Patent Publication Number: US-8994770-B2

Title: Optical scanning system for use in an imaging apparatus

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
     The present application is related to and claims benefit under 35 U.S.C. 119(e) from U.S. provisional application 61/792,288, filed Mar. 15, 2013, entitled, “Optical Scanning System for Use in an Imaging Apparatus,” the content of which is hereby incorporated by reference herein it is entirety. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     None. 
     REFERENCE TO SEQUENTIAL LISTING, ETC. 
     None. 
     BACKGROUND 
     1. Field of the Disclosure 
     The present disclosure relates generally to an optical scanning system in an imaging apparatus, and particularly to such a system utilizing a scan lens design and arrangement thereof which allow for a more compact scanning unit. 
     2. Description of the Related Art 
     In various imaging devices which utilize light to form images, optical scanning systems are typically employed to scan modulated light beams from one or more light sources onto at least one target surface on which images are to be formed. In an electrophotographic imaging device, for example, an optical scanning system typically includes a scanning mirror which reflects a modulated light beam towards a plurality of optical components. Such optical components may include lenses and mirrors which direct and focus the reflected light beam to form light spots upon a surface of a photosensitive member. As the scanning mirror moves, either in a reciprocating manner as with the case of a torsion oscillator or rotationally as with the case of a polygon mirror, the light beam reflected thereby is scanned across each of the optical components of the optical scanning system. Ultimately, the light beam impinges and is swept across the photosensitive member, which may itself be rotating, as scan lines so as to form latent images thereon. 
     A color laser printer, for example, may have four laser beam channels in its laser scanning unit (LSU), one for each of cyan, magenta, yellow, and black color planes. Scan lenses are used to focus the laser beams into small spot sizes on photosensitive members across all scan positions. In addition, the scan lenses keep a linear spot velocity during scanning and minimize the process and scan jitter induced by scanner mirror error. Scan lenses are complex optical components in the LSU and contribute a significant portion to the total size and cost of an LSU. 
     Some traditional optical designs for LSUs generally require one or two scan lenses per channel. Thus, the total quantity of scan lenses for all four channels for a color LSU may usually range from four to eight. Having such number of scan lenses may require a relatively large space requirement for the LSU. Moreover, the cost of the LSU also increases as the number of scan lenses increases. 
     In some existing designs, the number of scan lenses is reduced by allowing two channels to share one scan lens such that two laser beams enter the scan lens through opposite surfaces thereof. However, because two laser beams enter a single scan lens from opposite directions, the opposite lens surfaces must be symmetrical and the scan lens is typically large and thick in order to have a decent optical performance particularly on laser spot size. The cost of a plastic scan lens, for example, is mainly determined by the cycle time of the injection molding, and the cycle time is mainly determined by the thickness and size of the scan lens because a thicker lens requires much longer cooling time. As a result, the cost reduction due to the decrease in the quantity of scan lenses may be offset by increased cost per scan lens. Moreover, designs requiring two thick scan lenses may also add additional constraints on the optical layout of the LSU, such as requiring additional fold mirrors before the laser beams reach the scan lenses. This adds to the accumulated tolerances for the optical paths and makes it difficult to have precise optical alignment therein. 
     Accordingly, there is a need for an improved scanning unit which is more size and cost efficient. 
     SUMMARY 
     Example embodiments of the present disclosure provide a scanning system incorporating an optical design which allows for a more compact scanning unit. 
     In an example embodiment, a scanning system includes a scanning member having at least one reflective surface for reflecting light incident thereon. A first light source, a second light source, a third light source, and a fourth light source are controllable to emit first, second, third, and fourth light beams, respectively. Each of the first, second, third, and fourth light beams are configured to be incident on one planar surface portion of the at least one reflective surface of the scanning member at different angles with respect to a reference plane extending perpendicular to the planar surface portion such that the light beams are reflected off of the planar surface portion at different angles with respect to the reference plane. A first scan lens and a second scan lens are disposed downstream from the scanning member relative to the optical paths of the light beams. The first scan lens receives and focuses the reflected first and second light beams, and the second scan lens receives and focuses the reflected third and fourth light beams. A plurality of mirrors are disposed downstream the scanning member to direct the reflected and focused light beams to at least one surface. 
