JPH11301023A - Color image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a color image forming apparatus minimizing the color shift caused by the fluctuations of the rotation of the polygon mirror of an optical scanner to provide a sharp color image at a high speed. SOLUTION: In a color image forming apparatus having at least two optical scanners A, B equipped with rotary polygon mirrors 2, 20 allowing laser beams 1, 10 to be incident on a reflecting surface and deflecting laser beams to scan the surface of an image support and performing optical scanning at different positions or times of the surfaces 4, 40 of image carriers, the laser beams are allowed to scan in mutually reverse directions looking from the surfaces 4, 40 of the image carriers.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a color image forming apparatus having two optical scanning devices used as an exposure device for a laser beam printer or a digitally controlled copying machine.
In particular, the present invention relates to a color image forming apparatus that prevents color misregistration due to a precision error of a rotary polygon mirror, rotation fluctuation, and the like.

[0002]

2. Description of the Related Art Conventionally, print processing in offices and homes has been required to have a higher print speed. In recent years, color images have been required to have a higher speed in addition to monochrome images. I have. However, in contrast to the conventional monochrome printing machine that only had to output black,
Since a color printing machine needs to output many colors several times and superimpose them, the printing time is long, and therefore, there has been a great demand for high-speed printing.

[0003]

Generally, in digital copiers, laser printers, and the like, printing time is reduced by increasing the rotation speed of a polygon mirror installed in an optical scanning device. Exists.
However, since the rotation speed of the motor of the polygon mirror has already been usually high speed of 2000 rpm to 20,000 rpm, the motor needs to be increased in size in order to further increase the speed, and the assembly accuracy has been reduced due to measures against vibration. Must be improved, the production yield is low, and the cost is very high.

Therefore, in forming a color image, two types of latent images and different colors of different colors are irradiated by irradiating different positions on the photosensitive member using two optical scanning devices without increasing the rotation speed of the polygon mirror. Either a toner image is formed and color is superimposed on the intermediate transfer member, or two optical scanning devices irradiate the same position on the photoreceptor at different times so that toner images of different colors are superimposed. There is a technique for increasing the effective printing speed. However, in this case, if the scanning positions of the two light beams are displaced, the image will bleed or the color will be displaced. Although such a problem was not so serious in a single color, it becomes a serious problem in forming a color image in which colors are superimposed.

Although there are many causes of color misregistration, the cause of color misregistration that occurs particularly frequently is misregistration caused by the rotation fluctuation of the polygon mirror described above. FIG. 2 is a conceptual diagram when a vertical line is drawn by a single-color laser printer. It is assumed that the optical scanning device scans from left to right in the figure, the left side is the scanning upstream side, and the right side is the scanning downstream side (the same applies to the following description of other figures). The figure is a drawing in which a straight line is drawn in the vertical direction (perpendicular to the scanning direction) of the paper. Due to the accuracy error of the polygon mirror, rotation fluctuation, etc., the scanning starts slightly from the scanning start position, and the deviation slightly starts. Due to the accumulation of the amount, a large shift occurs on the downstream side of the scanning.

The rotation fluctuation of a general polygon mirror is 0.
Since it is about 03% PP, when vertical lines are drawn as shown in the figure, the following line swells occur on the upstream and downstream sides.

Upstream side: a = 50 × 0.03 / 100 = 0.015 (mm) Downstream side: b = (50 + 287) × 0.03 / 100 = 0.101 (mm) The amount of undulation on the upstream side is sufficiently small, Cannot be identified. On the downstream side, the standard unit of 600 dpi 0.042 (mm)
However, since the frequency of the undulation is large, it is inconspicuous in the case of a single color.

However, in the case of a color image in which colors are superimposed, the amount of deviation becomes large and conspicuous. That is, FIG. 3 is a conceptual diagram when a vertical line is drawn by a color laser printer when the scanning direction is the same. In the figure,
The laser scans from left to right. Here, a solid line indicates a line obtained by scanning with the first polygon mirror, and a broken line indicates a line obtained by scanning with the second polygon mirror. In this example, adjustment is made so that beams from the two polygon mirrors overlap near the center of the sheet.

In the figure, the displacement c between the two lines is not so large at the upstream end (left side in the figure) of scanning (2a = 0.
030 mm) at the downstream end (right side in the figure) of the scan, where the opening d between the two lines is maximum. In the worst case where the adjustment error and the undulation error overlap, the total amount is 2% of the undulation amount on the downstream side when a vertical line is drawn by the monochromatic laser printer.
Times (2b = 0.202 mm).

As described above, in the case of color, when two colors are overlapped or separated, the image is blurred or the color tone is changed, so that the color is conspicuous, and the amount thereof is required to be reduced.

