GB1589035A

GB1589035A - Optical record reader

Info

Publication number: GB1589035A
Application number: GB10887/78A
Authority: GB
Original assignee: Philips Gloeilampenfabrieken NV
Current assignee: Koninklijke Philips NV
Priority date: 1977-03-23
Filing date: 1978-03-20
Publication date: 1981-05-07
Also published as: NL7706753A; BE865140A; CA1116294A; ES468083A1; DD135654A5; SE7803140L; ATA200278A; IT1093249B; SE424677B; JPS6028055B2; DE2810566C2; AU3429278A; AR217276A1; DK124478A; BR7801692A; DE2810566A1; AT371275B; AU516903B2; JPS53118103A; FR2385173A1

Description

(54) OPTICAL RECORD READER (71) We, N. V. PHILIPS' GLOEILAMPENFABRIEKEN, a limited liability Company, organised and established under the laws of the Kingdom of the Netherlands, of Emmasingel 29, Eindhoven, the Netherlands do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The invention relates to an apparatus for reading an optical radiation-reflecting information carrier, which apparatus comprises a radiation source which produces a read beam, an objective system for focussing the read beam to a read spot on the information structure of the information carrier and for imaging the read spot on a radiationsensitive information detector whose output signal represents the information, and an opto-electronic focussing error detection system for determining a deviation between the desired and the actual position of the plane of focussing of the objective system, which focussing error detection system comprises two radiation-sensitive focussing detectors which co-operate with a narrow focussing beam, the difference in the output signals of the focussing detectors providing an indication about said deviation.

In this respect "focussing beam" is to be understood to mean an auxiliary beam with the aid of which the focussing errors of the read beam are detected. The "focussing detectors" are radiation-sensitive detectors which co-operate with said auxiliary beam.

Such apparatus is described in Patent Specification No. ,43 1,435,922. This apparatus is for example used for reading an information carrier on which a (colour) television programme is stored. The information structure then consists of a multitude or areas alternating with intermediate areas which are arranged in accordance with a spiral track, which areas and intermediate areas have a different influence on a read beam.

The information is then for example contained in the lengths of the areas and those of the intermediate areas. To obtain a sufficiently long playing time the details of the information structure will be very small for limited dimensions of the information carrier. For example, if a 30-minute television programme is stored on one side of a discshaped round information carrier in an annular area with an outer radius of approx. 15 cm and an inner radius of approx. 6 cm, the width of the tracks will be approx. 0.5 ijm and the average length of the areas and of the intermediate areas will be approximately 1 CLm In order to enable such minute details to be read an objective system with a fairly large numerical aperture must be employed.

However, the depth of focus of such an objective system is small. As in the read apparatus variations in the distance between the plane of the information structure and the objective system may occur which are greater than the depth of focus, steps must be taken to enable these variations to be detected and to enable the focussing to be corrected.

In the apparatus in accordance with the said Patent Specification a narrow beam is therefore split from the read beam before this beam enters the objective system. The narrow beam passes obliquely through the objective system. After this beam has been reflected by the information carrier, it traverses the objective system for a second time and then forms a radiation spot, the focussing spot, in the plane of the two focussing detectors. The symmetry of the focussing spot relative the focussing detectors then provides an indication of the degree of focussing of the read beam on the information structure.

In the known read apparatus a number of additional elements are needed, such as a semitransparent mirror, a fully reflecting mirror for the formation of the focussing beam, and an additional lens for focussing the auxiliary beam in the focal plane of the objective system. The positions of the additional elements are very critical.

