JP2000108399A - Method and device for thermal recording of multi- gradation image - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a thermal recorder for a multi-gradation image capable of recording a high quality image by compensating thermal accumulation in a thermal head. SOLUTION: There is disclosed an image recorder for thermally recording a multi-gradation image by a thermal line head 14. The recorder comprises a frame memory 11 for storing density data of an image, a calculation processing means 16 for calculating a temperature compensating quantity with respect to thermal accumulation in the thermal line head 14 and a gradation controlling means 13 that applies an application pulse to a heater on the thermal line head 14 corresponding to the calculated result of the calculation processing means 16. The calculation processing means 16 controls the temperature compensating quantity in a sub-scanning direction by a pulse width of a pulse applied to the heater and controls the temperature compensating quantity in a main scanning direction by the number of pulses applied to the heating resistor. The thermal accumulation on the thermal head in the sub-scanning and main scanning directions is compensated, thereby recording a high quality image.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for thermally recording a multi-tone image such as a full-color image and a recording apparatus therefor, and more particularly to a method for appropriately performing temperature compensation for heat storage of a thermal head. is there.

[0002]

2. Description of the Related Art In a thermal recording system, a thermal line head having a large number of minute heating elements on a line is brought into contact with a thermal sheet, and the heating elements are selectively energized in accordance with a recorded image. Is recorded and an image is recorded. In this method, a multi-color heat-sensitive sheet using a sublimable dye ink is used, and a color image can be recorded by supplying an energizing energy corresponding to a gradation to a heating element of a thermal head. It is used for many printing devices such as a printer and a video printer for hard-copying video images.

As shown in FIG. 3, a thermal head has a head base 27 made of a material such as aluminum having a low thermal resistance.
And a heating element substrate 25 bonded thereon with an adhesive layer 26.On the heating element substrate 25, via a glaze layer 24,
The heating element 23, the electrode 22, and the wear-resistant layer 21 are stacked, and a heating element corresponding to the number of dots in one line is formed in the longitudinal direction (main scanning direction) of the head base 27. At the center of the head base 27, a thermistor 28 for measuring the temperature of the head base 27 is embedded.

FIG. 4 is a view for explaining a method of designating the gradation of an image to be recorded in the thermal recording system.
Now, let us take as an example a case where the figure “A” in FIG. 4A is recorded. Here, it is assumed that the number of dots is 10, and the number of gradations is 4.

The gradation data of each dot (circle) of each line of the figure is held as image density data. The gradation data of each dot on the i-th line is shown in FIG.
As shown in (b), “0002230000”, that is, the tone of the fourth dot is 2, the tone of the fifth dot is 2, the tone of the sixth dot is 3, and the tone of other dots is Is 0.

At this time, as shown in FIG. 4 (c), the gradation data is gradation data representing the first gradation in which a dot less than one gradation is 0 and a dot more than one gradation is 1. The gradation data representing the second gradation in which the dots of less than 2 gradations are 0, the dots of 2 gradations or more are 1, and the 3rd floor is 0, the dots of less than 3 gradations are 0. Gradation data representing the key, and
0 for dots less than 4 tones and 1 for dots of 4 tones 4
It is decomposed into gradation data representing the gradation. And FIG.
As shown in (d), 1 and 0 of each gradation data are indicated by the presence or absence of a pulse, and data obtained by serially connecting them is output as data designating the gradation of the i-th line.

In the thermal head, a specified number of pulses are applied to a heating element corresponding to a specified dot based on this data, thereby recording the i-th line.

[0008] In this thermal recording method, since the heat generated when power is supplied to the heating element is used, the thermal head accumulates heat as recording proceeds. Therefore, if the pulse width of the pulse applied to the heating element is kept constant, the gradation of the image to be recorded changes, and the density of the image cannot be reproduced. ,
This leads to deterioration of image quality.

In the conventional thermal recording method, the pulse width applied to the heating element is measured by the thermistor 28 sequentially as the thermal head advances in the sub-scanning direction in order to eliminate the influence of the heat storage of the thermal head. One-dimensional temperature compensation for shortening based on the temperature of the head base 27 or the like is performed.

