Abstract:
An apparatus and method for stitching a at least two imagers together in a scanning system to produce accurate, wide and seamless images. The system includes a photoreceptor adapted to move in a process direction and at least two imagers for simultaneously forming a single scan line at a time, in a direction transverse to the process direction. The system also includes circuitry for stitching together the at least two imagers and for forming registration marks on the surface of the photoreceptor. Finally, the scanning system includes sensing circuitry for detecting displacements of successive registration marks and providing adjustment values to the stitching circuitry.

Description:
BACKGROUND OF THE INVENTION 
     A. Field of the Invention 
     The present invention relates generally to scanning systems. More particularly, this invention relates to the use of imagers to create of xerographic images. 
     B. Description of the Related Art 
     Electrophotographic printers wherein a laser scan line is projected onto a photoconductive surface are well known. In the case of laser printers, facsimile machines, and the like, it is common to employ a raster output scanner (ROS) as a source of signals to be imaged on a precharged photoreceptor (a photosensitive plate, belt, or drum) for purposes of xerographic printing. The ROS provides a laser beam which switches on and off according to digital image data associated with the desired image to be printed as the beam moves or scans, across the photoreceptor. Commonly, the surface of the photoreceptor is selectively, imagewise discharged by the laser in locations to be printed white, to form the desired image on the photoreceptor. The modulation of the beam to create the desired latent image on the photoreceptor is facilitated by digital electronic data controlling a modulator associated with the laser source. A common technique for effecting this scanning of the beam across the photoreceptor is to employ a rotating polygon surface (the surface of the polygon typically being a mirror or other reflective surface); the laser beam from the ROS is reflected by the facets of the polygon, creating a scanning motion of the beam, which forms a scan line across the photoreceptor. A large number of scan lines on a photoreceptor together form a raster of the desired latent image. Once a latent image is formed on the photoreceptor, the latent image is subsequently developed with a toner, and the developed image is transferred to a copy sheet and fixed, as in the well-known process of xerography. 
     FIG. 1 shows the basic configuration of a scanning system  100  used, for example, in an electrophotographic printer or facsimile machine. A laser source  10  produces a collimated laser beam, also referred to as a “writing beam”,  12  which is reflected from the facets of a rotating polygon  14 . Each facet of the polygon  14  in turn deflects the writing beam  12  to create an illuminated beam spot  16  on the pre-charged surface of photoreceptor  18 . The system may further include additional optical elements such as focusing lenses. The energy of the beam spot  16  on a particular location on the surface of photoreceptor  18 , corresponding to a picture element (pixel) in the desired image, discharges the surface for pixels of the desired image which are to be printed white. In locations having pixels which are to be printed black, the writing beam  12  is at the moment of scanning interrupted, such as by a modulator  11  controlled by imagewise digital data, so the location on the surface of photoreceptor  18  will not be discharged. It is to be understood that gray levels are typically imaged in like manner by utilizing exposure levels intermediate between the “on” and “off” levels. Thus, digital data input into laser source  10  is rendered line by line as an electrostatic latent image on the photoreceptor  18 . 
     When the beam spot  16  is caused, by the rotation of polygon  14 , to move across photoreceptor  18 , a scan line  20  of selectively discharged areas results on photoreceptor  18 . In FIG. 1, the photoreceptor  18  is shown as a rotating drum, but those skilled in the art will recognize that this general principle, and indeed the entire invention described herein, is applicable to situations wherein the photoreceptor is a flat plate, a moving belt, or any other configuration. The surface of photoreceptor  18 , whether it is a belt or drum, moves in a process direction (as indicated by the arrow drawn on the side of the drum  18 ); the motion of spot  16  through each scan line  20  is transverse to the process direction (as indicated by the arrow drawn on the surface of the drum  18  and below scan line  20 ). The periodic scanning of beam spot  16  across the moving photoreceptor  18  creates an array of scan lines  20 , called a raster  22 , on the photoreceptor  18 , forming the desired image to be printed. One skilled in the art will appreciate that such a configuration will typically further include any number of lenses and mirrors to accommodate a specific design. 
