JP2011013345A - Scanning optical system in image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a scanning optical system capable of reducing focus shift on an image face in the direction of main scanning due to fluctuation in environmental temperature and humidity, even when forming an fθ lens and a coupling lens by an optical transparent resin.SOLUTION: The scanning optical system satisfies expression (1) (wherein, coefficient of linear expansion of a case is α, coefficient of linear expansion of a coupling lens holder 21 is α, distance between a plane of incidence of the coupling lens (a collimator lens) 2 and a light source (semiconductor laser) 1 constituting a light source device is L, distance between a fixing point of the coupling lens holder on the case and the plane of incidence of the coupling lens is L, focal distance in the main direction of scanning of the fθ lens is f, focal distance of the coupling lens is f, L denotes focal depth, a, b, c denote constants, tdenotes design temperature, t denotes environmental temperature, Rdenotes radius of curvature on incoming side in the main direction of scanning of this scanning optical system at the environmental temperature, Rdenotes radius of curvature on outgoing side in the main direction of scanning of this scanning optical system at the environmental temperature, and S denotes thickness of the fθ lens).

Description

  BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a scanning optical system in an image forming apparatus such as a copying machine, a printer, a facsimile, or a composite machine using an electrophotographic method, and more particularly, a coupling lens (collimator lens) that uses light from a light source as a parallel light beam. ) And the fθ lens that makes the scanning speed of the light deflected by the polygon mirror constant on the surface to be scanned are optically transparent resin lenses and nevertheless on the image plane in the main scanning direction due to environmental temperature changes. The present invention relates to a scanning optical system in an image forming apparatus that can keep the focus shift at an allowable range.

  In image forming apparatuses such as copiers, printers, facsimiles, and composite machines using electrophotography, the photosensitive drum is uniformly charged by a charging device, and then is exposed to light modulated by a signal of an image to be formed. The photosensitive drum is exposed to form an electrostatic latent image, the electrostatic latent image formed on the photosensitive drum is developed by the developing device, and the toner image on the photosensitive drum formed by the development is transferred to a sheet by the transferring device. Image formation is performed by a process of transferring and fixing a toner image transferred onto a sheet by a fixing device.

  Among these, as a scanning optical system for exposing a uniformly charged photosensitive drum, in a high-speed machine, it is modulated by a signal of an image forming a laser diode or the like, and is scanned in a main scanning direction (of the photosensitive drum by a polygon mirror). A scanning optical system that scans the photosensitive drum by converting the constant angular velocity scanning into the constant velocity scanning by the scanning optical system using the fθ lens is used.

  In such a scanning optical system, generally, a laser beam emitted from a light source device such as a laser diode is converted into a substantially parallel light beam by a coupling lens such as a collimator lens, and then scanned on a photosensitive drum. In order to obtain a desired beam spot diameter above, the optical beam having a refractive power only in the sub-scanning direction (direction orthogonal to the main scanning direction), such as a cylindrical lens, by narrowing the light beam to a certain size by an aperture (aperture). The element is transmitted and imaged linearly on the polygon mirror in the main scanning direction, and imaged as a light beam spot on the scanning surface through an fθ lens that converts constant angular velocity scanning on the scanning surface to constant velocity scanning. It is configured as follows.

  Of these, the fθ lens is often composed of two elements in two groups. The first lens corrects aberrations in the main scanning direction and the conversion function from constant angular velocity scanning to constant velocity scanning, and the second lens tilts the polygon mirror. In general, the correction function is designed with the optical function separated.

  As the material of the fθ lens, optical glass (such as BK7) that is not easily affected by a change in refractive index, a change in refractive index distribution, thermal deformation, or the like due to environmental fluctuations has been used. In view of demands for reducing the cost of materials and parts, optical transparent resins such as optical acrylic resins (PMMA) and cycloolefin-based resins (COP) are increasingly used.

  However, these optical transparent resins such as optical acrylic resin (PMMA) and cycloolefin resin (COP) are more susceptible to changes in temperature and humidity than glass, and optical acrylic resin (PMMA) is hygroscopic. Therefore, the refractive index distribution in the lens is changed and the optical performance is likely to be deteriorated. On the other hand, cycloolefin resin (COP) has low hygroscopicity with respect to changes in humidity and is not easily affected by changes in the refractive index distribution. Cause deterioration.

