Kruithof's rule revisited using LED illumination

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Abstract
We have investigated the effect of colour temperature and illuminance on the visual response. Since Kruithof's (1941) work on general illumination, it has been proposed that users would find ‘pleasing’ high correlated colour temperature (CCT) illumination at high illuminance and low CCT illumination at low illuminance, although it is not unanimously accepted. Here, we question whether the pleasing sensation comes from facilitation of visual performance or from subjective appraisal. We have conducted experiments at various illuminance levels and various CCTs including performance tasks and subjective scaling of illumination quality. The change of illumination was obtained using adjustable LED clusters with high colour rendering index (R a > 90). Whereas the performance tasks yield results which depend on luminance and only slightly on CCT, observers reported that high CCT illuminations look brighter than low CCT illuminations. We investigate the effect of the various spectral power distributions on the intrinsically photoreceptive retinal ganglion cells and on the pupil response.