JP2006168176A - Image forming method and apparatus - Google Patents

Abstract

In an image forming apparatus using laser light, a small dot can be drawn at a focal position, and a high-definition image can be formed. However, it is difficult to ensure a solid density because a blank portion is generated between dots. At the position deviated from the focal position, the dot diameter becomes large and a high density image can be formed.
A focus mode in which the writing light beam is almost focused on a recording medium and a shift mode slightly shifted from the focus position are prepared. The focus mode is used for an image that requires high definition, and a high density is used. An image is formed by using the shift mode for an image that requires. Switching between modes may be performed during image formation, or one mode may be completed before the other mode is used. It is also possible to prioritize one of high definition and high density and form an image in only one mode corresponding to the entire image.
[Selection] Figure 2

Description

The present invention relates to an image forming technique using laser light on a medium, and more particularly, to an image forming method and apparatus capable of forming a high quality image having functions of a desired high-definition image forming mode and a high density image forming mode.
The image forming apparatus according to the present invention can be applied to a number of heat-sensitive coloring media such as a film for a plate, a medical film such as an X-ray photograph, a thermal paper film for office use, a ticket made of a heat-sensitive medium, a membership card, and a service ticket. On the other hand, it can be widely applied to applications for forming images.

FIG. 6 is a diagram for explaining the size of a spot when a laser beam is condensed by a lens. The figure (a) is a figure which shows a mode when a laser beam is irradiated from a different distance to the writing position, the figure (b) is an enlarged view of the vicinity of the writing position, and the figure (c) shows only the spot diameter. FIG.
In the figure, 1 is a laser light source, 2 is a lens, 3 is a writing position, L is a light beam, and S is a spot.
An image forming apparatus using laser light is excellent in that a high-definition image can be obtained in the case of characters and line drawings because the beam waist can be very small at the focal position of the laser beam. This is because the laser beam is an excellent coherent light and is a so-called Gaussian beam in which the energy distribution of the light beam cross section has a Gaussian distribution. Let us consider this point in more detail.
In the figure, five laser light sources 1a to 1e are shown. The luminous flux emitted from each light source becomes convergent light toward the writing position 3 by the corresponding lenses 2a to 2e. The distance between the laser light source 1 and the lens 2 is set to be equal to each other, but the distance from the lens 2 to the writing position 3 is set so as to be closer to the lower side in FIG. The distance is set such that the third light beam Lc from the top forms the minimum spot at the writing position 3.

In the energy distribution of the Gaussian beam, the energy is strongly concentrated at the center of the beam at a substantially focal position, but as the position deviates from the focal position, the difference in light intensity between the optical axis and the periphery decreases and the peak becomes broad. The state will be described with reference to FIGS.
The light beam La is a so-called front pin with respect to the writing position 3, and the light beam Lc is in focus. The light beam Lb is in an intermediate state between the two. The light beam Le is a so-called rear pin with respect to the writing position, and the light beam Ld is in an intermediate state between that and the in-focus state.
As shown in FIG. 6C, the spot size formed by each light beam at the writing position is larger than that of the front pin and the rear pin with Sc as a minimum. However, at the front focus and the rear focus, the spot changes to an ellipse, and the major axis directions are orthogonal to each other. In each spot, the light energy has a substantially Gaussian distribution in each cross section of the long axis and the short axis. When referring to the front pin state and the rear pin state collectively, it may be referred to as a defocused state. The positional deviation of the beam waist with respect to the writing position 3 is called defocusing.
Therefore, at the focal position, the light intensity is well above the medium sensitivity in the center region of the beam spot, and the periphery slightly away from the center is less than the sensitivity. Therefore, it becomes possible to draw a small dot, and it is possible to form a high-definition image. However, since the dot shape is small, the solid image is poorly filled, the overlap with adjacent dots is small, and a blank portion is generated between the dots, so that it is difficult to ensure a solid density.
On the other hand, at the position shifted from the focal position, the irradiation light peak becomes broad, and the dot diameter with respect to the medium sensitivity increases. Accordingly, the overlap between adjacent dots becomes large, and an image with good solidity can be obtained, so that a high density image can be formed. However, a high definition image cannot be formed.

