JP2001246774A - Method for correcting density variation in thermal printer, and thermal printer - Google Patents

Abstract

PROBLEM TO BE SOLVED: To eliminate occurrence of density variation even when a temperature of a thermal head or the ambient temperature is changed. SOLUTION: A head temperature sensor for measuring a head temperature Th is attached to a thermal head and an ambient temperature sensor for measuring the ambient temperature Ta is attached to a portion in the vicinity of the thermal head. In the assembling and adjusting operations in a factory, a plural sheets of test prints are formed by a 50% gray image print pattern by raising each of the head temperature and the ambient temperature by one degree by starting from a head temperature T0 and an ambient temperature T1 in a reference temperature condition. The recording densities of the test prints are measured and a constant D0 and coefficients dDh, dDa are determined to be stored in an SRAM. When the printing is performed by a user, the head temperature Th and the ambient temperature Ta from the head temperature sensor and the ambient temperature sensor are inputted to a CPU before the printing. The CPU reads the constant D0, the coefficients dDh, dDa and an operational expression from the SRAM to calculate density data D and to change table data of a gradation-energizing time LUT. A print driving section looks up the table data to convert a gradation level of each pixel to energizing time data and outputs it to the thermal head.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to a method for correcting uneven density caused by temperature conditions and a thermal printer for implementing the method.

[0002]

2. Description of the Related Art Thermal printers include a thermal transfer printer using an ink film and a thermal printer that records an image by directly heating a thermal recording paper. For example, a color thermosensitive printer uses a color thermosensitive recording paper in which a magenta thermosensitive coloring layer, a cyan thermosensitive coloring layer, and a yellow thermosensitive coloring layer are sequentially formed on a support. In this color thermosensitive recording paper, in order to selectively color each thermosensitive coloring layer,
Each of the thermosensitive coloring layers has a different coloring heat energy (mJ / mm ² ), and the deeper the thermosensitive coloring layer, the greater the coloring heat energy. Also, when the next thermosensitive coloring layer is thermally recorded, light fixing is performed by irradiating the recorded thermosensitive coloring layer with an electromagnetic wave peculiar to the thermosensitive coloring layer so that the recorded thermosensitive coloring layer thereabove does not recolor. Will be

[0003] In a thermal head, a large number of heating elements (resistance elements) are arranged in a line, and an image of one color is printed on one line.
Record line by line. When recording this one line, the energizing time and the number of pulses of each heating element are controlled in accordance with the gradation level so that a predetermined thermosensitive coloring layer develops a target density.

[0004]

The heating value of the heating element is:
It is determined from the applied voltage and the energizing time. However, even when the heating value of the heating element is accurately controlled, the heat storage of the thermal head and the contact pressure of the heating element change depending on the temperature of the thermal head and the environmental temperature around the thermal head, and the color thermal recording paper The applied thermal energy will be different. For this reason, the density varies depending on the temperature during printing, that is, so-called density unevenness occurs, which causes image quality deterioration.

An object of the present invention is to provide a method for correcting density unevenness of a thermal printer and a thermal printer capable of recording an image of good quality while suppressing density unevenness due to a temperature of a thermal head and an environmental temperature. It is the purpose.

[0006]

In order to achieve the above object, a method for correcting uneven density of a thermal printer according to the present invention comprises:
In a method for correcting density unevenness of a thermal printer in which an image is recorded on a recording sheet by driving a thermal head having a plurality of heating elements arranged in a line based on image data, the temperature of the thermal head and the environment near the thermal head are determined. A plurality of test prints on which a predetermined print pattern is recorded under a plurality of different temperature conditions consisting of a combination of temperature are prepared, and the density of these test prints is measured. Equations are obtained, and density unevenness data under this temperature condition is calculated from the temperature of the thermal head and the environmental temperature measured before printing using this relational equation, and the heat value of each heating element is corrected from the density unevenness data. Things. Further, the image data is corrected by the density unevenness data. Further, the predetermined print pattern is a 50% gray image.

