GB2286563A

GB2286563A - Optimising printing speed against grey scale density in printers with line print heads

Info

Publication number: GB2286563A
Application number: GB9501373A
Authority: GB
Inventors: David Edward Coons
Original assignee: Eastman Kodak Co
Current assignee: Eastman Kodak Co
Priority date: 1994-01-26
Filing date: 1995-01-24
Publication date: 1995-08-23
Anticipated expiration: 2015-01-24
Also published as: GB9501373D0; DE19501795B4; DE19501795A1; GB2286563B

Description

4L 2286563 PRINT SPEED OPTIMIZATION IN COLOR THERMAL PRINTERS

BACKGROUND OF THE INVENTION

Field of the inventicn t The present invention relates to the field of thermal printing. More particularly, it relates to improvements in thermal printers of the type adapted to produce continuous tone images on a line by-line basis.

2. Background Art:

Thermal printers of the above-mentioned type are well known. Typically, they comprise a cylindrical platen or drum for advancing a print receiver medium and a dye-bearing donor medium past a thermal print head. Commonly, the receiver medium is plain-or coated paper sheets, while the dye donor medium is in continuous web form. The dye donor medium is acted upon by the print head to transfer dye to the receiver medium.

The print head usually comp rises a linear array of closely spaced printing elements, and spans the transfer drum at the print zone. Each heating element can be an electrical resistor, selectively energized to raise the temperature of the heating element to the level required to cause dye to transfer to the receiver. Alternatively, the printing elements may comprise a linear array of electrically conductive elements which cooperate with a "resistor" donor web to thermally print image information. Such a print head is disclosed in U.S. Patent No. 4,800,399.

In either case, by selectively addressing the printing elements in the print head array, an entire line of image information is printed at once.

To produce continuous-tone images in which each 4W picture element (referred to as upixelll) of an image line exhibits any one of a multitude of different density levels, the duration of the current applied to each printing element is varied, either by.11 varying the width of the current pulse or by varying the total number of current pulses within the time interval allotted to print an image line. A full color image is formed by repeating the entire printing cycle for several color patches, or ucolor planes," included on the dye donor medium. The number of color planes used is typically three or four, depending on the printer.

The amount of dye transferred from the dye donor medium to the receiver medium by this process is a function of temperature, pressure, media efficiency, and the duration of time over which the temperature and pressure are applied.

In continuous-tone thermal printers, it is common to continuously advance the receiver medium relative to the head during the line printing operation. Such movement enhances the print quality by blending the pixels of adjacent image lines together. Moreover, it prevents sticking of the heating elements to the donor web. Continuous movement of the receiver media is usually effected by a precision stepper motor which advances the print drum at a substantially constant rate which is sufficiently slow to allow each pixel in a line to receive the maximum gray level density (e.g. 256 levels of gray).

In order to achieve an acceptably high print density, the printing line rate in color thermal printers must be rather slow in comparison to other printers, such as laser printers, used to - AV A produce mainly black and white text or simple binary graphics.

The line rate is usually fixed, and is chosen to accommodate the possibility that high., density will be called for somewhere in the image. As may be appreciated, the requirement to move the receiver medium at a rate no faster than required to produce a maximum density image on each line has a limiting effect on the rate at which prints can be produced.

In many cases, however, depending on the image being printed, none or very little dye is required from one or more of the available color planes. In these cases, the printing line rate is actually slower than is necessary to produce the desired image. That is, a faster line rate could be used to make the print while still accommodating the density requirements of the image data. Commonly assigned U.S. Patent 20 No. 4,990,930, which issued to Christopher A. Ludden and David A. Johnson on February 5, 1991, addresses the]roblem of increasing the productivity of a thermal printer. The patent discloses a method of accelerating the media transport to the start of the next print line as soon as the maximumdensity pixel has been printed in the present print line. While the system is effective for optimizing the speed of the printer, it results in an acceleration and deceleration cycle for substantially every print line. It would be desirable to provide a system for increasing the productivity of a thermal printer while maintaining a substantially constant media speed, as this would tend to enhance registration accuracy and decreases wear on the media-advance mechanism of the printer.

i i AW 4 - Another attempt at increasing the productivity of a printer is disclosed in U.S. Patent No. 5-,210,547, which issued to K. Watanabe et al. on May 11, 1993. in Watanabe et al., information relating to the size of a receiver medium sheet and the size and position of the image to be printed on the sheet are supplied as input data to a printer control circuit from an external apparatus. The media transport speed is increased at any blank regions of the image, whereby the time taken for printing is reduced. While this method would increase the efficiency of a printer, its application is restricted to any non-image region of the receiver medium sheet. Thus, its overall effect on the productivity of the printer would be limited.

