JPH0789235A - Thermal transfer recording - Google Patents

Abstract

PURPOSE:To provide a method of thermal transfer recording for enabling the stable representation of an intermediate color tone by density gradation by preventing an image quality from deteriorating by thermal destruction in the center of a laser beam emission part and restricting the area gradation of a recorded image attributed to the Gaussian characteristics of a laser beam. CONSTITUTION:This thermal transfer recording method is to irradiate a transfer sheet consisting of a dye layer provided on a transparent support surface, attached airtightly to the surface of a recording sheet, with a light flux emitted from a laser beam source, from the transparent support side. In this method, laser beam pulses P1, P2 for forming each one recording point D1, D2 are composed of continuous pulses divided to a specified length. In addition, the maximum pulse width of each pulse is set to such a short time that image data cannot be recorded by transfer merely by a single emission of the optical pulse.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention uses a thermal mode of a laser, that is, converting a laser beam into heat to heat and melt an ink layer of a transfer sheet or to sublimate a dye in the ink layer. The present invention relates to a thermal transfer recording method of forming an image by transferring ink, that is, dye, onto a recording sheet.

[0002]

2. Description of the Related Art Conventionally, as a laser thermal transfer recording technique, there is a method using a so-called raster scan. In this raster scan system, an image is composed of line segments by scanning a digital image signal in line synchronization. There is also known a thermal transfer recording method in which the movement of a digital image signal is stopped for each pixel to form an image as an aggregate of pixels. Furthermore, for example, Japanese Patent Laid-Open No. 3-1
As disclosed in Japanese Unexamined Patent Publication No. 97187, there is also known a thermal transfer recording method in which a plurality of laser beams to be irradiated are prepared and raster-scanned.

In each of the above thermal transfer recording methods, for example, a transfer sheet formed by laminating a light-heat conversion layer containing a light absorbing heat generating agent and an ink layer containing a dye on a transparent support is a recording sheet. Used as a dye applying means for forming an image thereon. The transfer sheet is superposed on the recording sheet, and in this state, a light beam such as a laser beam is emitted from the transparent support side of the transfer sheet. Then, the laser light applied to the transfer sheet is absorbed by the light-heat conversion layer to generate heat, and the ink or dye is melted or sublimated by the action of the heat and further diffused to transfer the desired image onto the recording sheet. To be done.

The beam diameter of the laser light can be narrowed down to about 10 to 50 μm by an optical system, so that an image with extremely high resolution can be obtained. On the other hand, in a recording method using laser light as a heat source for recording, the heat spread in a direction perpendicular to the thickness direction of the sheet, that is, in the width direction of the sheet is a normal thermal transfer recording method using a thermal head or the like. Compared with, it is accompanied by a sharp rise in temperature at the focus of light.

By the way, in the prior art, as shown in FIG. 5, an electric pulse for forming one recording point, that is, a writing pulse Pw was single. Moreover, the irradiated laser beam has a high energy density and its intensity distribution has a Gaussian distribution. Therefore, if the irradiation time becomes long, thermal destruction is likely to occur in the central portion of the irradiated laser beam, and further, the energy required for transfer can be obtained in the peripheral portion of the laser beam,
Therefore, the obtained recorded image has area gradation. This is an obstacle when an attempt is made to improve the recording speed by irradiating a light pulse with large energy in a short time.

In order to prevent thermal destruction from occurring in the center of the laser light irradiation portion, it is necessary to irradiate a light pulse having a relatively small energy for a long time. Furthermore, the range of conditions for the energy of the light pulse to maintain accurate transfer without thermal destruction is narrow. Therefore, it is very difficult to control the thermal energy in order to obtain a high-resolution and high-quality image.

Further, the proposal of the recording method equipped with a plurality of laser beams is intended only to increase the recording speed, and is a drawback of the thermal mode recording due to the characteristics of the laser beam described above, that is, laser light. It does not solve the problem that thermal destruction easily occurs in the central part of the irradiation part and that the obtained recorded image has area gradation.

[0008]

SUMMARY OF THE INVENTION The present invention has been made in order to solve the above-mentioned problems, and it is possible to prevent the image quality from being deteriorated due to thermal destruction in the central portion of the laser light irradiation portion. By preventing and suppressing the area gradation property of the recorded image due to the Gaussian characteristic of the laser light,
An object of the present invention is to provide a thermal transfer recording method capable of stably performing halftone expression by density gradation and thereby obtaining an image of high resolution and high quality.

