Patent Publication Number: US-7719556-B2

Title: Method and apparatus for imaging with multiple exposure heads

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This is a continuation of application Ser. No. 10/688,901, filed Oct. 21, 2003 now U.S. Pat. No. 7,256,811. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to imaging systems and more particularly to imaging systems which form an image on a recording media using multiple exposure heads. 
     BACKGROUND OF THE INVENTION 
     Imaging systems that are capable of imaging films, lithographic plates, flexographic plates, proofing materials and other media types are well known in the art. In the printing industry, laser based exposure heads are commonly used to form an image on a lithographic plate for subsequent use in a printing operation on a printing press. Some imaging systems are capable of printing on multiple media formats such as plates, films, and proofing media. 
     A common imaging system architecture provides an exposure head which generates one or more modulated beams or channels and an imaging media carrier for securing a media sheet. The beams are scanned over the media by a scanning means which produces relative motion between the media sheet and the beams. The scanning means may comprise, for example, an external drum, internal drum, or a flatbed scanning system. In an external drum system the media is held on a rotatable drum and the beams from the exposure head are scanned over the media surface by a combination of drum rotation and translation of the exposure head. 
     A common problem in the design of imaging systems is providing sufficient imaging speed to meet the media preparation requirements of the industry. Particularly in the printing industry, where a large capital investment in printing press equipment dictates that presses should be kept running at high duty cycles, the time taken to prepare a plate for press may be a limiting factor in the printer&#39;s overall workflow. 
     U.S. Pat. No. 5,887,525 to Okamura et al. describes a machine for simultaneously making two printing plates for newspaper printing. The machine has two exposure sections in series to speed up the production of plates for a newspaper press. In U.S. Pat. No. 5,795,689 to Okamura et al. the speed of a machine for making newspaper printing plates is increased by using two exposure heads in parallel to scan different areas of a plate, thus reducing the time taken to prepare a plate for use on the press. The exposure heads may each write images that are duplicates or the image written by each exposure head may be different. 
     U.S. Pat. No. 5,934,195 to Rinke et al. describes a flatbed system that is capable of simultaneously exposing two separate single-wide plates, each having the same or a different image thereon, or a single double-wide plate, each half of which has the same or a different image thereon. 
     There remains a need for better methods and apparatus for imaging with multiple exposure heads. 
     SUMMARY OF THE INVENTION 
     A first aspect of the invention provides an imaging apparatus comprising a media carrier and at least two exposure heads. Each exposure head is disposed to image a portion of a single sheet of media secured on the media carrier, or one of at least two sheets of media secured on the media carrier. An adjustable spacer is provided for moving the exposure heads relative to each other to change the spacing therebetween. 
     In another aspect of the present invention a method of imaging with at least two exposure heads is provided. The method comprises loading at least one sheet of media on a media carrier and adjusting the spacing between the exposure heads in accordance with the number and size of media loaded on the media carrier. A portion of a single sheet of media secured on the media carrier, or one of at least two sheets of media secured on the media carrier are then imaged by each exposure head. 
     In yet another aspect of the invention a method for aligning two exposure heads for imaging a unitary image on a media is provided. The unitary image is partitioned into two sub-images. The method comprises imaging a first test image with one of the exposure heads and imaging a second test image with the other exposure head, the second test image adjoining the first test image. The degree of misalignment between the exposure heads is determined by examining the adjoining portion between the test images. The traversing speed of at least one of the exposure heads is adjusted in accordance with the determined degree of misalignment. 
