JP2019184808A - Optical scanner and image formation device - Google Patents

Abstract

To provide an optical scanner which achieves cost reduction and high picture quality by appropriately constituting a lens.SOLUTION: An optical scanner 100 comprises: a deflector 6 deflecting a light flux from a light source 1 to scan a scanned surface 9 in a main scan direction; and an image formation optical system 70 including at least two lenses 7 and 8 to guide the light flux deflected by the deflector 6 to the scanned surface 9. Coefficient of water absorption of the first lens 7 arranged to be closest to the deflector 6, of the at least two lenses 7 and 8 is larger than coefficient of water absorption of the second lens 8. A conditional expression D<H is satisfied when a distance on an optical axis between an incidence surface and an emission surface of the first lens 7 is D and a value twice the shortest distance from an outer edge part in a sub-scanning cross section including the optical axis of the first lens 7 to the optical axis is H.SELECTED DRAWING: Figure 1

Description

  The present invention relates to an optical scanning device, and is particularly suitable for an image forming apparatus such as a laser beam printer (LBP), a digital copying machine, or a multifunction printer (MFP).

In recent years, an optical scanning device using a resin optical member has been known in order to reduce the cost.
Patent Document 1 discloses an optical scanning device that achieves cost reduction by using a resin lens in an imaging optical system.

Japanese Patent Laid-Open No. 2002-139689

Some resins generate a large amount of moisture absorption, and when moisture absorption occurs in a lens made from such a resin, there is a possibility that the optical performance changes and the image quality is deteriorated.
SUMMARY OF THE INVENTION An object of the present invention is to provide an optical scanning device that achieves low cost and high image quality by appropriately configuring a lens.

An optical scanning device according to the present invention includes a deflector that deflects a light beam from a light source and scans a surface to be scanned in a main scanning direction, and at least two lenses, and the light beam deflected by the deflector is applied to the surface to be scanned. The water absorption rate of the first lens that has an imaging optical system that guides light and is disposed closest to the deflector out of at least two lenses is greater than the water absorption rate of the second lens. D is the distance on the optical axis between the entrance surface and the exit surface of the lens, and H is twice the minimum distance from the outer edge to the optical axis in the sub-scan section including the optical axis of the first lens. When
D <H
The following conditional expression is satisfied.

  According to the present invention, it is possible to provide an optical scanning device that is low in cost and high in image quality by appropriately configuring a lens.

FIG. 3 is a main scanning sectional view of the optical scanning device according to the first embodiment. FIG. 7 is a main scanning sectional view of an optical scanning device according to a second embodiment. 1 is a cross-sectional view of main parts of a monochrome and color image forming apparatus according to an embodiment. FIG. 3 is a schematic sectional view of a lens.

  Hereinafter, an optical scanning device according to the present embodiment will be described in detail with reference to the accompanying drawings. It should be noted that the drawings shown below may be drawn at a scale different from the actual scale so that the present embodiment can be easily understood.

In the following description, the main scanning direction is a direction perpendicular to the rotation axis of the deflector and the optical axis of the optical system (the direction in which the light beam is deflected and scanned by the rotating polygon mirror). The sub-scanning direction is a direction parallel to the rotation axis of the deflector. A main scanning section is a section perpendicular to the sub-scanning direction. The sub-scanning cross section is a cross section perpendicular to the main scanning direction.
Accordingly, in the following description, it should be noted that the main scanning direction and the sub-scanning section are different between the incident optical system and the imaging optical system.

As the resin material of the lens used in the optical system, cycloolefin polymer (COP), cycloolefin copolymer (COC), acrylic and the like are often used.
In particular, as a resin material for a lens used in an optical system of an optical scanning device, COP and COC are often used from the viewpoint of environmental stability and the like.
However, although depending on the grade and the like, COP and COC often have a higher unit price per weight than acrylic, so it is desirable to use acrylic for cost reduction.
However, it should be noted that acrylic has high moisture absorption, particularly from the viewpoint of environmental stability.

When a lens is manufactured using acrylic, the lens takes in surrounding water or drains water from the lens to the surroundings depending on the humidity or the like of the place where it is used.
Thereby, the surface shape of the lens changes or the refractive index is biased, that is, a distribution is generated. In particular, if there is a refractive index deviation in the lens, the light beam bends toward the higher refractive index when passing through. Therefore, when the refractive index is higher off the axis than on the optical axis of the lens, it behaves as if it has negative power.
Further, when the surface shape of the lens changes, the position where the light beam is condensed, that is, the focus changes according to the shape.

