JP2006184914A - Scanning image-forming lens, optical scanner and image forming device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a scanning image-forming lens which forms a beam spot of a stable diameter by excellently correcting a field curvature in spite of presence of sag, to provide an optical scanner writing information with the beam spot of the stable beam spot diameter by using the scanning image-forming lens, and also to provide an image forming device forming an extremely satisfactory image by optical writing using the optical scanner. <P>SOLUTION: The scanning image-forming lens is used in the optical scanner which forms a light flux from a light source 10 into a long linear image in the scanning direction, deflects the linear image at an equal angular velocity by a light deflector 16 having deflecting/reflecting faces 16A in the neighborhood of the image-forming position of the linear image, condenses the deflected light flux as a beam spot on a scanned surface 19 to be scanned through scanning image-forming lenses 17 and 18, and optically scans the surface 19 to be scanned at an equal velocity. Here, this lens is composed of two or more lenses, wherein they have at least one special surface of which the sub-scanning curvature varies from the optical axis toward the periphery in the scanning direction, and at least one of the special surfaces has the sub-scanning curvature having asymmetric variation in the main scanning direction and the curvature has two or more extreme values. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a scanning imaging lens, an optical scanning device, and an image forming apparatus.

The light beam from the light source is deflected at a constant angular velocity by an optical deflector having a deflecting and reflecting surface, and the deflected light beam is condensed as a beam spot on the surface to be scanned by the scanning imaging lens, and the surface is scanned at a constant velocity. 2. Description of the Related Art Optical scanning devices that perform optical scanning are widely used for writing portions in image forming apparatuses such as laser printers, facsimiles, and digital copying machines.
A polygon mirror is widely used as an optical deflector for deflecting a light beam incident on a scanning imaging lens. Since the rotation center axis of the polygon mirror is generally set off from the optical axis of the scanning imaging lens, fluctuations in the positional relationship between the deflection reflection point and the scanning lens, so-called “sag”, occur as the polygon mirror rotates. To do.

When sag occurs, the field curvature in the sub-scanning direction (hereinafter referred to as “sub-scanning field curvature”) tends to deteriorate.
That is, in an optical scanning device using a polygon mirror, in general, the incident light flux to the scanning imaging lens is “not a parallel light flux in the sub-scanning direction” in order to correct the tilting of the polygon mirror, so that the deflection reflection point changes due to the sag. As a result, the “imaging position in the sub-scanning direction” by the scanning imaging lens shifts, so that the sub-scanning field curvature tends to deteriorate.

The deflected light beam incident on the scanning imaging lens can be “parallel light beam or divergent or convergent light beam” in the main scanning direction, and there is a technical merit in each case, When the deflected light beam incident on the scanning imaging lens is “focused light beam or divergent light beam in the main scanning direction”, the field curvature in the main scanning direction (hereinafter “ The main-scanning image surface curvature ”) and iso-velocity tend to deteriorate.
As a method for reducing the influence of sag, it is known that the scanning imaging lens is shifted or tilted in the main scanning plane (the plane in which the principal ray of the ideally deflected deflected light beam sweeps). This method cannot correct both the main scanning field curvature and the sub-scanning field curvature at the same time, and is insufficient to suppress the beam spot diameter fluctuation when the beam spot diameter is reduced to increase the writing density. Thus, it is impossible to answer the demand for higher density writing.
As a method of correcting the influence of sag in principle, a method of “setting the sub-scanning radius of curvature of the scanning imaging lens asymmetrically with respect to the optical axis” has been proposed (Patent Documents 1 to 5, etc.).

  In this method, the image forming position of the light beam can be matched to the scanned surface for each image height, and in principle, the sub-scanning field curvature can be completely corrected regardless of the presence of the sag. .

Recently, the density of writing has been remarkably increased, and the demand for reducing and stabilizing the beam spot diameter has been increasing. In order to meet such a demand, it is important to set the optical magnification of the scanning optical system as well as the field curvature as uniformly as possible regardless of the image height. When the optical magnification varies depending on the image height of the beam spot, the waist diameter of the beam spot varies substantially in proportion to the lateral magnification. Therefore, a “stable beam spot” having no variation in the beam spot diameter due to the image height cannot be obtained. .
In each of the inventions described in the above publications, the sub-scanning curvature radius is “monotonically changed”, and it is difficult to achieve both uniform optical magnification and field curvature correction. In addition, in “only the lens surface whose sub-scanning curvature radius changes monotonically”, in order to realize a desired optical performance, the number of scanning imaging lenses is three or more, or near the optical axis and the peripheral image height. Since the corresponding change in the radius of curvature is likely to be large, problems such as a “lens shape having a large uniform thickness and difficult to injection mold with plastic or the like” occur.
A scanning imaging lens having a biconvex cross section in the sub scanning direction near the optical axis and a plurality of extreme values in the sub curvature radius has been proposed (Patent Document 6).
However, since this scanning imaging lens has a biconvex shape, the principal point position for each image height cannot be freely set, and it is difficult to keep the optical lateral magnification constant for each image height. There is a problem that the beam spot diameter is likely to fluctuate.