  In addition, a coupling lens that converts diffused light from a light source device such as a semiconductor laser into a substantially parallel light beam has a large change in optical performance when a focus shift occurs due to a change in temperature and humidity. Conventionally, this coupling lens is optical. Glass was used. However, when optical glass is used, a cycloolefin resin may be used in order to increase the cost compared to the case of using an optical transparent resin. , Causing fluctuations in the focal length of the coupling lens. Therefore, there is a method of solving this problem by providing a diffraction grating in the coupling lens, or having two coupling lenses, each having positive and negative refractive powers, and canceling the focal length variation. It was taken.

  As described above, for example, in Patent Document 1, a cylindrical lens is made of an acrylic resin and an fθ lens is made of glass in order to prevent a focal position shift due to an environmental temperature change by using an optical transparent resin in the scanning optical system. In this case, a laser beam scanning optical system is shown in which the focal position deviation due to temperature change is suppressed within a practically allowable range. In the laser beam scanning optical system disclosed in Patent Document 1, the light source unit holding the cylindrical lens is deformed by a temperature change, and the imaging position in the vicinity of the polygon mirror fluctuates from the original position, and near the photoconductor. The image forming position moves from the original position, but as the temperature rises, the plastic cylindrical lens also deforms, and the fact that the image forming position changes in a direction to cancel the deformation of the light source unit is used to reduce the defocus. It is kept within a practically acceptable range.

  In Patent Document 2, in an optical scanning device using a plastic lens as the imaging lens B, a light source and a collimator lens are integrated with a lens barrel, and the lens barrel is a first lens barrel member and a second lens barrel member. When the temperature of the optical scanning device rises and the imaging position is displaced by Δx due to the refractive index change of the plastic lens B, the collimator lens is moved by ΔL due to the expansion of the lens barrel to return the imaging position. An optical scanning device is disclosed in which fine adjustment is possible and accurate temperature correction is possible by configuring the cylinder with a plurality of members having different linear expansion coefficients.

  Further, in Patent Document 3, an optical system of a scanning optical device includes a light source unit, a cylindrical lens that converges laser light emitted from the light source unit only in the sub-scanning direction, and a laser beam converged by the cylindrical lens dynamically. Consists of a polygon mirror that deflects in the main scanning direction and a scanning imaging lens that forms an image of the deflected laser beam on the surface to be scanned. The light source unit is a collimator that makes the semiconductor laser and the laser beam substantially parallel. These are fixed and held by a cylindrical plastic fixed holding member, and the effective length L of the fixed support member is shortened by reducing the back focus with respect to the focal length of the collimator lens. Thus, there has been proposed a scanning optical device capable of suppressing the shift of the focus position due to a temperature change.

Japanese Patent Laid-Open No. 5-19189 JP-A-10-293261 JP 2005-189500 A

  When an fθ lens or a coupling lens is configured with an optical transparent resin, since the refractive index variation with respect to the temperature change is larger than that of the optical glass as described above, the change in the focal length by each lens becomes large. . Also, a coupling lens that converts diffused light from a light source device such as a semiconductor laser into a parallel light beam has a distance in the optical axis direction between the semiconductor laser and the coupling lens when the temperature changes. It expands and contracts by an amount proportional to the linear expansion coefficient of the common fixing member installed.

When optically transparent resin is used as the lens material, the focal length variation due to temperature changes is larger than the variation in the distance between the semiconductor laser and the coupling lens due to thermal expansion, and the distance between the coupling lens and the semiconductor laser. Will be in the same state as is shortened. At this time, the surface to be scanned is proportional to the square of the ratio (lateral magnification) of f _col (t) representing the focal length of the coupling lens as a function of temperature t and the focal length f _fθ (t) of the fθ lens. There is a problem that the focal position shift in the optical axis direction occurs.

  In order to deal with these problems, the method of providing a diffraction grating in the coupling lens requires a separate design of the diffraction grating, and the production of the diffraction grating mold is special compared to ordinary lens molds. More man-hours are required. The method of canceling the configuration with two lenses uses two lenses, which increases the number of members and increases the cost.

  The laser beam scanning optical system disclosed in Patent Document 1 is a case where the cylindrical lens is made of acrylic resin and the fθ lens is made of glass. As described above, the apparatus is reduced in size and weight, and the member cost is reduced. In view of such demands, it cannot be applied when an optical transparent resin such as optical acrylic resin (PMMA) or cycloolefin resin (COP) is used for the fθ lens and the coupling lens.

