JP2018101028A - Optical scanner and image forming apparatus using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical scanner that can reduce the occurrence of an uneven distribution of light quantity between a plurality of light beams on a scanning target surface, while achieving a low cost, and an image forming apparatus using the same.SOLUTION: In an optical scanner comprising a light source that has a plurality of light emitting parts, a deflector, and an image forming optical system, light beams made incident on the deflector are linearly polarized light in which a vibration direction is inclined with respect to a main scanning cross section; in the main scanning cross section, principal rays made incident on first and second ends in a scanning area have angles different from each other with respect to an optical axis; of light beams immediately after being deflected by the deflector, a light beam directed to the first end has a smaller quantity of light than the quantity of light of a light beam directed to the second end; the value of the ratio of the quantity of light of the light beam directed to the first end immediately before being made incident on the scanning target surface to the quantity of light of the light beam immediately before being made incident on the image forming optical system, is larger than the value of the ratio of the quantity of light of the light beam directed to the second end immediately before being made incident on the scanning target surface to the quantity of light of the light beam immediately before being made incident on the image forming optical system.SELECTED DRAWING: Figure 1

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 (multifunction printer).

  In the optical scanning device, when the light beam emitted from the light source is linearly polarized light, the ratio of the P-polarized component and the S-polarized component is a specific value (depending on the reflection / transmission characteristics of each optical element of the deflector and imaging optical system). If it deviates from this, the light quantity distribution due to the light beam incident on the surface to be scanned is biased.

  There is known a method of adjusting the bias of the light amount distribution by rotating the light source to change the ratio of the S-polarized component and the P-polarized component of the light beam passing through each optical element to a specific value. However, when a multi-beam laser having a plurality of light emitting points is used as the light source, it is necessary to rotate the light source in order to adjust the interval in the sub-scanning direction of each light beam on the scanned surface. Cannot be adjusted.

  Patent Document 1 describes that the rotation direction of polarized light is regulated so as to cancel the inclination of the light amount distribution caused by the deflector and the inclination of the light amount distribution caused by the folding mirror included in the imaging optical system. Yes. Thereby, the bias of the light quantity distribution due to the light beam incident on the surface to be scanned is reduced.

JP2015-011116A

  However, in the method described in Patent Document 1, it is necessary to provide a dedicated reflection film in order to give the reflection mirror a predetermined reflection characteristic, and a general reflection film cannot be used. I will invite you.

  SUMMARY OF THE INVENTION An object of the present invention is to provide an optical scanning apparatus capable of suppressing the occurrence of a deviation in light amount distribution due to a plurality of light beams on a surface to be scanned and an image forming apparatus using the same while realizing cost reduction. To do.

  In order to achieve the above object, an optical scanning device according to the present invention includes a light source having a plurality of light emitting portions, and a deflector that deflects a light beam from the light source and scans a scanning region on a surface to be scanned in the main scanning direction. And an imaging optical system that guides the light beam deflected by the deflector to the scanning region, and the light beam incident on the deflector is linearly polarized light whose vibration direction is inclined with respect to the main scanning section. In the main scanning section, the angle formed between the principal ray and the optical axis incident on the first and second end portions in the scanning region is different from each other, and the light beam immediately after being deflected by the deflector is The light amount of the light beam toward the first end is less than the light amount of the light beam toward the second end, and the light amount immediately before entering the image forming optical system of the light beam toward the first end. The ratio of the amount of light just before entering the surface to be scanned is Being larger than the value of the ratio of the light amount immediately before entering the surface to be scanned with respect to the light quantity of the immediately before entering the second end the imaging optical system of the light beam directed to.

  An image forming apparatus according to the present invention includes the optical scanning device.

  According to the present invention, it is possible to provide an optical scanning device and an image forming apparatus using the same that can suppress the occurrence of a deviation in light amount distribution due to a plurality of light beams on a surface to be scanned while realizing cost reduction. it can.

