Patent Application
20050285930
This application claims priority under 35 USC 119 from Japanese Patent Application No. 2004-189832, the disclosure of which is incorporated by reference herein.
1. Filed of the Invention
The present invention generally relates to an exposure apparatus, and more particularly it pertains to an inner drum exposure apparatus (an inner surface scanning type light beam scanning exposure apparatus) wherein exposure processing is performed by scanning a photosensitive surface disposed on the inner surface of a cylindrical drum by means of a laser beam scanning optical system.
2. Description of Related Art
Generally, an inner drum exposure apparatus (an inner surface scanning type light beam scanning exposure apparatus) has widely been used, in which scanning exposure processing is performed by guiding a light beam such as a laser beam onto a photosensitive surface of a recording medium disposed on the inner circumferential surface of a cylindrical drum (for example, refer to JP-A No. 10-133132). Meanwhile, the recording medium which has been subjected to an image exposure-recording process is fed, as occasion demands, to an automatic processor and thus a latent image recorded on the recording medium is transformed to a visible image.
In such an inner drum exposure apparatus, ordinarily in order to express halftones by performing exposure processing with respect to the recording medium, an AM screen (a technique for forming a grayscale image from a halftone dot image) is used which expresses halftones in a meshed pattern (so-called micro-checked pattern). More specifically, in an AM screen, a halftone dot image constituting the smallest unit is comprised of a relatively large number of dots, such as for example a total of 196 dots, or 14 (the number of dots in a horizontal direction)×14 (the number of dots in a vertical direction), and a grayscale image is recorded by performing recording with such halftone dot images arrayed in the form of a 2-dimensional plane. However, when the AM screen is used in order to express halftone, it is possible that Moire fringes are formed, and tone jump is caused.
Another technique for forming a grayscale image from a halftone dot image is one called FM screen, in which the contrasting density of a recorded image is expressed based on an aggregate density of amorphous dots having no regularity. For example, an image comprised of a relatively small number of dots such as 2×2, a total of 4 dots, is dispersed in the form of a 2-dimensional plane, thereby conducting tone expression. The FM screen is advantageous in that it is possible in principle to suppress occurrence of Moire fringes
Thus, in an inner drum exposure apparatus, it is desired that halftone be created in the form of a small mesh by using an FM screen.
However, in a conventional inner drum exposure apparatus, since a structure is adopted in which an image is formed by causing light emitted from a single lateral mode laser light source to be focussed on a scanned surface, a beam spot is shaped in the form of a Gaussian distribution. For a case where a pixel is recorded by exposure to such a Gaussian distribution-shaped beam spot, the design is made such that the half width of the beam spot becomes substantially equal to or greater than the pixel size in order to prevent the spacing between scanlines from opening up. For this reason, conventional inner drum exposure apparatuses have a problem that difficulties are experienced in using an FM screen, since an image produced when an FM screen is recorded by exposure to a beam spot having a gentle edge portion. This is such a beam that makes it likely that the halftone dot image proportion (halftone dot coverage ratio characteristic) would rapidly be changed, and hence a density change would be caused, should the circumference of a recording pixel be changed even slightly by the recording conditions, such as light power change and number of printing sheets, and/or development conditions such as degree of development in an automatic processor.
In view of the foregoing, the present invention provides a novel and improved inner drum exposure apparatus which is capable of recording an image in a manner such that halftone can be stably expressed using an FM screen.
According to a first aspect of the present invention, there is provided an inner drum exposure apparatus wherein a light beam of a scanning unit, modulated in accordance with image information, is: deflected by a reflective mirror surface, which is rotationally driven; and scanned, with respect to a recording medium supported on an arcuate inner circumferential surface formed on a support body; thereby recording an image. The apparatus comprises a light source, that emits the light beam; a ¼ wavelength plate, disposed in a light path between the light source and the scanning unit and which converts a linearly-polarized light beam emitted from the light source to circularly-polarized light; and a single-axis crystal optical element disposed between the ¼ wavelength plate and the reflective mirror surface such that a plane in which the light beam is incident onto and outgoing from the reflective mirror surface and a crystal axis of the single-axis crystal optical element become substantially parallel to each other. The single-axis crystal optical element is mounted so as to rotate integrally with the reflective mirror surface, thereby forming a beam spot configuration such that the light beam is split into substantially equal light quantities of an ordinary ray and an extraordinary ray, and two beam spots are arrayed adjacent to each other in a sub-scanning direction so as to be partially superposed.
