JP4460865B2 - Optical scanning apparatus and color image forming apparatus - Google Patents

Description

  The present invention relates to an optical scanning device such as a digital color copying machine and a color laser printer, and a color image forming apparatus.

  In recent years, with the increase in the speed of color image forming apparatuses, for example, four photosensitive drums are arranged in the recording paper conveyance direction, and a latent image is exposed simultaneously by a plurality of scanning optical systems corresponding to the respective photosensitive drums. These latent images are visualized with a developing device using developers of different colors such as yellow, magenta, cyan, black (hereinafter referred to as Y, M, C, K), and then visible. Digital copying machines and laser printers that obtain color images by sequentially superimposing and transferring images on the same recording paper have been put into practical use (4-drum tandem system).

  The 4-drum tandem system is advantageous for high-speed printing because color and monochrome can be output at the same speed as the 1-drum system. On the other hand, since the four photoconductors (image carriers) have four scanning optical systems, there is a problem in downsizing the apparatus. Another problem is to reduce color misregistration when transferring a toner image developed on each photoconductor.

As such a tandem type image forming apparatus, a system in which a plurality of photosensitive media share a single optical deflector is disclosed.
(1) A system having a plurality of optical scanning devices for a plurality of photoconductors.
The number of optical scanning devices is the same as that of the photosensitive member, and scanning is performed by synchronizing the deflectors between the optical scanning devices (Patent Document 1).
(2) A counter scanning method in which a light beam is incident from both sides of the deflector, and the light beam is distributed in two directions and deflected and scanned.
A plurality of light beams that are substantially parallel and separated in the sub-scanning direction are incident on the deflector, and a plurality of scanning optical elements corresponding to the plurality of light beams are arranged in the sub-scanning direction and scanned (Patent Document 2).
(3) A one-side scanning method in which a light beam is incident from one side of the deflector and deflected and scanned in one direction.
A light beam is incident from one side of the deflector, and a plurality of scanning optical systems L1 and L2 pass through a plurality of light beams directed to different scanning surfaces, and L3 is provided for each scanning surface (patent). Documents 3, 4 and 5).

In recent years, in order to improve the scanning characteristics, a special surface represented by a non-circular arc can be easily formed on the optical element of the optical scanning device, and “resin-molded optical element” is frequently used to realize low cost. ing. In particular, in the tandem type image forming apparatus described above, since the number of optical elements used is large, the cost reduction effect by using resin optical elements is very large.
On the other hand, when a resin optical element is used in the optical scanning device, the temperature change causes a larger coefficient of thermal expansion than glass, resulting in a large change in shape, which changes the optical characteristics of the resin optical element. In particular, when the temperature in the optical box rises due to the deflection means such as a polygon mirror that generates a large amount of heat, the heat is uniformly transferred due to the air flow created by the rotation of the polygon mirror and the difference in the shape of the optical box. The temperature inside the optical box has a temperature distribution. Also in the scanning lens, a uniform temperature change does not occur due to a difference in heat transmission method, a difference in lens shape (a difference in installation area on the optical box), and the like, and a temperature difference due to the location of the scanning lens occurs.

  In the tandem type image forming apparatus, the light fluxes directed to the respective photoconductors pass through different scanning lenses, and due to the temperature distribution in the optical box holding the scanning lenses, different temperature distributions are generated between the respective scanning lenses. The shape change, the refractive index change, and the like are not uniform, and the amount of change in the scanning length and the change in isokineticity of each photoconductor are different. These latent images are visualized with a developing device using developers of different colors such as yellow, magenta, cyan, and black (Y, M, C, Bk), and then these visible images are the same. When a color image is obtained by sequentially superimposing and transferring on a recording sheet, a so-called “color shift” occurs. In particular, when the scanning lens closest to the deflecting means such as a polygon mirror that generates a large amount of heat in the optical box is made of resin, the change in optical characteristics becomes large.

  Further, in the case of continuous output, particularly when the number of continuous outputs is large, the internal temperature (temperature in the optical box) rises due to the heat generated by the deflection means. For this reason, the temperature distribution of each scanning lens changes, color shift occurs as described above, and the amount of change also changes. As a result, there is a problem that the color changes between the first output image and the last output image due to color misregistration.

