JP2015034870A - Optical scanning device and image forming device having the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce total size of an optical scanning device having common deflection means for deflecting a plurality of light beams for a plurality of scanning surfaces.SOLUTION: An optical scanning device includes; first and second imaging optical systems for focusing first and second light beams, respectively, coming via common deflection means on first and second scanning surfaces, respectively; and a plurality of reflective optical elements for bending first and second light paths such that each light path crosses itself in a sub-scanning cross-section. An effective image region W (mm) in a main scanning direction on each of the first and second scanning surfaces and a distance Dp (mm) between the first and second scanning surfaces in the sub-scanning cross-section satisfy a predetermined relationship. In a main scanning cross-section, a convergence degree m of each of the first and second light beams, which is expressed in terms of distance Sk (mm) from a rear principal plane of each of the first and second imaging optical systems to respective one of the first and second scanning surfaces and a synthetic focal length f (mm) of each of the first and second optical systems, shall be within a predetermined range.

Description

  The present invention relates to an optical scanning device and is suitable for an image forming apparatus such as a laser beam printer, a digital copying machine, or a multifunction printer.

  Conventionally, in an optical scanning device for a color image forming apparatus for forming a color image, a configuration in which an optical deflector is shared by a plurality of light beams corresponding to a plurality of scanned surfaces has been proposed for the purpose of downsizing the entire apparatus. (Patent Document 1).

  In this configuration, the optical path between the optical deflector and each of the plurality of scanned surfaces is folded back in order to arrange an optical component such as a lens or a mirror so as not to interfere with the light beam in a limited space. It is bent by.

JP 2004-317790 A

  Here, in the optical scanning device of Patent Document 1, in order to further reduce the size by narrowing the interval between the plurality of scanned surfaces in the sub-scanning section, the imaging lens on the optical deflector side is further replaced with the optical deflector. However, it is difficult to bring the optical deflector substrate and the lens abutting wall into account. At this time, a method of further reducing the substrate or a method of reducing the thickness of the wall may be considered. However, if the substrate size is reduced, the tilting accuracy of the rotation axis of the optical deflector deteriorates, and if the wall is made thinner, assembly is performed. The strength of the time cannot be maintained, and the lens easily falls.

  Although a method of reducing the size by bringing the light beam bent by a plurality of folding mirrors closer to the imaging lens on the optical deflector side is also conceivable, in the conventional optical scanning device, the thickness of the imaging lens on the optical deflector side is considered. Is thick, it tends to interfere with the light flux.

  An object of the present invention is to realize downsizing of the entire apparatus in an optical scanning apparatus having a common deflecting means for deflecting a plurality of light beams corresponding to a plurality of scanned surfaces.

In order to achieve the above object, an optical scanning device according to one aspect of the present invention deflects a first light beam emitted from a first light source means on a first deflection surface and emits the light from the second light source means. The second light beam to be deflected by a second deflecting surface different from the first deflecting surface, and the common deflecting means, and the first light beam deflected by the deflecting means to the first scanned surface A first imaging optical system for condensing, a second imaging optical system for condensing the second light beam deflected by the deflecting means on a second surface to be scanned, and the first deflection surface; A first optical path between the first scanned surface and a second optical path between the second deflection surface and the second scanned surface; And a plurality of reflective optical elements that bend each of the second optical paths so as to intersect with each other in the sub-scan section, And the effective image area in the main scanning direction on each of the second scanned surfaces is W (mm), and the distance between the first scanned surface and the second scanned surface in the sub-scan section is Dp (mm). )
0.25 <Dp / W <0.35
In the main scanning section, the convergence degree of each of the first and second light beams incident on the first and second deflection surfaces is m, and the first and second results are satisfied. Sk (mm) is the distance from each rear main plane of the image optical system to each of the first and second scanned surfaces, and the combined focal length of each of the first and second imaging optical systems. f (mm), and the convergence m is m = 1−Sk / f
When expressed as
0.1 <m <0.3
It satisfies the following condition.

(Function)
When considering the reverse optical path from the scanned surface to the deflecting means via the imaging optical system, the light beam exiting the imaging optical system and traveling to the deflecting means within the main scanning section is not a parallel light flux as in the prior art. By using a divergent light beam, the refractive power of the imaging optical system can be made weaker than before. This corresponds to the fact that the curvature of the refractive power of the imaging optical element in this imaging optical system becomes small (the radius of curvature becomes large) in the main scanning section. As a result, the imaging optical element in the imaging optical system can be brought closer to the deflection means side, and interference between the imaging optical element and the light beam whose optical path is bent can be avoided (a plurality of reflective optical elements). The elements M2 and M3 can be made closer to the imaging optical element side).

  According to the present invention, it is possible to provide an optical scanning device suitable for miniaturization using a convergent light beam having a predetermined convergence as an incident light beam, and an image forming apparatus using the same.

