System design is a method to realize system synthesis -- a special form of system identification -- that means, the determination of a system S corresponding to a given behavior. In synthesis this behavior is prescribed and S has to be constructed accordingly. The other form of identification is system recognition, where one has to find the structure of an existing system from a behavior given by measurements. In both cases the mathematical methods used won't differ essentially. For thin film systems the behavior is given by functions depending on the wavelength and/or the angle of incidence of a harmonic plane wave.