JP2014153600A - Optical scanner and color image forming apparatus using the same - Google Patents

Abstract

An optical scanning device and a color image forming apparatus using the same which can achieve a further reduction in thickness and a reduction in the number of parts to achieve a low cost.
A deflecting unit, an incident optical system that guides a first light beam and a second light beam respectively emitted from a first light source unit and a second light source unit to the deflecting unit, and a first light beam and a second light beam emitted from the deflecting unit. And an imaging optical system for guiding the first scanned surface to a second scanned surface whose spatial distance from the deflecting means is closer to the first scanned surface, and the deflecting means uses the first light beam. And an optical scanning device that scans the first scanned surface and the second scanned surface by deflecting the second light flux respectively, and the imaging optical system is a second that extends from the deflecting means to the second scanned surface. It includes an optical surface through which the light beam passes twice and the first light beam from the deflecting means to the first scanned surface passes only once.
[Selection] Figure 1

Description

  The present invention relates to an optical scanning device and a color image forming apparatus using the same, and is particularly suitable for a laser beam printer having an electrophotographic process, a digital copying machine, a multifunction printer, or the like.

  Conventionally, an optical scanning device has been used in a laser beam printer (LBP), a digital copying machine, a multifunction printer, and the like. In this optical scanning device, a light beam (light beam) that is light-modulated and emitted from a light source means in accordance with an image signal is periodically deflected by an optical deflector formed of, for example, a rotating polygon mirror (polygon mirror). The deflected light beam is focused in a spot shape on the surface of a photosensitive recording medium (photosensitive drum) by an imaging optical system having an fθ characteristic, and image recording is performed by optically scanning the surface.

  Conventionally, in an optical scanning device for a color image forming apparatus, an optical scanning device in which an optical deflector as a deflecting unit is shared by a plurality of light beams has been proposed for the purpose of making the entire device compact and cost-effective. (Patent Document 1). The optical scanning device disclosed therein employs a so-called sub-scanning oblique incidence optical system in which a light beam is incident on the optical deflector obliquely from the sub-scanning direction. The advantage of the sub-scanning oblique incidence optical system is that the light beam deflected and reflected can be separated without increasing the size of the deflection reflection surface of the optical deflector in the sub-scanning direction.

  FIG. 11A is a sub-scan sectional view of an optical scanning device for a color image forming apparatus employing a sub-scanning oblique incidence optical system. In the figure, among the four light beams reflected by the optical deflector 5, the light beams Ra and R'a far from the photosensitive drums 8A to 8D pass through the imaging lenses 6 and 6 'and the imaging lenses 7A and 7'A. After passing through each, it is reflected downward by the folding mirrors M1 and M′1, and forms an image on the scanned surfaces 8A and 8D. On the other hand, in the drawing, the light beams Rb and R′b on the side close to the photosensitive drums 8A to 8D pass through the imaging lenses 6 and 6 ′ and are reflected obliquely upward by the folding mirrors M2 and M′2.

  Then, the light passes through the imaging lenses 7B and 7'B, is again reflected downward by the folding mirrors M3 and M'3, and forms an image on the scanned surfaces 8B and 8C. In this configuration of the optical scanning device, the number of parts of the folding mirror is 6, and the number of parts of the lens is 6.

  In addition, as disclosed in Patent Document 2, a multistage lens (a lens surface is represented by a different expression) in which a plurality of light beams deflected and reflected by an optical deflector are integrated by overlapping a plurality of lens surfaces in the vertical direction. It is also proposed to pass a lens. Thereby, the number of lenses can be reduced.

  FIG. 11B is a sub-scanning sectional view of an optical scanning device using a multistage lens. Similarly to FIG. 11A, among the four light beams reflected by the optical deflector, the light beams Ra and R′a on the side far from the photosensitive drums 8A to 8D are the imaging lenses 6 and 6 ′. It passes through the image lenses 7A and 7'A, respectively. Then, the light is reflected downward by the folding mirrors M1 and M′1 and forms an image on the scanned surfaces 8A and 8D. On the other hand, in the figure, the light beams Rb and R′b on the side close to the photosensitive drums 8A to 8D pass through the imaging lenses 6 and 6 ′ and the imaging lenses 7B and 7′B. Then, the optical path is bent by the three folding mirrors M2, M3, M4 and M′2, M′3, M′4, and images are formed on the scanned surfaces 8B, 8C.

  The imaging lenses 7A and 7B are produced by plastic molding, and are multistage lenses in which two lens surfaces arranged above and below are integrated. By adopting such a configuration, the number of parts of the folding mirror is 8 and the number of parts of the lens is 4, so that the number of lenses having a relatively high cost can be reduced.

JP 2008-76586 A JP 2007-155838 A

  However, the conventional optical scanning device has the following problems to be solved. First, in the conventional optical scanning device shown in FIG. 11A, the way in which the optical paths of the light beams Rb and R′b directed to the scanned surfaces 8B and 8C determine the size in the height direction of the optical scanning device. ing.

  That is, the imaging lenses 7B and 7'B are in the optical path between the two folding mirrors (passing through the imaging lenses 6 and 6 'as the lower optical path) and passing through the imaging lenses 6 and 6' as the upper optical path. The imaging lenses 7B and 7'B are arranged so as to avoid interference with the luminous flux. Then, it becomes difficult to further reduce the thickness of the optical scanning device. In addition, the number of parts is large, and it is difficult to further reduce the cost.

