EP0627319B1 - Verfahren zum Korrigieren der Ungleichmässigkeit in einem Thermodrucksystem - Google Patents

Verfahren zum Korrigieren der Ungleichmässigkeit in einem Thermodrucksystem Download PDF

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Publication number
EP0627319B1
EP0627319B1 EP19940201310 EP94201310A EP0627319B1 EP 0627319 B1 EP0627319 B1 EP 0627319B1 EP 19940201310 EP19940201310 EP 19940201310 EP 94201310 A EP94201310 A EP 94201310A EP 0627319 B1 EP0627319 B1 EP 0627319B1
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Prior art keywords
density
heating element
power
printing
pixels
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French (fr)
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EP0627319A1 (de
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Eric C/O Agfa-Gevaert N.V. Kaerts
Paul C/O Agfa-Gevaert N.V. Verzele
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Agfa Gevaert NV
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Agfa Gevaert NV
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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 dye diffusion printing, further commonly referred to as sublimation printing, and more particularly to a method for correcting across-the-head uneveness in the printing density of a thermal sublimation print.
  • Thermal sublimation printing uses a dye transfer process, in which a carrier containing a dye is disposed between a receiver, such as a transparant or a paper, and a print head formed of a plurality of individual heat producing elements which will be referred to as heating elements.
  • the receiver is mounted on a rotatable drum.
  • the carrier and the receiver are generally moved relative to the print head which is fixed.
  • a particular heating element is energised, it is heated and causes dye to transfer, e.g. by diffusion or sublimation, from the carrier to an image pixel (or "picture element") in the receiver.
  • the density of the printed dye is a function of the temperature of the heating element and the time the carrier is heated.
  • the heat delivered from the heating element to the carrier causes dye to transfer to the receiver to make thereon an image related to the amount of heat.
  • Thermal dye transfer printer apparatus offer the advantage of true "continuous tone" dye density transfer. By varying the heat applied by each heating element to the carrier, a variable density image pixel is formed in the receiver. However, in systems utilising this type of thermal print head it is often observed that the printing density is not uniform across the page, but that lines, streaks, and bands are visible.
  • any banding is of special disturbance, because medical diagnosis executed by the radiologist is intrinsically based upon visual inspection of a radiographic image recorded on such a transparant film.
  • an improved eveness in the printing image is attained, which is remarkably better than the state of the art.
  • the main reason for the attained eveness may be given by the fact that in the present invention only strongly reduced lateral heatflows between neighbouring heating elements exist, because all heating elements are activated with exactly the same time averaged electrical power and so they all have the same temperature.
  • FIG 1 there is shown a global rempli scheme of a thermal printing apparatus that can be used in accordance with the present invention and which is capable to print a line of pixels at a time on a receiver or acceptor member 11 from dyes transferred from a carrier or dye donor member 12.
  • the receiver 11 is in the form of a sheet; the carrier 12 is in the form of a web and is driven from a supply roller 13 onto a take up roller 14.
  • the receiver 11 is secured to a rotatable drum or platen 15, driven by a drive mechanism (not shown for purpose of simplicity) which continuously advances the drum 15 and the receiver sheet 11 past a stationary thermal head 16. This head 16 presses the carrier 12 against the receiver 11 and receives the output of the driver circuits.
  • the thermal head 16 normally includes a plurality of heating elements equal in number to the number of pixels in the image data present in a line memory.
  • the imagewise heating of the dye donor element is performed on a line by line basis, with the heating resistors geometrically juxtaposed each along another and with gradual construction of the output density.
  • Each of these resistors is capable of being energised by heating pulses, the energy of which is controlled in accordance with the required density of the corresponding picture element.
  • the output energy increases and so the optical density of the hardcopy image 17 on the receiving sheet.
  • lower density image data cause the heating energy to be decreased, giving a lighter picture 17.
  • FIG 3 is a detailed crosssection of a thermal head, indicated as part 16 in figure 1 and containing a heatsink 31, a temperature sensor 32, a bonding layer 33, a ceramic substrate 34, a glazen bulb 35, a heating element (36 in Fig. 3, being equivalent to 28 in Fig. 2) and a wearresistant layer 37.
  • the activation of the heating elements is preferably executed pulsewise and preferably by digital electronics.
  • the different processing steps up to the activation of said heating elements are illustrated in the diagram of Fig 2.
  • First a digital signal representation is obtained in an image acquisition apparatus 21.
  • the image signal is applied via a digital interface 22 and a first storing means (indicated as MEMORY in Fig. 2) to a recording unit 23, namely a thermal sublimation printer.
  • the digital image signal is processed 24, which is explained more thoroughly in the next paragraph.
  • the recording head (16 in Fig. 1) is controlled so as to produce in each pixel the density value corresponding with the processed digital image signal value 24.
  • a stream of serial data of bits is shifted into another storing means, e.g. a shift register 26, representing the next line of data that is to be printed. Thereafter, under controlled conditions, these bits are supplied in parallel to the associated inputs of a latch register 27. Once the bits of data from the shift register 26 are stored in the latch register 27, another line of bits can be sequentially clocked into said shift register 26.
