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j. Cosmet. Sci., 53, 11-26 (January/February 2002) Prediction of sun protection factors by calculation of transmissions with a calibrated step film model BERND HERZOG, Ciba Specialty Chemicals Inc., P.O. Box 1266, G-9001.2.28, 79630 Grenzach-Wyhlen, Germany. Accepted for publication August 15, 2001. Synopsis 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 inhomo- geneous 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. INTRODUCTION The sun protection factor (SPF) is defined as the ratio of the minimal erythemal doses of solar radiation directed to human skin in the presence and in the absence of a sunscreening agent (1). Based on this definition, in vivo methods for sunscreen testing on volunteers have been established (2-4) which are applied when SPF claims on sunscreen products are made. However, for purposes of experimental screening, in vitro methods for determination of the SPF have been introduced (5-8), since testing on volunteers is time-consuming and expensive. In vitro methods are based on the assumption that the UV protection of sunscreens is merely caused by the attenuation of UV light according to the absorption characteristics and concentrations of the UV absorbers used in the sunscreen formulation. Any further effects that may be of relevance in the in vivo methods, such as antiinflammatory or antioxidative properties, are not considered in the in vitro methods. In most cases in vitro methods model in some way the inhomogeneous surface structure of the human skin by using appropriate substrates like Transpore © tape or quartz plates 11