J. Cosmet. Sci., 55, 49-63 CTanuary/February 2004) Studies of light scattering from ethnic hair fibers K. KEIS, K. R. RAMAPRASAD, and Y. K. KAMATH, TRI/Princeton, P.O. Box 625, Princeton, NJ 08542. Accepted for publication October 16, 2003. Synopsis One of the most desirable hair attributes to consumers, irrespective of ethnic background, is hair shine. The light reflected from a fiber has two components, specular and diffuse. The specular fraction of reflected light from the front surface of the fiber is generally recognized as a contributor to high luster. The distinction between specular and diffuse reflection is, however, not always clearly defined. In this study an attempt has been made to differentiate between specular and diffuse reflectance by analyzing mathematically goniopho­ tometric curves of light reflected from unaltered single hair fibers from European, African, and Asian ethnic groups. The effect of macroscopic characteristics of the hair fibers, such as fiber diameter, cross-sectional shape, and curvature on luster is demonstrated. Results indicate that broadening of the specular peak reduces luster values, and is related to these characteristics. Thus, specular peak broadening is one of the important features to take into account when evaluating luster. Therefore, a new method for luster evaluation from goniophotometric curves is proposed. Additionally, we present the general model for light scattering, showing how scattering by surface roughness of different origin and magnitudes, and the scattering and absorption processes by the hair's interior, affect the position of the specular reflectance peak and its broadening. INTRODUCTION During the last decade the cosmetic industry has increasingly formulated personal care products to meet the needs and expectations of ethnic consumers. For example, con­ sumers of African origin generally prefer straight hairstyles. As a result, they depend heavily on heat and chemical treatments to relax their hair, which result in significant damage to the hair. Therefore, multifunctional products are required to condition, balance moisture, repair damage, and add shine to the hair. Hair care products aimed at Asian and European consumers are required to endow different attributes, where addi­ tives to add shine to hair are also of primary importance. Obviously, shine is one of the most desirable hair attributes to consumers, irrespective of ethnic background. Generally, at TRI light scattering curves from goniophotometric measurements on single hair fibers are used to evaluate hair shine. In addition to quantitative information about hair shine, this method is also capable of providing information about the changes in hair shine due to product deposits and surface damage by various cosmetic treatments and combing (1-5). Apart from other factors, the appearance of hair and its shape is influenced by its ethnic origin. The differences in morphology and composition of some 49
50 JOURNAL OF COSMETIC SCIENCE ethnic hair are reported in the literature (6-8). However, to date there has been no major effort made in the direction of understanding how differences arising from ethnic back­ ground affect the luster of hair fibers. In this study unaltered hair from European, African, and Asian ethnic groups is investigated by means of goniophotometry. EXPERIMENT AL Samples (15-cm-long) of unaltered blond Piedmont hair, light brown European hair, dark brown European hair (purchased from DeoMeo Brothers), black Indian hair, black Japanese hair, and black African-American hair (obtained from a private source) were used. Additionally, we used black Chinese hair from a 35-year-old female with no known history of chemical treatments. A modified Brice-Phoenix goniophotometer (GP) was used to record the scattered light intensity as a function of the angle. Two light sources were used in this study: a He-Ne laser of 632-nm wavelength and a quartz tungsten halogen lamp emitting white light. Sections of hair at approximately the same distance from the fiber root were used in the measurements. Single hair fibers were mounted horizontally in the sample holder and held in place by clips. Most of the fibers studied here were naturally straight, except for the African-American hair, which was straightened carefully without extending the fiber. The measurements were performed in the root-to-tip direction of the hair fibers at an angle of incidence of 45°. For each hair type, measurements were made on 25 randomly chosen fibers. The reflected light was detected by the photomultiplier as a function of angle. Using peak-fit software, the GP curve was deconvoluted into specular and diffuse components, assuming a Gaussian distribution for the specular peak (see Figure 1). The luster was calculated by s L=-- (S +D) (1) where S is defined as the specular peak area obtained from the scattering curve and (S + D) is the total area under the curve. The goniophotometric measurements were also used to determine the scale angle. The scale angle, a, was calculated from the GP curves of the fiber in the root-to-tip and tip-to-root position, using the following expression: 8TR - 8RT a=--- (2) 4 where 8 7R and 8RT stand for the angle of specular peak for tip-to-root and root-to-tip positions, respectively (9). Fiber cross-sectional areas and the minor and major axis were measured by a laser scan micrometer (LSM-3100), from which ellipticity indices, i.e., the ratios of fiber major axes over fiber minor axes, were calculated. RESULTS AND DISCUSSION EFFECT OF ETHNIC ORIGIN OF HAIR ON FIBER STRUCTURE Human hair is a complex tissue almost entirely consisting of proteins. By means of
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