  On the other hand, also in the conventional optical scanning device shown in FIG. 11 (b), the direction of the optical path of the light beam Rb toward the scanned surfaces 8B and 8C determines the size in the height direction of the optical scanning device. That is, when the light beam passing through the imaging lenses 7B and 7'B as the lower optical path is turned back by the three mirrors, the three mirrors are arranged so as to avoid interference with the imaging lenses 7A and 7'A. Then, it becomes difficult to further reduce the thickness of the optical scanning device. Further, regarding the number of parts, the number of imaging lenses can be reduced from six to four with respect to the optical scanning device of FIG. 11A, but conversely, the number of folding mirrors is six. As a result, the total number of parts remained unchanged.

  SUMMARY OF THE INVENTION An object of the present invention is to provide an optical scanning apparatus and a color image forming apparatus using the same, which can achieve a further reduction in thickness and a reduction in the number of parts to achieve a low cost.

  In order to achieve the above object, an optical scanning apparatus according to the present invention includes a deflecting unit, and an incident optical system that guides the first light beam and the second light beam respectively emitted from the first light source unit and the second light source unit to the deflecting unit. And the first scanned light beam and the second luminous flux emitted from the deflecting unit, respectively, and a second scanned surface whose spatial distance from the scanned surface is closer than the first scanned surface. An imaging optical system that guides the light to the surface, and deflects the first light beam and the second light beam by the deflecting unit to scan the first scanned surface and the second scanned surface, respectively. In the scanning device, the imaging optical system may pass the second light flux from the deflecting unit to the second scanned surface twice, and the first optical system from the deflecting unit to the first scanned surface. It includes an optical surface through which the light beam passes only once.

  According to the present invention, it is possible to provide an optical scanning device and a color image forming apparatus using the same that can achieve a further reduction in thickness and a reduction in the number of parts to achieve low cost.

(a) is a sub-scan sectional view of the optical scanning device according to the first embodiment of the present invention, (b) is a main-scan sectional view of the optical system through which the light beam Ra passes in the optical scanning device according to the first embodiment. (C) is a main scanning sectional view of an optical system through which a light beam Rb passes in the optical scanning device according to the first embodiment. (a) is explanatory drawing which shows 2 light beams LA and LA 'of 4 light beams of the optical scanning device based on the 1st Embodiment of this invention, (b) shows 2 light beams LA and LB of 4 light beams. Explanatory drawing (c) is explanatory drawing which shows 2 light beam LA ', LB' of 4 light beams. It is an enlarged view near the imaging lens 7R in the first embodiment. (a) is a figure which shows the spot profile on the to-be-scanned surface of the optical scanning device based on 1st Embodiment, (b) is a figure which shows the f (theta) characteristic on a to-be-scanned surface, (c) is a to-be-scanned surface. It is a figure which shows the scanning line curve above. (a) is a sub-scan sectional view of an optical scanning device according to the second embodiment of the present invention, and (b) is a main-scan sectional view of an optical system through which a light beam Ra passes in the optical scanning device according to the second embodiment. (C) is a main scanning sectional view of the optical system through which the light beam Rb passes in the optical scanning device according to the second embodiment. It is an enlarged view near the imaging lens 7R in the second embodiment. (a) is a figure which shows the spot profile on the to-be-scanned surface of the optical scanning device based on 2nd Embodiment, (b) is a figure which shows the f (theta) characteristic on a to-be-scanned surface, (c) is a to-be-scanned surface. It is a figure which shows the scanning line curve above. (a) thru | or (f) are subscanning sectional drawings of the optical scanning device regarding the modification in this invention. (a) and (b) are sub-scanning sectional views of an optical scanning device for comparison explanation of further modifications in 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. (a) and (b) are sub-scan sectional views of a conventional optical scanning device of an oblique incidence optical system.

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

<< First Embodiment >>
(Color image forming device)
FIG. 10 is a schematic diagram of a main part of a color 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. 10, 60 is a color image forming apparatus, 12 is an optical scanning device, 21, 22, 23 and 24 are photosensitive drums as photosensitive members, 31, 32, 33 and 34 are developing units, and 51 is a conveyor belt. is there. In FIG. 10, a transfer device (not shown) for transferring the toner image developed by the developing device to a transfer material and a fixing device (not shown) for fixing the transferred toner image to the transfer material. Have.

  In FIG. 10, R (red), G (green), and B (blue) color signals are input to the color image forming apparatus 60 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. Then, image signals (image information) are recorded on the photosensitive drums 21, 22, 23, and 24 in parallel, and a color image is printed at high speed.

  In the color image forming apparatus according to the present embodiment, the optical scanning device 12 uses the light beam based on each image data as described above, and the corresponding photosensitive drums 21, 22, 23, and 24 are used for the latent images of the respective colors. Formed on top. 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)
FIG. 1A is a sub-scan sectional view of the optical scanning device according to the first embodiment of the present invention. In this embodiment, an optical scanning device of a type that scans both sides with respect to a plane that includes a rotation axis of an optical deflector as a deflecting unit and is perpendicular to the optical axis of the imaging lens is configured. FIG. 1B is a main scanning sectional development view of the optical system through which the light beam Ra passes in the optical system shown in FIG. FIG. 1C is a main scanning cross-sectional development view of the optical system through which the light beam Rb passes in the optical system shown in FIG. However, the optical path folded by the folding mirror M2 is written in an overlapping manner so that the light beam passes through the imaging lens 7B twice. FIG. 2 shows the incidence of all four light beams on the deflecting means.