[0009]

In order to achieve the above object, a thermal transfer recording method according to the present invention comprises a transfer sheet having a dye layer on the surface of a transparent support, which is adhered to the recording sheet and emitted from a laser light source. A thermal transfer recording method in which a light beam to be emitted from the transparent support side of the transfer sheet is radiated, and a laser light pulse for forming one recording point is composed of a pulse group divided into pulses of a predetermined length, The maximum time of each pulse forming the pulse group is such a short time that transfer recording is not performed by single irradiation of the light pulse.

[0010]

The irradiated laser beam is controlled by a large number of RZ (Return To Zero) pulse groups generated by the control circuit. When the first pulse of this pulse group is input to the laser drive circuit, the transfer sheet is irradiated with laser light,
The irradiated portion heats up due to light-heat conversion and rises in temperature. Subsequently, the temperature drop starts when the leading input pulse is turned off. Here, since the first pulse does not raise the temperature to the temperature required for transfer, image recording is not reached. This will be the preheat pulse.

Image recording is performed by the second and subsequent pulse inputs. At this time, if the movement amount of the beam per pulse time is selected to be sufficiently small with respect to the laser spot diameter, most of the laser beams irradiated corresponding to the respective pulses overlap with each other while slightly moving. Irradiation, that is, multiple irradiation.

By the sequential irradiation of the light pulse, the temperature of the transfer portion approaches the stable point while repeatedly rising and falling. Furthermore, since the irradiation energy per pulse is small, thermal destruction does not occur at the center of the laser beam. Further, since the diffusion of heat to the periphery of the irradiation portion is much smaller than that during continuous irradiation, area gradation is suppressed, and the quality of the output image can be improved. Further, since the light pulses of the RZ pulse group for controlling the laser light are made continuous, the irradiation of the laser light on the transfer sheet becomes pseudo multiple irradiation, so that the pulse number, pulse density or pulse width of the pulse group is generated. By increasing the value, it is possible to realize the density gradation property in which the recording density is improved, and thus the halftone expression is possible.

In the laser thermal transfer recording method, generally, the laser light is focused into a minute spot by the action of a condenser lens, and the laser light is irradiated onto the transfer sheet which is in close contact with the recording sheet in advance. The transfer sheet usually has a light-heat conversion layer and an ink layer each having a thickness of 2 to 3 μm on a transparent support.
The transfer sheet is irradiated with laser light from the transparent support side. The light-heat conversion layer and the ink layer may be collectively referred to as a transfer layer. In many cases, the base material of the recording sheet is made of a dielectric or organic material having a heat transfer coefficient smaller than that of the ink layer. Therefore, the heat generated in the transfer layer by the irradiation of the laser light is easily transferred in the in-plane direction of the transfer layer. Therefore,
When the transfer medium having such a structure is irradiated with continuous laser pulses, the laser energy which is photo-thermally converted is instantaneously conducted in the traveling direction of the laser light, and therefore the second laser pulse is used rather than the first laser pulse. The recording sensitivity is higher.

In the case of a pulse having a time width in which the heat conduction speed is not negligible as compared with the irradiation time of the laser pulse, the narrower the time interval of the laser pulse is, the more the influence of heat is exerted and the higher the temperature rises. When this laser pulse is divided into a plurality of pulses having a narrower time width and line recording of a desired length is performed, the recording density can be changed depending on the laser pulse interval. That is, the pulse interval is narrowed when the recording density is increased, and the pulse interval is widened when the recording density is decreased. In this way, the recording density can be controlled by performing the pulse density modulation.

In addition to the pulse density modulation, the recording density can be controlled by changing the number of pulses or the pulse width. After the temperature of the laser-irradiated portion exceeds a recording threshold value by irradiating a plurality of laser pulses, the number of pulses is changed to change the ink transfer amount according to the number of pulses. In thermal transfer recording, the transfer amount changes depending on the total amount of heat supplied for recording, so that the recording density can be controlled by changing the pulse width of the laser pulse.