  Further, in the optical scanning device and the scanning optical device disclosed in Patent Literature 2 and Patent Literature 3, the light source device and the collimator lens are integrated by the holding member, and therefore, the focal length of the scanning optical system varies. This makes it difficult to adjust the focus by changing the distance between the light source device and the collimator lens, and the desired beam spot diameter cannot be obtained.

  Therefore, in the present invention, even when an optical transparent resin such as an optical acrylic resin (PMMA) or a cycloolefin resin (COP) is used for the fθ lens and the coupling lens, the main scanning direction due to a change in environmental temperature and humidity. An object of the present invention is to provide a scanning optical system in an image forming apparatus capable of reducing the focus shift on the image plane.

ここで、Ｌは焦点深度、ａ、ｂ、ｃは定数、ｔ_０は設計温度、ｔは環境温度、Ｒ_１は環境温度ｔ_０での走査光学系の主走査方向入射側曲率半径、Ｒ_２は環境温度ｔ_０での走査光学系の主走査方向射出側曲率半径、Ｓはｆθレンズの厚みである。 In order to solve the above problems, the scanning optical system in the image forming apparatus according to the present invention is:
A light source device, a coupling lens that converts the diffused light from the light source device into a parallel light beam, and deflects light from the light source device that has been converted into a parallel light beam by the coupling lens, so that the surface to be scanned is in the main scanning direction. Image forming comprising: scanning optical means for scanning; and an fθ lens for condensing the light deflected by the scanning optical means as a light spot on the surface to be scanned so that the scanning speed on the surface to be scanned is constant. In the scanning optical system in the apparatus,
A coupling lens holder for holding the coupling lens is fixed in a focused state so as to be movable in the direction of the optical axis, and a housing made of a composite material made of resin and glass fiber holding the light source device. Having an fθ lens and a coupling lens in the scanning optical system made of an optical transparent resin,
The linear expansion coefficient of the housing is α ₁ , the linear expansion coefficient of the coupling lens holder is α ₂ , the distance between the incident surface of the coupling lens and the light source constituting the light source device is L ₁ , and the coupling lens. When the distance from the fixing point to the housing of the holder to the incident surface of the coupling lens is L ₂ , the focal length in the main scanning direction of the fθ lens is f _fθ , and the focal length of the coupling lens is f _col , (1) A scanning optical system in an image forming apparatus, wherein the optical system is configured to satisfy the expression (1).

Here, L is the depth of focus, a, b and c are constants, t ₀ is the design temperature, t is the ambient temperature, R ₁ is the curvature radius on the incident side in the main scanning direction of the scanning optical system at the ambient temperature t ₀ , and R _2. Is the exit side radius of curvature in the main scanning direction of the scanning optical system at the environmental temperature t ₀ , and S is the thickness of the fθ lens.

  As described above, the fθ lens and the coupling lens are made of the optical transparent resin, the light source device and the coupling lens holder for holding the coupling lens are held in the casing, and the coupling lens holder is optically By fixing in focus so that it can move in the axial direction, even if the focal length of the scanning optical system varies, it can be fixed after adjusting the focus by changing the distance between the light source device and the collimator lens. It becomes possible.

  Furthermore, when the scanning optical system lens is regarded as a synthetic single lens, the light source device held in the housing and the coupling lens composed of the optical transparent resin are designed so as to satisfy the expression (1). The focal shift amount of the coupling lens itself and the focal shift amount due to the deformation of the casing cancel each other, and the difference is the square of the ratio of the focal length of the fθ lens in the main scanning direction to the focal length of the coupling lens. By making the multiplied value and the focal length variation of the fθ lens in the main scanning direction substantially equal, the focal shift can be minimized. Further, if the focus shift amount at the time of environmental change is small, the design depth of focus can be made shallow, so that the design difficulty of the scanning optical system can be lowered. In addition, the coupling lens can use a conventional transparent optical resin without using expensive glass. Further, since it is not necessary to provide a diffraction grating, the cost can be reduced and the design of the diffraction grating can be realized. Optical design can be simplified because it becomes unnecessary.

