| 摘要 |
PURPOSE:To eliminate the adverse influence of plural diffracted light beams and to reduce the size of an encoder by making coherent luminous flux obtained by a light source means incident on a diffraction grating and setting the separation angle between diffracted light beams of adjacent orders among plural diffracted light beams from the diffraction grating to >=3 deg.. CONSTITUTION:The luminous flux emitted by the laser 1 is made incident on positions M1 and M2 of a radiation grating 7 provided with the diffraction grating on a disk 6 coupled with a rotating body to be measured. At this time, plural transmitted and diffracted light beams of 0th order, pi1st order, + or -2nd order, etc., are emitted from the positions M1 and M2. Then, when the beams of + or -1st order from the positions M1 and M2 are used as diffracted light beams to be superposed one over the other to form interference fringes, it is not prefer able that the diffracted light beams of + or -2nd order polarization are incident on reflecting means 8 and 9. For the purpose, the separation angle DELTA between the beams of adjacent degrees is so set preferably that ¦DELTA¦>3 deg. in consideration of usable coherent luminous flux, the constitution of an optical system, the diameter of the luminous flux, etc.
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