Refractive index is an important property for infrared optics applications. For most polymers, a refractive index of n = 1.6 is a typical limit, where sulfur-rich copolymers can have a refractive index as high as n = 1.9 . For 67-poly(S-r-CPD) made using the gas phase method, the refractive index was found to be in the range of n = 1.85-1.90 across the MWIR and LWIR region, as determined by specular reflectance spectroscopy .