JP-A-2-23313 Japanese Patent Publication No. 7-69521 Japanese Patent Laid-Open No. 7-113950 JP-A-8-122635 JP-A-8-297256 JP 10-148755 A

An object of the present invention is to realize a scanning imaging lens that can correct a curvature of field well regardless of the presence of a sag and can form a beam spot having a stable beam spot diameter.
In addition, the present invention can perform good writing with a beam spot having a stable beam spot diameter, and can perform high-density writing, and can perform good image formation using such an optical scanning device. Another object is to realize an image forming apparatus.

The scanning imaging lens according to the present invention “forms the light beam from the light source into a line image that is long in the main scanning direction, and deflects it at an equiangular velocity by an optical deflector having a deflection reflection surface in the vicinity of the imaging position of the line image. This is a scanning imaging lens used in an “optical scanning device that condenses a deflected light beam as a beam spot on a scanned surface by a scanning imaging lens and performs optical scanning at a constant speed on the scanned surface”.
Although the main scanning direction and the sub-scanning direction are originally directions that should be defined on the surface to be scanned, it is considered that there is no possibility of confusion. Therefore, in this specification, the light from the light source to the surface to be scanned The “direction corresponding to the main / sub-scanning direction” on the road will be referred to as the main / sub-scanning direction.
The optical deflector is such that "sag" is generated, that is, the deflecting reflecting surface does not coincide with the rotation axis thereof. Specifically, a polygon mirror, a rotating single mirror, a rotating dihedral mirror, etc. It is.

The scanning imaging lens has at least one “special surface”.
The special surface is “a surface in which the sub-scanning curvature changes as it goes from the optical axis to the periphery in the main scanning direction”.
The scanning imaging lens of the present invention is composed of “two or more balls”. Accordingly, it is possible to have a two-lens configuration as in the embodiments described later, and it is also possible to configure with three or more lenses.

  The scanning imaging lens according to claim 1 has the following characteristics.

  In other words, at least one of the special surfaces has an asymmetric change in the sub-scanning curvature in the main scanning direction, the sub-scanning curvature has an extreme value of 2 or more, and 1 of these two or more extreme values. One is near the optical axis, and the other extreme value is on one side of the optical axis.

At least one of the extreme values of the special surface is as follows: the position in the main scanning direction: he is the effective lens height from the optical axis: hmax on the + image height side or the −image height side:
(1) 0.62 <| (he) / (hmax) | <0.93
Is located at a position satisfying
Lateral magnification on the optical axis: β ₀ , Arbitrary image height lateral magnification: βh, conditions:
(2) 0.93 <| βh / β ₀ | <1.07
Satisfied.

“Sub-scanning curvature” refers to a curvature in a plane section when it is assumed that the lens plane is cut in a plane section perpendicular to the main scanning direction in the vicinity of the lens surface having a special surface. In the special surface, a line connecting the centers of curvature in the flat cross section in the main scanning direction is a curve.
“A change in sub-scanning curvature is asymmetric in the main scanning direction” means that a change in sub-scanning curvature in the main scanning direction does not have an axis of symmetry in the main scanning plane.
The “extreme value” is a maximum or minimum, and when the lens height (distance from the optical axis) in the main scanning direction is h and the sub-scanning curvature at the lens height: h is C (h), the curvature differentiation: dC (h) / dh lens height becomes 0 as h _0, curvature: or C (h) increases on both sides of h _0, refers to a site which decreases on both sides of h _0.

In general, if the optical magnification of the scanning imaging lens is kept constant, a higher-order curved field curvature is likely to occur. In particular, in an optical system with a small number of lenses, the image height of the beam spot is H, the coefficients are a and b, and the higher order curved sagittal field curvature (sub-scanning field curvature) is “a · H ² + b · H ⁴ ”. ) Is likely to occur.
Image height: Hn that gives such a “maximum bulge position” of sub-scanning image field curvature, where the effective writing height is Hm,
Hn = {1 / √ (2)} × Hm = 0.71 × Hm (7)
(Fumio Kondo, “Lens Design Techniques (Optical Industrial Technology Association)” PP. 146-148).

