Optical read-out system including asymetrical beam intensity and four detector cells

Optical readout system for optically reading a recording in the form of a support carrying a track of substantially constant width arranged in a reading surface;

• said track being made of a string of information items having non-uniform length and spacing, said information items ensuring by an alternative modification of a physical characteristic of said reading surface, the transcription of an angularly modulated waveform carrying the information stored in said recording;
• said optical readout system comprising:
• a source of coherent light, a lens arranged for causing said coherent light to converge into a reading spot illuminating a single area of said track extending across said width and photodetector means arranged for collecting, through said lens, a fraction of the modulated light emerging from said single area;
• the spatial distribution of the luminous intensities of the light beam incident upon the aperture of said lens being rendered asymmetrical in a direction coinciding with the direction of scanning of said track by said reading spot;
• said photodetector means comprising four cells arranged respectively along the quadrants of a detection plane;
• the crossed lines of separation of said quadrants forming a rectangular trihedron with the optical axis of said lens;
• one of said crossed lines of separation being oriented in said direction of scanning;
• each pair of said cells pertaining to two contiguous portions of said quadrants being electrically connected to a summation circuit;
• said summation circuit belonging to a set of four summation circuits delivering four electrical signals representative of the light energies respectively collected on each side of said crossed lines of separation;
• said electrical signals being applied to first and second subtraction circuits;
• the first of said subtraction circuits having two inputs fed respectively by those of said cells situated on the two sides of one of said crossed lines of separation;
• the second of said substraction circuits having two inputs fed respectively by those of said cells situated on the two sides of the other of said crossed lines of separation.