2003 年 40 巻 2 号 p. 97-108
When proofing spot colors on the direct digital color proof (DDCP), spot colors must be composed of existing DDCP colorants. However, if a spot color is outside the DDCP color gamut, the spot color cannot be reproduced colorimetrically. One of the optimum solutions is to find the perceptually closest color in the gamut using a color difference equation that accurately reflects visual estimates. In this research, five color-difference equations, CIELAB, CMC, BFD, CIE94, and CIEDE2000 were examined. Two types of psychophysical experiments were conducted. The first was a paired-comparison experiment which derived sets of interval scale values. Colordifference values calculated using each equation above were converted into interval color-difference values, and were compared with the interval scale values. The second was a categorical scaling experiment. Evaluation values were derived and compared directly with color-difference values. In addition, the composite-standard method was applied to derive interval scale values in the second experiment. It seemed to estimate perceptual color distances between target and reproduced colors using perceptibility criteria in the paired-comparison experiment and acceptability criteria for the categorical scaling experiment. In both experiments, BFD (1:1) and CIE94 (2:1) indicated the best correlation with the psychophysical evaluations. Additionally, the relationship between color difference values from BFD (1:1) and evaluation values from the categorical scaling experiment were examined to determine the limits of acceptable color difference for practical use.