     For an understanding of the invention, reference will now be made by way of example to a following detailed description in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In drawings which illustrate by way of example only preferred embodiments of the invention: 
         FIG. 1  is a perspective view of an imaging system for imaging two separate media sheets; 
         FIG. 2  is a perspective view of a system for imaging a single large media sheet; 
         FIG. 3  is a perspective view of a pair of exposure heads on a common leadscrew; 
         FIG. 4  is a perspective view of a pair of exposure heads each on an independent leadscrew; 
         FIGS. 5-A  to  5 -D are views of various aligning systems; 
         FIG. 6  is a process flowchart depicting a method of the present invention; 
         FIG. 7  is a schematic view of an imaging media and the relative positioning of the exposure heads; 
         FIGS. 8-A  to  8 -C are a series magnified views of a portion of the imaging media shown in  FIG. 7 ; 
         FIG. 9  is a schematic diagram showing a test image for aligning two exposure heads; and 
         FIG. 10  is a simulated moiré pattern illustrating one specific alignment method according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows an imaging engine  10  having a drum  12 . Drum  12  is rotatable about a central axis  14 . Two sheets of media  16  and  18  are secured on drum  12 . A first exposure head  20  is disposed to image media sheet  16  and a second exposure head  22  is disposed to image media sheet  18 . Exposure heads  20  and  22  are each attached to a corresponding carriage  24 . Carriages  24  are traversed by rotating leadscrew  26 , thus driving leadscrew nuts  28 , which are attached to carriages  24 . 
     Exposure heads  20  and  22  are independent imaging units, each responsive to separate data and control signals, but traversed by a common leadscrew  26 . Leadscrew rotational drive is provided by a suitable motor (not shown) such as a stepper motor or a servo motor. The position of the exposure heads  20  and  22  along the length of the leadscrew  26  may be determined by keeping a count of the number of steps applied to the stepper motor in relation to a previously established home position. Alternatively, other well-known linear or rotary mechanisms and linear or rotary encoding techniques may be employed to translate and keep track of the lateral position of an exposure head. 
     Imaging engine  10  is capable of imaging a media in two different modes. In a first imaging mode shown in  FIG. 1  exposure head  20  images media sheet  16  and exposure head  22  images media sheet  18 . The images may be different or identical. In a second imaging mode shown in  FIG. 2  each of the exposure heads  20  and  22  image a portion of a single large media sheet  40  loaded on drum  12 . This reduces the imaging time over that which would be required if the media sheet  40  were to be imaged by a single exposure head. 
     In an alternative third mode of operation, the two or more exposure heads may be used to provide some redundancy. In the event of a failure of one of the exposure heads the imaging can be done by the other exposure head until the failed exposure head is replaced or repaired. The imaging time in this mode will be slower, but this represents a useful system reliability feature to a user who is severely impacted by downtime. 
     In practice, there are some problems associated with the simple embodiments shown in  FIG. 1  and  FIG. 2  in that the spacing between the exposure heads  20  and  22  is fixed by leadscrew  26 . For a specific head spacing imaging speed is only maximized when imaging a media that has a width approximately twice the spacing between heads (for one large sized sheet or two smaller sheets, each smaller sheet approximately half the size of the large sheet). Any other sizes of media sheet will generally have less-than-optimal imaging times. 
     Another problem occurs when imaging a unitary image on a single large media sheet, with each exposure head imaging a portion of the unitary image. The line along which the two image portions join (the “stitch line”) will generally show some discontinuity unless the two exposure heads are very precisely spaced. The spacing may drift with time and environmental conditions making it necessary to periodically re-space the heads. 
     In an embodiment of the invention shown in  FIG. 3 , exposure heads  20  and  22  are traversed on a common leadscrew  26 . Exposure head  22  has a fixed leadscrew nut  50  while exposure head  20  has a rotatable leadscrew nut  52 . Nut  52  is connected to exposure head  20  via a bearing (not shown) allowing nut  52  to rotate freely while simultaneously preventing any relative longitudinal motion between the nut  52  and exposure head  20 . Exposure head  20  is additionally equipped with an auxiliary drive motor  54  which may be a stepper motor. Auxiliary motor  54  provides rotational drive to nut  52  via a pulley  56  driving a belt  58 . The spacing between heads  20  and  22  can be adjusted by rotating nut  52  in response to control signals from a motor controller  59 . 