As a method of coping with such a color misregistration problem, a method of arranging light receiving means on the writing start side and writing end side, respectively, and adjusting the image frequency of each light beam based on the light receiving time difference of each light receiving means However, although this method can correct the length of the entire width of the scanning line, it cannot correct the change in isokineticity due to the temperature distribution of each scanning lens. For this reason, for example, even if the dot positions in the main scanning direction at the start and end of writing are corrected by the respective photoconductors, the dot positions in the main scanning direction at the intermediate image height do not match and color misregistration occurs. End up.
JP 2001-305826 A JP-A-9-54263 JP 2001-4948 A Japanese Patent Laid-Open No. 2001-10107 JP 2001-33720 A Japanese Patent Laid-Open No. 9-58053 JP 2002-36625 A

The above tandem image forming apparatuses (1) to (3) have the following features and problems.
(1) A system having a plurality of optical scanning devices for a plurality of photoconductors.
Since the optical scanning devices are different, the shape variation of the optical elements and the temperature / humidity environment are different. Therefore, the beam position is relatively shifted on each surface to be scanned, and the “color shift” is likely to occur.
(2) A counter scanning method in which a light beam is incident from both sides of the deflector, and the light beam is distributed in two directions and deflected and scanned.
Compared with the above (1), the optical scanning device can be easily miniaturized, and a high cost weight polygon scanner can be used in common for four colors, which is advantageous for cost reduction. On the other hand, since the optical elements facing each other centering on the polygon have different shape variations and temperature / humidity environments of the optical elements, the beam positions are relatively shifted on each surface to be scanned, and the above-mentioned “color shift” tends to occur.
(3) A one-side scanning method in which a light beam is incident from one side of the deflector and deflected and scanned in one direction.
Each light flux corresponding to each color (Y, M, C, K) is transmitted through a scanning lens (first lens) mainly having power in the main scanning direction, so that there is little variation in the shape of the optical element, and temperature and humidity Since the environment is almost the same, there is a merit that it is difficult to cause beam spot position fluctuation (that is, color misregistration) in the relative main scanning direction of each color.

On the other hand, there are the following problems.
-Optical elements that share and transmit light beams corresponding to the respective colors (Y, M, C, K) have a thick lens height in the sub-scanning corresponding direction, so it is difficult to compensate for surface accuracy variations for each light beam. Therefore, there is a problem in reducing the beam spot diameter. Furthermore, it is a problem to reduce the cost increase due to the increase in molding time as the thickness increases.
-Since the light fluxes corresponding to the respective colors (Y, M, C, K) are incident on the same deflection mirror surface, the polygon mirror becomes thick, so that the apparatus becomes large in size, for example, the deflector becomes large. Furthermore, there are problems such as noise caused by windage loss, increase in power consumption, and durability deterioration.
In order to separate the luminous fluxes corresponding to the respective colors (Y, M, C, K), each luminous flux is configured to be obliquely incident on the deflection mirror surface. Therefore, due to the skew of the beam luminous flux accompanying the deflection of the polygon mirror, Wavefront aberration tends to deteriorate, and there is a problem in reducing the beam spot diameter. Further, since the scanning line is bent on the image carrier (photosensitive member), adjustment for correcting it is essential.

Therefore, the present invention
1. Good color image with little color misregistration by effectively correcting the relative color misregistration between each color even when the optical element is out of shape, temperature fluctuations during continuous printing, and disturbances such as temperature distribution. It is a first object of the present invention to provide an optical scanning device capable of outputting the above.
2. A second object is to provide a high-speed writing compatible optical scanning device that improves noise, power consumption, and durability due to polygonal windage damage.

In order to achieve the above object, an optical scanning device according to a first aspect of the present invention includes a scanning optical element that deflects and scans a plurality of light beams with a single deflector and forms an image on a plurality of image carriers. The deflector is composed of polygon mirrors divided into two stages, and light beams guided to at least two or more image carriers are angled with respect to the sub-scanning corresponding direction on the same mirror surface of each stage. The angle β formed in the sub-scanning corresponding direction of each light beam guided to at least two or more image carriers on the same mirror surface satisfies 1 ° <β <4 °, Of the light beams guided to the image carriers, at least one light beam is incident at an angle of 0.5 degrees or less with respect to a surface perpendicular to the polygon mirror rotation axis. In the scanning optical element, at least one surface is It has an eccentric surface that tilts in the sub-scanning corresponding direction, and Kokoromen is characterized in that the eccentric angle in response to the image height is varying surface shape.

