The number of colors discernible by normal trichromats has been estimated for the idealized object-color solid. How well these estimates apply to natural scenes is an open question, as it is unknown how much their colors approach the theoretical limits. The aim of this work was to estimate the number of discernible colors based on a database of hyperspectral images of 50 natural scenes. The color volume of each scene was computed in the CIELAB color space and was analyzed using the CIEDE2000 color-difference formula. It was found that the color volume of the set of natural scenes was about 30% of the theoretical maximum for the full object-color solid, and it corresponded to a number of about 2.3 million discernible colors. Moreover, when the lightness dimension was ignored, only about 26,000 (1%) could be perceived as different colors. These results suggest that natural stimuli may be more constrained than expected from the analysis of the theoretical limits.

The visual effects of lighting on art paintings is an important aspect that should be considered by museum curators. The aim of this work was to determine the correlated color temperature (CCT) of daylight illumination preferred by observers when appreciating art paintings. Hyperspectral images of 11 oil paintings were collected at the museum, and the appearance of the paintings under daylight illuminants with CCT from $\mathrm{25,000}\mathrm{K}$ to $3600\mathrm{K}$ was computed. In a psychophysical experiment using precise CRT reproductions of the paintings, observers had to adjust the CCT of the illuminant such that it produced the best visual impression. It was found that the distribution of observers’ preferences had a maximum at a CCT of about $5100\mathrm{K}$ and that this value did not depend on whether the observers were undergraduate students or museum visitors or on the degree of adaptation to the color of the illumination. These results suggest that observers prefer a more bluish-white light than that normally used in museums.

Painters reproduce some spatial statistical regularities of natural scenes. To what extent they replicate their color statistics is an open question. We investigated this question by analyzing the colors of 50 natural scenes of rural and urban environments and 44 paintings with abstract and figurative compositions. The analysis was carried out using hyperspectral imaging data from both sets and focused on the gamut and distribution of colors in the CIELAB space. The results showed that paintings, like natural scenes, have gamuts with elongated shapes in the yellow–blue direction but more tilted to the red direction. It was also found that the fraction of discernible colors, expressed as a function of the number of occurrences in the scene or painting, is well described by power laws. These have similar distribution of slopes in a log–log scale for paintings and natural scenes. These features are observed in both abstract and figurative compositions. These results suggest that the underlying chromatic structure of artistic compositions generally follows the main statistical features of the natural environment.

Simple color-difference formulae and pictorial images have traditionally been used to estimate the visual impact of color errors introduced by image-reproduction processes. But the limited gamut of RGB cameras constrains such analyses, particularly of natural scenes. The purpose of this work was to estimate visual sensitivity to color errors introduced deliberately into pictures synthesized from hyperspectral images of natural scenes without gamut constraints and to compare discrimination thresholds expressed in CIELAB and S-CIELAB color spaces. From each original image, a set of approximate images with variable color errors were generated and displayed on a calibrated RGB color monitor. The threshold for perceptibility of the errors was determined in a paired-comparison experiment. In agreement with previous studies, it was found that discrimination between original and approximate images needed on average a CIELAB color difference ΔE_ab* of about 2.2. Although a large variation of performance across the nine images tested was found when errors were expressed in CIELAB units, little variation was obtained when they were expressed in S-CIELAB units.

A variety of light sources are used in museum environments where the main concern is to prevent damaging effects of the light on paintings. Yet, the visual impression of an artistic painting is strongly influenced by the intensity and spectral profile of the illumination. The aim of this work was to determine psychophysically the spectral profile of the illumination preferred by observers when seeing paintings dated from the Renaissance époque and to investigate how their preferences correlate with the color temperature of the illumination and with the chromatic diversity of the paintings. Hyperspectral images of five oil paintings on wood were collected at the museum and the appearance of the paintings under five representative illuminants computed. Chromatic diversity was estimated by computing the representation of the paintings in the CIELAB color space and by counting the number of nonempty unit cubes occupied by the corresponding color volume. A paired-comparison experiment using precise cathode ray tube (CRT) reproductions of the paintings rendered with several illuminant pairs with different color temperatures was carried out to determine observers' preference. The illuminant with higher color temperature was always preferred except for one pair where no clear preference was expressed. The preferred illuminant produced the larger chromatic diversity, and for the condition where no specific illuminant was preferred the number of colors produced by the illuminant pair was very similar, a result suggesting that preference could have been influenced by chromatic diversity.