JP4250572B2 - Optical scanning device and image forming apparatus using the same - Google Patents

Description

  The present invention relates to an optical scanning device and an image forming apparatus using the same, and in particular, a light beam emitted from a light source means is reflected and deflected by a polygon mirror as an optical deflector and optically scanned on a surface to be scanned through a scanning optical system. For example, it is suitable for an image forming apparatus such as a laser beam printer (LBP) having an electrophotographic process, a digital copying machine, or a multi-function printer (multi-function printer) that records image information.

  2. Description of the Related Art Conventionally, in an image forming apparatus such as a laser beam printer or a digital copying machine, a light beam (beam) that is light-modulated in accordance with an image signal from a light source means made of, for example, a semiconductor laser, Periodically deflected by an optical deflector, and converged in a spot shape on a photosensitive recording medium (photosensitive drum) surface by a scanning optical system (scanning lens system) having fθ characteristics, and optically scanned on the recording medium surface Image recording.

  FIG. 15 is a sectional view (main scanning sectional view) of the main part in the main scanning direction of an optical scanning device used in this type of conventional image forming apparatus.

  In the figure, a parallel light beam emitted from a laser unit 91 including a semiconductor laser is incident on a cylindrical lens (condensing lens) 92 having a predetermined refractive power only in the sub-scanning direction. The parallel light beam incident on the cylindrical lens 92 is emitted in the state of the parallel light beam as it is in the main scanning section.

  On the other hand, the parallel light beam is focused in the sub-scan section, and is formed as a long line image in the main scanning direction in the vicinity of the deflection surface 93a of the optical deflector 93 composed of a rotating polygon mirror. The light beam reflected and deflected by the deflecting surface 93a of the optical deflector 93 is imaged as a light spot on the surface of the photosensitive drum 95, which is the surface to be scanned, by a scanning optical system (fθ lens system) 94 having fθ characteristics. . The surface of the photosensitive drum 95 is repeatedly scanned by this light spot. The scanning optical system 94 includes a spherical lens 94a and a toric lens 94b.

  In the optical scanning device described above, a BD (beam detector) sensor as a photodetector is used to adjust the timing for starting image formation on the surface of the photosensitive drum 95 before scanning the surface of the photosensitive drum 95 with a light spot. -98 is provided. The BD sensor 98 scans a BD light beam that is a part of the light beam reflected and deflected by the optical deflector 93, that is, an area outside the image forming area before the image forming area on the surface of the photosensitive drum 95 is scanned. Receive the light flux. The BD light beam is reflected by the BD mirror 96, condensed by the BD lens (condensing lens) 97, and enters the BD sensor 98. A BD signal (synchronization signal) is detected from the output signal of the BD sensor 98, and the image recording start timing on the surface of the photosensitive drum 95 is adjusted based on the BD signal.

  The photosensitive drum 95 rotates at a constant speed in synchronization with the drive signal of the semiconductor laser in the laser unit 91, and the surface of the photosensitive drum 95 moves in the sub-scanning direction with respect to the scanned light spot.

  In this way, an electrostatic latent image is formed on the photosensitive drum 95. The electrostatic latent image is developed by a well-known electrophotographic process and transferred to a transfer material such as paper to realize an image.

  In addition, a multiple image forming apparatus using a scanning optical system generally forms images of different colors in a plurality of image forming units, conveys paper by a conveying means such as a conveying belt, and transfers the image on this paper in an overlapping manner. Image formation is performed. In particular, when obtaining a full-color image to be subjected to multicolor development, the image is deteriorated even by a slight overlap shift. For example, if it is 400 dpi, even an overlap shift of a fraction of 63.5 μm per pixel appears as a change in color shift or color shift, and the image is remarkably deteriorated.

  Conventionally, color development is performed by using the same scanning optical system, that is, optical scanning is performed with the same optical characteristics to reduce image shift. However, this method has a problem that it takes time to output multiple images and full colors. In order to solve this problem, there is a method in which images are formed by separate optical scanning devices in order to obtain an image of each color, and the images are superimposed on the paper fed by the transport unit.

  However, a concern at this time is color misregistration when images are superimposed. On the other hand, as an effective method, there is a method of detecting the position of the image and controlling the image forming unit to correct the image according to the detection signal (see, for example, Patent Document 1).

  On the other hand, in an image forming apparatus that scans a plurality of photosensitive members with beams, the same number of scanning optical systems as that of the photosensitive members are usually used to form latent images on the plurality of photosensitive members. As this problem, the number of optical components is required as many as the number of scanning optical systems. In particular, the optical deflector (polygon mirror) is expensive, and therefore the cost is high. In particular, in the case of a high-speed and high-definition scanning optical system, the problem becomes more serious because the optical deflector needs to have the ability to deflect at high speed at the same time.

