EP0835760A1 - Korrektur der streifenförmigen Druckunregelmässigkeiten in einem thermischen Drucksystem - Google Patents

Korrektur der streifenförmigen Druckunregelmässigkeiten in einem thermischen Drucksystem Download PDF

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Publication number
EP0835760A1
EP0835760A1 EP96202801A EP96202801A EP0835760A1 EP 0835760 A1 EP0835760 A1 EP 0835760A1 EP 96202801 A EP96202801 A EP 96202801A EP 96202801 A EP96202801 A EP 96202801A EP 0835760 A1 EP0835760 A1 EP 0835760A1
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European Patent Office
Prior art keywords
surface temperature
heating element
data
thermal head
thermal
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Application number
EP96202801A
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English (en)
French (fr)
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designation of the inventor has not yet been filed The
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Agfa Gevaert NV
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Agfa Gevaert NV
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Priority to EP96202801A priority Critical patent/EP0835760A1/de
Publication of EP0835760A1 publication Critical patent/EP0835760A1/de
Withdrawn legal-status Critical Current

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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

Definitions

  • the present invention relates to thermal printing methods and, in particular, to the improvement of the print uniformity of a print produced by a thermal printing head.
  • thermal printing head a heat-sensitive receiving material or a combination of a heat-sensitive donor material and a receiving (or acceptor) material, and a transport device which moves the receiving material or the donor-acceptor combination relative to the thermal printing head.
  • the thermal head usually consists of a one-dimensional array of heating elements arranged on a ceramic base which is itself mounted on a heat-dissipating base element.
  • Systems of this kind generally do not reproduce original images that are uniformly coloured over their area, in a uniform colour shade, but additionally produce defects particularly in the form of streaks or bands perpendicular to the extension of the head. There is a multiplicity of possible causes of this, and there is a corresponding diversity in the number of published methods for eliminating these streak-like or band-shaped print non-uniformities.
  • Examples of such causes are i.a. inequality of the values of the resistors of the thermal head, variations in the thickness of the layers the thermal head is composed of, fluctuations of the coefficient of conductivity, variations of the thermal efficiency per pixel etc.
  • European Patent EP 0,627,319 of Eric Kaerts and Paul Verzele entitled “Method for Correcting Across-the-head Unevenness in a Thermal Printer” likewise describes a correction method based on density measurement values which have been obtained on a test print.
  • this test print is not produced by applying the same electrical control signal to all the heating elements, but by converting the same electrical power as a time average in all the heating elements. For this purpose, in a preceding step, the electrical resistance of each heating element is measured.
  • the uniformity of half-tone prints generated by means of a thermally operated printing method and a thermal printing head having a multiplicity of heating elements H i can be improved by applying a method comprising the steps of
  • half-tone prints can be produced by means of a thermally operated printing method and a thermal head having a multiplicity of heating elements H i , with well-corrected print non-uniformity in the direction of the thermal head, if the uncorrected image data record I i,u , which corresponds to an image to be printed, is converted into a corrected data record I i,c by means of a computing processor by converting each data item I i,u into the associated data item I i,c on the basis of a set of density correction values M i .
  • the density correction values M i are calculated on the basis of the surface temperature T i which the respective i-th heating element of the thermal head assumes when the thermal head is controlled by means of a data record I i,T that is selected in such a way that each heating element is heated approximately to the same surface temperature T.
  • the density correction values are generated in advance and off-line, for example in an in-factory calibration procedure.
  • the density correction values are then stored in memory to be used when producing a printed image.
  • 'half-tone print' is meant a reproduction of an original, which is defined in one or more colours and in which the colour intensity is defined in more than 2 gradations.
  • Single-colour or 3-colour image originals in at least 256 gradations of colour intensity are particular examples.
  • Thermally operated printing processes that are relevant in the context of the present application are all processes for producing a half-tone print, in which the temperature prevailing at a specific point of time and at a specific point of the printing head is a critical variable determining the optical colour density in the printed image.
  • thermal printers and processes performed in these printers are:
  • a receiving material is a reflecting or transparent material in sheet form.
  • the material has an edge length of between 40 mm and 2 m. Rectangular receiving materials with a smaller edge length of between 100 mm and 300 mm may be mentioned for preference.
  • Papers may be mentioned as examples of reflecting receiving material and polymer films, especially preferably films consisting of biaxially stretched polyethylene terephthalate, or polymer films of this type provided with additional coatings, are examples of transparent receiving materials.
  • Any type of digital computer may be used as a computing processor for carrying out the calculations according to the invention.
  • the computer may be part of the thermal printer.
  • Single-board microprocessors are preferred. It may also be expedient, in this case, to conduct the calculations, to be carried out for the method according to the invention, by means of distributed processors, for example with one processor for controlling the thermal head and for converting the uncorrected data record I i,u into the corrected data record I i,c on the basis of the already known set of density correction values M i and a second processor for all the other control functions and calculations.
  • the data record I i,T that is applied to the elements of the thermal head in order to enable measuring of the actual surface temperature they attain is preferably a signal which supplies the same heating power to each heating element as a time average.
