JP2008089704A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus that can form a desired image, even when an image of small dots is formed. <P>SOLUTION: The image forming apparatus is equipped with a small dot decision means 70 that detects the number of pixels of an exposure object, in a region having the width of a predetermined number of pixels in both the main scanning direction and the sub-scanning direction centering an arbitrary pixel of pixels of the exposure object and that decides that the arbitrary pixels are small dots, when the number of detected pixels of the exposure object is equal to or less than the predetermined number; and a bias light modulating means that increases bias light for a predetermined time, prior to and after exposing the pixel decided to be small dots by the small dot decision means 70. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an exposure apparatus that scans and exposes a photosensitive surface of a photosensitive material to form a two-dimensional image, and more particularly, to image formation capable of forming a good image even in an area having a small dot ratio. Relates to the device.

  In general, a photosensitive lithographic printing plate is used for offset printing (lithographic printing). In the field of lithographic printing, laser exposure processing is performed based on digital data from a computer, etc., and direct printing is performed by developing processing that converts the latent image formed on the photosensitive lithographic printing plate into a visible image using an automatic processor. A CTP (Computer to Plate) system for making a plate has been put into practical use.

  In such a CTP system, an exposure apparatus that conducts scanning exposure by introducing a light beam such as a laser beam onto the photosensitive surface of a photosensitive lithographic printing plate (hereinafter also simply referred to as a photosensitive material) disposed on the inner peripheral surface of a cylindrical drum. An internal scanning exposure apparatus is widely used.

  This inner surface scanning exposure apparatus generally includes a spinner mirror as a light beam deflector. The spinner mirror deflects and scans the photosensitive surface of the photosensitive material in the main scanning direction by causing the condensed beam to be incident and reflected on the reflection surface of the spinner mirror that is driven to rotate at high speed. . The spinner mirror performs deflection scanning and is moved at a constant speed in the axial direction of the central axis of the cylindrical support (hereinafter simply referred to as the central axis) by the sub-scanning moving means. Thus, the inner surface scanning type exposure apparatus performs main scanning and sub scanning, and performs scanning exposure on the entire photosensitive surface of the photosensitive material.

  In an exposure apparatus that exposes a photosensitive material using a light beam such as a laser, the rise time of the light beam is delayed. Due to the delay of the rise of the light beam, the exposure amount to the photosensitive material by the light beam may be reduced.

  In particular, when forming a halftone image, an image of an area having a relatively small halftone dot ratio of 1 to 5% (hereinafter also referred to as a small dot) has few adjacent pixels, and therefore the delay time of the rise of the semiconductor laser ( For example, it is particularly susceptible to 1-2 ns). That is, when a small dot image is formed, the exposure amount of the light beam is reduced due to the delay of the rise of the semiconductor laser, and the small dot image is not sufficiently exposed, resulting in poor dot image formation. There was a case.

In order to suppress the influence on the small dots due to the delay of the rise of the semiconductor laser, the timing at which the drive signal of the laser diode as the light source falls when forming an image of an image signal with a short pulse width. There is known an exposure method that compensates for a reduction in exposure amount due to a delay in the rise of the light beam emitted from the laser diode, and an exposure apparatus that uses this exposure method (Patent Document 1).
Further, there is provided an image recording method for modulating a light beam intensity of a semiconductor laser so as to increase the intensity of light when forming small dots, and performing exposure with the light beam having an increased intensity, and an image recording apparatus using the image recording method. Known (Patent Document 2).

JP-A-10-284781 JP 2001-265001 A

  In the exposure apparatus described in Patent Document 1, the length of the pulse width of the image signal is used as a reference in order to determine whether or not to delay the timing at which the drive signal of the laser diode falls. However, even when the pulse width is short, the image to be formed may not be an image forming a small dot. That is, even if there are few exposure target pixels in the main scanning direction, the exposure target pixels may continue in the sub scanning direction. Since pixels are not continuous, pixels that are not small points may be determined to be small points. As a result, even when there is no need to delay the timing at which the laser diode drive signal falls, this delay is executed. In this way, by delaying the falling timing of the drive signal of the laser diode at an unnecessary timing, the exposure amount increases, and the halftone dot ratio of the formed image matches the desired halftone dot percentage. In other words, there is a problem that image formation with a desired gradation may not be performed.

  Further, in the image recording apparatus described in Patent Document 1, since the fall timing of the light beam is delayed for the purpose of increasing the exposure amount at a small point, the time that the light beam is emitted from the light source is long. Thus, there is a problem that the life of the light source is shortened. Further, the exposure apparatus described in Patent Document 2 also has a problem that the life of the light source is shortened in the same manner in order to increase the intensity of the light beam from the light source when forming a small dot image.

  The present invention has been made in view of the above-described problems of the prior art, and an object thereof is to provide an image forming apparatus capable of forming a desired image even when an image with a small dot is formed. It is another object of the present invention to provide an image forming apparatus capable of improving the life of a light source by performing good image formation and reducing the load on the light source.

