WO1995032447A1

WO1995032447A1 - Scanning apparatus and method

Info

Publication number: WO1995032447A1
Application number: PCT/GB1995/001181
Authority: WO
Inventors: Keith William Gilbert
Original assignee: Imperial Chemicals Industries Plc
Priority date: 1994-05-25
Filing date: 1995-05-24
Publication date: 1995-11-30
Also published as: GB9410430D0

Abstract

A laser beam (14) is scanned across e.g. a dye sheet (11) to transfer dye to a receiver sheet (10) to produce a print. A helical mirror (1) downstream of focusing optics (16) scans the beam (14), the arrangement is such that the beam (14) travels a substantially constant path length to its focus in the dye sheet (11) throughout the scan. Mirror (1) comprises a disc on which is a stepped annular sloping wall with a top surface (4) from which the beam (14) is reflected. As the disc rotates, the point of incidence of the beam (14) on the surface (4) recedes to the right due to the surface's slope, and thus the surface (4) scans the beam (14) across the dye sheet (11). Instead of being incident radially on the surface (4), as shown, the beam (14) may be incident tangentially, e.g. out of the page.

Description

Scanning Apparatus and Method

The present invention relates to apparatus and a method for scanning a light beam, such as a laser beam, and particularly, but not exclusively, to a laser beam scanning system for use in dye transfer printing, including diffusion, sublimation and melt transfer. In dye diffusion thermal transfer printing, a dye sheet and a receiver sheet (or other such dye donor element and dye receiver element) are held in intimate contact with one another, and selected regions of the dye sheet are heated by a modulated scanning laser beam. This causes dye from the selected regions to diffuse into the receiver sheet to form a corresponding image therein.

Typically, scanning is achieved by reflecting the laser beam from a galvanometer mirror, i.e. a planar mirror mounted on a wire for rotation, or from a rotating, multifaceted mirror. The laser beam is scanned across the dye sheet as the mirror rotates.

In such a system, the focus of the laser beam is scanned in an arc. Accordingly, in order to ensure that the beam is focused at a desired constant depth within the dye sheet, the dye and the receiver sheets need to be arranged to lie in an arcuate plane corresponding to the arc traced out by the beam focus. Alternatively, compensating optics, such as an fθ lens, have to be used to correct the beam focus so that it is scanned in a flat plane. Such systems are somewhat complex and costly.

The present invention aims to provide an improved scanning apparatus and method, which overcome the above problems.

Viewed from one aspect, the present invention provides scanning apparatus comprising a light source, means for focusing a beam from the source such that the beam travels through a substantially constant path length to its focus, and rotating reflecting means downstream of the focusing means for scanning the beam, wherein the reflecting means comprises a helical reflecting surface.

In accordance with the invention, the helical surface may be arranged around the rotation axis of the reflecting means in such a manner that rotation of the reflecting means causes the focus of the reflected beam to scan back and forth in a substantially flat plane.

In use, the light beam from, for example, a laser source is incident at a suitable angle, for example 45", on the reflecting surface and comes to a focus downstream therefrom. As the reflecting means rotates, the incident laser beam is reflected from the helical surface at substantially the same angle of incidence but at a position further and further along the surface's slope. Thus, the incident beam is effectively reflected from a surface which recedes from the beam. Therefore, as the incident beam has to travel a continuously increasing distance to the reflecting surface, the reflected beam continually travels a correspondingly shorter distance from the reflecting surface before coming to its focus, and so the focus may be made to move in a straight line in a flat plane.

The invention allows an element to be scanned to be simply arranged to lie along this flat scan line, without the need for complex mounting of the element in an arc or for compensating optics.

A further problem with a galvanometer mirror scanning system is that there is a disadvantageous "dead time" in the scanning action between the end of one scan and the beginning of another, during which the galvanometer mirror must be rotated back to its home position.

With the present invention, however, the helical surface may slope for one complete pitch (i.e. may comprise a full 360" turn) , so that after one complete revolution of the reflecting means, the beam is reflected from the same portion of the surface as at the start of rotation. The beam is thus automatically returned to the start of the scan line, and scanning can be continuous, without a "dead time" during which the reflecting means must be returned to a home position.

In a preferred form, the reflecting means comprises more than one helical surface and may comprise, for example, two half-pitch helical surfaces (i.e. two surfaces each of which slopes about the rotation axis for 180") . In this latter case, on each rotation of the reflecting means, the beam is scanned twice along the scan line, once by each half-pitch surface, and is automatically returned to the start of the scan line as the beam stops being reflected by one surface and starts being reflected by the other. By increasing the number of reflecting surfaces, the number of scans per complete revolution of the reflecting means can be increased. Although not essential, it is preferable for the reflecting surfaces to be arranged so that a new surface begins directly after the preceding surface ends, without a gap therebetween. This provides the above-mentioned instant reset of the beam to the start of the scan line after each scan.