A thermal printer according to the present invention records an image on a recording sheet by driving a thermal head having a plurality of heating elements arranged in a line based on image data.
h, an environmental temperature measuring means for measuring an environmental temperature Ta near the thermal head, and a plurality of tests in which a predetermined print pattern is recorded with a plurality of different combinations of the head temperature Th and the environmental temperature Ta. Prints are made and the values D0, d obtained by these density measurements
A storage unit for storing Dh and dDa, and a calculation for calculating density unevenness data D by applying the head temperature Th and the environmental temperature Ta measured before printing by the head temperature measurement unit and the environmental temperature measurement unit to the arithmetic expression 3 And driving means for correcting the image data based on the density unevenness data D to drive each heating element.

D = D0 + dDh (Th-T0) + dDa (T
a-T1) T0: Head temperature virtually considered as the design center T1: Environmental temperature considered virtually as the design center D0: Density unevenness under standard temperature conditions where Th = T0 and Ta = T1 dDh: Head temperature Variation of uneven density when unit temperature rises dDa: Variation of uneven density when ambient temperature rises by unit temperature

In a thermal printer for recording an image on recording paper by driving a thermal head having a plurality of heating elements arranged in a line based on image data, a head temperature measurement for measuring a temperature Th of the thermal head is performed. Means, an environmental temperature measuring means for measuring an environmental temperature Ta near the thermal head, a head temperature Th, an environmental temperature T
storage means for creating a plurality of test prints in which a predetermined print pattern is recorded with a plurality of combinations different in a, and storing values D and dD obtained by measuring these densities;
A calculating means for calculating the density unevenness data D by applying the head temperature Th and the environmental temperature Ta measured before printing by the head temperature measuring means and the environmental temperature measuring means to the arithmetic expression 4; And a driving unit for driving each heating element by correcting image data.

D = D0 + dD (Th−Ta) D0: Density unevenness under standard temperature condition where Th = Ta dD: Amount of change in density unevenness when head temperature is higher than ambient temperature by unit temperature

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS In FIG. 2 showing an embodiment of a color thermal printer, a recording paper roll 11a in which a long color thermal recording paper (hereinafter, referred to as recording paper) 11 is wound in a roll form is set in a paper feeding section. Is done. As is well known, the recording paper 11 has a cyan thermosensitive coloring layer, a magenta thermosensitive coloring layer, and a yellow thermosensitive coloring layer sequentially formed on a support. The yellow thermosensitive coloring layer, which is the uppermost layer, has a high thermal sensitivity and develops yellow with small heat energy. The cyan thermosensitive coloring layer, which is the lowermost layer, has the lowest thermal sensitivity and develops cyan when a large amount of thermal energy is applied. Further, the yellow thermosensitive coloring layer loses its coloring ability when irradiated with near-ultraviolet rays of 420 nm. The magenta thermosensitive coloring layer is 36
The coloring ability disappears when irradiated with 5 nm ultraviolet rays.

The recording paper roll 11a is rotated by a paper feed roller 15 in contact with the recording paper roll 11a. The paper feed roller 15 is
The motor is rotated forward (counterclockwise) / reverse (clockwise) by a pulse motor. When the recording paper roll 11a rotates forward, the recording paper 11 is fed to the recording position, and when the recording paper roll 11a rotates reversely, the recording paper 11 is rewound.

At a recording position, a thermal head 16 for heating the recording paper 11 to form each thermosensitive coloring layer, and a platen roller 17 for supporting the recording paper 11 opposite to the thermal head 16 are arranged. . Downstream of the recording position, a transport roller pair 18 for nipping and transporting the recording paper 11 and an optical fixing device 20 are arranged.

The optical fixing device 20 radiates near-ultraviolet rays of 420 nm to the recording paper 11 to fix the yellow thermosensitive coloring layer, and emits near-ultraviolet rays of 365 nm to the recording paper 11 to magenta heat-sensitive. A magenta ultraviolet lamp 22 for fixing the color-forming layer is provided, and a reflector 23 for improving irradiation efficiency is provided behind these.