  • the heating elements 28 the upper terminals are connected to a positive voltage source (indicated as V in Fig. 2), while the lower terminals of the elements are respectively connected to the collectors of the driver transistors 29, whose emitters are grounded.
  • transistors 29 are selectively turned on by a high state signal (indicated as an "ANDed” STROBE in Fig. 2) applied to their bases and allow current to flow through their associated heating elements 28. In this way a thermal sublimation hardcopy (17 in Fig 1) of the electrical image data is recorded.
  • an image signal matrix is a twodimensional array of quantised density values or image data I(i,j) where i represents the pixel column location and j represents the pixel row location, or otherwise with i denoting the position across the head of the particular heating element and j denoting the line of the image to be printed.
  • an image with a 2880 x 2086 matrix will have 2880 columns and 2086 rows, thus 2880 pixels horizontally and 2086 pixels vertically.
  • the content of said matrix is a number representing the density to be printed in each pixel, whereby the number of density values of each pixel to be reproduced is restricted by the number of bits pro pixel.
  • the image signal matrix to be printed is preferably directed to an electronic lookup table (abbreviated as LUT) which correlates the density to the number of pulses to be used to drive each heating element (H i ) in the thermal print head; this number will further be referred to as input data (I i ).
  • LUT electronic lookup table
  • I i input data
  • FIG. 5 Before the invention is described in further detail, it is useful to illustrate (Fig. 5) the effect of feeding one activation pulse to a resistive heating element 28, showing the temperature on the vertical axis and the time on the horizontal axis.
  • T e the temperature of the resistive heating element
  • the resistive heating element rises from e.g. 20°C to 300°C, rising steeply at first and then more gradually.
  • the resistive heating element cools at an even more gradual rate.
  • Fig 6 illustrates how the printing density varies across the width of the printhead, and how this variation becomes more pronounced at higher density levels relating to higher amounts of heating.
  • the activating of the heating elements is preceded by retrieving, from the initial configuration settings stored in a memory means (abbreviated as MEM_0) in the printer, a predetermined power value (further represented by P ref ) and by adjusting the power of the heating element actually producing the lowest time averaged power (further represented by P min ) to said predetermined power value.
  • MEM_0 a memory means
  • said adjusting of the maximum power available for each heating element (P i,u ) to said predetermined power value (P ref ), is followed by equalising the printing power of all heating elements to a same time averaged power value, preferably equal to P ref .
  • FIGs. 17 and 18 are flowcharts illustrating all steps of a preferred embodiment, according to the method of the present invention.
  • the most important cause of vertical banding is the existing variation in the electrical resistance values of the heating elements.
  • the upper part of Fig 8 illustrates the variance in printing density (D i ) across a page of a flat field printed respectively with uncorrected input data D i,u and relates to the distribution across the thermal head of the "uncorrected" power P i,u available to each individual heating element H i as it may be determined during a preparatory power measurement. That's why, according to the present invention, first a correction will be made for the resistance variation. Therefor, the first step of the method consists of a power compensation calibration of the heating elements of the thermal head, which preferably can be executed according to our patent application EP-A-0 601 658, published 15.06.94.
  • the diffusion process for a pixel is a function of its temperature and of its transfertime
  • the printed density is a function of the applied energy.
  • the activation of the heating elements is preferably executed pulsewise, which will be further described in the next paragraphs, and thus the printing density has to be related to a time averaged power.
  • the activation of the heating elements is executed pulsewise in a special manner, further referred to as "duty cycled pulsing", which is indicated in Fig. 7 showing the current pulses applied to a single heating element (refs. Hi and 28 in Fig. 2).
  • the repetition strobe period (t s ) consists of one heating cycle (t son ) and one cooling cycle (t s - t son ) as indicated in the same Fig. 7.
  • the strobe pulse width (t son ) is the time an enable strobe signal (ref "ANDed” STROBE in Fig. 2) is on.
  • the strobe duty cycle of a heating element is the ratio of the pulse width (t son ) to the repetition strobe period (t s ).
  • the strobe period (t s ) preferably is a constant, but the pulse width (t son ) may be adjustable, according to a precise rule which will be explained later on; so the strobe duty cycle may be varied accordingly.
  • the line time (t l ) is divided in a number (N) of strobe pulses each with repetition strobe periods t s as indicated on Fig. 7.
  • N number of strobe pulses each with repetition strobe periods t s as indicated on Fig. 7.
  • the maximal diffusion time would be reached after 1024 sequential strobe periods.
  • the above mentioned power compensation calibration of the heating elements may occur fully automatically at some specific time intervals (e.g. at the power up of the system, after a change of consumable, after a number of prints, etc.) and may be realised in some consecutive steps, as schematically illustrated in the upper part of the flowchart of Fig. 17, which part now will be explained shortly. (For a more extensive description of this calibration step, one may refer to the application numbered EP-A-0 601 658 and published on 15.06.94).
  • the maximum time averaged power available for each heating element has to be restricted below a physical upper bound (P limit ) defined by the physical constraints of the printing system as regarding lifetime of the heating element, type of consumables to be used, melting or burning of the carrier or the receiver and loss of glossiness of the printing material.