  In order to cope with this problem, an optical scanning device that deflects a plurality of beams with a common optical deflector has been proposed. An optical scanning device that scans a photoconductor in the sub-scanning direction with a common optical deflector has a mechanism for shifting the drawing position of the beam in the sub-scanning direction in order to improve the accuracy of superimposing images in the sub-scanning direction. There is a need. In this method, adjustment is performed by selecting a deflection surface of an optical deflector that starts drawing of a beam in the sub-scanning direction and shifting the drawing position by one line in the sub-scanning direction.

More recently, there has been a demand for a compact, low-cost, full-color image forming apparatus with light image quality. One method for satisfying this requirement is to scan a plurality of beams using a single common polygon. A system for reducing the cost by reducing the number of points has been proposed.
Japanese Patent Publication No. 1-281468

  However, when a common polygon mirror is used, it is necessary to separate optical paths in order to guide a plurality of beams to different scanning surfaces. Therefore, it is necessary to keep the beam distance in the sub-scanning direction. As a result, there is a problem that the polygon mirror becomes thick and the cost increases.

  Even if the first optical element (scanning lens) of the scanning optical system is used in common, even if the cost is reduced, the light beam passing position of the lens passes through a position separated in the sub-scanning direction. The height of the direction is increased, and as a result, there is a problem that the effect of a large cost reduction is small.

  The present invention provides an optical scanning device having a simple and compact configuration capable of reducing the thickness of the optical deflector and suppressing scanning line bending on the surface to be scanned to obtain good optical performance, and an image forming apparatus using the optical scanning device. Objective.

In order to solve the above problems, in the present invention, a plurality of light source means, an optical deflector for deflecting a plurality of light beams emitted from the plurality of light source means, and the same deflecting surface of the optical deflector are deflected. A scanning optical system that forms an image of the plurality of light beams on different scanned surfaces,
A plurality of light beams incident on the same deflection surface of the optical deflector are incident at different angles with respect to the normal line of the deflection surface of the optical deflector in the sub-scan section,
The scanning optical system includes a first optical element that is commonly used for a plurality of light beams deflected by the same deflecting surface of the optical deflector, the first optical element, and the scanned surface. A second optical element disposed and disposed with respect to each of the plurality of light beams,
The power in the main scanning section of the first optical element and the second optical element is φ1m and φ2m, respectively, and the power in the sub-scanning section of the first optical element and the second optical element is φ1s, respectively. When φ2s
| Φ1m / φ2m |> 2.0
0 ≦ | φ1s | <0.001
| Φ1s / φ2s | <0.1
Satisfied,
In the sub-scan section, the principal ray of the light beam incident on the second optical element has an angle with respect to the optical axis in the sub-scan section of the second optical element, and
In the sub-scan section, the optical axis of the second optical element is decentered toward the deflection reflection point side with respect to the principal ray position in the sub-scan section of the light beam incident on the second optical element , and
In the second optical element, at least one of the light incident surface and the light emitting surface has a spherical shape in the main scanning section, and the second optical element has an axial power in the sub-scanning section. On the other hand, the configuration is such that the off-axis power is weak.

  According to the present invention, the thickness of the optical deflector can be reduced by setting the refractive power in the sub-scan section of the first optical element constituting the scanning optical system to substantially non-power as described above, and the scanning line. It is possible to achieve an optical scanning device having a simple and compact configuration capable of obtaining good optical performance with curving and an image forming apparatus using the same.

[Embodiment 1]
FIG. 1 is a cross-sectional view (sub-scanning cross-sectional view) of the main part in the sub-scanning direction of the optical scanning device (image forming apparatus) according to the first embodiment of the present invention.

  Here, the main scanning direction is a direction perpendicular to the rotation axis of the optical deflector and the optical axis of the scanning optical system (the direction in which the light beam is reflected and deflected (deflected and scanned) by the optical deflector), and the sub-scanning direction is The direction parallel to the rotation axis of the optical deflector is shown. The main scanning section indicates a plane parallel to the main scanning direction and including the optical axis of the scanning optical system. The sub-scanning cross section indicates a cross section perpendicular to the main scanning cross section.

  In the present embodiment, the light beam from the light source means that emits a plurality of light beams modulated according to the image signal is divided into two scanning groups (scanning optical systems) S1 and S2. The two scanning groups S1 and S2 are configured symmetrically with respect to the optical deflector (polygon mirror) 1, and the optical actions of the two scanning groups S1 and S2 are the same. The half scan group S1 will be described.

  In the figure, reference numerals 6M and 6Y denote photosensitive drums, in which a conductive layer is coated on a conductor, and an electrostatic latent image is formed by a light beam emitted from a scanning optical unit housed in an optical box 9.

  An optical deflector 1 is composed of, for example, a polygon mirror (rotating polygonal mirror), and is rotated at a constant speed by a driving means (not shown) such as a motor.

In this embodiment, when each element and each light beam are projected in the sub-scan section, the two light beams are incident obliquely at different incident angles with respect to the normal line of the deflection surface of the polygon mirror 1 (oblique incident scanning optics). system).

  The incident angle to the polygon deflection surface is the same as the angle θ1 having two different absolute values such as | Θ1 | ≠ | Θ2 | It may be Θ2.

