HAIR GLOSS 301 problems in binocular vision were assumed as a source for the appearance of gloss, meaning problems in combining the slightly different images of our environment seen by the left and the right eye. But this influence on gloss is difficult to measure and therefore was neglected for a long time (8). The research on gloss was mostly concentrated on the description of such photometric properties, which were easy to measure. A very comprehensive summary was formulated by Harrison (1). Although nearly 50 years old, it is still a useful source by information. Today's considerations of gloss are summarized in a CIE-research note (9). They are more or less related to the so-called reflection indicatrix in Figure 1. When a light source illuminates an element of the hair surface under an incidence angle, ½ 1, light is reflected in different receptor or viewing angles, ½2 (both angles count from the normal of the specimen surface). The amount of reflected light is indicated by the length of the arrows pointing into the designated directions. Connecting the peaks of all possible arrows by a curve generates the so-called indicatrix, as a kind of abbreviation for describing the arrows. Figure 2 shows some examples of indicatrix shapes. The horizontal axis represents the receptor angle, ½2, and the vertical axis the amount of reflected light in units of lumi- nance, L, for reasons explained later on. A very smooth surface often appears to be highly glossy, and a more or less sharp, bright image of the light source is seen on the surface. It generates an indicatrix similar to curve a, because it reflects nearly all the light into a certain angular region, which is identical with, or in the neighborhood of, the specular direction. With increasing roughness, the surface becomes more mat and less glossy. The peak intensity becomes lower, and the angular region in which the light is reflected increases. An example is given by indicatrix c. An ideal mat surface has no preferred direction for reflection and generates the indicatrix b. The relation between these measured photometric data and the perceived brightness is still a topic for research. According to CIE, for uniform bright areas the perceived specimen norma[ incidence angle light source receptor {viewing} angle scattering ,•• indicat • surface element t/F •-• Figure 1. Spatial distribution of reflected light.

302 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Figure 2. Schematic scattering indicatrices for a specular (a), a uniform diffusing (b), and a mixed reflecting (c) surface. brightness is a function of the third root of luminance (10). Gloss incorporates a brightness variation on the surface, and this variation may be important for the bright- ness scaling itself. For example, borderlines between bright and dark areas are enhanced in their contrast (11). This difficulty--besides others--is responsible for the lack of an unequivoca ! recom- mendation in respect to gloss scaling, but CIE considers maximal brightness •ind sharp- ness or clarity of the light source image on the sample as very important (9). If these parameters are described by maximal height and width of the luminance indicatrix (see CHARACTERIZATION OF THE PHOTOMETRIC DATA below), a nonlinear scal- ing of luminance such as the third root would not change the rank order. For reasons of simplification in this publication, the luminance itself is used as a suitable approxima- tion for the brightness. The dimension of gloss is still unknown, but according to investigations by O'Donnel (12), only one parameter is important for one subject at a certain moment. This parameter depends on the photometric properties of the sample and the personal condition of the subject. PHOTOMETRIC MEASUREMENTS Although many proposals for the photometric measurement of gloss exist (9), only a few have been accepted worldwide and standardized on a national or international level.