It is preferred that the mass of the reflecting means be well balanced about the rotation axis, so that there is no need to provide dynamic balance when it is rotated at what may be quite high speeds. This may be achieved by, for example, using multiple equal length reflecting surfaces, which may be contiguous with one another or evenly spaced apart.

It should be noted that the depth to which each helical surface slopes, measured in the direction of the rotation axis, determines the length of the scan line. Also, the slope angle of a surface and the speed of rotation of the reflecting means determine the scan speed produced by that surface.

The light beam may impinge on the reflecting surface from any suitable direction, and, in one embodiment, is incident on the surface in a "radial" direction, that is to say, the incident beam is in a plane which contains the rotation axis of the reflecting means, and the beam is incident on and reflects from the surface across the surface's width. When using a "radial" beam, the point on the helical surface from which the beam is reflected is not only further along the slope as the reflecting means Rotates, but also moves progressively radially outwardly across the surface from the rotation axis. Preferably, therefore, in this case, each helical surface is made wide enough in the radial direction to ensure that a beam will reflect from it over the whole of the desired scan length.

Further, because of the slope of the reflecting surface, a radial beam will, when reflected, be deflected slightly so that it no longer reflects in the radial direction. This deflection varies as the beam moves radially outwards across the surface's width, because, although there is a constant drop in height, the arc over which this height is dropped increases in length towards the surface's outer edge. The deflection has been found to move in a non-linear way with radius, so that the track followed by the focus of the reflected beam is somewhat curved. The deflection may be reduced by using a larger diameter reflecting means and by arranging for the beam to be brought to its focus closer to the reflecting means (the incident angle of the beam on the surface also has some effect, although not as great) . It is therefore possible to limit the curvature of the scanned beam (i.e. the percentage of deflection caused with respect to the line width) to a desired amount by suitably configuring the scanning apparatus. In a further embodiment, the laser beam is incident on the reflecting surface in a "tangential" direction, that is, the incident beam is perpendicular to the radius of the reflecting surface at the point of incidence, i.e. perpendicular to the plane of incidence of a "radial beam".

A tangential beam provides a number of advantages, in that the beam strikes the reflecting surface at a constant radius, i.e. at the same distance along the reflecting surface's width, and does not travel outwardly across the width during the scan. This enables the reflecting surface (which requires high precision machining) to be much narrower than when using a radial beam, and avoids the above-mentioned curvature problem.

When using a "tangential" beam, the point of incidence of the beam on the reflecting surface will tend to move along the surface's length, due to the slope of the surface, and this will introduce a slight non-linearity into the scan rate. This will, however, be negligible, and, in any case, if necessary, may be compensated for by suitable modulation of the laser beam.

Also, when using a "tangential" beam, it is preferred that the beam is incident on the reflecting surface in the same general direction as that in which the steps between the reflecting surfaces face on passing the beam, that is the beam fires over the top of the step onto the reflecting surface. This ensures that the reflected beam is never blocked by the step, and thus maximises the time during which the beam scans. If the beam were to be incident in the opposing direction, light reflected from a surface region directly in front of the step would be blocked by the step and scanning could not take place as the beam passed over that region.

Preferably, the helical surface is arranged such that, at any point along its slope, a line tangent to the surface and intersecting the rotation axis lies in a plane perpendicular to the rotation axis. This causes the focus of the beam to be scanned in a direction generally parallel to the rotation axis, although in the case of a radial beam the scan direction will be angled somewhat because of the incline of the helical reflecting surface along its length. This arrangement enables the various parts of a scanning system, such as the reflecting means and a platen roller, to be mounted in a scanning device in a simple manner, without interfering with one another. The arrangement is particularly useful where printing to a rigid flat element such as a credit or security card or 35 mm slide, which cannot be mounted about a roller but rather must be mounted on a flat support bed. In this case, it is again structurally much more simple to mount the bed in a generally parallel direction (although angled slightly for a radial beam) to the rotation axis of the reflecting means. Further, this arrangement facilitates the construction of a system in which the support bed moves at 90" to the beam scan direction to allow a card, etc., to be scanned along its length line-by-line.

The light beam may take any suitable form, and may, for example, be emitted by a laser source or from an LED source.

