JP2002067370A - Heat sensitive printing method and heat sensitive printer - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce generation of density variation at a portion where an image density changes very much. SOLUTION: A processor 12 converts image data into photographic energy through an energy conversion LUT memory 15. The processor 12 calculates a friction load added to a thermal head 5 at each line from a pressure distribution LUT memory 16 and a kinetic friction coefficient LUT memory 17, and a load variation volume is calculated based on a difference from the preceding line. When it moves from a high density line range to a low density line range, the photographic energy is converted to a value lower than usual so as not to increase the density rapidly and generation of linear density variation higher than the density around it can be controlled. When it moves from the low density line range to the high density line range, the photographic energy is converted to a value higher than usual so as not to lower the density rapidly and generation of linear density variation lower than the density around it can be controlled.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a thermal printing method and a thermal printer, and more particularly to a thermal printing method and a thermal printer for suppressing the occurrence of density unevenness even in a portion where the density of an original image changes rapidly.

[0002]

2. Description of the Related Art A thermal printer uses a thermal recording paper having at least one thermal coloring layer and a thermal head having a large number of heating elements formed in a line, and relatively moves the thermal head and the thermal recording paper. By driving each heating element of the thermal head based on the image data for one line, and applying printing energy corresponding to the image data to the thermosensitive recording paper for each pixel, the image is recorded on the thermosensitive recording paper in one line. Color recording is performed each time.

[0003] In such a thermal printer, it is known that unevenness occurs in the print density in a portion where the density of the original image changes rapidly. For example, as shown in FIG. 3, when printing an image in which a half-gray portion 4 surrounds a white background portion 3, the thermal recording paper 2 is conveyed at a constant pitch in the direction of the arrow during printing. During the transportation of the thermal recording paper 2, the thermal head 5 is pressed against the thermal recording paper 2. The thermal head 5 is provided with a heating element array in which a plurality of heating elements 5a are arranged in a line. Each heating element 5a
Is driven for the energizing time according to the image data, and the line A
The image is recorded line by line from to.

Lines A and B are high-density line areas having many high-density pixels. Line B to Line C
Until the low density line area has many white pixels.
Lines C to D are high density line areas.

In FIG. 4 showing the color density of a printed image on the XX line and the YY line in FIG. 3, the original image is faithfully reproduced on the XX line as shown in FIG. Has been reproduced. However, on the YY line, the density that should be uniform is higher than the original density near the line B, as shown in FIG. ing. In the vicinity of the line C, the image density is lower than the original image density, and linear density unevenness 8 occurs.

[0006] Such linear density unevenness is caused by frictional resistance between the thermal head and the thermal recording paper.
That is, although a protective layer made of resin is provided on the surface of the thermal recording paper, the coefficient of friction of the protective layer with respect to the thermal head changes depending on the average temperature of the thermal head. For example, when shifting from a high-density line region with many high-density pixels to a low-density line region with many white background pixels, the average temperature of the thermal head decreases and the frictional resistance suddenly increases. For this reason, the width of the conveyance pitch of the thermal recording paper is temporarily narrowed, and linear density unevenness having a higher density than the surroundings occurs. On the other hand, when the area shifts from a low density line area with many white pixels to a high density line area with many high density pixels, the frictional resistance suddenly decreases. The density becomes wider, and linear density unevenness having a lower density than surrounding areas occurs.

[0007] In order to suppress such density fluctuation due to the fluctuation of the transport pitch of the thermal recording paper, for example, Japanese Patent Application Laid-Open No.
In Japanese Patent No. 58806, the printing energy of each heating element driven based on the heating data of the line to be printed is obtained, and the load amount is calculated from each printing energy.
The load amount of each heating element is added to obtain the load amount for one line, and the heat start signal output in synchronization with the relative movement between the thermal head and the thermal recording paper is shifted according to the load amount. ing.

[0008]

The invention described in the above publication has a drawback that the circuit becomes complicated, for example, a timer circuit is required to speed up or slow down the output of the heat generation start signal.

