For the production of electronic devices one often needs a certain depth profile of a semiconductor's conductivity. To achieve this one growths epitaxially layers which differ from the substrate by their doping level. The obtained thickness of the so-called 'epilayer' can be checked non-destructively by infared analysis. This is the goal of this example. To make it simple we restrict ourselves to the case of an undoped epilayer on a highly doped substrate. Again, like in the previous section, we neglect any reflection from the wafer backside.