Abstract:
A method of reducing uneven use of the total number of printing elements on a print head in a printer, when selectively using the printing elements to make different size color image prints on respective similar size receiver mediums, comprises: selectively using the total number of printing elements to make color image prints substantially the same size as the receiver mediums; and selectively using a particular number of printing elements less than the total number of printing elements to make similar size color image prints smaller than the receiver mediums, but alternating which ones of the total number of printing elements can be selectively used to make each print so that the placement of each print on a receiver medium is alternated, whereby, since those printing elements that can be selectively used to make each print smaller than a receiver medium are alternated, uneven use of the printing elements is reduced.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     Reference is made to commonly assigned, co-pending application Ser. No. 10/274,352, entitled METHOD AND APPARATUS FOR REDUCING UNEVEN USE OF HEATING ELEMENTS ON THERMAL PRINT HEAD and filed Oct. 18, 2002 in the names of Robert F. Mindler and Charles S. Christ. 
    
    
     FIELD OF THE INVENTION 
     The invention relates generally to image printers, and in particular to thermal printers in which the selective use of individual heating or resistive elements on a thermal print head effects a color dye transfer from a dye donor medium to a dye receiver medium to create a color image print on the dye receiver medium. More specifically, the invention provides a method and corresponding apparatus for reducing uneven use of the heating elements on the thermal print head. 
     BACKGROUND OF THE INVENTION 
     A typical dye donor web that is used in a thermal printer includes a repeating series of three different primary color sections or patches such as a yellow color section, a magenta color section and a cyan color section. Also, there may be a transparent laminating section after the cyan color section. 
     To make a color image print using a thermal printer, respective color dyes in a single series of yellow, magenta and cyan color sections on a dye donor web are successively heat-transferred (e.g. by diffusion), one on top of the other, onto a dye receiver sheet. Then, optionally, the transparent laminating section is deposited on the color image print. The dye transfer from each color section to the dye receiver sheet is done one line of pixels at a time across the color section via a bead of selectively used heating or resistor elements on a thermal print head. The bead of heating elements makes line contact across the entire width of the dye donor web, but only those heating elements that are actually used for a particular line are heated sufficiently to effect a color dye transfer to the receiver sheet. The temperature to which a heating element is heated is proportional to the density (darkness) level of the corresponding pixel formed on the receiver sheet. The higher the temperature of the heating element, the greater the density level of the corresponding pixel. Various modes for raising the temperature of the heating element are described in prior art U.S. Pat. No. 4,745,413 issued May 17, 1988. 
     One example of a color print-making process using a thermal printer is as follows. 
     1. A dye donor web and a dye receiver sheet are advanced forward in unison, with a yellow color section of the donor web moving in contact with the receiver sheet longitudinally over a stationary bead of heating elements in order to effect a line-by-line yellow dye transfer from the yellow color section to the receiver sheet. A web take-up spool draws the dye donor web forward over the bead of heating elements, and a pair of pinch and drive rollers draw the dye receiver sheet forward over the bead of heating elements. A platen roller holds the dye receiver sheet in a dye receiving relation with the dye donor web at the bead of heating elements. 
     2. Once the yellow dye transfer is completed, the platen roller is retracted from adjacent the print head to allow the pair of pinch and drive rollers to return the dye receiver sheet rearward in preparation for a second pass over the bead of heating elements. 
     3. Then, the platen roller is returned to adjacent the print head, and the dye donor web and the dye receiver sheet are advanced forward in unison, with a magenta color section of the donor web moving in contact with the receiver sheet longitudinally over the bead of heating elements in order to effect a line-by-line magenta dye transfer from the magenta color section to the receiver sheet. The magenta dye transfer to the dye receiver sheet is in exactly the same area on the receiver sheet as was subjected to the yellow dye transfer. 
     4. Once the magenta dye transfer is completed, the platen roller is retracted from adjacent the print head to allow the pair of pinch and drive rollers to return the dye receiver sheet rearward in preparation for a third pass over the bead of heating elements. 
