Abstract:
An imaging system, which may be applied both to black and white and color imaging systems and comprising a platen member which defines an imaging portion and a calibration portion is disclosed. The imaging portion and the calibration portion comprise separate portions of the platen member. The platen member is supported by a movably mounted support member, and moves with the support member between a platen member calibrating position and a platen member imaging position. The platen member is dimensioned to support a sheet to be imaged. An imaging module with an imaging zone is supported in a position where the imaging zone is positioned to coincide with a portion of a sheet supported on the imaging portion of the platen member, when the platen member is in the platen member imaging position. A calibration member having a reference reflectivity is adhered to the calibration portion of the platen member. The imaging zone coincides with the calibration member, when the platen member is in the calibrating position. The calibration member lies substantially outside the imaging zone of the imaging module, when the platen member is in the platen member imaging position. A motor moves the support member between the calibrating position and the imaging position to allow performance of a calibration procedure on the imaging module.

Description:
BACKGROUND 
     1. Field of the Invention 
     This invention relates to intensity calibration targets and methods employed in imaging systems, which targets and methods are particularly useful in systems having a plurality, such as a pair of imaging arrays for scanning both sides of a document. 
     2. References 
     Acquisition of color, black and white, and shades of gray image information, adaptable for generating copies or electronic processing directly from printed original documents, has become greatly desirable as an adjunct to electronic document generation. An important reason for this desirability is the subsequent capability of manipulation of the electronically stored information for editing, compiling and using the information in forms other than those in which it was originally available. While such manipulation is, available for image information produced originally or otherwise available in an electronic format, it is desirable to have a similar capability for image information not so available. Accordingly, it is desirable to have automated image information input capability, which, coupled with available output devices, renders possible functions such as simplex and duplex copying, image rotation, cropping, editing, and the like, without the requirement of troublesome mechanical manipulation of originals and copies. 
     A number of document handling systems utilize image input devices for such purposes as archiving, printing and transmitting images. Typical applications include facsimile transmission, document reproduction, digital copying, inputting images into a database, and optical character recognition. Specifically a number of known systems utilize a linear array of solid-state light detecting devices to generate an image, linear area by linear area, until the entire image of, for example, a side of a sheet of paper is acquired. 
     In such devices, constant velocity relative movement, between the linear array and the document to be imaged, allows the linear array to scan the face of the document. This is usually achieved in one of two ways. Either the linear detector array may be held stationary and the document moved at a constant velocity past the linear array, or, alternatively, the document may be held stationary and the linear array scanned across the face of the document, carried by a constant velocity transport. 
     Each of the elements in the linear array receives one pixel of intensity information from the corresponding part of the image in the form of light from an illumination lamp reflected by that part of the image, and focused by an appropriate lens structure. In response, each of the elements outputs an electrical signal whose magnitude is indicative of the intensity of light falling on the element. 
     A primary problem associated with electronic input scanners is a periodic requirement for calibration of the sensor arrangement and its electronics and related optical imaging components. Because a large number of photosensitive elements comprise the scanning array, uniformity of response is of value for acceptable imaging quality. 
     The problem thus arises to determine what level of electrical output signal corresponds to white and black image portions. Black signal levels may be measured by simply turning the illumination lamp off. If the level of electrical output signal corresponding to a white image portion was always the same, the problem of determining what level of electrical output signal corresponds to a white image portion would be relatively simple to solve, by simply hard wiring a circuit which provides the desired response. However, a white image portion may cause a photodetector to emit a wide range of output voltages, depending upon numerous factors, such as ambient light, line voltage variations, pixel to pixel variations in the detector array, lamp age and manufacturing variations, optics, dirt, and other factors. Frequent calibration is required against a target having a known reflectance value. 
     In U.S. Pat. No. 5,280,368 to Fullerton, there is disclosed a dual purpose calibration/baffle member, which in a first position serves as a paper baffle along the sheet feeding path to the scanning station to support sheets at a first scanning element, and in a second position, supports a calibration target at a position for detection by the first scanning element. The calibration/baffle member is mounted on the input scanner frame for movement between the first and second positions. The baffle member is guided with pivoting movement for the purpose of calibration to a position where a calibration target is within the field of view of the fixed scanning element. 
     In U.S. Pat. No. 4,429,333 to Davis, a calibration strip cut from a sheet of ethylene propylene or other uniformly white material is disclosed. The calibration strip is pressed against the platen glass on the same side as the document. Prior to commencement of scanning, a dark calibration reference is established by scanning with the illumination off. Next, the illumination is turned on, and the scanner carriage is driven to a calibration position where the scanner views the calibration strip. At that time, the scanner is calibrated to produce output signals based on the known reflectivity of the calibration strip. 
