WO1998001786A1

WO1998001786A1 - Large-angle scan-objective system and scanning apparatus provided with such an objective system

Info

Publication number: WO1998001786A1
Application number: PCT/IB1997/000805
Authority: WO
Inventors: Josephus Johannes Maria Braat
Original assignee: Philips Electronics N.V.
Priority date: 1996-07-06
Filing date: 1997-07-01
Publication date: 1998-01-15
Also published as: JPH11513140A; NL1003530C2; EP0850434A1

Abstract

A large-angle scan-objective system (30) for transforming a collimated radiation beam varying in angular position to a scanning spot varying in location is described. The system comprises a first optical group (31, 32) for forming an intermediate spot varying in location, and a second optical group (33, 34, 35) for imaging the intermediate spot to the scanning spot. The first group is provided with an element (32) having a dioptrical power for correcting the image field curvature of the system. The second group preferably has a magnification of between -0.65 and -0.20.

Description

Large-angle scan-objective system and scanning apparatus provided with such an objective system.

The invention relates to a large-angle scan-objective system for transforming a collimated radiation beam varying in angular position to a scanning spot varying in location, comprising a first optical group having a positive power for forming an intermediate spot, varying in location, from the collimated beam, and a second optical group having a positive power for imaging the intermediate spot to the scanning spot. The invention also relates to a scanning apparatus provided with such an objective system. A collimated beam is understood to mean a parallel beam or a slightly-converging or slightly-diverging beam. An optical element or optical group having a positive power is an element or group that increases the convergence or decreases the divergence of an optical beam. The surface to be scanned may be a surface of an object whose dimensions or optically measurable properties must be determined. The surface may also be a medium for information storage or transmission such as a tape-shaped record carrier, a photosensitive layer in a printer or an image display panel inscribable with electromagnetic radiation, in which the information is formed by the written image. An optical tape-scanning apparatus may write and/or read information on an optical tape at great speed by means of an optical beam. When writing and reading with a rapidly rotating polygon used as a beam deflection unit, it is necessary to maintain a small size for the facets of the polygon. A small facet requires a small beam diameter d. Consequently, the scan angle θ of the beam must be chosen to be large. The product 20d of the beam varying in its angular position is converted by a scan-objective system into a value 2y(2NA) of a scanning spot varying in location, as is required at the area of the optical tape. In this case, 2y is the length of the scanning line, i.e. the field or scanning width on the tape. The numerical aperture NA at the tape side of the objective system is determined by the bit size on the tape. In an optical tape-scanning apparatus it holds that 2άθ = 4y(NA) « 1.0 mm. As a starting value for the optical design it holds that θ ~ 0.35, because d should not become too large due to the high number of revolutions per minute of the polygon. At the image side it is required that NA ■=• 0.50 due to the desired spatial density of the information on the tape. Consequently, the image field 2y is approximately equal to 1 mm. For the scan- objective this means that a large field angle must be used at the object side, while a large NA is required at the image side. Moreover, the entire system should be diffraction-limited, the (f-θ) relation during scanning should be reasonably taken into account, the image field should be flat and the imaging should be telecentric if the optical tape system is to operate in reflection A scan-objective system as described in the opening paragraph is known from the International Patent Application no. 93/24854. This objective system comprises a first optical group with five lenses for forming an intermediate spot varying in location. The intermediate spot is imaged by a telecentric second optical group with ten lenses. A drawback of the objective system is that the groups comprise many lenses. Moreover, the lenses are large and the objective system has small manufacturing tolerances, so that it is expensive.

It is an object of the invention to provide a large-angle scan-objective system with a flat image field and relatively simple optical groups. To this end, the scan- objective system according to the invention is characterized in that the first optical group is provided with an element for correcting the image field curvature of the scan-objective system. The known objective system corrects the image field curvature mainly in a double- Gauss system of the second group. The more image field curvature must be corrected, the larger the constriction, i.e. the ratio between the maximum and minimum beam diameter for the axis beam, in the double-Gauss system and, consequently, the larger the diameter of the lenses in the second group. In the known objective, the constriction is more than a factor of three. By largely or partially correcting the image field curvature according to the invention in the first group instead of in the second group, the second group may become considerably simpler, smaller and lighter. The element for the correction in the first group generally has a surface with a power which ensures an axial variation of the position of the scanning spot as a function of the position of the scanning spot on the scanning line. By performing the correction of the image field curvature in the first group, both the first and the second group may acquire a simple shape with smaller lenses and wide manufacturing tolerances. The element may be a refracting, reflecting or diffracting element. It may also be a refractive surface. A refractive element has preferably a negative power, a reflective element preferably a positive power. In the first-mentioned case, the element has a surface with which the correction takes place which is preferably located proximate to the intermediate spot. Since the surface then has the function of a field lens, the negative power of this surface may be used to change the axial position of the scanning spot without noticeably modifying the power of the objective system as a whole.

