CA1046636A - Plano-concave and plano-convex lens system for deflecting read beam in optical recording system - Google Patents
Plano-concave and plano-convex lens system for deflecting read beam in optical recording systemInfo
- Publication number
- CA1046636A CA1046636A CA219,523A CA219523A CA1046636A CA 1046636 A CA1046636 A CA 1046636A CA 219523 A CA219523 A CA 219523A CA 1046636 A CA1046636 A CA 1046636A
- Authority
- CA
- Canada
- Prior art keywords
- lens
- read
- plano
- record carrier
- lenses
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
Links
Classifications
-
- G—PHYSICS
- G11—INFORMATION STORAGE
- G11B—INFORMATION STORAGE BASED ON RELATIVE MOVEMENT BETWEEN RECORD CARRIER AND TRANSDUCER
- G11B7/00—Recording or reproducing by optical means, e.g. recording using a thermal beam of optical radiation by modifying optical properties or the physical structure, reproducing using an optical beam at lower power by sensing optical properties; Record carriers therefor
- G11B7/08—Disposition or mounting of heads or light sources relatively to record carriers
- G11B7/09—Disposition or mounting of heads or light sources relatively to record carriers with provision for moving the light beam or focus plane for the purpose of maintaining alignment of the light beam relative to the record carrier during transducing operation, e.g. to compensate for surface irregularities of the latter or for track following
-
- G—PHYSICS
- G11—INFORMATION STORAGE
- G11B—INFORMATION STORAGE BASED ON RELATIVE MOVEMENT BETWEEN RECORD CARRIER AND TRANSDUCER
- G11B7/00—Recording or reproducing by optical means, e.g. recording using a thermal beam of optical radiation by modifying optical properties or the physical structure, reproducing using an optical beam at lower power by sensing optical properties; Record carriers therefor
- G11B7/08—Disposition or mounting of heads or light sources relatively to record carriers
- G11B7/085—Disposition or mounting of heads or light sources relatively to record carriers with provision for moving the light beam into, or out of, its operative position or across tracks, otherwise than during the transducing operation, e.g. for adjustment or preliminary positioning or track change or selection
- G11B7/08547—Arrangements for positioning the light beam only without moving the head, e.g. using static electro-optical elements
Abstract
ABSTRACT:
An apparatus is described for optically reading a record carrier. Deflection of the read ???? over the record carrier can be achieved with the ??? of an as-sembly of a plano-concave and a plano-convex lens, the radiu??s of curvature of the curved surfaces of the len-ses being equal, and the distances between the curved surfaces being very small. The beam-deflection element is compact, may be disposed near the read objective, and need not ?eet stringent requirements as regards surface quality.
An apparatus is described for optically reading a record carrier. Deflection of the read ???? over the record carrier can be achieved with the ??? of an as-sembly of a plano-concave and a plano-convex lens, the radiu??s of curvature of the curved surfaces of the len-ses being equal, and the distances between the curved surfaces being very small. The beam-deflection element is compact, may be disposed near the read objective, and need not ?eet stringent requirements as regards surface quality.
Description
P~IN 7372 VMI/RJ
18.3.74 104t:~636 "Apparatus for reading a record carrier on which infor-mation is stored in an optically readable structure".
_ _ _ _ _ , The invention relates to an apparatus for read-; ing a record carrier on which information is stored in ; an optically readable structure of areas and interme-diate areas w~ch are arranged along tracks, which ap-paratus comprises a radiation source which supplies a read beam, an objective system for focussing the read beam to a read spot on the optical structure of the record carrier, a beam deflecting element for moving the read spot in at least one of` the directions:
'~ 10 transverse to the track direction and coincident with the track direction, and a radiation-sensitive detec-tion system for converting the read beam which is mo-dulated by the areas and intermediate areas into an electrical signal.
It has already been proposed~ for example in: "Philips' Technical Review" 33; No. 7, pages 178 -193, to store a colour television programme in a round disc-shaped record carrier. The information is con-tained in the lengths of the areas and intermediate 1 20 areas.
;~ A track of the record carrier can be read ,~ .
by projecting a read spot of a size of the order of magnitude of the smallest optical detail in the infor-~ .
, ,- , . . . -PHN. 7372.
104~636 mation structure onto the track and moving the read spot and the record carrier relative to each other in the longitudinal direction of the track. The read beam is then modulated in accordance with the sequence of the areas and intermeæiate areas in the track. Fcr read-ing the entire record carrier, the read spot and the record carrier must be mDved relative to each other in a direction transverse to the direction of the track.
For this use is made of a coarse control and a fine contrnl. Coaræ control is achieved in that a housing which accomnodates the cptical elements used for read-ing, is bodily moved relative to the reoDrd carrier.
For fine control for example a mirror may be disposed rotatably in the radiation path in front of the objec-; 15 tive system. By rotating the mirror the read spot can be mDved radially over the information structure.
When reading the record carrier it may fur-therm~re be necessaIy to correct the position of the read spot in the tangential direction, i.e. in the longitudinal direction of the track, in order to be ; able to oompensate for timerbase e D rs in the detect-~ ed signal. As proposed in our Canadian Patent 994,909 ; ~ which issued on August lO, 1976 a second mirrDr may be ` included in the radiation path in front of the objec-tive system, which mirrDr is rotatable about an axis which is perpendicular to the axis of rotation of the first mirrDr. ~
'. ~ '. ' _ 3 _ :
E?~ , , . .
1~.3-74 663~i The read beam impinges on the mirrors at angles of approximately 45. In order to prevent that upon re-flection from the mirrors the read beam becomes exces-sively astigmatlc, stringent requirements must be impos-ed on the planeness of the mirrors.
Furthermore, the mirrors may not be disposed in the pupil of the objective lens, which would be desirable for a stable control of the read beam fo-cussing.
The object of the present invention is to provide a read apparatus which mitigates said drawbacks.
The apparatus according to the invontion is charac-terizcd in that the bcam deflecting el0ment consist.s of an assemblyoBia plano-concave lens and a plano-convex lens, that the concave surface of the first lens faces the convex surface of the second l0ns, which surfaces have substantially the same radius of curvature and are disposed at a distance which is substantially smaller than said radius of curvature, at least one of the lenses being rotatable in such a way that the axes of rotation of the lenses are perpendicular to each other and are disposed in a plane perpendicular to the optical axis of the objec--;
tive system,-so that the projections of said axes on-to the record carrier extend in the direction of the track and transverse to the track direction.