Further, since the Gaussian beam has a high energy density at the center of the beam, a large temperature gradient is generated immediately after laser irradiation, and the temperature at the center of laser irradiation exceeds 500 ° C. After that, thermal diffusion to the periphery starts and the temperature gradient is relaxed. This rapid temperature rise and thermal diffusion are more noticeable as the energy of the irradiated laser is larger and the irradiation pulse width is longer, and the thermal destruction of the central portion and the increase of the transfer area are further promoted. In order to suppress the thermal destruction and the increase of the transfer area, the writing pulse should be composed of a plurality of consecutive pulses having a sufficiently short pulse width, and an appropriate pulse interval for reducing mutual thermal influence between the consecutive pulses. It is necessary to have. Therefore, it is possible to set the width of the light pulse so narrow that recording does not occur with a single pulse, and to set such a pulse to be emitted continuously. In this case, the temperature gradient of the transfer sheet due to the immediately preceding pulse irradiation is alleviated by the action of thermal diffusion, and therefore the temperature distribution becomes gentler than when single pulse irradiation is used. By doing so, the thermal energy due to the irradiation of the individual pulses forming the pulse group is reduced, and therefore, the increase of the transfer area due to the heat conduction is suppressed, and the thermal destruction is less likely to occur.

[0017]

EXAMPLE As shown in FIG. 4, the recording sheet 1 and the transfer sheet 2 were superposed in a closely contacted state, and then the circumferential length was set to 4
It was wound and fixed on a rotating drum of 00 mm. The transfer sheet 2 is formed by laminating the light-heat conversion layer 4 and the ink layer 5 on the surface of the transparent support 3. With the recording sheet 1 and the transfer sheet 2 fixed to the rotating drum, a wavelength of 830 n was measured from the transparent support 3 side of the transfer sheet 2.
Transfer recording was carried out by scanningly moving the laser head 6 having m semiconductor lasers mounted thereon. Reference numeral 7 indicates a condenser lens for condensing the laser light R at a proper position on the transfer sheet 2.

There are two possible methods of scanning movement of the laser head 6, for example, intermittent scanning movement and continuous scanning movement. The intermittent scanning movement referred to here is a scanning method in which the laser head 6 is intermittently moved one dot at a recording point and stopped there, and laser is emitted in the stopped state. Further, the continuous scanning movement mentioned here is a scanning method of irradiating a laser while moving the recording sheet 1 and the transfer sheet 2 together at a constant speed by rotating the rotary drum at a constant speed.

FIG. 1 schematically shows a recording state when laser recording is performed by the above intermittent scanning movement method, that is, when stationary spot recording is performed. In this still spot recording, the recorded image is expressed as a set of transfer dots. The position to be transferred and recorded is determined by a dot movement signal Sm generated by a recording control circuit (not shown). Then, recording is performed based on the write pulse Pw formed by a plurality of continuous pulses divided in a short time.

In the figure, the first recording D1 has a pulse width of 5
Recording is performed based on the write pulse P1 composed of a pulse group of 10 ns, a pulse interval of 5 ns, and a pulse number of 10. The second recording D2 shows the case where the recording density is lowered, and the pulse width is 5 ns and the pulse interval is 15 n.
s, and the recording is performed based on the write pulse P2 composed of a pulse group of 5 pulses. In the stationary spot recording method in which the recording portion is stopped at every recording, all the laser pulse groups are applied to the same recording portion, so the transfer recording dots formed by them are circular due to isotropic thermal diffusion. Become.

In the conventional recording method, since the write pulse Pw is single as shown in FIG. 5, if the pulse width is lengthened to increase the recording density, the center of the recording is damaged or the recording density is increased. When the write pulse width was changed to change the recording area, the recording area changed greatly. On the other hand, in the present embodiment, since the write pulse Pw is formed by a plurality of divided pulses as described above, there is no fear of such damage at the recording center portion or change in the recording area.

FIG. 2 schematically shows a recording state in the continuous scanning movement method, that is, the scanning recording in which the image is recorded while the rotating drum is rotated at a constant speed. With this recording method,
In synchronization with the rising of the dot position signal Sd used as recording timing indicating the position of the dot to be recorded,
The writing pulse Pw is output to perform transfer recording.

