JP5135482B2 - Optical scanning device and image forming apparatus - Google Patents

Description

  The present invention relates to an optical scanning apparatus that scans light emitted from a light source in one linear direction (main scanning direction) while forming an image on a surface to be scanned such as a surface of a photosensitive drum, and an image forming apparatus including the same. Is.

An electrophotographic image forming apparatus includes an optical scanning device that forms an image through an fθ lens while scanning light for writing an electrostatic latent image on a surface to be scanned (photosensitive drum surface) by an optical scanning unit such as a polygon mirror. ing. In such an optical scanning device, it is important to satisfy the following three optical characteristics in order to perform highly accurate image formation (writing of an electrostatic latent image).
That is, the first optical characteristic is that the curvature of field on the surface to be scanned is small (spot diameters are uniform). The second optical characteristic is that the scanning speed of light in the main scanning direction on the surface to be scanned is constant (the fθ characteristic is good). The third optical characteristic is that the magnification (sub scanning magnification) in the sub scanning direction orthogonal to the main scanning direction is small. This is because if the sub-scanning magnification is small, the spot diameter in the sub-scanning direction is uniform over the entire effective scanning range even if there is some variation in the angle of the light reflecting surface of the light scanning unit. This third optical characteristic is particularly important in making the pitch interval of the scanning lines constant in a multi-beam scanning device that simultaneously scans a plurality of light beams.
Furthermore, in order to meet the space saving needs of the image forming apparatus, it is desired to make the optical scanning device compact.
Conventionally, for example, Patent Document 1 discloses an fθ lens system composed of a combination of a glass toric lens having a cylindrical lens surface on the light incident surface side and a toric surface on the light exit surface, and a plastic toric lens.
In Patent Document 2, an optical system including a holographic fθ lens having power only in the main scanning direction and a cylindrical lens having power in the sub-scanning direction is used to correct fθ characteristics, surface tilt correction, and first-order diffracted light. A technique for separating the diffracted light of other orders is shown.

JP-A-7-318796 JP-A-3-125111

However, the fθ lens system disclosed in Patent Document 1 has a problem that since one surface is a cylindrical surface, the degree of freedom in aberration correction is small and the field curvature is large. Further, in the fθ lens system shown in Embodiment 1 of Patent Document 1, the power in the main scanning direction of the glass toric lens arranged on the surface to be scanned is the main scanning direction of the plastic toric lens arranged on the polygon mirror side. Therefore, there is a problem that the size of the glass toric lens in the main scanning direction needs to be relatively large, which is contrary to downsizing of the apparatus. Furthermore, the fθ lens system shown in Embodiment 2 of Patent Document 1 is a device in which the power in the sub-scanning direction of the plastic toric lens and the power in the sub-scanning direction of the glass toric lens are both positive. If both lenses are brought closer to the polygon mirror side for compactness, the problem arises that the sub-scanning magnification increases. When the sub-scanning magnification is large, the variation in the spot diameter of the light on the surface to be scanned with respect to the variation in the reflection surface of the optical scanning unit (polygon mirror or the like) increases. Note that Patent Document 1 does not specifically describe equalization of the sub-scanning magnification.
Further, in the optical system disclosed in Patent Document 2, for example, when the material of the cylindrical lens is glass, it is more cost-effective than a plastic lens, although it is more cost-effective than an fθ lens in which a plurality of lenses are combined. There was a problem that there was. In addition, in the optical system disclosed in Patent Document 2, for example, when the material of the cylindrical lens is plastic, a focus shift in the sub-scanning direction occurs due to a change in the oscillation wavelength of the light beam or an environmental change (particularly a temperature change). was there. In addition, if the cylindrical lens is brought close to the surface to be scanned in order to avoid the occurrence of the focus shift, it is necessary to increase the size of the cylindrical lens in the main scanning direction. The problem that it becomes high will arise.
Accordingly, the present invention has been made in view of the above circumstances, and an object of the present invention is to achieve variations in field curvature, sub-scanning magnification, and optical scanning speed in the main scanning direction on the surface to be scanned by a compact device configuration. An optical scanning device having a small focus movement amount in the sub-scanning direction on the surface to be scanned with respect to changes in the oscillation wavelength of the light beam and environmental changes, and an image forming apparatus including the same.

