Abstract: An optical position sensing apparatus utilizing a grating having two sets of opaque and transparent lines thereon, one of the sets being off-set from the other is associated with a source-detector assembly, the source and detector assembly including pairs of light sources and detection devices with at least one of the devices of one of the pairs being aligned with one set of light interrupting opaque lines and the other device of the pair being aligned with the other set of light interrupting opaque lines so that the image of the area illuminated by the opposing source is projected back upon the area of the grating in front of its respective detector as by an image reflecting surface, preferably a concave mirror. Inasmuch as the illuminated areas are of the same phase, the projected images will also be of a similar phase. The grating lines in front of the detectors, however, are 90.degree. off-set due to the off-set of the grating lines.