FIG. 1 is a main scanning sectional view of an optical scanning device according to an embodiment of the present invention. Explanatory drawing of the image forming apparatus carrying the optical scanning device which concerns on embodiment of this invention Schematic diagram of multi-beam laser (2 beam type) Schematic diagram of multi-beam laser (4-beam type) FIG. 3 is a sub-scanning sectional view of the optical scanning device according to the first embodiment. Main scanning sectional view of a conventional optical scanning device Sub-scan sectional view of the optical scanning device according to the second embodiment The figure which showed the incident angle dependence of the reflectance of S polarized light and P polarized light with respect to a reflective surface Graph showing light quantity distribution of light beam deflected and scanned by deflector The figure which showed the incident angle dependence of the transmittance | permeability of S polarized light and P polarized light with respect to a transmission surface The figure which showed the light quantity distribution by reflection by the deflector in the conventional optical scanning device, the light quantity distribution by passing through the imaging optical system, and the light quantity distribution on the scanned surface The figure which showed the light quantity distribution by reflection in the deflector in the optical scanning device concerning 1st Embodiment, the light quantity distribution by passing an imaging optical system, and the light quantity distribution on a to-be-scanned surface The figure which showed the main scanning cross-sectional shape of the 2nd imaging lens typically Graph showing the relationship between main scanning eccentricity and cost ratio The figure which showed the light source rotation angle dependence of the influence degree of S polarization component and P polarization component in a deflection | deviation reflective surface The figure explaining the application example of 1st Embodiment Diagram showing sub-scanning cross section of oblique incidence optical system In the second embodiment, when the light amount distribution by passing through the imaging optical system B has an inclination, the light amount distribution by reflection by the deflector, the light amount distribution by passing through the imaging optical system A, and the surface to be scanned The figure which showed light quantity distribution on A The figure which showed the light quantity distribution by reflection by the deflector in said 2nd Embodiment, the light quantity distribution by passing the imaging optical system B, and the light quantity distribution on the to-be-scanned surface B In the second embodiment, when the light amount distribution by passing through the imaging optical system B has a curve, the light amount distribution by reflection at the deflector, the light amount distribution by passing through the imaging optical system A, and the surface to be scanned The figure which showed light quantity distribution on A The figure which showed the light quantity distribution by reflection by the deflector in said 2nd Embodiment, the light quantity distribution by passing the imaging optical system B, and the light quantity distribution on the to-be-scanned surface B

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

<< First Embodiment >>
(Image forming device)
FIG. 2 is a schematic diagram of a main part of a color image forming apparatus 60 as an image forming apparatus equipped with an optical scanning device according to an embodiment of the present invention. This is a color image forming apparatus that records image information on the scanned surface, which is the drum surface of the photosensitive drums 71, 72, 73, and 74 that are image carriers in the optical scanning device 61. In FIG. 2, 31, 32, 33, and 34 are developing devices (developing devices), and 51 is a conveyor belt. Note that a transfer device (not shown) that transfers the toner image developed by each developing device to a transfer material and a fixing device (not shown) that fixes the transferred toner image onto the transfer material are provided.

  The color image forming apparatus 60 receives R (red), G (green), and B (blue) color signals from an external device 52 such as a personal computer. Each color signal output from an external device is converted into image data (dot data) of C (cyan), M (magenta), Y (yellow), and B (black) by the printer controller 53 in the apparatus. These image signals are input to the optical scanning device 61. From these optical scanning devices, light beams 41, 42, 43, 44 modulated in accordance with the respective image signals are emitted, and these light beams are irradiated on the photosensitive surfaces of the photosensitive drums 71, 72, 73, 74. Scans in the main scanning direction.

  As described above, the color image forming apparatus 60 uses the light scanning device 61 to convert the electrostatic latent images of the respective colors to the corresponding drum surfaces of the photosensitive drums 71, 72, 73, and 74 using the light beams based on the respective image signals. Form 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 apparatus 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.

(Optical scanning device)
In the following description, the main scanning direction corresponds to a direction perpendicular to the rotation axis of the deflector and the optical axis of the optical system, and the sub-scanning direction corresponds to a direction parallel to the rotation axis of the deflector. The main scanning section corresponds to a section perpendicular to the sub scanning direction, and the sub scanning section corresponds to a section including the optical axis of the optical system and the sub scanning direction.

  In FIG. 1 schematically showing a main scanning section, 1 is a light source, 2 is a condensing element, 3 is a cylindrical lens, 4 is a stop, 5 is a polygon mirror as a deflector, and 6 is an imaging lens group. A first imaging lens 6a and a second imaging lens 6b are provided. 7 is a cover glass, 8 is a surface to be scanned (photosensitive member), and M1 is a folding mirror. In the present embodiment, the imaging lens group 6 and the folding mirror M1 constitute an imaging optical system.

  The light source 1 is a laser light source having a plurality of light emitting points as a plurality of light sources, and is a multi-beam laser having a plurality of light emitting portions spaced apart in the sub-scanning direction on a single chip. Although it is an infrared laser with a wavelength λ = 790 nm, it is not necessarily limited to this, and a red laser with a wavelength λ = 650 nm or a blue laser with λ = 430 nm may be used. The divergent light beam emitted from the light source 1 is converted into substantially parallel light by the condensing element 2.

  The substantially parallel light converted by the condensing element 2 is converted into a light beam that converges in the sub-scanning direction by the cylindrical lens 3, and is condensed near the deflection surface 5 a of the deflector 5. Therefore, the light beam forms a line image in the vicinity of the deflection surface 5a.

  The diaphragm 4 has an opening of an elliptical shape, a rectangular shape, an oval shape, a rhombus shape, etc., and may be determined according to the wavelength of the light source used and the required size and shape of the beam spot. Further, the diaphragm has two members, for example, a slit member that extends in the sub-scanning direction to limit the light beam in the main scanning direction and a slit member that extends in the main scanning direction to restrict the light beam in the sub-scanning direction. You may divide and install in a slit member.

