Qiu, J. et al. Comparison of Object-Color and Illuminant Metamerism for Digital Image Color Correction Proceedings of 13th AIC Congress 2017 Comparison of Object-Color and Illuminant Metamerism for Digital Image Color Correction Jueqin Qiu, Haisong Xu * , Peng Xu State Key Laboratory of Modern Optical Instrumentation, College of Optical Science and Engineering, Zhejiang University, Hangzhou 310027, China * Corresponding Author’s Email Address: chsxu@zju.edu.cn ABSTRACT 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. KEYWORDS: object-color metamerism, illuminant metamerism, digital image color correction INTRODUCTION As the demand for accurate color reproduction grows, various color correction methods have been proposed to reduce the color distortion between the output digital images and the ground truths [1]. However, few studies concerned about the metamerism for the digital image color correction. Since the independent channel numbers of cameras are less than the number of bands required to represent the spectral information of the objects, the metamerism is an inherent uncertainty for the color correction [2]. The color correction module in the digital camera usually uses a transformation matrix to convert the device-dependent color signals to the device- independent color tristimulus values. Because of the failure to meet the Luther-Ives condition by the camera spectral sensitivity, two objects with the same perceptive color by the human visual system (HVS) under one condition may appear different in the output image after color correction, thus reducing the fidelity of color reproduction of the imaging system [3]. In this study, we employed a large set of spectral reflectance data and a spectrally tunable LED light source to evaluate image distortion caused by the metamerism during color correction. Two metamer-distorted images for each test scene of the object-color metamerism were reconstructed and compared with the ground truth by the image quality assessment (IQA) metric to investigate the color distortion caused by the object-color metamerism. Similarly, another two images for each test scene were reconstructed and compared to discuss the illuminant metamerism. METHODS In a typical 3-channel digital imaging system, the raw responses at pixel x can be simply written as ( , , ) (, ) ( ) ( ) , {, d ,} c c p RE R E S c rgb         x X (1)