Varnish layers applied to paintings often discolor as they age, upsetting the original colour relationships intended by the artist. The removal of aged varnish layers using physical and chemical means is a highly skilled and often time-consuming operation, which is not lightly undertaken. There are many aspects under consideration before embarking upon such treatment, including the visual result inferred by spot cleaning tests. In this article, we develop a technique for digital removal of discolored varnish that can help to envisage how a painting will look following cleaning treatment. The digital technique was applied to Vincent van Gogh's painting Field with Irises near Arles (May 1888), in parallel to the painting actually being cleaned, which allowed direct validation of the method developed. In the new method, we utilized not only hyperspectral data from parts of the painting with and without the varnish, but also experts' identification of spots on the painting where unmixed white pigment has been applied. The physical model that we use is based on Kubelka-Munk twoconstant theory, commonly used to model the optical properties of paint. We show that with this model it is possible to determine the transmittance and reflectance of the varnish layer as function of wavelength. Results from previous studies confirm the calculated values. With the new method, we created a high-resolution digital image of the painting, as it would look after varnish removal, at a moment when the actual varnish was still present on the painting. The new method may help conservators and others involved in decisions made regarding issues of varnish removal from paintings, or may help to visualize the colors of a painting without discolored varnish in cases where its physical removal cannot be safely accomplished and so is not an option. K E Y W O R D S cultural heritage, Kubelka-Munk, varnish wileyonlinelibrary.com/journal/col Color Res Appl. 2018;43:150-157.
Colors in many paintings of great art historical value have changed over time, due to the combined effects of natural ageing, accumulated surface grime, and materials added during later conservation treatments. The physical restoration of the colors in such paintings is not possible. This article describes one part of work done to digitally restore the colors of Van Gogh's painting Field with Irises near Arles, dating from May 1888. We have used multispectral reflectance data to estimate absorption K and backscattering S parameters of Kubelka-Munk 2-constant theory. This was done for all 13 pigments known to have been used by Van Gogh in this painting, and based on this the concentration maps for each of these pigments were calculated. We validated the calculated concentration maps in several ways. For some pigments, we were able to predict spots on the painting where the pigment is expected to occur in unmixed form based on visual examination. For several other pigments, the concentration maps could be shown to agree with XRF data. Finally, for some other pigments the concentration maps were supported by additional evidence from microscopic examinations, remarks in Van Gogh's letters and from early color reproductions. For the 1.7 million pixels for which multispectral data is available, the average color difference between the calculated and measured spectral reflectance curves is CIEDE2000 5 1.05. This further confirms that the Kubelka-Munk calculations are well suited to describe the variety of spectral reflectance on the painting. K E Y W O R D S Color formulation, cultural heritage, pigments wileyonlinelibrary.com/journal/col Color Res Appl. 2018;43:158-176.
The default method for color representation on displays involves sRGB as device-independent encoding color space. For improving color reproduction accuracy, we develop a device-specific display characterization model for the Apple iPad Air 2. In separate articles, we will evaluate the same method for other devices and for other display technologies. This model is combined with an easy-to-implement new method to account for the influence of illuminance from ambient light. The combination is called the mobile display characterization and illumination model (MDCIM) for the iPad Air 2, representing modern liquid crystal displays. Seven observers performed psychophysical tests at ambient illuminance levels from 600 to 3000 lx. They visually compared colors of calculated images with those of physical samples. The MDCIM model achieves similar color reproduction accuracy as when using the default method involving sRGB encoding at 1000 lx, while considerably improving color accuracy at other illuminance levels. At 600 lx, 98% of the observers prefer images directly generated with the MDCIM model over those created using the default method. The average color reproduction accuracy improves by two categories on a five-point scale. At 3000 lx, the percentage of colors that is represented at least reasonably well, according to the subjective assessment of visual observers, increased from 0% to 60%.
In earlier articles, we determined the spatial distributions and concentrations of all pigments used by Van Gogh in his painting Field with Irises near Arles. The colors of some pigments are expected to have changed over time, especially those of chrome yellow, cochineal, and eosin lake. For all pigments in this painting, we made physical paint reconstructions by following historical sources on raw materials and production processes, and we determined their optical properties. We combined this with pigment concentration maps to reconstruct the original colors of the painting digitally. When substituting the reconstruction paints into the calculations, we found that technical‐scientific data was not sufficient to resolve several issues. In those cases, discussions within the broad interdisciplinary team allowed us to make informed decisions. These issues refer to the representation of the sky area, and the original contributions of the red lake pigments to local colors. The digitally reconstructed colors of the painting show that due to discoloring of red lake pigments, the irises in the field have changed from a warm purple to purplish blue, and many pink spots in the field have turned to white. The range of yellows in the field has decreased and partly turned to dark brown. The digital reconstruction gives a better understanding of the color scheme used by Van Gogh when compared to remarks the artist made in letters when describing this painting. Also, the original color composition is seen to be aligned with color theories on which Van Gogh based his work.
For suitable illumination and observation conditions, sparkles may be observed in metallic coatings. The visibility of these sparkles depends critically on their intensity, and on the paint medium surrounding the metallic flakes. Based on previous perception studies from other disciplines, we derive equations for the threshold for sparkles to be visible. The resulting equations show how the visibility of sparkles varies with the luminosity and distance of the light source, the diameter of the metallic flakes, and the reflection properties of the paint medium. The predictions are confirmed by common observations on metallic sparkle. For example, under appropriate conditions even metallic flakes as small as 1 μm diameter may be visible as sparkle, whereas under intense spot light the finer grades of metallic coatings do not show sparkle. We show that in direct sunlight, dark coarse metallic coatings show sparkles that are brighter than the brightest stars and planets in the night sky. Finally, we give equations to predict the number of visually distinguishable flake intensities, depending on local conditions. These equations are confirmed by previous results. Several practical examples for applying the equations derived in this article are provided.
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