High duty cycle synchronized multi-line scanner

A scanner includes a single scanning element such as a holographic disc or a polygon mirror and an illumination system that provides a plurality of input beams that are directed to different areas on the scanning element. In one embodiment, a light source provides a single beam that is alternately directed to a first area and a second area. While an input beam is incident on the first area, a first scan line forms. While an input beam is incident on the second area, a second scan line forms. Duty-cycle and energy efficiency of this embodiment are high because the beam from the source is switched from one area to the other during dead time when a scan beam would not otherwise have been directed to the desired scan aperture. Separate scan lines are automatically synchronized since they originate from the same source. Thus, alignment of multiple scan lines to form an extended scan line is simplified. Another embodiment simultaneously directs input beams to different areas of a scanning element to generate a plurality of synchronized scan lines suitable for alignment to form extended scan lines or for use separately in applications such as color printing.

BACKGROUND
 1. Field of the Invention
 This invention relates to line scanners and methods for scanning multiple
 synchronized scan lines using scanners having high duty cycles that
 conserve illumination source energy.
 2. Description of Related Art
 Many devices such as printers, inspection devices, and medical equipment
 contain line scanners. For example, a typical laser printer contains a
 line scanner that scans a digitally modulated laser beam to form an image
 on a media such as paper, film, or plates, and in the medical industry, a
 line scanner in an X-ray machine scans an X-ray beam through a patient or
 sample to form lines of an image. A typical line scanner includes a
 scanning element such as a polygon mirror or a holographic disc on which
 an input beam from a laser or other light source is incident. The scanning
 element redirects the input beam, for example, by reflection or
 diffraction, to project a scan beam. Movement such as rotation or
 oscillation of the scanning element moves or scans the scan beam along a
 scan line in an image area. Line scanners often include pre-scan and
 post-scan optical systems that adjust the focus or collimation of the
 input and scan beams as well the linearity of the scan line and the
 uniformity of scan rate.
 An important property of a line scanner is the duty cycle which is defined
 by the ratio of the time that the scan beam is imaging to the total
 operating time. Generally, a line scanner has a periodic motion that
 includes a dead time during which the line scanner is not directing the
 scan beam toward the image area. Thus, duty cycles for line scanners are
 generally less than 100% and typically range from about 20% to 70%. The
 duty cycle of a line scanner can limit the useful output illumination per
 unit of input power from a laser or other light source. Accordingly, a low
 duty cycle line scanner requires either a higher power light source, more
 efficient optics, or a slower scan rate to provide a fixed amount of
 useful illuminating energy density. Accordingly, line scanners having high
 duty cycles are sought.
 Another important property of line scanners is the imaging area or scan
 line length. Conventionally, creating long scan lines to cover a large
 imaging area requires either a large line scanner or multiple smaller line
 scanners operated in series. Using multiple line scanners (or scan heads)
 to create a long scan line has a number of advantages including the
 ability to design smaller or more compact systems with less expensive
 scanning heads. However, multiple-head systems often require elaborate and
 expensive line-connecting optics or electronics to control the relative
 positions of the scan lines from the various scan heads and to synchronize
 the independently operating scan heads. This is due in part to the lack of
 synchronization of the motion of the light sources and the scanning optics
 in the multiple scan heads. U.S. Pat. No. 5,654,817 describes a system
 employing multiple scan heads for large area imaging. Line scanners that
 are compact and do not require expensive line connecting optics or
 electronics are desired.
 SUMMARY
 In accordance with an embodiment of the invention, a high duty scanner has
 a single compact scan head capable of creating multiple synchronized scan
 lines. The scan lines are easily connected because a single scanning
 element provides a common reference for the multiple scan lines and in
 some embodiments because a common light source is oriented relative to
 each input beam so that motion of the light source manifests itself in
 each scan line in both the same direction and magnitude to provide
 synchronization. One embodiment of the invention redirects an input beam
 to increase the duty cycle of a scanner. In particular, pre-scan optics
 move an input beam from a first area for incidence on a scanning element
 to a second area for incidence on a scanning element. In alternate
 embodiments, the first and second areas contain portions of a single
 scanning element or alternatively portions of two separate scanning
 elements. Moving the input beam from the first to the second area occurs
 when the scanning beam reaches the end of a scan line but the scanning
 element has not positioned another facet in the first area for the start
 of another scan line. Upon movement of the input beam, the second facet
 immediately directs the scan beam into the image area for another scan
 line or for an extension of a scan line originated from the first facet.
 Moving the input beam thus can increase the duty cycle by increasing the
 time during which the scan beam is in the image area. The scan line length
 increases when second facet extends a scan line the first facet previously
 started.
