Position Detection System for an Object With at Least Five Degrees of Freedom

The invention relates to a position detection system for detecting displacements, with respect to said system, of an object along the three axes of an orthogonal system of coordinates, and rotations of this object about two of these three axes, said object having a reflecting reference face on which a spherical element is provided, said system comprising a radiation source for supplying a measuring beam, an optical system for forming a first measuring sub-beam, focused in the center of curvature of the spherical element, for measuring said displacements, and a second measuring sub-beam, incident substantially outside the spherical element on the reference face, for measuring said rotations, and a radiation sensitive detection system arranged in the path of the measuring sub-beams reflected by the reference face and passing through the optical system which radiation-sensitive detection system comprises a first detector for the first measuring sub-beam and a second detector for the second measuring beam, the optical system being provided with a beam splitter towards the reference face.

The invention also relates to an optical apparatus for scanning a medium, which apparatus is provided with such a position detection system. The invention also relates to a mirror polygon used as an object in the position detection system and in the apparatus.

The term medium should be considered to have a wide meaning and comprises, for example a tape-shaped, record carrier, a photosensitive layer in a printer, or an image display panel which can be written with electromagnetic radiation and in which the information is constituted by the written image.

Since the introduction of the digital optical recording technique, there has been an increasing need of raising the storage capacity of the medium used so that, for example, a digital video signal can be stored on such a medium. In the known audio disc, or compact disc (CD) and media derived therefrom, such as CD-ROM, CD-I, etc., the storage capacity is determined by the size of the scanning spot formed in the information plane of these media, which scanning spot determines the resolving power of the scanning device and hence the minimum dimensions of the information details, for example information pits which can still be detected separately. It is true that the size of the scanning spot can be reduced by decreasing the wavelength of the scanning beam used and/or by increasing the numerical aperture of the objective system with which the scanning spot is formed, but this is only possible to a limited extent and cannot lead to an increase of the storage capacity by a factor of ten or more.

As described in the article "A Compact Optical Tape Recording System" in: SPIE, Vol 2338 Optical Data Storage, 1994, pp. 8-14, the storage capacity and scanning velocity may be increased by several orders of magnitude while maintaining the conventional scanning spot size by making use of a tape-shaped record carrier which, viewed in the longitudinal direction, is moved along an optical scanning device and in which the information is provided in information tracks extending in a direction perpendicular to the longitudinal direction of the tape. To scan these information tracks, the scanning device comprises a scanning element in the form of a mirror polygon having, for example six mirror faces, or facets. Upon rotation of the mirror polygon, each facet of the consecutive mirror facets ensures that an information track is scanned, hence is written or read.

In the scanning device described in the SPIE article, the mirror polygon is journaled and driven electromagnetically. The position of the mirror polygon in five degrees of freedom is continuously detected so as to be corrected, if necessary. To this end, the known device comprises a position detection system in which a radiation beam is generated, a part of which is reflected as a measuring beam by a beam splitter to the upper side of the polygon which functions as a reference face. A special lens which focuses the peripheral portion of the measuring beam in the center of curvature of a spherical element provided on the central part of the reference face is arranged between the beam splitter and the reference face. This peripheral portion constitutes a first measuring sub-beam. A second measuring sub-beam is constituted by the central portion of the measuring beam on which the lens does not perform a lens action, but gives only the chief ray of this measuring sub-beam a different direction than that of the chief ray of the first measuring sub-beam. The first and second measuring sub-beams reflected by the reference face are passed by the beam splitter to a first and a second detector, respectively, which detectors convert the beams into electric signals. Each of these detectors is constituted by a quadrant detector, i.e. a detector consisting of four detector segments arranged in four different quadrants.

In the known position detection system, only a part, for example half, of the measuring beam radiation is reflected towards the reference face and used for the measurements, while the rest of this radiation is lost. Moreover, the lens used therein for focusing the measuring beam is a very unconventional lens which is difficult to manufacture and renders the system expensive. Moreover, only the rotations about the X and Y axes are measured with this detection system and the rotation about the Z axis is not measured, i.e. the axis about which the mirror polygon must rotate to realize the desired scanning.

It is an object of the invention to provide a position detection system in which a normal lens can be used for focusing and in which also better use is made of the available measuring beam radiation. To this end, the position detection system according to the invention is characterized in that the optical system is further provided with a reflector which reflects the part of the measuring beam passed by the beam splitter as one of the measuring sub-beams towards the reference face, and in that the beam splitter reflects the other part of the measuring beam as the other measuring sub-beam towards the reference face.