A second aspect of the present invention is the optical scanning device according to the first aspect, further comprising a light beam separator composed of a transmission part and a reflection part, and a light beam guided to each of the image carriers. Of these, at least one light beam is optically separated by the reflecting portion.

Invention of claim 3, an optical scanning apparatus according to item 1 to any one of claims 1 to 2, in the scanning optical element, all of the light beam guided to the each image carrying body, The light beam is incident on a scanning optical element located closest to the deflecting reflection surface.

According to a fourth aspect of the present invention, a color image forming apparatus uses the optical scanning device according to any one of the first to third aspects.

  According to the present invention, even when a disturbance such as a shape shift of an optical element, a temperature variation during continuous printing, a temperature distribution, or the like occurs, a relative color shift between colors is effectively corrected, and the color shift It is possible to provide an optical scanning device and an image forming apparatus that can output a good color image with less image quality.

In order to facilitate understanding of the present invention, an optical system created on the basis of the present invention will be taken as a comparison target.
"Comparative Example 1"
In FIG. 6A, the light fluxes corresponding to the respective colors (Y, M, C, Bk) are incident obliquely on the polygon mirror, so that a thin polygon mirror can be used, and windage loss and consumption A deflector advantageous in terms of power and durability is realized. Each light beam is reflected at substantially the same location in the sub-scanning corresponding direction, and the polygon thickness is 3 mm.
The light beams corresponding to the respective colors (Y, M, C, Bk) are incident on the polygon mirror at a large incident angle because the light beams are separated by the separation mirrors M1 to M4 formed by long mirrors. In Comparative Example 1, the angle formed by Y and Bk is about 10 °.
In the above comparative example 1, since it is necessary to take a large incident angle to the polygon mirror, a large scanning line curve occurs. Scan line bending can be improved to some extent by means such as shifting the scanning lens L2 in the sub-scanning direction. However, it is difficult to improve the beam spot diameter increase due to the deterioration of the wavefront aberration, and the density unevenness on the image and the deterioration of the gradation are problems.

“Comparative Example 2”
In FIG. 6B, the light fluxes corresponding to the respective colors (Y, M, C, Bk) are configured to be incident substantially parallel to the deflection mirror surface in order to prevent deterioration of wavefront aberration due to oblique incidence. Each luminous flux is set at an interval of 5 mm and a polygon thickness is 17 mm.
The luminous fluxes corresponding to the respective colors (Y, M, C, Bk) are transmitted through the scanning lens L1 in common, are substantially parallel to L1, the luminous flux interval is 5 mm, and the L1 lens height is 20 mm. It is said.
-By setting it as the said structure, each light beam is isolate | separated by the separation mirrors M1-M4 comprised with a long mirror.
-In this comparative example, since the thickness of the polygon mirror is increased, the size of the apparatus is increased, such as an increase in the deflector. Furthermore, there are problems such as noise due to windage loss, increase in power consumption, and deterioration in durability.
Furthermore, since the height of the L1 lens is thick, it is difficult to compensate for variations in surface accuracy for each light beam, and there is a problem in reducing the beam spot diameter. Furthermore, it is a problem to reduce the cost increase due to the increase in molding time as the thickness increases.

The layout and geometric optical performance of this optical system are shown in FIGS. 7 (a) and 7 (b).
FIG. 7A shows an optical path diagram. This optical system includes a light source (semiconductor laser array) 701, a coupling lens 702, an aperture 703, a cylindrical lens 704, a cover glass 705, a deflector 706, a scanning lens L1 707, It has a scanning lens L2 708 and a non-scanning surface 709.

  FIG. 7B is a geometric aberration diagram. The solid line in the left graph indicates the main scanning field curvature, and the broken line indicates the sub-scanning field curvature. The solid line in the graph on the right indicates linearity, and the broken line indicates the Fθ characteristic. The y-axis of the graph is the image height.

  In geometric optical performance, the linearity is a little as large as about 5.7%. When the scanning optical system is assembled, it is corrected based on the measured linearity (or beam spot position deviation in the main scanning direction) data. Electrical correction is performed by shifting the phase of the pixel clock (for example, Patent Document 7).

