JPH05333281A - Image forming device - Google Patents

Abstract

PURPOSE:To obtain a good image having little picture element deviation over an entire recording area as to an image forming device simultaneously performing scanning by plural lines. CONSTITUTION:When an angle formed by the main light beam of a laser beam projected from an image-formation optical system 15 and the optical axis of the image-formation optical system is alpha on a maximum image area on a photosensitive drum 17, an integer >=1 expressing how many times a scanning line simultaneously scanned by the laser beam projected from the plural lasers is as long as a sub-scanning pitch is (m); the angle xsi formed by a flat surface formed by the laser beam scanning the photosensitive drum 17 and the normal of a surface to be scanned is set to xsi<=¦1/(2XmXtanalpha)¦ (rad), and positional deviation in the main scanning direction of the scanning line simultaneously formed is <=1/2 of the sub-scanning line pitch.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a multi-beam scanning device used in an image forming apparatus such as an electronic copying machine or a printer for forming an image by using a laser beam, and more specifically, it uses a plurality of laser beams. The present invention relates to a multi-beam scanning device capable of maintaining good image quality when forming one image.

[0002]

2. Description of the Related Art Most of image forming apparatuses such as electrophotographic copying machines and laser printers are adapted to form an electrostatic latent image on a photosensitive member and develop the electrostatic latent image with toner to record the image. .. In such an apparatus, it is usual to form one type of image using one laser beam, but it has also been proposed to form an image more quickly using a plurality of laser beams.

For example, in Japanese Examined Patent Publication No. 64-10805, a plurality of light sources arranged in an array are used to generate a plurality of laser beams. And
The plurality of beams are converged by a shaping optical system so as to be spatially concentrated as much as possible on a deflection mirror, and the plurality of obtained light beams are simultaneously deflected by a deflector so as to scan the photosensitive drum. ..

Further, in Japanese Patent Publication No. 1-45065,
A plurality of light source parts are used, and a plurality of beam spots by these light source parts scan the scanning lines on the photosensitive drum, which are separated from each other by a predetermined distance.

In the inventions described in the above publications, a plurality of beams are scanned in parallel at a certain interval in the sub-scanning direction in order to form an electrostatic latent image on a recording member such as a photosensitive drum at a high speed. I am trying to do it. By the way, in general, regardless of whether the laser beam is plural or singular, in the conventional image forming apparatus, the following consideration has been taken in determining the angle at which the laser beam is incident on the photosensitive drum.

(A) The incident angle of the laser beam with respect to the photosensitive drum is set in relation to the internal size of the image forming apparatus and the parts such as the developing device arranged around the photosensitive drum.

(B) When the laser beam incident on the photoconductor drum returns to its light source due to reflection on the surface of the photoconductor drum, oscillation becomes unstable. Therefore, in order to prevent this, the incident angle of the laser beam on the photoconductor drum is set. To a sufficiently large angle.

[0008]

For the above reasons, when the angle of the laser beam incident on the photoconductor such as the photoconductor drum is set to a large angle with respect to the normal line of the incident point,
When scanning a plurality of laser beams in parallel at the same time, the scanning magnification is different in each scanning line, which causes a problem that the image becomes unclear.

Therefore, an object of the present invention is to provide an image capable of obtaining a good image with less pixel shift over the entire recording area even in an image forming apparatus that simultaneously scans a plurality of lines for forming an image at high speed. It is to provide a forming apparatus.

[0010]

According to a first aspect of the present invention, a recording member such as a photosensitive drum for forming an image,
A plurality of lasers, a shaping optical system that shapes each of the laser beams emitted from the plurality of lasers into a desired shape, a deflecting unit that deflects each of the shaped laser beams toward a recording member, and this deflection In an image forming apparatus having an image forming optical system for forming an image on the recording member by each laser beam deflected by the means, the image forming optical system at the maximum scanning angle of view on the recording member is emitted. The angle between the principal ray of the laser beam and the optical axis of the imaging optical system is α, and how many times the scanning line simultaneously scanned by the laser beams emitted from the plurality of lasers represents the sub-scanning pitch. Where m is an integer greater than or equal to 1, the angle ξ between the plane formed by the laser beam scanning the recording member and the normal to the surface to be scanned is set to the range shown in equation (1).

By limiting the angle ξ to this angle range, the positional deviation in the main scanning direction of the scanning lines simultaneously formed on the recording member such as the photosensitive drum by the plurality of laser beams causes the sub scanning line pitch. By suppressing the displacement of the recording pixels in the main scanning direction,
A good image was obtained.