     5. Then, the platen roller is returned to adjacent the print head, and the dye donor web and the dye receiver sheet are advanced forward in unison, with a cyan color section of the donor web moving in contact with the receiver sheet longitudinally over the bead of heating elements in order to effect a line-by-line cyan dye transfer from the magenta color section to the receiver sheet. The cyan dye transfer to the dye receiver sheet is in exactly the same area on the receiver sheet as was subjected to the yellow and magenta dye transfers. 
     6. Once the cyan dye transfer is completed, the platen roller is retracted from adjacent the print head to allow the dye receiver sheet to be returned rearward in preparation for exiting the printer. 
     7. Then, the pair of pinch and drive rollers advance the dye receiver sheet forward to an exit tray. 
     When printing a 5×7 inch color image on a 6×8 inch dye receiver sheet, for example, a number of the heating elements closest to the opposite ends of the bead of heating elements are not selectively used, i.e. the heating elements closest to the opposite ends of the line are not selectively heated during the yellow, magenta and cyan dye transfers to the receiver sheet. This leaves a pair of 0.5 inch non-image (white) margin areas along opposite sides of the 5×7 inch color image print on the 6×8 inch receiver sheet. Alternatively, when printing a 6×8 inch color image (instead of a 5×7 inch image) on the 6×8 inch receiver sheet, the heating elements closest to the opposite ends of the bead of heating elements are selectively used, i.e. they are selectively heated during the yellow, magenta and cyan dye transfers to the receiver sheet. As a result, a color image print without any non-image margin areas, i.e. a borderless print, is formed. If the heating elements closest to the opposite ends of the bead of heating elements are used less often than the remainder of the heating elements along the bead, there can result an uneven deterioration between the two which causes the resistance values of the two to become materially different over time. Then, when printing the 6×8 inch color image, the material difference in the resistance values between a less-often-used heating element and an adjacent more-often-used heating element causes a corresponding difference in the density (darkness) levels of the dye transfer effected by the less-often-used heating element and the adjacent more-often-used heating element. As a result, an undesirable printing artifact appears as a white or gray line along the printed 6×8 inch color image. This can make the color image print unacceptable. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the invention, a method of reducing uneven use of the total number of printing elements on a print head in a printer, when selectively using the printing elements to make different size color image prints on respective similar size receiver mediums, comprises: 
     selectively using the total number of printing elements to make color image prints substantially the same size as the receiver mediums; and 
     selectively using a particular number of printing elements less than the total number of printing elements to make similar size color image prints smaller than the receiver mediums, but alternating which ones of the total number of printing elements can be selectively used to make each print so that the placement of each print on a receiver medium is alternated, whereby, since those printing elements that can be selectively used to make each print smaller than a receiver medium are alternated, uneven use of the printing elements is reduced. 
     According to another aspect of the invention an apparatus is provided for accomplishing each of the method steps. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic block diagram of a printer control assembly for a bead of heating elements on a print head in a thermal printer; 
     FIGS. 2-4 are illustrations of alternative placements of a 5×7 inch color image print on a 6×8 inch receiver medium, according to a preferred embodiment of the invention; and 
     FIG. 5 is an illustration of a 6×8 inch color image print on a 6×8 inch receiver sheet. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention is disclosed as being embodied preferably in a thermal printer in which the selective use, i.e. selective heating, of individual heating or resistive elements on a thermal print head effects a color dye transfer from a dye donor medium to a dye receiver medium to create a color image on the dye receiver medium. One example of such a printer is described in the “BACKGROUND OF THE INVENTION” and in prior art U.S. Pat. No. 4,745,413 issued May 17, 1988. The prior art patent is incorporated in the description of the invention which follows. 
     Because the features of a thermal printer are generally known, the description which follows is directed in particular only to those elements forming part of or cooperating directly with the invention. It is to be understood, however, that other elements not disclosed may take various forms known to a person of ordinary skill in the art. 
     Referring now to the drawings, FIG. 1 is a schematic block diagram of a printer control assembly for a bead of 1800 heating or resistor elements H 1 , H 2 , H 3 , H 4 , H 5 , H 6 , . . . , H 1800  arranged in a straight line on a thermal print head  10  in a thermal printer. 
     The printer control assembly is similar in many respects to one shown in incorporated U.S. Pat. No. 4,745,413 and includes: 