Specific numerical examples relating to the optical system of the present invention will be given below. The layout of the optical system is as shown in FIG.
Table 1 shows data of the optical system before the deflector. RY is the radius of curvature in the main scanning direction, RZ is the radius of curvature in the sub-scanning direction (lens center), N is the refractive index at the wavelength used (655 nm), and X is a surface perpendicular to the rotational axis of the rotating polygon mirror. This is the distance in the optical axis direction (being the L1 optical axis direction) when projected. Surface number 4 is a coaxial aspherical surface. Although the numerical value is not shown, the wavefront aberration emitted from the coupling lens is well corrected. The deflector is an A-size 18 mm, 6-sided polygon mirror.

  Table 2 shows data of the optical system after the deflector. β is a sub-scanning magnification between the deflector and the non-scanning surface, and is 0.38 in this embodiment. Each surface of surface numbers 1 and 2 has a non-arc shape in the main scanning direction, and is a flat surface in the sub-scanning direction. The lens surface shape is given by Equation 1. Each surface of surface numbers 3 and 4 has a non-arc shape in the main scanning direction, and the curvature radius in the sub-scanning direction continuously changes depending on the lens height. Each surface shape is given by Equation 1.

However, Cm = 1 / R _Y and Cs (Y) are
Cs (Y) = 1 / R Z + B 1 · Y + B 2 · Y 2 + B 3 · Y 3 + B 4 · Y 4 + B 5 · Y 5 + ···
Given in.

  Table 3 shows the aspheric coefficients of this example. In this optical system, a soundproof glass (refractive index of 1.51) having a thickness of 1.9 mm is inserted, and the soundproof glass is inclined and arranged in the 8 deg deflection plane.

A first embodiment of the present invention is shown in FIG.
A scanning optical element that deflects and scans a plurality of beam beams with one deflector and forms an image on a plurality of image carriers, and the deflector includes polygon mirrors divided into two stages, It is characterized in that light beams guided to at least two or more image carriers are incident on the same mirror surface in steps at an angle with respect to the sub-scanning corresponding direction.
The polygon mirror divided into two stages used here has six faces, an inscribed circle radius of 18 mm, a mirror center interval of 6 mm, a mirror thickness of each stage of 2 mm, and a total thickness of 8 mm. In addition, a gap of 4 mm is provided between the two-stage mirrors to form a cylindrical shape with a radius of 10 mm.

Employing the polygon mirror of this configuration has the following advantages.
Compared to “Comparative Example 1”, the optical path separation is easy even if the incident angle in the sub-scanning direction is reduced (in this embodiment, the angle formed by Y and M and C and Bk is 2 °), and the wavefront. Generation of beam spot thickening due to aberration degradation can be suppressed.
・ Compared to “Comparative Example 2”, the thickness of the polygon mirror can be reduced, and air gaps are provided in parts that are not used for unnecessary deflection, reducing windage loss when the polygon mirror rotates, and the center of gravity of the entire scanner. Since it can be set low, it is possible to realize a configuration that is advantageous for reducing noise and power consumption and improving durability.

In the second embodiment, the angle β formed by the sub-scanning corresponding direction of each light beam guided to at least two or more image carriers on the same mirror surface satisfies the following:
1 ° <β <4 °.
Here, if the upper limit of 4 ° is exceeded, the beam spot diameter increase due to the deterioration of the wavefront aberration cannot be ignored, resulting in density unevenness and gradation deterioration on the image.
If the lower limit of 1 ° is exceeded, it becomes difficult to separate the optical paths, and the light flux is likely to be kicked by the separation mirror due to fluctuations in the emission direction of the LD due to changes over time, etc., and color due to variations in the amount of light among Y, M, C, and Bk Taste tends to fluctuate.
Therefore, a favorable color image can be obtained by setting within the above range.

In the third embodiment, at least one of the light beams guided to each image carrier is incident substantially parallel to a plane perpendicular to the polygon mirror rotation axis. The inclination of the light beam to the plane perpendicular to the polygon mirror rotation axis is preferably up to ± 0.5 degrees.
In FIG. 1, the light beams guided to M and C are set substantially parallel to a plane perpendicular to the rotation axis of the polygon mirror.
By adopting this configuration, the angle β formed by Y and M and C and Bk can be set to a minimum, so that deterioration of the beam spot diameter can be suppressed. In addition, since the polygon thickness can be set thin, windage loss, noise, and durability can be improved.

