PATENT ABSTRACT
A pin registration system uses a plurality of registration pins having different thickness heads to align an edge of a printing plate when mounting the plate to a drum of an imaging system. The registration pins are optionally adjustable along the longitudinal axis of the drum. The registration system avoids the requirement to punch holes or notches in a printing plate and does not require adjusting the thickness of a common registration pin. The registration system allows printing plates having different widths to be mounted without necessitating adjustments of the registration pins.

PATENT DESCRIPTION
This application claims priority under 35 U.S.C. §119 (e) of U.S. Provisional patent application Ser. No. 60/184,880 filed on Feb. 25, 2000. 
    
    
     FIELD OF THE INVENTION 
     The present invention is in the field of imaging systems. More particularly, the present invention provides a media handling system for an external drum image recording apparatus for recording images onto sheets of recording material such as printing plates. 
     BACKGROUND OF THE INVENTION 
     In external drum image recording devices, a movable optical carriage is used to displace an image exposing or recording source in a slow scan direction while a cylindrical drum supporting recording material on an external surface thereof is rotated with respect to the image exposing source. The drum rotation causes the recording material to advance past the exposing source along a direction which is substantially perpendicular to the slow scan direction. The recording material is therefore advanced past the exposing source by the rotating drum in a fast scan direction. 
     An image exposing source may include an optical system for scanning one or more exposing or recording beams. Each recording beam may be separately modulated according to a digital information signal representing data corresponding to the image to be recorded. 
     The recording media to be imaged by an external drum imaging system is commonly supplied in discrete sheets and may comprise a plurality of plates, hereinafter collectively referred to as “plates” or “printing plates.” Each plate may comprise one or more layers supported by a support substrate, which for many printing plates is a plano-graphic aluminum sheet. Other layers may include one or more image recording (i.e., “imageable”) layers such as a photosensitive, radiation sensitive, or thermally sensitive layer, or other chemically or physically alterable layers. Printing plates which are supported by a polyester support are also known and can be used in the present invention. Printing plates are available in a wide variety of sizes, typically ranging, e.g., from 9″×12″, or smaller, to 58″×80″, or larger. The printing plate may additionally comprise a flexographic printing plate. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The features of the present invention will best be understood from a detailed description of the invention and embodiments thereof selected for the purpose of illustration and shown in the accompanying drawings in which: 
     FIG. 1 illustrates an external drum imaging system for recording images onto a supply of recording media such as a printing plate; 
     FIG. 2 illustrates an example of an imaging system including a movable optical carriage and scanning system, usable in the external drum imaging system of FIG. 1; 
     FIG. 3 is a perspective view of the external drum of the imaging system of FIG. 1, in accordance with an embodiment of the present invention 
     FIG. 4 is an end view of the external drum of FIG. 3; 
     FIG. 5 is a plan view of the external drum of FIG. 3; 
     FIG. 6 illustrates an external drum platesetter of the present invention; 
     FIG. 7 illustrates the media handling system of an external drum platesetter in accordance with the present invention; 
     FIG. 8 provides an end view of a leading edge clamping mechanism in an open orientation; 
     FIG. 9 provides an end view of the leading edge clamping mechanism of FIG. 8 in a closed orientation; 
     FIG. 10 is a plan view of the leading edge clamping mechanism of FIG.  8 . 
     FIG. 11 illustrates the location of the registration pins within the leading edge clamping mechanism of FIG. 8; 
     FIG. 12 is a partial perspective view of the trailing edge clamping mechanism; 
     FIG. 13A is a cross-sectional view of the clamping bar of the trailing edge clamping mechanism; 
     FIG. 13B is a cross-sectional view illustrating the operation of the clamping bar of FIG. 13A; 
     FIG. 14 is a cross-sectional view of a slidable support post and biasing system for supporting the clamping bar above the surface of the external drum; 
     FIG. 15 illustrates a friction wheel system for rotating the discs that carry the clamping bar of the trailing edge clamping mechanism; 
     FIGS. 16-21 illustrate the operation and media input flow of the media handling system of the external drum platesetter of FIG. 7; 
     FIGS. 22-25 illustrate the operation and media output flow of the media handling system of the external drum platesetter of FIG. 7; 
     FIGS. 26A and 26B illustrate the general input/output flow of the external drum platesetter of FIG. 7; 
     FIG. 27 illustrates an external drum platesetter in accordance with an alternate embodiment of the present invention; 
     FIG. 28 is a cutaway perspective view of the external drum platesetter of FIG. 27; 
     FIG. 29 is a perspective view of the external drum platesetter of FIG. 27, coupled to an on-line processor; 
     FIGS. 30A and 30B illustrate the general input/output flow of the external drum platesetter of FIG. 27; 
     FIGS. 31-44 illustrate the operation and media input/output process of the media handling system of the external drum platesetter of FIG. 27; 
     FIG. 45 illustrates an additional embodiment of the registration pins in accordance with the present invention; 
     FIG. 46 is a plan view of a registration pin arrangement in accordance with the present invention; 
     FIG. 47 illustrates the use of the registration pin arrangement of FIG. 46; 
     FIG. 48 illustrates a control system for controlling the media input/output operation; and 
     FIGS. 49 and 50 illustrate the operation of the edge sensors. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The features of the present invention are illustrated in detail in the accompanying drawings, wherein like reference numerals refer to like elements throughout the drawings. Although the drawings are intended to illustrate the present invention, the drawings are not necessarily drawn to scale. 
     An example of an imaging system  10  employing an external drum image recording system is illustrated in FIG.  1 . In this example, the imaging system  10  comprises an external drum platesetter configured to record digital data onto a printing plate. Although described below with regard to an external drum platesetter, many aspects of the present invention may be used in conjunction with a wide variety of other types of external drum, internal drum, or flatbed imaging systems, including imagesetters and the like, without departing from the intended scope of the present invention. 
     The imaging system  10  generally includes a front end computer or workstation  12  for the design, layout, editing, and/or processing of digital files representing pages to be printed, a raster image processor (RIP)  14  for further processing the digital pages to provide rasterized page data (e.g., rasterized digital files) for driving an image recorder, and an image recorder, such as an external drum platesetter  16 , for recording the rasterized digital files onto a printing plate or other recording media. The external drum platesetter  16  records the digital data (i.e., “job”) provided by the RIP  14  onto a supply of photosensitive, radiation sensitive, thermally sensitive, or other type of suitable printing plate  18 . In the present invention, the printing plate  18  is manually loaded onto a staging area of the external drum platesetter  16  by an operator. Alternately, or in addition to manual loading, the printing plate may be provided and loaded onto the external drum platesetter  16  by a media supply or autoloading system  60 . The media supply system  60  may accept a plurality of the same size printing plates  18 , and/or may accept a plurality of different size printing plates  18 . 
     The external drum platesetter  16  includes an external drum  20  having a cylindrical media support surface  22  for supporting the printing plate  18  during imaging. The external drum platesetter  16  further includes a scanning system  24 , coupled to a movable carriage  26 , for recording digital data onto the imaging surface  21  of the printing plate  18  using a single or multiple imaging beams  28 . An example of a scanning system  24  is illustrated in FIG.  2 . In particular, the scanning system  24  is displaced by the movable carriage  26  in a slow scan axial direction (directional arrow A) along the length of the rotating external drum  20  to expose the printing plate  18  in a line-wise manner when a single beam is used or in a section-wise manner for multiple beams. Other types of imaging systems may also be used in the present invention. 
     The external drum  20  is rotated by a drive system  36  in a clockwise or counterclockwise direction as indicated by directional arrow B in FIG.  1 . Typically, the drive system  36  rotates the external drum  20  at a rate of about 100-1000 rpm. In the present invention, the printing plate  18  is loaded onto the external drum  20  while rotating the drum in a first direction. The printing plate  18  is then imaged while the drum is rotated in the first, or in a second, opposite direction. Finally, the printing plate  18  is unloaded from the external drum  20  while rotating the drum in the second direction. The details of the loading, imaging, and unloading operations are discussed in further detail below. 
     As further illustrated in FIG. 2, the scanning system  24  typically includes a system  30  for generating the imaging beam or beams  28 . The system  30  comprises a light or radiation source  32  for producing the imaging beam or beams  28  (illustrated for simplicity as a single beam), and an optical system  34  positioned between the radiation source  32  and the media support surface  22  for focusing the imaging beam or beams  28  onto the printing plate  18 . It should be noted, however, that the system  30  described above is only one of many possible different types of scanning systems that may be used to record image data on the printing plate  18 . In the present invention, the system  30  comprises a multiple address grating light valve (GLV) or functionally similar modulator based system, or a multiple beam fiber optic coupled laser system. 
     In the external drum imaging system  10  shown in FIG. 1, the leading edge  38  of the printing plate  18  is held in position against the media support surface  22  by a leading edge clamping mechanism  40 . Similarly, the trailing edge  42  of the printing plate  18  is held in position against the media support surface  22  by a trailing edge clamping mechanism  44 . Both the trailing edge clamping mechanism  44  and the leading edge clamping mechanism  40  provide a tangential friction force between the printing plate  18  and the external drum  20  sufficient to resist the tendency of the edges of the printing plate  18  to pull out of the clamping mechanisms  40 ,  44 , at a high drum rotational speed. In accordance with the present invention, only a small section (e.g., 6 mm) of the leading and trailing edges  38 ,  42 , is held against the external drum  20  by the leading and trailing edge clamping mechanisms  40 ,  44 , thereby increasing the available imaging area of the printing plate  18 . 
     A stationary ironing roller system  46  flattens the printing plate  18  against the media support surface  22  of the external drum  20  as the external drum  20  rotates past the ironing roller  46  during the loading of the printing plate  18 . Alternately, or in addition, a vacuum source  45  may be used to draw a vacuum through an arrangement of ports and vacuum grooves  47  (see, e.g., FIG. 2) formed in the media support surface  22  to hold the printing plate  18  against the media support surface  22 . A registration system, comprising, for example, a set of registration pins or stops on the external drum  20 , and a plate edge detection system, may be used to accurately and repeatably position and locate the printing plate  18  on the external drum  20 . The plate edge detection system, as described infra, may comprise, for example, a plurality of sensors and/or the scanning system  24 . 
