2.3. Skin reflectances, PCA and ICA

Earliest studies on skin reflectances were made by Edwards and Duntley (1939), Buck and Froelich (1948), and Stimson and Fee (1953) according to Wyszecki and Stiles (2000). Recently, Angelopoulou (2001) has made noncontact measurement of skin at different places to separate skin objects from those which have skin coloured appearance. There are some shape differences in skin reflectances between her results and earlier measurements. Many models have also been presented for generating and simulating easily different skin reflectances, for example by Ohtsuki and Healey (1998), and So-Ling and Ling (2001). The reflectances obtained can be used for skin colour simulation as in Störring et al. (1999) who computed skin colour appearance under different light sources with one camera calibration. They also compare the calculated skin chromaticities to an average of those obtained from images and find the difference to be reasonable small.

Skin reflectances have been also subjected to principal component analysis, PCA. According to Imai et al. (1996) and Nakai et al. (1998), skin reflectances can be presented by just three basis functions which correspond to different skin colourants like melanin and carotene. PCA (Moon & Phillips 1998) and independent component analysis ICA (Hyvärinen et al. 2001) have also been applied to face images (a comparison between ICA and PCA for colour recognition has been presented by Laamanen et al. (2000). The eigenvectors produced by PCA are called eigenfaces and they are applied to face recognition but usually on grey scale images. Soriano et al. (1999) extended the eigenface approach to RGB images by applying PCA on each colour channel. They found that the first three eigenfaces contain information about the illumination and camera calibration, and are therefore useful for colour correction. ICA has been applied to colour face images to achieve different components like melanin concentration and to simulate the skin colour appearance with different degrees of components (Tsumura et al. 1999). ICA has been also used in medical analysis (Tsumura et al. 2001) and cosmetic research (Shimizu et al. 2001) of skin images.