In order to achieve the above object, an optical scanning device according to the present invention comprises the components shown in the following (1) to (4).
(1) Optical scanning means for scanning a light beam emitted from a light source in a main scanning direction which is a linear direction toward a predetermined scanning surface (such as the surface of an image carrier) while reflecting on a reflecting surface (for example, Polygon mirror, etc.).
(2) The scanning light disposed between the optical scanning unit and the scanned surface and scanned by the optical scanning unit forms an image on the scanned surface, and the scanning light on the scanned surface Imaging means for making the scanning speed of the lens substantially constant.
(3) A first lens that is provided in the imaging unit and has a positive power in the main scanning direction and a negative power in the sub-scanning direction orthogonal to the main scanning direction.
(4) The imaging means is provided and is disposed closer to the surface to be scanned than the first lens and has a negative power in the main scanning direction and a positive power in the sub-scanning direction. Second lens.
More specifically, in the optical scanning device according to the present invention, the power in the main scanning direction of the imaging means that combines the first lens and the second lens is φ _tm , When the power in the main scanning direction is φ _1m and the power of the second lens in the main scanning direction is φ _2m , at least one of the following conditions (a1) or (a2) is satisfied: It is desirable.
-1.20 ≦ φ _2m / φ _tm ≦ −0.48 (a1)
1.32 ≦ φ _1m / φ _tm ≦ 2.00 (a2)
Further, in the optical scanning device according to the present invention, when the power of the first lens in the sub-scanning direction is φ _1s and the power of the second lens in the sub-scanning direction is φ _2s , the following ( It is desirable that the condition of the formula b1) is satisfied.
-2.98 ≦ φ _1s / φ _2s ≦ −0.81 (b1)
The first lens and the second lens are preferably resin lenses because of their high workability (productivity).

In the optical scanning device according to the present invention, the image forming means including the first lens and the second lens constitutes an fθ lens system.
In general, in an fθ lens system composed of two lenses, the second lens on the downstream side in the light traveling direction (the side closer to the scanned surface) is brought closer to the scanned surface, that is, the width of the light beam in the sub-scanning direction is reduced. By disposing the second lens in a wider position, the sub-scanning magnification of the scanning light on the surface to be scanned can be reduced. However, it is necessary to increase the size of the second lens in the main scanning direction. This is contrary to the downsizing of the device.
In contrast, according to the present invention, in the imaging means, the width of the scanning light in the sub-scanning direction after passing through the first lens by the first lens having a negative power in the sub-scanning direction is conventionally ( The power of the first lens is wider than positive). For this reason, even if the second lens is arranged at a position far from the surface to be scanned (position near the reflecting surface of the optical scanning means), the sub-scanning magnification of the scanning light on the surface to be scanned can be reduced. it can. Further, since the light scanning range is relatively narrow at a position close to the light scanning means, the dimension in the main scanning direction of the second lens disposed at that position may be small. Therefore, according to the present invention, the field curvature in the sub-scanning direction on the surface to be scanned can be reduced while reducing the size of the apparatus.
When the positions of the first and second lenses are moved away from the surface to be scanned, the lens shape needs to be a rotationally symmetric aspherical surface in order to focus (converge) the scanning light on the surface to be scanned. However, by using resin lenses as the first lens and the second lens, the second lens can be manufactured (processed) relatively easily. Further, since the power in the sub-scanning direction of the first lens is negative, the size (height) of the second lens in the sub-scanning direction is slightly larger, but the main lens of the second lens is more than that. The fact that the size (width) in the scanning direction can be reduced greatly contributes to the downsizing of the apparatus.

In the image forming means, the condensing of the scanning light in the main scanning direction is performed only by the first lens, and therefore the curvature and thickness of the first lens are increased. However, even when the curvature and thickness of the first lens having a small size (width) in the main scanning direction are increased, a part of the condensed light is borne by the second lens having a large size in the main scanning direction (the first lens). The entire apparatus can be made more compact than making the power of the second lens in the main scanning direction positive.
In addition, since the second lens that mainly adjusts the focal length of the scanning light is a thin lens with a small curvature (power is negative), it is easy to suppress the curvature of field in the main scanning direction on the scanned surface. .
For example, if the power in the main scanning direction of each of the first lens and the second lens satisfies the condition of the above-described formula (a1) or (a2), the image plane in the main scanning direction on the scanned surface Variations in curvature and scanning speed of scanning light can be suppressed to such a level that the image quality when the electrostatic latent image is written by the scanning light becomes a sufficient level.
Similarly, if the power in the sub-scanning direction of each of the first lens and the second lens satisfies the condition of the above-described equation (b1), the field curvature in the sub-scanning direction on the surface to be scanned is It is possible to keep the image quality small enough to achieve a sufficient level when the electrostatic latent image is written by the scanning light.
The optical scanning device according to the present invention further includes a collimator lens, an aperture, and a cylindrical lens arranged in order from the light source side in the path of the light beam between the light source and the optical scanning unit, and further, the light beam It is even more preferable if a diffractive optical element is provided in the optical path from the light passing through the collimator lens to the light scanning means. For example, it is conceivable that the diffractive optical element is provided on the surface of the cylindrical lens.
As a result, when fluctuations in the oscillation wavelength of the light emitted from the light source or fluctuations in the environmental temperature occur, the diffraction is performed in a direction that cancels out the focus movement (focus shift) of the scanning light on the surface to be scanned due to the fluctuations. The focal length of the cylindrical lens can be changed by the optical element. In particular, when the first lens and the second lens are resin lenses, the diffractive optical element has a relatively large focus shift of scanning light due to environmental temperature fluctuations compared to a glass lens. A remarkable focus correction effect can be obtained.
The present invention is also regarded as an image forming apparatus that scans the surface of an image carrier (typically a photosensitive drum) with light for writing an electrostatic latent image by the optical scanning apparatus according to the present invention described above. You can also.