Moisture absorption in the lens first occurs on the surface, and then moisture permeates into the interior, so the effect depends on the outer shape of the lens.
For example, a lens used in an imaging optical system of an optical scanning device is long in the scanning direction (main scanning direction) by the deflector and short in the sub-scanning direction perpendicular to the optical axis direction, the optical axis direction, and the main scanning direction. There are often.
In the case of such a lens shape, the moisture that has permeated from the surface with the shortest distance to the center reaches the center the fastest, so the influence of moisture absorption from the surface with the shortest distance to the center becomes dominant.
In other words, since the influence of moisture absorption from a surface far from the center is small, the optical influence of moisture absorption along the main scanning direction is small.
Therefore, in the lens used in the imaging optical system, moisture absorption along the sub-scanning direction and the optical axis direction may be mainly considered.

Next, when comparing moisture absorption in each of the sub-scanning direction and the optical axis direction, the length of the lens in the optical axis direction (distance between the entrance surface and the exit surface on the optical axis, ie, the thickness) and the sub-scanning direction The size relationship with the length (twice the distance from the outer edge to the optical axis in the sub-scan section including the optical axis, that is, the lens height) differs depending on the product, but “thickness <lens height ”In the optical axis direction becomes dominant.
Here, even if the refractive index is deviated along the optical axis direction due to moisture absorption, the focus or the like is hardly affected.
Therefore, in order to suppress the influence of moisture absorption in the lens, it is better to design the lens outer shape so that the following conditional expression (1) is satisfied, where D is the lens thickness and H is the minimum lens height. it is conceivable that.
D <H (1)

In addition, a light beam having a finite width passes through a lens used in the imaging optical system of the optical scanning device. Therefore, even if the lens is designed to satisfy the relationship of “thickness <lens height” when the light flux width increases to some extent, the water that has penetrated from the optical axis direction of the lens will reach the center in the sub-scanning direction. Moisture that has permeated from the lens reaches the light beam passage portion of the lens. Thereby, moisture that has penetrated from the sub-scanning direction may affect the optical performance of the lens.
Therefore, the lens is designed to satisfy the following conditional expression (2) where d is the maximum diameter in the sub-scanning direction of the light beam passing through the lens, D is the thickness of the lens, and H is the minimum value of the lens height. More preferably.
D <Hd (2)
Here, as shown in FIG. 4, in consideration of the lens 400 having the collar portion 402 in addition to the lens portion 401, the minimum value H of the lens height is the light of the lens 400 including the collar portion 402. It may be defined as twice the minimum distance from the outer edge to the optical axis in the sub-scan section including the axis.

  However, in a lens having a thickness of more than 10 mm, if the height of the lens is increased so as to satisfy the relationship of the conditional expression (1), the cost is increased due to an increase in the amount of material used. In addition to this, in the production by molding, there is a possibility that the number to be obtained may be reduced, which is not preferable.

The influence of non-eccentric optical changes, such as refractive index bias, on the focus is relatively high on an optical surface with a large width of light passing therethrough, and conversely relatively low on an optical surface with a small light flux width.
As described above, since moisture absorption along the sub-scanning direction greatly affects the focus, it is desirable to manufacture a lens having an optical surface with a small width in the sub-scanning direction of a passing light beam using a low-cost material such as acrylic.
Specifically, in the optical scanning device, the light beam is once condensed in the sub-scanning direction in the vicinity of the deflector. Therefore, in the imaging optical system composed of a plurality of lenses, it is disposed closest to the deflector. In many cases, the width in the sub-scanning direction when the light beam passes through the lens is reduced.
Therefore, it is desirable to make the lens closest to the deflector with acrylic.

On the other hand, in the lens closest to the surface to be scanned and the lens having the most power in the sub-scanning section, the light flux width in the sub-scanning direction is often larger than that of other lenses. It becomes susceptible to moisture absorption along.
Therefore, for such a lens, it is desirable to employ a lens made of COP or COC having a very low water absorption rate in a saturated state, specifically 0.05% or less.

  From the above, when the imaging optical system of the optical scanning device is composed of two lenses, the first lens closer to the deflector is acrylic and the second lens closer to the scanned surface Is made of COP or COC. Thereby, it is possible to reduce the cost while maintaining the optical performance.

  In addition, the water absorption rate in the saturated state of acrylic varies depending on the manufacturer and grade manufactured, but is generally 1% or more in many cases.

[First embodiment]
FIG. 1 is a main scanning sectional view of the optical scanning device 100 according to the first embodiment.
An optical scanning device 100 according to this embodiment includes a light source 1, an anamorphic collimator lens 3, an aperture stop 5, a deflector 6, a first imaging lens 7 (first lens), and a second imaging lens 8 (first lens). 2 lenses).

