Edwards [8], using human stratum corneum and Transpore as substrates for SPF evaluation, observed that in 10 out of 11 sunscreens studied the SPF values measured with Transpore were systematically higher than those obtained using human stratum corneum. In another study, Lott et al. [9] adopting three different substrates obtained significant differences in the SPF values; these results have been related to the non-uniformity of product application. Other physical effects related to the coupling between substrate and sunscreen can partially account for such discre- pancy,assuggestedbyFerreroetal.[10],whoshowedacorrelation between substrates roughness and SPF outcomes in case of PMMA plates. We have analyzed the variability in term of absorbance values by using different substrates as follow. Let’s try to design the interaction between two media, A (sunscreen) and B (the substrate), and light taking into account different physical effects. If the two media have both smooth faces and low scattering properties,thetransmittedradiationpathisstraightforwardwhile part of radiation is straight back reflected (Fig. 9a). If medium A is diffusing, both the in-coming radiation and the back reflected at interfacearethenscatteredinalldirections,andpartofthelastone is again re-directed in the substrate (Fig. 9b); if the substrate has a roughened surface, as in case of PMMA plates and roughened
Fig. 7. Absorption, computed as A =1TR, of the different substrates.
Fig. 8. Sunscreen absorbance measurements on the seven sunscreens tested. (a) Sunscreen applied on Transpore attached on polished quartz; (b) sunscreen applied on hydrated Vitro-Skin; (c) sunscreen applied on roughened quartz; (d) sunscreen applied on PMMA; (e) sunscreen applied on PTFE; (f) sunscreen applied on self-standing Transpore.
D. Garoli et al./Journal of Dermatol