The substrate / plate is the material to which the sunscreen product sample is ap-plied. It must be UV-transparent, non-fluorescent (i.e., give no detectable fluorescence when exposed to UVR, as measured with the spectrophotometer), photostable and inert towards all ingredients of the preparations to be tested. Furthermore, to approximate the application of a thin film of sunscreen product to skin topography, the substrate should have a textured upper surface.
For this method, PMMA plates (polymethylmethacrylate) with one side of the substrate roughened, have proven to be satisfactory. The topography of the recom-mended PMMA plates has been characterized by 6 roughness parameters and the resulting targets and upper / lower limits of these parameters are described in Ap-pendix III.
The size of the substrate should be chosen such that the application area is not less than 16cm² and a square shape is preferred.