OPTICAL DISCRIMINATION OF SKIN 129 origin of the reflected light, i.e., surface or bulk tissue of skin, is not a problem in subjective assessment since training allows the expert to look at, or into, the skin as required. Using polarized illumination and viewing the polarized and depolarized re- flected light, there is available for the first time a means of preferential selection of pictorial information from different levels in the skin. A clearer understanding of the mechanisms involved can be obtained by considering the schematic representation in Figure 4. Polarized light incident on the skin can reflect from the surface or penetrate into the skin. The former is simply regular reflection from an air-dielectric interface of refractive index 1.5 (19). At near normal incidence no change in the polarization will occur and 100% polarized light will be reflected. There will be some attenuation of the reflected light intensity where the angle of incidence with the non-planar skin surface approaches the polarization angle (20), i.e., in regions showing glare. Light that passes through the air-stratum corneum interface can only return to the surface by reflection from the bulk tissue of the skin. Anderson et al. (21), using diffuse reflection data, concluded that the principal contribution to the remit- tance of light originated in scattering from collagen fibres in the dermis. In the current investigation not only is the scattering changing the direction of the incident light beam through approximately 180 degrees but it is also depolarizing the light. Teale (22) has shown that this is exactly what happens in turbid media, with each scattering event reducing the polarization, expressed as anisotropy, by 0.7 of its orig- inal value. Anisotropy is defined as r = (Ill-IL)/(II1 = 211) where I 11 is the measured light intensity with polarizer and analyser parallel (parallel A B C STRATUH COFLNEUH EP JDE RH ! S DE RH ! S E D Figure 4. Schematic representation of optical paths through skin with polarized light: ¸, Light polarized perpendicular to the plane of the page."O7', Depolarized light. A) Surface reflection. B) Epidermal remit- tance. C) Dermal remittance. D) Forward scattered. E) Dermal scattered. X) Absorption.

130 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS polaroids). I L is the measured light intensity with polarizer and analyser perpendicular (crossed polaroids). Where scattering is most efficient, in the dermis, leading to depolarization, visible light has the highest probability of being absorbed by the hemoglobin in the peripheral blood supply to this region. Each change in direction with a scattering event amounts to an increase in the optical pathlength in an absorbing medium. It is to be expected, therefore, that progressive depolarization would be accompanied by increased spectral attenuation by hemoglobin. As a consequence, there would be a preponderance of re- flected red light and an enhanced contrast of those regions that are normally pink or erythematous. This is exactly the effect demonstrated by the color photography of the depolarized light component of the remitted light. Surface reflected light is all but eliminated by the crossed polaroid on the camera lens since there is a dramatic loss of surface detail when comparing Figure 1C with lB. The enhanced redness in the latter cannot be a photographic artefact or the reference white in each image would show different hues. Such an effect can be seen when comparing the reference whites in Figure lB and 1C with that in 1A where no polaroid filters were present in the optical train. The calculated anisotropy of the remitted radiation at 650 nm (taken from Figure 3), at approximately 0.06, requires multiple scattering to reduce the residual polarization to such a low value. As many as seven scattering events would be required if the anisotropy is reduced to 0.7 for each scattering interaction in the skin. It is not possible to be precise since the calculated anisotropy does not refer uniquely to the dermal remittance but includes polarized reflection from the surface of the stratum corneum. The site-spe- cific sources of the polarized and depolarized light cannot be included in any spectro- photometric corrections that can be undertaken. It must be presumed that the anisot- ropy value is an upper limit. Although the depolarization component has been discussed with respect to only one orientation, the isotropic nature of the light requires an equal intensity for the oppo- site or perpendicular orientation. Both spectral and photographic recordings of the po- larized reflected light therefore contain a depolarized light component. This is consis- tent with the residual erythema shown in Figure lB and the hemoglobin spectrum in Figure 3, contour B. The difference between the polarized and depolarized spectra, contour C, is therefore closer to the corrected polarized reflection spectrum if depolar- ization is complete before remittance of the light from the skin. Although there may be some contribution from an inverted hemoglobin absorption spectrum to the difference spectrum, it is small. No significance can be attached to the absolute intensity, since spectral normalization was undertaken with the polarized and depolarized contours. The featureless nature of the reflection is consistent with the epidermal reflection spectrum published by Anderson et al. (21). The lack of spectral detail in the difference contour gives it a superficial similarity to the polarized reflection spectrum obtained with the integrating sphere, Figure 2, contour B. There is a major inconsistency, however, in that the latter shows much higher absolute reflection intensity, but it must be remem- bered that the difference contour was generated from normalized spectra. Part of the problem relates to the instrumental background correction using a standard white refer- ence overlaid by a sheet of polaroid film. However, application of a sheet of polaroid to the skin with sufficient pressure to ensure a light-tight seal to the integrating sphere would result in compression of the skin. Pressure constriction would reduce blood flow