In this position detection system, the measuring sub-beams are no longer formed by a lens having a special shape but by the beam splitter, so that it has a dual function and does not necessitate the use of a specially formed lens. Moreover, a maximum quantity of radiation of the measuring beam is now used for the measurements so that stronger measuring signals are obtained. The beam may be a polarization-neutral beam splitter, but it may alternatively be constituted by the combination of a radiation-sensitive beam splitter and a quarter-wavelength plate.

A preferred embodiment of the position detection system is further characterized in that the second measuring sub-beam also measures the rotation of the object about the third axis, and to this end, the reference face is provided with at least an area having a reflection which is different from its surroundings.

This embodiment is particularly important in a scanning apparatus for a tape-shaped record carrier. Since the position detection system also supplies a measuring signal which represents the rotation in the Z axis, or the axis of rotation of the mirror polygon, it is not necessary to use a separate detection system for generating this signal.

A first embodiment of the position detection system according to the invention is further characterized in that the reflector extends at such an angle to the chief ray of the measuring beam that the measuring sub-beam reflected by the reflector is incident on the reference face at an acute angle to the normal, in that said measuring sub-beam is the second measuring sub-beam, and in that a converging lens system is arranged in the path of the first measuring sub-beam reflected by the beam splitter.

Since the second measuring sub-beam is incident on the reference face at a small angle to the normal, the chief ray of this measuring sub-beam, after reflection by the reference face, has a different direction than the chief ray of the first measuring sub-beam, so that these beams are sufficiently separated in the detection plane to enable them to be incident on separate detectors.

A preferred embodiment of the position detection system, having a larger capturing range than the first embodiment, is further characterized in that the beam separating face of the beam splitter extends at such an angle to the chief ray of the measuring beam that the measuring sub-beam reflected by this face is incident on the reference face at an acute angle to the normal, in that said measuring sub-beam is the second measuring sub-beam, and in that a converging lens system is arranged in the path of the first measuring sub-beam reflected by the reflector.

In this embodiment, both measuring sub-beams pass obliquely through the system after reflection by the beam splitter so that, with respect to the first embodiment, the distance between the radiation spots formed by the beams in the detection plane is doubled so that a large capturing range can be realized without vignetting occurring on lens elements in the system.

The invention also relates to an optical apparatus for electromagnetically scanning a medium, which apparatus is provided with an optical scanning device which, in operation, scans the medium in at least one direction by means of an electromagnetic radiation spot and is provided with a rotating scanning element, as well as a position detection system for this scanning element. This apparatus is characterized in that the position detection system is a system as described hereinbefore and the object is the scanning element.

This apparatus may be, for example a laser printer in which a scanning laser beam is used for initially providing signs, symbols or graphic representations in a photosensitive layer which is subsequently passed through an ink bath and then printed on paper. In the laser printer, a scanning spot moves with great velocity across the successive lines and the transition from a first line to the next is realized by moving, at a smaller velocity, the photosensitive layer in a direction perpendicular to the radiation spot scanning direction.

The apparatus may also be a picture display apparatus which is provided with at least a reflecting liquid crystalline picture display panel comprising a photoconducting layer. The picture to be displayed is written by scanning the photoconducting layer by means of the scanning spot in accordance with a matrix pattern, the beam forming the scanning spot being modulated in conformity with the picture information to be displayed. For scanning in a first direction at a high velocity, the above-mentioned fast scanning element is used for which the position is controlled by means of the position detection system described hereinbefore. A second scanning element, for example a vibrating mirror may be used for the slower scanning in the second direction.

The invention finds a special application in an apparatus in which the medium is a tape-shaped optical record carrier for recording a structure of optically detectable information areas arranged in information tracks which extend in a first direction transversely to the longitudinal direction of the tape, said apparatus being provided with a supply reel and a take-up reel for transporting the tape in a second direction being the longitudinal direction of the record carrier, and a radiation source for supplying a scanning beam, the scanning element being constituted by a rotatable mirror polygon having mirror facets arranged around its circumference and realizing a scanning movement of the scanning beam in the first direction. The position of this mirror polygon is controlled by the position detection system as described hereinbefore.

