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).

IN VITRO SPF MEASUREMENTS 315 (2) R. M. Sayre, P. P. Agin, G. J. LeVee, and E. Marlowe, A comparison of in vivo and in vitro testing of sunscreening formulations, Photochem. Photobid., 29, 559-566 (1979). (3) B. L. Diffey and J. Robson, A new substrate to measure sunscreen protection factors throughout the ultraviolet spectrum, J. Soc. Cosmet. Chem., 40, 127 (1989). (4) J.J. O'Neill, Effect of skin irregularities on sunscreen efficiency, J. Pharm. Sci., 73, 888-891 (1984). (5) S. Brown and B. L. Diffey, The effect of applied thickness on sunscreen protection: In vivo and in vitro studies, Photochem. Photobiol., 44, 509-513 (1986). (6) S.R. Spruce and J.P. Hewitt, In vitro SPF: Methodology and correlation with in vivo data, Euro Cosmetics, 14-20 (June 1995). (7) A. Beer, Ann. Physik Chem. (J. C. Poggendorff), 86, 78 (1852). (8) L. E. Rhodes and B. L. Diffey, Quantitative assessment of sunscreen application technique using in vivo fluorescence spectroscopy, J. Soc. Cosmet. Chem., 47, 109-115 (1996). (9) N.J. Turro, Molecular Photochemistry (W. A. Benjamin Inc., New York, 1967), p. 45.