     In a rotating-polygon scanning system, there is a practical limit to the rate at which digital information may be processed to create an electrostatic latent image on a photoreceptor. One practical constraint on the speed of a system is the maximum polygon rotation speed. It can be appreciated that high quality images require precision placement of the raster scan lines as well as exact timing to define the location of each picture element or pixel along each scan. In a conventional polygon scanner, this precision is achieved by holding very close mechanical tolerances on the polygon geometry and the rotational bearings supporting the polygon body and drive motor. Experience has shown that beyond about 20,000 RPM, precision ball bearings with the required closeness of fit have limited life and are impractical in many scanner applications. As a result, exotic alternatives such as air bearings are sometimes used, but these represent a substantial increase in engineering complexity and maintenance, and hence cost. Another constraint is the size of the polygon itself; it is clear that the forces associated with high speed rotation increase with the diameter of the object being rotated. In particular, both the stored energy and the gyroscopic forces that must be restrained by the bearings increase with the square of the diameter. It is therefore prudent to limit the polygon size to maximize bearing life as well as reduce the potential for damage should a bearing fail at high speed. 
     In addition to practical constraints, the speed of a printer must be considered in conjunction with other competing desirable characteristics of a printer, particularly resolution. In purely optical terms, there is a trade-off between speed and resolution in a scanning system. The higher the resolution, that is, the more pixels that are designed to form a latent image of a given size, the lower the numerical aperture of the optical system required in order to define the pixels accurately. This trade off can be summarized by a derived equation for an under filled system relating the angular velocity ω of a polygon having a mean diameter D to the desired pixel size (that is, the inverse of resolution) Δx: 
     
       
           Dω   2   =[LλP   2   /Δx   3 ][(60/π) 2   k/ 2 ][E/χ]   
       
     
     The other variables in this equation are as follows: L is the length of the intended scan path, which in this context is the width of the photoreceptor across which the scan line is formed. P is the process speed, in inches per second, of the photoreceptor motion in the machine. k is a constant which depends on the intensity profile of the beam (for example, under a certain convention, the usable pixel size is dependent on a focused concentration where 86% of the total power of the beam is focused within a circular area of a given size). E is an efficiency, factor relating to the proportion of the “circumference” of the polygon which is practically usable for scanning purposes, i.e., because the numerical aperture for a given resolution Δx requires a specific beam width at the polygon, the beam will not be reflected usefully for a certain portion of the time when the beam is focused near the ends of the facets of the polygon. The larger the ratio of facet length to beam width, the larger the proportion of the polygon rotation which is usable for scanning purposes. χ is the ratio of reflected scan angle to rotational scan angle, which depends on whether the facets of the polygon are parallel or oriented at 45 degrees to the axis of the polygon. If the facets are parallel, as in the illustrated case, then χ is equal to 
     2. There are some designs in which the facets of the polygon are set at 45 degrees relative to the axis so that the polygon has the general appearance of a truncated cone. In that case, the beam from the source is incident on the facets parallel to the axis of the polygon, and is reflected in a direction perpendicular to the axis; for this geometry, χ is equal to 1. 
     Looking at the most important system design variables in the above equation, the scan length L, the process speed P, and the spot size Δx, it is clear that the desire for a longer scan, faster throughput, and higher quality image (smaller spot size) all increase the value of the right hand side of the equation and are at cross-purposes with the need to keep the left hand side of the equation, representing the demands on the polygon, as small as possible. As a practical matter, it has been discovered that for electrostatographic printers, the largest practical polygon from a cost and safety standpoint is one having a diameter of about five inches, although diameters of about two inches are generally preferred from a standpoint of machine compactness. Simultaneously, system cost and engineering difficulties are rapidly compounded at rotational speeds of more than 20,000 rpm. The above equation, it should be remembered, has been derived strictly on the basis of optical laws and without consideration of practical limitations. There is, therefore, a distinct advantage in any arrangement which facilitates a substantial increase in the possible rate of digital data that may be imaged with a scanning apparatus, thereby providing the possibility of enhanced resolution, increased scan length, or faster process speed, in various proportions without violating the necessary relationship defined in the equation. 
     Therefore, there is a need in the art for a system that can produce fast, wide images, without increasing the size of the imager nor its distance from the photoresistive material. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to meet the foregoing needs by providing systems and methods that allow the “stitching” of imagers (described more fully hereinafter) in a scanning system to produce accurate, wide and seamless images. 
     Specifically, a scanning system for meeting the foregoing needs is disclosed. The scanning system comprises a surface moving in a process direction; means for stitching together at least two imagers; imager means, corresponding to the at least two imagers, for simultaneously forming a single scan line at a time, transverse to the process direction; means for forming registration marks on the surface; and sensing means for detecting displacements of successive registration marks. Further, the detected displacements can be used by an actuator to align the imagers. 
