Two basic designs of reflectors

Only a perfectly smooth surface exhibits directed (specular) reflection, i.e., where the angle of reflectance equals the angle of incidence. This, however, is not the case under real-life conditions, where the surface micro-roughness cannot be avoided, and consequently, the UV can be slightly diffused over a finite range of reflection angles. 

UV reflectors also reflect radiation energy in the visible and infrared (IR) energy spectral regions at least as efficiently as in the UV region. This is undesirable in such cases where the substrate or the piece being irradiated is heat sensitive. To overcome this, the surface of the reflector is coated with 

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