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We propose a small field-of-view color image acquisition system for the imaging and measurement of skin lesion and its properties in dermatology. The system consists of a 3 chip CCD camera, a frame grabber, a high-quality halogen annular light source and a pentium PC. The output images are in a standard device-dependent color space called sRGB or ITU-R BT.709 which has a known relation to the device-independent CIE XYZ color space and provides a fairly realistic view on a modern CRT- based monitor. In order to transform the images from the unknown and variable input RGB color space of the acquisition system to the sRGB space a profile of the acquisition system is determined based on 24 color targets with known properties. Determination of this profile is simple and quick, and it remains valid for many hours of operation (weeks or even months of normal use). Precision or reproducibility of the system is very good, both short-term (consecutive measurements) <(Delta) E*ab> equals 0.04 with (Delta) E*ab less than 0.1, medium-term (measurements under one profile but on different warm-up cycles) <(Delta) E*ab> equals 0.34 with (Delta) E*ab less than 1.2. Long-term precision (measurements under different profiles) is of the same order. Accuracy was evaluated for profiles based on different RGB to sRGB polynomial transforms computed both by linear least-squares in the sRGB space and by non-linear optimization in CIE L*a*b* color space. Results show that, using a set of test targets consisting of 15 paper color targets and 12 real measurements of human skin, the simple linear transform outperforms higher order polynomials and has <(Delta) E*ab> equals 6.53, with (Delta) E*ab less than 11.21. A small study of the pigmentation of the human skin after UV-radiation shows that when measuring areas of at least a few hundred pixels differences of more than 2 - 3 dE units are statistically significant.
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