Trinitrofluorenone (TNF) and poly-n-vinylcarbazole (PVK) are commonly used in thermoplastic-photoconductor recording materials. The optical characterization of the layers of these films is of great interest in achieving maximum diffraction efficiency. Interference measurements of the optical thickness of absorbing media can be rapid and accurate. The real part of the refractive index is computed from the ellipsometric data in the range from 360 nm to 480 nm, It is extrapolated up to 780 nm and used with spectral transmittance and reflectance measurements to compute the imaginary part of the refractive index and the thickness of the sample of PVK:TNF. The sample is considered as a Fabry-Perot interferometer with surface defects. The defects arise from spatial inhomogeneities in the thickness. Non-linear regression analysis is applied to obtain the parameters of interest.