  In order to achieve the above object, the present invention emits a light beam modulated based on an image signal from a light source unit, and scans and exposes a photosensitive material two-dimensionally with the light beam emitted from the light source unit to form an image. In the image forming apparatus to be formed, the light source unit emits bias light having a light intensity that the photosensitive material does not sensitize to a non-image forming area where the image of the photosensitive material does not form an image during the scanning exposure, Detecting the number of pixels to be exposed in an area having a predetermined number of pixel widths in both the main scanning direction and the sub-scanning direction centering on an arbitrary one of exposure target pixels forming an image; When the number of detected pixels to be exposed is equal to or smaller than a predetermined number, a small point determination unit that determines that the arbitrary pixel is a small point, and a pixel that the small point determination unit determines as the small point Before and after exposure To provide an image forming apparatus characterized by comprising a bias light modulating means for increasing the bias light only time.

  Here, the small dot discriminating means detects the area detected in an area having a predetermined number of pixel widths in both the main scanning direction and the sub-scanning direction centering on any one of the exposure target pixels. If the number of pixels to be exposed is the number of pixels in which the halftone dot value in the region is 2% or less, the arbitrary one pixel may be determined to be a pixel constituting a small dot. preferable.

  The photosensitive material is preferably one in which the photopolymerization reaction that occurs in the region irradiated with the light beam is inhibited by oxygen.

  Also, a support having an arc-shaped inner peripheral surface and holding the photosensitive material on the inner peripheral surface, and a reflective surface for reflecting the light beam emitted from the light source unit toward the inner peripheral surface of the support And deflecting the light beam in a main scanning direction that coincides with the circumferential direction of the inner peripheral surface of the support by rotating the reflecting surface about the center line of the inner peripheral surface of the support. It is preferable to include a main scanning unit having an element, and a sub-scanning unit that relatively moves the main scanning unit and the support in a sub-scanning direction that coincides with the center line direction of the inner peripheral surface of the support. .

The bias light is preferably a light beam having an intensity of 0 to 1/100 of the intensity of the light beam that exposes the pixel to be exposed.
Further, the bias light modulation means sets the intensity of the bias light for a predetermined time before and after the exposure of the pixel determined to be the small point to 1/200 to the intensity of the light beam that exposes the pixel to be exposed. It is preferable to increase the light beam to 1/50 intensity.

According to the present invention having the above-described configuration, the small dot discriminating means is used to discriminate the pixel constituting the small dot from the pixel to be exposed, and the bias is applied for a predetermined time before and after the pixel discriminated as the pixel constituting the small dot. By increasing the light, it is possible to prevent the image formation failure of small dots due to the delay of the rise of the light beam emitted from the light source and to prevent the image quality from deteriorating. That is, it is possible to prevent density unevenness at small points due to the delay of the rise of the light beam, and high-quality image formation can be performed.
Further, since the small point is determined by the small point determination unit and the bias light is increased only for a predetermined time before and after the small point, the load on the light source can be reduced, and the life of the light source can be improved.

  Hereinafter, an inner surface scanning type exposure apparatus of the present invention will be described in detail based on a preferred embodiment shown in the accompanying drawings.

FIG. 1 shows a conceptual diagram of an example of an inner surface scanning type exposure apparatus of the present invention.
An inner surface scanning exposure apparatus 10 (hereinafter referred to as an exposure apparatus 10) shown in FIG. 1 holds a photosensitive material P in an arc shape (cylindrical inner surface shape), and scans a light beam L scanned in the circumferential direction from the inner side of the arc. Thus, the photosensitive material P is subjected to scanning exposure.
Such an exposure apparatus 10 basically includes a support 12 indicated by an imaginary line (two-dot chain line) in the drawing, a central control means 14, a laser driver 16, a spinner driver 18, a laser light source 20, The optical path deflecting unit 26 including a pair of reflecting mirrors 26 a and 26 b, a condensing lens 28, and a main scanning unit 30 are configured.

In the present embodiment, the photosensitive material P uses a photopolymer type CTP plate. This photopolymer type CTP plate is a photosensitive material in which a photosensitive layer made of a photopolymer is formed on a support of a metal substrate such as aluminum, and a transparent overcoat layer that blocks oxygen is formed on the photosensitive layer. is there. In this photopolymer type CTP plate, a portion irradiated with light is cured by a photopolymerization reaction. Thereafter, heat treatment and development treatment are performed, and a portion irradiated with light and cured remains as an image.
The photosensitive material P is not limited to the photopolymer CTP plate, and various photosensitive lithographic printing plates (so-called PS plates) can be used.

The central control unit 14 includes a CPU and the like, and controls the driving and operation of the entire exposure apparatus 10. Further, the central control means 14 supplies the laser driver 16 with a drive signal of the laser light source 20 corresponding to the image to be formed, and further instructs the spinner driver 18 to rotate the spinner mirror 36 in the main scanning means 30 and to perform sub-scanning. Put out.
In the present invention, the central control means 14 has an exposure control block 60. The exposure control block 60 will be described in detail later.

The laser driver 16 is a known laser driver corresponding to the laser light source 20, and drives the laser light source 20 in accordance with a drive signal supplied from the central control means 14, and modulates the light beam according to the image to be formed. L is emitted.
As will be described later, in the exposure apparatus 10, the main scanning unit 30 moves in the direction of the center line of the arc-shaped inner peripheral surface of the support 12 while rotating the spinner mirror 36 (sub-scanning in the sub-scanning direction). The spinner driver 18 rotates the motor 38 of the main scanning unit 30 and performs sub-scanning in response to an instruction from the central control unit 14.