     A perspective view of the external drum  20  in accordance with the present invention is illustrated in FIG.  3 . An end view and a plan view of the external drum  20  are illustrated in FIGS. 4 and 5, respectively. As shown, the external drum  20  comprises an outer wall  48  that includes the media support surface  22 , a hollow cylindrical hub  50 , and a plurality of radial spokes  52  extending between the cylindrical hub  50  and the outer wall  48 . The external drum  20  is rotated by the drive system  36  (FIG. 1) via shafts  53  coupled to the ends of the hub  50 . 
     The external drum  20  is formed in a single piece using an extrusion process from a lightweight and strong material such as an aluminum alloy. Suitable aluminum alloys may include, for example, aluminum alloy 6063-T5. Other aluminum alloys, or alloys formed of metals other than aluminum, that can be suitably extruded, may also be used to form the external drum  20 . In other embodiments of the present invention, however, the external drum  20  may be formed of a material such as steel or other ferromagnetic alloy using other processes. Such a material may be required if the leading edge clamping mechanism  40  and/or trailing edge clamping mechanism  44  utilize magnetic or electromagnetic clamping components. 
     The present invention provides a stiff external drum  20  having low rotational inertia. This allows the external drum  20  to be accelerated and decelerated more rapidly than other currently available drums, using smaller and less expensive motors, power supplies, etc., thereby further increasing the throughput of the imaging system  10  of the present invention. 
     The outer wall  48  of the external drum  20  further includes a section  54  containing a groove  56  that provides an interface for the leading edge clamping mechanism  40 . The leading edge clamping mechanism  40  is attached within the groove  56  by inserting and securing suitable mounting hardware (e.g., bolts, etc.) through the leading edge clamping mechanism  40  and corresponding apertures  58  formed in the bottom of the groove  56 . In the present invention, the groove  56  is disposed above one of the radial spokes  52 . The relative thickness of the outer wall  48  is increased below the groove  56  to maintain minimum wall thickness requirements, and to offset any change in drum balance as a result of removing material to form the groove  56 . By forming the groove  56  in this location, the stiffness and strength of the external drum  20  are not compromised. The groove  56  may be formed as part of the extrusion process, and/or may be machined into the external drum  20  after extrusion. 
     To compensate for the weight of leading edge clamping mechanism  40 , and other adjacent system components, thereby balancing the external drum  20  during rotation, the section  60  of the external drum  20  opposite the groove  56  is provided with extra material (i.e., extra mass). This is achieved by increasing the extruded thickness of the outer wall  48  opposite the groove  56 . Thus, the present invention nominally and inexpensively balances the external drum  20  and leading edge clamping mechanism  40  by adding extruded material opposite the clamping mechanism. Proper balancing of the external drum  20  helps to prevent the introduction of vibration-induced artifacts into the images recorded on the printing plate  18  by the imaging system  10 . 
     The basic structure of the media handling system  70  of an external drum platesetter  16  in accordance with the present invention is illustrated in FIG.  7 . The external drum platesetter  16  includes an external drum  20  (see, e.g., FIGS. 3-5) having a cylindrical media support surface  22  for supporting a printing plate  18  during imaging. The external drum  20  is supported by a frame  72 . A drive system  36  rotates the external drum  20  during imaging. A scanning system  24 , carried by a movable carriage  26 , travels axially along the rotating external drum  20  to record digital data onto the imaging surface of the printing plate (see, e.g., FIG.  2 ). The external drum  20  and scanning system  24  are positioned on a base  74 . The base  74  is formed of heavy material, such as a polymer-concrete mixture, granite, or the like, to vibrationally isolate the external drum  20  and scanning system  24  from external vibrations, thereby reducing artifacts in the recorded image. 
     In order to load and hold the printing plate  18  in intimate contact with the cylindrical media support surface  22  of the external drum  20  while the external drum  20  is rotated and an image is recorded onto the printing plate  18 , a leading edge clamping mechanism  40  is provided to hold a leading edge of a printing plate  18  in position against the media support surface  22 . The clamping system of the present invention, comprising the leading edge clamping mechanism  40  and the trailing edge clamping mechanism  44 , is capable of holding a variety of different printing plate widths either left, right, or center justified on the external drum  20 . An actuating system  75 , including an actuator  76  (e.g., a pneumatic actuator, solenoid, etc.), selectively opens and closes the leading edge clamping mechanism  40  to receive, capture, and release the leading edge  38  of the printing plate  18 . The actuating system  75  of the leading edge clamping mechanism  40  is mounted to a frame member (not shown) of the external drum platesetter  16  such that the actuating system  75  is positioned above the media support surface  22  of the external drum  20 . 
     The leading edge clamping mechanism  40  is fixed in position on the external drum  20 . The leading edge clamping mechanism  40  is positioned within a groove  56  (see, e.g., FIGS. 3-5) formed in the external drum  20 . A set of registration pins or stops  78  (hereafter referred to as “registration pins”) are incorporated into the leading edge clamping mechanism  40  to accurately and repeatably position, or “register,” the leading edge  38  of a printing plate  18  at a predetermined location on the external drum  20 , and to align the leading edge  38  of the printing plate  18  along an axis which is substantially parallel to the longitudinal axis of the external drum  20 . The registration pins  78  may also be incorporated into an electrical or other mechanical structure for other purposes, such as to electronically or mechanically detect the presence of the leading edge  38  of the printing plate  18  within the leading edge clamping mechanism  40 . 
     An embodiment of the leading edge clamping mechanism  40  is illustrated in greater detail in FIGS. 8-11. FIGS. 8 and 9 provide end views of the leading edge clamping mechanism  40  in open and closed positions, respectively. FIGS. 8 and 9 additionally illustrate the operation of the actuator  76  and the function of the registration pins  78 . FIG. 10 provides a plan view of the leading edge clamping mechanism  40 . FIG. 11 illustrates the set of registration pins  78  and the mounting portion  80  of the leading edge clamping mechanism  40 . 
     As shown in FIGS. 8 and 9, the leading edge clamping mechanism  40  includes a mounting portion  80  and a clamping portion  82 . The mounting portion  80  is used to secure the leading edge clamping mechanism  40  within the groove  56  of the external drum  20 . As described with reference to FIG. 5, the leading edge clamping mechanism  40  may be attached within the groove  56  by inserting and securing suitable mounting hardware (e.g., bolts, etc.) through the mounting portion  80  and corresponding apertures  58  formed in the bottom of the groove  56 . 
     The clamping portion  82  of the leading edge clamping mechanism  40  is attached to the mounting portion  80  by a biasing system  84 . The biasing system  84 , which may comprise a spring system including one or more springs, biases the clamping portion  82  of the leading edge clamping mechanism  40  closed against the mounting portion  80  with sufficient force to prevent the printing plate  18  from moving during rotation of the external drum  20 . The actuator  76  is used to selectively open the leading edge clamping mechanism  40  to receive or release the leading edge  38  of the printing plate  18 . In particular, as shown in FIG. 8, the actuator  76  includes an extendable member  86  that is configured to selectively engage and press against the clamping portion  82 , thereby counteracting the biasing force of the biasing system  84  and opening the leading edge clamping mechanism  40 . When the clamping portion  82  of the leading edge clamping mechanism  40  is in an open position as illustrated in FIG. 8, a printing plate  18  may be loaded against the registration pins  78  until two points of the leading edge  38  of the printing plate  18  are in contact with two registration pins  78 . 
     As further illustrated in FIG. 8, during the loading of a printing plate  18  onto the external drum  20 , the leading edge  38  of the printing plate  18  is accurately and repeatably positioned and aligned on the external drum  20  using the registration pins  78 . As will be further illustrated below, only two axially spaced registration pins  78  contact the leading edge  38  of the printing plate  18  to ensure that the leading edge  38  is correctly positioned along an axis which is substantially parallel to the longitudinal axis of the external drum  20 . A plate edge detection system (not shown), comprising, for example, an optical sensor, a mechanical sensor, etc., is used to electronically and/or mechanically sense or detect a perpendicular edge of the printing plate  18  (i.e., an edge perpendicular to the axis of the leading edge  38 ) to determine the axial position of the printing plate  18  on the external drum  20 . Once the exact position of the printing plate  18  is determined on the external drum  20 , the scanning system  24  (see, e.g., FIGS. 1,  2 , and  7 ) can be accurately positioned by the movable carriage  26  to record image data in predetermined locations on the printing plate  18  with respect to the leading and perpendicular edges thereof. 
     As illustrated in FIG. 9, the leading edge  38  of the printing plate  18  is secured in position on the external drum  20  by closing the leading edge clamping mechanism  40 . In particular, to close the leading edge clamping mechanism  40 , the actuator  76  retracts the member  86  away from the clamping portion  82 . This removes the force that previously counteracted the biasing force applied against the clamping portion  82  by the biasing system  84 . Accordingly, the clamping portion  82  is now forced toward the external drum  20  by the biasing system  84 , thereby securing the leading edge  38  of the printing plate  18  against the external drum  20 . 
     After the leading edge  38  of the printing plate  18  has been properly positioned against the registration pins  78  and secured to the external drum  20  by the leading edge clamping mechanism  40 , subsequent media handling operations may then be performed to completely load the printing plate  18  onto the external drum  20 . Thereafter, image data may be recorded on the printing plate  18  by the scanning system  24  as the printing plate  18  is rotated on the external drum  20 . Upon completion of the imaging process, the printing plate  18  is unloaded from the external drum  20  as will be described supra with reference to FIGS. 22-25. A layer of rubber or other nonabrasive material  83  may be applied to the clamping portion  82  to prevent damage to the imaging surface of the printing plate  18 , to take-up the tolerances of the location of the clamping surface of the clamping portion  82 , and to increase friction between the clamping portion  82  and the printing plate  18 . 