Downstream of the optical fixing device 20, a cutter 24 for cutting the recording paper 11 on which heat recording and optical fixing have been completed to a predetermined length, and the cut recording paper 11 is sent out of the printer from an outlet 25. A discharge roller pair 26 is provided.

As shown in FIG. 3, the thermal head 16
Has an insulating substrate 29 mounted on an aluminum base plate 28. A ceramic plate 30 is mounted on the substrate 29, and a glaze layer 31 is formed thereon. On the glaze layer 31, a heating element array in which a large number of heating elements 32 are arranged in a line is provided. The heating element array extends in a direction perpendicular to the drawing and has a length substantially equal to the width of the recording paper 11. Further, a control circuit for driving each of the heating elements 32 is provided on the insulating substrate 29, and a plastic protective cover 33 for covering and protecting the heating elements 32 and guiding the recording paper 11 is attached. Further, the thermal head 16 is provided so as to be swingable about a shaft 34, and the recording paper 1 on the platen roller 17.
1 and a retracted position away from the platen roller 17.

An aluminum base plate 28 includes a thermal head 16
A head temperature sensor 35 for measuring the temperature of the head is attached. An environmental temperature sensor 36 for measuring an environmental temperature near the thermal head is attached near the thermal head 16. These temperature sensors 3
For example, thermistors 5 and 36 are used.

In FIG. 4, which shows the electrical configuration of the color thermal printer, the output signals from the head temperature sensor 35 and the environmental temperature sensor 36 are digitally converted and then input to the CPU 40 as temperature information. CPU
In addition to the temperature sensors 35 and 36, a frame memory 41, a line memory 42, a print driving unit 43, a gradation-conduction time LUT 44, and an SRAM 45 are connected to 40.

In the frame memory 41, image data for one frame captured from a digital camera or the like is written in a state where the image data is separated for each of yellow, magenta, and cyan. These image data are read out line by line according to the color to be printed and sent to the line memory 42. The image data written in the line memory 42 is read out for each pixel and sent to the print driving unit 43.

The print driving section 43 is provided with a gradation-current supply time L
With reference to table data representing the relationship between the gradation level stored in the UT 44 and the energization time data for each heating element,
The energization time data for each heating element corresponding to the gradation level of each pixel is sent to the thermal head 16.

When the temperature of the aluminum base plate 28 rises due to the driving of each heating element, the temperature of the protective cover 33 attached thereto also rises. Aluminum base plate 28
Easily dissipates heat, but the resin protective cover 33 is unlikely to dissipate heat, so the protective cover 33 stores heat. The heat energy stored in the protective cover 33 returns to the glaze layer 31 through the aluminum base plate 28 and is given to the recording paper 11 via the heating element 32. The contact pressure between the heating element 32 and the recording paper 11 changes according to the heat storage of the protective cover 33, the insulating substrate 29, and the like. As a result, even if the heat generation amount of the heating element 32 is the same, the coloring density of the recording paper 11 is different.

In order to suppress density unevenness due to temperature conditions, the table data of the gradation-conduction time LUT 44 is changed by density unevenness data D obtained immediately before printing starts. The uneven density data D is calculated by the following arithmetic expression 5 stored in the SRAM 45.

[0021]

D = D0 + dDh (Th-T0) + dDa (T
a-T1)

The meaning of each symbol in the arithmetic expression 5 is as follows. Th: head temperature immediately before the start of printing Ta: environmental temperature immediately before the start of printing T0: head temperature virtually considered to be the design center T1: environmental temperature virtually considered to be the design center D0: standard where Th = T0, Ta = T1 Density unevenness under temperature condition dDh: Change in density unevenness when head temperature increases by unit temperature dDa: Change in density unevenness when environmental temperature increases by unit temperature

The constant D0 and the coefficients dDh and dDa are determined for each color thermal printer based on a test print prepared at the time of assembly adjustment at a factory.
For example, image data for printing a 50% gray print pattern is input to the frame memory 41, and a unit temperature, for example, 1 degree, is calculated from the head temperature T0 and the environmental temperature T1 (standard temperature conditions), which are virtually considered as the design center. Make 10 test prints while raising each. Also, a test print may be performed each time the thermal head is used for a predetermined period, or when the thermal head is repaired or replaced.