  • P ref 65 mW
  • said power P min may be adjusted to equal the predetermined power value P ref retrieved from the initial configuration settings of the printer (MEM_0).
  • this adjustment of the power preferably is executed by adjusting the pulse duration of the strobe pulses (t son ) and thus adjusting the strobe duty cycle (being t son : t s ) accordingly (cf. Fig. 7).
  • All heating elements may now be activated with a reduced, but common duty cycle and preferably such that P min equals the above mentioned predetermined power value P ref , stored in a memory means of the printer (MEM_0).
  • P min the above mentioned predetermined power value P ref , stored in a memory means of the printer (MEM_0).
  • a very favorable method for determination of the heating element (H i ) actually producing the lowest time averaged power (P min ), is described in our patent application with application number EP-A-0 601 658, being published on 15.06.94.
  • the resistance values of the heating elements may change and consequently the dissipated power may change, as the applied voltage and the applied number and strobe duty cycle of the activation pulses are constants.
  • said eventually increased power of the actual reference element has nevertheless to be kept constant and equal to the predetermined power value (P ref ), which, according to the present invention, can be obtained by adjusting the pulse duration of the strobe pulses (t son in Fig.7 ) and thus adjusting the strobe duty cycle and the time averaged power accordingly.
  • P ref predetermined power value
  • the maximum power available for each heating element (P i,u ) is already limited to said predetermined power value (P ref ), but said power is not yet necessarily equal for all heating elements (H i ), which thus leaves some banding in the printing image.
  • the method of the present invention prevents such uneveness by a successive step of equalising the available printing power over all heating elements to a same time averaged power value, preferably P ref .
  • P ref time averaged power value
  • This equalising aim may preferably be attained by ommitting an apt number of heating pulses and applies as well in duty cycled pulse systems as in non duty cycled pulse systems, as e.g. pulsewidth or pulsecount systems.
  • the further and individual reduction of the power of said other elements may preferably be done by equidistant skipping a number of heating pulses (see Fig. 9 and Fig. 17.2).
  • equidistant skipping a number of heating cycles of those heating elements that generate too much instantaneous power namely where P i,u > P min
  • a pulsetrain is drawn as activating the reference heating element (with P min ).
  • a corrected pulsetrain is drawn as activating another heating element which in the abscence of the present invention, would dissipate e.g. 25 percent of power above said reference, thus dissipating 125% P ref .
  • every fifth strobe pulse may be skipped.
  • the available time averaged power (P ave ) for every heating element may be made equal and preferably equal to the power of the heating element actually having the lowest time averaged power (P min ).
  • the wording "equidistant skipping” is meant not to be restricted in a mathematical exact relation, whereby each time precisely the same time distance or the same number of pulses between successive skippings has to occur.
  • all following skipping cases are included by the present invention: a) the case of exact equidistant skipping (e.g. successive skippings on the 4th, the 4th, the 4th ... strobe pulse), b) the case of average equidistant skipping (e.g. successive skippings on the 4th, the 3th, the 5th ... strobe pulse), and c) even the broader case of time spread skipping (e.g.
  • the wording "equidistant skipping” thus mainly excludes the skipping cases wherein all skipped pulses are grouped, as it is often, in the present state of the art, at the end of the line time; but other possible cases of “grouped skipping” are also excluded, as e.g. grouped skipping at the start or (nearly) in the midde of the line time.
  • Figs. 10 and 11 Both figures, showing the temperature on the vertical axis (indicated as T e in °C) and the time on the horizontal axis (indicated as t in ms), are graphs of the heating and cooling curves of two distinctive heating elements heated by heating pulses corresponding to one line-time and including duty cycle activation with time equidistant skipping.
  • Fig. 10 relates to an activation with a duty cycle of 100 % and is meant as a comparative example, whereas Fig. 11 is an exemplary practical example according to the method of the present invention, with a duty cycle of 75%; all other circumstances being constant, as e.g. same time constant and same accumulated heat pro line.
  • the left parts of said Figs. 10 and 11 give the temperature evolution during a complete line time of e.g. 16 milliseconds; the right parts of said Figs. 10 and 11 give a detailed view of the temperature evolution during a small interval within said line time, e.g. from 2 to 4 milliseconds.
  • the upper curves represent the temperature evolution for a heating element with an electrical resistance of e.g. 2000 ⁇ .
  • the lower part of said Figs. 10 and 11 comprises two curves, wherein the smoother curve of the lower curves represents the temperature evolution for a heating element with an electrical resistance of e.g. 2500 ⁇ , and wherein the dented curve of the lower curves represents the temperature evolution for a heating element with an electrical resistance of e.g. 2000 ⁇ but now corrected by equidistant skipping in order to equalise the available power to P min .
  • the upper curve and the smoother curve of the lower curves of Fig. 10 may also be interpreted as representing the temperature evolution if a conventional breaking off LUT would be used; the dented curve of said lower curves relating to a skipping LUT being used. From this Fig. 10 it is seen very clearly that the state of the art with a conventional LUT breaks off the consecutive heating pulses at the end of a number of pulses required to reach a predetermined optical density.
