EP1582362B1 - Thermisches Druckverfahren - Google Patents

Thermisches Druckverfahren Download PDF

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Publication number
EP1582362B1
EP1582362B1 EP20040101372 EP04101372A EP1582362B1 EP 1582362 B1 EP1582362 B1 EP 1582362B1 EP 20040101372 EP20040101372 EP 20040101372 EP 04101372 A EP04101372 A EP 04101372A EP 1582362 B1 EP1582362 B1 EP 1582362B1
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EP
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Prior art keywords
values
function
mtw
parameters
thermal
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Expired - Fee Related
Application number
EP20040101372
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English (en)
French (fr)
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EP1582362A1 (de
Inventor
Bruno c/o AGFA-GEVAERT Van Uffel
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Agfa HealthCare NV
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Agfa HealthCare NV
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Priority to DE200460010062 priority Critical patent/DE602004010062T2/de
Priority to EP20040101372 priority patent/EP1582362B1/de
Priority to US11/078,092 priority patent/US7190385B2/en
Priority to JP2005086405A priority patent/JP2005289060A/ja
Publication of EP1582362A1 publication Critical patent/EP1582362A1/de
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    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B41—PRINTING; LINING MACHINES; TYPEWRITERS; STAMPS
    • B41J—TYPEWRITERS; SELECTIVE PRINTING MECHANISMS, i.e. MECHANISMS PRINTING OTHERWISE THAN FROM A FORME; CORRECTION OF TYPOGRAPHICAL ERRORS
    • B41J2/00—Typewriters or selective printing mechanisms characterised by the printing or marking process for which they are designed
    • B41J2/315—Typewriters or selective printing mechanisms characterised by the printing or marking process for which they are designed characterised by selective application of heat to a heat sensitive printing or impression-transfer material
    • B41J2/32—Typewriters or selective printing mechanisms characterised by the printing or marking process for which they are designed characterised by selective application of heat to a heat sensitive printing or impression-transfer material using thermal heads
    • B41J2/35—Typewriters or selective printing mechanisms characterised by the printing or marking process for which they are designed characterised by selective application of heat to a heat sensitive printing or impression-transfer material using thermal heads providing current or voltage to the thermal head
    • B41J2/355—Control circuits for heating-element selection
    • B41J2/36—Print density control
    • B41J2/365—Print density control by compensation for variation in temperature