  In order to correct the bulge in the vicinity of the effective writing height: Hm = 0.71, it is effective to have an extreme value in the vicinity of the lens surface position corresponding to the position, and the height exceeding the fourth order. Considering that the next sub-scanning image surface curvature is also corrected, the position of the extreme value is set to a range that satisfies the above equation (1).

  The “hmax” is an effective lens height on the + image height side when “he ≧ 0”, and an effective lens height on the −image height side when “he <0”. Further, it is assumed that the + side of the image height is the side where the light beam from the light source side enters the deflecting reflection surface.

By making the change in the sub-scanning curvature of the special surface “having a plurality of extreme values”, the power in the sub-scanning direction on the lens surface is higher than the higher-order curved sub-scanning field curvature. Correction is performed by changing it functionally, and the sub-scanning field curvature is corrected well.
When using an optical deflector whose deflection reflection surface and its rotation axis do not coincide with each other like a polygon mirror, the sub-scanning field curvature is particularly likely to deteriorate due to the influence of sag. The sub-scan field curvature that occurs in this case has coefficients c, d,. . . As a result, the field curvature deterioration of the shape of the odd function “c · H + d · H ³ +. In order to effectively correct such sub-scanning field curvature, the change in the sub-scanning curvature is made "asymmetric in the main scanning direction".
In the present invention, since the scanning imaging lens is composed of two or more balls, the degree of freedom of design particularly regarding the optical characteristics in the main scanning direction is increased, and the main scanning field curvature and the constant velocity can be corrected well. For this reason, it is possible to realize high-density writing of, for example, 1200 dpi or more, which cannot be achieved with a single ball.

Since the beam waist diameter of the light beam forming the beam spot varies substantially in proportion to the fluctuation of the lateral magnification of the imaging, in order to obtain a beam spot with a stable beam spot diameter, the lateral magnification for each image height is constant. It is important to make it. The lateral magnification of imaging is determined by the imaging magnification of the entire optical system from the light source to the scanned surface, but the lateral magnification varies depending on the image height of the beam spot because the lateral magnification in the main scanning direction of the scanning imaging lens Therefore, stable beam spot diameter fluctuations can be effectively prevented by making the lateral magnification of the scanning imaging lens satisfy the above condition (2).
In the scanning imaging lens according to claim 1, the change in the sub-scanning curvature is asymmetric in the main scanning direction and the extreme value of the sub-scanning curvature is 1 on the special surface having the sub-scanning curvature of 2 or more. One is “near the optical axis”. Thus, by providing an extreme value in the vicinity of the optical axis, a good field curvature can be obtained over the entire image height.
In the scanning imaging lens of the present invention, a lens having a “special surface whose sub-scanning curvature change is asymmetric in the main scanning direction” is generally used as an optical axis because this lens surface is asymmetric in the main scanning direction. Does not have a rotational symmetry axis. In this specification, the term “optical axis” refers to “a reference axis orthogonal to the main / sub-scanning direction in the reference coordinate system for determining the lens surface shape” for such an asymmetrical lens. To do.

  The scanning imaging lens according to claim 1 has two “special surfaces whose change in sub-scanning curvature is asymmetric in the main scanning direction”, and in one of these surfaces, the extreme value of the sub-scanning curvature is the optical axis. One of the vicinity and the other one on one side of the optical axis are two in total, and on the other surface, the extreme value of the sub-scanning curvature is one in the vicinity of the optical axis. ).

  Even if only one “special surface” is used, the curvature of the sub-scanning image surface can be suppressed to a very small level. However, depending on the lens configuration, the optical magnification for each image height may not be kept constant. If two or more special surfaces are used as in the invention of claim 2, the front and rear principal point positions can be arbitrarily changed, so that the magnification for each image height is kept constant, and a stable beam spot It becomes possible to get.

  The scanning imaging lens according to claim 1 also has a "special surface in which the change in the sub-scanning curvature is asymmetric in the main scanning direction, and the extreme value of the sub-scanning curvature on this special surface is one near the optical axis. , A total of three with the other two on one side of the optical axis ”(claim 3).

  The scanning imaging lens according to claim 2 can include “a lens surface whose shape in the main scanning section is asymmetric with respect to the optical axis” (claim 4).

The “main scanning section” is a virtual section by the main scanning plane described above.
The scanning imaging lens according to claim 3 or 4 can “tilt and / or shift one or more lenses constituting the scanning imaging lens” (claim 5).
By giving tilt and / or shift to the lens, it is possible to satisfactorily correct deterioration of main scanning field curvature and isovelocity due to sag.
In some cases, the tilt or shift alone cannot be sufficiently corrected. In particular, when the deflected light beam is a focused light beam in the main scanning direction, not only the sub-scanning field curvature but also the main scanning field curvature and isovelocity are greatly degraded by the sag. Although aberrations remain, as in claim 4, the inclusion of a lens surface whose shape in the main scanning section is asymmetric with respect to the optical axis effectively reduces the main scanning field curvature and isovelocity due to the sag. Can be corrected.