In order to improve the resolution, there is a proposal to arrange the photoconductor at a position shifted from the focal position of the lens (see, for example, Patent Document 1). In this proposal, laser light is input to an optical fiber, and light beams from a plurality of fibers are arranged to form an apparent spot.
However, since this conventional technique generates a single beam with a plurality of laser emission sources and optical fibers, the number of components is increased and the control system is complicated, resulting in high costs. In particular, the problem becomes more prominent when a multi-beam is used.
In order to make the recording density in the sub-scanning direction variable, there is a proposal to arrange the recording surface at a position shifted from the focal position of a lens that forms an image of a plurality of parallel laser beams on the recording surface (see, for example, Patent Document 2). . In this proposal, a so-called beam compressor is used to reduce the interval (inter-beam pitch) between a plurality of parallel light beams in parallel, and the interval is made variable by using a zoom lens in part. However, although the recording surface is shifted from the focal position of the lens, it is fixed at a position that coincides with the beam waist and is not configured to obtain a high density image.
A spot diameter that tends to occur on the recording surface by arranging multiple beams from a light source, which is a monolithic array of light emitting units, with a uniform shift in the optical axis direction with respect to the focal position of the common coupling lens. There is a proposal to reduce the variation of the above (for example, see Patent Document 1). The relative shift of the light source and the coupling lens in the optical axis direction finally appears as a relative shift between the recording surface and the focal position of the light beam. As a result, the optical system is fixed by selecting the shift amount that minimizes the variation among the plurality of spots. In this case, the smallest spot is likely to be near the beam waist, and the largest spot is likely to be out of the beam waist position. In any case, since the optical system is fixed in that state, a high-definition image or a high-density image cannot be selected consciously.

Japanese Patent Laid-Open No. 9-218568 Japanese Examined Patent Publication No. 6-46305 JP 2001-31894 A

  When the present invention recognizes what should be a high-definition image and what should be a high-density image depending on the state of the image, the spot diameter is made variable accordingly.

According to the first aspect of the present invention, there is provided a method for forming an image by irradiating light onto a recording medium, wherein the recording medium is irradiated with light at a substantially focal position (hereinafter referred to as a focal mode), and from the focal position. It has a mode in which the recording medium is irradiated with light at a shifted position (hereinafter referred to as a shifted mode).
According to a second aspect of the present invention, in the image forming method according to the first aspect, when the image to be formed is an image for which high-definition image formation should be prioritized, image formation is performed in the focus mode, and high-density image formation is performed. When the image is to be prioritized, the image is formed in the shift mode.

According to a third aspect of the present invention, in the image forming method according to the first aspect, among the image information to be formed, image information suitable for forming a high-definition image is formed in the focus mode. Image information suitable for forming a density image is formed in the shift mode.
According to a fourth aspect of the present invention, in the image forming method according to any one of the first to third aspects, the light source optical system for executing the focus mode is a light source optical for executing the shift mode. It also serves as a system, and switching between the two modes is based on movement of the light source optical system.
According to a fifth aspect of the present invention, in the image forming method according to any one of the first to third aspects, the light source optical system for executing the focus mode is a light source optical for executing the shift mode. Also serving as a system, the switching between the two modes is by changing the optical path length of the light reaching the recording medium.

According to a sixth aspect of the present invention, in the image forming method according to any one of the first to third aspects, a light source optical system for executing the focus mode and a light source optical for executing the shifting mode. And switching between the two modes depends on switching of the light source optical system to be used.
According to a seventh aspect of the present invention, in the image forming method according to any one of the first to sixth aspects, the switching between the two modes is performed as needed during the formation of one image. .
According to an eighth aspect of the present invention, in the image forming method according to any one of the first to sixth aspects, the switching between the two modes is an image in the other mode after the image formation in one of the modes is completed. It is characterized by forming.