  In the following description of the present embodiment, the optical axis or axis of the imaging optical system refers to 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.

(Incoming optical system and optical deflector)
In FIG. 2B, the plurality of incident light beams emitted from the plurality of light source means are inclined obliquely with respect to the plane including the surface normal of the deflecting reflection surface of the deflecting means 5. In FIGS. 1B and 1C, reference numerals 1A and 1B denote first light source means and second light source means, which are made of, for example, a semiconductor laser. Reference numerals 3A and 3B denote collimator lenses, which convert divergent light beams emitted from the light source means 1A and 1B into substantially parallel light. Then, by passing through the cylinder lenses 4A and 4B, an image is formed as a longitudinal line image in the main scanning direction on the deflecting surface 5a of the optical deflector 5 as the deflecting means described later in the sub-scanning section.

  Reference numerals 2A and 2B denote aperture stops, which limit the beam width so as to obtain a desired spot shape on the surface to be scanned. In the present embodiment, the aperture stops 2A and 2B are rectangular-shaped stops, so that a substantially rectangular spot is formed on the surface to be scanned.

  Further, as shown in FIGS. 1B and 2B, each element of the diaphragm 2A, the collimator lens 3A, and the cylinder lens 4A for the light beam Ra (first light beam) constitutes an incident optical system LA. . On the other hand, as shown in FIGS. 1C and 2B, the incident optical system LB with respect to the light beam Rb (first light beam) has the same configuration, and is incident on the optical deflector 5 in the sub-scanning direction. The direction of is opposite (symmetric with respect to the plane Po described later). Here, the optical deflector 5 is composed of a polygon mirror having four surfaces with a circumscribed circle radius of 10 mm. The optical deflector 5 is rotated at a constant speed in the direction of arrow A shown in FIGS. 1B and 1C by a driving means (not shown).

(Imaging optics SR, SL)
The optical scanning device of this embodiment includes two imaging optical systems SR and SL with an optical deflector 5 interposed therebetween, and four optical beams Ra, Rb, R′a, and R′b are generated by one optical deflector 5. The deflection scanning is performed, and the corresponding photosensitive drum surfaces 8A (Bk), 8B (C), 8C (M), and 8D (Y) are scanned.

  Here, in the imaging optical system SR, the first light beam Ra which is a deflected light beam deflected and reflected by the deflecting surface 5a of the optical deflector 5 which is a deflecting means passes through the imaging lenses 6 and 7A and is reflected by the folding mirror M1. Wrapped. Then, an image is formed (condensed in a spot shape) on the photosensitive drum 8A (Bk) as the first photosensitive member which is the first scanned surface.

  The second light beam Rb, which is a deflected light beam deflected and reflected by the deflecting surface 5a of the optical deflector 5, passes through the imaging lenses 6 and 7B and is then folded by a folding mirror M2 serving as a first reflecting means. Then, after passing again through the imaging lens 7B from the opposite direction to the first pass, it is folded back by a folding mirror M3 as a second reflecting means, and used as a second photoconductor as a second scanned surface. Is imaged (condensed in a spot shape) on the photosensitive drum 8B (C).

  On the other hand, the imaging optical system SL is also operated in the same optical path as the imaging optical system SR. The deflected light beam R′a deflected and reflected by the deflecting surface 5′a of the optical deflector 5 passes through the imaging lenses 6 ′ and 7′A, is folded by the folding mirror M′1, and is a photosensitive drum that is the scanned surface. An image is formed on 8D (Y) (condensed in a spot shape). 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 lenses 6 ′ and 7 ′ B, and is then folded back by the folding mirror M ′ 2. Then, after passing again through the imaging lens 7'B from the direction opposite to the first pass, it is folded back by the folding mirror M'3 to form an image (spot-like) on the photosensitive drum 8C (M) which is the scanning surface. Is condensed).

  Note that an optical system (an optical system that scans the scanned surface) that forms an image on the scanned surfaces 8A and 8D far from the optical deflector 5 via the folding mirrors M1 and M′1 as the third reflecting means. These are called optical systems SA and SD. Further, optical systems that form images on the scanned surfaces 8B and 8C close to the optical deflector 5 (optical systems that scan the scanned surfaces) are referred to as imaging optical systems SB and SC. Further, being close to the optical deflector 5 means that the spatial distance (physical distance) is closest to the deflection surface of the optical deflector 5, and being far from the optical deflector 5 is spatial. This means that the distance (physical distance) is farthest from the deflection surface of the optical deflector 5.

  The configuration and optical action of the two imaging optical systems SR and SL in the present embodiment are the same as each other, and will be described below with the imaging optical system SR. The imaging optical systems SA and SB in the present embodiment are each composed of a plurality of imaging lenses, and the imaging lens 6 (second imaging lens) closest to the deflection means is a plurality of imaging optical systems SA, Shared by SB.

  The imaging lenses 7A and 7B close to the surface to be scanned constitute a multi-stage lens (detailed later in FIG. 3) in which a plurality of lens surfaces are arranged side by side in the sub-scanning direction and the lens surfaces are integrally formed. doing. That is, the lens surfaces of the imaging lenses 7A and 7B used in the scanning unit SR have different surface shapes represented by different expressions as described later. By doing so, the number of imaging lenses is reduced, and a reduction in size and cost is achieved.