In a preferred embodiment, a beam of elliptical cross-section is used, for example from a laser diode, with the beam orientated such that it is scanned with the minor axis of its elliptical cross-section lying in the scan direction. This can be advantageous because the scanning beam of the present invention is not normal to the scan line, but rather lies at an angle to it (generally the angle of the beam to the scan line is equal to the angle of incidence of the beam on the helical reflecting surface) . This causes the size of the scanning spot along the scan line to be greater than the width of the beam in that direction, and, by having an elliptical beam's narrower width in the scan direction, the shape of the scanning spot may be made more symmetric. Preferably, the angle of incidence of the beam on the scan surface, and so on the reflecting surface, is set to provide a good spot shape (having regard to any other considerations, such as mentioned above in relation to the tangential beam, and, for example, in order to prevent excessive travel of the spot across the reflecting surface) .

The reflecting means may take any suitable form and may be made from any suitable materials. For example, it could take the form of one or more helical strips mounted on a central rod by radial struts.

In one preferred embodiment, the reflecting means comprises a disc rotatable about its central axis, with a raised wall extending from a surface of the disc about the central axis,^' the top surface of the wall reflecting the light beam and sloping in the wall's circumferential direction. Preferably, the wall extends circumferentially about the whole of the disc, the wall having at least one step therein between a maximum and a minimum slope height, with the number of steps depending on the number of reflecting surfaces.

Regarding the manufacture of the disc, the substrate may be made from any suitable engineering materials, and fabricated by casting or injection moulding followed by optical finishing. Stainless steel, aluminium or ABS may be used. Engineering plastics such as ABS have the benefit of low mass, but metal may be preferred to avoid the possibility of creep. The reflecting surface may comprise a series of thin-film coatings designed to provide high reflectivity at the wavelengths of interest. Typically a silver or aluminium coating could be used for visible wavelength laser beams, silver being preferred over aluminium at wavelengths around 800 nm.

The disc may be driven by an suitable drive means, such as an electric motor through, for example, suitable gearing. Overall, the invention is able to provide a reliable fast-action scanning system of low cost and few moving and wearing parts, and is particularly suitable for use with inexpensive laser diodes by allowing some degree of compensation for the beam's elliptical nature. It is also simple to synchronise the rotation of the reflecting means with the rotation of a platen roller. Furthermore, scanning may be substantially linear (i.e. the beam is scanned at a constant speed) , which is not always the case with the more inexpensive galvanometer scanners.

The invention may be used in any application requiring the scanning of a light beam, such as a laser beam, and is particularly applicable to dye transfer printing, because it enables efficient use of laser power. It is especially useful in the printing of small formats, such as 35 mm transparencies, as the reflecting means need not be overly large. The invention may be used in data storage/reading equipment in general, and is especially suited to use with optical tapes, which tend to have narrow widths.

A further aspect of the invention provides a beam scanning apparatus in which a light beam travels a constant path length to its focus during a scan, the ^■ apparatus comprising scanning means from which the light beam reflects, the surface region of the scanning means from which the beam reflects reciprocating back and forth at a constant angle to the incident beam.

As the reflecting region reciprocates back and forth at a constant angle, the beam focus moves correspondingly back and forth in a straight scan line in a flat plane, and an element to be scanned can be mounted simply along this plane. The reflecting means could comprise a planar mirror movable back and forth in a direction normal to the mirror surface so that the beam scans in a line parallel to the direction of movement. This enables the mounting of an element to be scanned to be particularly simple. The reflecting means may be driven by any suitable means, such as a crank, or any suitably shaped cam arrangement, and could, for example, be driven by a cam configured similarly to the above-mentioned helical disc. Preferably, the light beam is of elliptical cross-section, such as from a laser diode, and is scanned with its minor cross-sectional axis in the scan direction to provide the advantages mentioned above. Further preferably, the beam is reflected from the reflecting means at an angle of about 45".

The invention extends to a method of scanning a light beam using any of the above systems, and especially to laser dye thermal transfer printing using such a method, where the system's low cost and high speed scanning are especially suitable to small format high resolution imaging, such as 35 mm transparencies. Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 shows a helical mirror for scanning a laser beam comprising two half-pitch helical portions;

Figure 2 shows a top view of the helical mirror of Figure 1;

Figure 3 shows, schematically, a radial beam scanning system incorporating the helical mirror of Figure 1; and

Figures 4 and 5 shows schematically side and top views respectively of a scanning system utilising a tangential beam. Referring to Figures 1 and 2, the helical mirror 1 comprises a disc 2 on the front surface of which is provided a stepped annular sloping wall 3, the top surface 4 of which is optically finished to reflect laser light.

