightness in the two rooms. No relationship was found between CCT and perceived room brightness in either experiment. Evidence from linear brightness models, nonlinear color appearance models, and the results of the psychophysical experiments reported here suggests that that there is no relationship between CCT and the perception of interior brightness. Although brightness perception is related to the spectral power distribution of an illuminant, CCT is too limited to characterize the relationship between source color and perceived brightness.