  According to the present invention, in the two lenses constituting the fθ lens system, the lens on the surface to be scanned (the second lens) can be arranged on the upstream side in the traveling direction of the scanning light. Since the positive power for increasing the curvature and thickness of the first lens is borne only by the first lens having a small size in the main scanning direction, the entire apparatus can be made compact. Furthermore, when the powers of the two lenses constituting the fθ lens system satisfy the conditions of the above-described formulas, variations in field curvature, sub-scanning magnification, and optical scanning speed in the main scanning direction on the surface to be scanned can be reduced. In addition, the action of the diffractive optical element can reduce the amount of focus movement (focus shift) in the sub-scanning direction on the surface to be scanned with respect to changes in the oscillation wavelength of the light beam and environmental changes. As a result, when an electrostatic latent image is written with light scanned by the optical scanning device according to the present invention, high-precision image formation can be performed.

1 is a schematic configuration diagram of a main part (image forming unit) of an image forming apparatus X including an optical scanning device Y according to an embodiment of the present invention. Schematic of the optical scanning device Y and the optical path of beam light seen from the sub-scanning direction. FIG. 3 is a schematic diagram of a cross section of an optical scanning device Y and an optical path of scanning light viewed from the main scanning direction. The figure which shows an example of the implementation conditions of the optical scanning device Y. The graph showing the experimental result which evaluated the influence which it has on the linearity of the change of the 1st power condition of the main scanning direction of the scanning light lens in the optical scanning device Y on the scanning light. The graph showing the experimental result which evaluated the influence which the 1st power condition of the main scanning direction of the scanning light lens in the optical scanning device Y has on the quadrant curvature of the main scanning direction. The graph showing the experimental result which evaluated the influence which it has on the change of the 1st power condition of the main scanning direction of the scanning light lens in the optical scanning apparatus Y on the quadrant curvature of a subscanning direction. The graph showing the experimental result which evaluated the influence which it has on the linearity of the change of the 2nd power condition of the scanning light lens in the optical scanning device Y in the main scanning direction. The graph showing the experimental result which evaluated the influence which the 2nd power condition of the main scanning direction of the scanning light lens in the optical scanning device Y has on the quadrant curvature of the main scanning direction. The graph showing the experimental result which evaluated the influence which it has on the change of the 2nd power condition of the main scanning direction of the scanning light lens in the optical scanning apparatus Y on the quadrant curvature of a subscanning direction. 6 is a graph showing experimental results for evaluating the influence of scanning light lens on the linearity of scanning light in the optical scanning device Y in the sub-scanning direction. The graph showing the experimental result which evaluated the influence which the power condition of the scanning light lens of the optical scanning device Y changes in the subscanning direction in the main scanning direction. Change of power condition in the sub-scanning direction of the scanning light lens in the optical scanning device Y is a graph showing the experimental results of evaluating the influence on the quadrant curvature in the sub-scanning direction The spot diagram of the scanning light by the optical scanning device Y. The spot diagram (1) of the scanning light when the first power condition in the main scanning direction of the scanning light lens in the optical scanning device Y is changed. The spot diagram (2) of the scanning light when the first power condition in the main scanning direction of the scanning light lens in the optical scanning device Y is changed. The spot diagram (3) of the scanning light when the first power condition in the main scanning direction of the scanning light lens in the optical scanning device Y is changed. FIG. 6 is a spot diagram (4) of scanning light when the first power condition in the main scanning direction of the scanning light lens in the optical scanning device Y is changed. The spot diagram (1) of the scanning light when the second power condition in the main scanning direction of the scanning light lens in the optical scanning device Y is changed. The spot diagram (2) of the scanning light when the second power condition in the main scanning direction of the scanning light lens in the optical scanning device Y is changed. The spot diagram (3) of the scanning light when the second power condition in the main scanning direction of the scanning light lens in the optical scanning device Y is changed. The spot diagram (4) of the scanning light when the second power condition in the main scanning direction of the scanning light lens in the optical scanning device Y is changed. The spot diagram (1) of the scanning light when the power condition in the sub-scanning direction of the scanning light lens in the optical scanning device Y is changed. The spot diagram (2) of the scanning light when the power condition in the sub-scanning direction of the scanning light lens in the optical scanning device Y is changed. The spot diagram (3) of the scanning light when the power condition in the sub-scanning direction of the scanning light lens in the optical scanning device Y is changed. The spot diagram (4) of the scanning light when the power condition in the sub-scanning direction of the scanning light lens in the optical scanning device Y is changed.

Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings so that the present invention can be understood. The following embodiment is an example embodying the present invention, and does not limit the technical scope of the present invention.
FIG. 1 is a schematic configuration diagram of a main part (image forming unit) of an image forming apparatus X including an optical scanning device Y according to an embodiment of the present invention. FIG. 2 is an optical scanning device Y viewed from the sub-scanning direction. 3 is a schematic view of the optical path of the beam light, FIG. 3 is a schematic view of the cross section of the optical scanning device Y and the optical path of the scanning light viewed from the main scanning direction, and FIG. 5 is a graph showing experimental results for evaluating the influence of the change of the first power condition in the main scanning direction of the scanning light lens in the optical scanning device Y on the linearity of the scanning light, and FIG. Change in first power condition in main scanning direction Graph showing experimental results for evaluating influence on quadrant curvature in main scanning direction, FIG. 7 shows first power in the main scanning direction of the scanning light lens in optical scanning device Y Changing the conditions The effect on the quadrant curvature in the sub-scanning direction FIG. 8 is a graph showing experimental results for evaluating the influence of the second power condition in the main scanning direction of the scanning light lens in the optical scanning device Y on the linearity of the changed scanning light, and FIG. FIG. 10 is a graph showing experimental results of evaluating the influence of the scanning light lens on the quadrant curvature in the main scanning direction of the scanning light lens in the optical scanning device Y. FIG. 10 shows the scanning light lens in the optical scanning device Y. FIG. 11 is a graph showing experimental results of evaluating the influence of the second power condition in the main scanning direction on the ellipsoidal curvature in the sub-scanning direction. FIG. 11 shows the power condition in the sub-scanning direction of the scanning light lens in the optical scanning device Y. FIG. 12 is a graph showing an experimental result of evaluating the influence of the changed scanning light on the linearity, and FIG. 12 shows the power condition in the sub-scanning direction of the scanning light lens in the optical scanning device Y. FIG. 13 is a graph showing an experimental result of evaluating the influence on the music, and FIG. 13 shows an experimental result of evaluating the influence of the scanning light lens in the optical scanning device Y on the quadrant curvature in the sub-scanning direction. 14 is a spot diagram of scanning light by the optical scanning device Y, and FIGS. 15 to 18 are spot diagrams of scanning light when the first power condition in the main scanning direction of the scanning light lens in the optical scanning device Y is changed. 19 to 22 are spot diagrams of scanning light when the second power condition in the main scanning direction of the scanning light lens in the optical scanning device Y is changed, and FIGS. 23 to 26 are scanning light lenses in the optical scanning device Y. 5 is a spot diagram of scanning light when the power condition in the sub-scanning direction is changed.

First, the configuration of the main part of the image forming apparatus X including the optical scanning device Y according to the embodiment of the present invention will be described with reference to the schematic configuration diagram shown in FIG.
The image forming apparatus X is an electrophotographic image forming apparatus such as a copying machine, a printer, or a facsimile machine that forms an image using toner.
The image forming apparatus X includes an image forming unit (a portion shown in FIG. 1) that forms a toner image and forms an image on a recording paper, a paper feeding unit (not shown) that supplies the recording paper to the image forming unit, and A paper discharge section (not shown) for discharging recording paper on which image formation has been performed is provided.
As shown in FIG. 1, the image forming apparatus X includes a photosensitive drum 1 (image carrier) that carries a toner image, a charging device 3 that uniformly charges the surface of the photosensitive drum 1, and the photosensitive drum 1. Optical scanning device Y that scans the surface with beam light and writes an electrostatic latent image by exposing it to the scanning light, developing device 2 that develops a toner image by supplying toner to the electrostatic latent image, and the toner A transfer roller 4 for transferring the image onto the recording paper, and a charge removal device 5 for removing the charge on the surface of the photosensitive drum 1 after transferring the toner image onto the recording paper are schematically configured.
The charging device 3 uniformly charges the surface of the photosensitive drum 1 along its axial direction.
The developing device 2 includes a developing roller that supplies toner to the photosensitive drum 1 and visualizes the electrostatic latent image on the photosensitive drum 1 with toner. In accordance with the potential gap between the potential applied to the developing roller (developing bias potential) and the potential on the surface of the photosensitive drum 1, the toner on the developing roller is drawn onto the surface of the photosensitive drum 1, and The electrostatic latent image is visualized as a toner image.
The image forming apparatus X includes other well-known constituent elements included in a general electrophotographic image forming apparatus, but the description thereof is omitted here.

Next, the configuration of the optical scanning device Y according to the embodiment of the present invention will be described with reference to the plan view shown in FIG. 2 (viewed from the sub-scanning direction) and the sectional view shown in FIG.
As shown in FIGS. 2 and 3, the optical scanning device Y includes a light source 10, an incident light adjusting optical device 20, a polygon mirror 30, and an imaging optical device 40 (fθ lens system).
The light source 10 is a light source such as a semiconductor laser that emits beam light (light beam) for writing an electrostatic latent image.
The incident light adjusting optical device 20 includes a collimator lens 21, an aperture 22, and a cylindrical lens 23 arranged in order from the light source 10 side in the path of the light beam (light beam) between the light source 10 and the polygon mirror 30. These optical devices are used to shape the emitted light (beam light) from the light source 10 before entering the polygon mirror 30.
The beam light emitted from the light source 10 is converted into parallel light by passing through the collimator lens 21 and then shaped by passing through the aperture 22, and the light diameter is adjusted by the cylindrical lens 23. Later, the polygon mirror 30 is reached.
A diffractive optical element 24 is provided on the surface of the cylindrical lens 23. The diffractive optical element 24 may be disposed at another position as long as the light beam (light beam) passes from the collimator lens 21 to the polygon mirror 30. The operation of the diffractive optical element 24 will be described later.

