304 JOURNAL OF COSMETIC SCIENCE goniophotometry (2), by allowing a large number of measurements in a short period of time. This enables one to arrive at significant results despite the inherent large vari- ability of the hair material. Due to the nature of the parameters to be discussed here, no calibration of the instrument with respect to luminance (9) was conducted. Influences on the GP curves by diffraction effects are expected at large incidence angles only (=60 ø ) (3). The surface of hair is not smooth but features cuticle cells in a tile-like arrangement, where the cell scale edges point towards the fiber tip. In the case of human hair, multiple layers of cuticle cells are observed. They amount to about ten layers in the root region and are progressively worn off towards the tip through combing and brushing (10). Figure 3 shows an SEM micrograph of a Caucasian hair of typical appearance. The section originates from the middle part of a medium-length (25 cm), brown hair from a Caucasian female. The hair shows typical, though minor, damage of the scale edges and a few lifted scales due to grooming. In view of the surface and overall morphological structure of human hair, the reflection of light will be subject to a special type of geometry, which, in turn and with the principles of geometrical optics, leads one to expect three principal components of light reflection, as schematically shown in Figure 4. Such a three component model has been proposed and investigated by Stamm et al. (11) and subsequently by Guiolet et al. (4). The model is based on the simplifying, but reasonable assumption that the relevant reflection and refraction processes occur at the air/hair interface. Differences in the refractive indices of the morphological components are considered in a first approxima- tion as being of minor importance. The incident beam hits the fiber surface in the root-to-tip (RT) direction at the incident angle ½i, which is given with respect to the direction normal to the fiber axis. Figure 3. Scanning electron micrograph of a typical Caucasian human hair. The section originates from the middle part of a medium-length (25 cm) brown hair taken from a Caucasian female.

304 JOURNAL OF COSMETIC SCIENCE goniophotometry (2), by allowing a large number of measurements in a short period of time. This enables one to arrive at significant results despite the inherent large vari- ability of the hair material. Due to the nature of the parameters to be discussed here, no calibration of the instrument with respect to luminance (9) was conducted. Influences on the GP curves by diffraction effects are expected at large incidence angles only (=60 ø ) (3). The surface of hair is not smooth but features cuticle cells in a tile-like arrangement, where the cell scale edges point towards the fiber tip. In the case of human hair, multiple layers of cuticle cells are observed. They amount to about ten layers in the root region and are progressively worn off towards the tip through combing and brushing (10). Figure 3 shows an SEM micrograph of a Caucasian hair of typical appearance. The section originates from the middle part of a medium-length (25 cm), brown hair from a Caucasian female. The hair shows typical, though minor, damage of the scale edges and a few lifted scales due to grooming. In view of the surface and overall morphological structure of human hair, the reflection of light will be subject to a special type of geometry, which, in turn and with the principles of geometrical optics, leads one to expect three principal components of light reflection, as schematically shown in Figure 4. Such a three component model has been proposed and investigated by Stamm et al. (11) and subsequently by Guiolet et aL (4). The model is based on the simplifying, but reasonable assumption that the relevant reflection and refraction processes occur at the air/hair interface. Differences in the refractive indices of the morphological components are considered in a first approxima- tion as being of minor importance. The incident beam hits the fiber surface in the root-to-tip (RT) direction at the incident angle ½1, which is given with respect to the direction normal to the fiber axis. Tip Root Figure 3. Scanning electron micrograph of a typical Caucasian human hair. The section originates from the middle part of a medium-length (25 cm) brown hair taken from a Caucasian female