Abstract: A rotational binary output encoder comprises a housing (1) defining an enclosed space (12), a shaft (3) extending through an end wall (1b) of the housing into the space (12) and having mounted on the inner end thereof a transparent planar disc member (7) having grid means (8) on one surface thereof and reflector means (6) on the other surface thereof, the grid means (8) having alternate areas (22,23) which allow or prevent the passage of light, light transmitting means (5) and light receiving means (4a,4b) mounted in said housing so that the grid means (8) intermittently interrupts the light passing from the light transmitting means (5) to the light receiving means (4a,4b) to produce opto-digital signals, and electric circuit means (10) for providing a digital electrical output from the opto-digital signals.