50 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS 1.55 1.33 n (dry horny layer) I fallin moisture content n 260 300 310 350 WAVELENGTH [NM] Figure 16. Hypothetical scheme of moisture-induced changes of refraction indices. noise-ratio is lower on account of lower lamp power and the sensitivity of the photo- multiplier. For improvement the attenuator used in the preceding study should be eliminated, increasing the signal intensity (300 nm). The wavelength-dependent intraindividual variation coefficient generally shows a max- imum around 330-340 nm (Figures 5 and 6). Because this maximum is coincident with the main absorption of NADH, this could mean that changing metabolic states of skin are responsible for this phenomenon. The individual intensity values irrespective of wavelength can be better described as logarithmic distributions than as linear ones evaluated by the Kolmogoroff-Smirnow test. This seems to fit into the idea that absorption processes play a major role in forming the general shape of the standardized spectra: Lambert-Beer's law relates the logarithm of light intensity to penetration pathlength and concentration of absorber. The low reflectivity below 310 nm is probably mainly determined by low-molecular-weight components of the horny layer. Occlusion and stripping increase reflectivity between 300-340 nm and decrease it below 300 nm. The reason might be the anomalous dispersion of the refraction coefficient n (8) around main absorption peaks (•. max. "• 295 nm) (Figure 16). Rising reflection may be caused by increased scattering, and rising differences at interfaces will increase reflection (Fresnel's law). Moisturization such as stripping removes dry levels of horny layer, leaving moist layers. Increasing water content (ni% o = 1.33) of corneocytes will decrease the ni• of hydrophylic cell content and leave constant n L of lipophilic parts like membranes or hydrophobic proteins (nsc= 1.55), resulting in rising n = n•. - n• with falling n•. In the UVB/C-range, n after moisturizing is apparently smaller than in

HUMAN SKIN UV/VIS REFLECTION SPECTRA 51 the drier state therefore scattering will be less, optical paths will be longer, and the chance of absorption will be larger. Such phenomena will repeat at each absorption peak but will be negligible above 340 nm on account of increasing penetration depth and the resultant falling influence of moisturization of the upper horny layer. Light-absorbing compounds very often show shifting absorption and fluorescence peaks, depending on the polarity of the solvent and the resulting stabilizing effects on ground and excited states (13). Adsorption of endogenous UV filters (such as aromtic amino acid or urocanic acid) to surfaces of corneocytes, modified by moisture content, might be another important factor influencing light-absorption characteristics of human skin (14,15). Erythema either induced by UVB, pressure (dermographism), or nicotinic acid ester shows the normal O2-hemoglobin absorption at 410 nm, 540, and 580 nm. The absorption at 460 nm is probably caused by bilirubin. Increased pressure during mea- surement decreases these absorptions. Skin with insufficient venous flow (the lower legs of elderly people) is characterized by intense absorptions at 440 and 560 nm, mainly the absorption of desoxyhemoglobin. Water extracts low-molecular-weight ingredients from the horny layer. The spectra of Figures 13 and 14 show an overlapping of the increased reflectivity caused by increased H20 content (300-340 nm) and caused by the increasing potential of soap and SDS to elute water-soluble substances absorbing in the spectral range of 250-300 nm. This is especially well recognized when the hydrated stratum corneum loses water (Figure 12) and the primarily lowered reflectivity is changing into increased reflectivity, caused by deficits of absorbing compounds (Figure 14). Reflection spectroscopy, especially in the UV range, opens many new possibilities to answer unsolved questions referring to skin moisturization and to the content of low- molecular-weight compounds in the horny layer and their light absorption, which differ in the horny layer of changing moisture content from the well known optical behavior in definite solvents. REFERENCES (1) R. Anderson and J. Parrish, The optics of human skin, J. Invest. Dermatol., 77, 13-19 (1981). (2) L. Grossweiner, "Photophysics" in The Science of Photobiology, K. C. Smith, Ed. (Plenum Press, New York, 1989). (3) H. Ippen, in Biological Effects of Ultraviolet Radiation, F. Urbach, Ed. (Pergamon Press), p. 683. (4) N. Kollias and A. H. Baqer, Quantitative assessment of UV-induced pigmentation and erythema, Photodermatol., 5, 53-60 (1988). (5) K. F. K•51mel, B. Sennhenn, K. Giese, Investigation of skin by ultraviolet remittance spectroscopy, Brit. J. Dermatol., 122, 209-216 (1990). (6) Cheryl F. Rosen and R. Gange, Immediate pigment darkening: Visual and reflectance spectropho- tometric analysis of action spectrum, Photochem. Photobid., 51, 583-588 (1990). (7) G. Sauermann and U. Hoppe, "Intrinsische Fluoreszenz yon Haut-Integrale und bildmiiBige Erfas- sung" in Empfindliche Haut (Diesbach Verlage, Berlin, 1992), pp. 95-1105. (8) I. H. Blank, Factors which influence the water content of the stratum corneum, J. Invest. Dermatol., 18, 433-440 (1952). (9) S. Dikstein, M. Katz, A. Zlotogorski, Y. Broun, D. Wilson, and H. Maibach, Comparison of different instruments for measuring stratum corneum moisture content, Int. J. Cosmet. Sci., 8, 289- 292 (1986).