  In the present embodiment, two light beams are incident on the same deflection surface of the polygon mirror 1, but three or more light beams may be incident.

  Reference numeral 2A denotes a first scanning lens as a first optical element, which has non-power refractive power in the sub-scanning section and refractive power in the main scanning section. The first scanning lens 2A has a surface with the same radius of curvature of the light incident surface and the light exit surface in the sub-scan section, and includes one or more aspheric surfaces in the main scan section. The first scanning lens 2A is mainly responsible for imaging in the main scanning direction and constant speed scanning (fθ characteristics) for the incident light beam.

  The second optical elements 3M and 3Y use a scanning lens in the present embodiment, but may be replaced with a diffraction element or a curved mirror. Further, in the present embodiment, the second optical element is configured by one scanning lens as described below, but may be configured by two or more scanning lenses.

  The first optical element 2A uses a scanning lens in this embodiment, but may be replaced with a diffraction element or a curved mirror.

  Reference numerals 3M and 3Y denote second scanning lenses as second optical elements each having a light incident surface having a spherical shape in the main scanning section and a light emitting surface having no inflection point in the main scanning section. It is made of a plastic material having a weak off-axis refractive power with respect to the scanning axis in the sub-scanning section.

  The second scanning lens as the second optical element may be a diffraction element or a curved mirror.

  Further, the optical axes in the sub-scanning sections of the second scanning lenses 3M and 3Y are decentered from the principal ray positions in the sub-scanning section of the light rays incident on the second scanning lenses 3M and 3Y. The second scanning lenses 3M and 3Y are set so that the on-axis and off-axis magnifications are substantially constant in the sub-scan section. The second scanning lenses 3M and 3Y are mainly responsible for correcting the field curvature in the sub-scanning direction with respect to the incident light beam.

  In the present embodiment, the first scanning lens system is constituted by the first scanning lens 2A and the second scanning lens 3M, and the second scanning lens is constituted by the first scanning lens 2A and the second scanning lens 3Y. A scanning optical system is constituted by the first and second scanning lens systems. In the present embodiment, the imaging magnification in the sub-scan section of the scanning optical system is set to be 1.3 times or less.

  The first and second scanning lens systems each form light beams E1 and E2 based on image information reflected and deflected by the polygon mirror 1 on the photosensitive drums 6M and 6Y as scanning surfaces, and in the sub-scanning section. 3 has a tilt correction function by providing a conjugate relationship between the deflection surface of the polygon mirror 1 and the surfaces of the photosensitive drums 6M and 6Y.

  Reference numerals 4A and 5M denote first and second folding mirrors (optical members) in this order, which are provided in the optical path of the light beam E1, and reflect the light beam in a predetermined direction. Reference numeral 5Y denotes a third folding mirror (optical member), which is provided in the optical path of the light beam E2, and reflects the light beam in a predetermined direction.

  These first, second, and third folding mirrors are configured to be movable or / and deformable as will be described later, and the scanning line bending is adjusted on the surface to be scanned by the adjusting member.

  In this embodiment, when each element and each light beam are projected in the sub-scan section, the first folding mirror 4A separates the two light beams E1 and E2 deflected by the polygon mirror 1 into two optical paths, and The incident light beam E1 is reflected to the side opposite to the photosensitive drums 6M and 6Y. An optical box 9 stores each component of the scanning optical unit.

  In the present embodiment, a scanning optical unit is disposed below the photosensitive drum, and the scanning optical unit causes two light beams to enter the polygon mirror 1 on both sides, respectively, on each corresponding photosensitive drum surface. Light beams E1 to E4 are guided to color images at high speed.

  The scanning optical system in this embodiment is an oblique incidence scanning optical system in the sub-scan section as described above. The oblique incidence scanning optical system is an optical system in which a light beam is incident from an oblique direction with respect to a surface (main scanning section) perpendicular to the rotation axis of the polygon mirror 1 in a sub-scanning section (plane parallel to the paper surface). . Such oblique incidence makes it possible to reduce the width of the deflecting / reflecting surface of the polygon mirror 1 in the sub-scanning direction. Similarly, since the first scanning lens 2A is close in position to each light beam in the sub-scanning direction, the width in the sub-scanning direction can be reduced.

  Next, the optical action of this embodiment will be described.

  In the present embodiment, two light beams E1 and E2 incident on the deflecting surface of the polygon mirror 1 from two incident optical systems to be described later are reflected at an angle ± θ with respect to the main scanning section and deflected and scanned. Thereafter, the two light beams E1 and E2 are incident on the common first scanning lens 2A. The two light beams E1 and E2 that have passed through the first scanning lens 2A are separated into respective optical paths by the first folding mirror 4A. After passing through the second scanning lens 3M, the light beam E1 reflected by the first folding mirror 4A is reflected upward in the drawing by the second folding mirror 5M, and intersects its own optical path in the space. By being folded by the first and second folding mirrors 4A and 5M, the light beam crosses the optical path of another light beam E2 twice and reaches the photosensitive drum 6M.

