JP2003075751A - Tandem scanning optical system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a low-priced tandem scanning optical system having excellent image surface performance. SOLUTION: This scanning optical system is equipped with a scanning system which dividedly guides four laser beams (LY, LM, LC and LB) deflected by a polygon mirror (1) to four photoreceptors (IY, IM, IC and IB) and scans to form an image on the respective photoreceptors (IY, IM, IC and IB). The scanning system has 1st optical systems (11 and 12) arranged in common to the four laser beams (LY, LM, LC and LB) deflected on the same polygon surface (S) of the mirror (1), and four 2nd optical systems (21Y, 21M, 21C and 21B) arranged corresponding to the four laser beams (LY, LM, LC and LB) passing through the 1st optical systems (11 and 12). The 1st optical systems (11 and 12) have at least one free curved surface non-axially-symmetric in a main scanning direction, and the 2nd optical systems (21Y, 21M, 21C and 21B) have at least one free curved surface non-axially-symmetric in a subscanning direction.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a tandem scanning optical system, for example, a color laser printer,
The present invention relates to a tandem scanning optical system for exposing and recording an image on a plurality of scanned surfaces while scanning a plurality of laser beams in an image forming apparatus such as a color digital copying machine.

[0002]

2. Description of the Related Art Usually, when a color image is formed by laser scanning, four photoconductors (for example, Y: yellow,
(M: magenta, C: cyan, B: black photoconductors for each color) are required. A tandem scanning optical system for recording an image on four photoconductors is a tandem system in which four laser beams are incident on a polygon mirror at different angles in the sub-scanning direction, and then optical paths are separated in the sub-scanning direction. Scanning optics are known. For example, JP-A-11-6475
Japanese Patent Publication No. 4 has proposed a tandem scanning optical system of one polygon mirror and two fθ lenses, each fθ lens being divided into two optical systems to perform optical path separation between them. Further, in JP-A-8-122673, 1
There has been proposed a tandem scanning optical system of a type that includes one polygon mirror and one fθ lens and performs optical path separation behind the fθ lens.

[0003]

[Patent Document 1] Japanese Patent Application Laid-Open No. 11-6475
In the tandem scanning optical system proposed in Japanese Patent Publication No.
The cost increases because two fθ lenses are required. Even if the tandem scanning optical system is configured by one fθ lens by the extension of the technique, two types of optical systems after the optical path separation are required, which also increases the cost. Further, in this tandem scanning optical system, since the fθ lens is composed only of rotationally symmetric surfaces, when the incident angle in the sub-scanning direction becomes large, aberration cannot be sufficiently corrected, and high performance is obtained. There is also the problem that it cannot be secured.

In the tandem scanning optical system proposed in Japanese Patent Application Laid-Open No. 8-122673, the polygon mirror has one
The cost is low because there is one fθ lens and the fθ lens is composed of only one optical system before the optical path separation. However, if the fθ lens is composed of only one optical system before optical path separation, the sub-scanning magnification of the tandem scanning optical system becomes high. If the sub-scanning magnification is high, the polygon mirror malfunctions (for example, the polygon surface goes in and out due to processing errors,
The influence of (face tilt etc.) is magnified on the image plane, and it becomes difficult to maintain good image plane performance.

The present invention has been made in view of such circumstances, and an object thereof is to provide a low-cost tandem scanning optical system having good image plane performance.

[0006]

In order to achieve the above object, the tandem scanning optical system of the first invention comprises a plurality of light sources, and a single deflecting means for deflecting a light beam from each light source.
A tandem scanning optical system comprising: a scanning system that guides a plurality of light beams deflected by the deflecting means separately to a plurality of scanned surfaces corresponding to each light source, and performs an image scanning on each scanned surface, A first optical system in which the scanning system is commonly arranged for a plurality of light beams deflected on the same surface of the deflecting means;
A plurality of second optical systems arranged so as to correspond to a plurality of light beams that have passed through the first optical system, wherein the first optical system has at least one free-form surface that is non-axisymmetric in the main scanning direction. And the second optical system has at least one free-form surface that is non-axisymmetric in the sub-scanning direction.

