304 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS luminanee. luminous intensity maximum reflectometer value •'[•) A •' [•) I oe1 oe1-d oe, +d oe1-• oe1+• Figure 4. Parametric characterization of a scattering indicatrix. half-value angle receptor angle But there are other possibilities. Light may be reflected by the cuticles before entering the hair, and because these cuticles are tilted against the fiber axis, the orientation of the main reflection direction is also tilted (ray C). If the light has the chance to pass the whole hair, a reflection at the opposite lying cuticles may occur (ray D). They are tilted in the opposite direction and therefore cause an opposite tilt of the reflected light. To measure the reflection maximum in all these cases, it is useful to move at least the photometer head in different receptor angles ½2. GONIOPHOTOMETER--PROPERTIES For this purpose, a more universal instrument is necessary, a so-called goniophotometer. In a basic version it is standardized as "abridged goniophotometer" in ISO 7759 (15). A precise computer-controlled instrument is shown in Figure 6 and explained more in detail in reference 16. The mechanical parts were designed and supplied by the Halle Company, Berlin the optical and electronic ones were purchased from several sources and assembled in BAM (Federal Institute for Materials Research and Testing, Berlin). This instrument was used for our research on hair gloss. The fixed horizontal arm carries the equipment for illuminating the hair sample on the central turntable. The illumi- nated part of the hair is nearly circular, with an area of about ! cm 2, and the incident angie ½• is variable (- 75 ø • ½• • + 75ø), but for better comparison with published data (2) for all measurements, ½• = + 30 ø was choseq.

HAIR GLOSS 305 Figure 5. Possible directions for maximal reflection of light by hair. Ray A: reflection by a smoothing substance, covering the hair ray B: reflection by a cortex cell ray C: reflection by a cuticle before light enters the fiber ray D: reflection by a cuticle before hight leaves the fiber. The other arm, seen behind the turntable, has a horizontal working region (horizontal receptor angle) of about 180 ø and a vertical one (vertical receptor angle) of about 90 ø, but most hair indicatrices are measured in the horizontal plane only, the optical inci- dence plane (see CHARACTERIZATION OF THE PHOTOMETRIC DATA below). Therefore, if not otherwise indicated, the vertical receptor angle is fixed at 0 ø, and the receptor angle given in the figures is identical with the horizontal receptor angle. The movable arm carries the photometer head, the main equipment for detecting the re- flected light. It simulates a person who moves around the hair sample and evaluates the gloss from different points of view. When measuring corresponding hair indicatrices, conformity with natural conditions for gloss evaluation is important--for example, with respect to the illumination. Because the angular diameter of the sun as a natural light source is about 0.5 ø (17), a similar size for the chosen light source was chosen. Then an exclusively specular reflecting sample generates an indicatrix according to "a" in Figure 2, with a half-value angle of 0.5 ø (see CHARACTERIZATION OF THE PHOTOMETRIC DATA below). Therefore, no indicatrix details smaller than 0.5 ø can be resolved. When measuring hair strands, the chosen angular resolution of about 1 ø results in neglectable errors. The angular resolu- tion is limited by the mechanical parts of the goniophotometer and reaches 0.01 ø at its best. According to the section VISUAL GLOSS EVALUATION, the luminance, L, is used as a substitute for the visual brightness stimulus. To measure luminance the goniopho- tometer has to be calibrated with BaSO 4 as a good approximation of an ideal diffuse- reflecting surface. It is also illuminated under ½• = 30 ø.