It is an object of the present invention to provide an apparatus of the type mentioned in the preamble in which a minimal number of additional elements is needed for focussing detection. The invention provides apparatus for reading an optical radiationreflecting information carrier, which apparatus comprises a radiation source which produces a read beam, an objective system for focussing the read beam to a read spot on the information structure of the information carrier and for imaging the read spot on a radiation-sensitive information detector whose output signal represents the information and an opto-electronic focussing error detection system for determining a deviation between the desired and the actual position of the plane of focussing of the objective system, which focussing error detection system comprises two radiationsensitive focussing detectors which cooperate with a narrow focussing beam, the difference in the output signals of the focussing detectors providing an indication of said deviation, wherein, in order to produce the focussing beam the radiation path of the read beam on one side of the optical axis of the objective system includes a radiationdeflecting element, whose surface area is substantially smaller than the cross-sectional area of the read beam.

Owing to the radiation-deflecting element a small portion of the read beam is given an other direction from the rest of the read beam. This portion is focussed on the focussing detectors by the objective system, the position of the radiation spot, which is formed in the plane of the focussing detectors, relative to the detectors being governed by the degree of focussing of the read beam on the information surface of the information carrier.

Preferably, the information detector and the focussing detectors are disposed in the same plane perpendicular to the optical axis.

The radiation-deflecting element may be constituted by an optical wedge or by a diffraction grating.

The radiation deflecting element may be included in the radiation path of the radiation beam directed towards the information carrier in such a way that the radiation which is incident on the radiation deflecting element forms an additional radiation spot on the information structure besides the read spot, which additional radiation spot is imaged on the focussing detectors by the objective system.

The radiation source may be a gas laser, such as a helium-neon laser. In that case the distance between the objective system and the plane of the detectors is comparatively great. The focussing spot is then situated at a comparatively great distance from the read spot image.

It is alternatively possible to employ a (semi-conductor) diode laser as radiation source. Such a laser may also be used as information detector. In that case the radiation which is reflected by the information carrier need not be separated from the radiation which is directed towards the information carrier. The optical read unit can then be kept simple and compact. Furthermore, the objective system may then have a low magnification. If in such a read apparatus a focussing beam is formed by means of a deflecting element, the focussing spot may be so close to the image of the read spot that the focussing detectors cannot be arranged within the required distance from the diode laser. If it were possible to arrange the focussing detectors in the desired position, a part of the read beam would then be incident on the focussing detectors in the case of a slight focussing error of the read beam, resulting in an error in the focussing control signal.

In order to avoid these problems where the radiation-deflecting element is an optical wedge, a second optical wedge may be included in the radiation path of the subbeam which is formed by the first optical wedge and which is reflected by the information carrier.

Preferably, the second optical wedge is then disposed within the image of the first optical wedge which image is formed with the aid of the information carrier and the lens element of the objective system nearest the information carrier. This means that the area of the second optical wedge is smaller than or equal to the area of the first optical wedge.

The objective system may comprise a plurality of lens elements or one lens element. In the last-mentioned case the "lens element of the objective system nearest the information carrier" is the objective system itself.

The second optical wedge, whose angle of refraction is preferably greater than that of the first optical wedge, deflects the focussing beam reflected by the information carrier additionally relative to the read beam, so that the distance between the focussing spot and the read spot is increased.

In order to ensure that the second wedge always remains in the image of the first wedge independently of he position of the information carrier relative to tlle objective system, the optical wedges may be disposed in the back focal plane of the lens element of the objective system nearest the information carrier.

The radiation-deflecting element may be disposed in the path of the read beam which is reflected by the information carrier and which originates from the read spot, in such a way that the radiation which is incident on the radiation deflecting element is deflected to the focussing detectors.

The dividing line between the focussing detectors may be arranged so as to make an acute angle with the direction in which the focussing spot moves owing to focussing errors. As a result, the position of the focussing detector is rendered uncritical.

The radiation-deflecting elements which are used are substantially smaller than the cross-section of the read beam. As a result of this the size of the read spot and thus the actual information read-out is not affected significantly. The slight influence of the radiation-deflecting elements on the readout can further be reduced by arranging for the line of interconnection between the optical axis of the objective system and the radiation-deflecting element to make an angle of 45" with the direction in which an information track of the information carrier is read.