The support (drum) 12 has an arc-shaped inner peripheral surface (cylindrical inner surface), and holds the photosensitive material P in close contact with the inner peripheral surface.
The photosensitive material P is held (fixed) on the support 12 by using an inner drum type (cylindrical inner surface scanning) such as a method using suction, a method using static electricity, a method using a fixing member using a magnet or fitting. A known method used in a type exposure apparatus may be used. Further, the supply and discharge of the photosensitive material P to the support 12 may be performed manually or automatically using a known method used in an inner drum type exposure apparatus.

  The laser light source 20 is a light source that emits laser light that can sensitize the photosensitive material P, and is modulated and driven by the laser driver 16 to emit a light beam L that is modulated in accordance with an image to be formed.

Here, in the exposure apparatus 10 of the present invention, the laser light source 20 has a strength (when an image is not formed on the photosensitive material P even when scanning a non-image forming area on the photosensitive material P where an image is not formed during scanning exposure). Bias light that is a light beam of light intensity, that is, bias light having intensity (light intensity) that does not start the photopolymerization reaction is emitted from the photopolymer CTP plate used as the photosensitive material P in the present embodiment. By emitting this bias light, when the scanning position is moved from an area where there is no image to be formed to an area where an image is formed, i.e., an area where a pixel to be exposed exists, the rise of the light beam is delayed. As a result, it is possible to suppress the shortage of the exposure amount.
Note that the bias light for scanning the non-image forming area where no image is formed is hereinafter also referred to as steady-state bias light. As will be described later, in the present embodiment, a light beam having an intensity of about 1/400 of the intensity of the light beam at the time of image exposure is used as the bias light at the steady state.

  The optical path deflecting means 26 is composed of a pair of reflecting mirrors 26a and 26b, which reflects the light beam L, deflects the optical path, and forms an arc shape of the support 12. The light is incident on the main scanning means 30 through the condenser lens 28 as a predetermined optical path that travels along the center line of the inner peripheral surface (center line of the cylinder). In the following description, the center line of the arc-shaped inner peripheral surface is simply referred to as the center line.

The condensing lens 28 constitutes a light beam optical system of the exposure apparatus 10 together with the optical path deflecting unit 26. The condensing lens 28 is arranged with its optical axis aligned with the center line, condenses the light beam L reflected on the optical path polarization means 26 and traveling on the center line, and has a predetermined size on the inner peripheral surface of the support 12. A beam spot.
The light beam L collected by the condenser lens 28 travels on the center line and then enters the main scanning unit 30.

The spinner mirror 36 is fixed to the tip of the rotating shaft of the motor 38 at an angle of 45 ° with respect to the center line with the reflecting surface (in the illustrated example, the center position of the reflecting surface) positioned at the center line. The motor 38 rotates the rotation shaft around the center line.
Accordingly, the light beam incident on the spinner mirror 36 is reflected by the spinner mirror 36 on the inner peripheral surface of the support, and is scanned in the circumferential direction of the inner peripheral surface indicated by the arrow m in the figure (the main scan is performed in the main scanning direction). )

Although not shown, the exposure apparatus 10 is integrated with the main scanning means 30 and the condenser lens 28 and moves in the direction of extension of the center line (sub scanning direction) indicated by the arrow s in the drawing. A scanning means is arranged.
As described above, since the light beam is main-scanned in the circumferential direction of the inner peripheral surface of the support 12 by the spinner mirror 36 (main scanning means 30), the sub-scanning in the center line direction by this sub-scanning means. The entire surface of the photosensitive material P held on the inner peripheral surface of the support 12 can be two-dimensionally scanned and exposed.
The sub-scanning means is not particularly limited, and various known linear movement methods such as a screw transmission method can be used.

  The exposure apparatus 10 configured as described above supports a light beam L modulated according to an image to be formed by deflecting it with a spinner mirror 36 (main scanning means 30) that moves in the sub-scanning direction while rotating at high speed. By performing main scanning in the circumferential direction of the inner circumferential surface of the body 12 and sub-scanning in the center line direction, the entire surface of the photosensitive material P held on the inner circumferential surface of the support 12 is two-dimensionally scanned and exposed. And an image can be formed on the photosensitive material P.

Here, when the exposure apparatus 10 performs scanning exposure of the photosensitive material P to form an image, that is, when exposing the exposure target pixel, the exposure target pixel may not be sufficiently exposed. is there. For example, the exposure target pixel may not be sufficiently exposed due to a shortage of exposure due to the rise delay of the light beam. In this case, when the photosensitive material P, which is a photopolymer type CTP plate, is subjected to development processing, pixels in a region where the exposure amount is insufficient may be lost, and the image quality may deteriorate.
In particular, when scanning exposure is performed with a spinner that rotates at a high speed as in an inner surface scanning exposure apparatus, the exposure time (for example, about 10 ns) in one pixel is shortened, so that the rise time of the light beam L from the laser light source is delayed. There is also a problem that image formation defects are likely to occur due to a slight change in exposure conditions due to the light beam, and image quality is likely to deteriorate due to this.