For example, a semiconductor laser is used as the light source 1 and is protected by a cover glass (not shown).
The anamorphic collimator lens 3 converts the light beam emitted from the light source 1 into a parallel light beam in the main scanning section and a convergent light beam in the sub-scanning section. Here, the parallel light beam includes not only a strict parallel light beam but also a substantially parallel light beam such as a weak divergent light beam or a weakly convergent light beam. Further, temperature compensation is performed by providing a diffractive surface on the incident surface side of the anamorphic collimator lens 3.

The aperture stop 5 shapes the light beam that has passed through the anamorphic collimator lens 3 into a predetermined beam shape. Specifically, the aperture stop 5 limits the light beam diameter in the main scanning direction and the sub-scanning direction, so that the spot diameter on the scanned surface 9 is reduced. Is set to a desired size.
The deflector 6 is a four-sided rotary polygon mirror, and deflects and scans the light beam that has passed through the aperture stop 5 in the main scanning direction. The deflector 6 is rotated at a constant speed in the direction of arrow A in the figure by a driving means (not shown) such as a motor.
The first and second imaging lenses 7 and 8 have fθ characteristics, and collect (guide) the light beam deflected and scanned by the deflection surface 6a of the deflector 6 on the surface 9 to be scanned. Yes.

In the optical scanning device 100 according to the present embodiment, an incident optical system 80 is configured by the anamorphic collimator lens 3 and the aperture stop 5, and an imaging optical system 70 is configured by the first and second imaging lenses 7 and 8. Has been.
Then, the imaging optical system 70 performs surface tilt correction in a sub-scanning section with a conjugate relationship between the deflection surface 6a of the deflector 6 and the surface 9 to be scanned.

  In the optical scanning device 100 according to the present embodiment, a divergent light beam that is light-modulated according to image information and emitted from the light source 1 is converted into a parallel light beam in the main scanning section and in a sub-scanning section by the anamorphic collimator lens 3. Is converted so as to be condensed near the deflection surface 6a of the deflector 6. Thereby, a long line image in the main scanning direction is formed on the deflection surface 6a. A part of the light beam that has passed through the anamorphic collimator lens 3 is shielded by passing through an opening provided in the aperture stop 5.

The light beam deflected and reflected by the deflecting surface 6a of the deflector 6 is condensed on the scanned surface 9 by the first and second imaging lenses 7 and 8, and a spot-like image is formed. Then, by rotating the deflector 6 in the direction of arrow A, optical scanning is performed on the surface 9 to be scanned in the direction of arrow PB (main scanning direction) at a constant speed.
As a result, image recording is performed on the photosensitive surface of the photosensitive drum 9 disposed at the position of the scanned surface 9 as a recording medium.

  Next, specification values such as a radius of curvature, a surface interval, and a refractive index of each optical element provided in the optical scanning device 100 according to the present embodiment are shown in Table 1 below.

In Table 1, Ry is a radius of curvature in the main scanning section, Rz is a radius of curvature in the sub-scanning section, D is a distance between optical surfaces or optical elements, and N is a refractive index for a light beam having a wavelength of 780 nm.
Further, in the aspheric coefficient, the suffix u indicates the aspheric coefficient on the light source side, and l indicates the aspheric coefficient on the anti-light source side.

A diffractive surface is provided on the incident surface side of the anamorphic collimator lens 3 and is defined by the following phase function φ.

Here, λ is the design wavelength (= 790 nm), Y is the coordinate in the main scanning direction, Z is the coordinate in the sub scanning direction, C is the phase coefficient in the main scanning direction (Y direction), and E is the sub scanning direction (Z direction). Of the phase coefficient.
This diffraction surface is a diffraction grating provided with a step having a height that causes a difference corresponding to the wavelength of the optical path length at coordinates where the phase function φ is an integral multiple of 2π.

In addition, the aspherical shape X (bus shape) in the main scanning section and the aspherical shape S (child line shape) in the sub-scanning section of each optical surface of the first and second imaging lenses 7 and 8, respectively. It is represented by the following formulas (4) and (5).

Here, k is an eccentricity, Y is a coordinate in the main scanning direction, Z is a coordinate in the sub scanning direction, _RY is a radius of curvature in the main scanning section, and r _Z ′ is a radius of curvature in the sub scanning section.
Further, the radius of curvature r _Z ′ in the sub-scanning section is expressed as the following formula (6), where r _Z is the radius of curvature in the sub-scanning section on the optical axis.

