OPTICAL APPARATUS FOR USE WITH LASER PRINTERS AND THE LIKE

Abstract: Optical apparatus for scanning a beam across and object field comprising, a source of radiation (21, 22, 23) for providing a beam (15) of said radiation, first moveable reflective means (11A), first optical means (26) for passing said beam to said first moveable reflective means (11A) so that said beam is reflected from said first moveable reflective means (11A), reflective means (28) to reflect said once reflected beam a second time to said first moveable reflective means (11A) where it is reflected a third time, second optical means (24, 26, 31, 32, 33) to pass said thrice reflected beam (15E) from said first moveable reflective means (11A), to a second moveable reflective means (11B) whereby said beam is reflected and is scanned along a focus line, the optical path length between said first (26) and second optical means (24, 26, 31, 32, 33) being variable as said first moveable reflective means (11A) moves, whereby to provide a variable distance from said second moveable reflective means (11B) to said focus line.

Description: OPTICAL APPARATUS FOR USE WITH LASER PRINTERS AND THE LIKE The present invention relates to an optical apparatus for use with, for example, laser printers, photocopiers, and other scanning apparatus. One arrangement comprises optical apparatus such as a scanning apparatus of variable focal length. In one arrangement optical apparatus is provided for scanning a beam across an object field (eg, laser printers and photocopiers), usually in the form of a surface eg: a photosensitive drum) and in another arrangement apparatus may be provided for receiving radiation (eg, detection scanning apparatus for detecting, for example, faults in a web material such as paper, or plastic film), the area from which the radiation is received being scanned. Another arrangement comprises optical apparatus for moving a beam so as to track a moveable reflective surface such as a facet of a mirror drum. For convenience, in this specification we will generally refer to optical wavelengths, and in particular will use expressions such as "optical", "beam", "light" and so on but it will be understood that the invention is applicable to radiation of other wavelengths, such as infrared and ult raviol et . Simple laser scanners (shown diagrammat ical 1y in Figure 1 ) scan an incident focused laser beam 10A via a polygon mirror drum 11 having a plurality of mirror facets 11A, 11B etc which is rotatable about an axis 12. The reflected laser beam 10B is truly in focus on the arc of a circle centered at the scan origin. We will refer to this arc as the focal line 13. In practice if a flat surface 14 is to be scanned the laser spot diameter is limited to a size where the depth of focus is great enough to cover the excursion of the focal line 13 from the flat surface 14 (a +- b in Figure 1 ) . If greater resolution is required in the scan, ie, a smaller spot size than is allowed in a simple scanner, then a field flattening lens 16 is usually sited between the mirror drum 11 and the scanned surface 14 as shown in Figure 2. A multi element lens system 16 can be designed to produce up to approximately 30,000 spots resolved in a line. However, if the flattening of the field is to be accurate this requires a very expensive special lens system 16 if scanning takes place over a large angle. Another difficulty is that in a confocal laser scanning system 16 in which the reflected beam from the flat surface 14 is collected by the mirror drum 11 both the outgoing scan beam 10B and the returned light 10C are on the same oath so that the returned light 10C is descanned at the same mirror facet. If the returned light level is low then this will be confused with the back reflections from the multiple surfaces of lens system 16. Furthermore if a scanning apparatus is required to scan different widths or a surface 14 which is of a shape other than planar, then a different lens system 16 is required for each situation. It would be preferable to provide some means for providing variable focusing to easily provide a focal line of a particular shape (i.e. straight for a planar surface) and to easily change the position of the focal line. The first aspect of the invention is based on the realisation that as a reflective member rotates about an axis out of its plane (such as a mirror facet 11A, 11B of a rotatable mirror drum 11, about the axis 12) the distance from a fixed point in the optical path of an incoming incident beam to the reflective member changes slightly and hence the optical path length varies slightly. If there are a plurality of, usually, plane mirrors (the various facets of the mirror drum) as is provided in many conventional scanners then, as the mirror drum 11 rotates, this optical path length of the incident beam varies in a regular manner in synchronism with the scanning action as each facet 11A, 11B passes across the incoming beam. The reflective beam may be collected and passed through an optical system which multiplies this change of optical path length so as to provide a change in the position in which the optical system brings the beam to a focus, that is a change in the focal line 13. This therefore provides a simple variable focus as the beam is scanned by the scanning apparatus and may be used to replace the lens system 16. The present invention provides optical ' apparatus for scanning a beam across an object field comprising, a source of radiation