According to a second aspect of the present invention, there is provided an inner drum exposure apparatus wherein a light beam of a scanning unit is: modulated in accordance with image information; deflected by a reflective mirror surface, which is rotationally driven; and, scanned with respect to a recording medium supported on an arcuate inner circumferential surface formed on a support body, thereby recording an image. The apparatus comprises a random polarization unit that causes a randomly polarized light beam to be incident on the reflective mirror surface of the scanning unit; and a single-axis crystal optical element disposed such that a plane in which the randomly polarized light beam is incident onto and outgoing from the reflective mirror surface and a plane containing a crystal optical axis of the single-axis crystal optical element and a normal to the incident plane of the single-axis crystal optical element become substantially coplanar. The single-axis crystal optical element is mounted so as to rotate integrally with the reflective mirror surface, thereby forming a beam spot configuration such that the light beam is split into substantially equal light quantities of an ordinary ray and an extraordinary ray, and two beam spots are arrayed adjacent to each other in a sub-scanning direction so as to be partially superposed.
According to a third aspect of the present invention, there is provided an inner drum exposure apparatus wherein a recording medium is supported on an arcuate inner circumferential surface formed on a support body, and a plurality of light beams, modulated in accordance with image information, are deflected and scanned onto the recording medium arcuately supported to record images. The apparatus comprises a light beam output unit, that emits the plurality of light beams; a scanning unit, including a reflective mirror surface having a rotation axis that coincides with a center axis of the arcuate inner circumferential surface of the support body, and which scans onto the recording medium by reflecting the plurality of light beams by rotating the reflective mirror surface; a light deflecting unit that causes the light beams to be deflected relatively and 2-dimensionally in a plane orthogonal to the rotation axis of the scanning unit, and shifted on the recording medium in a main-scanning direction and in a sub-scanning direction; a control unit that controls the positions where the light beams are shifted on the recording medium by the light deflecting unit, in synchronism with the rotation of the reflective mirror surface; a ¼ wavelength plate, disposed in a light path between the light beam output unit and the scanning unit, and which converts linearly-polarized plural light beams emitted from the light beam output unit to circularly-polarized light; and a single-axis crystal optical element, disposed between the ¼ wavelength plate and the reflective mirror surface, disposed such that a plane in which the randomly polarized light beams are incident onto and outgoing from the reflective mirror surface and a plane containing a crystal optical axis of the single-axis crystal optical element and a normal to the incident plane of the single-axis crystal optical element become coplanar. The single-axis crystal optical element is mounted so as to rotate integrally with the reflective mirror surface, thereby forming beam spot configurations such that each of the plurality of light beams is split into substantially equal light quantities of ordinary rays and extraordinary rays, and each two beam spots are arrayed adjacent to each other in a sub-scanning direction so as to be partially superposed.
Other aspects, features and advantages of the present invention will become apparent from the following description taken in conjunction with the accompanying drawings.
Preferred embodiments of the present invention will be described in detail based on the following figures, in which:
The inner drum exposure according to a first embodiment of the present invention will be described with reference to
Referring to
Further, in the inner drum exposure apparatus 10, an unrecorded recording medium 14 is fed by a recording medium feed/discharge device (not shown) and subjected to exposure processing after having been disposed along the inner circumference of the support body 12 and positively in close contact therewith. The recording medium 14 which has been subjected to exposure processing, is discharged from the support body 12 to the exterior.
In the inner drum exposure apparatus 10, a spinner mirror device 16, which serves as a scanning unit, is provided at the center position of the arcuate configuration of the support body 12. The spinner mirror device 16 includes a column-shaped rotary shaft 18 which can be rotated by a motor 20 about the center axis thereof (which corresponds to the center axis of the arcuate configuration of the support body 12). Further, the rotary shaft 18 of the spinner mirror device 16 is provided on its fore end with a reflective mirror surface 18A which forms an angle of 45 degrees with respect to the rotation axis of rotary shaft 18.
The spinner mirror apparatus 16, which is adapted to serve as a scanning unit, is scanningly moved at a constant speed by a sub-scanning moving unit (not shown) in the direction of the center axis of the arcuate configuration of the support body 12 (the left right direction of
Furthermore, the inner drum exposure apparatus 10 is structured such that beam splitting is performed and main-scanning is carried out on a recording surface of the recording medium 14, which is disposed on the inner circumferential surface of the support body 12.