  6. The scanning imaging lens according to claim 3, wherein the change in the sub-scanning curvature is asymmetric in the main scanning direction, and the lens having a special surface having three extreme values in the sub-scanning curvature is an optical axis. In the vicinity, the shape in the sub-scan section can be a positive meniscus shape.

  In this way, the principal point position of the optical system can be arbitrarily set for each image height by the combination of the curvature radii of the two surfaces, so that the lateral magnification for each image height can be kept constant. The principal point position cannot be changed greatly with a biconvex or biconcave lens.

The scanning imaging lens according to any one of claims 1 to 6, wherein the effective writing width: W and the width of the sub-scanning image surface curvature within the effective writing width: Fs are:
(3) Fs / W <0.005
Is preferably satisfied (claim 7).

  By satisfying the condition (3), the fluctuation of the sub-scanning image surface curvature is sufficiently suppressed, and the fluctuation of the beam spot diameter due to the image height can be effectively reduced.

It is preferable that the scanning imaging lens according to any one of claims 1 to 7 "at least the lens having a special surface is a plastic lens" (claim 8).
When using a plastic lens, there is a concern that the curvature of field will deteriorate due to lens deformation due to environmental fluctuations, etc., but by setting the curvature of field sufficiently small in advance, the influence of environmental fluctuations can be allowed. it can.

  The optical scanning device according to claim 9 forms a light beam from a light source into a line image that is long in the main scanning direction, and deflects it at an equiangular velocity by an optical deflector having a deflecting reflection surface near the imaging position of the line image. In the optical scanning device for condensing the deflected light beam as a beam spot on the surface to be scanned by the scanning imaging lens and performing optical scanning on the surface to be scanned at a constant speed, the scanning imaging lens is used as a scanning imaging lens. Any one of (1) is used.

  The image forming apparatus according to claim 10, wherein a latent image is formed by performing optical scanning on the image carrier, and the formed latent image is developed to form an image. The optical scanning device according to claim 9 is used as the optical scanning device for scanning. In this case, a photoconductive photosensitive member is used as the latent image carrier, the latent image carrier is uniformly charged, an electrostatic latent image is formed by optical scanning, and the formed electrostatic latent image is developed to form a toner. An image can be obtained and the toner image is fixed on a recording medium to form an image.

In the image forming apparatus according to claim 10, for example, a silver salt photographic film can be used as the image carrier. In this case, the latent image formed by the optical scanning by the optical scanning device can be visualized by a developing method of a normal silver salt photographic process. Such an image forming apparatus can be implemented as, for example, a “laser plate making apparatus”.
The image forming apparatus according to the eleventh aspect can be implemented specifically as a laser printer, a laser plotter, a digital copying machine, a facsimile machine, or the like.

As described above, according to the present invention, a novel scanning imaging lens, optical scanning device, and image forming apparatus can be realized.
The scanning imaging lens of the present invention can satisfactorily correct the curvature of field regardless of the presence of sag and can form a beam spot with a stable beam spot diameter. The optical scanning device of the present invention can perform good writing with a beam spot having a stable beam spot diameter by using the scanning imaging lens, and can perform high-density writing.
The image forming apparatus of the present invention can form an extremely good image by optical writing by the optical writing device.

FIG. 1 schematically shows an embodiment of an optical scanning device.
A light beam from the semiconductor laser 10 that is a “light source” is coupled to a subsequent optical system by a coupling lens 12 to become a parallel light beam or a weakly divergent or weakly convergent light beam that passes through the aperture 13 aperture. The beam periphery is shielded from light and “beam shaped”. The beam-shaped light beam is then focused by the cylinder lens 14 in the sub-scanning direction and reflected by the mirror 15 to form a long line image in the main scanning direction.

The polygon mirror 16 as an “optical deflector” has a deflecting / reflecting surface 16A in the vicinity of the line image forming position, and deflects the reflected light beam at a constant angular velocity by constant speed rotation. The deflected light beam passes through the lenses 17 and 18 constituting the “scanning imaging lens” and is scanned by a scanning imaging lens 19 (substantially, for example, a photosensitive surface of a photoconductive photosensitive member). The light beam is condensed as a beam spot, and the surface to be scanned 19 is optically scanned at a constant speed. As the scanning imaging lens constituted by the lenses 17 and 18, the one described in claim 1 is used as shown in Example 1 described later.
That is, the optical scanning device shown in FIG. 1 forms an optical beam from the light source 10 into a line image that is long in the main scanning direction, and has an optical deflection having a deflection reflection surface 16A in the vicinity of the imaging position of the line image. An optical scanning device that deflects light at a constant angular velocity by a detector 16, condenses the deflected light beam as a beam spot on a scanned surface 19 by scanning imaging lenses 17 and 18, and optically scans the scanned surface 19 at a uniform velocity. In this case, the scanning imaging lens is the one described in claims 1 and 2.

