Realization of a shearing interferometer with LED multipoint illumination for form characterization of optics

Within a Deutsche Forschungsgemeinschaft (DFG) joint project between PTB and BIAS, a new approach to interferometric form measurement based on shearing interferometry with low coherence light is investigated. Since shear interferometry only requires the coherence of field amplitudes at positions separated by the shear, it permits multibeam LED illumination. Artefacts due to the coherent amplification of parasitic reflections are significantly reduced by using partially coherent light. We make use of a multi-spot light source which comprises an LED coupled into seven fibres that can be individually adapted to the specimen. Each light source yields an interferometric pattern at the camera section. The individual patterns are combined into an overall pattern. This method secures optimal illumination and well resolvable data all over the measurement area. A high degree of flexibility in surface form measurement can be reached by applying the required number of illumination spots with individual orientation. A realization of a shearing interferometer in combination with sevenfold LED point illumination and corresponding experiments measuring the form of optics will be shown.