SUMMARY OF THE INVENTION The present invention has been made in view of the above-described problems, and minimizes color misregistration caused by accuracy errors, rotation fluctuations, and the like of a polygon mirror of an optical scanning device, and provides a high-speed and clear color image. It is an object to provide a color image forming apparatus capable of providing an image.

[0012]

SUMMARY OF THE INVENTION In order to solve the above-mentioned problems, the present invention provides a rotating polygon mirror which irradiates a light beam toward a reflecting surface, deflects the light beam and scans the image carrier surface. A color image forming apparatus comprising at least two optical scanning devices, wherein each of the optical scanning devices performs optical scanning at a different position on the image carrier surface or at a different time. The scanning is performed in the opposite directions as viewed from the body surface. The rotating polygon mirrors may be rotated in different directions so as to scan in opposite directions when viewed from the image carrier surface.

On the other hand, a rotating polygonal mirror for irradiating a light beam toward the reflecting surface, deflecting the light beam and scanning on the surface of the image carrier, and provided between the rotating polygonal mirror and the surface of the image carrier. A scanning lens that forms an image of the light beam deflected by the reflection surface on the surface of the image bearing member, and at least two optical scanning devices are arranged, and each of the optical scanning devices is In a color image forming apparatus configured to perform optical scanning at different positions or at different times on the image carrier surface, the two optical scanning devices may be configured such that a light beam to the reflecting surface is formed with respect to an optical axis of each of the scanning lenses. The incident directions are made to coincide with each other, and scanning is performed in directions opposite to each other when viewed from the image carrier surface. Also in this case, the rotation directions of the rotary polygon mirrors may be different from each other so that scanning is performed in directions opposite to each other when viewed from the image carrier surface.

[0014] According to the above-described configuration, by utilizing the fact that the amount of deviation is small for the scanning on the upstream side of the scanning and the amount of deviation is large as the scanning is performed on the downstream side, the scanning direction is opposite to that of the image carrier. , The amount of displacement on the downstream side can be suppressed as much as possible.

[0015]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a schematic diagram of a color printer according to the present invention. A is photoconductor 4
The optical scanning device optically scans the surface of the optical scanning device, in which the light source 1 is disposed. The light source 1 emits a light beam such as a semiconductor laser. Reference numeral 2 denotes a polygon mirror that deflects the light beam from the light source 1 and scans the surface of the photoconductor 4, includes a rotating polygon mirror, and is configured to rotate the rotating polygon mirror by a polygon motor (not shown). The surface of the photoconductor 4 is optically scanned in the X direction. Reference numeral 3 denotes a scanning lens which collects light on the surface of the photoconductor 4. Reference numeral 5 denotes a beam detector for setting a writing start position on the photoconductor 4. Reference numeral B denotes an optical scanning device that optically scans another photosensitive member 40, and scans the surface of the photosensitive member 40 with a light beam in the Y direction toward the same side as the scanning surface of the photosensitive member 4. Similarly to the optical scanning device A, the optical scanning device B has a polygon mirror 20 and a scanning lens 3.
0, a beam detector 50. The semiconductor lasers 1 and 10 scan the incident direction of the scanning lenses 3 and
The direction of incidence of the light beam incident on the reflecting surface of the rotary polygon mirror is made to coincide with the 30 optical axes. Also, the rotation direction of the polygon mirrors 3 and 30, which is a feature of the present invention, is rotated in directions opposite to each other so that the scanning directions on the photoconductors 4 and 40 are opposite to each other in the X or Y direction. doing. The position for scanning the surface of each of the photoconductors 4 and 40 is irradiated to different positions when two photoconductors are used as described in the present embodiment, but the present invention is not limited to this. A plurality of developing devices may be arranged around the device to optically scan the same position at different times.

Reference numeral 6 denotes an intermediate transfer member (not shown).
A toner image is formed on each of the photoconductors 4 and 40 by a developing device containing different colors disposed therearound, and a color image is formed by superimposing the toner image on the surface of the intermediate transfer body 6.

Next, image formation will be described. The luminous flux emitted from the two light sources 1 and 2 is converted to polygon mirrors-3 and 4
Is reflected by each optical path. The latent images on the surfaces of the photoconductors 4 and 40 formed by the irradiation of the laser light are developed by a plurality of developing devices (not shown), transferred to the intermediate transfer member 6, and transferred to paper by the second transferring device 7. You.

In a color image forming apparatus having such a configuration, an experiment similar to that shown in FIG. FIG. 4 is a conceptual diagram when a vertical line is drawn by the color printer of the present invention when the scanning directions of the photoconductors 4 and 40 are reversed as in the feature of the present invention. Here, a solid line indicates a line X obtained by scanning the polygon mirror 2 from left to right, and a broken line indicates a line Y obtained by scanning the polygon mirror 20 from right to left. Also in this example, adjustment is made so that beams from the two scanners overlap near the center of the sheet.

