LIGHT SCATTERING FROM ETHNIC HAIR FIBERS 0.02 O.Q15 --:- 0.0 125 ➔···· ········································ ······ :::::, � j 0.01 C QJ - 0.0075+···················· ·· 0.005 0.0025 20 40 60 Scattering angle (degrees) 80 51 Figure 1. A typical goniophotometric curve and its deconvolution into specular and diffuse components. electrophoresis, the chemical composition of the hair proteins has not been found to depend on racial origin (7). Yet, a significant variation is found in the morphology of the hair of different ethnic groups. Generally, the hair fiber can be qualitatively divided into distinct regions such as the outermost cuticle layers, inner cortical cells bonded together with the cell membrane complex, and the porous medulla. However, depending on the ethnic origin, the relative dimensions of these layers could differ significantly. The outermost cuticle, consisting of flat overlapping scales is influenced by the sur­ rounding environment and surface modifying treatments that cause the thinning and fusion of the surface cuticle cell (10). These changes have a significant effect on the optical properties of hair. The cuticle of European and Asian hair consists generally of six to eight layers, whereas the cuticle of African-American hair has a variable thickness with six to eight layers at the ends of the minor axis of the fiber and only one to two layers at the ends of the major axis (11). In this region the structure is weakened and vulnerable during grooming procedures. The structure of the medulla can appear as a continuous or a discontinuous channel, with significant differences in the packing of the medullary cells. It is important to note that in some cases the medulla is completely absent. Often, the absence of the medulla occurs for fine hair with a small diameter, whereas for medium- and large-diameter hair the medulla is generally present (the diameter of hair fibers varies from 40 to 100 µm). Thus, as the diameter of the hair can be related to its ethnic background, as shown below, the amount of medullation can change as well. The medulla is known to have a large effect on optical properties and hair shine (12).

52 JOURNAL OF COSMETIC SCIENCE The geometric configuration of hair fibers is known to depend on the relative amount of para- and orthocortical cells and their distribution in the fiber cross section. Obser­ vations by Swift suggest the relationship between the curliness of different ethnic hair and their bilateral structure (8). For example, straight Asian hair has only paracortex and slightly curly European hair has a thin one-cell-layer orthocortex at the periphery of the cortex, whereas the most curly African hair has approximately equal amounts of the two cell types. ELLIPTICITY AND CROSS-SECTIONAL AREAS FOR ETHNIC HAIR In Table I the data from measurements are presented. The results are organized into three groups as follows: First, hair of European origin with different pigmentation levels (denoted as the European group below) second, hair of Asian origin (denoted as the Asian group below) and finally, the African-American hair, forming the third separate group due to its unique configuration with twists and kinks along the fiber axis. For a given number of fibers from the tresses used in this study, the cross-sectional area was found to be smallest for the fine light-brown European hair, followed by Piedmont hair and Chinese hair. The dark brown European hair had the largest cross-sectional area of the three differently colored hair samples of European origin. Indian and African­ American hair had similarly large cross-sectional areas, whereas the largest cross­ sectional area was measured for Japanese hair. Measurement reveals that Chinese hair fibers were the most regular, with almost circular cross sections given by an ellipticity index of 1.16. In the Asian hair group, Indian hair had the most asymmetrical shapes, with an ellipticity index of 1.44. Hair of European origin was found to be oval in cross section. Within this ethnic group the ellipticity index increased in the order Piedmont hair (1.36), light brown European hair (1.46), and finally dark brown European hair (1.52). African-American hair fiber had twisted ribbon shapes, which in cross section appeared as flattened or curved ovals. The average ellipticity index for the African hair was found to be 1.6. The variations in the ellipticity indices found in our study are in good agreement with the general trend reported in the literature (13,14). The hair of Asian background is generally reported to be the closest to having a circular cross section, with an ellipticity index around 1.25, oval European hair having a ratio of 1.35, and African-American hair, with the greatest deviation from circularity, having an average ellipticity index of 1.75. We note that the average ellipticity index of 1.6 obtained in our study for African hair is at the low end of the wide range of ellipticity indices (1.6-1.9) reported in the literature. From Table I we observe several trends. Within the European hair group, with increas­ ing hair pigmentation levels, luster increases and the width of the specular peak at half height (W 112 ) decreases. There is also a systematic increase in the ellipticity index and a decrease in W 112 from measurements performed with both laser and white light illumination. Luster increases with the ellipticity index. In the two remaining ethnic groups the hair fibers are all black in color, and thus we assume that the effect of pigmentation level can be neglected in our discussion. Comparable high luster values were obtained for African and Indian hair, followed by Japanese and Chinese hair. As in the case of European hair, luster was found to increase with the ellipticity index. Within the Asian group, under white light illumination, the mean W 1I2 followed the same trend observed for European hair, i.e., W 112 increasing with the decreasing ellipticity index.