For each of the test prints, the recording density is measured with a densitometer, and the average of the measured values is determined as D. Based on the test results of a plurality of test prints, a statistical method is used to calculate the density unevenness D0 under standard temperature conditions, and the variation d of the density unevenness when the temperature of the thermal head is raised by one degree.
Dh, the variation d of the density unevenness when the environmental temperature rises by 1 degree
Find Da. Obtained constant D0, coefficients dDh and dD
a is stored in the SRAM 45.

The operation of the thus constituted color thermal printer will be described with reference to FIG. At the time of assembling adjustment at a factory, image data for printing a 50% gray print pattern is input to the frame memory 41, and ten test prints are created while increasing the temperature once each from the standard temperature condition. For each of the test prints, the recording density of each heating element is measured by a densitometer, and a constant D0, coefficients dDh and dDa are determined by a statistical method, and these are stored in the SRAM 45. This operation is performed for each color thermal printer.

At the time of printing by a user, image data for one frame desired to be printed is taken into the frame memory 41 from a digital camera or the like. When the print key is operated, the head temperature Th and the environmental temperature T for each heating element are obtained from the head temperature sensor 35 and the environmental temperature sensor 36.
a is input to the CPU 41. The CPU 40
The constant D0, the coefficients dDh, dDa, and the arithmetic expression 5 are called from the SRAM 45, and the head temperature Th and the environmental temperature Ta are applied to the arithmetic expression 5, and the density unevenness data D is calculated. CP
U40 is determined by the calculated density unevenness data D.
The table data stored in the energization time LUT 44 is changed.

On the other hand, the CPU 40 operates the paper feed roller 15, rotates the recording paper roll 11a in the normal direction, and feeds the recording paper 11 toward the recording position. During the paper feeding, the thermal head 16 is set at the retracted position away from the platen roller 17. The recording paper 11 is a thermal head 16
Between the feed roller pair 1
8, the recording paper 11 is conveyed by the conveying roller pair 18.

When the leading edge of the recording area of the recording paper 11 reaches below the thermal head 16, the yellow print process is started. First, the thermal head 16 swings clockwise and moves to the print position, and presses the recording paper 11.

The CPU 40 reads out one line of image data of a yellow image from the frame memory 41 and writes it once into the line memory 42. Print driver 43
Reads the pixels from the line memory 42 in order and outputs the energization time data corresponding to the gradation level of each pixel to the thermal head 16 with reference to the corrected table data of the gradation-energization time LUT 44.

Each heating element of the thermal head 16 is driven in accordance with the given energization time data, and causes the yellow thermosensitive coloring layer of the recording paper 11 to develop a color corresponding to the gradation level.

When the first line of the yellow image is recorded, the conveying roller pair 18 rotates stepwise by one line, and at the same time, the second line of the yellow image is read from the frame memory 41.
The image data of the line is read. Based on the image data of the second line of this yellow image, the recording paper 11
The second line is thermally recorded on the yellow thermosensitive coloring layer.
When the portion where the yellow image is thermally recorded reaches the optical fixing device 20, near ultraviolet rays of 420 nm are irradiated on the recording paper 11 from the ultraviolet lamp 21, and the yellow thermosensitive coloring layer is optically fixed.

When a yellow image for one frame is thermally recorded, the paper feed roller 15 and the transport roller pair 18 are rotated in the reverse direction, and the recording paper roll 11a is rotated clockwise.
Is rewound. When the leading end of the recording area of the recording paper 11 comes to the position of the thermal head 16 again, the sheet feeding roller 15 and the conveying roller pair 18 rotate forward, and the magenta image is recorded line by line on the magenta thermosensitive coloring layer. The coloring heat energy of the magenta image is larger than the coloring heat energy of the yellow image, but since the yellow coloring layer has already been optically fixed, the yellow coloring layer does not recolor. The recording paper 11 on which the magenta image is recorded is
Ultraviolet light near 365 nm is irradiated from the ultraviolet lamp 22 and is optically fixed.

