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.