  Further, in the photopolymer type CTP plate used as the photosensitive material P in the present embodiment, a region subjected to scanning exposure by a light beam is cured by a photopolymerization reaction. At this time, in the photopolymer type CTP plate, the photopolymerization reaction may be inhibited by oxygen and the photopolymerization reaction may not sufficiently proceed. In this case, when the photosensitive material P is subjected to development processing, pixels in which the photopolymerization reaction has not sufficiently progressed are lost, and the image quality is deteriorated.

  The deterioration of the image quality due to the above-described delay of the rise of the light beam and the inhibition of image formation by oxygen in the image forming process tends to become more prominent particularly in the region where the dot ratio is small. Therefore, especially in the area where the dot ratio is small, that is, in the area where the small dot image is formed, the deterioration of the image quality due to the light beam rise delay or the small dot image formation failure caused by oxygen inhibition in the image forming process is ignored. Can not do it.

  Here, inadequate exposure is compensated for by increasing the intensity of the light beam during image formation or delaying the fall timing of the light beam. However, when such a method is used, there is a problem that the life of the light source is shortened because the load on the light source increases.

  The exposure apparatus of the present invention determines whether each pixel to be exposed is an image of a region having a relatively small halftone dot ratio, that is, a pixel constituting an image of a small dot. When a pixel determined to be a point is exposed, the bias light is increased for a predetermined time before and after the pixel determined to be a small point is exposed. As a result, when performing scanning exposure (image formation) of a small dot image with a relatively small dot ratio, a small dot image is caused by oxygen beam inhibition of the photopolymerization reaction of the photosensitive material during the image forming process. It is possible to prevent the formation failure, and to perform good image formation, and it is possible to improve the life of the light source by reducing the load on the light source.

Hereinafter, a method for determining small points in the exposure apparatus of the present invention will be described.
In the present invention, in order to determine whether the pixel to be exposed is a pixel constituting a small dot, the number of pixels to be exposed is detected in a predetermined area centered on the pixel to be determined as a small dot. Here, the predetermined area centered on the pixel to be identified with a small dot is a predetermined number (k, k is an integer equal to or greater than 1) in the main scanning direction and the sub-scanning direction with the pixel to be identified as a small dot. This region is a pixel number detection target region. That is, the pixel number detection target area is an area composed of (2k + 1) × (2k + 1) pixels of (2k + 1) pixels in the main scanning direction and (2k + 1) pixels in the sub-scanning direction centering on the pixel for which the small dot is to be determined. It is.

  In the present invention, when the number of exposure target pixels in this pixel number detection target region is detected and the detected number of exposure target pixels is equal to or smaller than a predetermined value, the small dot discrimination target pixel is a small dot (small dot If the pixel is larger than the predetermined value, it is determined that the small dot determination target pixel is not a small dot, thereby determining the small dot.

  That is, the small dot discrimination method of the present invention is a pixel number detection target area composed of (2k + 1) × (2k + 1) pixels of 2k + 1 pixels in the main scanning direction and 2k + 1 pixels in the sub-scanning direction. In this method, the number of pixels is detected, and when the detected value is equal to or smaller than a predetermined value (small point determination threshold value), it is determined that the center pixel of the pixel number detection target region is a pixel constituting the small point. The center pixel of the pixel number detection target area is the (k + 1) th pixel in the main scanning direction and the (k + 1) th pixel in the sub-scanning direction from the end of this pixel number detection target area.

Here, in this embodiment, a halftone image is formed using the FM network. 2A to 2D show halftone dot images using the FM network. Here, the used FM network has a resolution of 2400 dpi, a size of one pixel of 10.6 × 10.6 μm, and a minimum dot of 2 × 2 pixels. 2A to 2D show halftone dot images with halftone dot ratios of 1%, 2%, 3%, and 4%, respectively.
Here, one square in FIGS. 2A to 2D represents one pixel, a black square represents an exposure target pixel, and a white square represents a non-exposure target pixel. 2 is the main scanning direction, the horizontal direction is the sub-scanning direction, the pixel on the near side in the main scanning direction is scanned, and the pixel on the left side in the sub-scanning direction is scanned. It is a pixel on the upstream side of exposure.

FIG. 3 shows a specific example of the small point discrimination method. In the present embodiment, the small dots are determined by using the pixels in the area where the dot ratio is 2% or less as the pixels constituting the small dots.
FIG. 3A is an explanatory diagram of small dot determination using an image with a halftone dot rate of 2%. The pixel for which the small dot is to be determined is the pixel D1, and the pixel number detection target region centered on the pixel D1. Is indicated by a region A1.
FIG. 3B is an explanatory diagram of small dot determination using an image with a halftone dot ratio of 3%. The pixel for which the small dot is to be determined is the pixel D2, and the pixel number detection target region centered on the pixel D2 Is indicated by a region A2.

  In the specific example of the small dot discrimination method shown in FIG. 3, the pixel consists of 7 pixels in the main scanning direction and the sub scanning direction around the small dot discrimination target pixel, that is, a total of 225 pixels when k = 7. The region is set as a pixel number detection target region. That is, the small dot discrimination target pixel is the eighth pixel in the main scanning direction and the sub-scanning direction in the pixel count detection target region, and the pixel count detection target region is centered on the small dot discrimination target pixel. This is an area composed of 15 pixels in the scanning direction and 15 pixels in the sub-scanning direction.

