Towards Consistent Color Image Acquisition in Dermatology

We 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 ringlight, frame grabber and PC. The calibration procedure involves building a profile of the acquisition system based on the 24 color targets. This profile is easily checked before a set of image acquisitions and remains valid over 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 the CIE L*a*b* space, allowing percetual color differences ∆E∗ ab to be computed. The field of view (FOV) of the system is 1.5 by 2.0 cm, and the resolution is around 38 pixels/mm for a 570 by 760 pixel image. Although the accuracy of the proposed procedure is not very high (∆E∗ ab > 10 for some of the MBCCC targets), the precision or reproducibility for those targets is quite good with an average error of 〈∆E∗ ab〉 = 0.34, and a maximal error of ∆E∗ ab < 1.2. For in vivo measurements typical maximal error is more like ∆E∗ ab < 2.2 based on preliminary findings, mainly because of the difficulty in determining consistent regions of intrest (ROI’s) in different images. Keywords— Color calibration, Dermatology, Color image acquisition, camera calibration, CCD camera