 Alternatively, multiple input beams either derived from a common light
 source and beam splitters or from multiple light sources, are directed to
 multiple separated areas on a single scanning element to generate multiple
 synchronized scan lines. Timing of the multiple scan lines depends on the
 relative positions of the incident areas of the multiple input beams so
 that scan lines can be simultaneously or sequentially scan.
 The synchronized scan lines can be used separately in applications such as
 color laser printing or can be aligned for formation of a single extended
 scan line. Whether simultaneous input beams or alternating input beams are
 used, a single scanning element and/or light source simplifies alignment
 of multiple scan lines for formation of an extended scan line because many
 variations of the scanning element and light source are common to all of
 the scan lines that constitute the extended scan line.
 In one embodiment of the invention a scanner includes a scanning element
 such as a holographic disc or a polygon mirror and an illumination system
 that directs a first input beam to a first area and a second input beam to
 a second area. The first and second areas are separated from each other
 and situated so that portions of the scanning element move through the
 first and second areas during a scanning operation. First and second scan
 beams respectively originate from deflections of the first and second
 input beam by portions of the scanning element in the first and second
 areas. The input beams encountering the scanning element are oriented so
 that motion of both scan beams are synchronized for a given light source
 motion, and thus motion of the light source does not affect
 synchronization or alignment of the scan beams. Post-scan optics direct
 the first and second scan beams to illuminate first and second scan lines
 which can be used separately or aligned for joining that forms of an
 extended scan line. When the scan lines are aligned, a beam detector
 assembly can be positioned between the scanning element and an image plane
 in which the extended scan line is formed and in a gap between a path of
 the first scan beam to the end of the first scan line and a path of the
 second scan beam to the start of the second scan line. In one embodiment,
 the detector assembly is mounted on the back of a mirror having a first
 mirrored section positioned to reflect the first scan beam, a second
 mirrored section positioned to reflect the second scan beam, and a
 transparent section between the first and second mirrored sections.

Use of the same reference symbols in different figures indicates similar or
 identical items.
 DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
 In accordance with an embodiment of the invention, a high duty cycle
 scanner uses a single scanning element such as a polygon mirror or
 holographic disc, to create multiple synchronized scan lines. The
 synchronized scan lines are easily connected to create an extended scan
 lines of greater length because the multiple scan lines have common
 characteristics that arise from the common scanning element and the common
 orientation of the light source. The scanner can switch an input beam from
 one area containing a portion of the scanning element to another area
 during a "dead zone" when a scan beam would not be directed into the image
 area. As used herein, the dead zone is defined as the period of time
 between the end of one scan line and the start of the next scan line, and
 the downtime is defined as the time during which the laser beam is
 performing no useful function. Redirecting the input beam from one area to
 another can place a scan beam back in the image area and thus reduce the
 downtime of the scanner. The incident areas could contain portions of the
 same scanning element or correspond to and contain portions of different
 scanning elements. By deflecting the input beam from a first area to a
 second area during the dead zone of the first area and back to the first
 area during the dead zone of the second area, a scanner can exhibit a duty
 cycle of greater than 90%.
 If the input beam switches to an area containing another facet of the same
 scanning element, the two scan lines are synchronized. In addition, if the
 same light source generates the input beams to multiple areas, the motion
 of each scan line due to pointing error of the light source is
 synchronized. Synchronized scanning and light source pointing allow for
 better in-scan and cross-scan control of where scan lines connect. Thus,
 less costly correction techniques can align the multiple scan lines.
 FIGS. 1A and 1B respectively show a top view and a side view of a line
 scanner 100. Line scanner 100 includes a light source 110, path selection
 optics 120, pre-scan optics, a scanning element 140, and post-scan optics.
 Light source 110 generates an input beam such as a laser beam which path
 selection optics 120 and the pre-scan optics direct onto scanning element
 140 with proper input beam orientation. In the specific embodiment shown
 in FIG. 1B, light source 110 includes a laser 112, beam shaping elements
 114 and 116, and an acousto-optic modulator (AOM) 118 that digitally
 modulates the input beam's intensity. Such light sources are well known
 for line scanners used in laser printers. An alternative embodiment of the
 invention uses laser diode light sources that can be modulated internally,
 eliminating the need for the AOM 118. An alternative embodiment of the
 invention uses a light source other than lasers, for example, an LED
 (light emitting diode) or an x-ray source and eliminates AOM 118 if
 modulation of the input beam is not required. Beam shaping elements 114
 and 116 are selected as required to provide a beam profile for the
 application of line scanner 100.