[0012]

EXAMPLES The present invention will be described in detail below with reference to examples.

FIG. 1 shows an optical system and a photosensitive drum as essential parts of an image forming apparatus according to an embodiment of the present invention. In this image forming apparatus, a laser beam output from a laser light source 11 is shaped by a beam shaping optical system 12 including a collimator lens, and then the polygon mirror 1 is formed.
The light is incident on one surface of the mirror forming part 3. The polygon mirror 13 is rotated at a high speed by a polygon mirror driving motor (not shown), and the incident laser beam is 1
It is deflected in units of scanning lines. The laser beam reflected from the polygon mirror 13 passes through an imaging optical system 15 including an fθ lens and the like, is further reflected by a cylindrical mirror 16, and is scanned on the photosensitive drum 17.

By the way, a laser beam array is used for the laser light source 11 of this image forming apparatus. The laser beam array does not output a single laser beam, but outputs three laser beams at predetermined intervals in the sub-scanning direction. These laser beams pass through the same image forming optical system 15 and the same cylindrical mirror 16, and three scanning lines 18 are simultaneously formed on the photosensitive drum 17. At this time, the laser beam passing through the cylindrical mirror 16 has a normal 19
And is incident on the photoconductor drum 17 with a predetermined angle ξ.

FIG. 2 is a view of the optical system and the photoconductor drum shown in FIG. 1 as seen from a plane perpendicular to the rotation axis of the photoconductor drum, and FIG. 3 is seen as a plane perpendicular to the rotation axis of the polygon mirror. It is a thing. Further, FIG. 4 shows a state in which three scanning lines are scanned in parallel on the photosensitive drum. In these figures, consider a Cartesian coordinate system consisting of an X axis, a Y axis and a Z axis. Here, the X axis is the moving direction of the member to be scanned, that is, the photosensitive drum 17, and the Y axis is the main scanning direction. The Z axis is the normal line of the photosensitive drum 17.

As shown in FIG. 3, the angle of view between the principal ray 21 of the laser beam and the optical axis 22 of the imaging optical system 15 at the maximum scanning angle of view is α, and the YZ plane and the photosensitive drum 1 are set.
The angle with the plane formed by the laser beam incident on 7 is ξ
And At this time, the maximum relative optical path difference in the Z direction of the three laser beams is defined as the maximum spacing in the sub-scanning direction between the scanning lines simultaneously scanned on the photosensitive drum 17 as shown in FIG. , Can be expressed by the following equation (2).

[0017]

[Equation 2]

Further, the deviation of the incident position in the Y direction from the principal ray of each laser beam incident on the photosensitive drum 17 is ΔY (in the figure, the deviation of the incident position between the first and the second beams is Δa, and the second position is the second beam). If the incident position shift for the third line is indicated by Δb), this can be expressed by equation (3).

[0019]

[Equation 3]

Further, when the sub-scanning pitch is p and "m" (m is an integer of 1 or more) is used as a number indicating how many times the scanning line simultaneously scanned represents the sub-scanning pitch, the sub-scanning pitches are simultaneously scanned. The pixel shift of the scanning line in the main scanning direction is shown in (4) below.
It can be represented by a formula.

[0021]

[Equation 4]

Here, in order to set ΔY to 1/2 of the sub-scanning pitch, the following expression (5) must be satisfied.

[0023]

[Equation 5]

Based on the actual optical system, the angle of view α is set to 22.38 de
g, the sub-scanning pitch was 63.5 μm, and the maximum scanning line interval for simultaneous scanning was 444.5 μm. The angle formed by the YZ plane and the plane formed by the laser beam incident on the photosensitive drum 17 was changed from 0 deg to 15 deg. Table 1 below shows an example of calculating the pixel shift amount on the recording member at the same scanning angle of the principal ray by ray tracing.

[0025]

[Table 1]

Table 2 below shows an example of calculation based on the equation (4).

[0027]

[Table 2]

In the present embodiment, the angle of view α is set to 22.38 deg, and the number m indicating how many times the scanning line simultaneously scanned represents the sub-scanning pitch is set to 7. At this time, if the angle ξ between the YZ plane and the plane formed by the laser beam incident on the photoconductor drum 17 is 0.173 rad, that is, 10 deg or less, the error in the main scanning direction may be less than 1/2 of the sub-scanning density. Shown and is in agreement with actual calculations. Further, in this range, there is almost no relative difference in the bending of each scanning line in the sub-scanning direction.

