LIGHT REFLECTION FROM HAIR 313 ,• 5o .0 [•5] 0 blonde g 45 0 • 0 [] [] brown • 40 o 35 ß • y= 43.1 -0.36x o 3O •' 0 5 10 15 20 25 30 35 Distance fro• Hair Root [c•] Ei•urc tO. •ea•s Cot [he posido• o½ [he diffuse re•ecdo• peak a[ hM½ hei•b[, •, a[ vadous posido•s ato• •he hair, •ive• as distance ½rom •he hair tool ß btack hair, • brow• hair, • bto•de hair. The soiM is d•e H•ear re•ressio• i•e •hrou•h •he da•a Cot black hair, Cot which •he e•uadom retad• •o Mt resuks, is •Ne• o• •he •rap•. The horizontal t•es o• •he ri•h• mark •he •roup mea•s Cot brow• a•d hair, respecdvety (see Table D. fiber surface, that is, in the multilayered structure of the cuticle, where systematic and pronounced differences in the refractive index and/or gaps between the cuticle layers are expected to play an important role (11). It is interesting to note, however, that this effect is similar in size but opposite in direction to the shift of the specular reflection angle, due to the cuticle inclination. This effect will be the objective of further inves- tigations. WIDTH OF THE DIFFUSE REFLECTION PEAK The diffuse peaks show very similar overall widths, with group means between 47 ø and 50 ø, thus being, by a factor of about five, broader than the specular peak (see Table I). For the black Asian hair this width shows an apparent slight increase from root (43.7 ø ) to tip (48.9ø), where the slope of the linear regression line just misses to be significant at the 95% level (or -- 0.053). This effect corresponds to the systematic decrease of •/d towards the tip region for this type of hair. The data are summarized in Figure 11. For the brown hair, w d increases significantly from root to tip (LR: ot = 0.002). The regression line through the data is shown in Figure 11. For the black and the brown hair, where diffuse reflection will mainly occur at or close to the fiber surface, this change in the width of the related peak is attributed to increased damage of the hair from root to tip due to grooming, which leads to damage to the cuticle edges, chipping of exocuticle, and exposure of rough fracture surfaces in the endocuticle (10). For the blonde hair, the width of the diffuse reflectance peak has a group mean of 46.4 ø and is independent of the position on the hair. Analysis of variance shows that group-wise there are significant differences between the

314 JOURNAL OF COSMETIC SCIENCE 55 50 45 4O 0 black 0 blonde o o y= 45,1 +0.31x 5 10 15 20 25 30 35 Distance from Root [cm] Figure 11. Means for the width of the diffuse reflection peak at half height, wd, at various positions along the hair, given as distance from the hair root. O black hair, ß brown hair, 0 blonde hair. The solid line is the linear regression line through the data for brown hair, for which the equation, relating to all individual results, is given on the graph. The horizontal lines mark the group means for black and blonde hair, respectively (see Table I). hair types (ANOVA: o• = 0.03). This inhomogeneity originates from the w d values of the brown and the blonde hair being significantly different (LSD: o• = 0.011). LOCATION AND WIDTH OF THE INTERNAL REFLECTION PEAK A second, broader peak besides the one for specular reflection was observed ih a com- paratively small number of cases when measuring in the lightly colored tip region of the blonde hair. This peak is attributed to internal reflection. The frequency of occurrence of the effect was, in our experiments, much lower than would have been expected from the work of Stamm et al. (2,11). The location of this peak, given by % = 64.0 ø + 2.96 ø (N = 8), is significantly shifted towards higher angles compared to the ray-tracing prediction (55ø). This deviation is in good agreement with the observations by Stamm et al. (11) and is attributed to the repeated passage of the refracted and then internally reflected light through sheets of lower and higher refractive index. They propose, as a primary source of the effect, the existence of gaps between cuticle layers. In view of the highly cooperative structure of the hair cuticle, which on straining delaminates from the cortex in rings rather than in sheets within the cuticle layers (20), we are rather inclined to assume systematic dif- ferences in the refractive properties of the layered morphological structure of the cuticle cell, namely exo- and endocuticle. In this context, the large differences in the moduli of exo- and endocuticle, determined by Parbhu et al. (21), using atomic force microscopy, should be noted. Similar considerations apply for the shift of the peak for diffuse reflection, as discussed above. The width of the peak for internally reflected light w i is 24.9 ø + 3.02 ø. Due to the nature of this light, this value is, not unexpectedly, by about a factor of roughly two, higher than that for the specularly reflected light and by the same factor smaller than the width of the diffuse peak in the GP curve.