  • each heating element enforces for each heating element a same "temperature profile", meaning that independent from possible variations in the individual characteristics of the distinctive heating elements, each heating element will have a same temperature rise during the heating time.
  • a same optical density meaning that independent from possible variations in the individual characteristics of the distinctive heating elements, each heating element will have a same temperature rise during the heating time.
  • the heating power available for heating elements with different electrical resistances are made equal to a same time averaged power, indicated as P min .
  • P min a time averaged power
  • the common reduction of the strobe duty cycle may be replaced by a correspondingly enlarged individual equidistant skipping (as indicated in Fig. 17.2).
  • the common reduction of the duty cycle could eventually be based on the heating element producing the highest time averaged power (indicated by P max ).
  • P max the power of the heating element producing the highest time averaged power
  • P ref predetermined power value
  • said calibration can be executed in one of several ways, the common data flow of which is given in Fig. 13.
  • a first embodiment incorporates an adjusting of the power P min by adjusting the duty cycle and a consequential equalising of the power P i,u of each heating element H i to a predetermined power value P ref by equidistant skipping an apt number of heating pulses.
  • a second embodiment preferably relating to a power compensation calibration at maximal density, incorporates an adjusting of the power P max by adjusting the duty cycle and a consequential equalising of the power P i,u of each heating element Hi to a predetermined power value P ref by equidistant skipping an apt number of heating pulses.
  • This specific embodiment of the present invention is not indicated in Fig. 17, solely for sake of simplicity).
  • a third embodiment incorporates said adjusting of the power P i,u not by adjusting the duty cycle, but replaces said common reduction of the duty cycle by a correspondingly enlarged individual equidistant skipping, such that adjusting and equalising of the power P i,u of each heating element H i to a predetermined power value are obtained both together by equidistant skipping an individual apt number of heating pulses via the datapath and related to each individual heating element.
  • an array of power corrections 121 may be obtained, also referred to as "power map", to obtain power corrected image signals.
  • This array gives for each heating element (H i ) and for each uncorrected input data (I i,u ), the "power corrections" R i,p (as illustrated schematically in Fig. 17.1) intended for equidistant skipping of the strobe pulses according to the present invention.
  • This thus guarantees an equal time averaged power available to the heating elements (H i ), although their individual characteristics, as resistance value (Ref. 28 in Fig. 2) and time delay in the switching circuit (Ref. 29 in Fig. 2), may be different. So, eventual heat flows between neighbouring heating elements are principally eliminated, or at least reduced significantly, which is a great advantage above the prior art of the field and is probably the cause of an improved eveness in the print image.
  • Such power map 121 may be implemented in the form of a lookup table, as it is in some preferred embodiments of the present invention.
  • a power compensation R i,p is memorised, comprising pro density level a row of binary 0 's and 1 's such that te heating element with the highest resistance and which, per consequence, could only dissipate a rather low power, is allowed to dissipate fully naturally, in order to attain the above mentioned P ref , and hence all R i,p 's (with i having a fixed value) equal 1.
  • the power map will present a R i,p value consisting of 1024 times 1 (thus 111...111).
  • the power map will present a R i,p value 11101110.... All other heating elements will have R i,p values in between them, as e.g. 10101010...
  • thermomechanical nonuniformities e.g. variations in the mechanical or thermal contact between the thermal head and the back of the dye donor sheet, or variations in the thermal contact between the ceramic base of the head assembly and the heatsink, etc.
  • Fig. 14 gives a general overview of the basic blocks of the present invention, which, in the further description, will be described in much more detail and in different embodiments. In general, Fig.
  • FIG. 14 shows a clock pulsed strobe path (indicated as STROBE), a main data path (from the uncorrected input data I i,u to the final input data I i,h supplied to the heating elements), a power map 121 resulting from the power compensation calibration, and density correction means M i,d , comprising density correction rows R i,d (ref 141) or density correction factors C i,d (ref 142).
  • the optical transmission (indicated by the symbol T) of a print is the ratio of the light intensity of the transmitted light through the print to the light intensity of the incident light
  • the optical density (indicated by the symbol D) equals the logarithm to the base 10 of the reciprocal of the transmission
  • the input data (I i ) for each heating element (H i ), representing the number of strobe pulses (N i ) to be applied, can be corrected in order to improve the eveness by the method described in the next paragraphs.
  • the next step in the density compensation calibration makes a flat field on a receiver, preferably a transparent receiver.
  • a flat field comprises at least a heigth, e.g. 50 mm, which can be measured correctly by a transmission or reflection densitometer or microdensitometer, the transmittance or a relative transmittance of the transparent receiver versus the position accross the head direction may be measured by said densitometer or microdensitometer.
  • a microdensitometer may be used advantageously if the measuring of the printing density in a flat field print is carried out on individual pixels.
  • a conventional densitometer may be used advantageously if the measuring of the printing density in a flat field print is carried out on socalled "clustered" pixels, which are pixels aggregated or clinged together.
  • clustered pixels are pixels aggregated or clinged together.
  • the output from the microdensitometer is a plurality of transmittance data, from which a set of density values may be calculated, according to formula [1a] or [1b].
