In optical device for measuring displacement an optical grating is formed of alternating transparent and opaque stripes having equal widths. The grating is in an aperture plane with the stripes parallel to a central plane that is perpendicular to the aperture plane. Respective subpluralities of the stripes are located in an aperture area in the aperture plane, so that incident light rays on the aperture area are at least partially passed as beams by the transparent strips in the aperture area. A concave mirror has it center of curvature at the aperture area so as to return the beams back to the aperture area. The return beams are further passed by the transparent strips in the aperture area as return light. The grating is displaceable laterally to the cental plane, such that a corresponding responsiveness of the return light intensity to the lateral position provides a measure of displacement of the grating relative to the central plane. Alternative device with reflecting strips function similarly.