  Further, in the present embodiment, the number of folding mirrors of the imaging optical system SB that forms an image on the scanned surface 8B close to the optical deflector 5 is set to the minimum two. Therefore, the total number of lenses in the scanning unit SR is 2, and the total number of folding mirrors is 3. Thus, the conventional example shown in FIGS. 11A and 11B (the number of lenses in the scanning unit SR is 3, the number of folding mirrors is 3, or the number of lenses is 2, the number of folding mirrors) Compared to 4), the number of parts in the scanning unit SR is reduced by one point.

  Accordingly, the total number of parts of the scanning unit SR and the scanning unit SL is reduced by two points, and the cost and the size of the optical scanning device can be reduced by reducing the cost of the parts and the occupied space.

  In FIG. 1A, 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, and the deflection reflection point C0 serves as a reference point for the imaging optical system. Each lens surface is optically designed to satisfy a desired optical performance by using the following aspherical expression. The surface P0 is a surface that passes through the deflection reflection point C0 and is perpendicular to the rotation axis of the polygon mirror.

(Specific configuration of incident optical system and imaging optical system)
The optical arrangement of the incident optical system LA and the imaging optical system SA in this embodiment is shown in Table 1, the lens surface shape of the incident optical system LA is shown in Table 2, and the lens surface shape of the imaging optical system SA is shown in Table 3. Similarly, the optical arrangement and lens surface shape of the incident optical system LB and the imaging optical system SB in this embodiment are shown in Table 4, the lens surface shape of the incident optical system LB is shown in Table 5, and the lenses of the imaging optical system SB. Table 6 shows the surface shape.

(Temperature compensation optics)
The collimator lens 3 and the cylinder lens 4 of the present embodiment are diffractive surfaces in which a diffraction grating is formed on one of the surfaces. The collimator lens 3 and the cylinder lens 4 are molded by injection molding using a plastic material, and are a so-called temperature compensation optical system that compensates for changes in refractive power due to environmental fluctuations by changes in diffraction power due to wavelength changes of the semiconductor laser. Yes. Here, the diffraction surface is defined by the phase function expressed below.

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.

(Aspherical shape of imaging optical system)
The generatrix shapes of the lens entrance surfaces and the lens exit surfaces of the imaging lenses 6, 7A, and 7B are both aspherical shapes that can be expressed as functions up to the 10th order. The intersection of the lens surfaces of the imaging lenses 6, 7A, 7B and the optical axes of the imaging lenses 6, 7A, 7B is the origin, the optical axis direction is the X axis, and the axis is orthogonal to the optical axis in the main scanning section. Is the Y axis, 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 , and B ₁₀ are aspheric coefficients)
The aspheric coefficients B ₄ , B ₆ , B ₈ , and B ₁₀ are the side (B _4U , B _6U , B _8U , B _10U ) where the light source means 1 of the optical scanning device is not arranged and the light source means 1 are arranged. It is also possible to make the numerical values different on the side (B _4L , B _6L , B _8L , B _10L ). By doing so, 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 curvature radius (sub-wire curvature radius) Rs ^* in the sub-scanning direction at a position Y away from the optical axis in the main scanning direction is expressed by the following equation.

(Where Rs is the radius of curvature of the sub-wire on the optical axis, and D ₂ , D ₄ , D ₆ , D ₈ and D ₁₀ are the coefficient of change of the sub-wire)
Also in this case, as in the main scanning shape, the aspherical coefficients D _{2 to} D ₁₀ are the side where the light source means 1 of the optical scanning device is not arranged (D _{2U to} D _10U ) and the side where the light source means 1 is arranged (D _{2L to} D _10L ), it is possible to express an asymmetric shape in the main scanning direction.

The second term on the right side of the equation relating to S is a term that gives the tilt amount in the direction of the child line composed of a linear function of Z, and the third term is the direction of the child line composed of a quartic function of Z. This term gives the amount of non-arc. With respect to the amount of tilt of the child line, M _{0,1 to} M _10,1 are arranged on the side where the light source means 1 of the optical scanning device is not arranged (M _{0,1U to} M _10,1U ) and the light source means 1 is arranged. _Different numerical values are used on the side (M _{0,1L to} M _10,1L ). As a result, the shape can be asymmetric in the main scanning direction. In this way, a quaternary non-arc amount can be given in the child line direction, but in this embodiment, a quartic non-arc amount is not given in the child line direction.

  In the present embodiment, the function of the surface shape is defined by the above definition formula, but the scope of the right of the present invention is not limited thereto.

(Image formation of light flux on the scanned surface)
FIG. 4A shows a spot shape at a position of zero defocus (on the surface to be scanned). The contour lines are 2%, 5%, 13.5%, and 50% of the peak light amount from the outside. As shown in the figure, a side lobe of 5% of the peak light amount occurs in the main scanning direction at a position where Y = ± 80 mm and Y = ± 105 mm. Although the spot shape was confirmed at image heights other than the five image heights shown here, side lobes exceeding 5% did not appear, and the wavefront aberration was corrected at all image heights. I know.

