340 JOURNAL OF COSMETIC SCIENCE Figure 4. Scheme of the image analysis procedure employed to determine light intensity as a function of tress length. 250 200 150 100 50 Tress 1 Tress 2 ! ! 1 0 10 20 30 40 50 Distance (rnm) Figure 5. Light distribution curve showing light intensity (luminance) as a function of distance along the hair tress The data were obtained for untreated dark brown hair.

LUSTER QUANTIFICATION OF HAIR 341 S - Dstamm LSt ..... -- S (2) Similar to the data of Stamm et aL (1,2), we found that the use of equation 2 leads to larger differences in luster values for different hair types and hair treatments. Reich and Robbins (4) determined the diffuse reflection in the same manner as Stamm et aL, but they utilized a different relationship to define luster: S Lz•ei"h-zeøaains = Dst ..... * W•/2 (3) where Wz/2 represents the width of the specular peak at half of its maximum intensity. Repeated measurements of various tresses of untreated hair yielded the following values of luster parameters for exposure settings of f8 and 1/13 s: Lst ...... = 0.72 + 0.02 and Lnei,_h_noaai,z, = 0.67 ß 0.006 for f8 and 1/6 s: Lst ....... = 0.72 ß 0.03 and Ln•i,_•_noaai• s = 0.54 ß 0.07 and the values N c = 0.80 ß 0.02, Ls• ...... = 0.67 ß 0.02, and L•i,_3_noaai• , = 0.37 ß 0.02 for exposure values off8 and 1/3 s. These numbers give an •sessment of the precision and reproducibility of the luster parameters determined by the described measurement method. In order to validate the measurement method and provide a reference point to the literature data, we have studied the luster for various types of hair (1,2). It can be seen in Figure 6 that hair pigmentation has a significant effect on the optical properties of hair and in particular on its luster. Figure 6 presents images of reflected light from natural white, light blonde, light brown, and dark brown hair. These images were obtained by selecting the exposure values in such a way as to visualize the details of the specular reflection band (f8 and 1/13 s). The light distribution cu•es are presented in Figure 7, and they are consistent with the visual representation of the images shown in Figure 6. For example, one can clearly see two specular reflection bands for natural white and light blonde hair (Figure 6), which are evident by two peaks in the light distribution cu•es (Figure 7). The peak at 16 mm gets progressively smaller with an increase in the Natural Light Blonde Light Brown Dark Brown White Figure 6. Images of light reflections from various types of hair. Exposure values: f8 and 1/15 s.