A reflecting element preferably has the shape of a concave mirror. The positive optical power of such an element may be used for forming an intermediate spot, varying in location, from the collimated beam. The image field curvature introduced by the mirror has a sign which is opposed to the image field curvature introduced by a positive lens. The mirror can thereby reduce the image field curvature of the entire objective system. The first group is preferably provided with an aspherical surface to perform the (f-θ) correction as satisfactorily as possible by means of a relatively low number of elements. In order to maintain the imaging quality throughout the image field well within the diffraction limit, an aspherical surface is preferably added in the second group proximate to the position in the second group where the pupil is imaged so as to give all beams a comparable correction for, inter alia, the spherical aberration.

The focal length f_j of the first group and the magnification M of the second group are preferably chosen to be such that the objective system is as compact as possible and has a minimum building length. This is important when the entire objective system should be fit in, for example an air bearing for actuation for the purpose of focusing or radial tracking. The magnification M is defined as the ratio between the image height and the object height. The following relation exists between the parameters {-. , M and the focal length f of the objective system: f = M f_l. The value of M preferably satisfies -0.65 < M < -0.20. This provides the possibility of maintaining the total volume of the objective system, hence of the first and the second group, relatively small so that the system may be inexpensive and have a light weight. A further improvement can be obtained if M satisfies -0.50 < M < -0.30. In that case, the diameters of the first group and the second group are substantially equal, which simplifies the structure of the objective system. The value of f is given by | f | = d/(2NA). The beam size d is limited by the facet size which in turn is limited by the required rotational speed of the polygon mirror. The numerical aperture NA is determined by the required resolution of the system. A suitable value of f is -1.25 mm. Due to the choice of the values of M and f, the value of fj is fixed. 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 a first embodiment of the objective system with refracting elements;

Fig. 3 shows a second embodiment with refracting elements; Fig. 4 shows a third embodiment with refracting and reflecting elements; Fig. 5 shows a fourth embodiment with refracting and reflecting elements; Fig. 6 shows a fifth embodiment with refracting and reflecting elements; Fig. 7 shows a sixth embodiment with refracting and reflecting elements, and

Fig. 8 shows a seventh embodiment with refracting and reflecting elements.

In Fig. 1, the reference numeral 1 denotes a tape-shaped record carrier.

This tape 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 collimated beam b, a rotating mirror polygon 20 which reflects the, for example parallel, beam to an objective system 30 focusing the beam to a radiation spot V on the tape. Instead of the mirror polygon, any other beam-deflection unit may be used, such as a rotating mirror or an acousto-optical deflection unit. The mirror polygon comprises, for example ten mirror facets f_j-f_jo 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 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 system. The scanning spot formed by this lens then scans a track extending perpendicularly to the direction 5. Consecutive tracks are successively scanned by means of the consecutive facets.

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° to the central position of the scanning beam which is moved, for example through an angle of 48°. The objective lens, in the form of an f-0 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, read and/or erase 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 this unit 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 system 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/μm 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 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 μm. 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. The objective system 30 according to the invention may be completely built up from refracting elements or from a combination of refracting and reflecting elements. Two embodiments of the first type and three embodiments of the second type are described below.

The two embodiments of an objective system having refracting elements only have a focal length of the total system of 1.25 mm, a scan angle 20 •= 48° of the polygon and an NA of 0.47 at the image side. Fig. 2 shows a first embodiment. The system comprises five lenses 31 to 35, viewed from the side of the objective system where the substantially collimated beam enters. Point H in the Figure is the object-sided main point and point F is the object-sided focus of the objective system. Point F is the location where the entrance pupil of the objective system intersects the optical axis 29 and where the polygon facet is present which deflects the beam at a given moment. Point H* in the Figure is the image-sided main point, and point F* is the image-sided focus of the objective system. The optical tape to be scanned runs through the point F*. The Figure shows the passage of a single beam by means of the central ray of the beam and two marginal rays.