It is an object of the present invention to provide a thermal printing method and a thermal printer capable of reducing the occurrence of density unevenness even in a portion having a large density change such as a sudden change in the load of a drive system, with a simple configuration. is there.

[0010]

In order to achieve the above object, in the thermal printing method according to the present invention, while the thermal head and the thermal recording paper are relatively moved, the heat generation of the thermal head is performed based on one line of image data. By driving the element, the printing energy corresponding to the image data is applied to the thermal recording paper for each pixel, and in the thermal printing method of performing color recording on the thermal recording paper line by line, the image data of each pixel for one line is converted. The printing energy is converted into printing energy to be applied to the thermal recording paper, and a table showing a relationship between a kinetic friction coefficient and a printing energy of each heating element with respect to the thermal recording paper and a table showing a pressing distribution of each heating element with respect to the thermal recording paper are referred to. Then, the load amount applied to the thermal head is calculated for each line, and the load amount of the i-th line and (i)
-1) The load variation of the i-th line is obtained from the difference from the load of the line, and each printing energy of the i-th line is corrected according to the load variation before and after the i-th line.

The thermal printer of the present invention drives each heating element of the thermal head based on the image data for one line while relatively moving the thermal head and the thermal recording paper, thereby converting the image data for each pixel. An energy conversion table indicating the relationship between image data and the printing energy to be applied to the thermal recording paper in a thermal printer that applies the corresponding printing energy to the thermal recording paper and color-records the thermal recording paper line by line. A dynamic friction coefficient table showing the relationship between the dynamic friction coefficient and the printing energy of each heating element, a pressure distribution table showing the pressure distribution of each heating element on the thermal recording paper, and an energy conversion table for each pixel in one line. After converting the image data into printing energy, refer to the dynamic friction coefficient table and the pressure distribution table. The load amount for each pixel is obtained, and these are added to calculate the load amount applied to the thermal head for each line, and the difference between the load amount of the i line and the load amount of the (i-1) line before the i line Computing means for determining the load fluctuation amount of the i-line and correcting each printing energy of the i-line according to the load fluctuation amount before and after the i-line. Further, the amount of load fluctuation is obtained from five lines including two lines before and after.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS In FIG. 1, image data read from an image input device (not shown) such as a video camera is written into a frame memory 10. The image data of the frame memory 10 is stored in the line memory 11 line by line.
Sent to The line memory 11 stores a plurality of, for example, five lines of image data. The image data input to the line memory 11 is sent to the arithmetic processing unit 12 line by line.

The arithmetic processing unit 12 includes an energy conversion LU
A T memory 15, a press distribution LUT memory 16, a dynamic friction coefficient LUT memory 17, and a work memory 18 are connected. In the energy conversion LUT memory 15, a table is written in which the gradation level of the image data is used as an address, and the printing energy to be given from the heating element 5a of the thermal head 5 to the thermal recording paper 2 is used as data. This printing energy is a value in consideration of the resistance unevenness of each heating element.

In the pressing distribution LUT memory 16, a table is written in which each heating element 5 a of the thermal head 5 is used as an address and each pressing force on the thermal recording paper 2 is used as data. This pressing force is measured at the time of final adjustment at the time of shipment from the factory, and is written in the pressing distribution LUT memory 16.

The dynamic friction coefficient LUT memory 17 stores a table showing the relationship between the printing energy of each heating element 5a and the dynamic friction coefficient between the surface of the thermal recording paper 2 and each heating element 5a. As shown in FIG. 2, this table has a relationship in which the higher the printing energy of the heating element 5a, the lower the dynamic friction coefficient. The table is obtained in advance for each type of the thermal recording paper 2 by an experiment or the like.
The dynamic friction coefficient is written to the LUT memory 17. And
The corresponding table is selected according to the type of the thermal recording paper 2 to be used.