     In Buchar, U.S. Pat. No. 4,967,233, it is indicated that the scanning element is rotated out of the scanning position to view a calibration target. More particularly, the scanning element is rotated about an axis transverse to the direction of paper travel through the scanning station, and parallel to the paper path, with the axis through the scanning element. 
     U.S. Pat. No. 4,574,316 to Wilman et al. discloses a document scanner unit which rotates into at least one other scanning position to receive light reflected from a remote source. 
     U.S. Pat. No. 4,464,681 to Jacobs et al. discloses an optical scanning system comprising a linear photodiode array which can be adjusted in position to view an optical test pattern. U.S. Pat. No. 4,605,970 to Hawkins discloses a calibration arrangement which moves an optical scanning head assembly from a reference location into a testing position to view an optical test pattern. U.S. Pat. No. 4,706,125 to Takagi discloses an image reading device comprising an integrated image reading unit and an optical sensitivity checking member which concurrently translate in unison from an inoperative position into an operative position during the scanning of an original. 
     U.S. Pat. No. 4,806,977 to Mizutani et al. discloses a movable carriage housing for a scanning-type optic apparatus wherein a rack and pinion arrangement allows an upper body portion apparatus to pivot outwardly to expose a transfer station and scanning head for maintenance. 
     It is desirable to provide a scanning device for scanning duplex documents, for example, original documents having image information on both sides, for simplex documents having image information only on a single side, and for material not adaptable to be passed through sheet handling devices. In the past, this feature has been achieved in input scanners in a variety of ways, for example, there is disclosed in U.S. Pat. No. 4,536,077 to Stoffel, an arrangement provided with an optical system to direct light reflected from a first side of the document to a single scanning array, while the document is moving past a first position, and subsequently directing light from the second side of the document to the scanning array when it has reached a second position. 
     A disclosure entitled “Automatic Duplex Document Electronic Scanning” by Richard E. Smith, and published in the Xerox Disclosure Journal, Vol. 8, No. 3, May/June, 1983 at page 263, discloses both side scanning of a document with two spaced apart scanning arrays arranged on opposite sides of a document path, and platen scanning by a movable carriage supporting one of the arrays. All the patents and publications cited hereinabove are incorporated herein by reference. 
     Another solution to the calibration problem involves the reading of the intensity of light from a selected portion or portions of a white calibration target. In one arrangement the calibration target is on the same side of the platen glass as the document to be imaged. In accordance with the invention, it is noted that because contact between the calibration target and the glass may result in uneven intensity at one or more points, an air gap is introduced between the calibration target and the platen glass. 
     SUMMARY 
     In accordance with embodiments of the invention, a calibration target painted on the platen glass substantially avoids, or minimizes, the uneven intensity problem. For example, this problem may take the form of variations of apparent reflectivity of two to four percent. In accordance with the invention, this is done without introducing an air gap and associated contamination problems centering on introduction of airborne materials into parts of the system, such as the gap. These materials can cause calibration errors resulting in overexposure or underexposure. 
     More particularly, the inventive imaging system, which may be applied both to black and white and color imaging systems, comprises a platen member which defines an imaging portion and a calibration portion. In the preferred embodiment, the imaging portion and the calibration portion take up adjacent areas on the platen member. The platen member is supported by a movably mounted support member, and moves with the support member between a platen member calibrating position and a platen member imaging position. The platen member is dimensioned to support a sheet to be imaged. An imaging module with an imaging zone is supported in a position where the imaging zone is positioned to coincide with a portion of a document such as a sheet supported on the imaging portion of the platen member, when the platen member is in the platen member imaging position. A calibration member having a reference reflectivity is adhered to the calibration portion of the platen member. The calibration member lies substantially outside the imaging zone when the platen member is in the platen member imaging position. A motor moves the support member between the calibrating position and the imaging position to allow performance of a calibration procedure on the imaging module. 
     In accordance with an embodiment of the invention, the imaging module is disposed over the top surface of the platen member, to image the side of the sheet bearing against the bottom surface of the platen member and the calibration member is disposed over the bottom surface of the platen. 
     In accordance with an embodiment of the invention, the calibration member comprises an opaque paint on the bottom surface of the platen member covered by a layer of at least partially transparent film to prevent the paint from wearing as the document passes by the calibration member. 
