The amount of the light beam passing through two diffraction gratings is output as an electrical signal by a photo detector. The amount of relative displacement of the diffraction gratings is detected according to the electric signal (output signal) that changes in proportion to the relative displacement....http://www.google.com.tw/patents/US5801378?utm_source=gb-gplus-share專利 US5801378 - Optical encoder diffraction gratings for eliminating diffracted light components 
