GB2312115A

GB2312115A - Optical image writing arrangement

Info

Publication number: GB2312115A
Application number: GB9607307A
Authority: GB
Inventors: Paul Christopher Cook; Edward James Judd
Original assignee: ICG Ltd
Current assignee: ICG Ltd
Priority date: 1996-04-09
Filing date: 1996-04-09
Publication date: 1997-10-15
Anticipated expiration: 2016-04-09
Also published as: GB2312115B; GB9607307D0

Abstract

An imaging arrangement for an image writing apparatus, such as a platesetter, comprises a field lens 30 which directs light from a scanning arrangement to a relay lens 22 and on to a recording medium 24 (eg photosensitive film). The focal length of the field lens 30 is selected to direct the principal rays through a common point 32, which point is at a distance from the relay lens equal to its focal length. The arrangement prevents any variation in distance between points in the scan line if variation of distance between film 24 and relay lens occurs, however it does not prevent the location of the scan line from wandering. Thie may be overcome by (Figure 5) displacing the field lens 30 such that the principal rays (P, Q, R) are deflected so as to be parallel after passing through the relay lens (22). The scanning light may be in the visible, infra-red or ultra-violet regions of the spectrum.

Description

AN IMAGING ARRANGEMENT FOR AN IMAGE WRITING APPARATUS The present invention relates to an image writing arrangement for an image writing apparatus, such as a platesetter, in which a modulated light beam is scanned over a photosensitive material.

Figure 1 is a schematic illustration of a flat bed platesetter. The platesetter consists of a rectangular flat bed 2 supporting a travelling arm 4 which carries a scanning head 6.

The arm 4 is supported on guide rails 8 and is movable along the direction indicated by the arrow X so as to traverse the bed 2. The motion of the arm 4 is controlled such that the longitudinal axis of the arm remains parallel with the direction indicated by the arrow Y. The scanning head 6 contains a light beam deflection mechanism arranged to write a line of picture elements, herein referred to as a "sub-scan", parallel to the direction indicated by the arrow X. The light beam is modulated during scanning in order to record a portion of an image on a photosensitive film. Once a sub-scan has been completed, the position of the scanning head 6 is moved a predetermined distance along the arm 4 in the direction of arrow Y so as to write a further sub-scan parallel with the most recently completed sub-scan. Each sub-scan comprises a predetermined number of picture elements (pixels) and the sub-scans are grouped into swathes. The sub-scans within an individual swathe are parallel with one another and not longitudinally displaced with respect to one another. Thus, as shown in Figure 2, each of the sub-scans 10 within an individual swathe 12 have common start and finish Co- ordinates along the X direction but are displaced from one another in a regular arrangement along the Y direction.

Once a swathe has been completed, the arm 4 is moved along the X direction to a new position such that another swathe can be written onto the photosensitive material substantially contiguously with the most recently completed swathe.

The scanning head 6 may incorporate a scanning device, such as an acousto-optic deflector, which deflects the light beam so as to produce an array of image points wherein the direction of the scanning light beam is inclined with respect to the normal of the surface of the bed 2, and consequently the light beam is also inclined with respect to the normal of the photosensitive material supported by the bed 2.

The distance between the surface of the photosensitive material and the scanning head 6 may vary by a small amount due to variations in film thickness, the film not lying flat on the bed 2, the bed 2 not being flat or not being correctly aligned with the carriage rails 8, or due to distortion in the carriage rails. The quality of the image formed by raster scanning of the deflected light beam is very sensitive to position and variations in the separation of the exposed picture elements. Each of these variations can cause the width and/or path of the individual swathes to vary.

It is both difficult and expensive to eliminate the mechanical causes of swathe width/path degradation.

According to the present invention there is provided an imaging arrangement for an image writing apparatus having a deflector arranged to scan a light beam over a predetermined path so as to produce an array of image points, the arrangement comprising a relay lens and focusing means for directing the light beam from the deflector through a first position to the relay lens, said first position being separated from the relay lens by substantially the focal length of the relay lens.