Definitions

  • the present invention relates to a thermal printing method, in particular to a method of generating a sharpened image by means of a thermal printer.
  • the printing process is typically divided into print cycles whereby in each print cycle the heating elements receive different amounts of energy appropriate to cause the wanted densities on the part of the media that is in contact with the thermal head during that cycle.
  • the thermally sensitive material maybe composed of a donor sheet and an acceptor sheet as in the diffusion transfer process or maybe film or sheet of paper that is thermally sensitive by itself.
  • the present invention relates in particular to a printing method to be used in a thermal printer intended for applications that require high image quality such as printers for medical diagnosis.
  • images are supplied by a host computer, hereafter called “host-images” that have digital values directly or indirectly corresponding to the wanted density of a certain area on the printed media. This area is called a “host-pixel” further on.
  • host-pixel In case of a colour printer, each value corresponds directly or indirectly to the density of one specific colorant of the colour image.
  • host-image For each "host-pixel” and for each colour printed, one value is required.
  • the host-image is typically organised in rows and columns, whereby in one specific case each column corresponds to one heating element of the head and each row corresponds to one print cycle of the printing process.
  • the source image has to be transformed into a new image with a different number of values so that each value corresponds to a heater element in a specific print cycle. This transformation is well known as "interpolation”.
  • the resulting pixels of the printer-image are not directly corresponding to the densities wanted on the media, there is usually a translation mechanism that translates the "user" meaning of the values into a value that corresponds to the wanted density effect on the media (see figure 1). Because the media can only be influenced by temperature of the heating elements, the density values must be translated into a temperature value for the heating elements. This is done using the sensitometry information of the media. This sensitometry information is obtained usually during a media calibration before using such a printer. Most printers have automatic means to perform a media calibration when entering new media into the printer.
  • the image is available as "wanted temperature” for each heating element during each heating cycle.
  • the only driving parameter the printer has available is the power it injects into each heating element during each heating cycle.
  • the amount of power that a printer needs to reach the wanted temperature for a certain density is highly dependent on different temperatures in the system. Therefore, in state of the art printers each printing cycle a line of the wanted temperature image is fed into a 'thermal model' along with measured temperatures and a state variable to calculate a line of power values which then are used to drive the heating elements.
  • the State variable is a set of values that assist the thermal model to compensate for the temperature rise in the thermal head and the lateral heat distribution (see figure 2).
  • An example of such a thermal model can be found in European patent application 671 276 .
  • This thermal model has to run real-time and should therefore not be complicated, especially if it runs on a computer that has several other tasks to perform.
  • thermal printers would render very poor image quality and be unstable in density reproduction.
  • state of the art printers are capable of producing images that are acceptable for medical application.
  • thermal printers Although the quality of thermal printers is accepted, images of wet laser printers compared to state of the art thermal printers are still easier to read.
  • a density profile corresponding with a step function is printed, both on a state of the art thermal printer and on a laser printer and then the printed result is measured using a scanning micro densito meter.
  • curve A represents the wanted density for a transition from low to high density in transport direction
  • curve B is the density curve obtained by means of a typical wet laser printer
  • curve C is the resulting density curve of a typical state of the art thermal printer.
  • Curve C is the result of the printer behaviour combined with the sensitometric transformation characteristic of the recording material.
  • Curve B of the laser printer has a typical shape due to the Gaussian beam distribution of the laser spot used to write the image. Already at a distance equal to a few times the spotsize of the laser from the edge, the density profile has reached its final value. Curve C pertaining to the thermal printer clearly shows that at the first 2mm after increasing the wanted density, the thermal model is not capable to predict the needed amount of power correctly.
  • Figure 5 pertains to the direction parallel with the thermal head.
  • Curve A represents the wanted density for a transition from low to high density in a direction parallel with the thermal head.
  • Curve B is the resulting density curve of a typical wet laser printer. It is basically the same as the laser curve in the transport direction.
  • Curve C is the resulting density curve of a typical state of the art thermal printer. Curve C pertaining to the thermal printer clearly shows that in the first 1 mm distance from the edge of the low to high density transition, the thermal model is not capable to predict the needed amount of power correctly.
  • a way of solving this problem could be to simulate the heat distribution in the head using a Finite Element Model that is capable of predicting the temperatures in the neighbourhood of the heating elements very accurately and therefore also enables a computer system to calculate the required power to heat the elements to a desired value to any degree of accuracy.
  • a FEM model is capable of performing the task, a very expensive computer would be needed to run it at sufficient speed to serve as part of the thermal model in the printer.
  • a module is developed which can be placed between the "wanted temperature image” and the “thermal model” of the state of the art printer as shown in figure 2 and that improves the sharpness of the images without requiring expensive hardware upgrades of the printer.
  • TML Temporal Modification Layer
  • the "wanted temperature image” can be written as a matrix of digital values Mtw whereby each row Ktw of the matrix represents the “wanted temperature” values for a given print cycle and the columns Ttw of the matrix represent the "wanted temperature” values for a given heating element of the head for all print cycles of the image to be printed.
  • a print cycle is a period of time corresponding to an area on the recording medium during which wanted densities along the print head have a certain value.
  • a function F(Ktw,b,c,%) is defined that accepts a number of fixed parameters b,c ... and an array of values Ktw, representing the wanted temperatures for one cycle of the printing process.
  • the function F(Ktw,b,c,...) returns a vector Ft of the same size as Ktw.
  • the function F can be any function but is preferably a simple one.
