Prediction of sun protection factors by calculation of transmissions with a calibrated step film model.
Measurements of in vitro sun protection factors (SPFs) are a common way of assessing sunscreen formulations at the stage of screening. The aim of the present investigation is to provide an alternative tool for the estimation of SPF values using a calculation based on the UV spectroscopic properties of the individual UV absorbers. As with in vitro measurements, the crucial step is to work out realistic values of transmissions of UV light through a film of the sunscreen formulation in the important spectral range between 290 and 400 nm. Once these transmissions are given, the SPF can be calculated. Since the human skin is an inhomogeneous substrate, a step film model for the calculation of such transmissions had been proposed by J.J. O'Neill. The step film geometry in this model is a function of two parameters that characterize the fraction of the thin and thick parts of the film and their difference in thickness. The transmissions and therefore the SPF are sensitive functions of the step film parameters. In order to use the model for the prediction of realistic SPF values, the step film parameters are calibrated using three sunscreen standard formulations with well-known in vivo SPF. A satisfactory correlation of in vivo SPF values and SPF values calculated with the calibrated step film model using an additional 36 different sunscreen formulations (in vivo SPF values between 3 and 36) is demonstrated.
Herzog
Bernd
B
Ciba Specialty Chemicals Inc., P.O. Box 1266, G-9001.2.28, 79630 Grenzach-Wyhlen, Germany.
D002138:Calibration; D008956:Models, Chemical; D013056:Spectrophotometry, Ultraviolet; D013472:Sunlight; D013473:Sunscreening Agents / Q000737:chemistry*
D013473:Sunscreening Agents