The invention also relates to a mirror polygon to be used as an object in the above-mentioned position detection system or optical apparatus, the side faces of said polygon being constituted by reflective facets and the upper or lower face used as a measuring reference face being reflective and provided with a spherical element. According to the invention, the reference face is provided with at least one area having a reflection which is different from its surroundings.

By providing an area or strip or a pattern of strips with a deviating reflection on the reference face, the detector signal generated by the second measuring sub-beam is periodically modulated, from which the speed of rotation or the frequency can be derived.

These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.

In the drawings:

FIG. 1 shows the circuit diagram of an apparatus for scanning an optical tape,

FIG. 2 shows an embodiment of a radiation source detection unit used in this apparatus,

FIG. 3 shows the principle of the optical detection system in accordance with the invention for checking the position of the mirror polygon in this apparatus,

FIG. 4 shows an embodiment of a strip pattern to be provided on the reference face,

FIG. 5 shows the rotation speed signal obtained by means of this pattern,

FIGS. 6 and 7 show different embodiments of the position detection system according to the invention,

FIG. 8 shows the circuit diagram of a laser printer in which the position detection system can be used,

FIG. 9 shows the circuit diagram of a picture display apparatus with a picture display panel which can be written by means of radiation and in which the position detection system is usable, and

FIG. 10 shows the construction of such a panel.

In FIG. 1, the reference numeral 1 denotes a tape-shaped record carrier. This record carrier is directly transported from a supply reel 3 to a take-up reel 2 across a stationary guiding element 4. The apparatus does not have to comprise any further tape-guiding elements. Both reels are driven by separate motors (not shown). The motors may be driven in such a way that the tape tension remains constant. The tape travel direction is denoted by means of the arrow 5.

The scanning device of the apparatus comprises a radiation source detection unit 10 which supplies a scanning beam b, a rotating mirror polygon 20 which reflects the, for example parallel, beam to an objective lens 30 focusing the beam to a radiation spot V on the tape. The mirror polygon comprises, for example ten mirror facets f.sub.1 -f.sub.10 which are, for example parallel to the axis of rotation of the mirror polygon. During operation, this polygon rotates in the direction of the arrow 22. Each facet rotating in the radiation path of the beam, facet f.sub.2 in the drawing, will move the beam in the direction of the arrow 25, perpendicularly to the tape travel direction 5, across the entrance pupil of the objective lens. The radiation spot V formed by this lens then scans a track extending in the direction perpendicular to the direction 5. A second, a third, etc. track are consecutively scanned by means of the facets f.sub.1, f.sub.10, etc.

The beam coming from the unit 10 and incident on a mirror facet is located in the plane defined by the scanning beam coming from the mirror polygon and extends at an angle of, for example 38.degree. to the central position of the scanning beam which is moved, for example through an angle of 48.degree.. The objective lens, in the form of an f-.theta. lens has, for example an effective focal length of 1.25 mm and a numerical aperture of 0.45. The scanning spot can then be moved, for example through a distance of 1 mm in the vertical direction. In this way, it is possible to write and read tracks having a length of 1 mm in the direction perpendicular to the tape travel direction.

A plurality of horizontal strips of vertical information tracks may be written on a tape. To this end, tracks with a length of 1 mm are first written from the beginning to the end of the tape. Then the travel direction of the tape is reversed, the tape and the optical system are displaced through a distance of slightly more than 1 mm with respect to each other and the next horizontal strip of vertical tracks is written. Thus, 12 strips with information tracks can be provided on a tape having a width of 12.7 mm. The apparatus is also suitable for recording tapes having a width of 8 mm. Reading a written tape is effected in a manner analogous to that for writing. Then the beam reflected by the tape traverses the same optical path in the reverse direction to the radiation source detection unit. The information signal, the focus error signal and the tracking error signal are obtained in a similar way as in an optical audio disc (CD) player.

The radiation source detection unit comprises a high power diode laser having a wavelength of, for example 780 nm. If the objective lens has an NA of 0.45, a resolving power which is comparable to that of the Compact Disc system is obtained. Then an information density of 1 bit/.mu.m can be achieved, and a tape having a width of 12.7 mm and a length of 42 m may comprise 50 Gbytes of information.

