Abstract: An optical encoder irradiating light emitted from a light emitting unit includes a rectangular collimator lens having a rectangular face onto a main scale which moves in the longitudinal direction, receives the collimated light beam passing the main scale in a light receiving unit, and optically reads the displacement of the main scale. Since the collimator lens is rectangular, the light emitting unit can be made compact without decreasing the irradiation area of a light beam which is effective in the detection of the displacement. Furthermore, it is possible to increase the irradiation area of the light beam by dividing a collimated light beam from the collimator lens into a plurality of diffracted light beams by a transmitting or reflecting diffracton grating.

Abstract: A parallel light ray measuring apparatus invention measures the parallelism of a light beam by: receiving a light beam which has passed through two gratings having the same grating period by means of at least two photoelectric converting elements, and by detecting the light intensity obtained from those aforementioned photoelectric converting elements and the phase difference between the periodic signals obtained when either of the gratings is moved, using the at least photoelectric converting elements. Further, the parallel light ray measuring apparatus includes: a first grating; a second grating having the same grating period as that of the first grating, and being disposed such that the grating lines of the second grating are in parallel with those of the first grating; a light pick-up screen for receiving a light beam which has passed through both gratings, whereby the light intensity distribution appearing on the light pick-up screen is detected so as to measure the parallelism of the light beam.