     a suitably programmed microcomputer 12; 
     a control interface circuit 14 
     a series of 1800 AND gates A 1 -A 1800 ; 
     a latch register 16 having a series of 1800 latch stages L 1 -L 1800 ; and 
     a shift register  18  having 1800 serial shift stages S 1 -S 1800 . 
     As described in incorporated U.S. Pat. No. 4,745,413, the control interface circuit  14  under the programmed direction of the microcomputer  12  provides an ENABLE signal to the AND gates A 1 -A 1800 , a LATCH signal to the latch register  16 , and IMAGE DATA and CLOCK signals to the shift register  18 . The IMAGE DATA signal is loaded, based on the CLOCK signal, as a serial data stream of binary 1 &#39;s  (highs) and 0 &#39;s  (lows) into the shift register  18  until all of the serial shift register stages S 1 -S 1800  have the image data, i.e. a “1” or a “0” at each one of the shift register stages. When the image data has been completely loaded into the shift register  18 , the LATCH signal causes the image data in each shift register stage S 1 -S 1800  to be latched at the latch stages L 1 -L 1800  in order to temporarily save the image data. The latched data then serves to determine whether each one of the heating elements H 1 -H 1800  in the print head  10  is to be used or not used, i.e. is energized (ON) or not energized (OFF) to be heated or not heated. The ENABLE signal causes the latched data to be gated at the AND gates A 1 -A 1800  to energize or not energize each one of the heating elements H 1 -H 1800 . In other words, a “1” loaded into the shift register stage S 1  and latched at the latch stage L 1  causes the heating element H 1  to be energized (ON) when the AND gate A 1  is enabled. Conversely, a 0″ loaded into the shift register stage S 1  and then latched at the latch stage L 1  permits the heating element H 1  to remain not energized (OFF) when the AND gate A 1  is enabled. This is commonplace in known thermal heaters. See incorporated U.S. Pat. No. 4,745,413. 
     To make a color image print, the respective color dyes in a single series of yellow, magenta and cyan color sections on a dye donor web  20  are successively heat-transferred (e.g. by diffusion), one on top of the other, onto a dye receiver sheet  22  which, as is typical, is white. The dye transfer from each color section to the dye receiver sheet  22  is done one line of pixels at a time across the color section via the bead of 1800 heating elements H 1 -H 1800  on the thermal print head  10 . See FIG.  1 . The heating elements H 1 -H 1800  make line contact across the entire width of the dye donor web  20 , but only those heating elements that are actually used for a particular line are energized to be heated to effect a color dye transfer to the receiver sheet  22  When any one of the heating elements H 1 -H 1800 , is energized, the temperature to which it is heated must be high enough so that the color dye transfer to the receiver sheet  22  causes the corresponding pixel to have the desired density (darkness) level. The temperature of the heating element can be raised to increase the magnitude of the color dye transfer in order to obtain the desired color density level for the corresponding pixel. As described in incorporated U.S. Pat. No. 4,745,413, this can be done by a pulse width modulation or pulse count modulation of the heating element. According to the pulse width modulation mode, a single constant current pulse is applied to the heating element for a variable time, controlled by the ENABLE signal, in order to vary the time the heating element is energized to effect a color dye transfer to the receiver sheet  22 —depending on the desired density level for the corresponding pixel. According to the pulse count modulation mode, a variable number of constant current pulses are applied to the heating element, controlled by the number of times an IMAGE DATA signal is loaded into the shift register  18 , in order to vary the number of times the heating element is energized to effect a color dye transfer to the receiver sheet  22 —depending on the desired density level for the corresponding pixel. If as we assume, as in incorporated U.S. Pat. No. 4,745,413, there are N possible dye density levels, an IMAGE DATA signal is loaded into the shift register  18  the same number of times, so that the heating element can be energized N different times depending on the desired density level for the corresponding pixel. Each time an IMAGE DATA signal is loaded into the shift register  18 , the serial data stream of binary 1 &#39;s  (highs) and 0 &#39;s  (lows) is typically different to vary the density level from pixel to pixel along one line. 
     By way of example, the heating elements H 1 -H 1800  can be selectively used, i.e. selectively heated, to make a 5 (width)×7 (length) inch color image print  24  on a larger 6 (width)×8 (length) inch receiver sheet  22  or to make a 6 (width)×8 length) inch color image print  26  on the 6×8 inch receiver sheet. 
