Patent ID: 6154278
Filing Date: 2000-11-28
Classification: G01D

Abstract:
An apparatus for measuring a displacement of a moving body comprising:a light source for providing a coherent collimated beam of wavelength .lambda. and diameter ""D"";a fixed diffraction plate and a movable diffraction plate each comprising a grating of pitch ""p"", said grating having main diffraction components of orders .+-.1, said fixed and movable diffraction plates being arranged in parallel to each other with a distance ""g"" between them, said plates being provided in an optical path of the collimated beam so as to be perpendicular to an optical axis of the collimated beam so that the collimated beam is diffracted by the gratings in the fixed and movable diffraction plates successively; andan optical detector for detecting an amount of a light substantially caused by interference of diffraction components of orders .+-.1, said diffraction components being generated by the successive diffraction by the first and second diffraction plates, said optical detector comprising a condenser lens and a photosensor, said condenser lens having an entrance pupil size which is within D-2g.lambda./p, wherein said condenser lens condenses the light transmitted through said fixed and movable diffraction plates, and said photosensor detecting light condensed by said condenser lens;whereby a displacement of the movable diffraction plate can be detected from the detected amount of light.