  • this signal is called 'a power compensated signal' (denoted by I i,p ). This signal can be obtained by applying the method described in European patent application 0 601 658.
  • Application of a power compensated signal provides a compensation for electrical non-uniformities between individual elements of the thermal head. Compensation for the non-uniform thermal behaviour of the head is obtained by application of the method of the present invention.
  • the compensation process comprises two successive steps. First the element in the thermal head which dissipates the minimum of the power dissipated by all elements is identified. Then a global compensation is performed. More particularly, for all elements the power is augmented by a value which is determined so that the power dissipated by the identified element (dissipating minimum power) becomes equal to the envisaged power value. This is performed by changing the pulse width of strobe pulses gating the data signal to the elements of the thermal head.
  • the power dissipated by each element is individually adapted so as to become equal to the envisaged power value.
  • This second step is performed by skipping a number of pulses in the data signal applied to an element. Pulses are preferably skipped in an equidistant manner so that the averaged power value becomes equal to the envisaged power value.
  • the image data record I i,T used is either the image data record I i,p as explained higher or an image data record which is calculated from this image data record I i,p and from previously obtained density corrections (during an iterative process).
  • a correction value M i for an element H i of the thermal head is calculated as follows.
  • First element H i is activated by means of a signal value I i,T . Then, the surface temperature T i of that element is measured. This is performed for a number of elements of the head so that a number 'i' of values T i are obtained. Next, the average value T av of 'i' values T i is calculated.
  • the correction values M i can be applied to an uncorrected signal I i,u to obtain corrected data to be fed to the thermal head.
  • Corrected image data I i,c are e.g. obtained by skipping pulses in the data path of a pulse-wise controlled thermal head. The number of pulses that is to be skipped is determined by the values M i .
  • Pulse skipping basically consists of a decrease of the number of pulses that would normally be applied to the driver of a given element of the thermal head in order to generate an envisaged density on a printing material.
  • the number of pulses that is skipped is such that the time averaged power dissipated by an element is changed so that non-uniformities of electrical origin are compensated electrically (i.e. through control of the dissipated power in an element of the head instead of through control of the dissipation time).
  • non-electrical non-uniformities for example non-uniformities that are due to the non-uniform thermal characteristics of the thermal head, can be corrected in the above-way.
  • Pulses are preferably skipped in an equidistant manner.
  • Pulse skipping will be explained in greater detail hereinbelow with reference to a specific embodiment of a pulse-wise controlled thermal head.
  • the described example is only given as an example and is not limitative.
  • the recording head of a thermal printer comprises a number of individually energisable resistors (for example 4352).
  • the head further commonly comprises shift registers each providing data signal values for the resistor elements. The output of each of the registers is applied via a latch register and a strobe controlled gating means to the drivers of the elements of the thermal head.
  • a single line of an image is printed by activating the elements of the thermal head during a number of successive strobe periods.
  • the line time (period required to print a single line) takes for example 20 msec and is composed of 960 consecutive strobe periods.
  • the corresponding element of the thermal head is activated during a number of said strobe periods.
  • Maximum density for example corresponds with activation during 960 strobe periods (no correction being applied). This is implemented by feeding the gate which is connected to the driver of that specific element of the thermal head consecutively with 960 logical '1' values.
  • the number and sequence of logical binary values to be applied to the driver of each element of the thermal head to print each line of an image is in the following called 'an image matrix'.
  • the data referring to an element having maximum density comprises 960 consecutive logical '1' values.
  • a density value 480 would be represented by 480 logical '1' values.
  • the image matrix would for example comprise for that specific element 480 consecutive logical 'one' values followed by 480 logical '0' values.
  • compensation for electric non-uniformities is implemented by first generating so-called power map.
  • the power map is generated by measuring the resistor values, measuring the power dissipated by these resistors under predetermined operational conditions, determining the congestionual deviation of dissipated power to the minimum dissipated power value (as has been described higher), and finally generating a power map on the basis of this information.
  • Such a power map comprises for each element in a line and for each strobe period within said line either a logical 'zero' or a logical 'one'.
  • the values in said power map are determined so that when a value in the power map is applied to an 'AND' logical gate together with the corresponding value out of the image matrix, a number of logical 'one' values in the data path are turned to zero.
  • the number of logical 'one' values that is turned to zero is such that the effective time averaged power dissipation becomes equal to a 'set' value.
  • This second power map is preferably combined with the first power map to generate a combined power map the values of which are applied to an AND gate together with the corresponding value out of the image matrix so as to generate compensated data wherein when compared with the original data signal pulses are skipped.
  • corrected image data I i,c are thus obtained by skipping a number of pulses in the pulse wise signal representing uncorrected image data I i,u .
  • the number of pulses that is skipped is determined by the correction values M i .
  • the surface temperature of the heating elements is preferably obtained by measuring the radiation power emitted by the respective heating element.
  • this measurement of the surface temperature takes place in a wavelength range of 1 to 20 micrometres.
  • one of the heating elements may be activated by means of a heating current. It is however preferable to activate a number of elements around an element under evaluation because in this way secondary temperature influencing factors such as the heating of the substrate and the cooling via the heat sink are also taken into account.