     As shown in FIGS. 2-4, the placement of a 5×7 inch color image print  24  on a 6×8 inch receiver sheet  22  can be alternated or varied according to the invention. In FIG. 2, a 5×7 inch color image print  24  is offset leftward on the 6×8 inch receiver sheet  24  to a first side  28  of the receiver sheet so that a 1 inch (width) non-image (white) margin area  30  is left inwardly adjacent a second side  32  of the receiver medium, i.e. along a first side  34  of the color image print. Alternately, in FIG. 3, a 5×7 inch color image print  24  is offset rightward on the 6×8 inch receiver sheet  24  to the second side  32  of the receiver sheet so that a 1 inch (width) non-image (white) margin area  30  is left inwardly adjacent the first side  28  of the receiver medium, i.e. along a second side  36  of the color image print. Alternately, in FIG. 4, a 5×7 inch color image print  24  is centered on the 6×8 inch receiver sheet  22  between the first and second sides  28  and  32  of the receiver sheet so that separate 0.5 inch (width) non-image (white) margin areas  38  are left inwardly adjacent the first and second sides of the receiver medium, i.e. along the first and second sides  34  and  36  of the color image print. Each non-image margin area  30  or  38  along the first and/or second sides  34  and  36  of a 5×7 inch color image print  24  can be manually or automatically trimmed or cropped from the receiver medium (although trimming is not mandatory) using known trimming or cutting means. 
     On the other hand, when a 6×8 inch color image print  26  is made on the 6×8 inch receiver sheet  22 , as in FIG. 5, no non-image margin area is created on the receiver sheet. Thus, the 6×8 inch color image print  26  on the 6×8 inch receiver sheet  22  is a borderless print. 
     To achieve the alternate placement of a 5×7 inch color image print  24  on a 6×8 inch receiver sheet  22  as in FIGS. 2-4 (as compared with making a 6×8 inch color image print  26  on the 6×8 inch receiver sheet  22  as in FIG. 5) the print-making methodology is as follows, using a pulse count modulation mode. 
     To place a 5×7 inch color image print  24  on a 6×8 inch receiver sheet  22  as in FIG. 2, digital image data in the form of binary 1 &#39;s  and 0 &#39;s  is inputted from an image data source, such as a work station, into the microcomputer  12 . The microcomputer  12 , in turn, formulates and processes the digital image data to assemble it in a memory as respective sets or pages of yellow, magenta and cyan image data for the three color dyes in a single series of yellow, magenta and cyan color sections on the dye donor web  20 . Within each data set, the image data is stored line-by-line as binary 1 &#39;s  and 0 &#39;s  to be used one line at a time to cause the corresponding color dye to be successively heat-transferred by the heating elements H 1 -H 1800  onto the receiver sheet  22 . When one line of the yellow image data is transferred to the control interface circuit  14 , the interface outputs a first IMAGE DATA signal to be loaded into the shift register  18  as a serial data stream of binary 1 &#39;s  and 0 &#39;s until all of the serial shift register stages S 1 -S 1800  have the image data, i.e. a “1” or a “0” at each one of the shift register stages. The heating elements H 1 -H 1800 , in turn, are individually energized or not energized, to be heated or not heated. This is done again, successively, with N minus 1 IMAGE DATA signals, each IMAGE DATA signal representing a further stream of binary 1 &#39;s  and 0 &#39;s , to vary the number of times a heating element is energized, in order to print one line of yellow dye image content as pixels at varying desired density levels on the receiver sheet  22 . Once all of the lines of yellow dye image content are printed on the receiver sheet  12 , the sequence is repeated line-by-line to print all of the lines of magenta dye image content and then to print all of the lines of cyan dye image content on the receiver sheet  12  (in the same area, i.e. superimposed). Each time an IMAGE DATA signal is loaded into the shift register  18  as a serial data stream of binary 1 &#39;s  and 0 &#39;s , the shift register stages S 1500 -S 1800  receive a “0”. The remaining shift register stages S 1 -S 1499  receive a combination of “1” &#39;s  and “O” &#39;s . As a result, the heating elements H 1500 -H 1800 , i.e. the ones closest to a first end  40  of the line of the heating elements H 1 -H  1800 , are not to be selectively used, i.e. they cannot be selectively energized or not energized to be heated or not heated. Instead, they all remain not energized during the yellow, magenta and cyan dye transfers to the receiver sheet  22 . The remaining heating elements H 1 -H 1499 , including the heating elements H 1 -H 300 , i.e. the ones closest to a second end  42  of the line of the heating elements, are selectively used, i.e. they can be selectively energized or not energized to be heated or not heated. Thus, as in FIG. 2, the 5×7 inch color image print  24  is offset leftward on the 6×8 inch receiver sheet  22  to the first side  28  of the receiver sheet so that a 1 inch (width) non-image (white) margin area  30  is left inwardly adjacent the second side  32  of the receiver sheet, i.e. along the first side  34  of the color image print. 
