A position detector includes a diffraction grating provided on the surface of an object, an illumination system for illuminating the diffraction grating, a detection system for detecting diffracted light diffracted from the diffraction grating, and a processing system for detecting positional information...http://www.google.co.uk/patents/US5369486?utm_source=gb-gplus-sharePatent US5369486 - Position detector for detecting the position of an object using a diffraction grating positioned at an angle