Arranged in front of the first lens 31 is a plane plate 36 which functions as the exit window of a housing for polygon mirror 20. A first optical group with lenses 31 and 32 having a positive power forms an intermediate image from the beam 37 varying in angular position, in the form of an intermediate spot 38 moving along a line and thereby limits the lens diameters. Lens 32 has a surface 39 with a negative power which is proximate to the intermediate image and has, inter alia, the task of reducing the image field curvature. A second optical group with lenses 33, 34 and 35 has a positive power and substantially provides a re-image of the intermediate spot 38. Lens 31 ensures an image of the polygon facet, which is active as a diaphragm, at the location of the second, negative lens 32 for obtaining a telecentric pupil image at the image side. The entrance pupil of the objective system coincides with the diaphragm. The second lens thus has little influence on the pupil image. The second positive group provides a real image of the pupil to the image space. The objective system is made of five elements of high-refractive glass with two aspherical surfaces provided in the form of replica layers. An aspherical surface 40 is present in the first group and particularly plays a role in reaching the rigorous telecentricity throughout the image field and in reaching the (f-0) scan relation. A second aspherical surface 41 is chosen proximate to the internal pupil image so as to effectively combat the spherical aberration of all field beams. The aspherical surfaces may be made by means of the replica technique known from, inter alia, European Patent Specification no. 0 156 430.

Fig. 3 shows a second embodiment of an entirely refracting objective system. The objective system consists of five elements 42-46, in which the lenses 42 and 43 constitute the first optical group and the lenses 44, 45 and 46 constitute the second optical group. The objective system has three aspherical surfaces 47, 48 and 49 on the lenses 42, 44 and 45. the lenses are made of polycarbonate (PC). The last, negative surface 46' of the system contributes to the further reduction of astigmatism and image field curvature and ensures the correct telecentricity.

The objective system according to the invention can also be realized by means of a reflecting element. A reflecting element in the form of a concave mirror may realize the desired planeness of the image field of the objective system. A concave mirror has a positive optical power, while the image field curvature of the mirror is opposed to that of a positive lens.

The concave mirror should be tilted to a slight extent so as to pass the reflected beam without obstruction. If necessary, the field-independent astigmatism caused by the tilted mirror is compensated in one of the aspherical surfaces. Then, such an aspherical surface does not exhibit any circular symmetry. The non-circular symmetrical correction is, however, very small, of the order of 0.1 to 0.2 μm. The curvature of the scan line caused by the tilted mirror can be simply compensated by irradiating the polygon obliquely. In practice, it will generally be necessary to provide a device with which the curvature of the scan line is compensated or, in contrast, can be introduced.

Fig. 4 shows a third embodiment of the objective system, provided with a concave mirror. The facet of the rotating polygon is in the object-sided focus F. The collimated beam reflected by the facet goes to the right in the Figure and is reflected by the concave mirror 50. The mirror is rotated about an axis in the plane of the drawing perpendicular to the line F*H*HF, so that in reality, the reflected beam comes from the plane of the drawing. The reflected beam forms an intermediate spot 51 moving along a line. The intermediate spot is focused by two lens elements 52 and 53 on the optical tape at the location of the image-sided focus F*. Lens element 52 is a bi-asphere; lens element 53 may be a simple plane-convex lens. The first optical group consists of the concave mirror, the second optical group consists of the lens elements 52 and 53. In practice, the tilt of mirror 50 should be relatively large in this construction, because the reflected beam must be passed through or along the polygon housing.

Fig. 5 shows a fourth embodiment of the objective system, provided with a reflecting element. The first optical group consists of a plane-convex lens 54 which is used as a reflecting element by providing a part of the rear side 55 as well as the plane front side 56 with a silver coating. The convex rear side of the lens is slightly tilted again. The beam reflects at the concave rear side of the lens, subsequently reflects at the plane front side and exits through the part of the rear face of the lens which is not provided with the silver coating. The second group comprises two focusing lens elements 57 and 58, both being concave-convex mono-aspheres with the concave side being aspherical. This construction provides a sufficiently large image field and has a fairly compact building length.

