A laser interferometer for calibrating displacement has a linear array of elongate parallel photosensitive elements (70) situated in a spatial fringe field created from interfering beams having non-parallel directions of propagation. The elements are grouped into sets in an interleaved repeating pattern. Movement of a retroreflector (48) causes a detectable shift in the fringe field. The apparatus includes a beam splitter (44) creating a pair of orthogonally polarised beams. One of the beams is diverted into relatively fixed retroreflector (46), the other passes undiverted into retroreflector (48). The beams recombined at beam splitter (44) are partly overlapping and parallel. A birefringent prism (60) refracts the beams through different angles causing them to converge and polarising element (62) mixes their polarisation states so they interfere and generate a spatial fringe field. The apparatus may also calibrate angular displacement, straightness, squareness, pitch roll or yaw on a machine tool. <IMAGE>