     In accordance with the preferred embodiment, the movably mounted support member comprises a frame secured to the platen member. The frame is supported by a guide rail mechanism, comprising a pair of rails, for movement of the platen member between the platen member calibrating position and the platen member imaging position. Such movement is accomplished by a motor whose output shaft is mechanically coupled to the frame by a gear train comprising a rack gear and a pinion gear. 
     The inventive method of imaging sheets comprises moving, between a first imaging position and a second calibration position, the platen with a calibration element secured to the platen. The calibration element has been applied to the platen in a painting operation. The platen is moved to the calibration position prior to imaging and a calibration operation can be performed with the platen in the calibration position or preferably as the calibration element is moved past the imaging zone. An imaging array head images the calibration element during the calibration operation. After performance of the calibration operation, the platen is moved to the imaging position. After the platen is in the imaging position, sheets are advanced through the system to be imaged at a position where the sheets to be imaged are viewed by the imaging array head. In accordance with the preferred embodiment, the array head is held stationary during imaging and the sheets are transported by a constant velocity transport past the imaging array head, as the imaging array head is scanning the sheets. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be described in detail with reference to the following drawings, wherein: 
         FIG. 1  is a schematic front view of an image input terminal constructed in accordance with the present invention; 
         FIG. 2  is a schematic view of the inventive image input terminal in a position to input images of single sheets or books with the document handler not shown for clarity of illustration; 
         FIG. 3  is a detail, illustrating the side-one image capture module during fixed platen operation; 
         FIG. 4  is a detailed view of the side-two image module during scanning of an input image; 
         FIG. 5  is a perspective view of the side-two platen glass carrier in the image scanning position; and 
         FIG. 6  is a view similar to  FIG. 4 , illustrating the side-two image module during a calibration operation; 
         FIG. 7  is a detailed view, similar to  FIG. 4 , of an alternative side-two imaging subsystem during scanning of an input image; 
         FIG. 8  is a perspective view of the side-two platen glass carrier in the alternative embodiment of  FIG. 7 ; 
         FIG. 9  is a perspective view, from the input end, of the side-two platen glass carrier and its supporting structure in the image scanning position in the alternative embodiment of  FIG. 7 ; 
         FIG. 10  is a perspective view of the side-two platen glass carrier and its supporting structure in the image scanning position in the alternative embodiment of  FIG. 7 ; and 
         FIG. 11  is a perspective view, from the output end, of the side-two platen glass carrier and its supporting structure in the image scanning position in the alternative embodiment of  FIG. 7 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     An image input terminal  10 , constructed in accordance with the present invention and comprising an automatic document handler  12 , is illustrated in  FIG. 1 . Image input terminal  10  may be used for the input of black and white as well as color images. Document handler  12  is secured to the platen cover of terminal  10 , which is, in turn, secured to a pivotally mounted frame assembly (not shown) which may employ conventional pivoting structure. Document handler  12  comprises a multiple sheet input tray  16 , a paper feeder comprising a document feed roller  30 , a cooperating transport roller  36  and a cooperating idler roller  38 , a constant velocity transport  18 , a side-two image module  20 , and an output tray  22 . 
     Image input terminal  10  also comprises a base  24 . Platen cover  14 , incorporating document handler  12 , is pivotally mounted on the rear of the photocopier for movement between the cover down position illustrated in  FIG. 1  and a raised cover position.  FIG. 2  shows the use of terminal  10  to copy pages of a book in the raised cover position, with the document handler removed for clarity of illustration. Input tray  16  is adapted to receive a stack of sheets, such as two-sided paper documents  26 , to be copied. The stack of documents  26  is driven upwardly in the direction of arrow  28 , by a conventional motor and linkage assembly (not shown), toward and into engagement with a document feed roller  30 . Documents  26  are maintained in position between a pair of paper guides  32 , which engage opposite sides of documents  26 . 
     The top sheet  34  in the stack of documents  26  is fed into the system by feed roller  30 . Feed roller  30  is rotated to pull successive top sheets, starting with top sheet  34 , into a baffle assembly  35 . Baffle assembly  35  serves to guide the documents along the initial portion of a desired path as the documents advance, one by one, through the system. In baffle assembly  35 , documents are engaged between cooperating transport roller  36  and cooperating idler roller  38  and advanced toward cooperating idler roller  40  and transport roller  42 . Continued advancement of a sheet of paper taken from the stack of documents  26  is provided by transport rollers  44 ,  46 , and cooperating rollers  44   a  and  46   a . As a sheet advances through baffle assembly  35 , it is accelerated to the speed of constant velocity rollers  48  and  50 . Roller  52  cooperates with roller  54  to further advance the sheet of paper. Rollers  48  and  50  are contained within a drive on glass paper guide  53 , which also performs the function of guiding the paper sheet being fed. Rollers  30  and  36 – 46  are mounted in a conventional manner at the positions indicated in  FIG. 1 . 
