JP2011158912A - Optical scanner - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve the image quality of an image by suppressing variation in the sectional beam diameter of a laser beam imaged on a photoreceptor drum, in an optical scanner of an over illumination type. <P>SOLUTION: In this optical scanner, an imaging optical system 60 includes an imaging lens 61 having at least one surface unsymmetrical to a first direction using, as a boundary point, the position where a luminous flux passes the optical axis of a system defined at a position having the shortest distance between each reflecting surface of a polygon mirror 50 and the photoreceptor drum 23. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to an optical scanning device used in, for example, a laser printer, a digital copying machine, and the like. In particular, the width of a light beam incident on a polygon mirror in the main scanning direction (the direction along the rotation direction of the polygon mirror) The present invention relates to an over-illumination type optical scanning device that is wider than the width of a reflecting surface in the main scanning direction.

  In laser printers and digital copying machines, which are electrostatic copying image forming apparatuses that form an electrostatic latent image using a light beam and develop the electrostatic latent image to obtain a visible (developer) image. The image to be output (original image) is decomposed into a first direction and a second direction orthogonal to the first direction, and the decomposed first direction or the second direction An optical scanning device that repeatedly outputs, that is, scans, a light beam whose light intensity has been changed based on image data in a substantially linear manner at a predetermined time interval is used. The recording medium and the latent image holding member are scanned at a predetermined speed while the light beam for one line and the light beam for the next line are scanned or while one line is being scanned. By moving in the direction orthogonal to the light beam, an image corresponding to the original image is obtained.

  In such an optical scanning device, the direction in which the light beam is scanned, that is, the first direction is usually referred to as the main scanning direction. Also, the second direction orthogonal to the first direction is usually called the sub-scanning direction.

  The optical scanning device includes a semiconductor laser element that is a light source that emits a light beam (laser beam), and a first lens that adjusts a cross-sectional beam diameter and a cross-sectional shape of the light beam emitted from the semiconductor laser element to a predetermined size and shape. The light beams adjusted to a predetermined size and shape by the first lens group and the first lens group are continuously reflected and deflected (scanned) in a direction orthogonal to the direction in which the recording medium or the latent image holder is moved. A deflecting device and a second lens group that forms an image of the light beam deflected by the deflecting device at a predetermined position of the recording medium or the latent image holding member are included.

  The image forming apparatus has a recording medium or latent image holding body for holding an image (latent image), a recording medium or a latent image holding body for one line every time one line of image data is irradiated by the optical scanning device. By moving the image, an image in which an image for one line is arranged in order in a direction orthogonal to the image for one line is formed. Image forming devices can be classified into wet (liquid type), dry type, direct type and transfer type (indirect type) depending on the method of visualizing the latent image and whether the latent image is directly formed on the recording medium. It is.

In the above-described image forming apparatus and optical scanning apparatus, process speed (speed at which the recording medium or latent image holding member is moved), image resolution (number of dots per unit length, generally dots per inch), Between the rotational speed (number of rotations) of the polygon mirror motor of the deflection device and the number of polygon mirror surfaces of the deflection device,
P (mm / s) is the process speed,
R (dpi) is the image resolution (number of dots per inch),
Vr (rpm) is the number of revolutions of the polygon mirror motor, and
N is the number of polygon mirror faces,
And when
P × R = 25.4 × Vr × N / 60 (1)
There is a relationship.

  From equation (1), it is recognized that the image forming speed and the image resolution are proportional to the number of rotations of the polygon mirror (usually the same as the number of rotations of the polygon mirror motor) and the number of surfaces of the polygon mirror. Therefore, in order to increase the speed of the image forming apparatus (increase the image forming speed per unit time) and increase the resolution (improving the image resolution), the number of polygon mirrors is increased or the number of rotations of the polygon mirror is increased. There is a need.

  In an under-illumination type (a general term for comparison with an over-illumination type) optical scanning device used in many image forming apparatuses today, the width of a light beam (light beam) incident on a polygon mirror in the main scanning direction ( In the case where the cross-sectional beam diameter is different between the main scanning direction and the sub-scanning direction, the beam diameter in the main scanning direction is limited to be smaller than the width in the main scanning direction of an arbitrary reflecting surface of the polygon mirror. Therefore, the light beam guided to each reflecting surface of the polygon mirror is totally reflected by the reflecting surface.