  • the measurement of the surface temperature is carried out by means of a radiation detector movable parallel to the thermal head.
  • a particular way of positioning the radiation detector comprises the steps of activating a number of elements in the thermal head on both sides neighbouring the element that will be examined, whereby the elements in the immediate neighbourhood of the element under examination are however not activated.
  • the detector is then moved parallel to the thermal head and from the detected surface temperature values the location of the element to be examined can exactly be deduced.
  • This particular method is advantageous in that (i) the positioning is not complex yet very accurate and (ii) care is taken to keep the introduction of additional thermal non-uniformities to a minimum.
  • Figure 1 shows a direct thermal printer generally comprising a rotable drum 6 and a thermal head 5 .
  • a recording material 7 is secured between head 5 and drum 6 .
  • Drum 6 is rotated by a driving mechanism which is not shown and which continuously advances the drum 6 and the recording material 7 secured to the drum past a stationary thermal head 5.
  • the head comprises a number of individually energisable heating elements arranged in a row.
  • an original image to be printed is transmitted as a digital image data record I i,u , via the data transfer lead 1 , to the image memory 2 of a thermal printer.
  • the image information is transmitted line by line from this memory to the processor 3 of the printer and, is converted according to the correction values M i determined in a calibrating step executed in advance and recalled from a memory 8 , into the corrected image data signal I i,c .
  • This data signal is transmitted to the line memory of the thermal-head control electronics 4 .
  • the printing operation is triggered in the thermal head 5 and the recording material is moved on mechanically by the amount of one line spacing. Thereafter, the subsequent line is read out of the image memory 2 and into the processor 3 and is continued in the same way.
  • a calibration unit comprising an infrared radiation detector 9 and controlling and data processing electronic circuitry 11-13 is positioned in front of the thermal head 5 so that an activated zone of the thermal head is imaged onto the surface of the infrared detector 9 .
  • the elements of the thermal head are controlled by means of an input signal I i,T .
  • I i,T is determined so that a heating element that is activated by means of a value I i,T roughly attains an envisaged temperature T.
  • I i,T is a power compensated image signal, which means that approximately the same heating power is applied to each heating element. The value of the signal applied to an individual element of the thermal head has thus already been compensated for non-uniformities of electrical origin as has been described higher.
  • the surface temperature of an activated element is detected by means of the radiation detector and a correction value pertaining to the activated heating element is determined on the basis of the measured surface temperature T i .
  • the actual measurement of the surface temperature of a heating element is preceded by a step wherein the detector is exactly positioned in front of the element under examination.
  • the positioning of the radiation detector 9 relative to the elements of the thermal head 5 is illustrated in figure 2 and works as follows.
  • the radiation detector is mounted on a mechanism 10 which allows it to be shifted parallel to the row of thermal elements of the thermal head.
  • the detector 9 is shifted past the thermal head while measuring the surface temperature of the thermal head.
  • the signal generated by the detector is amplified by amplifier 11 , converted into a digital signal by analog-to-digital convertor 12 and applied to a controlling processor unit 13 .
  • Processor unit 13 selects the measured value which corresponds to the location of an element of the thermal head in between non-activated neighbouring elements and controls the positioning of the detector 9 so as to be positioned in that location.
  • a correction value is determined pertaining to said element.
  • heating element 'i' as well as a number of heating elements (for example 50 elements on each side of the element under examination) neighbouring heating element 'i' are activated because in this way the influence of the substrate onto which the heating elements are mounted in the thermal head as well as of a heat sink can be taken into account.
  • the calibration is performed at the production stage. It would however also be possible to built the printer so that it has a built-in calibration station which is in front of the thermal head during calibration and is swung away during actual printing so that it becomes possible for the receiving material to be pressed down onto the thermal head. As has already been mentioned, this embodiment would allow on-line calibration in between actual printing cycles to be performed.
  • thermo head is cleaned (by means of for example alcohol, cloth..).
  • the thermal head on which the thermographic measurements have been performed possesses a silicon nitride layer of about 8 micrometres as a scratch-resistant coating.
  • This material has extremely high absorption precisely in the wavelength range of 8 to 12 micrometres, the measuring range of the thermographic camera explained below. Despite this small layer thickness, extinction is already above 5.
  • the characteristic thermal radiation of this object is therefore determined solely by the temperature of the silicon nitride layer.
  • thermographic camera was the model 600 of Inframetrics, Billeria, MA USA, with a 10 ⁇ telescope lens as macrooptics.
  • the instrument measures two-dimensional temperature distributions I in a temperature resolution of at least 0.1°C. In the configuration used here, a local resolution of less than 0.1 mm is achieved. Since the emission coefficient of the measurement object was not known sufficiently precisely, it was not possible to measure absolute temperatures, but only temperature differences.
  • the method according to the invention for the correction of print non-uniformities allows a constructively simple and operationally reliable implementation of the measuring unit.
  • the assignment of the temperature measurement value and heating-element number is necessarily ensured by the sequential control.
  • the requirements placed on the mechanical precision of the detection unit are reduced to the normal degree for precision machine building as a result of the regulation of position by means of the amplitude of the measurement signal.
  • the calibrating step requires no activities by the operator and is independent of any fluctuations in quality of the receiving material.