     Rollers  52  and  54  are mounted as shown in  FIG. 1  to receive sheets from baffle assembly  35 . Transport rollers  30  and  36 – 54  are made of a rubber-like urethane while cooperating rollers are made of a hard plastic. The result is to effectively engage, grip and advance a sheet of paper guided by baffle assembly  35 . Baffle assembly  35  comprises a radially inner guide assembly  56  and a radially outer guide assembly  58 . A paper path  60  is defined between radially inner guide assembly  56  and radially outer guide assembly  58 . Paper path  60  is also defined by registration frame  62  ( FIG. 3 ). In particular, registration frame  62  includes an inclined guide surface  64  for guiding fed documents toward cooperating rollers  52  and  54 . 
     Registration frame  62  also includes a calibration target  66  with a painted surface  68 . Calibration target  66  has a reflectivity of about 80% to 85%, as measured by an X-Rite (tm) Model 938 spectrodensitometer. Referring to  FIG. 3 , calibration target  66  is held stationary within a recess  70  in registration frame  62 . Stationary calibration target  66  has a thickness which is less than the depth (for example 4 mm) of recess  70 . Accordingly, a gap  71  is defined between painted surface  68  and a stationary transparent platen glass  72 . A side-one image module  74  is disposed beneath the surface formed by platen  72 , registration frame  62  and a side-one platen glass  76  ( FIG. 3 ). 
     Because there is no contact between painted surface  68  and the top surface  78  of platen  72 , irregularities in optical density resulting from varying contact between a painted target surface and the top side of a platen glass are avoided, thus enabling successful calibration, as is more fully described below. However, gap  71  presents the possibility of airborne contamination by airborne dust, oil or other materials, small amounts of which could cause an inaccurate calibration. Accordingly, interfaces  80  are sealed with a suitable sealant disposed between the surface of the platen glass  72  and the primary surface of registration frame  62 . 
     Referring to  FIG. 4 , downstream portions of paper path  60  are defined by a guide surface  82 , which guides fed sheets between a lower guide member  84  and an upper guide member comprising side-two platen glass  86 . Lower guide member  84  and side-two platen glass  86 , together, define a document transport channel  88 , which extends over a pair of constant velocity transport document advancing rollers  90  and  92 . Rollers  90  and  92  are mounted within a drive on glass paper guide  94 . 
     Documents traveling through transport channel  88  are guided into an output channel  96  defined between a guide surface  98  and transport bottom plate  100 , where they are engaged by idler roller  102  and driven roller  104 . Rollers  102  are mounted on arms  103 . Arms  103  are biased by springs  105  to urge rollers  102  against rollers  104 . Torsion spring  105  bears against a catch  103   a  at one of its ends and the body of imaging module  20  at its other end. Imaging module  20  is secured to assembly frame  15 . Rollers  102  and  104  form another roller pair and output fully scanned documents into output tray  22 . 
     Referring to  FIGS. 4 and 5 , side-two platen glass  86  bears a painted calibration strip  106  (located in painted calibration portion  107 ), which is covered with a protective plastic coating  108  (which may be, for example, a polyethylene coating), which extends beyond the edge of painted calibration strip  106 . Protective plastic coating  108  is desirable, as it protects calibration strip  106  from the abrasive effects of passing sheets as the sheets are passed through the inventive system. 
     Painted calibration strip  106  has a reflectivity of about 80 to 85% measured using an X-Rite  938  spectrodensitometer. Painted calibration strip  106  is painted on side-two platen glass  86 , and thus provides a highly uniform calibration area which is very robust against dirt and wear. It is noted that the reflectivity of the painted calibration strip in the inventive system can vary widely with higher reflectivity being preferred. For example, target reflectivity in the range between 50 and 92 percent (92% is the reflectivity of the highest reflectivity paper commonly available) can be accommodated to the inventive system in accordance with known calibration techniques. 
     Side-two platen glass  86  is supported within a box  109 . Box  109  is formed integrally with guide surfaces  82  and  98 . Box  109  defines a pair of support wings  110 . Wings  110  are supported by a pair of complementary track members  112 . Wings  110  roll on track member  112  supported by four rollers  113  which act as wheels and thus reduce friction. Complementary track members  112  are, in turn, supported within platen cover  14 , being secured to assembly frame  15 . Each of the support wings  110  rolls, supported by rollers  113 , on its respective rail  114 , each of which is formed integral with its respective track  112 . 