It is thus possible to provide an imaging arrangement in which the light rays emerging from the relay lens are substantially parallel to one another irrespective of the position of the light beam along the predetermined path. The predetermined path corresponds to an individual sub-scan, and the length of each sub-scan is substantially unaffected by variations in the distance between the relay lens and the photosensitive material.

As used herein, the term light is to be broadly construed to include radiation selected from the infra-red, visible and ultra-violet regions of the electromagnetic spectrum.

Preferably the focusing means is a field lens (also known as a pupil control lens). The field lens is interposed in the optical path between the deflector and the relay lens.

Preferably the field lens is located away from the object plane of the relay lens, such that the relay lens does not form focused images of dirt or other matter on the field lens at the photosensitive material.

Preferably the imaging arrangement also comprises a ray path corrector for deflecting the ray paths of light emerging from the deflector such that the first position lies substantially on the optical axis of the relay lens. Thus the principal ray paths emerging from the relay lens are perpendicular to the image plane. Consequently, variations in the distance between the relay lens and the photosensitive material do not give rise to variations in the width or path of a swathe.

The present invention will further be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic plan view of a platesetter; Figure 2 is a diagrammatic representation of the sub-scans within a single swathe; Figure 3 is a schematic illustration of a known imaging arrangement within a platesetter; Fiaure 4 is a schematic illustration of an imaging arrangement constituting a first embodiment of the present invention; and Figure 5 is a schematic diagram of an imaging arrangement constituting a second embodiment of the present invention.

In Figures 3, 4 and 5, three principal rays P, Q and R representing the ray paths to a start, a centre point, and an end of a sub-scan, respectively, are illustrated. A principal ray (also known as a chief ray) is the central ray of a bundle of light rays that form an image at a point. A central ray can be viewed as a reference ray indicating the general direction of the image forming bundle and indicating the positions at which images are formed or where pupils are positioned. Chain lines 34 in Figures 3 to 5 schematically illustrate the limits of an image forming bundle of rays related to the central principal ray Q.

In each of Figures 3, 4 and 5, the lens shown therein are illustrated in schematic form only and in practice are likely to be compound lenses comprising two or more elements.

Compound lenses introduce less distortion within the image than an equivalent single lens.

As shown in Figure 3, light rays P, Q and R originating from the acousto-optic deflector (not shown) are focused by the acousto-optic deflector towards and pass through an object plane 20 of a relay lens 22. The focal length of the relay lens 22 is selected such that the ray bundle is brought to a focus at a photosensitive film 24 supported on the bed 2. The relay lens controls the size of the final image as well as bringing the image into focus at the photosensitive film.

A principal axis 26 (indicated by a chain-dot line) can be regarded as extending perpendicularly to the plane of the photosensitive film 24. The principal rays P, Q and R originating from the deflector are initially not parallel to the principal axis 26.

On passing through the relay lens 22, the principal rays P, Q and R pass through a common point 28 who's distance from the relay lens 22 is equal to the focal length of the relay lens. This in itself is of no significance, but does serve to illustrate why the principal rays arrive at the film 24 from a range of directions. If the distance between the film 24 and the relay lens 22 varies during writing of a sub-scan, it can be seen that the positions of the image points relative to the principal axis would vary and that the separation of the image points within the sub-scan would also vary. Thus the width of the swathe of exposed points can vary and the path of the swathe can wander.

Figure 4 shows an imaging arrangement constituting a first embodiment of the present invention. A field lens 30 is interposed between the deflector (not shown) and the relay lens 22. The field lens 30 is illustrated as being coincident with the object plane 20.