  When the number of light emitting points of the laser used for the light source 1 is large, jitter can be reduced by providing a slit member that restricts the light beam in the main scanning direction in the vicinity of the deflector. Similarly, when the number of laser emission points used in the light source 1 is large, a slit member that restricts the light beam in the sub-scanning direction is provided at a position conjugate with the imaging lens (approximately between the light source 1 and the light condensing element 2). However, the uniformity of the printing intervals of the plurality of beams is improved. In consideration of such a technical viewpoint, a diaphragm divided into a plurality of members may be employed.

  The deflector 5 composed of a plurality of deflection surfaces (reflection surfaces) is rotationally driven by a rotation shaft by a drive system (not shown). The incident light beam guided to the deflector 5 by the incident optical system provided in the optical path from the light source 1 to the deflector 5 is deflected and scanned by an arbitrary deflection surface that is rotationally driven, and is applied to the imaging lens group 6 (fθ lens). Led.

  Next, the operation of the imaging lens group 6 will be described. The imaging lens group 6 (fθ lens) has a two-lens configuration of a first imaging lens 6a and a second imaging lens 6b made of glass or plastic. The imaging lens group 6 condenses the light beam reflected and deflected by the deflector 5 on the scanned surface 8 to form a beam spot. This beam spot scans the surface to be scanned 8 at a constant speed. The imaging lens made of plastic is manufactured by a known molding technique in which a mold is filled with a resin and is taken out of the mold after cooling. Thereby, it can be manufactured at a lower cost than a conventional scanning lens using a glass lens.

  The imaging lens group 6 is designed with a known power arrangement. For example, the first imaging lens 6a is configured as a lens having power mainly in the main scanning direction. The lens surface shape is a shape expressed by a known function. The shape in the main scanning section is symmetric with respect to the optical axis, and in the sub-scanning direction, the entrance surface and the exit surface may be substantially non-power (refractive power is substantially zero) with the same curvature. Power may be given accordingly.

  One second imaging lens 6b is an aspherical lens having power mainly in the sub-scanning direction, and the lens surface shape is an aspherical shape expressed by a known function. In the second imaging lens 6b, the power in the sub-scanning section is larger than the power in the main-scanning section, the shape in the main-scanning section is symmetric with respect to the optical axis, and the main scanning direction near the axis is It is almost non-power (refractive power is almost zero).

  The shape of the sub-scanning cross section is a substantially flat surface where the curvature of the entrance surface is extremely loose, and the exit surface is a convex shape whose curvature gradually changes from on-axis to off-axis, and is symmetrical with respect to the optical axis. The incident light beam is mainly responsible for image formation in the sub-scanning direction and correction of some distortion in the main scanning direction.

  The imaging relationship in the sub-scanning direction by the imaging lens group 6 is a so-called tilt correction system in which the deflecting / reflecting surface 5a and the scanned surface 8 are substantially conjugate. The number of imaging lenses (imaging optical elements) constituting the imaging lens group 6 is not limited to two, and may be one or three or more.

  The imaging optical system is not necessarily configured as described above. For example, the imaging optical system may be asymmetric with respect to the optical axis in order to improve the imaging performance.

  The cover glass 7 is inclined so as to have an angle with respect to the incident light beam in the sub-scan section. This is to prevent the surface reflected light from the cover glass 7 from returning to the light source 1. If the surface reflected light from the cover glass 7 returns to the light source 1, the laser oscillation of the light source 1 may become unstable and the light quantity may fluctuate.

  In this embodiment, in order to control the light emission amount to a predetermined amount, APC (Auto Power Control) is performed in which the light flux emitted from the laser light source is monitored by the light amount detection sensor and the drive current is controlled. Further, for synchronization detection, a synchronization detection light is guided to a synchronization detection light receiving sensor via a synchronization detection optical system (not shown) with respect to a light beam having an image height outside the effective image area on the scanned surface 8, and a print writing timing. Is controlling.

  The incident optical system, the imaging optical system, the synchronization detection optical system, and the like are accurately held in an optical box (not shown) to form an optical scanning device 61.

(Bias suppression of light quantity distribution incident on the scanned surface)
In FIG. 1, the principal ray when the light beam deflected and scanned by the deflector 5 is orthogonal to the scanning area of the scanned surface 8 is defined as a reference line (optical axis). In the scanning area of the surface to be scanned 8, the maximum scanning position on the side where the light source 1 is located with respect to the reference line, the opposite side of the light source 1 is the first end, and the maximum scanning position on the other side is the second end And In the present embodiment, as described below, in the main scanning section, the first angle formed by the principal ray incident on the first end and the reference line, the principal ray incident on the second end, and the reference The second angle made with the line is made different.