On the other hand, the cross-sectional beam diameter of the light beam guided to the recording medium or the latent image holding body (image surface) (the beam diameter in the main scanning direction when the main scanning direction and the sub scanning direction are different) is the second lens. It is proportional to the F number Fn of the group (imaging optical system). F number Fn is
The focal length of the imaging optics is f, and
D is the diameter in the main scanning direction of the light beam at an arbitrary reflecting surface in the polygon mirror.
Then,
Fn = f / D
It is represented by Therefore, in order to increase the resolution, if an attempt is made to reduce the cross-sectional beam diameter of the light beam on the scanning object (image surface), that is, the recording medium or the latent image holding member (photoreceptor), It is necessary to increase the cross-sectional beam diameter in the main scanning direction. For this reason, when the surface width of each reflecting surface of the polygon mirror is increased and the number of reflecting surfaces is further increased, the size of the polygon mirror becomes large.

  However, in order to rotate a large polygon mirror at high speed, a large motor with a large torque is required. In this case, the motor cost naturally increases. At the same time, noise and vibration are increased and more heat is generated, so these measures are necessary.

  As a countermeasure, there is an over illumination type optical scanning device. The principle of the optical scanning device using over illumination is disclosed in, for example, Laser Scanning Notebook (Leo Beiser, SPIE OPTICAL ENGINEERING PRESS).

  In the over illumination type optical scanning device, the width in the main scanning direction of the light beam applied to each reflecting surface of the polygon mirror is set larger than the width in the main scanning direction of each reflecting surface of the polygon mirror. The light beam can be reflected on the entire surface of each reflecting surface. Accordingly, the number of reflecting surfaces of the polygon mirror can be increased in order to increase the image forming speed and resolution without increasing the size, particularly the diameter, of the polygon mirror more than necessary.

  In this way, by applying over illumination to the relationship between the cross-sectional beam diameter of the light beam applied to the polygon mirror and the width of each reflecting surface of the polygon mirror in the main scanning direction, the load on the polygon motor can be reduced. Even if the polygon mirror is rotated at high speed, heat generation is also suppressed. This also reduces the cost of the polygon motor. Further, since the shape of the main scanning surface of the polygon mirror approaches a perfect circle by increasing the number of reflection surfaces of the polygon mirror, noise, particularly wind noise and vibration can be reduced. Since noise and vibration are reduced to a certain level, the number of parts such as dust-proof glass required for soundproofing and airtightness is also reduced, so that a merit of further cost reduction can be expected. Also, a high duty cycle is possible.

  In the above-described over-illumination type optical scanning device, in the under-illumination type optical scanning device, the width in the main scanning direction of the light beam deflected by the polygon mirror is constant regardless of the scanning angle (position angle). On the other hand, there is a problem that it varies depending on the scanning angle.

  As described above, in the over-illumination type optical scanning device, the angle at which the light beam guided to the arbitrary reflecting surface of the polygon mirror is reflected by the arbitrary reflecting surface of the polygon mirror, that is, the scanning angle changes. Since the F number Fn changes, a variation (deviation) occurs in the main scanning direction of the cross-sectional beam diameter of the light beam that has reached the photoconductor (image plane). In particular, when the light beam incident on the polygon mirror has an angle with the optical axis of the imaging optical system in the main scanning plane (when entering from an oblique direction), the variation in cross-sectional beam diameter is related to the main scanning direction. There is a problem of left-right asymmetry with respect to the optical axis center of the optical system.

  In order to reduce the variation in the cross-sectional beam diameter of the light beam on the image surface (photosensitive member), if the light beam is incident on the arbitrary reflecting surface of the polygon mirror from the front (near) in the main scanning direction, the polygon The light beam reflected (deflected) by the individual reflecting surfaces of the mirror is reflected by the second lens group, returned to an arbitrary reflecting surface of the polygon mirror, and again reflected toward the second lens group. There is. When the light beam reciprocating between the second lens group and an arbitrary reflecting surface of the polygon mirror is imaged in the image area of the photoreceptor, there is a problem that the image quality of the image deteriorates.

  Further, by providing an antireflection film (a coating for improving light transmittance) on the lens surface of the lens included in the second lens group, the lens surface is reciprocated between the second lens group and each reflecting surface of the polygon mirror. However, there is a problem that the cost of a single lens increases. In addition, when an antireflection film is provided on the lens surface of a lens having a low glass transition temperature, the surface accuracy of the lens surface may be lowered and optical characteristics may be deteriorated.

  An object of the present invention is to deteriorate image quality caused by a light beam reflected by a lens surface of an imaging lens for imaging a light beam on a photosensitive member being scanned again on the photosensitive member through the imaging lens. It is an object of the present invention to provide an over-illumination type optical scanning apparatus that can prevent the above-described problem and reduce the size of the apparatus.