     In another example embodiment, each of the scan lenses has a light incident surface that is substantially planar, and a light exit surface having two curved surface sections. A first curved surface section and a second curved surface section of the light exit surface define therebetween a junction line extending between opposed longitudinal ends of the scan lens. The junction line is non-linear and, in particular, substantially bowed in a sub-scan direction perpendicular to a main scan direction extending longitudinally across the scan lens. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above-mentioned and other features and advantages of the disclosed embodiments, and the manner of attaining them, will become more apparent and will be better understood by reference to the following description of the disclosed embodiments in conjunction with the accompanying drawings, wherein: 
         FIG. 1  is a side elevational view of an imaging device according to an example embodiment; 
         FIG. 2  illustrates an optical layout of a laser scanning unit of the imaging device in  FIG. 1  according to an example embodiment; 
         FIG. 3A  illustrates a side view of a scan lens used in the laser scanning unit of  FIG. 2  according to an example embodiment; 
         FIG. 3B  illustrates a top view of the scan lens in  FIG. 3A ; 
         FIG. 3C  illustrates a front view of the scan lens in  FIG. 3A ; 
         FIG. 4  illustrates a perspective view of the scan lens in  FIG. 3A-3C ; 
         FIG. 5  illustrates two sets of ray traces through the scan lens in  FIG. 3A ; 
         FIG. 6  is a graph showing vertical position of a discontinuity line between two curved surface sections of the scan lens of  FIGS. 3A-3C  relative to two light beam channels; and 
         FIG. 7  is a graph illustrating step sizes of discontinuity points between the two curved surface sections associated with the discontinuity line of  FIG. 6 . 
     
    
    
     DETAILED DESCRIPTION 
     It is to be understood that the present disclosure is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the drawings. The present disclosure is capable of other embodiments and of being practiced or of being carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of “including,” “comprising,” or “having” and variations thereof herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. Unless limited otherwise, the terms “connected,” “coupled,” and “mounted,” and variations thereof herein are used broadly and encompass direct and indirect connections, couplings, and mountings. In addition, the terms “connected” and “coupled” and variations thereof are not restricted to physical or mechanical connections or couplings. 
     Spatially relative terms such as “top”, “bottom”, “front”, “back” and “side”, “above”, “under”, “below”, “lower”, “over”, “upper”, and the like, are used for ease of description to explain the positioning of one element relative to a second element. Terms such as “first”, “second”, and the like, are used to describe various elements, regions, sections, etc. and are not intended to be limiting. Further, the terms “a” and “an” herein do not denote a limitation of quantity, but rather denote the presence of at least one of the referenced item. 
     Furthermore, and as described in subsequent paragraphs, the specific configurations illustrated in the drawings are intended to exemplify embodiments of the disclosure and that other alternative configurations are possible. 
     Reference will now be made in detail to the example embodiments, as illustrated in the accompanying drawings. Whenever possible, the same reference numerals will be used throughout the drawings to refer to the same or like parts. 
       FIG. 1  illustrates a color image forming device  100  according to an example embodiment. Image forming device  100  includes a first toner transfer area  102  having four developer units  104  that substantially extend from one end of image forming device  100  to an opposed end thereof. Developer units  104  are disposed along an intermediate transfer member (ITM)  106 . Each developer unit  104  holds a different color toner. The developer units  104  may be aligned in order relative to the direction of the ITM  106  indicated by the arrows in  FIG. 1 , with the yellow developer unit  104 Y being the most upstream, followed by cyan developer unit  104 C, magenta developer unit  104 M, and black developer unit  104 K being the most downstream along ITM  106 . 
     Each developer unit  104  is operably connected to a toner reservoir  108  for receiving toner for use in a printing operation. Each toner reservoir  108  is controlled to supply toner as needed to its corresponding developer unit  104 . Each developer unit  104  is associated with a photoconductive member  110  that receives toner therefrom during toner development to form a toned image thereon. Each photoconductive member  110  is paired with a transfer member  112  for use in transferring toner to ITM  106  at first transfer area  102 . 
     During color image formation, the surface of each photoconductive member  110  is charged to a specified voltage, such as −800 volts, for example. At least one laser beam LB from a printhead or laser scanning unit (LSU)  130  is directed to the surface of each photoconductive member  110  and discharges those areas it contacts to form a latent image thereon. In one embodiment, areas on the photoconductive member  110  illuminated by the laser beam LB are discharged to approximately −100 volts. The developer unit  104  then transfers toner to photoconductive member  110  to form a toner image thereon. The toner is attracted to the areas of the surface of photoconductive member  110  that are discharged by the laser beam LB from LSU  130 . 