     Wings  110  are formed with rack gears  116 . Each rack gear  116  cooperates with its respective one of pinon gears  118   a  and  118   b . Pinon gears  118   a  and  118   b  are, for cooperative synchronized motion, secured to and mounted on drive shaft  120 , which is mounted for rotation between two bearings  119  in facing track members  112 , as illustrated in  FIG. 5 . Facing track members  112  are fixedly secured to frame assembly  15 . Drive shaft  120  is driven by a DC motor  122 , which is coupled to drive shaft  120  by a gear train comprising three cooperating gears  124 ,  126  and  128 . Pinon gears  118   a  and  118   b  are kept in contact with their respective rack gears  116  by being mounted on drive shaft  120 , which, in turn, is mounted for rotation in bearings  119 , which are integral with supported track members  112 . 
     The operation of rack gears  116  and pinon gears  118   a  and  118   b  provide for a compact mechanism for enabling calibration of a document imaging system. More particularly, side-two platen glass  86  with the painted calibration strip  106  is moved into and out of the field of view of the image module  20  during a calibration cycle, as will be described in detail below. 
     The bottom of platen cover  14  is formed by a pressure pad  132 . Pressure pad  132  serves the function of providing pressure on single documents while they are being copied. 
     When it is desired to input images of documents into the system, whether for copying, archiving or other purposes, one or more of the same are put into input tray  16  to form the stack of documents  26 . Side one  134  of the top sheet  34  in the stack of documents  26  is in engagement with roller  30  after the documents have been input into input tray  16 . 
     When copying begins, it is done one sheet at a time, with side one  134  being removed from the stack by rotation of roller  30 . This feeds sheet  34  into paper path  60 , where it is engaged by rollers  36  and  38 , and then by rollers  40  and  42 . As sheet  34  continues to be accelerated through paper path  60 , its movement is continued next by roller pairs  44  and  44   a  and then by roller pairs  46  and  46   a . This continues advancement of sheet  34  through paper path  60 . 
     After engagement by roller  46 , sheet  34  is engaged by a pair of rollers  48  and  50 , which function as a constant velocity transport. Both rollers  48  and  50  are motor driven and very precisely control and maintain the desired constant speed of sheet  34 . Such maintenance of uniform speed is important because variations in speed will result in magnification variations in the direction of feed. Rollers  48  and  50  push sheet  34  against the side-one platen glass  76 . Sheet  34  slides over platen glass  76  in a so-called “drive on glass” arrangement. 
     Because paper path  60  is U-shaped, sheet  34  is inverted and side one  134  faces downwardly toward image module  74 , when sheet  34  is under rollers  48  and  50 . When sheet  34  is under rollers  48  and  50 , side one  134  is illuminated by a light source  138 , thus illuminating the linear area of information on side one  134 . Light source  138  comprises a fluorescent lamp of conventional design and cooperates with a reflector  139 , to send direct light rays  137  and reflected light rays  141  to the area on the document to be imaged. The illuminated linear area of information on side one  134  is imaged by a gradient indexed lens array  140  ( FIG. 3 ). An appropriate lens would be a Selfoc brand lens made by Nippon Sheet Glass. 
     Gradient indexed lens array  140  is a part of image module  74  and performs the function of imaging the illuminated linear area on a linear array of photodiode or equivalent elements, which may be of conventional design (not illustrated). The photodiode or equivalent elements may typically take the form of a linear array of a single row of 5000 to 8000 photodiodes. Alternatively, multiple row detectors with two, three, or four rows of photodiodes, (for example, a 4 by 5000 detector array) may be used, because the lenses that form the lens array may image an area that is several pixels wide. Imaging is done on a one-to-one basis, that is, substantially without enlargement or magnification. The image detected by the photodiode elements is then downloaded as a linear area image, later to be combined with other linear area images to constitute a complete side-one image. More particularly, as sheet  34  is advanced over image module  74 , image module  74  images successive linear areas and sends the same to memory so that a complete image of side one  134  of sheet  34  is obtained. 
     Before sheet  34  comes within the field of view of image module  74 , the system performs a calibration of image module  74  by causing image module  74  to move (at a velocity of about 140 mm/sec) under a central portion of painted surface  68  of side one calibration target  66 . Such velocity is not critical. Such movement is achieved by moving image module  74 , from the position illustrated in  FIG. 1 , in the direction indicated by arrow  142 , so that module  74  is imaging region  144  of painted surface  68  ( FIG. 3 ), where several readings at several points along the path of movement of module  74  are taken for the purpose of calibration of image module  74 . After this, the system is ready to image side one  134  of sheet  34 . 