  • This set of density values further simply indicated by D i solely but also implicitely including if relevant the meaning of D' i , corresponds to each individual heating element. From said set of density values, a correction may be made to the applied energy to each heating element in order to improve the eveness.
  • the deviation ( ⁇ i ) of the printing density in relation to a printing density intensionally aimed at by the power applied to each heating element may now be calculated for each heating element in one of following ways.
  • the above mentioned determining for each heating element of the deviation in printing density ( ⁇ i ) may be represented by the difference to a desired density, or calculated relative to D min,p and/or to D max,p or calculated relative to the ratio (D i,p -D min,p )/(D max,p -D min,p ) .
  • D i,p is the individual pixel related optical density realised by activating the heating elements with power compensated input data I i,p
  • D min,p is the minimum of all D i,p on a printline
  • D max,p is the maximum of all D i,p on same said printline.
  • the variation in printed density ( ⁇ i ) may be calculated from a single set of density measurements on one line of a flat field print.
  • several lines may be measured on at least one printed flat field and the average values of the densities are calculated, which gives a higher statistical reliability.
  • several lines on at least one printed flat field may be measured and the median values of the densities are calculated, which results in statistical more robust results, thus being less influencable by possible outliers, as possibly originating from dust or scrathes.
  • the above-mentioned density measurements and correction calculations may be used to correct for remaining uneveness in the printing density.
  • said practical use can be carried out in several ways, two of which are are described hereinafter.
  • said correction of the applied energy is made in reference to the number and the time spread of the strobe pulses, e.g. by additional skipping an apt number of pulses; said skipping being preferably distributed over the total number of strobe pulses (which Sach was already explained above in reference to Fig. 9)
  • the density deviation factor ⁇ i may adapt the contents of the abovementioned power map 121 (Fig. 12 & Fig. 13). More practically, from said ⁇ i may result pro heating element H i a row vector consisting of logical 0's and 1's, as e.g. [11111111001111101 ...], called "density correction row" R i,d . It is stated that this correction row R i,d not necessarily has to be time equidistant, as it may be illustrated by a power map 121 intended for maximal 1024 density levels relating to a heating element with index i and to a density level d.
  • further steps include calculating for each heating element a density correction row R i,d taking into account said deviation ( ⁇ i ) in printing density, and storing each of said density correction rows R i,d individually to each heating element (H i ) into a memory means (POWER MAP_D, ref. 151).
  • further steps may include transforming the input data (I i,u ) to each heating element taking into account said deviation ( ⁇ i ) in printing density, the thus transformed data further being indicated as "density-corrected input data" I i,d , and storing each of said density corrected input data (I i,d ) individually to each heating element (H i ) into a memory means (LUT_D, ref 161).
  • each of said transformed input data (I i,d ) or of each of said density correction factors (Ci,d), both individually related to each heating element can preferably be implemented in the form of a look up table (indicated as LUT_D, ref 161).
  • LUT_D look up table
  • the use of a specific LUT embodiment brings an additional advantage. While such a table consists of an ordered pair of input and output values, the LUT is very efficient in performing repetitive operations. Indeed, rather than calculating every time the density corrected input data I i,d from the power compensated input data I i,p , these power and density corrected data I i,d are directly retrieved from said LUT. Especially when dealing with large size images, this can save a significant amount of time.
  • each heating element is activated by power and density corrected signals available at the output of the ENABLE AND gate 131 (see Fig. 15 and 16), which thus guarantees an equal density printed by the heating elements 29, although their individual characteristics, as e.g. resistance value and mechanical or thermal contacts, may be different.
  • Said fitting parameter ⁇ is generally not a constant over the entire density range.
  • said fitting parameter ⁇ may be defined (see Fig. 18) empirically by the following method:
  • Fig. 17 there is illustrated a principal flow chart of all main steps of the method of the present invention according to a preferred embodiment, including as well the power compensation calibration (see Fig. 17.1) as the density compensation calibration (see Fig. 17.2).
  • Fig. 18 there is illustrated a principal flow chart of all steps of the method for the experimental defining of the fitting parameter ( ⁇ ) according to a preferred embodiment of the present invention. Because the arrangements of Fig. 17 and of Fig. 18 are similar in structure and operation to the above identified steps in the full description, they do not need to be described once again. As already mentioned above, some of these steps may be modified or even omitted, within the same scope of the present invention.
  • the printing system is ready to perform the steps of correcting an input image. While printing, said correction may be carried out by replacing each initially uncorrected input data signal (I i,u ) by its power and density corrected input data signal (I i,d ).
  • a method for correcting across-the-head uneveness in the printing density (D i ) of a thermal sublimation printer containing a head having a plurality of heating elements (H i ) and containing storing means for holding density corrected values I i,d or density correction factors C i,d or density correction row R i,d for each heating element I i,d so that while printing said density corrected values I i,d or said density correction factors C i,d or said density correction row R i,d can be used to print input image data, characterised in that said density corrected values I i,d or said density correction factors C i,d or said density correction row R i,d are obtained according to the method described hereabove.
