a super high resolution rotary optical encoder having as its disk an optical disk , i . e ., a disk employing the optical principle of non - contact readout based on diffraction theory . the encoder is adapted to be coupled to a device of which it is desired to measure the rotational motion thereof . the encoder includes an optical disk having , at a fixed radial distance , at least one track of pits and lands ; a coupler for coupling the optical disk to the device such that rotary motion of the device results in a corresponding rotary motion of the optical disk ; and an optical disk pick - up for generating an output in response to the pits and lands indicative of the rotary motion of the device .