The beam deflecting element according to the ."`, ' .
,. . .- .. .
18.3.74 ~0~6636 invention is compact and can be disposed substantially within the pupil of the objective system. The lens sur-faces may for example, at the same angle of incidence of the read beam, be a factor 4 less accurate than the surfaces of the said mirrors.
The invention will now be described in more detail with reference to the drawing, in which:
Fig. 1 shows a known record carrier provid-ed with an optically readable structure, F~g. 2 shows an apparatus according to the invention for reading such a structure, Fig. 3 illustrates the operation of a beam deflecting element according to thc invention, Fig. 4 shows an embodiment of the mechanical construction of such an element, and ;~; Figs. 5 and 6 show how the lenses of the beam deflecting element can be moved.
i- Fig. 1 shows a part of a disc-shaped round "
record carrier 1. The record carrier is provided with a multitude of concentric tracks 2, of which only a ; . . .
few are shown. The tracks 2 comprise a multiplicity of areas g alternating with intermediate areas t.
The lengths of the areas and of the intermediate areas are determined by the information stored. B~-tween the informatiGn tracks 2 structureless lands 3 are disposed. The tracks can be read with a beam of radiation, which is focussed to a read spot V on the ~, ',~ ' , 18.3.74 ~ 0~ 6 information structure. The areas may be distinct from the intermediate areas and the lands as regards transmission coefficient or as regards reflection coefficient, in which cases the read beam is amplitude modulated. The read beam may also be phase modulated. For this, the areas g must be disposed at a different level in the record carrier than the intermediate areas t and the lands 3. Such a phase structure may consist of a mul-tiplicity of pits which are pressed along the tracks in a reflecting record carrier.
An apparatus for reading such a record car-rier is shown in Fig. 2. In said Figure the reference numeral 6 denotes a radiation source, for example a . ~ laser source. The beam 30 supplied by said radiation ~, 15 source travorses a first lens 10, is subsequently re-flected by a plane mirror 11 and is then focussed to ; a read spot V on the information structure by an ob-,, ~:
; ~ ~ jective lens 15. As an example, the information struc-ture i9 d~sposed at the upper surface of the record ¦~ 20 carrier. The lens 10 ensures that the entire pupil of , . . . .
the objective lens 15 is filled. A round disc-shaped ,: ~
record carrier 1 can be rotated by a shaft 5 which extends through a central opening 4 in the record carrier, so that the radiation spot is consecutively projected onto all areas and intermediate areas of a track.
, ~
~ After reflection at the information struc-! , ~
. ~, .
,, ~ .
.: , . ~
18.3.74 .
10~t;6~6 ture the read beam 30 traverses the objective lens 15 for a second time and is subsequently reflected by the plane mirror 11. Next, the read beam is reflected towards a radiation-sensitive detection system 21, for example, by a semi-transparent mirror 20. The electrical signal supplied by said detection system, which signal is mo-dulated in accordance with the sequence of areas and intermediate areas in a track, is fed to an e]ectronic circuit 22. In said circuit the signal is processed, in known manner, into for example a video and/or audio signal Si~ which may be reproduced for example by means ~`f a conventional television receiver ~3.
In order to be able to read all tracks of - the record carrier after each other, a control system, not shown, is provided by means of which the optical read unit is bodily moved in a radial direction. The coarse control is not rendered operative until the read spot is to be moved over a distance greater than a certain minimum distance. For smaller displacements use is made of a fine control. Said fine control is also used for correcting centring errors of the read spot relative to the centre of the track to be read.
Owing to for example out-of-roundness of the disc-shaped record carrier or an eccentricity of the centre of rotation of the disc-shaped record carrier it may happen that the path of an information track ; is no longer concentric or splral-shaped relative to ,.
~, . .
- 7 ~
PHN. 7372.
the pivot. m is may give rise to both a deviation in the radial direction tthe x-direction in Fig. 1) and in the tangential direction (the y-direction in Fig. 1) of the position of the read spot relative to the track to be read. A deviation in the tangential direction results in a time base error of the detected signal, whilst owing to a deviation in the radial direction the mDdu-lation depth of the detector signal may decrease and crosstaIk may occur between adjacent tracks.
A aentring error of the read spot may for example, as previously proposed in our Canadian ; Patent 987,029 which issued on April 6, 1976, be detected with the aid of tw~ additional radiation spots which are projected onto the information struc-ture. m e tWD additional radiation spots are projected on the edge of the track to be read and, viewed in the longitudinal direction of the track, are offset in opposite directions relative to the read spot. me tWD
additional radiation spots may, for example, be obtained by including a grating, not shown, in the radiation path in front of the lens 10. Said grating diffracts the beams produced by the radiation souroe into a zero-order beam and tWD first-order beams. Said beams are fccussed by the objective lens 15 to radiation sp~ts on the information structure at different locations. In the detection system 21 a separate detector is provided for each radiation spot. me detector element 21a ;., ~`
- . . . -PHN. 7372.
'104~636 supplies a high-frequency information signal. By com!
paring the electrical signals supplied by the detector ~ -elements 21b and 21c an indication can be obtained of the magnitude and direction of a possible centring error of the read spot. The electronic circuit 22 can derive a control signal Sr for centring correction from the two last-mentioned signals.
As is described in the previously mentioned Canadian Patent 994,909, it is also possible to derive from the signals supplied by the detectors 21b and 21c an indication of a deviation in the tangential ~;rection, with the aid of a phase-shifting element which causes a phase shift equal to one fourth of the revDlution period of the record carrier. The electro-nic circuit 22 then also derives a signal St for cor-recting the tangential position of the read spot.
In the previously proposed read app~ratus ~I the mirror 11 was disposed rDtat~bly for correcting ;1, the radial position of the read spot. The angular position of said mirror was determined by the signal !
~ Sr. Fbr correcting the tangential position of the read i spot a second rotatable mirror, n~t shcwn, was pro-; vided, ~hose angular position was determined by the signal St. The read beam was incident on the mirrors at an angle of approximately 45~. me mirrors were disposed in a dive~ging beam. If the mirrors were not ~, equally plane over their entire surface, besides the '1` ~ ~ , ' ;, ' . t _ 9 _ ' ,' . , .