The information density in the track direction is, for example 0.6 .mu.m/bit so that a track may comprise approximately 1600 bits. The nominal rotation frequency of the mirror polygon is, for example 2000 revolutions per sec. The scanning frequency of a mirror polygon with ten facets is then 20 kHz. At 1600 bits per track, a bitrate of 32 Mbits per second is achieved. The track period is, for example of the order of 1.6 .mu.m. At a scanning frequency of 20 kHz, the tape speed is then 3.2 cm/sec during reading and writing. This is a relatively low speed so that no complicated tape transport mechanism is required.

FIG. 2 shows a cross-section of an embodiment of the radiation source detection unit 10. This embodiment resembles that used in the Compact Disc systems. The unit comprises a radiation source 11 in the form of a diode laser supplying a diverging beam b. This beam is split by a diffraction grating 12 into a central main beam, which is used for writing and reading information, and two auxiliary beams (not shown) which are used for tracking. These beams traverse a polarization-sensitive beam splitter 13 and are converted into parallel beams by a collimator lens 14. Via a quarter-wavelength plate 16, the beams leave the radiation source detection unit on their way to the mirror polygon 20. A folding mirror 15 for reducing the length of the unit may be arranged between the collimator lens 14 and the plate 16. If necessary, a telescopic system consisting of an objective lens 17 and a collimator lens 18 may be arranged behind the plate 16 so as to reduce the beam diameter to a desired value of, for example 1.1 mm. After reflection on the mirror polygon, the beams are focused by the objective lens 30 on the tape so that a main spot, or scanning spot, and two auxiliary spots are produced on this tape.

After passing the objective lens 30, the beams reflected by the tape are reflected to the unit 10 by the mirror polygon and traverse this unit in the reverse direction as far as the beam splitter 13. There, the beams are reflected from the path of the on-going beams in a direction perpendicular to the plane of the drawing so as to reach the radiation-sensitive detection system 60. A cylindrical lens 19 with which notably the main beam can be rendered astigmatic is arranged between the beam splitter 13 and the system 60. This beam is incident on a four-quadrant detector with which the shape of the main spot formed in the detection plane can be determined, which main spot shape is determined by the extent of focusing of the beam on the tape. For tracking, two auxiliary beams, which cooperate with separate detectors, are used in this embodiment, because a satisfactory tracking error detection can be realized therewith, independent of the nature of the information bits in the tracks. As the facets of the mirror polygon are parallel to the axis of rotation of this polygon, the auxiliary spots move along straight lines parallel to the path traversed by the main spot. Consequently, use can be made of the standard three-spot tracking system as used in the Compact Disc systems.

It is to be noted that the radiation source detection unit shown in FIG. 2 is only one of the possible embodiments. This unit may be modified in various ways known from the Compact Disc technology. For example, for the focus detection, the cylindrical lens may be replaced by a roof prism which splits the beam reflected by the tape into two sub-beams each cooperating with a detector pair. The position of each sub-beam with respect to the associated detector pair is then a measure of focusing. The tracking signal may not only be generated by using two auxiliary spots, but may alternatively be generated from the scanning beam only, by splitting the detector for this beam into two parts. By subtracting the signals of the detector from each other, a tracking signal is obtained. This method is known as the push-pull method. The beam splitter 13 may be replaced by a diffraction grating which deflects the scanning beam reflected by the tape towards the detection system. This grating may be divided into two parts or have an astigmatic implementation.

The objective, or scanning lens 30 may be arranged in an actuator (not shown) so that this lens can be moved in two directions, one movement serving for focus setting and the other for tracking. In order to define an average focus position, the tape is transported via the guiding element 4. However, a low-frequency change of the distance between the tape surface and the lens 30 may then still occur, for example, due to variations in tape thickness. This may be compensated by said first movement, so that the distance between the lens and the tape surface can be varied in such a way that the focal plane of the lens always coincides with the tape surface. This focus control may have a very small bandwidth. If the focal plane of the lens 30 is tilted with respect to the tape surface, a focus tilt correction must be performed. To this end, the guiding element may have a tiltable implementation.

When writing an empty tape, the objective lens is fixed in a stationary position. The system has a sufficiently high stability to write perfectly straight tracks with a constant track period. No vibrations at a frequency of the order of the scanning frequency occur in the system.