To this end, on the spinner mirror device 16 side, a single-axis crystal optical element 26 is provided on a holder 24 which is secured to the rotary shaft 18 so as to rotate therewith. Meanwhile, the holder 24 is configured, for example, in a cylindrical shape and formed with a through-hole 14A (shown on
The single-axis crystal optical element 26 may be configured such that it is disposed in an optical path between a condensing lens 28 on the light source side and the reflective mirror surface 18A of the spinner mirror device 16, and mounted so as to rotate with the reflective mirror surface 18A. Meanwhile, the inner drum exposure apparatus 10 may be structured in various ways using, as the single-axis crystal optical element 26, an optical element which is capable of performing parallel splitting by a beam displacing prism.
As the single-axis crystal optical element 26, as shown in
On entering the single-axis crystal optical element 26, a circularly-polarized light beam (or a randomly-polarized light beam which is modulated to an image signal by an unillustrated random polarization unit) is split into equal light quantities of an ordinary ray Po and an extraordinary ray Pe, as shown in
When a beam spot configuration is formed in which a light beam is split into equal light quantities of an ordinary ray Po and an extraordinary ray Pe (a system in which a light beam is split angle-wise may be adopted) and the light rays Po and Pe are in turn shifted in parallel with each other and superposed upon each other (when a beam is split so as to form beam spots), a state occurs such that two Gaussian beams, each having a half width of 5 μm, are shifted 5.5 μm and superposed upon each other (a split beam state), as shown in
When this is compared with a comparative example of
As shown in
As shown in
The optical system on the light source side includes a semiconductor laser light source (LD) 30 which emits a laser beam L consisting of substantially linearly polarized light, and a light focussing optical system which focuses the laser beam emitted by the semiconductor laser light source 30 onto the exposure surface of the recording medium 14. The semiconductor laser light source 30 may use a single lateral mode semiconductor laser having an intensity distribution such that the light intensity is high at the center and becomes gradually lower as the distance from the center increases.
The optical system on the light source side is structured by arranging a ¼ wavelength plate, a reflective mirror 34, a reflective mirror 36, and a condensing lens 28 in that order from the semiconductor laser light source 30.
The ¼ wavelength plate 32 is arranged such that a laser beam L consisting of linearly polarized light emitted from the semiconductor laser light source 30 is converted to circularly polarized light as a result of having passed through the ¼ wavelength plate 32. Further, the circularly polarized laser beam L which has been converted to circularly polarized light is condensed by the condensing lens 28 and then passed through the single-axis optical element 26; thus the circularly polarized laser beam L is split into beams of equal light quantity and the split laser beams are shifted to be parallel with respect to each other so that two beam spots can be superposed in the sub-scanning direction. In such a state, the split beams are caused to be incident in a direction such that the direction in which the circularly polarized laser beam L is split in equal proportions and the split laser beams are shifted to be parallel with respect to each other is parallel with a predetermined incident reflection surface at the reflective mirror surface 18A of the spinner mirror device 16. Further, the laser beams are reflected at the reflective mirror surface 18A, distributed in an approximately rectangular shape with respect to the sub-scanning direction, and focussed at focussing spots on the recording medium 14 disposed on the inner circumferential surface of the support body 12, and perform recording processing.
For a case where the semiconductor laser light source 30 emits a laser beam L of circularly polarized light or a randomly polarized light, the optical system on the light source side is structured as an optical system in which the ¼ wavelength plate 32 is not used.
As shown in
Further, at the same time, the central control unit 40 controls rotation of the motor 20 to turn the reflective mirror surface 18A such that the laser beam L entering the reflective mirror surface 18A from the optical system on the light source side is reflected at the reflective mirror surface 18A. In this way, a scanning exposure in the main-scanning direction is carried out with respect to the recording medium 14, and a control signal is transmitted to the spinner driver 22. The spinner driver 22 which has received the control signal controls the unillustrated sub-scanning moving unit, thereby permitting the spinner mirror device 16 to perform a moving scan at a uniform speed in the axial direction of the center axis of the arcuate configuration of the support body 12 (in the directions of arrows C, i.e., in the leftward and rightward directions as viewed in
Description will next be made of the performance and operation of the inner drum exposure apparatus according to the first embodiment of the present invention.