P~ 7372 18.3.74 1046~36 occurrence of a displacement of the eventual radiation spot on the information structure, the read beam would become astigmatic, so that even after reduced imaging onto the record carrier, the read spot would not be suited for a correct reading. As a result of the de-focussing, the modulation depth of the detec-ted signal decreases, whilst moreover cross-talk between adjacent tracks may occwr. The focussing error ~ is given by~ = 2 a where v is the magnification factor of the object:Lve lens, whilst a is the displacement of the focal point 32 of the lens 10. The geometric diamoter of the circle of u~sharpnessAI on the information structure is propor-tional to ~.xN.A, in whlch N.A. is the numerical aper-ture of the objective lens 15. Even for a slight de-1 15 viation in the planeness of the mlrror the unsharpness ~ is no longer permissible.
; As it must be rotatable and in its rest po-sition makes an angle of 45 with the optical axis of the read system, the mirror which is nearest to the objective lens may not be placed in the pupil of said lens. Of course, the other mirror is even fur-ther away from said pupil. For a stable focussing control of the read beam this is undesirable, as will appear from the following.
- For detecting focussing errors use can be made of an auxiliary beam 31 of smaller diameter, as is shown in Fig. 2. ~aid beam, of which for clarity ~, ., .: ' P~N 7372 18.3.74 ~04~6~6 only one ray is shown, is split from the main beam by means of a semi-transparent mirror 7 and a fully re-flecting mirror 8. The beam 31 falls onto the mirror 11 through a slit of a diaphragm 17 and after reflec-tion traverses the objective lens 15 in a point out-side the optical axis of said lcns. The auxiliary beam is subsequently incident on the information structure at an acute angle. After reflection at the information structure the beam 31 passes the objective lens again outside the optical axis, and is then reflected towards the diaphragm slit by tho plane mirror. When the distance from the plane of the information structure to the ob-jective lens -is correct, the image of the diaphrilgm slit is symmetrical relative to t;ho actual diaphragm slit. As a result, two radiation-sensitive detectors disposed at either side of said slit receive the same amount of radiation.
If t~e plane of the information structure is moved relative to the objective lens, the reflected sub-beam 31 traverses ano-ther part of said lens. As a result, the beam is refracted through a different angle than in case that the plane of the information structure is in the desired position. The image of the diaphragm slit then moves over the detectors 18 and 19. By comparing $he output signals of said de-~- tectors an indication can be obtained about the magnitude and the direction of~a read-beam defocussing.
.
.
, . , . : . .
18.3.74 1 04~
The auxiliary beam 31 also traverses the ele-ments for deflecting the read beam. If said elements are disposed at some distance from the pupil ~ the ob-jective lens 15, as will be the case for the mirrors, the auxiliary beam will be if deflected over the pupil of the objective lens 15 when said elements are ro-tated. The direction of the auxiliary beam is then no longer determined exclusively by the position of the plane of the information structure, so that no cl.CCU-rate focussing detection is possible any longer.
According to the invention an assembly 12 of two lenses 13 and 14 is uscd for deflccting thc read beam. The lens 14 is a plano-convex lens and the lens 13 is a plano-concave lens. The curvature of the concave surface of the lens 13 is substantially equal to that of the convex surface of the lens 14. In prin-ciple, the lenses may be placed against each other.
However, in practice, there will be an air gap between the lenses, which air gap is very small, for example 50 - 100 ~um. Each lens of the assembly is rotatable about an axis through the centre of curvature of the curved lens surface.
Fig. 3 illustrates the operation of the lens assembly 12. A radiation beam r, schematically re-presented by one ray only, traverses the lens system without refraction if the plane surfaces of the lenses 13 and 14 are in parallel. When the lens 13 is rotated 18.3.74 ~046~3~
through an angle ~ about an axis which passes through the centre of cur~ature M and which is perpendicular to the plane of drawing, the beam upon emergence from the lens system is refracted in a direction which is located in the planc of the drawing (case b). By such a rotation of the lens 13 the beam will be deflected - in a radial direction in the apparatus of Fig. 2. The magnitude of the deflection is determined by the wedge angle ~ between the plane surfaces of the lenses 13 ; 10 and 14.
If, as is shown in Figo 3 under c), the plano-convex lens 1~ is rotated throughan angle ~ , not shown, about an axis through the cenlre of curvature M and dis-."
posed in the plane Or drawin~, the radiation beanl r upon entering the lens system wlll be refracted in a direction which is disposed in a plane perpendicular to the plane of drawing. Owing to such a rotation of the lens 14 the read beam in the apparatus of Fig. 2 will be deflected tangentially~ i.e. in the direction of the tracks 2. In Fig. 3 the plane surface of the lens 14, which becomes partly visible upon rotation ~ of said lens, is denoted by 25.
- In an embodiment of a lens system according to the invention the minimum thickness of the plano-conca~e lens 13 was 1 mm, whilst the maximwn thickness of the plano-convex lens 14 was 2 mm. The diameter of the lensQs was 8 mm. The lenses could be rotated thrnugh .
~ 13 1~.3.71~
~046~3~;
an angle of + 5. In the apparatus uf ~ig. 2 the lens 13 could be disposed at a distance of 3 mm from the objec-tive lens.
When passing from a first medium with a refrac-tive index n1 to a second medium with a refractive index n2, the deviation in the wave front of a beam of radia-tion owing to irregularities at the interface is propor tional to (n1-n2). When a radiation beam which propagates through a:ir is reflected, n1 may be assumed to be -1 (-sign owing to the reflection) and n2 to be +1. The said deviation is thcn proport.ional to 2. In thQ case of refrac-tion of the radiat:ion beam by a lens, n1 is approximately-1.5 and n2 is again +1, so that the de-viationis then proportional to 0.5. The requirements to be imposed on the surface of a mirror must therefore be a factor 4 more stringent than the requirements to be imposed on the surfaces of the lenses for the same angle of incidence of the read beam. Because the angle of incidence of the read beam on the lens surface of the lens 14 is near 90 the requirements imposed on the lens surfaces are even less stringent.
~or greater wedge angles C~ and ~ between the plane surfaces of the lenses 13 and 14 asymmetry errors may occur, mainly coma. This may give rise to a non-
18.3.74 104t:~636 "Apparatus for reading a record carrier on which infor-mation is stored in an optically readable structure".