     To place a 5×7 inch color image print  24  on a 6×8 inch receiver sheet  22  as in FIG. 3, the steps are the same as for FIG. 2, except that each time an IMAGE DATA signal is loaded into the shift register  18  as a serial data stream of binary 1 &#39;s  and 0 &#39;s , the shift register stages S 1 -S 300  (instead of S 1500 -S 1800 ) receive a “0”. The remaining shift register stages S 301 -S 1800  receive a combination of “1” &#39;s  and “0” 3 s . As a result, the heating elements H 1 -H 300 , i.e. the ones closest to the second end  42  of the line of the heating elements H 1 -H 1800 , are not to be selectively used, i.e. they cannot be selectively energized or not energized to be heated or not heated. Instead, they all remain not energized during the yellow, magenta and cyan dye transfers to the receiver sheet  22 . The remaining heating elements H 301 -H 1800 , including the heating elements H 1500 -H 1800 , i.e. the ones closest to the first end  40  of the line of the heating elements, are selectively used, i.e. they can be selectively energized or not energized to be heated or not heated. Thus, as in FIG. 3, the 5×7 inch color image print  24  is offset rightward on the 6×8 inch receiver sheet  22  to the second side  32  of the receiver sheet so that a 1 inch (width) non-image (white) margin area  30  is left inwardly adjacent the first side  28  of the receiver sheet, i.e. along the second side  36  of the color image print. 
     To place a 5×7 inch color image print  24  on a 6×8 inch receiver sheet  22  as in FIG. 4, the steps are the same as for FIG. 2, except that each time an IMAGE DATA signal is loaded into the shift register  18  as a serial data stream of binary 1 &#39;s  and 0 &#39;s , both the shift register stages S 1 -S 150  and S 1650 -S 1800 receive a “0”. The remaining shift register stages S 151 -S 1649  receive a combination of“1” &#39;s  and “0” &#39;s . As a result, the heating elements H 1 -H 150  and H 1650 -H 1800 , are not to be selectively used, i.e. they cannot be selectively energized or not energized to be heated or not heated. Instead, they all remain not energized during the yellow, magenta and cyan dye transfers to the receiver sheet  22 . The remaining heating elements H 151 -H 1649  are selectively used, i.e. they can be selectively energized or not energized to be heated or not heated. Thus, as in FIG. 4, the 5×7 inch color image print  24  is centered on the 6×8 inch receiver sheet  22  between the first and second sides  28  and  32  of the receiver sheet so that separate 0.5 inch (width) non-image (white) margin areas  38  are left inwardly adjacent the first and second sides of the receiver sheet, i.e. along the first and second sides  34  and  36  of the color image print. 
     The microcomputer  12  is programmed, using known programming techniques, to automatically alternate the placement of each 5×7 inch color image print  24  on a receiver sheet  22  as in FIGS. 2-4. In other words, the microcomputer  12  is programmed to alternate which of the shift register stages S 1500 -S 1800 , S 1 -S 300 , or S 1 -S 150  and S 1650 -S 1800  receive a “0” so that the heating elements H 1500 -H 1800 , H 1 -H 300 , or H 1 -H 150  and H 1650 -H 1800  cannot be selectively used, i.e. they cannot be selectively energized or not energized to be heated or not heated. 
     When a 6×8 inch color image print  26  is made on the 6×8 inch receiver sheet  22 , as in FIG. 5, the steps are the same as for FIG. 2, except that each time an IMAGE DATA signal is loaded into the shift register  18  as a serial data stream of binary 1 &#39;s  and 0 &#39;s , the shift register stages S 1 -S 1800  receive a combination of “1” &#39;s  and “0” &#39;s . As a result, all 1800 of the heating elements H 1 -H 1800  (as compared to 1500 for FIGS. 2-4) are selectively used, i.e. they can be selectively energized or not energized to be heated or not heated. Thus, as in FIG. 5, no non-image margin area is created on the receiver sheet  22 . Instead, the color image print  26  is borderless. 
     The invention has been described in detail with particular reference to a preferred embodiment thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention. 
     For example any number of different size color image prints, besides 5×7 inch and 6×8 inch color image prints  24  as in FIGS. 2-5, which are smaller than the receiver medium  22  can be made according to the invention. 
     Also, all of the heating elements H 1 -H 1800 , can be initially energized to be heated, but in this instance they are all heated below the respective dye transfer thresholds for the yellow, magenta and cyan dye transfers onto the receiver sheet  22 . Then, selected ones of the heating elements are further energized to be heated sufficiently to cause the color dyes to be successively heat-transferred onto the receiver sheet  22 . 
     Also, when there is a transparent laminating section (after the cyan color section) included in each single series of yellow, magenta and cyan color sections on the dye donor web  20 , the transparent laminating section can be deposited on the 5×7 inch color image print  24  or the 6×8 inch color image print  26 . Preferably, the transparent laminating section is always deposited on the 6×8 receiver sheet  22  from its first side  28  to its second side  32 . Alternatively, when making the 5×7 inch color image print  24 , the transparent laminating section can be deposited only on the color image print (rather than on the 6×8 receiver sheet  22  from its first side  28  to its second side  32 ). 
     