  • Parameter 'b' controls the dynamic behaviour of the function and parameter 'c' controls the output amplitude of the function.
  • additional parameters further controlling the function behaviour may be available.
  • An example of such a function is a function that creates a new vector out of Ktw whereby each element of the new vector is the sum of b elements of Ktw, multiplied by c/b.
  • This 'matching function' has the important property that it does not alter the steady state values of Ktw and that it has a shape which to a certain extent simulates the behaviour of the printer when applying a vector Ktw during a printcycle.
  • the parameters b,c,.. of function F () are set so that Fm () combined with the sensitometry of the recording material optimally simulates the (thermal) behaviour of the printer.
  • a test pattern is printed for each print cycle using a vector KTW_test (which represents a test pattern), for example an image corresponding with a step function (a step-wise transition from a low value to a high value) evolving in the direction of the thermal head is printed using the printer with TML switched off.
  • the step function must be such that both low value and high value produce a measurable density on the media.
  • the printed film-sheet is accurately measured, and a step function response measured in densities is obtained.
  • This step function response in densities is transformed into values proportional to wanted heating element temperatures using sensitometry information of the recording material that is used.
  • the vector of these values is indicated with symbol Ktm.
  • Ktw_test a .
  • the matching function is a prediction of the printer's behaviour.
  • Ktwc 1 / aopt * Ktw - F Ktw ⁇ bopt ⁇ copt ...
  • formula (2) does not change overall densities and that the module can be inserted without changing the density stability of the system. Furthermore, because the behaviour of the changing function is matched to the measured behaviour of the printer, the changed temperature vector Ktwc will result in a more accurately printed density pattern and therefore a sharper image.
  • the result of the above described TML process is input of the thermal model of the printer and adapted driving power values for each of the elements of thermal head are generated.
  • a function F(Ttw,b,c,%) is defined that accepts a number of fixed parameters b,c... and a vector of values Ttw, representing the wanted temperatures for a given heating element for all printing cycles of the printing process.
  • the function F () returns a vector Ft of the same size as Ttw. It can be any function but preferably a simple one. Parameter 'b' controls the dynamic behaviour of the function and parameter 'c' controls the output amplitude of the function. Also additional parameters further controlling the function behaviour may be available.
  • next steps are similar to the embodiment that provided a improved temperature in head direction.
  • a function F(Mtw,b,c,d,%) is defined which accepts a number of fixed parameters b,c,d... and a matrix of values Mtw, representing the wanted temperatures for all heating elements and all printing cycles of the printing process and that generates a new matrix Ft as output.
  • the function F () influences both details along the head direction as details along the transport direction whereby the behaviour in head direction is mainly determined by a subset of parameters of the parameters b, c, d.... and the behaviour in transport direction is mainly determined by (another) subset of parameters of b, c, d...
  • the function F () can be any function but again preferably a very simple one.
  • An example of such a two dimensional function is a function which renders the sum of b.d elements of the matrix Mtw. (See formula 4). In the border-regions the missing elements of Mtw are substituted by zero's.
  • next steps are similar to the embodiments that provided a correction in head and transport direction separately.
  • the parameters of the function F () are determined, e.g. by performing the following steps.
  • a image corresponding with a test pattern such as a step-wise evolving pattern is printed. Generated densities are measured and a reference matrix Mtm is determined of values proportional to wanted heating temperatures using the sensitometry of the printing material.
  • an optimizing process is run to obtain an optimal match between Mtp and Mtm thereby modifying the parameters b, c, d, ....
  • the parameters of the function F () are set to the values bopt, copt, dopt, ... obtained at the end of the optimizing process.
  • Mtwc 1/aopt (Mtw-F(Mtw, bopt,copt, dopt, ).
  • the function F () has an internal state St.
  • St can be a single value, an array of values, a two-dimensional matrix of values or any combination thereof.
  • the state of the function St is initialised before calculation. After start of the calculation, the state St of the function enables the function to calculate the values Mtwc of a printing cycle based on the state St and the row of Mtw corresponding to a current printing cycle and the parameters b,c,d,...
  • Formula 4 is implemented in this example as a function using a state.
  • the state St of this function consists of a vector S and a matrix B and a pointer p.
  • Vector S has as many elements as Mtw has columns.
  • Matrix B has as many columns as Mtw and has b rows.
  • the array of sums is updated by subtracting the element that falls outside the range over which the sum has to be taken and this element is replaced by the proper element of Mtw, both in the buffer B and in the sum S.
  • Mtw the proper element of Mtw
  • the meaning of S is guaranteed at all times and the formula is correct for all values of i.
  • the same function is obtained, but 'b' additions are replaced by a subtraction, an addition and two memory operations per heating element.
  • the setting of the pointer has to be done only once per line. With large 'b' this 'state' approach will enable substantially faster calculation.
  • the function F() used both as part of the matching function Fm () and in the TML step consists of a function Fk(Ktw,St,b,c,d,%) and a function Fs (Ktw, St, b, c, d, ...) .
  • the function Fs () uses the changed temperatures to calculate the state.
  • a set of identical functions F () of any of the types discussed in previous sections, with or without a state variable is defined, whereby all functions accept the same set of parameters a,b,c,d,... but with different values for these parameters and each function also accepts Mtw or a row of Mtw in case the state approach is used.
  • Each function generates new matrices F1,F2,F3, whereby, if Mtw has a stable value V, the function values of F1,F2,F3 are c1.V,c2.V,c3.V respectively.
  • a measurement can be done on a known input pattern Mtw resulting in a density pattern which can be measured. Measured values can be converted to an equivalent temperature pattern Mtm. Then a curve fitting process can be executed to fit Mtp to Mtm and thereby defining an optimal set of parameters.
  • F1 (),F2 (),F3 ()... can be of any of the forms discussed in the embodiments above.
  • ALTIVEC is a trade name of Motorola. Processing speed can be enhanced by keeping the functions F very simple and accuracy can be enhanced by including as much components as needed.