_ _ _ _ _ , The invention relates to an apparatus for read-; ing a record carrier on which information is stored in ; an optically readable structure of areas and interme-diate areas w~ch are arranged along tracks, which ap-paratus comprises a radiation source which supplies a read beam, an objective system for focussing the read beam to a read spot on the optical structure of the record carrier, a beam deflecting element for moving the read spot in at least one of` the directions:
'~ 10 transverse to the track direction and coincident with the track direction, and a radiation-sensitive detec-tion system for converting the read beam which is mo-dulated by the areas and intermediate areas into an electrical signal.
It has already been proposed~ for example in: "Philips' Technical Review" 33; No. 7, pages 178 -193, to store a colour television programme in a round disc-shaped record carrier. The information is con-tained in the lengths of the areas and intermediate 1 20 areas.
;~ A track of the record carrier can be read ,~ .
by projecting a read spot of a size of the order of magnitude of the smallest optical detail in the infor-~ .
, ,- , . . . -PHN. 7372.
104~636 mation structure onto the track and moving the read spot and the record carrier relative to each other in the longitudinal direction of the track. The read beam is then modulated in accordance with the sequence of the areas and intermeæiate areas in the track. Fcr read-ing the entire record carrier, the read spot and the record carrier must be mDved relative to each other in a direction transverse to the direction of the track.
For this use is made of a coarse control and a fine contrnl. Coaræ control is achieved in that a housing which accomnodates the cptical elements used for read-ing, is bodily moved relative to the reoDrd carrier.
For fine control for example a mirror may be disposed rotatably in the radiation path in front of the objec-; 15 tive system. By rotating the mirror the read spot can be mDved radially over the information structure.
When reading the record carrier it may fur-therm~re be necessaIy to correct the position of the read spot in the tangential direction, i.e. in the longitudinal direction of the track, in order to be ; able to oompensate for timerbase e D rs in the detect-~ ed signal. As proposed in our Canadian Patent 994,909 ; ~ which issued on August lO, 1976 a second mirrDr may be ` included in the radiation path in front of the objec-tive system, which mirrDr is rotatable about an axis which is perpendicular to the axis of rotation of the first mirrDr. ~
'. ~ '. ' _ 3 _ :
E?~ , , . .
1~.3-74 663~i The read beam impinges on the mirrors at angles of approximately 45. In order to prevent that upon re-flection from the mirrors the read beam becomes exces-sively astigmatlc, stringent requirements must be impos-ed on the planeness of the mirrors.
Furthermore, the mirrors may not be disposed in the pupil of the objective lens, which would be desirable for a stable control of the read beam fo-cussing.
The object of the present invention is to provide a read apparatus which mitigates said drawbacks.
The apparatus according to the invontion is charac-terizcd in that the bcam deflecting el0ment consist.s of an assemblyoBia plano-concave lens and a plano-convex lens, that the concave surface of the first lens faces the convex surface of the second l0ns, which surfaces have substantially the same radius of curvature and are disposed at a distance which is substantially smaller than said radius of curvature, at least one of the lenses being rotatable in such a way that the axes of rotation of the lenses are perpendicular to each other and are disposed in a plane perpendicular to the optical axis of the objec--;
tive system,-so that the projections of said axes on-to the record carrier extend in the direction of the track and transverse to the track direction.
The beam deflecting element according to the ."`, ' .
,. . .- .. .
18.3.74 ~0~6636 invention is compact and can be disposed substantially within the pupil of the objective system. The lens sur-faces may for example, at the same angle of incidence of the read beam, be a factor 4 less accurate than the surfaces of the said mirrors.
The invention will now be described in more detail with reference to the drawing, in which:
Fig. 1 shows a known record carrier provid-ed with an optically readable structure, F~g. 2 shows an apparatus according to the invention for reading such a structure, Fig. 3 illustrates the operation of a beam deflecting element according to thc invention, Fig. 4 shows an embodiment of the mechanical construction of such an element, and ;~; Figs. 5 and 6 show how the lenses of the beam deflecting element can be moved.
i- Fig. 1 shows a part of a disc-shaped round "
record carrier 1. The record carrier is provided with a multitude of concentric tracks 2, of which only a ; . . .
few are shown. The tracks 2 comprise a multiplicity of areas g alternating with intermediate areas t.
The lengths of the areas and of the intermediate areas are determined by the information stored. B~-tween the informatiGn tracks 2 structureless lands 3 are disposed. The tracks can be read with a beam of radiation, which is focussed to a read spot V on the ~, ',~ ' , 18.3.74 ~ 0~ 6 information structure. The areas may be distinct from the intermediate areas and the lands as regards transmission coefficient or as regards reflection coefficient, in which cases the read beam is amplitude modulated. The read beam may also be phase modulated. For this, the areas g must be disposed at a different level in the record carrier than the intermediate areas t and the lands 3. Such a phase structure may consist of a mul-tiplicity of pits which are pressed along the tracks in a reflecting record carrier.
An apparatus for reading such a record car-rier is shown in Fig. 2. In said Figure the reference numeral 6 denotes a radiation source, for example a . ~ laser source. The beam 30 supplied by said radiation ~, 15 source travorses a first lens 10, is subsequently re-flected by a plane mirror 11 and is then focussed to ; a read spot V on the information structure by an ob-,, ~:
; ~ ~ jective lens 15. As an example, the information struc-ture i9 d~sposed at the upper surface of the record ¦~ 20 carrier. The lens 10 ensures that the entire pupil of , . . . .
the objective lens 15 is filled. A round disc-shaped ,: ~
record carrier 1 can be rotated by a shaft 5 which extends through a central opening 4 in the record carrier, so that the radiation spot is consecutively projected onto all areas and intermediate areas of a track.
, ~
~ After reflection at the information struc-! , ~
. ~, .
,, ~ .
.: , . ~
18.3.74 .
10~t;6~6 ture the read beam 30 traverses the objective lens 15 for a second time and is subsequently reflected by the plane mirror 11. Next, the read beam is reflected towards a radiation-sensitive detection system 21, for example, by a semi-transparent mirror 20. The electrical signal supplied by said detection system, which signal is mo-dulated in accordance with the sequence of areas and intermediate areas in a track, is fed to an e]ectronic circuit 22. In said circuit the signal is processed, in known manner, into for example a video and/or audio signal Si~ which may be reproduced for example by means ~`f a conventional television receiver ~3.
In order to be able to read all tracks of - the record carrier after each other, a control system, not shown, is provided by means of which the optical read unit is bodily moved in a radial direction. The coarse control is not rendered operative until the read spot is to be moved over a distance greater than a certain minimum distance. For smaller displacements use is made of a fine control. Said fine control is also used for correcting centring errors of the read spot relative to the centre of the track to be read.
