Abstract There is described a device for determining the relative displacement of an object by means of a diffraction grating which is mechanically secured to the object and the grooves of which extend at right angles to the direction of displacement, the radiation from the source of radiation once or several times traversing the grating and an associated optical system which includes an optically anisotropic element and ultimately falling on at least one photoelectric detector, in which signals are generated in the case of a displacement. It is stated that for this purpose the optically anisotropic element is included in the image-forming ray path of the diffraction grating and comprises the series arrangement of two birefringent uniaxial plate-shaped crystals the principal sections of which taken at right angles to the crystal surfaces are normal to one another, while in these principal sections the angle between the optic axis and the crystal surface is the same in both crystals. It is stated that the angle lies between 30* and 45*.
A device for determining the relative displacement of an object, comprising a diffraction grating mechanically secured to the object and having grooves orientated at right angles to the direction of displacement of the object, means for directing a beam of radiation onto a first area of the grating whereby the beam is diffracted into angularly displaced sub-beams, means for redirecting the sub-beams back to a second area of the grating at a mutual angular displacement equal to the angular displacement of the sub-beams emanating from the grating, whereby the subbeams are recombined into an output beam, photoelectric means in the path of the output beam for generating a signal corresponding to the displacement of the grating, and an anisotropic element in the path of the sub-beams emanating from and impinging on the grating and comprising at least two birefringent uniaxial plateshaped crystals of equal thickness, the principal sections of which taken at right angles to the crystal surface are mutually perpendicular, the angle between the optic axis and the crystal surface in the principal sections of each of the crystals being equal, so that variations in the frequency of the radiation are compensated for.
Claim 2
Independent Claim
194 words
A device for determining the relative displacement of an object, comprising a diffraction grating mechanically secured to the object and having grooves orientated at right angles to the direction of displacement of the object, means for directing a beam of radiation onto a first area of the grating whereby the beam is diffracted into angularly displaced sub-beams, means for redirecting the sub-beams back to a second area of the grating at a mutual angular displacement equal to the angular displacement of the sub-beams emanating from the grating, whereby the subbeams are recombined into an output beam, photoelectric means in the path of the output beam for generating a signal corresponding to the displacement of the grating, and an anisotropic element in the path of the sub-beams emanating from and impinging on the grating and comprising at least two birefringent uniaxial plateshaped crystals of equal thickness, the principal sections of which taken at right angles to the crystal surface are mutually perpendicular, the angle between the optic axis and the crystal surface in the principal sections of each of the crystals being equal, so that variations in the frequency of the radiation are compensated for.
Claim 3
Dependent Claim
26 words
A device as claimed in claim 1, wherein the angle between the optic axis and the crystal surface of each crystal lies between 30* and 45*.