     A distal end  88  of the clamping portion  82  of the leading edge clamping mechanism  40  is weighted such that the center of gravity of the clamping portion  82  is located between the biasing system  84  and the distal end  88 . In FIGS. 8 and 9, for example, the center of gravity is to the “right” of the biasing system  84 . By forming the clamping portion  82  in this manner, the clamping force applied by the clamping portion  82  against the external drum  20  and printing plate  18  increases as the rotational speed of the external drum  20  increases. This helps to prevent the clamping portion  82  from inadvertently releasing the leading edge  38  of the printing plate  18  during high speed rotation (e.g., 1000 rpm) of the external drum  20  during imaging. 
     A plan view of the leading edge clamping mechanism  40  is illustrated in FIG.  10 . As shown, the leading edge clamping mechanism  40  may include a single clamping portion  82  that extends the length of the external drum  20 , which may be opened and closed by an actuating system comprising a single actuator  76 . Alternately, an actuating system comprising a plurality of actuators  76  (shown in phantom) may be positioned (e.g., distributed along) the external drum  20  to open and close the single clamping portion  82 . The clamping portion  82  may also comprise a plurality of discrete sections  90  (shown in phantom), collectively operated by an actuating system including a single actuator  76 , or a plurality of actuators  76 . 
     A plan view of the mounting portion  80  of the leading edge clamping mechanism  40  is illustrated in FIG.  11 . The mounting portion  80  includes a channel  92  through which the registration pins  78  extend away from the external drum  20 , and along which the registration pins  78  may be selectively positioned and secured according the size or sizes of the printing plates  18  to be imaged. The channel  92  extends along the entire length of the external drum  20  to allow the registration pins  78  to be positioned at any axial location along the external drum  20 . Generally, the registration pins  78  are positioned on the external drum  20  such that the leading edge  38  of a printing plate  18  contacts two of the registration pins  78 . The registration pins  78  may be fixed in position along the channel  92  in any suitable manner. Each registration pin  78  includes a base  94  having a width W 1  larger than the width W 2  of the channel  92 . 
     Referring again to FIG. 7, a stationary ironing roller system  46  is used to flatten the printing plate  18  against the media support surface  22  of the external drum  20  as the external drum  20  rotates past the ironing roller system  46  during the loading of the printing plate  18 . In particular, the stationary ironing roller system  46  applies a force that keeps the printing plate  18  in contact against the media support surface  22  of the external drum  20  as the external drum  20  is rotated and the printing plate  18  is applied. The stationary ironing roller system  46  comprises an ironing roller assembly  96 , including one or more rollers, and an actuating system  98  for extending or retracting the ironing roller assembly  96  toward or away from the media support surface  22  of the external drum  20 . The ironing roller assembly  96  is retracted away from the external drum  20  prior to the imaging of the printing plate  18 . The stationary ironing roller system  46  is mounted to a frame member (not shown) of the external drum platesetter  16  such that the stationary ironing roller system  46  is positioned above the media support surface  22  of the external drum  20 . 
     A trailing edge clamping mechanism  44  is provided to hold the trailing edge  42  (see, e.g., FIG. 1) of the printing plate  18  in place against the media support surface  22  of the external drum  20  during rotation of the external drum  20  and imaging of the printing plate  18 . Operationally, the trailing edge clamping mechanism  44  is held against the external drum  20  with enough force to resist the forces resulting from wrapping the normally flat printing plate  18  around the cylindrical external drum  20  and to counteract the centrifugal forces which act to lift the printing plate  18  and the trailing edge clamping mechanism  44  off the external drum  20  during rotation of the external drum  20 . The same functionality is also provided by the leading edge clamping mechanism  40 . 
     If a single length printing plate  18  is to be imaged by the external drum platesetter  16 , the trailing edge clamping mechanism  44  may be located at a fixed position on the external drum  20  corresponding to the location of the trailing edge  42  of the single length printing plate  18 . The external drum platesetter  16  of the present invention, however, is intended to be used to image printing plates  18  having different lengths. As such, the position of the trailing edge clamping mechanism  44  is configured to be movable around the outer diameter of the external drum  20  with respect to the leading edge clamping mechanism  40  to accommodate different plate lengths. 
     In the present invention, the trailing edge clamping mechanism  44  employs a vacuum to hold the trailing edge  42  of the printing plate  18  against the external drum  20 . Other configurations that utilize a magnetic force, an electromagnetic force, a mechanical force, etc., to hold the trailing edge  42  of the printing plate  18  against the external drum  20 , may also be used. 
     As illustrated very generally in FIG. 7, and in greater detail in FIGS. 12-14, the trailing edge clamping mechanism  44  includes a clamping bar  100  that extends past both ends of the external drum  20 . The clamping bar  100  includes a plurality of chambers  102  (FIG. 12) each formed by a continuous flexible peripheral gasket  104 . A single chamber formed by a single continuous peripheral gasket extending across the external drum may also be used. A vacuum source  106  is connected to the clamping bar  100  via a rotary union (not shown) and tubing  108 . A check valve  110  is provided to prevent a sudden loss of clamping force should the vacuum source  106  become disconnected or otherwise inoperative. This allows the external drum  20  to be safely brought to a stop automatically or by an operator before the vacuum clamping force is lost. The vacuum is distributed to each of the plurality of chambers  102  through one or more hollow sections  112  (FIG. 13A) of the clamping bar  100 . The hollow sections  112  extend along the length of the clamping bar  100  and into each of the chambers  102 , thereby acting as a manifold. A pressure relief valve  113  is located between the hollow sections  112 . Alternately, the pressure relief valve  113  may be disposed at any suitable location within the pneumatic circuit of the clamping bar  100 . In operation, as shown in FIG. 13B, the trailing edge  42  of the printing plate  18  extends beneath the clamping bar  100  and is forced against the media support surface  22  of the external drum  20  by the gasket  104  when a vacuum is supplied to the chamber(s)  102  by the vacuum source  106 . 
     As further illustrated in FIG. 12, the clamping bar  100  is supported at each end by a guided support post  114  that provides the clamping bar  100  with a radial degree of translational freedom with respect to the media support surface  22  of the external drum  20 . This allows the clamping bar  100  to be radially displaced away from or toward the media support surface  22  of the external drum  20  during the loading and unloading of the printing plate  18 . Each support post  114  is coupled to a disc  116  that is mounted coaxially to an end of the external drum  20  (one such disc  116  is shown in FIG.  7 ). Thus, one disc  116  is mounted coaxially to a first end of the external drum  20  and a second disc  116  is mounted coaxially to a second, opposing end of the external drum  20 . A biasing system  118  (e.g., a compression spring), biases each end of the clamping bar  100  radially away from the media support surface  22  of the external drum  20 , allowing the clamping bar  100  to be freely rotated over and around the external drum  20 . In another embodiment of the present invention, the clamping bar  100  may further be supported at a plurality of locations along the external drum  20  using retainers  120  and attached to the support posts  122  that slide within undercut slots  124  formed in the external drum  20  (FIG.  14 ). The slots  124  extend at least partially about the circumference of the external drum  20 . A biasing system  126  (e.g., a compression spring) is provided at each support post  122  to bias the clamping bar  100  radially away from the media support surface  22  of the external drum  20 . The small allowable motion of the clamping bar  100  combined with the plurality of support posts  114 ,  122 , prevent any part of the clamping bar  100  from deflecting into an area occupied by imaging optics, system electronics, media transport mechanisms, etc., even if the vacuum force is lost while the external drum  20  is rotating. 
     A counterweight  128  (FIGS. 7 and 12) may be attached to, or integrally formed with, each of the discs  116  to counterbalance the overall mass of the trailing edge clamping mechanism  44  with respect to the axis of the external drum  20 . This helps to properly balance the external drum  20  to prevent the introduction of vibration-induced artifacts into the images recorded on the printing plate  18 . 
     As illustrated in FIG. 15, a drive system  130 , including, for example, a motor driven friction wheel  131 , is selectively actuated into a drive position against at least one (typically both) of the discs  116  using a pneumatic actuator  132  or other actuating system (e.g., solenoid, etc.). The pneumatic actuator  132  causes a normal force between the drive system  130  and the disc  116 . The discs  116  and attached clamping bar  100  may be rotated to any position around the external drum  20  by a rotation of the drive system  130 . A spring  133  is provided to bias the drive system  130  away from the disc  116  to a non-drive position in the event of a failure of the pneumatic actuator  132 . 
     The trailing edge clamping mechanism  44  further includes an actuation system  134  for selectively forcing the clamping bar  100  against the external drum  20  and over the trailing edge  42  of the printing plate  18 . The actuation system  134  is used to counteract the forces of the biasing systems  118 ,  126 , to push the clamping bar  100  against the external drum  20  creating a seal between the gaskets  104 , the trailing edge  42  of the printing plate  18 , and the external drum  20  (see also FIG.  13 B). 
     As shown in FIG. 7, the actuation system  134  includes an actuator  136  (e.g., a pneumatic actuator, solenoid, etc.) and an extendible arm  138 . After the clamping bar  100  is forced against the trailing edge  42  of the printing plate  18  by the actuation system  134 , the air within the chambers  102  is evacuated by vacuum source  106  (see, e.g., FIG.  12 ), thereby holding the clamping bar  100  against the external drum  20  and the trailing edge  42  of the printing plate  18 . The actuation system  134  of the trailing edge clamping mechanism  44  is mounted to a frame member (not shown) of the external drum platesetter  16  such that the actuation system  134  is positioned above the media support surface  22  of the external drum  20 . 
     The operation and media input flow of the media handling system  70  of the external drum platesetter  16  is illustrated in FIGS. 16-21. It should be noted that although the following steps are described in a specific order, many of the steps (or sets thereof) may be performed in a different order (or omitted) without departing from the scope of the present invention. 
