Comparison of object-color and illuminant metamerism for digital image color correction


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Metamerism is the phenomenon that two object colors, which are colorimetrically indistinguishable under one lighting and viewing condition, become distinguishable under another condition. Since the number of channels of an RGB camera is less than that required to represent the spectral information, the variation of either the captured object or the illuminant may introduce color reproduction errors when transforming device-dependent RGB values to device-independent stimuli. In this study, we collected and utilized a large spectral reflectance database to investigate the color reproduction errors corresponding to the object-color metamerism, and employed a spectrally tunable LED light source to generate spectral power distributions (SPDs) that were metameric to a specific illuminant to analyze the reproduction errors corresponding to the illuminant metamerism. The image quality assessment (IQA) metric was adopted to evaluate the degree of image distortion caused by the two types of metamerism. The IQA results indicate that, compared with the illuminant metamerism, the object-color metamerism has little impact on the accuracy of color correction, and consequently the acquisition of the SPD of the illuminant is the critical factor for high-fidelity color reproduction.


Figure 1. Schematic diagram of the reconstruction of the color corrected images $I_\textrm{ideal}$ as well as the distorted image $I_\textrm{OCM}^k$ by the object-color metamerism.
Figure 2.The SPDs of the 8 test illuminants (blue solid line) with the daylight metamers (black dashed line) and 4-channel LED metamers (red dotted line). From left to right, top to bottom: CIE-A, D50, D100, CWF, F8, TL84, LED, and iPhone Flash. Note that CIE-A and TL84 have no daylight metamer since their CCT are below 4000K.
Figure 3. The DCSCI scores of two degrees of the object-color metamerism and two types of the illuminant metamerism.



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