  FIG. 4B shows the fθ characteristic and plots the amount of deviation of the imaging position in the main scanning direction from the ideal image height. The image forming lens 6 is shared by the light beam Ra and the light beam Rb, but the image forming lens 7A and the image forming lens 7B have different shapes, so that the fθ characteristics match between the image forming optical systems SA and SB. Not. The image formation positions are separated in the main scanning direction by 1.065 mm at the most shifted position. However, it is possible to obtain a color misregistration amount with no problem in the actual image by controlling the light emission timing of the light source using an electrical correction technique developed in recent years.

  FIG. 4C shows scanning line bending, which is plotted with the on-axis imaging point set to zero. As for the scanning line bending, the imaging optical systems SA and SB do not coincide with each other as in the fθ characteristic. The image forming positions are separated in the sub-scanning direction by 110 μm at the most shifted position. Also in this case, by using the electric correction technique, it is possible to obtain a color misregistration amount at a level with no problem in the actual image.

(Imaging lens 7R)
FIG. 3 is an enlarged sub-scanning view around the imaging lens 7R as the first imaging lens in the imaging optical system SR of the present embodiment. The imaging lens 7R is a multistage lens in which an imaging lens 7A through which the light beam Ra passes and an imaging lens 7B through which the light beam Rb pass are overlapped vertically in the sub-scanning direction. Since lenses having different shapes are stacked in the main scanning direction and the sub-scanning direction, a step is generated at the boundary between the two lens surfaces.

  If the stepped portions P1 and P2 exist, sink marks may occur on the lens surface at the time of lens molding starting from those points, and the spot shape will be disturbed if the light beam passes through the spot portions. Therefore, in this embodiment, the light beam is deflected and scanned at an oblique incident angle of ± 3 ° in the sub-scanning direction, and the light beam Ra and the stepped portions P1 and P2 on the both surfaces of the imaging lens 7R reach the region where there is no influence of sink marks. Rb is sufficiently separated. The imaging lens 7B is configured such that the light beam reflected by the folding mirror M2 is incident again.

  In this way, in the imaging optical system SR, the second light beam (Rb) from the deflecting means to the second scanned surface (8B) passes twice, and the deflecting means reaches the first scanned surface (8A). An optical surface through which the first light beam (Ra) passes only once is included.

  In this embodiment, the lens shape of the imaging lens 7B that passes through the optical surface at the first time and the lens shape of the imaging lens 7B that passes through the optical surface at the second time are integrated so as to be the same. Designed as That is, in this embodiment, regarding the imaging lens 7R, the first surface shape in the sub-scanning direction in the region where the second light beam Rb toward the second surface to be scanned passes through the first time and the region where the second light beam Rb passes through the second time. The second surface shape in the sub-scanning direction is represented by the same mathematical expression (expression).

  For this reason, there is no need to worry about the step of the lens surface between them. When the light beam Rb emitted from the imaging lens 6 and passing through the imaging lens 7B and the part of the light beam Rb reflected by the folding mirror M2 and again passing through the imaging lens 7B are configured to overlap in the sub-scan section. The height of the lens is suppressed, and it is not necessary to use useless materials.

  In order to do so, the imaging lens 7B may be closer to the folding mirror M2. That is, as shown in FIG. 3, the distance L1 between the surface of the imaging lens 7B on the folding mirror M2 side and the folding mirror M3, and the distance L2 between the surface of the imaging lens 7B on the folding mirror M2 side and the folding mirror M2. Are preferably configured such that L1> L2. Further, when the imaging lens 7R is arranged at a position where L1> L2, the length of the lens in the main scanning direction becomes shorter than when the lens is arranged at a position where L1 <L2. Therefore, the condition of L1> L2 is an important condition for suppressing the height and length of the lens.

  The condition of L1> L2 is that the imaging lens 7R is brought as close to the folding mirror M2 as possible so that the imaging optical system SA (FIG. 1B) and the imaging optical system SB (FIG. 1C). Is also an important condition that the imaging magnification β in the sub-scanning direction is set to a close value. That is, since the imaging lens 6 (second imaging lens) is shared by the imaging optical systems SA and SB, the imaging magnification in the sub-scanning direction greatly depends on the arrangement position of the imaging lenses 7A and 7B. To do.

(Adjustment degree of imaging magnification in the sub-scanning direction)
If the imaging magnification is too far away, the configuration of the incident optical system must be made different between the light beam Ra and the light beam Rb (for example, changing the focal length of the collimator lens), preventing the commonality of components. This is disadvantageous in terms of cost reduction. In the present embodiment, imaging in the sub-scanning direction of the imaging optical system SA (FIG. 1B) with respect to the light beam Ra along the lower optical path (FIG. 1A) from the deflecting surface 5a of the optical deflector 5 is performed. The magnification β is β = −2.15. On the other hand, the imaging magnification β in the sub-scanning direction of the imaging optical system SB (FIG. 1 (c)) with respect to the light beam Rb along the upper optical path (FIG. 1 (a)) from the deflecting surface 5a of the optical deflector 5 is β = −1.77, which is almost the same.

(Relationship with photosensitive drum)
Finally, in this embodiment, as shown in FIG. 1A, the photosensitive drum is arranged on the lower side of the drawing, and among the plurality of light beams deflected by the deflecting means, spatially (physically B) The light beam Rb on the far side is imaged as follows. That is, after passing through the imaging lens twice, an image is formed on the scanned surface 8B spatially (physically) close to the deflecting means. On the other hand, after passing the light beam Ra spatially (physically) close to the scanning surface once through the imaging lens through which the light beam has passed twice, it is spatially (physically) far from the deflecting means. An image is formed on the scanned surface 8A.