[0010]

However, when the thermal recording apparatus is used for full-color printing on large-size paper such as A4, the recording speed needs to be increased, and the applied voltage per unit time increases. There is a problem that the effect of heat storage in the thermal head appears more remarkably, and image quality deteriorates unless more precise temperature compensation is performed.

When recording an image having a rapid change in density in the main scanning direction, the heat generated by the individual heating elements of the thermal head arranged in the main scanning direction is not uniform. Is non-uniform, and the conventional one-dimensional temperature compensation method in which the energizing pulse width of one line is constant cannot accurately correct the recording density fluctuation.

The present invention is to solve such a conventional problem, and appropriately compensates for the influence of the thermal storage of the thermal head on image recording, so that a multi-tone image capable of recording a high-quality image can be recorded. It is an object of the present invention to provide a thermal recording method and to provide a thermal recording apparatus for performing the method.

[0013]

Therefore, in the thermal recording method of the present invention, the weighted addition of the applied pulse energy supplied to each heating element of the thermal line head and the heating elements in the vicinity thereof is sequentially repeated to store the heat of each heating element. The amount of heat is determined, the average amount of heat storage of all the heating elements is used as a parameter to determine the applied pulse width for one gradation of the next line, and the image given to each heating element is also determined. The density data is corrected based on the ratio between the heat storage amount of the heating element and the averaged heat storage amount.

Therefore, temperature compensation in the sub-scanning direction and temperature compensation in the main scanning direction can be performed with high accuracy.

Further, in the thermal recording apparatus of the present invention, a frame memory for storing density data of an image, an arithmetic processing means for calculating the amount of temperature compensation for heat storage of the thermal line head, and a calculation result of the arithmetic processing means for the thermal line Gradation control means for reflecting the pulse applied to the heating element of the head, and the arithmetic processing means adjusts the amount of temperature compensation in the sub-scanning direction by the pulse width of the pulse applied to the heating element of the thermal line head. The temperature compensation amount in the scanning direction is adjusted by correcting the number of pulses applied to the heating element.

In this apparatus, high-quality images can be recorded by compensating for the influence of the heat stored in the thermal head in the sub-scanning direction and the main scanning direction.

[0017]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The invention according to claim 1 of the present invention is directed to an image recording method for thermally recording a multi-tone image using a thermal line head. The heat accumulation of each heating element is determined by sequentially repeating the weight addition of the applied pulse energy to be supplied, and the average heat storage amount obtained by averaging the heat accumulation of all the heating elements is used as a parameter, and one gradation of the next line is used. The application pulse width is determined so that the temperature compensation in the sub-scanning direction can be performed with high accuracy.

According to a second aspect of the present invention, the image density data given to each heating element is corrected based on the ratio between the heat storage amount of the heating element and the averaged heat storage amount. Directional temperature compensation can be appropriately performed.

According to a third aspect of the present invention, a group consisting of a plurality of heating elements is regarded as one heating element, and arithmetic processing is performed using this group as a unit.
The amount of calculation can be reduced.

According to a fourth aspect of the present invention, there is provided an image recording apparatus for thermally recording a multi-tone image using a thermal line head, wherein a frame memory for storing image density data and a temperature compensation amount for heat storage of the thermal line head. An arithmetic processing means for performing an arithmetic operation; and a gradation control means for reflecting an arithmetic result of the arithmetic processing means on a pulse applied to a heating element of the thermal line head. The temperature compensation amount in the main scanning direction is adjusted by correcting the number of pulses applied to the heating element by adjusting the pulse width of the pulse applied to the heating element of the head. High-quality images can be recorded by compensating for the effects of the scanning direction and the main scanning direction.

According to a fifth aspect of the present invention, the arithmetic processing means uses, as a parameter, an average heat storage amount obtained by averaging the heat storage amounts of the respective heating elements of the thermal line head, and calculates one gradation of the next line. The pulse width to be applied is determined, and temperature compensation in the sub-scanning direction can be performed with high accuracy.