  3 and 4 are diagrams schematically showing a multi-beam laser having a plurality of light emitting portions on a single chip. The rotation angle θe is adjusted and attached so that the distance in the sub-scanning direction on the surface to be scanned 8 of each light beam emitted from each light emitting unit 102a, 102b (or 104a to 104d) becomes a desired amount. . At this time, with respect to the deviation in the main scanning direction on the surface to be scanned 8 of each light beam, the writing position is adjusted by controlling the light emission timing.

  Each light beam is linearly polarized light, and the polarization direction coincides with the direction in which the light emitting units are arranged. That is, a linearly polarized light beam in the direction of the rotation angle θe with respect to a plane (main scanning section) perpendicular to the rotation axis of the deflector 5 is emitted. When the polarization direction of the linearly polarized light emitted from the light source 1 is tilted with respect to the main scanning section of the deflector 5, the ratio of the S-polarized component and the P-polarized component of each light beam with respect to the deflecting surface 5a according to the tilt angle. Changes.

  FIG. 8 is a graph showing the incident angle dependence of the S-polarized reflectance and the P-polarized reflectance of the deflecting surface 5a which is a reflective surface. Here, a general protective film is provided on the deflection surface 5a (reflected by the aluminum surface) in the present embodiment. The reflectance of the P-polarized light having the oscillation direction of the electric field in the plane of the incident surface including the surface normal of the deflecting surface 5a and the incident light on the deflecting surface 5a is substantially constant with respect to the change in the incident angle. On the other hand, the reflectance of S-polarized light having the direction of vibration of the electric field in the direction orthogonal to the surface normal of the deflecting surface 5a and the surface of the incident surface including the incident light on the deflecting surface 5a increases with changes in the incident angle. Change.

  As shown in FIG. 3B, in the conventional configuration shown in Table 1 using a multi-beam laser having two light emitting portions and a light emitting point interval Le of 90 μm, the rotation angle θe was 3.88 deg. At this time, the inclination of the polarization direction of the linearly polarized light emitted from the light source 1 with respect to the main scanning section of the deflector 5 is 3.88 deg equal to the rotation angle θe, and the P-polarized component with respect to the deflection surface 5a is dominant. Therefore, the light amount distribution of the light beam deflected and scanned by the deflector 5 is substantially uniform over the entire scanning region (FIG. 8).

  In contrast, in the present embodiment, as shown in FIG. 4, four light emitting units are provided to cope with an increase in printing speed. In order to avoid an increase in the size of the apparatus, a narrow pitch multi-beam laser is used in which the distance Le ′ between the light emitting points is 30 μm so that the distance Le between the light emitting points at both ends is 90 μm, which is the same as the conventional one. Further, in order to suppress the influence of manufacturing errors and the like and contribute to high image quality, the lateral magnification in the sub-scanning direction is set lower than that in the conventional case, and the configuration shown in Table 2 is used, and the rotation angle θe is 21.37 degrees.

  At this time, the inclination of the polarization direction of the linearly polarized light emitted from the light source 1 with respect to the main scanning section of the deflector 5 is 21.37 deg equal to the rotation angle θe, and includes an S-polarized component and a P-polarized component with respect to the deflecting surface 5a. It is.

  As shown in FIG. 8, since the S-polarized light reflectance of the polarization plane 5a changes greatly according to the change of the incident angle, the light amount distribution of the light beam deflected and scanned by the deflector 5 is scanned as shown in the graph of FIG. There is an inclination in the region, and a magnitude relationship occurs between the scan start position and the scan end position. Specifically, the amount of light reflected by the deflector 5 is such that the light beam reflected by the deflector 5 travels to the first end (first maximum scanning position), while the second end (first It is higher when heading to the maximum scanning position (2). That is, the light amount distribution by reflection by the deflector corresponding to the image height (main scanning direction) has the first inclination.

  Next, the ratio of the light amount immediately before entering the scanning surface after passing through the imaging optical system with respect to the light amount immediately before entering the imaging optical system (passage rate in the imaging optical system) will be described. In the imaging optical system according to the present embodiment, the distribution corresponding to the image height (main scanning direction) has a second inclination (opposite to the first inclination), and the first inclination and the second inclination. By canceling out, the bias of the light quantity distribution incident on the surface to be scanned is suppressed.

  That is, the amount of light A incident on the surface to be scanned at each image height position is the amount of light B directed to each image height position with the light beam immediately after being deflected by the deflector, and the image forming optical system of the light beam directed to each image height position. Since it is represented by the product of the passing rate C, the above-described bias can be suppressed.

  In this embodiment, the first ray formed by the principal ray and the reference line at the first end is arranged so that the distribution corresponding to the image height position in the imaging optical system has the second inclination. And the second angle formed by the principal ray at the second end and the reference line are made different from each other. Specifically, the pass rate in the imaging optical system is such that the light beam immediately after being reflected by the deflector is directed toward the first end, and is lower when directed toward the second end. The angle of 1 is different from the second angle.