  If the photosensitive drum shown in FIG. 1A is arranged on the upper side instead of the lower side of the drawing, a light beam (Ra) that is spatially (physically) far from the surface to be scanned is Thus, the image is formed. That is, after passing through the imaging lens through which the light beam has passed twice, an image is formed on the scanned surface 8A which is spatially (physically) far from the deflecting means. In this case, after the light beam (Rb) that is spatially (physically) closer to the scanning surface among the plurality of light beams deflected by the deflecting unit is passed through the imaging lens twice, the deflecting unit The image is formed on the scanned surface 8B that is spatially (physically) close to.

  In such an arrangement, the folding mirror M3 is rotated by 90 ° so that the reflected optical path is directed upward, and the folding mirror M3 and a light beam (Rb) directed toward the folding mirror M2 along the upper optical path. A new problem called interference occurs. Therefore, in order to make the image forming apparatus main body more compact, among the plurality of light beams deflected by the deflecting unit, the light beam spatially (physically) closer to the surface to be scanned is spatially transmitted from the deflecting unit. It is desirable to form an image on a surface to be scanned (physically) far away.

(Effect of this embodiment)
As described above, the present embodiment has predetermined performance in terms of fθ characteristics, scanning line bending, the degree of coincidence of imaging magnification in the sub-scanning direction, etc., and the optical scanning device can be thinned without degrading image quality. In addition, low cost can be achieved.

<< Second Embodiment >>
FIG. 5A is a sub-scan sectional view of the optical scanning device according to the second embodiment of the present invention. FIG. 5B is a main scanning cross-sectional development view of the optical system through which the light beam Ra passes among the optical system shown in FIG. FIG. 5C is a main scanning cross-sectional development view of the optical system through which the light beam Rb passes in the optical system shown in FIG. However, the optical path folded by the folding mirror M2 is written in an overlapping manner so that the light beam passes through the imaging lens 7B twice.

(Imaging lens 7R)
FIG. 6 is an enlarged sub-scan view around the imaging lens 7R. As can be seen from this, the imaging lens 7B is composed of two lens portions 7BU and 7BL. Therefore, the imaging lens 7R includes the imaging lens 7A through which the light beam Ra passes, the imaging lens 7BL through which the light beam Rb passes for the first time, and the imaging lens 7BU through which the light beam Rb reflected by the folding mirror M2 passes the second time. It consists of three lenses. That is, the imaging lens 7R is a multistage lens (lens whose lens surface is expressed by different expressions) stacked vertically in the sub-scanning direction.

  The imaging lens 7BL and the imaging lens 7A are symmetrical with respect to the plane P0. That is, in the present embodiment, regarding the imaging lens 7R, the surface shape in the sub-scanning direction in the region where the first light beam Rb toward the first scanned surface passes once, and the first light beam toward the first scanned surface. The surface shape in the sub-scanning direction in the region where Ra passes only once is expressed by the same mathematical formula. Thus, the imaging lens 7R has an integrated lens surface shape.

  For this reason, there is no step at the boundary portions P3 and P4 between the imaging lens 7BL and the imaging lens 7A. On the other hand, at the boundary portions P5 and P6 between the imaging lens 7BU and the imaging lens 7BL, the lens surface shape is different, so that a step is generated. Therefore, the imaging lens 7R must be disposed so that the light beam Rb does not pass near the boundaries P5 and P6. For this reason, on the surface of the imaging lens 7B on the side of the folding mirror M2, the distance between the two positions through which the light beam Rb passes is narrowed in the first embodiment, but in the present embodiment, it is necessary to widen it.

  That is, as shown in FIG. 6, the distance L1 between the surface on the folding mirror M2 side of the imaging lens 7B and the folding mirror M3, and the distance L2 between the surface on the folding mirror M2 side of the imaging lens 7B and the folding mirror M2. Is preferably configured so that L1 <L2. By disposing the imaging lens 7B at such a position, it is possible to widen the interval between the light beams without providing a folding angle at the folding mirror M2. Therefore, the light beam does not have to pass through the sunken lens surface in the vicinity of the step portion, and a good spot shape can be obtained.

(Specific configuration of incident optical system and imaging optical system)
Next, Table 7 shows the optical arrangement of the incident optical system LA and the imaging optical system SA in this embodiment, Table 8 shows the lens surface shape of the incident optical system LA, and Table 9 shows the lens surface shape of the imaging optical system SA. Shown in Similarly, the optical arrangement and lens surface shape of the incident optical system LB and the imaging optical system SB in this embodiment are shown in Table 10, the lens surface shape of the incident optical system LB is shown in Table 11, and the lenses of the imaging optical system SB. Table 12 shows the surface shape.

(Image formation of light flux on the scanned surface)
FIG. 7A shows a spot shape at a position of zero defocus (on the surface to be scanned). The contour lines are 2%, 5%, 13.5%, and 50% of the peak light amount from the outside. As shown in this figure, a side lobe of 5% of the peak light amount occurs in the main scanning direction at a position where Y = ± 80 mm. Although the spot shape was confirmed at image heights other than the five image heights shown here, side lobes exceeding 5% did not appear, and the wavefront aberration was corrected at all image heights. I know.