(A) is a sub-scan sectional view of the optical scanning device according to the embodiment of the present invention, (B) is a developed view in the main scanning direction, and (C) is a main scanning sectional view in the vicinity of the deflecting means. It is a figure explaining the relationship between the convergence of the optical scanning device which concerns on embodiment of this invention, and a lens shape. (A) is a figure explaining generation | occurrence | production of the jitter of the optical scanning device based on embodiment of this invention, (B) is explanatory drawing of the scanning optical system in a convergence system. It is a figure explaining the relationship between the convergence and the jitter of the optical scanning device concerning the embodiment of the present invention. 1 is a schematic view of a main part of a color image forming apparatus equipped with an optical scanning device according to an embodiment of the present invention.

  Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings.

<< First Embodiment >>
(Image forming device)
FIG. 5 is a schematic diagram of a main part of an image forming apparatus equipped with an optical scanning device according to an embodiment of the present invention. The present embodiment is a tandem type color image forming apparatus in which four optical scanning devices (optical imaging optical systems) are arranged in parallel and image information is recorded on a photosensitive drum surface as an image carrier. In FIG. 5, reference numeral 60 denotes a color image forming apparatus, 12 denotes an optical scanning device shown in the first embodiment, 21, 22, 23, and 24 denote photosensitive drums as photosensitive members as image carriers, and 31, 32, and 33, respectively. , 34 are developing units, and 51 is a conveyor belt.

  In FIG. 5, a developing device (not shown) that develops an electrostatic latent image formed on the photosensitive surface of the photoreceptor as a toner image, and the toner image developed by the developing device is a transfer material (recording material). And a fixing device for fixing the transferred toner image to the transfer material.

  In FIG. 5, the color image forming apparatus 60 receives color signals as R (red), G (green), and B (blue) code data from an external device 52 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 53 in the apparatus. These image data are respectively input to the optical scanning device 12. From these optical scanning devices, light beams 41, 42, 43, 44 modulated according to each image data are emitted, and the photosensitive surfaces of the photosensitive drums 21, 22, 23, 24 are caused by these light beams. Scanned in the main scanning direction.

  The color image forming apparatus according to the present embodiment emits scanning light corresponding to each color of C (cyan), M (magenta), Y (yellow), and B (black) from the optical scanning device 12. In parallel, image signals (image information) are recorded on the surfaces of the photosensitive drums 21, 22, 23, and 24, and color images are printed at high speed.

  The color image forming apparatus according to the present embodiment uses the light beam based on each image data by the optical scanning device 12 as described above, and converts the latent images of the respective colors onto the corresponding photosensitive drums 21, 22, 23, and 24, respectively. Is formed. Thereafter, a single full color image is formed by multiple transfer onto a recording material.

  As the external device 52, 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 60 constitute a color digital copying machine.

  A color image forming apparatus having a configuration in which a photosensitive drum is disposed above the optical scanning device may be used. In that case as well, it is possible to achieve a compact color image forming apparatus.

(Optical scanning device)
1A is a sub-scan sectional view of an optical scanning device according to an embodiment of the present invention, FIG. 1B is a development view in the main scanning direction, and FIG. 1C is a main scanning sectional view in the vicinity of a deflecting unit. Here, in the following description of the present embodiment, the optical axis or axis of the imaging optical system or imaging optical element is an axis that passes through the center of the scanned surface and is perpendicular to the scanned surface. . The sub-scanning direction (Z direction) is a direction parallel to the rotation axis of the deflecting unit. The main scanning section is a section having the normal in the sub scanning direction. The main scanning direction (Y direction) is the direction in which the light beam deflected and scanned by the deflecting means is projected onto the main scanning section. The sub-scanning cross section is a cross section whose normal is the main scanning direction.

  In FIG. 1B, reference numeral 1 denotes light source means (first and third light source means) that are provided two apart from each other in the sub-scanning direction, and is made of, for example, a semiconductor laser. Reference numeral 3 denotes an aperture stop which shapes the beam shape by limiting the passing light flux. An anamorphic lens 2 converts the divergent light beam emitted from the light source means 1 into weakly convergent light in the main scanning section, and extends the main scanning direction in the sub-scanning section on a deflecting surface 5a of an optical deflector 5 to be described later. So that it is formed as a line image. Note that the anamorphic lens 2 may have two configurations: a collimator lens that converts weakly convergent light in the main scanning section and the sub-scanning section, and a cylindrical lens that has power only in the sub-scanning direction.

  In FIG. 1B, regarding the light source means 1, each element of the aperture stop 3 and the anamorphic lens 2 with respect to the light beam Ra from the first light source means is incident so that the light beam is incident on the deflecting means from above in the sub-scan section. An optical system LA is configured. Then, the light beam deflected by the deflecting unit by the incident optical system LA becomes a light beam Ra from the first light source unit heading downward in the sub-scan section (FIG. 1A). On the other hand, the incident optical system LB with respect to the light beam Rb from the third light source means has the same configuration, and the direction of incidence in the sub-scanning direction to the optical deflector is reversed (the light beam is incident on the deflecting means from below in the sub-scan section). ) Only. The light beam deflected by the deflecting means becomes a light beam Rb directed upward in the sub-scan section and forms a first optical path.

