Consistent digital color image acquisition of the skin

The authors propose a simple, quick and practical color calibration procedure that results in a precise (reproducible), albeit not very accurate (close to values obtained with a reference device), color description in a standard device-dependent color space (sRGB). The system consists of a 3-chip CCD camera, a continuous ring-light, frame grabber and PC. The calibration procedure involves building a profile of the acquisition system based on 24 color targets with known characteristics. This profile is easily checked before a set of image acquisitions and remains valid for a long period. The acquired images are transformed from the device-dependent RGB camera space to the gamma corrected sRGB (or ITU-R BT.709) space and are readily displayable on a CRT-based monitor. Moreover, sRGB tristimulus values are readily transformed to the CIE L*a*b* space, allowing perceptual color differences (ΔE_ab*) to be computed. Although the accuracy of the proposed procedure is not very high (ΔE_ab *>10 for some of the color targets), the precision or reproducibility is quite good. The short-term precision based on 20 consecutive measurements of a white patch is (ΔE_ab*)=0.04, with ΔE_ab*<0.1. The medium-term precision, based on 10 measurements of the 24 color patches made during different warm-up cycles of the acquisition system under one profile is (ΔE_ab*)=0.34, with ΔE_ab*<1.2. Long-term or inter-profile precision is of the same order, even when the color temperature of the light source is changed between profiles