190 JOURNAL OF COSMETIC SCIENCE and heat setting. At this time we do not have a mechanistic interpretation for these findings. It should be noted that water uptake measurements are typically performed gravimetri- cally under isothermal conditions with RH as the only variable. The gravimetric method, by its very nature, is susceptible to interference by the convection currents set up during changes in temperature. Volumetric methods, which are less frequently used, are also performed under isothermal conditions. Furthermore, these methods cannot be performed on live heads. Our measurements, on the other hand, are not "real" equilib- rium uptake measurements. The heated sample is allowed to relax to a new final RH and temperature. Two variables, temperature and RH, are changed simultaneously, so in that sense it is a composite of a temperature-jump and an RH-jump experiment, both of which affect the water content of hair. Superimposed on this complexity is the hysteresis associated with the heating of hair, which is well documented in the literature (1,2). Nevertheless, this protocol models the real-life situation better than the isothermal measurements. Also, the "infinite time" measurements represent true equilibrium. The data presented above show that this method has the sensitivity to allow identifi- cation of materials that would alter the water-binding property of hair. In the later stages of product development, such experiments can be performed either on hair tresses or directly on live heads in a controlled-humidity environment, for product optimization and for claims substantiation. We have also found this technique to be extremely useful in the characterization of oxidative hair-coloring products. The measurement of "lift" (melanin bleaching) during oxidative coloring, particularly with dark brown and black shades, is not possible with a typical colorimeter due to interference from the dyes themselves. A way around this problem is to use a dyeless base. This strategy does not work when evaluating com- petitive products. As noted in the Introduction section above, the synthetic hair dyes do not affect the reflectance properties of hair beyond 750 nm, while the natural hair pigment also absorbs the NIR radiation. This difference in the light absorption char- acteristics of the natural hair pigment and the synthetic dyes can be exploited to measure bleaching or "lift" produced during oxidative hair coloring, without interference from the deposited hair color, using NIR spectroscopy. Figure 5 shows the tail end of the absorption of human hair. Spectrum 1 is from black hair from one of the authors (C.P., Asian Indian) while spectra 2 and 3 are from commercial dark brown and blended gray hair, respectively. The effect of pigmentation in this region of NIR wavelengths is clear when one compares the above spectra with spectrum 6, which is due to Piedmont hair that lacks melanin pigment.* These data reveal that the darker the natural hair color, the higher the absorption in this region. It would follow that this region of the spectrum could be used to follow changes in the natural pigment concentration in hair. Indeed, spectra 4 and 5 represent blended gray hair (spectrum 3) dyed with dark brown shades of two oxidative hair-coloring products. It is clear from the spectra that oxidative coloring reduced the intrinsic melanin con- centration in the blended gray hair sample. Furthermore, the product corresponding to * The band at ca. 1285 nm, seen clearly in the Piedmont hair spectrum and only as a shoulder in the other spectra, is due to protein backbone.

NEAR-INFRARED SPECTROSCOPY 191 (1) .40 .30 ,) .10 (6) .20 1050 1100 1150 1200 1250 1300 1350 Wavelength (nrn) Figure 5. Absorption spectra of human hair. The spectra, from the top, represent: (1) hair from one of the authors (C.P.) (2) commercially blended medium brown hair (3) commercially blended gray hair (4) commercially blended gray hair dyed with product A (5) commercially blended gray hair dyed with product B and (6) Piedmont hair. The spectra represent the tail end of melanin absorption. Comparison of spectra I and 6 reveals the absorption due to melanin pigment, while comparison of spectra 3, 4, and 5 reveals the "lift" produced by permanent hair dyes. Notice that product B provides slightly more "lift" compared to A, even though they are both dark brown shades. Such subtle differences between products translate into differences in the appearance of hair color and wearing properties. spectrum 5 produced more bleaching (lift) than the one responsible for spectrum 4. Thus, this methodology can be used during the product development process to opti- mize lift to the desired level, based on the market positioning of the product. It should be recognized that conventional UV/visible reflectance measurements would be more sensitive than the NIR methodology described above for monitoring chemical or photochemical bleaching, due to higher extinction in this region of the spectrum. Such instrumentation, however, cannot distinguish between natural pigment color and syn- thetic colors. In summary, we have evaluated NIR spectroscopy for use in hair research. We show that it will prove to be a valuable tool in hair care and hair color product research, devel- opment, and claims substantiation. This fiber-optic-based instrumentation is capable of measurements on live heads, which also makes it suitable for salon use. REFERENCES (1) C. Robbins, Chemical and Physical Behavior of Human Hair (Springer-Verlag, New York, Berlin, Heidelberg, 1988).