According to a sixth aspect of the present invention, the arithmetic processing means corrects the image density data of each heating element based on the ratio between the heat storage amount of each heating element and the averaged heat storage amount. In addition, temperature compensation in the main scanning direction can be appropriately performed.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.

As shown in FIG. 1, the multi-tone image thermal recording apparatus according to the embodiment includes an interface box 10 connected to an image apparatus such as a personal computer or NTSC, or an image apparatus such as a digital still camera, and an interface box. A frame memory 11 for storing data obtained by decomposing an image signal input from 10 into yellow, magenta, and cyan (and, if necessary, black) elements; a multi-gradation thermal head 14 for performing thermal recording; A thermistor 15 for measuring the temperature of the head base, a microcomputer 12 for determining the pulse width applied to the thermal head 14 and calculating the correction of the image density data, and receiving the data determined by the microcomputer 12 to the thermal head 14. And a gradation control circuit section 13 for controlling the output of.

The microcomputer 12 includes a memory 17 for storing heat storage amount data of each heating element of the thermal head 14, a line memory 18 for temporarily storing image recording data read from the frame memory 11, an applied pulse width and an image pulse width. An arithmetic processing unit 16 that calculates the correction amount of the density data is provided, and the gradation control circuit 13 includes a shift register that receives the data output from the arithmetic processing unit 16.

The operation of this device will be described.

The amount of heat stored in the j-th heating element of the thermal head 14 when the thermal head 14 is recording the i-line is expressed as the amount of heat stored in the j-th heating element of the i-line.
(I, j). This S (i, j) is
It is represented by the recurrence formula of (Equation 1).

S (i, j) = Σαk · S (i−1, j−k) + Σβk · P (i−1, j−k) (Equation 1) where P (i−1, j−k) is the applied pulse energy applied to the (i−1) th (j−k) th heating element based on the image data, (I-1) Line (j-
It corresponds to the calorific value of the k) th heating element. Also, α
k and βk are coefficients.

S (i, j) is given by (Equation 1)
A value (first term) obtained by weighting the amount of heat stored in the front line (i-1) of the nearby heating element and a value obtained by weighting addition of the applied pulse energy in the front line (i-1) of the nearby heating element (first term). 2). The initial value S of this S (i, j)
Since (0, j) is 0, S (i, j) is obtained by sequentially repeating weighted addition of applied pulse energies of nearby heating elements.

Further, the sum of the applied pulse energies of the respective heating elements can be represented by (Equation 2). ΣP (i, j−k) = βP (i−1) + Σβ′k · P (i−1, j−k) (Equation 2) (Every Σ adds k from −m to m.) Here, P (i-1) is the value obtained when the added value of the applied pulse energies of the heating elements from j-m to j + m near j in line (i-1) is changed while j is sequentially changed. ,
This is an addition value representing the minimum level. Therefore, ΔP representing the sum of the applied pulse energies of the heating elements near j
(I, jk) is P (i-1) representing the basal level at the (i-1) line, and the second term relating to the applied pulse energy of each heating element from j-m to j + m. And can be expressed as the sum of The initial value of P (i) is 0.

By representing the weighted sum of the applied pulse energies of the nearby heating elements by P (i),
S (i, j) in (Equation 1) can be simplified as in the following (Equation 3). S (i, j) = {αk · S (i−1, jk) + β ″ P (i−1) (Equation 3) (Σ adds k from −m to m)

After calculating the heat storage amount S (i, j) of each heating element by (Equation 1) or (Equation 3), the multi-gradation thermal head
The pulse width applied to 14 is determined by (Equation 4). Pulse (i) = Pb · e (i) (Equation 4) Here, Pb is a reference pulse, and the head temperature and the head base temperature reach the reference temperature (usually around 30 degrees of normal temperature), and are saturated and stable. The applied pulse width when in the
b. The coefficient e (i) is represented by (Equation 5). e (i) = {func (T (i), S (i-1, j))} / j (Equation 5) (Σ is added for j) where T (i) is the measurement of the thermistor 15 Temperature and f
unc (T (i), S (i-1, j)) is a function of T (i) and S (i-1, j). This function is obtained from data of an experiment performed at the time of shipping the thermal recording apparatus. That is, various values of the function corresponding to each value of T (i) and S (i-1, j) are selected, and the recording state of the thermal recording apparatus at that time is checked by, for example, printing a test pattern. I do. The recording state is evaluated by visual observation and a density measuring device to determine whether the printed matter has image blur or a change in hue, and its function is determined so as to obtain an optimum recording state. T (i) and S determined by these experiments
The relationship between (i-1, j) and the function is described in a table,
It is held in the microcomputer 12.

