DEVICE FOR SCANNING AN OPTICALLY READABLE RECORD CARRIER

摘要

A DEVICE IS DESCRIBED FOR OPTIONALLY SCANNING AN INFORMATION LAYER (2'') BY MEANS OF A FOCUSED RADIATION BEAM (5). RADIATION REFLECTED FROM THE INFORMATION FROM THE INFORMATION LAYER IS MADE ASTIGMATIC BY AN OBLIQUE PLANE PARALLEL PLATE (50) AND SUBSEQUENTLY DETECTED BY A DETECTION SYYSTEM (51). AN ELECTRONIC CIRCUIT 26 DERIVES TWO FOCUS SIGNALS, EACH REPRESENTING THE DISTANCE BETWEEN THE LOCAL POINT (7) OF THE RADIATION BEAM (5) AND THE INFORMATION PLANE (2''). THE FIRST FOCUS ERROR SIGNAL IS FORMED FROM THE HIGH-FREQUENCY COMPONENTS IN THE ELECTRICAL DETECTOR SIGNALS, THE SECOND FOCUS ERROR SIGNAL IS FORMED FROM THE LOW-FREQUENCY COMPONENTS IN THE SAME DETECTOR SIGNALS. A SECOND ELECTRONIC CIRCUIT (27) COMBINES THE TWO FOCUS ERROR SIGNALS TO A COMBINED FOCUS ERROR SIGNAL TO CONTROL A FOCUS SERVO LOOP. THE CIRCUIT CHANGES THE CONTROL GRADUALLY FROM ONE FOCUS ERROR SIGNAL TO ANOTHER. (FIG. 6)