     Both the foregoing general description and the following detailed description provide examples and explanations only. They do not restrict the claimed invention. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and, together with the description, explain the advantages and principles of the invention. In the drawings, 
     FIG. 1 illustrates a prior art scanning system; 
     FIG. 2 illustrates one embodiment of the scanning system of the present invention; 
     FIG. 3 broadly outlines the sensing system of the present invention; 
     FIG. 4 illustrates an example of a registration mark pattern used by the sensing system of FIG. 3; 
     FIG. 5A illustrates a preferred embodiment of the sensor used in the sensing system of FIG. 3; and 
     FIG. 5B is the graph of a function that corresponds to signals produced by detectors in the sensing system. 
    
    
     DETAILED DESCRIPTION 
     Reference will now be made to preferred embodiments of this invention, examples of which are shown in the accompanying drawings and will be obvious from the description of the invention. In the drawings, the same reference numbers represent the same or similar elements in the different drawings whenever possible. 
     Systems and methods consistent with the present invention allow the efficient stitching of imagers in a scanning system. For purposes of the following description, the systems and methods consistent with the present invention are only described with respect to an ROS imager and split detectors. The description should also be understood to apply in the cases where other imagers and detectors are used. 
     FIG. 2 shows the preferred embodiment of a scanning system  200  according to the present invention. Among the major differences between the scanning system  200  and the scanning system  100  of FIG. 1, is that system  200  includes an intermediate belt  19 , instead of the photoreceptor  18 , and most notably, that system  200  incorporates the use of two additional polygons. The number of adjacent polygons (three, i.e.  14   a-c ) illustrated in FIG. 2, and thus, the number of ROSs, serves only as an example and does not limit the invention. 
     To reduce the complexity of the drawing illustrated in FIG. 2, the laser source  10  and the modulator  11  are shown only for one rotating polygon  14   a.  It will be appreciated that a laser  10  and a modulator  11  are needed for each additional polygon (e.g.,  14   b  and  14   c ) operating in the scanning system  200 . 
     The facets  13   a-c  from polygons  14   a-c  reflect writing beams  12   a-c  to form illuminated beam spots  16   a-c  on the intermediate belt (IB)  19 . The energy of each of the beam spots  16   a-c  on a particular location on the surface of the IB  19  corresponds to a pixel in the desired image. These beam spots are produced simultaneously to form a scan line  20  on the IB  19 . The location of the polygons  14   a-c  is important in that they must be able to simultaneously form the beam spots  16   a-c  and as long as those beam spots are aligned so as to produce a single scan line at a time. The arrangement of polygons discussed above is more generally described herein as a “stitching” of the ROSs. The range covered by each of the simultaneously formed spot beams  16   a-c  is referred to as a writing field. When three polygons are used as part of the scanning system  200 , three adjacent writing fields are formed on the IB  19 . 
     The advantages of the adjacent writing fields arrangement, as disclosed in the present invention, include producing a scan line length greater than that produced by a single ROS, resulting in wider images; in decreasing the distance between the imager (ROS) and the belt, resulting in a compact scanning system; and in the ability to produce images in less time, by reducing the scanning time of each line (as compared to the scanning, time achieved by a single ROS). 
     A general “stitching problem” arises in the prior art of direct marking when writing fields of multiple printing heads (or of a single head making multiple passes) are positioned adjacent to one another and do not produce an accurate image because of misalignment of the printing heads, lack of synchronization among the printing heads, etc. This is an issue for ink jet printers, for instance. The degree of success in solving the stitching problem determines directly the overall image quality. 
     The present invention solves the general stitching problem, and allows the ROSs to accurately print in a direction transverse to the process direction (in parallel). The present invention concerns the stitching of adjacent writing fields of photonic imagers, in particular, but not exclusively the Motor Polygon Assembly (MPA) ROS and the vertical cavity laser diode array printer, and utilizes a marks-on-belt (MOB) sensor to accurately measure the displacement of adjacent fields by looking at the toned image of a chevron pattern on the IB  19 . With the relative position of adjacent writing fields known precisely, electro-mechanical actuators are used to position the adjacent writing fields to form a “seamless” latent image. Actuator means other than electro-mechanical actuator means could also use the relative position information determined by the methods of the present invention. 