  Reference numeral 5 denotes an optical deflector serving as a deflecting means, which is composed of a polygon mirror having four faces with a circumscribed radius R5 = 10 mm. This optical deflector 5 is rotated at a constant speed in the direction of arrow A in the figure by a driving means (not shown).

  The optical scanning device according to the present embodiment includes scanning units SR (including two imaging optical systems as one set) and opposed to each other with the optical deflector 5 interposed therebetween, and SL (similarly, two imaging optical systems as a different set). ). Similarly to the light source means 1, two light source means (second and fourth light source means) provided in the sub-scanning direction are provided. Then, the light beam deflected by the deflecting unit becomes a light beam R′a from the second light source unit heading downward in the sub-scan section (FIG. 1A).

  On the other hand, the incident optical system LB for the light beam Rb from the fourth light source means has the same configuration, and the direction of incidence in the sub-scanning direction on the optical deflector is reversed (the light beam is incident on the deflecting means from below in the sub-scan section). ) Only. The light beam deflected by the deflecting means becomes a light beam R′b directed upward in the sub-scan section, and forms a second optical path.

  Accordingly, the four light beams Ra, Rb, R′a, and R′b are deflected and scanned by one optical deflector 5. Then, the photosensitive drum surfaces 8A (Bk), 8B (C), 8C (M), and 8D (Y), which are scanned surfaces corresponding to the respective light beams, are scanned.

  Here, in the scanning unit SR, the deflected light beam Ra deflected and reflected by the deflecting surface 5a of the optical deflector (four-sided polygon mirror) 5 serving as a deflecting means passes through the imaging lenses 6A and 7A and is then turned back by the turning mirror M1. Then, it is guided to the photosensitive drum 8A (Bk) which is the surface to be scanned. The imaging optical system guided to the photosensitive drum 8A (Bk) via the imaging lenses 6A and 7A as the imaging optical elements and the reflection mirror M1 as the reflection optical element is referred to as a first imaging optical system. To do.

  Further, the deflected light beam Rb deflected and reflected by the deflecting surface 5a of the optical deflector 5 passes through the imaging lens 6A, is folded back by the reflecting mirror M2, passes through the imaging lens 7B, and is folded back by the reflecting mirror M3. Then, after intersecting with its own light beam that has passed through the imaging lens 6A within the sub-scanning cross section, it reaches the photosensitive drum 8B (C) that is the surface to be scanned.

  Here, the position that intersects with its own light beam that has passed through the imaging lens 6A is more optically than the imaging lens 6A as the first imaging optical element that is optically closest to the deflecting means constituting the imaging optical system. This is the scanned surface side (photosensitive drum 8B (C) side). The reflecting mirrors M2, M3 and M′2, M′3 bend the first and second optical paths (optical paths toward the photosensitive drums 8B and 8C) so as to intersect with each other in the sub-scan section. .

  The imaging optical system guided to the photosensitive drum 8A (Bk) via the imaging lenses 6A and 7B as the imaging optical elements and the reflection mirrors M2 and M3 as the reflection optical elements is the second imaging optical system. And

  On the other hand, in the imaging optical system SL, the same optical path is used as in the imaging optical system SR. That is, the deflected light beam R′a deflected and reflected by the deflecting surface 5′a of the common optical deflector 5 passes through the imaging lenses 6′A and 7′A, and is then folded by the folding mirror M′1 to be scanned. It is guided to the photosensitive drum 8D (Y) which is the surface. Further, the deflected light beam R′b deflected and reflected by the deflecting surface 5′a of the optical deflector 5 passes through the imaging lens 6′A and then is folded back by the folding mirror M′2 and passes through the imaging lens 7′B. It is folded by the folding mirror M′3 and guided to the photosensitive drum 8C (M) that is the surface to be scanned.

  Here, in the following description, an optical system (an optical system that scans the scanned surface) that forms an image on the scanned surfaces 8A and 8D farthest from the optical deflector 5 is defined as the imaging optical system SA (the first imaging). Optical system), referred to as SD. An optical system that forms an image on the scanned surfaces 8B and 8C closest to the optical deflector 5 (an optical system that scans the scanned surface) is referred to as an imaging optical system SB (the second imaging optical system), SC. .

  Further, “closest to the optical deflector 5” means optically closest to the deflecting surface of the optical deflector 5, and “farthest to the optical deflector 5” means optically the optical deflector 5. The farthest from the deflection surface. Since the configuration and optical action of the two imaging optical systems SR and SL in this embodiment are the same as each other, the following description will be made with the imaging optical system SR.

