5 February 1993 Speckle reduction by virtual spatial coherence
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Abstract
Using highly coherent laser sources in interferometry often leads to speckles in the interferograms. These speckles constitute a noise on the fringe phase and, hence, lead to a reduction of wavefront measurement precision. They arise from the light scattered by random imperfections of the optical surfaces. A new technique was developed at Carl Zeiss to reduce the effects of speckles in the laser interferometer DIRECT 100 by a virtual reduction of the spatial coherence regarding the speckle contrast. In the technique presented here the direction of the illuminating light beam in the interferometer is modulated while averaging wavefronts (not intensities) with the real-time wavefront averaging capability of DIRECT 100, resulting in a virtually larger extent of the light source. The fringe contrast is independent of this beam modulation, whereas the speckle contrast in the accumulated wavefront is determined by the virtual extent of the light source. Thus, speckle effects not only from the imaging part of the optical train but also from the illuminating part are reduced.
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Klaus R. Freischlad, Klaus R. Freischlad, Michael F. Kuechel, Michael F. Kuechel, } "Speckle reduction by virtual spatial coherence", Proc. SPIE 1755, Interferometry: Techniques and Analysis, (5 February 1993); doi: 10.1117/12.140784; https://doi.org/10.1117/12.140784
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