     The adjustment will generally be made before or after an imaging operation and may advantageously be executed during a retrace cycle while the exposure heads are returning to a home position on completion of an image. The adjustment is preferably performed automatically in response to a control signal from controller  59  but this is not mandated and some of the benefits of the invention may be realized in a manually adjusted system. 
     As may be appreciated by a person skilled in the art, many other mechanisms for driving nut  52  may be employed to effect the adjustment. Other mechanisms for adjusting the spacing between exposure heads  20  and  22  may also be provided instead of, or in addition to, a nut  52  which is rotatable relative to its carriage  24 . For example, the adjustment between nut  52  and exposure head  20  may be provided by a separate translation stage, such as a secondary leadscrew or other linear translation stage employed to move exposure head  20  relative to nut  52 . In the embodiment shown in  FIG. 3 , the main traversing drive is still provided by leadscrew  26 . Alternatively it is also possible to provide rotational drive to both nuts  50  and  52 , while holding leadscrew  26  stationary. In another alternative embodiment shown in  FIG. 4 , a pair of exposure heads  60  and  62  are each independently driven by leadscrews  64  and  66 . The drive to leadscrews  64  and  66  is provided by separate motors (not shown). The separate motors may nevertheless be synchronized to operate from a common system synchronization clock (which may also be used to control drum rotation). Conveniently, stepper motors may be used for the leadscrew drive since they allow both precise stepping and control, but any other suitable type of motor or motor/encoder combination may also be used. 
     In another alternative embodiment, only one of the exposure heads is driven by the leadscrew. The other exposure head is coupled to the first exposure head via a coupling to space them apart. The second head moves in tandem with the first. The spacing between heads is adjusted by varying the length of the coupling. In one embodiment the coupling comprises a bar having a length which can be varied by heating or cooling the coupling bar to thermally expand or contract it. The spacing is accurately maintained by controlling the temperature of the bar. A heater controlled by a controller (not shown) may be used to control the length of the coupling bar. Thermal adjustment provides very fine control of the spacing and a fixed bar provides a rigid connection between exposure heads removing any leadscrew effects from the spacing. 
     In a method according to the invention the imaging operation comprises the following steps:
         (a) determining the format of the loaded media—e.g. a single media sheet or a pair of media sheets;   (b) adjusting the spacing between the exposure heads to correspond to either half the width of the single media sheet or to align with the spacing between a pair of media sheets, depending upon which format is present; and,   (c) imaging either the single media sheet, with each exposure head imaging approximately half of the media sheet, or a pair of media sheets with each exposure head imaging one of the sheets. If there are more than two exposure heads then, for printing on a single media sheet, the exposure heads may be space apart by 1/0 times the width of the media sheet, where 0 is the number of exposure heads.       

     Advantageously, by adjusting the spacing between two exposure heads in accordance with the size of the media being imaged, the overall imaging time is reduced for any combination of media. 
     In the case where two or more separate media are imaged, each with a separate exposure head, the adjustment between the exposure heads need not be particularly precise. Many imaging systems have edge detection hardware for detecting the edge of the media sheet, optically or otherwise. One common optical edge detection method senses the discontinuity in surface reflectivity between the media and the drum surface. A precision of roughly 5:m can be achieved, which is quite adequate for most printing. In  FIG. 5-A , media sheets  16  and  18  are secured on drum  12 . Each of exposure heads  20  and  22  are equipped with an edge detection beam  72 . The exposure heads  20  and  22  are traversed over the edges  90  and  92 , and the edge locations recorded. The imaging data may then be arranged such that each image is correctly located on the media  16  and  18 . 