  FIG. 7B shows the fθ characteristic, and plots the amount of deviation of the imaging position in the main scanning direction from the ideal image height. Since the imaging lens 6 is shared by the luminous flux Ra and the luminous flux Rb, and the imaging lens 7A and the imaging lens 7B are lenses having a symmetrical shape with respect to the plane P0, the imaging optical systems SA and SB The fθ characteristics are almost the same. The image formation positions are separated in the main scanning direction by 0.026 mm at the most shifted positions. As described in the first embodiment, the image formation position shift may be further reduced by using the electric correction technique.

  FIG. 7C shows scanning line bending, which is plotted with the on-axis imaging point set to zero. As for the scanning line bending, the imaging optical systems SA and SB substantially coincide with each other in the same manner as the fθ characteristic. The image forming positions are separated in the sub-scanning direction by 1.6 μm at the most shifted position. In this case, it is fully consistent up to a level that does not require electrical correction.

(Adjustment degree of imaging magnification in the sub-scanning direction)
Further, the imaging magnification β in the sub-scanning direction of the imaging optical system SA is β = −2.31, which substantially coincides with the imaging magnification β in the sub-scanning direction of the imaging optical system SB = −2.35. This is because the imaging lens 6 is shared by the imaging optical systems SA and SB, and the imaging lenses 7A and 7BL having main power in the sub-scanning direction are symmetrical with respect to the plane P0. Since the imaging lens 7BU is a lens having no power (refractive power) in the sub-scanning direction, it hardly contributes to the sub-scanning magnification. Therefore, since the imaging magnifications in the sub-scanning direction are almost the same, it is not necessary to make the configuration of the incident optical system different between the light beam Ra and the light beam Rb, which is advantageous from the viewpoint of cost reduction such as common use of parts. To work.

(Effect of this embodiment)
As described above, the present embodiment is superior to the first embodiment in terms of fθ characteristics, scanning line bending, the degree of coincidence of image forming magnifications in the sub-scanning direction, and the like. Therefore, in the optical scanning device that does not have the function of correcting the imaging position by electrical correction, the configuration of this embodiment can achieve a reduction in thickness and cost of the optical scanning device without degrading image quality. it can.

(Modification)
The present invention is not limited to the above-described embodiment, and various modifications can be made. In the above-described embodiment, the multistage lens is formed on the imaging lens far from the optical deflector. However, as shown in FIG. 8, the multistage lens is formed on the imaging lens near the optical deflector. May be. Then, as shown in FIG. 8A, the imaging lenses 6A and 6B constituting the lens 6R can be constituted by the same lens surface having one expression.

  Further, as shown in FIG. 8B, the imaging lenses 6A and 6B constituting the multistage lens 6R may be different lens surfaces having different expressions. Further, as shown in FIG. 8C, the surface of the imaging lenses 6A and 6B far from the optical deflector is a lens surface having a different expression, and the surface of the imaging lens 6A on the optical deflector side is also used. A configuration may be adopted in which the surface shape is different between the lens surface passing the first time and the lens surface passing the second time. Further, as shown in FIG. 8 (d), the imaging lens 7A may be deleted from the configuration of FIG. 8 (c).

  Further, the light beams Ra and Rb can be wavelength separated instead of spatially separated. That is, as shown in FIG. 8E, the light beams Ra and Rb having different wavelengths that have passed through the multistage lens 6R whose lens surfaces are expressed by different expressions are separated by the dichroic mirror D2. The light beam Ra may be transmitted through the dichroic mirror D2, and the light beam Rb may be reflected by the dichroic mirror D2. In this case, it is possible to adopt a configuration in which incident light beams having different wavelengths are incident on the deflecting / reflecting surface of the optical deflector vertically rather than obliquely in the sub-scanning direction.

  Further, as shown in FIG. 8 (f), two light beams may be vertically incident on the deflection reflection surfaces of the optical deflectors 5A and 5B arranged in two stages in the sub-scanning direction.

  Further, instead of the configuration shown in FIG. 9A (corresponding to FIG. 8A), as shown in FIG. 9B, two optical deflectors are arranged side by side and deflected and reflected only on one side. An optical scanning device of the type described above may be configured. In this case, even if there is ghost light reflected by each lens surface when passing through each lens surface with respect to the light beam reflected by one deflection reflection surface, it does not reach the other scanned surface.

  It can design using such various modifications according to the design specification given to an optical scanning device. In any case, the light beam Ra directed to the scanned surface 8A far from the optical deflector passes once through the lens 6R through which the light beam Rb directed to the scanned surface 8B close to the optical deflector passes twice. As a result, the number of lenses or the number of folding mirrors can be reduced, and a compact and low-cost optical scanning device can be provided.

(Further different modifications)
1) In FIG. 6, the first surface shape in the sub-scanning direction in the region where the second light beam Rb toward the second scanned surface passes once on the surface near the deflecting unit of the imaging lens 7R, passes again. A second surface shape that is a surface shape in the sub-scanning direction in the region to be performed may be configured as follows. That is, the refractive power of the third surface shape in the sub-scanning direction in the region where the first light beam directed to the first scanned surface passes only once is equal to the sum of the refractive powers of the first surface shape and the second surface shape. You may do it.

  2) In the above-described embodiment, the folding mirror is used as the reflecting means. However, the present invention is not limited to this, and an internal reflecting member can be used. For example, an imaging lens instead of the folding mirror shown in FIGS. A reflective film may be formed on the surface far from the 7R deflecting means.