The invention will now be described in more detail on the basis of apparatus which employs a diode laser as radiation source and optical wedges as radiation-deflecting element. In this description reference is made to the accompanying drawings, in which: Fig. 1 shows a first embodiment of apparatus in accordance with the invention, Figs. 2a and 2b show different orientations of the focussing detectors relative to the directions of movement of the focussing spot, Figs. 3a and 3b show how the focussing spot moves relative to the focussing detectors when the optical wedges are rotated relative to the optical axis, and Fig. 4 shows a second embodiment of apparatus in accordance with the invention.

Fig. 1 shows a part of a round disc-shaped information carrier 1 in radial cross-section.

The information structure is for example a phase structure and comprises a multitude of concentric or quasi-concentric tracks 2, which tracks consist of a sequence of areas and intermediate areas. The areas may for example be situated at a different level in the information carrier than the intermediate areas. The information may for example be a colour television programme, but it may alternatively be other information, such as a multitude of different images or digital information. Preferably, the information structure is situated at the back of the information carrier 1.

The information carrier is illuminated by a read beam 3 produced by a diode laser 4. An objective system, which consists of a single lens, or as shown in Fig. 1, of two lenses L1 and L2 focusses the read beam to a read spot Vi on the information structure. The read beam 3 is then reflected by the information structure and upon rotation of the information carrier it is modulated in accordance with the information which is contained in a track portion to be read. After reflection the read beam traverses the objective system for a second time, an image V' i being formed of the read spot Vi. At the location of the radiation spot V' i a detector is situated which converts the modulated read beam into an electrical signal Si.

If the radiation source is a diode laser it is possible, as is described in the published German Patent Application No. 2,244,119, to use this diode laser as a detector. Depending on the intensity of the reflected read beam the electrical resistance across the diode laser or the intensity of the radiation emitted from the rear of the diode laser will vary. When a diode laser is used as radiation source no beam-splitting element is necessary to separate the modulated read beam reflected by the information carrier from the unmodulated read beam which is directed towards the information carrier.

In accordance with the invention a small optical wedge 5 is disposed in the path of the read beam3. This wedge splits-off a subbeam 6 (represented by dashed lines in Fig.

1) from the read beam. This sub-beam is focussed to a radiation spot Vf on the information structure by the lens L,. After reflection at the information structure and a second passage through the objective system the focussing beam forms a radiation spot Vtf (focussing spot) on an assembly of two focussing detectors 7 and 8. If the distance between the plane of the tracks 2 and the objective system is correct, the focussing spot is symmetrical relative to the focussing detectors, so that both detectors receive an equal amount of radiation and the output signals S7 and S8 are equal. If the plane of the information structure moves downwards relative to the objective system, the point where the principal ray of the reflected beam 6 enters the lens L1 will be shifted towards the optical axis 00,. The deflection of the beam 6 by the objective system is then slightly less and the focussing spot V'f moves to the left. The detector 7 then receives more radiation than the detector 8. If the plane of the tracks 2 moves upwards, the reverse takes place, and the detector 7 receives less radiation than the detector 8.

The signals S7 and S8 from the detectors are applied to an electronic circuit 9. In this circuit the signals are subtracted from each other in a manner known per se. At the output of the circuit 9 a focussing control signal rf is then obtained with which the focussing of the objective system can be corrected, for example by moving this system along the optical axis 00'. If the radiation source is a diode laser, the optical read unit may also be moved along the optical axis.

The optical wedge, or a diffraction grating, is disposed in the path of the read beam which is directed towards the information carrier, and the focussing beam which passes through the lens L1 is narrow. Thus it is ensured that the spot Vf is appreciably larger than the spot Vi. The details of the information structure cannot then be distinguished with the focussing beam, so that the signals S7 and Ss will not exhibit any high-frequency variations.

