NEAR-INFRARED SPECTROSCOPY 203 0,26 0.24 0.16 0.14 r = 0.98 n =52 p 0.0001 I i I I I I 0 5 10 15 20 25 3o WATER CONTENT (ø7o) Figure 3. In vitro experiment: Influence of the water content of stratum corneum samples on the NIR absorption measured at 1936 nm and 1100 nm. near-infrared absorbance, change in conductance, and change in clinical dryness score. The change in the near-infrared parameter (Figure 9a) showed that all the treatments except preparation D had statistically significant activity. Products A, C, and E were significantly more effective than product B, which, in turn, was more effective than product D. In terms of conductance, product D was again statistically ineffective, but products A, C, and E were significantly more effective than products B and D, which had comparable effects (Figure 9b). Finally, the reduction in the dryness score (Figure 9c) was significant for products A, B, C, and E, which showed similar activity, but not for product D. On the basis of near-infrared absorbance, which correlated with the clinical score, product B was distinguished from products A, C, D, and E. DISCUSSION With the equipment described, the near-infrared radiation penetrated deeply into the skin, although the superficial layers had a clear influence, as shown in Figures 2 and 4. The study of isolated stratum corneum samples with varying degrees of relative humid- ity showed that the difference in absorbance at 1936 and 1100 nm indeed reflected water content, since, for values between 0 and 30% (at which water is bound), there was a linear relationship between absorbance and water content. It was also this difference in absorbance that was best correlated with the overall skin dryness score, which, as mentioned above, was the sum of several clinical parameters (roughness, scaling, "cig- arette paper" aspect, etc.). The correlation was far better than that between conductance values and the degree of

204 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS 1.400 1.200 • 1.000 z 0 .800 .600 .400 .200 I I I I I I I I I r I I I 1200 1400 1600 1800 2000 2200 2400 WAVELENGTH (nm) Figure 4. NIR absorption spectra of skin showing different scores of dryness (intermediate scores not presented). 0.94 0.92 1.10 1.08 r = - 0.536 n = 310 pO,001 1.06 o • 1.04 (• 1,02 C) 1.00 r,... 0.98 0 0.96 0.90 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 GLOBAL SCORE Figure 5. Linear correlation between the NIR absorbance (measured at 1936-1100 nm) and the global score. skin dryness: above an overall score of 3, no further changes in conductance were observed. A closer examination of the clinical data shows that electrical conductance was related to the roughness of the skin: skin roughness no longer varied above an overall