  In the present embodiment, the plurality of imaging optical systems SA and SB are each composed of an imaging lens as a plurality of imaging optical elements, and the imaging lens 6A closest to the deflecting unit includes a plurality of imaging optical systems SA, Shared by SB. Further, the imaging lenses 7A and 7B close to the scanned surface use lenses (eccentric lenses) having different outline center lines from the line connecting the surface vertices of the entrance surface and the exit surface.

  Therefore, the imaging lenses 7A and 7B used in the scanning unit SR are differently shaped lenses having different lens optical axis shifting methods. Similarly, the imaging lenses 7′A and 7′B used in the scanning unit SL are lenses having different shapes with different lens optical axis shifting methods. By doing so, it is possible to avoid interference between the imaging lens 7B (7B ') and the light beam Rb (Rb') due to the eccentricity of the imaging lens 7B (7B '). However, the imaging lenses 7A and 7B can be made to be the same lens by making the optical parameters such as the degree of convergence m, the Kθ coefficient, and the focal length of each lens, which will be described later, the same in the optical system for the light beams Ra and Rb.

  In the present embodiment, the number of folding mirrors as the reflection type optical elements of the imaging optical system SB that forms an image on the scanned surface 8B closest to the optical deflector 5 is set to the minimum two. In this embodiment, the imaging lens 7A closest to the scanned surface 8A of the imaging optical system SA (first imaging optical system) that forms an image on the scanned surface 8A farthest from the optical deflector 5, It is arranged closer to the optical deflector 5 than the folding mirror M1 closest to the scanned surface 8A. Thereby, the length of the imaging lens 7A in the main scanning direction is shortened.

  On the other hand, a configuration in which the imaging lens 7A closest to the scanned surface 8A is arranged closer to the scanned surface 8A than the folding mirror M1 closest to the scanned surface 8A is also conceivable, but the length of the imaging lens 7A in the main scanning direction is also conceivable. This is not preferable because the overall length of the apparatus becomes large. Therefore, by adopting the optical design values and arrangement as described above, it is possible to achieve both cost reduction and downsizing of the apparatus with the minimum necessary number of parts.

  1A shows a housing (optical box) of the optical scanning device, and a dotted line 10 'shows an outer peripheral portion of the housing of the conventional device. In this embodiment, the distance Dp of the surface to be scanned (equal to the distance between the condensing positions of the light beams) can be 58 mm (11 mm shorter than the conventional apparatus), and the horizontal length of the optical scanning apparatus is also 11 ×. 3 = 33 mm could be shortened. In general, considering that the length in the drum arrangement direction of a color image forming apparatus that outputs an A4 size image is about 400 to 500 mm, a compactness of less than 10% is achieved.

  Note that the above-described scanning surface interval (equal to the light beam condensing position interval) Dp is a synchronous control of the rotation of the photosensitive drums 21, 22, 23, and 24 and the movement of the conveying belt 51 shown in FIG. In order to facilitate, it is made equal to a plurality of scanned surfaces.

  Here, FIG. 1B is a main scanning sectional view of the imaging optical system SA with respect to the light beam Ra among the light beams Ra and Rb deflected and scanned to the same side by the optical deflector 5. The imaging optical system SB for the light beam Rb is also a similar main scanning sectional view, and is not shown. In the figure, C0 is a deflection reflection point (reference point) of the principal ray of the axial light beam. In the sub-scanning direction, the light beams Ra and Rb intersect at the deflection reflection point C0. The deflection reflection point C0 is a reference point of the imaging optical system, and the distance from the deflection reflection point C0 to the surface to be scanned is hereinafter defined as “optical path length L of the imaging optical system”. Tables 1 and 2 show lens surface shapes and optical arrangements of the optical scanning device according to the present embodiment.

The incident surface of the anamorphic lens 2 of this embodiment is a diffractive surface in which a diffraction grating is formed on a plane, and the exit surface is an anamorphic refracting surface having different radii of curvature in the main scanning direction and the sub-scanning direction. The anamorphic lens 2 is formed by injection molding using a plastic material, and is a so-called temperature compensation optical system that compensates for changes in refractive power due to environmental fluctuations with changes in diffraction power due to wavelength changes of the semiconductor laser. The diffraction surface is defined by the phase function expressed below.
φ = 2πM / λ (C ₃ Z ² + C ₅ Y ² )
Here, φ is a phase function, M is a diffraction order, and this embodiment uses first-order diffracted light (M = 1). λ is a design wavelength, and in this embodiment, λ = 790 nm.

  The generatrix shapes of the lens entrance surfaces and lens exit surfaces of the imaging lenses 6A and 7A are both aspherical shapes that can be expressed as functions up to the 12th order. The intersection between the lens surfaces of the imaging lenses 6A and 7A and the optical axes of the imaging lenses 6A and 7A is the origin, the optical axis direction is the X axis, and the axis orthogonal to the optical axis in the main scanning section is the Y axis. In this case, the bus direction corresponding to the main scanning direction is expressed by the following equation.

