Tham khảo tài liệu 'modern developments in x-ray and neutron optics episode 6', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 188 A. Rommeveaux et al. be shown that the spurious signal resulting from a slightly offset sinusoidal fringe is in quadrature with the signal resulting from the centered fringe. The depth of the image minimum is affected but its position does not change. The position of the minimum is interpolated from nine bracketing points. In some cases namely when measuring gratings 7-9 the SUT reflectivity will be different for polarization along or perpendicular to the track direction. In order to minimize the loss of fringe contrast in this case we use a specially cut Wollaston prism arrangement where the optical axes of the two prisms are set at 45 to the wedge direction and therefore parallel to the quarterwave plate axes instead of being parallel and perpendicular to the wedge as it is usually constructed. Due to the symmetry the reflected components for the two principal directions of polarization are equal and the fringe contrast is preserved. Finally the direction of the probe beam can be chosen by different arrangements of the mirrors and prisms in the moving head. By rotating P2 by 180 around the X-axis before gluing we obtain an upward pointing stabilized beam. The actual configuration used to measure downward facing surfaces is obtained by inserting between Ml and P1 a periscope composed of two flat and parallel mirrors which brings the beam up without changing its direction. Side illumination is realized using the same principle with M1 and the following prisms in an upward pointing configuration turned 90 around the incoming beam so that the lateral direction of the equivalent roof reflector is now along Y instead of Z. A 500 m long instrument of the type described above is able to measure slopes in the range of about 5mrad corresponding to a radius of 10 m in a 100 mm long mirror 14 15 . When this range is not enough it is still possible to extend the measurement length by stitching a series of successive scans with different inclinations of the surface. A .