     Sheet  34 , after imaging by image module  74 , is deflected by guide surface  64  ( FIG. 3 ) toward rollers  52  and  54 . Rollers  52  and  54  cooperate to advance sheet  34  toward guide  82 , which guides sheet  34  into document transport channel  88 . 
     Before sheet  34  comes within the field of view of side-two image module  20 , a calibration sequence may be performed by advancing side-two platen glass  86  in the direction of arrow  148  ( FIG. 4 ), from the position illustrated in  FIG. 4  to the position illustrated in  FIG. 6 . The mechanics for achieving this may be understood with reference to  FIG. 5 . 
     More particularly, as illustrated in  FIG. 5 , when it is desired to perform a calibration sequence, motor  122  is actuated, causing the transmission of rotary motion through gears  124 ,  126  and  128  to drive shaft  120 . Drive shaft  120  rotates, rotating pinon gears  118   a  and  118   b . This causes the application of forces to rack gears  116 , with the effect of causing movement in the direction of arrow  150 . Because side two platen glass  86  is contained within box  109 , movement of box  109 , on rollers  113 , in the direction of arrow  150  results in moving side two platen glass  86  in the direction of arrow  148  in  FIG. 4  to the position illustrated in  FIG. 6 . During this movement, side two platen glass  86  slides on three low friction feet  147 , only two of which are visible in  FIG. 4 , the third being hidden behind another of the other low friction feet  147 , as indicated by dashed lines in the figure. Similar to the side one calibration operation, as this occurs, the central area of painted calibration strip  106  passes under side-two image module  20  and several readings are taken to calibrate the system. It is noted that guide surface  98  is an integral part of box  109 . Rollers  102  and  104  are mounted to frame assembly  15  and remain in place, protruding through holes  153  and surface  98 . 
     As noted above, painted calibration strip  106  extends across portion  107 , which, in the document scanning position of platen glass  86 , is out of the field of view of image module  20 , as illustrated in  FIG. 4 . During calibration, portion  107  moves about 28 mm at a speed in the range of 40 mm/sec to 50 mm/sec, for example 42 mm/sec, to the position illustrated in  FIG. 6 , where multiple readings are taken to calibrate the system. Such velocity is not critical to the invention. After calibration has been accomplished, platen glass  86  returns to the imaging position illustrated in  FIG. 4 , and the system is ready to image side two of sheet  34 . Such return to the imaging position illustrated in  FIG. 4  is accomplished by driving motor  122  in the reverse direction. 
     Alternatively, motor  122  can operate against the biasing force of a spring during the calibration operation and power removed, or the gears disengaged, or other mechanical artifice employed after calibration is completed allowing the biasing spring to return the platen to the original position. 
     It is contemplated that when the system is instructed to image a stack of documents  26 , both the side one and side two calibration subroutines will be implemented simultaneously, and that upon the completion of the calibration subroutines for image modules  20  and  74 , roller  30  will initiate the feeding of sheets from the stack of documents  26 . 
     Returning to the advancement of sheet  34  through the system, in document transport channel  88 , rollers  90  and  92  press sheet  34  against side-two platen glass  86 . Rollers  90  and  92  impart a constant velocity motion to sheet  34 , as sheet  34  passes under side-two image module  20 . Sheet  34  is illuminated on side two by a suitable light source  149 , which cooperates with reflector  151 . Side-two image module  20  performs the function of creating a side-two image on a linear area by linear area basis, in much the same manner that image module  74  creates a side-one image one linear area at a time. It is noted that modules  20  and  74  and associated light sources and focusing optics are substantially identical and function in the same way during imaging and calibration. 
     In particular, a gradient indexed lens array  152  images side two of sheet  34  through transport imaging portion  155  of platen glass  86 , linear area by linear area as it passes under gradient indexed lens array  152 , until the entire document is imaged. This information is sent to any suitable electronic memory where the image is available for printing, data processing, transmission by facsimile or any other purpose. 
     After the imaging of sheet  34  has been completed, the system continues to advance sheet  34  through the action of roller  104 , which delivers the fully imaged sheet to output tray  22 . 