Of the first and second imaging lenses 7 and 8 provided in the imaging optical system 70 of the optical scanning device 100 according to the present embodiment, as a material of the first imaging lens 7 on the deflector 6 side. In addition, the use of an acrylic lippet (a 24-hour water absorption rate of 0.3% and a saturated water absorption rate of about 1%), which is a kind of acrylic, aims to reduce costs.
Then, by setting the distance from the deflector 6 to the first imaging lens 7 to 10.5 mm, sub-scanning on the optical axis when passing through the incident surface and the exit surface of the first imaging lens 7 is performed. The light flux width d in the direction is 0.48 mm and 0.76 mm.
Further, by setting the thickness D of the first imaging lens 7 to 6.5 mm and the minimum value H of the lens height to 10 mm, the conditional expressions (1) and (2) can be applied to both the entrance surface and the exit surface. Thus, the influence of moisture absorption on the optical performance of the first imaging lens 7 is suppressed.

On the other hand, as a material for the second imaging lens 8 on the scanned surface 9 side, E48R (a water absorption rate of 0.05% or less in a saturated state) of Nippon Zeon Co., Ltd., which is a kind of COP resin, is used. The influence of moisture absorption on the optical performance of the second imaging lens 8 is suppressed.
As can be seen from Table 1, the power in the sub-scanning section of the first imaging lens 7 is negative and smaller than that of the second imaging lens 8.

[Second Embodiment]
FIG. 2 shows a main scanning sectional view of the optical scanning device 200 according to the second embodiment.
The optical scanning device 200 according to this embodiment includes first, second, third, and fourth light sources 1A, 1B, 1C, and 1D, first and second sub-scanning apertures 2A and 2B, and first and first light sources. 2, third and fourth collimator lenses 3A, 3B, 3C and 3D. The optical scanning device 200 according to the present embodiment includes the first and second integrated cylindrical lenses 4A and 4B, the first and second main scanning diaphragms 5A and 5B, the deflector 6, and the first imaging lens. 7A and 7B, and second imaging lenses 8A, 8B, 8C and 8D.

As the first, second, third and fourth light sources 1A, 1B, 1C and 1D, for example, semiconductor lasers are used and are protected by a cover glass (not shown). The first and second light sources 1A and 1B are arranged in the sub-scanning direction, and the third and fourth light sources 1C and 1D are arranged in the sub-scanning direction.
The first sub-scanning stop 2A and the second sub-scanning stop 2B are respectively luminous fluxes LA and LB emitted from the first and second light sources 1A and 1B and luminous fluxes emitted from the third and fourth light sources 1C and 1D, respectively. The beam diameter in the sub-scanning direction of LC and LD is limited.

  The first and second collimator lenses 3A and 3B respectively convert the light beams LA and LB that have passed through the first sub-scanning stop 2A into parallel light beams in the main scanning section. Further, the third and fourth collimator lenses 3C and 3D respectively convert the light beams LC and LD that have passed through the second sub-scanning stop 2B into parallel light beams in the main scanning section. Here, the parallel light beam includes not only a strict parallel light beam but also a substantially parallel light beam such as a weak divergent light beam or a weakly convergent light beam.

  The first integrated cylindrical lens 4A and the second integrated cylindrical lens 4B are respectively the light beams LA and LB that have passed through the first and second collimator lenses 3A and 3B, and the third and fourth collimator lenses 3C and 3D. The light beams LC and LD that have passed through are converted so as to become convergent light beams in the sub-scan section. Note that the first and second integrated cylindrical lenses 4A and 4B have power in the main scanning section in order to suppress the generation of ghost light. Further, temperature compensation is performed by providing a diffractive surface on the exit surface side of each of the first and second integrated cylindrical lenses 4A and 4B.

The first main scanning stop 5A and the second main scanning stop 5B respectively include the light beams LA and LB that have passed through the first integrated cylindrical lens 4A and the light beams LC and LD that have passed through the second integrated cylindrical lens 4B. The beam diameter in the main scanning direction is limited. The first, second, third, and fourth scanned surfaces 9A, 9B, and 9C are formed by the first and second sub-scanning apertures 2A and 2B and the first and second main scanning apertures 5A and 5B. And the spot diameter on 9D is set to a desired size.
The deflector 6 is a rotary polygon mirror having a five-surface configuration, and deflects and scans the light beams LA to LD that have passed through the first and second main scanning stops 5A and 5B in the main scanning direction. Further, the deflector 6 is rotated at a constant speed in the direction of arrow PA in the figure by a driving means (not shown) such as a motor.
The first imaging lenses 7A and 7B and the second imaging lenses 8A, 8B, 8C and 8D have an fθ characteristic, and are formed by the first and second deflection surfaces 6a and 6b of the deflector 6. The deflected and scanned light beams LA to LD are condensed (guided) on the first, second, third and fourth scanned surfaces 9A, 9B, 9C and 9D.