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EP96202801A 1996-10-09 1996-10-09 Korrektur der streifenförmigen Druckunregelmässigkeiten in einem thermischen Drucksystem Withdrawn EP0835760A1 (de)

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Application Number Priority Date Filing Date Title
EP96202801A EP0835760A1 (de) 1996-10-09 1996-10-09 Korrektur der streifenförmigen Druckunregelmässigkeiten in einem thermischen Drucksystem

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Application Number Priority Date Filing Date Title
EP96202801A EP0835760A1 (de) 1996-10-09 1996-10-09 Korrektur der streifenförmigen Druckunregelmässigkeiten in einem thermischen Drucksystem

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Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JPS6328667A (ja) * 1986-07-22 1988-02-06 Rohm Co Ltd 熱印字ヘツド検査装置
JPH04152151A (ja) * 1990-10-17 1992-05-26 Oki Electric Ind Co Ltd 印字濃度補正データ生成装置およびこの装置の補正データを適用した感熱記録装置
JPH0679906A (ja) * 1992-09-01 1994-03-22 Hitachi Koki Co Ltd サーマルヘッドの濃度補正方法
EP0601658A1 (de) * 1992-12-09 1994-06-15 Agfa-Gevaert N.V. Kalibrierungsverfahren für Heizelemente eines thermischen Kopfes in einem Thermodrucksystem
EP0627319A1 (de) * 1993-05-28 1994-12-07 Agfa-Gevaert N.V. Verfahren zum Korrigieren der Ungleichmässigkeit in einem Thermodrucksystem

Patent Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JPS6328667A (ja) * 1986-07-22 1988-02-06 Rohm Co Ltd 熱印字ヘツド検査装置
JPH04152151A (ja) * 1990-10-17 1992-05-26 Oki Electric Ind Co Ltd 印字濃度補正データ生成装置およびこの装置の補正データを適用した感熱記録装置
JPH0679906A (ja) * 1992-09-01 1994-03-22 Hitachi Koki Co Ltd サーマルヘッドの濃度補正方法
EP0601658A1 (de) * 1992-12-09 1994-06-15 Agfa-Gevaert N.V. Kalibrierungsverfahren für Heizelemente eines thermischen Kopfes in einem Thermodrucksystem
EP0627319A1 (de) * 1993-05-28 1994-12-07 Agfa-Gevaert N.V. Verfahren zum Korrigieren der Ungleichmässigkeit in einem Thermodrucksystem

Non-Patent Citations (3)

* Cited by examiner, † Cited by third party
Title
PATENT ABSTRACTS OF JAPAN vol. 012, no. 236 (M - 715) 6 July 1988 (1988-07-06) *
PATENT ABSTRACTS OF JAPAN vol. 016, no. 440 (M - 1310) 14 September 1992 (1992-09-14) *
PATENT ABSTRACTS OF JAPAN vol. 018, no. 335 (M - 1627) 24 June 1994 (1994-06-24) *

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