 Path selection optics 120 switch the input beam back and forth between two
 paths, path A and path B. In the embodiment shown, path selection optics
 120 include an adjustable deflector 122 and a fixed deflector 124 having
 two facets 124A and 124B. Adjustable deflector 122 can be a
 piezo-electrically driven deflector mirror, acousto-optical deflector
 (AOD) or any device that allows control over the exit angle of a light
 beam. Fixed deflector 124 is a prism, a pair of mirrors oriented at a
 90.degree. angle, or any device that deflects a beam along sufficiently
 separated paths. In operation, deflector mirror 122 deflects the beam from
 source 110 onto one or the other of facets 124A and 124B of defector 124.
 From facet 124A, the beam reflects to an adjustable deflector 126A and
 along path A. From facet 124B, the beam reflects to an adjustable
 deflector 126B and along path B. FIG. 1C illustrates how path A and path B
 are widely separated, e.g., in opposite directions. Light source 110 and
 path selection optics 120 are oriented so that movement of or changes in
 light source 110 or path selection optics 120 causes synchronized movement
 of path A and path B at the image plane. In particular, the images of
 light source 110 formed from beams along path A and path B should have the
 same orientation, for example, both upright or both inverted. In scanner
 embodiments employing a common light source for multiple beams, optics
 that synchronize movement of the scan beams reduce the effects of pointing
 error because movement of the light source causes matching changes in all
 of the scan beams.
 A control circuit (not shown) periodically switches the orientation of
 adjustable deflector 122 from a configuration directing the input beam
 along path A to a configuration directing the input beam along path B. As
 described further below, the orientation of adjustable deflector 122 for
 the two configurations is selected to align a scan line generated when the
 beam is incident scanning element 140 from path A with a scan line
 produced when the beam is incident on scanning element 140 from path B.
 Deflector 122 selects a path A or B by positioning the beam on facet 124A
 or 124B and positions the beam on the selected facet for alignment along
 the in-scan direction. Deflectors 126A and 126B position their respective
 beams for cross-scan alignment of the two scanned lines A and B relative
 to each other.
 From selection optics 120, the input beam travels through pre-scan optics
 along the selected path. For path A or B, the beam passes through a first
 beam shaping lenses 132A or 132B, reflects from two fold-mirrors 134A and
 136A or 134B and 136B, and then passes through a second beam shaping lens
 138A or 138B before reaching holographic scan element 140. FIG. 1A does
 not show mirrors 136A and 136B and lenses 138A and 138 which overlie and
 would obscure the top view of scanning element 140. For simplicity of
 illustration, FIG. 1B omits lens 132A, mirror 134A, and the pre-scan
 optical elements along path B.
 The input beam is incident on a portion of scanning element 140 that is
 currently within an area 146A or 146B, depending on whether optics 120
 selected path A or B. During scanning, scanning element 140 rotates so
 that the portions of scanning element 140 within areas 146A and 146B
 constantly change. Changes in the properties of the portions of scanning
 element 140 within areas 146A and 146B change the angle at which the scan
 beam emerges and scan the scan beam along a scan line. Adjusting the
 orientations of deflectors 122 and 126A or 126B selects the location of
 incident area 146A or 146B, the radius at which the input beam strikes
 scanning element 140, and the cross-scan position of a scan line 170 in an
 image plane 180.
 In the post-scan optics, the scan beam initially reflects from fold mirrors
 152A and 154A or 152B and 154B onto a first aspheric curved mirror 156A or
 156B. Mirror 156A or 156B in turn reflects the scan beam onto a second
 aspheric curved mirror 158A or 158B. The size of mirrors 156A, 156B, 158A,
 and 158B and gaps between mirrors 156A and 156B or mirrors 158A and 158B
 along with the property of the portions of scanning element 140, control
 scan line apertures and the lengths of scan lines 170A and 170B. The
 focusing properties of curved mirrors 156A, 156B, 158A, and 158B are
 selected according to the application of line scanner 100. In particular,
 curved mirrors 156A, 156B, 158A, and 158B provide a linear scan (i.e., a
 constant scan speed at a constant rotation speed of scanning element 140),
 correct bow caused by scanning element 140, flatten the focus field (so
 that all points along the scan line are in focus), and create a
 quasi-telecentric scanned beam at the focal plane. Many alternative
 embodiments of the post-scan optics are known in the art or can be
 created. For example, curved mirrors 156A, 156B, 158A, and 158B could be
 replaced by more complex reflective and/or transmissive optics that
 provide scan lines of any desired characteristics.