  • figure 8 illustrates the variance in printing density (D i ) across a page of a flat field printed respectively with uncorrected input data D i,u , with power corrected input data D i,p and with power and density corrected input data D i,pd .
  • the illustrated curves may progressively be obtained by the consecutive steps of the present invention, which steps were hereabove described one by one. Note that for the density corrected values the same densities are achieved for many more heating elements than for uncorrected values of input data.
  • the correcting method of the present invention can be carried out either as an integrated part of the power correction or as a separate and consecutive input data transformation.
  • the power and density correction of the present invention may occur at the power up of the system, after a change of consumable, after a number (e.g. 1000) of prints, etc.
  • the method for printing an image by thermal sublimation is characterised in that the pixels wherefrom the printing density in a flat field print is measured, correspond to individual pixels.
  • Such embodiment is schematically illustrated in Figs. 19.1 to 19.5.
  • the method for printing an image by thermal sublimation is characterised in that the initial pixel on a line wherefrom the printing density in a flat field print is measured, is located either in a fixed position (see Figs. 19.1 and 19.2), or in a (phase-) shifted position (see Figs. 19.3 to 19.5)
  • the method for printing an image by thermal sublimation is characterised in that the pixels wherefrom the printing density in a flat field print is measured, correspond to individual pixels which are distant pixels, either periodically distant (see Figs. 19.1 and 19.2) or variably distant (see Figs. 19.3 to 19.5),
  • the estimating for each heating element of the individual deviation ( ⁇ i ) of the printing density from a printing density aimed at by said power applied to each heating element (H i ) may preferably be carried out by curve fitting.
  • this technique is well known to the people skilled in the art, it does not require any additional description.
  • the method for printing an image by thermal sublimation is characterised in that the distant pixels wherefrom the printing density in a flat field print is measured are variably distant in one direction, e.g. in either horizontal direction (see Fig. 19.4) either in vertical direction (see Fig. 19.3), or are variably distant in two perpendicular directions, preferably in horizontal and in vertical direction (see Fig. 19.5).
  • the method is characterised in that the pixels wherefrom the printing density in a flat field print is measured, correspond to clustered pixels _comprising individual pixels aggregated or clinged together (see Fig. 20)_, having either a fixed number (see Fig. 20.1 to 20.3, 20.5 to 20.7) of pixels or a variable number (see Fig. 20.4) of pixels.
  • a more conventional densitometer _e.g. with a conventional circular spot of a diametre 3 to 5 mm_ may be used and that the capacity of the memory may be reduced economically. It also may be clear that in case all individual pixels are measured, the highest accuracy may result. It also follows that in case distant clusters are measured, the capacity of the memory further may be reduced even more economically; and that in case of variably distant clusters, possible sytematic faults also may be reduced.
  • the method is characterised in that the clustered pixels are aggregated into a rectangular or a quasi-rectangular spot (see Figs. 20.1 to 20.6) or into a circular spot (see Fig. 20.7).
  • the method for printing an image by thermal sublimation is characterised in that the initial cluster on a line wherefrom the printing density in a flat field print is measured, is located either in a fixed position (see Figs. 20.1 and 20.2; 20.4 to 20.7), either in a (phase) shifted position (see Figs. 20.3)
  • the method is characterised in that consecutive sets of clustered pixels are distant, either periodically distant (see Figs. 20.1 and 20.2, 20.4 or 20.5) or variably distant (see Fig. 20.3 and 20.6).
  • the consecutive sets of clustered pixels are variably distant in one direction, e.g. in horizontal direction (see Fig. 20.3) or in vertical direction, or are variably distant in two perpendicular directions, preferably in horizontal and in vertical direction.
  • the method is characterised in that consecutive sets of clustered pixels are partly overlapping (see Figs. 20.6 and 20.7).
  • the method for printing an image by thermal sublimation is characterised in that a memory means (MEM_C) for holding a density correction means M i,d comprises a floppy disk drive fitted for cooperating with a floppy disk for holding a density correction means M i,d for each heating element H i to be used to correct the input image data while printing.
  • a memory means (MEM_C) for holding a density correction means M i,d comprises a floppy disk drive fitted for cooperating with a floppy disk for holding a density correction means M i,d for each heating element H i to be used to correct the input image data while printing.
  • the method for printing an image by thermal sublimation is characterised by the step of storing the estimates for each heating element of the deviation ( ⁇ i ) of the printing density (D i,p ) from a printing density D i,t aimed at by said power applied to each heating element in a memory means that comprises a floppy disk.
  • the method for printing an image by thermal sublimation is characterised by the step of storing the values (e.g. 2000 ⁇ , 2500 ⁇ ) of the electrical resistances of the (e.g. 2880) different heating elements in a memory means that comprises a floppy disk.
  • Both last mentioned embodiments have the specific advantage that, if the thermal head is accurately measured while leaving the manufactury, it may be dispatchied with a floppy disk holding only a moderate number of measured values (e.g. 2880).
  • this floppy disk has to be introduced in a floppy disk drive of the printer, and preferably copied on a hard disk of the printer.