     In the case where each exposure head images a portion of a single media sheet the spacing between exposure heads should be more precisely adjusted to avoid a visible discontinuity between the joined image portions. Simple edge detection may not be sufficiently accurate for other than low resolution imaging. It has been found that even errors of around ⅕th of a pixel may be discernable on some sensitive media. At 2400 dpi this translates into a sensitivity of around 2:m, which is an almost impossible accuracy to hold through mechanical tolerancing alone. A practical approach is to periodically align each exposure head to a target located on the drum.  FIG. 5-D  shows a drum  12  with a target  70  positioned at a fixed location on the surface of drum  12 . The target provides a common alignment point for each of the exposure heads  20  and  22 . Exposure head  22  is shown with an auxiliary beam  72  impinging on target  70 . By first aligning exposure head  22  to target  70 , and then moving exposure head  20  to align with target  70 , the spacing between the exposure heads may be determined and adjusted. Alternatively, the imaging heads could be aligned to separate targets, spaced a known distance apart, albeit with potentially lower accuracy. 
     One specific embodiment of the target is shown in  FIG. 5-B . Beam  80  from exposure head  20  is directed towards lens  82 . Lens  82  is recessed into the surface of drum  12 . The light gathered by lens  82  is directed to a position sensitive detector (PSD)  86  via mirror  84 . PSD  86  generates a signal  88  responsive to the position of a beam  90  on the sensitive area of the PSD  86  and is able to indicate movement of the beam  90  in the direction of arrow  92 . Lens  82  magnifies the displacement to increase the sensitivity of the target thus amplifying the motion  92  occurring at the surface of PSD  86 . 
     In an alternative embodiment shown in  FIG. 5-C , a target  94  has non-reflective areas  96  and reflective areas  97 . Reflective areas  97  are located in the shape of a “Y” (on its side). The geometry of the reflective target  94  and specifically the angle between the “Y” branches is accurately determined prior to installing the target. A suitable target  94  may be constructed from a thin sheet of stainless steel using a lithography and chemical etching process to pattern the “Y” shape, guaranteeing a precise, known, geometry. Alternately the target may be separately characterized using well known measuring techniques. 
     In operation an auxiliary laser beam from the imaging head  20  or  22  is scanned over target  94  along line  98 , the laser beam traversing two branches of the “Y” in succession. The reflection of the laser beam from the target  94  is monitored by a light sensor such as a photodiode (not shown) that converts the light intensity reaching the light sensor into an electrical signal. As each reflective branch of the “Y” target is traversed, the light sensor signal changes sharply defining a transition from non-reflective area  96  to reflective area  97  and back again to non-reflective area  96 . The signal from the light sensor representing this transition is used to precisely determine the location of the Y branch. 
     Advantageously target  94  allows both X and Y co-ordinates of the laser beam to be simultaneously determined in a single traversing of the target along direction  98 . The Y co-ordinate is determined as the half way point between the encoder readings at the two signal transitions. The X co-ordinate is 
     determined from the following formula: 
               X   ′     =       d   2     ·     tan   ⁡     (     θ   2     )               
where X′ is the X displacement of the beam (at line  98 ) from intersection point  99  of the two branches of the “Y”, d is the distance between the signal transitions, and □ is the angle between the branches of the Y. For □=90E the tan term equates to 1 and X′=d/2.
 
     Alternatively, the target  94  may be viewed by a video camera. The resulting image is analyzed using pattern matching software (systems that include a video camera and pattern matching software are available, for example from Cognex Corp, USA). 
     Advantageously, it is not necessary for the beam to traverse the reflector target at any specific location, as long is it traverses both branches of the “Y”. The third branch of the “Y”, is used in as a convenient Y co-ordinate determination when there is no need for an X co-ordinate determination. It should be readily appreciated that the target may also be constructed from two angled reflective lines, not necessarily intersecting and not necessarily oriented as shown. 
     In some high resolution imaging systems, a discontinuity may still occur at the join between the two sub-images of a unitary image imaged on a single media sheet, even when the exposure heads are precisely spaced. For the best results it may be more practical to do a final fine adjustment based on inspection of a test pattern imaged on the media. In the embodiment shown in  FIG. 2  the beam (or beams) from exposure heads  20  and  22  are scanned over the media  40  by simultaneously rotating drum  12  while translating exposure heads  20  and  22 , each exposure head thus circumscribing a helical pattern around the drum. The discontinuity may be caused by a simple displacement between the end of one sub-image and the start of the other sub-image, or it may be caused by slight differences between the imaging beams that write the image in the adjoining area. A discontinuity is more likely to be apparent when imaging at high resolution. Another factor that influences the appearance of the discontinuity is the media. Some media are more likely to reveal or accentuate imaging artifacts than others. 