  As described above, the scanning optical system in the image forming apparatus according to the present invention can minimize the focus shift amount due to the environmental temperature of the scanning optical system even if the fθ lens and the coupling lens are made of transparent optical resin. It is possible to suppress degradation of optical performance when the environmental temperature changes, and because the focus shift amount at the time of environmental fluctuation is small, the design depth of focus can be made shallow. The difficulty level at the time of design can be lowered. Even if the focal length of the scanning optical system varies, the focus adjustment can be easily performed because the distance between the light source device and the collimator lens can be varied before being fixed.

1 is a perspective view of a schematic configuration in which an optical scanning optical device according to the present invention is applied to a photoconductor exposure apparatus in an image forming apparatus. FIG. 2 is a cross-sectional view in the sub-scanning direction of the optical scanning optical device shown in FIG. 1. It is a schematic sectional drawing of the housing | casing holding the light source and the coupling lens holder which accommodated the coupling lens. the focal length is 100mm of fθ lens, the _{L 2} to a temperature change in the case of a 5 mm, it is a graph showing changes in [Delta] L _image. To a temperature change in the case of optimizing L _2, it is a graph showing changes in [Delta] L _image.

  Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the drawings. However, the dimensions, materials, shapes, relative arrangements, and the like of the components described in this embodiment are not intended to limit the scope of the present invention unless otherwise specified, but are merely illustrative examples. Absent.

  FIG. 1 is a perspective view of a schematic configuration of a scanning optical system in an image forming apparatus according to the present invention, FIG. 2 is a sectional view in the sub-scanning direction of the optical scanning optical apparatus shown in FIG. 1, and FIG. FIG. 3 is a schematic cross-sectional view of a housing that holds a coupling lens holder that accommodates a lens and that can move the coupling lens holder in the optical axis direction. 1 to 3 are denoted by the same reference numerals.

  First, the optical scanning optical device according to the present invention will be described with reference to FIGS. 1 and 2. For example, a light source 1 made of a semiconductor laser having a wavelength of 670 nm and light from the light source 1 made of the semiconductor laser are made parallel light. An aspherical collimator lens 2 as a coupling lens, an aperture stop (aperture) 3 having an opening 31 having the parallel light of a predetermined size, and light from the light source 1 converted into parallel light in the main scanning direction 10 A linear condensing element such as a cylindrical lens 4 that forms an image on the reflecting surface 5a of a rotating reflecting mirror (polygon mirror) 5 that scans the surface 9 to be scanned as a long linear shape, and light deflected by the polygon mirror 5 It is composed of a scanning optical system such as fθ lenses 6 and 7, a reflecting mirror 8 and the like that make the scanning speed on the surface to be scanned constant. In the present invention, the cylindrical lens 4 is an optical glass S-BSL7 (manufactured by OHARA INC.). The reflecting mirror 8 may be provided as necessary, and may be omitted depending on the arrangement configuration (layout) of each optical member.

  The fθ lenses 6 and 7 have a positive power in the main scanning direction, a first lens 6 having a negative power in the sub scanning direction orthogonal to the main scanning direction, and a first lens 6 having a positive power in the sub scanning direction. 2 lenses 7 and one of the lenses has an aspherical shape. In the present invention, the collimator lens 2 and the fθ lenses 6 and 7 are formed using an optical transparent resin such as ZEONEX330R (manufactured by Nippon Zeon Co., Ltd.) which is a cycloolefin resin (COP), for example. Details of the lens 2 will be described later with reference to FIG. 3. The lens 2 is housed in a coupling (collimator lens) lens holder, and is made of a composite material of polycarbonate, glass fiber, and ABS together with the light source 1, such as RN7740D (manufactured by Teijin Limited). It is held in a housing made of material.

  Next, FIG. 3 which is a schematic sectional view of the housing 20 holding the light source 1 and the coupling lens holder 21 containing the coupling lens (collimator lens) 2 will be described.

  As described above, the casing 20 is made of a composite material of polycarbonate, glass fiber, and ABS, such as RN7740D (manufactured by Teijin Ltd.), and the like, and a coupling lens holder that houses the coupling lens (collimator lens) 2 21 and a light source fixing member 23 to which the light source (semiconductor laser) 1 is fixed.

  The coupling lens holder 21 is formed in a cylindrical shape with resin, and a coupling lens holder fixing rib 22 is provided at the center. After the focus adjustment is performed by moving the casing 20 in the optical axis direction, The adhesive is deposited at the position indicated by, and is fixed by the coupling lens holder fixing rib 22.