In the inner drum exposure apparatus 10, the semiconductor laser light source 30, which is controlled by the central control unit 40 and the laser driver 42, provides a laser beam L which is modulated in accordance with image information and caused to be incident on the ¼ wavelength plate 32. After being incident on the ¼ waveguide plate 32, the laser beam L, which is substantially linearly polarized light, is converted to circularly polarized light, which in turn is reflected by the reflection mirror 34 and the reflection mirror 36, condensed by the condensing lens 28, and caused to enter the single-axis crystal optical element 26. The laser beam L, which is now circularly polarized light, is split to ordinary ray Po and extraordinary ray Pe when passing through the single-axis crystal optical element 26. Meanwhile, the refraction angle for the extraordinary ray Pe can be arbitrarily tailored depending on the thickness in the direction of the optical axis and/or the material of the single-axis crystal optical element 26. Further, the single-axis crystal optical element 26 is arranged such that the ordinary ray Po and the extraordinary ray Pe, which are split in the sub-scanning direction, are set so as to have an equal intensity.
The ordinary ray Po and the extraordinary ray Pe, which are split in the sub-scanning direction by the single-axis crystal optical element 26 and adjusted in intensity, are reflected at the reflection mirror surface 18A and then focussed at focussing spots, distributed in an approximately rectangular shape with respect to the sub-scanning direction, on the recording medium 14. More specifically, in this inner drum exposure apparatus 10, exposure processing is performed while a state is constantly maintained in which a beam is split and distributed in an approximately rectangular configuration, while at the same time the beam spot having a sharp edge portion is elongated in the sub-scanning direction (that is, a state in which the longitudinal direction of the approximately rectangular distribution of a beam spot coincides with the sub-scanning direction (the direction orthogonal to the main-scanning direction)). Thus, an image produced when an FM screen is recorded can be recorded in a manner such that halftones are stably expressed using an FM screen, while the circumference of a recording pixel is not changed at all by recording conditions, such as a light power change and number of printing sheets, and/or development conditions, such as degree of development in an automatic processor. Thereby the halftone dot image proportion (halftone dot coverage ratio characteristic) is prevented from being rapidly changed, so that causing changes in density becomes more difficult.
Next, the inner drum exposure apparatus according to a second embodiment of the present invention will be described with reference to FIGS. 6 to 9. Firstly, the principles of the second embodiment of the present invention will be explained with reference to
When the reflective mirror surface 18A of the spinner mirror device 16 serving as a scanning unit is oriented as shown in
Here, by causing a laser beam #1 to be shifted by a predetermined amount in the negative X-axis direction and in the positive Y-axis direction, reflected laser beams #1 and #2 can be modified to array along the Z-axis direction on a X-Z plane as shown in
As can be seen, by 2-dimensionally adjusting the direction in which the laser beam #1 is incident on the reflective mirror surface 18A, spots of the respective laser beams #1 and #2 on a recording sheet S can always be arrayed in the Z-axis direction. In this way, as shown in
Now, when the spots of laser beams #1 and 2# incident on the reflective mirror surface 18A of the spinner mirror device 16 are projected onto a plane S′, which is conjugate to the recording medium 14, as shown in
X=−W cos ωt
Y=−W cos ωt
where ω is the angular speed of the spinner mirror device 16, and W is the width of split of the laser beams #1 and #2 on the scanning surface (the distance between the position of the beam spot of the laser beam 1# on the scanning surface and the position of the beam spot of the laser beam #2 on the scanning surface).
Consequently, by causing the laser beam #1 to be deflected in accordance with the above equations and guided to the scanning unit, it is possible to form on the scanning surface plural scanlines which are separated by a constant spacing corresponding to the divided width W, as shown in
Next, the structure of the inner drum exposure apparatus according to the second embodiment of the present invention will be explained with reference to
The laser beam La emitted from the first semiconductor laser light source 30A is guided to a polarized beam splitter 48 and combined with the second laser beam Lb. As in the above-described inner drum exposure apparatus shown in
Further, in the light source side optical system for the second semiconductor laser optical source 30B, the second laser beam Lb, emitted from the second semiconductor laser light source 30B and converted to a parallel beam by the collimating lens 46, is passed through an acousto-optic element 52X serving as a light deflecting unit which deflects the second laser beams Lb in the X-axis direction of the spinner mirror device 16 and then through an acousto-optic element 52Y serving as a light deflecting unit which deflects the second laser beam Lb in the Y-axis direction. Subsequently, the second laser beam Lb is guided to a deflected beam splitter 48 and combined with the first laser beam La. The respective light beams La and Lb thus combined are converted to right-handed circularly polarized light and left-handed circularly polarized light which in turn are irradiated toward the spinner mirror device 16 by the light source side optical system as described above.
