Capability to simulate the coherence function is important when tuning an interference microscope in an effort to reduce sidelobes in interference signals. The coherence function cannot directly be derived from the light source spectrum since the microscope's effective spectrum is affected by e.g. spatial coherence effects. We show this by comparing the true system spectrum measured using a spectrometer against the effective system spectrum obtained by Fourier analysis of the interference data. The results show that a modulation function that describes the scattering-induced spatial coherence dampening in the system is needed to correct the observed difference between these two spectra. The validity of this modulation function is further verified by quantifying the arithmetic mean roughness of two specified roughness standards. By providing a spectral transfer function for scattering, our method can simulate a sample specific coherence function, and thus shows promise to increase the quality of interference microscope images.