Variations in photopic and melanopic total reflectance of interior colors under different light sources: Which implications for lighting design?

Surfaces’ optical characteristics contribute to determining the spatial and spectral distribution of light within spaces, in turn affecting visual and non-visual responses to light and the general room’s appraisal. Photopic and melanopic reflectance values, expressing surfaces’ potential in reflecting the parts of the radiation able to stimulate human visual and circadian systems respectively, help designers in choosing colors and materials for indoor applications. These values are reported in technical sheets or in online databases referring to the CIE D65 illuminant. However, since both photopic and melanopic total reflectance depend not only on the material but also on the spectral irradiance, the aforesaid values may not be valid anymore under spectra other than D65. Here, this topic is deepened through a systematic analysis: the total photopic and melanopic reflectance values of 195 color samples selected from the Natural Colour System are calculated considering 18 LED spectra grouped in warm, intermediate, and cool, and 4 CIE standard daylight illuminants besides D65. Then, for each sample, the reflectance values (both photopic and melanopic) under each light source are compared against the corresponding ones under D65 obtaining percentage differences. Results show that, for each sample, reflectance values under sources belonging to the same group are very similar to each other, and that those under all daylight spectra are almost equal to the corresponding values under D65. Regarding electric light sources, low percentage differences occur under cool LED, whereas high ones under warm LED. These results are valid for both photopic and melanopic reflectance. Also, color attributes are crucial. Specifically, hue defines the amplitude of percentage difference ranges and if they are positive or negative, with different effects on photopic and melanopic reflectance. On the contrary, chromaticness affects photopic and melanopic reflectance in the same way: the higher the chromaticness, the higher the percentage difference.