In a device for recording or reproducing information by using a light beam, there is provided an optical disk comprising a base plate, a recording layer formed on the base plate, tracks provided for recording information on the recording layer, and a focus check area for checking if the light beam is...http://www.google.com.tw/patents/US4755980?utm_source=gb-gplus-share專利 US4755980 - Optical disk and recording/reproducing device for optical disk