In the tandem scanning optical system of the second invention, in the configuration of the first invention, a plurality of light beams from the light source are incident on the deflecting means at different angles in the sub-scanning direction. Characterize.

A tandem scanning optical system of a third invention is characterized in that, in the structure of the second invention, the incident positions of all the light beams on the deflecting means are substantially coincident with each other in the sub-scanning direction.

In the tandem scanning optical system of the fourth invention, in the structure of the first invention, the light beam passing position in the sub-scanning direction in the second optical system differs depending on the light beam incident angle in the sub-scanning direction. Characterize.

In the tandem scanning optical system of the fifth invention, in the structure of the fourth invention, the lens surface constituting the second optical system has one lens surface for different light beam incident angles in the sub-scanning direction. It is characterized by having a surface shape.

A tandem scanning optical system of a sixth invention is characterized in that, in the configuration of the first invention, the second optical system is axially symmetrical in the main scanning direction.

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS A tandem scanning optical system embodying the present invention will be described below with reference to the drawings. FIG. 1 is a sub-scan sectional view schematically showing an embodiment of a tandem scanning optical system, and a through system (a state in which an optical path is not folded) showing this is a main scanning sectional view and a diagram of FIG. 3 is a sub-scanning sectional view of FIG. In FIGS. 1 to 3, 1 is a polygon mirror, S is a polygon surface (that is, a deflecting / reflecting surface composed of a mirror surface), W is a polygon window, 2Y, 2M, 2C are optical path separating mirrors, 3
Y, 3M, 3C and 3B are folding mirrors, 4 is a light source, 5 is a collimator lens, 6 is a cylinder lens, 11 is a first lens, 12 is a second lens, 21Y, 21M, 21C and 21B are third lenses, IY , IM, IC, IB
Is a photoconductor that constitutes the surface to be scanned (Y: yellow, M: magenta, C: cyan, B: photoconductor for each color of black), L
Y, LM, LC and LB are laser beams, and AX is an optical axis (virtual) of the first optical system including the first and second lenses (11, 12).

2 and 3, the mirrors (2Y, 2
(M, 2C; 3Y, 3M, 3C, 3B) are omitted in FIG.
Then, the optical configuration before the polygon mirror (1) in FIG. 2 is omitted in the drawing. In addition, in FIG. 3, four laser beams
Only 2 (LM, LY) of (LY, LM, LC, LB) are shown,
The other two (LB, LC) are not shown. In this tandem scanning optical system, since the oblique incident angle in the sub-scanning direction is divided into ± (see Table 6 described later), the remaining two laser light beams (LB, LC) are relative to the optical axis (AX). It is a mirror image relationship. It should be noted that X, Y and Z in FIGS. 2 and 3 indicate directions orthogonal to each other, and the direction parallel to the rotation axis of the polygon mirror (1) is the Z direction, and the photoconductors (IY, IM, IC, I
The main scanning direction on B) is the Y direction, and the direction parallel to the optical axis (AX) is the X direction. The main scanning direction is the direction in which the laser beam (LY, LM, LC, LB) scans each photoconductor (IY, IM, IC, IB), and the sub-scanning direction is the direction perpendicular to the main scanning direction. Is.