FIG. 2 schematically shows another form of the optical scanning device. In order to avoid confusion, the same reference numerals as in FIG. 1 are used for those which are not likely to be confused, and the description thereof will be omitted.
The deflected light beam deflected by the polygon mirror 16 serving as an “optical deflector” is transmitted through the lenses 20 and 21 constituting the “scanning imaging lens”, and a beam spot on the scanned surface 19 by the action of the scanning imaging lens. And the scanned surface 19 is optically scanned at a constant speed. As the scanning image forming lens constituted by the lenses 17 and 18, the one described in claims 1 and 3 is used as shown in Example 2 described later.
That is, in the optical scanning device shown in FIG. 2, the light beam from the light source 10 is formed into a line image that is long in the main scanning direction, and the optical deflection having the deflecting reflection surface 16A in the vicinity of the imaging position of the line image. An optical scanning device that deflects the light beam at a constant angular velocity by the detector 16, collects the deflected light beam as a beam spot on the scanning surface 19 by the scanning imaging lenses 20 and 21, and optically scans the scanning surface 19 at a constant velocity. Then, the scanning imaging lens according to claim 1 or 11 is used.

Specific examples will be given below.
Of Examples 1 and 2, Example 1 is a specific example of the embodiment shown in FIG. 1. Among the lenses 17 and 18 constituting the scanning imaging lens, the first lens 17 is the incident side surface (the first side). 1 surface) and the exit side surface (second surface) both have a “coaxial aspheric shape”, where the radius of curvature at the optical axis position is R and the distance from the optical axis is H, Yes.
X = (H ² / R) / [1 + √ {1- (1 + K) (H / R) ² }] +
A ₄・ H ⁴ + A ₆・ H ⁶ + A ₈・ H ⁸・ ・ (8)
The second lens 18 on the scanning surface side has a “non-arc-shaped” surface shape in the main scanning surface, and the radius of curvature in the main scanning surface at the optical axis position is Rm, in the main scanning direction from the optical axis. distance Y, the conic constant Km, the higher coefficient _{Am 1, Am 2, Am 3} , Am 4, Am 5, Am 6, when the .., represents the optical axis direction X by the following polynomial Yes.
X = (Y ² / Rm) / [1 + √ {1- (1 + Km) (Y / Rm) ² }] +
Am ₁・ Y + Am ₃・ Y ³ + Am ₄・ Y ⁴ + Am ₅・ Y ⁵ + Am ₆・ Y ⁶ + ・ ・ (9)
Here, the one in which “a numerical value other than zero” is substituted for at least one of odd-order Am ₁ , Am ₃ , Am ₅ ... Has an asymmetric shape in the main scanning direction.

In the first embodiment, the odd order of the expression (9) is not 0, and is asymmetric in the main scanning direction (claim 4).
The incident side surface (third surface) and the exit side surface (fourth surface) of the second lens 18 have shapes in which the radius of curvature in the sub-scanning surface: Rs (the reciprocal thereof is “sub-scanning curvature”) changes according to Y. Where the radius of curvature in the sub-scanning plane on the optical axis is Rs ₀ , and the higher order coefficients are As ₁ , As ₂ , As ₃ , As ₄ , As _5. It is represented by
Cs = 1 / Rs
= 1 / (Rs ₀ + As ₁ · Y + As ₂ · Y ² + As ₃ · Y ³ + As ₄ · Y ⁴ + As ₅ · Y ⁵ + ··). . . (Ten)
Here, when “a numerical value other than zero” is substituted into at least one of odd-order As ₁ , As ₃ , As ₅ ..., The sub-scanning curvature becomes asymmetric in the main scanning direction.

