Abstract:
An apparatus ( 10 ) for adjusting the calibration of an imaging system to correct for geometric distortion comprises a rotating drum for receiving recording media ( 17 ), a plurality of carriages ( 18 ) each having an imaging head ( 16 ) with a plurality of addressable imaging channels, and a controller programmed with a lookup table, analytical relationship or algorithm that relates corrective adjustments to be made to the imaging of a given imaging head based on the positions of others of the imaging heads. A method is described for obtaining the lookup table, analytical relationship or algorithm. The method has particular relevance to large imaging systems.

Description:
FIELD OF THE INVENTION 
     The invention relates to imaging systems with multiple imaging heads for forming images on recording media. More particularly, the invention relates to alignment and calibration of such imaging systems. 
     BACKGROUND OF THE INVENTION 
     Various imaging systems are used to form images on recording media. For example, computer-to-plate systems (also known as CTP systems) are used to form images on printing plates. A plurality of imaged printing plates is subsequently provided to a printing press where images from the printing plate are transferred to paper or other suitable surfaces. It is important that the plurality of images be accurately aligned with respect to one another to ensure an accurate registration among the images. It is important that each image be geometrically correct and free from distortion to ensure desired quality characteristics of the finished printed article. Geometric characteristics of an image include but are not limited to: a desired size of an image portion or a desired alignment of one image portion with another image portion. 
     The geometric accuracy of the images formed on a recording media is dependent on numerous factors. For example, images are formed on a recording media by mounting the media on a support and directing imaging beams towards the media to form the images thereupon. Scanning the recording media with the imaging beams during a plurality of scans typically forms the images. The positioning accuracy of the imaging beams with respect to the recording media impacts the geometric correctness of the formed images. Deviations in required positioning of the imaging beams during each scan can lead to imaging errors. 
     In order to reduce imaging errors, imaging systems are typically calibrated. Test images are typically formed on recording media and are analyzed to determine deviations. Deviations associated with a desired geometric characteristic of a test image are typically corrected by performing various adjustments in the imaging systems. The adjustments can be electronic or mechanical in nature. Analysis of the test images is typically performed on specialized and dedicated equipment that can include various image sensors. For example, CCD sensors can be used to capture various images of the test images and a controller can be used to analyze the captured images and determine positional information therefrom. 
     Specialized and dedicated calibration systems are costly and require regular calibration themselves to insure their integrity. Such systems are typically employed at the factory where the imaging systems are manufactured. Factory based calibration systems complicate the calibration of an imaging system in the field. For instance, test images would need to be made in the field and then shipped to another site for analysis. This increases the time required for calibration and increases the chances for imaging errors to occur. 
     Co-pending U.S. Patent Publication 2008/0299470 shows a system and method for changing the calibration of an imaging machine to adjust for geometric distortion while the machine is serving in the field. The machine addressed by U.S. Patent Publication 2008/0299470 comprises a single imaging head. 
     There remains a need for effective and practical methods and systems that permit the calibration of imaging machines with multiple imaging heads to correct geometric distortions of images formed by such multiple-head imaging machines. 
     SUMMARY OF THE INVENTION 
     The present invention is a method for changing the calibration of an imaging system comprising a plurality of imaging heads. The method comprises adjusting the imaging of a first imaging head of the plurality of imaging heads based on a position of at least a second imaging head of the plurality of imaging heads. The first imaging head comprises a first plurality of addressable channels. Adjusting the imaging of the first imaging head can comprise one or more of adjusting the activation timing of at least one of the first plurality of channels, adjusting the speed of the first imaging head, and adjusting the position of the first imaging head. 
     The method of the invention comprises placing the second imaging head in a first position and determining a first adjustment to be made to the imaging of the first imaging head. This is then repeated for different positions of the second imaging head. Interpolation is used to obtain the adjustments that need to be made for positions of the second imaging head between those for which physical determinations are made. The adjustment made to the first imaging head, along with the position of the second imaging head, are entered in a lookup table that can be used to correct the imaging of the first imaging head. 