The work memory 18 is used for temporarily storing the result of the operation performed by the operation processing unit 12, and the like.

The arithmetic processing unit 12 includes an energy conversion LUT
The image data of the i-th line and the J-th dot taken from the line memory 11 with reference to the memory 15 is converted into printing energy E (i, j). And this printing energy E
After performing edge enhancement and stain correction on (i, j), the corrected print energy E (i, j) is stored in the work memory 18.

The arithmetic processing unit 12 reads the dynamic friction coefficient μ (E (i, j)) corresponding to the printing energy E (i, j) from the dynamic friction coefficient LUT memory 17, and reads the J-th dot from the pressing distribution LUT memory 16. The pressing force N (j) at which the J-th heating element 5a presses the thermal recording paper 2 from one end of the thermal head 5 is read. Then, by multiplying them, the load applied to the thermosensitive recording paper 2 by the corresponding heating element 5a is obtained for the image data of the i-th line and the J-th dot. By adding this load for all the heating elements, the load F (i) received by the thermal recording paper 2 when the thermal head 5 thermally records the image of the i-th line on the thermal recording paper 2 is calculated. This calculation is represented by the following Equation 1.

[0019]

(Equation 1)

Then, by calculating the difference from the load F (i-1) of the previous line (i-1), the load variation .DELTA.F (i) of the i-th line is obtained. This is represented by Equation 2 below.

[0021]

(Equation 2)

If the load variation ΔF (i) obtained by the equation (2) is not “0”, the load applied to the transport system of the thermal recording paper 2 varies, so that the transport pitch of the thermal recording paper 2 is temporarily changed. change. For example, when shifting from a high-density line region with many high-density pixels to a low-density line region with many white pixels, the average temperature of the thermal head 5 decreases, and the frictional resistance suddenly increases. Therefore, ΔF (i)>
As a result, the width of the conveying pitch of the thermal recording paper 2 becomes temporarily narrow, and linear density unevenness higher than the surrounding density occurs. Further, when the area shifts from a low-density line area with many white pixels to a high-density line area with many high-density pixels, the frictional resistance suddenly decreases.
As a result, the width of the conveyance pitch of the thermal recording paper 2 temporarily increases, and linear density unevenness lower than the surrounding density occurs.

Since the load variation often extends over several lines, it is preferable to smooth the density unevenness in the range of the several lines. Therefore, it is preferable to correct the printing energy of the i-th line in consideration of the load fluctuation of the a and b lines before and after the i-th line (for example, a = b = 2). Therefore, the load variation ΔF (i)
, The printing energy E (i, j) before the load fluctuation correction
Is converted to the corrected printing energy Out (i, j) as shown in the following Expression 3. Note that k is the relative number of lines before and after the i line (k = −a to 0−b), and Coef
(k) is a correction coefficient for the relative position. This Coef (k)
Can be both positive and negative for k = -a to 0-b, respectively, but the total value in Equation 3 below is negative. Note that k = 0 is the i-th line.

[0024]

(Equation 3)

As is apparent from Equation 3, when there is no load fluctuation amount on the i-th line and the a and b-lines before and after the i-th line, ΔF (ik) = 0, and therefore Out (i, j) ) = E
(i, j), and the printing energy E (i, j) is not corrected. When shifting from the high density line area to the low density line area, ΔF (ik)> 0, and the printing energy E (i, j) is smaller than the original printing energy Out (i, j).
Is converted to The degree of this conversion is greatest at the i-th line and decreases toward the end a and b lines. As a result, the printing energy E (i, i,
j) is the smallest at the i-th line and approaches the original uncorrected value toward the a and b lines. Thereby, linear density unevenness in which the density is higher than the surrounding area is suppressed.

