Abstract:
A method and a device for aligning sheets. The device includes a first and a second supporting stop for supporting the sheet against gravity in a first non-aligned position of the sheet. The device further includes two alignment stops and an actuator for moving the sheet from the first non-aligned position to a second aligned position. In the second aligned position, a substantially straight edge of the sheet contacts the two alignment stops and the first supporting stop supports the sheet while the second supporting stop does not support the sheet.

Description:
[0001]    The application claims the benefit of U.S. Provisional Application No. 60/345,386 filed Oct. 26, 2001.  
         FIELD OF THE INVENTION  
         [0002]    The present invention relates to a method and a device for aligning sheets. More specifically the invention relates to a method and a device for aligning thermal-sensitive sheets to be used in a thermal printer.  
         BACKGROUND OF THE INVENTION  
         [0003]    Thermal imaging or thermography is a recording process wherein images are generated by the use of thermal energy.  
           [0004]    In thermography three approaches are known:  
           [0005]    1. Direct thermal formation of a visible image pattern by image-wise heating of a recording material containing matter that by chemical or physical process changes color or optical density.  
           [0006]    2. Image-wise transfer of an ingredient necessary for the chemical or physical process bringing about changes in color or optical density to a receptor element containing other of the ingredients necessary for said chemical or physical process followed by uniform heating to bring about said changes in color or optical density.  
           [0007]    3. Thermal dye transfer printing wherein a visible image pattern is formed by transfer of a colored species from an imagewise heated donor element onto a receptor element.  
           [0008]    Thermographic materials of type  1  can be rendered photothermographic by incorporating a photosensitive agent which after exposure to UV, visible or IR light, e.g. by means of a laser, is capable of catalyzing or participating in a thermographic process bringing about changes in color or optical density.  
           [0009]    A survey of direct thermal imaging methods is given in the book “Imaging systems” by Kurt I. Jacobson-Ralph E. Jacobson, The Focal Press—London and New York (1976), Chapter VII under the heading “7.1 Thermography”.  
           [0010]    Common thermal printers that do no use a laser light source comprise a rotatable drum and an elongate thermal head which is spring-biased towards the drum to firmly line-wise contact a heat-sensitive material which is passed between the head and the drum. The thermal head includes a plurality of heating elements. The image-wise heating of a sheet is performed on a line by line basis, with the heating elements geometrically juxtaposed along each other in a bead-like row running parallel to the axis of the drum. Each of these elements is capable of being energized by heating pulses, the energy of which is controlled in accordance with the required density of the corresponding picture element. The sheet is advanced between the head and the drum by frictional contact of its rear side with the drum.  
           [0011]    Patent application EP-A-0 846 565 discloses such a thermal printer having a thermal head.  
           [0012]    The images that are printed on such a thermal printer are often used for diagnostic purposes, medical diagnosis in particular. Customarily such images for medical diagnosis are printed on a transparent support. Examples of such images are echograms, CT scans, NMR images. These images are negative-type images, which means that their background is substantially black, the image details having lesser optical densities. FIG. 1 shows two sheets  10  that bear images that are printed by a thermal printer having a thermal head. The image areas E are substantially black and the margins A, B, C and D are transparent. The image cannot be printed on the sheet up to the edge since otherwise the thermal sensitive layer of the sheet would be squeezed at the edge due to the pressure between head and drum, which would soil the thermal head and the transport rollers.  
           [0013]    These images are viewed on a light box for diagnosis. On the light box, the images can be positioned so that the transparent margins B are outside of the illuminated area while black screens can be moved in the light box, like curtains, so that they cover the margins C and D. However, if two sheets  10  are positioned alongside each other, as shown in FIG. 1, a transparent area between the two image areas E remains.  
           [0014]    Radiologists are unfamiliar with such a transparent area, which does not exist in conventional AgX X-ray images. Moreover, a large transparent area has a dazzling effect. FIG. 1 shows two mammographic images. In mammography, it is customary to view the images of the right and of the left breast on a light box, positioned with respect to each other as shown in FIG. 1 (reference sign  15  in FIG. 1 indicates the contours of the breasts). Both sheets  10  are pushed against each other so that no space is left between them (for clarity, in FIG. 1 an open space is shown between the sheets  10 ). Thus, a transparent area of twice margin A remains between the images. Up till now, such mammographic images did not have a transparent margin, because they were made e.g. by a conventional AgX apparatus or in a photothermographic printer wherein the laser can expose the sheet up to its edges.  
