JOURNAL OF COSMETIC SCIENCE 384 assemblies do not have a continuous solid surface, each z-dimension reading is an average of measurements obtained from multiple refl ections corresponding to fi bers at various depths below the outermost hair surface. The utility of this technique lies in its ability to measure changes in the volume of hair tresses subjected to chemical or physical treatment intended to reshape hair. Utilizing this approach, a hair tress may be positioned on the platform of the translation stage and a surface plot corresponding to the three-dimensional volume occupied by the hair assembly may be constructed. In addition, data are also presented in the form of contour plots or cross-sectional representations of the hair tress allowing one to view the tress along its primary axis. As an example of the technique relevant to reshaping hair, Figure 4 contains images and three-dimensional plots of virgin frizzy hair and the same hair that underwent a hair straightening treatment. In both the images and the contour plots, it is clearly evident that a reduction in the overall volume of the tress occurs due to reduction of frizz. Regardless of the reshaping procedure employed, laser stereometry provides a useful modality for monitoring three-dimensional shape and volume occupied by a hair fi ber assembly. SPECTROFLUORESCENCE TECHNIQUES TO CHARACTERIZE BIOCHEMICAL CHANGES IN HAIR Steady-state spectrofl uorescence is a useful tool to monitor the health of biological tissues as it can measure the level of tryptophan (Trp), which is representative of protein integrity. In addition to Trp fl uorescence, several other fl uorophores are also present in hair and are believed to be attributed to kynurenenine, N-formylkynurenine, and 3-hydroxykynurenine, which are known metabolic and degradation products of Trp that are affected by environ- mental stresses normally experienced by hair (15). Nowadays, we are able to construct an endogenous fi ngerprint of fl uorescent compounds present in hair by employing a range of excitation wavelengths from 270 nm to 450 nm with a resolution of 2 nm. Figure 3. Images of hair with various degrees of curvature in the relaxed and fully extended states.

HAIR SHAPE AND DAMAGE FROM RE-SHAPING HAIR 385 As a result, we can generate surface plots of fl uorescence emission as a function of excita- tion and emission wavelengths (excitation–emission matrices). Thus, we are able to profi le the levels of various structural molecules in hair before and after exposure to ther- mal straightening irons as well as to chemical treatments such as permanent waving systems or relaxers. SPECTROFLUORESCENCE BACKGROUND Previous studies of the intrinsic fl uorescence of hair yielded information about the effects of chromophores on the fl uorescent behavior of Trp and its metabolic/degradation prod- ucts: kynurenenine, N-formylkynurenine, and 3-hydroxykynurenine (15). The effects of melanin on the fl uorescence of these molecules can be monitored by comparing highly pigmented dark brown hair with nonpigmented Piedmont hair or white hair. Piedmont hair contains higher levels of yellow pigmentation than pure white hair. Gray hair may also contain yellowish coloration, presumably present due to exposure to ultraviolet (UV) radiation, which is believed to be attributed to higher levels of 3-hydroxykynurenine (16). In general, hair containing greater quantities of melanin has much lower emission characteristics, most likely because more light absorbed by melanin will result in less light available to interact with Trp and other fl uorophores. Further, Trp fl uorescence is highly dependent on the moisture content of hair, with greater Trp fl uorescence occuring Figure 4. Images and three-dimensional plots of (A and C) virgin frizzy hair and the same hair that under- went a hair straightening treatment (B and D).