124 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS photodiode detection system (Model 7052, Oriel Scientific). In this way there was no restriction on the size of filter or on the location over the surface where spectral trans- mission was to be measured. Multi-reflection in the fibre optic reduced the inherent polarization from the grating monochromator to an acceptably low level (dichroic ratio = 1.04). IN VIVO TEST SPECIMENS Human volunteers showing a wide range in the clinical condition of their hands were recruited. Particular attention was paid to the degree of redness exhibited. Photography was undertaken with either palmar or dorsal surface uppermost. Reflection spectroscopy was always undertaken on the palmar surface close to the base of the thumb or near the edge, i.e., thenar and hypothenar eminence, respectively. These fleshy parts of the hand made it easy to obtain a light-tight seal to the spectrometer. RESULTS The spectral transmission characteristics of the various polaroid filters used throughout this study were found to be very similar. Representative values for absolute transmission are given in Table I. Photography with such filters would tend to skew the color ren- dering towards the red since the constancy of transmission from green to red (550-700 nm) is not maintained in the blue (400-500 nm), where the transmission changes by a factor of approximately 2, i.e., 32% to 14%. This is not an unexpected result with a filter showing a predominantly brown color. COLOR PHOTOGRAPHY OF HUMAN SKIN USING POLARIZED LIGHT The changes in the optical discrimination of skin condition when viewed under unpo- larized and polarized illumination are clearly demonstrated in Figure 1 for a hand showing a minimal erythemal condition. Figure 1A is a conventional color photograph taken with unpolarized light. The picture is composed of sheen from light reflected near to the critical angle and with modulated light intensity from the fine surface structure superimposed on the pink to red of the underlying tissue. Figure lB is the same feature photographed in polarized light with the transmission vector of all polaroid filters par- allel. As can be seen, there is attenuation of the underlying red and enhancement of the sheen and the surface detail. Finally, the complementary image, Figure 1C, where only Table I Wavelength-Dependent Light Transmission Characteristics of Visible-Grade Polaroid Filters Wavelength % Transmission 700 39 650 36 600 34 550 34 500 32 450 26 400 14

OPTICAL DISCRIMINATION OF SKIN 12 5 Figure I. Color photography of the hand: A) Using conventional flash illumination. B) Using polarized flash illumination and a polaroid filter on the camera lens oriented parallel to the incident light. C) As (B) but with the polaroid filter turned through 90 ø and crossed with respect to the incident light. the orientation of the polaroid filter on the camera is changed to perpendicular with respect to those on the lamps, give a record of those areas where depolarization has occurred. It is clear that the distribution of red in the skin does not have any obvious superimposed contribution from surface reflected light. Not only has the sheen gone but also the bulk of the fine detail from the stratum corneum. These differences are more obvious if the photographs are scrutinized with a x 3 to x 5 magnifying lens. The surface sulci are more clearly delineated in Figure lB than in Figure 1A. Figure 1C, however, shows clearer delineation of underlying redness than Figure 1A. A mottled pattern is obvious in the depolarized image which is not at all clear in the unpolarized image. Major folds in the palmar surface and that of the fingers, where the stratum corneum is thinner, are clearly outlined in red with greater contrast than in Figure 1A. Under magnified examination a subtle network of fine white lines