  The present invention has been made in order to achieve the above-described object. An optical system before deflection for shaping a light beam from a light source to form a long line image in a first direction, and the light beam on a scanning object. An optical scanning unit that scans to a predetermined position; and an imaging optical system that forms an image of the light beam scanned by the optical scanning unit on a scanning object, wherein the light beam incident on the optical scanning unit In the optical scanning device wider than the width of the single reflecting surface of the means in the first direction, the imaging optical system has a center in the longitudinal direction in the first direction with respect to the optical axis. Is decentered or rotated at a position intersecting the optical axis by a predetermined amount with respect to the rotation center of the surface rotated about the axis in the second direction orthogonal to the first direction. An optical scanning device having at least one surface is also provided. It is.

The present invention also provides a pre-deflection optical system that shapes a light beam from a light source to form a long line image in a first direction, an optical scanning unit that scans the light beam at a predetermined position of a scanning object, An imaging optical system that forms an image of the light beam scanned by the optical scanning unit on a scanning target, and the light beam incident on the optical scanning unit is the first reflection surface of the single reflecting surface of the optical scanning unit. In the optical scanning device wider than the width in the direction, the imaging optical system includes an optical component having a surface having at least one power, and the shape of the surface having the power of at least one of the optical components is the shape of the surface When the first direction is the y-axis, the second direction orthogonal to the first direction is the z-axis, and the direction in which the light beam travels is the x-axis, the shape of the surface having the power of at least one surface is not The spherical term is
A _m Σy ^m (m = 0, 1, 2, 3,...) Or
If A _mn Σy ^m z ⁿ (m = 0, 1, 2, 3,..., N = 0, 1, 2, 3,...),
The present invention provides an optical scanning device including a term in which A _m ≠ 0 or A _mn ≠ 0 when m = 2 × k + 1 (k = 0, 1, 2, 3,...).

  Furthermore, the present invention provides a pre-deflection optical system that shapes a light beam from a light source to form a long line image in a first direction, an optical scanning unit that scans the light beam to a predetermined position of a scanning object, An imaging optical system that forms an image of the light beam scanned by the optical scanning unit on a scanning target, and the light beam incident on the optical scanning unit is the first reflection surface of the single reflecting surface of the optical scanning unit. In the optical scanning device wider than the width in the direction, the imaging optical system includes a lens having power, and the lens having power has at least a surface without a surface treatment for suppressing reflection when the light beam is incident. The present invention provides an optical scanning device having one surface, wherein the incident light to the optical scanning means and the optical axis of the imaging optical system have an angle in a main scanning plane or a sub-scanning plane.

  Still further, the present invention provides a pre-deflection optical system that shapes a light beam from a light source to form a long line image in a first direction, an optical scanning unit that scans the light beam at a predetermined position of a scanning object, An imaging optical system that forms an image of the light beam scanned by the optical scanning unit on a scanning object, and the light beam incident on the optical scanning unit is the first reflection surface of the single reflecting surface of the optical scanning unit. In the optical scanning device wider than the width in the direction, the imaging optical system includes a plurality of optical components, and the number of optical components having power among the plurality of optical components is 2 or less. A device is provided.

  Furthermore, the present invention provides a photoconductor that receives an optical beam to generate an image, a pre-deflection optical system that shapes a light beam from a light source and forms a long line image in a first direction, and the light beam as described above. An optical scanning unit that scans a predetermined position of the photosensitive member; and an imaging optical system that forms an image of the light beam scanned by the optical scanning unit on the photosensitive member. The light beam incident on the optical scanning unit An optical scanning device wider than the width of the first reflecting surface of the optical scanning means in the first direction, wherein the imaging optical system has its center in the first direction with respect to the optical axis. With respect to the rotation center of the surface rotated about the axis in the second direction perpendicular to the first direction at a position where the center in the longitudinal direction is decentered or intersects the optical axis. An optical scanning device characterized in that it has at least one surface that is rotated by a fixed amount; There is provided a developing apparatus for developing an image formed on the photoreceptor by the optical scanning apparatus, an image forming apparatus characterized by having a.

  As described above, according to the present invention, in the over-illumination type optical scanning device, since the shape of the surface having the power of the imaging optical system is asymmetric in the main scanning direction, the reflected light from the lens surface of the imaging optical system It is possible to prevent image deterioration due to the image quality, and to reduce the beam diameter variation in the image area, thereby improving the image quality of the output image.