       FIG. 6  is a flowchart depicting of a method for imaging a unitary image on a media with two exposure heads. Data defining a unitary image is received in step  140 . In step  142 , the data is partitioned to define two sub images  144   a  and  144   b . The unitary image data file is split into two independent files, each containing a sub-image  144   a  or  144   b . Each of the sub-images  144   a  and  144   b  are sent to a corresponding exposure head in step  146   a  and  146   b . Preferably, each exposure head will image approximately 50% of the image but this is not mandated. 
     In step  148  the sub-images are imaged on a single media sheet to form a unitary image on the media. It should be evident that the goal is that there should be no easily discernable difference between an image written by two or more exposure heads and an image written conventionally by a single exposure head. 
       FIG. 7  depicts an imaging media  40  that has been imaged by exposure heads  20  and  22 . As previously described the scanning action may produce a series of slanted helical bands  110  across the imaging media. Each band may be a few mm in width or more and is imaged by a number of parallel independent beams or channels. It is well known in the art to re-arrange the data transmitted to the exposure head to ensure that, while the imaging bands may be tilted by some angle to the edge  116  of the media  40 , the actual image imparted is orthogonal to the imaging media. 
     The first exposure head  20  starts imaging sub-image  124  at band  112 . The second exposure head  22  starts imaging sub-image  126  at band  122 . If it is required to image right to the edge  116  of the imaging media  40 , band  112  may also span across the edge  116  of imaging media  40 . The last full width band imaged by the exposure head  20  is band  118 . The sub-images  124  and  126  are divided along line  100  according to the previously described partition point in the unitary image. Line  100  may be called a stitch line or a stitch. Since line  100  may not have been chosen exactly at the end of full band  118 , exposure head  20  may be required to image partial band  120  in order to complete the first sub-image. When partial band  120  is being imaged by exposure head  20 , band  122  has already been imaged by exposure head  22 . The partial band  120  must be precisely aligned with band  122  to avoid the appearance of a discontinuity at the boundary therebetween. 
     In order to align the end of partial band  120  with the beginning of band  122  it is necessary to calculate how many individual beam widths are in the first sub-image  124 , and then arrange for exposure head  20  to plot a pre-determined number of full bands, followed by a partial band with the last imaged beam being close to, but not necessarily overlapping, the beginning of band  122 . Since the minimum width that can be imaged is an individual beam width, the alignment will generally be in error by less than one individual beam width. Unfortunately, at higher resolutions and for some imaging media types an error of a single beam width or less may be clearly apparent as a discontinuity in the resulting image. 
     This effect is further explained with reference to  FIGS. 8-A  to  8 -C, which are magnified views of region  8  indicated in  FIG. 7 . In  FIG. 8-A  the last full band  118  and the partial band  120  of sub-image  124  are shown, as is the first band  122  of the second sub-image  126 . The end of band  118  joins partial band  120  along line  119 . Lines  130  do not define the bands but rather define the extents of individual imaging beams  132 . Each band comprises a plurality of such individual imaging beams  132 . The gap indicated at  134 , which is smaller than the width of individual beam  132 , results from not imaging an individual beam over the gap  134 . If an individual beam were written in gap  134  it would also overlap the beginning of band  122 . This situation is depicted in  FIG. 8-B  where the beginning of band  122  has been overwritten. This is shown schematically as a dark line  136 , which results from the double exposure. In the cases shown in  FIG. 8-A  and  FIG. 8-B  the discontinuity may be discernable. The size of the gap  134  or the overwritten portion  136  can always be arranged to be less than the width of one individual imaging beam, since if the gap were more than this width it would be a simple matter to write one more individual beam to reduce the width of gap  134 . In this way, the misalignment may be always restricted to an individual beam width or less. 