for providing a beam of said radiation, first moveable reflective means, first optical means for passing said beam to said first moveable reflective means so that said beam is reflected from said first moveable reflective means, reflective means to reflect said once reflected beam a second time to said first moveable reflective means where it is reflected a third time, said optical means to pass said thrice reflected beam from said first moveable reflective means, to a second moveable reflective means whereby said beam is reflected and is scanned along a focus line, the optical path length between said first and second optical means being variable as said first moveable reflective means moves, whereby to provide a variable distance from said second moveable reflective means to said focus Tine. There is preferably provided a focusing means in the beam path between the second moveable reflective means and the first movable reflective means whereby said changes in optical path length are translated into said variation of distance from said second moveable reflective means to said focus 1 ine. Each said moveable reflective means may comprise different facets of a multi-faceted mirror drum. Said reflective means may comprise a spherical mirror mounted adjacent the mirror drum, the centre of the spherical mirror being generally coincident with the surface of the first facet. The invention provides, according to a second aspect a variable focusing means comprising, moveable reflective means pivotal ly mounted so as to be rotatable about an axis remote from the reflective surface of said moveable reflective means, first optical means to direct a beam of radiation at said moveable reflective means, second optical means to receive radiation reflected from said moveable reflective means and optical focusing means, the arrangement being such the optical path length from the first to the second optical means varies as the moveable reflective means rotates about said axis, said change in optical path length being multiplied by said focusing means whereby to vary the focus of the beam of radiation after it passes through said focusing means. Whereas in the arrangement shown in Figure 2 the fecal line 13 is of a predetermined shape to match the flat object field 12, in the preferred arrangement of the invention, this predetermined shape can be changed so as to match an alternative object field, for example, the surface of an object which is not flat. In a third aspect the present invention provides detection apparatus comprising, first moveable reflective means to scan and receive radiation from a radiation source provided along a focus line of said first moveable reflective means, first optical means to pass and define a beam reflected from said first moveable reflective means to a second moveable reflective means; reflective means to reflect said beam after it is reflected by said second moveable reflective means back to said second moveable reflective means, second optical means for passing said beam reflected twice by said second moveable reflective means to a photo detector, whereby the optical path length between said first and second optical means is variable as said second moveable reflective means moves, whereby to provide a variable distance from said first moveable reflective means to said focus line. The invention also provides according to a further aspect a novel apparatus for displacing a beam to one side, for example for the purpose of "facet tracking" ie., maintaining the beam on a facet of a rotating mirror drum as it rotates. The present invention also provides, according to a fourth aspect optical apparatus for scanning a beam across an object field comprising a source of radiation for providing a beam of said radiation, first moveable reflective means mounted to pivot about an axis remote from its respective surface, first optical means for passing said beam to said first moveable reflective means so that said beam is reflected from said first moveable reflective means, reflective means comprising a spherical or cylindrical mirror mounted adjacent the first moveable reflective means, the centre of curvature of the spherical or cylindrical mirror being displaced from the reflective surface of the first moveable reflective means by a predetermined distance, said reflective means being adapted to reflect said once reflected beam a second time to said first moveable reflective means where it is reflected a third time, said optical means to pass said thrice reflected beam from said first moveable reflective means, to a second moveable reflective means which pivots about an axis in synchronism with said first moveable reflective means whereby said beam is reflected and is scanned along a focus line, the lack of coincidence between the centre of curvature of said spherical or cylindrical mirror and the surface of the first moveable reflective means causing said twice reflected beam to oscillate from side to side as the twice reflected beam moves across the first moveable reflective means, said predetermined distance being chosen so that said side to said movement of the beam causes it to follow the second moveable reflective means as it rotates. The present invention also provides, according to a fifth aspect a laser printer comprising a photosensitive drum, ra optical apparatus as aforesaid, means for modulating said beam, whereby said photosensitive drum comprises said object field and the arrangement being such