Finnegan, Henderson, Farabow, Garrett, and Dunner
Device for detecting the position of an optical head
 A position-detecting device comprising a scale, a first photo-interrupter, and a second photo-interrupter. The scale is attached to a movable member, and has a plurality of slits arranged at regular intervals. Each photo-interrupter includes a light source and a light detector, which are located at opposing sides of said scale, respectively. The first and second photo-interrupters are spaced apart such that when the scale moves, they generate, respectively, a first signal and a second signal which have a phase difference of .pi./2. These signals are processed, thereby to determine the position of the movable member with high accuracy. 
Kabushiki Kaisha Toshiba
G
G11B
inventional
Hiroshi
Yamashita