     In FIG. 16, a single printing plate  18  is positioned, or “staged,” on an input tray  140  above a pair of input nip roller assemblies  142 , wherein each input nip roller assembly  142  includes at least one resilient input nip roller  144 . The input tray  140  is positioned adjacent a front area of the external drum platesetter  16  such that the printing plate  18  is loaded on the input tray  140  from a front of the external drum platesetter  16 . At least a portion of the input tray  140  is positioned directly over the external drum  20 . In FIG. 7, for example, the input tray  140  is positioned directly over, and completely within a space defined by the diameter D of (see FIG.  7 ), the external drum  20 . By positioning the input tray  140  in this manner, the overall floor space (i.e., “footprint”) of the external drum platesetter  16  is reduced. 
     The leading edge  38  of the printing plate  18  is positioned by the input tray  140  to rest substantially between the input nip roller assemblies  142 . The input nip roller assemblies  142  are positioned above the external drum  20  and are oriented such that the common tangent of the rollers  144  is tangent to the media support surface  22  of the external drum  20 . The input tray  140  is oriented such that the loading path of the printing plate  18  supported thereon extends along a line that is tangent to the external drum  20  at the leading edge clamping mechanism  40 . 
     A curved input/output guide platen  150 , mounted to a frame member (not shown) of the external drum platesetter  16  may be provided to direct the leading edge  38  of the printing plate  18  toward the leading edge clamping mechanism  40  during the loading of the printing plate  18  onto the external drum  20 . In addition, the curved input/output guide platen  150  is configured to direct the printing plate  18  off of the external drum  20  toward a plate output area, such as an output tray  200 , after imaging is complete. Depending upon the specific arrangement of the various components of the external drum platesetter  16 , the input/output guide platen  150  may be fixed in position relative to the external drum  20 , or may be selectively displaced between a plate loading and unloading location by a drive system (not shown). Other guide means may be used in lieu of, or in conjunction with, the input/output guide platen  150 , to guide the printing plate  18  onto and off of the external drum  20  during the loading and unloading of the printing plate  18 . 
     Prior to loading the printing plate  18  on the external drum  20 , several steps are performed. First, the external drum  20  is rotated, if necessary, by the drive system  36 , until the leading edge clamping mechanism  40  is in position to receive the leading edge  38  of the printing plate  18 . The clamping portion  82  of the leading edge mounting mechanism  40  is held in an open position by the actuator  76 , thereby exposing the registration pins  78  (see, e.g., FIG.  8 ). The trailing edge clamping mechanism  44  is rotated by the drive system  130 , if necessary, to position the clamping bar  100  out of the way of the loading path of the printing plate  18 . The actuation system  134  for the trailing edge clamping mechanism  44 , and the ironing roller system  46 , are also retracted away from the media support surface  22  of the external drum  20 , if necessary, out of the way of the loading path of the printing plate  18 . 
     The input nip rollers  144  of one of the input nip roller assemblies  142  are mounted on a rotatable transport drive shaft  148 . A drive system (not shown) is provided to selectively rotate the transport drive shaft  148  and the input nip rollers  144  mounted thereon to propel the leading edge  38  of the printing plate  18  toward and into the leading edge clamping mechanism  40 , until the leading edge  38  of the printing plate  18  comes to rest against two of the registration pins  78  (shown in phantom in FIG.  16 ). The input nip roller assemblies  142  are configured to automatically deskew the printing plate  18 , if necessary, to ensure that two of the registration pins  78  are contacted by the leading edge  38  of the printing plate  18 . 
     As shown in FIG. 17, after the leading edge  38  of the printing plate  18  is properly positioned against two of the registration pins  78 , the leading edge clamping mechanism  40  is closed, thereby pinching the printing plate  18  against the external drum  20  while the leading edge  38  of the printing plate  18  remains in contact with the registration pins  78 . In particular, as previously described in detail in reference to FIG. 9, as the actuator  76  retracts the member  86  away from the clamping portion  82  of the leading edge mounting mechanism  40 , the biasing system  84  biases the clamping portion  82  closed against the external drum  20 . This traps the leading edge  38  of the printing plate  18  in a registered position between the clamping and mounting portions  80 ,  82 , of the leading edge clamping mechanism  40 . 
     As illustrated in FIG. 18, after the leading edge clamping operation, the external drum  20  is rotated a few degrees by the drive system  36 . Next, the ironing roller assembly  96  of the stationary ironing roller system  46  is extended and positioned against the printing plate  18  by the actuating system  98 . The force applied by the ironing roller assembly  96  keeps the printing plate  18  in contact against the media support surface  22  of the external drum  20 . The external drum  20  continues to rotate, while the printing plate  18  is forced against the external drum  20  by the ironing roller assembly  96 , thereby further drawing the printing plate  18  from the input tray  140  and wrapping the printing plate  18  around the media support surface  22  of the external drum  20 , until the trailing edge  42  of the printing plate  18  is positioned adjacent the actuation system  134  of the trailing edge clamping mechanism  44 . This configuration is shown in FIG.  19 . As the external drum  20  is rotated, the discs  116 , and the trailing edge clamping mechanism  44  and clamping bar  100  mounted thereon, remain stationary. 
     The transport drive shaft  148  is deactivated after the trailing edge  42  of the printing plate  18  passes completely through the input nip roller assemblies  142 . At this point in the media input/output flow, as shown in FIG. 19, an operator may place another printing plate  18 ′ in a staged position on the input tray  140  above the pair of resilient input nip roller assemblies  142 . In accordance with the present invention, the printing plate  18 ′ may be positioned on the input tray  140  even as the pre-imaging loading and clamping steps described below for the printing plate  18  are being performed. This increases the throughput of the external drum platesetter  16 , when compared to existing external drum imaging systems, since an operator no longer has to wait to load the next printing plate  18 ′ to be imaged until the previous printing plate  18  has been completely loaded, imaged and removed from a single, common input/output location. 
     When the trailing edge  42  of the printing plate  18  is positioned adjacent the actuation system  134  of the trailing edge clamping mechanism  44 , the discs  116  are rotated by the drive system  130  to position the clamping bar  100  over the trailing edge  42  of the printing plate  18 . This configuration is illustrated in FIG.  20 . The location of the trailing edge  42  of the printing plate  18  is determined based on the dimensions of the printing plate  18 , which are typically input into a control system of the external drum imagesetter  16  via an operator control terminal (OCT) (not shown), the workstation  12  (FIG.  1 ), or other input system. Alternately, or in addition, a plate edge sensing system (not shown) may be used to automatically detect the trailing edge  42  of the printing plate  18 . 
     The clamping bar  100 , which is normally biased away from the external drum  20 , is then forced against the external drum  20  by the actuation system  134 . A vacuum is subsequently introduced in each of the plurality of chambers  102  underneath the clamping bar  100  by the vacuum source  106  (FIG.  12 ), causing the clamping bar  100  to be firmly attached to the external drum  20  and over the trailing edge  42  of the printing plate  18 . Next, as illustrated in FIG. 21, the actuation system  134  of the trailing edge clamping mechanism  44  and the ironing roller assembly  96  of the stationary ironing roller system  46  are retracted away from the external drum  20 . The drive system  130  is also retracted away from the discs  116 . The printing plate  18  is now held in place for subsequent imaging by the leading and trailing edge clamping mechanisms  40 ,  44 . A vacuum may then be introduced between the printing plate  18  and the external drum  20  (e.g., in vacuum grooves  47  (FIG.  2 )) by vacuum source  45  (FIG. 1) to hold the body of the printing plate  18  firmly against the external drum  20 . Alternately, the vacuum may be applied during the loading of the printing plate  18  to reduce the time required to evacuate the air between the printing plate  18  and the external drum  20 . The printing plate  18  is now fully applied to the external drum  20  and ready for side edge registration and subsequent imaging by the scanning system  24 . 
     The external drum  20  is then rotated up to speed. The scanning system  24  is linearly advanced by the movable carriage  26  to record an image onto the printing plate  18 . After imaging, the scanning system  24  is “parked” in a home position (e.g., adjacent an end of the external drum  20 ), and the external drum  20  is braked to a stop. 
     The operation and media output flow of the media handling system  70  of the external drum platesetter  16  is illustrated in FIGS. 22-25. Again, it should be noted that although the following steps are described in a specific order, many of the steps (or sets thereof) may be performed in a different order (or omitted) without departing from the scope of the present invention. 
     As illustrated in FIG. 22, upon completion of imaging, the external drum  20  is brought to a stop with the clamping bar  100  positioned beneath the retracted actuation system  134 . The ironing roller assembly  96  is lowered onto the printing plate  18  to prevent the printing plate  18  from releasing or dropping off the external drum  20  upon the subsequent release of clamping bar  100 . Next, the vacuum applied to the clamping bar  100  and the external drum  20  is turned off. Residual vacuum is released from under the clamping bar  100  by an actuator (not shown) that selectively actuates the pressure relief valve  113  located between the hollow section  112  of the clamping bar  100 . The actuator of the pressure relief valve  113  may be disposed on the actuation system  134  of the trailing edge clamping mechanism  44 , or may be formed separately therefrom. 
     As shown in FIG. 23, when the biasing force applied by the biasing system  118  (FIG. 12) exceeds the vacuum force holding the clamping bar  100  against the external drum  20 , the clamping bar  100  is displaced radially away from the external drum  20 . At this point, the actuation system  134  of the trailing edge clamping mechanism  44  is retracted, and the clamping bar  100  is rotated away (shown in phantom in FIG. 23) from the output path of the printing plate  18  in response to a rotation of the discs  116  by the drive system  130  (FIG.  15 ). With the printing plate  18  still held against the external drum  20  by the ironing roller assembly  96 , and the leading edge  38  of the printing plate  18  still clamped within the leading edge clamping mechanism  40 , the external drum  20  is rotated in an opposite direction (the input/output guide platen  150  may also be displaced), to advance the trailing edge  42  of the printing plate  18  toward an output area such as output tray  200 . The trailing edge  42  of the printing plate  18  is guided by the input/output guide platen  150  between a pair of output nip roller assemblies  202  (FIG. 24) each including at least one resilient nip roller. The support shafts of the output nip roller assemblies  202  are mounted to a frame member (not shown) of the external drum platesetter  16 . At least one of the output nip roller assemblies  202  is mounted on a rotatable drive shaft  204 . The driven output nip rollers force the printing plate  18  between the output nip roller assemblies  202  toward the output tray  200 . The printing plate  18  is also forced toward the output tray  200  by the rotation of the external drum  20 . 