A hole 5 in the centre of the mirror 1 allows it to be mounted on a drive shaft, so that it may be spun about its centre axis 6.

This particular mirror 1 has two helical portions 8a and 8b, each of half-pitch (i.e. sloping about the centre axis 6 for 180"), which meet at steps 9. The helical axis of each portion 8a, 8b corresponds with the rotational axis 6 of the mirror 1.

A complete scanning system using the mirror of Figs. 1 and 2 is shown in Fig.3. A receiver sheet 10 is mounted below a dye sheet 11 about a platen roller 12. The roller 12 is rotated by an electric motor 13. A laser beam 14 from a laser source 15, such as a laser diode, passes through focusing optics 16 of constant focal length and is reflected from the sloping surface 4 of the rotating helical mirror 1, so as to be brought to a focus at a desired depth within the dye sheet 11. The mirror 1 is mounted on the drive shaft 17 of an electric motor 18. As explained below, the beam 14 is scanned across the dye sheet 11 by rotating the mirror 1 about its central axis 6. By modulating the beam 14 during its scan, selected pixel regions of the dye sheet 11 can be heated to cause dye to transfer from these regions into the receiver sheet 10. Thus, by scanning the beam 14 and rotating the roller 12, a desired image can be formed line-by-line in the receiver sheet 10.

Concentrating on the scanning action, the laser beam 14 is shown to be incident on the helical mirror 1 at the highest point of the top surface 4, which occurs at the top of either one of the steps 9. The beam 14 is then reflected to a point A within the dye sheet 11. As the mirror 1 is rotated by the electric motor 18, the point at which the sloping surface 4 intercepts the laser beam 14 and reflects it onto the dye sheet 11 moves to the right in a direction normal to the reflecting surface, parallel to the central mirror axis 6. Thus, after a quarter turn of the mirror 1, the reflecting surface 4 lies along a line B (as shown by the chain line) , and the beam 14 is reflected to a point C within the dye sheet 11.

After the mirror 1 has completed a half a turn, the reflecting plane lies along line D (as shown by the dot- chain line) , and the beam 14 is reflected to a point E within the dye sheet 11. As the mirror 1 continues to rotate, the beam 14 passes across the other of the two steps 9, and automatically begins the scan again from points A to E. Thus, after one complete 360° revolution, the beam 14 is scanned twice across the dye sheet 11 from left to right.

Throughout the scan, the beam 14 remains in focus at a constant depth within the dye sheet 11, because the distance the beam 14 travels from the focusing optics 16 to the dye sheet 11 remains constant, with the continual increase in the distance which the incident beam travels to the mirror surface 4 being compensated for by the correspondingly shorter distance the reflected beam has to travel to the dye sheet 11.

It should be noted that, in practice, the platen roller 12 is angled slightly from a direction parallel to the rotational axis 6, to compensate for the fact that, because the surface 4 slopes along its circumferential length, the reflected beam does not scan in a direction parallel to the axis 6. It should also be noted that, in this example, as the mirror 1 rotates the laser beam 14 is intercepted by the sloping surfaces 4 at a point which moves radially outwardly from the rotation axis 6 of the mirror 1. The surfaces 4 should, therefore, be made wide enough to allow for this.

As the beam moves radially outwardly, the actual scan line may be slightly curved due to the increase in the arc over which the slope height drops towards the outer edge of the surface. However, this may be minimised by increasing the radius of the disc 2 and by moving the roller 12 nearer to the disc. The angle of incidence of the beam 14 on the mirror surface 4, and so on the dye sheet 11, is shown as 45°. If a laser beam of circular cross-section were used, the laser spot profile in any particular layer of the dye sheet 11 would therefore be elliptical. To compensate for this and to provide a more symmetric spot profile, a laser source which emits an elliptical beam may be used, such as a laser diode, with the minor axis of the elliptical cross-section lying in the direction of the scan. The angle of incidence may be set to provide an optimal beam profile, within any limitations set by other considerations.

The depth X to which each helical portion 8a and 8b extends in the direction of the rotational axis 6 determines the beam scan length from A to E, and the slope angle of each helical portion 8a, 8b, as well as the speed of rotation of the mirror 1, determines the scan rate.

Although the top surface 4 slopes in the circumferential direction, it does not slope in the radial direction (i.e. at any point along its slope a line tangent to the surface 4 and intersecting the rotational axis 6 lies in a plane perpendicular to the axis 6), as best seen in Fig.3.

A second embodiment of the invention is shown in Figs. 4 and 5, which uses identical elements as in the first embodiment, but in which the laser beam 14 is incident on the reflecting surface 4 at a tangent to its direction of rotation, and in which the platen roller 12 is suitably mounted to receive the reflected scanning beam 9.