  As for the imaging optical system, FIG. 1 schematically shows a main scanning section of this embodiment, and FIG. 5 schematically shows a sub-scanning section. An imaging optical system that guides the light beam deflected and scanned by the deflector 5 onto the scanned surface 8 includes an imaging lens group 6 including an imaging lens 6a and an imaging lens 6b, a folding mirror M1, and a cover glass 7. It is configured. Regarding the pass rate of the imaging optical system, the amount of light after passing through the imaging optical system is determined by the accumulation of transmittance and reflectance as the pass rate of each optical element, and the light amount distribution in the scanning area of the scanned surface The bias is mainly determined by the following three factors.

  The first element is caused by the change in the incident angle due to the deflection scanning of the pass rate of each optical element in the imaging optical system, which is a change symmetrical to the reference line. As an example, FIG. 10 is a graph showing the incident angle dependence of the S-polarized light transmittance and the P-polarized light transmittance of the cover glass 7 (refractive index 1.5). When attention is paid to one polarized light, the transmittance gradually changes as the distance from the reference line (incidence angle 0 deg) increases (incident angle increases), and the light quantity distribution of the light flux that has passed through the cover glass 7 is scanned. It has a curved component that is substantially symmetrical with respect to the reference line within the scanning region of the surface.

  The second element is a change according to the incident angle of the ratio of the S-polarized light component and the P-polarized light component generated by the inclination of the polarization direction of the linearly polarized light that is deflected and scanned. An inclination component is given by a change in the relative angle of the direction. Thereby, an inclination component is added to the light quantity distribution mainly passing through the optical element within the scanning region of the surface to be scanned.

  The third element is a characteristic distribution within each optical element including manufacturing errors. In the folding mirror, in the process of generating the reflection film, unevenness of the film thickness occurs in the reflection surface, and the reflectance characteristics may partially change. Due to the deflection scanning, the reflection position continuously changes simultaneously with the change of the incident angle with respect to the folding mirror, and therefore, the light amount distribution in the scanning region of the surface to be scanned may be irregularly changed.

  In addition, birefringence is likely to occur in a plastic imaging lens manufactured by injection molding. When the light beam passes through the birefringent lens, the polarized light rotates, the ratio of the S-polarized component and the P-polarized component changes according to the scanning position, and the transmittance and reflectance of the subsequent optical element change.

  Birefringence due to the shape of the imaging lens occurs almost symmetrically with respect to the reference line (optical axis), but birefringence due to the mold structure and molding conditions is independent of the reference line (optical axis). Occurs. For this reason, in an imaging optical system including a plastic imaging lens, the light amount distribution in the scanning region may be changed irregularly and simultaneously with a symmetric change with respect to the reference line.

  Furthermore, since it is difficult to predict birefringence generated in a plastic imaging lens, there are cases where measures cannot be taken sufficiently at the design stage and must be handled at the prototype stage. At the same time, the birefringence may change over time due to changes in material lots or wear of the mold.

  As described above, the deviation of the light amount distribution of the light beam that has passed through the imaging optical system has a curved component as the first element, an inclination component as the second element, and an irregular and difficult to predict component as the third element. It will be. In this embodiment, an effect is exhibited especially for the first element, and a solution can be provided for the third element.

(Contrast with comparative example)
FIG. 11A shows a conventional optical scanning device (FIG. 6) as a comparative example, and the scanning range is set to θ _u = θ _l . θu is an angle formed between the principal ray and the reference line at the maximum scanning position on the same side as the light source 1, and θ _l is an angle formed between the principal ray and the reference line at the maximum scanning position on the side opposite to the light source 1. As described above, the polarization direction of linearly polarized light emitted from the light source 1 is inclined by the rotation angle θe of the multi-beam laser. Then, as shown in FIG. 11B, the light amount distribution of the light beam immediately after being deflected by the deflector 5 is inclined in the scanning region (the light amount distribution corresponding to the image height has a first inclination).

  On the other hand, in the imaging optical system, a light quantity distribution having a curved component as shown in FIG. In addition, in each graph of FIG. 11, the dash-dot line which approximated the light quantity distribution in a scanning area | region with the linear function was added for easy understanding.

  Here, the light amount distribution on the surface to be scanned 8 includes a light amount distribution generated by the deflector 5 (FIG. 11B) and a light amount distribution generated by the imaging optical system corresponding to the pass rate distribution of the imaging optical system (FIG. 11). 11 (c)). Therefore, the light amount distribution on the scanned surface 8 has an inclination as the light amount distribution as shown in FIG.

With respect to such a comparative example, the present embodiment will be described with reference to FIG. As shown in FIG. 12A, the scanning range is set to θ _u > θ _l . Then, the polarization direction of the linearly polarized light emitted from the light source 1 is inclined by the rotation angle θe of the multi-beam laser, and the light quantity distribution of the light beam immediately after being deflected by the deflector 5 as shown in FIG. Lean on. That is, the light amount distribution corresponding to the image height has a first inclination).

  Specifically, in the present embodiment, the light amount of the light beam directed to the first end is smaller than the light amount of the light beam directed to the second end (small at the scanning start position (minus image height side), and the scanning end position ( It is larger on the positive side image height side)).