When reading a written tape, three unwanted relative movements of the scanning spot and a scanned track might occur. The first is a movement of the tape in a direction parallel to the track direction. This movement is not troublesome because the scanning path length is chosen to be larger than the track length. This ensures that the track is always within the scanning area. The only effect of this movement is that a small delay occurs in the signal which has been read. The second unwanted movement is caused by small variations of the tape travel speed. These variations may result in a mean tracking error. During reading and rewriting the tape, this error can be compensated by the second movement of the lens made possible by the lens actuator, namely a rotation of the lens about an axis which is parallel to, and preferably located in, the rear focal plane of this lens. This axis has a direction which is parallel to the track direction. In this way it is possible to shift the scanning path in a direction perpendicular to the track direction for the purpose of compensating said tracking error. The third unwanted movement is caused by a tilt of the tracks with respect to the scanning path. To compensate this, the scanning path may be tilted in a corresponding manner by tilting the mirror polygon.

In this way, all irregular tape movements can be compensated by active servocontrols so that a very accurate tape-guiding system with microprecision is not necessary. The tape transport mechanism is very simple: the tape is directly transported from one reel to the other via only one guiding element 4.

The mirror polygon is journaled electromagnetically and may move in six degrees of freedom. These movements must be detected so that they can be corrected, if necessary. The invention provides a position detection system with which movements of the mirror polygon along three axes and tilts about two of these axes can be measured. This system also provides the possibility of measuring the rotation of the mirror polygon about its rotation axis. This system has a simple set-up and the available measuring radiation is used efficiently so that measuring signals having the maximum strength are obtained.

FIG. 3 illustrates the principle of the position detection system 80. The system is arranged at the side of the mirror polygon where the spherical element 23, see FIG. 1, is present. In FIG. 3, the reference numeral 33 denotes a diode laser emitting a radiation beam 40. This beam is first converted by a collimator lens 35 into a parallel beam. Subsequently, the beam 40 is incident on a splitting cube 36 with a separating face 37 which reflects a part of the measuring beam 40 as measuring sub-beam 50 towards the polygon. This polygon is denoted by the face 31, which is visible in FIG. 1 and is indicated hereinbefore as the reference face. This face is reflective and is provided in the center with a half sphere element 23 which is also shown in FIG. 1 and is also reflective. The part of the measuring beam 40 which is not reflected by the splitting cube is passed towards a reflector 38 which reflects this part as a measuring sub-beam 45 towards the spherical element 23 on the reference face 31. This, first, measuring sub-beam is focused by an objective lens 39 in the center of curvature of the spherical element 23. The measuring sub-beam reflected by the element 23 traverses the objective lens 39 again and is reflected by the reflector towards the beam splitter 36 which reflects a part of the beam 45 towards a radiation-sensitive detection system 60 comprising a plurality of detection elements. A lens 41 converting the measuring sub-beam into a converging beam 55 is arranged between the beam splitter and the detection system 60. When the mirror polygon is moved in the X direction, in the plane of the drawing, and in the Y direction, perpendicular to the plane of the drawing, the radiation spot formed by the first measuring sub-beam in the detection plane moves in the X and Y directions, respectively, with respect to the elements of the detection system 60. This movement can be measured by combining the output signals of the detection elements in known manner. A cylindrical lens 34 is arranged in the radiation path for measuring the movement of the polygon in the Z direction, in the plane of the drawing. This lens converts the diode laser beam into an astigmatic beam. After reflection by the reference face, such a beam forms a radiation spot in the detection plane, which spot has a shape which is dependent on the extent to which the beam is focused in the center of curvature of the spherical element 23. If the beam is sharply focused at this point, i.e. if the reference face has the correct position with respect to the position detection system, said radiation spot is round. When the position of the reference face deviates from the desired position, i.e. when the beam is no longer sharply focused in said center of curvature, said radiation spot has an elliptical shape. The shape of the radiation spot, and hence the Z position of the reference face of the mirror polygon, may be detected in known manner by means of a four-quadrant detector accommodated in the detection system 60.

Alternatively, the cylindrical lens 34 may be arranged between the beam splitter and the detection system 60, instead of between the radiation source 33 and the beam splitter 36. The neutral beam splitter 36 may also be replaced by the combination of a polarization-sensitive beam splitter and a quarter-wavelength plate. Then, in principle, all radiation from the reference face 31 is passed towards the detection system 60. Instead of a splitting cube which is polarization-sensitive or not sensitive, a plane-parallel plate having a separating layer which is polarization-sensitive or not sensitive may be arranged at an angle of 45.degree. to the chief ray of the beam 40. Such a plate also introduces astigmatism in the beam so that the cylindrical lens 34 is then no longer required.