When shifting from the low-density line area to the high-density line area, ΔF (ik) <0, and the printing energy E (i, j) becomes larger than the original printing energy Out (i, j).
j). The degree of this conversion is greatest at the i-th line and decreases toward the end a and b lines.
As a result, the printing energy E given to the thermal recording paper 2
(i, j) is the largest at the i-th line and approaches the original uncorrected value toward the a and b lines. Thereby, linear density unevenness in which the density is lower than the surroundings is suppressed.
If the printing energy E (i, j) is increased or decreased and converted to printing energy Out (i, j), the load on the transport system of the thermal printer fluctuates, but this load is negligible. The transport pitch is not affected.

The printing energy Out (i, j) output from the arithmetic processing unit 12 is input to the energization time arithmetic control unit 19. The energization time calculation control unit 19 converts the current into an energization time consisting of a bias heating time and a gradation heating time corresponding to the printing energy Out (i, j), and sends the result to the head driver 6. The head driver 6 drives each heating element 5a of the thermal head 5 in accordance with the energizing time in synchronization with the conveyance of the thermal recording paper 2 by the capstan roller 21 and the nip roller 22. Each heating element 5a applies printing energy Out (i, j) to the thermosensitive recording paper 2 and color-records the thermosensitive recording paper 2 line by line. Note that the heating element 5a may be driven by a pulse instead of being driven continuously.

At a position facing the thermal head 5 via the thermal recording paper 2, a platen roller 23 for pressing the thermal recording paper 2 against the thermal head 5 is provided. The transport system of the thermal recording paper 2 by the capstan roller 21 and the nip roller 22 transports the thermal recording paper 2 at a constant pitch as long as the friction load generated between the thermal head 5 and the thermal recording paper 2 does not fluctuate. .

The operation of the thus configured thermal printer will be described. Image data is read from the frame memory 10 line by line and written to the line memory 11. Image data of five adjacent lines is written in the line memory 11. This line memory 11
The image data is read out line by line and sent to the arithmetic processing unit 12. The arithmetic processing unit 12 calculates the energy conversion L
After converting the image data into the printing energy E (i, j) to be applied to the thermal recording paper 2 with reference to the UT memory 15, the image is subjected to edge enhancement and stain correction, and then stored in the work memory 18. .

The arithmetic processing unit 12 reads the printing energy E (i, j) from the work memory 18 and
Referring to the UT memory 16 and the dynamic friction coefficient LUT memory 17, the load fluctuation amount ΔF (i)
Is calculated. Then, the printing energy E (i, i,
j) is converted into printing energy Out (i, j).

In a portion where the high density line area shifts to the low density line area, for example, in the line B shown in FIG. 3, the average temperature of the thermal head 5 sharply decreases and the frictional resistance sharply increases. For this reason, the width of the conveyance pitch of the thermal recording paper is temporarily narrowed, and linear density unevenness with high density occurs. However, in the present invention, the printing energy E (i, j) is converted into the printing energy Out (i, j) that is smaller than the original so that the density does not suddenly increase. The occurrence of density unevenness is suppressed.

In the line C shown in FIG. 3, the average temperature of the thermal head 5 sharply increases, so that the frictional resistance sharply decreases. For this reason, the width of the conveyance pitch of the thermal recording paper temporarily increases, and linear density unevenness lower than the surrounding density occurs. However, in the present invention, the printing energy E (i, j) is set so that the density does not drop sharply.
Is converted into the printing energy Out (i, j) which is larger than the original, so that the occurrence of linear density unevenness lower than the surrounding density can be suppressed.

The printing energy Out (i, j) output from the arithmetic processing unit 12 is sent to the energization time arithmetic control unit 19,
The current is converted into an energizing time including a constant bias heating time and a gradation heating time corresponding to the printing energy Out (i, j), and is sent to the head driver 6. The heating elements 5a of the thermal head 5 start generating heat at a timing synchronized with the conveyance of the thermal recording paper 2, and are driven in accordance with the power-on time. The printing energy Out (i, j) is given to the thermosensitive recording paper 2 by the heating element 5a, and an image is color-recorded on the thermosensitive recording paper 2 line by line.

In the embodiment described above, 1
The present invention relates to a monochrome thermosensitive recording paper in which one thermosensitive coloring layer and a protective layer are sequentially formed, and the present invention relates to a cyan thermosensitive coloring layer, a magenta thermosensitive coloring layer, a yellow thermosensitive coloring layer, and a protective layer. It can also be applied to a set color thermal recording paper. In this color thermosensitive recording paper, since the thermal sensitivity is lower as the layer is deeper, the recording is sequentially performed sequentially from the yellow thermosensitive coloring layer on the front side based on the difference in thermal sensitivity. The correction of the printing energy may be performed on all the thermosensitive coloring layers. However, since the streaks are not noticeable in the yellow thermosensitive coloring layer, the correction may be performed only on the recording of the magenta thermosensitive coloring layer and the cyan thermosensitive coloring layer.

[0035]

As described above, according to the present invention,
From the relationship between the kinetic friction coefficient and the printing energy of each heating element and the pressure distribution of each heating element, the load amount is calculated for each line to be recorded, and the amount of load fluctuation with the previous line is calculated. Since the printing energy is corrected in accordance with the load fluctuation amount, it is possible to suppress the occurrence of density unevenness even in a portion where the image density changes suddenly without using a complicated circuit.

[Brief description of the drawings]

FIG. 1 is a block diagram schematically illustrating a thermal printer embodying the present invention.

FIG. 2 is a graph showing a relationship between printing energy of a heating element and a dynamic friction coefficient of the heating element with respect to a thermosensitive recording sheet.

FIG. 3 is an explanatory diagram illustrating printing of an image having a high density line area and a low density line area.

FIG. 4 is a diagram showing a change in density on a line XX and a line YY in FIG. 3;

[Explanation of symbols]

 2 Thermal recording paper 5 Thermal head 5a Heating element 6 Head driver 7, 8 Density unevenness 11 Line memory 12 Operation processing unit 15 Energy conversion LUT memory 16 Press distribution LUT memory 17 Dynamic friction coefficient LUT memory 19 Energization time operation processing unit

Claims (3)

[Claims] 1. A printing energy corresponding to image data for each pixel is driven for each pixel by driving each heating element of the thermal head based on image data for one line while relatively moving the thermal head and the thermal recording paper. In a thermal printing method of applying color to a thermal recording paper and recording the thermal recording paper one line at a time, the image data of each pixel for one line is converted into printing energy to be applied to the thermal recording paper, and each heat generation to the thermal recording paper is performed. The load applied to the thermal head is calculated for each line with reference to a table showing the relationship between the kinetic friction coefficient and the printing energy for each element and a table showing the distribution of pressure applied to the heat-sensitive recording paper for each heating element. Is obtained from the difference between the load amount of the i-th line and the load amount of the preceding (i-1) line, and the load fluctuation amount before and after the i-th line is calculated. Thermal printing method characterized by correcting the respective printing energies of the i line Flip. 2. A method according to claim 1, wherein each of the heating elements of the thermal head is driven based on one line of image data while the thermal head and the thermal recording paper are relatively moved. In a thermal printer which applies color to the thermal recording paper and color-records the thermal recording paper line by line, an energy conversion table indicating the relationship between image data and printing energy to be applied to the thermal recording paper; A dynamic friction coefficient table showing the relationship between the dynamic friction coefficient and the printing energy, a pressing distribution table showing the pressing distribution of each heating element on the thermal recording paper, and an image conversion energy table for printing image data for each pixel in one line. After that, the load amount for each pixel is calculated with reference to the dynamic friction coefficient table and the pressure distribution table. Calculating a load applied to the thermal head for each by adding these lines, the i-line loading of the previous (i
-1) calculating means for calculating the load variation of the i-th line from the difference from the load of the line, and correcting each printing energy of the i-line according to the load variation before and after the i-line. Thermal printer. 3. The thermal printer according to claim 2, wherein the amount of load fluctuation is determined from five lines including two lines before and after.