Sharp and rectified imaging of plane test objects in diffractive grazing incidence interferometers

Fine-ground plane surfaces can be interferometrically tested in grazing incidence with laser illumination in the visible. Since the roughness of the surfaces is in the micrometer-range, the incidence angle of the probing wave front is very close to 90°, causing a strong anamorphic distortion of the length of the test sample on an imaging detector. In contrast to this the width of the sample remains undistorted in the image plane. Here, the case of a diffractive grazing incidence solution will be discussed. In particular, the rectification of the anamorphic distortion combined with sharp imaging of the whole length of the test sample will be described together with solutions for matching the illuminating and imaging beam cross section to the lateral extension of the sample.