(Where R is the radius of curvature of the bus, and K, B ₄ , B ₆ , B ₈ , B ₁₀ , B ₁₂ are aspheric coefficients)
The numerical values of the aspheric coefficients B ₄ , B ₆ , B ₈ , B ₁₀ and B ₁₂ are different on the side where the semiconductor laser 1 is arranged and the side where the semiconductor laser 1 is not arranged. B _4U , B _6U , B _8U , B _10U , B _12U are values on the side where the semiconductor laser 1 is arranged, B _4L , B _6L , B _8L , B _10L , B _12L are not arranged with the semiconductor laser 1. It is the value of the side. As a result, an asymmetric shape in the main scanning direction can be expressed.

  Further, the sub line direction corresponding to the sub scanning direction is expressed by the following expression.

S is a child line shape defined in a plane perpendicular to the main scanning plane, including the normal line of the bus bar at each position in the bus bar direction. Here, the radius of curvature (sub-wire curvature radius) Rs ^* in the sub-scanning direction at a position Y away from the optical axis in the scanning direction is expressed by the following equation.
1 / Rs ^* = 1 / Rs + D ₂ × Y ² + D ₄ × Y ⁴ + D ₆ × Y ⁶ + D ₈ × Y ⁸ + D ₁₀ × Y ¹⁰
(Where Rs is the radius of curvature of the strand on the optical axis, D ₂ , D ₄ , D ₆ , D ₈ , D ₁₀ are the strand changing coefficients)
As in the main scanning shape, the aspherical coefficients D _{2 to} D ₁₀ are determined on the side where the semiconductor laser 1 of the optical scanning device is disposed (D _{2U to} D _10U ) and on the side where the semiconductor laser 1 is not disposed (D _2L to D _10L) and de varying numbers. Thereby, an asymmetric shape in the main scanning direction can be expressed. In the present embodiment, the function of the surface shape is defined by the above definition formula, but the present invention is not limited to this.

  By the way, normally, in an optical scanning device in which a light beam is incident from an oblique direction in the sub-scanning section, a phenomenon is observed in which a spot collapses due to a twist of wavefront aberration. In the present embodiment, the twist of wavefront aberration is reduced by optimizing the power arrangement of each surface and the shift amount of the imaging lens 7A. That is, in the imaging optical system SA of the present embodiment, the wavefront aberration is corrected by shifting the imaging lens 7A in the sub-scanning direction by −1.638 mm with respect to the surface P0.

(Convergent light flux)
In the present embodiment, the light beam incident on the imaging lens 6A as the imaging optical element is set as a convergent light beam in the main scanning section, so that the refractive power (power) of the imaging lens 6A in the main scanning direction is described as follows. ) Is weakened to reduce the lens thickness. Further, in the present embodiment, since the deflecting surface of the optical deflector 5 as the deflecting means is a flat surface, the incident light beam to the optical deflector is set as a convergent light beam, and all the incident light beams directed to the optical deflector 5 are subjected to main scanning. A converged light beam having the same convergence m in the cross section is used. Accordingly, the color image forming apparatus is miniaturized by shortening the optical path length L of all the imaging optical systems.

(Lens thickness reduction by convergent luminous flux)
Hereinafter, the effect of reducing the lens thickness by the convergent light beam will be described. The light beam deflected by the deflecting unit and incident on the imaging lens serving as the imaging optical element is converted into a convergent beam within the main scanning section, and the reverse optical path from the scanned surface to the deflecting unit via the imaging optical element is reduced. Considering the following: That is, the light beam that exits the imaging optical element within the main scanning section and travels toward the deflecting means is a divergent light beam, not a conventional parallel light beam. As a result, the curvature of the imaging optical element becomes loose, and the thickness of the main scanning end becomes thick when maintaining the center thickness. If the thickness of the main scanning end is minimized, the center thickness is reduced. Will be reduced.

(Convergence m)
Here, when considering the reverse optical path from the scanned surface to the deflecting means through the imaging optical element, the incident angle α of the central ray of the light beam incident on the imaging optical element in the reverse optical path with respect to the optical axis of the imaging optical element Assuming that the emission angle is β, tan β / tan α indicates the degree of convergence. The degree of convergence is defined as follows as the degree of convergence m.

m = 1-Sk / f
Sk: distance from the rear main plane of the imaging optical element to the scanned surface in the main scanning section (mm)
f: Focal length (mm) in the main scanning section of the imaging optical element
Further, when the imaging optical system is composed of a plurality of imaging optical elements, the following expression is established.

m = 1-Sk / f
Sk: distance from the rear main plane of the imaging optical system to the scanned surface in the main scanning section (mm)
f: Composite focal length (mm) in the main scanning section of the imaging optical system
Here, the convergence m preferably satisfies the following conditions.

