It has been shown that the optical transmission of microwave-plasma CVD produced diamond films is a strong function of the surface roughness. Polycrystalline diamond films consisting of pm-size crystallites and deposited at relatively high substrate temperatures (~700-1000 °C) have significant surface roughness with values ranging from a few hundred Angstroms to a few thousand Angstroms. The discrepancy in the transmission spectra between single crystal type IIA natural diamond and polycrystalline diamond films can be fully accounted for by optical scattering and difference in the thicknesses of the two types of samples. For sufficiently long wavelengths in the infrared, the optical scattering is predominantly due to surface roughness. Our data show that for diamond films with surface roughness values of about 500 Å or less, the infrared transmission of such films at wavelengths about 5μm and greater is comparable to that of single crystal type IIA natural diamond.
"Correlation Of Optical Properties And Surface Roughness Of Polycrystalline Diamond Films", Proc. SPIE 1146, Diamond Optics II, (15 January 1989); doi: 10.1117/12.962073; https://doi.org/10.1117/12.962073