258 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS 1 rear ß , treatment beforet•. treatmen 1•oo 16'00 ' 18'00 ' 20'00 nm Figure 8. Apparent absorbance spectra before and after product application. 60 i i i 40 20 -6O -80 1879 1890 1909 1927 wavelengfh (nm) Figure 9. Net concentration changes after product application. r 2200 into the deeper tissues. According to Scheuplein (14), IR radiation at the water absorp- tion bands that is not absorbed or scattered by the SC will be absorbed by the first few viable cell layers of the epidermis. Radiation that penetrates deeper into the skin is less likely to be returned to the surface and thus will not be measured.

MOISTURE MEASUREMENT BY NIR SPECTROSCOPY 259 180 160 140 .120 • 100 '- 60 40 20 1879 [•i•A•B •C 1890 i i 1909 1927 wavelength (nm) Figure 10. Net scattering effect after product application. b a epidermis dermis Figure 11. Possible pathways for NIR radiation in the skin: a) specular reflectance b) diffuse reflectance c) absorption d) multiple internal scattering. Changes in the depth of penetration of radiation depend on changes in the refractive index or in skin surface morphology. The refractive index of the stratum corneum is about 1.55 at the sodium D line (14), but has been found to decrease upon hydration

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