A diffuse reflector of radiation in the near and mid infrared regions includes (i) an assembly that has a reflecting element and a diffusing element that is made of one or more layers of calcium fluoride, sapphire, or alumina; or (ii) a diffusively reflective surface configured as a metallic layer with a rough surface. The diffuse reflector can be incorporated into systems for measuring properties of sheet materials and particularly into optical sensors that include a measurement window configured with one or more of the diffuse reflectors that cause incident radiation from a sensor light source to be diffused and reflected a plurality of times within a layer of material before being detected by the sensor receiver.