 Curved mirrors 158A and 158B form scan lines 170A and 170B on the same
 image plane 180. An optional line-correction mirror 160 which monitors the
 position of the scan beams and the resulting scan lines is inserted
 between image plane 180 and mirrors 158A and 158B. If line-correction
 mirror 160 is used, the scan beam reflects from correction mirror 160 to
 image plane 180.
 In the exemplary embodiment, scanning element 140 is a multi-faceted
 holographic scan disc attached to a motor assembly. The holographic scan
 disc has four facets 141, 142, 143, and 144, each of which spans
 90.degree. and diffracts the beam into post-scan optics when selection
 optics 120 select a portion of the facet in area 146A or 146B. Of the
 90.degree. of each facet, about 40.5.degree. beginning about 24.75.degree.
 from a leading edge of the facet is for scanning and directs the beam to a
 path ending in a desired portion of the image plane. For the exemplary
 embodiment, scanning of extended scan line 170 requires a rotation of
 about 90.degree. (one facet of scanning element 140). Each extended scan
 line 170 includes two scan lines 170A and 170B that two separate facets of
 scanning element 140 provide.
 In an example scanning process, facets 141 and 144 scan the scan beam
 respectively along scan lines 170A and 170B which are aligned and
 concatenated to create extended scan line 170. Selection optics 120 select
 area 146A when scanning element 140 is oriented so that facet 141 is in
 area 146A and directs a scan beam along a path 191 to the start 171 of
 scan line 170A. At this point, the portion of facet 141 in area 146A is
 about 24.75.degree. from the edge of facet 141 in the exemplary
 embodiment. Scanning element 140 then rotates about 20.25.degree. while
 the scan beam in image plane 180 moves until facet 141 directs the scan
 beam along a path 192 to the midpoint 172 of scan line 170A. Rotation
 through the next 20.25.degree. moves the scan beam the same distance
 further to the end point 173 of scan line 170A where the scan beam is
 along a path 193 passing under scanning element 140. At this point, scan
 line 170A from facet 141 is complete, and path selection optics 120 switch
 the input beam from path A to path B.
 Areas 146A and 146B are separate from each other by an odd number of half
 facets, which in FIG. 1A is an angle .theta. of 135.degree.. Scanning
 element 140 must rotate another 4.5.degree. before facet 144 is in
 position to direct the scan beam to the start of scan line 170B.
 Accordingly, path selection optics 120 have the time required for a
 4.5.degree. rotation to switch the path of the input beam. The portion of
 facet 144 is in area 146B, at that point, directs the scan beam along a
 path 194 which ends at the starting point 174 of scan line 170B.
 Accordingly, scanning element 140 rotates through 45.degree. (half a
 facet) between the start of scan line 170A and the start of scan line
 170B. If scanner 100 is properly adjusted, the start point 174 of scan
 line 170B is at substantially the same location as the end point 173 of
 scan line 170A. Adjustment of the positions of the scan lines is discussed
 below. The scan beam moves from starting point 174 to end point 176 of
 scan line 170B while scanning element 140 rotates through 40.5.degree.. At
 this point, extended scan line 170 is complete, and path selection optics
 120 switch the input beam back to path A for the start of the next scan
 line. For the next extended scan line 170, facet 142 sweeps the image beam
 across scan line 170A, and facet 141 sweeps the image beam across scan
 line 170B.
 Alternative systems and methods are available for monitoring the position
 of scan lines 170A and 170B. One system uses a start-of-scan (SOS)
 position detector assembly 181, a central position detector assembly 182,
 and an end-of-scan (EOS) position detector assembly 183, which are located
 at or near the image plane 180. Detector assemblies 181, 182, and 183 can
 be mounted above scan line 170 to allow mounting in the dual head casting
 which is a frame for mounting of the various optical components. SOS
 detector assembly 181 and EOS detector assembly 183 are located outside of
 the scan aperture (i.e., the boundaries of scan line 170) and are similar
 or identical to SOS and EOS detector assemblies found in conventional
 laser printers, image setters and many other scanning devices. Central
 position detector assembly 182 is at or near the center of scan line 170.
 During typical operation of scanner 100, media being scanned blocks
 central position detector assembly 182. However, prior to scanning media,
 central detector assembly 182 can accurately determine the relative
 positions of the end of scan line 170A and the start of scan line 170B in
 both the cross-scan and in-scan directions. Detector assemblies 181 and
 183 then control the pixel clock data rate and the scan rate of scan lines
 170A and 170B when scanning media.