The tandem scanning optical system of this embodiment is
As shown in Fig. 2, four light sources (4) that emit laser beams (LY, LM, LC, LB) one by one, a collimator lens (5),
Light source optical system consisting of cylinder lens (6) and each light source (4)
It is equipped with a single polygon mirror (1) for deflecting a total of four laser beams (LY, LM, LC, LB) emitted from. Laser beam emitted from each light source (4) (LY, LM, LC, L
B) is beam-shaped by the collimator lens (5) and the cylinder lens (6), respectively, and then is subjected to optical path synthesis in the main scanning direction by an optical path synthesis mirror (not shown). As a result of beam shaping, each laser beam (LY, LM, LC, LB) becomes a substantially parallel light in the main scanning direction and a polygon mirror in the sub scanning direction.
The light will be condensed near the polygon surface (S) of (1). 4 laser beams (LY, LM, LC,
LB) is simultaneously deflected and reflected by one polygon surface (S) at the same position on the polygon mirror (1). At this time, four laser beams (LY, LM, LC, LB) are incident on the polygonal surface (S) at different angles in the sub-scanning direction, and are deflected and reflected at different angles.

The four laser beams (LY, LM, LC, LB) deflected and reflected by the polygon mirror (1) are the polygon window.
After passing through (W), it enters the scanning system including the first and second optical systems. The first optical system is the first lens (11) and the second lens
(12) and are arranged in common for the four laser beams (LY, LM, LC, LB) deflected by the same polygonal surface (S). On the other hand, the second optical system is the third lens (21Y, 21Y
M, 21C, 21B) only, and each light source (4) corresponds to a total of four laser beams (LY, LM, LC, LB) that have passed through the first optical system (11, 12). It is arranged. In this tandem scanning optical system, one laser beam from one light source (4)
Since (LY, LM, LC, LB) is emitted, the laser beam (LY, LM, L
The second optical system (21Y, 21M, 21C, 21B) is arranged for each C, LB).

The four light beams (LY, L) that have passed through the first optical system (11,12)
Of the M, LC, LB), the laser beam (LB) is the folding mirror (3
After being reflected by (B), it passes through the third lens (21B) and the photoconductor (I
B) Image on top. Laser beam (LC) is a mirror for optical path separation
After being reflected in order of (2C) and the folding mirror (3C), an image is formed on the photoconductor (IC) through the third lens (21C). Laser light flux (LM) is a mirror for optical path separation (2M), folding mirror (3M)
After being reflected in this order, the photoconductor passes through the third lens (21M).
Image on (IM). The laser beam (LY) is reflected in the order of the optical path separation mirror (2Y) and the folding mirror (3Y), then the third
An image is formed on the photoconductor (IY) through the lens (21Y).

As described above, the scanning system uses the optical path separation mirrors (2Y, 2M, 2C) to transmit the four laser beams (LY, LM, LC, LB) to the four photosensitizers corresponding to the respective light sources (4). While guiding one by one to the body (IY, IM, IC, IB), the first to third lenses (11; 12; 21
Y, 21M, 21C, 21B) first and second optical system, each laser beam (LY, LM, LC, LB) is focused in a spot shape
The exposure scan for (IY, IM, IC, IB) is performed. In addition, in this embodiment, one light source emits one laser light beam, and exposure scanning for one photoconductor is performed by one laser light beam, but the present invention is not limited to this. For example, a multi-beam type light source that emits two or more laser light beams may be used, and exposure scanning for one photoconductor may be performed with two or more laser light beams.

Tables 1 to 6 show construction data of this embodiment. The surface vertex coordinates in Table 1 and the eccentricity vector data in Table 2 (only those with tilt eccentricity are shown) correspond to the through system laser light flux (LY, LM) (Fig. 3). , A global coordinate system in which the optical axis (AX) direction is the X direction, the main scanning direction is the Y direction, and the sub-scanning direction is the Z direction.
The origin of the local coordinate system (x, y, z) in (X, Y, Z) and the vector represent the arrangement of each optical surface (reference to the vertex of the surface) {Note that the numerical values are given in (). The core thickness is a linear distance when eccentricity is not taken into consideration. }. The optical surface described as "free curved surface" in the column of radius of curvature in Table 1 is a free curved surface whose surface shape is expressed by the following formula (FS). Free-form surface coefficient C of free-form surface used
_ij is shown in Tables 3 to 5 (where En = × 10 ⁻ⁿ ).

