| 摘要 |
PURPOSE:To prevent a measuring accuracy from being decreased due to a grating pitch error by superposing diffracted light beams of specific orders different from one another from a grating and leading the superposed, diffracted light beams to light receiving means. CONSTITUTION:The light beam irradiated from a laser 1 is changed to a parallel light beam by a collimator lens 2 and projected upon a beam splitter 3 and then upon a position M1 on one 4 of the radial diffraction gratings provided on a disk connected to a rotating object to be measured. Two diffracted light beams of specific orders different from each other, for example, +n-th and -n-th orders or n-th and m-th orders, in the transmitted and diffracted light beams that are transmitted through and reflected from the grating 4 on which the light beams are projected are reflected from optical means 5 and 5' composed of reflecting mirrors and returned in the same light path as in advance to be again projected upon approximately the same position M1 on the grating. The same two diffracted light beams of such specific orders as, for example, n-th and m-th orders as the previous ones which are diffracted again by the grating 4 are projected upon the beam splitter 3. The two diffracted light beams reflected from the beam splitter 3, that is, the diffracted light beams wherein the diffraction of n-th order is conducted two times and the diffracted light beams wherein the diffraction of m-th order is conducted two times are superposed on each other and led to light receiving means 6.
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