A device for detecting a position of an edge of a disk-like workpiece having an outer periphery whose shape includes a portion bearing positional information related to the workpiece, the device including a system for rotationally moving the workpiece; an illuminating system; and a photodetecting system;...http://www.google.ca/patents/US4887904?utm_source=gb-gplus-sharePatent US4887904 - Device for positioning a semi-conductor wafer