This (Equation 5) indicates that the average of the heat storage amounts of all the heating elements is used as the coefficient e (i), and (Equation 4) is expressed by the coefficient e (i). The average value is i
This shows that the heat storage amounts of all the heating elements in the line are used, and this value is used as a parameter to determine the pulse width for one gradation, which is the unit of the applied pulse width, in the next line.

The gradation control circuit 13 supplies a pulse to the thermal head 14 in a form reflecting the applied pulse width determined in this way.

Next, compensation for non-uniform temperature distribution in the main scanning direction will be described.

The unevenness of the temperature distribution in the main scanning direction is dealt with by correcting the image density data.
That is, compensation in the sub-scanning direction is performed by changing the pulse width of a unit pulse for one gradation, while compensation in the main scanning direction is performed by changing the number of pulses.

In this case, the gradation data of the original image is corrected by (Equation 6). data ′ (j) = data (j) · ed (j) · f (Equation 6) where data (j) represents original image density data;
data ′ (j) represents the corrected image density data. Further, f represents the reciprocal of e (i) obtained by (Equation 5), and the coefficient ed (i) is represented by (Equation 7). ed (i) = func (T (i), S (i−1, j)) (Equation 7) (Equation 6) and (Equation 7) express the image density data of the target heating element by This indicates that correction is performed based on the ratio between the heat storage amount and the averaged heat storage amount of all the heating elements.

The image density data corrected in this way is supplied to the thermal head 14 through the gradation control circuit 13.

FIG. 2 is a flowchart showing an operation procedure in the microcomputer 12 of the thermal recording apparatus.

Step 21: arithmetic processing section 16 of microcomputer 12
Represents the heat storage amount S (i, j) of the j-th heating element in the i-line,
Each time a transition from the i-th line to the i + 1-th line is performed, (Equation 1)
Alternatively, it is updated by (Equation 3), and the result is stored in the memory 17. By the time the data is sent to the multi-gradation thermal head 14, the arithmetic processing unit 16 calculates P (i) in (Equation 3).
Is determined, the value is stored in the memory 17, and the next (i +
1) Used when determining the heat storage amount S (i + 1, j) in the line.

Step 22: Next, the heat storage amount S of each heating element
Based on (i, j) and temperature information T (i) from the thermistor 15 attached to the multi-gradation thermal head 14, the correction coefficient (func (Tunc (T
(I), S (i-1, j)) is determined. Step 23: Using the correction coefficient for each heating element, the pulse width correction coefficient e for the entire heating element is calculated by (Equation 5).
Step 24: The pulse width of the line is determined by (Equation 4).

In this manner, the applied pulse width of the i-th line is determined for all gradations from the heat storage amount of the (i-1) -th line and the head temperature information.

Step 25: Next, image density data is read from the frame memory 11 to the line memory 18, and Step 26: the correction coefficient (= ed
Using (j)), the image density data is corrected according to (Equation 6). Step 27: The data is output to the gradation control circuit unit 13.

The gradation control circuit section 13 gives a pulse, which is output from the microcomputer 12 and reflects the corrected image density data and the applied pulse width data, to the corresponding heating element of the thermal head 14. To realize this, the gradation control circuit unit 13 has a control mechanism for turning on the strobe pulse for the set time.

The thermal head 14 executes printing by holding the data sent from the gradation control circuit section 13 by a latch circuit.

In the embodiment, the case where the calculation is performed in consideration of the heat storage amount of each heating element has been described. However, a plurality of heating elements are treated as one heating element, and the number of data of the heat storage amount is calculated. It is also possible to reduce the amount of calculation associated with the determination of the applied pulse width and the data correction. However, in this case, it is necessary to finally determine where to find a compromise point visually with a trade-off with the actual print data.