           [0015]    It would be advantageous to print such mammographic images by means of a printer with a thermal head, since this is less expensive than using a photothermographic printer. However, when printed by a conventional printer with a thermal head, on a light box the transparent area between the two images is disturbing.  
         OBJECTS OF THE INVENTION  
         [0016]    It is therefore an object of the invention to provide a thermal printer having a thermal head that can print mammographic images that are suitable for diagnosis on a light box.  
           [0017]    It is a further object of the invention to provide a method that allows obtaining, by means of a thermal printer having a thermal head, mammographic images that are suitable for diagnosis on a light box.  
         SUMMARY OF THE INVENTION AND DEFINITION OF TERMS  
         [0018]    The above-mentioned objects are realised by a thermal printer including a device as claimed in claim  1  and claim  5  and by a thermal printer performing a method as claimed in claim  17  and claim  19 . The dependent claims set out preferred embodiments of the invention.  
           [0019]    A sheet  10  having a substantially straight edge  11 , as shown in FIG. 1, is accurately aligned in accordance with the invention. An image can then be printed on the sheet leaving only a small margin A, of e.g. 1.1 mm, between the image area E and the substantially straight edge  11 . A transparent area of twice such a small margin A, between two image areas E, is not disturbing when viewed on a light box. Because of the accurate alignment, the margin A has a nearly constant width so that there is no risk of the image area E coming too close to the sheet edge  11 , which would result in soiling the thermal head as mentioned above. One margin, margin A in FIG. 1, has a small width; the other margins, margins B, C and D in FIG. 1, may have a larger width.  
           [0020]    In a preferred embodiment of the invention, shown in FIG. 2, the substantially straight edge  11  of sheet  10  is aligned with respect to an alignment axis  25  that is substantially perpendicular to the axis  45  of the drum of the thermal printer. Sheet  10  as shown in FIG. 1 may be a substantially rectangular sheet having the standard dimensions of 10″×12″. The image is then printed line-wise with the image lines substantially perpendicular to edge  11 , i.e. substantially parallel to edge  12 . Preferably, edge  12  is the short, 10″, sheet edge and edge  11  is the long, 12″, sheet edge. An advantage of this embodiment is that a shorter and hence less expensive thermal head may be used than if the printed image lines would be substantially parallel to the longer sheet edge  11 .  
           [0021]    In this text, a “substantially straight edge” of a sheet is defined as follows. LS is the straight line segment that is the least squares fit of the edge. An edge is substantially straight if, for all points PT in line segments S belonging to the edge, so that the total length of the line segments S is at least 80% of the length of the edge and preferably at least 90% of the length of the edge, the distance d between PT and LS is d&lt;5 mm, preferably d&lt;3 mm, more preferably d&lt;1 mm and most preferably d&lt;0.5 mm. The distance between points PT and straight least squares segment LS may be larger over portions of the edge (of relative length 20% or 10%) to allow for e.g. notches which are quite customary in medical film sheets.  
           [0022]    A first line L 1  is “substantially parallel” to a second line L 2  if, when L 1  is the line parallel to L 1  through an arbitrary point O taken as origin and L 2 * is the line parallel to L 2  through O, the smallest angle α between L 1 * and L 2 * is a &lt;15°, preferably α&lt;10°, more preferably α&lt;5°.  
           [0023]    A first line L 1  is “substantially perpendicular” to a second line L 2  if, when L 1 * is the line parallel to L 1  through an arbitrary point O taken as origin and L 2  is the line parallel to L 2  through O, the smallest angle β between L 1 * and L 2 * is β&gt;75°, preferably β&gt;80°, more preferably β&gt;85°.  
           [0024]    A line is “substantially vertical” if it is substantially parallel to a vertical line; a vertical line has the same direction as the force of gravity.  
           [0025]    A “substantially horizontal” line is substantially perpendicular to a vertical line.  
           [0026]    A sheet is “substantially rectangular” if it has four substantially straight edges and if the adjoining edges are substantially perpendicular to each other, as defined above.  
