Table I Experimentally Determined Hair Fiber Characteristics by Ethnic Origin Cross-sectional area Ellipticity Luster (%) (He-Ne Hair origin (mm2) x 10- 3 index laser illumination) W112 (degrees) Scale angle Piedmont 4.53 1.36 17.35 ± 1.37 5.79 ± 0.3 2.8 ± 0.2 Light brown European 3.75 1.46 21.53 ± 2.3 4.79 ± 0.4 2.9 ± 0.3 Dark brown European 5.08 1.52 27 .91 ± 1.47 4.38 ± 0.2 3.0 ± 0.2 Indian 5.65 1.44 35.6 ± 3.8 3.67 ± 0.4 3.7 ± 0.3 Japanese 6.64 1.33 34.25 ± 4.6 5.04 ± 0.4 3.6 ± 0.3 Chinese 4.88 1.16 24.05 ± 2.2 4.29 ± 0.3 3.6 ± 0.4 African-American 5.27 1.6 35.95 ± 4.3 4.83 ± 0.4 2.3 ± 0.4 Luster (%) (white light illumination) 48.2 ± 2.3 55.3 ± 1 63 ± 3.1 65 ± 2.26 59.5 ± 3.34 62.4 ± 1.9 64.5 ± 2.8 W112 (degrees) 20.0 ± 1.5 17 .6 ± 1.1 15.5 ± 0.8 12.8 ± 0.6 14.8 ± 0.9 15.6 ± 1.9 14.1 ± 1.1 I:'""" - Q ::c ,.., C/'J n � ,.., l:l1 ::,:, - z Q "'Ij ::,:, 0 � l:l1 ,.., ::c z - n ::c � "'Ij - b:l l:l1 ::,:, C/'J VI \..)J

54 JOURNAL OF COSMETIC SCIENCE With the laser illumination the Indian hair had indeed the smallest W112 , whereas for Japanese and Chinese hair the W112 values were higher. It seems that specular peak broadening is one of the important characteristics to take into account when evaluating the luster of hair. On the basis of our observations described above, we suggest that specular peak broadening could be related to changes in the following hair characteristics: 1. Color (or transparency) of the hair fiber 2. Fiber diameter and ellipticity (if the illuminating light beam is larger compared to fiber) 3. Fiber curvature (presence of twists and kinks) 4. Surface roughness, either microscopic or macroscopic (scale angle) The effect of these characteristics on luster and peak broadening are discussed in more detail in the following paragraphs. EFFECT OF SURFACE ROUGHNESS ON LUSTER As mentioned earlier, luster is greatly affected by the surface condition of the hair fiber. The modification in GP curve shape and angular position can arise, depending on the origin of roughness and its magnitude. A general model for light scattering can be successfully used to explain the changes in the GP curve caused by surface roughness. Figure 2 schematically shows the light scattering from surface roughness of different magnitudes. In the uppermost graph, A, specular reflection from the smooth surface occurs at the angle of incidence. Hair fiber can reflect light in a specular manner, almost like a mirror, when a shine spray or oil layer covers the scale structure completely, forming a smooth surface. The surface with microscopic roughness results in an isotropic surface scattering in addition to specular reflection. In the case of the hair fiber, this situation is caused by Figure 2. Schematic representation of light scattering model for surfaces with different roughness mag­ nitudes and its effect on goniophotometric intensity scan.