     The method can be extended to imaging apparatus with more than two imaging heads. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments and applications of the invention are illustrated by the attached non-limiting drawings. The attached drawings are for purposes of illustrating the concepts of the invention and may not be to scale. 
         FIG. 1  is a partial schematic view of an imaging apparatus as per an example embodiment of the invention; 
         FIG. 2  is a flow chart representing a method as per an example embodiment of the invention; 
         FIG. 3  is a schematic plan view of a target image to be formed on a recording media; 
         FIG. 4  is a schematic plan view of a the target image of  FIG. 3 , as formed on a recording media mounted on a media support; 
         FIG. 5A  shows a possible causes for sub-scan deviations of the projection point of imaging beams; 
         FIG. 5B  shows possible causes for main-scan deviations of the projection points of imaging beams; and 
         FIG. 6  is a flow chart representing a method as per an example embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Throughout the following description specific details are presented to provide a more thorough understanding to persons skilled in the art. However, well-known elements may not have been shown or described in detail to avoid unnecessarily obscuring the disclosure. Accordingly, the description and drawings are to be regarded in an illustrative, rather than a restrictive, sense. 
       FIG. 1  schematically shows an apparatus  10  for forming an image  19 A on a recording media  17  as per an example embodiment of the invention. Apparatus  10  includes a media support  12 , which in this example includes an external drum configuration. Other example embodiments of the invention can include other forms of media supports such as internal drum configurations or flat surface configurations. Recording media  17  is supported on a surface  13  of media support  12 . One or more edge portions of recording media  17  are secured to surface  13  by clamps  28 A and  28 B. Other example embodiments of the invention can secure recording media  17  to media support  12  by other methods, including but not limited to, providing a low-pressure source between the surface  13  and recording media  17 . 
     Apparatus  10  includes imaging head  16 , which is movable with respect to media support  12 . In this example embodiment of the invention, imaging head  16  is mounted on movable carriage  18 . Carriage  18  is moved with respect to support  20  in manner in which imaging head  16  is moved along a path aligned with an axis of the drum of media support  12 . In this example embodiment of the invention, imaging head  16  moves along a path aligned with sub-scan axis  24 . 
     Apparatus  10  includes at least one more imaging head  46 , equipped with sensor  62  and illuminator  65 , and which is movable with respect to media support  12 . In this example embodiment of the invention, imaging head  46  is mounted on movable carriage  48 . Carriage  48  is moved with respect to support  20  in a manner in which imaging head  46  is moved along a path aligned with an axis of the drum of media support  12 . In this example embodiment of the invention, imaging head  46  moves along a path aligned with sub-scan axis  24 . 
     Media support  12  rotates with respect to support  20 . Motion system  22  is used to provide relative motion between imaging head  16  and media support  12 . Motion system  22  (which can include one or more motion systems) can include any suitable prime movers needed for the required motion. In this example embodiment of the invention, motion system  22  is used to move media support  12  along a path aligned with main-scan axis  26  while moving imaging head  16  and imaging head  46  along paths aligned with sub-scan axis  24 . Guide system  32  is used to guide carriage  18  and carriage  48 , which are both moved independently. Carriage  18  is moved under the influence of transmission member  33  and carriage  48  is moved under the influence of transmission member  34 . In this example embodiment of the invention, transmission members  33  and  34  include precision lead screws. Those skilled in the art will realize that other forms of motion can be used in accordance with the present invention. For example, imaging heads  16  and  46  can be stationary while media support  12  is moved. In other cases, media support  12  is stationary and imaging heads  16  and  46  are moved. In still other cases, the imaging heads  16  and  46 , as well as the media support  12 , are moved. Imaging heads  16  and  46  on the one hand, or media support  12  on the other, or all three, can reciprocate along corresponding paths. Separate motion systems can also be used to operate different systems within apparatus  10 . 
