On the dynamics of UV bleaching of epidermis autofluorescence
A.B. Pravdin, S.P. Chernova, A.A. Kudryashov
Department of Optics, Saratov State University, Russia
Autofluorescence spectroscopy, as a method for monitoring status of tissue and diagnosis of diseased tissue, has increased rapidly during recent years. Autofluorescence of skin tissue is phenomenon known since the early 20th century. Nowadays the fluorescence response of skin forms the basis of some diagnosis of skin diseases [1]. Epidermis autofluorescence on the hand could reveal alterations of natural fluorophores composition in epidermis pathologies (e. g. psoriasis) and on the other might distort precise measurement of tissue transmittance or reflectance, especially over UV range.
It is essential, that there exists an obstacle to in vivo measurement of epidermis autofluorescence
because of intensive dermis fluorescence (it is so at least for 442 nm excitation [2]), so we modified
surface epidermal stripping technique [3] as to employ in spectrophotometric study of human epidermis.
In early experiments [4] the decrease in autofluorescence intensity of epidermal strippings under
continuous 
laser irradiation has been observed. That autofluorescence decay was attributed to
exponential photobleaching (irreversible photochemical transition into nonfluorescent molecule) of two
epidermal fluorophores (plus one non-bleaching) under continuous UV irradiation. But in the present
work in the course of studing epidermis autofluorescence under 
nm excitation we obtained the
kinetic curves (fluorescence intensity vs time of irradiation) one of which is shown in the picture.
The form of the curves does not allow approximation by the sum of exponential decays, i.e. bleaching rate can not be represented as a linear combination of different fluorophores contents; as a tentative approximation of rate vs fluorophore content curve we suggested a polynomial of degree four in fluorophore.