  On the other hand, in the optical path of the light beam E2, the light beam E2 that has passed through the first scanning lens 2A passes by the side of the first folding mirror 4A, and the light beam E1 and the optical path are separated. Then, after passing through the second scanning lens 3Y, the light beam E2 is reflected upward in the drawing by the third folding mirror 5Y and reaches the photosensitive drum 6Y.

  The light fluxes directed to the photosensitive drums 6C and 6K similarly form an electrostatic latent image, and form a multicolor image on paper in an unillustrated electrophotographic process of development, transfer, and fixing.

  In the present embodiment, the first scanning lens 2A is shared by the two light beams E1 and E2, and the second scanning lenses 3M and 3Y are used for the light beams E1 and E2, respectively.

  2 is a sectional view (main scanning sectional view) of the main scanning direction of one scanning group (scanning optical system) S1 shown in FIG. 1, and FIG. FIG. 2 and 3A, the same elements as those shown in FIG. 1 are denoted by the same reference numerals. Note that the folding mirror is omitted.

  FIG. 3A shows only the magenta station (M) in FIG. 1, and the yellow station (Y) is omitted. Note that the yellow station is an optical system arranged symmetrically with respect to the normal 31 of the deflecting / reflecting surface.

  In the figure, reference numeral 11 denotes a plurality of light source means for emitting a light beam modulated in accordance with an image signal, and is composed of, for example, a semiconductor laser. In the present embodiment, the light source unit includes a plurality of light source units. However, the present invention is not limited to this. For example, the light source unit may include a plurality of light emitting units. A conversion optical element (for example, a collimator lens) 12 converts a light beam emitted from the plurality of light source means 11 into a substantially parallel light beam (or a substantially divergent light beam or a substantially convergent light beam). Reference numeral 13 denotes an aperture stop which shapes a beam shape by limiting a plurality of passing light beams. Reference numeral 14 denotes a cylindrical lens as a condensing lens, which has a predetermined refractive power (power) only in the sub-scanning direction, and a plurality of light beams that have passed through the aperture stop 13 will be described later in the sub-scanning section. Is once formed as a substantially linear image in the vicinity of the deflection surface 1a. Each element such as the collimator lens 12, the aperture stop 13, and the cylindrical lens 14 constitutes an element of the incident optical system.

  A scanning lens system 7 includes the first scanning lens 2 (2A) having the shape described above and the second scanning lens 3 (3M, 3Y), and is based on image information reflected and deflected by the polygon mirror 1. A plurality of light beams are imaged on different photosensitive drum surfaces 6 (6M, 6Y) as scanning surfaces, and the deflection surface 1a of the polygon mirror 1 and the photosensitive drum surface 6 have a conjugate relationship in the sub-scanning section. By doing so, it has a tilt correction function.

  Reference numeral 6 (6M, 6Y) denotes a photosensitive drum surface as a surface to be scanned, and 15 denotes scanning light deflected and scanned by the polygon mirror 1.

  In FIG. 3A, 31 is a normal line at the deflection reflection point 36 of the deflection surface 1a. Reference numeral 33 denotes an optical axis of the second scanning lens 3, which is decentered toward the deflection reflection point 36 side of the deflection surface 1 a with respect to the light beam passage position 34. The light beam passage position 34 corresponds to the principal light beam. The optical axis 33 is parallel to the normal 31 of the deflection surface 1a.

  In the present embodiment, a plurality of light beams from the cylindrical lens 14 enter the normal line 31 of the deflecting surface 1a with an angle α in the sub-scan section. For this reason, the scanning line incident on the second scanning lens 3 is incident in a bent state, and the scanning line is likely to be bent on the photosensitive drum surface. Therefore, it is necessary to suppress the occurrence of scanning line bending, and the scanning optical system 7 is configured so that the imaging magnification is 1.3 times or less in the sub-scanning section so that the scanning line bending does not occur. In other words, the influence of the change in the main scanning direction position of the optical axis height of the second scanning lens, which is a main cause of the scanning line bending in manufacturing the scanning optical system, is suppressed to a small level. At this time, by decentering the second scanning lens 3 (parallel decentering and / or rotational decentering), the image forming magnification in the sub-scanning section is 1.3 times or less, and spot rotation and scanning line bending on the photosensitive drum are removed. Making it easy to do.

  That is, in this embodiment, the optical axis 33 of the second scanning lens 3 is decentered toward the deflection reflection point 36 side of the deflecting surface 1a with respect to the light beam passing position (principal light position) 34, and at the same time, the second scanning lens 3 Is incident on the optical axis 33 at an angle in the sub-scan section.

  FIG. 3B is an explanatory diagram of the in-company optical system of the present invention. In the figure, 11Y and 11M are light sources, 12Y and 12M are collimator lenses, 13Y and 13M are diaphragms, and 14Y and 14M are cylindrical lenses. The cyan station and black station have the same optical system arranged symmetrically with respect to the polygon rotation axis.