     FIG. 3 demonstrates the stitching problem and the sensing solution. The arrow  301  indicates the process direction of the IB  19 . As the IB  19  moves at a velocity V IB  in the process direction, chevron patterns (e.g.,  312 - 314  and  324 - 326 ) are formed on the IB  19 . The two rows of chevron patterns are formed by the adjacent writing fields  302 ,  304 , and  306 . Each writing field corresponds to a different imager (e.g., a ROS). The reason that the writing fields  302 ,  304  and  306  appear as curvilinear traces is because the corresponding beam spots  16   a-c  that form the traces are incident on the belt with a light intensity that is more dense in the center than at the edges of the writing field. 
     Note that in order to form the chevron patterns, the writing fields must overlap. For example, this overlap could be of  400  pixels. The left column of chevron patterns (i.e.,  312 ,  313 ,  314 , etc.) is produced by the overlap of adjacent writing fields  302  and  304 . The left column is centered about the intersection point  303  of the writing fields  302  and  304 . Likewise, a right column of chevron patterns ( 324 - 326 ) is formed by the overlap of the adjacent fields  304  and  306 , and is centered about the intersection point  305  of the fields  306  and  304 . 
     There are three types of chevron patterns that are formed (a “chevron element”) on the IB on a per-column basis. Taking the left column as an example, the first chevron element  312  is formed entirely by a beam spot corresponding to writing field  302 . The second chevron element  313  is formed by both of the adjacent writing fields  302  and  304 . The left half of chevron element  313 , positioned (in the figure) right below chevron element  312 , is formed by the writing field  302 , while the right half is formed by the writing field  304 . The third chevron element  314  is formed entirely by the writing field  304 . 
     In order to determine the relative displacement of writing fields  302  and  304  and the relative displacement of writing fields  304  and  306 , two MOB sensors  3   18  are used. The sensors are positioned close to the IB  19 , and detect the chevron elements as they pass over the sensors  318 , due to the movement of the IB  19 . The operation of sensors  318  will be explained with reference to FIG.  5 A. For now it suffices to mention that both vertical and horizontal displacement of chevron elements with respect to one another is determined by an algorithm, disclosed as part of this invention, which takes into account the relative time differences between three successive marks, corresponding to the three types of chevron patterns (the chevron elements) mentioned above. The three successive marks as a whole are alternatively referred to as a registration mark pattern. The reason that the group consisting of the three chevron elements is repeated over and over again, to form column registration mark patterns, is that by doing so, an average of time differentials can be computed, which yields a better signal-to-noise ratio. The “signal”, as used in this context refers to the signal detected and produced by the MOB sensor  318 . This signal is used by ROS actuators (not shown), while the “noise” refers to noise sources, such as stray deposits due to imperfections on the IB surface and other factors, that cause the signal produced by the MOB sensors  318  to represent an inaccurate time of passage of a chevron element by the sensor  318 . It is desirable, therefore, to have a system in which the signal-to-noise ratio is fairly high, in order to improve printing accuracy. 
     Although the idea behind a scanning system  200  is to have a final print in which the adjacent imager field contributions butt up to each other to within a few microns of positional accuracy, the actual fields must overlap sufficiently to be able to measure the relative displacement of one imager field to its neighbors. The registration mark pattern produced by the overlapping writing fields of the present invention is similar to the pattern disclosed in U.S. Pat. No. 5,287,162 (de Jong et at.), assigned to the present assignee. The major difference between the two registration mark patterns lies in the manner in which they are formed. Specifically, in the prior art reference the pattern is produced by several printers aligned in the process direction, without requiring an overlap of their writing fields, while in the present invention the writing fields must overlap in order to allow the stitching of the imagers in the parallel direction (transverse to the process direction). 
     FIG. 4 shows a magnified registration mark pattern formed of three chevron elements  312 - 314 . By comparing the chevron elements  312  and  314 , respectively formed by writing fields  302  and  304  (FIG.  2 ), one can readily notice a relative displacement in the horizontal direction. A displacement in the vertical direction does not become apparent until a third chevron element  313  is formed by both writing fields  302  and  304 . The registration mark pattern of FIG. 4 can be formed due to the capacity of writing fields  302  and  304  to overlap. The arrows  402  show the horizontal displacement while arrows  404  show the vertical displacement. Also shown in FIG. 4 are the time differences T 1 −T 4  between the chevron elements. Time T 2  corresponds to the time difference between the right-hand side “legs” of chevrons  313  and  314 , while time T 1  corresponds to the lefthand side legs. Time T 4  corresponds to the time difference between the right-hand side legs of chevrons  312  and  314 , while time T 3  corresponds to the left-hand side legs. Although the arrows associated with times T 1 −T 4  only extend from one edge of a chevron element to a second edge of another chevron element, the actual times T 1 −T 4  are measured with respect to the relative position of the centroid of the marks. 