 Another system for monitoring scan line position includes optional line
 correction mirror 160 which is between a final focusing element of scanner
 100 and the image plane containing scan line 170. FIG. 2 shows an
 embodiment of mirror 160 which includes two mirrored sections 262 and 264
 and three transparent sections 261, 263, and 265. Mirror 160 can be made
 up of two separate mirrors mounted on a rigid platform with transparent
 sections between the mirrors or one continuous mirror that is silvered in
 only the desired sections. Mirrored section 262 reflects the scan beam
 during formation of scan line 170A and lies between an area 291 where path
 191 reflects from mirror 160 and an area 293 where path 193 reflects from
 mirror 160. Mirrored section 264 reflects the scan beam during formation
 of scan line 170B and lies between an area 294 where path 194 reflects and
 an area 296 where path 196 reflects. Scanner 100 is not perfectly
 telecentric and causes beam paths 191, 193, 194, and 196 to "toe out."
 This allows transparent section 263 to be in the gap between areas 293 and
 294.
 Beam position detector assemblies 211, 213, and 215 are mounted behind
 mirror 160 in transparent areas 261, 263, and 265, respectively. Detector
 assemblies 211 and 215 are respectively the start-of-scan (SOS) and
 end-of-scan (EOS) detector assemblies. Detector assembly 213 is a central
 detector assembly that monitors the positions of the end of scan line 170A
 and the start of scan line 170B. An advantage of central detector assembly
 213 when compared to central detector assembly 182 of FIG. 1A, is that
 central detector assembly 213 is upstream of the image plane and not
 blocked by media in position for scanning. A control system (not shown) is
 connected to position detector assemblies 211, 213, and 215 and monitors
 the in-scan timing and the cross-scan positions of scan lines 170A and
 170B. In particular, detector assemblies 211, 213, and 215 provide
 feedback on position and timing of the scan lines to the control system
 which in turn adjusts scanner 100 (e.g., deflectors 122, 126A and 126B of
 FIG. 1 A) to align scan lines 170A and 170B. Based on the information from
 detector assemblies 211, 263, and 265, the control system also provides a
 signal to the pixel clock to synchronize pixel speed and location within
 scanned beams 170A and 170B.
 FIG. 3 illustrates a scanner 300 in accordance with another embodiment of
 the invention. Scanner 300 differs from scanner 100 primarily in that a
 scanning element 340 of scanner 300 is a polygon mirror instead of a
 holographic disc as used for scanning element 140 of scanner 100. In
 scanner 300, light source 110 and path selection optics 120 direct an
 input beam to pre-scan optics including lenses 332A and 332B and mirrors
 334A, 334B, 336A, and 336B. As described below in regard to other
 embodiments of the invention, instead of selection optics 120 which switch
 the path of the input beam, input beams can be simultaneously directed
 along paths A and B using multiple light sources or a single light source
 and a beam splitter. In the embodiment shown in FIG. 3, the pre-scan
 optics direct an input beam to an incident area 346A or 346B containing a
 portion of scanning element 340. Scan element 340 directs a scan beam into
 post scan optics including flat mirrors 354A and 354B and focusing mirrors
 356A, 356B, 358A, and 358B. The post scan optics may also include optional
 line correction mirror 160. Scanning element 340 rotates so that while the
 input beam is incident area 346A, the scan beam scans scan line 170A.
 While the input beam is incident area 346B, the scan beam scans scan line
 170B.
 In an exemplary embodiment of the invention, scanning element 340 is
 hexagonal. For scan line 170A, path selection optics 120 direct the input
 beam to area 146A while scanning element rotates about 30.degree. (i.e.,
 about half the angular extent of a facet of scanning element 240.) Path
 selection optics 120 then switch the input beam to area 146B while
 scanning element 340 rotates through another 30.degree. rotation (i.e.,
 another half facet rotation). Path selection optics 120 then switch the
 input beam back to incident area 346 at which point another facet of
 scanning element 340 is back in position for the start of another scan
 line.
 FIGS. 4A and 4B illustrate a line scanner 400 which simultaneously directs
 input beams along paths A and B. Scanner 400 includes a common light
 source 410 that contains a laser 112 and beam shaping elements 114 and 116
 as described for light source 110 of FIG. 1B but does not include an
 acousto-optical modulator. Light source 110 directs a source beam to beam
 separation optics 420 which split the source beam into two input beams,
 one directed along path A and another directed along path B. Beam
 separation optics 420 include a beam splitter 422 (e.g., a half silvered
 mirror or other means of dividing the source beam into two equal energy
 input beams) and deflectors 124, 126A, 126B, and 428. Beam splitter 422
 simultaneously directs a first input beam via deflector 428 to facet 124A
 of deflector 124 and a second input beam to facet 124B of deflector 124.