Fig. 6 shows a fifth embodiment of the objective system, provided with a reflecting element. A slightly tilted concave mirror 59 is used in air. The reflected beam is received by a plane mirror 60 and reflected in the forward direction again. Due to the extra large correction of the image field curvature by a mirror in air, the last two imaging lenses 61 and 62 will be simpler, namely, a substantially plane-convex bi-asphere and a simple spherical plane-convex lens, and the optical tolerances are favorable. The optical diameter of the assembly is slightly larger than that of the objective system shown in Fig. 5.

An objective system in a tape-scanning apparatus used for mastering, i.e. writing a tape of which subsequently many copies are made, is subjected to more stringent requirements than the objective systems shown in Figs. 2 to 6 which are suitable for normal tape-scanning apparatuses intended for writing and reading normal tapes. Since a mastering apparatus operates with ultraviolet radiation, it is not possible to use synthetic material optical elements. Consequently, refracting aspherical surfaces can be realized only with great difficulty. An objective system for a mastering tape-scanning apparatus therefore preferably comprises spherical lenses only. The refractive index of most glasses is relatively small at wavelengths in the ultraviolet range. The curvature of the refracting surfaces should therefore be stronger than in lenses suitable for radiation having longer wavelengths. Due to the required accuracy with which the master tape is to be written, the objective system should be better corrected than an objective system for a normal tape-scanning apparatus. An objective system meeting these requirements has a concave lens in the first optical group and a double- Gauss system in the second optical group. The concave lens corrects the greater part of the image field curvature of the objective system. The double-Gauss system now only needs to correct the remaining image field curvature. Consequently, the constriction in the double- Gauss system is not much larger than 1.5 and 2 at a maximum. This contributes to the building tolerances and provides the possibility of using lenses with a relatively small diameter in the second group. Fig. 7 shows a sixth embodiment of the objective system suitable for use in a mastering apparatus. The first optical group comprises a concave spherical mirror 63 and a convex mirror 64, both having approximately the same center of curvature and mirror 63 having a radius of curvature which is approximately twice as large as that of mirror 64. The combination of the two mirrors is also referred to as an Offner system. Such a system has an extremely good imaging quality throughout the field. The second optical group comprises a double-Gauss imaging system 65 with ten spherical lenses. The double-Gauss imaging system has the advantage that coma compensation and the introduction of distortion required to change from a (f-tanø) relation to a (f-0) relation can be realized relatively easily by means of an equal distribution of refracting surfaces on both sides of the image of the pupil halfway the double-Gauss imaging section. The beam in Fig. 7 varies in location in a plane perpendicular to the plane of the drawing. The Figure shows the beam in an extreme position which is projected in the Figure on the plane of the drawing. The plane of the drawing passes through the optical axis of the double-Gauss imaging section and the center of curvature of the Offner system.

Fig. 8 shows a seventh embodiment of the objective system, also suitable for use in a mastering apparatus. A spherical mirror 66 has a small tilt. The mirror is irradiated by means of a collimated beam from its center of curvature. By choosing a large radius of curvature, the beam separation in the mirror system is relatively simple. A plane mirror 67 reflects the beam reflected by the mirror 66 towards a double-Gauss imaging section 68 with ten spherical lenses. The maximum OPD in the field is smaller than 20 mλ, while the manufacturing tolerances of the imaging section are very acceptable.

The following six pages state design parameters of the first, second, third, fourth, fifth and seventh embodiment of the objective system according to the invention.