The second measuring sub-beam 50 reflected by the beam splitter is incident as a parallel beam on a flat portion of the reference face 31. This beam is reflected by the reference face and a part thereof is passed by the beam splitter to the detection system 60, which beam is also converged by the lens 41. Upon tilting the mirror polygon and the reference face 31 about the X and/or Y axis, the radiation spot formed by the second measuring sub-beam in the detection plane is moved across the detection elements of the system 60 in the X and/or Y direction, so that these tilts can be measured.

By giving an area or strip of the flat portion of the reference face a lower or higher reflection coefficient than the rest of this face, as is denoted by means of the strip 24 in FIG. 1, the intensity of the second measuring beam can be decreased or increased for a short time interval so that a further measuring possibility is created for the detection system. If a single area having a deviating reflection is present, the intensity of the second measuring beam will be decreased or increased once per revolution of the mirror polygon, and the detector signal generated by this beam will have a minimum or maximum value once per revolution. Then, the number of revolutions of the mirror polygon per unit of time, second or minute, hence the speed of rotation or rotation frequency of the mirror polygon can be determined. Said area or strip may consist of a dark or dull area or strip, or of a diffusing area obtained by roughening or the provision of a grating. The size of this area is preferably equal to the cross-section of the second measuring sub-beam for obtaining a maximal effect.

Instead of a single area or strip, a plurality of radially varying strips or lines may be provided on the flat portion of the reference face. Then, the detector signal generated by the second measuring sub-beam has a corresponding number of minima or maxima per revolution. The relative angle of rotation of the mirror polygon can then be measured. The number of strips is determined by the desired accuracy with which the angle of rotation is to be measured, and by the size of the radiation spot formed by the second measuring sub-beam on the reference face. Each strip may consist of a dark or dull line or of a diffusing groove.

Instead of providing a strip or a number of strips having a deviating reflection in the reference face, for the purpose of measuring the speed of rotation, the reference face may be implemented in such a way that it has a monotonous linear variation of the reflection through 360.degree.. Then, the detector signal is a sawtooth-shaped signal with a period corresponding to one revolution. The linearly increasing or decreasing reflection can be obtained by linearly varying the extent of dullness or by linearly varying the strip density if the reference face is provided with strips through 360.degree..

FIG. 4 shows a strip pattern to be provided on the reference face for measuring the speed of rotation, which is specially suitable for combination with the polygon drive motor described in the above-mentioned article "A Compact Optical Tape Recording System" in: SPIE, VOL 2338, Optical Data Storage, 1994, pp. 8-14, and which is provided with two layers of four magnet coils each. As is shown in FIG. 4, the strip structure comprises two groups of strips 85 and 86 each extending through 180.degree.. In each group, the density of the strips 87 initially increases and then decreases. If such a strip pattern rotates under the second measuring sub-beam, the intensity of this beam will vary and hence the detector signal generated by this beam, as is shown in FIG. 5. In this Figure, the time t is plotted horizontally and the value of the detector signal So is plotted vertically in arbitrary units. The time interval between the vertical lines 83 and 84 corresponds to one revolution. During a revolution, the sinusoidal signal So changes sign four times. This signal is very suitable for directly driving the polygon motor described in said SPIE article.

In the position detection system according to the invention, the first measuring sub-beam 45 is used for detecting displacements along the three axes, X, Y and Z, and the second measuring sub-beam 50 is used for measuring rotations about these three axes, in which both the radiation reflected by the beam splitter and the radiation passed by the beam splitter is used. A separate detection system for measuring the relative angle of rotation of the mirror polygon is then no longer required.

To separate the first and second measuring sub-beams from each other in a satisfactory manner, use is made of the embodiment of FIG. 6. In this embodiment, the first measuring sub-beam 45 is reflected by the beam splitter 36 towards the spherical element 23, while the second measuring sub-beam 50 is passed by the beam splitter towards the reflector 38. This reflector is arranged at such an angle, different from 45.degree. with respect to the chief ray of the beam 40, that the second measuring sub-beam 50 is incident on the reference face 31 at an angle different from 90.degree.. The beam 50 reflected by this face passes obliquely through the system on its way to the detection system 60, which beam is focused to a radiation spot 62 on the detection plane by the lens 41. The first measuring sub-beam 45 is perpendicularly incident on the spherical element and the reflected beam 45 is focused by the lens 41 in the radiation spot 63 in the detection plane. Since the beam 45 passes straight through the system and the beam 50 passes obliquely through the system and are notably perpendicularly or obliquely incident on the lens 41, the radiation spots 62 and 63 are separated from each other so that the separate detectors for these beams can be arranged at some distance from each other.