 The first and second beams respectively reflect from deflectors 126A and
 126B and propagate along respective paths A and B. Acousto-optical
 modulators 418A and 418B are in respective paths A and B and control beam
 intensity into the pre-scan optics and the portions of scanning element
 140 in respective areas 146A and 146B. In paths A and B, scanner 400
 includes: pre-scan optical elements 132A, 132B, 134A, 134B, 136A, 136B,
 138A, and 138B; scanning element 140; and post-scan optical elements 152A,
 152B, 154A, 154B, 156A, 156B, 158A, and 158B which are the same as the
 elements described above in regard to FIGS. 1A and 1B.
 With scanning element 140 as described above and areas 146A and 146B
 separated by an odd number of half facets, only one of areas 146A and 146B
 at a time contain a portion of scanning element 140 that directs a scan
 beam into extended scan line 170. Accordingly, separation optics 420
 always directs one of the input beams down a path (A or B) that currently
 does not lead to scan line 170. This reduces the percentage of
 illumination power utilized for imaging. However, scanning element 140
 automatically alternates between directing a scan beam from area 146A to
 scan line 170A and directing a scan beam from area 146B to scan line 170B
 and does not require control logic and beam path switching that is
 synchronized with the rotation of scanning element 140. The delay between
 the scan beam from area 146A reaching the end of scan line 170A and the
 scan beam from area 146B starting scan line 170B can be adjusted by
 changing the angle between areas 146A and 146B. In one embodiment, areas
 146A and 146B are positioned so that area 146A and area 146B
 simultaneously direct scan beams along respective paths 193 and 194. This
 eliminates the delay between the end of scan line 170A and the start of
 scan line 170B. The elimination of the delay can simplify cross-scan
 corrections required in applications where a medium moves at constant
 velocity in the cross-scan direction relative to scanner 400 during
 scanning. Larger changes in the relative positions of areas 146A and 146B
 can implement a variety of scanning options including simultaneous
 scanning of scan lines 170A and 170B.
 FIG. 5 illustrates a scanner 500 which simultaneously scans scan lines 570A
 and 570B. Scanner 500 has two input beams derived from a common light
 source using separation optics such as describe in regard to FIG. 4B, a
 scanning element 510 with four substantially identical facets, and
 incident areas 520A and 520B that are an integer number of facets apart,
 e.g., 180.degree. apart relative to a rotation axis of scanning element
 510. With this orientation, incident areas 520A and 520B simultaneously
 contain portions of scanning element 510 that direct scan beams to scan
 lines 570A and 570B. The post-scan optics include flat mirrors 552A, 552B,
 554A, and 554B and focusing elements such as curved mirrors 556A, 556B,
 558A, and 558B which simultaneously direct scan beams 530A and 530B from
 areas 520A and 520B to the same image plane 580.
 FIG. 6 illustrates a scanner 600 which has three input beams (not shown)
 that simultaneously illuminate three synchronized areas 620A, 620B, and
 620C. Either separate light sources or a common light source with a beam
 splitter can generate the three input beams. Areas 620A, 620B, and 620C
 are situated so that identical portions of three different facets of a
 scanning element 610 are simultaneously in areas 620A, 620B, and 620C.
 Post-scan optics direct scan beams from areas 620A, 620B, and 620C to an
 image plane 680 where scan lines 670A, 670B, and 670C form. The post-scan
 optics include six flat mirrors, 652A, 652B, 652C, and 654A, 654B, and
 654C which direct scan beams from respective areas 620A, 620B, and 620C to
 three identical focusing systems (curved mirrors) 656A and 658A, 656B and
 658B, and 656C and 658C, respectively. Mirrors 652A, 652B, 652C, and 654A,
 654B, and 654C are positioned so that the optical path length from areas
 620A, 620B, and 620C to respective focusing systems 656A and 658A, 656B
 and 658B, and 656C and 658C are the same. When a common light source is
 used, proper orientation of the input beams before deflection in area
 620A, 620B or 620C causes the image of the light source to have the same
 orientation at each of the scan lines 670A, 670B, and 670C. This allows
 changes in the orientation of a common light source (not shown) to cause
 matching shifts in each of the scan lines 670A, 670B, and 670C and thereby
 improves the scanner's tolerance of pointing error.