     ITM  106  is disposed adjacent to each of developer unit  104 . In this embodiment, ITM  106  is formed as an endless belt disposed about a drive roller and other rollers. During image forming operations, ITM  106  moves past photoconductive members  110  in a clockwise direction as viewed in  FIG. 1 . One or more of photoconductive members  110  applies its toner image in its respective color to ITM  106 . For mono-color images, a toner image is applied from a single photoconductive member  110 K. For multi-color images, toner images are applied from two or more photoconductive members  110 . In one embodiment, a positive voltage field formed in part by transfer member  112  attracts the toner image from the associated photoconductive member  110  to the surface of moving ITM  106 . 
     ITM  106  rotates and collects the one or more toner images from the one or more developer units  104  and then conveys the one or more toner images to a media sheet at a second transfer area  114 . Second transfer area  114  includes a second transfer nip formed between at least one back-up roller  116  and a second transfer roller  118 . 
     Fuser assembly  120  is disposed downstream of second transfer area  114  and receives media sheets with the unfused toner images superposed thereon. In general terms, fuser assembly  120  applies heat and pressure to the media sheets in order to fuse toner thereto. After leaving fuser assembly  120 , a media sheet is either deposited into output media area  122  or enters duplex media path  124  for transport to second transfer area  114  for imaging on a second surface of the media sheet. 
     Image forming device  100  is depicted in  FIG. 1  as a color laser printer in which toner is transferred to a media sheet in a two step operation. Alternatively, image forming device  100  may be a color laser printer in which toner is transferred to a media sheet in a single step process—from photoconductive members  110  directly to a media sheet. In another alternative embodiment, image forming device  100  may be a monochrome laser printer which utilizes only a single developer unit  104  and photoconductive member  110  for depositing black toner directly to media sheets. Further, image forming device  100  may be part of a multi-function product having, among other things, an image scanner for scanning printed sheets. 
     Image forming device  100  further includes a controller  140  and memory  142  communicatively coupled thereto. Though not shown in  FIG. 1 , controller  140  may be coupled to components and modules in image forming device  100  for controlling same. For instance, controller  140  may be coupled to toner reservoirs  108 , developer units  104 , photoconductive members  110 , fuser  120  and/or LSU  130  as well as to motors (not shown) for imparting motion thereto. It is understood that controller  140  may be implemented as any number of controllers and/or processors for suitably controlling image forming device  100  to perform, among other functions, printing operations. 
     Referring now to  FIG. 2 , an optical layout of LSU  130  is shown according to an example embodiment of the present disclosure. LSU  130  may include a light assembly  202 , pre-scan optics  204 , a scanning device  206 , and post-scan optics  208 . 
     Light assembly  202  may include light sources  202 A,  202 B,  202 C, and  202 D associated with cyan, magenta, yellow and black (CMYK) color image planes, respectively, such that each light source generates a light beam for use in forming a latent image on the surface of a corresponding photoconductive member  110 . Each light source of light assembly  202  may be implemented, for example, using a laser diode or any other suitable device for generating a beam of light. LSU  130  may also include driver circuitry (not shown) communicatively coupled to controller  140  for receiving video/image information and/or control data that may be utilized to set and/or vary the laser power used by each light source of light assembly  202 . 
     Pre-scan optics  204  may include one or more collimating lenses  210  and/or pre-scan lens  212  to direct and focus each of the modulated beams LB emitted by light sources  202 A- 202 D towards scanning device  206 . In one example, pre-scan lens  212  may be a cylinder pre-scan lens. 
     Scanning device  206  may include at least one reflective surface  214  for receiving and reflecting light incident thereon. In one example embodiment, scanning device  206  may comprise a scanning oscillator, such as a torsion oscillator, controlled to operate bi-directionally at a scanning frequency to create scan lines on photoconductive members  110  in both forward and reverse directions along a main scan direction. The main scan direction may refer to the direction of scanning of a laser beam by scanning device  206  across an optical component or a photoconductive member  110 . With respect to LSU  130  of  FIG. 2 , the main scan direction may be seen to be either into or out of the sheet on which  FIG. 2  appears and generally extends between longitudinal end portions of each optical component in the post-scan path of each laser beam. On the other hand, a sub-scan direction may refer to a direction perpendicular to the main scan direction. The sub-scan direction may, in some cases, correspond to a direction along the height of an optical component. In another example embodiment, scanning device  206  may include a polygon mirror having a plurality of facets and controlled to rotate at a rotational velocity during an imaging operation so as to uni-directionally scan laser beams LB emitted by light sources  202 A- 202 D to create scan lines on photoconductive drums  110  in a forward direction. 
