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Error influences of the shear element in interferometry for form characterization of optics

A shearing interferometer combined with an LED multispot illumination provides a high flexibility form characterization of optical surfaces as it is needed for aspheres and freeforms. Core element of the setup is the spatial light modulator as shearing element (SLM). Error influences due to the used blazed grating of the SLM need to be investigated. We show results of wavefront measurements with a Shack-Hartmann sensor which demonstrate residual structures of the grating at the wavefront under test. Additionally, simulated data are compared to the measurements to get a better understanding of the expected effects. These investigations help to correct the wavefront under test for this static error and improve the accuracy of the form characterisation.