When a magenta image for one frame is thermally recorded, the paper feed roller 15 and the transport roller pair 18 rotate in reverse, and the recording paper 11 is rewound. When the leading end of the recording area of the recording paper 11 reaches the position of the thermal head 16 again, the cyan image is recorded line by line on the cyan thermosensitive coloring layer. This cyan thermosensitive coloring layer does not have light fixing property to the cyan thermosensitive coloring layer since the coloring heat energy has a value that does not cause coloring in a normal storage state. Therefore,
In the thermal recording of the cyan thermosensitive coloring layer, the optical fixing device 20 is off.

After the completion of the thermal recording of the yellow, magenta and cyan images, the paper feed roller 15 and the transport roller pair 18 continuously rotate forward. After the leading end of the recording paper 11 is nipped by the discharge roller pair 26, the recording paper 11 is conveyed by a predetermined length, and the cutter 24 operates when the rear end of the recording area passes the position of the cutter 24 by a predetermined length. The recorded recording paper 11 is cut. The cut recording paper 11 is discharged to an external tray through a discharge port 25.

In the above embodiment, the density unevenness data D is calculated using the arithmetic expression 5, but the following arithmetic expression 6, which is a simplified version of the arithmetic expression 5, may be used.

[0036]

D = D0 + dD (Th−Ta)

The meaning of each symbol in the arithmetic expression 6 is as follows. D0: Density unevenness under standard temperature condition where Th = Ta dD: Amount of change in density unevenness when unit temperature is higher than ambient temperature by unit temperature

In the same manner as in the above embodiment, a test print is prepared at the time of assembly and adjustment in a factory, the density of a recorded image for each heating element is measured, and a constant D is determined by a statistical method.
0, the coefficient dD is determined. Then, the constant D0 and the coefficient d
D is stored in the SRAM 45. At the time of printing by the user, the CPU 40 sends the head temperature T from the head temperature sensor 35 and the environmental temperature sensor 36 immediately before starting printing.
h and the environmental temperature Ta, the constant D0, the coefficient dD, and the arithmetic expression 6 are called from the SRAM 45, and the head temperature Th and the environmental temperature Ta are applied to the arithmetic expression 6, and the density unevenness data D is calculated. Hereinafter, since it is completely the same as the above embodiment, the description is omitted.

In the embodiment described above, the timing for calculating the density unevenness data is immediately before the start of printing.
For example, the density unevenness data may be created immediately before recording an image of one color, and the density unevenness data may be obtained for each color.

In the above embodiment, the density unevenness data common to each heating element is calculated. However, the density of a test print is measured for each heating element, and a constant D0, a coefficient dDh, a coefficient dDh, If dDa is obtained, density unevenness data for each heating element can be created.

In the above embodiment, the SRAM is used as the storage means. However, an arbitrary data rewritable storage device such as an EEPROM can be used. In addition, although the test print is created using the print pattern of 50% gray, a test print may be created using another print pattern as long as the print pattern shows uneven density.

Further, the color thermal printer of the embodiment described above thermally prints an image while reciprocating the recording paper linearly.
Although the image was fixed by light, for example, the recording paper was wound around the peripheral surface of the platen drum, and the image was thermally recorded while rotating.
Light fixing may be used.

In the above embodiment, a color thermal printer is taken as an example. However, the present invention can be applied to a monochrome thermal printer, a color thermal transfer printer using an ink film, and the like. Also, the line printer that moves the color thermal recording paper and the thermal head relatively one-dimensionally has been described. However, the present invention can also be used for a serial printer whose relative movement is two-dimensional.