1 is a schematic diagram illustrating an example of an image forming apparatus in which an optical scanning device according to an embodiment of the present invention is incorporated. FIG. 2 is a schematic diagram illustrating an optical scanning device incorporated in the image forming apparatus illustrated in FIG. 1. FIG. 3 is a schematic block diagram showing an example of a drive circuit of a digital copying apparatus including the optical scanning device shown in FIG. 2. FIG. 3 is a schematic diagram illustrating a state in which a laser beam reciprocates between individual reflecting surfaces of a polygon mirror and an imaging lens of the over illumination type optical scanning device illustrated in FIG. 2. FIG. 3 is a schematic diagram for explaining a relationship between a scanning position of a laser beam scanned by rotation of individual reflecting surfaces of a polygon mirror of the over illumination type optical scanning device shown in FIG. 2 and a light beam width in the main scanning direction of the laser beam. By using the over-illumination type optical scanning device shown in FIG. 2 and reducing the variation in the convergence angle in the entire scanning region in the main scanning direction, the variation in the cross-sectional beam diameter of the laser beam imaged on the photosensitive drum can be reduced. Schematic explaining the principle which can suppress a grade. In the optical scanning device described with reference to FIG. 2, the state of variation in the convergence angle ψ with respect to the position in the main scanning direction is expressed on the photosensitive drum as a known optical scanning device with an image region ranging from −160 mm to 160 mm. The graph which shows the result of the comparison. In the optical scanning device described with reference to FIG. 2, the variation in the cross-sectional beam diameter of the laser beam imaged on the photosensitive drum is known on the photosensitive drum as a range from −160 mm to 160 mm as an image region. The graph which shows the result compared with the optical scanning device. In the optical scanning device described with reference to FIG. 2, the fθ characteristic in the main scanning direction is a graph showing a result of comparison with a known optical scanning device on the photosensitive drum as an image region in a range from −160 mm to 160 mm. . Schematic explaining another embodiment of the optical scanning device described with reference to FIG. In the optical scanning device described with reference to FIG. 10, the state of variation of the convergence angle ψ with respect to the position in the main scanning direction is defined as the range of −160 mm to 160 mm on the photosensitive drum as an image region. The graph which shows the result of the comparison. In the optical scanning device described with reference to FIG. 10, the variation in the cross-sectional beam diameter of the laser beam imaged on the photosensitive drum is known on the photosensitive drum as a range from −160 mm to 160 mm as an image region. The graph which shows the result compared with the optical scanning device. In the optical scanning device described with reference to FIG. 10, the fθ characteristic in the main scanning direction is a graph showing the result of comparison with a known optical scanning device on the photosensitive drum, with the range from −160 mm to 160 mm as an image region. .

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

  FIG. 1 shows a digital copying machine as an image forming apparatus having an optical scanning device according to an embodiment of the present invention.

  As shown in FIG. 1, the digital copying apparatus 1 includes, for example, a scanner unit 10 as an image reading unit and a printer unit 20 as an image forming unit.

  The scanner unit 10 has predetermined imaging characteristics for light from the first carriage 11 formed to be movable in the direction of the arrow, the second carriage 12 that is moved by following the first carriage 11, and the light from the second carriage 12. An optical lens 13 to be applied, a photoelectric conversion element 14 that photoelectrically converts light given predetermined imaging characteristics by the optical lens 13 and outputs an electric signal, a document table 15 that holds the document D, and a document D on the document table 15 A document fixing cover 16 to be pressed is provided.

  The first carriage 11 is provided with a light source 17 that illuminates the document D, and a mirror 18 a that reflects the reflected light reflected from the document D by the light emitted from the light source 17 toward the second carriage 12. Yes.

  The second carriage 12 has a mirror 18b for bending the light transmitted from the mirror 18a of the first carriage 11 by 90 °, and a mirror 18c for further bending the light bent by the mirror 18b by 90 °.

  The document D placed on the document table 15 is illuminated by the light source 17 and reflects reflected light in which the brightness of light corresponding to the presence or absence of an image is distributed. The reflected light of the document D enters the optical lens 13 via the mirrors 18a, 18b and 18c as image information of the document D.

  The reflected light from the document D guided by the optical lens 13 is condensed on the light receiving surface of the photoelectric conversion element (CCD sensor) 14 by the optical lens 13.