10

EMBODIMENT 1

*** CONSTRUCTIONAL DATA OF THE OPTICAL SYSTEM ***

nr thickness curvature radius index labels diameter medium mm mm^A(-l) mm mm

1 infinity .00000000 infinity* 1.000000 0 0 0 1.2000

2 3.7565 .00000000 *infinity* 1.000000 0 0 0 10.0000

3 1.2000 .00000000 *infinity* 1.511083 0 0 0 10.0000 BK7

4 .5000 .00000000 *infinity* 1.000000 0 0 0 5.7000

5 3.7500 -.18750000 -5.3333 1.785350 0 0 0 6.9000 SF6

6 .0450 -.21859225 -4.5747 1.558774 1 0 0 6.9000 DIACRYL

7 .5000 .22675200 4.4101 1.000000 0 0 0 6.1000

8 6.5000 .69579400 1.4372 1.785350 0 0 0 1.8000 SF6

9 4.8000 -.11838700 -8.4469 1.000000 0 0 0 2.9000

10 4.5000 -.16553600 -6.0410 1.785350 0 0 0 5.4000 SF6

11 .1000 .14134737 7.0748 1.000000 2 0 0 5.9000

12 .0250 .12800000 7.8125 1.558774 0 0 0 5.9000 DIACRYL

13 2.8000 .00000000 *infmity* 1.785350 0 0 0 5.6200 SF6

14 .1000 .15842500 6.3121 1.000000 0 0 0 5.4600

15 2.8000 .00000000 infinity* 1.785350 0 0 0 4.3000 SF6

16 3.2074 .00000000 infinity* 1.000000 0 0 0 1.0400

**** END OF CONSTRUCTIONAL DATA OF THE OPTICAL SYSTEM ****

even terms of the power series expansion of aspheric surface nr 6 (label 1) coef 6 2 -.10929613E+00 coef 6 4 .53332454E - 03 coef 6 6 -.17945889E - 05 coef 6 8 -.42137262E - 06 coef 6 10 .16929783E - 07

even terms of the power series expansion of aspheric surface nr 11 (label 2) coef 11 2 .70673684E - 01 coef 11 4 -.64752963E - 03 coef 11 6 -.17796626E - 04 coef 11 8 .11875254E - 05

TABLE 1 EMBODIMENT 2

*** CONSTRUCTIONAL DATA OF THE OPTICAL SYSTEM ***

nr thickness curvature radius index labels diameter medium mm mm^λ(-l) mm mm

1 infinity .00000000 *infmity* 1.000000 0 0 0 4.0000

2 3.7565 .00000000 *infinity* 1.000000 0 0 0 10.0000

3 1.2000 .00000000 *infιnity* 1.511083 0 0 0 10.0000 BK7

4 1.0000 .11717800 8.5340 1.000000 0 0 0 6.6000

5 3.0000 -.20248795 -4.9386 1.573080 1 0 0 6.6000 PC

6 .5000 .20766300 4.8155 1.000000 0 0 0 6.0000

7 8.2000 .58349000 1.7138 1.573080 0 0 0 2.0000 PC

8 5.2000 -.31253413 -3.1997 1.000000 2 0 0 3.4000

9 4.5000 -.22590000 -4.4267 1.573080 0 0 0 6.6000 PC

10 .1000 .10229930 9.7752 1.000000 7 0 0 7.2000

11 2.8000 -.08378160 -11.9358 1.573080 0 0 0 7.2000 PC

12 .1000 .23098500 4.3293 1.000000 0 0 0 6.6000

13 4.5000 .22000000 4.5455 1.573080 0 0 0 3.6000 PC

14 2.9823 .00000000 infinity* 1.000000 0 0 0 10.0000

**** END OF CONSTRUCTIONAL DATA OF THE OPTICAL SYSTEM ****

even terms of the power series expansion of aspheric surface nr 1 (label 1) coef 5 2 -.10124398E+00 coef 5 4 .14388469E - 02 coef 5 6 -.93949192E - 05 coef 5 8 .24679611E - 06 even terms of the power series expansion of aspheric surface nr 8 (label 2) coef 8 2 -.15626707E+00 coef 8 4 -.36067533E - 02 coef 8 6 -.63029798E - 03 coef 8 8 -.79118996E - 04 even terms of the power series expansion of aspheric surface nr 10 (label 7) coef 10 2 .51149648E - 01 coef 10 4 -.63188605E - 03 coef 10 6 .87476848E - 05 coef 10 8 -.26582718E - 06

TABLE 2 EMBODIMENT 3

*** CONSTRUCTIONAL DATA OF THE OPTICAL SYSTEM ***

nr thickness curvature radius index labels diameter medium mm m ^A(-l) mm mm

1 infinity .00000000 *infιnity* 1.000000 0 0 0 4.0000

2 23.7500 .00000000 infinity* 1.000000 0 0 0 10.0000

3 .7940 .00000000 *infιnity* 1.000000 0 0 0 10.0000

4 3.8713 -.10517700 -9.5078 1.000000 0 0 0 10.0000

5 -16.0000 -.05301543 -18.8624 -1.000000 1 0 0 4.0000

6 -4.0443 .26388953 3.7895 -1.785350 2 0 0 4.0000 SF6

7 -.2000 -.18355900 -5.4478 -1.000000 0 0 0 4.0000

8 -4.7982 .00000000 infinity* -1.785350 0 0 0 4.0000 SF6

9 -.5000 .00000000 *infinity* -1.000000 0 0 0 10.0000

**** END OF CONSTRUCTIONAL DATA OF THE OPTICAL SYSTEM ****

even terms of the power series expansion of aspheric surface nr 5 (label 1) coef 5 2 -.26507716E - 01 coef 5 4 .90624280E - 02 coef 5 6 -.14340911E - 02 coef 5 8 .13875028E - 02 coef 5 10 -.27916294E - 03