For the sake of clarity the reflected focussing beam is shown passing through the border of the lens L1 in Fig. 1. In reality the point where the principal ray of this beam enters the lens L, will be nearer the optical axis.

In the apparatus in accordance with the invention the focussing beam is formed with very simple means, namely with a wedgeshaped element only or a small diffraction grating only. The wedge or the diffraction grating may for example be mounted on a transparent plate. This plate may be fixed relative to the lens L1 in the direction of the optical axis 00,.

The angle of refraction of the wedge 5 is subject to an upper limit, so that this is also the case for the deflection of the focussing beam by said wedge. It is desirable that the point of the information structure to which the focussing is adjusted is nearest the point of the information structure where read-out is effected. The distance between Vi and Vf is for example 100 m. In cases where the information carrier is oblique relative to the optical axis or where variations in thickness of the information carrier occur it is then also possible to maintain a correct focussing of the read beam.

In order to have a sufficient distance between the focussing spot V' f and the read spot V'i deflection by the wedge 5 alone suffices if the magnification of the objective system is sufficiently high, or if the radiation source does not at the same time constitute the information detector, so that the radiation reflected by the record carrier can be mirror-diverted and the detectors can be arranged at a suitable distance from the information carrier.

When a diode laser is used as radiation source (see Fig. 1) and an objective system which images the diode laser onto the information structure with a reduction ratio of 2:1, the distance between the objective system and the diode laser being preferably small, the distance, resulting from the deflection by the wedge 5, between the spots V' and V'f is too small. In that case it is possible in accordance with the invention to employ a second optical wedge 10. This wedge is then disposed in the path of the reflected focussing beam. The wedge 10 may have a greater angle of refraction than the wedge 5, because it does not affect the distance between the spots Vi and Vf.

Even if a satisfactory distance between the spots V'i and Vtf can be obtained with a wedge 5, a second wedge 10 may still be used. By means of the second wedge it is then possible to prevent radiation of the read beam from being incident on the focussing detectors when the information structure is out of focus, resulting in the read spot V' being "blown up".

The wedge 10 should then be disposed in the shadow of the wedge 5 or, in other words, the wedges 5 and 10 must be imaged onto each other by the lens L1 via the information carrier. In Fig. 1 the marginal rays of said imaging are represented by dash-dot lines.

If the plane of the wedges were situated at an arbitrary height between the lenses L1 and L2, the image of the wedge 5 would depend on the distance between the plane of the information structure and the objective system. Therefore care may be taken that the plane of the wedges coincide with the back focal plane F of the lens L1.

In order to ensure that all the radiation which is deflected by the first wedge 5 passes through the second wedge 10, the second wedge would have to be slightly larger than the first wedge. However, a small portion of the read beam 3 itself would then pass through the second wedge and result in a separate radiation spot Vn on the surface of the detectors; compare the small beam 3' indicated by the uninterrupted lines in Fig. 1.

In the situation of Fig. 1, in which the read beam is correctly focussed on the information structure the radiation spot Vn is situated close to the focussing detectors. If the plane of the tracks 2 were to move upwards, the radiation spot Vn might then fall onto the detector 7 in the event of a small focussing error, thus giving rise to an erroneous signal rf.

Therefore, the area of the wedge 10 should at the most be equal to that of the wedge 5 and the wedge 10 is disposed in the shadow of the wedge 5. As a result of this, a part of the focussing beam, the beam 6' represented by the dashed lines, will not be incident on the detectors 7 and 8. However, this merely results in the signals S7 and S8 being slightly smaller. The sensitivity of the detection system for focussing errors is not significantly affected thereby.

Furthermore, care is taken that the distance d between the optical axis 00' and the point where the focussing beam enters the lens L, is approximately 0.7 times the radius r of the lens pupil. For the read method shown in Fig. I, where the read beam traverses the information carrier twice, the influence of spherical aberration in the objective system on the shape of the spot Vi in the case of variations in the thickness of the information carrier is then minimal for the focussing control method described.

