| Summary: | Abstract The growing importance of publishing art collections online has led to increasingly strict tolerances on digital photography of art objects. There are two internationally recognized sets of guidelines for creating high-quality digital images, Metamorfoze and FADGI. These guidelines require using sets of standardized color patches in museal photography. The X-Rite ColorChecker SG chart with 140 color patches is often used. Recent studies showed that even in standardized conditions it is often difficult to satisfy the strictest guidelines on color accuracy for camera profiling, with no indications for improvements. We report results of our investigation into the bottlenecks in achieving high color accuracy. We show that a large part of the color deviations originates from the 15 black color patches of the ColorChecker SG chart. These patches have a large impact on the average color deviation and the maximum color deviation that are the performance measures for color accuracy in the FADGI and Metamorfoze guidelines. We show that spectrophotometer measurements for the black patches produce color deviations dE(CIE 1976) ranging from 3.7 to 5.2 with respect to reference data, making it impossible to meet the strictest Metamorfoze guidelines. The black patches push the average color difference CIEDE2000 from 0.59 to 0.82 with respect to reference data already when using spectrophotometer data. Since the strictest FADGI guidelines prescribe an average CIEDE2000 = 2.0, this leaves little tolerance for errors due to lighting and camera profile. Our results indicate that the common practice of manually tweaking camera profiles until software suggests sufficient color accuracy is obtained with respect to suppliers’ reference data often does not improve color representation but makes it worse, especially for representing dark nuances. This is unfortunate for example for the digital photography of seventeenth century Dutch paintings, where dark passages occupy large areas of the art works. We show that the key step in achieving color accurate digital photography is to use customer reference data rather than commonly used generic reference data. We explain the results by investigating not only the color properties of the ColorChecker SG chart, but also its glossiness.
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