[0044]

As described above in detail, according to the present invention, a plurality of test prints on which a predetermined print pattern is recorded in different combinations of the temperature of the thermal head and the environmental temperature near the thermal head are prepared. By measuring the density of these test prints, the relational expression between the temperature condition and the density unevenness data is obtained, density unevenness data is calculated from the head temperature and the environmental temperature measured before printing, and each heat generation is calculated from the density unevenness data. Since the calorific value of the element is corrected, it is possible to suppress density unevenness due to the temperature of the thermal head and the environmental temperature, and record a good image.

[Brief description of the drawings]

FIG. 1 is a flowchart showing the creation of density unevenness correction data.

FIG. 2 is a schematic view illustrating an example of a color thermal printer.

FIG. 3 is an enlarged view of a thermal head.

FIG. 4 is a block diagram illustrating an electrical configuration of the color thermal printer.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 11 Recording paper 16 Thermal head 20 Optical fixing device 28 Aluminum base plate 31 Glaze layer 32 Heating element 33 Protective cover 35 Head temperature sensor 36 Environmental temperature sensor 40 CPU 43 Print drive unit 44 Gradation-conduction time LUT 45 SRAM

Claims (5)

[Claims] 1. A method for correcting density unevenness of a thermal printer in which an image is recorded on a recording sheet by driving a thermal head having a plurality of heating elements arranged in a line based on image data. By creating a plurality of test prints on which a predetermined print pattern is recorded under a plurality of different temperature conditions consisting of a combination with the ambient temperature in the vicinity of the head, and measuring the density of these test prints, the temperature conditions and the density are measured. Find the relational expression with uneven data,
Using this relational expression, the density unevenness data under this temperature condition is calculated from the temperature of the thermal head measured before printing and the environmental temperature, and the calorific value of each heating element is corrected from the density unevenness data. To correct uneven density of thermal printer. 2. The method according to claim 1, wherein the image data is corrected using the density unevenness data. 3. The method according to claim 1, wherein the predetermined print pattern is a 50% gray image. 4. A thermal printer for recording an image on recording paper by driving a thermal head in which a plurality of heating elements are arranged in a line based on image data, wherein a temperature Th of the thermal head is measured. Means, an environmental temperature measuring means for measuring the environmental temperature Ta near the thermal head, and a plurality of test prints in which a predetermined print pattern is recorded by a plurality of combinations of different head temperatures Th and environmental temperatures Ta. A storage unit for storing values D0, dDh, and dDa obtained by the density measurement, and a head temperature Th and an environmental temperature Ta measured before printing by the head temperature measuring unit and the environmental temperature measuring unit are applied to an arithmetic expression 1. Calculating means for calculating density unevenness data D; correcting image data based on density unevenness data D to generate heat Thermal printer characterized in that a drive means for driving the child. D = D0 + dDh (Th−T0) + dDa (T
a-T1) T0: Head temperature virtually considered as the design center T1: Environmental temperature considered virtually as the design center D0: Density unevenness under standard temperature conditions where Th = T0 and Ta = T1 dDh: Head temperature Variation of uneven density when unit temperature rises dDa: Variation of uneven density when ambient temperature rises by unit temperature 5. A thermal printer for recording an image on recording paper by driving a thermal head in which a plurality of heating elements are arranged in a line in accordance with image data, wherein a temperature Th of the thermal head is measured. Means, an environmental temperature measuring means for measuring the environmental temperature Ta near the thermal head, and a plurality of test prints in which a predetermined print pattern is recorded by a plurality of combinations of different head temperatures Th and environmental temperatures Ta. Storage means for storing the values D and dD obtained by the density measurement, and applying the head temperature Th and the environmental temperature Ta measured before printing by the head temperature measuring means and the environmental temperature measuring means to the arithmetic expression 2 to obtain density unevenness; Calculating means for calculating data D;
And a driving means for driving each heating element by correcting the image data based on the density unevenness data D. ## EQU2 ## D = D0 + dD (Th-Ta) D0: Density unevenness under standard temperature condition where Th = Ta dD: Amount of change in density unevenness when the head temperature becomes higher than the ambient temperature by a unit temperature