CA1066412A - Double mirror beam steering in video disc playback assembly - Google Patents

Abstract

ABSTRACT
The present invention relates to the steering of a light beam with an articulated and fixed mirror pair to the surface of a video disc. Prior art attempts to adjust the reading beam involved in the coarse manipulation of the light source to direct it to the video disc surface. The present invention provides an improved apparatus and method for steering the light beam. In accordance with the invention, an optical system having a source for producing a reading beam of radiation is pro-vided. The beam follows an optical path to the surface of the video disc. A pair of mirrors disposed in the optical path are arranged in opposing spaced relation to one another to cause the reading beam to be reflected at least once from each of the mirrors. At least one of the mirrors is articulated to steer the reading beam to a precisely selected location on the surface of the video disc. The mirrors may be planar and have their respective planar reflecting surfaces lying in parallel planes and at an angle to the reading beam. The articulated mirror can be arranged to steer the reading beam radially and/or circumferentially of the video disc.

Description

BACKGROU~D OF THE INVENTION
Field Of The Invention Systems have heretofore been developed for repro-ducing signals at video frequencies from information recorded on discs, tapes, or other media. Such systems have utilized, among other things, optical recordings upon photosensitive discs, electron beam recording on thermo plastic surfaces and, in prior patents assigned to the assignee of the present invention, systems utilizing a rotating disc which is respon-sive to impinging radiation to reflect or transmit radiation corresponding to and representative of the information stored on the surface of the disc.
For example, in U. S. Patent No. 3,530,258, issued to David Paul Gregg and Keith O. Johnson on September 22, 1970, there was shown and described a system in which a video signal transducer included a servo controlled pair of flexible, fiber optic elements. An air bearing supported an objective lens system. A light source of radiant energy was positioned below the disc and the transducer was responsive to transmitted light.
Other patents have shown the use of a radiant source which directed an energy beam to the surface of the disc and provided a transducer that was responsive to reflected energy~
One of the major problems to be encountered in the recording and reproduction of video information, arises directly from a consideration of the energy levels involved in such a process and the restraints imposed by the considerations of size, weight and operating conditions.
To be commercially desirable as a home instrument, the system should be able to store and reproduce a "program"
of at least 15 to 30 minutes in length. The record disc should be of an easily handled size, comparable to the phonograph records currently in use. If the playback turntable was dg/~ _ -2-~ 1066412 operated at 1800 rpm, some 54,000 revolutions would provide 30 minutes of playback. Assuming a 1 micron track width and 1 micron spacing between adjacent tracks, a circular band approximately 4.25 inches wide is required. Assuming that the smallest radius at which information can be stored is approximately three inches, the resultant disc is about 15 inches in diameter. The duration of the program or the speed of the turntable can change the dimensions of the recorded area, as can the width of the individual track and the spacing be-tween adjacent tracks.
Assuming that the video information has been recordedin some digital fashion, the presence or absence of a signal can be detected at an appropriate information rate. If the width of the track is approximately one micron, and that the space between adjacent tracks is also one micron, the quantity of energy necessary to impart information from the disc can be determined. It is necessary to provide sufficient radiant energy to "illuminate" a "spot" of approximately one micron in diameter and, at the same time, provide sufficient radiant energy at the detector, so that the "presence" or "absence" of a signal can be distinguished.
It has been discovered, in attempting to utilize the transmitted radiation techniques of the prior art, that the provision of an inordinately large amount of radiation into the system is required in order to "transmit" a sufficiently useful increment of energy for detection through the record. It has also been determined that a substantial magnification ls re-quired to enable a state-of-the-art transducer to respond to a one micron diameter radiant spot.
If a light source illuminates the entire field which can be scanned by the detector under control of the servo system, dg/~3' -3-`` 1066412 it will be seen that an extraordinary light intensity must be provided before the light transmitted through or reflected from the disc will be of sufficient intensity to register upon the photosensitive device.
According to one aspect of the present invention, there is provided an apparatus for use in a system of optically reading information recorded on a surface of a disc, the system including source means for producing a reading beam of radiation to impinge upon the surface of the disc and optical means for directing the reading beam along a prescribed optical path from the source to the surface of the disc. The apparatus includes a pair of mirrors disposed in the optical path between the source means and the disc, the mirrors being arranged in an opposing space relation to one another to cause the reading beam to be reflected at least once from each of the mirrors, and means for articulating at least one of the mirrors to steer the reading beam to a precisely selected location on the surface of the disc.
According to another aspect of the present invention, there is provided a method for optically reading information recorded on a surface of a disc, the method including the steps of producing a reading beam from a source of light radiation to impinge upon the surface and directing the reading beam along a predescribed optical path from the source to the surface of the disc. In the method of the present invention, there is imposed in the optical path a pair of mirrors, which mirrors are arranged in opposing spaced relation to one another to cause the reading beam to be reflected at least once from each of the mirrors. At least one of the mirrors is articulated to steer the reading beam to a precisely selected location on the surface of the disc.

