The lateral resolution of an interferometer is limited mainly by the design of the optical arrangement as well as the size of the beam stop. For its characterization the MTF1,2 is not very useful. The height of a structure normal to the surface under test is transferred into a phase of a reflected wavefront. Since imaging mechanisms for intensity and phase are very different, we propose a Height Transfer Function (HTF) to describe the lateral resolution of interferometers. The HTF shows the quotient of the reconstructed and the original height of a sine-modulated surface structure as a function of the spatial frequency. The HTF can be measured with a test sample of varying periodical surface profiles and spacings. Simulations can be made using a combination of geometrical ray tracing and Fourier transformation techniques. Two different layouts of null systems for the test of an asphere are compared. A device to measure the HTF is shown along with results for a variety of different interferometers.© (2005) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.