Thin films are used in many energy conversion applications, ranging from photodetectors to solar cells. Accurately predicting the radiative properties of thin films when they possess rough surfaces is critical in many instances, but can be challenging due to the complexity arising from light scattering and interferences at the microscale. This work describes measurements of the spectral transmittance and reflectance of several thin-film materials (including crystalline silicon wafers and a polycrystalline diamond film) in the mid-infrared spectral region (2 20 m) using a Fourier-transform infrared (FT-IR) spectrometer. The transmittance and reflectance were calculated using thin-film optics for the double-side polished samples and scalar scattering theory for the single-side polished samples. The effects of partial coherence are considered using a fringe smoothing technique. The interval used for fringe smoothing was assumed to be linearly dependent on the wavenumber. Good agreement between the predicted and measured transmittance was achieved for the double-side polished silicon wafers and for the diamond film. The disagreement for some single-side polished silicon wafers may be inherently related to their surface microstructures, as suggested from surface topographic data and images obtained from surface profilometry and microscopy. By comparing the intervals used for fringe smoothing with the instrumental resolution, beam divergence in the spectrometer was found to be a major factor contributing to the partial coherence. Future research is proposed to investigate the correlation between the detailed surface characteristics and the conical-conical transmittance and reflectance.