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~066412 The novel features which are believed to be charac-teristic of the invention, both as to organization and method of operation, together with further objects and advantages thereof will be better understood from the following descrip-tion considered in connection with the accompanying drawings in which several preferred embodiments of the invention are illustrated by way of example. It is to be expressly under-stood, however, that the drawings are for the purpose of illustration only and are not intended as a definition of the limits of the invention.

BRIEF DESCRIPTION OF THE DR~WINGS
Fig. 1 is an idealized side view of a playback assembly according to the present invention;
Fig. 2 is a more detailed block diagram of the elements in the optical playback system;
Fig. 3 is an idealized view of an alternative articulated mirror assembly;
Fig. 4 is a block diagram of a suitable detector and tracking circuit of the prior art;
Fig. 5 is a block diagram of an optical detector of the prior art suitable for use in the present invention;
Fig. 6 is an enlarged side view of the optical head and air bearing assembly;
Fig. 7 is a top idealized view of a cam and follower assembly for controlling the bias on the air bearing assembly;
and Fig. 8 is a side view of another alternative artic-ulated mirror arrangement useful in the system of the present invention.

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` 1066412 DESCRIPTION OF THE PREFERRED EMBODIMENTS
Turning first to FIG. 1, there is shown, in side view, a playback assembly 10 suitable for use in the present invention. The playback assembly 10 includes a laser element 12 which moves with the playback assembly 10.
It is, however, within the state-of-the-art to provide a stationary laser which is coupled optically to the movable assembly 10. Preferably, the laser 12 provides coherent, polarized light. A read head 14 is mounted in arm 16 of the playback assembly 10.
A video disc 20, which has video information recorded upon it is mounted on a turntable 22, which is adapted to rotate the disc 20 at a relatively high speed.
In the preferred embodiment, the turntable speed is set at 1800 rpm.
Suitable video discs have been described and claimed in the patents to Gregg, Johnson, supra.
The playback assembly 10 is mounted on a rotatable element 24 which, in the view of FIG. 1, translates the reading head in the radial direction relative to the disc 20 and in an arc that is generally orthogonal to the plane of the drawing.
The laser 12 generates a reading beam 26 which generally passes from the laser 12 through an optical system including an arrangement of mirrors 9 to the playback head 14. The beam is then directed to the surface of the disc 20 and returns through the playback head 14 along the same optical path until a read assembly 28 is encountered.

