In a laser recording system, a high energy intensity laser beam is focused to form a circular write-spot on a rotating thermoplastic disc. This spot creates tracks of grooves and flat areas or lands in accordance with input data. To read the data from the disc a reduced energy beam is focused on the...http://www.google.com/patents/US3983317?utm_source=gb-gplus-sharePatent US3983317 - Astigmatizer for laser recording and reproducing system