As shown in FIG. 3 (a), there are four pixels in the pixel number detection target area A1, which is composed of a total of 225 pixels of seven pixels in the main scanning direction and the sub-scanning direction centering on the pixel D1 that is a small dot determination target. There are pixels to be exposed. Here, when the number of exposure target pixels is 4, the halftone dot ratio in the pixel number detection target area A1 is 2% or less, and therefore, it is determined that the pixel D1 is a pixel constituting a small dot.
Further, as shown in FIG. 3B, there are six exposure target pixels in the pixel number detection target area A2. When the number of exposure target pixels is 6, the halftone dot ratio in the pixel number detection target area A2 is larger than 2%, and therefore it is determined that the pixel D2 is not a pixel constituting a small dot.
In this way, it is determined whether or not the pixel for which a small point is to be determined is a small point.

  In the present invention, when exposing a pixel determined to be a pixel constituting a small dot by the above-described small dot discrimination method, as shown by a solid line in FIG. 4, a predetermined time (τns) before exposing the small dot is The bias light is increased by a predetermined amount for a predetermined time (τns) after the exposure. This can prevent a shortage of exposure due to the delay of the rise of the light beam when exposing a small dot image, and thus can prevent image quality deterioration in the small dot image. In addition, it is possible to prevent a small-point image formation failure caused by the inhibition of the photopolymerization reaction of the photosensitive material A by oxygen, and to prevent deterioration in image quality.

In the present embodiment, the intensity of the bias light when increasing the bias light by a predetermined amount during a predetermined time (τns) before and after exposing the small spot is the light beam intensity during scanning exposure (image formation). The strength is about 1/100.
The predetermined time during which the bias light is increased by a predetermined amount before and after exposing the small spot is preferably 2 ns to 10 ns. Note that 10 ns is an exposure time of one pixel. This predetermined time can be determined according to the characteristics of the light source and the photosensitive material used, the type of halftone dot to be used, the resolution of the halftone dot, the size of the minimum dot of the halftone dot, and the like.

Here, as indicated by a one-dot chain line in FIG. 4, normally, bias light having about 1/200 of the intensity of the light beam at the time of exposure (during image formation) is used. This is set in consideration of the suppression of small-point image formation caused by the delay of the rise of the light beam and the inhibition of the photopolymerization reaction due to oxygen, the reduction of the load on the light source, and the like.
However, even when the bias light is set as described above, when a small dot image is exposed, the small dots are not sufficiently exposed and image quality may be deteriorated. In addition, scanning exposure is performed with a constant bias light in all non-image forming regions, which is inefficient.

On the other hand, in the present invention, the small spot is discriminated and the bias light is increased at a predetermined time before and after the exposure of the small spot image. As a result, the bias light can be reduced as shown by the solid line in FIG. 4 when the small dot image is not exposed or when the exposure is not performed, that is, when the image data is zero.
Further, according to the present invention, when performing scanning exposure of a non-image forming region other than a predetermined time before and after exposure of a small dot image, bias light can be reduced, thereby reducing the load on the laser light source. Therefore, the lifetime of the laser light source can be improved.

  Hereinafter, an exposure control block 60 according to the present invention will be described in which the pixels constituting the dot are determined by the dot determination method, and the bias light is modulated based on the determination result. FIG. 5 is a block diagram showing the configuration of the exposure control block 60. FIG. 6 shows a timing chart for explaining the operation of the exposure control block 60.

  The exposure control block 60 includes a k pixel delay circuit 62, delay circuits 64, 66, and 68, a small dot discriminating unit 70, an OR circuit 72, an AND circuit 74, and an analog switch 76. The pixel constituting the dot is discriminated from the image data stored in the image memory using the dot discriminating unit 70, and when the pixel discriminated as the pixel constituting the dot is exposed, the pixel before the exposure is exposed. The bias light is increased during τns and τns after exposure.

As shown in FIG. 5, the k pixel delay circuit 62 and the small dot discriminating unit 70, when receiving a pixel clock signal from a pixel clock generating unit (not shown) included in the central control unit 14, receives the image data stored in the image memory. Image data for one pixel is received from the image data of line i _o . Here, the image data of the line _io is a main scanning line that is the center in the sub-scanning direction of the area to be discriminated when performing the small dot discrimination in the small dot discriminator 70 described later.

The k pixel delay circuit 62 delays the exposure timing of the read image data of one pixel by k pixels. That is, the exposure timing is delayed by the exposure time of k pixels. The signal a output from the k pixel delay circuit 62 is input to the delay circuit 64 and the OR circuit 74.
Here, k is the same value as k used in the description of the small point determination method in the small point determination. That is, the k pixel delay circuit 62 delays the image data by k pixels in order to determine a small point.

