The number of colors perceived by dichromats when appreciating art paintings under standard illuminants

The chromatic content and diversity experienced by normal observers when observing art paintings vary with the spectral composition of the illumination and can be estimated by quantities such as color rendering indices and the number of discernible colors. Can these estimates be extended to color deficient observers? The aim of this work was to investigate how the number of discernible colors perceived by dichromats in art paintings varies when the paintings are rendered under CIE standard illuminants. Hyperspectral images of eleven oil paintings were collected at the museum and the number of discernible colors perceived by trichromats and dichromats under 55 CIE illuminants was estimated for each painting. It was found that the number of discernible colors varies considerably across illuminants for all classes of observers analyzed. When comparing with CIE standard illuminant A, substantial enhancement of about 14%, 30%, 20% and 10%, could be obtained with specific illuminants for trichromats, protanopes, deuteranopes and tritanopes, respectively. These results suggest that color deficient observers may require personalized lighting conditions. Introduction The rich chromatic diversity of art paintings and its appreciation by normal trichromats is very dependent on the spectral composition of the illumination used [1-3]. The chromatic diversity perceived by dichromats in the context of natural scenes is considerable impaired and of about 7% [4] of the number of discernible colors perceived by normal trichromatic observers. In spite of the severe impairment in color vision due to the lack of one cone type, some chromatic enhancements can be obtained with colored filters in the context of natural scenes [4, 5]. The availability of modeling allowing the conversion of dichromatic color vision to perceptions experienced by normal trichromats [6] enables computational estimations of the chromatic diversity perceived by dichromats and to study the influence of colored filters or illuminants. The aim of this work was to investigate how the number of discernible colors perceived in art paintings by dichromats varies with standard illuminants. Methods Eleven oil paintings were digitalized in the Museu Nogueira da Silva, Braga, Portugal, with a hyperspectral system. The system consisted of a low-noise Peltier-cooled digital camera with a spatial resolution of 1344×1024 pixels and 12-bit output (Hamamatsu, C4742-95-12ER, Hamamatsu Photonics K.K., Japan) and a fast-tunable liquid-crystal filter (VariSpec, model VS-VIS2-10HC-35-SQ, Cambridge Research & Instrumentation, Inc., MA, USA) mounted in front of the lens (for more details on the hyperspectral system see [7]). The hyperspectral digitalization was carried out over the range 400720 nm at 10 nm intervals. The paintings were illuminated with low level SoLux® illumination to avoid over exposition to high intensity levels and consequent painting damage. The spectral reflectance of each pixel of the paintings was estimated from a gray reference surface present near the painting at the time of digitalization. Illuminant spatial non-uniformities were compensated using hyperspectral measurements of a uniform surface imaged in the same location and under the same illuminating conditions as the paintings [2]. 400 480 560 640 720 0.0 0.5 1.0 HP2 R e la ti v e P o w e r