Owing to for example out-of-roundness of the disc-shaped record carrier or an eccentricity of the centre of rotation of the disc-shaped record carrier it may happen that the path of an information track ; is no longer concentric or splral-shaped relative to ,.
~, . .
- 7 ~
PHN. 7372.
the pivot. m is may give rise to both a deviation in the radial direction tthe x-direction in Fig. 1) and in the tangential direction (the y-direction in Fig. 1) of the position of the read spot relative to the track to be read. A deviation in the tangential direction results in a time base error of the detected signal, whilst owing to a deviation in the radial direction the mDdu-lation depth of the detector signal may decrease and crosstaIk may occur between adjacent tracks.
A aentring error of the read spot may for example, as previously proposed in our Canadian ; Patent 987,029 which issued on April 6, 1976, be detected with the aid of tw~ additional radiation spots which are projected onto the information struc-ture. m e tWD additional radiation spots are projected on the edge of the track to be read and, viewed in the longitudinal direction of the track, are offset in opposite directions relative to the read spot. me tWD
additional radiation spots may, for example, be obtained by including a grating, not shown, in the radiation path in front of the lens 10. Said grating diffracts the beams produced by the radiation souroe into a zero-order beam and tWD first-order beams. Said beams are fccussed by the objective lens 15 to radiation sp~ts on the information structure at different locations. In the detection system 21 a separate detector is provided for each radiation spot. me detector element 21a ;., ~`
- . . . -PHN. 7372.
'104~636 supplies a high-frequency information signal. By com!
paring the electrical signals supplied by the detector ~ -elements 21b and 21c an indication can be obtained of the magnitude and direction of a possible centring error of the read spot. The electronic circuit 22 can derive a control signal Sr for centring correction from the two last-mentioned signals.
As is described in the previously mentioned Canadian Patent 994,909, it is also possible to derive from the signals supplied by the detectors 21b and 21c an indication of a deviation in the tangential ~;rection, with the aid of a phase-shifting element which causes a phase shift equal to one fourth of the revDlution period of the record carrier. The electro-nic circuit 22 then also derives a signal St for cor-recting the tangential position of the read spot.
In the previously proposed read app~ratus ~I the mirror 11 was disposed rDtat~bly for correcting ;1, the radial position of the read spot. The angular position of said mirror was determined by the signal !
~ Sr. Fbr correcting the tangential position of the read i spot a second rotatable mirror, n~t shcwn, was pro-; vided, ~hose angular position was determined by the signal St. The read beam was incident on the mirrors at an angle of approximately 45~. me mirrors were disposed in a dive~ging beam. If the mirrors were not ~, equally plane over their entire surface, besides the '1` ~ ~ , ' ;, ' . t _ 9 _ ' ,' . , .
P~ 7372 18.3.74 1046~36 occurrence of a displacement of the eventual radiation spot on the information structure, the read beam would become astigmatic, so that even after reduced imaging onto the record carrier, the read spot would not be suited for a correct reading. As a result of the de-focussing, the modulation depth of the detec-ted signal decreases, whilst moreover cross-talk between adjacent tracks may occwr. The focussing error ~ is given by~ = 2 a where v is the magnification factor of the object:Lve lens, whilst a is the displacement of the focal point 32 of the lens 10. The geometric diamoter of the circle of u~sharpnessAI on the information structure is propor-tional to ~.xN.A, in whlch N.A. is the numerical aper-ture of the objective lens 15. Even for a slight de-1 15 viation in the planeness of the mlrror the unsharpness ~ is no longer permissible.
; As it must be rotatable and in its rest po-sition makes an angle of 45 with the optical axis of the read system, the mirror which is nearest to the objective lens may not be placed in the pupil of said lens. Of course, the other mirror is even fur-ther away from said pupil. For a stable focussing control of the read beam this is undesirable, as will appear from the following.
- For detecting focussing errors use can be made of an auxiliary beam 31 of smaller diameter, as is shown in Fig. 2. ~aid beam, of which for clarity ~, ., .: ' P~N 7372 18.3.74 ~04~6~6 only one ray is shown, is split from the main beam by means of a semi-transparent mirror 7 and a fully re-flecting mirror 8. The beam 31 falls onto the mirror 11 through a slit of a diaphragm 17 and after reflec-tion traverses the objective lens 15 in a point out-side the optical axis of said lcns. The auxiliary beam is subsequently incident on the information structure at an acute angle. After reflection at the information structure the beam 31 passes the objective lens again outside the optical axis, and is then reflected towards the diaphragm slit by tho plane mirror. When the distance from the plane of the information structure to the ob-jective lens -is correct, the image of the diaphrilgm slit is symmetrical relative to t;ho actual diaphragm slit. As a result, two radiation-sensitive detectors disposed at either side of said slit receive the same amount of radiation.
If t~e plane of the information structure is moved relative to the objective lens, the reflected sub-beam 31 traverses ano-ther part of said lens. As a result, the beam is refracted through a different angle than in case that the plane of the information structure is in the desired position. The image of the diaphragm slit then moves over the detectors 18 and 19. By comparing $he output signals of said de-~- tectors an indication can be obtained about the magnitude and the direction of~a read-beam defocussing.
.
.
, . , . : . .
18.3.74 1 04~
The auxiliary beam 31 also traverses the ele-ments for deflecting the read beam. If said elements are disposed at some distance from the pupil ~ the ob-jective lens 15, as will be the case for the mirrors, the auxiliary beam will be if deflected over the pupil of the objective lens 15 when said elements are ro-tated. The direction of the auxiliary beam is then no longer determined exclusively by the position of the plane of the information structure, so that no cl.CCU-rate focussing detection is possible any longer.
According to the invention an assembly 12 of two lenses 13 and 14 is uscd for deflccting thc read beam. The lens 14 is a plano-convex lens and the lens 13 is a plano-concave lens. The curvature of the concave surface of the lens 13 is substantially equal to that of the convex surface of the lens 14. In prin-ciple, the lenses may be placed against each other.
However, in practice, there will be an air gap between the lenses, which air gap is very small, for example 50 - 100 ~um. Each lens of the assembly is rotatable about an axis through the centre of curvature of the curved lens surface.