  In the figure, the laser beams emitted from the light sources 11Y and 11M are refracted by the collimator lenses 12Y and 12M and enter the cylindrical lenses 14Y and 14M having power in the sub-scanning direction as parallel beams. The cylindrical lenses 14Y and 12M are condensed in the sub-scanning direction and the light beam width is restricted by the apertures 13Y and 13M, and then enter the deflecting reflection surface of the polygon mirror at different incident angles of + Θ and minus Θ.

  FIG. 4 is a sub-scan sectional view showing the state of light rays in the vicinity of the deflection surface of the polygon mirror when the present invention is not carried out.

  In the figure, 41 is a polygon mirror, 42 is a first scanning lens, 43 is a normal line in the sub-scan section of the deflecting reflecting surface, 44 is the first scanning light after being reflected by the deflecting surface, and 45 is the deflecting surface. The second scanning light 47 and 48 after being reflected by the second scanning lens 47 and 48 are second scanning lenses, and 49 (49M and 49Y) are scanned surfaces (photosensitive drum surfaces).

  The first scanning lens 42 shown in the figure has a positive power with a biconvex shape in the sub-scan section. When the first scanning lens 42 is arranged eccentrically above the normal line 43 due to a manufacturing error as indicated by a broken line, the first scanning light 44 is refracted and passes a position deviated from the design value. The amount of shift of the light beam varies depending on the image height in the main scanning direction, and the shift amount becomes larger as the image height in the main scanning direction is higher.

  Therefore, a light beam having a U-shaped locus from the design reference light beam is incident on the second scanning lens 47, and as a result, a convex scanning line is formed on the surface to be scanned 49. Similarly, the second scanning light 45 is also refracted and passes through a position deviated from the design value. As a result, a scanning line having a convex shape is formed on the scanned surface 49.

  FIG. 5 is a sub-scan sectional view showing the state of light rays near the deflection surface of the polygon mirror when the present invention is implemented. In the figure, the same elements as those shown in FIG.

  In the figure, 53 is a normal line in the sub-scan section of the deflection surface, 54 is the first scanning light after being reflected by the deflection reflection surface, 55 is the second scanning light after being reflected by the deflection surface, 33 Are the optical axes of the second scanning lenses 3M and 3Y, respectively.

  As shown in the figure, both lens surfaces in the sub-scan section of the first scanning lens 2A are configured with a loose radius of curvature (the same radius of curvature), that is, substantially non-powered.

That is, in this embodiment, when the refractive power in the sub-scanning section of the first scanning lens 2A is φ1s,
0 ≦ | φ1s | <0.001 (1)
It is formed to satisfy.

In this embodiment, when the refractive powers in the main scanning section of the first scanning lens 2A and the second scanning lens (3M, 3Y) are φ1m and φ2m, respectively.
| Φ1m / φ2m |> 2.0 (2)
It is configured to satisfy.

  Conditional expression (1) defines the refractive power in the sub-scanning section of the first scanning lens 2A. If the conditional expression (1) is not satisfied, the first scanning lens 2A has power in the sub-scanning section. In other words, the amount refracted in the sub-scanning direction changes depending on the rotation angle of the polygon, which contributes to the occurrence of scanning line bending. Therefore, when the lens position is changed due to a lens arrangement error or the like, the scanning line is bent, which is not good because the scanning line is increased in manufacturing the optical scanning device.

  Conditional expression (2) relates to the ratio of refractive power in the main scanning section of the first scanning lens 2A and the second scanning lens, and if the conditional expression (2) is not satisfied, the main scanning of the second scanning lens is performed. In order to increase the power in the cross section and to improve the fθ characteristic, it is necessary to change the refractive power in the main scanning cross section according to the angle of view. As a result, the shape of the second scanning lens in the main scanning cross section tends to be wavy. The lens shape swells when the lens placement error occurs, and the scanning line becomes undulated. Even if the scanning line bend is corrected by another optical member, the uniform bend can be corrected and the bend cannot be corrected. Not good for that.

More preferably, the conditional expressions (1) and (2) are
0 ≦ | φ1s | <0.0001 (1a)
| Φ1m / φ2m |> 4.0 (2a)
It is good to do.

  In the present embodiment, even if the first scanning lens 2A is shifted in the sub-scanning direction due to a manufacturing error or the like, the position of the light beam in the sub-scanning cross section hardly changes. Therefore, the second scanning lenses 3M, 3Y The state of the light incident on the lens does not change. Therefore, since the scanning light does not bend, the optical system can be made insensitive to the lens and lens surface shift, which is a major cause of the occurrence of the scanning line bending due to a manufacturing error. It becomes.

  FIG. 6 is an explanatory view showing the shape of the second scanning lens (47, 48) in the main scanning section when the present invention is not carried out.

  In this figure, 62 is the lens surface on the incident side of the second scanning lens (47, 48), 63 is the lens surface on the exit side, and 64 is the optical axis in the main scanning section of the second scanning lens 47, 48. is there.

  If the conditional expressions (2) and (2a) are not satisfied, it is necessary to make the shape of the second scanning lens (47, 48) undulate in order to correct the fθ characteristic of the scanning optical system.