[0019]

[Equation 1]

[0020]

[Table 1]

[0021]

[Table 2]

[0022]

[Table 3]

[0023]

[Table 4]

[0024]

[Table 5]

[0025]

[Table 6]

First lens (11) constituting the first optical system
Is composed of free-form surfaces, both surfaces of which are non-axisymmetric in the main scanning direction. In other words, the free-form surface forming the fourth surface and the fifth surface is symmetrical with respect to the optical axis (AX) in the sub-scanning direction (there is no odd-order item), but in the main scanning direction. Is the optical axis
It is asymmetric with respect to (AX) (there is an odd order term). In addition, the second lens (21Y, 21M, 21
C, 21B) has a surface on the polygon mirror (1) side which is a free-form surface that is non-axisymmetric in the sub-scanning direction. That is, the eighth
The free-form surface forming the surface is the optical axis (AX) in the main scanning direction.
Is symmetric with respect to (no odd-order terms),
Asymmetric with respect to the optical axis (AX) in the sub-scanning direction
(There is an odd order term). Thus, the first optical system has at least one free-form surface that is non-axisymmetric in the main scanning direction, and the second optical system has at least one free-form surface that is non-axisymmetric in the sub-scanning direction.
It is desirable to have a face. This allows the polygon mirror
Even if the problem (1) occurs, good image plane performance can be maintained, and cost reduction of the tandem scanning optical system can be achieved. This will be described in detail below.

As shown in FIG. 1 and Table 6, four laser beams (LY, LM, LC, LB) are separated by four laser beams by the optical path separation mirrors (2Y, 2M, 2C). Luminous flux (LY, LM, LC, LB)
May be incident on the polygon mirror (1) at different angles in the sub-scanning direction. And the laser beam (LY, L
To make it easier to separate the optical path of (M, LC, LB) in the sub-scanning direction,
The oblique incident angle in the sub-scanning direction with respect to the polygon mirror (1) may be increased. As a result, even if the oblique incidence angle in the sub-scanning direction with respect to the scanning system becomes large, good image plane performance can be obtained by disposing a surface that is not axially symmetric in the sub-scanning direction in the scanning system. . Also, by disposing a surface that is not axially symmetric in the main scanning direction in the scanning system, it is possible to correct performance deterioration (image surface waviness and the like) due to polygon reflection point movement.

In order to make the sub-scanning magnification low, a second optical system may be provided after the optical path separating position as in the present embodiment, but a second non-axisymmetric surface in the main scanning direction is used as the second optical system. When arranged in the optical system, a lens of a type according to the number of light sources (for example, the number of types when the incident angle in the sub-scanning direction is divided by ± is 1/2 of the number of light sources) is the second optical element. It becomes necessary for the system. In other words, the third lens (21Y, 21
(M, 21C, 21B) cannot be composed of one type of lens, resulting in high cost.

Therefore, the first surface is a non-axisymmetric surface in the main scanning direction.
It is desirable that the second optical system is arranged in the optical system and is composed of one type of lens defined by one surface shape even for different incident angles in the sub-scanning direction. A non-axisymmetric surface in the scanning system is provided in the first optical system in the main scanning direction and in the second optical system in the sub scanning direction, and the constituent lens surfaces of the second optical system are defined by one surface shape. By doing so, the first optical system can be a common optical system for all laser light fluxes (LY, LM, LC, LB), and the second optical system can be used for all laser light fluxes (LY, LM, LC, LB). LB) can be configured with one type of lens.

FIG. 3 shows a state in which two laser beams (LY, LM) are incident on different positions at different incident angles with respect to the third lens (21Y, 21M) having one lens surface shape. Shows. As described above, it is desirable that the light beam passage position in the sub-scanning direction in the second optical system be different depending on the light beam incident angle in the sub-scanning direction, and the lens surface forming the second optical system is different in the sub-scanning direction. It is desirable to have one surface shape with respect to the light beam incident angle. By using an optical configuration in which the light flux passage positions are different, it is possible to secure performance for each laser light flux (LY, LM, LC, LB), and by using a single lens surface shape, it is possible to reduce manufacturing costs. Can be realized.

