58 JOURNAL OF COSMETIC SCIENCE Figure 4. Picture illustrating the effect of color on luster. From the left: blond Piedmont hair tress, black carbon fiber tape, and tress of dark brown European hair. A C 0.10 0.025 0.08 0.020 :::i :::i • 0.06 • 0.015 � � In Ill ii 0.04 C 0.010 B .!! 0.02 0.005 0.00 0.000 20 40 60 60 20 40 60 80 Scattering angle (degrees) Scattering angle (degrees) B D 0.05 0.025 0.04 0.020 :::i l'i 0.03 l'i 0.015 � Ill C C 0.02 .!! 0.010 .5 0.01 0.005 0.00 0.000 20 40 60 80 20 40 60 80 Scattering angle (degrees) Scattering angle (degrees) Figure 5. GP curves under white light illumination for (A) Indian, (B) Japanese, (C) Chinese, and (D) African-American hair. available for specular reflection, observed by higher specular reflection intensities. Thus, with other parameters held constant, the increase in the fiber major axis facing the light source increases the luster as a result of decreased contribution from diffuse scattering. In the case of a hair fiber with a scale structure, the effect of the increased fiber major axis is more complicated, affecting both the W112 and the nature of the diffuse peak associated with the light emerging from the far side of the fiber. EFFECT OF CURL ON LUSTER Permanent waves and hair relaxers function primarily by changing fiber curvature to

LIGHT SCATTERING FROM ETHNIC HAIR FIBERS 59 0.4 Pressure 2 Pressure 1 0.3 ::::, ca � 0.2 "iii Nylon/ 0.1 / P1 0.0 20 40 60 80 Scattering angle (degrees) Figure 6. Nylon fiber ellipticity effect on GP intensity scan under white light illumination. Intensities are normalized. Cylindrical I fiber diameter I Minor axis of �� ttened fiber --------�---!-�--.... -- ,' ..... I I I I 1 GP curves ' .... - - -- -- ,, Figure 7. The effect of ellipticity on GP intensity scan. Simplified model for light reflection from cylin­ drical nylon fiber (dotted lines) and flattened nylon fiber (solid lines). either a curlier or a straighter form. When the curvature is large, it dominates many physical fiber properties like fiber friction, flyaway hair, luster, combing ease, manage­ ability, and hair body (19 ,20). Generally, the low luster of curly hair is related to poor fiber alignment. The ethnic hair of African origin used in this study has a natural distinctive and unique configuration, with twists along the fiber leading to a curly structure. As mentioned in the Experimental section, the African-American hair fibers were pulled straight in order to record the GP spectra. Therefore, twists and kinks are more important than a curly structure with large coils. In order to demonstrate the effect of the fiber twist on luster, the goniophotometric intensity curves were recorded first for the nylon fiber with a high ellipticity index and then for the same fiber twisted repeatedly over its longitudinal axis. The result is presented in Figure 8. The following features are observed: shift of the position of the specular peak towards the higher angles, increased W 112 , and increased diffuse reflectance. The reason is that upon illumination of the surface of a twisted fiber, there are number of planes of incidence, i.e., the light reflection occurs at various angles of incidence. Thus, along the fiber, luster is reduced