Abstract: In a system for security marking a product, a surface of the product 10 is covered by a layer 14 of light-transmitting material which exhibits variations in refractive index throughout the material such that for most angles of view the material acts as a conventional diffuser of light and thereby conceals or obscures a marking 12 applied to a surface of the product and covered by layer 14, but for a critical angle of view, or a limited range of angles close to that critical angle, the layer is effectively transparent allowing unimpeded viewing of the marking. In a variant, the marking is itself incorporated in the layer in the form of refractive index variations in the layer.