As described above, in this embodiment, since the oblique incident angle in the sub-scanning direction is divided into ± (Table 6, FIG. 3), the surface shape of the first lens (11) is axial in the sub-scanning direction. It is symmetrical, but depending on the oblique incident angle condition, the first lens
The surface shape of (11) may be non-axisymmetric in the sub-scanning direction. However, it is desirable that the second optical system be axially symmetric in the main scanning direction. In order to use the same type of third lens (21Y, 21M, 21C, 21B) for all laser beams (LY, LM, LC, LB), make the second optical system axially symmetric in the main scanning direction. It becomes inevitable.

Further, it is desirable that the incident positions of all the laser light fluxes (LY, LM, LC, LB) on the polygon mirror (1) are substantially matched in the sub-scanning direction. As shown in FIG. 1 and Table 6, the incident position in the sub-scanning direction (that is, the polygon reflection point position) on the polygon mirror (1) is determined by the total laser beam (LY, LM, LC, L).
By making the same in B), the polygon mirror (1) can be thinned in the sub-scanning direction. Polygon mirror
By reducing the thickness required in (1), it becomes possible to use the low-cost polygon mirror (1).

[0033]

As described above, according to the present invention, it is possible to realize a low-cost tandem scanning optical system that maintains good image plane performance even if there is a defect in the polygon mirror.

[Brief description of drawings]

FIG. 1 is a sub-scanning sectional view showing an embodiment of a tandem scanning optical system.

FIG. 2 is a main-scan sectional view showing the tandem scanning optical system of FIG. 1 as a through system.

FIG. 3 is a sub-scanning sectional view showing the tandem scanning optical system of FIG. 1 as a through system.

[Explanation of symbols]

1 ... Polygon mirror (deflecting means) S ... Polygon surface 4 ... Light source 11 ... First lens (part of first optical system, part of scanning system) 12 ... Second lens (part of first optical system, scanning) 21Y, 21M, 21C, 21B ... 3rd lens (second optical system, part of scanning system) IY, IM, IC, IB ... Photoreceptor (scanned surface) 2Y, 2M, 2C ... Optical path Separation mirrors 3Y, 3M, 3C, 3B ... Folding mirrors LY, LM, LC, LB ... Laser beam AX ... Optical axis of the first optical system

Claims (6)

[Claims] 1. A plurality of light sources, a single deflecting means for deflecting the light flux from each light source, and a plurality of light fluxes deflected by the deflecting means are separately guided to a plurality of scanned surfaces corresponding to the respective light sources. A tandem scanning optical system comprising: a scanning system for image-forming scanning on each surface to be scanned, wherein the scanning system is arranged in common for a plurality of light beams deflected on the same surface of the deflecting means. First optical system and its first
A plurality of second optical systems arranged so as to correspond to a plurality of light beams that have passed through the optical system, wherein the first optical system has at least one free-form surface that is non-axisymmetric in the main scanning direction, Second
A tandem scanning optical system, wherein the optical system has at least one free-form surface that is non-axisymmetric in the sub-scanning direction. 2. The tandem scanning optical system according to claim 1, wherein a plurality of light beams from the light source are incident on the deflecting means at different angles in the sub-scanning direction. 3. The tandem scanning optical system according to claim 2, wherein the incident positions of all the light beams on the deflecting unit are substantially coincident with each other in the sub-scanning direction. 4. The tandem scanning optical system according to claim 1, wherein the light beam passage position in the sub-scanning direction in the second optical system differs depending on the light beam incident angle in the sub-scanning direction. 5. The tandem scanning optical system according to claim 4, wherein the lens surface forming the second optical system has one surface shape for different light beam incident angles in the sub-scanning direction. . 6. The tandem scanning optical system according to claim 1, wherein the second optical system is axially symmetric in the main scanning direction.