     This remaining misalignment cannot be easily corrected since imaging occurs on a pixel-by-pixel basis, the pixel being the minimum addressable element defined by an individual imaging beam  132 . Returning to  FIG. 7  it should readily be appreciated that the spacing between adjacent bands  110  is determined by the speed of translation of exposure heads  20  and  22 . This is usually adjusted so that no separation between the bands is evident when the individual beams are correctly spaced for the chosen imaging resolution. The ability of an imaging engine to produce such a geometrically accurate image is important, particularly in the printing industry, where color separations must be accurately registered to print properly. The required registration accuracy may vary for different printing presses and printing resolutions. At 1200 dpi an accuracy of around 30:m is generally sufficient. At this resolution individual beams having widths of approximately 20:m are typically used. By fractionally increasing or decreasing the speed of translation of exposure heads  20  and  22 , the gap  134  or overwritten portion  136  shown in  FIGS. 8-A  and  8 -B may be effectively eliminated. The fractional increase in speed need only account for the width of an individual beam or less. Consequently, the effect on the geometric accuracy of the final image is negligible. The spacing between adjacent bands is affected by only a very small amount. 
     As an example, considering a 22-inch wide plate where each of the sub-images are 11 inches wide, at 1200 dpi there would be approximately 13,200 individual beam widths in each sub image. For an exposure head with 240 parallel channels, this corresponds to 55 bands. The maximum correction required for eliminating the gap or dark band at the stitch is 20:m (one individual beam width or less). This corresponds to an adjustment of 0.36:m at each band or a speed change of ∀ 0.007%, which is undiscernible from band-to-band but corrects for the discontinuity at the adjoining area. 
     In practice, the actual speed change required may be determined empirically by writing a number of images on one or more imaging media sheets, each with successive small changes in speed of translation. The speed that produces the least visible discontinuity is chosen for use in subsequent imaging operations. Advantageously, as shown in  FIG. 9 , a single sheet of imaging media  40  may be imaged with a test set comprising a plurality of test strips  150  made at different speeds of translation. The test strip  152  with the least visible discontinuity near line  100  indicates the optimal translation speed. This process has the added advantage that if there is some difference between the beams produced by exposure heads  20  and  22 , this difference may at the same time be at least partially corrected by the choice of the visually best image in the test set shown in  FIG. 9 . 
     Another method for determining the required speed change is to deliberately overwrite a set of vertical lines from each exposure head. The resulting moiré interference pattern may be examined to determine the required speed change. This method is explained with reference to  FIG. 10 , which shows a first set of lines  160  imaged at a small angle to a second set of lines  162 . In the depicted example the lines are imaged using an imaging system of the type that effects a helical scanning of the drum. The first set of lines  160  are imaged by the first exposure head and the second set of lines  162  by the second exposure head. The small angle between the lines may be introduced by disabling a number of channels on one of the exposure heads. This changes the helix angle for that head as the traversing speed is automatically increased by the system to compensate for having fewer imaging channels. The two sets of lines  160  and  162 , offset at a small angle to each other, will produce a moiré pattern as shown in  FIG. 10 . For the situation where the pitch between the lines and the angle between the sets of lines is known, as is the case here, the position of the dark band or fringe  166  is indicative of the misalignment between the two patterns and thus the offset between the two exposure heads. Advantageously a scale  168  may be imaged alongside the sets of lines so that the spacing can be directly read off the imaging media at the location of fringe  166 . Alternatively, the position of the light fringe  164  may be used to calculate the offset. As will be readily apparent to a person of skill in the art the use of lines to generate moiré patterns is convenient but not mandated. Any repetitive feature will create a moiré pattern that is usable for the purposes of the method described. For example, a plurality of dots in a regular grid when overlapped with another plurality of dots on a regular grid will also produce a moiré pattern. 