0.1 <m <0.3 (Formula A)
If the lower limit of Formula A is exceeded, the effect of the present invention on the conventional apparatus will not be significant. On the other hand, if the upper limit value of the expression A is exceeded, the problem of cardiac jitter peculiar to the convergence system (or diverging system) described below arises. That is, increasing the degree of convergence m has the effect of reducing the lens thickness, but jitter δY (convergence system surface eccentricity jitter) shown in FIG. That is, in the converging system, when the deflecting surface 5a of the optical deflector is decentered due to a manufacturing error or the like, the position where the light beam is condensed on the scanned surface 8A is generated (Patent Document 2). This is a problem peculiar to the convergence system (or diverging system), and does not occur in principle if the incident optical system is parallel light.

  In FIG. 3A for explaining the principle of generation of the converging system surface eccentricity jitter, only the principal ray of the reflected light beam from the deflecting surface 5a of the optical deflector 5 and the deflected deflecting surface 5b is shown. Here, the two principal rays Rap0 and Rap1 are parallel in the main scanning section before entering the scanning imaging system SA. The two parallel principal rays intersect in the main scanning section at the focal position 13A of the imaging optical system SA. On the other hand, since the surface to be scanned is a position 8A on the optical deflector 5 side from the focal position 13A of the scanning imaging system, an image is formed on the surface to be scanned 8A at a position shifted by δY shown in the figure.

  FIG. 3B depicts a light beam (principal ray and two marginal rays) reflected by the deflecting surface 5a, and is thus condensed on the surface to be scanned 8A. When the degree of convergence is m = 0, that is, when the incident light beam is parallel light within the main scanning section, 8A and 13A coincide with each other, so that no jitter is generated even if the deflection surface 5a is decentered.

  FIG. 4 is a graph plotting the amount of converging system surface decentering jitter δY at each image height Y generated when the deflecting surface is decentered by 10 μm in the focusing optical system shown in FIG. . As described above, the parallel light flux has zero jitter, and the amount of jitter increases as the degree of convergence m increases.

  This jitter causes an image defect as an image moire when the amount of eccentricity of each surface of the deflection surface is different. As a countermeasure, it is possible to manufacture an optical deflector with a reduced amount of eccentricity. However, there is a limit to the processing accuracy, and when using a method such as selecting a polygon mirror with a small amount of eccentricity, it is obvious. Incurs an increase in cost. In a normal processing method, the relative surface eccentricity of the polygon is about 10 μm.

  Here, since the jitter amount δY is generally allowed to be about 10 μm, the image quality within the allowable level is maintained at the convergence m that satisfies (Expression A). In this embodiment, a weakly convergent light beam of m = 0.199 is used, and the imaging lens 6A can shift the exit surface of the imaging lens 6A by 2.7 mm toward the deflecting unit as compared with the case where a parallel light beam is incident. did it. As a result, the imaging lens 6A can be brought closer to the deflecting means, and interference between the imaging lens 6A and the light beam whose optical path is bent can be avoided (imaging optical elements of the folding mirrors M2 and M3). To the side).

  Furthermore, in this embodiment, the convergence m of the two incident optical systems LA and LB to the optical deflector 5 is made the same on the assumption that the deflecting surface of the optical deflector 5 is a plane. Depending on the sign of the degree of convergence m, the light beam incident on the optical deflector is divided into the following three cases. That is, when m = 0, a parallel light beam enters the optical deflector in the main scanning direction, when m <0, a divergent light beam enters the optical deflector in the main scanning direction, and when m> 0, in the main scanning direction. The convergent light beam enters the optical deflector.

  FIG. 2 is a diagram for explaining the change in the thickness t of the imaging lens 6A having the main power in the main scanning direction when the design is performed with the convergence degree m. As the graph moves from right to left, the convergence increases as m = 0.000, m = 0.105, m = 0.199, and m = 0.293. m = 0.199 is the imaging lens shown in the numerical embodiments of Tables 1 and 2.

  While the thickness t = 8.0 (mm) in the case of the parallel system (m = 0.00) as the conventional example, when the convergence m is increased, t = 7.0 (mm), t = 5.5 (mm), t = 5.0 (mm) and the lens thickness can be reduced. The more the power in the main scanning direction is applied to the incident optical system, the more the power in the main scanning direction of the scanning optical system can be reduced, resulting in a lens having a large curvature radius.

(Imaging lens thickness t and scanning surface distance Dp)
Here, as a result of intensive studies, it is preferable that the scanning surface interval Dp related to the downsizing of the apparatus and the imaging lens thickness t related to the achievement of the downsizing are configured to satisfy the following expressions. It has been found.