     Imaging heads  16  and  46  comprise radiation sources (not shown), such as lasers. Imaging heads  16  and  46  are controllable to direct one or more imaging beams  21  (shown in  FIG. 5A  for imaging head  16 ) capable of forming image  19 A on recording media  17 . Imaging beams  21  generated by imaging head  16  are scanned over recording media  17  while image-wise modulated according to image data specifying the image to be written. One or more imaging channels are driven appropriately to produce imaging beams  21  with active intensity levels wherever it is desired to form an image portion. Imaging channels not corresponding to the image portions are driven so as not to image corresponding areas. Imaging head  46  is operated in the same way. 
     Image  19 A can be formed on recording media  17  by different methods. For example, recording media  17  can include an image modifiable surface, wherein a property or characteristic of the modifiable surface is changed when irradiated by an imaging beam to form an image. An imaging beam can be used to ablate a surface of recording media  17  to form an image. An imaging beam can be used to facilitate a transfer of an image forming material to a surface of recording media  17  to form an image. Imaging heads  16  and  46  can include pluralities of channels that can be arranged in an array. An array of imaging channels can include a one-dimensional or two-dimensional array of imaging channels. An imaging beam can traverse a direct path from a radiation source to the recording media or can be deflected by one or more optical elements towards the recording media. 
     Groups of channels can form an image swath having a width related to the distance between a first pixel imaged and a last pixel imaged during a given scan. Recording media  17  is typically too large to be imaged within a single imaged swath. Multiple imaged swaths are typically formed to complete an image on recording media  17 . 
     Controller  30 , which can include one or more controllers is used to control one or more systems of apparatus  10  including, but not limited to, various motion systems  22  used by media support  12  and carriages  18  and  48 . Controller  30  can also control media handling mechanisms that can initiate the loading or unloading of media  17  to and from media support  12 , respectively. Controller  30  can also provide image data  37  to imaging heads  16  and  46  and control imaging heads  16  and  46  to emit imaging beams  21  in accordance with this data. Various systems can be controlled using various control signals or implementing various methods. Controller  30  is programmable and can be configured to execute suitable software and can include one or more data processors, together with suitable hardware, including by way of non-limiting example: accessible memory, logic circuitry, drivers, amplifiers, A/D and D/A converters, input/output ports and the like. Controller  30  can comprise, without limitation, a microprocessor, a computer-on-a-chip, the CPU of a computer or any other suitable microcontroller. Controller  30  can be associated with a materials handling system, but need not necessarily be, the same controller that controls the operation of the imaging systems. 
     In larger machines in particular, guide systems  32  and the transmission members  33  and  34  are long and can deflect significantly under the weight of imaging heads  16  and  46 . The geometric correction of a first imaging head  16  of the system is, therefore, affected by the position of a second imaging head  46 . Thus, while the intended motion of the two imaging heads  16  and  46  can very well be independent, the positioning and orientation errors induced in the actual positioning and orientation of the first imaging head  16  by the positioning of the second of the imaging heads  46  are significant and, for a given position of the first imaging head  16 , have to be compensated for all possible positions of the second imaging head  46 . 
     Thereupon the procedure is repeated for second imaging head  467  the errors of which have to be compensated for as a function of the position of first imaging head  16 . 
     A representative set of positions for each of the imaging heads can be selected and the geometric correction can be performed for those positions of the two imaging heads  16  and  46 . The corrections for positions between the selected ones can then be obtained to a good approximation by interpolation. 
       FIG. 2  shows a flow chart representative of a method of calibrating a first imaging head  16  of an image recording apparatus  10  whilst allowing for the deviations caused by the positioning of a second imaging head  46 , followed by calibrating a second imaging head  46  of image recording apparatus  10  whilst allowing for the deviations caused by the positioning of the first imaging head  16 , as per an example embodiment of the invention. The various steps illustrated in  FIG. 2  are described with reference to apparatus  10  shown in  FIG. 1 . This is for the purposes of illustration only and other suitable imaging apparatus can be used in the present invention. 
