A study of the photolightening mechanism of blond hair with visible and ultraviolet light.  Takahashi T1, Nakamura K. Abstract

In this study the photolightening behavior of blond hair was investigated. The results demonstrated that visible (VIS) and ultraviolet (UV) light lighten blond hair through different mechanisms. VIS light was found to contribute much more to the lightening of blond hair than UV light, and acted directly, while UV light only lightened blond hair that had been washed following irradiation. VIS and UV light both, however, lightened to a similar degree isolated melanin granules and decomposed melanin granules that were exposed on a cross section of blond hair. These results indicate that melanin granules are imageedit_6_8063544284equally sensitive to both forms of light while blond hair is most sensitive to VIS light. The results also indicate that hair tissues, excluding melanin granules, are damaged by UV light but not by VIS light. Based on these facts, the hypothetical lightening mechanism of UV light is assumed to be that UV light preferentially attacks and damages hair tissues rather than melanin granules. This occurs only after the hair is washed, as the washing process removes the melanin granules that effuse from loose hair fibers. In contrast, VIS light preferentially attacks and decomposes the melanin granules rather than other tissues, and also results in the lightening of blond hair but without the need for subsequent washing. We also found that while VIS light destroys the structure of isolated melanin granules, UV light does not act in a similar manner. Consequently, it is proven that VIS and UV light attack different sites of the melanin granule, even though the lightening rates from both light sources are similar. (https://www.ncbi.nlm.nih.gov/pubmed/15264056)