0.15 <2t / Dp <0.25 (Formula B)
At the degree of convergence satisfying (Formula A), the ratio of the center thickness t of the imaging lens 6A to the distance Dp between the scanning surfaces is as follows.
2t / Dp = 2 × 7.0 / 58 = 0.24 (when m = 0.105)
2t / Dp = 2 × 5.5 / 58 = 0.19 (when m = 0.199)
2t / Dp = 2 × 5.0 / 58 = 0.17 (when m = 0.293)
FIG. 1C shows a main scanning section in the vicinity of the optical deflector 5 and the imaging lens 6A. It can be seen that the light-reflecting light beam from above the paper surface with the optical path bent does not interfere with the thinned imaging lens 6A (6′A) even when the optical deflector substrate and lens abutting wall are the same as in the conventional apparatus. .

(Effective image area W and scanning surface distance Dp)
In the present embodiment, the effective image area in the main scanning direction on the surface to be scanned is W (mm), and the interval (interval of the light beam condensing position) in the sub-scan section of the plurality of surfaces to be scanned is Dp (mm). In this case, it works effectively when the scanning surface interval is narrow enough to satisfy the following expression.

0.25 <Dp / W <0.35 (Formula C)
When the lower limit of (Formula C) is exceeded, unless the convergence m is made very strong, interference between the imaging lens 6A and the light beam whose optical path is bent cannot be avoided, and the above-described deterioration due to jitter cannot be ignored. Therefore, it is not preferable. Further, if the upper limit value of (Formula C) is exceeded, it is not preferable since the distance Dp to be scanned is widened and the image forming apparatus itself cannot be made compact. In the present embodiment, since Dp = 58 (mm) and W = 220 (mm), Dp / W = 0.26, which satisfies Expression (C).

(Ratio of focal length between imaging optical element and imaging optical system)
Moreover, it is preferable that the following conditions are satisfied with respect to the ratio of the focal lengths of the imaging optical element and the imaging optical system.

0.7 <f1 / f <0.9 (Formula D)
When the lower limit of (Formula D) is exceeded, the main scanning power of the imaging lens 6A as the imaging optical element increases, and as a result, the radius of curvature decreases and the lens thickness increases. If the upper limit of (Formula D) is exceeded, the on-axis negative power of the imaging lens 7A becomes weak, and if the thickness of the main scanning end is to be secured, the center thickness increases. As a result, the lens material cost increases and the resin molding time increases, leading to a total lens cost increase. Therefore, if the power ratio of the two imaging lenses is set so as to be within (Equation D), both the imaging lenses 6A and 7A can be designed as an optimally thick lens, resulting in cost reduction. Connected.

  By the way, the ratio K (Kθ coefficient, Y = Kθ) of the scanning field angle θ (rad) with respect to the scanning image height Y (mm) becomes the same value as the main scanning focal length of the imaging optical system in the case of parallel light incidence. However, in the case of the convergence system, the main scanning focal length of the imaging optical system becomes longer than K. In this embodiment, while K = 123 (mm / rad), the main scanning focal length of the imaging optical system is as long as f = 152.16 (mm). In the present embodiment, the main scanning focal length f1 of the imaging lens 6A as the imaging optical element is 117.02 (mm), and the main scanning focal length f2 of the imaging lens 7A is −349.67 (mm). f1 / f = 0.769, which satisfies the expression D.

(The circumscribed circle radius R5 of the deflection surface and the distance Dp to be scanned)
Further, regarding the circumscribed radius R5 of the deflection surface and the distance Dp to be scanned, it is preferable to satisfy the following expression.

0.14 <R5 / Dp <0.20 (formula E)
If the lower limit of (Equation E) is exceeded, the reflecting surface of the polygon mirror is too small to perform wide-angle scanning, and the distance L from the optical deflector to the surface to be scanned becomes long. If the upper limit of (Equation E) is exceeded, the polygon mirror is larger than the distance Dp of the surface to be scanned, and interference between the imaging lens and the light beam is likely to occur. Therefore, by setting the circumscribed circle radius R5 of the polygon mirror surface, which is the deflection surface, so as to be within (Equation E), both miniaturization by wide-angle scanning and avoidance of interference between the imaging lens and the light beam can be achieved. Can do. In the present embodiment, R5 / Dp = 0.172 satisfies (Equation E).

(Modification)
As mentioned above, although embodiment of this invention was described, this invention is not limited to this, A various deformation | transformation is possible within the range of the identity of this invention.

(Modification 1)
In the above-described embodiment, the oblique incidence optical system that causes the light beam to be incident on the deflection surface of the optical deflector from the oblique direction within the sub-scan section has been described, but the present invention is not limited to this. For example, polygon mirrors as deflecting means may be stacked in two stages in the sub-scanning direction so that the light beam is incident vertically on the deflection surface of each polygon mirror within the sub-scanning section.