     In step  100 , as shown in  FIG. 2 , the method of the present invention proceeds by positioning second imaging head  46  at a known first of N positions, where N is the integer number of different positions of second imaging head  46  for which corrections are to be made for the deviations induced by the varying positioning of second imaging head  46  in the imaging performed by first imaging head  16 . 
     In step  110 , apparatus  10  is used to form a target image on recording media  17 . Various target images can be used in step  110 . One such image is shown in  FIG. 3 . In this example, target image  40  comprises a regular grid pattern made up of target cells  41  which are defined by image boundaries of a desired size. In this example embodiment, target cells  41  are square shaped. Target image  40  is represented in a desired alignment with various edges of recording media  17 . Specifically, it is desired to form target image  40  by a distance X from edge  35  and by a distance Y from edge  36 . It is desired to form target image  40  in an aligned relationship with main-scan axis  26  and sub-scan axis  24 . 
     Target image  40  is represented by image data  37  (see  FIG. 1 ) and is provided to controller  30  to form an image on recoding media  12 . Controller  30  controls imaging heads  16  and  46  to direct imaging beams  21  to form image  19 A while scanning over recording media  17 . In this example embodiment of the invention, controller  30  controls motion system  22  to create relative motion between imaging heads  16  and  46  on the one hand, and recording media  17  on the other during the imaging. In this example embodiment of the invention, imaging heads  16  and  46  are translated in a coordinated manner with the rotation of media support  12  to form helically-oriented image swaths. 
       FIG. 4  schematically shows an example calibration image  19  formed on recording media  17  in response to the desired imaging of target image  40  of  FIG. 3  by first imaging head  16 . While first imaging head  16  is singled out here for the sake of clarity, the method also holds for imaging head  46 . Recording media  17  is shown mounted on surface  13  of media support  12 . For the sake of clarity, recording media  17  and media support surface  13  are depicted in a “flat” orientation. It is to be understood that media support surface is cylindrical in nature in this example embodiment of the invention. As shown in  FIG. 4 , calibration image  19  does not correspond exactly to target image  40 . Various imaging distortions appear in different areas of calibration image  19 . Imaged cells such as imaged cells  42 A,  42 B,  42 C and  42 D (collectively referred to as imaged cells) do not correspond exactly to the pattern of target cells  41 . For example, imaged cell  42 A is shifted in a main-scan direction with respect to imaged cell  42 B.  FIG. 4  also shows that image cell  42 D is elongated in size as compared to image cell  42 C. Further, all of the imaged cells  42 A-D are elongated in size in a sub-scan direction as compared with target cells  41 .  FIG. 4  shows that the overall scale of imaged cells  42 A-D does not match the required scale of target cells  41 .  FIG. 4  also shows that calibration image  19  is respectively displaced from edges  35  and  36  by distances X 1  and Y 1  which differ from desired distances X and Y. 
     Positional and size distortions can occur for several reasons. For example, overall scaling problems can arise from temperature variances. One, or more of recording media  17 , media support  12  and various transmission components such as transmission member  33  can include different material compositions that have different coefficients of thermal expansion. Different expansion rates can lead to scaling problems. Carriage  18  moves along a guide system  32  that is not perfect in form. Guide system  32  can include various suitable guide tracks and guided members that can include sliding or rotational moving bearing elements. Imperfections can be present even when high precision components are used. Mechanical factors, such as guided member-to-guide track play, guide track straightness and sag in support  20  can lead to imaging imperfections 
     As carriage  18  moves along a path aligned with sub-scan axis  24 , mechanical factors can subject first imaging head  16  to various additional motions that can adversely impact the projection of imaging beams  21  onto recording media  16 . Imaging imperfections can be visualized with reference to  FIGS. 5A and 5B .  FIG. 5A  shows a possible cause for sub-scan deviations in the projection of imaging beams  21  onto recording media  17 . In this case, carriage  18  undergoes small yawing rotations (exaggerated for the sake of clarity) along yaw directions  27  as it moves along a path aligned with sub-scan axis  24 . This causes variations in the projection of imaging beams  21  onto recording media  17  which can cause distortion of formed images in a sub-scan direction. Yaw motions of carriage  18  can arise for numerous reasons including play in guide system  32 . Further, imperfections in other components can lead to sub-scan deviations. For example, transmission member  33  can comprise a precision lead screw, which can have slight pitch irregularities at various points along its length. Pitch irregularities can cause distortion of formed images in a sub-scan direction. 