The respective acousto-optic elements 52X and 52Y which are light deflecting units are controlled by a circuit arrangement shown in
Next, description will be made of the performance and operation of the inner drum exposure apparatus structured as above in accordance with the second embodiment of the present invention.
The second laser beam Lb emitted from the second semiconductor laser light source 30B and converted to a parallel beam by the collimating lens 46 is deflected in the X-axis direction by the acousto-optic element 52X which is a light deflecting unit, and subsequently deflected in the Y-axis direction by the acousto-optic element 52Y which is a light deflecting unit (see
The spinner mirror device 16 reflects and deflects the second laser beam Lb (#1) by the reflective mirror surface 18A which rotates about the Z-axis, and guides the second laser bean Lb to the recording medium 14.
On other hand, the first laser beam La (#2) is introduced along the axis of rotation of the spinner mirror device 16, reflected and deflected by the reflective mirror surface 18A, and guided to the recording medium 14.
Next, the controlling of the second laser beam Lb (#1) emitted from the second semiconductor laser light source 30B will be described in detail with reference to
Firstly, the control circuit 54 as a control unit supplies a control clock signal to the cosine wave signal generating circuit 56a, based on a positional signal which is derived from the unillustrated encoder provided in the spinner mirror device 16. The cosine wave voltage signal (X=−a·cos ωt) outputted from the cosine wave signal generating circuit 56a is converted to a frequency-modulated signal by the voltage-controlled oscillator 58a, and then supplied to the acousto-optic element 52X via the amplifier 60a. In this case, based on this cosine wave voltage signal, the acousto-optic element 52X deflects the first laser beam Lb (#1) in the X-axis direction as shown in
Further, the control circuit 54 as a control unit supplies a control clock signal to the sine wave signal generating circuit 56b. The sine wave voltage signal (Y=−a·sin □t) derived from the sine wave signal generating circuit 56b is converted to a frequency-modulated signal by the voltage-controlled oscillator 58b, and then supplied to the acousto-optic element 52Y via the amplifier 60b. In this case, based on this sine wave voltage signal, the acousto-optic element 52Y deflects the second laser beam Lb (#1), which is deflected in the X-axis direction by the acousto-optic element 52X, in the Y-axis direction as shown in
As a result of this, the second laser beam Lb (#1) guided to the reflective mirror surface 18A of the spinner mirror device 16 draws an approximately circular locus on a plane S′, which is orthogonal to the rotary shaft 18 of the spinner mirror device 16, in synchronism with the rotation of the spinner mirror device 16, as shown in
On the other hand, the first laser beam La (#2) emitted from the first semiconductor laser light source 30A is guided to the recording medium 14 without moving on the plane S′, which is orthogonal to the rotary shaft 18.
Subsequently, as shown in
More specifically, two parallel scanlines which are uniform in spacing and in length are formed on the recording medium 14, as shown in
In the inner drum exposure apparatus according to the second embodiment of the present invention, exposure processing is performed using beam-split two scanlines as described above, and consequently an image produced when an FM screen is recorded can be rapidly recorded in a manner such that a halftone is stably expressed using an FM screen while the circumference of a recording pixel is not changed at all by recording conditions, such as a light power change and number of printing sheets, and/or development conditions, such as a degree of development in an automatic processor. Thereby rapid changes in the halftone dot image proportion (halftone dot coverage ratio characteristic) are prevented, so that a density change is prevented from being caused. Further, according to the second embodiment of the present invention, it is possible to enhance the reliability of recording accuracy based on scanning, by increasing the number of light beams and setting the rotation speed of the scanning unit at a low value.
Although in the second embodiment described above, the acousto-optic elements 52X and 52Y, which are light deflecting units, are structured seperately, it is to be noted that these acousto-optic elements may be structured integrally such that the deflections in the X-axis and Y-axis directions can be achieved by a single acousto-optic element. Further, an electro-optical element may be used instead of an acousto-optic element. Still further, three or more semiconductor laser light sources may be provided such that an image can be recorded using laser beams emitted from those light sources. Furthermore, an arrangement may be adopted in which a laser beam emitted from a semiconductor laser light source is split into a predetermined number of laser beams by a beam splitter and an image is recorded using such split laser beams
While the present invention has been illustrated and described with respect to specific embodiments thereof, it is to be understood that the present invention is by no means limited thereto and encompasses various modifications and changes which are possible within the scope of the appended claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2004-189832 | Jun 2004 | JP | national |
