200 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS adapted lenses, as well as under the internal sphere by a special optical system. The beam geometry (solid angles and incidence) in the external sphere is identical to that in the internal sphere. In preliminary studies, these modifications did not lead to a wavelength shift or change the absorption spectra of test materials. The fibers permit measurements to be made over the range 900-2500 nm with sufficient sensitivity, something that has not always been the case (6). The spectra were acquired from 1100 to 2500 nm in 4-nm steps and expressed as absorbance (log l/R), where R is the ratio between the energy reflected on the walls of the sphere and that retrodiffused by the sample. For homogeneous samples with little absorbance and no diffusion, the concentration of an analyte can be considered directly proportional to the absorbance (Ai) at a given wavelength (M) according to an equation based on Beer-Lambert's law: C = KAi = K log (1/Ri) In practice, particularly for measurements of the skin, the phenomena are far more complex. This law is no longer applicable because of interference due to analytes other than that under study and, above all, to diffusion due to the granulometry or surface state of the skin. The effects of this diffusion can be partly eliminated by calculation based on the difference in the absorbance at two wavelengths. The resulting equation is as follows: C = K (A2 - A1) = K log (R1/R2) (equation 1) Conductance measurements. Skin conductance was determined using a DermoDiag © apparatus (8), which operates at 10 MHz the result is related to the degree of hydration, the surface state (contact impedance between the skin and the electrode), and the thickness of the stratum corneum. EXPERIMENTAL STUDIES In vitro. Samples of stratum corneum isolated by heat-trypsin treatment were equili- brated at various degrees of relative humidity and analyzed by means of near-infrared spectroscopy. The difference in absorbance at 1936 and 1100 nm was recorded, and the water content was deduced from the sorption isotherms determined by differential calorimetry and weighing (9). Spectroscopic analysis of the skin in vitro was performed before and after removal of the epidermis by heat treatment, in order to study the influence of the most superficial layers of the skin. In vivo. Study population. Dry skin was characterized by studying the external surface of both legs (just below the middle of the calf) in a panel of 159 women with a mean age of 40 years (range, 18-67), who had given their informed consent. The efficacy of five cosmetic preparations was determined by studying five groups of about 20 women per group matched for age and the dryness score (2). The treatment was applied twice daily for four weeks to one leg, the other serving as a control. Products were different O/W (products B, C, D) and W/O (products A, E) formulations con- taining various proportions of moisturizing agents except product D. The measurements

NEAR-INFRARED SPECTROSCOPY 201 and clinical evaluations were made at the beginning (TO) and end of the treatment period (T4). Clinical scores. The aspect of the dry skin was evaluated by a trained expert on the basis of the following five criteria: ß "papyrac6" state of the skin ("cigarette paper" aspect) ß roughness (tactile evaluation) ß presence of squames ß presence of scales ("snakeskin" aspect) ß irritation (subclinical inflammation: redness) Each criterion was scored from 0 to 4 according to the degree of severity half points were permitted. The average of the five criteria has been taken as the overall score of each individual. The score for each item was also calculated. Statistical analysis. Correlations between the experimental values were sought by using Spearman's test for nonparametric variables and Pearson's test for parametric variables. Regression lines were constructed from the mean scores (half-point steps), together with the corresponding standard error of the mean (SEM). Three-factors analysis of variance, including a hierarchical analysis [time, product subject (product)], was used to deter- mine the statistical significance of the treatment effects, followed by the Newman-Keul test. The results are presented as means -+ SEM. RESULTS IN VITRO STUDIES The absorbance of the dermis was greater than that of total skin (Figure 2), the differ- ence, i.e., the spectrum of the epidermis, showing the two characteristic water peaks at 1450 and 1936 nm. As expected, the absorbance of the skin increased after removal of the epidermis, partly because of the removal of the least-hydrated layer, the stratum corneum, and partly because of an increase in the volume analyzed due to the greater infrared penetration. Figure 3 shows the correlation (r = 0.98, p 0.001) between the difference in absorbance at 1936 and ! !00 according to the water content of the stratum corneum. The difference in absorbance at the wavelengths used clearly increased with water content, as did the water peak (!936 nm). This difference in absorbance is computed according to the modified Beer-Lambert's law (equation !) and is better than any other one computed using the water peak wavelengths and references. CHARACTERIZATION OF DRY SKIN IN VIVO Figure 4 shows the mean spectra obtained for the study population according to the overall clinical score. Absorbance fell gradually with the increase in the clinical score, and this was particularly marked for the two water peaks (!450 and !936 nm) and less marked in the region between 2000 and 2500 nm. Finally, a multiple correlation study has shown that the absorbance (1936--1100 nm) was better correlated to the dry skin score than the 1450-1100 nm one (r = 0.789