SUMMARY OF THE INVENTION

In light of the foregoing discussion, an object of this invention is to increase the printmaking rate of high-resolution, continuous-tone thermal printers of the above-mentioned type without substantially sacrificing print quality.

According to one feature of the present invention, this and other objects are accomplished by changing the printing rate for each color plane to accommodate the highest required-density in that color plane. In this manner, the shortest duration of time is spent by the operator waiting for a print to be produced.

In a preferred embodiment of the present invention, a process for achieving fa"ster print times includes analyzing the image data prior to printing a color plane, making calculations to determine the optimal line rate value needed for that color plane. When printing is initiated, the line rate for the entire color plane is set to this optimal value.

Such data analysis might include determining the highest density requirement any,lere in the color plane. Having determined this value, a computation or table look-up is used to determine the best line rate to use.

In accordance with one feature of the present invention, a thermal printer having a printing cycle adapted to produce continuous tone images on a line-by- line basis (1) by imparting relative motion between a receiver medium and a print head having selectively-addressable printing elements to substantially simultaneously print an entire line of associated pixels at a printing line rate and (2).by selectively addressing the printing elements for variable time durations depending on the desired density levels of an associated pixel; is provided with drive control apparatus for setting the printing line rate to be substantially that rate required to just accommodate the maximum density of the image.

In accordance with another feature of the present invention, the thermal printer further includes a drive.(1) for rapidly advancing receiver medium through the print station until a region of the receiver medium that is to receive the first image line to be printed is aligned with the print head and (2) for thereupon slowing the receiver medium to the determined line rate.

In accordance with yet another feature of the present invention, the thermal printer further includes a drive for rapidly advancing print receiver medium through the print station after the or 6 - last image line to be printed passes alignment with the print head.

In accordance with still another feature of the present invention, a full color image is, formed by repeating the entire printing cycle for several color planes; and by adjusting the printing rate between color plane printing cycles to just accommodate the highest required density in each color plane.

Analysis of the image data could occur in one of several points in the image chain. The analysis might be done by software in a computer which drives the printer, such as in the computer's print driver. The computer would communicate the optimization information to the printer before initiating a color plane print. Alternatively, the printer itself could analyze the image data after it is received from the host computer. In some printers, this could be done by a software stage 20 known as the "Raster Image Processor" (RIP).

The invention and its various objects and advantages will become more apparent to those skilled in the art from the ensuing detailed description of the preferred embodiments presented below, the accompanying drawings, and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a schematic of a thermal printer which can be employed to make color images in a dye 30 receiver medium in accordance with this invention; Figure 2 is a schematic perspective of several heating elements used in the print head of the printer of Figure 1; Figure 3 shows a portion of a typical dye donor medium; and - jp 7 Figures 4a, 4b and 4c are logic flow charts showing the operation of the print speed optimization techniques according to a preferred embodiment of the present invention. A BEST MODE FOR CARRYING OUT THE INVENTION

Referring to Figure 1, a thermal printer 10 according to a preferred embodiment of the present invention includes a cover mechanism 12 having several major components attached to it. These components include a head positioning arm 14, a print head assembly 16 and dye donor medium supply and take-up spools 18 and 20, respectively. A main printer support structure 22 includes a roller platen assembly 24, a dye receiver medium transport mechanism 26 and a dye receiver medium supply 28. Thermal printer 10 is shown as it operates, with cover mechanism 12 in a closed position.

The cover mechanism is mounted to main printer support structure 22 on a cover mechanism mounting shaft 32. When a latch 34 is released, cover mechanism 12 can rotate to an open position. Head positioning arm 14 is mounted to cover mechanism 12 on a head positioning arm mounting shaft 36.

Normal thermal printer operations include loading dye receiver medium, printing information upon the dye receiver medium and ejecting the finished print. Each of these operations is fully described in coassigned U.S. Patent No. 5,176,458, which issued to H.G. Wirth on January 5, 1993. The disclosure of that patent is hereby incorporated into this specification by reference, and therefore only a brief description will be herein given of the illustrated embodiment of the thermal printer.

Printer operation begins with a loading phase, in which print head assembly 16 moves to a 0 8 - loading position, a sheet of dye receiver medium advances to a printing location, and print head assembly 16 is positioned in preparation for the printing operation. Priint head assembly 16 andhead positioning arm 14 begin in their ejecting positions, which are at the highest point of travel of head positioning arm 14's range of motion. A first cam 38 is driven to a loading position which is between the ejecting and the printing positions. A first cam follower 40 attached to head positioning arm 14 maintains contact with first cam 38, and head positioning arm 14 rotates about its mounting shaft 36 to a loading position.