と表せる。 In the present invention, as shown in FIG. 3, the linear expansion coefficient of the housing 20 is α ₁ , the linear expansion coefficient of the coupling lens holder 21 is α ₂ , and the incident surface of the coupling lens (collimator lens) 2 and L ₁ the distance between the light source 1 of the light source _device. distance L ₂ from the fixed point 24 to the housing of the coupling lens holder 21 to the entrance surface of the coupling lens _2, the focal length of the coupling lens 2 _Is f _col , and the focal length in the main scanning direction of the fθ lens is f _fθ . Further, the design temperature is t ₀ and the temperature after the environmental temperature change is t. All the above parameters the value of the design temperature t _0.
The distance L _Col between the laser diode (light source) 1 and the collimator lens 2 when the environmental temperature changes from t _{0 to} t corresponds to the difference in expansion change between the housing 20 and the coupling lens holder 21 due to temperature change. Using the above parameters,

It can be expressed.

となる。
横倍率Ｍと縦倍率Ｍ^２を求めると、

と表せる。これは上記の焦点距離で構成される光学系において、物点での光軸方向に移動した場合の像点での焦点位置の移動量を示す。
従って、環境温度がｔ_０→ｔに変化した場合のレーザーダイオード１とカップリングレンズ２の距離がＬ_ｃｏｌになった時の像点での焦点位置の移動量ΔＬ_１は

となる。
ここで、式４の符号が負になっているのは、Ｌ_ｃｏｌが大きくなると、像点の焦点位置が光進行方向に対して逆向きに移動するからである。 Next, the position of the laser diode 1 in the main scanning direction and the vertical magnification of the image plane are obtained.
When the environmental temperature is t, the refractive index when all the lenses of the scanning optical system are regarded as a synthetic single lens is n (t), the lens center thickness is S (t), and the main scanning direction incident-side radius of curvature is R ₁ (t ) When the main scanning direction exit side radius of curvature is R ₂ (t), the main scanning focal length f (t) of the scanning optical system is

It becomes.
When determining the lateral magnification M and the longitudinal magnification M ^2,

It can be expressed. This indicates the amount of movement of the focal position at the image point when moving in the optical axis direction at the object point in the optical system configured with the focal length.
Therefore, the movement amount ΔL ₁ of the focal position at the image point when the distance between the laser diode 1 and the coupling lens 2 becomes L _col when the environmental temperature changes from t _{0 to} t is

It becomes.
Here, the sign of Expression 4 is negative because the focal position of the image point moves in the opposite direction to the light traveling direction when L _col increases.

ここで、αはｆθレンズの材料の線膨張係数、Ｒ_１、Ｒ_２、Ｓはそれぞれ、環境温度ｔ_０での走査光学系の主走査方向入射側曲率半径、主走査方向射出側曲率半径、レンズ厚みであり、ａ、ｂ、ｃは温度変化に対する屈折率の変化を近似した時の定数である。
式５〜式８を式２に代入して

を得る。
式９にｔ＝ｔ_０を代入して

を得る。
環境温度がｔ_０→ｔに変化した場合に像点での焦点移動量ΔＬ_２は、

となる。
式４と式１１から、環境温度がｔ_０→ｔに変化した場合の像点での焦点位置移動量は、

である。
そして、環境温度がｔ_０→ｔに変化した場合の像点での焦点位置移動量ΔＬ_{ｉｍａｇｅ}が、所望の光学性能（ビームスポット径等）を満たす焦点位置の範囲（焦点深度）Ｌの範囲内であれば温度変化しても焦点ずれが無いことになるので、

を満たす様に各パラメータを設定すれば、温度変化しても、焦点ずれの少ない走査光学系を設計する事ができる。 Next, the amount of movement of the focal length when the environmental temperature of the fθ lens changes is obtained.
The variables in equation 2, n (t), R ₁ (t), R ₂ (t), and S (t) are each rewritten as a function of temperature t.