The field lens does not have to be in the object plain 20, but illustrating the lens at this position helps to simplify the description of the imaging process. In practice, the field lens 20 is situated away from the object plane 20 so as to ensure that an image of the surface of the field lens 30 and any dust or debris collected thereon is not formed in focus at the film 24. An in-focus image of dust on the field lens 30 would be capable of obliterating one of the picture elements written on the film 24. The focal length of the field lens 30 is selected to ensure that the principal rays pass through a common point 32 located between the field lens 30 and the relay lens 22. The distance of the common point 32 from the relay lens 22 is selected to equal the focal length of the relay lens. This ensures that the principal rays P, Q and R are parallel to each other when they arrive at the film 24. A variation in the distance between the relay lens 22 and the film 24 (for whatever reason) does not result in a variation of the picture element separations. Thus, the length of the sub-scan is unaffected by changes in the position of the film 24. However, because the principal rays are inclined to the principal axis 26, changes in separation between the film and relay lens cause the path of the swathe to wander. This can give rise to errors in abutment between adjacent swathes.

Figure 5 illustrates a further refinement of the arrangement shown in Figure 4. In Figure 4, the central principal ray Q passes through the centres of both the field lens 30 and the relay lens 22. Consequently, the central principal ray Q is not deflected by the lens 22 and 30 and arrives at the film 24 at an angle defined by the acousto-optic deflector. In the arrangement shown in Figure 5, the field lens 30 is laterally displaced such that the central principal ray Q no longer passes through the optical axis thereof.

Consequently, the central principal ray Q is deflected as it passes through the field lens 30. Similarly, all of the other principal rays are also deflected as they pass through the field lens 30. The lateral displacement of the field lens 30 is selected such that the central principal ray Q is deflected to be parallel to the principal axis 26. This means that it and, indeed, all of the principal rays P, Q and R arrive at the film 24 at normal incidence. Thus variations in the distance between the film 24 and the relay lens 22 do not induce changes in the width or path of the swathe of sub-scans. The principal rays can be described as being telecentric and perpendicular to the image plane at the film 24.

It is noted that only a portion of the field lens 30 is used. In practice, the lens 30 could be truncated in order to remove the unused portion.

It should be noted that it is the relative displacement between the relay lens and the field lens which gives rise to the normal illumination. Thus a similar result could be achieved by displacing the relay lens 22. However, because the image forming ray bundles have a significant aperture at the relay lens, as illustrated by the chain lines 34 for the central principal ray Q, displacing the relay lens can introduce significant image aberrations and therefore requires a more complex design in order to correct for these aberrations. Conversely, it is to be noted that the aperture of the image forming bundle is small at the field lens 30 (even when the field lens 30 is displaced to one side of the image plane 20), and consequently abberations of the image introduced by the field lens are small. Thus, the field lens controls the size of pupil surrounding the point 32, but does not affect the imaging properties of the system.

Deflection of the principal rays could be achieved by an optical element separated from the field lens 30. For example, a prism might be employed to deflect the principal rays.

It is thus possible to provide an imaging system for use in a platesetter which renders the quality of the image written onto a photosensitive film located on a flat bed of the platesetter substantially invariant to changes in the distance between the surface of the film and a relay lens of the imaging system. Thus the quality of the image is substantially unaffected by the thickness of the film or by relatively small imperfections in the surface of the bed supporting the film or by the film not lying completely flat on the bed.

Claims

1. An imaging arrangement for an image writing apparatus having a deflector arranged to scan a light beam over a predetermined path so as to produce an array of image points, the arrangement comprising a relay lens and focusing means for directing the light beam from the deflector through a first position to the relay lens, said first position being separated from the relay lens by substantially the focal length of the relay lens.

2. An imaging arrangement as claimed in claim 1, in which light rays emerging from the relay lens are substantially parallel to one another irrespective of the position of the light beam along the predetermined path.

3. An imaging arrangement as claimed in claims 1 or 2 in which the focusing means is a field lens.

4. An imaging arrangement as claimed in claim 3, in which the field lens is interposed in the optical path between the deflector and the relay lens.

5. An imaging arrangement as claimed in claim 3 or 4, in which the field lens is located away from an object plane of the relay lens.

6. An imaging arrangement as claimed in any one of the preceding claims, further comprising a ray path corrector for deflecting ray paths of light emerging from the deflector, such that the first position lies substantially on an optical axis of the relay lens.

7. An imaging arrangement as claimed in any one of the preceding claims in which the light belongs to at least one of the infra-red, visible and ultra violet regions of the electromagnetic spectrum.