The fourth embodiment is characterized in that at least one surface of the scanning optical element has an eccentric surface tilted in the sub-scanning corresponding direction. Here, an example in which an eccentric surface is adopted for the L1 lens will be described.
Except for the shape of the exit surface of the scanning lens L1, it is exactly the same as the above data. L1 exit surface (second surface) is constituted by an eccentric surface of the four stages has a shape tilt component X _t is added of the following formula with respect to the data. The second surface of the scanning lens L1 (the surface of the exit side) _{X t (Y, Z) =} (F 0 + F 1 · Y + F 2 · Y 2 + F 3 · Y 3 + F 4 · Y 4 + F 5 · Y 5 + F 6・ Y ⁶ + ...) Z
It is. The coefficients are given in Table 4. In this embodiment, the value βh / β0 at the image height having the largest magnification difference from the central image height is 0.99.

Where X _t represents the shape according to the tilt component in depth. Y indicates main scanning, and Z indicates sub-scanning coordinates.
By arbitrarily setting the tilt coefficients F ₁ , F ₂ , F ₃ ..., The tilt angle can be changed according to the image height. Adopting an eccentric surface having this configuration has the following advantages.
If only the eccentric surface is set, a large scanning line curve (about 0.2 to 0.5 mm) is generated on the photoconductor (on the image carrier). This scanning line bending can be corrected to some extent by β tilt (tilt in the sub-scanning direction) or Z shift (shift in the sub-scanning direction) of the scanning lens L2 mainly having power in the sub-scanning direction. However, the scanning line curve cannot be completely corrected to zero, and a non-negligible sub-scanning color misregistration of about 0.02 to 0.05 mm may occur between Y, M, C, and Bk. .

Further, the wavefront aberration is also easily deteriorated by the β tilt (sub-scanning corresponding direction tilt) and Z shift (sub-scanning corresponding direction shift) of L2, and it becomes difficult to narrow down to a small-diameter beam spot.
Therefore, the present invention uses an eccentric surface whose tilt angle changes according to the image height, so that the scanning line bending can be completely zeroed in principle and the occurrence of scanning line bending can be suppressed. Less likely to cause color misregistration. In addition, since the deterioration of the wavefront aberration is very small, it is possible to realize a small-diameter beam spot of 50 μm or less.
The beam spot diameter in this embodiment is shown in FIG. The beam spot diameter is also reduced. The numerical value shown in the right frame of the graph indicates the image height.

In the fifth embodiment, there is a light beam separating means composed of a transmission part and a reflection part, and at least one light beam among the light beams guided to each image carrier is optically separated by the reflection part. It is characterized by that.
An embodiment of the present invention is shown in FIG.
The luminous fluxes corresponding to the respective colors (Y, M, C, Bk) are transmitted through the scanning lens L1, and the L1 lens height is 10 mm.
The tilt angle of each eccentric surface is set corresponding to each of the Y, M, C, and Bk light beams on the exit surface side of L1 (four steps), and the light beams refracted according to the tilt angle are transmitted to the separation mirrors M1 to M4 Guided and can separate the luminous flux.
・ By performing such light beam separation, the polygon mirror thickness can be reduced, so that the windage loss during rotation of the polygon mirror can be reduced, and the center of gravity position of the entire scanner can be set low, reducing noise and power consumption. A configuration advantageous for reduction and improvement of durability can be realized.
・ Furthermore, since the scanning lens can be lowered in height, the molding time can be shortened, making it possible to realize low cost and suppressing the occurrence of sink marks, etc., and obtaining a highly accurate surface shape. Can do.

  Here, the height of the L1 lens is 10 mm. However, for example, by superimposing or pasting 5 mm high lenses in two stages, the cost can be further reduced and the surface accuracy can be improved (see FIG. 4). .

In the sixth embodiment, in the scanning optical element used in the previous embodiments, all the light beams guided to the respective image carriers are incident on the scanning optical element located closest to the deflecting reflection surface. It is characterized by that.
According to the present invention, a one-side scanning method in which a light beam enters from one side of a deflector and deflects and scans in one direction.
Each light flux corresponding to each color (Y, M, C, Bk) is transmitted through a scanning lens (first lens) mainly having power in the main scanning direction, so that there is little variation in the shape of the optical element, and temperature and humidity Since the environment is almost the same, there is a merit that it is difficult to cause beam spot position fluctuation (that is, color misregistration) in the relative main scanning direction of each color.

