Patent GB-1502630-A (GB)
Title: Apparatus for reading a record carrier on which information is stored in an optically readable structure
IPC: G11B7/12
Filed: 1975-02-12
Abstract: 1502630 Photo-electric record readers PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd 12 Feb 1975 [15 Feb 1974] 5951/75 Heading G1A [Also in Division G2] Apparatus for reading a recorded track comprises: a source supplying a radiation beam; objective means for focusing the beam at a spot on the track; beam-directing means for moving the spot in at least one of the directions respectively along and across the track; means for converting radiation, after modulation by the track, into an electrical signal; and at least two concave mirrors in the radiation path, one said mirror receiving an image of the spot from the objective means and imaging the entrance pupil of the objective means on to the beam-directing means, and the other mirror relaying the image of the spot towards the radiation converting means. A laser 4, Fig. 1 provides a scanning beam 30 which, after being focused by a lens 5 and reflected by concave mirrors 8-11, is reflected perpendicularly by a fixed plane mirror 12 and focused by an objective lens 13 on a spot V on a recorded track 20 (carrying e.g. a video, audio or digital information signal) on a disc 1 rotatable about a shaft 3. The beam, returning along the same path after modulation by the track, is polarized in a plane perpendicular to that of the scanning beam by a #/4 plate 7 and directed by birefringent means 6 to form an image of the spot on a photo-cell 14 which provides an electrical signal to an electronic converter 15 connected to display means 16. The concave mirrors 9, 11, which are fixed, each image the entrance pupil of the objective lens 13 with unity magnification at the location of, and in a direction substantially normal to, a respective one of mirrors 8, 10, which are rotatable. The rotatable mirrors 8, 10, each image the scanning beam at a point 28, 26 near to a respective mirror 9, 11, but spaced from that mirror sufficiently to ensure that the final sharpness of the scanning spot V is not impaired by irregularities in the mirror 9, 11. The mirror 8, rotatable about an axis 23 parallel to the disc 1, is usable to adjust the spot V in a direction athwart the track 20, The mirror 10, rotatable about an axis 24 perpendicular to the disc 1, will move the spot V in a direction substantially tangential to the track 20. The mirrors 8, 10 may each be mounted on a moving-coil actuator driven by signals representing deviations of the spot from a required position. Transverse deviation may be detected by the use of a pair of sub-beams (not shown). Tangential deviation may be detected by means of phase-separated signals S t , S r  derived from the electronic converter 15 and representing angular displacement of the disc 1. In a modification, Fig. 2, only two concave mirrors 11, 21 are used, the beam-directing elements being constituted by a thin plano-convex lens 18 and a thin plano-concave lens 19 of equal radii of spherical curvature, separated by a negligible gap, each lens being rotatable about a respective one of mutually-perpendicular axes through the common centre of curvature. The lenses 18, 19 are located at the image of the entrance pupil of the objective lens (not shown), formed by the mirror 11. The mirror 21 forms an image of the spot upon the photo-cell. A preferred beam-splitter, Fig. 3 comprises a symmetrical block 32 constituted by birefringent prisms 33, 35 having parallel optical axes 36, 38 and flanking a central isosceles birefringent prism 34 having an optical axis 37 perpendicular to axes 36, 38.