  On the other hand, in the imaging optical system, a light quantity distribution having a curved component is obtained as shown in FIG. In addition, in each graph of FIG. 12, for the sake of clarity, a one-dot chain line that approximates the light amount distribution in the scanning region by a linear function and a broken line that indicates the light amount distribution in the region that is not actually scanned are added. Yes.

Here, the light quantity distribution of the light flux that has passed through the imaging optical system, that is, the cumulative transmitted / reflected light quantity (passing light quantity corresponding to the passing rate of the imaging optical system) in the imaging optical system is as shown in FIG. Are curved symmetrically with respect to the reference line (image height 0). For this reason, the value of the ratio of the light quantity just before entering the surface to be scanned with respect to the light quantity just before entering the imaging optical system has a shape symmetrical to the optical axis in the main scanning direction. However, by biasing the scanning area in the main scanning direction, that is, | θ _u −θ _l |> 0, the light amount distribution in the scanning area is inclined to a one-dot chain line graph approximated by a linear function (first A second slope opposite to the slope) can be provided.

That is, the ratio of the amount of light just before entering the surface to be scanned with respect to the amount of light just before entering the imaging optical system of the light flux toward the first and second ends is a distribution symmetrical to the optical axis in the main scanning direction. A straight line connecting the both has a second inclination. Specifically, in the present embodiment, the magnitude relationship between the scan start position (minus image height side) and the scan end position (plus side image height side) is opposite to the magnitude relationship by the deflector 5 described above. θ _u > θ _l is set.

  Then, the light amount distribution on the surface to be scanned 8 includes a light amount distribution generated by the deflector 5 (FIG. 12B) and a light amount distribution generated by the imaging optical system corresponding to the transmission rate distribution of the imaging optical system (FIG. 12). (C)) is integrated. For this reason, as shown in FIG. 12D, the light amount distribution on the surface to be scanned 8 has a small inclination of a one-dot chain line graph obtained by approximating the light amount distribution in the scanning region with a linear function. That is, the difference in the light amount distribution on the scanned surface 8 is improved in FIG. 12D in which the present embodiment is adopted compared to FIG. 11D in the comparative example.

(Preferred form on implementation)
Here, the preferable form on implementation is demonstrated as a guideline for implementing this embodiment more effectively. 13A and 13B are diagrams schematically showing a main scanning cross-sectional shape of the second imaging lens 6b, in which FIG. 13A is a conventional shape example, and FIG. 13B is a scanning region in the main scanning direction as in the present embodiment. This is a shape example in the case of bias, | θ _u −θ _l |> 0. In the present embodiment, the thickness at the center (reference line) position is increased in order to ensure the thickness at the end portion in the main scanning direction on the side where the scanning region is widened.

Here, when the second imaging lens 6b is a plastic lens, increasing the thickness requires increasing the cooling time at the time of molding in addition to increasing the volume of the material, thereby increasing the cost of the lens. It becomes a factor. FIG. 14 is a graph showing the main scanning eccentricity ratio and the cost ratio based on the time θ _u = θ ₁ in the present embodiment. Here, the main scanning eccentricity is defined as follows.

  As shown in FIG. 14, the cost of the lens does not increase until the main scanning eccentricity is about 0.1, and the cost increases as the main scanning eccentricity increases above this. Therefore, the main scanning eccentricity of this embodiment is 0.48, and the cost ratio is 1.5 times.

  The relationship between the main scanning eccentricity and the cost ratio varies depending on various factors such as the shape, size, material, and manufacturing method of the lens. Although it depends on the configuration of the electrical correction means to be compared, the main scanning eccentricity is approximately 0.5 or less in light of the objective of obtaining a high-speed, high-quality optical scanning device at low cost. Is good.

  Next, a preferred embodiment of the multi-beam laser rotation angle θe will be described. First, a guideline for simply determining the inclination (first inclination) of the light amount distribution by the deflector will be described. Referring to FIG. 8, the absolute value of the differential coefficient of the angle dependency of the S-polarized light at the incident angle when the principal ray of the light beam reflected by the polarization surface 5a is orthogonal to the surface to be scanned 8 is abS and the angle dependency of the P-polarized light. The absolute value of the differential coefficient is abP.

  Then, the influence degree efS and P polarization of the S polarization component are obtained by using the polarization direction of the linearly polarized light, that is, the ratio of the S polarization component to the polarization plane 5a when the rotation angle of the light source 1 is θe, and abS and abP. The influence degree efP of the component is defined as follows.

  In FIG. 1, which is a schematic diagram of the main scanning section of the present embodiment, the angle formed between the reference line and the principal ray of the incident light beam guided to the deflector 5 by the incident optical system is 90 degrees, and is reflected by the polarization plane 5a. The incident angle when the principal ray of the luminous flux is orthogonal to the scanned surface 8 is 45 deg. At this time, from the graph of FIG. 8, the absolute values of the differential coefficients of the angle dependency of the S-polarized light and the P-polarized light at the incident angle of 45 deg are abS = 0.00215 and abP = 0.00031. The graph of FIG. 15 shows the dependency of efS and efP on the rotation angle θe.