  • said moderate number of measured values e.g. 2880
  • said moderate number of measured values may be read on the hard disk of the printer and may be followed by an automatically generating of the density correction means M i,d described hereabove.
  • the method of the present invention provides a remarkable eveness in the printing density across the headwidth, said method is very well suited to be used in medical diagnosis.
  • the printing may be applied in graphic representations, in facsimile transmission of documents etc.
  • This invention may be used as well for greyscale thermal sublimation printing as well as for colour thermal sublimation printing.

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Claims (17)

  1. Verfahren zum Korrigieren der quer zum Kopf verlaufenden Ungleichmäßigkeit in einem Thermodrucksystem, das folgende Schritte umfasst:
    1) Liefern eines Stroms unkorrigierter Eingangsdaten (Ii,u) an eine Verarbeitungseinheit eines Thermodruckers mit einem zeilenartigen Thermokopf mit mehreren Heizelementen (Hi);
    2) Erhalten von Dichtekorrekturmitteln (Mi,d) zum Verbessern der quer zum Kopf verlaufenden Ungleichmäßigkeit bei der Druckdichte gemäß den folgenden Schritten:
    a) tastverhältnismäßig impulsförmiges Aktivieren jedes Heizelements mit leistungsmäßig kompensierten Eingangsdaten (Ii,p), so dass unabhängig von einzelnen Unterschieden in den Eigenschaften von Heizelementen eine gleiche, über die Zeit gemittelte Leistung in jedem Heizelement erzeugt wird, um einen Flachfeldausdruck zu erhalten; umfassend die folgenden Teilschritte:
    (i) aus einem Speichermittel in dem Drucker einen vorbestimmten Leistungswert abrufen;
    (ii) Verstellen der für jedes Heizelement zur Verfügung stehenden Leistung auf den vorbestimmten Leistungswert durch gemeinsames Verstellen des Strobetastverhältnisses zu allen Heizelementen, so dass die zur Verfügung stehende Druckleistung jedes Heizelements nicht diejenige Leistung übersteigt, die in dem Heizelement mit dem höchsten Wert an Widerstand von allen Heizelementen abgeleitet werden kann;
    (iii) Ausgleichen der zur Verfügung stehenden Druckleistung jedes Heizelements durch äquidistantes Überspringen einer individuellen Anzahl an Strobeimpulsen zu jedem Heizelement;
    b) Messen von Druckdichten (Di,p) von Bildpunkten, die den Heizelementen entsprechen, in dem Flachfeldausdruck;
    c) Schätzen der einzelnen Unterschiede in den Eigenschaften der Heizelemente durch Schätzen einer Abweichung (δi) der Druckdichte für jedes Heizelement von einer durch die an jedes Heizelement angelegte Leistung angestrebten Druckdichte;
    d) Berechnen eines Dichtekorrekturmittels (Mi,d) für jedes Heizelement unter Berücksichtigung der Abweichung (δi) in der Druckdichte; und
    e) Speichern jedes der Dichtekorrekturmittel Mi,d in einem Speichermittel (MEM_C) individuell für jedes Heizelement;
    3) Kombinieren der jeweiligen nichtkorrigierten Eingangsdaten (Ii,u) mit den jeweiligen Dichtekorrekturmitteln (Mi,d) für jedes einzelne Heizelement; und
    4) Zuführen der so korrigierten Daten Ii,d zu dem Thermokopf zum Widergeben des Bilds.
  2. Verfahren nach Anspruch 1, bei dem die Bildpunkte, deren Druckdichte gemessen wird, individuellen Heizelementen entsprechen.
  3. Verfahren nach Anspruch 1, bei dem die Bildpunkte, deren Druckdichte gemessen wird, zusammengeballten Bildpunkten entsprechen, umfassend Bildpunkte, die angehäuft sind oder aneinander haften, mit entweder einer festen Anzahl von Bildpunkten oder einer veränderlichen Anzahl von Bildpunkten.
  4. Verfahren nach Anspruch 3, bei dem die zusammengeballten Bildpunkte, deren Druckdichte gemessen wird, eine rechteckige, eine quasi rechteckige oder eine kreisförmige Zusammenballung von Bildpunkten bilden.
  5. Verfahren nach Anspruch 3 oder 4, bei dem aufeinander folgende Mengen von zusammengeballten Bildpunkten sich teilweise überlappen.
  6. Verfahren nach einem der Ansprüche 2 bis 4, bei dem der anfängliche Bildpunkt oder die anfängliche Zusammenballung auf einer Zeile, anhand dem bzw. derer die Druckdichte in einem Flachfeldausdruck gemessen wird, entweder an einer festen Stelle oder an einer verschobenen Stelle liegt.
  7. Verfahren nach einem der Ansprüche 2 bis 4, bei dem die Bildpunkte, deren Druckdichte gemessen wird, entfernten Bildpunkten entsprechen, die entweder periodisch entfernt oder veränderlich entfernt sind, oder zusammengeballten Bildpunkten entsprechen, die entweder periodisch entfernt oder veränderlich entfernt sind.