This arrangement has the advantage that the beam 14 does not move across the width of the surface 4 during the scan. Also the reflected beam is scanned in a straight line, parallel to the rotational axis of the disc 1.

The beam 14 is incident on the surface 4 in the same general direction in which the step 9 faces on rotating passed the laser (the beam 14 shines over the top of the step 9) . This arrangement ensures that the beam is continually reflected from the mirror 1, and so may be made full use of. If the beam were directed in the other direction, there would be a region of the mirror surface 4, just in front of the face of step 9, from which the reflected light would be blocked by the step's face. Accordingly, the beam would not be available to scan as it passed over this region, and scanning time would be lost.

Various modifications on the above embodiment are possible. For example, the number of helical portions may be increased. Also, the mirror could take any other suitable form, for example, it could comprise one or more thin helical strips mounted by radial struts to a central rod. It could also be a cam member configured in a similar manner to the above helical mirror, but driving a separate reflecting means such as a planar mirror. Indeed, the invention would also extend to, for example, a planar mirror reciprocating back and forth in a direction normal to the mirror surface driven by any suitable means such as cams or cranks. This would provide a scanning action parallel to the reciprocating direction of the mirror. Although especially applicable to laser dye transfer systems, the invention is also applicable to scanning systems in general.

Claims

Cl aims

1. Scanning apparatus comprising a light source, means for focusing a beam from the source such that the beam travels through a substantially constant path length to its focus, and rotating reflecting means downstream of the focusing means for scanning the beam, wherein the reflecting means comprises a substantially helical reflecting surface.

2. The apparatus of claim 1, wherein the helical surface slopes for one complete pitch.

3. The apparatus of claim 1, wherein the reflecting means comprises more than one helical surface.

4. The apparatus of claim 3, wherein the reflecting means comprises two half-pitch helical surfaces.

5. The apparatus of claim 3 or 4, wherein the reflecting surfaces are arranged so that the beam reflects from one surface directly after having been reflected from a preceding surface.

6. The apparatus of any preceding claim, wherein the light beam is incident on the helical reflecting surface in a radial direction.

7. The apparatus of any of claims 1 to 5, wherein the light beam is incident on the helical reflecting surface in a tangential direction.

8. The apparatus of claim 7, wherein the beam is incident on the reflecting surface in the same general direction as that in which a step at the end of the reflecting surface faces on passing the beam.

9. The apparatus of any preceding claim, wherein the helical surface is arranged such that, at any point along its slope, a line tangent to the surface and intersecting its rotation axis lies in a plane perpendicular to the rotation axis.

10. The apparatus of any preceding claim, wherein the reflecting means comprises one or more helical strips mounted on a central rod by radial struts.

11. The apparatus of any of claims 1 to 9, wherein the reflecting means comprises a disc rotatable about its central axis, with a raised wall extending from a surface of the disc about the central axis, the top surface of the wall reflecting the light beam and sloping in the wall's circumferential direction.

12. The apparatus of claim 11, wherein the wall extends circumferentially about the whole of the disc, the wall having at least one step therein between a maximum and a minimum slope height.

13. Beam scanning apparatus in which a light beam travels a substantially constant path length to its focus throughout a scan, the apparatus comprising scanning means from which the light beam reflects, a surface region of the scanning means from which the beam reflects reciprocating back and forth at a constant angle to the incident beam.

14. The apparatus of claim 13, wherein the reflecting surface comprises a planar mirror movable back and forth in a direction normal to the mirror surface so that the beam scans in a line parallel to the direction of movement.

15. The apparatus of claim 13 or 14, wherein the reflecting surface is driven by a cam comprising a disc rotatable about its central axis, with a raised wall extending from a surface of the disc about the central axis, the top surface of the wall sloping in the wall's circumferential direction.

16. The apparatus of any preceding claim, wherein the light beam is of elliptical cross-section, and is scanned with its minor cross-sectional axis in the scan direction.

17. The apparatus of any preceding claim, wherein the beam is reflected from the reflecting means at an angle of about 45".

18. Dye transfer printing apparatus including scanning apparatus according to any preceding claim.

19. A method of scanning a light beam using any of the above scanning apparatus.

20. A method of laser dye thermal transfer printing using any of the above scanning apparatus.

21. An element for scanning a light beam comprising a disc rotatable about its central axis, with a raised wall extending from a surface of the disc about the central axis, the top surface of the wall sloping in the wall's circumferential direction and being able to reflect the light beam.