     Post-scan optics  208  may include post-scan lenses  218 A,  218 B and a plurality of mirrors  220  used to focus and direct each modulated beam LB to its corresponding photoconductive member  110 . It is understood that components forming post-scan optics  208  may be provided within and/or as part of the LSU  130  or alternatively be provided separately therefrom, such as being directly mounted to a frame within image forming device  100  external to LSU  130 . 
     During an imaging operation, image data corresponding to an image to be printed may be converted by controller  140  into laser modulation data. The laser modulation data may be utilized by the driver circuitry to modulate at least one of light sources  202 A- 202 D so that LSU  130  outputs modulated laser beams LB. Each laser beam LB emitted from its corresponding light source  202  may be collimated by corresponding collimation lenses  210  and pass through pre-scan lens  212  so that the laser beams LB converge to strike the reflective surface  214  of the scanning device  206 . 
     In  FIG. 2 , a reference horizontal plane  230  passes through a normal of the reflective surface  214  of the scanning device  206  from a central portion thereof, and extends into and out of the sheet. According to an example embodiment, the light sources  202 A- 202 D may be arranged vertically offset from each other on the same side of scanning device  206 . In the example layout shown, two upper light sources  202 A,  202 B are disposed above reference plane  230  and two lower light sources  202 C,  202 D are disposed below reference plane  230 . After passing through pre-scan lens  212 , the laser beams emitted by each of the light sources  202 A- 202 D converge into scanning device  206 . The vertically offset arrangement between the light sources allows each of the emitted laser beams LB to be incident on the reflective surface  214  of the scanning device  206  at different angles with respect to reference plane  230 . In particular, laser beam LB 1  emitted by light source  202 A becomes incident on the reflective surface  214  from above the reference plane at an angle θ 1 , laser beam LB 2  emitted by light source  202 B becomes incident on the reflective surface  214  from above reference plane  230  at an angle θ 2 , laser beam LB 3  emitted by light source  202 C becomes incident on the reflective surface  214  from below the reference plane  230  at an angle θ 3 , and laser beam LB 4  emitted by light source  202 D becomes incident on the reflective surface  214  from below the reference plane  230  at an angle θ 4 . Thus, laser beams LB emitted by the light source  202  become incident on the reflective surface  214  from the same side of the scanning device  206  and at different angles with respect to the reference plane  230 . In an example embodiment, the laser beams LB may strike the reflective surface  214  at overlapping reflection points. In other alternative embodiments, the laser beams LB may strike the reflective surface  214  without overlapping with each other. 
     As a further consequence of the vertically offset arrangement between light sources  202 A- 202 D, laser beams LB are also reflected off of the reflective surface  214  of the scanning device  206  at different angles with respect to the reference plane  230 . In the example shown, the upper channels consisting of laser beams LB 1 , LB 2  are reflected off of the reflective surface  214  towards a direction below the reference plane  230 , while the lower channels consisting of laser beams LB 3 , LB 4  are reflected off of the reflective surface  214  towards a direction above the reference plane  230 . Reflected laser beams LB 3 , LB 4  may directly enter first scan lens  218 A disposed above the reference plane  230  while reflected laser beams LB 1 , LB 2  may be picked off by fold mirror  220 A disposed below the reference plane  230 . Fold mirror  220 A may direct reflected laser beams LB 1 , LB 2  toward second scan lens  218 B disposed above the reference plane  230 . The first and second scan lenses  218  may focus the reflected laser beams into small spot sizes on corresponding photoconductive members  110  with the aid of the plurality of mirrors  220  positioned downstream of the first and second scan lenses  218 . In this example, two laser beams LB share a single scan lens  218  such that the optical system requires only two scan lenses  218 , and only a single fold mirror  220 A is used upstream of the scan lens  218 B relative to laser beam direction. 
     In the example embodiment of  FIG. 2 , reference plane  230  is depicted as being horizontal and light sources  202 A- 202 D are depicted as being vertically offset from each other. However, it will be appreciated that the above-described orientations have been presented for ease of description and should not be considered limiting, and that other orientations may be implemented. For example, in an alternative embodiment, light sources  202 A- 202 D may be horizontally offset from each other, and reference plane  230  may be a vertical plane. 