Further, in the embodiment, the printer of the multi-color thermal thermal head has been described. However, it is needless to say that the present invention is also effective in printing a single color, for example, only black.
In the embodiment, a printer using a multicolor heat-sensitive sheet is described. However, the present invention is naturally effective in a multi-gradation thermal printer provided with a separate ink sheet.

Further, the frame memory 11 can be replaced with a ring buffer for one color or less when there is a restriction on cost.

Also, a DSP can be used as a multiprocessor instead of the microcomputer 12. Although there is a method of implementing hardware, it is necessary to determine constants according to the individual differences of the multi-tone thermal head, to determine the constants by experiments, and to change the control output function, so that there is considerable freedom. Degree is required, and it is difficult to realize by hardware.

[0050]

As is apparent from the above description, the image recording method of the present invention can perform the temperature compensation in the main scanning direction and the sub-scanning direction with respect to the heat storage of the thermal head with high accuracy, and can reduce the image density. Can be faithfully reproduced.

Further, the thermal recording apparatus of the present invention can record a high-quality image by eliminating the influence of the heat accumulation of the thermal head in the sub-scanning direction and the main scanning direction.

[Brief description of the drawings]

FIG. 1 is a block diagram illustrating a configuration of an image recording apparatus according to an embodiment of the present invention.

FIG. 2 is a flowchart showing an operation of the image recording apparatus according to the embodiment of the present invention;

FIG. 3 is a perspective view illustrating a configuration of a thermal head.

FIG. 4 is a diagram for explaining a gradation specifying method in a thermal recording method.

[Explanation of symbols]

 10 Interface box 11 Frame memory 12 Microcomputer 13 Gradation control circuit 14 Multi-gradation thermal head 15, 28 Thermistor 16 Arithmetic processing unit 17 Memory 18 Line memory 21 Wear-resistant layer 22 Electrodes 23 Heating element 24 Glaze layer 25 Heating element substrate 26 Adhesive layer 27 head base

Continued on the front page (72) Inventor Taichi Ito 1006 Kazuma Kadoma, Osaka Prefecture Matsushita Electric Industrial Co., Ltd.F-term (reference)

Claims (6)

[Claims] 1. An image recording method for thermally recording a multi-tone image by a thermal line head, wherein the weighted addition of applied pulse energy supplied to each heating element of the thermal line head and a heating element in the vicinity thereof is sequentially repeated for each of the heating elements. The heat storage amount of the heating element is determined, and the average heat storage amount obtained by averaging the heat storage amounts of all the heating elements is used as a parameter to determine an applied pulse width for one gradation of the next line. Image recording method. 2. The image recording method according to claim 1, wherein the image density data given to each of the heating elements is corrected based on a ratio between a heat storage amount of the heating element and the averaged heat storage amount. . 3. A group comprising a plurality of said heating elements,
The image recording method according to claim 1, wherein the plurality of heating elements are regarded as one, and the arithmetic processing is performed using the group as a unit. 4. An image recording apparatus for thermally recording a multi-tone image using a thermal line head, a frame memory for storing density data of the image, an arithmetic processing means for calculating a temperature compensation amount for heat storage of the thermal line head, Gradation control means for reflecting the calculation result of the calculation processing means to the applied pulse to the heating element of the thermal line head, wherein the calculation processing means sets the temperature compensation amount in the sub-scanning direction to the heating element of the thermal line head. An image recording apparatus comprising: adjusting a pulse width of an applied pulse; and adjusting a temperature compensation amount in a main scanning direction by correcting the number of pulses applied to the heating element. 5. The arithmetic processing means determines an applied pulse width for one gradation of the next line by using, as a parameter, an average heat storage amount obtained by averaging the heat storage amounts of the respective heating elements of the thermal line head. The image recording apparatus according to claim 4, wherein: 6. The image processing apparatus according to claim 5, wherein the arithmetic processing unit corrects image density data of each heating element based on a ratio between a heat storage amount of each heating element and the averaged heat storage amount. Image recording device.