A pin receiving recess 42 located on head positioning arm 14 constrains a first pin 44 of print head assembly 16 to movement along one axis, which in the case of this embodiment is the vertical axis. In the loading position, first pin 44 contacts a side of pin receiving recess 42 and an adjustment screw, not shown, for setting the point at which head positioning arm 14 contacts first pin 44 as arm 14 moves from the print position to other positions, and for modifying the point in the head positioning arm,s motion at which a second cam follower 46 contacts a second cam surface 47.

As head positioning arm 14 moves to a loading position, print head assembly 16 moves to its loading position. Second cam follower 46 and cam surface 47 allow print head assembly 16 to tilt toward roller platen assembly 24, until a guide 48 contacts platen assembly 24. When print head assembly reaches its loading position, the print head assembly 16 is spaced away from platen assembly 24.

A sheet of dye receiver medium 50 moves forward into a dye receiver medium guide 52, where it follows a curved path toward a gap between print head assembly 16 and platen assembly 24. As the dye receiver medium moves into this gap, it contacts a dye donor medium 54 and is guided toward dye receiver medium transport mechanism 26 and associated pinch roller. While this embodiment describes dye receiver medium in sheet form, dye receiver medium supplied in roll form could also be utilized. Once dye receiver medium 50 is firmly held by dye receiver medium transport mechanism 26, print head assembly 16 moves toward platen assembly 24, pressing dye donor medium 54 and dye receiver medium 50 against platen assembly 24 to form a sandwich for thermal printing.

When the loading phase is completed, printer 10 enters a printing phase, during which print head assembly 16 presses dye donor medium 54 and the dye receiver medium into platen assembly 24, and prints information on the dye receiver medium.

When the printing phase is completed, printer 10 enters an ejecting phase, during which the print head assembly is retracted from the platen assembly and the finished print is ejected from the printer. When print head assembly 16 is in the ejecting position, dye receiver medium transport mechanism 26 captures the dye receiver medium between it and a pinch roller 55 to drive the completed print out of thermal printer 10. When the ejecting phase of the printer operation is finished, the printer is ready to begin another printing operation.

Referring to Figure 2, the print head of print head assembly 16 includes a plurality of heating elements 56, such as electrical resistors, which are pressed against dye donor medium 54 to force the dye donor medium against dye receiver medium. When one of a plurality of switches 58 is closed, the associated - heating element 56 is connected to a voltage potential source VS" The amount of dye transferred is a function of the time period that switch 58 is closed.

Dye donor medium 54 comprises a leaden portion followed by a repeating series of dye frames. The dye frames may be contiguous as shown or spaced by interframe regions, and, as shown in Figure 3, each series includes in sequence yellow, magenta, and cyan dye frames. A single series is of course used to print one color plane on dye receiver medium 28.

In this disclosure, the term "dye,, refers to a colored material which transfers from the dye donor medium to a dye receiver medium in response to energy applied by individual elements of the print head.

Although the print head is shown as having electrically resistive heating elements 56, those skilled-in the art will understand that other sources of energy such as, diode laser array and individual lasers have been and can be effectively used in accordance with this invention. After a color plane is formed on the dye receiver medium, the dye receiver medium will be referred to as a print.

As shown, there are two LEDs 60 and 62 which illuminate the dye donor medium from above. LED 60 emits yellow light and LED 62 emits-red light. Two photodetectors 'W' and "B" are disposed below the dye donor medium and receive light which passes through the dye donor medium. Photodetectors 'W' and "B" provide a signal for identifying the start of series and each individual color dye frame in such series. For a more complete discussion of this identification, reference is made to commonly assigned U.S. Patent No. 4,710,781 to S. Stephenson, the disclosure of which is incorporated by reference herein. It will be understood to those skilled in the art that other types

1 of well known apparatus can be used to identify the start of each series of colored dye frames. See for example U.S. Patent No. 4,893,951.

Referring l,-..o Figures 4a, 4b and 4c, a preferredprocess according to the present invention starts with the generation of image data at 70 such as by means of a scanner, a computer drawing program, an electronic camera, an imager, etc. Using a computer, an operator may modify, edit or combine image data as desired to produce the image to be printed.

The computer will break down the print data into color planes to match the color sequence expected by the printer, such as for example, yellow, magenta, and cyan; or yellow, magenta, cyan, and black (logic function block 72). When a color plane of data is sent to the printer, various counters and flags are initialized to zero; including ones for the FIRST LINE number, the LAST LINE number, maximum density "DMAXii, and the present line number (logic function block 74). At 76, the preseht line count is incremented. At logic function block 78, the counter for the present pixel I'PIXELn11 and a PIXEL USED flag are reset to ZERO and NO, respectively.

