Evaluation of Museum Lightings by Indices of Color Variety

This paper evaluates museum lighting sources by proposing indices of color variety perceivable in artworks under illumination. We adopt three types of white LED with high Ra values of 90, 96 (excited by blue ray, B-LED), and 97 (excited by purple ray, P-LED) where correlated color temperatures (CCTs) are 3000K and 4000K, and we use some other supplementary LEDs. These LEDs are applied to two groups of oil paintings. The first depicts the same hydrangea motif drawn under two different lightings, and the second is two masterpieces in the National Museum of Western Art, Tokyo. We propose and test three indices of color variety. Two are color standard deviations σ376 and σ300 of CIELAB color space measured by CIE1976 color difference ΔE*ab and CIEDE2000 color difference ΔE00, respectively. The third index is color diversity S obtained via color entropy of an artwork’s probability distribution in the CIELAB color space discretized with ΔE00. A color reference chart and a numerical model of color distribution are preliminarily examined to show basic properties of the indices. Experimental results of the paintings reveal the effectiveness of σ300 and S and show that P-LEDs may yield higher color variety than B-LEDs with Ra values nearly equivalent to those of P-LEDs with considerable certainty. We discuss similarity between the properties of σ300 and those of S from the viewpoint of distribution bias in the CIELAB color space and mention a kind of correlation between the similarity and Ra value.