In the first embodiment, the sub-scanning curvature is asymmetrical on both the third surface and the fourth surface.
In the optical arrangement shown in FIGS. 1 and 2, when the light beam coupled by the coupling lens 12 is a “weakly focused light beam”, this weakly focused light beam is “a light beam that is naturally focused as it is. When the “focusing position” is called a natural focusing point, the position of the natural focusing point represents the degree of focusing of the coupled light beam.
The angle θ shown in FIGS. 1 and 2 is called a polygon mirror incident angle. In FIGS. 1 and 2, the distance indicated by the symbol ζ is “the misalignment of the central axis of the polygon mirror with respect to the reference beam (if there is no scanning imaging lens, the principal beam of the deflected beam orthogonal to the scanned surface)” "
As a countermeasure against sag, the lens surface can be tilted. The tilt angle (unit: degree) is represented by α, and the counterclockwise tilt angle is positive. In the second embodiment, the shift: y is given to the first and second lenses.
The shift is a displacement in the main scanning direction of the lens optical axis with respect to the reference beam, and an upward displacement in FIGS. Shift: y can be given for each lens surface, but is common to scanning imaging lenses in the following embodiments.
The lens data given below is in a state where no shift / tilt is given. The unit of the quantity having the dimension of length is “mm”.

"Example 1"
Natural focusing point: 700 mm from the deflecting reflecting surface to the scanned surface side Polygon mirror incident angle: θ = 60 degrees Number of deflecting reflecting surfaces of the polygon mirror: 6
Inscribed circle radius of polygon mirror: 18mm
Center axis deviation of polygon mirror ξ: 7.80mm
Angle of view: −39.95 ° to + 40.1 ° Lens data surface number below the deflecting / reflecting surface (surface number: 0) of the polygon mirror Rm Rs x (surface interval) y α n (refractive index)
0 ∞ ∞ 26.150 0
1 -100.912 -100.912 18.000 0 1.52441
2 -76.404 -76.404 13.062 0
3 2895.637 100.694 15.000 1.52441
4 -163.649 -29.884 143.188
The first and second surfaces are “coaxial aspherical surfaces”, and the third and fourth surfaces are “extreme values where the change in the sub-scanning curvature is asymmetric in the main scanning direction and the change in curvature is 1 or more. And a special surface whose shape in the main scanning section is asymmetric with respect to the optical axis.
The data of each coefficient regarding the first to fourth surfaces is as shown in Table 1 below.

3A shows a change in the sub-scanning curvature of the third surface, and FIG. 3B shows a change in the sub-scanning curvature of the fourth surface. As shown in (a), the third surface is “a change in the sub-scanning curvature is asymmetric in the main scanning direction, and the change in curvature has one extreme value in the vicinity of the optical axis. "Special surface with another extreme value".
FIG. 3C shows changes in the lateral magnification in the sub-scanning direction in the scanning imaging lens of Example 1 depending on the image height.
As shown in the figure, the variation of the lateral magnification due to the image height is small, and the condition (2) is satisfied.
Parameter (1): The value of | (he) / (hmax) | is as follows for each point a and b in FIG.
Point a: | (-36) / (-44.9) | = 0.80
b point: | (0) / (-44.9) | = 0
And the condition (1) is satisfied at the extreme value at point a.
Condition (3) parameter: The value of Fs / W is 0.027 / 210 = 0.0001, which satisfies condition (3).
A diagram of field curvature and isovelocity with respect to Example 1 is shown in FIG. The “distortion aberration” in the diagram of isovelocity usually corresponds to the fθ characteristic. In Example 1, the curvature of field in the main and sub-scanning directions is corrected extremely well, and the isovelocity is very good.

Example 2 described below relates to the embodiment shown in FIG.
The second embodiment is an embodiment of the first and third aspects of the invention.

The first surface of the first lens 20 is a coaxial aspheric surface and is represented by the above-described equation (8).
The surface shapes of the second and third surfaces of the scanning imaging lens, that is, the exit side surface of the first lens 20 and the incident side surface of the second lens 21 in the main scanning surface are non-arc shapes, and are described above. This is represented by Expression (9). In the second embodiment, the shapes of the second and third surfaces in the main scanning plane use only even-order terms in the equation (9), and are therefore symmetrical with respect to the X axis, which is the optical axis in the main scanning direction. is there.
The second and third surfaces of the scanning imaging lens have a shape in which the radius of curvature Rs in the sub-scanning surface changes according to the coordinate Y in the main scanning direction, and the sub-scanning curvature Cs (Y) is changed to the optical axis. The curvature in the sub-scanning plane including Cs ₀ is represented by the following equation, where Cs _{0 is} the higher order coefficient, and As ₁ , As ₂ , As ₃ , As ₄ , As ₅ . Cs = Cs ₀ + As ₁ · Y + As ₂ · Y ² + As ₃ · Y ³ + As ₄ · Y ⁴ + As ₅ · Y ⁵ + (11)
Expression (11) differs from expression (10) in terms of curvature. In Example 2, the sub-scanning curvature of the second surface is symmetric with respect to Y = 0 (the odd-order term in the equation (11) is 0), and the third surface is a special surface whose sub-scanning curvature is asymmetric in the Y direction. is there.

