The substrate / plate is the material to which the sunscreen product sample is applied. 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.