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The read assembly 28 is mounted on the arm 16.
In operation, the laser directs a reading light beam 26 to the surface of the disc 20 through the optical system.
The information recorded upon the disc interacts with the im-pinging beam and a reflected beam is produced which contains the recorded information. The reflected light beam is returned to the optical system which "analyzes" the returned beam to de-termine whether the beam is properly tracking the signal channel.
If the electronics determine that the laser spot is not being directed to a predetermined area of the information channel, appropriate servo signals are derived which, when ap-plied to the read head 14, cause the point of impingement of the laser beam to shift in the radial direction to retain alignment with the track that is being read.
In an alternative embodiment, the driver for the rotatable element 24 for the playback assembly 10 can also be controlled by the servo signals which changes the position of the laser spot. In yet other embodiments, a motor can be coupled to the turntable driver to provide a predetermined increment of radial motion for each revolution of the turntable 22. In any case, the playback head 10 can be made to track the information channel recorded on the disc 20 with a "course"
adjustment being applied to the driver 24 and a "fine" adjust-ment being applied to an articulated mirror, described in greater detail below. A cam and follower assembly 132, des-cribed in greater detail in conjunction with FIG. 7 below, is located on the rotatable element 24 and communicates with the arm 16 through a flexible cable 130.
Turning next to FIG. 2, there is shown a diagram of the elements of the reading system. The reading laser beam 26 is applied to a beam splitting prism 30. The prism 30 is ro-tated slightly with respect to the optical path. A lens 32 is provided to better form the beam 26 at the surface 20 and to dg/~f' -7---` 106641Z
optimize the resolving power of the system. The transmitted portion of the beam 26 is applied through a quarter wave plate 36 and is then directed through the reading head 14 to the disc 20.
A returning beam 38 containing the information from the disc 20 follows substantially the identical path.
At the quarter wave plate 36, the returning beam is now given an additional quarter wave shift for a total polarization of one-half wavelength. The returning beam 38 reaches the beam splitter 30 and is reflected therefrom to a suitable optical system 40. Light from the laser 12 that is initially reflected in the prism 30 and re-reflected from the base of the prism will, due to the slight rotation of the prism 30, be aimed at a point that wholly misses the detector 40. Moreover, the cumulative effect of the quarter wave plate which polarizes the returning beam by A/2 substantially attenuates any transmitted component. What is transmitted is cross polarized with respect to the laser 12.
The read head 14 includes a fluid-bearing member 50 which is adjacent to and supportive of a microscope objective lens 52. A limited amount of vertical adjust-ment is available in the objective lens 52. Directing the illumination to the objective lens 52 is the arrangement of mirrors 9, which arrangement includes an articulated mirror 54 which is mounted adjacent to and cooperates with a second or fixed mirror 56 that is sub-stantially parallel with the articulated mirror 54. The fixed mirror receives the reading beam 26 and directs it to the articulated mirror 54.
The reading beam 26 undergoes at least one reflection from the articulated mirror 54 before the beam sb/ ~

`" 106641Z
is applied to the objective lens 52. Two such reflections are illustrated in the embodiment of FIG. 2. Similarly, the beam path is - 8a -sb/ ~

.such that a reflected beam 38 returning from the surface of the

2 ¦ disc 20 would also undcrgo two reflcctions from the articulated 5 ¦ mirror 54 and two rcflections from the fixed mirror 56 bcfore 4 ¦ proceeding into the optical path including an additional fixed 5 ¦ mirror 57 which ultimately leads to the read assembly 2~.
6 ¦ In the embodiment illus~rated, the articulated mirror 54 7 ¦ is mounted on a point pivot 58 that is centrally located with 8 ¦ respect to the mirror 54. The mirror 54 may have an oblong 9 ¦ shape with the long axis in the plane of the drawing and the 10 ¦ short axis orthogonal to the plane of the drawing. As shown, 11 ¦ a mirro~ driver 60 is connected to one end of the mirror 54 12 ¦ and is operable to impart motion about the central pivot 58.
13 ¦ If the driver 60 rotates the mirror 54 in the clockwise 14 ¦ direction, as viewed in FIG. 2, the point of impingement of 15 ¦ the read beam 26 will be shifted to the left. This would 16 ¦ represent a deflection of the beam in a first radial direction.
17 ¦ If the driver 58 rotates the mirror 54 in the counter-clockwise 18 ¦ direction, then thc point of impingement of the transmitted 19 ¦ beam 26 will be shifted to the right, as seen in FIG. 2, or in 20 ¦ a second, opposite radial direction.
21 ¦ It will be obvious that the reflected beam 38 and the 22 ¦ reading beam 26 trace identical paths between the surface of 23 ¦ the disc 20 and the beam splittcr 30. The articulated mirror 54 24 serves to "steer" the reading spot to a desired location and thcn ~reads" only thc illuminatcd area, iransmitting that 26 information back to the read assembly 28.
87~ In altern-tive emocdiments, the articul~ted mirror 54 and B ,~