even terms of the power series expansion of aspheric surface nr 6 (label 2) coef 6 2 .13194476E+00 coef 6 4 .15884798E - 02 coef 6 6 .18619518E - 03 coef 6 8 -.64747077E - 05 coef 6 10 .59251015E - 06

TABLE 3

EMBODIMENT 4

*** CONSTRUCTIONAL DATA OF THE OPTICAL SYSTEM ***

nr thickness curvature radius index labels diameter medium mm m ^A(-l) mm mm

1 infinity .00000000 *infmity* 1.000000 0 0 0 4.0000

2 16.2500 .00000000 *infinity* 1.000000 0 0 0 10.0000

3 2.2000 .00000000 infinity* 1.000000 0 0 0 8.0000

4 12.0000 .03762810 -26.5759 1.785350 0 0 0 8.0000 SF6

5 -12.0000 .00000000 infinity* -1.785350 0 0 0 8.0000 SF6

6 12.0000 .03762810 -26.5759 1.785350 0 0 1 8.0000 SF6

7 16.0000 .22268585 -4.4906 1.000000 1 0 0 3.4000

8 5.0000 .21505500 -4.6500 1.785350 0 0 0 6.4000 SF6

9 .2000 .25769200 3.8806 1.000000 0 0 0 6.4000 10 4.5000 .14534170 6.8803 1.785350 2 0 0 3.2000 SF6 11 2.1346 .00000000 infinity* 1.000000 0 0 0 10.0000

**** END OF CONSTRUCTIONAL DATA OF THE OPTICAL SYSTEM ***

even terms of the power series expansion of aspheric surface nr 7 (label 1) coef 7 2 -.11134293E+00 coef 7 4 -.79266587E - 02 coef 7 6 -.28103418E - 03 coef 7 8 -.18358366E - 03 coef 7 . 10 .0OOO0OO0E+00 coef 7 12 .OOOOOOOOE+00

even terms of the power series expansion of aspheric surface nr 8 (label 2) coef 10 2 .72670849E - 01 coef 10 4 .10889008E - 01 coef 10 6 -.37180355E - 03 coef 10 8 .74663208E - 03 coef 10 10 .OOOOOOOOE+00 coef 10 12 .OOOOOOOOE+00

TABLE 4 EMBODIMENT 5

*** CONSTRUCTIONAL DATA OF THE OPTICAL SYSTEM ***

nr thickness curvature radius index labels diameter medium mm mm^A(-l) mm mm

1 infinity .00000000 *infinity* 1.000000 0 0 0 40.0000

2 23.7500 .00000000 *infinity* 1.000000 0 0 0 40.0000

3 .7940 .00000000 infinity* 1.000000 0 0 0 10.0000

4 15.6813 -.05000000 -20.0000 1.000000 0 0 0 12.4000

5 -13.0000 .00000000 *infιnity* -1.000000 0 0 0 8.0000

6 17.0000 -.17238000 -5.8011 1.000000 1 0 0 2.6000

7 6.8624 -.20504138 -4.8771 1.785350 2 0 0 4.8000 SF6

8 .2000 .23758000 4.2091 1.000000 0 0 0 4.8000

9 6.6406 .00000000 ""infinity* 1.762486 0 0 0 4.8000 LAFN28

10 .5000 .00000000 *infιnity* 1.000000 0 0 0 10.0000

**** END OF CONSTRUCTIONAL DATA OF THE OPTICAL SYSTEM ****

even terms of the power series expansion of aspheric surface nr 6 (label 1) coef 6 2 -.86189998E - 01 coef 6 4 -.53245708E - 02 coef 6 6 .36459395E - 03 coef 6 8 -.29517799E - 03 coef 6 10 .33102331E - 04

even terms of the power series expansion of aspheric surface nr 7 (label 2) coef 7 2 -.10252069E+00 coef 7 4 -.39529992E - 03 coef 7 6 .21868555E - 04 coef 7 8 -.11263245E - 04 coef 7 10 .15010718E - 05