Fig. 3 illustrates the operation of the lens assembly 12. A radiation beam r, schematically re-presented by one ray only, traverses the lens system without refraction if the plane surfaces of the lenses 13 and 14 are in parallel. When the lens 13 is rotated 18.3.74 ~046~3~
through an angle ~ about an axis which passes through the centre of cur~ature M and which is perpendicular to the plane of drawing, the beam upon emergence from the lens system is refracted in a direction which is located in the planc of the drawing (case b). By such a rotation of the lens 13 the beam will be deflected - in a radial direction in the apparatus of Fig. 2. The magnitude of the deflection is determined by the wedge angle ~ between the plane surfaces of the lenses 13 ; 10 and 14.
If, as is shown in Figo 3 under c), the plano-convex lens 1~ is rotated throughan angle ~ , not shown, about an axis through the cenlre of curvature M and dis-."
posed in the plane Or drawin~, the radiation beanl r upon entering the lens system wlll be refracted in a direction which is disposed in a plane perpendicular to the plane of drawing. Owing to such a rotation of the lens 14 the read beam in the apparatus of Fig. 2 will be deflected tangentially~ i.e. in the direction of the tracks 2. In Fig. 3 the plane surface of the lens 14, which becomes partly visible upon rotation ~ of said lens, is denoted by 25.
- In an embodiment of a lens system according to the invention the minimum thickness of the plano-conca~e lens 13 was 1 mm, whilst the maximwn thickness of the plano-convex lens 14 was 2 mm. The diameter of the lensQs was 8 mm. The lenses could be rotated thrnugh .
~ 13 1~.3.71~
~046~3~;
an angle of + 5. In the apparatus uf ~ig. 2 the lens 13 could be disposed at a distance of 3 mm from the objec-tive lens.
When passing from a first medium with a refrac-tive index n1 to a second medium with a refractive index n2, the deviation in the wave front of a beam of radia-tion owing to irregularities at the interface is propor tional to (n1-n2). When a radiation beam which propagates through a:ir is reflected, n1 may be assumed to be -1 (-sign owing to the reflection) and n2 to be +1. The said deviation is thcn proport.ional to 2. In thQ case of refrac-tion of the radiat:ion beam by a lens, n1 is approximately-1.5 and n2 is again +1, so that the de-viationis then proportional to 0.5. The requirements to be imposed on the surface of a mirror must therefore be a factor 4 more stringent than the requirements to be imposed on the surfaces of the lenses for the same angle of incidence of the read beam. Because the angle of incidence of the read beam on the lens surface of the lens 14 is near 90 the requirements imposed on the lens surfaces are even less stringent.
~or greater wedge angles C~ and ~ between the plane surfaces of the lenses 13 and 14 asymmetry errors may occur, mainly coma. This may give rise to a non-
2~ uniform .intensity distribution over a radiation spot formed on the record carrier for different positions of the lenses 13 and 11l. The occurrence of asymmetry _ 14 .
18.3.74 ~04~i36 errors can be prevented in a simple manner by including a single positive lens 16 inthe radiation path as a cor-rection element. The lens 16 may be disposed both in front of' and behind the lens system 12. Preferably, i~
is disposed between the mirror 11 and the lens system 12, so that said system 12 may be placed closc to the objective 15. By using mater:;als with a high refractive index (n = 1.7 for example) for the lenses 13 and 14, a certain deflection of the radiation beam can be ob- , tained at smaller wedge anglesC~ and ~ , than ir the , lenses are made of materials with a low refractive index.
The f`act that the invention has been described `, with reference to the apparatus of Fig. 2, by no means implies that the scope of the invention is limited to ' said specific appara,tus. The signals Sr and St can be obtained in various manners, but the manner in which falls beyond the scope of the present invention. Also when reading other than disc-shaped record carriers, ,~ 2 20 such as record carriers in the form of a tape or cy-',~ lindrical record carriers, errors may occur in the centring or in the tangential guidance of the read spot ' relative to the track to be read, so that also in these cases a deflection element 12 according to the invention may be employed. Of course, the in~ormation stored on the record carrier' may be other than a television programme.
' `' '.
P~ 7372 18.3.74 ~ o~36 Fig. 4 illustrates a possible method of sus-pendin~ a lens system according to the invention. By means of two rods 40 and 42 the lens 13 is moved in a direction perpendicular to the plane of drawing. Said rods are rotatable in the bearings 41 and 43. The lens 14 is movable in the direction indicated by arrows 54 in the plane of drawing. Furthermore, two rods 44 and 1l5 are attached~to said lens, which rods are disposed before and behind the plane of drawing. Only the rod 44 which is disposed behind the plane of drawing is shown. The connection line of the bearings 41 and 43 and the connection line of the bearings in which the rods 44 and 45 are mountcd pass throllgh the centres of curvature M13 and M1l~ respectively, which substan-tially coincide.
The lenses 13 and 14 can be moved with the aid of magnet fields as is shown in Figs. 5 and 6.
- Fig. 5 is a section taken on the line 5, 5~ of Fig. 4, whilst ~ig. 6 is a section on the line 6, 6~ of Fig. 4.
On the lens 13 filamentary windings 46 and 47 are dis-~ ~ posed. Said filamentary windings are located in a per-,~ manent-magnet field which is produced by two magnet poles 48 and 49. The control signal Sr for the radial position of the read spot, derived in the apparatus of Fig. 2, may be applied to the filamentary windings 46 and 47. By means of the signal St the lens 13 in Fig.
,. . .
~ i 5 can be moved to the left or to the right.
f ~ , .
~' ~ ' ..
~ 16 ..
,. . . . . . . . . .. . . ..
18.3.74 ~0~636 The control signal St of the tangential guidance of the read spot may be applied to the windings 50 and 51, which are disposed on the lens 14, and which are located in a magnetic field produced by the magnet poles 52 ancl 53. By mQans of thQ signal St the lens 14, in Fig. 6, can be moved upwards or downwards.
; By means of the rods 40 and 41, and 44 and ll5 respectively the movement to the left or to the right in Fig. 5 or the up or down movemcnt in Fig. 6, can be converted into a rotation o:f the lens 13 about the centre of curvature M13, or of the lens 14 about the celltre oi cuFv~tu~a M1l~ resp~CtiV~lY~
~' ' ' ' ` .
:.
.
18.3.74 ~04~i36 errors can be prevented in a simple manner by including a single positive lens 16 inthe radiation path as a cor-rection element. The lens 16 may be disposed both in front of' and behind the lens system 12. Preferably, i~
is disposed between the mirror 11 and the lens system 12, so that said system 12 may be placed closc to the objective 15. By using mater:;als with a high refractive index (n = 1.7 for example) for the lenses 13 and 14, a certain deflection of the radiation beam can be ob- , tained at smaller wedge anglesC~ and ~ , than ir the , lenses are made of materials with a low refractive index.