10664~Z
the stationary mirror 56 can be adjusted and repositioned to provide a greater plurality of reflections between the two mirrors before the beam continues either to or from the disc surface 20. In such an arrangement, the magnitude of mirror deflection required to steer the reading spot appropriately can be greatly reduced. The driver 60 therefore, need only impart small, incremental motions to the articulated mirror 54.
In an alternative embodiment, as shown in FIG. 3, a first articulated mirror 54' is provided which is mounted on a central pivot member 58', and is driven about an axis orthogonal to the plane of the FIGURE and in the clockwise and counter-clockwise direction by a first driver 60' that is coupled to the mirror 54' at the end of a long axis.
A second driver 60" is coupled to one end of a third mirror 54" for imparting rotational motion to the third mirror 54" about the long axis that is in the plane of the FIGURE.
In operation, the first driver 60' permits trans-lation of the beams in the "radial" direction to permit "fine"
tracking of the information channel. The second driver 60" is used to translate the beam in the circumferential direction, to provide time synchronization, if desired, and to compen-sate for eccentricity.
In other embodiments, the problem of time synchron-ization can be handled mathematically, as a step in the process of electronically compensating for eccentricity of the disc 20 and in such embodiments, only the single articulated mirror is used.
Turning next to FIG. 4, there is shown a preferred embodiment of the optical detector assembly 40 which utilizes some of the electronics of the Munro patent, supra. As shown in FIG. 4, the returned optical image 38 is directed to impinge dg//~ 10-upon a photocell 70 when a channel is being tracked properly with the spot on the outer half of the track, a predetermined output signal is generated. The output of the photocell 70 is applied to a compaxator 72. An adjustable bias 74 is applied to the other input of the comparator 72 and is adjusted to provide a null when the predetermined output signal is being applied. The error signals resulting from drift can be integrated, and the output of the integrator can be applied to an appropriate circuit to urge the movable playback assembly 10 relative to the center of the disc 20. The error signal can also be used to apply a signal directly to the mirror driver 60 of FIG. 2 to urge the beam to follow the track.
If, however, the track is not being followed properly, depending, of course, upon the characteristics of the disc surface, a condition will be presented in which the energy impinging upon the photocell 70 will be different than the bias provided by bias circuit 74, and accordingly, the error signal of appropriate polarity will be provided to correct the position of the light spot relative to the information channel.
The integrator output then is applied to the movable playback assembly 10, and if the bias signal is greater, a forcing function is generated tending to send the spot toward the per-iphery of the disc. If the received signal is greater, the spot is directed to the center of the disc. As the spot follows the spiral track properly, the differential output tends toward the null. For this example, it is assumed that an appropriate mechanism drives the rotatable element 24 so that the arm moves in the radial direction at a predetermined rate. The output of the integrator would then provide a correcting signal tending to correct the rate at which the arm is moving toward the center.
Alternatively, if the arm is to be driven entirely by the output of the integrator, the convention observed is substantially immaterial. If the bias signal being greater urges the spot toward the center dg/,~-',~ -11-of thc disc, thcn thc s~ot will follow the track on thc "inncr"
edge. On the oth~r hand, if a greater bias signal drives the spot toward the pcriphcry, thcn the spot will follow the outer edge of the track. In either case, the error signal, when integrated, will provide an appropriate forcing function to the arm driver circuits so that the arm generally follows the track.
In FIG. 5, there is illustrated the prior art optical detector electronics utilized and shown as FIG. 10 in the previously issued Gregg, et al., U.S. Patent No. 3,530,258, assigned to the assignee of the present invention. For convenience, the same reference numbers are used in Gregg, et al and herein.
A pair of photo detectors 96, 98 are employed which, in combina-tion, provide an additive information signal and, when differenced, an error signal which controls servo elements that redirect the reading elements. As applied to the present invention, the radial error signal could be applied to either of the drivers 60, 60' of the articulated mirror assemblies of Figures2 and 3, respectively.
As shown in FIG. 5, a dual photo detector has two sections 96, 98 whose outputs are applied to respective ampli-fiers 100, 101. The outputs of the amplifiers 100, 101 are summed in a summing network 106. me output from the summing net-work represents the sum signal from the two photo detector sec-tions 96, 98 and constitutes the modulated signal output of the transducer.
The signal amplitude fro~ the first photo detector section is applied to a detector 102, and this detector produces a negative unidirectional signal representative thereof. The signal amplitude from the second photo detector section is applied to a detector 103, and the latter detector produces a negative unidirectional signal in response thereto. The two signals are added algebraically in a summing network 105 which produces an error signal.