       FIG. 5B  shows possible causes for main-scan deviations in the projection of imaging beams  21  onto recording media  17 . While we single out first imaging head  16 , the deviations and corrective method hold also for imaging head  46 . Main-scan deviations can arise for different reasons. For example, as carriage  18  moves along guide system  32  it may undergo small displacements aligned with main-scan axis  26 . The small displacements can be caused by various factors, which can include play in guide system  32 , and deviations in the guide tracks including gravitational sag in the tracks and gravitational sag in support  20 . Main-scan deviations can be caused as carriage  18  undergoes small pitching rotations, (exaggerated for the sake of clarity) along pitch directions  29  and also as the carriage  18  moves along a path aligned with sub-scan axis  24 . Pitch displacements can be caused by numerous reasons including play in guide system  32 . 
     The mass of first imaging head  16  itself, particularly in large machines, induces a displacement along the main-scan direction of the image formed by first imaging head  16 . The chief distortion in the image produced by first imaging head  16  as a result of the positioning of second imaging head  46  is likewise that of displacement of the image along the main-scan direction, being due to the mass of second imaging head  46 . 
     Those skilled in the art will realize that the image distortions described are exemplary in nature and that other types of distortion can occur. In the dual imaging head arrangement of  FIG. 1 , the image distortions associated with first imaging head  16  will be different for different positions of second imaging head  46 . While most of the variation occurs along the main-scan direction as a result of the weight of carriage  48  and second imaging head  46 , the effect is not limited to the main-scan direction and some sub-scan direction displacement can occur. Similarly, in the dual imaging head arrangement of  FIG. 1 , the image distortions associated with second imaging head  46  will be different for different positions of first imaging head  16 . 
     In step  120 , apparatus  10  is adjusted to correct for the deviations in calibration image  19  produced by first imaging head  16 . Deviations can be corrected by various methods. In some example embodiments of the invention, main-scan distortions such as the shifts between imaged cells  42 A and  42 B can be corrected by adjusting an activation timing of the imaging channels. Although the activation of a given imaging channel to form or not form an image pixel is dependent on image data, the timing of the activation of the given channel can be adjusted. Adjustments of the activation timing of various channels can be used to delay or advance the activation of those channels to form or not form one or more image pixels. Various channels can be controlled to offset a first portion of an imaged swath with respect to an additional portion of the imaged swath in a main-scan direction. A portion of a given imaged swath can be offset from a portion of an additional imaged swath. A portion of an imaged swath can include the entirety of the imaged swath. The activation timing of various channels of first imaging head  16  can be adjusted at various positions as first imaging head  16  is moved along a path aligned with sub-scan axis  24 . The distortion of images formed at these various positions can be corrected by activation timing adjustments at these positions. In this example embodiment of the invention, activation-timing changes are made independently of image data  37 . Controller  30  can be programmed to provide signals to first imaging head  16  to adjust activations timing of its imaging channels. Motion system  22  can include suitable sensors, which can generate various signals representative of a position of first imaging head  16  and/or media support  12 . In some example embodiments of the invention, sensor signals can be used by controller  30  to adjust activation timings of various channels. Activation timing adjustments can also be made to adjust the position of calibration image  19  from edge  36 . 