As a seventh embodiment, a case will be described in which the numerical value of the optical system after the deflector is changed and an eccentric surface is used for the scanning lenses L1 and L2.
A semiconductor laser used as a light source has an emission wavelength of 655 nm. A divergent light beam emitted is converted into a “substantially parallel light beam” by a coupling lens (focal length: 15 mm), and a cylindrical lens (focal length: 96 mm), an image is formed as a “line image long in the main scanning direction” at the position of the deflection reflection surface of the polygon mirror.
The polygon mirror has a deflecting reflection surface number of 6 and an inscribed circle radius of 18 mm.
Further, the light beam is incident on the polygon mirror obliquely at 2 ° in the sub-scanning direction, and is incident at about 60 ° with respect to the light flux toward the image height 0 in the main scanning direction.
As the aperture for restricting the light beam emitted from the coupling lens, a rectangular aperture of 6.4 mm in the main scanning direction and 0.9 mm in the sub scanning direction is used.
L1 (surface numbers 1 and 2) is arranged parallel to the deflecting and reflecting surface (the light beam is obliquely incident on the lens at 2 °), and L2 (surface numbers 3 and 4) is the optical axis of the lens and the incident light beam. They are arranged so that they are aligned (tilted by 2 ° so that the light beam is not obliquely incident on the lens).

  Table 5 shows data of the optical system after the deflector. The way of reading the table is the same as in Table 2. Each surface of surface numbers 1, 2, and 3 has a non-arc shape in the main scanning direction, and is a flat surface in the sub-scanning direction. Surfaces 2 and 3 are special tilt eccentric surfaces.

The lens surface shape is given by Equation 2. However, C _m = 1 / RY and C _S (Y) = 1 / RZ.

The surface of surface number 4 has a non-arc shape in the main scanning direction, and the curvature radius in the sub-scanning direction continuously changes depending on the lens height. Each surface shape is given by Equation 2. However, C _S (Y) is given by Equation 3.

  Table 6 shows the aspheric coefficients. In this optical system, a soundproof glass (refractive index of 1.511) having a thickness of 1.9 mm is inserted, and the soundproof glass is disposed so as to be inclined within the 8 deg deflection plane.

  The beam spot diameter according to this example is shown in FIG. 8A shows the beam spot diameter in the main scanning direction, and FIG. 8B shows the beam spot diameter in the sub scanning direction. It can be seen that the beam spot diameter does not spread with respect to the change in image height.

The problems of this optical system ・ Noise due to windage loss caused by increasing the thickness of the polygon mirror, increased power consumption, durability deterioration, and By adopting the oblique incidence configuration, the beam spot diameter reduction accompanying the deterioration of the wavefront aberration can be improved by the first to seventh embodiments.

In this embodiment, as an example of a color image forming apparatus having the optical scanning apparatus of the present invention, an example implemented as a tandem type color image forming apparatus is shown in FIG.
First, on the lower side in the apparatus, a conveying belt 2 is provided that conveys transfer paper (not shown) that is disposed in the horizontal direction and is fed from the sheet feeding cassette 1. On the conveying belt 2, a photosensitive body 3Y for yellow Y, a photosensitive body 3M for magenta M, a photosensitive body 3C for cyan C, and a photosensitive body 3K for black K are sequentially arranged at equal intervals from the upstream side. Yes. Hereinafter, the subscripts Y, M, C, and Bk (K) with respect to the reference numerals are appropriately added for distinction. These photoreceptors 3Y, 3M, 3C, and 3K are all formed to have the same diameter, and process members are sequentially disposed around the photoreceptors in accordance with an electrophotographic process. Taking the photoconductor 3Y as an example, a charging charger 4Y, a scanning imaging optical system 5Y, a developing device 6Y, a transfer charger 7Y, a cleaning device 8Y, and the like are arranged in this order. The same applies to the other photoconductors 3M, 3C, and 3K. That is, in the present embodiment, the photoconductors 3Y, 3M, 3C, and 3K are irradiated surfaces set for the respective colors, and the scanning imaging optical systems 5Y, 5M, 5C, and 5K are provided for the respective colors. They are provided in a one-to-one correspondence.

