Infrered fresnel lens is molded in a flexible, 0.015" (0.457mm) thick, milky white plastic. Advantages of this product are: least absorption loss in the 8-14 micron region, extremely thin with consistent thickness across the lens, large apertures and minimal thermal expansion. The design of an infrared-transmitting fresnel lens involves many complex considerations. The grooved side of a fresnel lens should face the longer conjugate (away from the detector when used to collect radiation).