     Sub-scan distortions such as elongated imaged cells  42 D can be corrected by various methods. In some example embodiments of the invention, the movement of carriage  18  is adjusted. In some example embodiments of the invention, the movement of transmission member  33  is adjusted. Adjusting the movement of carriage  18  or transmission member  33  can include adjusting a speed of carriage  18  or transmission member  33 . For example, in a drum based imaging system (e.g. apparatus  10 ) spiral or helical image swaths are formed as the carriage  18  is translated while media support  12  is rotated. By adjusting the speed of transmission member  33  or carriage  18 , the pitch of the helical swaths can be adjusted to scale the image to a desired size. In some example embodiments of the invention, uniform speed adjustments can be used to adjust the overall size of a formed image. In some example embodiments of the invention, non-uniform speed adjustments can be used to adjust the size of a part of the formed image. For example, adjusting a speed of transmission member  33  at various points along the motion path of carriage  18  can be used to correct image deviations corresponding to those points. Adjusting a speed of transmission member  33  at various points along the motion path of carriage  18  can be used to correct for pitch discrepancies. Adjusting a speed of transmission member  33  at various points along the motion path of carriage  18  can be used to correct head yaw displacements. Adjustments can also be made to adjust the position of an image in a sub-scan direction. For example, adjustments can be used to adjust a position of calibration image  19  from edge  35 . 
     Controller  30  can be programmed to provide signals to motion system  22  to adjust a movement of carriage  18  or transmission member  33 . In some example embodiments of the invention, sensor signals can be used by controller  30  to adjust a movement of carriage  18  or transmission member  33 . 
     In step  130 , it is ascertained whether the deviations in the imaging by first imaging head  16  has been corrected for all N positions of second imaging head  46 . If it has, then the method proceeds to step  150 . If it has not, then the counter n is increased to n+1 in step  140 , and the corrective calibration process as described by step  120  is performed for this new position of second imaging head  46 . The process is repeated until the imaging by first imaging head  16  has been corrected for all chosen positions of second imaging head  46 , at which point step  130  registers a result of n=N. 
     In step  150 , the corrections to the imaging by first imaging head  16  at positions of second imaging head  46  between n and n+1 are determined by interpolation. The corrections may be stored as lookup tables or may be stored as corrective algorithms or functions. The method may now be repeated for different positions of first imaging head  16  so that, for every position of first imaging head  16 , imaging corrections are known for all possible positions of second imaging head  46 . 
     The method of the invention can be extended to imaging systems having more than two imaging heads, in which case the imaging for a given imaging head had to be adjusted for the positions of more than one other imaging head. 
     A reduction in the complexity of the operation may be devised by operating the two imaging heads  16  and  46  so that they are always in a known mutual positional relationship along sub-scan axis  24 . The corrections for the two imaging heads  16  and  46  are then only required as a function of the position of the particular imaging head being calibrated, the position of the other imaging head being functionally known, even if it is not at a fixed distance from the first imaging head. This reduces the access to any lookup tables involved. 
     In one embodiment of the present invention, carriages  18  and  48  are not advanced continuously in the subscan direction to cause imaging heads  16  and  46  to write a helical path, but are, instead, moved to discrete subscan positions and the media support  12  is rotated to cause imaging heads  16  and  46  to write a circular path. These are referred to as “step-and-repeat” systems. Adjustments to the imaging of imaging heads  16  and  46  for “step-and-repeat” systems are performed not by adjusting the subscan speed of carriages  18  and  48 , but by adjusting the positions to which they are instructed to relocate themselves. 
     Co-pending U.S. Patent Publication 2008/0299470, herewith incorporated in full, describes a method to adapt the calibration of apparatus  10  by adjusting the imaging corrections to allow for geometric distortion. To this end permanent reference features  50  are provided on surface  13  of media support  12 . In an embodiment of the present invention shown in  FIG. 6 , the imaging corrections for first imaging head  16 , located at a given subscan position, due to second imaging head  46  being in a particular subscan position are determined by detecting a reference feature  50  at or near to the position of first imaging head  16  using illuminator  55  and sensor  52 . Controller  30  compares the newly determined positions of reference features  50  against the previously determined positions. If during this comparison, a change in an expected position of a detected reference feature  50  is noted, then controller  30  adjusts the imaging corrections in accordance with this change. A determined position of reference feature  50  is thereby obtained. The determined position of the reference feature is then compared with an expected position for the same reference feature. The imaging corrections of imaging head  16 , while the latter is located at the given subscan position, are then derived from the difference between the determined position and the expected position. The method is repeated for different positions of second imaging head  46 , while first imaging head  16  is kept in the given subscan position. The process is repeated for first imaging head  16  placed at other subscan positions. By the same method, imaging corrections for second imaging head  46  due to first imaging head  16  being at different possible positions may be made for all possible positions of second imaging head  46 . 