 Systems similar to scanner 600 can generate four or more simultaneous
 synchronized scan beams in an image plane. For four or more simultaneous
 scan beams, an illumination system has pre-scan optics that direct four or
 more input beams to four or more respect incidence areas. The incidence
 areas simultaneously contain the same portions of different facets of a
 scanning element. Post-scan optics can include, for example, four or more
 similar or identical focusing systems and injection optics that direct
 scan beams from the incidence areas to corresponding focusing systems. The
 injection optics may differ for each incidence area but provide the same
 optical path lengths from incidence areas to the focusing systems. The
 geometry of the injection optics depends on the number of facets on the
 scanning element and the locations of the incidence areas relative to the
 scanning element. Pre-scan optics can compensate for differences in the
 injection optics so that each image formed by a scan beam has the same
 orientation as images formed by the other scan beams.
 An advantage of above scanners where multiple scan beams simultaneously
 originate from a common light source and a scanning element/motor assembly
 is the high degree of synchronization of the scan beams. Known
 line-scanner effects such as wedge (deviation in parallelism of two
 surfaces of the disc), wobble of motor bearings, disc eccentricity, and
 variations in the rotational speed of the scanning element have
 substantially less effect on the synchronization of scan lines from a
 common scanning element and a common light source than on the
 synchronization of scan lines from separate scanning elements.
 Additionally, timing synchronization between the two scan lines is
 automatic for a single scanning element, whereas in the two element
 system, the rotational orientation of the two scanning elements must be
 synchronized every time the device is powered up. In particular, when
 starting a system having two scanning elements, the rotational speed of
 each element is adjusted in an iterative fashion to match not only the
 speeds but also the relative phases of the elements. A scanner using a
 single scanning element has the further advantages of being relatively
 compact and inexpensive because a single scanning head provides multiple
 scan beams without requiring costly components for scan line
 synchronization. Prior systems have required a separate scan head for each
 scan beam and complicated synchronization systems.
 FIG. 7 shows a scanner 700 in accordance with an embodiment of the
 invention that incorporates multiple input beams and/or switching of the
 paths of the input beams to provide a high duty cycle. Scanner 700
 includes a scanning element (holographic disc) 710 having four facets and
 an illumination system (not shown). In a first configuration, the
 illumination system simultaneously directs four input beams to incidence
 areas 720A, 720B, 720C, and 720D. In a second configuration, the
 illumination system simultaneously directs two input beams to two of four
 incidence areas 720A, 720B, 720C, and 720D at a time. In a third
 configuration, the illumination system serially directs a single input
 beam to areas 720A, 720B, 720C, and 720D. Post-scan optics direct scan
 beams from areas 720A, 720B, 720C, and 720D for formation of respective
 scan lines 770A, 770B, 770C, and 770D. In scanner 700, scan lines 770A and
 770B are aligned in an image plane 780A to form an extended scan line.
 Similarly, the post scan optics form scan lines 770C and 770D in an image
 plane 780C to form a second extended scan line. Alternatively, the
 post-scan optics can direct each scan line 770A, 770B, 770C, and 770D to
 four different image planes to form four separate scan lines or direct all
 four scan lines 770A, 770B, 770C, and 770D to the same image plane. Scan
 lines in a common image plane can be aligned and synchronized to form one
 long scan line or simultaneously illuminate two or more parallel scan
 lines.
 In the first illumination configuration, each of areas 720A, 720B, 720C,
 and 720D directs a scan beam to the respective scan line only when a
 suitable portion of scanning element 710 is in the area. As illustrated,
 areas 720A and 720C are 180.degree. apart on a four-facet scanning element
 and simultaneously direct scan beams to respective scan lines 770A and
 770C. Similarly, areas 720B and 720D are 180.degree. apart and
 simultaneously direct scan beams to respective scan lines 770B and 770D.
 Scanner 700 in the first illumination configuration automatically switches
 from starting scan lines 770A and 770C to starting scan lines 770B and
 770D after about a one-half facet revolution of scanning element 710.
 In the second illumination configuration, areas 720A, 720B, 720C, and 720D
 are paired so that each input beam corresponds to a pair of areas, and the
 lighting system switches each input beam back and forth from a path to one
 incident area to path to the other incident area in the pair. For example,
 area 720A or 720D can be paired with area 720B or 720C. In the exemplary
 embodiment, areas 720A and 720B are paired, and areas 720C and 720D are
 paired so that aligned scan lines share the same light source and possible
 misalignments of that light source. In operation, when input beams are
 simultaneously directed to areas 720A and 720C, scanning element 710
 directs scan beams to respective sets of flat mirrors 752A and 754A and
 752C and 754C which reflect scan beams to respective focusing systems 756A
 and 756C. Scanning element 710 rotates through about half the angular span
 of a facet (e.g., somewhat less than 45.degree. for a four-facet scanning
 element) while the scan beams scan along scan lines 770A and 770C. The
 illumination system then switches the input beams to paths to areas 720B
 and 720D. Areas 720B and 720D are positioned so that portions of facets in
 areas 720B and 720D are the same as the portions that were in areas 720A
 and 720C before the rotation by one half of a facet. While the input beams
 are directed to areas 720B and 720D, scanning element 710 rotates through
 about half a facet, and scan beams sweep along scan lines 770B and 770D.
