Abstract: A measuring device is disclosed which can measure an angle or length and which employs either an angle or length measuring plate. The angle or length measuring plate has thereon at least one measuring scale which has a regular pattern along the majority of the lengths thereof and an irregular pattern on two portions thereof. A light beam shines through an optical system to pass through the measuring scale at two points and from there into a photo-detector. The light path is arranged to pass through the two irregular portions at the same time. Several embodiments of the irregular patterns are shown. A circuit is also shown to detect the irregular portions and employ it as a reference signal.