In Figs. 2a and 2b the two focussing detectors 7 and 8 are shown with the focussing spot V' f projected thereon. It is assumed that, in the event of a variation of the focussing of the read beam, the focussing spot Vtf moves in the x-direction. For an optimum sensitivity to focussing errors of the detection system the lineg separating the detectors 7 and 8 should be perpendicular to the x-direction, as is shown in Fig. 2a. However, the derived focussing control signal rf would then greatly depend on the position of the focussing detectors in the x-direction.

The detectors 7 and 8 may be arranged so that the line of separation g makes an acute angle, for example 45 , with the x-direction as is shown in Fig. 2b. The null of the signal rf can then be adjusted by rotating the wedge 5 or the wedges 5 and 10 about the optical axis 00,. In Figs. 3a and 3b the path described by the focussing spot V' f if the wedges are rotated is represented by the curve c. In the case of Fig. 3a, in which the detectors have the orientation of Fig. 2b, the radiation distribution over the focussing detectors will change when the focussing spot moves over the detectors in accordance with the curve c.

During assembly of the read apparatus, after the plate with the wedges has been mounted between the lenses L, and L2 and the focussing has been adjusted correctly, the plates can then be rotated so that the focussing spot is symmetrical relative to the detectors 7 and 8. This is not possible if the focussing detectors have the orientation in accordance with Fig. 2a. In that case the radiation distribution over the focussing detectors cannot be influenced by rotating the wedge plate through small angles. Compare Figure 3b.

If the focussing detectors have the orientation of Fig. 2b, moving the focussing spot V' in the x-direction, i.e. a movement as a result of the focussing errors, will result in a smaller variation of the signals S7 and S8 then if these detectors were oriented in accordance with Fig. 2a. Consequently, the sensitivity of the detection system is reduced. However, this presents no problems. The sensitivity still remains adequate in the case of the arrangement of Fig. 2b. The advantage obtained in respect of the positional tolerance of the focussing detectors is more important than the loss of sensitivity.

As a focussing beam is derived from the read beam, this beam will no longer fill the pupil of the lens L, in an optimum manner.

As a result, the radiation spot Vi will become slightly larger in the direction of the line connecting the optical axis 00' to the centre of the deflecting element (a wedge or a grating).

The resolution of the read beam in this direction is then slightly reduced. The influence of this, in itself minor effect, may further be reduced by arranging the line which connects the optical axis and the deflection element at an angle of approximately 45" with the direction of a track portion to be read.

The two radiation-deflecting elements 5 and 10 in Figure 1, which are necessary to obtain an adequate distance between the radiation spots V'i and V' should be correctly aligned relative to each other.

Moreover, the elements 5 and 10 together should be correctly aligned relative to the objective system. This is because the element 10 must be disposed in the shadow of the element 5.

Figure 4 shows an embodiment of apparatus in accordance with the invention in which an adequate spacing is obtained between the focussing spot V' and the reimaged read spot with the aid of only one radiation-deflecting element whose position is not very critical. In Figure 4 the elements which correspond to those of Figure 1 bear the same reference numerals.

In the arrangement of Figure 4 a small optical wedge 10 is disposed so that a subbeam, or focussing beam 6 is deflected from the read beam which has been reflected by the information carrier. The dashed lines in Figure 4 indicate which part of the read beam passes through the wedge. The lenses L1 and L2 ensure that the focussing beam 6 is concentrated on the focussing detectors to a radiation spot, or focussing spot, V' f.

In this arrangement only one radiation spot on the information structure is used for reading the information and for generating a focussing error signal. The area of the information structure on which the read beam is focussed is then always the area which is being read.

The wedge 10 also deflects a part from the read beam which is directed towards the information carrier. However, this part is focussed on the information structure to an additional radiation spot to the right of the read spot Vi. The lens system L1, L2 reimages the additional radiation spot in a position to the left of the optical axis 00', i.e. not on the focussing detectors.