Here, α is the linear expansion coefficient of the material of the fθ lens, R ₁ , R ₂ , and S are the main scanning direction entrance radius of curvature, the main scanning direction exit radius of curvature of the scanning optical system at the environmental temperature t ₀ , respectively. It is the lens thickness, and a, b, and c are constants obtained by approximating the change in the refractive index with respect to the temperature change.
Substituting Equation 5 to Equation 8 into Equation 2

Get.
Substituting t = t ₀ into Equation 9

Get.
When the environmental temperature changes from t _{0 to} t, the focal shift amount ΔL ₂ at the image point is

It becomes.
From Equation 4 and Equation 11, the focal position movement amount at the image point when the environmental temperature changes from t _{0 to} t is

It is.
The focal position movement amount ΔL _image at the image point when the environmental temperature changes from t _{0 to} t is within a focal position range (focus depth) L that satisfies the desired optical performance (beam spot diameter, etc.). Then there will be no defocus even if the temperature changes.

If each parameter is set so as to satisfy the above, it is possible to design a scanning optical system with little defocus even if the temperature changes.

表１でＲ_２が負になっているのは光の進行方向とは逆方向に曲率中心がある事を示している。即ち、これは凸レンズになっている事を示す。
ｔ_０は常温（２０±１５℃）の代表値として中心値である２０℃を採用した。
また、屈折率の温度変化に対する係数ａ、ｂ、ｃについてはシクロオレフィン系樹脂（ＣＯＰ）のＺＥＯＮＥＸＥ４８Ｒ（日本ゼオン株式会社製）の実験値から算出したものである。また、αは同樹脂の線膨張係数である。
α_１はポリカーボネート、ガラス繊維、ＡＢＳの複合材料であるＲＮ７７４０Ｄ（帝人株式会社製）の線膨張係数である。
α_２は一般的なポリカーボネートの線膨張係数である。
表１に示した数値のうちＬ_１はコリメータレンズの焦点距離から確定する距離である。従ってここでは最適化できるパラメータとしてはＬ_２が好適である。 Next, an actual optical design example will be described. First, an embodiment will be described for a case where the focal length is relatively short. The parameters are as shown in Table 1 below.

In Table 1, R ₂ is negative, indicating that the center of curvature is in the direction opposite to the light traveling direction. That is, this indicates that it is a convex lens.
For t ₀ , 20 ° C., which is the center value, was adopted as a representative value at room temperature (20 ± 15 ° C.).
Further, the coefficients a, b, and c with respect to the temperature change of the refractive index are calculated from experimental values of ZEONEEXE 48R (manufactured by Nippon Zeon Co., Ltd.), a cycloolefin resin (COP). Α is a linear expansion coefficient of the resin.
α ₁ is a linear expansion coefficient of RN7740D (manufactured by Teijin Limited), which is a composite material of polycarbonate, glass fiber, and ABS.
α ₂ is a linear expansion coefficient of general polycarbonate.
L ₁ of the numerical values shown in Table 1 is the distance to determine the focal length of the collimator lens. Here as a parameter that can be optimized accordingly it is preferably L _2.

表２はｆθレンズの焦点距離が１００ｍｍで、Ｌ_２を５ｍｍとした場合の温度変化に対するΔＬ_{ｉｍａｇｅ}の変化を示している。図４は、ｆθレンズの焦点距離が１００ｍｍで、Ｌ_２を５ｍｍとした場合の温度変化に対する、ΔＬ_{ｉｍａｇｅ}の変化を示したグラフであり、縦軸はΔＬ_{ｉｍａｇｅ}、横軸は環境温度を示している。
画像形成装置の一般的な使用環境は１０℃〜３５℃の範囲であり、その場合画像形成装置中の走査光学系の環境温度は最大で約５０℃程度になるため、表２における環境温度ｔを１０℃〜５０℃としている。
表２及び図４から、温度が上がるにつれて、焦点位置が感光ドラムの光進行方向側へ１０℃〜５０℃の範囲で１ｍｍ程ずれている事がわかる。

Table 2 shows changes in ΔL _image with respect to temperature changes when the focal length of the fθ lens is 100 mm and L ₂ is 5 mm. FIG. 4 is a graph showing changes in ΔL _image with respect to temperature changes when the focal length of the fθ lens is 100 mm and L ₂ is 5 mm. The vertical axis indicates ΔL _image and the horizontal axis indicates the environmental temperature. Yes.
The general use environment of the image forming apparatus is in the range of 10 ° C. to 35 ° C. In this case, the environmental temperature of the scanning optical system in the image forming apparatus is about 50 ° C. at maximum. Is set to 10 ° C to 50 ° C.
From Table 2 and FIG. 4, it can be seen that as the temperature rises, the focal position shifts by about 1 mm in the range of 10 ° C. to 50 ° C. toward the light traveling direction of the photosensitive drum.