  FIG. 7 shows the light beam arrival position on the surface to be scanned when the scanning lens is misaligned in the sub-scanning direction due to a manufacturing error or the like. FIG. 7 is a graph showing scanning line bending by a conventional scanning optical system.

  In the figure, the horizontal axis Y indicates the drawing position (image height), and the vertical axis Z indicates the light beam arrival position in the sub-scanning direction. Reference numeral 71 is a graph showing the scanning position in the sub-scan section when the second scanning lens (47, 48) is tilted. As shown in the graph 71, the scanning position in the sub-scanning section has a wavy shape depending on the drawing position, which is caused by the lens shape. At this time, a graph 72 shows the correction amount of the scanning line when the scanning line bending adjustment is performed as described later.

  Normally, when an optical member other than the second scanning lens (47, 48) is deformed or displaced by scanning line bending adjustment, the scanning line correction amount shows a locus without waviness. Therefore, even if the scanning line bending adjustment is performed, the undulation of the scanning line remains as in the line of the graph 72.

  In addition, even when the scanning line bending adjustment is performed by the inclination of the second scanning lens (47, 48), the swell component generated by the second scanning lens (47, 48) can be adjusted. Since the bend shape generated by the factor (for example, the nod of the polygon mirror) is a shape having no undulation, the undulation component becomes large as a result, and the color shift of the superimposed image is deteriorated.

  FIG. 8 is an explanatory diagram showing the shape of the second scanning lens in the main scanning section when the present invention is used.

  In the figure, 3 (3M, 3Y) is the second scanning lens made of a plastic material as described above, 82 is the incident-side surface of the second scanning lens 3 (3M, 3Y), 83 is the exit-side surface, 84 Is the optical axis in the main scanning section of the second scanning lens 3.

  In the present embodiment, the incident-side surface (light incident surface) 82 of the second scanning lens 3 has an R-surface shape (spherical shape), and the emission-side surface (light-emitting surface) 83 has an aspheric shape having no inflection point. It has become. In order for the lens surface in the main scanning section to have a shape with less waviness as shown in the figure, it is necessary to satisfy the above conditional expression (2), and most of the fθ characteristic is the first. It is necessary to correct by the scanning lens 2A.

  The state of the scanning line bending at this time is shown in FIG. FIG. 9 is a graph showing scanning line bending by the scanning optical system of the present embodiment.

  In the figure, the horizontal axis Y indicates the drawing position (image height), and the vertical axis Z indicates the light beam arrival position in the sub-scanning direction. Reference numeral 91 is a graph showing the state of scanning line bending when the second scanning lens 3 is tilted. As shown in the figure, the trajectory of the scanning line has a straight shape without undulations. A graph 92 shows the correction amount when the scanning line bending adjustment is performed in this state. As shown in the graph 92, the shape after adjusting the scanning line bending is a shape without undulation.

  In this embodiment, the light incident surface of the second scanning lens 3 has a spherical shape and the light exit surface has an aspheric shape. However, the present invention is not limited to this. For example, the light entrance surface has an aspheric shape and the light exit surface has a spherical shape. The shape may be formed, or both the light incident surface and the light emitting surface may be formed in a spherical shape or an aspherical shape. The incident angle to the polygon deflection surface is the same as the angle θ1 having two different absolute values such as | Θ1 | ≠ | Θ2 | It may be Θ2.

  FIG. 10 is an explanatory diagram showing a method of adjusting the scanning line bending according to the present embodiment.

  As a method of adjusting the scanning line bending in the present embodiment, the adjustment is performed by making the folding mirror 5 disposed in the optical path deformable (for example, bending). That is, in FIG. 1, the scanning line bending adjustment can be performed by elastically deforming one or more of the plurality of mirrors of the folding mirror 5 (5K, 5C, 5M, 5Y) by the adjusting member 104.

  That is, in the figure, 5 (5K, 5C, 5M, 5Y) is a folding mirror, 102 and 103 are fixed points of the folding mirror 5, 104 is an adjustment member (pressure member) for deforming the folding mirror, and 105 is The reflection point 106 of the folding mirror is the shape of the folding mirror after deformation.

  As shown in the figure, the folding mirror 5 is elastically deformed by being pushed by the adjusting member 104 to form a shape 106 indicated by a dotted line. Thereby, the shape of the reflecting surface of the folding mirror 5 becomes a convex shape. By using the folding mirror in this state, the trajectory of the scanning line on the surface to be scanned can be changed, and the scanning line bending can be adjusted.

  In this embodiment, the folding mirror 5 can be deformed. However, the present invention is not limited to this. For example, the folding mirror 5 may be movable, or may be deformable and movable.

  In the present embodiment, as described above, the magnification in the sub-scan section of the scanning optical system is reduced to 1.3 times or less, the second scanning lens 3 is disposed close to the surface to be scanned, and the first By making the air space between the scanning lens 2A and the second scanning lens 3 large, the scanning light is easily separated between the first scanning lens 2A and the second scanning lens 3. As a result, in this embodiment, the optical path length can be shortened compared to separating the optical path after the second scanning lens 3, and as a result, the entire scanning optical system can be made compact.

