A relationship between Jones and Mueller matrices originally derived for random media is applied to incoherence effects in photometric ellipsometry. Such effects are for example depolarization after reflection from a sample with varying film thickness or from a layer which is thicker than the coherence length of the incident light. The main task is to calculate the expectation value of a statistical ensemble. For the important case of thick layers, this expectation value is derived in a symbolic form. Results calculated with this method for transmission ellipsometry, ellipsometry at the back surface of the substrate and the determination of the optical constants at the substrate-layer interface are compared with measurements.