The delay circuit 64 delays the input signal by τns and outputs it, and further delays the signal a output from the k pixel delay circuit by τns and outputs the signal b. The signal b output from the delay circuit 64 is input to the delay circuit 66 and the OR circuit 74. Here, similarly to the delay circuit 64, the delay circuit 66 outputs the received signal after being delayed by τns, and further delays the signal b output from the delay circuit 64 by τns. Is output.
The signal b output from the delay circuit 64 is input to the laser driver 16. This signal b is a light source ON / OFF signal for controlling the ON / OFF timing of the laser light source 20.

The signal c output from the delay circuit 66 is input to the OR circuit 72 together with the signal a output from the k pixel delay circuit and the signal b output from the delay circuit 64.
As shown in FIG. 6, the signal a output from the k pixel delay circuit input to the OR circuit 72, the signal b (light source ON / OFF signal) output from the delay circuit 64, and the signal output from the delay circuit 66. The signal c is logically ORed to output a logical OR signal d.
The output logical sum signal d is input to the AND circuit 74.

On the other hand, the small dot discriminating unit 70 has ± k lines in the sub-scanning direction centered on the line i _o of the image data in the main scanning direction, that is, main scanning lines i _{o + j} (j = −k,... Each time a pixel clock is received from each main scanning line of 2k + 1 columns (0,..., + K), image data of one pixel is read. The small dot discriminating unit 70 discriminates from the read image data whether the (k + 1) th pixel of the main scanning line _io is a pixel constituting the small dot.
Hereinafter, the small point discriminating unit 70 that performs the above-described small point discriminating method will be described in detail.

FIG. 7 is a block diagram schematically showing the configuration of the small dot discriminating unit 70.
As shown in FIG. 7, the small dot discriminating unit 70 stores 2 kbit image data, 2k + 1 shift registers 80 _{o + j} (j = −k,..., 0,..., + K), An adder 82 _{o + j} that reads and adds each bit value of the shift register 80 _{o + j} , an adder 84 that further adds and calculates the operation values of a plurality of adders 82 _{o + j} , and a small point determination that outputs a small point determination threshold value Threshold transmission unit 86, a comparator that compares the operation value output from adder 84 with the transmission determination threshold output from small point determination threshold transmission unit 86, and outputs a small point determination signal based on the comparison result 88.

The 2k + 1 shift registers 80 _{o + j} are associated with each other so as to read image data of pixels of the main scanning line i _{o + j} corresponding to the subscripts, and each of the 2k + 1 shift registers 80 _{o + j} is 2k + 1 columns. Image data of any one column of the main scanning lines i _{o + j} (j = −k,..., 0,..., + K) is read without overlapping with other shift registers.
In the present embodiment, the shift registers 80 _{o + j} are the same except that they are associated with different main scanning lines i _{o + j.} Except for the shift register 80, the same reference numerals are used. In addition, each shift register _{80 o + j,} adders _{82 o + j} are connected. Similarly, the adder 82 _{o + j} is denoted by the same reference numeral as the adder 82 unless it is particularly necessary to distinguish.

Each shift register 80 is a shift circuit that stores 2 kbit data and can shift the digit of the stored data in one direction (from left to right in FIG. 7).
Upon receiving the pixel clock, each shift register 80 uses this as a trigger to shift the stored image data of each pixel in one direction, and one pixel from the main scanning line corresponding to each shift register 80. When the read image data is the image data of the exposure target pixel, 1 is stored as the image data. When the read image data is the image data of the non-exposure target pixel, 0 is stored as the image data.

  The adder 82 combines 2k + 1 bit image data (2k + 1), which is a combination of 2 kbit image data (2k pixel image data) stored in the shift register 80 and one pixel of image data input from the image memory. The number of exposure target pixels is calculated from the image data of individual pixels). That is, the number of pixels whose image data value is 1 is calculated from each digit of 2k + 1 bit image data, and this calculated value is output. An adder 82 connected to each of the 2k + 1 shift registers 80 calculates the number of pixels to be exposed from the pixels stored in each shift register 80 and one pixel input from the image memory, respectively. 84.

  The adder 84 adds the calculated value of each adder 82, that is, the number of exposure target pixels in 2k + 1 main scanning lines, thereby calculating the pixel stored in each 2k + 1 shift register 80. The number of exposure target pixels is calculated from the data and the image data of the pixels of each main scanning line received from the image memory, that is, from the image data of (2k + 1) × (2k + 1) pixels. Output the value. The output calculated value of the number of exposure target pixels is input to the comparator 88.

Here, in this embodiment, the small dot discrimination is performed on the pixel of the (k + 1) -th digit of the main scanning line _io , that is, the pixel read before k pixels of the pixel to which image data is currently transmitted from the image data memory. It is set as a target pixel, that is, a small point discrimination target pixel.
The shift register 80, the adder 82, and the adder 84 described above start from (2k + 1) × (2k + 1) pixels including the pixel transmitted from the image data memory centered on the small dot discrimination target pixel. The number of exposure target pixels is calculated.

  In addition, the small point determination threshold value transmission unit 86 outputs a predetermined small point determination threshold value and transmits it to the comparator 88. Here, the small point determination threshold value is used to determine whether the small point determination target pixel is a small point based on the number of exposure target pixels in the image data of (2k + 1) × (2k + 1) pixels. Is the threshold value of the number of exposure target pixels in the image data of (2k + 1) × (2k + 1) pixels.