FIG. 5 shows the arrangement of the pixels at the image end of the photosensitive drum in the image forming apparatus of this embodiment, and FIG. 6 shows the image end of the conventional photosensitive drum for comparison with this. 3 illustrates a state of arrangement of pixels of a part. In these figures, the three pixels 31 ₁ , 31 ₂ , from the top,
31 ₃ is simultaneously scanned at a predetermined scanning timing 3
The first pixel in the image region of the laser beam of the book, the next three pixels, three pixels 32 ₁ , 32 ₂ , 32
₃ is the first pixel in the image region by the three laser beams scanned simultaneously at the next scan timing.

As shown in FIG. 6, conventionally, the above-mentioned incident position deviation ΔY in the Y direction is large. For this reason, the pixel shift is conspicuous at the edge of the image, and the thin line orthogonal to the main scanning direction becomes jagged and is represented as a thick and blurred line. On the other hand, if the correction is performed as in the present embodiment, the pixel arrangement shown in FIG. 5 is obtained, and the thin line orthogonal to the main scanning direction is also clearly expressed.

In the embodiment described above, the polygon mirror 1
3 shows a configuration in which the laser beams are arranged in a straight line perpendicular to the deflection plane formed by 3. Other than this, the present invention can be applied even to a configuration described in Japanese Patent Publication No. Sho 64-10805 in which the direction in which the laser beams are arranged is slightly inclined with respect to the deflection plane. That is,
In order to keep the scanning magnification deviation of each scanning line (the error in the length of the scanning lines simultaneously scanned on the scanned recording member despite the same scanning angle of view) below a certain amount, the present invention The relationship between the incident angle of the laser beam on the recording member and the sub-scanning interval is significant.

Although the laser beam array is used as the light source in the embodiment, it is not limited to this. For example, a combination of laser beams or a beam splitter using a He-Ne (helium-neon) laser or the like, splitting, separately modulating, synthesizing again, and scanning them simultaneously may be used. Good. Further, although the photosensitive drum is used as the recording member to be scanned in the embodiment, it goes without saying that other recording members such as a photosensitive belt can be used.

[0033]

As described above, according to the first aspect of the invention, the angle ξ formed by the plane formed by the laser beam for scanning the recording member and the normal to the surface to be scanned is expressed by equation (1). Since the range is limited, the positional deviation in the main scanning direction of the simultaneously formed scanning lines becomes 1/2 or less of the sub scanning line pitch,
There is an effect that a good image with less pixel shift can be obtained at high speed over the entire recording area.

[Brief description of drawings]

FIG. 1 is a perspective view showing a main part of an image forming apparatus according to an embodiment of the present invention.

FIG. 2 is an explanatory diagram showing the optical system and the photoconductor drum shown in FIG. 1 on a plane perpendicular to the rotation axis of the photoconductor drum.

FIG. 3 is an explanatory diagram showing the optical system and the photoconductor drum shown in FIG. 1 on a plane perpendicular to the rotation axis of a polygon mirror.

FIG. 4 is a perspective view showing a state in which three scanning lines are scanned in parallel on a photosensitive drum.

FIG. 5 is an explanatory diagram showing an arrangement state of pixels at an image edge portion in the present embodiment.

FIG. 6 is an explanatory diagram showing a state of arrangement of pixels at an image edge portion in the related art for comparison with the present embodiment.

[Explanation of Codes] 11 ... Laser light source, 12 ... Beam shaping optical system, 13 ... Polygon mirror, 15 ... Imaging optical system, 16 ... Cylindrical mirror, 17 ... Photosensitive drum, 19 ... Normal line, 21 ... Principal beam, 22 ... Optical axis, 31, 32 ... Pixel

Claims (1)

[Claims] 1. A recording member for forming an image, a plurality of lasers, a shaping optical system for shaping a laser beam emitted from each of the plurality of lasers into a desired shape, and each shaped laser beam. A deflection means for deflecting the laser beam toward the recording member, and an image forming optical system for forming an image on each of the laser beams deflected by the deflection means on the recording member. An angle formed by the chief ray of the laser beam emitted from the imaging optical system and the optical axis of the imaging optical system at the scanning angle of view is α, and scanning is performed by the laser beams emitted from the plurality of lasers simultaneously. When m is an integer greater than or equal to 1 indicating how many times the line represents the sub-scanning pitch, the angle ξ between the plane formed by the laser beam scanning the recording member and the normal to the scanning surface is An image forming apparatus characterized in that the range of the formula (1) is satisfied. [Equation 1]