The f`act that the invention has been described `, with reference to the apparatus of Fig. 2, by no means implies that the scope of the invention is limited to ' said specific appara,tus. The signals Sr and St can be obtained in various manners, but the manner in which falls beyond the scope of the present invention. Also when reading other than disc-shaped record carriers, ,~ 2 20 such as record carriers in the form of a tape or cy-',~ lindrical record carriers, errors may occur in the centring or in the tangential guidance of the read spot ' relative to the track to be read, so that also in these cases a deflection element 12 according to the invention may be employed. Of course, the in~ormation stored on the record carrier' may be other than a television programme.
' `' '.
P~ 7372 18.3.74 ~ o~36 Fig. 4 illustrates a possible method of sus-pendin~ a lens system according to the invention. By means of two rods 40 and 42 the lens 13 is moved in a direction perpendicular to the plane of drawing. Said rods are rotatable in the bearings 41 and 43. The lens 14 is movable in the direction indicated by arrows 54 in the plane of drawing. Furthermore, two rods 44 and 1l5 are attached~to said lens, which rods are disposed before and behind the plane of drawing. Only the rod 44 which is disposed behind the plane of drawing is shown. The connection line of the bearings 41 and 43 and the connection line of the bearings in which the rods 44 and 45 are mountcd pass throllgh the centres of curvature M13 and M1l~ respectively, which substan-tially coincide.
The lenses 13 and 14 can be moved with the aid of magnet fields as is shown in Figs. 5 and 6.
- Fig. 5 is a section taken on the line 5, 5~ of Fig. 4, whilst ~ig. 6 is a section on the line 6, 6~ of Fig. 4.
On the lens 13 filamentary windings 46 and 47 are dis-~ ~ posed. Said filamentary windings are located in a per-,~ manent-magnet field which is produced by two magnet poles 48 and 49. The control signal Sr for the radial position of the read spot, derived in the apparatus of Fig. 2, may be applied to the filamentary windings 46 and 47. By means of the signal St the lens 13 in Fig.
,. . .
~ i 5 can be moved to the left or to the right.
f ~ , .
~' ~ ' ..
~ 16 ..
,. . . . . . . . . .. . . ..
18.3.74 ~0~636 The control signal St of the tangential guidance of the read spot may be applied to the windings 50 and 51, which are disposed on the lens 14, and which are located in a magnetic field produced by the magnet poles 52 ancl 53. By mQans of thQ signal St the lens 14, in Fig. 6, can be moved upwards or downwards.
; By means of the rods 40 and 41, and 44 and ll5 respectively the movement to the left or to the right in Fig. 5 or the up or down movemcnt in Fig. 6, can be converted into a rotation o:f the lens 13 about the centre of curvature M13, or of the lens 14 about the celltre oi cuFv~tu~a M1l~ resp~CtiV~lY~
~' ' ' ' ` .
:.
.
Claims (3)
OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. An apparatus for reading a record carrier on which information is stored in an optically readable structure of areas and intermediate areas which are arranged along tracks, which apparatus includes a ra-diation source which supplies a read beam, an objective system for focussing the read beam to a read spot on the optical structure of the record carrier, a beam deflect-ing element for moving the read spot in at least one of the directions: transverse to the track direction and coincident with the track direction, and a radiation-sensitive detection system for converting the read beam which is modulated by the areas and intermediate areas into an electrical signal, characterized in that the beam-deflecting element consists of an assembly of a plano-concave lens and a plano-convex lens, that the concave surface of the first lens faces the convex surface of the second lens, which surfaces have sub-stantially the same radius of curvature and are dis-posed at a distance which is substantially smaller than said radius of curvature, and at least one of the lenses being rotatable in such a way that the axes of rotation of the lenses are mutually perpendicular and are disposed in a plane perpendicular to the optical axis of the objective system, so that the projections of said axes onto the record carrier extend in the track direction and transverse to the track direction.
2. An apparatus as claimed in Claim 1, charac-terized in that the radiation path from the radiation source to the beam deflecting element includes a cor-rection element in the form of a single positive lens.
3. An apparatus as claimed in Claim 1 or 2, characterized in that the lenses are rigidly secured by rods, which are rotatable in bearings, the line between the bearings associated with a lens passing through the centre of curvature of the curved surface of the rele-vant lens, and that on each lens filamentary windings are disposed which face magnet poles, a control signal for centring the read spot relative to a track to be read being applied to the filamentary windings of one of the lenses, and a control signal for positioning the read spot in the longitudinal direction of a track to be read to the filamentary winding of the other lens.