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In the present example, the resulting error signal is amplified in an amplifier 104, and it is applied to the circuits of FIG. 3 and driver 60'. The error signal applied to the driver 60' causes the mirror 54' to shift the beams in a radial direction with respect to the disc 20, as explained above. The direction and amount of the shift depends on the polarity and amplitude of the error signal, so as to maintain the spot in perfect registry with the recording track on the record 20.
The output signal from the summing network 106 is applied to appropriate video detection and reproducing circuitry such as is illistrated in FIGS. 17 and 18 of Gregg, et al, supra, and described therein.
The DC component of the output of the amplifier 104, when properly processed, may be used in several ways to move the pick-up arm of FIG. 1 across the disc 20 at very nearly the rate which makes the signal approach zero. One method is to integrate this component over short intervals until it reaches a predetermined value, at which it triggers a solenoid. This solenoid, in turn, actuates a light-duty friction ratchet which then turns the pick-up arm through a very small angle as is taught in Gregg, et al, supra.
Another method, also suggested in Gregg, et al, supra, is to use an inexpensive electric clock movement with a reduc-tion gear to drive the arm continuously across the disc at a rate just slightly above 2 microns for each 1/30 second or revolution of the disc. In this case, the integrated signal of the first method is used to interrupt the motor voltage occasionally. To assist the process, the arm 16 of FIG. 1 may be biased slightly towards the center of the disc 20.

dg/ i_ ~ -13-, 1066412 In FIG. 6, there is shown an enlarged side view of the lens and air bearing assembly of the playback head 14.
The movable arm 16 connects to the playbac~ head 14 through a pair of parallel leaf springs 120, 122. The spring force of the leaf springs 120, 122 is generally insufficient to maintain the springs in the horizontal position with the playback head 14 unsupported by the fluid bearing that is generated by the rotating disc 20. Within the read head 14 is the fluid bearing member 50 and the microscope type objective lens 52. Also contained in the read head 14 are the fixed and articulated mirrors 54, 56, 57 necessary to direct the beam of light from the source to the lens 52 and back from the surface of the disc 20.
A support post 124 extends outward of the read head 14 toward the inner end of the arm 16. Mounted to this support post 124 is a bias spring 126, the other end of which is fastened to a lever 128. The lever 128 is coupled to the arm 16 and, through a flexible cable 130, connected to a cam and follower assembly 132, to be described in connection with FIG. 7 below.
Also included, but not described in detail, are dg//~ 14-appropriate interlocking solenoid asscmblies operating in 2 conjunction with the cam and follower assembly to maintain the

