OPTICAL DISCRIMINATION OF SKIN 123 grating sphere attachment with quartz fibre optics. The attachment was fitted to a custom-made support frame (Neo-Tech Engineering) to ensure a light-tight contact of the sample port of the sphere onto the skin of human volunteers. Polarized reflection spectroscopy was undertaken with a polaroid filter fitted over the sample port, thus allowing only polarized reflected light, from the skin, to re-enter the integrating sphere. With this arrangement it was not possible to measure the spectrum of the perpendicularly polarized reflected light and an alternative procedure was used. In vivo polarized reflection spectroscopy of human skin was undertaken with a modified Jobin-Yvon JY3CS computer-controlled spectrofluorimeter. An incident illumination assembly for use with thin-layer chromatography plates replaced the conventional cell compartment. An off-axis concave mirror was placed as close as possible to the normal axis of the incoming incident light beam to collect reflected light efficiently. In this way very small changes in the sample-to-mirror distance (5mm) did not critically affect the measured intensity ([2.5%]), as was the case with the design originally supplied ([10%]). Visible-grade polaroid filters were mounted immediately after the exit slit of the excitation monochromator and before the entrance slit of the emission monochromator. Reflection spectra from sites on the palmar surface of the hand were recorded by running both monochromators synchronously over the same spectral range. Four nanometer slits were used throughout, and in this way spectral bandpass compati- bility was maintained with the Perkin-Elmer Lambda 7 spectrophotometer. Parallel and perpendicular polarized reflection spectra were recorded and are referred to by the orientation vectors of the polaroid filters in the incident and reflected light beams. Since this was a single-beam system, all reflection spectra were corrected for the wavelength-dependent transmission of the polaroid filters by recording the spectrum of a diffuse white standard. This standard was prepared by painting a thick film of Kodak white reflectance coating onto a bright aluminium plate 7 cm by 7 cm. The recorded, paired spectra of the skin were then normalised by setting the parallel polarized reflec- tion value at 700 nm to unity. In this way only relative spectral intensity changes could be compared. TRANSMISSION SPECTROSCOPY The methods used to measure the spectral transmission characteristics of polaroid filters were determined by their size. Filters small enough to cover a camera lens were measured with the Perkin-Elmer Lambda 7 operating in transmission mode. In order to eliminate contributions from wavelength-dependent polarization changes from the grating monochromator, the in- strument background was normalized with a sheet of polaroid in the sample beam. A second (test) polaroid filter, set to the same transmission vector orientation as the first, was introduced into the cell compartment but closer to the exit window. In this way, spectral intensity changes would be due to light absorption only, as expected for an isotropic light source. Large sheets of polaroid filter were measured using a monochromatic illumination as- sembly composed of a 2.5 kW Xenon arc lamp and collecting optics (Oriel Scientific) and a Monospek 600 monochromator (Rank-Precision). A 1.8-metre-long quartz fibre optic (Schott) conducted the light to the polaroid filter placed in front of a UV/visible

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