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j. Cosmet. Sci., 53, 387-402 (November/December 2002) Influence of internal structure of hair fiber on hair appearance. II. Consideration of the visual perception mechanism of hair appearance SHINOBU NAGASE, NAOKI SATOH, and KOICHI NAKAMURA, Kao Corporation, Hair Care Research Laboratories, 1-3, Bunka 2-chome, Sumida-ku, Tokyo, 131-8501 Japan. Accepted for publication May 29, 2002. Based on a presentation given at the 12th International Hair-Sdence Symposium of DWI, Heidelberg, Germany, September 7, 2001. Synopsis The optical properties of hair fibers were studied, focusing on the reflections (highlights) from both the front and the back surfaces of the fiber in consideration and on the effect these have on the perceptions of hair appearance. The two reflections are distinguished from each other by sight, because only the back surface reflection is colored by melanin granules and/or dyestuffs inside the fiber. When we observe a flat plate as a model for hair without a cuticle structure, the visual angle between the two light loci correlates with the thickness of the model plate and gives an impression of depth. In the case of hair with a cuticle angle, the visual angle is maintained even when the fiber thickness is reduced. This visual angle causes an overesti- mation of the thickness and enhances the impression of depth. The visual angle changes dramatically with a curl curvature change of the hair tress, meaning that the impression of depth is also dynamically changed by a small change in hairstyle. The dynamic change in the impression of depth probably causes a vibrant impression. The following are required for beautiful hair appearance along with an impression of depth and vibrancy: (a) Internal structure without light scattering origins is essential to observe vivid colored high- lights from the back surface. (b) Well-ordered cuticles are essential to get intensive double highlights from the front and back surfaces. (c) A properly curved hairstyle is essential to obtaining a more vibrant impression. INTRODUCTION Since hair shine is one of the most important concerns of the consumer, much research has been conducted in order to understand the mechanism of hair shine and how to improve it. Hair shine has been mostly understood as an optical phenomenon of fiber surfaces, and technology to improve surface optical properties has been developed. Light penetrates, however, into hair fiber and is reflected at the back surface of the fiber, and so the internal hair structure must also affect hair appearance when the hair fiber is not completely black. The authors confirmed that the hair appearance is influenced by the 387