Considerations of a consistent model for the reflection of light from human hair

Gloss, shine, lustre of hair relate to one of the most important and most widely used marketing claims for hair cosmetic products. For the consumer the quality of hair shine is an important indicator of hair ‘health’. This corresponds to applications for computer graphics, where the successful implementation of shine for hair collectives on animals or on humans in dynamic situations of body and light movements is an important condition for the realistic, ‘natural’ appearance of, e.g., a video character.Computer graphics, when starting from single hairs rather than from heuristic models for hair collectives, generally apply a model based on the basic geometrical optics of hair fibres, leading to two components of hair shine, which derive from the surface and the backside of the fibre, respectively (Marschner model). Experiments using multiangle-goniophotometry (MALGOP) have confirmed, in line with general light reflection models, that a minimum of three, symmetrical components of light reflection are in fact necessary to model experimental goniophotometric curves for hair, namely, specular, backside and directional diffuse reflection. To further our understanding of light reflection from human hair a series of Asian hair samples was investigated, starting with the virgin, black material and then proceeding to samples with increasingly lightened colour through repeated bleaches. The results show that with bleaching a fourth component of light reflection develops, which is attributed to ideal diffuse, angle-independent reflection. The frequency of occurrence as well as the intensity of this effect increases with increasing lightness of colour. Microscopic investigations reveal that this effect can to some extent be attributed to the medulla in Asian hair, providing internally a rough surface of reflection. A four component model for light reflection is thus established for hair, which provides a sensitive tool for probing the surface as well as the internal structure of hair and gives a comprehensive view of gloss formation. The changes of the parameter values of the model as well as of gloss formation imparted by bleaching Asian hair are discussed.