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Claims (7)

  1. Ein Verfahren zur Erzeugung eines Bildes mit verbesserter Bildschärfe auf einem Druckmaterial in einem Thermodrucker, der mit einem Modul mit integriertem thermischem Modell versehen ist, das Eingangswerte in Leistungswerte für jedes Element eines Thermokopfes und für jeden Druckzyklus umrechnet, wobei das Verfahren folgende Schritte umfasst:
    - Definieren einer Funktion F(Mtw,b,c,d,...), in der Mtw eine Matrix erwünschter Temperaturwerte für jedes Heizelement eines Thermokopfes und für alle Druckzyklen bedeutet, wobei zumindest ein Subsatz von Parametern b,c,d,... das Verhalten der Funktion in die Thermokopfrichtung beeinflusst und ein Subsatz der Parameter b,c,d das Verhalten der Funktion in eine Richtung, in die das Druckmaterial am Thermokopf vorbeigefördert wird, beeinflusst,
    - Definieren einer Vergleichsfunktion Fm = aMtw + F(Mtw,b,c,d,...), wobei a = 1-c,
    - Bestimmung optimaler Werte aopt, bopt, copt und dopt,... für die Parameter a,b,c,d,..., so dass Fm (aopt, bopt, copt, dopt,...) das Druckerverhalten simuliert, und
    - Einbeziehung der Mtwc-Werte in das integrierte thermische Modell, wobei die Mtwc-Werte gleich 1/aopt*(Mtw-F(Mtw, bopt, copt, dopt,...)) sind.
  2. Verfahren nach Anspruch 1, dadurch gekennzeichnet, dass optimale Werte aopt, bopt, copt und dopt für die Parameter a, b, c und d wie folgt bestimmt werden :
    - Beaufschlagung des Moduls des thermischen Modells mit einem Testmuster zur Erzeugung eines Dichtemusters,
    - Messung der Dichten auf dem erzeugten Dichtemuster,
    - Berechnung einer Bezugsmatrix Mtm von Werten, die den erwünschten Heizelementtemperaturen proportional sind, unter Einbeziehung der sensitometrischen Daten des Druckmaterials,
    - Berechnung einer Matrix Mtp, indem das Testmuster in die Vergleichsfunktion eingeführt wird,
    - Durchführen eines Optimierungsprozesses zum Erhalten einer optimalen Übereinstimmung zwischen Mtp und Mtm, wobei die Parameter a,b,c,d,... so geändert werden, dass Werte aopt, bopt, copt, dopt,... erhalten werden, und
    - Einstellen der Parameter der Funktion F() auf die gleichen Werte wie die Werte aopt, bopt, copt, dopt, ....
  3. Verfahren nach Anspruch 1, dadurch gekennzeichnet, dass 'a' gleich '1-c' ist und c den Beharrungswert der Funktion F ist.
  4. Verfahren nach Anspruch 2, dadurch gekennzeichnet, dass das Testmuster ein Stufenmuster ist.
  5. Verfahren nach Anspruch 1, dadurch gekennzeichnet, dass Mtw eine eindimensionale Matrix ist, deren Ktw-Werte erwünschte Temperaturwerte für einen einzelnen Druckzyklus des Druckprozesses des Bildes mit verbesserter Bildschärfe bedeuten.
  6. Verfahren nach Anspruch 1, dadurch gekennzeichnet, dass Mtw eine eindimensionale Matrix ist, deren Ttw-Werte erwünschte Temperaturwerte für ein Element des Druckkopfes des Thermodruckers bedeuten.
  7. Verfahren nach Anspruch 1, dadurch gekennzeichnet, dass F = ∑ i G b i , c i , d i … .
    Figure imgb0014
EP20040101372 2004-04-02 2004-04-02 Thermisches Druckverfahren Expired - Fee Related EP1582362B1 (de)

Priority Applications (4)

Application Number Priority Date Filing Date Title
DE200460010062 DE602004010062T2 (de) 2004-04-02 2004-04-02 Thermisches Druckverfahren
EP20040101372 EP1582362B1 (de) 2004-04-02 2004-04-02 Thermisches Druckverfahren
US11/078,092 US7190385B2 (en) 2004-04-02 2005-03-11 Thermal printing method
JP2005086405A JP2005289060A (ja) 2004-04-02 2005-03-24 感熱式プリンテイングの方法

Applications Claiming Priority (1)

Application Number Priority Date Filing Date Title
EP20040101372 EP1582362B1 (de) 2004-04-02 2004-04-02 Thermisches Druckverfahren

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EP1582362B1 true EP1582362B1 (de) 2007-11-14

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Family Cites Families (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JP3209797B2 (ja) * 1992-07-03 2001-09-17 松下電器産業株式会社 階調プリンタ
DE69419072T2 (de) * 1993-05-28 2000-02-17 Agfa Gevaert Nv Verfahren zum Korrigieren der Ungleichmässigkeit in einem Thermodrucksystem

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DE602004010062D1 (de) 2007-12-27
JP2005289060A (ja) 2005-10-20
EP1582362A1 (de) 2005-10-05

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