     Once accomplished, the adjustment may be susceptible to drift due to changes in the environmental temperature. Many imaging systems use a steel leadscrew for advancing the exposure heads, the steel having an expansion coefficient of around 12 ppm/EC. For a 500 mm distance between exposure heads the leadscrew will thus expand or contract by ˜12:m for every 2EC change in temperature. Such a minor change in environmental conditions would have the effect of completely negating the alignment. The change may be accommodated by precisely measuring the temperature of the leadscrew and adjusting the scanning speed to compensate for any changes. The temperature measurement may need to take account of temperature gradients in the imaging system and will possibly require two or more temperature measurements at different points along the leadscrew. Alternatively, the expansion of the leadscrew with respect to the frame supporting the engine may be measured directly using a measuring device such as a Linear Variable Displacement Transducer (LVDT). The drum, leadscrew and carriage ways are typically all held in a frame, which may be of a different material than the leadscrew. It is particularly important to measure the difference between the expansion of the frame and the leadscrew. Thus an LVDT or the like attached to the frame at the floating end of the leadscrew and contacting the end thereof is ideally disposed to measure the quantity of interest. 
     Another factor that may affect the alignment is the pointing stability of the imaging beams produced by the exposure heads. The pointing direction is typically a property of the optical systems used to form the imaging beams. In some instances, it may be necessary to provide intermittent or continuous monitoring and adjustment of the beam pointing to ensure that the image-to-image alignment is maintained for a reasonable time. 
     Yet another factor that may need to be taken into account is the overall scaling of the images. Many imaging systems are carefully adjusted to provide accurately scaled images by imaging and measuring test images on an XY measuring table or the like. Scaling factors are calculated and applied to the imaging system as a machine calibration. It should be understood that such a calibration and the alignment of the images will generally be interrelated and will need to be performed together so that images are aligned and appropriately scaled. 
     There may also be a requirement to duplicate a portion of the data in the region of the partition point to deliberately overlap the images at the partition point. This need arises in the imaging of some types of media wherein adjacent bands are commonly overlapped by one or more beam widths. This feature is particularly useful for some types of thermally sensitive imaging media where subsequent exposures are not additive. Overlapping has been found to even out the boundaries between adjacent bands. Overlapping may also be useful in aligning sub-images produced by different exposure heads. The overlapped data is a repeat of the previously written data and writing may occur at full beam power or at reduced beam power. The duplication of the data is preferably taken into account when partitioning the image file into sub-images. 
     Although the foregoing discussion has been focused on a specific embodiment of an imaging engine the method may be applied to a wide range of imaging architectures where it is desired to write a single image with two or more exposure heads. Where the exposure heads share a common translation means the same translation speed change is applied to both exposure heads equally, thus limiting the correction to being performed with two such exposure heads. However, where the exposure heads are independently translated the invention may be applied to systems having two or more exposure heads. Similarly the method is also applicable to the situation where the distance between exposure heads is not adjustable, in which case the size of the imaging media and/or the size of the image to be written will determine the proportion of the image to be written by each exposure head. 
     The method is also applicable to other imaging architectures such as internal drum systems and flatbed systems. In such cases, while the scanning may be different the requirement still exists to stitch together two or more sub-images and as such, the translation speed may be altered in the manner described to reduce the appearance of the discontinuity. 
     The data partitioning may be achieved in a variety of different ways depending on the data format and the configuration of the system. For example instead of splitting the image into two separate sub-image files, a pointer may be used to indicate the point of partition between the two sub-images. It should also be understood that other formatting steps may follow the partitioning step. 
     It should also be noted that other methods of scanning beams across an imaging media are well known. One example of an alternative scanning method is to image a circumferential band while the exposure head is held stationary, whereafter the exposure head is indexed to a new position to image the next circumferential band. During the indexing operation the imaging ceases until the exposure head is in position to image another circumferential band, lined up alongside the previous one. While the invention has been described in relation to a helical scanning system, it is also applicable to other scanning methods employed in the industry. 
     As will be apparent to those skilled in the art in light of the foregoing disclosure, many alterations and modifications are possible in the practice of this invention without departing from the spirit or scope thereof.