"Example 2"
Natural focusing point: ∞ (coupled beam becomes parallel beam)
Polygon mirror incident angle: θ = 60 degrees Number of polygon mirror deflection reflection surfaces: 6
Inscribed circle radius of polygon mirror: 13mm
Center axis deviation of polygon mirror ξ: 5.22mm
Angle of view: -42 degrees to +42 degrees Lens data surface number below deflecting / reflecting surface (surface number: 0) of polygon mirror Rm Rs x (surface distance) yn (refractive index)
0 ∞ ∞ 52.390 1.588
1 -312.597 -312.597 31.400 1.52716
2 -82.915 -82.238 78.0
3 -500.00 -47.55 3.5 1.52716
4 -1000.00 -23.38 143.377
The first surface is a “coaxial aspheric surface”, the second surface is a “special surface”, and the third surface is “an extreme value in which the change in the sub-scanning curvature is asymmetric in the main scanning direction and the change in the curvature is 1 or more. The fourth surface is a “normal toroidal surface”.
The data of each coefficient relating to the first to third surfaces is as shown in Table 2 below.

FIG. 5A shows a change in the sub-scanning curvature of the third surface. The third surface is a special surface in which the change in the sub-scanning curvature is asymmetric in the main scanning direction and the change in the curvature has three extreme values (and four inflection points), and one of the extreme values. Is near the optical axis.
FIG. 5B shows “change in lateral magnification in the sub-scanning direction due to image height” in the scanning imaging lens of the third embodiment. As shown in the figure, the variation of the lateral magnification due to the image height is small, and the condition (2) is satisfied.
Parameter (3): The value of Fs / W is 0.203 / 320 = 0.0006, which satisfies condition (3). The parameter of condition (1): | (he) / (hmax) | is the value of each of points b and c in FIG.
b point: | (+87.5) / (+ 106) | = 0.83
c point: | (+97.5) / (+ 106) | = 0.92
These extreme values satisfy the condition (1).

  A diagram of field curvature and isovelocity with respect to Example 2 is shown in FIG. Also in Example 2, the curvature of field in the main and sub-scanning directions is corrected extremely well, and the isovelocity is very good.

Finally, an embodiment of the image forming apparatus will be described with reference to FIG.
This image forming apparatus is a laser printer.
The laser printer 100 has a “cylindrical photoconductive photosensitive member” as the image carrier 111. Around the image carrier 111, a charging roller 112 as a charging unit, a developing device 113, a transfer roller 114, and a cleaning device 115 are provided. A “corona charger” can also be used as the charging means.

In addition, an optical scanning device 117 using a laser beam LB is provided, and “exposure by optical writing” is performed between the charging roller 112 and the developing device 113.
In FIG. 20, reference numeral 116 denotes a fixing device, reference numeral 118 denotes a cassette, reference numeral 119 denotes a registration roller pair, reference numeral 120 denotes a paper feed roller, reference numeral 121 denotes a conveyance path, reference numeral 122 denotes a discharge roller pair, reference numeral 123 denotes a tray, reference numeral P Indicates transfer paper as a recording medium.
When forming an image, the image carrier 111, which is a photoconductive photosensitive member, is rotated at a constant speed in the clockwise direction, the surface thereof is uniformly charged by the charging roller 112, and the optical beam of the laser beam LB of the optical scanning device 117 is written. An electrostatic latent image is formed upon exposure to the image. The formed electrostatic latent image is a so-called “negative latent image”, and the image portion is exposed.
This electrostatic latent image is reversely developed by the developing device 113, and a toner image is formed on the image carrier 111.

The cassette 118 storing the transfer paper P can be attached to and detached from the main body of the image forming apparatus 100. When the transfer paper P is mounted as shown in the drawing, the uppermost sheet of the stored transfer paper P is fed by the paper supply roller 120. The transfer paper P that has been fed and fed is fed to the registration roller pair 119 at its leading end. The registration roller pair 119 feeds the transfer paper P to the transfer portion in time with the toner image on the image carrier 111 moving to the transfer position. The transferred transfer paper P is superimposed on the toner image at the transfer portion, and the toner image is electrostatically transferred by the action of the transfer roller 114. The transfer paper P to which the toner image is transferred is sent to the fixing device 116, where the toner image is fixed by the fixing device 116, passes through the conveyance path 121, and is discharged onto the tray 123 by the discharge roller pair 122.
The surface of the image carrier 111 after the toner image is transferred is cleaned by a cleaning device 115 to remove residual toner, paper dust, and the like.
By using the optical scanning device (specifically, the first and second embodiments) shown in FIGS. 1 and 2 as the optical scanning device 117, extremely good image formation can be performed.