The comparator 88 receives the calculated value of the number of exposure target pixels transmitted from the adder 84 from the negative input terminal, and the predetermined small point determination threshold transmitted from the small point determination threshold transmission unit 86 is the positive side. Input from the input terminal. The comparator 88 compares the calculated value of the number of exposure target pixels with the small point determination threshold value, and outputs 1 as the small point determination signal if the number of exposure target pixels is equal to or smaller than the small point determination threshold value. Further, the comparator 88 outputs 0 as the small point determination signal if the number of exposure target pixels is larger than the small point determination threshold value. The small dot determination signal output from the comparator 88 is input to the AND circuit 74 as a small dot determination signal (signal e) delayed by τns by the delay circuit 68.
Note that the delay circuit 68 synchronizes the timing at which the logical sum signal d and the small dot determination signal (signal e) are input to the AND circuit 74.

  The AND circuit 74 calculates a logical product of the input logical sum signal d and the small dot determination signal to calculate a bias selection signal. That is, when it is determined that the small dot discrimination target pixel is a small dot and the small dot discrimination signal is 1, the logical sum signal d is valid. On the other hand, when it is determined that the pixel for determining the small point is not a small point and the small point determination signal is 0, the logical sum signal d is invalid. The output bias selection signal is input to the analog switch 76.

  The analog switch 76 selects either the high bias setting signal or the low bias setting signal in accordance with the bias selection signal input from the AND circuit 74 and transmits the selected signal to the laser driver 16 as a bias setting signal. When it is determined that the small dot discrimination target pixel is a small dot, the logical sum signal d is validated in the AND circuit 74 and the bias selection signal rises. When the bias selection signal rises, the analog switch 76 selects the high bias setting signal as the bias setting signal. When it is determined that the pixel for which the small point is to be determined is not a small point, the logical sum signal d is invalidated in the AND circuit 74 and the bias selection signal does not rise. When the bias selection signal does not rise, the analog switch 76 selects the low bias setting signal as the bias setting signal.

  The laser driver 16 receives a light source ON / OFF signal and a bias setting signal as a control signal for controlling the operation of the light source, and provides a light source driving current based on the control signal. At this time, the central control unit 14 causes the spinner driver 18 to perform the operation of the laser light source 20 based on the control signal of the light source transmitted to the laser driver 16 and the main scanning and sub-scanning of the spinner mirror in synchronization. Send a synchronized drive signal.

  When the laser driver 16 receives the high bias setting signal, the laser driver 16 applies a light source driving current to the laser light source 20 so as to increase the bias current during τns before and after pixel exposure, and receives the low bias setting signal. A normal bias current is applied to the laser light source 20.

  As described above in detail, according to the present invention, the exposure control block 60 included in the central control unit 14 determines the pixels constituting the small points using the small point determination method of the present invention, and determines the small points. When exposing pixels determined to be configured, the bias light is increased by a predetermined time τns before and after the exposure. As a result, it is possible to prevent the deterioration of the image quality by preventing the delay of the rise of the light beam and the small-point image formation failure due to the oxygen inhibition of the photopolymerization reaction of the photosensitive material.

  In addition, according to the present invention, since the bias light is increased only when the pixels constituting the dot are reliably determined and the pixels constituting the dot are exposed, the pixels other than the pixels constituting the dot are increased. When exposing the pixel to be exposed, exposure can be performed without increasing the bias light. Therefore, the exposure amount does not increase in the exposure target pixels other than the pixels constituting the small dots, and an image can be formed with a desired halftone dot ratio.

  Further, according to the present invention, since the intensity of the bias light is increased only when a small dot is determined and a pixel that causes image formation failure of the small dot is exposed, a point other than the normal small point is exposed. At the same time, since the bias light can be reduced when scanning the non-image forming region, the load on the laser light source can be reduced, and the life of the light source can be improved.

  In the present embodiment, k = 7 is used when the small dot is discriminated by the small dot discriminating method of the present invention, but the present invention is not limited to this, and the value of k is the value of the photosensitive material to be used. It can be set as appropriate according to the characteristics, the type of halftone dot to be used, the resolution of the halftone dot, the size of the minimum dot of the halftone dot, etc., and this makes it possible to reliably determine the pixels constituting the small dot. it can.

  Further, in the present embodiment, the image in the area where the halftone dot ratio is 2% or less is set as a small dot. However, the present invention is not limited to this, and the characteristics of the light source and photosensitive material used, the type of halftone dot used, By appropriately setting the threshold value of the halftone dot to be determined as a small dot in accordance with the resolution of the image and the minimum dot size of the halftone dot, it is possible to reliably determine the small dot, and the image formation failure of the small dot And good image formation can be performed.

In this embodiment, the intensity of the bias light at a predetermined time (τns) before and after exposing the small spot is set to 1/100 of the intensity of the light beam at the time of image exposure. However, the present invention is not limited to this. Can be determined according to the characteristics of the light source and photosensitive material used, the type of halftone dot used, the resolution of the halftone dot, the size of the minimum dot of the halftone dot, etc., and the intensity of the light beam during image exposure It is preferably 1/200 to 1/50.
In the present embodiment, the intensity of the bias light in the steady state is 1/400 of the intensity of the light beam at the time of image formation. However, the present invention is not limited to this, and the characteristics of the light source and the photosensitive material to be used, It can be determined according to the type of halftone dot to be used, the resolution of the halftone dot, the size of the minimum dot of the halftone dot, etc., and is preferably 0 to 1/100 of the intensity of the light beam at the time of image exposure. .
Needless to say, the intensity of the bias light at the time of increasing before and after exposing the pixel at the small point is larger than the intensity of the bias light at the normal time.