In the optical scanning device 200 according to the present embodiment, the first incident optical system 80A includes the first sub-scanning aperture 2A, the first collimator lens 3A, the first integrated cylindrical lens 4A, and the first main scanning aperture 5A. Is configured.
The first sub-scanning aperture 2A, the second collimator lens 3B, the first integrated cylindrical lens 4A, and the first main scanning aperture 5A constitute a second incident optical system 80B.
The second sub-scanning aperture 2B, the third collimator lens 3C, the second integrated cylindrical lens 4B, and the second main scanning aperture 5B constitute a third incident optical system 80C.
The second sub-scanning aperture 2B, the fourth collimator lens 3D, the second integrated cylindrical lens 4B, and the second main scanning aperture 5B constitute a fourth incident optical system 80D.

  In the optical scanning device 200 according to the present embodiment, the first imaging optical system 70A is configured by the first imaging lens 7A and the second imaging lens 8A. Further, the first imaging optical system 70A performs surface tilt correction in a conjugate relationship between the first deflection surface 6a of the deflector 6 and the first scanned surface 9A in the sub-scan section. Yes.

The first imaging lens 7A and the second imaging lens 8B constitute a second imaging optical system 70B. The second imaging optical system 70B is a deflector in the sub-scan section. The surface tilt correction is performed with a conjugate relationship between the first deflection surface 6a and the second scanned surface 9B.
The first imaging lens 7B and the second imaging lens 8C constitute a third imaging optical system 70C. The third imaging optical system 70C is a deflector in the sub-scan section. The surface tilt correction is performed with a conjugate relationship between the second deflection surface 6b 6 and the third scanned surface 9C.
The first imaging lens 7B and the second imaging lens 8D constitute a fourth imaging optical system 70D. The fourth imaging optical system 70D is a deflector in the sub-scan section. The surface tilt correction is performed with a conjugate relationship between the second deflecting surface 6b 6 and the fourth scanned surface 9D.

  In the optical scanning device 200 according to the present embodiment, the divergent light beam LA, which is light-modulated according to image information and emitted from the first light source 1A, has a light beam diameter in the sub-scanning direction limited by the first sub-scanning aperture 2A. The Then, the light beam LA that has passed through the first sub-scanning stop 2A is converted by the first collimator lens 3A into a parallel light beam in the main scanning section. Then, the light beam LA that has passed through the first collimator lens 3A is converted by the first integrated cylindrical lens 4A to become a converged light beam in the sub-scan section. The light beam LA that has passed through the first integrated cylindrical lens 4A has its light beam diameter limited in the main scanning direction by the first main scanning stop 5A, and is condensed near the first deflection surface 6a of the deflector 6. Is converted as follows. Thereby, a long line image is formed on the first deflection surface 6a in the main scanning direction by the light beam LA.

  Further, the divergent light beam LB that is light-modulated according to the image information and emitted from the second light source 1B is limited in the light beam diameter in the sub-scanning direction by the first sub-scanning aperture 2A, and is mainly controlled by the second collimator lens 3B. It is converted so as to be a parallel light beam in the scanning section. The light beam LB that has passed through the second collimator lens 3B is converted by the first integrated cylindrical lens 4A to become a converged light beam in the sub-scan section. The light beam LB that has passed through the first integrated cylindrical lens 4A has its light beam diameter limited in the main scanning direction by the first main scanning stop 5A, and is condensed in the vicinity of the first deflection surface 6a of the deflector 6. Is converted as follows. Thereby, a long line image in the main scanning direction by the light beam LB is formed on the first deflection surface 6a.

  Further, the divergent light beam LC, which is light-modulated according to the image information and emitted from the third light source 1C, is limited in the light beam diameter in the sub-scanning direction by the second sub-scanning stop 2B, and is mainly controlled by the third collimator lens 3C. It is converted so as to be a parallel light beam in the scanning section. Then, the light beam LC that has passed through the third collimator lens 3C is converted by the second integrated cylindrical lens 4B to become a converged light beam in the sub-scanning section. The light beam LC that has passed through the second integrated cylindrical lens 4B is confined in the vicinity of the second deflection surface 6b of the deflector 6 with the light beam diameter in the main scanning direction being limited by the second main scanning stop 5B. Is converted as follows. Thereby, a long line image in the main scanning direction by the light beam LC is formed on the second deflection surface 6b.

  Further, the divergent light beam LD that is light-modulated according to the image information and emitted from the fourth light source 1D is limited in the light beam diameter in the sub-scanning direction by the second sub-scanning aperture 2B, and is mainly controlled by the fourth collimator lens 3D. It is converted so as to be a parallel light beam in the scanning section. Then, the light beam LD that has passed through the fourth collimator lens 3D is converted by the second integrated cylindrical lens 4B to become a converged light beam in the sub-scan section. The light beam LD that has passed through the second integrated cylindrical lens 4B has its light beam diameter limited in the main scanning direction by the second main scanning stop 5B, and is condensed in the vicinity of the second deflection surface 6b of the deflector 6. Is converted as follows. Thereby, a long line image in the main scanning direction by the light beam LD is formed on the second deflection surface 6b.

