OPTICAL DISCRIMINATION OF SKIN 131 and hence the measured contribution from hemoglobin absorption to the reflection spectrum. Depolarization due to multiple scattering would therefore give higher avail- able light intensities in the absence of hemoglobin, with the parallel oriented vector of the isotropically reflected light contributing to the polarized spectrum. Any additional spectral absorption must be attributed to other light-absorbing moieties that are nor- mally lost under the hemoglobin spectrum. Absence of pressure on the skin is therefore essential if quantitative data are to be compared with the pictorial contrasts shown in Figures lB and 1C. Since no window was present when the spectrofluorimeter was used to record polarized and depolarized spectra, the differences shown in Figure 2 are probably better semiquantitative evalua- tions of the relative pictorial differences of Figures lB and 1C. This result is confirma- tion of the skill of the trained assessor in that all surface topography information is based on evaluating small differences in the amounts of reflected light. When trained assessors score a clinical condition, they can orient the specimen or change the viewing angle in order to optimize the contrast in the feature of interest. The major advantage of polarized and depolarized image recording is in the optimiza- tion of contrast without the need to orient specific regions. Since the polarized image is a composite of polarized and depolarized reflected images, pictorial subtraction of the superimposable depolarized image should leave a difference image of polarized reflected light only. This would have little color and contain only detail of the surface topography of the stratum corneum. If depolarization of light due to surface scattering from rough or dry skin were present, as is suggested by scrutiny of the magnified fine surface detail from Figure 1C, then these features would be atten- uated or lost from the difference image. This may appear to reduce the optical segregating power of the technique but could be used to advantage by pictorial manipulation with an image processor capable of working in color. A measure of the enhanced redness in the depolarized image would allow in vivo quanti- tative analysis to be undertaken with the minimum contribution from skin surface reflection. The latter, however, could still be present as superimposed white or off- white detail since it would arise from fine surface detail depolarization. If discriminating white from pink or red was possible, then the fine surface scattering sites on the skin could be distinguished from the gross surface topography, an operation which the trained assessor does well. The advantage of objective image analysis, how- ever, lies in the very small changes which can be quantitatively determined using an electronic sensor with a finer discrimination of light intensity than is possible by eye. ACKNOWLEDGEMENTS The authors wish to thank Justin North of Graphic Design Section, Unilever Research, Colworth House, for the color photography and preparation of the prints. REFERENCES (1) I. Kanton, W. G. Ballinger, and R. C. Sevin, Severely dry skin: Clinical evaluation of a highly effective therapeutic lotion, Cutis, 30, 410-424 (1982).

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