The purpose of this study is to demonstrate the influence of plate roughness on well‐known absolute or relative UV spectroscopic indices such as the in vitro SPF, in vitro UVA PF (PPD), UVA/UVB ratio, critical wavelength and SPF/UVA PF (PPD) ratio. Firstly, a roughness index was measured by non‐contact surface topography analysis on different PMMA plates, which are available today on the European market. A large difference between maximum and minimum plate roughness values could be measured ranging from 1.88 to 6.76 μm. Then, the PMMA plates were grouped into five different roughness classes. To measure the different spectroscopic indices, the same sunscreen preparation was applied at the same amount rate (1 mg cm–2) to each of the selected plates. Large differences were noted for the absolute in vitro indices SPF and UVA PF (PPD), with the SPF varying in one instance from 13.5 to 40.7. On the other hand, the relative absorbance indices UVA/UVB ratio and critical wavelength showed a small but significant variation in relation to roughness. The SPF/UVA PF (PPD) ratio index based on the direct in vitro values was found to be very sensitive to roughness variation. However, adjusting the UV absorbance curve to reach equality between the in vitro SPF and in vivo SPF is a recognized and efficient mathematical process to limit the variation. All these results can be interpreted using the irregular sunscreen film theory. Mathematical developments can be achieved by modelling the height distribution with a gamma function.

The efficacy of sun protection, mostly realized by the application of sunscreen formulations, is commonly described by the sun protection factor (SPF). Previous investigations have shown that the efficacy of the sun protection inter alia depends on the homogeneity of the distribution of the topically applied sunscreen formulation on the human skin. Therefore, suitable methods are required to determine the homogeneity of topically applied substances on the skin surface. This study provides and compares two different methods, which enable this determination. Laser scanning microscopy allows the analysis of tape strips removed from skin treated with a sunscreen. These reflect the inhomogeneous distribution on the skin that can complementary be determined directly, utilizing a dermatological laser scanning microscope. For the second method, a chromatic confocal setup was utilized, which enables the study of the microtopography of skin replicas before and after the application of a sunscreen product. The two methods were applied for the evaluation of three different sunscreen formulations for each method. A correlation of the homogeneity of distribution with the in vivo SPF could be confirmed. Both methods are suitable to investigate the homogeneity of the tested sunscreen formulations, although they provide different advantages and disadvantages.