  The light beam LA deflected and reflected by the first deflecting surface 6a of the deflector 6 is condensed on the first scanned surface 9A by the first and second imaging lenses 7A and 8A, and a spot-like image is formed. It is formed. Then, by rotating the deflector 6 in the arrow PA direction, the first scanned surface 9A is optically scanned in the arrow PB direction (main scanning direction) at a constant speed.

The light beam LB deflected and reflected by the first deflecting surface 6a of the deflector 6 is condensed on the second scanned surface 9B by the first and second imaging lenses 7A and 8B, and is spot-like. An image is formed. Then, by rotating the deflector 6 in the direction of the arrow PA, the second scanned surface 9B is optically scanned at a constant speed in the direction of the arrow PB (main scanning direction).
The light beam LC deflected and reflected by the second deflecting surface 6b of the deflector 6 is condensed on the third scanned surface 9C by the first and second imaging lenses 7B and 8C, and is spot-like. An image is formed. Then, by rotating the deflector 6 in the direction of the arrow PA, the third scanning surface 9C is optically scanned in the arrow PB direction (main scanning direction) at a constant speed.
The light beam LD deflected and reflected by the second deflecting surface 6b of the deflector 6 is condensed on the fourth scanned surface 9D by the first and second imaging lenses 7B and 8D, and is spot-like. An image is formed. Then, by rotating the deflector 6 in the arrow PA direction, the fourth scanned surface 9D is optically scanned in the arrow PB direction (main scanning direction) at a constant speed.

  As a result, the first, second, third and fourth photosensitive drums arranged as the recording medium at the positions of the first, second, third and fourth scanned surfaces 9A, 9B, 9C and 9D, respectively. Image recording is performed on the photosensitive surfaces 9A, 9B, 9C, and 9D.

In the optical scanning device 200 according to the present embodiment, the optical axes of the first and second incident optical systems 80A and 80B are inclined by + 3 ° and −3 ° with respect to the main scanning section, respectively. The optical axes of the third and fourth incident optical systems 80C and 80D are inclined by -3 ° and + 3 ° with respect to the main scanning section, respectively.
Thereby, the light beams LA to LD emitted from the first to fourth light sources 1A to 1D are obliquely incident on the deflector 6, respectively, so that each light beam can be separated and guided to each scanning surface. ing.

  Next, specification values such as a radius of curvature, a surface interval, and a refractive index of each optical element provided in the optical scanning device 200 according to the present embodiment are shown in Table 2 below.

In Table 2, Ry is a radius of curvature in the main scanning section, Rz is a radius of curvature in the sub-scanning section, D is a distance between optical surfaces or optical elements, and N is a refractive index with respect to a light beam having a wavelength of 790 nm.
Further, in the aspheric coefficient, the subscript u indicates the non-light source side and l indicates the aspheric coefficient on the light source side.

  Further, a diffractive surface is provided on the exit surface side of the first and second integrated cylindrical lenses 4A and 4B, and is defined by the phase function φ of the above equation (3).

ここで、副走査断面内の曲率半径ｒ_Ｚ’は、上記の式（６）で表される。 Further, the aspherical shape (bus shape) in the main scanning section of each optical surface of the first imaging lenses 7A and 7B and the second imaging lenses 8A, 8B, 8C and 8D is expressed by the above equation (4). It is represented by
Further, the aspherical shape (sub-wire shape) in the sub-scan section of each optical surface of the first imaging lenses 7A and 7B is expressed by the above equation (5).
In addition, the aspherical shape (sub-wire shape) in the sub-scan section of each optical surface of the second imaging lenses 8A, 8B, 8C, and 8D is expressed as the following Expression (7).

Here, the radius of curvature r _Z ′ in the sub-scanning section is expressed by the above equation (6).

  Of the first imaging lens 7A and the second imaging lenses 8A and 8B provided in the first and second imaging optical systems 70A and 70B of the optical scanning device 200 according to the present embodiment, a deflector. As a material for the first imaging lens 7A on the 6th side, an acrylic lippet (a water absorption rate of 0.3% in 24 hours, a water absorption rate of about 1% in a saturated state), which is a kind of acrylic, is adopted. So we are trying to cut costs.