3 read head 14 out of contact with.the disc 20 as the arm 16

4 swings out of engagcmcnt with the disc 20, and which act to prevcnt damage if, for any rcason, thc disc 20 should slow 6 appreciably while being trac~ed by the read head 14.
7 The bias spring 126, when compressed, acts like a solid 8 rod, enabling the lever 128 to directly cam the read head 14 9 upward and away from the disc 20, if this configuration is desired. Alternatively, when the read head 14 is in position 11 over the disc, the lever 128 rotates in the opposite direction, 12 relieving thc compression on the spring 126. Under normal 13 circumstances, the weight of the read head 14 is supported by 14 the fluid bearing member 50 on the disc, thereby enabling the leaf springs 120, 122 to be substantially parallel and horizontal.
16 According to the present invention, an additional bias is 17 provided through the use of the bias spring 126 to maintain a 18 substantially constant separation between the read head 14 and 19 the fluid bearing member 50 and the surface of the disc 20. The relative surface velocity changes as the moving arm 16 progresses 21 toward the cènter of the disc and the fluid bearing is less able 22 to support the read head. Therefore, at the outset, the lever 23 128 is rotated in the downward direction, applying a stretch to 24 the spring 126 which, in turn, imparts a downward force to the support arm 124, thereby increasing the bias on the fluid 26 bcaring 50 whilc the fluid prcssurc is at its greatest.
27 As the arm 16 moves inwardly of the disc 20 and the surface 28 velocity is reduccd, a cam follower arrangement gradually 29 rotates the lever 128 in the upward direction, reducing the 31 tension of the spring 126, thcreby lessening thc bias on the 1~' -10664~Z
`read head 14. By sclecting an appropriatc c~m contour, I:he bias on the fluid bearing 50 can be maintained at an optimum value for constant separation from the disc 20 for the surface velocity of the disc at any radial location.
Turning now to FIG. 7, there is shown one form of cam and follower asse~Tbly 132 that can drive the lever 128 through the flexible cable 130 (also shown in FIG. 1). A cam 140 is cut so that at the outermost position of the arm 16, a follower 142 rests on a high lobe which maintains the head 14 in an "up"
position, safely out of contact with the edge of the rotating disc 20.
As the arm 16 tracks inwardly, the follower 142 immediately proceeds to the innermost point on the cam 140 surface, applying maximum bias to the read head 14. As the arm then continues inwardly in the radial direction, the follower 142 gradually rides outwardly from the center of the cam 140, thereby reducing the bias forces on the read head 14.
It is clear that techniques are readily available for transmitting simple mechanical motion from the cam follower assembly 132 to the arm 16, and the specific details are un-necessary in the present application.
In FIG. 8, there is shown an alternative configuration for the articulated mirror assembly that is mounted on the read head 14. In this alternative embodiment, a fixed mirror 150 and an articulated mirror 152 are arranged on converging planes.
An incoming beam in the horizontal direction impinges upon the articulated mirror 152, and through multiple reflectioll betwcc the fixed mirror 150 and the articulated mirror 152, the beam is ultimately rotated through 90 and is directed downward into the reading assembly. Similarly, the returning beam re-traces the same path. The mirror 152 is articulated to rotate about an axis ~b 1 that is in the plane of the drawing to deflcct the transmittcd 2 beam in a direction that is perpendicular to the plane of the 3 drawing.
4 The angle of incidence of the mirror 150 and the angle of convergence between the mirrors 150 and 152 are controlled so 6 that the incoming beam makes a plurality of reflections off of 7 the two mirrors before being directed into the disc. Moreover, 8 sincc the pair of mirrors, in addition to providing a "folded"
9 light path, also rotates the beam-through 90, a separate 45 mirror can be omitted, thereby increasing the intensity of 11 available light to the disc. Of course, this would permi~ at 12 least one extra reflection bet~een the mirror pair without in 13 any way degrading the quality of the light beam. The same number 14 of internal reflections as in the embodiment of FIG. 2 could be employed with less light loss in the mirror system.
16 Thus, there has been shown an improved video disc reading 17 assembly which steers the illuminating radiation to the informatio I8 track on the surface of the disc and steers the return signal 19 from the track to an optical detector. An articulated mirror enables the steering of both the transmitted and the returned 21 light beam.
22 An improvcd optical dctector is utilized in combination 23 with a fi~c~ bias sourcc so that a single detector provides both 24 the information signal and the servo signals necessary to track the information channel.
26 A novel air bearing assembly has ~lso been disclosed, 227 which enables a microscope lens to travel at a fixed distance above the disc supported on a fluid bearing, and maans are 29 providcd to impart a variable bias to the fluid bearing as a function of relativc velocity between the disc and the bearing 31 mcmber.

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Claims (27)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. Apparatus for use in a system for optically reading information recorded on a surface of a disc, said system including source means for producing a reading beam of radiation to impinge upon said surface of said disc and optical means for directing said reading beam along a prescribed optical path from said source means to said surface of said disc, said apparatus comprising: a pair of mirrors disposed in said optical path between said source means and said disc, said mirrors being arranged in opposing spaced relation to one another to cause said reading beam to be reflected at least once from each of said mirrors; and means for articulating at least one of said mirrors to steer said reading beam to a precisely selected location on said surface of said disc.

2. Apparatus as claimed in Claim 1, wherein said mirrors are arranged to cause said reading beam to be reflected more than once from each of said mirrors.

3. Apparatus as claimed in Claim 2, wherein said mirrors are planar and have their respective planar reflecting surfaces lying in parallel planes and at an angle to said reading beam.

4. Apparatus as claimed in Claim 1, wherein said information is recorded in a spiral track on said surface of said disc, and said articulated mirror is arranged to steer said reading beam radially of said disc.

5. Apparatus as claimed in Claim 4, and fur-ther including: servo means for detecting the location of said reading beam on said surface of said disc and for selectively controlling said articulated mirror to effect radial steering of said reading beam to said precisely selected location on said disc.

6. Apparatus as claimed in Claim 4, wherein both said mirrors are articulated.

7. Apparatus as claimed in Claim 1, wherein said information is recorded in a substantially circular track on said surface of said disc, and said articulated mirror is arranged to steer said reading beam circumferentially of said disc.

8. Apparatus as claimed in Claim 7, and further including: servo means for detecting the location of said reading beam on said surface of said disc and for selectively controlling said articulated mirror to effect circumferential steering of said reading beam to a precisely selected location on said surface of said disc.

