Typical thin film semiconductor transmission (or reflection) spectra in the visible and near-infrared regions are composed of a transparency region with interference spectral fringes and an absorption region. Often, the position and contrast of the fringes are used to compute the film thickness and optical constants. In this work, we show experimentally that the transmission is strongly altered by the size of the spot if the film is not homogeneous in thickness, and also by the apparatus resolution if the optical thickness of the film is big enough. The experimental evidence is brought out using 1-2 mu m thick amorphous silicon films, the transmission being carried out in a double-beam spectrophotometer from 0.5 to 3.2 mu m. The results are analyzed using a theoretical mathematical model of the transmission of thickness inhomogeneous thin films previously developed by the authors, and briefly recalled in the present paper. The experiments confirm the model predictions particularly the alteration of fringe contrast and quasi-conservation of incoherent transmission.