Then, by setting the distance from the deflector 6 to the first imaging lens 7A to be 17 mm, the sub-scanning direction on the optical axis when passing through the incident surface and the emitting surface of the first imaging lens 7A, respectively. The beam width d is set to 0.41 mm and 0.45 mm.
Further, the thickness D of the first imaging lens 7A is 7.29 mm, and the minimum value H of the lens height on the entrance surface and the exit surface is 10.6 mm and 12.2 mm, respectively. As a result, the conditions of the conditional expressions (1) and (2) are satisfied on both the entrance surface and the exit surface, and the influence of water absorption on the optical performance of the first imaging lens 7A is suppressed.

On the other hand, as a material for the second imaging lenses 8A and 8B on the first and second scanned surfaces 9A and 9B side, K22R of Nippon Zeon Co., Ltd., which is a kind of COP resin (water absorption 0.05 in a saturated state). % Or less), the influence of moisture absorption on the optical performance of the second imaging lenses 8A and 8B is suppressed.
As can be seen from the calculation from Table 2, the power of the first imaging lens 7A in the sub-scan section is negative and smaller than those of the second imaging lenses 8A and 8B.
The above configuration is the same in the third and fourth imaging optical systems 70C and 70D.

[Monochrome image forming apparatus]
FIG. 3A is a cross-sectional view of the main part of the monochrome image forming apparatus 104 on which the optical scanning apparatus 100 according to the first embodiment is mounted.

The code data Dc output from the external device 117 such as a personal computer is input to the image forming apparatus 104. The input code data Dc is converted into image data (dot data) Di by a printer controller 111 in the apparatus. Then, the image data Di is input to the optical scanning device 100 according to the first embodiment.
A light beam 103 modulated according to the image data Di is emitted from the optical scanning device 100, and the photosensitive surface of the photosensitive drum 101 is scanned along the main scanning direction by the light beam 103.

The photosensitive drum 101 serving as an electrostatic latent image carrier (photoconductor) is rotated clockwise by a motor 115. With this rotation, the photosensitive surface of the photosensitive drum 101 moves in the sub-scanning direction perpendicular to the main scanning direction with respect to the light beam 103.
A charging roller 102 that uniformly charges the photosensitive surface of the photosensitive drum 101 is provided on the upstream side in the rotation direction of the photosensitive drum 101 so as to contact the photosensitive surface of the photosensitive drum 101.
A light beam 103 scanned by the optical scanning device 100 is irradiated onto the photosensitive surface of the photosensitive drum 101 charged by the charging roller 102.

As described above, the light beam 103 is modulated based on the image data Di, and by irradiating the light beam 103, an electrostatic latent image is formed on the photosensitive surface of the photosensitive drum 101.
The electrostatic latent image is developed as a toner image by a developing unit 107 which is a developing unit disposed so as to be in contact with the photosensitive drum 101 further downstream in the rotation direction of the photosensitive drum 101 than the irradiation position of the light beam 103. Is done.
The toner image developed by the developing unit 107 is transferred onto a sheet 112 as a transfer material by a transfer roller 108 (transfer unit) disposed below the photosensitive drum 101 so as to face the photosensitive drum 101.

The paper 112 is stored in the paper cassette 109 in front of the photosensitive drum 101 (on the right side in FIG. 3A), but can be fed manually.
A paper feed roller 110 is disposed at the end of the paper cassette 109, and the paper 112 in the paper cassette 109 is fed into the transport path by the paper feed roller 110.
As described above, the sheet 112 on which the unfixed toner image has been transferred is further conveyed to the fixing unit 118 behind the photosensitive drum 101 (on the left side in FIG. 3A).

The fixing device 118 includes a fixing roller 113 having a fixing heater (not shown) therein and a pressure roller 114 disposed so as to be in pressure contact with the fixing roller 113.
The unfixed toner image on the sheet 112 is fixed by heating the sheet 112 transferred and conveyed while being pressed at the pressure contact portion between the fixing roller 113 and the pressure roller 114.
Further, a paper discharge roller 116 is disposed behind the fixing device 118, and the fixed paper 112 is discharged to the outside of the image forming apparatus 104 by the paper discharge roller 116.

Although not shown in FIG. 3A, the printer controller 111 is not only the data conversion described above, but also each part in the image forming apparatus 104 including the motor 115 and a deflector in the optical scanning apparatus 100. It also controls some polygon motors.
The recording density of the image forming apparatus 104 is not particularly limited. However, considering that the higher the recording density is, the higher the image quality is required, the optical scanning apparatus according to the first embodiment in the image forming apparatus 104 of 1200 dpi or more. The configuration of 100 is more effective.

[Color image forming apparatus]
FIG. 3B shows a sub-scan sectional view of the main part of the color image forming apparatus 260 on which the optical scanning devices 211, 212, 213 and 214 according to the first embodiment are mounted.