  3) Further, in FIG. 8E, an imaging lens having a lens surface shape represented by a different mathematical expression is shown as the multistage lens 6R, but an imaging lens having a lens surface shape represented by the same mathematical expression is used. It can also be replaced.

1A, 1B ··· Light source means, LA, LB ··· Incident optical system, ··· Deflection means, 7A ·· Imaging lens, 8A ··· First scanned surface, 8B ··· Second scanned surface

Claims (19)

Deflection means;
An incident optical system for guiding the first light beam and the second light beam respectively emitted from the first light source means and the second light source means to the deflecting means;
The first light beam and the second light beam emitted from the deflecting unit are respectively a first scanned surface and a second scanned surface whose spatial distance from the deflecting unit is closer than the first scanned surface. An imaging optical system that leads to
An optical scanning device that scans the first scanned surface and the second scanned surface by deflecting the first light beam and the second light beam by the deflecting unit, respectively.
In the imaging optical system, the second light beam from the deflecting unit to the second scanned surface passes twice, and the first light beam from the deflecting unit to the first scanned surface passes only once. An optical scanning device characterized by including an optical surface that does not.   The optical scanning apparatus according to claim 1, wherein the imaging optical system includes a first imaging lens including the optical surface.   The optical scanning device according to claim 2, wherein the optical surface is a surface spatially close to the deflecting unit of the first imaging lens.   4. The optical surface according to claim 1, wherein the first light beam passes through a position closer to the sub-scanning direction from the second scanned surface than the second light beam. 5. Optical scanning device.   The region of the optical surface through which the second light beam passes for the second time is further away from the second surface to be scanned in the sub-scanning direction than the region through which the second light beam passes for the first time. Item 5. The optical scanning device according to any one of Items 1 to 4.   On the optical surface, a first surface shape in the sub-scanning direction in a region where the second light flux passes through the first time, and a second surface shape in the sub-scanning direction in a region where the second light flux passes through the second time. Is represented by the same mathematical formula, and has a refractive power different from that of the third surface shape in the sub-scanning direction in a region where the first light flux passes only once. The optical scanning device according to item 1.   On the optical surface, a first surface shape in the sub-scanning direction in a region where the second light flux passes for the first time, and a third surface shape in a sub-scanning direction in a region where the first light flux passes only once. Are expressed by the same mathematical formula and have refractive power, while the second surface shape in the sub-scanning direction in the region where the second light beam passes for the second time has no refractive power. Item 6. The optical scanning device according to any one of Items 1 to 5. On the optical surface, a first surface shape in the sub-scanning direction in a region where the second light flux passes through the first time, and a second surface shape in the sub-scanning direction in a region where the second light flux passes through the second time. The third surface shape in the sub-scanning direction in the region where the first light flux passes only once has different refractive powers,
6. The optical scanning device according to claim 1, wherein a sum of refractive powers of the first surface shape and the second surface shape is equal to a refractive power of the third surface shape. 7.   The imaging optical system includes a first reflecting unit in an optical path from the first passage to the second passage of the optical surface with respect to the second light flux from the deflecting unit to the second scanned surface. The optical scanning device according to claim 1, wherein:   The imaging optical system has a second reflection that bends an optical path toward the second scanned surface after the second passage of the optical surface with respect to the second light flux from the deflecting unit to the second scanned surface. The optical scanning device according to claim 9, further comprising: means. The imaging optical system includes a first imaging lens including a surface on the side spatially close to the deflecting unit as the optical surface,
The distance between the surface of the first imaging lens that is spatially far from the deflecting means and the first reflecting means is:
11. The optical scanning device according to claim 10, wherein the distance is smaller than a distance between a surface of the first imaging lens far from the deflecting unit and the second reflecting unit. The imaging optical system includes a first imaging lens including a surface on the side spatially close to the deflecting unit as the optical surface,
The distance between the surface of the first imaging lens that is spatially far from the deflecting means and the first reflecting means is:
11. The optical scanning device according to claim 10, wherein the distance is larger than a distance between a surface of the first imaging lens that is spatially far from the deflection unit and the second reflection unit.   In the imaging optical system, the first light beam and the second light beam pass on a side spatially close to the deflection unit or on a side spatially far from the deflection unit with respect to the first imaging lens. The optical scanning device according to claim 1, further comprising a second imaging lens.   The optical scanning device according to claim 1, wherein the first light flux and the second light flux are spatially separated or wavelength separated.   15. The optical scanning device according to claim 1, wherein the first light flux has two or one third reflecting means for reflecting.   The first light beam and the second light beam are incident on the deflecting unit from both sides with respect to a plane that includes the rotation axis of the deflecting unit and is perpendicular to the optical axis of the imaging optical system. Item 16. The optical scanning device according to any one of Items 1 to 15.   The said 1st light beam and the said 2nd light beam are inclined diagonally with respect to the plane containing the surface normal of the deflection | deviation reflective surface of the said deflection | deviation means, The one of Claim 1 thru | or 16 characterized by the above-mentioned. Optical scanning device.   18. The optical scanning device according to claim 1, and a first photosensitive member and a second photosensitive member arranged on each of the first scanned surface and the second scanned surface, respectively. And a color image forming apparatus.   The color image forming apparatus according to claim 18, further comprising a printer controller that converts color signals input from an external device into image data of different colors.