198 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS state of the skin was tested after four weeks of daily application by means of the near-infrared and conductance methods as well as by using the clinical scoring system. The results indicate that the water content of the skin and its conductance changed in the direction opposite to the macroscopic aspect. Near infrared spectroscopy emerged as the most sensitive method for evaluating the efficacy of the preparations. We present the overall results in terms of the clinical description of dry skin, together with the actual efficacy of the moisturizing preparation relative to the expert evaluation. INTRODUCTION Despite thorough descriptions of dry skin, its causes are still poorly understood. Several biophysical methods are now available to characterize the state of the skin in vivo (1), but although progress has been made in our understanding of it functional properties (2), little is known of its precise nature, particularly with regard to lipid content, the reported variations of which are only minor (3,4). The fundamental question is whether or not dry skin is dehydrated. The lack of informative data is due to the fact that the numerous noninvasive methods used to described skin condition and to determine the efficacy of cosmetic preparations are based on measurements of physical parameters that are themselves dependent on the presence of water, lipids, and other components. In addition, the variation of the physical parameters studied is rarely directly related to differences in water content. In other words, these methods can provide adequate answers to the question "How much?" but not to the question "Why?" Although water is the main determinant of the overall physical properties of the stratum corneum, other components can play an important role. This is particularly the case of lipids, which modify the relationship between light diffusion, reflection, and transmis- sion, and which also influence the surface properties of the skin in addition, they can give a smooth or rough feeling according to their nature. Cosmetics are complex compositions of numerous ingredients, but all are composed of water and oil. Most cosmetic products improve the smoothness of the skin and its overall condition, but we are not yet in a position to describe how they do so. Infrared spectroscopy (5) can, in principle, provide valuable information on the water content of the skin. However, the infrared spectrum of water is wide and poorly defined. The amide I and II bands, the amplitudes of which are modified by the presence of water, are only indirect markers. In practice, measurements are made via a prism within which the rays undergo total reflection and show only poor penetration (less than one micron) into the tissue studied. Furthermore, the measurements take a considerable time, and the occlusive effect of the prism on the skin influences the water content. The near-infrared spectrum (1100-2500 nm) appears to be more practical, since the water bands are precise. Two types of apparatus--an infrared sensor and a data analy- ser-are required in addition, the radiation re-emitted by the skin cannot be deter- mined precisely because of the nature of the applicator (6). To overcome these difficulties, we have totally modified the optical elements of an integrating sphere of a near-infrared spectrophotometer (Infra-Alyser 500) in such a way as to record spectra at any skin site in a routine manner. We have conducted a study of the true nature of winter xerosis and the efficacy of various treatments, comparing the results with those provided by a simple and widely used electrical conductance method. In addition, the skin was characterized by trained experts in terms of several clinical

NEAR-INFRARED SPECTROSCOPY 199 criteria, both before and after four weeks of treatment. On the basis of the results obtained, we attempted to answer the following questions: ß Is dry skin insufficiently hydrated, or is it simply rough? ß How good is the agreement between clinical evaluation, and results provided by the physical measurement? ß Are the results of the two physical methods redundant or complementary? MATERIALS AND METHODS MATERIALS Near-infrared spectrophotometer. Diffuse-reflection infrared spectroscopy has been considerably developed since the initial work by Norris in the 1970s (7). This rapid, noninvasive method can be used to obtain spectra of solid, opaque samples. In the agro-alimentary field, it is mainly used for in vitro analysis of water, lipids, and proteins (7). Apart from applications aimed at identifying raw materials and analyzing finished products, most uses in cosmetology (e.g., the assessment of hydration and the efficacy of moisturizing agents) involve in viva measurements. The apparatus we have adapted to spectral measurements in viva is an integrating sphere spectrophotometer (Infra-Alyser 500) coupled to a PC AT microcomputer. An external integrating sphere has been added (Figure 1) so as to acquire a full energy spectrum and to optimize the signal-to-noise ratio, without modifying the basic functions. The external integrating sphere is identical to the internal sphere and is connected to the optical elements of the Infra-Alyser 500 via two special optical fibers with a very low attenuation coefficient (0.5 db/m) throughout the study spectrum. The reference beam and the analytical beam are collected by the optical elements through a group of EXTERNAL MEASUREMENTS Internal integrating sphere "-'• • Sample beam Chopper • Reference beam :. %.. , /,,,, Optic •t fibers t• • ,• II • •'" ß II , •'/ External •,• , • Integrati Detectors • [ sphere Sample Figure 1. Modification scheme of the Infra-Alyser 500 to obtain in vivo measurement on all body sites.