Synopsis A rapid method has been developed in order to compare the photostability of several sunscreen agents incorporated in the same type of emulsion. Thin films of the different preparations were spread on quartz plates and irradiated with a solar simulator. Differences in energy distribution according to wavelengths observed with solar simulator and sun radiation were taken into account. A kinetic study of absorption properties was carried out under solar simulator irradiation. A simple calculation gave the results which would be obtained under sunlight. This technique differentiated the reversible transformations of photoisomerizable compounds from an irreversible disappearance which resulted in a significant loss of protective power. Applied to benzylidene camphor derivatives, this technique showed the excellent photostability of these sunscreen agents.

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The reactions of indole semiquinone radicals produced following one-electron oxidation of 5,6dihydroxyindole (DHI) and its N-methyl-substituted analogue (MeDHI) have been studied using pulse radiolysis with spectrophotometric detection in the p H range 5-1 0 using different dose/pulse values (1-20 Gy/pulse). Using a dose/pulse of 18.5 Gy the semiquinone radicals of DHI and MeDHl decay predominantly by second order kinetics. The second order rate constants for disappearance of the semiquinone radicals are dependent upon the pH. Values of rate constants for decay of the semiquinone radical of DHI (pK, = 6.8) at p H 5.5 and 9.1 are 3.8 x lo9 and

Abstract— The variation with temperature, pH and light of the ESR signal of hydrated melanin powders from Japanese black hair has been studied. An explanation of the results is proposed on the basis of quinhydrone type complexes and of acid‐base equilibria of melanin and its semiquinone radicals. During exposure to light of wavelengths 254–600 nm, both stable and unstable radicals have been observed. The action spectrum for the formation of stable melanin radicals has been determined.