IN VITRO SPF MEASUREMENTS 311 2.5 n 1.5 0.5 I I I I I I 290 300 310 320 330 340 350 360 Wavelength [nm] Figure 7. Comparison of measured and calculated high and low individual MPF scans for 1 wt % OMC on Vitro-Skin © substrate. High, measured --, calculated I low, measured ....... , calculated O. highlights the sensitivity of the MPF curves at high concentrations of absorber to the fractions at the smallest thicknesses. This allows the fractions at small thicknesses to be quite accurately defined and provides a useful point to commence initial analysis of the data to find the complete thickness profile. By routine application of the analysis, experience could be built up about how particular formulations behave on the substrates. Improved accuracy could be achieved by selecting more appropriate absorber concentrations or perhaps using four or five concentrations instead of three. Vitro-Skin © produces much more uniform data than Transpore © tape by virtually eliminating the possibility of producing small areas of uncovered substrate. It produces a very simple thickness profile that does not permit wide fluctuations in thicknesses or their surface fractions. Both substrates develop SPF values similar to that of skin by providing a "sink" for a large fraction of the formulation in relatively deep crevices in their surfaces, Transpore © tape taking up 56% of the formulation in this fashion and Vitro-Skin © 81%. The change in SPF noted for the Transpore © tape after allowing the formulations to dry implies a modification of the profiles on drying. If the profile had remained unchanged, then the concentration change of the absorber on drying would not have altered the MPF curve. There is scope here for assessing the effects of the drying process on the thickness profiles. The transmission equations permit a detailed analysis to be made of the variation of SPF with concentration of active ingredient. These calculations are presented in Figure 9,

312 JOURNAL OF COSMETIC SCIENCE 7O 30 20 ' ..... '*"-,..•.,% • 0 T 290 300 310 320 330 340 350 360 Wavelength [nm] Figure 8. Comparison of measured and calculated high and low individual MPF scans for 10 wt % OMC on Transpore © tape substrate. High, measured --, calculated I low, measured ....... , calculated where it can be seen that the Transpore © tape curve flattens out at high absorber concentrations, producing a range of SPFs that agrees roughly with the Vitro-Skin © data. This is a direct result of the presence of a small fraction of uncovered substrate. The Vitro-Skin © data are displaying the characteristic, fortuitous linearity of SPF variation with concentration that is generally noted in sunscreens. The plot also contains the measured SPF values, showing excellent agreement between calculated and measured SPFs. A distinctive recurring deviation of the calculated curves from the measured ones occurs mainly at the two lower concentrations in the 290-310 nm wavelength regions (see, for instance, Figures 6 and 7). The calculated curve shapes parallel that of the measured absorption properties of OMC (Figure 1). It would appear that the absorption charac- teristics of OMC are slightly different in the formulation from those applying to its alcohol solution. It is known that the solvent can influence the absorption characteristics of organics (9). It is also possible that at these lower wavelengths other components in the formulation have significant absorption properties. These effects are demonstrated in Figure 10, where the absorption curves for OMC in alcohol and mineral oil, a component in the formulation, are compared with an absorption curve obtained from a very thin film of the 1% OMC formulation. It is clear that the thickness profile analysis could be improved if a reliable thin film method could be established for measuring the absorp- tion characteristics of the actual formulations. However, separate derivation of the profiles using all three absorption curves shows that the curve differences in Figure 10 have only relatively minor impact on the profile.