A combination of ellipsometric and photometric measurements provides a convenient and accurate method for the determination of the optical properties of thick polymer films. Of course, a reasonably good surface and layer quality is necessary. Multiple reflections in the thick film and thus incoherent superposition causes partial depolarization of the reflected or transmitted light. Therefore, the Mueller matrix elements of these layers are measured and compared to calculated ones. A relationship between Jones and Mueller matrices originally given for random media, is used to derive the formulas for the Mueller matrix elements of these anisotropic thick films. This relationship can also be applied to other incoherent effects in spectroscopic ellipsometry, such as depolarization upon reflection by a sample with a varying film thickness or by a layer with fluctuating optical constants.