To have a sufficiently large capturing range for the various measuring signals, the detection elements of the system 60 must have a given size and the radiation spots 62 and 63 must be sufficiently spaced apart. In order to prevent that the distance between the beams 45 and 50 at the location of the lens 41 becomes so large that vignetting may occur on this lens, the embodiment of FIG. 7 is preferably used.

This embodiment largely corresponds to that of FIG. 3, but now the beam splitter 36 is arranged obliquely in the radiation path so that the beam-separating face 37 extends at an angle different from 45.degree. to the chief ray of the beam 40. It is thereby achieved that the second measuring sub-beam 50 reflected by the beam splitter is incident on the flat portion of the reference face 31 at an angle different from 90.degree.. The reflected beam 50 then passes obliquely through the system and the lens 41 focuses this beam in a radiation spot 62 which is located to the left of the system axis. The first measuring sub-beam 45 is perpendicularly incident on the spherical element 23. The reflected beam 45 is now obliquely incident on the separating face 37 of the beam splitter 36 and is consequently deflected to the right so that this beam is also obliquely incident on the lens 41. The radiation spot 63 formed by this beam in the detection plane is then located to the right of the system axis. The radiation spots 62 and 63 are now spaced apart so far that a sufficiently large capturing range is realized, while the beams 45 and 50 still enter the lens 41 sufficiently far from the edges of this lens.

The detection system 60 is shown in a bottom view in the upper part of FIG. 7. This system comprises two quadrant detectors 70 and 75 with detection elements 71, 72, 73 and 74 and 76, 77, 78 and 79, respectively. If the signals of the detection elements 71, 72, 73 and 74 are represented by a, b, c and d, and those of the detection elements 76, 77, 78 and 79 by p, q, r and s, then the displacements Mx, My and Mz in the X, Y and Z directions, respectively, are given by:

and the rotations .phi..sub.x, .phi..sub.y, .phi..sub.z about the X, Y and Z axes are given by:

in which the signal .phi..sub.z has, for example, a jump where a strip with a reduced reflection passes underneath the beam 50, or where the signal has a sinusoidal variation.

Instead of about the Y axis, as is shown in FIG. 7, the beam splitter 36 may alternatively be tilted about the chief ray of the beam 40 so as to achieve that the radiation spots 62 and 63 are spaced sufficiently far apart.

The invention may not only be used in a tape scanning apparatus but also in any scanning apparatus in which a rotating mirror polygon which can move in six degrees of freedom is used. A laser printer and a picture display apparatus may be mentioned as examples.

FIG. 8 shows the principle of a laser printer 90. In such a printer, a photosensitive layer is first written with a scanning laser beam. Subsequently, this layer is passed through an ink bath and then a print on paper is made. The photosensitive layer 92 may be wound around a roll 91 which is rotated about a shaft 93 for describing consecutive lines. The line scanning is realized by means of a mirror polygon 20 having, for example six reflecting facets f and rotating about a shaft 95. The reference numeral 30 denotes an objective lens, for example again an f-.theta. lens which focuses the radiation coming from a radiation source 11, for example, a high-power diode laser, and reflected by the mirror facets to a radiation spot V on the medium 92. The intensity of the laser beam is modulated in accordance with the information to be written by modulating the current through the diode laser or by means of a separate, for example acousto-optical or electro-optical modulator 96. To detect the position of the mirror polygon in six degrees of freedom, the apparatus is provided with a position detection system 80 which is implemented as described with reference to FIGS. 3, 6 and 7.

FIG. 9 shows the circuit diagram of a picture display apparatus 100 in which the picture is generated by a reflecting, radiation-sensitive, i.e. radiation-inscribable, picture display panel 110. The use of such a panel in an image projection apparatus is described in European Patent Application 0,517,517. The advantage of a radiation-inscribable panel as compared with a conventional active matrix panel is that a high light efficiency can be achieved therewith because it is not necessary to provide a matrix of electronic switches and conducting electrodes on the panel surface and because this panel absorbs hardly any radiation.