According to a ninth aspect of the present invention, in the image forming method according to any one of the first to eighth aspects, the light applied to the recording medium comprises a plurality of light beams.
According to a tenth aspect of the present invention, in the image forming method according to any one of the first to ninth aspects, the recording medium includes a heat-sensitive recording material having at least a photothermal conversion material, a leuco dye, and a developer. It is a medium.
According to an eleventh aspect of the present invention, there is provided an image forming apparatus using the image forming method according to any one of the first to tenth aspects.

  The present invention satisfies each requirement by changing the image forming method between a case where a high-definition image such as characters and line drawings is required and a case where a high-density image such as a solid image is required. Can be.

FIG. 1 is a schematic diagram showing a configuration for carrying out the present invention.
In the figure, reference numeral 10 denotes a light source optical system, 11 denotes a drum, 12 denotes a recording medium, D denotes a distance from the light source optical system to the recording medium, and M denotes an arrow indicating a direction in which the light source optical system is moved.
In the figure, a recording medium 12 is a medium that directly develops color when irradiated with laser light, such as thermal paper. However, a method of developing a latent image once, such as a silver salt film, or a method of developing an electrostatic latent image by irradiating laser light and developing it with a developing means, such as an electrophotographic method It does not matter.
A light source optical system including a laser light source is moved forward and backward in the direction of the recording medium by a driving unit (not shown) to switch the spot on the recording medium between a focused state and a defocused state. In the out-of-focus state, whether the front pin is better or the rear pin is better depends on the design convenience, so it is not uniquely determined which end of the movement should be in focus. .
The case where a focused spot is formed on the recording medium by moving the light source optical system is referred to as a focus mode, and the case where a defocused spot is formed on the recording medium is referred to as a shifted mode.

  However, when an image is formed at a position shifted from the focal position (defocus position), the image is not formed if the peak value of the light intensity becomes lower than the sensitivity of the medium by shifting. Therefore, as will be described later, the density temporarily rises by shifting from the focal position, but the density rapidly decreases when the defocus amount is increased by shifting the focus position as it is. Therefore, the distance D between the light source optical system capable of forming a high density image and the recording medium (hereinafter simply referred to as the light source distance D) has a small allowable width, and is more accurate than the case of forming an image at a substantially focal position. High writing means position control is required.

FIG. 2 is a diagram showing a change in state due to movement of the light source optical system. FIG. 4A is a diagram showing how the aspect ratio of the beam diameter changes, and FIG. 4B is a diagram showing the change in image density on a certain heat-sensitive recording medium. Details of the recording medium used here will be described later. Details of the measurement will also be described later.
In the figure, symbol JF indicates a range that can be regarded as being substantially in focus, Gr indicates a graph that indicates the aspect ratio of the beam diameter, and Gd indicates a graph that indicates the image density.
The horizontal axis of the figure is common to both graphs and indicates the light source distance D. The left side of the range JF indicates a front pin state region, and the right side indicates a rear pin state region. When viewed from the center position of the range JF, the left-right distance corresponds to a so-called defocus amount.
In the range JF, the density is stable with respect to the light source distance D, but the absolute value of the density is low. On the other hand, the density is high at a position deviated from the focal point, but the tolerance for the light source distance D is narrow and unstable.
Compared to electrophotographic photoreceptors and silver halide films, the heat-sensitive recording medium has a very low sensitivity and the allowable width is narrower. Therefore, a highly accurate writing means position control function and laser light source output control function are provided. Required.
It is necessary to grasp the above characteristics and use the focus mode and the shift mode properly.
For both modes, the line mode and text image are formed with high definition in the focus mode, and the solid image is formed with high density in the shift mode, and the entire image is either in the focus mode or the shift mode. Various methods may be used depending on the purpose, such as the method of forming the film.