     The rotation of the external drum  20  and the output nip roller assemblies  202  is temporarily halted when the leading edge clamping mechanism  40  is located adjacent the ironing roller assembly  96 . The ironing roller assembly  96  is then retracted, and the external drum is further rotated, if necessary, until the leading edge clamping mechanism  40  is located at its home position (i.e., under the actuator  76 ). The clamping portion  82  of the leading edge clamping mechanism  40  is then opened by the actuator  76 , as shown in FIG. 25, thereby completely freeing the printing plate  18  from the external drum  20 . The output nip roller assemblies  202  are subsequently rotated until the printing plate  18  is expelled onto the output tray  200 , trailing edge  42  first (shown in phantom in FIG.  25 ). The actuator  76  of the leading edge clamping mechanism  40  is then retracted. As soon as the printing plate  18  is out of the loading path, the next printing plate  18 ′, previously staged on the input tray  140 , may be loaded onto the external drum  20  for imaging. 
     The output tray  200  is located at the rear of the external drum platesetter  16 . Thus, a printing plate  18  is loaded onto the input tray  140  at the front of the external drum platesetter  16 , while an imaged printing plate  18  is removed from the output tray  200  from a rear area (i.e., behind the input tray  140 ) of the external drum platesetter  16 . As shown in FIG. 25, the input tray  140  is formed separately from the output tray  200 . In particular, the input tray  140  is positioned in front of (looking from right to left in FIG. 25 (i.e., from the front to the rear of the external drum platesetter  16 )). In addition, at least a portion of the input tray  140  is located above the output tray  200 . Further, at least a portion of the input tray  140  may extend over the output tray  200  toward the rear of the external drum platesetter  16 . In FIG. 25, for example, the input tray  140  is positioned such that it is completely above the level of the output tray  200 . As shown in FIG. 6, however, a portion of the input tray  140  may extend to, or below, the level of the output tray  200 . By positioning the input tray  140  in this manner relative to the output tray  200  the overall footprint of the external drum platesetter  16  is reduced further. 
     The output tray  200  is illustrated in FIG. 25 as having a horizontal orientation (i.e., substantially parallel to a floor surface on which the external drum platesetter  16  is supported). The output tray  200 , however, may also be angled relative to the floor such that the printing plate  18  is directed upward along the output tray  200  during the unloading process. This configuration of such an output tray  200 ′ is shown in phantom in FIG.  25 . This further reduces the overall footprint of the external drum platesetter  16 . 
     The relative positions and/or orientations of the input tray  140  and output tray  200  also may apply to the input tray  440  and output tray  510  of the external drum platesetter  300  described infra with regard to FIGS. 27-44. 
     The imaged printing plate  18  may be manually removed by an operator from the output tray  200  from the sides or rear of the output tray  200 . Alternately, the output tray  200  may include a conveying apparatus to automatically unload the imaged printing plate  18  from the sides or rear of the output tray  200  into, for example, a plate storage area, or an online processor. As shown in FIG. 6, the imaged printing plate  18  may also be removed from the output tray  200  from the front of the external drum platesetter  16  (directional arrow  141 ) through an opening  143  formed in the input tray  140 . Removal of the printing plate  18  through the opening  143  may be more convenient, for example, if the external drum platesetter  16  is positioned in a corner of a room, or if the rear of the external drum platesetter  16  is positioned against a wall. 
     In the external drum platesetter  16  of the present invention, throughput is increased since a operator may load a printing plate  18 ′ on the input tray  140  while a previously loaded printing plate  18  is being loaded on the external drum  20  (see, e.g., FIG.  19 ), while the printing plate  18  is being imaged (see, FIG.  21 ), and/or while the imaged printing plate  18  is being unloaded onto the output tray  200  (see, e.g., FIG.  25 ). Indeed, the printing plate  18 ′ may be loaded on the input tray  140  as soon as, or anytime after, the trailing edge  42  of the printing plate  18  has passed through the input nip roller assemblies  142 . Further, the actuating systems  75 ,  46 , and  134 , of the leading edge clamping mechanism  40 , the ironing roller assembly  96 , and the trailing edge clamping mechanism  44 , respectively, are each located in the same position relative to the external drum  20  during the loading, imaging, and unloading of the printing plate  18 ; no additional displacements are required, thereby reducing the cost and complexity of the actuating systems. Also, the clamping bar  100  and the leading edge clamping mechanism  40  never leave the vicinity of the external drum  20 , and therefore cannot be lost. 
     A control system  800  for controlling the media input/output operation of the external drum platesetter  16  is illustrated in block form in FIG.  48 . Although not described, a similar control system is utilized to control the media input/output operation of the (VLF) external drum platesetter  300 . 
     As illustrated in FIG. 48, the control system  800  generally comprises a computer or workstation  802  used for system diagnostics, and a media motion control board  804 , comprising, for example, a stand-alone PC board or similar system, for controlling all media input/output tasks (media motion) except for the motion (i.e., rotation) of the external drum  20 . A separate drum motion control board  806 , again comprising, for example, a stand-alone PC board, is provided to control the motion of the external drum  20 . Alternately, a single PC board may be used to control all media input/output tasks including the motion of the external drum  20 . In the following description of the control system  800 , the term “servo” is defined as the combination of a drive motor, an encoder (e.g., position or velocity), and a feedback (e.g., position or velocity) control loop. 
     The media motion control board  804  sends and/or receives control information to/from the servos associated with the drive systems of the input nip roller assemblies  142 , the output nip roller assemblies  202 , and the discs  116  for selectively positioning the clamping bar  100  about the external drum  20 . The media motion control board  804  also sends and/or receives power, control, actuation, and/or other operational signals to/from various components of the external drum platesetter  16 , including, for example, the actuator  76  of the leading edge clamping mechanism  40 , the actuating system  98  of the ironing roller system  46 , the actuation system  134  of the trailing edge clamping mechanism  44 , the vacuum sources  45  and  106  (i.e., the drum and clamping bar vacuum sources), the drive system (if present) of the input/output guide platen  150 , the drive systems  130  of the discs  116 , etc. The media motion control board  804  also receives information from various sensors (described infra) that are used, for example, to determine the position of the printing plate  18  within, and relative to various components of, the external drum platesetter  16 . 
     The drum motion control board  806  sends and/or receives information (e.g., control, power, position, velocity, etc.) to/from the servo(s) associated with the drive system  36  of the external drum  20 . The drum motion control board  806  also receives information from various sensors (described infra) that is used, for example, to control the rotation of the external drum  20  during media input/output and imaging. 
     The present invention employs several sensors to track the location of the printing plate  18  within the input/output path of the external drum platesetter  16 . (Similar sensors may also be used where appropriate in external drum platesetter  300 .) For example, a pair of sensors  810 ,  812  (see FIG. 49) are suitably located adjacent the input nip roller assemblies  142  to detect the presence (or absence) of a printing plate  18  loaded in the input tray  140 . A sensor  814  (see FIG. 16) is also used to indicate when the clamping bar  100  of the trailing edge clamping mechanism  44  is positioned under the actuation system  134  of the trailing edge clamping mechanism  44 . There is also a sensor  816  to detect when the trailing edge  42  of the printing plate  18  has passed through the output nip roller assemblies  202  (see FIG. 24) during the unloading of the imaged printing plate  18 . Each of the sensors  810 ,  812 ,  814 , and  816 , may be an optical, electrical, or mechanical switch or other device capable of detecting the presence of the printing plate  18 . 
     The operation of the sensors  810 ,  812 , is illustrated in FIG.  49 . In particular, sensor  810  provides an indication that the left edge (LE) of the printing plate  18  is at least to the left of the sensor  810  to ensure that the printing plate  18  is located in a proper zone for imaging. Indicia, and/or a mechanical stop  818  (see FIG. 6) may be provided on the input tray  140  to assist the operator in loading the printing plate  18  on the input tray  140  such that the left edge LE will be located to the left of the sensor  810 . If the sensor  810  does not sense the printing plate  18 , while the sensor  812  located to the right of sensor  810  does, an error message will result and the input and subsequent imaging of the improperly positioned printing plate  18  will not occur. 
     The sensors  810 ,  812 , may also be used in cooperation with the servo (encoder) associated with the driven input nip roller assembly  142 , and/or the servo (encoder) associated with the drive system  36  of the external drum  20 , to measure the plate wrap dimension (i.e., the distance between the leading edge  38  and trailing edge  42 ) of the printing plate  18 . For example, the plate wrap dimension may be determined by measuring the rotational travel of either or both of the driven input nip roller assembly  142  and/or external drum  20  between the detection of the leading edge  38  of the printing plate  18  and the detection of the trailing edge  42  of the printing plate  18  by the sensors  810 ,  812 . The sensor based determination of the plate wrap dimension serves as a double-check of the plate wrap dimension input value provided by an operator of the external drum platesetter  16 . An error message will result, for example, if the plate wrap dimension value provided by the operator does not match the plate wrap dimension detected by the sensors. This may occur, for example, if the wrong size printing plate is inadvertently loaded into the input tray  140 . The value of the plate wrap dimension is used to advance the trailing edge  42  of the printing plate  18  adjacent the actuation system  134  of the trailing edge clamping mechanism  44 . 
     The sensors  810 ,  812 , may be used for other purposes. For example, as illustrated in FIG. 50, the sensors  810 ,  812 , may be used to determine the degree of skew ( )E) of the printing plate  18  as it passes through the input nip roller assemblies  142 . In particular, skew may be determined by detecting the time between the detection of different portions of the trailing edge  38  of the printing plate  18  by the sensors  810 ,  812 . Since the size of the printing plate  18  and the velocity of the printing plate  18  through the input nip roller assembly  142  are known, the skew can be easily calculated. Once determined, the skew can be compensated for by the scanning system  24  in a known manner. Alternately, if the skew exceeds a predetermined amount, an error message will be generated, and the loading process will be terminated. 