 Thus from the start of scan line 770A or 770C to the end of scan line 770B
 or 770D, scanning element 710 rotates by one facet and positions the next
 facets in areas 720A and 720C for the start of another scan line. The
 second illumination configuration utilizes illumination power more
 efficiently than the first illumination configuration but requires path
 selection optics and control circuits that are synchronized with the
 rotation of scanning element 710.
 In the third illumination configuration, path selection optics direct an
 input beam along a path to area 720A for scanning of scan line 770A. (This
 requires a one half facet rotation of scanning element 710.) The path
 selection optics can then switch the input beam to a path to area 720B or
 720D. Both areas 720B and 720D then contain the proper portions of
 scanning element 710 to begin scanning respective scan line 770B or 770D.
 Once the second scan line 770B or 770D is complete, the path selection
 optics direct the input beam to area 720C for scanning of the third scan
 line 770C. Following scanning of scan line 720C, the path selection optics
 direct an input beam to area 720D or 720B for scanning of the last of the
 four scan lines 770D or 770B. The third illumination configuration has the
 advantage of high duty cycle without beam splitting since a single input
 beam (and a single light source) are used for all of scan lines 770A,
 770B, 770C, and 770D, but the third illumination configuration is slower
 than the first or second illumination configurations because scan lines
 are scanned serially (one at a time). However, this illumination is
 attractive for applications that require high energy density since all
 energy from the source is directed to a single scan line.
 FIG. 8 shows a scanner 800 in accordance with an embodiment of the
 invention that uses multiple light sources 810A and 810B rather than path
 selection optics and path switching as in scanners 100 and 300 of FIGS.
 1A, 1B, and 1C or a beam splitter as in scanner 400 of FIGS. 4A and 4B.
 Light sources 810A and 810B may be, for example, lasers, laser diodes, or
 other sources of electromagnetic radiation of suitable wavelength,
 intensity, and collimation. Light sources 81 OA and 810B simultaneously
 direct separate input beams through respective pre-scan optics 830A and
 830B to respective areas 846A and 846B. A scanning element 840, which is a
 holographic disc in this embodiment, and areas 846A and 846B are
 positioned so that the same portions of two different facets of scanning
 element 840 are simultaneously in areas 846A and 846B. This causes
 symmetric deflections of scan beams 890A and 890B. The portions of
 scanning element 840 in areas 846A and 846B direct scan beams 890A and
 890B into post-scan optics which include flat mirrors 852A and 852B,
 focusing elements such as curved mirrors 854A and 854B, and holographic
 elements 856A and 856B.
 In FIG. 8, the post-scan optics direct synchronized scan beams 890A and
 890B to separate image planes so that scan beams 890A and 890B scan along
 the surfaces of drums 870A and 870B respectively. Systems having multiple
 image planes and drums are commonly employed, for example, in color laser
 printers. For color printing, beams corresponding to different colors must
 be synchronized for drums 870A and 870B to apply corresponding lines and
 pixels of different colors to the same locations on media being printed.
 Alternatively, both scan beams can be directed to the same image plane and
 conjoined to form a single extended scan line. Scanner 800 has the
 advantages of using a single scanning element 840. In particular,
 variations in rotational speed of scanning element 840 have the same
 effects on scan lines on drums 870A and 870B. Use of separate light
 sources 810A and 810B may provide a more economical method of achieving a
 desired scan beam intensity than would using a single light source and
 beam splitting. According, separate light source can be employed in any of
 the above described embodiments employing multiple input beams.
 Although the invention has been described with reference to particular
 embodiments, the description is only an example of the invention's
 application and should not be taken as a limitation. In particular, even
 though much of preceding discussion was aimed at holographic scanning
 elements having four facets, alternative embodiments of the invention
 include scanning elements including holographic discs and polygon mirrors
 having any number of facets. Additionally, due to the position and time
 synchronization provided by a single deflecting element, each of the
 embodiments described can be used in conjunction with the line correction
 techniques described to concatenate two or more scan lines to create
 longer extended scan lines. Various other adaptations and combinations of
 features embodiments disclosed are within the scope of the invention as
 defined by lowing claims.