  In the present embodiment, the bias light in the steady state is 1/400 even in the non-image forming region, but the present invention is not limited to this. In the present invention, non-image formation excluding the region where scanning exposure is performed with the increased bias light is performed by increasing the bias light for a predetermined time (τns) before and after exposing the pixels constituting the small dots. Even if the bias light is set to 0 in the region, it is possible to prevent poor image formation at small points and perform good image formation.

In this embodiment, the photopolymer type CTP plate is used as the photosensitive material. However, the present invention is not limited to this. Even in the case where image formation is performed using various photosensitive materials, the pixels constituting the small dots are used. , It is possible to prevent small-point image formation defects due to the delay of the rise of the light beam, and to prevent deterioration in image quality.
In the present embodiment, the halftone image is formed using the FM network. However, the present invention is not limited to this, and even when the halftone image is formed using the AM network, the small dot can be reliably determined in the same manner. It is possible to obtain the same effect.

  Further, in the present embodiment, the inner surface scanning exposure apparatus using the present invention has been described, but the present invention is not limited to this, and an outer surface scanning exposure apparatus (outer drum type exposure apparatus) or a planar scanning exposure apparatus. The present invention can also be applied to a (flatbed exposure apparatus).

  The inner scanning exposure apparatus of the present invention has been described in detail above. However, the present invention is not limited to the above embodiments, and various improvements and modifications may be made without departing from the spirit of the present invention. Of course.

It is a conceptual diagram of an example of the inner surface scanning exposure apparatus of this embodiment. FIG. 2 is a conceptual diagram for explaining a halftone dot image formed using an FM net in the inner surface scanning exposure apparatus shown in FIG. 1. It is a conceptual diagram for demonstrating the Example of a small point discrimination | determination method. It is a conceptual diagram explaining the exposure method of the pixel which comprises a small point. It is a block diagram which shows typically the exposure control block of this invention. 6 is a timing chart for explaining the operation of the small dot discriminating section shown in FIG. FIG. 6 is a block diagram schematically illustrating a small point determination unit illustrated in FIG. 5.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Inner surface scanning exposure apparatus 12 Support body 14 Central control means 16 Laser driver 18 Spinner driver 20 Light source 26 Optical path deflection means 26a, 26b Reflection mirror 28 Condensing lens 30 Main scanning means 36 Spinner mirror 38 Motor 52 Light beam optical system 60 Exposure control Block 62 k pixel delay circuit 64, 66, 68 Delay circuit 70 Small point discriminating unit 72 OR circuit 74 AND circuit 76 Analog switch 80 Shift register 82, 84 Adder 86 Small point discriminating threshold transmission unit 88 Comparator

Claims (4)

An image forming apparatus that emits a light beam modulated based on an image signal from a light source unit, and forms an image by two-dimensionally scanning and exposing a photosensitive material with the light beam emitted from the light source unit,
The light source unit emits bias light having a light intensity at which the photosensitive material is not exposed to a non-image forming region where an image of the photosensitive material is not formed during the scanning exposure,
The number of pixels to be exposed is detected in an area having a predetermined number of pixel widths in both the main scanning direction and the sub-scanning direction centering on an arbitrary pixel of the exposure target pixels forming the image. A small point discriminating means for discriminating that the arbitrary one pixel is a small point when the number of detected pixels to be exposed is equal to or less than a predetermined number;
An image forming apparatus, comprising: the small point discriminating unit; and a bias light modulating unit that increases the bias light for a predetermined time before and after performing exposure of a pixel discriminated as the small point.   The small dot discriminating means detects the exposure target detected in an area having a predetermined number of pixel widths in both the main scanning direction and the sub-scanning direction centering on an arbitrary pixel of the exposure target pixels. 2. The image formation according to claim 1, wherein if the number of pixels is the number of pixels in which the value of the halftone dot ratio in the region is 2% or less, the arbitrary one pixel is determined to be a pixel constituting a small dot. apparatus.   The image forming apparatus according to claim 1, wherein the photosensitive material is such that a photopolymerization reaction occurring in a region irradiated with the light beam is inhibited by oxygen. A support having an arc-shaped inner peripheral surface and holding the photosensitive material on the inner peripheral surface;
A reflection surface that reflects the light beam emitted from the light source unit toward the inner peripheral surface of the support, and rotating the reflection surface around the center line of the inner peripheral surface of the support; Main scanning means having a deflecting element that scans a light beam in a main scanning direction coinciding with the circumferential direction of the inner peripheral surface of the support;
Sub-scanning means for relatively moving the main scanning means and the support in a sub-scanning direction coinciding with the direction of the center line of the inner peripheral surface of the support;
An image forming apparatus according to any one of claims 1 to 3.

Images