       FIGS. 3-4 , show an example shape and profile of each of scan lenses  218  according to an example embodiment.  FIGS. 3A ,  3 B, and  3 C illustrate side, top, and front views, respectively, of each of scan lenses  218 , while  FIG. 4  illustrates a perspective view thereof. 
     As shown in  FIG. 3A , scan lens  218  may include a light incident surface  302  and a light exit surface  304 . The light incident surface  302  may be substantially continuous and in the example embodiment is substantially planar. The light exit surface  304 , on the other hand, may be partitioned into two aspherical surfaces shown in  FIGS. 3A and 4  as two curved surface sections  304 A and  304 B. The two curved surface sections  304 A and  304 B may have different surface equations which may be derived or selected based on several factors to provide desired focal lengths for each of the two lens sections, such as, for example, an index of refraction of the scan lens  218 , thickness, and radius of curvatures of the light incident surface  302  and respective curved sections of light exit surface  304 . In an example embodiment, the first curved surface section  304 A may be defined by the surface equation:
 
 z =−(9.531×10 −4 ) x   2 −(2.335×10 −2 ) y   2 +(1.551×10 −7 ) x   4 +(4.161×10 −6 ) x   2   y   2 −(1.340×10 −11 ) x   6 −(7.501×10 −10 ) x   4   y   2 ;
 
while the second curved surface section  304 B may be defined by the surface equation:
 
 z =−(9.788×10 −4 ) x   2 −(2.324×10 −2 ) y   2 +(1.604×10 −7 ) x   4 +(3.846×10 −6 ) x   2   y   2 −(1.551×10 −11 ) x   6 −(4.078×10 −10 ) x   4   y   2 ;
 
where z is the surface sag, x is along the main scan direction, and y is along the sub-scan direction, all in units of millimeters.
 
     The two curved surface sections  304 A and  304 B may further have different optical axes. For example, as shown in  FIG. 3A , the first curved surface section  304 A may have an optical axis indicated by arrow  306 A and the second curved surface section  304 B may have an optical axis indicated by arrow  306 B. In  FIG. 5 , two sets of laser beams LB-X, LB-Y are shown entering the light incident surface  302  of scan lens  218  but exiting the scan lens  218  separately at the two curved surface sections  304 A,  304 B, respectively. Because the optical axes for the first curved surface section  304 A and the second curved surface section  304 B have different angles relative to a reference axis  340  ( FIG. 3A ), the two sets of laser beams LB-X, LB-Y entering the light incident surface  302  may diverge upon separately exiting the scan lens  218  at the two curved surface sections  304 A and  304 B, respectively. This may allow for easier separation of laser beams LB-X, LB-Y by pickoff mirrors  220  positioned downstream of the scan lenses  218 . 
     Since four laser beams LB 1 -LB 4  share one reflective surface  214  of the scanning device  206  by which the beams are deflected, the optical systems involved are off-axis systems. More particularly, the upper and lower channels depicted in  FIG. 2  may have a relatively large off-axis angle relative to the optical axis of the reflective surface  214 . Because of this, the beam tracing on the surface of the scan lenses  218  during scanning may exhibit an optical bow. For example, as shown in  FIG. 6 , a first channel  320 A which may correspond to a first laser beam exiting the scan lens  218  at the upper first curved surface section  304 A, and a second channel  320 B which may correspond to a second laser beam exiting the same scan lens  218  at the lower second curved surface section  304 B, may be curved or bowed in the vertical or sub-scan direction across the main scan direction from one side portion of scan lens  218  to an opposed side portion thereof. In order to reduce cross-talk between adjacent channels or laser beams entering a common scan lens  218 , a discontinuity line  324  defined by the junction formed between the first curved surface section  304 A and the second curved surface section  304 B of the light exit surface  304 , may be bent or bowed to substantially follow the shape of the laser beams scanned across the scan lens  218 . For example, the vertical position of each point of the discontinuity line  324  may lie between the first channel  320 A and the second channel  320 B as shown in  FIG. 6 . In addition, the discontinuity line  324  extending across the light exit surface  304  of the scan lens  218  may also lie between the lowest vertical ray position  326  of first channel  320 A (0 mm scan position) and the highest vertical ray position  328  of second channel  320 A, at which respective rays of channels  320 A and  320 B leave the light exit surface  304 . In this way, by having a bowed discontinuity line  324  largely matching the bowed shape of light beams exiting scan lens  218  and positioned between the channels  320 A and  320 B for curved surface sections  304 A and  304 B, cross-talk between the two channels  320 A and  320 B may be substantially avoided. 