     The method comprises applying the same process with the roles of first imaging head  16  and second imaging head  46  interchanged, thereby providing imaging corrections for imaging head  46  at a set of subscan positions for different positions of first imaging head  16  in each case. In this case illuminator  65  and sensor  62  are employed. 
       FIG. 6  is a flow diagram of this embodiment of the method of the present invention. In step  200 , second imaging head  46  is positioned at an n th  position, where n=1, 2, . . . N. The deviation of a reference feature  50  from an expected position is determined in step  210  using sensor  52 . The image corrections to imaging head  16  are made in step  220  to correct for the deviation of reference feature  50  due to the positioning of first imaging head  46 . 
     In step  230 , it is ascertained whether the deviation in the imaging by first imaging head  16  has been corrected for all N positions of second imaging head  46 . If it has, then the method proceeds to step  250 . If it has not, then the counter n is increased to n+1 in step  240 , and the corrective calibration process as described by steps  210  and  220  is performed for this new position of second imaging head  46 . The process is repeated until the imaging by first imaging head  16  has been corrected for all chosen positions of second imaging head  46 , at which point step  230  registers a result of n=N. 
     In step  250 , the corrections to the imaging by first imaging head  16  at positions of second imaging head  46  between n and n+1 are determined by interpolation. The corrections may be stored as lookup tables or may be stored as corrective algorithms or functions. The method may now be repeated for different positions of first imaging head  16  so that, for every position of first imaging head  16 , imaging corrections are known for all possible positions of second imaging head  46 . By the same method, imaging corrections for second imaging head  46  due to first imaging head  16  being at different possible positions may be made for all possible positions of second imaging head  46 . 
     In various embodiments of the invention, sensors  52  and  62  can include any suitable sensor for detecting a reference feature  50 . In some example embodiments of the invention, illuminators  55  and  65  are used to illuminate a reference feature  50  while it is detected. In some example embodiments of the invention, one or more imaging beams  21  emitted by imaging head  16  are used to detect reference features  50 . Imaging beams from imaging head  46  may also be employed to detect reference features  50 . Reference features  50  can include various shapes and forms suitable for detection by sensors  52  and  62 . Without limitation, reference features  50  can include various registration marks or fiducial marks. Reference features  50  can include cross-hairs, diamond shapes, circular shapes and the like. 
     It is to be understood that the exemplary embodiments of the invention are merely illustrative and that those skilled in the art can devise many variations of the described embodiments without departing from the scope of the invention. 
     PARTS LIST 
     
         
           10  apparatus 
           12  media support 
           13  surface 
           16  imaging head 
           17  recording media 
           18  carriage 
           19 A image 
           19  calibration image 
           20  support 
           21  imaging beams 
           22  motion system 
           24  sub-scan axis 
           26  main-scan axis 
           27  yaw directions 
           28 A clamps 
           28 B clamps 
           29  pitch directions 
           30  controller 
           32  guide system 
           33  transmission member (for carriage  18 ) 
           34  transmission member (for carriage  48 ) 
           35  edge 
           36  edge 
           37  image data 
           40  target image 
           41  target cells 
           42 A imaged cells 
           42 B imaged cells 
           42 C imaged cells 
           42 D imaged cells 
           46  imaging head 
           48  carriage 
           50  reference features 
           52  sensor 
           55  illuminator 
           62  sensor 
           65  illuminator 
           100  step 
           110  step 
           120  step 
           130  step 
           140  step 
           150  step 
           200  step 
           210  step 
           220  step 
           230  step 
           240  step 
           250  step 
         X distance 
         X 1  distance 
         Y distance 
         Y 1  distance