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| NLAANVRAGE7401937,A NL171641C (en) | 1974-02-13 | 1974-02-13 | DEVICE FOR READING A REGISTRATION BEARER ON WHICH INFORMATION IS INCLUDED IN AN OPTICALLY READABLE STRUCTURE. |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1046636A true CA1046636A (en) | 1979-01-16 |
Family
ID=19820737
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA219,523A Expired CA1046636A (en) | 1974-02-13 | 1975-02-06 | Plano-concave and plano-convex lens system for deflecting read beam in optical recording system |
Country Status (17)
| Country | Link |
|---|---|
| US (1) | US3924063A (en) |
| JP (1) | JPS5622062B2 (en) |
| AT (1) | AT337268B (en) |
| AU (1) | AU496928B2 (en) |
| BE (1) | BE825416A (en) |
| CA (1) | CA1046636A (en) |
| CH (1) | CH584418A5 (en) |
| DE (1) | DE2503952C2 (en) |
| DK (1) | DK137355B (en) |
| ES (1) | ES434625A1 (en) |
| FR (1) | FR2260843B1 (en) |
| GB (1) | GB1502151A (en) |
| IT (1) | IT1031582B (en) |
| NL (1) | NL171641C (en) |
| NO (1) | NO750418L (en) |
| SE (1) | SE398789B (en) |
| ZA (1) | ZA75234B (en) |
Families Citing this family (18)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| FR2280150B1 (en) * | 1974-07-26 | 1977-01-07 | Thomson Brandt | DEVICE FOR DETECTION OF FOCUS DEVIATION OF AN OPTICAL READING HEAD FROM A READING SURFACE |
| NL182594C (en) * | 1975-03-12 | 1988-04-05 | Philips Nv | PLAYING DEVICE, IN PARTICULAR VIDEO PLAYER, FOR PLAYING A PLATE PROVIDED FROM TRANSPARENT MATERIAL, AND ADJUSTING PLATE FOR USE WITH THIS DEVICE. |
| NL181060C (en) * | 1975-10-03 | 1987-06-01 | Philips Nv | ELECTROMAGNETICALLY CONTROLLABLE RADIUS DEFLECTION DEVICE. |
| NL176315C (en) * | 1976-05-12 | 1985-03-18 | Philips Nv | OPTICAL SCANNER. |
| NL7703123A (en) * | 1977-03-23 | 1978-09-26 | Philips Nv | DEVICE FOR READING AN OPTICAL RADIATION-REFLECTING REGISTRATION CARRIER. |
| NL7703232A (en) * | 1977-03-25 | 1978-09-27 | Philips Nv | OPTICAL SCANNER. |
| JPS5826088B2 (en) * | 1977-12-12 | 1983-05-31 | 日本ビクター株式会社 | Movable lens device in optical information recording medium disc playback device |
| US4385373A (en) * | 1980-11-10 | 1983-05-24 | Eastman Kodak Company | Device for focus and alignment control in optical recording and/or playback apparatus |
| US4432085A (en) * | 1981-10-30 | 1984-02-14 | Rca Corporation | Dual input telescope for multi-beam optical record and playback apparatus |
| JPH0675298B2 (en) * | 1981-12-19 | 1994-09-21 | 富士通株式会社 | Optical information reproducing device |
| JPS58114334A (en) * | 1981-12-26 | 1983-07-07 | Fujitsu Ltd | Optical head |
| US4550249A (en) * | 1983-04-21 | 1985-10-29 | At&T Bell Laboratories | Optical disc read/write apparatus |
| JPS60131648A (en) * | 1983-12-19 | 1985-07-13 | Toshiba Corp | Optical head |
| US4760565A (en) * | 1986-09-15 | 1988-07-26 | International Business Machines Corporation | High speed track access for optical disks using acousto-optic deflector |
| US4827365A (en) * | 1987-07-24 | 1989-05-02 | Olympus Optical Co., Inc. | Assembly for accessing information recording medium |
| US5090002A (en) * | 1989-03-07 | 1992-02-18 | International Business Machines Corporation | Positioning systems employing velocity and position control loops with position control loop having an extended range |
| EP0996951A2 (en) * | 1998-05-13 | 2000-05-03 | Koninklijke Philips Electronics N.V. | Device for optically scanning a record carrier |
| JP3666632B2 (en) * | 1998-12-11 | 2005-06-29 | パイオニア株式会社 | Aberration correction apparatus and optical pickup apparatus using the same |
Family Cites Families (5)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US3381086A (en) * | 1962-08-16 | 1968-04-30 | Minnesota Mining & Mfg | Reproduction of television signals from photographic disc recordings |
| US3302543A (en) * | 1963-10-21 | 1967-02-07 | Karl F Ross | Wide-angle dioptric system |
| US3530258A (en) * | 1968-06-28 | 1970-09-22 | Mca Technology Inc | Video signal transducer having servo controlled flexible fiber optic track centering |
| FR2174353A5 (en) * | 1972-02-29 | 1973-10-12 | Thomson Csf | |
| US3829622A (en) * | 1972-10-24 | 1974-08-13 | Mca Disco Vision | Video disc player with variably biased pneumatic head |
-
1974
- 1974-02-13 NL NLAANVRAGE7401937,A patent/NL171641C/en active Search and Examination
- 1974-04-29 US US464732A patent/US3924063A/en not_active Expired - Lifetime
-
1975
- 1975-01-13 ZA ZA00750234A patent/ZA75234B/en unknown
- 1975-01-31 DE DE2503952A patent/DE2503952C2/en not_active Expired
- 1975-02-06 CA CA219,523A patent/CA1046636A/en not_active Expired
- 1975-02-10 DK DK48175AA patent/DK137355B/en unknown
- 1975-02-10 NO NO750418A patent/NO750418L/no unknown
- 1975-02-10 GB GB5489/75A patent/GB1502151A/en not_active Expired
- 1975-02-10 AT AT97975A patent/AT337268B/en not_active IP Right Cessation
- 1975-02-10 IT IT20090/75A patent/IT1031582B/en active
- 1975-02-10 AU AU78027/75A patent/AU496928B2/en not_active Expired
- 1975-02-10 SE SE7501409A patent/SE398789B/en unknown
- 1975-02-10 CH CH156575A patent/CH584418A5/xx not_active IP Right Cessation
- 1975-02-11 ES ES434625A patent/ES434625A1/en not_active Expired
- 1975-02-11 BE BE153261A patent/BE825416A/en unknown
- 1975-02-13 JP JP1752775A patent/JPS5622062B2/ja not_active Expired
- 1975-02-13 FR FR7504507A patent/FR2260843B1/fr not_active Expired
Also Published As
| Publication number | Publication date |
|---|---|
| IT1031582B (en) | 1979-05-10 |
| US3924063A (en) | 1975-12-02 |
| FR2260843A1 (en) | 1975-09-05 |
| JPS5622062B2 (en) | 1981-05-22 |
| JPS50115842A (en) | 1975-09-10 |
| FR2260843B1 (en) | 1978-02-03 |
| SE7501409L (en) | 1975-08-14 |
| AU496928B2 (en) | 1978-11-09 |
| DK137355B (en) | 1978-02-20 |
| DK48175A (en) | 1975-10-06 |
| DE2503952C2 (en) | 1982-12-09 |
| ES434625A1 (en) | 1977-02-01 |
| DK137355C (en) | 1978-07-17 |
| SE398789B (en) | 1978-01-16 |
| NL171641C (en) | 1983-04-18 |
| NO750418L (en) | 1975-08-14 |
| ATA97975A (en) | 1976-10-15 |
| DE2503952A1 (en) | 1975-08-14 |
| BE825416A (en) | 1975-08-11 |
| NL7401937A (en) | 1975-08-15 |
| ZA75234B (en) | 1976-08-25 |
| CH584418A5 (en) | 1977-01-31 |
| AU7802775A (en) | 1976-08-12 |
| AT337268B (en) | 1977-06-27 |
| GB1502151A (en) | 1978-02-22 |
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