     The time differences T 1 −T 4  are measured by a timer, not shown. What triggers the timer is the passage of the marks, specifically their centroids, over the MOB sensors. In the preferred embodiment of the present invention two split detectors are used to detect the position of the marks. The general operation of the split cell detectors will be explained with reference to FIG.  5 A. 
     Once the time differences T 1 −T 4  are recorded, the horizontal displacement  402  of a first ROS with respect to a second ROS is determined, as well as the vertical displacement  404 . The following algorithm calculates the respective displacements: 
     
       
         Vertical Displacement= V   IB *(( T   3 +2 *T   1 )−( T   4 +2 *T   2 ))/2 
       
     
     and 
     
       
         Horizontal Displacement= V   IB *( T   3 − T   4 )/2; 
       
     
     where V IB  is the assumed velocity of the IB. It is important to notice that the time differences T 1 −T 4  can be calculated for a single mark registration pattern or can be calculated for an average of mark registration patterns, as mentioned above (to increase signal-to-noise ratio). 
     FIG. 5A shows two split detectors  501  and  505  used as a MOB sensor  318  for detecting the chevron element  314 . Although the chevron element  314  has been drawn to the side of the MOB sensor  318 , in a normal sensing operation the element  314  moves in the direction indicated by the arrow  301  so as to pass over and superimpose (“block”) the detectors  501  and  505 . Depending on the size of the detectors or chevron element  314 , the detectors  501  and  505  can be blocked either partially or completely, depending on the instantaneous position of the chevron element  314 . 
     The detector  501  is positioned so as to detect the right-hand side “leg” of the chevron element  314 , and the detector  505  is positioned so as to detect the left-hand side “leg”. Each detector is split into two cells. The detector  505  splits into cells Br ( 506 ) and Ar ( 508 ), separated by a narrow line  507 , and the detector  501  splits into cells B 1  ( 502 ) and A 1  ( 504 ), separated by a narrow line  503 . The explanation that follows assumes the ideal situation, not necessarily expressed in the drawings, where each detector is positioned at the same angle as the corresponding leg of the chevron element, and also has approximately half the same width as the corresponding element. The invention would still work even if the limitations just mentioned were not present. Those limitations only allow a more efficient detection of the chevron element  314 . 
     The operation of the sensor is explained with relation to only one detector (e.g.,  501 ). Detector  501  produces two electrical signals, such as a voltage, that correspond to the area of each cell  502  and  504  that has been blocked by a mark. The electrical signal produced by cell  502  is subtracted from the electrical signal produced by cell  504 . The subtraction is performed by electronic means not shown in the drawings. In FIG. 5A, because neither cell ( 502  and  504 ) is originally blocked by a mark, both cells produce electrical signals of the same amplitude (and phase), and the result of the subtraction will be zero. 
     As the chevron element  314  moves in the process direction  301 , its left leg starts to block cell  502 . As a result, the amplitude of the signal produced by cell  502  is less than the amplitude of the signal produced by cell  504 , which remains unblocked. Thus, the subtractions of the signals, dominated as Va-b, increases in value until the left leg of the chevron  314  starts blocking cell  504 . As the left leg of the chevron  314  moves to block cell  504 , the signal produced by cell  504  decreases in value, and thus, the resulting signal Va-b also starts to decrease. 
     When the centroid C 1  of the left leg of the chevron crosses the line  503  dividing the cells  503  and  504 , the blocked area of each cell is roughly the same (even if the chevron covers the entire area of the cells or only partially covers the area of the cells), and the value of the signal Va-b becomes zero. As the left leg continues to move into the cell  504  area and out of the cell  502  area, the signal produced by cell  502  becomes greater than the signal produce by cell  504  and the signal Va-b assumes a negative value. Finally, when the left leg of the chevron no longer blocks cells  504  and  502 , the signal Va-b returns to a value of zero. 
     The signal Va-b produced as a result of the events discussed above is plotted as a function of time in FIG.  5 B. Point  510  represents the time tc at which the centroid C 1  crosses the line  503 , as explained above. Time tc is recorded in order to determine the relative time differences between the left legs of chevrons passing over the detector  501 . For example T 2  represents the difference between the time recorded for the left legs of chevron  314  and chevron  313 . 