FIG. 3 is a diagram for explaining the writing means using multi-beams. FIG. 4A shows an example using a linear array of light sources, and FIG. 4B shows an example using a staggered array of light sources.
In the figure, reference numerals 20 and 30 denote light source optical systems including a plurality of light sources, reference numerals 21 and 31 denote drums, reference numeral 22 denotes an optical recording medium, and reference numeral 33 denotes a moving stage.
In FIG. 3A, a light beam modulated by image information in time from a light source optical system 20 that emits irradiation light from a plurality of light sources at equal intervals, with the drum 21 with the recording medium 22 attached rotating at high speed. Irradiate. Each time the drum 21 rotates, the light source optical system 20 moves one dot in a direction parallel to the drum axis to form an image. Image formation ends when the image is formed by the adjacent light source, and the light source optical system 20 returns to the original position. In this configuration, the rotation direction of the drum is the main scanning direction, and the moving direction of the light source optical system is the sub-scanning direction. In this configuration, when the light source optical system 20 itself is moved back and forth to switch between the focus mode and the shift mode, it is very difficult to move back and forth at high speed during image formation due to inertia problems. Therefore, in the case of such a configuration, it is considered that one image is completed by scanning twice.

Image processing is performed in advance, and information that requires high definition, such as character information and line drawings, and image information that requires high density, such as a solid image, are separated on the memory. For example, the first time In the focus mode, writing is performed using high-definition information such as character information, and after returning the light source optical system, the mode is switched to the shift mode, and writing is performed using the high-density information for the second time.
Since it is difficult to move the entire light source optical system back and forth because of its large inertia, it is preferable to change only the optical path length of each light source without moving the entire light source optical system. For example, there is a method of changing the optical path length by inserting a member having an electro-optic effect in the optical path and changing the refractive index by turning on and off the applied voltage. In addition, various known techniques such as those using liquid crystal can be used.
In the case of such a configuration, it is possible to instantaneously switch between the focus mode and the shift mode for each light source during image formation so that an arbitrary position on the image can be made high definition or high density.

In FIG. 5B, the light sources arranged in two rows in the upper and lower rows in a staggered arrangement all have the same light source distance D with respect to the recording medium. In this case, the interval between the light sources adjacent to each other is very small, so that the time required for sub-scanning can be shortened. The other usage is the same as the configuration shown in FIG.
Here, a modification of FIG. In the modified example, the moving stage 33 is allowed to move only in the x direction. The two upper and lower rows of light sources in the staggered arrangement have different light source distances D. For example, the upper row is set to the focus mode distance, and the lower row is set to the shift mode distance. As described above, the image information is divided into high-definition information and high-density information in advance, and high-definition information is given to the upper row and high-density information is given to the lower row. Since the positions of the light sources in each row are shifted in both the main scanning direction and the sub-scanning direction, timing adjustment corresponding to the shift amount is performed at the time of writing. By doing so, both modes of image can be formed as desired in one sub-scan. If a simple two-row arrangement is used instead of the staggered arrangement and the upper and lower light sources are not displaced in the sub-scanning direction, the writing timing only needs to be corrected in the main scanning direction. Becomes easier.
The present invention can also be applied to a scanning optical system using a polygon mirror. Details will be described later with reference to FIG.