  Hereinafter, when the start of image formation is input from an operation panel (not shown) or an external device, the first carriage 11 and the second carriage 12 are in a predetermined positional relationship with respect to the document table 15 by driving a carriage drive motor (not shown). After being moved to the home position determined to be, it is moved along the document table 15 at a predetermined speed, so that the image information of the document D, that is, the image light reflected from the document D is reflected by the mirror 18a. The mirror is cut out with a predetermined width along the extending direction, i.e., the main scanning direction, reflected toward the mirror 18b, and the mirror perpendicular to the extending direction of the mirror 18a, i.e., the sub-scanning direction. The image data is sequentially taken out in units of the width cut out by 18a, and all image information of the document D is guided to the CCD sensor 14. The electrical signal output from the CCD sensor 14 is an analog signal, converted into a digital signal by an A / D converter (not shown), and temporarily stored as an image signal in an image memory (not shown).

  As described above, the image of the document D placed on the document table 15 is imaged by the CCD sensor 14 for each line along the first direction in which the mirror 18a is extended. Is converted into, for example, an 8-bit digital image signal indicating the density of the image.

  The printer unit 20 includes an optical scanning device 21 serving as an exposure device, which will be described later with reference to FIGS. 2 and 3, and an electrophotographic image forming unit 22 capable of forming an image on a recording paper P as an image forming medium. have.

  The image forming unit 22 is rotated by the main motor described with reference to FIG. 3 so that the outer peripheral surface moves at a predetermined speed, and the laser beam L is emitted from the optical scanning device 21, thereby image data, that is, the image of the document D. A drum-shaped photoconductor (hereinafter referred to as a photoconductor drum) 23 on which an electrostatic latent image corresponding to 1 is formed, a charging device 24 for applying a surface potential of a predetermined polarity to the surface of the photoconductor drum 23, and the photoconductor drum 23 A developing device 25 that selectively supplies toner as a visualization material to the electrostatic latent image formed by the optical scanning device and develops the toner image formed on the outer periphery of the photosensitive drum 23 by the developing device 25. Is transferred to the recording paper P, and the recording paper P onto which the toner image has been transferred by the transfer device and the toner between the recording paper P and the photosensitive drum 23 are statically transferred to the photosensitive drum 23. The separation device 27 that releases from the adsorption (separates from the photosensitive drum 23) and the transfer residual toner remaining on the outer peripheral surface of the photosensitive drum 23 are removed, and the potential distribution of the photosensitive drum 23 is changed to the surface potential by the charging device 24. A cleaning device 28 and the like for returning to a state before being supplied are provided. Note that the charging device 24, the developing device 25, the transfer device 26, the separation device 27, and the cleaning device 28 are arranged in order along the direction of the arrow in which the photosensitive drum 23 is rotated. Further, the laser beam L from the optical scanning device 21 is irradiated to a predetermined position X on the photosensitive drum 23 between the charging device 24 and the developing device 25.

  An image signal read from the document D by the scanner unit 10 is converted into a print signal by an image processing unit (not shown) by, for example, gradation processing for contour correction or halftone display, and the like. The light intensity of a laser beam emitted from a semiconductor laser element to be described below is determined so that the electrostatic latent image can be recorded on the outer periphery of the photosensitive drum 23 to which a predetermined surface potential is applied by the charging device 24. The image is converted into a laser modulation signal for changing to any one of the non-recording intensity.

  Each of the semiconductor laser elements shown below of the optical scanning device 21 is intensity-modulated according to the laser modulation signal described above, and records an electrostatic latent image at a predetermined position on the photosensitive drum 23 corresponding to predetermined image data. So that it emits light. The light from the semiconductor laser element is deflected in a first direction that is the same as the reading line of the scanner unit 10 by a deflecting device described below in the optical scanning device 21, and Irradiated to a predetermined position X on the outer periphery.

  Thereafter, when the photosensitive drum 23 is rotated in the direction of the arrow at a predetermined speed, the first carriage 11 and the second carriage 12 of the scanner unit 10 are moved along the document table 7 in the same manner as in the document table 7. The laser beam from the deflected semiconductor laser element is exposed on the outer circumference of the photosensitive drum 23 at predetermined intervals for each line.

  In this way, an electrostatic latent image corresponding to the image signal is formed on the outer periphery of the photosensitive drum 23.

  The electrostatic latent image formed on the outer periphery of the photosensitive drum 23 is developed with toner from the developing device 25, conveyed to a position facing the transfer device 26 by the rotation of the photosensitive drum 23, and supplied from the paper cassette 29. One sheet is taken out by the paper roller 30 and the separation roller 31, and is transferred by the electric field from the transfer device 26 onto the recording paper P supplied with the timing aligned by the aligning roller 32.