The optical elements are aligned so that if the distance between the plane of the information tracks 2 and the objective system Ll, L2 is correct, the radiation which is incident on the optical wedge is directed as indicated by the dashed lines in Figure 4. The optical wedge then deflects the focussing beam 6 so that the focussing spot is symmetrical relative to the focussing detectors. These focussing detectors then receive the same amount of radiation, and the output signals 87 and S8 of the detectors 7 and 8 are then equal.

If the plane of the information structure moves relative to the objective system L,L2, the convergence of the read beam which is reflected by the information carrier changes.

As a result of this, that part of the read beam which is used as focussing beam will be incident on the wedge 10 at an angle which differs from that indicated in Figure 4. As a result of this the direction of the beam 6 which passes through the wedge 10 and thus the position of the focussing spot V' f relative to the focussing detectors also changes. If the plane of the information structure moves towards the objective system, the detector 7 will receive more radiation than the detector 8. However, if the plane of the information structure moves away from the objective system, the detector 7 will receive less radiation than the detector 8.

The additional steps described with reference to Figure 1 may also be applied to the arrangement of Figure 4.

Preferably, the distance a between the centre of the wedge 10 and the optical axis 00' is approximately 0.7 times the radius of the read beam at the location of the wedge.

In the case of a variation in the thickness of the information carrier the influence of the spherical aberrations in the objective system on the shape of the spot V'i is then again minimal.

Furthermore, the line which separates the focussing detectors preferably makes an acute angle, of for example 45 , with the direction in which the radiation spot which is formed in the plane of the focussing detec tors moves upon a change in the position of the plane of the information structure.

Finally, the line which interconnects the optical wedge 10 and the optical axis prefer ably makes an angle of approximately 45" with the direction of a track portion to be read.

The fact that the invention has been described on the basis of a wedge as a radiation-deflecting element does not mean that the invention is limited to the use of such a wedge. Instead of a wedge it is alternatively possible to use any other radiation-deflecting element, such as a diffraction grating.

Steps may also be taken to deflect the focussing beam 6 in a direction opposite to that indicated in the Figures, so that the focussing detectors can be arranged on the same side of the optical axis 00' as the radiation-deflecting element 10. For this purpose the wedge 10 may for example be rotated 1800 about its own axis.

WHAT WE CLAIM IS: 1. Apparatus for reading an optical radiation-reflecting information carrier, which apparatus comprises a radiation source which produces a read beam, an objective system for focussing the read beam to a read spot on the information structure of the information carrier and for imaging the read spot on a radiation-sensitive informa tion detector whose output signal represents the information and an opto-electronic focussing error detection system for determining a deviation between the desired and the actual position of the plane of focussing of the objective system, which focussing error detection system comprises two radiation-sensitive focussing detectors which co-operate with a narrow focus

Claims

**WARNING** start of CLMS field may overlap end of DESC **. reflected by the information carrier changes. As a result of this, that part of the read beam which is used as focussing beam will be incident on the wedge 10 at an angle which differs from that indicated in Figure 4. As a result of this the direction of the beam 6 which passes through the wedge 10 and thus the position of the focussing spot V' f relative to the focussing detectors also changes. If the plane of the information structure moves towards the objective system, the detector 7 will receive more radiation than the detector 8. However, if the plane of the information structure moves away from the objective system, the detector 7 will receive less radiation than the detector 8. The additional steps described with reference to Figure 1 may also be applied to the arrangement of Figure 4. Preferably, the distance a between the centre of the wedge 10 and the optical axis 00' is approximately 0.7 times the radius of the read beam at the location of the wedge. In the case of a variation in the thickness of the information carrier the influence of the spherical aberrations in the objective system on the shape of the spot V'i is then again minimal. Furthermore, the line which separates the focussing detectors preferably makes an acute angle, of for example 45 , with the direction in which the radiation spot which is formed in the plane of the focussing detec tors moves upon a change in the position of the plane of the information structure. Finally, the line which interconnects the optical wedge 10 and the optical axis prefer ably makes an angle of approximately 45" with the direction of a track portion to be read. The fact that the invention has been described on the basis of a wedge as a radiation-deflecting element does not mean that the invention is limited to the use of such a wedge. Instead of a wedge it is alternatively possible to use any other radiation-deflecting element, such as a diffraction grating. Steps may also be taken to deflect the focussing beam 6 in a direction opposite to that indicated in the Figures, so that the focussing detectors can be arranged on the same side of the optical axis 00' as the radiation-deflecting element 10. For this purpose the wedge 10 may for example be rotated 1800 about its own axis. WHAT WE CLAIM IS:

1. Apparatus for reading an optical radiation-reflecting information carrier, which apparatus comprises a radiation source which produces a read beam, an objective system for focussing the read beam to a read spot on the information structure of the information carrier and for imaging the read spot on a radiation-sensitive informa tion detector whose output signal represents the information and an opto-electronic focussing error detection system for determining a deviation between the desired and the actual position of the plane of focussing of the objective system, which focussing error detection system comprises two radiation-sensitive focussing detectors which co-operate with a narrow focussing beam, the difference in the output signals of the focussing detectors previding an indication of said deviation, wherein, in order to produce the focussing beam, the radiation path of the read beam on one side of the optical axis of the objective system includes a radiation-deflecting element, whose surface area is substantially smaller than the crosssectional area of the read beam.

2. Apparatus as claimed in Claim 1, wherein the radiation deflecting element is constituted bv a diffraction grating.

3. Apparatus as claimed in Claim 1, wherein the radiation-deflecting element is constituted by an optical wedge.

4. An apparatus as claimed in Claim 1, wherein the radiation-deflecting element is included in the radiation path of the read beam directed to the information carrier in such a way that the radiation which is incident on the radiation-deflecting element forms an additional radiation spot on the information structure besides the read spot which additional radiation spot is imaged on the focussing detectors by the objective system.

5. An apparatus as claimed in Claim 4, wherein the radiation-deflecting element is an optical wedge, and wherein the radiation path of the sub-beam which is formed by a first optical wedge and which is reflected by the information carrier includes a second optical wedge.

6. An apparatus as claimed in Claim 5, wherein the second optical wedge is disposed within the image of the first optical wedge formed with the aid of the information carrier and the lens element of the objective system nearest the information carrier.

7. An apparatus as claimed in Claim 6, wherein the optical wedges are disposed in the back focal plane of the lens element of the objective system nearest the information carrier.

8. An apparatus as claimed in Claim 4, wherein the deflecting element is arranged so that the distance between the optical axis of the objective system and the point where the focussing beam enters the lens element of the objective system nearest the information carrier for the first time equals approximately 0.7 times the radius of the pupil of said lens element.

9. An apparatus as claimed in Claim 1, wherein the radiation-deflecting element is included in the path of the read beam which is reflected by the information carrier and

which originates from the read spot, in such a way that the radiation which is incident on the radiation-deflecting element is deflected to the focussing detectors.

10. An apparatus as claimed in Claim 9, wherein the distance between the centre of the radiation-deflecting element and the optical axis is equal to approximately 0.7 times the radius of the read beam at the location of the radiation-deflecting element.

11. An apparatus as claimed in any of the preceding Claims, wherein the dividing line between the focussing detectors makes an acute angle with the direction in which the radiation spot formed in the plane of the focussing detectors moves as a result of focussing errors.

12. An apparatus as claimed in any of the preceding Claims, wherein the connecting line between the optical axis of the objective system and the radiation-deflecting element makes an angle of approximately 45C with the direction in which an information track of the record carrier is read.

13. Apparatus substantially as described with reference to Figure 1 or Figure 4 of the accompanying drawings.