  In addition, a registration roller 9 and a belt charging charger 10 are provided around the transport belt 2 on the upstream side of the photoconductor 5Y, and a belt separation charger 11 is provided on the downstream side of the photoconductor 5K. A static elimination charger 12, a cleaning device 13, and the like are provided in this order. Further, a fixing device 14 is provided on the downstream side of the belt separating charger 11 in the transport direction, and is connected to a paper discharge tray 15 by a paper discharge roller 16.

  In such a schematic configuration, for example, in the full color mode (multiple color mode), each of the photoconductors 3Y, 3M, 3C, and 3K is based on image signals of colors Y, M, C, and K, respectively. An electrostatic latent image is formed by optical scanning of the light beams 5Y, 5M, 5C, and 5K by the scanning imaging optical system. These electrostatic latent images are developed with the corresponding color toners to form toner images, which are superposed by being sequentially transferred onto transfer paper that is electrostatically attracted onto the transport belt 2 and transported. After being fixed as an image, it is discharged. In the black mode (monochrome mode), the photoconductors 3Y, 3M, and 3C and their process members are in an inoperative state, and only the photoconductor 3K is provided with a scanning imaging optical system based on a black image signal. An electrostatic latent image is formed by optical scanning of the light beam with 5K. This electrostatic latent image is developed with black toner to become a toner image, and is transferred onto a transfer sheet that is electrostatically attracted onto the transport belt 2 and then transferred onto a transfer sheet, thereby being fixed as a black and white image. The paper is ejected.

  Reference numerals 31M and 32M are two fθ lenses. Each fθ lens is fixed to the optical housing 31 and is placed on the plate 33M. The plate 33M is in contact with the entire surface or a part of the contact surface side of the fθ lenses 31M and 32M. The material of the fθ lenses 31M and 32M is made of a plastic material that is easily aspherical and low in cost, and specifically, a synthetic resin excellent in low water absorption, high transparency, and moldability is suitable.

  Here, when a large number of color images are continuously printed, rapid temperature fluctuations occur due to heat generated by the polygon motor and the fixing device in the optical scanning device. Therefore, there is a problem that the color tone fluctuates in the color image after the first print and a plurality of sheets are output. According to the present invention, it is possible to satisfactorily correct such a problem.

  By using a color image forming apparatus as shown in the above embodiment, a high quality image can be obtained.

It is a figure which shows the optical system in a 1st Example. It is a graph which shows the relationship between the beam spot diameter in a 4th Example, and a defocus. It is a figure which shows the optical system in a 5th Example. It is a figure which shows the structure of the scanning lens in a 5th Example. It is a figure which shows the structure of the color image forming apparatus in an 8th Example. It is the figure which showed the comparative example for making an understanding of this invention easy. It is a figure which shows the layout of the comparative example and the optical system of this invention, and the geometrical aberration of a comparative example. It is the figure which showed the relationship between the image height and beam spot diameter in a 7th Example.

Claims (4)

A scanning optical element that deflects and scans a plurality of beam beams with one deflector and forms an image on a plurality of image carriers, and the deflector includes polygon mirrors divided into two stages, In the optical scanning device in which light beams guided to at least two or more image carriers are incident on the same mirror surface of the stage at an angle with respect to the sub-scanning corresponding direction ,
The angle β formed in the sub-scanning corresponding direction of each light beam guided to at least two or more image carriers on the same mirror surface satisfies 1 ° <β <4 °,
Among the light beams guided to each image carrier, at least one light beam is incident at an angle within 0.5 degrees with respect to a plane perpendicular to the polygon mirror rotation axis,
In the scanning optical element, at least one surface has an eccentric surface tilted in the sub-scanning corresponding direction,
The optical scanning device according to claim 1, wherein the eccentric surface has a surface shape in which an eccentric angle changes in accordance with an image height .   2. A light beam separator composed of a transmission part and a reflection part, further comprising: a light path separating at least one light beam among the light beams guided to each image carrier by the reflection part. The optical scanning device according to 1. In the scanning optical element, wherein all the light beam guided to each image carrier is most deflected first term to any one of claims 1 to 2, characterized in that entering the scanning optical element located on the reflecting surface side The optical scanning device according to 1. Color image forming apparatus characterized by using the optical scanning device according to claim 1-3.

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