IN VITRO SPF MEASUREMENTS 313 20 n 10 ,/ 0 I I I I I 0 5 10 15 20 25 30 OMC Concentration [wt.%] Figure 9. Comparison of measured and calculated SPFs against OMC concentration. Vitro-Skin ©, calcu- lated --., measured I Transpore © tape, calculated ....... , measured O. Analysis of excised skin data is now required to determine which substrate behaves most similarly to human skin. Rhodes and Diffey (8) have already shown that sunscreen application on human skin at the macro scale is a very hit-and-miss affair. Analysis of organic-based formulations on excised skin would complete the picture down to the micro scale. The thickness profile analysis will also permit comparisons of the surface profiles of different skin types. It will allow different formulations to be analyzed for their detailed "spreading" qualities, related to their viscosity properties, and will thus provide under- lying reasons for the occurrence of varying SPFs from different formulations containing identical absorber concentrations. Although it has been shown that it is possible to derive an equation for MPF at a fixed application rate, thereby allowing calculation of the MPF curves at all concentrations at that rate, it is not possible to calculate the effects of varying the application rate itself. Each rate of application will have its own thickness profile, requiring the full analysis of formulations at varying concentrations. Only in this way will the relationship that eluded Brown and Diffey (5) be explained. CONCLUSIONS A calculation method has been applied to measured spectral mono-protection factors for

314 JOURNAL OF COSMETIC SCIENCE lOO 90 80 -• 70 - • ',,,\ • 50 $0 - 20 • ""• 10 - ",, \,, 0 290 300 310 320 aao 340 350 360 370 380 390 400 Wavelength [nm] Figure 10. Comparison of the spectral absorption cu•es of OMC in ethanol and mineral oil with an absorption curve for the 1% OMC formulations. Ethanol .... mineral oil ,, 1% OMC formulation ....... . a formulation containing varying amounts of OMC supported on Transpore © tape and Vitro-Skin © substrates. Relatively simple thickness profiles were evolved that simulate the widely varying thicknesses of formulation which occur on these substrates. Excellent agreement has been obtained between measured and calculated SPFs. The wide vari- ability between individual spectral mono-protection curves obtained on Transpore © tape compared with Vitro-Skin © can be explained. The profiles allow calculation of the SPFs for all concentrations of absorber in the formulation on the two substrates. The vari- ability of the spectral absorption curves for OMC in different solvents is illustrated, but it can be shown that such variation has minor impact on the thickness profiles. ACKNOWLEDGMENTS The author is indebted to Uniqema (ICI), Solaveil, for the preparation of the formula- tions and the use of their laboratories and staff, and to Labsphere for making staff and equipment available for the measurements. REFERENCES (1) R. M. Sayre, Correlation of in vivo tests, in vivo SPF predictions. A survey of published studies, Cosmet. Toiletr., 108, 111-114 (1993).