FIG. 4 is a schematic diagram showing the structure of a heat-sensitive recording medium that can be suitably used in the present invention. FIG. 4A is a diagram showing a layer structure, and FIG. 4B is a schematic diagram of a recording layer.
In the figure, reference numeral 40 denotes a recording medium, 41 denotes a substrate, 42 denotes a recording layer, 43 denotes a photothermal conversion layer, 421 denotes a leuco dye, and 422 denotes a developer.
The recording medium 40 has a three-layer structure, and the uppermost layer is made of a material including at least a dye that absorbs laser light and converts it into heat (in this example, a dye having an absorbance around 820 to 840 nm) and an ultraviolet curable resin. This is a photothermal conversion layer 43, and this layer also serves as a protective layer. The intermediate layer is a recording layer 42 that includes at least a leuco dye 421 and a developer 422 and has a function of developing a color when melted by heating. The lowermost layer is a substrate 41 such as PET. The substrate material may be any material as long as it is a material capable of coating the coloring layer, such as resin, metal, paper, and synthetic paper, and does not decompose or break due to heat generated by laser light irradiation.
The material of the uppermost light-to-heat conversion layer 43 and the recording layer 42 of the intermediate layer are mixed to form a two-layer structure as a medium.

In the graph shown in FIG. 2, the recording medium 40 having the above configuration is irradiated with the output light of a 150 mW semiconductor laser light source through a condenser lens using the apparatus shown in FIG. 3B, and image formation is performed at 2400 dpi. It is a measurement result in the case of.
FIG. 4A shows the aspect ratio of the dot image actually drawn on the recording medium. The aspect ratio is approximately 1 when the defocus amount included in the range JF is around zero. If the energy distribution of the beam cross section is always Gaussian, the aspect ratio graph should increase monotonically in the figure. In the actual measurement value, a decreasing tendency appears near the right end of the range JF, and the increase starts again. This is presumably because the energy distribution of the luminous flux deviates from the Gaussian distribution.
The spot shape does not change greatly in the vicinity of the focal position. Therefore, if the aspect ratio in the vicinity of the focal position is the same, the beam diameter is considered to be substantially the same. On the other hand, at the position where the aspect ratio changes, either the main scanning direction or the sub-scanning direction rapidly increases.

FIG. 5 is a diagram for explaining an example using a focus mode light source and a shift mode light source.
In the figure, reference numeral 60 is a beam combining means, 61 is a focus mode light source, 62 is a shift mode light source, 61a and 62a are semiconductor lasers, 63 is a combining prism, 63a is a polarization separation film, and 64 is a half wavelength. Each plate is shown.
The beam synthesizing unit 60 is configured so that the focus mode light source 61 and the shift mode light source 62 can be irradiated onto the recording medium by superimposing their optical paths using the synthesizing prism 63 which is a beam synthesizing unit. Only the corresponding light source is caused to emit light depending on whether it is high-definition information or high-density information. Since the amount of light loss is 50% or more with a normal semi-transparent mirror, as a method that does not involve light amount loss, the colors of the two laser light sources are made different, for example, red and green, and the optical path is synthesized with a dichroic mirror. You may do it.
However, the use of two laser light sources having different colors is not so preferable in terms of commonality of parts, so that the semiconductor lasers 61a and 62a in the figure have the same characteristics.

The beam combining means 60 is formed by integrating a half-wave plate 64 and a combining prism 63, and the combining prism 63 has a polarization separation film 63a. The polarization separation film 63a transmits P-polarized light and reflects S-polarized light.
Laser beams from the focus mode light source 61 and the shift mode light source 62 are emitted so as to be “P-polarized with respect to the polarization separation film”. Therefore, when the beam coupled by the coupling lens 62b enters the prism, it passes through the polarization separation film 63a as it is. The beam coupled by the coupling lens 61b is transmitted through the half-wave plate 64, so that the polarization plane is rotated by 90 degrees and becomes S-polarized light with respect to the polarization separation film 63a. This beam is totally reflected by the inclined surface of the prism, and then reflected by the polarization separation film 63a and exits from the prism. Thus, the beams from the focus mode light source and the shifted mode light source are combined. The optical axes of the coupling lenses 61b and 62b are parallel to each other, and the distance between these optical axes is determined so that the optical axes of both coupling lenses coincide with each other when the beams are combined by the beam combining means.