The polygon mirror 30 is a rotating body having a plurality of reflecting surfaces 30a, and reflects the beam light (light beam) for writing an electrostatic latent image emitted from the light source 10 on the surface to be scanned while reflecting the reflecting surface 30a. Scanning is performed in the main scanning direction (direction parallel to the rotation axis 1g of the photosensitive drum 1) which is one linear direction toward the surface of a certain photosensitive drum 1 (an example of the optical scanning unit). In addition to the polygon mirror 30, a MEMS mirror or the like may be employed as the beam light scanning means.
The imaging optical device 40 is disposed between the polygon mirror 30 and the surface of the photosensitive drum 1, and beam light (hereinafter referred to as scanning light) scanned by the polygon mirror 30 is used as the surface of the photosensitive drum 1. Is a lens system that functions as a so-called fθ lens that adjusts the scanning speed of the scanning light on the surface of the photosensitive drum 1 to be substantially constant (irradiates with a desired spot diameter). An example of image means). As shown in FIGS. 2 and 3, the imaging optical device 40 includes a first scanning light lens 41 and a second scanning light lens 42 arranged in order from the upstream side in the traveling direction of the scanning light. .
FIG. 3 is a cross-sectional view at the center position of the light scanning range in the main scanning direction (the center position of the first scanning light lens 41 and the second scanning light lens 42 in the main scanning direction). The optical path of the scanning light shown in FIG. 2 is an optical path when the scanning light from the polygon mirror 30 is at the center position of the main scanning range (all scanning ranges in the main scanning direction).

The first scanning light lens 41 is a lens that is disposed at a position closer to the polygon mirror 30 than the second scanning light lens 42 and is formed to extend in the main scanning direction so as to allow the scanning light to pass therethrough. . The first scanning light lens 41 has a positive power in the main scanning direction and a direction orthogonal to the main scanning direction (a direction perpendicular to the rotation shaft 1g of the photosensitive drum 1, hereinafter referred to as a sub-scanning direction). And a lens having negative power (corresponding to the first lens).
The second scanning light lens 42 is a lens which is disposed on the surface of the photosensitive drum 1 with respect to the first scanning light lens 41 and is formed to extend in the main scanning direction so as to allow the scanning light to pass therethrough. is there. The second scanning light lens 42 has a wider scanning range of scanning light than the first scanning light lens 41 because the position of the second scanning light lens 42 is farther from the polygon mirror 30 than the first scanning light lens 41. The dimension (width) in the main scanning direction is large. The second scanning light lens 42 is a lens having negative power in the main scanning direction and positive power in the sub-scanning direction (corresponding to the second lens).
Here, the first scanning light lens 41 and the second scanning light lens 42 are closer to the reflective surface 30a of the polygon mirror 30 than the intermediate position between the reflective surface 30a of the polygon mirror 30 and the surface of the photosensitive drum 1. Arranged on the side. That is, the distance from the reflecting surface 30a of the polygon mirror 30 to the surface of the photosensitive drum 1 is L, and the distance from the reflecting surface 30a of the polygon mirror 30 to the surface 42b of the second scanning light lens 42 on the photosensitive drum 1 side. When the distance is d, (d <L / 2).

（ｃ１）式において，ｘは走査光（ビーム光）の光軸方向のサグ量，ｙは主走査方向の走査範囲の中心線（二等分線）を基準とする主走査方向の位置，ｚは副走査方向の位置，ｒ_mは走査光（ビーム光）の主走査方向の走査範囲の中心線における主走査方向の断面の曲率，ｒ_s0は走査光（ビーム光）の主走査方向の走査範囲の中心位置における副走査方向の断面の曲率，Ｋ_mはコーニック係数，Ａ３，Ａ４，Ａ６，Ａ８，Ａ１０，Ｂ１〜Ｂ４は面ごとに適宜設定される係数（非球面係数）である。
なお，前記第１の走査光レンズ４１の副走査方向のパワーが負であるため，前記第２の走査光レンズ４２の副走査方向の寸法（高さ）が若干大きくなるが，それよりも前記第２の走査光レンズ４２の主走査方向の寸法（幅）を小さくできることの方が，装置のコンパクト化への寄与が大きい。 In the imaging optical device 40 of the optical scanning device Y, as shown in FIG. 3, the first scanning light lens 41 having a negative power in the sub-scanning direction passes through the first scanning light lens 41. The width of the subsequent scanning light in the sub-scanning direction is widened. For this reason, in the optical scanning device Y, even if the second scanning light lens 42 is disposed at a position relatively far from the surface of the photosensitive drum 1 (a position close to the polygon mirror 30), the surface of the photosensitive drum 1 It is possible to reduce the sub-scanning magnification (magnification in the sub-scanning direction) of the scanning light. Further, at a position close to the polygon mirror 30 (d <L / 2), the light scanning range is relatively narrow, and therefore the size of the second scanning light lens 42 disposed at that position is small. I'll do it. Therefore, the optical scanning device Y is compact and has a small field curvature in the sub-scanning direction on the surface of the photosensitive drum 1.
Here, when the positions of the first scanning light lens 41 and the second scanning light lens 42 are moved away from the surface of the photosensitive drum 1, they are formed in order to form an image on (converge) the scanning light on the surface of the photosensitive drum 1. The shape of the lenses 41 and 42 needs to be a rotationally symmetric aspherical surface. On the other hand, by using resin lenses as the first and second scanning light lenses 41 and 42, both lenses 41 and 42 can be manufactured (processed) relatively easily.
Each surface of the first scanning light lens 41 (the surface 41a on the scanning light incident side and the surface 41b on the same exit side) and each surface of the second scanning light lens 42 (the surface 42a on the scanning light incident side and the same exit surface). The cross-sectional shape (rotationally symmetric aspherical surface) of the side surface 42b) in the sub-scanning direction can be expressed by the following equation (c1).

