SENSITIVE PROBE OF HAIR SURFACE 227 The incident light in this experiment was set at 30 ø, and one can see a maximum in the measured curve near this angle. This is termed specular reflection. Light is also scattered at angles other than the specular this is termed diffuse reflection. This latter type of scattering is caused by light hitting the scale edges of the hair. It can also be caused by small imperfections on the hair surface and, in addition, by deposited particles. An excellent discussion of the morphological features of hair that give rise to observed light-scattering patterns can be found in reference 1. The question now arises as to how one can use the goniophotometric results to measure shine or luster. From the definition of shine, it appears obvious that luster increases with increasing specular reflection and decreases with increased diffuse scattering. Any func- tions used to estimate shine must therefore take these two relationships into account. Several workers have developed shine functions, employing these relationships for ap- plications such as textile fibers, polymer surfaces, etc. [see, for example, (2,6-10)]. In this work, several functions were tested both from the literature and also devised by ourselves. The best agreement with subjective evaluations was found using the rela- tionship L = S/DW(V2) (Eq. 1) where L equals luster or shine. D in this expression is the integrated diffuse reflectance and is obtained, as in reference 2, by connecting the scattered light intensities at 0 ø and 75 ø and measuring the area under the resulting line. S in equation 1 is the integrated specular reflectance and is obtained by measuring the area of the specular peak, while W(Vp) is the width of the specular peak at half-height. All three of these quantities are illustrated in Figure 2. It has been pointed out (2) that use of expressions such as equation 1 with D in the denominator are valueless for cases where diffuse reflectance goes to zero. For most cases involving hair, however, scattering off the scale edges insures a minimum value for D, so that equation 1 is broadly applicable. In the current experiments, hair fibers from treated tresses were scanned one at a time in the goniophotometer. There is tremendous variation from hair to hair, even from a single head of hair, so that in order to obtain meaningful shine values for a particular treatment, an average of many hairs must be taken. In the current case, 21 hairs were taken from each tress, while three tresses were employed for each treatment. Each shine value, therefore, represents an average taken from 63 hairs. RESULTS AND DISCUSSION SINGLE-FIBER SCREENING TESTS Figures 4-6 show typical light-scattering scans taken after a series of shampoo treat- ments of single hair fibers. These types of single-fiber experiments are useful as a means of rapidly screening the effects of various treatments on hair. The results from these experiments can only be treated qualitatively, however, since they represent treatments on single hairs and there is too much variation among hairs for

228 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS z uJ I-- z LEAN HAIR SHAMPOO BI (#1) SHAMPOO B I (#2) SHAMPOO B I (#3} SHAMPOO B I (#4) SLS 15 30 45 60 SCATTERING ANGLE Figure 4. A single-fiber experiment testing the effect on hair of shampoo B 1. The hair was washed with SLS at the beginning of the experiment (clean hair) and at the end. The same curve was obtained in both cases. quantitative results from a single hair to be meaningful. A particular result must therefore be repeated several times on different hairs in order to be considered real. Bearing this in mind, and also the fact that single-fiber treatment conditions are very different from tress and in-shower treatments, one can use single-fiber screening tests as a convenient means of determining the possible effects of many different products on hair. This method is especially useful when hairs are undergoing a series of treatments, since after each particular step in the series, a light-scattering scan can be run in order to determine the effect of that particular step. This can be very helpful in elucidating the mechanism of a particular effect. Figure 4 shows the results of a series of treatments of an Oriental hair with shampoo B 1. This is a commercial product that, at the time these experiments were performed, contained Polyquaternium-10, a polycationic well known to be substantive to hair, in a trideceth-7 carboxylic acid detergent system. The hair was first washed with 20% sodium lauryl sulfate (SLS). The resultant light- scattering curve is considered that of a clean hair. Following this, successive treatments with shampoo B 1 (followed by water rinses) caused dulling, indicated by decreases in the peak height along with increases in diffuse scattering. This loss of shine was caused by deposition of shampoo residue, probably Polyquaternium-10, on the hair surface. After four treatments with B 1, the hair was again treated with 20% SLS. The resulting light-scattering curve was congruent with the original, clean hair curve, implying that