[Numerical Example 1]
Numerical Example 1 of the present invention will be shown below. Table 1 shows optical parameters of the BK station of FIG. 1 of the present invention. FIGS. 11 and 12 are a main scanning sectional view and a sub-scanning sectional view of the optical scanning device according to Numerical Embodiment 1, respectively. 11 and 12, the same elements as those shown in FIGS. 2 and 3 are denoted by the same reference numerals. 11 and 12 describe one station (the BK station in FIG. 1), but the other stations (Y, M, C) can also achieve good performance by arranging similar optical systems. It is possible to realize a scanning optical system for a compact and color image forming apparatus. That is, only the BK station among the four stations in FIG. 1 is shown in FIGS. The other three stations (Y, M, C) are not shown in FIGS. 11 and 12, and only the BK station in FIG. However, the other three stations (Y, M, C) also take optical parameters.

The surface shapes of the refractive surfaces of the first and second scanning lenses 22 and 23 in Numerical Example 1 are expressed by the following shape expression. The intersection of the lens surface and the optical axis is the origin, the optical axis direction is the x axis, the axis orthogonal to the optical axis in the main scanning section is the y axis, and the axis orthogonal to the optical axis in the sub scanning section is the z axis. When
The bus direction corresponding to the main scanning direction is

(Where R is the radius of curvature, and K, B ₄ , B ₆ , B ₈ , and B ₁₀ are aspheric coefficients)
The sub-scanning direction (the direction including the optical axis and orthogonal to the main scanning direction) and the sub-line direction are

(Where r ′ is the radius of curvature on the optical axis, D ₂ , D ₄ , D ₆ , D ₈ , D ₁₀
Is aspheric coefficient)

  In this embodiment, a light beam is incident at an oblique incident angle of 2.2 degrees with respect to the normal line of the deflecting surface 1a of the polygon mirror 1 (oblique incident optical system). At this time, the second scanning lens 23 has the optical axis of the lens at a position shifted in the 1.46 (mm) Z direction (sub-scanning direction) with respect to the plane perpendicular to the deflection reflection point. The paraxial image plane position at this time is shown in FIG. As shown in the figure, the optical performance is good with respect to imaging performance and image height deviation.

  Further, the uniformity of the sub-scan magnification of the scanning optical system at this time needs to be within 10%, but in this numerical example, it is within ± 1% within the effective scanning region, and the scanning of 600 dpi. When used in an optical system, the level is completely satisfactory. Further, by shifting the optical axis of the second scanning lens 23 with respect to the incident beam toward the 1.3 (mm) deflection reflection point side, the rotation of the beam is removed, and a favorable spot shape is obtained. That is, the magnification of the scanning optical system in the sub-scanning direction is constant within the effective image area. The present invention is more effective at a resolution of 1200 dpi or higher where high image quality is required.

At this time, the power in the sub-scan section of the first scanning lens 22 as the first optical element is 1.08e-6 (1.08 × 10 ⁻⁶ ), and the power in the sub-scan section is substantially non-power. It is. This satisfies the conditional expressions (1) and (1a). The power ratio in the main scanning section of the first and second scanning lenses 22 and 23 is | φ1m / φ2m | = 4.45, which satisfies the conditional expressions (2) and (2a).

  In Numerical Example 1, the first scanning lens 22 mainly has power in the main scanning section, and the shape of the first surface (light incident surface) of the second scanning lens 23 in the main scanning section is spherical. It has a shape and is made of a plastic material having a weak off-axis refractive power with respect to the scanning axis in the sub-scanning section. Accordingly, the scan line is hardly bent due to the eccentricity of the first scanning lens 22, and the scan line undulation due to the eccentricity of the second scanning lens 23 as the second optical element is very small. If the scanning is performed, the amount of bending of the scanning line can be made very small.

Further, the scanning optical system includes a second scanning lens 23 having power in the sub-scanning direction provided for each of the plurality of light beams, and the first scanning lens 22 and the second scanning lens 23 are connected to each other. When the power in the scanning section is φ1s and φ2s, respectively
| Φ1s / φ2s | <0.1
Is preferably satisfied.

[Embodiment 2]
FIG. 14 is a schematic diagram of a main part of a color image forming apparatus according to Embodiment 2 of the present invention.

  The present embodiment is a tandem type color image forming apparatus that scans four beams by the optical scanning device shown in the first embodiment and records image information on a photoconductor as an image carrier in parallel.

  In FIG. 14, 130 is a color image forming apparatus, 141 is an optical scanning apparatus having the configuration shown in the first embodiment, 151, 152, 153 and 154 are photosensitive drums as image carriers, 161, 162, 163 and 164, respectively. Are developing units, and 131 is a conveyor belt.