このとき、Ｌ_２は負の値を示すが、これは、接着点がコリメートレンズ２よりも光源１側にある事を示す。Ｌ_２を最適化する事で温度変化に対する焦点位置変化を略０にする事ができる。焦点深度Lが４ｍｍあれば、温度に対する焦点位置のずれを最小限にする事が可能となり、画像品質を維持する事が可能となる。 Next, the parameters in the case of optimizing the L ₂ in Table 3. Also, it is shown in Table 4 and FIG. 5 a variation of [Delta] L _image to a temperature change in the case of optimizing the _{L 2.} In FIG. 5, the vertical axis represents ΔL _image and the horizontal axis represents temperature.

At this time, L ₂ indicates a negative value, which indicates that the adhesion point is closer to the light source 1 than the collimating lens 2. The focal position for temperature changes can be substantially zero by optimizing the L _2. If the focal depth L is 4 mm, it is possible to minimize the deviation of the focal position with respect to the temperature, and it is possible to maintain the image quality.

  According to the present invention, even if the fθ lens and the coupling lens in the scanning optical system are made of an optical transparent resin, the focus shift can be within an allowable range even if there is a change in environmental temperature. It can be manufactured at low cost and so as not to deteriorate the image quality even when the environmental temperature changes.

1 Light source (semiconductor laser)
2 Collimator lens (coupling lens)
3 Aperture (aperture stop)
31 Aperture 4 Cylindrical lens 5 Polygon mirror 6, 7 fθ lens 8 Reflecting mirror 9 Photosensitive member (scanned surface)
10 housing a distance L ₂ coupling lens holder and the incident surface of the linear expansion coefficient L ₁ coupling lens in linear expansion coefficient alpha ₂ coupling lens holder in the scanning direction alpha ₁ housing 20 and a light source constituting the light source device The distance from the fixed point to the incident surface of the coupling lens f _{fθh The} focal length in the main scanning direction of the fθ lens f _col The focal length of the coupling lens Δf _fθ The focal length variation in the main scanning direction of the fθ lens per unit temperature

Claims (3)

A light source device, a coupling lens that converts the diffused light from the light source device into a parallel light beam, and deflects light from the light source device that has been converted into a parallel light beam by the coupling lens so that the surface to be scanned is in the main scanning direction. Image forming comprising: scanning optical means for scanning; and an fθ lens for condensing the light deflected by the scanning optical means as a light spot on the surface to be scanned so that the scanning speed on the surface to be scanned is constant. In the scanning optical system in the apparatus,
A coupling lens holder for holding the coupling lens is fixed in a focused state so as to be movable in the direction of the optical axis, and a housing made of a composite material made of resin and glass fiber holding the light source device. Having an fθ lens and a coupling lens in the scanning optical system made of an optical transparent resin,
The linear expansion coefficient of the housing is α ₁ , the linear expansion coefficient of the coupling lens holder is α ₂ , the distance between the incident surface of the coupling lens and the light source constituting the light source device is L ₁ , and the coupling lens. When the distance from the fixing point to the housing of the holder to the incident surface of the coupling lens is L ₂ , the focal length in the main scanning direction of the fθ lens is f _fθ , and the focal length of the coupling lens is f _col , (1) A scanning optical system in an image forming apparatus, wherein the optical system is configured to satisfy the expression (1).

Here, L is the depth of focus, a, b and c are constants, t ₀ is the design temperature, t is the ambient temperature, R ₁ is the curvature radius on the incident side in the main scanning direction of the scanning optical system at the ambient temperature t ₀ , and R _2. Is the exit side radius of curvature in the main scanning direction of the scanning optical system at the environmental temperature t ₀ , and S is the thickness of the fθ lens.   2. The scanning optical system in an image forming apparatus according to claim 1, wherein the casing is made of a composite material made of polycarbonate, glass fiber, and ABS resin. 3. The scanning optical system in an image forming apparatus according to claim 1, wherein the design temperature t ₀ is normal temperature, and the environmental temperature t is 10 ° C. or more and 50 ° C. or less.

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