     Once the printing plate  18  is mounted on the external drum  20 , the scanning system  24  can be advanced across the printing plate  18  to detect the left edge and right edge of the printing plate  18 , thereby locating the printing plate  18  on the external drum  20  for subsequent imaging, and determining the longitudinal dimension of the printing plate  18  across the external drum  20 . The longitudinal dimension of the printing plate  18  serves as a double-check of the plate width input value provided by an operator of the external drum platesetter  16 . Also, given the position of the left edge of the printing plate  18 , image data may be recorded at a predetermined distance from the left edge of the printing plate  18  using the servo/encoder of the movable carriage  26 . The same is also true for placing the image data with respect to the leading edge  38  of the printing plate  18 . Specifically, the servo of the drive system  36  of the external drum  20  can be used to position the edge of the image data at a predetermined distance from the leading edge  38  (from the registration pins  78 ) of the printing plate  18 . 
     The general input/output flow of the external drum platesetter  16  is summarized in FIGS. 26A and 26B (refer to FIGS. 16-25 for the specific component reference numbers). In step  210 , an operator places a printing plate  18  onto the input tray  140 , with the leading edge  38  of the printing plate  18  positioned between the input nip roller assemblies  142 . The sensors  810 ,  812 , detect the presence of the printing plate  18  at the input nip roller assemblies  142 . In step  212 , the external drum  20  is rotated, if necessary, to position the leading edge clamping mechanism  40  for plate loading. The actuating system  75  opens the clamping portion  82  of the leading edge clamping mechanism  40 , exposing the registration pins  78 , and the external drum  20  is held stationary. In step  214 , the input nip roller assemblies  142  deskew and propel the printing plate  18  into the external drum imagesetter  16  until the leading edge  38  of the printing plate  18  contacts the registration pins  78 . In step  216 , the actuating system  75  closes the clamping portion  82  of the leading edge clamping mechanism  40  against the leading edge  38  of the printing plate  18 . In step  218 , the external drum  20  is rotated a few degrees by the drive system  36  and the actuating system  98  forces the ironing roller assembly  96  against the printing plate  18 . In step  220 , the external drum  20  is rotated, with the ironing roller assembly  96  forcing the printing plate  18  against the external drum  20 , until the trailing edge  42  of the printing plate  18  is positioned adjacent the actuation system  134  of the trailing edge clamping mechanism  44  (as determined by sensor  814 ). The input sensors  810 ,  812  measure the skew of the trailing edge  42 . During the rotation of the external drum  20 , the plate wrap dimension is determined. In step  222 , which may be performed as soon as the trailing edge  42  of the printing plate  18  passes completely through the input nip roller assemblies  142 , the rotation of the input nip roller assemblies  142  is stopped. At this point, or during or after any of the following steps  224 - 244 , the operator may place another printing plate  18 ′ onto the input tray  140 . In step  224 , the discs  116  are rotated to position the clamping bar  100  over the trailing edge  42  of the printing plate  18  and under the actuation system  134 . In step  226 , the clamping bar  100  is forced against and over the trailing edge  42  of the printing plate  18  by the actuation system  134 , and a vacuum is introduced in each of the chambers formed underneath the clamping bar  100 . This causes the clamping bar  100  to be firmly attached against the external drum  20 , thereby pinching the trailing edge  42  of the printing plate  18  against the external drum  20 . In step  228 , the actuation system  134  and the ironing roller assembly  96  are moved away from the external drum  20 , and a vacuum is introduced between the printing plate  18  and the external drum  20 . The printing plate is now fully applied to the external drum  20 . In step  230 , after side edge registration of the printing plate  18 , the external drum  20  is rotated and the printing plate  18  is imaged. 
     After imaging is complete, the external drum  20  is brought to a stop in step  232 , with the clamping bar  100  positioned under the actuating system  134 . In step  234 , the ironing roller assembly  96  is extended against the printing plate  18 , and the vacuum to the clamping bar  100  and the external drum  20  is turned off. Residual vacuum beneath the clamping bar  100  is released by opening the pressure relief valve  113  (e.g., using the actuating system  134 ). In the absence of the vacuum, the clamping bar  100  is automatically biased away from the trailing edge  42  of the printing plate  18  by the biasing system  118  (see, e.g., FIG.  12 ). In step  236 , the clamping bar  100  is rotated out of the output path of the printing plate  18 . In step  238 , the external drum  20  is rotated to position and direct the trailing edge  42  of the printing plate  18  between and through the rotating output nip roller assemblies  202  toward the output tray  200 . The trailing edge  42  of the printing plate  18  is guided into the output nip roller assemblies  202  by the input/output platen  150 . The trailing edge  42  of the printing plate  18  is detected exiting the output nip roller assemblies  202  by the sensor  816 . In step  240 , rotation of the external drum  20  and the output nip roller assemblies  202  is temporarily halted when the leading edge clamping mechanism  40  is located beneath the actuating system  75 . The ironing roller assembly  96  is then retracted. In step  242 , the clamping portion  82  of the leading edge clamping mechanism  40  is opened by the actuator  76 , completely freeing the printing plate  18  from against the external drum  20 . Finally, in step  244 , the output nip roller assemblies  202  are again rotated until the printing plate  18  is expelled onto the output tray  200 . The sensor  816  provides a signal indicating that the printing plate  18  has passed completely through the output nip roller assemblies  202 . The printing plate  18 ′ previously loaded on the input tray  140 , may now be mounted on the external drum  20  starting at step  212 . 
     A very large format (VLF) external drum platesetter  300  in accordance with an alternate embodiment of the present invention is illustrated in FIG.  27 . Although this embodiment of the present invention provides a very large format (VLF) external drum platesetter  300  capable of handing and imaging printing plates having dimensions of up to 58″×80″, or greater, the platesetter  300  may be used to record image data onto almost any size printing plate. Examples of plate sizes that may be imaged on the external drum platesetter  300  include, inter alia, 45″×50″, 45″×57″, 50″×60″, 55″×70″, and 58″×80″. A range of possible plate sizes is from 20″×28″ to 58″×80″. 
     The external drum platesetter  300  is similar to the external drum platesetter  16  described above with reference to FIG.  7 . In particular, the external drum platesetter  300  includes an external drum  320  having a cylindrical media support surface  322  for supporting a printing plate  318  during imaging. The external drum  320  is supported by a frame  372 . A drive system  336  rotates the external drum  320  during imaging. A scanning system  324 , carried by a movable carriage  326 , travels axially along the rotating external drum  320  to record digital data onto the imaging surface of the printing plate  318 . The external drum  320  and scanning system  324  are positioned on a heavy, stable base  374 . 
     A leading edge clamping mechanism  340  is provided to hold and register a leading edge  338  of the printing plate  318  in position against the media support surface  322 . The leading edge clamping mechanism  340  has a configuration similar to that of the leading edge clamping mechanism  40  employed in the external drum platesetter  16 , and operates in a similar manner. In particular, the leading edge clamping mechanism  340  is selectively opened and closed by an actuating system  375 , including an actuator  376  (e.g., a pneumatic actuator, solenoid, etc.) and extendible member  386 . The leading edge clamping mechanism  340  is provided to selectively receive, capture, and release the leading edge  338  of the printing plate  318 . The leading edge clamping mechanism  340  is fixed in position on the external drum  320 , within a groove (see, e.g., FIGS. 3-5) formed in the external drum  320 . A set of selectively positionable registration pins  378  are incorporated into the leading edge clamping mechanism  340  to register the leading edge  338  of the printing plate  318 . 
     A stationary ironing roller system  346  is used to selectively force the printing plate  318  against the media support surface  322  of the external drum  320  as the external drum  320  rotates past the ironing roller system  346  during the loading of the printing plate  318 . The stationary ironing roller system  346  includes an ironing roller assembly  396 , including one or more rollers, and an actuating system  398  for selectively extending or retracting the ironing roller assembly  396  toward or away from the external drum  320 . The ironing roller assembly  396  is retracted away from the external drum  320  prior to the imaging of the printing plate  318 . 
     There are several differences between the external drum platesetter  16  and the external drum platesetter  300 . These differences are primarily due to the fact that the printing plates  318  imaged by the external drum platesetter  300  are generally much larger, heavier, and more unwieldy, than the printing plates  18  imaged by the external drum platesetter  16 . For example, the external drum platesetter  300  includes a landing zone  400  which provides a broad surface for an operator to initially rest a printing plate  318  as it is brought to the platesetter  300 . Once the plate is “landed” on the landing zone  400 , the operator can easily slide the printing plate  318  up onto an input tray  440  to a staged position. 
     The input tray  440  is pivotable about a pivot point P between a landing position (shown in solid lines), where the input tray  440  is aligned with the landing zone  400  (e.g., coplanar with, or parallel to, the landing zone  400 ), and a loading position (shown in phantom), where the input tray  440  and the printing plate  318  are angled more steeply down toward the external drum  320 . The input tray  440  may be manually or automatically pivoted between the landing and loading positions. A guard  442  prevents the printing plate  318  from sliding off the input tray  440  as the input tray  440  is pivoted between the landing and loading positions. 
     When the input tray  440  is in the loading position, the weight of the printing plate  318  may cause the printing plate  318  to slide downward toward the external drum  320  (i.e., the printing plate  318  is fed by gravity toward the external drum  320 ). A door  414 , or similar escapement mechanism, which is selectively activated (e.g., extended or retracted) by an actuator  416  (e.g., a pneumatic actuator, solenoid, etc.), may be provided to regulate the displacement of the printing plate  318 . Alternately, the printing plate  318  may be allowed to slide toward the external drum  320  as soon as the leading edge  338  of the printing plate  318  clears the guard  442 . A roller system, such as the input nip roller assembly  144  (FIG.  16 ), or other suitable system, may also be used to controllably direct the printing plate  318  toward and onto the external drum  320 . 