     It is important to keep in mind that the description makes only reference to the left-side of the chevron and detector, but that the same description applies to the right-hand side. It is also important to note that the split detector as well as the mechanisms connected to the detector, in order to measure the time at which the centroids cross the line dividing the cells, are only preferred embodiments of the present invention and do not limit the invention. 
     Once the vertical and horizontal displacements are calculated using the algorithm disclosed above, these displacements can be used in combination with the knowledge that a first ROS is using, for example, its last 400 pixels to write a chevron element (or part of it), and that the second ROS was writing the chevron with its first 400 pixels, the relative position of the last pixel of the first ROS and the first pixel of the second ROS is easily calculated. As an example, consider the following scenario of how the relative displacement measurements could be used: 
     A) Electro-mechanical actuators could be used to appropriately move the ROSs so that vertical and horizontal relative displacements of adjacent ROS fields are integer multiples of the basic pixel spacing, say  {fraction (1/600+L )} of an inch for a  600 spots per inch (spi) ROS. 
     B) With relative displacements now given in known multiples of the basic pixel displacement, straightforward manipulations of the image in the (upstream) image path are all that is necessary to stitch together adjacent imager fields. The following strategies allow the parsing of an image in the horizontal and vertical directions: 
     1) In the horizontal direction knowing that pixel  2458  of the first ROS overlays pixel  317  of the second would easily establish that if the first pixel of the image started with the pixel  458  of the first ROS, then pixel  2001  of the image should correspond to pixel  318  of the second physical imager, etc. 
     2) In the vertical direction, line buffers of sufficient size are needed to handle the relative vertical offset. If the second ROS were vertically offset by  {fraction (1/600+L )} inch in the direction of IB travel relative to the first ROS, the ( 8-bit) pwm commands for the first 2 lines would go from 
     012 010 046 128 148 173 182 212 172 165 123 098 083 074 063 053 025 
     013 011 048 117 137 179 185 217 175 159 124 092 088 080 059 060 029 to become, assuming that the horizontal intersection occurs between the 8 th  and 9 th  pixels, 000 000 000 000 000 000 000 000 172 165 123 098 083 074 063 053 025 012 010 046 128 148 173 182 212 175 159 124 092 088 080 059 060 029 
     Note that the size of the (whole) line buffer depends upon the maximum vertical relative displacement after actuation, i.e., if actuation brings all imagers within a range of 0.15″, vertically, then a 90 line buffer is needed. 
     This application is not limited, however, to these types of actuators, or to the necessity of having all relative displacements as multiples of the basic pixel spacing. 
     Electronic registration is an alternate, even preferred, actuator embodiment. The important point is that the sensing means discussed herein is not linked solely to any actuator type. Likewise, it is not required that the relative displacement be integer multiples of pixel size. As noted in the example given above for purposes of illustration, this made the image processing step easy to comprehend. 
     The stitching method proposed above also provides an effective means to stitch together VCSEL arrays. There are several advantages to stitching together multiple VCSEL printbars. First, the die size for the VCSEL arrays is limited by the wafer size of the GaAs, and hence this sets an upper limit of about 14 inches for the width of a single VCSEL printbar operating in projection. Second, utilizing more imagers and making each imager shorter would increase the yield of the resulting smaller laser die, and also simplify the assembly and handling. Third, since the field angle of the optics is generally fixed, using several, instead of one imager, reduces the track length and hence the overall size of the print engine. 
     Further related to the stitching of VCSEL printbars, adding about 400 extra pixels in order to allow for overlapping MOBs would add about 1.2 mm to the size of the GaAs die, and would permit using existing MOB sensors and methods. It may be feasible to shrink the size of the MOB and thereby the number of additional pixels considerably. Given that the proposed GaAs die length for the VCSEL bar is currently over 40 mm long, having the means to stitch together images from 2 to 4 smaller die would significantly shorten the array segments, while at the same time require only a small percentage of additional pixels. 
     In summary, the above two examples illustrate how the proposed sensing method could extend the applicability of xerographic images to wider architectures. In general, a scanning system with multiple subscanners that each write information onto overlapping MOBs is disclosed. The applicability to direct marking systems has been mentioned above, and requires only that the marks be generated and sensed on a substrate rather than a photoreceptor. This concept may also be extensible to very small subscanners. 
     The foregoing description of preferred embodiments of the present invention provides an exemplary illustration and description, but is not intended to be exhaustive or to limit the invention to the precise form disclosed. Modifications and variations are possible in light of the above teachings or may be acquired from practice of the invention.