The importance of signal-to-noise ratio (SNR) calculations for an optimum Fourier transform-infrared spectroscopy (FT-IR) detection of thin, isotropic layers on dielectric substrates is discussed; some illustrative examples are given. It is found that the SNR increases with increasing refractive index of the substrate whereas the use of p-polarized light is preferable. Optimum measurement conditions for oriented layers were found for a Ge substrate at an angle of incidence of 50-55 degrees.