JPH07156432A - Thermal medium contrast recording method - Google Patents

Abstract

PURPOSE:To provide a method suitable for a sub-scanning dividing system preventing the generation of streak noise in a printing image and not affected by the heat generating state of adjacent pixels or the pixels of a previous line. CONSTITUTION:A heating resistor recording one pixel is heated by a short pulse row (e.g; 88 pulses per 4 printing cycle T=25ms) of the same width. The first pulse becomes the rising pulse of the heating resistor and printing is executed from a part where image data shows the min. density. The number of pulses is successively increased to increase the printing width in the sub-scanning direction of one pixel to successively increase printing density. For example, when medium contrast density is recorded by 22 pulses, printing is applied to the part of 1/4 in the sub-scanning directions of each pixel. When the printing start position of one pixel is shifted within a printing cycle with respect to the printing start position of adjacent pixels and the pixel 92 adjacent to a pixel 91 is shifted by 11/88 quantity and recording start is shifted by 11/88 quantity in the order of respective adjacent pixels, a recording start period is successively shifted and streak noise is not generated in a printing image in medium contrast recording.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a thermal halftone recording method in a thermal head in which a plurality of heating resistors are arranged side by side in the main scanning direction.

[0002]

2. Description of the Related Art Conventionally, sublimation type thermal transfer recording has been known as a color halftone recording method using a thermal head, but it requires a large amount of energy and therefore requires a long printing time.
Further, there is a problem that the cost is high because special paper is used. On the other hand, in the fusion type thermal transfer recording, printing can be performed with a small amount of energy and the cost is low, but since the ink donor film itself cannot obtain gradation even if the applied energy is changed, it is difficult to perform multi-gradation recording. A method has been proposed in which a heat generation area such as sub-scanning division and heat concentration is reduced to obtain gradation. For example, the heat-sensitive halftone recording method disclosed in JP-A-60-248074 and JP-A-3-219969 will be described with reference to the drawings. This heat-sensitive halftone recording method uses a heating element whose width in the sub-scanning method is smaller than that in the main scanning direction (hereinafter, this method is referred to as a sub-scanning division method).

FIG. 14 is a plan view of a heat generating portion of a thermal head used in this system. The electrode structure is a well-known alternating lead type. The alternating lead type electrode is a comb-shaped electrode 11.
To 16 and comb-shaped selection electrodes 31 to 35
Are arranged so as to face each other, and the strip-shaped resistors 20 arranged in one line in the main scanning direction are arranged on both poles. Printing by the thermal head is performed by selecting and energizing the selection electrode, for example, the selected selection electrode, for example, the selection electrode 31 and the resistor portion 20a sandwiched by the common electrodes 11 and 12 on both sides of the selection electrode 31 generate heat. .

FIG. 15 shows an example of print recording when halftone recording is performed using this thermal head. In halftone recording in color recording, yellow, magenta, and cyan are overprinted, and this print record shows the print state for each color. For example, (a) is the printing state of each pixel of the first color (pixels 41, 42, 43, 44, 45), (b)
Shows the printing state of the second color, and (c) shows the printing state of the third color. As shown in this figure, each pixel is often printed in halftone recording, and since the recording start timing of each pixel 41 to 45 is the same, the recording start of each pixel does not start when overprinting in color. They were aligned and formed a line in the main scanning direction. This line formation causes streak noise in the printed image and deteriorates the printing quality. Therefore, if registration is not performed accurately for each color, the degree of overlap of the lines formed by each color will change, causing a problem of color misregistration.

Further, FIGS. 16 and 17 show how the print dot size changes due to heat accumulation due to heat generation of adjacent pixels in the case of printing by the sub-scanning division method. In FIG. 16, the print dots of the same gradation are to be reproduced, but the print dots 411, 421, 431 are part 400, part 41 in the sub-scanning direction as the number of heat generation of the adjacent pixels of the resistor 20 increases.
It is expanding like 0. In addition, in FIG. 17, the print dot sizes 411, 42
2, 432 changes. In this print example, the print dots 412, 422, and 432 are trying to reproduce print dots of the same gradation, but the print interval with the previous line becomes narrower as the number of gradations increases, and it depends on the heat history of the previous line. It can be seen that the heat storage increases and the print dots expand in the portions 430 and 433 in the sub-scanning direction. As described above, when the size of the print dot reproduced in the heat generation condition of the adjacent pixel or the previous line pixel is changed, there is a problem that the halftone reproducibility and the color reproducibility are deteriorated and the print quality is deteriorated.

[0006]

SUMMARY OF THE INVENTION In view of the above problems, the present invention is capable of preventing streak noise from being generated in a printed image.
In addition, the present invention proposes a thermal halftone recording method suitable for the sub-scanning division method, which is not affected by the heat generation status of the adjacent pixels or the preceding line pixels.

[0007]

According to the heat-sensitive halftone recording method of the first invention, a heating resistor for recording one pixel is caused to generate heat by a large number of pulse trains of the same width so that the recording width of one pixel in the sub-scanning direction is reduced. The modulated and minimum density recording pixel is recorded using a plurality of initial pulses out of a large number of pulses, and after the minimum density recording pixel, the number of pulses is sequentially increased and recording is performed.
The recording start position of the pixel in the sub-scanning direction is different from that of the adjacent pixel within the width of one pixel. Furthermore, when the thin line or character detection function detects a thin line or a character, a recording start position of dots indicating the thin line or the character is provided at the same time.

In the thermal halftone recording method of the second aspect of the invention, the heating resistor for recording one pixel is heated by a large number of pulse trains of the same width to modulate the recording width of one pixel in the sub-scanning direction to obtain the minimum density. Recording pixels are recorded using a plurality of initial pulses out of a large number of pulses, and after recording the pixel with the minimum density, the number of pulses is sequentially increased and the heat is accumulated in the heating resistor due to the history before recording. The initial number of pulses is changed and corrected, and the heat accumulation of the heating resistor by the adjacent pixels that are generating heat during recording is corrected by changing the number of pulses that are sequentially applied after recording the pixel with the minimum density. It has a configuration.

[0009]

In the heat-sensitive halftone recording method of the first aspect of the invention, since the heating resistor for recording one pixel is heated by a large number of pulse trains of the same width, the recording start position can be freely selected and each heating resistor can be selected. Since the print start position of the print dots printed by the body in the sub-scanning direction is different from that of adjacent pixels within the width of one pixel in the sub-scanning direction, even if halftone is performed, the print dots are printed in the main scanning direction. Almost no lines are generated. Therefore, the problem of image quality deterioration such as streak noise and color shift due to line formation in the main scanning direction is solved, and good halftone recording can be realized.

In the heat-sensitive halftone recording method of the second invention, further, the influence of the history of the previous line and the heat accumulated by the adjacent pixels is almost eliminated, and the size of the print dot recorded by each heating resistor is substantially constant. Become. Therefore, the problem of deterioration in gradation reproducibility and color reproducibility due to heat storage is solved, and good halftone recording can be realized.

[0011]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The thermal halftone recording method according to the present invention will be described in detail below with reference to the embodiments shown in the drawings. Example 1 FIG. 1 is a diagram for explaining halftone expression by a heat-sensitive halftone recording method of the present invention, in which (a) is an explanatory view of a printed state and (b) is a resistor corresponding to printing. The pulse application state of is shown. In this embodiment, the width of one pixel in the main scanning direction is 84.7 μm and the width in the sub-scanning direction is 170 μm (84.
7 μm × 2), a case of executing halftone recording of approximately 64 gradations will be described. The thermal head is an alternating read type, the resolution in the main scanning direction is 300 dpi, and the width of the resistor in the sub-scanning direction is about 43 μm and about 1/4 of the width of one pixel in the sub-scanning direction of 170 μm. High-resolution ink donor film (PET substrate thickness 4.5 μm, ink application amount 2.0 g
/ M ² ) was used for printing and recording on synthetic paper. The recording paper is fed at a constant speed, and the printing cycle T is set to T = 25 ms so that the recording paper is fed by the width of one pixel in the sub-scanning direction. The heating resistor is, as shown in (b),
It is driven by 88 short pulses per printing cycle T. As shown in FIG. 3, the short pulse has a period of about 284 μsec. Then, about 284 μsec. The resistor heats up in 1/3 of the above.
The applied power is about 0.05 watt.

The reason why the pulse condition is selected will be described from the surface temperature of the resistor. The temperature of the resistor surface was measured using an infrared microscope RM-2A manufactured by Nippon Burns.
The position of the measurement spot 50 to be measured is, as shown in FIG. 4, the heating resistor 20 between the common electrode 11 and the selection electrode 31.
The center position of the measurement spot 50 is 34 μm in diameter.
It was φ. The measurement result of the resistor surface temperature is shown in FIG. As the measurement results show, under these printing conditions, the initial
The temperature of the resistor surface rises up to the pulse, but after that, it is in a steady state even if the applied pulse is increased. Since the melting point of the ink of the ink donor film is about 70 ° C., printing on the recording paper can be performed by maintaining the heating resistor at a temperature of 70 ° C. or more for about several ms. From this matter, the surface temperature of the heating resistor 20 is melted to raise it to a printable temperature in the vicinity of the 20th pulse from the initial stage, and in the subsequent pulse, the temperature of the heating resistor 20 is melted by the ink. It may be maintained near the temperature where it exists.

On the other hand, in the thermal transfer recording, stable recording reproducibility cannot be obtained unless the size of the printed dots is about the size of the heating resistor. Therefore, in order for the initial printed dot size to be about the size of the heating resistor, the width of the resistor used for printing in the sub-scanning direction is about 1/4 of the width of one pixel in the sub-scanning direction. It is sufficient to start printing with about 20 pulses, which is about ¼ of the total number of 88 pulses in one cycle. Further, in the sub-scanning division method, which is this printing method, one pixel is divided into fine pieces for printing, so that tailing or the like occurs if the temperature of the heating resistor does not immediately drop after the end of printing heat generation, and gradation Make the characteristics worse. However, in this embodiment, after the initial gradation is printed, the resistor is maintained near the ink melting temperature, so when the printing pulse is stopped, the resistance rapidly drops to below the ink melting temperature, causing problems such as tailing. Can be prevented.
Moreover, the heating resistor reaches the end of its life due to repeated stress caused by the rapid rise and fall of temperature, but in this method, the maximum temperature is low and the range of rise and fall of the temperature is narrow, as can be seen from the temperature waveform in FIG. The thermal stress is less than that of the thermal transfer recording, and the reliability of the thermal head is improved. further,
Since the temperature change of the thermal head is small, the thermal stress on the ink donor film is also reduced, the heat resistance of the ink donor film can be lowered, and the cost of the ink donor film can be reduced. In addition, the substrate thickness of the ink donor film can be reduced, and the resolution of the ink donor film can be improved.

Next, halftone recording will be described with reference to a recording example shown in FIG. In actual printing, pixel 51, pixel 52
As described above, printing is not performed in about 0 to 10 pulses, and printing is started from about 10 pulses, and in 15 pulses, one pixel 53 is divided into two and printed due to the alternate lead electrodes.
A printed dot having a resistor size (1/4 of one pixel) can be reproduced in the vicinity of 20 pulses. When the number of print pulses applied sequentially is further increased, the print dots are enlarged in the sub-scanning direction as shown by the pixels 56 and 57, and halftone recording can be performed. Here, the relationship between the number of pulses and the print density is examined (Fig. 5
reference). Concentration measurement is made by X-Rite Densitometer (Model No. 9
38) was used to measure a 2 cm square intermediate length patch. As can be seen from the graph showing the measurement results, the concentration is low when the applied pulse is small, and the concentration is gradually increased as the applied pulse is increased. That is, it was found that a smooth halftone characteristic was obtained by increasing or decreasing the number of pulses. The gradation expression by this method can be 64 gradations or more.

Next, a pulse application method (driving method) for setting the recording start position of the recording pixel in the sub-scanning direction to an arbitrary position within the width of one pixel is performed by performing halftone recording with print dots by a thermal head. An example will be described (see FIG. 6).
If the recording is started at the first timing of the printing cycle, the same recording as that of the recording example shown in FIG. 1 is performed. However, in this printing recording, the recording timing is 0/88 to 66 of the printing cycle.
The recording start position is changed within one pixel by changing / 88. For example, when printing is performed at the first timing of the printing cycle, the pixels are printed from the beginning in the sub-scanning direction as indicated by the first pixel 91. The second pixel 92 is the first pixel 9
By starting the printing by shifting 11 pulses from the start of one printing, the printing space of the first pixel 91 is shifted by a half space. That is, the pulse is shifted by a constant pulse with respect to the print start pulse of the adjacent pixel.
By starting the printing by shifting by one pulse, the recording start position of the recording pixel in the sub-scanning direction is sequentially changed.

By changing the recording timing within the printing cycle as described above, the recording start position of the recording pixel in the sub-scanning direction can be set at any position within the width of one pixel, and each heating resistor can be used. The print dots to be recorded can be recorded at any position within the width of one pixel in the sub-scanning direction,
Recording pixels do not continue in the main scanning direction. FIG. 7 shows an example of print recording of recording pixels by 22 pulses actually using this method. The recording start timing of this recording example uses a microprocessor or the like to generate a random number, and this value is used as the recording start timing. By varying the recording timing of each recording pixel in the range of 0/88 to 66/88 of the printing cycle, the print dot recorded by each heating resistor can be set within the width of one pixel in the sub-scanning direction. Recorded in position. That is, when halftone recording is performed, the first
Print dots 10 of the pixel 110 and the adjacent second pixel 120
Since 0s are not continuously connected linearly in the main scanning direction, no line in the main scanning direction is generated. As a result, the problem of streak-like noise and color misregistration that cause deterioration of image quality due to line formation in the main scanning direction is solved, and good halftone recording can be realized. In this case, since the heating resistor is heated by a large number of pulse trains having the same width, the recording start timing can be freely manipulated. Further, in this recording example, the shift amount with respect to the adjacent pixel is set to an arbitrary value, but it may be shifted with a constant amount, for example, the shift amount with respect to the adjacent pixel may be printed as half of one pixel. The problem of deterioration of image quality such as noise and color shift is solved.

Next, the resolution in the sub-scanning direction is 150 dpi,
A method of generating print data by the gradation conversion circuit when performing halftone recording with 88 pulses will be described with reference to the control flow of FIG. In step 200, image data of 1 to 256 gradations is input, and in step 210, it is converted into a numerical value of 0 to 88 while being .gamma.-corrected so as to match the gradation characteristic by this method. In step 220, the shift amount of the recording start timing is calculated from the n pulse number data (0 ≦ n ≦ 88) and the random number generated using the microprocessor, and 0/88 to (88) -N) / 8
Since the shift amount can be varied in the range of up to 8, a variation value is generated in the range of 0 to (88-n) using random numbers. Next, in step 230, binary data (88 bits) for the thermal head is generated in the print data generation circuit from the converted n pulse number data and the generated shift amount, and this data is rearranged and parallelized. -Serial conversion etc. is performed and it transfers to a thermal head, and printing is performed in step 240.

Next, a method of generating print data in the case of performing print recording by separating the fine line (line) / character portion of the document will be described with reference to the control flow of FIG. The resolution of the image data is 300 dpi in both the main scanning direction and the sub-scanning direction in order to express the character (line). 1 to 25 in step 300
Input 6-gradation image data. In step 310, it is determined from the image data whether the image is a character (line) or halftone. This determination is made by, for example, the same gradation data in the main scanning direction.
If the data continues for x bits or more, a detection algorithm for detecting a thin line or a character is added. When it is determined that the tone is a halftone, the process is performed by the above-described halftone control flow.
However, since the above-described processing shows the case where the resolution is 150 dpi, the resolution in the sub-scanning direction is converted to the processing when the resolution becomes 300 dpi. That is, in step 340, the resolution conversion in the sub-scanning direction is performed by averaging the two pixels in the sub-scanning direction. Then, in step 350, γ correction is performed so as to match the gradation characteristics, and the value is converted into a numerical value of 0 to 88. A shift amount is generated in step 360, and step 3
At 70, binary data for printing is generated and halftone processing is performed. When it is determined in step 310 that the same gradation continues for x bits and is a character (line), the process proceeds to step 320 and 300
If there is a recording pixel of dpi, it is set to 44 pulses, and if there is no recording pixel, it is set to 0.
Use pulse. Then, in order to match with the halftone in step 330, print data of 150 dpi and 88 pulses (44 pulses x 2) are generated at one time, converted into binary data, and printed by the thermal head in step 380. Is executed.

Here, as the resolution conversion method, the average of two pixels is used in this embodiment, but other neighboring pixels may be added for averaging, or a conversion method by thinning without averaging may be used. It is possible. Further, the method of varying the position of the printing of each resistor in the sub-scanning direction is not limited to this embodiment, and random numbers or the like are not used, and the amount of deviation in the sub-scanning direction from an adjacent resistor is different. Rules may be set.

Further, in the case of this embodiment, the printing start position of each resistor in the sub-scanning direction changes within a range of about 170 μm in width in the sub-scanning direction of one pixel. However, if the pixel density is low or if the position is changed by the width of one pixel in the sub-scanning direction, the deviation of each print dot on the first line may deteriorate the image quality depending on the intended use. In addition, the recording position may be changed within a range smaller than the width of one pixel in the sub-scanning direction. Further, although the halftone recording is realized by 88 pulses in this embodiment, the number of pulses is not limited to this example. The number of pulses is the pixel density
It is possible to change the size of the resistor, the printing cycle, the number of gradations, etc. In that case, it is necessary to further adjust the pulse width, power, etc. A series of processing is performed by a look-up table using a microprocessor for controlling the thermal head or a ROM.

In the recording method of this embodiment, both the character (line) area and the halftone area can be simultaneously printed with 88 pulses, and the offset amount is eliminated in the character (line) area so that the halftone area can be removed. It is possible to print with an offset amount. Further, in the halftone recording, since the print dots are not continuously connected linearly, a line in the main scanning direction does not occur, and a good halftone image can be obtained. Further, in the recording of characters (lines), the start positions of the respective print dots are aligned, and the reproducibility of fine lines and characters is improved.

Embodiment 2 This embodiment shows a recording method in which the influence of the heat generation condition of the adjacent pixel and the preceding line pixel in the sub-scanning direction division method is removed. FIG. 10 shows the relationship between the actually measured number of pulses and the area ratio of the printed dots from the recording example when printing was performed under the same conditions as in Example 1 (resistor size, pulse application conditions, etc.). The area ratio of the printed dots was measured by recording the printed dots in an isolated state in which the effect of heat storage can be ignored and digitizing the dots with a CCD camera through a microscope. From this measurement result, it was difficult to obtain a stable dot when the number of pulses was less than 10 and several pulses. Therefore, the graph shown in FIG. As can be seen from this graph, in the isolated dot state, a smooth halftone characteristic is obtained from around ten or more pulses, and gradation expression can be 64 gradations or more.

Next, FIG. 11 shows the relationship between the number of measured pulses and the area ratio of print dots when the same gradation is recorded on continuous lines without heating adjacent pixels under the same printing conditions. The measurement result when there is no effect of heat storage is shown by the dotted line, and the solid line is the measurement result when recorded by a continuous line.
As can be seen from this graph, an increase in the area ratio can be confirmed when recording is performed on a continuous line from the vicinity of more than 66 pulses. In other words, this increase in area ratio is considered to be due to the effect of heat storage in the previous line. Therefore, in this embodiment, an attempt was made to correct the increase in the area ratio due to heat storage by decreasing the pulse applied initially. That is, the number of pulses was gradually decreased within the range of 66 pulses or more and the increase of the area ratio due to heat storage.

Table 1 shows the optimized conditions for the number of pulses to be reduced depending on the area ratio.

[Table 1]

The pulse number was corrected under these conditions, and when the same gradation was recorded on continuous lines without causing adjacent pixels to generate heat, the measured pulse number and the area ratio of the printed dots were obtained. The result is shown by a triangle in the graph of FIG. As can be seen from this measurement result, the corrected measurement result almost agrees with the measurement result when there is no influence of heat storage, and this correction method can almost eliminate the increase in the printing area ratio due to heat storage in continuous lines. did it.

FIG. 12 shows the relationship between the number of measured pulses and the dot area ratio when the same gradation is recorded by heating only the adjacent pixels without heating the preceding line. The dotted line is the measurement result when there is no effect of heat storage, and the ∘ mark indicates the measured value when the adjacent pixels on one side are heated, and the ■ mark indicates the measured value when the adjacent pixels on both sides are heated. As can be seen from this measurement result, the influence of the heat storage of adjacent pixels on the area ratio is
It is large at 20 gradations or more and does not much depend on the number of pulses.

In the present embodiment, an attempt was made to reproduce the increase of the area ratio due to this heat storage by reproducing the smallest dot and reducing the number of pulses after the pulse (15 pulses) to correct it. For the purpose of aligning the thermal characteristics after the last pulse was applied, the last two pulses were left as the pulses to be reduced, and the pulses before the last pulse were reduced. Table 2 shows the results obtained by optimizing the relationship between the number of pulses and the area ratio.

[0028]

[Table 2]

Under this condition, the relationship between the measured pulse number and the dot area ratio when the same gradation is recorded by heating only the adjacent pixels without heating the preceding line will be examined. The measurement result after the one-sided heat generation correction is represented by the mark X in FIG. 12, and the measurement result after the both-sided heat generation correction is represented by the triangle mark. As can be seen from the graph of FIG. 12, both the X mark and the Δ mark are at the same position as the measurement result of the isolated dot state represented by the dotted line. From these facts, it is understood that the influence of heat generation of the adjacent pixels is almost eliminated by adopting this correction method.

Next, in order to confirm simultaneously the corrections from both the heat accumulation due to the preceding line and the heat accumulation due to the heat generation of the adjacent pixels, the patch of each gradation of 2 mm square is printed while performing the above correction, and the number of pulses is increased. And the area ratio was measured. The result is shown in FIG. The case of an isolated dot is shown by a dotted line, and the measurement result after correction in a 2 mm square patch of each gradation is shown by a straight line. The measurement result of the isolated dot represented by the dotted line and the measurement result of the patch of each gradation represented by the straight line are almost the same, and the effects of both the heat accumulation by the previous line and the heat accumulation by the heat generation of the adjacent pixels are almost corrected and eliminated by the correction of this method. You can see that In this measurement result, there is a slight difference in the measured values in the region where the number of pulses is large, but this is because the effect of heat accumulation due to the heat generation of the adjacent pixels on the previous line is ignored, and to improve the accuracy. Further, these influences may be corrected.

Further, although the number of pulses is reduced in order to correct the effect of heat storage, the position and number of pulses to be reduced are not limited to those in this embodiment. Further, in the present embodiment, the halftone recording is realized by 88 pulses, but the number of pulses is not limited to this example, and the number of pulses is changed according to the pixel density, the resistor size, the printing cycle, the number of gradations, etc. However, in that case, it is necessary to further adjust the pulse width, the power, and the like.

As in this embodiment, the heat accumulation by the previous line is corrected by changing the number of the initial plurality of pulses, and the heat accumulation by the adjacent pixel which is generating heat during recording is performed after the minimum density pixel is recorded. Since the number of pulses that are sequentially applied is changed and corrected, the size of the print dot recorded by each resistor is substantially constant without being affected by the history of the previous line and the heat accumulation by the adjacent pixels.

[0033]

In the thermal halftone recording method of the sub-scanning direction division method according to the first aspect of the invention, since the heating resistor is heated by a large number of pulses of the same width, the recording start timing can be set freely. The print dots recorded by each heating resistor can be recorded at any position within the width of one pixel in the sub-scanning direction, and the print dots of each line are not continuously connected linearly but in the main scanning direction. Does not cause line formation. Therefore, the streak-like noise in the printed image due to the line formation in the main scanning direction, which is a conventional problem, can be eliminated. Further, even if there is no accurate registration in color recording, the overlap of each color in each print dot is arbitrary, and color misregistration does not occur in the entire image. As described above, according to the present invention, the problem of deterioration of image quality such as streak noise and color misregistration in the main scanning direction is solved, and good halftone recording can be realized.

Further, by adding a thin line or character detection algorithm, when thin lines or characters are detected, the recording start timing of dots indicating the thin lines or characters is made the same to improve the reproducibility of the thin lines or characters. be able to.
As described above, not only the halftone image but also the image including characters and fine lines can obtain good image quality without unevenness of fine lines. Further, since the heating resistor is not excessively heated depending on the applied printing conditions, it is possible to prevent tailing and the like in terms of image quality, and improve the reliability of the thermal head. Further, it is possible to reduce the cost of the ink donor film and improve the resolution. Further, the thermal head using this method can be applied to any conventional thermal recording system, but has a remarkable effect in the melt transfer recording, which has not been able to express multi-gradation in the past, especially in the color halftone recording.

In the second aspect of the invention, the heat accumulation due to the history of the preceding line is further corrected by changing the number of the initial plurality of pulses, and the heat accumulation due to the adjacent pixels which generate heat at the time of recording is the minimum density pixel. After that, the number of pulses sequentially applied is changed to correct. Therefore, the history of the previous line and the heat accumulation by the adjacent pixels are almost corrected, and the size of the print dot recorded by each heating resistor is substantially constant without being affected by the history of the previous line and the heat accumulation by the adjacent pixels. For this reason,
It is possible to solve the problem of gradation reproducibility and color reproducibility image quality deterioration due to heat storage and realize good halftone recording.

[Brief description of drawings]

FIG. 1 is an explanatory diagram of a halftone recording example and pulse application by a heat-sensitive halftone recording method of the present invention.

FIG. 2 is a graph showing the relationship between pulse application and the temperature of a resistor.

FIG. 3 is an explanatory diagram of a pulse application state.

FIG. 4 is an explanatory diagram of a method for measuring a surface temperature of a heating resistor.

FIG. 5 is a diagram for explaining halftone expression by the heat-sensitive halftone recording method of the present invention.

FIG. 5 is a diagram for explaining gradation characteristics of the present invention.

FIG. 6 is an explanatory diagram of a halftone recording example and pulse application by the heat-sensitive halftone recording method of the present invention.

FIG. 7 is an explanatory diagram of a recording example of the thermal head of the present invention.

FIG. 8 is a control flow chart in the halftone recording method of the present invention.

FIG. 9 is a control flow to which the pictogram determination processing of the present invention is added.

FIG. 10 is a graph showing the relationship between the number of pulses and the area ratio of print dots.

FIG. 11 is a graph showing the relationship between the number of pulses and the area ratio of print dots when the same gradation is recorded in continuous lines without causing adjacent pixels to generate heat.

FIG. 12 is a graph showing the relationship between the number of pulses and the area ratio of print dots when the same gradation is recorded by heating adjacent pixels without heating the preceding line.

FIG. 13 is a graph showing the relationship between the number of corrected pulses and the area ratio of print dots according to the present invention.

FIG. 14 is a plan view of a conventional thermal head.

FIG. 15 is a diagram showing a printing example of a conventional thermal head.

FIG. 16 is a diagram showing a printing example of a conventional thermal head.

FIG. 17 is a diagram showing a printing example of a conventional thermal head.

[Explanation of symbols]

10 common electrodes, 20 heating resistors, 31 selection electrodes, 50 measurement spots, 51-57, 91-97
Pixels, 100 print dots, 110-130 pixels.

Continuation of the front page (51) Int.Cl. ⁶ Identification code Office reference number FI Technical display location H04N 1/52 4226-5C H04N 1/40 B 4226-5C 1/46 B

Claims (4)

[Claims] 1. A thermal halftone recording method using a thermal head in which a plurality of heating resistors whose width dimension in the sub-scanning direction is shorter than the width dimension of one pixel in the sub-scanning direction are arranged in parallel in the main scanning direction. The heating resistor for recording is heated by a large number of pulse trains of the same width to modulate the recording width of one pixel in the sub-scanning direction, and the recording pixel with the minimum density has a plurality of initial pulses among a large number of pulses. A thermal halftone recording method in which halftone recording is performed by recording by using a pixel of the minimum density and sequentially increasing the number of pulses to modulate the recording width of one pixel in the sub-scanning direction. 2. The recording start position of one pixel in the sub-scanning direction is set to 1
2. The heat-sensitive halftone recording method according to claim 1, wherein the pixel is different from the adjacent pixel within the width of the pixel. 3. The thermal halftone recording method according to claim 2, wherein a fine line or character detection function is provided, and when a thin line or a character is detected, the recording start positions of the dots indicating the fine line or the character are set at the same time. 4. A plurality of heating resistors each having a width in the sub-scanning direction shorter than that of one pixel are applied with a plurality of pulse trains of the same width to modulate the recording width of one pixel in the sub-scanning direction, and the intermediate width is obtained. In the halftone recording method that expresses the tonal density, the recording pixel with the minimum density is recorded by using the initial multiple pulses of a large number of pulses, and after the pixel with the minimum density is recorded, the number of pulses is sequentially increased and recorded. At the same time, the heat accumulation of the heating resistor due to the history before recording is corrected by changing the number of multiple initial pulses, and the heat accumulation of the heating resistor due to the adjacent pixels that generate heat during recording is recorded with the minimum density pixel recording. A thermal halftone recording method, characterized in that the number of pulses applied subsequently is changed and corrected.