Based on the theory of partially coherent light, an analytical scheme is established to determine the radiative properties of multilayer thin films. Accurate knowledge of the magnitude of interference effects is absolutely indispensable when measurements of the spectral transmissivity and reflectivity of thin films are used to derive optical constants of the film material. As illustrated by experiments, in many cases of practical interest neither a geometrical nor a wave-optics model gives satisfactory results. A general formulation is introduced that covers this intermediate partially coherent regime as well as the limiting cases of geometrical and wave optics. The complex degree of coherence provides a direct measure of the varying influence of interference effects on spectral measurements. An analytical approximation of the numerical approach is developed that gives a good physical understanding of the occurring phenomena. Experiments on the transmissivity of a one-layer glass film using a Fourier transform infrared spectrometer in the medium infrared range (λ = 2 to 20 μm) confirm the theoretical approach and the relevance of the addressed issue.