In the equation (c1), x is a sag amount in the optical axis direction of the scanning light (beam light), y is a position in the main scanning direction with reference to the center line (bisector) of the scanning range in the main scanning direction, z Is the position in the sub-scanning direction, r _m is the curvature of the cross section in the main scanning direction at the center line of the scanning range of the scanning light (beam light), and r _s0 is the scanning in the main scanning direction of the scanning light (beam light). The curvature of the cross section in the sub-scanning direction at the center position of the range, K _m is a conic coefficient, and A 3, A 4, A 6, A 8, A 10, B 1 to B 4 are coefficients (aspherical coefficients) that are appropriately set for each surface.
Since the power in the sub-scanning direction of the first scanning light lens 41 is negative, the dimension (height) in the sub-scanning direction of the second scanning light lens 42 is slightly larger, but the size is higher than that. The fact that the size (width) of the second scanning light lens 42 in the main scanning direction can be reduced greatly contributes to the compactness of the apparatus.

Further, in the imaging optical device 40, the condensing of the scanning light in the main scanning direction is performed only by the first scanning light lens 41, and for this purpose, the curvature and thickness of the first scanning light lens 41 are reduced. Becomes larger. However, even if the curvature and thickness of the first scanning light lens 41 having a small size (width) in the main scanning direction are increased, the second scanning light lens having a large size in the main scanning direction is partially collected. The entire apparatus can be made more compact than if it is carried by 42 (the power in the main scanning direction of the second scanning light lens 42 is made positive).
Further, since the second scanning light lens 42 that mainly adjusts the focal length of the scanning light is a thin lens having a small curvature (power is negative), the curvature of field in the main scanning direction on the surface of the photosensitive drum 1. Is easy to keep small.
FIG. 4 shows an example of implementation conditions for the optical scanning device Y.
In the conditions shown in FIG. 4, the surface numbers “1” to “4” of the optical scanning light lens are the surface 41a on the scanning light incident side, the surface 41b on the same emission side of the first scanning light lens 41, respectively. It is an identification number that indicates the scanning light incident side surface 42 a and the outgoing side surface 42 b of the second scanning light lens 42. Further, the aspheric coefficient and the conic coefficient are coefficients applied to the equation (c1).

Next, appropriate conditions for the two scanning light lenses 41 and 42 will be described with reference to FIGS.
FIG. 14 shows a spot diagram of the scanning light in the optical scanning device Y that satisfies the implementation condition shown in FIG. 4 (variation of the spot center position on the surface of the photosensitive drum 1).
FIGS. 5 to 7 and FIGS. 15 to 18 show conditions (hereinafter referred to as the first main condition), which will be described later, regarding the power in the main scanning direction of the two scanning light lenses 41 and 42 based on the implementation conditions shown in FIG. (Scanning direction power condition) is changed, linearity of the scanning light, quadrature curvature in the main scanning direction (main scanning elephant curvature), field curvature in the sub-scanning direction (sub-scanning field curvature), spot center position It is a graph showing dispersion | variation.
FIGS. 8 to 10 and FIGS. 19 to 22 show the conditions (hereinafter referred to as the second main condition) described later regarding the power in the main scanning direction of the two scanning light lenses 41 and 42 based on the implementation condition shown in FIG. (Scanning direction power condition) is changed, linearity of the scanning light, quadrature curvature in the main scanning direction (main scanning elephant curvature), field curvature in the sub-scanning direction (sub-scanning field curvature), spot center position It is a graph showing dispersion | variation.
FIGS. 11 to 13 and FIGS. 23 to 26 show conditions (hereinafter referred to as sub-scanning direction powers), which will be described later, regarding the power in the sub-scanning direction of the two scanning light lenses 41 and 42 on the basis of the implementation conditions shown in FIG. Scanning light linearity (relative deviation from the ideal image height), main scanning direction ellipsoidal curvature (main scanning ellipsoidal curvature), subscanning direction field curvature (subscanning image plane) It is a graph showing the variation of the curvature and the spot center position. The linearity of the scanning light is based on a state (linear state) in which the scanning light is scanned in the main scanning direction at a constant target speed on the surface of the photosensitive drum 1, and the scanning position (in the reference state) This represents the amount of deviation of the actual scanning position with respect to (imaginary height).
5 to 13, the horizontal axis represents the image height (position in the main scanning direction with reference to the center of the main scanning range). 5 to 26, the range indicated as “OK range” is visually observed even when the geometric optical aberration of the beam light (scanning light) for writing the latent image is shifted from the ideal state within the range. This is a range (allowable range) that does not lead to deterioration of image quality, which is a problem. 14 to 26, (a) “on the axis” means the center position of the main scanning range, and (b) “periphery” means the scanning start position (or scanning end) in the main scanning range. It is a position in the vicinity of (position).
Hereinafter, the power in the main scanning direction of the imaging optical device 40 including the first scanning light lens 41 and the second scanning light lens 42 is φ _tm , and the main scanning direction of the first scanning light lens 41 is the same. the power in the phi _{1 m,} the power of the phi _2m in the main scanning direction of the second scanning lens 42, the power of the phi _1s in the sub-scanning direction of the first scanning lens 41, the second scanning lens 42 _{Is 2 s} in the sub-scanning direction.

