Photopic luminance does not always predict perceived room brightness

The perception of room brightness over photopic luminances ranging from 30 cd m -2 to 67 cd m -2 was judged by 12 subjects in an almost uniformly white experimental chamber. Two different illuminants were compared which had different spectral compositions, but were colour matched. Brightness judgements were often opposite to large differences in photopic luminance. These results are inconsistent with models of brightness perception that depend solely on cone receptors. At the luminance levels considered here subjective evaluation of light intensity depends upon both photopic and scotopic spectral contributions. These results imply that aspects of the visual system operate mesopically under most interior lighting conditions.