The influence of surface roughness on the optical performance of diamond turned components has been investigated in the wavelength range 7.3μm to 12.6,μm, with special regard to the Modulation Transfer Function (M.T.F.). Specifically, diamond turned, planar mirrors, and conventionally polished mirrors have been inserted in a selection of optical trains, in order to perform comparative measurements. The optical trains employed, corresponded to two commonly used assessment techniques, (i) Fizeau Interferometry, (ii) Measurement of the Line-Spread Function. It is known that surface roughness can give rise to a finite degradation in performance and may influence the M.T.F. It has been shown that method (i) is largely insensitive to such a degradation. However, method (ii) exhibits a significant dependence on surface roughness. Results are presented for mirrors constructed from aluminium alloy, and brass.