Patent ID: 8730483

Claim:
A method for measuring displacement, comprising: turning light emitted from a light source into parallel light by a collimator lens; dividing the parallel light by a first transmissive diffraction grating disposed on an optical axis of the parallel light into zeroth-order light, which travels in the same direction as that of said parallel light, and ±nth-order light having diffraction angles relative to said zeroth-order light, where n is a positive integer of 1 or greater; dividing, by a second transmissive diffraction grating, the zeroth-order light and the ±nth-order light that have passed through the first transmissive diffraction grating into zeroth-order light and ±nth-order light, the second transmissive diffraction grating having a same grating pitch as the first transmissive diffraction grating and being disposed so as to face and be movable relative to said first transmissive diffraction grating; receiving, by a first optical sensor, among diffracted light rays that have passed through both the first and second transmissive diffraction gratings, at least one pair of interfering light rays travelling along an optical axis of one of the zeroth-order light, the positive nth-order light, and the negative nth-order light from the first transmissive diffraction grating to detect an amount of light received, said one pair of interfering light rays having mutually overlapping portions when emerging from the second transmissive diffraction grating, optical path lengths of said one pair of interfering light rays reaching the first optical sensor being different from each other, and a difference in the optical path lengths being dependent on a displacement between the first and second transmissive diffraction gratings in the direction of the optical axis of the parallel light so that destructive or constructive interference occurs between said one pair of interfering light rays in said mutually overlapping portions depending on the displacement between the first and second transmissive diffraction gratings, wherein said first optical sensor is disposed at a location where said mutually overlapping portions of the one pair of interference light rays from the second transmissive diffraction grating are present; and determining an amount of the displacement between the first and second transmissive diffraction gratings in the direction of the optical axis of the parallel light on a basis of said amount of light received by the first optical sensor.