as to maintain the incident beam on said photosensitive drum in focus as it scans across the photosensitive drum. Preferred arrangements of the invention will now be described by way of example and with reference ' to the accompanying drawings in which:- Figure 1, already described, shows in diagrammatic form a prior arrangement of scanner, Figure 2 shows an alternative prior art arrangement of scanner, Figure 3 shows a diagrammatic view, similar to Figures 1 to 2, of a scanner according to a first aspect of the invention , Figure 4 shows an enlargement of part of Figure 3, Figure 5 shows an alternative arrangement of Dart of Figure 4, Figure 6 shows a diagrammatic perspective view of the optical arrangement of a laser printer including an optical arrangement according to the invention, Figure 7 shows a diagrammatic view, similar to Figure 3 of an alternative aspect of the invention, showing apparatus for facet tracking, and, Figure 8 shows a diagrammatic view of part of the apparatus of Figure 7 enlarged. We now refer to Figure 3 which shows an arrangement of the invention. The beam 10 has been renumbered to save confusion with Figure 1, different sections of the beam being numbered 15A to 15J. Laser 21 provides an initial laser beam 15A which is shaped by doublet lenses 22,23. After passing through a polarising cube beam splitter 24 which includes a quarter wave plate 25, the beam 15B is focused down by a doublet lens 26 to a spot P at or near the reflective inner surface of a spherical mirror 28, the beam 15B passing through an aperture 29 in the spherical mirror 28, being incident on a facet 11A of the mirror drum 11 , and being reflected therefrom (as beam 15C) to the reflective inner surface of the spherical mirror 28. The arrangement is such that the axis of the spherical mirror 28 is at or adjacent the surface of the relevant facet of the mirror drum 11. Thus the beam 15C incident on the spherical mirror 28 is reflected (as beam 15D) therefrom, is reflected by the same facet 11A of the mirror drum 11 (as beam 15E) and passes back through the aperture 29 and doublet lens 26 to the beam splitter 24 where it is reflected (as beam 15F) away from the incident beam 15A. The beam 15E is ' further shaped by two further doublet lenses 31 and 32. The reshaped beam (now 15G) is reflected by mirror 33 on to a facet 11B of mirror drum 11 opposite facet 11A so that the beam is then scanned (as beam 15H) by the mirror drum as in the arrangement of Figures 1 and 2. Figure 4 shows the spherical mirror 28 and mirror drum 11 in greater detail. In this case the width of the beams 15B, 15C, 15D, 15E is exaggerated and two alternative positions of the mirror drum and hence the facet 11A are shown. Referring to Figure 4, when the facet 11A of the mirror drum 11 rotates, the reflected beam 15C is rotated through twice the angle of rotation of the facet 11A but more importantly the point of incidence of the incoming beam 15B on the facet 11A moves forward because of the displacement d of the point on which the beam 15B is incident as shown in Figure 4. In fact, the position of the focus point P moves with respect to the su ' rface of the spherical mirror 28 from the position P1 to P2, that is with respect to the axis of the- surface of the spherical mirror 28, by a distance equalling 2 x d. For ease of illustration we have indicated P1 as oeing on the surface of the spherical mirror 28 and P2 being a distance 2 x d behind the spherical mirror 28. In practice the exact point P at which the beam comes to a focus may be in a slightly different radial position but the difference between the two point P1 and P2 in the radial direction wi 11 st i 11 be 2 x d. The distance d is given by where r is the radius of the mirror drum and θ is the rotation angle. Thus overall, the optical path length for the beam from a fixed point such as the aperture 29 to the facet 11 A to the spherical mirror surface 28 back to the facet 11A and back to the aperture 29 changes by 4 x d as the mirror drum 11 rotates, this movement being in synchronism, of course, with rotation of the mirror drum 11. This change in optical path length can be translated into a change of optical path length between the lens 32 and the point of which the beam 15H is bought to focus. By suitable choice of the lenses 26,31 ,32 one can arrange for a variety of shapes of the focus line 13 from a flat line to match the surface 14 to a more concave shape than that shown in Figure 1, or to a convex shape. An alternative arrangement is shown in Figure 5 which has the additional advantage of providing a substant ally linear scan speed of the laser spot along the focus line 13 in Figure 5. In place of the rotating mirror drum 11 there is provided an oscillating scanner 41 in the form of two opposed mirror facets 42A,42B corresponding generally to the facets 11A, 11B of Figure 3 and 4 which are resonantly oscillated back and forth about an axis 43. The two facets 42A,42B will act like the opposite facets of the mirror drum 11 except that the scan speed because of the oscillation will be approximately linearised. The same focusing arrangement of the spherical mirror 28 can be used but the focus beam is scanned along the surface 14 at a substantially linear rate. Figure 6 shows in diagrammatic form a laser printer incorporating an optical apparatus according to the invention. Similar parts have similar reference numerals. Thus the laser 21 provides a beam which is shaped by lens 22 to form