           [0027]    Further advantages and embodiments of the present invention will become apparent from the following description and drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0028]    The invention is described with reference to the following drawings without the intention to limit the invention thereto, and in which:  
         [0029]    [0029]FIG. 1 shows two mammographic images;  
         [0030]    [0030]FIG. 2 shows an embodiment of a device according to the invention;  
         [0031]    [0031]FIG. 3 shows another embodiment of a device according to the invention.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0032]    [0032]FIG. 2 shows a first embodiment of a device according to the present invention. A sheet  10 , which may be fed from a sheet tray in a thermal printer, is dropped onto two supporting stops  27  and  28 . The sheet is now in a first, non-aligned position, which is shown in FIG. 2. To accurately align sheet  10 , it is moved from this first non-aligned position to a second aligned position (not shown) wherein the substantially straight edge  11  of sheet  10  contacts two alignment stops  21 ,  22 . In the embodiment of FIG. 2, in the second aligned position sheet  10  contacts the alignment stops  21 ,  22  in points Q 1  and Q 2 . These two points define an alignment axis  25 . In the second aligned position, sheet  10  is thus aligned with its substantially straight edge  11  with respect to alignment axis  25 .  
         [0033]    In order to obtain an accurately defined position of the aligned sheet  10 , only one of the two supporting stops  27 ,  28  supports sheet  10  in its second aligned position. In the embodiment of FIG. 2, sheet  10  contacts in its second aligned position the first and second alignment stops  21 ,  22  and the first supporting stop  27 ; it does not contact the second supporting stop  28 . Furthermore, sheet  10  makes contact with contact element  34  which pushes sheet  10  against the alignment stops  21 ,  22 . Sheet  10  as shown in FIG. 2 is substantially rectangular. Moreover alignment axis  25  is substantially vertical. The first supporting stop  27  is positioned higher than the second supporting stop  28 , i.e. y 27 &gt;y 28  wherein y 27  and y 28  are the coordinates with respect to vertical axis y of respectively the first alignment stop  27  and the second alignment stop  28 . In this way, when sheet  10  contacts the alignment stops  21  and  22 , it does not contact the second supporting stop  28 .  
         [0034]    In a preferred embodiment of the invention, the first supporting stop  27 , which supports sheet  10  in its second aligned position, is nearer the alignment axis  25  than the second supporting stop  28 , i.e. in FIG. 2 distance d 27 &lt;d 28 . In this way, the second aligned position is more stable than if d 27 &gt;d 28 .  
         [0035]    [0035]FIG. 3 shows another embodiment in accordance with the invention. Contrary to the embodiment shown in FIG. 2, in the embodiment of FIG. 3 the supporting stops  27  and  28  each contact a different edge of sheet  10  in its first non-aligned position: the  5  first supporting stop  27  contacts edge  14  while the second supporting stop  28  contacts edge  13 .  
         [0036]    Sheet  10  is moved to its second aligned position by actuator  30 . An embodiment of actuator  30  is shown schematically in FIG. 2; it comprises an electromagnet  31 , a resilient element  32  such as a spring, a lever  33  that can pivot around point P and a contact element  34  on lever  33 . To move sheet  10 , electromagnet  31  is energized and pulls lever  33 , and contact element  34  on lever  33 , in the direction of arrow R. Contact element  34  contacts edge  13  of sheet  10  and pushes sheet  10  against alignment stops  21  and  22 . Lever  33  is used to increase the stroke of electromagnet  31 . An advantage of resilient element  32  is that a given force is applied to edge  13  without enforcing edge  13  to move over a fixed displacement, which would be the case if the resilient element  32  would be omitted. Enforcing a fixed displacement would cause a deformation of sheet  10  by pressing sheet  10  against alignment stops  21  and  22 . Applying the force through resilient element  32  on the other hand gently pushes sheet  10  against the alignment stops  21  and  22 . Instead of the actuator  30  shown in FIG. 2, any other actuator as known in the art may be used.  
         [0037]    Preferably, before moving sheet  10  against alignment stops  21  and  22 , sheet  10  is given a slight touch. This touch may be given by actuator  30 . The purpose of this touch is to obtain a good first non-aligned position of sheet  10 , since dropping sheet  10  may e.g. have caused the sheet to be not well supported by the supporting stops  27 ,  28 . In the embodiment of FIG. 2, touching the sheet is accomplished by shortly energizing the electromagnet  31  just before the electromagnet  31  is energized again to move the sheet. The complete cycle of touching and moving sheet  10  may be quite short, e.g. less than 1 second.  