       
         
               
             
               
               
             
           
               
                   
               
               
                 PARTS LIST 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 10. 
                 print head 
               
               
                 H 1 , H 2 , H 3 , H 4 , H 5 , H 6 , . . . , H 1800.   
                 heating elements 
               
               
                 12. 
                 microcomputer 
               
               
                 14. 
                 control interface circuit 
               
               
                 A 1 -A 1800 . 
                 AND gates 
               
               
                 16. 
                 latch register 
               
               
                 L 1 -L 1800 . 
                 latch stages 
               
               
                 18. 
                 shift register 
               
               
                 S 1 -S 1800 . 
                 serial shift stages 
               
               
                 20. 
                 dye donor web 
               
               
                 22. 
                 dye receiver sheet 
               
               
                 24. 
                 color image print 
               
               
                 26. 
                 color image print 
               
               
                 28. 
                 first side of receiver sheet 
               
               
                 30. 
                 non-image margin area 
               
               
                 32. 
                 second side of receiver sheet 
               
               
                 34. 
                 first side of color image print 
               
               
                 36. 
                 second side of color image print 
               
               
                 38. 
                 non-image margin area 
               
               
                 40. 
                 first end of the line of the heating 
               
               
                   
                 elements 
               
               
                 42. 
                 second end of the line of the 
               
               
                   
                 heating elements