     With further reference to  FIGS. 3A-3C , the discontinuity line  324  may extend across the length L of scan lens  218  between a first end  330 A and a second end  330 B. As shown, the discontinuity line  324  has a height that gradually decreases in a direction from the opposed longitudinal ends  330  towards a central portion between the opposed longitudinal ends  330 . More particularly, the discontinuity line  324  has a height H 1  at or near the first and second ends  330  that is greater than a height H 2  located at or near the central portion between the first and second ends  330 . 
     Each of the scan lenses  218 A and  218 B may be made of plastic material, such as polymethyl methacrylate (PMMA) or Zeonex resins, by injection molding. Alternatively, scan lenses  218  may be made of glass material. In some cases, a relatively large discontinuity between the two curved surface sections  304  may make it difficult to have good molding flow which may potentially increase the cooling time, hence the cost, or make the lens surfaces surrounding the discontinuity line  324  less accurate. In order to mitigate this, the discontinuity between the two curved surface sections  304 A and  304 B may be kept as small as possible while still meeting a desired optical performance. For example, in  FIG. 3A , the thickness D 1  of scan lens  218  between the light incident surface  302  and the first curved surface section  304 A, the thickness D 2  between the light incident surface  302  and the second curved surface section  304 B, as well as the tilt angle of each curved surface section  304  relative to the reference axis  340 , may be selected so that the step between the two curved surface sections  304 A and  304 B may be reduced to less than about 10 um.  FIG. 7  shows a graph illustrating step sizes at discontinuity points between the two curved surface sections  304 A and  304 B according to an example embodiment. As shown, the step sizes of points of discontinuities between the two curved surface sections  304  may vary between about 0 um and about 10 um across the various scan positions of the light exit surface  304 , but does not exceed 10 um. 
     According to an example embodiment, the overall thickness of scan lens  218  may vary between about 2 mm and about 20 mm, and more particularly between about 2 mm and 10 mm, such as about 4.5 mm. The length L (seen in  FIG. 3C ) of scan lens  304  across the main scan direction may be between about 50 mm and about 110 mm, and particularly between about 65 mm and about 75 mm, such as about 70 mm. Furthermore, in order for the laser beams LB to achieve a substantially uniform spot size on the photoconductive members  110  for a given amount of laser power, the arrangement of the various optical components may be in a manner such that the overall beam path lengths of the laser beams LB are substantially the same. In one example embodiment, the beam path length of a laser beam LB from reflective surface  214  of scanning device  206  to a corresponding photoconductive member  110  may be between about 180 mm and about 300 mm, and more particularly between about 200 mm and about 250 mm, such as about 204 mm. In another example embodiment, the beam path length of a laser beam LB from the reflective surface  214  of the scanning device  206  to a corresponding scan lens  218  may be between about 40 mm and about 120 mm, more particularly between about 50 mm and about 80 mm, such as about 60 mm. 
     By having the thickness of the scan lenses  218  relatively thin and length L thereof substantially reduced, and by having the overall beam path length of each laser beam shorter than conventional designs, as described in the above example embodiments, LSU  130  can be made more compact which may consequently reduce the volume and size of LSU  130  in the imaging apparatus. In addition, because of the compactness and simplicity of the optical layout requiring less optical components, such as requiring only two scan lenses and eight mirrors downstream the scanning device  216  in the example optical layout of  FIG. 2 , the above example design may reduce the tolerance stack up caused by accumulated variation of size and/or position of individual downstream optical components, and improve alignment robustness. Furthermore, the use of only two relatively thin scan lenses  218  and the decrease in the number of optical components may provide significant savings with respect to the overall cost of LSU  130 , and consequently the cost of imaging apparatus  100 . 
     The description of the details of the example embodiments have been described in the context of electrophotographic imaging devices. However, it will be appreciated that the teachings and concepts provided herein are applicable to other systems employing optical scanners for scanning light beams. 
     The foregoing description of several methods and an embodiment of the invention have been presented for purposes of illustration. It is not intended to be exhaustive or to limit the invention to the precise steps and/or forms disclosed, and obviously many modifications and variations are possible in light of the above teaching. It is intended that the scope of the invention be defined by the claims appended hereto.