Heating time and temperature distribution of a semi-finished pre-product are key factors for a satisfying final shape in the thermoforming process. The heating is in most cases done by radiation, which is modelled in simulation by the law of Bouguer Beer Lambert. Two material parameters, the reflection of the surface and the optical penetration depth are needed for the application of this law. One prerequisite for this is that the optical penetration depth is independent of the material thickness. In literature it is very often common to trust on that assumption and to determine it via single measurement of a film or plate with one thickness. The validity of this presumption is investigated for two different polystyrene compounds as part of this work. This assumption was fulfilled for a clear unfilled polystyrene (PS) in a wide range of wavelengths. In contradiction to this, the optical penetration depth of the white filled PS compound is dependent on film thickness. Moreover it increases slightly with film thickness. This deviation is correlated to the strong scattering effect of titanium dioxide contained in this compound.