Multiple wavelength interferometric testing of aspherical mirrors

Multiple wavelength interferometry is applied to the testing of general aspheric surfaces - smooth ones as well as special surfaces with edges or steps. A HeNe-laser and different frequency stabilized diode lasers are used as coherent light sources. The phase shifting technique is applied for the analysis of the single-wavelength interferograms. Different analysis techniques for the combination of single-wavelength phase-maps have been tested. Due to the high spatial frequency of the fringe pattern and the limited modulation transfer function of the camera the signal/noise ratio of the calculated phase maps is low. A phase unwrapping procedure based on a cellular automata is used to restore the noisy phase maps. A specific problem in multiple wavelength interferometry is the chromatic aberration of the interferometer optics. An interferometer with minimum chromatic aberration was designed and built up. Special calibration techniques for multiple wavelength interferometry are presented. A discussion of systematic errors is given and an error-compensating approach is outlined.