     The inventive image input terminal may also be used to input images from books or from single sheets on a sheet-by-sheet, hand-fed basis. This is in contrast to the constant velocity mode of multiple-page automatic document image input described above. In particular, in the hand-fed or stationary document mode, after calibration, image module  74  is advanced to the position illustrated in  FIG. 3 , where the imaging point  154  of lens array  140  is at the edge of a document  156  to be imaged. Image module  74  then moves in the direction of arrow  158 , until the entire area to be imaged has been covered. As image module  74  moves from, for example, the position illustrated in solid lines in  FIG. 2  to the position illustrated in phantom lines in  FIG. 2 , the photodiode array associated with lens array  140  produces images of successive linear areas of the image printed on the underside  160  of document  156 , until the entire image on the underside  160  of document  156  has been scanned and sent to memory. Calibration of image module  74  for stationary platen copying is performed in the same manner as calibration for the constant velocity transport scanning imaging process described above. 
     Once calibration of image module  74  has been completed, in the hand-fed mode, a sheet of paper or other object, such as a book  162 , may be imaged. Preferably, this is done after platen cover  14  has been rotated to the down position ( FIG. 1 ). This results in pressure pad  132  bearing against the sheet of paper, book or other object being imaged and applying pressure to insure a quality imaging operation. 
     The operation of movable platen system  1010 , as illustrated in  FIGS. 7–11 , is similar to the operation of the system illustrated in  FIGS. 1–6 . To the extent practical, analogous parts in the embodiment of  FIGS. 7–11  are given numbers one thousand higher than the corresponding parts in the embodiment illustrated in  FIGS. 1–6 . 
     Referring to  FIG. 7 , downstream portions of a paper path  1060  are defined by a guide surface  1082  which is an integral part of box  1109 , which guides fed sheets (coming from roller pair  1052  and cooperating roller pair  1054 ) between a lower guide member  1084  and an upper guide member comprising side-two platen glass  1086 . Roller pair  1054  is biased by spring  1055  toward roller pair  1052 . Lower guide member  1084  and side-two platen glass  1086 , together, define a document transport channel  1088 , which extends over a pair of constant velocity transport document advancing rollers  1090  and  1092 . Rollers  1090  and  1092  are mounted within a drive on glass paper guide  1094 . 
     Documents traveling through transport channel  1088  are guided into an output channel  1096  defined between a guide surface  1098  ( FIG. 8 ) and transport bottom plate  1100 , where they are engaged by idler rollers  1102  and driven rollers  1104 . Rollers  1102  are mounted on bearings  1103 , as illustrated in  FIG. 9 . Referring to  FIG. 7 , bearing  1103   a  is biased by coil spring  1105  to urge rollers  1102  against rollers  1104 . Coil spring  1105  is tensioned between a pair of clips  1105   a , on a support  1105   b , one at each of its ends. Imaging module  1020  is secured to the assembly frame of the cover of the system. Rollers  1102  and  1104  form another roller pair and output fed documents into the output tray, not illustrated. 
     Referring to  FIG. 7 , side two platen glass  1086  bears a painted calibration strip  1106 , which is covered with a protective plastic coating, which comprises an ultra high molecular weight polyethylene film member secured over painted calibration strip  1106  by a self-adhesive layer, acting to protect calibration strip  1106  from abrasion from passing sheets, as the sheets are passed through the inventive system. 
     Painted calibration strip  1106 , which is painted on side-two platen glass  1086  provides a highly uniform calibration area which is very robust against dirt and wear. Painted calibration strip  1106  is formed using a high gloss opaque white paint such as Sherwin-Williams Hydralon Chemaqua white F82W582 with part B V66V580 catalyst. 
     Side-two platen glass  1086  is supported within a box  1109 . Box  1109  is formed integrally with guide surface  1098 . Box  1109  defines a pair of support wings  1110 , as illustrated in  FIG. 10 . Wings  1110  are supported by a pair of complementary track members  1112 . Wings  1110  roll on track members  1112  supported by four rollers  1113 , which act as wheels and thus reduce friction. Complementary track members  1112  are, in turn, supported within the platen cover by being secured to the assembly frame. Each of the support wings  1110  rolls, supported by rollers  1113 , on its respective rail  1114 , each of which is formed integral with its respective track  1112 . 
     Wings  1110  are formed with rack gears  1116 . Each rack gear  1116  cooperates with its respective one of pinon gears  1118   a  and  1118   b . Pinon gears  1118   a  and  1118   b  are, for cooperative synchronized motion, secured to and mounted on drive shaft  1120 , which is mounted for rotation between two bearings  1119  in facing track members  1112 , as illustrated in  FIG. 10 . Facing track members  1112  are fixedly secured to the frame assembly. Drive shaft  1120  is driven by a DC motor  1122  ( FIG. 11 ), which is coupled to drive shaft  1120  by a gear train comprising a stationarily mounted gear train  1124 . Pinon gears  1118   a  and  1118   b  are kept in contact with their respective rack gears  1116  by being mounted on drive shaft  1120 , which, in turn is mounted for rotation in bearings  1119 . 