  The range of the rotation angle θe where efS> efP, that is, the influence of the S-polarized component that causes the gradient of the light amount distribution by the deflector in the following conditional expression is dominant. In this embodiment, the rotation angle θe is recommended to be 8.25 deg or more. It becomes a condition to be.

  Further, as an application example, on the premise of an optical scanning device capable of scanning a wider range than the actual scanning region as shown in FIG. 16, a configuration that can move in the main scanning direction along the scanned surface 8 is adopted. it can. The third factor (manufacturing error) is different from the previous prediction (design), or even if the light amount distribution on the scanned surface changes due to a change over time. That is, for example, by changing only the setting of the scanning range from the scanning region A to the scanning region B, it is possible to suppress the bias of the light amount distribution on the surface to be scanned without relying on an electrical correction unit or the like.

  Furthermore, even when the rotation angle θe of the multi-beam laser changes due to a change in resolution, pitch interval, etc., the electrical correction means simply changes the setting of the scanning range in accordance with the change in the amount of light distribution by the deflector 5. The bias of the light quantity distribution can be suppressed without depending on the above.

  As described above, according to the present embodiment, it is possible to suppress the occurrence of a deviation in light amount distribution due to a plurality of light beams on the surface to be scanned while realizing cost reduction. That is, according to Patent Document 1 instead of this embodiment, it is necessary to provide a dedicated reflection film instead of a general reflection film in order to give the folding mirror a predetermined reflection characteristic (In Patent Document 1, imaging optics is used). It cannot be applied when the system does not have a folding mirror), resulting in high cost. Further, not only in the present embodiment, but also in the case of using a known technique of partially changing the transmittance of the cover glass or using a folding mirror having special reflectance characteristics, the cost is increased.

<< Second Embodiment >>
The present embodiment is an optical scanning device that includes first and second imaging optical systems corresponding to first and second scanned surfaces, respectively, and in which a deflector is shared. FIG. 7 is a diagram schematically showing the sub-scanning section of the imaging optical system of the present embodiment, and FIG. 17 is a diagram schematically showing the sub-scanning section of the oblique incidence optical system. Here, the oblique incident optical system refers to an incident optical system that allows a light beam to be incident on one deflection surface obliquely from the sub-scanning direction.

  In order to reduce the number of deflectors, a plurality of light beams are deflected in the same direction by one deflecting surface of the deflector. Further, in order to reduce the size of the deflector in the sub-scanning direction, the deflector is incident on the same deflection surface at an angle in the sub-scanning direction.

  A plurality of light beams deflected and scanned on the same deflection surface pass through the first imaging lens 6a, and then one of them passes through the second imaging lens 6b via the folding mirrors M1 and M2, and further passes through the cover glass 7. Then, the light is condensed on the photosensitive drum 8B which is the surface to be scanned. The other passes through the second imaging lens 6b and is condensed on the photosensitive drum 8A, which is the surface to be scanned, via the folding mirror M3 and the cover glass 7. In this way, a plurality of light beams deflected by the same deflection surface are separated by a folding mirror or the like in the imaging optical system, and the corresponding scanned surface is scanned with each light beam.

  In an optical scanning device including an oblique incidence optical system, the number of folding mirrors and the folding angle or the arrangement order of the imaging lens and the folding mirror are different in each optical path. For this reason, the light quantity distribution of the light beam that has passed through the imaging optical system, that is, the cumulative transmitted / reflected light quantity (passing light quantity) in the imaging optical system may be different for each optical path with respect to the incident light quantity incident on the imaging optical system.

  FIGS. 18 to 21 are optical scanning devices adopting the present embodiment, and FIGS. 18 and 19 related to the case where the light amount distribution by passing through the imaging optical system B has an inclination, respectively. , Corresponding to the scanned surface B. Also, FIGS. 20 and 21 related to the case where the light amount distribution by passing through the imaging optical system B has a curve correspond to the scanned surface A and the scanned surface B, respectively. The graph (b) shows the light amount distribution by the deflector 5, (c) shows the light amount distribution by the imaging optical system, and (d) shows the light amount distribution on the scanned surface 8.

  For easy understanding, each graph includes a one-dot chain line obtained by approximating the light amount distribution in the scanning region with a linear function and a broken line indicating the light amount distribution in the region not actually scanned.

In this embodiment, the light quantity distribution by the deflector 5 shared by a plurality of optical paths has an inclination. Then, as described in the first embodiment, the optical path toward the scanned surface 8A is set as θ _u > θ _l to suppress the deviation of the light amount distribution on the scanned surface 8A.