In the conventional recording method, the write pulse is 1
The line segment recording was one in which the laser irradiation was continuous at one dot or where the recording information was continuous.
In this conventional method, the area gradation becomes dominant, the drawing start of line segment recording is thinner than the following line width, and the line width gradually increases from the beginning of drawing and after the line is thermally balanced There was a problem of heat effect that the temperature was stable. Therefore, in this embodiment, as in the case of FIG. 1, the recording pulse for forming one dot is set to a pulse group having a width of 5 ns to suppress the influence of heat.
The width of each pulse forming the pulse group is set to be equal to or less than the threshold energy of transfer recording by itself, so that the first pulse of the pulse group is not directly involved in transfer recording. It is a preheat pulse.

FIG. 3 shows the relationship between the writing pulse for laser irradiation and the temperature change of the laser irradiation portion of the transfer sheet when the thermal transfer recording method according to the present invention is used. The emitted laser light is controlled by a large number of RZ (Return To Zero) pulse groups generated by an output controller (not shown). When the first pulse Ps of this pulse group is input to a laser driving circuit (not shown), laser light is irradiated to the inside of the transfer sheet from the transparent support side, and the irradiated portion is heated by light-heat conversion. Raise the temperature. Then, the temperature drop starts when the first pulse Ps is turned off. But first
With the pulse Ps of 1, there is no temperature rise to the temperature required for transfer, so image recording cannot be achieved.

Image recording is performed by the second and subsequent pulses Px. At this time, the amount of movement of the beam in the time corresponding to one pulse is sufficiently small with respect to the laser spot diameter, so that the laser beam irradiated based on each pulse is slightly moved, but the spots overlap each other. Is large, so multiple irradiation is used. By the sequential irradiation of pulses, the temperature of the transfer portion approaches the stable point while repeatedly rising and falling. Furthermore, since the irradiation energy per pulse is small, thermal destruction does not occur in the central part of the laser beam, and the diffusion of heat to the periphery of the irradiation part is much smaller than during continuous irradiation, and the area gradation is reduced. Suppressed. Thereby, the quality of image output can be improved.

Further, since the laser light irradiation is pseudo multiple irradiation by controlling the laser light with a plurality of divided continuous RZ pulses, the number of pulses of the pulse group,
By increasing the pulse density or the pulse width, the recording density is improved and the density gradation is obtained, whereby halftone expression is possible.

The present invention has been described above with reference to the preferred embodiments, but the present invention is not limited to these embodiments and can be variously modified within the technical scope described in the claims.

[0028]

According to the thermal transfer recording method of the present invention,
In thermal transfer recording using a laser beam whose light intensity has a Gaussian beam distribution as an energy source, it is possible to suppress area destruction characteristics due to thermal destruction and thermal diffusion of the central portion of the recording dot due to energy concentration of the laser beam. High resolution and high quality images are obtained. Furthermore, since the recording pulse is a pulse group consisting of a pulse having a predetermined length and an extremely short time, any of pulse number modulation, pulse density modulation or pulse width modulation can be performed, and good density gradation is mainly used. A halftone expression can be realized.

[0029]

[Brief description of drawings]

FIG. 1 is a schematic view showing an embodiment of a thermal transfer recording method according to the present invention.

FIG. 2 is a schematic view showing another embodiment of the thermal transfer recording method according to the present invention.

FIG. 3 is a diagram showing changes in temperature of a portion irradiated with laser light when the thermal transfer recording method according to the present invention is used.

FIG. 4 is a schematic side view schematically showing a recording sheet and a transfer sheet on which thermal transfer recording is performed.

FIG. 5 is a diagram showing an example of a write pulse used in a conventional thermal transfer recording method.

[Explanation of reference numerals] 1 recording sheet 2 transfer sheet 3 transparent support 4 light-heat conversion layer 5 ink layer 6 laser head 7 condenser lens

Claims (2)

[Claims] 1. A thermal transfer recording method in which a transfer sheet having a dye layer on the surface of a transparent support is brought into close contact with the recording sheet, and a light beam emitted from a laser light source is irradiated from the transparent support side of the transfer sheet. A laser light pulse for forming one recording point is formed by a plurality of consecutive pulses divided into a predetermined length, and the maximum pulse width of each pulse is such that transfer recording is performed by single irradiation of the light pulse. A thermal transfer recording method characterized by a short time that is not performed. 2. The thermal transfer recording method according to claim 1, wherein the halftone expression is performed by modulating the pulse number, pulse density or pulse width of each pulse.