1 is a diagram schematically showing an embodiment of an optical scanning device according to the present invention. It is a figure which shows schematically another form of implementation of the optical scanning device of this invention. It is a figure which shows the change of the subscanning curvature of the special surface in Example 1, and the horizontal magnification change of a subscanning direction. It is a figure which shows the field curvature regarding Example 1, and isokineticity. It is a figure which shows the change of the subscanning curvature of the special surface in Example 2, and the horizontal magnification change of a subscanning direction. It is a figure which shows the curvature of field regarding Example 2, and isovelocity. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram for explaining one embodiment of an image forming apparatus according to the present invention.

Explanation of symbols

16 Optical deflector 17, 18 Scanning imaging lens 19 Scanned surface

Claims (11)

The light beam from the light source is formed into a line image that is long in the main scanning direction, and deflected at an equiangular speed by an optical deflector having a deflecting reflection surface in the vicinity of the image forming position of the line image. A scanning imaging lens used in an optical scanning device that collects light as a beam spot on the surface to be scanned, and optically scans the surface to be scanned at a constant speed,
It consists of two or more balls,
The sub-scanning curvature has at least one special surface that changes from the optical axis toward the periphery in the main scanning direction,
At least one of the special surfaces has an asymmetric change in the sub-scanning curvature in the main scanning direction, the curvature has an extreme value of 2 or more, and one of the two or more extreme values is Near the optical axis, the other extrema are on one side of the optical axis,
At least one of the extreme values of the special surface is as follows: the position in the main scanning direction: he is the effective lens height from the optical axis: hmax on the + image height side or the −image height side:
(1) 0.62 <| (he) / (hmax) | <0.93
Is located at a position satisfying
Lateral magnification on the optical axis: β ₀ , Arbitrary image height lateral magnification: βh, conditions:
(2) 0.93 <| βh / β ₀ | <1.07
A scanning imaging lens characterized by satisfying The scanning imaging lens according to claim 1.
There are two special surfaces in which the change in the sub-scanning curvature is asymmetric in the main scanning direction. In one of these surfaces, the extreme value of the sub-scanning curvature is one near the optical axis and the other on one side of the optical axis. A scanning imaging lens, wherein there are two in total, and the extreme value of the sub-scanning curvature is one near the optical axis on the other surface. The scanning imaging lens according to claim 1.
There is one special surface whose change in the sub-scanning curvature is asymmetric in the main scanning direction, and the extreme value of the sub-scanning curvature on this special surface is one near the optical axis and the other two on one side of the optical axis. A total of three scanning imaging lenses. The scanning imaging lens according to claim 2.
A scanning imaging lens comprising a lens surface whose shape in the main scanning section is asymmetric with respect to the optical axis. The scanning imaging lens according to claim 3 or 4,
A scanning imaging lens, wherein one or more lenses constituting the scanning imaging lens are tilted and / or shifted. The scanning imaging lens according to claim 3, 4 or 5,
A lens having a special surface in which the change in the sub-scanning curvature is asymmetric in the main scanning direction and the sub-scanning curvature has three extreme values has a positive meniscus shape near the optical axis in the sub-scan section. A scanning imaging lens characterized by the above. The scanning imaging lens according to any one of claims 1 to 6,
Effective writing width: W, width of sub-scanning image surface curvature within the effective writing width: Fs is a condition:
(3) Fs / W <0.005
A scanning imaging lens characterized by satisfying The scanning imaging lens according to any one of claims 1 to 7,
A scanning imaging lens, wherein at least the lens having a special surface is a plastic lens. The light beam from the light source is formed into a line image that is long in the main scanning direction, and deflected at an equiangular speed by an optical deflector having a deflecting reflection surface in the vicinity of the image forming position of the line image. In the optical scanning device that collects the light as a beam spot on the surface to be scanned and optically scans the surface to be scanned at a constant speed,
An optical scanning apparatus using the scanning imaging lens according to any one of claims 1 to 8.   An image forming apparatus for forming a latent image by performing optical scanning on an image carrier and developing the formed latent image to form an image, wherein the image carrier is optically scanned. An image forming apparatus using the described optical scanning device. The image forming apparatus according to claim 10.
A photoconductive photosensitive member is used as the latent image carrier. After the latent image carrier is uniformly charged, an electrostatic latent image is formed by optical scanning, and the formed electrostatic latent image is developed to obtain a toner image. An image forming apparatus, wherein the toner image is fixed on a recording medium to form an image.

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