Patent ID: 7470892

Claim:
An optical encoder comprising: an incoherent light source; a first grating, which is an amplitude grating having a first grating period, for spatial amplitude modulation of the incoherent light from the light source; a second grating, which is a phase grating having a second grating period, for spatial phase modulation of light from the first grating; a third grating, which is an amplitude grating having a third grating period, for spatial amplitude modulation of light from the second grating; and a light detecting element for detecting light from the third grating, wherein the encoder detects relative displacement between respective gratings, the second grating is a reflective phase grating, and the first and third gratings are arranged on the same side of the second grating, and the second grating has, in cross-section, ridges and valleys, a duty ratio of substantially 50%, and an optical path difference for light incident at a ridge and light incident at a valley is substantially equal to λ/4, where λ is wavelength of light.