FIG. 10 shows the structure of the radiation-addressable panel 110. Viewed from the side of the write radiation 130, this panel comprises a first transparent substrate 111 of, for example glass, a transparent electrically conducting layer 112, preferably consisting of indium tin oxide (ITO), a photoconducting layer 113 having a high resistance in the dark and a good conductance if it is exposed and preferably consisting of silicon, possibly a light-obstructing layer 114, a light-reflecting layer 115 preferably consisting of a packet of dielectric layers having a thickness of a quarter wavelength, a first orientation layer 116, a layer of liquid crystalline material 117, a second orientation layer 118, a second transparent electrically conducting layer 119 and a second transparent substrate 120.

This panel is line sequentially scanned by a write beam 130 coming from a unit 125 accommodating a radiation source, preferably a laser, as well as a beam shaping optical system, the information to be displayed, for example a video signal, is applied to the laser so that the laser beam is intensity-modulated in conformity with this information. The laser beam 130 is incident on a rapidly rotating mirror polygon 20 and subsequently on a second scanning element 131 which moves more slowly and is formed by, for example a vibrating plane mirror or by a second mirror polygon. The scanning element 131 reflects the beam towards the panel 110. The mirror polygon 20 reflects the converging beam 130 in such a way that the radiation spot formed on the photosensitive layer of the panel describes a line. The second scanning element 131 ensures a relatively slow movement of this radiation spot in a second direction perpendicular to the line direction. Thus, the photosensitive layer 113 of the panel 110 is scanned in two dimensions and a two-dimensional matrix of pixels is written. The use of a mirror polygon for scanning a picture display panel by means of a write beam is known from the English language abstract of Japanese Patent Application 62-56931.

The electrodes 112 and 119 are connected to a voltage source 121. As long as the photoconducting layer 113 is dark, the electrode 112 is isolated from the liquid crystalline layer 117. At any position of the photoconducting layer 113, on which the beam 130 switched at the high intensity level is consecutively incident, the layer becomes conducting and an electric field is locally produced across the liquid crystalline layer so that the orientation of the molecules in this layer changes locally and thus the state of polarization of a corresponding part of a read or projection beam 145 incident from the left changes. This change of polarization is converted in known manner by means of a polarizing analyzer into an intensity variation in the beam 146 reflected by the panel.

As is shown in FIG. 9, the panel 110 with the write system may be used in an image projection apparatus. This apparatus is provided with an illumination unit comprising a radiation source 140 and a beam-shaping optical system 141, which unit supplies an illumination beam 145. This beam illuminates the panel 110 via a polarization-sensitive beam splitter 142. The image formed in this panel is projected on a projection screen 144 by the beam 146 reflected by the panel and by a projection lens 143.

The inscribable panel with the write system may alternatively be used in a direct-vision apparatus in which a viewer directly watches the panel.

To determine the movements of the mirror polygon 20 in six degrees of freedom, the position detection system 80 described above with reference to FIGS. 3, 6 and 7 may be used.

Also in the picture display system which is known as laser TV and with which an image is directly written on a projection screen or on a wall functioning as such by means of a scanning laser beam, or three laser beams in the case of a color image, the position detection system for a mirror polygon may be used. A laser TV apparatus is described in, for example European Patent Application 0,374,857. In such an apparatus, scanning must take place at a high velocity so that also a small mirror polygon is preferably used in this case which rotates at a high speed and free floating in vacuum as described in the above-mentioned SPIE article. The position of the mirror polygon in the laser TV apparatus should also be determined with great accuracy, for which again the position detection system according to the invention may be used.

Another use of the invention is in the field of scanning cameras, notably infrared cameras, in which an image of a scene or object formed by an objective lens is moved across a detector or a row of detectors. Such a camera is described in, for example U.S. Pat. No. 3,706,484. A rapidly rotating mirror polygon is used for line-sequentially moving the image across the detector. The position detection system according to the invention may be used to great advantage for measuring the positions of this polygon.

Mirror polygons are also used in apparatuses for inspecting objects or workpieces during or after their manufacture, or for reading codes, for example bar codes, on objects, so that the position detection system according to the invention may also be used in these apparatuses.