     The operation of rack gears  1116  and pinon gears  1118   a  and  1118   b  provide for a compact mechanism for enabling calibration of a document imaging system. More particularly, side-two platen glass  1086  with painted calibration strip  1106  is moved into and out of the field of view of the imaging module  1020  during a calibration cycle, as will be described in detail below. 
     Imaging module  1020  receives and/or transmits power, control information and output video along multiconductor ribbon cable  1021 , which is kept in place by clips  1023 , as illustrated in  FIG. 11 . 
     Before a sheet  1034  ( FIG. 7 ), being advanced through the system, comes within the field of view of side-two image module  1020  ( FIG. 11 ), a calibration sequence is performed by advancing side two platen glass  1086  in the direction of arrow  1148  ( FIG. 7 ), from the position illustrated in  FIG. 7 , to a position analogous to that illustrated in  FIG. 6 . The mechanics for achieving this may be understood with reference to  FIG. 10 . 
     More particularly, as illustrated in  FIG. 10 , when it is desired to perform a calibration sequence, motor  1122  is actuated, causing the transmission of rotary motion through gear train  1124  to drive shaft  1120 . Drive shaft  1120  rotates, rotating pinon gears  1118   a  and  1118   b . This causes the application of forces to rack gears  1116 , with the effect of causing movement in the direction of arrow  1150 . Side-two platen glass  1086  is contained within box  1109 , being supported by top surfaces  1149  of springy foam pads  1153 , between airflow preventer  1157  (which reduces the accumulation of dirt deposits), and wall  1159 . Box  1109  includes cutaway areas  1161  and  1163 , which provide space for low friction feet  1147 . Movement of box  1109 , on rollers  1113 , in the direction of arrow  1150  results in moving side-two platen glass  1086  in the direction of arrow  1148  in  FIG. 7  to a position analogous to that illustrated in  FIG. 6 , in the above description of the previously described embodiment. 
     During this movement, side-two platen glass  1086  slides on three low friction feet  1147 , only two of which are visible in  FIG. 7 , the third being hidden behind another one of the other low friction feet  1147 , as indicated by dashed lines in  FIG. 7 . Low friction feet  1147  are made of acetal, such as that sold under the trademark Delrin. Similar to the side-one calibration operation, as this occurs, the central area of painted calibration strip  1106  passes under side-two image module  1020  and several readings are taken to calibrate the system. Rollers  1102  and rollers  1104  protrude through slots  1153  in surface  1098  as illustrated in  FIG. 8 . 
     As noted above, painted calibration strip  1106  in the document scanning position of platen glass  1086  is out of the field of view of image module  1020 , which, instead is aimed at transparent document scanning area  1155 . During calibration, calibration strip  1106  moves to a calibration position analogous to that illustrated in  FIG. 6 , where multiple readings are taken to calibrate the system. After calibration has been accomplished, platen glass  1086  returns to the imaging position illustrated in  FIG. 7 , and the system is ready to image side two of sheet  1034 . Such return to the imaging position illustrated in  FIG. 7  is accomplished by driving motor  1122  in the reverse direction. 
     As sheet  1034  advances through the system, in document transport channel  1088 , rollers  1090  and  1092  press sheet  1034  against side-two platen glass  1086  and impart a constant velocity motion to sheet  1034 , as sheet  1034 , illuminated on side two by a suitable light source  1149 , which cooperates with reflector  1151 , passes under side-two image module  1020 , which performs the function of creating a side-two image one linear area at a time. 
     A gradient indexed lens array  1152  images side two of sheet  1034 , linear area by linear area as it passes under gradient indexed lens array  1152 , until the entire document is imaged. This information is sent to any suitable electronic memory where the image is available for printing, data processing, transmission by facsimile or any other purpose. 
     After the imaging of sheet  1034  has been completed, the system continues to advance sheet  34  through the action of roller  1104 , which delivers the fully imaged sheet to an output tray on the system. 
     It is, therefore, evident that there has been provided, in accordance with the present invention, an image input terminal that fully satisfies the aims and advantages hereinbefore set forth. While this invention has been described in conjunction with several embodiments thereof, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, it is intended to embrace all such alternatives, modifications and variations as may fall within the spirit and broad scope of the appended claims.