  8. Verfahren nach Anspruch 7, bei dem die entfernten Bildpunkte oder die entfernten Zusammenballungen, anhand derer die Druckdichte in einem Flachfeldausdruck gemessen wird, in einer Richtung entfernt sind oder in zwei senkrechten Richtungen entfernt sind.
  9. Verfahren nach Anspruch 4 oder 8, bei dem das Schätzen, für jedes Heizelement, der Abweichung (δi) der Druckdichte von einer durch die an jedes Heizelement (Hi) angelegte Leistung angestrebten Druckdichte durch Kurvenanpassung durchgeführt wird.
  10. Verfahren nach Anspruch 1, bei dem das Dichtekorrekturmittel Mi,d eine Dichtekorrekturreihe Ri,d ist und dass das Speichermittel eine Leistungsabbildung (LEISTUNGSABBILDUNG_D, Bez. 151) ist.
  11. Verfahren nach Anspruch 1, bei dem das Dichtekorrekturmittel Mi,d ein Dichtekorrekturfaktor Ci,d ist und dass das Speichermittel eine Nachschlagetabelle (LUT_D, Bez. 161) ist.
  12. Verfahren nach einem der vorhergehenden Ansprüche, bei dem die Aktivierung der Heizelemente tastverhältnismäßig impulsförmig ausgeführt wird.
  13. Verfahren nach Anspruch 10 oder 11, bei dem das Schätzen, für jedes Heizelement, der Abweichung (δi) der Druckdichte (D) durch die Differenz von einer Solldichte dargestellt wird oder relativ zu Dmin und/oder Dmax berechnet wird oder relativ zu dem Verhältnis (Di,p-Dmin,p) / (Dmax-Dmin,p) berechnet wird.
  14. Verfahren nach Anspruch 11, bei dem der Dichtekorrekturfaktor Ci,d zum Transformieren der Eingangsdaten Ii,u zu jedem Heizelement berechnet wird gemäß der Formel Ii,d = Ci,d × Ii,u oder gemäß der Formel Ii,d = Φ × Ii,u + (1-Φ) × Ii,u × (δi) oder gemäß der Formel Ii,d = Φ × Ii,u + (1-Φ) × Ii,u × [(Di,p-Dmin,p) / (Dmax,p-Dmin,p)], wobei Φ ein Anpassungsparameter ist.
  15. Verfahren nach einem der Ansprüche 1 bis 14, bei dem das Speichermittel zum Halten eines Dichtekorrekturmittels Mi,d ein Diskettenlaufwerk umfasst, das zum Zusammenwirken mit einer Diskette zum Halten eines Dichtekorrekturmittels Mi,d für jedes Heizelement zum Einsatz bei der Korrektur der Eingangsdaten während des Drucks installiert ist.
  16. Verfahren nach einem der Ansprüche 1 bis 14, weiterhin mit dem Schritt des Speicherns in einem Speichermittel, das eine Diskette umfasst, der Schätzwerte für jedes Heizelement der Abweichung (δi) der Druckdichte von einer durch die an jedes Heizelement angelegte Leistung angestrebten Druckdichte.
  17. Verfahren nach einem der Ansprüche 1 bis 14, weiterhin mit dem Schritt des Speicherns in einem Speichermittel, das eine Diskette umfasst, der Werte der elektrischen Widerstände der verschiedenen Heizelemente.
EP19940201310 1993-05-28 1994-05-10 Verfahren zum Korrigieren der Ungleichmässigkeit in einem Thermodrucksystem Expired - Lifetime EP0627319B1 (de)

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US5786837A (en) * 1994-11-29 1998-07-28 Agfa-Gevaert N.V. Method and apparatus for thermal printing with voltage-drop compensation
EP0835760A1 (de) * 1996-10-09 1998-04-15 Agfa-Gevaert N.V. Korrektur der streifenförmigen Druckunregelmässigkeiten in einem thermischen Drucksystem
EP1104700B1 (de) 1999-12-01 2005-10-12 Agfa-Gevaert Thermodruckkopf

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JPS6256161A (ja) * 1985-09-06 1987-03-11 Sharp Corp 印字装置
JPS62256575A (ja) * 1986-04-30 1987-11-09 Fuji Xerox Co Ltd 感熱記録装置
JPH01192561A (ja) * 1988-01-29 1989-08-02 Toshiba Corp 画情報処理方式
JPH01310971A (ja) * 1988-06-08 1989-12-15 Eastman Kodatsuku Japan Kk サーマルプリンタの濃度ムラ補正方法
US4827279A (en) * 1988-06-16 1989-05-02 Eastman Kodak Company Process for correcting across-the-head nonuniformity in thermal printers
NL8803152A (nl) * 1988-12-23 1990-07-16 Philips Nv Prediktieve kodeer- en dekodeerschakeling voor beeldelementwaarden.
US5010491A (en) * 1988-12-27 1991-04-23 International Business Machines Corp. Automated system for machining parts to close tolerances
GB9007014D0 (en) * 1990-03-29 1990-05-30 Dowty Maritime Systems Ltd Thermal recording apparatus
JPH048561A (ja) * 1990-04-26 1992-01-13 Sanyo Electric Co Ltd サーマルヘッドの駆動方法

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