At logic function block 80, the pixel count is incremented, and at logic decision block 82 a decision is made concerning whether PIXELn is to be printed. If PIXELn has a non-zero density value, the PIXEL USED flag is set YES at logic function block 84, and the density of PIXELn is compared to the value DMAX at logic decision block 86. If PIXELn is greater than DMAX, the value of DMAX is updated to the density of PIXELn at logic function block 88.

12 - Referring to Figures 4a, 4b, and 4c, if the present line is the first line which contains a non-zero density pixel, the first line flag is updated to the present 'ine number at logic bloc ks 90 and 92. Once the first line flag has been updated, it will not be changed for the rest of the color plane.

If there are more pixels to be examined in the presen t print line as determined by logic decision block 94, the process is repeated until the last pixel of the line has been examined. once the last pixel of a line has been examined, that line is designed as the LAST LINE if any pixel in the line has a non- zero density value; logic blocks 96 and 98.

If the present print line is not the last line of the color plane as determined by logic decision block 100, the process is repeated for the next print line. Otherwise, the color plane is ready for printing.

The fastest printing line rate that can be used for the color plane is computed using the DMAX value, as this represents the pixel which requires the longest duration energy pulse. Color correction tables for thermal printers are commonly generated assuming a particular line rate. Accordingly, new table values are preferably generated for the line rate determined in accordance with DakX.

When a sheet of receiver medium is sent to the print station to receive a color plane image, the sheet can be rapidly advanced through the print station until the region of the sheet that is to receive the image corresponding to the flagged FIRST LINE is aligned with the print head. Now the sheet 1 13 is slowed to the computed fastest printing line rate, and printing of the color plane begins.

When the flagged LAST LINE is reached and printed, the receiver medium sheet is either A:

reversed to prepare for the next color plane; or, if this is the last color plane, the sheet is ejected as a finished print. Note that the time normally taken to advance the receiver sheet from the last line to be printed to the trailing edge of the sheet at the slow print line rate has been saved; greatly increasing the productivity of a printer.

The invention has been described in detail with particular reference to a certain preferred embodiment thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.

14

Claims

I claim:

1. A thermal print-er having a printing cycle adapted to produce continuous tone images.,on a line-by-line basis by imparting relative motion between a receiver medium and a print head having selectivelyaddressable printing elements to substantially simultaneously print an entire line of associated pixels at a printing line rate, and by selectively addressing the printing elements for variable time durations depending on the desired density levels of an associated pixel; characterized by drive control apparatus for setting the printing line rate to be substantially that rate required to just accommodate the maximum density of the image.

2. A thermal printer as defined in Claim 1, further characterized by means for rapidly advancing receiver medium through the print station until a region of the receiver medium that is to receive the first image line to be printed is aligrfed with the print head, and for thereupon slowing the receiver medium to the determined line rate.

3. A thermal printer as defined in Claim 2, further characterized by means for rapidly advancing print receiver medium through the print station after the last image line to be printed passes alignment with the print head.

4. A thermal printer as defined in Claim 1, wherein the duration of the address time for the printing elements is varied by changing the 1 - width of an electrical pulse to the printing elements.

5. A thermal printer as defined in, Claim 1, wherein the duration of the address time for the printing elements is varied by changing the width of an electrical pulse to the printing elements.

6. A thermal printer as defined in Claim 1, wherein: a full color image is formed by repeating the entire printing cycle for several color planes; and the printing rate is adjustable between color plane printing cycles to just accommodate the highest required density in each color plane.

7. A thermal printer as defined in Claim 6, further characterized by means for analyzing image data prior to printing a color plane, calculating the optimal line rate needed for thatcolor plane, and setting the printing line rate for the entire color plane in accordance with the calculated optimal line rate.

8. A thermal printer as defined in Claim 6, further characterized by: means for rapidly advancing receiver medium through the print station until a region of the receiver medium that is to receive the first image line to be printed is aligned with the print head, and for thereupon slowing the receiver medium to the determined line rate; and means for rapidly reversing receiver medium through the print station after the last image line 4 16 - 1 of a color plane to be printed passes alignment with the print head, whereby the next color plane may be received without first requiring the full receiver medium to pass the print head.

9. A process for producing continuous tone images on a line-by-line basis; said process comprising the steps of imparting relative motion between a receiver medium and a print head having selectively-addressable printing elements to substantially simultaneously print an entire line of associated pixels at a printing line rate, and selectively addressing the printing elements for variable time durations depending on the desired density levels of an associated pixel; and characterized by setting the printing line rate to be substantially that rate required to just accommodate the maximum density of the image.

10. A process for producing continuous tone images on a line-by-line basis as defined in Claim 9, further characterized by:

rapidly advancing receiver medium through the print station until a region of the receiver medium that is to receive the first image line to be printed is aligned with the print head; and thereupon slowing the receiver medium to the determined line rate.

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