In this case, the shift amounts e and f of the two lines are independent of the position, and the amount is equal to the sum of the amount of undulation on the upstream side and the amount of undulation on the downstream side when a vertical line is drawn by a monochromatic laser printer. Therefore, a + b = 0.116 mm. In the case of a color printer, the deviation amount of the line drawn in each color is
Since it is desirable that the thickness be 0.15 mm or less, it can be understood that this goal can be achieved by using the present invention.

It is preferable that the direction of irradiation of the reflecting mirror with respect to the optical axis of the scanning lens of the present invention is the same. The reason is that, with respect to the reflecting surface of the rotating polygon mirror, a configuration in which the light is applied obliquely to the optical axis of the scanning lens,
Since the reflection point on the reflecting surface of the rotating polygon mirror moves asymmetrically with respect to the optical axis, the fθ characteristic and the curvature of field become asymmetrical with respect to the optical axis. This is because the image quality deteriorates due to the problem of the asymmetry even if the problem of the amount of displacement caused by the rotation fluctuation or the like is solved because the image height position and the laser beam diameter scanned by the scanning device do not match.

However, it is not an essential requirement of the present invention to make the direction of irradiation on the reflecting mirror the same. That is, as means for solving the above-mentioned problem of asymmetry, Japanese Patent Application Laid-Open No.
It is known that an aspheric lens is formed asymmetrically with respect to the optical axis as shown by 3313. If such a lens solves the problems of fθ characteristics and field curvature, the above-mentioned lens is formed. There is no need to make the same direction because there is no such problem. Further, in the present invention, the direction of irradiating the reflecting mirror with respect to the optical axis of the scanning lens is set to be the same, but it is more preferable that the direction is substantially equal to the optical axis of the scanning lens on the reflecting surface. Even better results are obtained if the light is incident at the same angle.

Also, as in the present embodiment, the rotation direction of the polygon mirror need not necessarily be reversed. In other words, an optical scanning device configured to scan in the opposite direction to the photoconductor surface for optical scanning can scan the photoconductor surface in the opposite direction even if the rotation direction of the polygon mirror is the same. Further, the position of the optical scanning device is not necessarily arranged such that the surfaces of the photoconductors 4 and 40 for optical scanning scan on the same side as in the present embodiment. May be arranged so as to face each other.

[0023]

According to the present invention, the directions of scanning the photoconductor surface by a plurality of laser beams scanned at different positions or at different times on the photoconductor from the two optical scanning systems are opposite to each other.
It is possible to minimize the color shift due to the rotation fluctuation of the polygon mirror and the like.

Further, by irradiating the reflecting mirror with the same direction with respect to the optical axis of the scanning lens, optical performance (field curvature / distortion) can be achieved while minimizing color shift.
Color shift due to the asymmetry of the image can be minimized.

[Brief description of the drawings]

FIG. 1 is a schematic explanatory view of a color printer of the present invention.

FIG. 2 is an explanatory diagram when a vertical line is drawn by a single-color laser printer.

FIG. 3 is an explanatory diagram when a vertical line is drawn by a color laser printer when the scanning direction is the same.

FIG. 4 is an explanatory diagram when a vertical line is drawn by a color printer when the scanning direction of the present invention is reversed.

[Explanation of symbols]

 1, 10: Light source 2, 20: Polygon mirror 3, 30: Scanning lens 4, 40: Photoreceptor 5, 50: Beam detector 6: Intermediate transfer member 7: Second transfer device

Claims (4)

[Claims] An optical scanning device having a rotary polygonal mirror for irradiating a light beam toward a reflecting surface and deflecting the light beam to scan on the surface of an image carrier; A color image forming apparatus in which each of the optical scanning devices performs optical scanning at a different position on the image carrier surface or at a different time, wherein the optical scanning devices are configured to scan in opposite directions when viewed from the image carrier surface. A color image forming apparatus. 2. The color image forming apparatus according to claim 1, wherein the rotating polygon mirrors are rotated in different directions so as to scan in opposite directions when viewed from the image carrier surface. 3. A rotating polygon mirror for irradiating a light beam toward a reflecting surface, deflecting the light beam and scanning on the image carrier surface, and provided between the rotating polygon mirror and the image carrier surface. A scanning lens for forming an image of the light beam deflected by the reflection surface on the surface of the image bearing member, wherein at least two of the optical scanning devices are arranged, and each of the optical scanning devices is In a color image forming apparatus configured to perform optical scanning at different positions or at different times on the image carrier surface, the two optical scanning devices are configured to transmit a light beam to the reflecting surface with respect to the optical axis of each scanning lens. A color image forming apparatus, wherein incident directions are made to coincide with each other and scanning is performed in mutually opposite directions when viewed from the image carrier surface. 4. The color image forming apparatus according to claim 3, wherein the rotating polygon mirrors are rotated in different directions so as to scan in opposite directions when viewed from the image carrier surface.