�L 12 10 8 6 4 2 0 0 PHOTOLIGHTENING MECHANISM OF RED HAIR 55 ---vis light - -o - UV light 10 20 30 40 50 60 70 80 90 100 irradiation units Figure 7. Lightening degree of the granules isolated from red hair. Solid line is the result of VIS-light irradiation. Dotted line is that of UV-light irradiation. not necessary for the lightening of red hair by UV light. The difference in the photo­ lightening behaviors of red and blond hair should be due to differences in their melanin types. It is proven that pheomelanin, which is predominant in red hair, is highly sensitive to UV light. In the cases of both red and blond hair, hair protein absorbs UV light to some extent, so that melanin granules inside hair fibers show less sensitivity to UV light than they do when exposed in cross sections. However, phenomelanin is degraded due to its quite high UV sensitivity, regardless of any attenuation effects. Hair in which eumelanin is predominant has low sensitivity to UV light, and is not lightened by UV light alone during the short irradiation periods used in this experiment. Conversely, hair in which pheomelanin is predominant has high UV sensitivity and is lightened by it. Controversy regarding the photosensitivity of pheomelanin and eumelanin has lasted for some decades. Some researchers have argued that pheomelanin is more photolabile than eumelanin (11-13) other researchers have offered a contrary view (14). One of the reasons considered for this inconsistency is the state of the melanin granules that they investigated. When we study isolated or synthetic melanin, we see the photosensitivity of directly irradiated melanin. On the other hand, when studying the lightening behavior of hair fiber, especially regarding UV irradiation, we see the photosensitivity of melanin gran­ ules that were subject to light attenuation through absorption by hair protein. It follows that the sensitivity in this case is evaluated at a lower-than-real level. Actually, the researchers who argued that pheomelanin is more UV-labile than eumelanin performed their study mainly by using isolated and synthesized melanin (11-13 ), while other

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