Hereinafter, an appropriate range of the first main scanning direction power condition will be described.
The first power condition in the main scanning direction is a condition on how φ _2m / φ _tm is set.
More specifically, the first main scanning direction power condition d1M in the implementation condition shown in FIG. 4 is φ _2m / φ _tm = −0.75. Further, in FIGS. 5 to 7 and FIGS. 15 to 18, when (φ _2m / φ _tm ) is −1.22 as other first main scanning direction power conditions (condition d1L ′ ), −1.20 (condition d1L), −0.48 (condition d1H), and −0.50 (condition d1H ′).
As can be seen from the graph shown in FIG. 6, when the power condition in the first main scanning direction is (φ _2m / φ _tm <−1.20) or (φ _2m / φ _tm > −0.48) (condition d1L (Or see the graph of condition d1H)), the quadrant curvature in the main scanning direction of the scanning light on the surface of the photosensitive drum 1 becomes larger than the appropriate range (OK range).
As can be seen from FIGS. 17 and 18, when the first main scanning direction power condition is (φ _2m / φ _tm <−1.20) or (φ _2m / φ _tm > −0.48), The variation of the spot position of the scanning light on the surface of the photosensitive drum 1 becomes larger than the appropriate range (OK range).
On the other hand, as can be seen from the graphs shown in FIG. 5 to FIG. 7 and FIG. 14 to FIG. 16, if the power condition in the first main scanning direction satisfies the following expression (a1), Linearity, ellipsoidal curvature in the main scanning direction, field curvature in the sub-scanning direction, and spot position variations are all within the appropriate range.
-1.20 ≦ φ _2m / φ _tm ≦ −0.48 (a1)

Next, the appropriate range of the second power condition in the main scanning direction will be described.
The power condition in the second main scanning direction is a condition on how φ _1m / φ _tm is set.
More specifically, the second main scanning direction power condition d2M in the implementation condition shown in FIG. 4 is φ _1m / φ _tm = 1.65. Further, in FIGS. 8 to 10 and FIGS. 19 to 22, when (φ _1m / φ _tm ) is 1.30 as other second main scanning direction power conditions (condition d2L ′). , 1.32 (condition d2L), 2.00 (condition d2H), and 2.02 (condition d2H ′).
As can be seen from the graph shown in FIG. 8, when the power condition in the second main scanning direction is (φ _1m / φ _tm > 2.00) (see the graph of condition d2H ′), the scanning on the surface of the photosensitive drum 1 is performed. The linearity of light deteriorates beyond the appropriate range (OK range).
As can be seen from the graph shown in FIG. 9, when the power condition in the second main scanning direction is (φ _1m / φ _tm <1.32) (see the graph of condition d2L ′), the surface of the photosensitive drum 1 The ellipsoidal curvature of the main scanning light in the main scanning direction becomes larger than the appropriate range (OK range).
Further, as can be seen from FIG. 21, when the power condition in the second main scanning direction is (φ _1m / φ _tm <1.32) (see the graph of the condition d2L ′), the scanning light on the surface of the photosensitive drum 1 is obtained. The spot position variation increases beyond the appropriate range (OK range).
On the other hand, as can be seen from the graphs shown in FIGS. 8 to 10, 14 and 19 to 22, if the power condition in the second main scanning direction satisfies the following expression (a2), The linearity of the scanning light, the ellipsoidal curvature in the main scanning direction, the field curvature in the subscanning direction, and the spot position variation are all within the appropriate range.
1.32 ≦ φ _1m / φ _tm ≦ 2.00 (a2)