     As soon as the trailing edge  342  of the printing plate  318  is pulled off the input tray  440  as the printing plate  318  is loaded onto the external drum  320 , the input tray  440  may be manually or automatically pivoted back to its landing position. Once the loading tray  440  is returned to the landing position, an operator may place the next printing plate  318 ′ (FIG. 39) to be imaged on the landing zone  400 , and then slide the printing plate  318 ′ up onto the input tray  440  to a staged position. The next printing plate  318 ′ may be positioned on the input tray  440  even as the pre-imaging loading and clamping steps describe below for the printing plate  318  are being performed, thereby increasing the throughput of the external drum platesetter  300 . 
     In the external drum platesetter  16  (see, e.g., FIG.  21 ), enough vacuum force is generated between the clamping bar  100  and the external drum  20  to securely clamp the trailing edge  42  of the printing plate  18  against the external drum  20  during imaging, even when the external drum  20  is rotating at a high rate of speed (e.g., 100-1000 rpm). In the VLF external drum platesetter  300 , however, it is much more difficult (though not impossible) to produce a vacuum force sufficient to counteract the forces which act to lift the printing plate  318  off of the external drum  320  during imaging. These forces are due, for example, to the increased weight, greater thickness, and higher resultant resilience of the printing plates  318 . To overcome these forces, the external drum platesetter  300  includes a trailing edge clamping mechanism  344  that employs a set of magnetic clamps  350  to securely clamp the trailing edge  342  of the printing plate  318  against the external drum  320 . 
     The set of magnetic clamps  350  employed by the trailing edge clamping mechanism  344  may include a single, elongated magnetic clamp that extends across the length of the external drum  320 , or may comprise a plurality of discrete magnetic clamps which are distributed across the external drum  320 . The set of magnetic clamps  350  may be collectively or individually positioned against, and removed from, the external drum  320  by an actuating system  352  comprising at least one actuator. An underside of each of the magnetic clamps  350  is covered with a deformable layer of rubber  351  or other suitable nonabrasive material to prevent damage to the printing plate  318 . 
     As illustrated in FIGS. 27-28, a conveying system  500  may be provided to help propel an imaged printing plate  318  toward and onto an output tray  510  of the external drum platesetter  300 . In addition, or alternately, a roller system  501  (FIG.  27 ), such as the output nip roller assemblies  202  (FIG.  24 ), or other suitable system, may be used to controllably direct the printing plate  318  toward and onto the output tray  510 . The conveying system  500  includes a plurality of belts  502  that extend around sets of pulleys  504  mounted on shafts  506 . At least one of the shafts  506  is rotated by a drive system (not shown), which causes a rotation of the pulleys  504  and the belts  502  mounted thereon. The belts  502  comprise rubber or other suitable material. Other conveying systems may also be used. In FIGS. 27-28, the belts  502  are rotated in a counterclockwise direction to position an imaged printing plate  318  onto the output tray  510  (directional arrow  512 ). Once fully unloaded into the output tray  510 , the rotation of the belts  502  is terminated, and the printing plate  318  may be manually or automatically removed from the rear (directional arrow  514 ) or from either side (directional arrows  516 ,  518 ) of the output tray  510   
     A roller arrangement  520 , shown in FIG. 28, may be provided to displace the printing plate  318  off of the output tray  510 . As illustrated, the roller arrangement  520  may comprise a plurality of rollers  522 , some or all of which are rotated by suitable drive systems (not shown) to displace the printing plate  318  toward either side (directional arrows  516 ,  518 ) of the output tray  510 . As illustrated in FIG. 29, the printing plate (not shown) is selectively directed through a slot  530 , or other suitable opening, formed in either or both sides  532 ,  534  (FIG.  29 ), of the housing  536  of the external drum platesetter  300 . Alternately, a slot or opening (not shown) may be formed in the rear of the housing  536 . In this case, the printing plate  318  is directed through the rear slot by the conveying system  501  (FIG.  27 ). In FIG. 29, for example, a printing plate (not shown) may be directed through the slot  530  into an on-line processor  540  which chemically develops, fixes, and washes the printing plate  318 . Also shown in FIG. 29 is a cover  542  for covering the output tray  510 , and an output tray  544  for the online processor  540 . The cover  542  may be a light-tight cover for preventing exposure of the printing plate  318  prior to developing the image recorded thereon. 
     The general input/output flow of the external drum platesetter  300  is summarized in FIGS. 30A and 30B (refer to FIGS. 27-29 and  31 - 44  for the specific component reference numbers). It should be noted that although the following steps are described in a specific order, many of the steps (or sets thereof) may be performed in a different order (or omitted) without departing from the scope of the present invention. In step  610 , an operator positions and rests a printing plate  318  on the landing zone  400 . The landing zone  400  is provided to initially support the printing plate  318  as it is brought to the external drum platesetter  300 . By providing the landing zone  400 , the operator is not required to immediately maneuver the often large, heavy, and bulky printing plate  318  into position on the input tray  440 . Instead, the operator may initially rest the printing plate  318  on the landing zone  400 , and then easily and accurately slide the printing plate  318  up onto the input tray  440  to a staged position. 
     In step  612 , the printing plate  318  is slid onto the input tray  440  and positioned in a staged position (see also FIG.  31 ). During step  612 , the printing plate  318  may also be center justified, left justified, right justified, or otherwise suitably oriented in a loading position within the input tray  440 . The loading position of the printing plate  318  may be indicated by indicia or mechanical stops formed on or adjacent the input tray  440 . 
     In step  614 , the input tray  440  is pivoted from the landing to the loading position (see also FIG.  32 ). In step  616 , the external drum  320  is rotated by the drive system  336 , if necessary, to position the leading edge clamping mechanism  340  for plate loading. The actuating system  375  then opens the leading edge clamping mechanism  340 , exposing the registration pins  378 . This step may be performed before, during, or after step  614 . The door  414 , if present, is then opened by the actuator  416 , thereby allowing the printing plate  318  to slide down the input tray  440  until the leading edge  338  of the printing plate  318  contacts two of the registration pins  378  (see also FIG. 33 (pins not visible)). In step  618 , the actuating system  375  closes the leading edge clamping mechanism  340  against the leading edge  338  of the printing plate  318  (see also FIG.  34 ). In step  620 , the actuating system  398  forces the ironing roller assembly  396  against the printing plate  318  (see also FIG.  35 ). In step  622 , the external drum  320  is rotated by the drive system  336 , with the ironing roller assembly  396  forcing the printing plate  318  against the external drum  320  (see also FIG.  36 ), until the trailing edge  342  of the printing plate  318  is positioned below the set of magnetic clamps  350  of the trailing edge clamping mechanism  344  (see also FIG.  37 ). 
     In step  624 , which may be performed as soon as the trailing edge  342  of the printing plate  318  is pulled completely off of the input tray  440 , or during or after any of the following steps  626 - 642 , the input tray  440  may be manually or automatically pivoted back to the landing position (see also FIG.  38 ). The operator may now slide another printing plate  318 ′ onto the input tray  440  from a landed position on the landing zone  400  to a staged position on the input tray  440  (see also FIG.  39 ). 
     In step  626 , the set of magnetic clamps  350  of the trailing edge clamping mechanism  344  are forced against and over the trailing edge  342  of the printing plate  318  by the actuating system  352  (see also FIG.  40 ). In step  628 , the actuation system  352  and the ironing roller assembly  396  are moved away from the external drum  320  (see also FIG.  41 ), and a vacuum is introduced between the printing plate  318  and the external drum  320 . The printing plate  318  is now fully applied to the external drum  320 . In step  630 , after side edge registration of the printing plate  318 , the external drum  320  is rotated and the printing plate  318  is imaged (see also FIG.  42 ). 
     After imaging is complete, the external drum  320  is brought to a stop in step  632 , with the set of magnetic clamps  350  of the trailing edge clamping mechanism  344  positioned under the actuating system  352 . In step  634 , the ironing roller assembly  396  is extended against the printing plate  318 , the set of magnetic clamps  350  are pulled off the external drum  320  by the actuating system  352 , and the vacuum to the external drum  320  is turned off. In step  636 , the external drum  320  is rotated to position and direct the trailing edge  342  of the printing plate  318  onto the conveying system  500  and/or into and through the roller system  501  (see also FIG.  43 ). In step  638 , rotation of the external drum  320  is temporarily halted when leading edge  338  of the printing plate  318 , which is fixed in position by the leading edge clamping mechanism  340 , is located beneath the actuating system  375 . The ironing roller assembly  396  is then retracted. In step  640 , the leading edge clamping mechanism  340  is opened by the actuating system  375 , completely freeing the printing plate  318  from against the external drum  320 . Finally, in step  642 , the conveying system  500  and/or roller system  501  are operated to fully eject the printing plate  318  onto the output tray  510  (see also FIG.  44 ). The imaged printing plate  318  may be manually or automatically unloaded from the output tray  510 , or may be directed into the on-line processor  540 . The printing plate  318 ′ previously loaded on the input tray  440 , may now be mounted on the external drum  320  starting at step  614 . 
     The media input/output scheme of the external drum platesetter  300  of the present invention provides, inter alia, the following: 
     1. The floor space (i.e., footprint) required by the external drum platesetter  300  is conserved by forming the input tray  440  separately from, and positioning the input tray  440  over, the output tray  510 . Other related art input/output schemes often require at least twice the floor space as the present invention. 
     2. The separate input and output trays  440 ,  510 , allow prestaging of a second printing plate  318 ′ while a first printing plate  318  is being imaged. 
     3. The landing zone  400  is optimally located; an operator is not required to blindly locate an input slot or registration point as is often required by the related art. After the printing plate  318  is “landed,” the plate is easily slid to a staged position on the input tray  440 . 
     4. After a printing plate  318  has been staged on the input tray  440 , the leading edge  338  of the printing plate  318  is relatively close to the registration pins  378  on the external drum  320 . Since the printing plate  318  is not required to travel a great distance to reach the external drum  320 , transport errors are prevented, and transport time is reduced. 