beam 15A; this is passed through the rear of the spherical mirror 28 and then passes to an oscillating scanner 41 driven by motor 40 which scanner includes two opposed mirror facets 42A.42B as described with reference to Figure 5. The beam is reflected from facet 42A to the spherical mirror 28 and thence back to the facet 42A as in Figures 3 and 4. The thrice reflected beam 15F is then passed via two mirrors 30,33 and lenses 31 ,32 to the opposite mirror facet 42B from whence it is scanned across a photosensitive laser printer drum 14. The optical apparatus of the invention is particularly useful with a laser printer because the angle β is relatively large. Furthermore an oscillating drive system for the scanner 41 can easily be arranged to be sinusoidal which provides a substantially constant scan speed along the surface of the drum 14. The beam is modulated by conventional means, (not shown) to "write" a line on the printer drum 14. It will be understood that the focal line 13 can be easily changed by suitable choice of lenses 31 and 32 in particular. These lenses are simple doublet lenses and are economical and so the focal line can be matched to any particular surface at minimal cost. We have described the arrangement with respect to a beam being scanned from the mirror drum 11 across a surface 14. Of course a similar optical arrangement may be used whereby radiation from the surface 14 is picked up by the mirror ' drum scanner 11 and transmitted through the optical system to a detector in place of the laser 21. Furthe r more the apparatus from 21-32 may be used in other circumstances to provide a variable focusing means without the scanning apparatus. Some of the lenses may be replaced by zoom lenses which allows for continuous adjustment of the focus line. We now refer to the arrangement set out in Figures 6 and 7. There is a requirement to scan at a greater rate " , for example up to about 750,000 scans per minute, and with a twelve facet mirror drum, this requires a rotation of 60,000 revolutions per minute. The mirror drums are normally made of beryllium and because of the high speed or rotation there are physical constraints on the size of the mirror drum. At this speed, it is necessary to have a mirror drum of about 35mm diameter maximum, which means each facet is approximately 9mm wide. In order to provide the necessary spot diameter where the beam meets the focus line 13, and where the distance from the mirror drum to the focus line 13 is approximately 1500mm, the width of the beam where it strikes the facet 11B of the mirror drum 11 on the return from the surface 14 is, say, of the order of 8mm in diameter. Referring now to Figure 8 the incoming beam 15G to the facet 11B is, as can be seen, of the same order as the width of the facet 11B. Assuming the incoming beam 15G1 strikes the facet 11B in the position shown in a continuous line, then when the facet 11B has moved to the position shown in dashed line, then part of the beam (that is the part indicated B) will not strike the facet 11 B and this causes difficulties. It would be desirable if the incoming beam 15G could "track" the facet so as to move from the position 15G1 when the facet 11B is in the position shown in continuous line in Figure 8, to the position shown in 15G2 when the facet 11B has moved to its dashed lined posit ion. The arrangement of Figure 7 is intended to provide a movement of the beam in the manner required in Figure 8 (ie, "facet tracking"). The arrangement of the remainder of the optical system is as shown in Figure 3 except that a further lens is required between lenses 31,32 or another mirror between mirror 33 and the drum 11. Referring to Figure 7, therefore, it will be seen that the arrangement is generally similar to that of Figure 3 except that the spherical mirror surface 28 is moved further towards the axis 12 of the mirror drum 11 by a predetermined distance. The axis of the spherical mirror surface 28 is (exaggeratedly) arranged t * be at the position shown at 20 in Figure 7. The effect of this movement is that for the incoming and reflected beams to P1 in Figure 7, the situation is the same as in Figure 3 (except the beams are labelled 15C1 , 15D1, 15E1). However when the beam is reflected to the position P2 by movement of the mirror drum, it will be understood that that part of the spherical mirror surface 28 will tend to reflect the incoming beam 15C2 not back to along the line of 15C2, but to one side, in fact to a point below the axis 20. This is indicated by beam 15D2 which is thereby reflected by the facet 11A to form a beam 15E2. In Figure 7, 15E2 is to the right of the reflected beam 15E1. This sideways movement of the beam (exaggerated in Figure 7, in practice the beam 15E2 is likely to only be displaced by half the beam width at the extreme of the scanning) passes through the optical system as before, that is through the lenses 26, beam splitter 24, lenses 31 and 32 (new lens not shown) to provide the movement of the beam illustrated in Figure 8. Although not restrictive, the distance from the mirror 28 to the facet 11A is approximately 10mm and the distance from the facet 11A to the new position of the axis 20 is approximately 1mm. It will be understood that the movement to the side of beam 15E2 is produced in synchronism with rotation of the mirror drum as is desired to provide the relative movement of the beam shown in Figure 8, The invention is not restricted to the details of the foregoing examples.