By adopting such a configuration, a focus mode light source is used for an image that needs to reproduce one dot or one dot width thin line, and a shifted mode light source is used for an image that needs to increase the solid density. By using this, it is possible to form a high-quality image. When a printing plate is produced, the required image quality differs depending on the level of the apparatus, and also differs depending on the type of printing image, the level of the plate material used, the recording target such as paper, and the application. Therefore, the initial state is the focus mode, and the selection of the shift mode, or both the focus mode and the shift mode is performed by the platemaker who manages the plate image and the printer who manages the print quality, or for each application. In the manufactured apparatus, the mode corresponding to the request is set from the beginning.
However, it is also necessary to have a mode in which the device determines by looking at the contents of the image.
i. When there is a 1-dot image or a 1-dot narrow line, the focus mode is set.
ii. Line images and graphic images are separated, and line images are created in the focus mode, and graphic images are created in the shifted mode.
iii. If there is an image line number or resolution in the header information of the image file,
Shift mode is used for low line count and low resolution, and focus mode is used for high line count and high resolution.
Judge.
For example, in the case of a circuit board manufacturing application, it is desirable that the wiring drawing is performed in the focus mode and the solid drawing is performed in the shift mode.
Needless to say, the above-described configuration for changing the optical path length can be applied even to a scanning optical system using a polygon mirror. In this case, it is not necessary to prepare a laser light source for each mode, and both modes can be shared.

It is a schematic diagram which shows the structure for implementing this invention. It is a figure which shows the change of the state by the movement of a light source optical system. It is a figure for demonstrating the writing means by a multi beam. It is a schematic diagram which shows the structure of the thermosensitive recording medium which can be used suitably for this invention. It is a figure for demonstrating the example using the light source for focus modes, and the light source for shift modes. It is a figure for demonstrating the magnitude | size of the spot when a laser beam is condensed with a lens.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Laser light source 2 Lens 10 Laser light source 11 Drum 12 Recording medium 20 Laser light source 21 Drum 22 Recording medium 30 Laser light source 31 Drum 40 Recording medium 42 Recording layer 43 Photothermal conversion layer 421 Leuco dye 422 Developer

Claims (11)

  A method of forming an image by irradiating a recording medium with light, wherein the recording medium is irradiated with light at a substantially focal position (hereinafter referred to as a focal mode), and light is applied to the recording medium at a position shifted from the focal position. An image forming method characterized by having a mode for irradiating (hereinafter referred to as a shift mode).   2. The image forming method according to claim 1, wherein when the image to be formed is an image for which high-definition image formation should be prioritized, the image is formed in the focus mode, and when high-density image formation is to be prioritized. An image forming method comprising forming an image in the shifting mode.   2. The image forming method according to claim 1, wherein image information suitable for forming a high-definition image among image information to be formed is suitable for forming an image in the focus mode and forming a high density image. An image forming method characterized in that image information is formed in the shift mode.   4. The image forming method according to claim 1, wherein a light source optical system for executing the focus mode also functions as a light source optical system for executing the shift mode, and switching between the two modes. 5. Is a method of forming an image characterized by moving the light source optical system.   4. The image forming method according to claim 1, wherein a light source optical system for executing the focus mode also functions as a light source optical system for executing the shift mode, and switching between the two modes. 5. Is an image forming method characterized by changing the optical path length of light reaching the recording medium.   4. The image forming method according to claim 1, further comprising: a light source optical system for executing the focus mode; and a light source optical system for executing the shift mode; Is switched by switching the light source optical system to be used.   7. The image forming method according to claim 1, wherein switching between the two modes is performed as needed during the formation of one image.   7. The image forming method according to claim 1, wherein the switching between the two modes is performed by performing image formation in the other mode after image formation in one of the modes is completed. Forming method.   9. The image forming method according to claim 1, wherein the light applied to the recording medium includes a plurality of light beams.   10. The image forming method according to claim 1, wherein the recording medium is a sensible recording medium having at least a photothermal conversion material, a leuco dye, and a developer. Forming method. An image forming apparatus using the image forming method according to claim 1.

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