     5. The loading of a printing plate  318  is assisted by gravity, due to the angled orientation of the input tray  440 . 
     6. The external drum  320  and clamping mechanisms  340 ,  344 , are easily accessed for service, since the input and output trays  440 ,  510 , are positioned to the rear of the external drum platesetter  300 . This configuration also provides space at the front of the external drum platesetter  300  for an optional automated multiple plate loader. 
     Either or both of the external drum platesetters  16 ,  300  may include an indicator light arrangement for providing visual signals to an operator regarding the operational and error status of the platesetter. An example of an indicator light arrangement  550  for the external drum platesetter  300  is illustrated in FIGS. 28 and 29. 
     The indicator light arrangement  550  comprises a pair of light columns  552  positioned on opposite sides of the input tray  440 . Each light column  552  includes a plurality of individual light segments  554 , each outputting one or more predetermined colors of light, and/or type of light (e.g., steady illumination, strobe, etc.). 
     The individual light segments  554  of the indicator light arrangement  550  of the present invention may be relatively large, thereby providing a visual indication of the operating status of the platesetter  16 ,  300 , that is visible from a great distance. This may be useful, for example, in large printing shops with high noise levels, since the indicator light arrangement  550  allows an operator to view the status of the platesetter  16 ,  300 , from as far away as 100 feet or more. 
     The indicator light arrangement  550  may be used to provide an operator with a wide variety of visual information, including, for example: 
     1. Platesetter on-line (ready); 
     2. Platesetter off-line (paused); 
     3. Error requiring operator intervention (e.g., clear jam, reboot, etc.); 
     4. Plate input allowed (i.e., a printing plate  318  may be loaded onto the input tray  440 ). This may occur, for example, if a job has not yet arrived from the RIP  14  (FIG.  1 ); 
     5. Plate input requested (e.g., a job has arrived from the RIP  14 , but a previously received printing job has not finished imaging); 
     6. Plate input required (e.g., the platesetter is stalled until an operator loads a printing plate  318 ); and 
     7. Job progress percentage. 
     The job progress percentage may be provided, for example, by sequentially actuating individual light segments  554 , from the bottom toward the top of each light column  552  (or vice versa), as a printing plate  318  is being imaged. The color sequence of the light segments  554  along each light column  552  may change from red to yellow to green as the printing plate  318  is being imaged, with green indicating that imaging is complete. Many other illumination schemes are also possible. 
     Many imaging systems, including internal and external drum imagesetters and platesetters, utilize a registration system comprising a pair of registration pins to accurately and repeatably position and locate several different sizes of printing plates or other recording media on a drum surface. To ensure that the registration pins properly align the leading edge of the printing plate to the longitudinal axis of the drum of the imaging system, the contact points of registration pins should be positioned as far apart as possible along the leading edge of the printing plate. To accommodate a wide variety of plate sizes, and provide the necessary separation (contact distance) of the registration pins, the registration pins of related art registration systems are often manually repositioned for each different plate size, or are automatically repositioned using a complex and expensive, active-powered adjustment system. 
     As illustrated in FIGS. 45-47, the present invention provides a registration system  700  that obviates the need for the manual or automatic relocation of the registration pins. In particular, the registration system  700  of the present invention provides a unique set of registration pins that is capable of supporting a plurality of different plate sizes and combinations of registration pin contact distances. 
     The registration system  700  includes a plurality of flanged registration pins  702  that are located within the groove  56  formed in the external drum  20  (or  320 ). The registration pins  702  may be located along the groove  56  at any axial position. A scale or fixture (not shown) may be used to accurately position the registration pins  702  along the groove  56 . As described supra, the registration pins  702  are incorporated into the leading edge clamping mechanism  40 . It should be noted, however, that the registration system  700  may also be used independently of the leading edge clamping mechanism  40 , and in other imaging systems, to accurately and repeatably position and locate a printing plate or other recording media on a drum surface. 
     As shown most clearly in FIG. 45, each registration pin  702  is formed in an “L” shape that is configured to fit within and against a corner  704  of the groove  56 . The leg  706  of the registration pin  702  includes an angled face  708 . A bar  710  is provided to clamp the registration pins  702  against the corner  704  of the groove  56 . The bar  710  includes a complementary angled face  712  that mates with, and wedges against, the angled face  708  formed on the leg  706  of each registration pin  702 . A single bar  710  is used to collectively clamp the plurality of registration pins  702  within the groove  56 . Alternately, several individual bar sections may be employed to clamp one or more of the registration pins  702  within the groove  56 . The registration pins  702  are locked into position along the groove  56  by securing one or more fasteners  714  (e.g., bolts, screws, etc.) into the external drum  20  through the bar  710 . As the bar  710  is tightened against the bottom of the groove  56 , the angled face  712  of the bar  710  is wedged against the angled face  708  formed on the leg  706  of each registration pin  702 , thereby securely clamping the registration pins  702  against the corner  704  of the groove  56 . If axial adjustment of the registration pins  702  is required, the fasteners  714  may be loosened without removing the bar  710  from the groove  56 , thereby allowing the registration pins  702  to be slid to the desired location within the groove  56 . 
     The head  716  of each registration pin  702  extends radially beyond the media support surface  22  (or  322 ) of the external drum  20  (or  320 ). During the loading of the printing plate  18  onto the external drum  20 , the leading edge  38  of the printing plate  18  contacts, and is positioned against, the heads  716  of a plurality of the registration pins  702 . Only two of the registration pins  702  are contacted by each different size printing plate  18 . 
     To ensure that only two of the registration pins  702  are contacted by any size (i.e., width, a.k.a., longitudinal dimension) of printing plate  18 , the heads  716  of the registration pins  702  are provided with variable thicknesses (e.g., in about 0.005″ increments) along a direction D that is parallel to the direction of rotation of the external drum  20 . For example, a plan view of an arrangement  718  of registration pins  702  capable of supporting four different plate sizes, with each plate size contacting only two of the registration pins  702 , is illustrated in FIG.  46 . Of the five registration pins shown in FIG. 46, registration pin  702   1  has the smallest head thickness, registration pins  702   2A  and  702   2B  have the same head thickness that is Δt larger than registration pin  702   1 , registration pin  702   3  has a head thickness that is 2Δt larger than registration pin  702   1  and Δt larger than registration pins  702   2A  and  702   2B , and registration pin  702   4  has a head thickness that is 3Δt larger than registration pin  702   1 , 2Δt larger than registration pins  702   2A  and  702   2B , and Δt larger than registration pin  702   3 . Thus, the head thicknesses of the registration pins  702   1 ,  702   2A  and  702   2B ,  702   3 , and  702   4  increase gradually in increments of Δt. An analogous arrangement of registration pins  702  may be provided for the case in which the different size printing plates  18  are right justified, rather than left justified. 
     In an alternate embodiment, the heads  716  of the registration pins  702   1 ,  702   2A  and  702   2B ,  702   3 , and  702   4  may have a cylindrical configuration (shown in phantom in FIG. 46) with radii that vary in increments of Δt. Other configurations capable of providing the necessary variable thicknesses are also possible. 
     The use of the registration pin arrangement  718  is illustrated in FIG.  47 . In FIG. 47, it should be noted that the relative sizes of the printing plates  18  and the registration pins  702 , and the degree of tilt, if any, of the printing plates  18 , have been exaggerated in order to more clearly depict the use of the registration pin arrangement  718  of the present invention. In this example, it is assumed that each different size (i.e., width) printing plate  18 A,  18 B, . . . ,  18 E, is left justified at location LJ (i.e., the bottom left edge of each printing plate  18 A,  18 B, . . . ,  18 E is positioned at location LJ. Printing plate  18 A, the smallest printing plate, contacts registration pins  702   2A  and  702   1 . The leading edge  38 A of printing plate  18 A angles downward toward registration pin  702   1  due to the difference in thickness Δt between registration pins  702   2A  and  702   1 . Printing plate  18 B, the next larger printing plate, contacts the registration pins  702   2A  and  702   2B , which have the same thickness, and the leading edge  38 B of printing plate  18 B is maintained in a level orientation. Printing plate  18 C, the next larger printing plate, contacts registration pins  702   2A  and  702   3 . The leading edge  38 C of printing plate  18 C angles upward toward registration pin  702   3  due to the difference in thickness Δt between registration pins  702   2A  and  702   3 . Finally, printing plate  18 D, the largest printing plate, contacts registration pins  702   2A  and  702   4 . The leading edge  38 D of printing plate  18 D angles upward toward registration pin  702   4  due to the difference in thickness 2Δt between registration pins  702   2A  and  702   4 . The small amount of tilt, having a maximum of 2Δt in the example illustrated in FIG. 47, is typically within system tolerances, and may be easily compensated for, if necessary, by other components of the imaging system. 
     It should be noted that the increments in thickness between adjacent pairs of the registration pins  702  do not have to be uniform (i.e., equal to ±Δt). Rather, and more generally, the thicknesses of the registration pins  702  may have any suitable value as long as the lowest point of a line connecting the plate contact points of the registration pins  702  is located at the second registration pin (e.g.,  702   1  in FIG. 47) from the edge used for justification (e.g., LJ in FIG.  47 ). It should also be noted that each different size printing plate  702  contacts a common registration pin (e.g.,  702   2A  in FIG.  47 ), namely the registration pin closest to the edge used for justification. 
     In FIGS. 46 and 47, the head  716  of each registration pin  702   1 ,  702   2A ,  702   2B ,  702   3 , and  702   4 , is provided with a large radius cylindrical registration surface  720  (e.g., a radius of about 1″). The registration surfaces  720  minimize contact stress with the leading edge  38  of the printing plate  18 , thereby preventing plate edge damage. 
     The foregoing description of the present invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and many modifications and variations are possible in light of the above teaching. Such modifications and variations that may be apparent to a person skilled in the art are intended to be included within the scope of this invention.