         [0038]    In a preferred embodiment of the invention, at least one of the alignment stops  21 ,  22  is adjustable. In the embodiment of FIG. 2, alignment stop  21  is adjustable. Point Q 1  is the contact point of alignment stop  21  with sheet  10  in its second aligned position. By rotating alignment stop  21  around its pivot point  24 , contact point Q 1  moves towards or away from pivot point  24 , since arc  23  on which Q 1  is located is positioned eccentrically with respect to pivot point  24 .  
         [0039]    In another embodiment of the invention, the first and second alignment stops  21 ,  22  are both part of a single element that contacts sheet  10  in its second aligned position by means of these first and second alignment stops  21 ,  22 .  
         [0040]    Advantages of a device in accordance with the invention are that it is simple and inexpensive, yet it allows accurate sheet alignment.  
         [0041]    After aligning the sheet, an image may be printed on the sheet in a thermal printer having a thermal head. It is preferred, as shown in FIG. 2, that the drum axis  45  of the thermal printer is substantially perpendicular to the alignment axis  25 . Printing may proceed as follows. The aligned sheet is seized by a transport mechanism in the thermal printer—the transport system may include the thermal head and the drum. Actuator  30  is now switched off; i.e. in the embodiment of FIG. 2 electromagnet  31  is de-energized so that contact element  34  is withdrawn from sheet  10 . The image is printed line-wise, while sheet  10  is advanced between the thermal head and the drum. In the embodiment shown in FIG. 2, the image lines are substantially parallel to edge  12  of sheet  10  (after alignment). The image is printed with a small and nearly constant margin that is adjacent to substantially straight edge  11 . Preferably—as shown in FIG. 2, wherein y represents a vertical axis—the alignment axis  25  is substantially vertical and the drum axis  45  is substantially horizontal.  
         [0042]    To adjust the alignment device, a special test image may be written, preferably in the factory during production of the thermal printer. Using measurements of this test image, the alignment device is then adjusted, e.g. by adjusting alignment stop  21  in FIG. 2. In this way, the small margin of the sheet—i.e. margin A in FIG. 1—will have a nearly constant width. To set the magnitude of the margin width, the position of the thermal head along its axis may be adjusted (the axis of the thermal head is substantially parallel to the drum axis  45 ).  
       EXAMPLE  
       [0043]    An aligning device as shown in FIG. 2 is used with the following coordinates with respect to axis y:  
         [0044]    y 28 =0;  
         [0045]    y 27 =0.2 mm;  
         [0046]    y 24 =41.6 mm;  
         [0047]    y 22 =206.6 mm;  
         [0048]    y 34 =131.1 mm;  
         [0049]    y 45 =295.4 mm;  
         [0050]    and with the following distances:  
         [0051]    d 27 =42 mm;  
         [0052]    d 28 =214 mm.  
         [0053]    Sheet  10  is a thermal-sensitive sheet:  
         [0054]    having a support of poly(ethylene terephtalate) with a thickness of 0.18 mm;  
         [0055]    having dimensions 302.5 mm (=the length of edges  11  and  13 )×252 mm (=the length of edges  12  and  14 ) and a perpendicularity not larger than 1.5 mm over 300 mm.  
         [0056]    Those skilled in the art will appreciate that numerous modifications and variations may be made to the embodiments disclosed above without departing from the scope of the present invention. LIST OF REFERENCE SIGNS  
         [0057]    [0057] 10  sheet  
         [0058]    [0058] 11 ,  12  edge  
         [0059]    [0059] 13 ,  14  edge  
         [0060]    [0060] 15  contour  
         [0061]    [0061] 21 ,  22  alignment stop  
         [0062]    [0062] 23  arc  
         [0063]    [0063] 24  point  
         [0064]    [0064] 25  alignment axis  
         [0065]    [0065] 27 ,  28  supporting stop  
         [0066]    [0066] 30  actuator  
         [0067]    [0067] 31  electromagnet  
         [0068]    [0068] 32  resilient element  
         [0069]    [0069] 33  lever  
         [0070]    [0070] 34  contact element  
         [0071]    [0071] 45  drum axis  
         [0072]    d 27 , d 28  distance  
         [0073]    y vertical axis  
         [0074]    y 22 ,y 24 ,y 27 ,y 28 ,y 34 ,y 45  coordinate with respect to y-axis  
         [0075]    A,B,C,D margin  
         [0076]    E image area  
         [0077]    P, Q 1 , Q 2  point  
         [0078]    R arrow