(Modification 2)
In the above-described embodiment, the imaging optical system includes two imaging lenses (imaging optical elements). However, the present invention is not limited to this, and the imaging optical system includes a plurality of imaging optical systems including a plurality of imaging lenses. It may be configured to include an imaging lens or only one imaging lens. Further, although the light beam whose optical path is bent is formed by the folding mirror as two reflection type optical elements, the folding mirror as three or more reflection type optical elements may be used. However, in general, the larger the number of folding mirrors, the easier it is to create streaks in the image due to dust and scratches adhering to the mirrors, the banding of scanning lines due to mirror vibrations, and higher costs. It should be considered separately.

(Modification 3)
In the above-described embodiment, the convergence m of the light beams incident on the deflecting unit is all set to the same value on the assumption that the deflecting surface of the deflecting unit is a plane, but may be set to different values. In addition, when the deflecting surface of the deflecting unit is not a flat surface and has refractive power in the main scanning section, the light beam incident on the deflecting unit can be a parallel light beam.

(Modification 4)
In the above-described embodiment, the light beam whose optical path is bent is passed through the space on the side away from the optical deflector of the imaging lens closest to the optical deflector. However, the optical path is provided in the space between the optical deflector and the optical deflector. It is also possible to pass a bent light beam.

5 .. Optical deflector (deflection means), 6A, 7B .. Imaging lens (imaging optical element), 8A .. Scanned surface (photosensitive drum surface), M2M3 .. Folding mirror (reflection optical element)

Claims (11)

The first light beam emitted from the first light source means is deflected by the first deflection surface, and the second light beam emitted from the second light source means is different from the first deflection surface. A common deflection means for deflecting on the surface;
A first imaging optical system for condensing the first light beam deflected by the deflecting unit on the first scanned surface, and the second light beam deflected by the deflecting unit on the second scanned surface. A second imaging optical system for focusing;
A first optical path between the first deflection surface and the first scanned surface, and a second optical path between the second deflection surface and the second scanned surface, respectively. A plurality of reflective optical elements that bend each of the first and second optical paths so as to intersect with each other in the sub-scan section,
An optical scanning device comprising:
The effective image area in the main scanning direction on each of the first and second scanned surfaces is W (mm), and the interval between the first scanned surface and the second scanned surface in the sub-scanning section is defined as W (mm). When Dp (mm),
0.25 <Dp / W <0.35
Satisfying the following conditions, and
In the main scanning section, the convergence degree of each of the first and second light beams incident on the first and second deflection surfaces is m, and the rear side of each of the first and second imaging optical systems. The distance from the main plane to each of the first and second scanned surfaces is Sk (mm), and the combined focal length of each of the first and second imaging optical systems is f (mm). Convergence m is m = 1−Sk / f
When expressed as
0.1 <m <0.3
An optical scanning device characterized by satisfying the following conditions.   The first optical path intersects itself on the first surface to be scanned side with respect to the first imaging optical element that is optically closest to the deflecting means and that constitutes the first imaging optical system. And the second optical path is itself closer to the second surface to be scanned than the second imaging optical element optically closest to the deflecting means constituting the second imaging optical system. The optical scanning device according to claim 1, wherein When the focal length in the main scanning section of each of the first and second imaging optical elements is f1 (mm),
0.7 <f1 / f <0.9
The optical scanning device according to claim 1, wherein the following condition is satisfied. When the thickness of each of the first and second imaging optical elements is t (mm),
0.15 <2t / Dp <0.25
The optical scanning device according to claim 1, wherein the following condition is satisfied.   5. The convergence of each of the first and second light beams incident on the first and second deflecting surfaces in the main scanning section is the same. 6. The optical scanning device according to 1. The deflection means includes four deflection surfaces, and when the circumscribed circle radius of each of the four deflection surfaces is R5 (mm),
0.14 <R5 / Dp <0.20
The optical scanning device according to claim 1, wherein the following condition is satisfied.
  The first imaging optical system includes a first imaging optical element and a third imaging optical element, and the second imaging optical system includes a second imaging optical element and a fourth imaging optical element. The optical scanning device according to claim 1, comprising: an imaging optical element.   The deflecting means deflects the third light beam emitted from the third light source means on the first deflecting surface, and the fourth light beam emitted from the fourth light source means on the second deflecting surface. The first and third light beams pass through the first imaging optical element, and the second and fourth light beams pass through the second imaging optical element. The optical scanning device according to claim 1, wherein:   9. The optical scanning device according to claim 1, wherein there are two reflective optical elements in each of the first optical path and the second optical path.   An electrostatic latent image formed on the photosensitive surface of the photosensitive member disposed on the scanned surface by the optical scanning device according to any one of claims 1 to 9 and a light beam emitted from the optical scanning device. A developing unit that develops the toner image as a toner image; a transfer unit that transfers the developed toner image onto a transfer material; and a fixing unit that fixes the transferred toner image onto the transfer material. An image forming apparatus.   The optical scanning device according to claim 1, and a printer controller that converts code data input from an external device into an image signal and inputs the image signal to the optical scanning device. Image forming apparatus.