Next, the appropriate range of the power condition in the sub-scanning direction will be described.
The power condition in the sub-scanning direction is a condition on how φ _1s / φ _2s is set.
More specifically, the sub-scanning direction power condition d3M in the implementation condition shown in FIG. 4 is φ _1s / φ _2s = −2.52. Further, in FIGS. 11 to 13 and FIGS. 23 to 26, as the other power condition in the sub-scanning direction, (φ _1s / φ _2s ) is −3.00 (condition d3L ′), − The state is shown when 2.98 (condition d3L), -0.81 (condition d3H), and -0.81 (condition d3H ').
As can be seen from the graph shown in FIG. 13, when the power condition in the sub-scanning direction is (φ _1s / φ _2s <−2.98) or (φ _1s / φ _2s > −0.81) (condition d3L ′, condition d3H ′ (see the graph of d3H ′), the quadrant curvature in the sub-scanning direction of the main scanning light on the surface of the photosensitive drum 1 increases beyond the appropriate range (OK range).
Further, as can be seen from FIGS. 25 and 26, when the power condition in the sub-scanning direction is (φ _1s / φ _2s <−2.98) or (φ _1s / φ _2s > −0.81) (condition d3L ′ , See the graph of condition d3H ′), the variation in the spot position of the scanning light on the surface of the photosensitive drum 1 becomes larger than the appropriate range (OK range).
On the other hand, as can be seen from the graphs shown in FIGS. 11 to 13, 14 and 23 to 26, if the power condition in the sub-scanning direction satisfies the following expression (b1), the scanning light on the surface of the photosensitive drum 1 Linearity, ellipsoidal curvature in the main scanning direction, field curvature in the sub-scanning direction, and spot position variations are all within the appropriate range.
-2.98 ≦ φ _1s / φ _2s ≦ −0.81 (b1)
Note that the large variation in the spot center position in FIGS. 24 to 26 is caused by the variation in the aberration of the scanning light.

As described above, in the optical scanning device Y, the power in the main scanning direction of each of the first scanning light lens 41 and the second scanning light lens 42 is at least the above-described formula (a1) or (a2). If either condition of the equation is satisfied, the surface curvature (linearity) in the main scanning direction on the surface of the photosensitive drum 1 (scanned surface) and the scanning light scanning speed variation (linearity) are reduced by the scanning light. It is possible to keep the image quality small enough to achieve a sufficient level when writing.
Similarly, if the power in the sub-scanning direction of each of the first scanning light lens 41 and the second scanning light lens 42 satisfies the condition (b1), the sub-scanning direction on the surface of the photosensitive drum 1 is obtained. Can be suppressed to such a level that the image quality when the electrostatic latent image is written by the scanning light becomes a sufficient level.

Next, the operation of the diffractive optical element 24 will be described.
In the light source 10 such as a semiconductor laser, the oscillation wavelength of the beam light emitted varies depending on the use conditions. Further, the refractive index of the first scanning light lens 41 and the second scanning light lens 42 changes to an extent that the refractive index cannot be ignored in accordance with a change in environmental temperature. For this reason, fluctuations in the usage conditions of the optical scanning device Y cause the scanning light to move (focus out of focus) on the surface of the photosensitive drum 1.
On the other hand, the diffractive optical element 24 arranged in the optical path of the beam light between the collimator lens 21 and the polygon mirror 30 has a variation in the oscillation wavelength of the beam light emitted from the light source 10 and a variation in the environmental temperature. When this occurs, it is possible to change the focal length of the beam light that has passed through the cylindrical lens 23 in a direction that cancels the focus movement of the scanning light due to the fluctuation. In particular, since the first scanning light lens 41 and the second scanning light lens 42 are resin lenses, the refractive index changes due to environmental temperature fluctuations (as compared to the case where they are glass lenses) ( That is, since the scanning light focus movement amount) is relatively large, the diffractive optical element 24 provides a remarkable focus correction effect.

  The present invention is applicable to an optical scanning device and an image forming apparatus having the same.

X: Image forming apparatus according to an embodiment of the present invention Y: Optical scanning device 10: Light source 1: Photosensitive drum 2: Developing device 3: Charging device 5: Charge removing device 10: Light source 20: Incident light adjusting optical device 21: Collimator Lens 22: Aperture 23: Cylindrical lens 24: Diffraction optical element 30: Polygon mirror 40: Imaging optical device 41: First scanning light lens 42: Second scanning light lens

Claims (4)

An imaging optical system that forms an image of scanning light scanned in the main scanning direction on a surface to be scanned and substantially constants the scanning speed of the scanning light on the surface to be scanned,
A first lens having a positive power in the main scanning direction and a negative power in a sub-scanning direction orthogonal to the main scanning direction;
A second lens disposed closer to the scanned surface than the first lens and having a negative power in the main scanning direction and a positive power in the sub-scanning direction;
Comprising
When the power of the first lens in the sub-scanning direction is φ _1s and the power of the second lens in the sub-scanning direction is φ _2s , −2.98 ≦ φ _1s / φ _2s ≦ −2. An imaging optical system characterized by satisfying the condition of 52. When the power in the main scanning direction including the first lens and the second lens is φ _tm , and the power in the main scanning direction of the second lens is φ _{2 m} , −1.2 ≦ φ an imaging optical system according to claim 1 comprising qualify for _2m / _{φ tm} ≦ -0.48. When the power in the main scanning direction of the first lens and the second lens combined is φ _tm and the power in the main scanning direction of the first lens is φ _{1 m} , 1.32 ≦ φ _{1 m} The imaging optical system according to claim 1, wherein the condition of / φ _tm ≦ 2.00 is satisfied.   The imaging optical system according to claim 1, wherein the first lens and the second lens are resin lenses.

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