Claims: CLAIMS 1. Optical apparatus for scanning a beam across an object field comprising: a source of radiation (21,22,23), for providing a beam (15) of said radiation, first moveable reflective means (11A), first optical means (26) for passing said beam to said first moveable reflective means so that said beam is reflected from said first moveable reflective means, reflective means (28) to reflect said once reflected beam a second time to said first moveable reflective means where it is reflected a third time, said optical means (26,24,31,32,33) to pass said thrice reflected beam from said first moveable reflective means, to a second moveable reflective means (11B) whereby said beam is reflected and is scanned along a focus line (13), the optical path length between said first and second optical means being variable as said first moveable reflective means moves, whereby to provide a variable distance from said second moveable reflective means to said focus line. 2. Optical apparatus as claimed in claim 1 characterised in that said first and second mo* eable reflective means are facets of a mult -faceted rmrr ** r drum. 3. Optical apparatus as claimed in claims 1 or 2 characterised in that there is provided a focusing means (31 ,32) in the beam path between the first moveable reflective means and the second movable reflective means whereby said changes in said optical path length are translated into said variation in the distance from said second movable reflective means to said focus line. 4. Optical apparatus as .claimed in claims 1 to 3, characterised in that said reflective means comprises a spherical or cylindrical mirror mounted adjacent the first moveable reflective means, the centre of curvature of the spherical mirror being generally coincident with the surface of the first moveable reflective means. 5. A variable focusing means comprising: moveable reflective means (11A), pivotally mounted so as to be rotatable about an axis (12) remote from the reflective surface of said moveable reflective means (26), first optical means (26) to direct a beam of radiation (15A) at said moveable reflective means, second optical means (26) to receive radiation reflected from said moveable reflective means (31 ,32), and optical focusing means (31 ,32), the arrangement being such the optical path length from the first to the second optical means varies as the moveable reflective means rotates about said axis, said change in ootical path length being multiplied by said focusing means whereby to vary the focus z : the oeam of radiation after it passes through said focusing means. 6. A variable focusing means as claimed in claim 5 characterised in that there is provided a scanning apparatus (11) for receiving the beam after it has passed through said focusing means to pass the beam and to focus it on a focal 1 ine. 7. Detection apparatus comprising: first moveable reflective means (11B) to scan and receive radiation from a radiation source provided along a focus line (13) of said first moveable reflective means, first optical means (33,32,31,24,26) to pass and define a beam reflected from said first moveable reflective means to a second moveable reflective means (11A); reflective means (28) to reflect said beam after it is reflected by said second moveable reflective means back to said second moveable reflective means, second optical means (26) for passing said beam reflected twice by said second moveable reflective means to a photo detector, whereby the optical path length between said first and second optical means is variable as said second moveable reflective means moves, whereby to provide a variaole distance from said first moveable reflective means to said focus 1 ine. 3. Optical apparatus for scanning a beam across an object field comprising: a source of radiation (21 ,22,23) for providing a beam (15) of said radiation, first moveable reflective means (11A) mounted to pivot about an axis remote from its respective surface, first optical means (22,23,25,26) for passing said beam to said first moveable reflective means so that said beam is reflected from said first moveable reflective means, reflective means (28) comprising a spherical or cylindrical mirror mounted adjacent the first moveable reflective means, the centre of curvature of the spherical or cylindrical mirror being displaced from the reflective surface of the first moveable reflective means by a predetermined distance, said reflective means being adapted to reflect said once reflected beam a second time to said first moveable reflective means where it is reflected a thi rd t ime, said optical means (26,24,31 ,32,33) to pass said thrice reflected beam from said first moveable reflective means, to a second moveable reflective means (11B) which pivots about an axis in synchronism with said first moveable reflective means whereby said beam is reflected and is scanned along a focus line (13), the lack of coincidence between the centre of curvature of said spherical or cylindrical mirror and the surface of the first moveable reflective means causing said twice reflected beam to oscillate from side to side as the twice reflected beam moves across the first moveable reflective means, said predetermined distance being chosen so that said side to said movement of the beam causes it to follow the second moveable reflective means as it rotates, 9. A laser printer comprising: a photosensitive drum (14), an optical apparatus as claimed in any of claims 1 to 4, means for modulating said beam, whereby said photosensitive drum (14) comprises said object field and the arrangement being such as to maintain the incident beam on said photosensitive drum (14) in focus as it scans across the photosensitive drum (14). 10. Apparatus as claimed in claim 9 characterised in that said first moveable - reflective means (42A) is pivotally mounted so as to be rotatable about an axis (12) remote from the reflective surface of said first moveable reflective means in an oscillatory manner.

Inventors: WEST ROBERT NOEL, WEST, ROBERT, NOEL
Applicants: SIRA LTD, SIRA LIMITED
Application year: 1993
Main classification: G02B26
IPC codes: G02B26, H04N1