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 discrepancy,
as suggested by Ferrero et al. [10], who showed a correlation
between substrates roughness and SPF outcomes in case of PMMA
plates.