On the other hand, with respect to the optical path toward the scanned surface 8B, even if θ _u > θ _l is set, the bias of the image surface illuminance distribution on the scanned surface 8B does not change (FIG. 19 (d)), or θ _u. > with the setting theta _l increases the difference between the light quantity distribution on the scanned face 8B (FIG. 21 (d)).

When the optical path toward the scanned surface 8B in FIG. 21 is limited, the difference in the light amount distribution can be reduced by setting the scanning range as θ _u <θ _l . However, on the scanned surface 8A in FIG. This increases the difference in the light quantity distribution.

Compared to the conventional case where the scanning range is set as θ _u = θ ₁ and the difference in the light amount distribution on each scanning surface exceeds the allowable amount, and electrical correction means are required in both optical paths. In the embodiment, electrical correction means can be omitted in one optical path. The difference in the light amount distribution on the other surface to be scanned increases, but only this need be electrically corrected.

  As described above, according to the present embodiment, as in the first embodiment, it is possible to suppress the occurrence of bias in the light amount distribution due to a plurality of light beams on the surface to be scanned while realizing cost reduction.

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

(Modification 1)
In the above-described embodiment, the first angle θ _u and the second angle θ _l are set. However, as described with reference to FIG. 16, at least one of them can be configured to be variable. As specific means, light emission control means for controlling the start and end timings of the light emission of the light source 1 (that is, the timing of printing start and end of writing) is used.

(Modification 2)
In the embodiment described above, the imaging optical system includes the folding mirror, but the present invention can be applied even if the imaging optical system does not include the folding mirror.

1 .... light source, 5 .... deflector, 6 .... imaging lens group, 8 .... scanned surface

Claims (12)

A light source having a plurality of light emitting portions, a deflector that deflects a light beam from the light source and scans a scanning region on the surface to be scanned in the main scanning direction, and a light beam deflected by the deflector is guided to the scanning region. An imaging optical system,
The light beam incident on the deflector is linearly polarized light whose vibration direction is inclined with respect to the main scanning section,
In the main scanning section, the angle formed between the principal ray incident on the first and second end portions in the scanning region and the optical axis is different from each other,
Of the light beam immediately after being deflected by the deflector, the light amount of the light beam toward the first end is less than the light amount of the light beam toward the second end,
The value of the ratio of the amount of light just before entering the scanned surface to the amount of light just before entering the imaging optical system of the light beam going to the first end is the concatenation of the light beam going to the second end. An optical scanning device characterized by being larger than a value of a ratio of a light amount immediately before entering the scanned surface to a light amount immediately before entering the image optical system.   Among the light beams immediately after being deflected by the deflector in the S-polarized light component of the linearly polarized light, the light amount of the light beam directed to the first end portion is smaller than the light amount of the light beam directed to the second end portion. The optical scanning device according to claim 1.   The ratio of the light amount immediately before entering the surface to be scanned with respect to the light amount immediately before entering the imaging optical system of the light beam traveling toward the first and second ends is symmetric with respect to the optical axis in the main scanning direction. The optical scanning device according to claim 1, wherein the optical scanning device is on a line having a distributed shape. An optical scanning device including first and second imaging optical systems corresponding to first and second scanned surfaces, respectively, and sharing the deflector,
4. The optical scanning device according to claim 1, wherein at least one of the first and second imaging optical systems is the imaging optical system. 5. When the angles formed between the principal ray incident on the first end and the second end and the optical axis are θ _u and θ _l ,
The optical scanning device according to claim 1, wherein the following relationship is satisfied. The angle formed by the vibration direction of the linearly polarized light and the main scanning section is θe,
When the principal ray of the light beam scanned by the deflector is orthogonal to the surface to be scanned, the absolute values of the differential coefficients of the incident angle dependence of the reflectance of the S-polarized light and the P-polarized light in the deflector are abS and abP, respectively. And when
The optical scanning device according to claim 1, wherein the following relationship is satisfied.   7. An incident optical system comprising a condensing element for condensing a light beam and a slit member for limiting the light beam in an optical path from the light source to the deflector. The optical scanning device according to claim 1.   8. The optical scanning device according to claim 1, wherein at least one of an angle formed between the principal ray incident on the first and second end portions and the optical axis is variable. 9. .   9. The device according to claim 1, wherein in the main scanning section, the first end portion is an end portion on the light source side with respect to the optical axis of the imaging optical system. The optical scanning device described.   The angle formed between the principal ray incident on the first end and the optical axis is greater than the angle formed between the principal ray incident on the second end and the optical axis. The optical scanning device according to 1.   The optical scanning device according to claim 1, a developing device that develops an electrostatic latent image formed on the surface to be scanned by the optical scanning device as a toner image, and the developed toner An image forming apparatus comprising: a transfer device that transfers an image to a transfer material; and a fixing device that fixes the transferred toner image to the transfer material.   An optical scanning device according to any one of claims 1 to 10, and a printer controller that converts a color signal output from an external device into image data and inputs the image data to the optical scanning device. Image forming apparatus.