| 摘要 |
1504001 Measuring displacement photoelectrically ERNST LEITZ WETZLAR GmbH 15 May 1975 [6 June 1974] 20540/75 Heading G1A In displacement measuring apparatus, rays l -1 , l +1 diffracted by a first optical element 1 are recombined by a penultimate one 2 of a plurality of elements so as to form an interference pattern at a final element 3, whereby photo-electric means (not shown) may provide phase-separated signals representing relative displacement of at least one optical element relative to the others. The spacing e between two elements, the diffraction constant d of at least one of them and the wavelenght # and bandwidth ## of the light, are so related that disturbances due to an adjacent diffraction order, e.g. l 0 are substantially avoided without the need of a diaphragm to obstruct the adjacent order. The relationship is calculated from is a symmetry factor, equal to 2 in the embodiment, where three equally-spaced elements are used. The overlap factor K is the ratio of the actual difference in path length between the orders l 0 and l -1 (or l +1 ) to the coherence length l c of the light, given by Under ideal conditions K would be not less than 1; but for practical purposes, where the apparatus is required to be compact, the disturbance due to the lo order may be tolerable even for values of K of less than 1. In an alternative embodiment, Fig. 2, the penultimate optical element comprises a pair of mirrors. |