9. Apparatus as claimed in Claim 2, wherein said mirrors are planar and have their respective planar reflecting surfaces lying in converging planes, the angle of convergence between said reflecting surfaces being such as to cause said reading beam to have multiple reflections in the direction of convergence followed by multiple reflections in the direction opposite the direction of convergence.

10. Apparatus as claimed in Claim 1, wherein said source means comprises a laser beam generator for producing said reading beam of radiation.

11. Apparatus as claimed in Claim 2, wherein said surface of said disc is reflective and said reading beam is modulated by reflecting said reading beam from said information recorded on the surface of said disc.

12. Apparatus as claimed in Claim 11, wherein said modulated beam is directed by said optical means to a sensing means along a reflected beam path, at least a portion of said reflected beam path being common to said reading beam and said reflected beam, said mirrors being positioned in said common path portion.

13. Apparatus as claimed in Claim 12, wherein said apparatus includes beam separating means in said path for directing said modulated beam out of the path of said reading beam.

14. Apparatus as claimed in Claim 13, wherein said beam separating means comprises: a beam splitting prism; and a quarter wave plate disposed between said prism and said surface of said disc.

15. An improvement in a method for optically reading information recorded on a surface of a disc, said method including producing a reading beam from a source of light rad-iation to impinge upon the surface of the disc, and directing the reading beam along a prescribed optical path from the source to the surface of the disc, the improvement comprising: inter-posing a pair of mirrors in said optical path; arranging said mirrors in opposing spaced relation to one another to cause the reading beam to be reflected at least once from each of said mirrors; and articulating at least one of the mirrors to steer the reading beam to a precisely selected location on the sur-face of the disc.

16. The improvement in the method as claimed in Claim 15, wherein said arranging step includes arranging said mirrors to cause the reading beam to be reflected more than once from each of the mirrors.

17. The improvement in the method as claimed in Claim 16, wherein said mirrors are planar and are arranged to have their planar reflecting surfaces lying in parallel planes and at an angle to the reading beam.

18. The improvement in the method as claimed in Claim 15, wherein said information is recorded in a spiral track on the surface of the disc, and said articulating step includes articulating said one mirror to steer the reading beam radially of the disc.

19. The improvement in the method as claimed in Claim 18, and further including: detecting the location of the reading beam on the surface of the disc; and selectively controlling said articulated mirror to effect radial steering of the reading beam to said precisely selected location on said disc.

20. The improvement in the method as claimed in Claim 18, wherein said articulating step includes articulating both said mirrors to effect beam steering.

21. The improvement in the method as claimed in Claim 15, wherein said information is recorded in a substantially circular track on the surface of the disc, and said articulated mirror is articulated to steer said reading beam circumfer-entially of said disc.

22. The improvement in the method as claimed in Claim 21, and further including: detecting the location of the reading beam on the surface of the disc, and selectively con-trolling the articulated mirror to effect circumferential steering of the reading beam to a precisely selected location on the surface of the disc.

23. The improvement in the method as claimed in Claim 16, wherein said arranging step includes arranging said mirrors to have their respective planar reflecting surfaces lying in converging planes, the angle of convergence between said reflecting surfaces being such as to cause the reading beam to have multiple reflections in the direction of conver-gence followed by multiple reflections in the direction opposite the direction of convergence.

24. The improvement in the method as claimed in Claim 15, wherein the reading beam is modulated by reflecting the reading beam from the information recorded on the surface of the disc.

25. The improvement in the method as claimed in Claim 24, including optically directing the modulated beam to a beam sensor along a reflected beam path, at least a portion of the reflected beam path being common to the reading beam and the reflected beam, said mirrors being positioned in the common path portion.

26. The improvement in the method as claimed in Claim 25, wherein said step of directing the reflected beam to a beam sensor includes optically separating the reflected beam out of the path of the reading beam.

27. The improvement in the method as claimed in Claim 26, wherein said beam separating step includes: passing the reading beam through a beam splitting prism and passing the beam emerging from the beam splitting prism through a quarter wave plate prior to impingement of the beam on the disc; passing the reflected beam through said quarter wave plate in opposing direction to that of the reading beam; and passing the reflected beam emerging from the quarter wave plate to a reflective surface of the beam splitting prism.