338 JOURNAL OF COSMETIC SCIENCE (shark squalene, Arista Oils) 5% cholesterol (Aldrich) and 5% linoleic acid (Emersol 315, Cognis Corporation). RESULTS AND DISCUSSION MEASUREMENTS OF UNTREATED HAIR The phenomena accompanying the interaction of light with objects include reflection, scattering, refraction, diffraction, interference, and absorption. The most important for luster is reflection, which can be further categorized as "specular reflection" and "diffuse reflection." Other phenomena, such as scattering, refraction, and absorption, also play a critical role in the interaction of light with hair. Refer to R. Stamm's work for an explanation of these terms (1). Every measurement set of untreated hair consisted of two images, one recorded with the polarizers parallel (Figure 2) and the other with the polarizers perpendicular (Figure 3) to each other. In the case of parallel polarizers, all reflected radiation is captured in the image, while in the case of perpendicular polarizers, the specular reflection is eliminated and the observed image is due to diffuse scattering. It should also be added that all images shown in Figures 2, 4, 6, 8, 10, 12, and 14 are exposed in such a way that one can view the details in the highlight area (f8 and 1/13 s or f8 and 1/6 s as aperture and shutter speed settings, respectively). These images do not show as much detail in the dark areas, making this region of the tress more difficult to evaluate by visual exami- nation. In contrast to this, we have also employed exposure values corresponding to the Figure 2. Digital image obtained for untreated dark brown hair with polarizers oriented parallel to each other. Exposure values: f8 and 1/13 s.

LUSTER QUANTIFICATION OF HAIR 339 Figure 3. Digital image obtained for untreated dark brown hair with polarizers oriented perpendicular to each other. Exposure values: f8 and 1/3 s. average light intensity for the entire image (f8 and 1/3 s). With such an exposure value, the specular reflection falls into slightly overexposed highlights, with no details visible in this area however, differences in the regions of the tress peripheral to the specular reflection are accentuated. In order to quantify the luminance along the hair tress, light intensity measurements were performed with image analysis software. This is demonstrated in Figure 4, in which the four red lines, 50 pixels in width, represent the region from which the luminance readings were obtained for each tress. A plot of luminance as a function of distance along each hair tress is provided in Figure 5 for both the specular (Figure 2) and diffuse (Figure 3) light images. The specular curves in Figure 5 are characterized by the presence of one sharp peak corresponding to the reflected light from the dark brown hair. It is note- worthy that a similar examination of diffuse images provides a curve in which two peaks could be resolved, one for diffuse reflection from the front face (22 mm) of the hair fibers and the other for diffuse reflection from the back face (17 mm) of the fibers. The luster parameters were calculated using three different models, one of which is the Nickerson contrast gloss: S-D No- S (1) where S represents the area underneath the specular curve and D is the area under the diffuse curve. Stamm et al. (2) proposed another model, based on equation 1, in which the area under a straight line obtained by connecting the first and last points of the specular reflection curve was used as a measure of diffuse reflection, Dstamm: