JPH09123509A - Method and apparatus for thermally halftone recording - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a thermal halftone record which makes it possible to improve the dot reproducibility in the case of printing with low density without influence to the printing of high density. SOLUTION: A selecting circuit 14 fetches respective input image signals sequentially sent from data input means 15, selects the converting means allocated with the image from input and output converting means 13 by referring to the pixel disposition of a plurality of previously specified converting means, and transfers density information. The respective converting means send the transferred density information according to different rules for the low density part, convert the intermediate or high density except it according to the same rule to deal with the number of pulses, and sends it to pulse applying means 12. The means 12 generates power pulses of the sent number of pulses, and applies it to the heating element 2 of a thermal head 1. The ink doner film 7 corresponding to the heating part of the element 2 is melted, brought into contact with a recording sheet 10 and transferred.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to thermal transfer recording using a thermal head, and more particularly, to a thermal halftone recording method capable of obtaining a gradation excellent in dot reproducibility in a low density portion when performing halftone recording. Regarding the device.

[0002]

2. Description of the Related Art Conventionally, as a color halftone recording method using a thermal head, there are sublimation type thermal transfer recording and fusion type thermal transfer recording. The sublimation type thermal transfer recording has problems that a large amount of energy is required and it takes a long time for printing, and a special paper is used, which causes a cost. On the other hand, melt-type thermal transfer recording can be printed with a small amount of energy and is low in cost, 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. Also, a method has been proposed in which a gradation is obtained by subdividing a heat generation area such as sub-scanning division and heat concentration.

A sub-scanning division method for obtaining halftone recording uses a thermal head in which a plurality of heating elements each having a heating element whose length in the sub-scanning direction is shorter than the length of one pixel in the sub-scanning direction are arranged in parallel in the main scanning direction. A power pulse according to an input image signal is applied to the thermal head, and paper is fed by a length shorter than one pixel in the sub-scanning direction for printing. FIG.
Is the print result of the halftone recording by the sub-scanning division method, and 12 from 0 to 127 for the adjacent 9 pixels.
It shows how to reproduce 8 gradations. In this case, as shown in FIG. 9A, the number of power pulses applied to the heating element of the thermal head is proportional to the density information (gradation) indicated by the input image signal and corresponds to the conversion rule. And the printing energy is changed by this.

That is, the number of power pulses corresponding to the density information (gradation) indicated by the input image signal is increased in proportion to the gradation by using the same input / output conversion rule from low density to high density. Then, a power pulse of the above-mentioned number of pulses is applied to the heating element for printing the corresponding pixel, and the hot-melt ink melted by the heat generated by the heating element is transferred to the recording paper to perform printing.

[0005]

FIG. 9B shows the gradation represented by the input image signal and the reflection density of the corresponding reproduced pixel when halftone recording is performed by using the above conversion rule. It is a figure. Letting D be the reflection density, let R be the reflectance that represents the ratio of the amount of reflected light to the amount of incident light when light is incident on the image, and the relationship of D = log ₁₀ (1 / R) holds. In the figure, in the low density portion, especially in 24 gradations or less, the dots are hardly printed or the dots are printed although the power pulse is applied, but the reflection density is 0.07 or less and it is substantially visible. There is no state. This is because the heat-melting ink cannot be melted / transferred unless the heat generating element of the thermal head is applied with a power pulse a certain number of times or more and sufficient heat is generated.
This is because, in the highlight portion of two gradations or less, the ink may not be transferred to the recording paper and may not appear as dots even though the power pulse is applied. Therefore, there is a problem that dot reproducibility is reduced and printing is likely to occur in printing in a low density portion. Further, the rise of the reflection density becomes steep from around 24 gradations where dots start to occur, and there is a problem that an ideal gradation that changes smoothly cannot be obtained.

In order to improve the deterioration of dot reproducibility in the low density portion as described above, the dither size (pixel size)
Is set to a large value and one pixel is printed with a larger number of gradations to obtain a smooth gradation even in the printing of low density areas. In this case, however, the resolution is insufficient in the printing of high density areas. There was a point. Further, attempts have been made to further subdivide the size of the heating element, but there is a limit due to the difficulty of production and the manifestation of variations in resistance value.

The present invention has been made in view of the above circumstances, and a thermal halftone recording method and apparatus capable of improving dot reproducibility in printing in a low density portion without affecting printing in a high density portion. The purpose is to provide.

[0008]

In order to achieve the above object, the thermal halftone recording method according to the first aspect of the present invention is arranged such that a plurality of heating elements having a length in the sub-scanning direction shorter than the length of one pixel in the sub-scanning direction are arranged in parallel in the main scanning direction. A power pulse consisting of the number of pulses corresponding to the density information of the input image signal is applied to the thermal head, the paper is fed shorter than the length of one pixel in the sub-scanning direction for printing, and the pulse of the applied power pulse is applied. A thermal halftone recording method that expresses a halftone of a printed pixel by area gradation that changes the printed area in each pixel by the number includes the following contents. When converting the input image signal into the number of power pulses, two or more different conversion rules are set. At least one of the plurality of conversion rules corresponds to a power pulse having a pulse number of a certain number or more regardless of the density information indicated by the input image signal in printing the low density portion. When printing the low-density portion, halftone in the low-density portion is expressed by printing in which print pixels according to the plurality of conversion rules are mixed.

According to the above method, the pixels printed by the heating element for applying the power pulse consisting of a certain number or more of pulses to the pixels forming the low density portion, regardless of the density information indicated by the input image signal. Since it exists, it is possible to alleviate a sharp decrease in the reflection density of the reproduced image due to the decrease in the density indicated by the input image signal in the low density portion, and obtain a gradation that changes smoothly.

According to another aspect of the thermal halftone recording method of the present invention, a thermal head in which a plurality of heating elements each having a length in the sub-scanning direction shorter than one pixel in the sub-scanning direction are arranged side by side in the main scanning direction is used as density information of an input image signal. A power pulse consisting of a corresponding number of pulses is applied, paper feeding is performed by feeding paper that is shorter than the length of one pixel in the sub-scanning direction, and the printing area in each pixel is changed according to the pulse number of the applied power pulse. The thermal halftone recording method for expressing the halftone of the print pixel by the area gradation to be performed includes the following contents. At the time of printing in the low density portion, printing is performed by a heating element that applies a power pulse consisting of a certain number or more of pulses regardless of the density information indicated by the input image signal, and print pixels are printed in the main scanning direction and the sub scanning direction. Periodically arranged at intervals. Except for the print pixels arranged periodically, printing is performed by a heating element that applies a power pulse consisting of a pulse number that increases or decreases according to the density information of the input image signal, thereby expressing the halftone of the low density portion. To do.

According to the above method, a pixel (hereinafter referred to as a normal pixel) to be printed by a heating element that applies a power pulse consisting of a pulse number that increases or decreases according to the density information indicated by the input image signal is input to the input image signal. The pixels to be printed (hereinafter referred to as constant density pixels) are periodically arranged by the heating element that applies the power pulse consisting of a certain number or more of pulses regardless of the density information shown in FIG. The sudden decrease in the reflection density of the reproduced image due to the decrease in the density of the input image signal in the density part is mitigated, and the number of pulses to the corresponding heating element is adjusted for normal pixels according to the density of the input image signal. Since it is decreased, the overall density can be gradually decreased, and a smoothly changing gradation can be obtained. Further, due to the periodic arrangement of the constant density pixels, reproduction dots are constantly generated for the density indicated by the input image signal during low density printing, and gradation reproduction can be stabilized.

A heat-sensitive halftone recording method according to a third aspect is the heat-sensitive halftone recording method according to the second aspect, wherein a power pulse composed of a certain number or more of pulses applied to the heating element during printing of the low density portion. Has a reflection density of 0.15 or more when printing is performed on all pixels by applying the power pulse to the heating element, and printing is performed on the periodically arranged pixels by applying the power pulse to the heating element. The number of power pulses is set so that the reflection density is 0.07 or less.

The number of power pulses set as described above is generated due to the difference in the number of pulses applied to the heating element that prints both when constant density pixels are periodically arranged with respect to normal pixels. Regarding the inflow of heat from the fixed density pixel to the normal pixel, the heat inflow is relatively small when printing the part with relatively high density in the low density part, and the number of pulses is sufficient for the constant density pixel to appear as a dot. Is. On the other hand, when printing a relatively low density portion in the low density portion, as the number of power pulses applied to the heating element for printing a normal pixel decreases as the density indicated by the input image signal decreases, When the flow of heat from the constant density pixel to the normal pixel increases and the dot area of the constant density pixel decreases, and when no electric power pulse is finally printed on the heating element that prints the normal pixel, the constant density pixel becomes a dot. Since the number of pulses does not occur, the low density portion can be reproduced as a smooth gradation in which the density gradually changes from 0 gradation.

According to another aspect of the thermal halftone recording apparatus of the present invention, a thermal head in which a plurality of heating elements each having a length in the sub-scanning direction shorter than one pixel in the sub-scanning direction are arranged side by side in the main scanning direction is used for density information of an input image signal. An energy supply means for applying a power pulse composed of a corresponding number of pulses to the thermal head, printing is performed by feeding a paper shorter than the length of one pixel in the sub-scanning direction, and the pulse number of the applied power pulse By the area gradation that changes the printed area in each pixel by
A thermal halftone recording apparatus that expresses halftones of print pixels includes the following configuration. The energy supply means has two or more different conversion means when converting the input image signal into a pulse number. At least one of the plurality of converting means is a converting means for generating a pulse number of a certain number or more regardless of the density information indicated by the input image signal when printing the low density portion.

According to the above arrangement, as a means for converting the input image signal into the pulse number, there is provided a converting means for generating a pulse number of a certain number or more irrespective of the density information indicated by the input image signal. Since the pixels for printing by applying the power pulse of the generated pulse number to the heating element are present in the pixels forming the low density portion, the reflection density of the reproduced image due to the decrease in the density of the input image signal in the low density portion It is possible to alleviate a sharp drop in the gradation and obtain a gradation that changes smoothly.

[0016]

BEST MODE FOR CARRYING OUT THE INVENTION An example of an embodiment of the present invention will be described below with reference to the drawings. FIG. 1 is a schematic diagram of the configuration of a thermal halftone recording apparatus. The thermal halftone recording apparatus includes a thermal head 1 including a plurality of heating elements 2 arranged in a line in the front and back directions of the figure, a lever 3 for supporting the thermal head 1, and a lever-3 at a predetermined pressure. The elastic member 4 that presses against the platen roller 5 is provided.
A base material 6 and an ink donor film 7 are connected to a heating element 2 and a platen roll from an ink supply medium 8 formed by superposing an ink donor film 7 made of a heat-meltable ink on a film-shaped base material 6 and winding the ink donor film 7 in a roll shape. The substrate 6 is conveyed between the la 5 and the substrate 5, and is collected by the roller 9. The recording paper 10 is inserted between the heating element 2 and the platen roller 5 on the ink donor film 7 side, and is conveyed by the platen roller 5. Each heating element 2 is selectively energized by the pulse applying means 12 to generate heat, and the ink donor film 7 corresponding to the heating element 2 is transferred to the recording paper side to form an image 11. . The platen roller 5 and the take-up roller 9 are rotationally driven by driving means (not shown).

The pulse applying means 12 includes an input / output converting means 1
3, the input / output conversion means 13 is connected to the selection circuit 14, and the selection circuit 14 is connected to the data input means 15. The data input means 15 takes in an input image signal for printing with a thermal head. The selection circuit 14 selects the conversion means in the input / output conversion means 13 when converting each input image signal from the data input means 15 into the number of pulses applied to the heating element 2.
The input / output conversion unit 13 has a plurality of conversion units (for example, the first conversion unit 13a and the second conversion unit 1) having different conversion rules when converting the input image signal into the pulse number.
3b, ..., Nth converting means 13n), and outputs any one of the outputs to the pulse applying means 12. The pulse applying means 12 applies the power pulse of the designated number of pulses to the heating element 2. The number of signal lines connected to the heating elements 2 from the data input means 15 through the selection circuit 14, the input / output conversion means 13 and the pulse applying means 12 is the same as that of the heating elements, but it is simplified in the figure. Only one is shown.

Next, the operation of the thermal halftone recording apparatus will be described. The selection circuit 14 takes in each input image signal sequentially sent from the data input means 15, refers to the pixel arrangement of a plurality of conversion means defined in advance, and inputs / outputs the conversion means to which the corresponding pixel is assigned. One of the converting means 13 is selected and the density information is transferred. Input / output conversion means 1
Each conversion means in 3 converts the density information transferred from the selection circuit 14 into the number of pulses according to each conversion rule, and sends it to the pulse application means 12. Each conversion unit performs conversion of the low density portion according to different rules, but performs conversion of medium density or high density other than that according to the same rule.
The pulse applying means 12 generates power pulses of the number of pulses sent and applies them to the heating element 2 of the thermal head 1.

The substrate 6 and the ink donor film 7 are
As the platen roller 5 rotates, the platen roller 5 is conveyed in the direction of the arrow and pressed against the recording paper 10 by the pressure contact force between the platen roller 5 and the thermal head 1 and selectively energized by applying the above-mentioned power pulse. The ink donor film 7 corresponding to the heat generating portion of the heat generating element 2 is melted and transferred to the recording paper 10 to form an image 11. On the other hand, the base material 6 on which the reverse image of the image 11 remains is collected by the winding roller 9.

When printing, the resolution is 300 dpi, the size of the heating element is 84.7 μm in width, and the length (length in the sub-scanning direction).
Using a 50 μm strip-shaped lift-off thermal head 1,
A power pulse with an applied power of 0.086 W was applied at the timing shown in FIG. That is,
194μsec cycle, 19.4% duty for thermal head 1
A width pulse of 100 pulses was applied, and the printing cycle of one pixel was set to 25 msec. Also, the length of one pixel (length in the sub-scanning direction)
Is set to 169 μm and the paper feed is 169 μm to 128 by microstep feed of a stepping motor.
The gear ratio of the platen roller 5 was adjusted so that the platen roller 5 was moved by one pulse.

Here, even when a power pulse having a large number of pulses is applied to the heating element 2, as shown in the left side of FIG. 2B, until about 24 pulses elapse until the heating element 2 heats sufficiently. Only the paper feed is performed without the dots appearing, and dots are not generated for about 32 to 33 μm from the position of the heating element 2 at the start of pulse application for one line. In consideration of this blank portion, the dot length when reproducing the highest gradation with the highest density is one pixel length 1
The number of applied pulses may be determined so as to match 69 μm, and 100 pulses are set here. Incidentally, since the printing is usually performed by the residual heat even after the application of the power pulse to the heating element 2 is stopped, the number of applied pulses is set in consideration of the residual heat. The reproduced image in this case is shown in FIG.
(B) As shown on the right, the heating element 2 at the start of pulse application
It is considered that one pixel starts from a position deviated from the position by about 32 to 33 μm, and the printing results of the subsequent examples also follow this.

A specific embodiment will be described below. The reflection density is 0.15 or more when printing is performed on all pixels by applying a power pulse to the heating element, and the same power pulse is applied to the heating element. The number of power pulses having a reflection density of 0.07 or less when applied and printed periodically for pixels is set to 32.

[0023]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS As a first embodiment, the input / output conversion means 13 includes two conversion means, a first conversion means 13a and a second conversion means 13b.
Three conversion means are provided, and the conversion rule of the number of pulses with respect to the gradation is set as shown by the solid line (conversion 1) and the dotted line (conversion 2) in FIG. The conversion 1 performed by the first conversion unit 13a is performed when the gradation indicated by the input image signal is 32 gradations or less,
The number of pulses is made to correspond to 32 regardless of the density, and in the case of 32 gradations or more, the number of pulses is increased or decreased according to the gradation shown by the input image signal. The conversion 2 performed by the second conversion unit 13b is made to correspond to the number of pulses that is increased or decreased according to the gradation indicated by the input image signal from low density to high density as in the conventional case. At this time, as shown in FIG. 3B, conversion 1 and conversion 2
The selection circuit 14 is set in advance so that the pixels to which is applied are arranged at intervals of one pixel in the main scanning direction and the sub scanning direction. The size of one pixel is 84.7 in width.
μm and length 169 μm. Ink donor film 7
As the recording paper 10, a wax ink having a base thickness of 4.5 μm, an ink amount of 2.0 g / m ² , and an ink melting point of about 70 ° C. was used, and as the recording paper 10, a peach coated paper manufactured by Nisshinbo was used.

As a result of printing, print dots as shown in FIG. With respect to the pixels for which the conversion 2 is applied and printing is performed as usual, no dots appear at 24 gradations or less. Heating element 2 corresponding to the pixel to which conversion 1 is applied
32 pulses are always applied in the case of 32 gradations or less, but when printing 20 gradations, heat flows from the pixel to which the conversion 1 is applied to the pixel to which the conversion 2 is applied, and the actual application The dot area becomes smaller with respect to the pulse, and it does not appear as a dot at a density lower than that. Therefore, it is possible to reproduce up to around 20 gradations with respect to the density of 24 gradations or less, which has not been reproduced conventionally, and to reduce the sharp decrease of the reflection density of 32 gradations or less as shown in FIG. 4B. The characteristics were obtained.

[0025]

EXAMPLE As a second example, a recording apparatus having the same thermal head as that of the first example was used, and the input / output conversion means 13 was also configured to print. However, this time, in order to increase the stability of the minute dots, the size of one pixel is set to a width of 169 μm and a length of 169 μm, and one pixel is printed by two heating elements adjacent in the main scanning direction. That is, as shown in FIG. 5A, selection is made such that two pixels to which the conversion 1 and the conversion 2 are applied are arranged in the main scanning direction and one pixel is arranged in the sub scanning direction at intervals of one pixel. The settings were made beforehand in the circuit 14. As the recording paper 10, Xerox L paper was used, and a wax layer having a thickness of about 10 μm was printed on the entire surface before printing, and printing was performed with the surface smoothed.

As a print result, print dots as shown in FIG. 5 (b) were produced, and it was possible to reproduce up to around 20 gradations as in the first embodiment. Therefore, FIG.
As shown in, the density characteristics that alleviate the sudden decrease in the reflection density of 32 gradations or less were obtained.

[0027]

EXAMPLE As a third example, a recording apparatus having a thermal head similar to that of the second example is used, three conversion means are provided in the input / output conversion means 13, and each conversion rule is as shown in FIG.
The procedure shown in (a) is indicated by the one-dot chain line (conversion 1), the solid line (conversion 2), and the dotted line (conversion 3). First conversion means 1
The conversion 1 performed in 3a is the same as the conversion 1 in the first embodiment. The conversion 2 performed by the second conversion unit 13b and the conversion 3 performed by the third conversion unit 13c moderate the decrease of the pulse at 32 gradations or less as compared with the conversion 2 in the first embodiment, and the conversion 2 is performed. It is assumed that 3 reduces the pulse more gently, and both of them correspond to the pulse number in the same way as the conversion 1 in the case of 32 gradations or more. The size of one pixel is the same as that in the second embodiment, and one pixel is printed by two heating elements adjacent in the main scanning direction. That is, as shown in FIG. 6B, in the main scanning direction, a column in which two pixels to which conversion 1 and conversion 2 are applied are arranged, and two pixels to which conversion 3 and conversion 1 are applied are arranged. Columns are arranged alternately, and in the sub-scanning direction, the conversion 1 and the conversion 3 are arranged at intervals of one pixel and the conversion 2
The selection circuit 14 is set in advance so that two columns each of which is a column in which the conversion 1 and the conversion 1 are arranged at intervals of one pixel are alternately arranged. As the recording paper 10, a postcard manufactured by a government agency was used. Before printing, a wax layer having a thickness of about 10 μm was printed on the entire surface and printing was performed with the surface being smooth.

As a result of printing, print dots as shown in FIG. 7 (a) were produced. In the conversion 3, the number of pulses decreases more gradually with the decrease in the gradation represented by the input image signal than the conversion 2. Therefore, the dot area of the pixel to which the conversion 2 is applied is
It decreases before the dot area of the pixel to which the conversion 3 is applied. Therefore, due to the inflow of heat from the pixel to which the conversion 1 is applied, which is always applied with 32 pulses, to the adjacent pixel, the pixel adjacent to the pixel to which the conversion 2 is applied is adjacent to the pixel to which the conversion 3 is applied. The dot area decreases before that. By this conversion, it is possible to reproduce even the low density portion from 0 gradation, and as shown in FIG.
A density characteristic was obtained in which the reflection density smoothly changed from 32 gradations to 0 gradation.

[0029]

According to the present invention, at the time of printing a low density portion, a plurality of means for converting the density information indicated by the input image signal into the pulse number of the power pulse is applied, and at least one converting means is the input image signal. Since the number of pulses of a certain number or more is made to correspond regardless of the density information shown by, the sharp decrease of the reflection density of the reproduced image due to the decrease of the density of the input image is alleviated, and other conversion means By making the number of pulses gradually decrease in accordance with the decrease in density, the overall density can be gradually decreased. Therefore, it is possible to improve the dot reproducibility in the printing of the low density portion and reproduce the smoothly changing gradation without affecting the printing of the high density portion.

Further, by periodically arranging the printing by each conversion means in the main scanning direction and the sub scanning direction, reproduction dots are constantly generated for the input image signal, and gradation reproduction can be stabilized. In the gradation reproduction of the present invention,
It is possible to effectively use the power pulse that has been applied even at the time of printing a low density portion that does not appear as a dot in the past.

[Brief description of the drawings]

FIG. 1 is a schematic diagram of the configuration of a heat-sensitive halftone recording apparatus according to the present invention.

FIG. 2A is a diagram showing a power pulse application state to a thermal head according to the present invention, and FIG. 2B is a diagram showing an actual printing position according to the present invention.

FIG. 3A is a diagram showing an input / output conversion rule of the first embodiment of the present invention, and FIG. 3B is a diagram showing a pixel arrangement for printing according to each conversion rule.

4A is a diagram showing a printing result according to the first embodiment, and FIG. 4B is a diagram showing a density characteristic of printing according to the first embodiment.

5A is a diagram showing a pixel arrangement for printing according to each conversion rule of the second embodiment, FIG. 5B is a diagram showing a printing result according to the second embodiment, and FIG. 5C is a second diagram. FIG. 6 is a diagram showing a density characteristic of printing according to an example.

FIG. 6A is a diagram showing an input / output conversion rule according to a third embodiment of the present invention, and FIG. 6B is a diagram showing a pixel arrangement for printing according to each conversion rule.

7A is a diagram showing a printing result according to the third embodiment, and FIG. 7B is a diagram showing a density characteristic of printing according to the third embodiment.

FIG. 8 is a diagram showing gradation reproduction by a conventional sub-scan division method.

FIG. 9A is a diagram showing a conventional input / output conversion rule,
FIG. 6B is a diagram showing the density characteristic of printing according to the conventional input / output conversion rule.

[Explanation of symbols]

1 ... Thermal head, 2 ... Heating element, 3 ... Lever,
4 ... Elastic member, 5 ... Platen roller, 6 ... Base material,
7 ... Ink donor film, 8 ... Ink supply medium,
Reference numeral 9 ... Winding roller, 10 ... Recording paper, 11 ... Image, 12 ... Pulse applying means, 13 ... Input / output converting means, 14 ... Selection circuit, 15 ... Data input means

Claims (4)

[Claims] 1. A thermal head in which a plurality of heating elements each having a length in the sub-scanning direction shorter than the length in the sub-scanning direction of one pixel are arranged in parallel in the main scanning direction, and an electric power pulse having a number of pulses corresponding to density information of an input image signal is supplied. Printing is performed by applying a paper feed shorter than the length of one pixel in the sub-scanning direction, and by changing the printed area in each pixel according to the pulse number of the applied power pulse, the area gradation changes the print pixel In the thermal halftone recording method for expressing halftone, in converting the input image signal into the number of power pulses, two or more different conversion rules are set, and at least one of the plurality of conversion rules is In the printing of the low density portion, the power pulse consisting of a certain number or more of pulses is made to correspond regardless of the density information indicated by the input image signal, and when printing the low density portion, Thermal halftone recording method characterized by expressing a halftone in the low density portion by printing in which a mix of each print pixel by 換規 law. 2. A thermal head in which a plurality of heating elements each having a length in the sub-scanning direction shorter than one pixel in the sub-scanning direction are arranged side by side in the main scanning direction, and an electric power pulse having a number of pulses corresponding to density information of an input image signal is supplied. Printing is performed by applying a paper feed shorter than the length of one pixel in the sub-scanning direction, and by changing the printed area in each pixel according to the pulse number of the applied power pulse, the area gradation changes the print pixel In a thermal halftone recording method that expresses halftone, when printing a low density portion, printing is performed by a heating element that applies a power pulse consisting of a certain number or more of pulses regardless of the density information indicated by the input image signal. At the same time, the print pixels are periodically arranged at intervals in the main scanning direction and the sub-scanning direction, and the print pixels other than the periodically arranged print pixels are set according to the density information of the input image signal. By printing the heat generating element for applying a power pulse to a pulse number be increased or decreased, thermal halftone recording method is characterized in that to represent the halftone of the low density portion. 3. A power pulse composed of a certain number or more of pulses applied to a heating element when printing a low density portion is a reflection when all the pixels are printed by applying the power pulse to the heating element. The density is 0.15 or more, and the reflection density when the power pulse is applied to the heating element to print the periodically arranged pixels is 0.
The thermal halftone recording method according to claim 2, wherein the number of pulses of the power pulse is set so as to be 07 or less. 4. A thermal head in which a plurality of heating elements having a length in the sub-scanning direction shorter than one pixel in the sub-scanning direction are arranged side by side in the main scanning direction, and a power pulse comprising a number of pulses corresponding to density information of an input image signal. An energy supply unit for applying to the thermal head is provided, and printing is performed by feeding paper that is shorter than one pixel in the sub-scanning direction, and the printed area in each pixel is determined by the number of pulses of the applied power pulse. In a thermal halftone recording apparatus that expresses a halftone of a print pixel by changing the area gradation, the energy supply means has two or more different conversion means when converting the input image signal into a pulse number. However, at least one of the plurality of converting means is a converting means for generating a pulse number of a certain number or more regardless of density information indicated by the input image signal in printing of a low density portion. Thermal halftone recording apparatus wherein there.