  In FIG. 14, the color image forming apparatus 130 receives R (red), G (green), and B (blue) color signals from an external device 132 such as a personal computer. These color signals are converted into C (cyan), M (magenta), Y (yellow), and B (black) image data (dot data) by a printer controller 133 in the apparatus. These image data are input to the optical scanning device 141. The optical scanning device 141 emits beams 171, 172, 173, and 174 that are modulated in accordance with each image data, and the photosensitive surfaces of the photosensitive drums 151, 152, 153, and 154 are driven by these beams in the main scanning direction. Scanned.

  The color image forming apparatus in this embodiment scans four beams by the optical scanning device 141, and each corresponds to each color of C (cyan), M (magenta), Y (yellow), and B (black), and is parallel to each other. Thus, image signals (image information) are recorded on the photosensitive drums 151, 152, 153, and 154, and a color image is printed at high speed.

  In the color image forming apparatus according to this embodiment, the latent image of each color is formed on the corresponding photosensitive drums 151, 152, 153, and 154 by using the beam based on each image data by the optical scanning device 141 as described above. ing. Thereafter, a single full color image is formed by multiple transfer onto a recording material.

  As the external device 132, for example, a color image reading device including a CCD sensor may be used. In this case, the color image reading apparatus and the color image forming apparatus 130 constitute a color digital copying machine.

FIG. 3 is a sub-scan sectional view of the optical scanning device according to the first embodiment of the present invention. FIG. 1 is a main scanning sectional view of a scanning optical system according to a first embodiment of the present invention. FIG. 3 is a sub-scan sectional view of the scanning optical system according to the first embodiment of the present invention. Sub-scan sectional view of a conventional scanning optical system Sub-scan sectional view of the scanning optical system of the present invention Explanatory drawing of the shape of the main scanning section of the 2nd lens of the conventional scanning optical system Graph showing scanning line bending in a conventional scanning optical system Explanatory drawing of the shape of the main scanning cross section of the 2nd lens of Embodiment 1 of this invention The graph which showed the scanning line curve in the scanning optical system of Embodiment 1 of this invention Explanatory drawing which showed adjustment of scanning line bending of Embodiment 1 of the present invention Main scanning sectional view of optical scanning device of numerical embodiment of the present invention Sub-scan sectional view of optical scanning device of numerical embodiment of the present invention Explanatory drawing which shows the optical performance of numerical embodiment of this invention FIG. 5 is a sub-scanning sectional view of a color image forming apparatus according to a second embodiment of the present invention. Main scanning sectional view of an optical scanning device used in a conventional image forming apparatus

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Deflection element 2 1st scanning lens 3 2nd scanning lens 4, 5 Folding mirror 6 Scanning surface

Claims (7)

A plurality of light source means, a light deflector for deflecting a plurality of light beams emitted from the plurality of light source means, and a plurality of light beams deflected by the same deflection surface of the light deflector are respectively connected to different scanned surfaces. An optical scanning device having a scanning optical system for imaging,
A plurality of light beams incident on the same deflection surface of the optical deflector are incident at different angles with respect to the normal line of the deflection surface of the optical deflector in the sub-scan section,
The scanning optical system includes a first optical element that is commonly used for a plurality of light beams deflected by the same deflecting surface of the optical deflector, the first optical element, and the scanned surface. A second optical element disposed and disposed with respect to each of the plurality of light beams,
The power in the main scanning section of the first optical element and the second optical element is φ1m and φ2m, respectively, and the power in the sub-scanning section of the first optical element and the second optical element is φ1s, respectively. When φ2s
| Φ1m / φ2m |> 2.0
0 ≦ | φ1s | <0.001
| Φ1s / φ2s | <0.1
Satisfied,
In the sub-scan section, the principal ray of the light beam incident on the second optical element has an angle with respect to the optical axis in the sub-scan section of the second optical element, and
In the sub-scan section, the optical axis of the second optical element is decentered toward the deflection reflection point side with respect to the principal ray position in the sub-scan section of the light beam incident on the second optical element , and
In the second optical element, at least one of the light incident surface and the light emitting surface has a spherical shape in the main scanning section, and the second optical element has an axial power in the sub-scanning section. On the other hand , an optical scanning device characterized in that the off-axis power is weak . The light incident surface of the second optical element has a spherical shape in the main scanning section, and the light emitting surface of the second optical element has a shape having no inflection point in the main scanning section. 2. An optical scanning device according to 1. 2. The optical scanning device according to claim 1, wherein the light incident surface and the light emitting surface of the second optical element are spherical in the main scanning section . The optical scanning device according to any one of claims 1 to 3 ratio of the sub-scan section of the scanning optical system is less than 1.3 times. 2. The optical axis of the second optical element is decentered parallel to the deflection reflection point side of the principal ray position in the sub-scanning section of the light incident on the second optical element in the sub-scanning section. 5. The optical scanning device according to any one of claims 1 to 4 . An image forming apparatus having a plurality of image carriers each of which is disposed on a surface to be scanned of the optical scanning device according to any one of claims 1 to 5 and forms images of different colors. The image forming apparatus according to claim 6 , further comprising a printer controller that converts color signals input from an external device into image data of different colors and inputs the converted image data to each optical scanning device.

Images