TABLE 5

EMBODIMENT 7

*** CONSTRUCTIONAL DATA OF THE OPTICAL SYSTEM ***

nr thickness curvature radius index labels diameter medium mm mm^A(-l) mm mm

1 -8.0200 .00000000 *infιnity* 1.000000 0 0 0 44.0000

2 75.0000 -.01333330 -75.0002 1.000000 0 0 0 62.0000

3 -55.0000 .00000000 *infιnity* -1.000000 0 0 0 20.0000

4 10.8000 .00000000 *infmity* 1.000000 0 0 0 8.8000

5 .0000 .05466170 18.2943 1.000000 0 0 0 8.8000

6 2.0000 .14558900 6.8687 1.599569 0 0 0 7.2000 BSM51Y

7 5.1666 .00000000 infinity* 1.000000 0 0 0 5.0000

8 2.0000 .14461400 6.9150 1.599569 0 0 0 5.0000 BSM51Y

9 3.3652 .00000000 *infinity* 1.000000 0 0 0 4.2000

10 2.0000 .12974600 7.7074 1.599569 0 0 0 4.2000 BSM51Y

11 .8211 .00000000 *infιnity* 1.000000 0 0 0 4.2000

12 2.5000 -.11505000 -8.6919 1.599569 0 0 0 4.2000 BSM51Y

13 .8289 .04480150 22.3207 1.000000 0 0 0 4.2000

14 2.5000 .00000000 *infιnity* 1.599569 0 0 0 4.0000 BSM51Y

15 .5000 .13458400 7.4303 1.000000 0 0 0 3.8000

16 2.5000 .26023200 3.8427 1.599569 0 0 0 3.4000 BSM51Y

17 1.0114 .00000000 *infmity* 1.000000 0 0 0 3.2000

18 3.3488 .00000000 *infιnity* 1.000000 0 0 0 4.0000

19 2.5000 -.13468100 -7.4250 1.599569 0 0 0 4.6000 BSM51Y 0 1.0000 .15644300 6.3921 1.000000 0 0 0 5.0000

21 2.5000 .00000000 infinity* 1.599569 0 0 0 4.4000 BSM51Y 2 .5000 .19666800 5.0847 1.000000 0 0 0 4.0000 3 2.5000 .00000000 *infinity* 1.599569 0 0 0 3.0000 BSM51Y 4 .3000 -.08686980 -11.5115 1.000000 0 0 0 2.5000 5 1.4000 .00000000 * infinity* 1.599569 0 0 0 2.2000 BSM51Y 6 .9007 .00000000 *infinity* 1.000000 0 0 0 1.2000 7 .0000 .00000000 *infιnity* 1.000000 0 0 0 *********

(BSM51Y is provided by the OH ARA-company /Japan)

**** END OF CONSTRUCTIONAL DATA OF THE OPTICAL SYSTEM ****

TABLE 6

Claims

CLAIMS:

1. A large-angle scan-objective system for transforming a collimated radiation beam varying in angular position to a scanning spot varying in location, comprising a first optical group having a positive power for forming an intermediate spot, varying in location, from the collimated beam, and a second optical group having a positive power for imaging the intermediate spot to the scanning spot, characterized in that the first optical group is provided with an element having a power for correcting the image field curvature of the scan-objective system.

2. A large-angle scan-objective system as claimed in Claim 1 , characterized in that the element is refracting.

3. A large-angle scan-objective system as claimed in Claim 2, characterized in that the element has a surface located proximate to the intermediate spot.

4. A large-angle scan-objective system as claimed in Claim 2 or 3, characterized in that the element has a negative power.

5. A large-angle scan-objective system as claimed in Claim 1 , characterized in that the element is reflecting.

6. A large-angle scan-objective system as claimed in Claim 5, characterized in that the first optical group comprises the element only.

7. A large-angle scan-objective system as claimed in Claim 1, characterized in that the second group has a magnification of between -0.65 and -0.20.

8. An optical apparatus for radially scanning a surface, which apparatus is provided with a scanning device which, in operation, scans the surface in at least one direction by means of a scanning spot of the radiation and which is provided with a beam- deflecting unit and a large-angle scan -objective system as claimed in any one of Claims 1 to 7.