  The image forming apparatus 260 according to the present embodiment arranges four optical scanning apparatuses according to the first embodiment, and each tandem type color recording image information on a photosensitive surface of a photosensitive drum as an image carrier in parallel. An image forming apparatus.

As shown in FIG. 3B, the image forming apparatus 260 receives R (red), G (green), and B (blue) color signals output from an external device 252 such as a personal computer. Is done. These color signals are converted into C (cyan), M (magenta), Y (yellow), and K (black) image data (dot data) by a printer controller 253 in the apparatus. These image data are input to the optical scanning devices 211, 212, 213, and 214, respectively.
Then, light beams 241, 242, 243, and 244 modulated in accordance with each image data are emitted from each of the optical scanning devices 211 to 214. The emitted light beams 241, 242, 243, and 244 respectively scan the photosensitive surfaces of the photosensitive drums 221, 222, 223, and 224 along the main scanning direction.

In the color image forming apparatus 260 according to the present embodiment, four optical scanning devices 211, 212, 213, and 214 are arranged.
Each optical scanning device corresponds to each color of C (cyan), M (magenta), Y (yellow), and K (black), and the photosensitive drums 221, 222, 223, and 224 are exposed in parallel. An image signal (image information) is recorded on the surface, and a color image is printed at high speed.
In other words, the image forming apparatus 260 according to the present embodiment uses the light beams 241, 242, 243, and 244 based on the respective image data emitted by the four optical scanning devices 211, 212, 213, and 214 as described above. Thus, electrostatic latent images of the respective colors are formed on the photosensitive surfaces of the corresponding photosensitive drums 221, 222, 223, and 224 that are charged by a charger (not shown).

Thereafter, the electrostatic latent images of the respective colors are developed as toner images by the developing units 231, 232, 233, and 234, and the developed toner images are conveyed from the paper cassette 238 by the conveying belt 251 in a transfer unit (not shown). Multiple transfer is performed on a sheet as a transfer material.
Then, the toner image that has been multiplex-transferred on the paper is fixed by the fixing device 254, and one full-color image is formed.

As the external device 252, for example, a color image reading device including a CCD sensor may be used. In this case, the color image reading apparatus and the color image forming apparatus 260 constitute a color digital copying machine.
Further, instead of the four optical scanning devices according to the first embodiment, the optical scanning device according to the second embodiment can be used for the color image forming apparatus 260.

DESCRIPTION OF SYMBOLS 1 Light source 6 Deflector 7 1st imaging lens (1st lens)
8 Second imaging lens (second lens)
9 Scanned Surface 70 Imaging Optical System 100 Optical Scanning Device

Claims (11)

A deflector that deflects the light beam from the light source and scans the surface to be scanned in the main scanning direction;
An imaging optical system that includes at least two lenses and guides the light beam deflected by the deflector to the surface to be scanned;
Of the at least two lenses, the water absorption rate of the first lens disposed closest to the deflector is greater than the water absorption rate of the second lens,
The distance on the optical axis between the entrance surface and the exit surface of the first lens is D, and the minimum distance from the outer edge to the optical axis in the sub-scan section including the optical axis of the first lens When H is 2 times
D <H
An optical scanning device characterized by satisfying the following conditional expression:   2. The optical scanning device according to claim 1, wherein the second lens is a lens that is disposed closest to the surface to be scanned among the at least two lenses.   2. The optical scanning device according to claim 1, wherein the second lens is a lens having the largest power in a sub-scanning section among the at least two lenses. When the maximum diameter in the sub-scanning direction of the light beam passing on the optical axis of the first lens is d,
D <H-d
The optical scanning device according to claim 1, wherein the following conditional expression is satisfied.   5. The optical scanning device according to claim 1, wherein the water absorption rate of the first lens in a saturated state is 1% or more. 6.   6. The optical scanning device according to claim 1, wherein a water absorption rate of the second lens in a saturated state is 0.05% or less.   The optical scanning device according to claim 1, wherein the first and second lenses are lenses made of a resin.   The optical scanning device according to claim 1, wherein the first lens is a lens made of acrylic.   9. The optical scanning device according to claim 1, wherein the second lens is a lens made of a cycloolefin polymer or a cycloolefin copolymer. 10.   The optical scanning device according to claim 1, a developing device that develops an electrostatic latent image formed on the surface to be scanned by the optical scanning device as a toner image, and the developed toner An image forming apparatus comprising: a transfer device that transfers an image to a transfer material; and a fixing device that fixes the transferred toner image to the transfer material.   An optical scanning device according to claim 1, and a printer controller that converts code data output from an external device into an image signal and inputs the image signal to the optical scanning device. Image forming apparatus.

Images