  The recording paper P onto which the toner image has been transferred is separated together with the toner by the separation device 27 and guided to the fixing device 34 by the transport device 33.

  The recording paper P guided to the fixing device 34 is discharged onto the tray 36 by the paper discharge roller 35 after the toner (toner image) is fixed by heat and pressure from the fixing device 34.

  On the other hand, the photosensitive drum 23 after the toner image (toner) is transferred to the recording paper P by the transfer device 26 is opposed to the cleaning device 28 as a result of the subsequent rotation, and the transfer residual toner (residual toner) remaining on the outer periphery. ) Is removed, and the state before the surface potential is supplied by the charging device 24 is returned to the initial state, and the next image formation becomes possible.

  By repeating the above process, a continuous image forming operation can be performed.

  As described above, the document D set on the document table 15 is read by the scanner unit 10, and the read image information is converted into a toner image by the printer unit 20 and output to the recording paper P. Copied.

  In the above description of the image forming apparatus, a digital copying machine is taken as an example. However, for example, a printer without an image reading unit may be used.

  2A and 2B are schematic diagrams for explaining the configuration of the optical scanning device shown in FIG. In FIG. 2A, the optical elements arranged between the light source (semiconductor laser element) and the photosensitive drum (scanning object) included in the optical scanning device are exposed from the optical deflection device (polygon mirror). FIG. 3 is a schematic plan view illustrating a light beam traveling toward a body drum viewed from a direction orthogonal to a main scanning direction (first direction) that is parallel to a direction scanned by an optical deflecting device and unfolding by a mirror. FIG. 2B is a schematic cross-sectional view showing the sub-scanning direction (second direction) perpendicular to the main scanning direction, that is, the direction shown in FIG.

  2A and 2B, the optical scanning device 21 includes, for example, a semiconductor laser element (light source) 41 that emits a laser beam (light beam) L of 780 nm, and a semiconductor laser element 41. The cross-sectional beam shape of the emitted laser beam L is adjusted to a predetermined shape and size, and the light amount (light flux width) of the laser beam L that has passed through the finite focus lens or collimator lens 42 is set to a predetermined value. A cylindrical lens 44 to which a positive power is applied only in the sub-scanning direction in order to adjust the cross-sectional shape of the laser beam L limited in size to a predetermined cross-sectional beam shape. , And a finite focal lens or collimating lens 42, an aperture 43, and a cylindrical lens Sectional shape has a laser beam L, the pre-deflection optical system 40 having a mirror 45 or the like for bending in a predetermined direction from the semiconductor laser element 41 which is adjusted to a predetermined sectional beam shape by 4.

  In the direction in which the laser beam L to which the predetermined cross-sectional beam shape is given by the pre-deflection optical system 40 travels, it is formed integrally with the polygon mirror motor 50A that rotates at a predetermined speed. A polygon mirror (light deflecting device) 50 is provided for scanning the laser beam L adjusted to a predetermined shape toward the photosensitive drum (scanned surface) 23 located at the subsequent stage.

  Between the polygon mirror 50 and the photosensitive drum 23, the laser beam L continuously reflected by each reflecting surface of the polygon mirror 50 is imaged substantially linearly along the axial direction of the photosensitive drum 23. An imaging optical system 60 is provided.

  The imaging optical system 60 is configured so that the position on the photoconductive drum 23 when the photoconductive drum 23 is irradiated with the laser beam L continuously reflected by the respective reflecting surfaces of the polygon mirror 50 and the polygon mirror 50. Irradiating from one end to the other end in the longitudinal (axis) direction of the photosensitive drum 23 at the exposure position X shown in FIG. 1 while making the rotation angle of each reflecting surface proportional, which position in the longitudinal direction on the photosensitive drum 23 In FIG. 5, an imaging lens (usually called an fθ lens) 61 that can provide convergence with a predetermined relationship based on an angle at which the polygon mirror 50 is rotated so as to have a predetermined cross-sectional beam diameter. And a dust-proof glass 62 that prevents toner, dust, paper dust, and the like floating in the image forming unit 22 from entering a housing (not shown) of the optical scanning device 21.

  The optical path of the laser beam L from the semiconductor laser element 41 in the optical scanning device 21 to the photosensitive drum 23 is bent in a housing (not shown) of the optical scanning device 21 by a plurality of mirrors (not shown). Further, by optimizing the curvature of the imaging lens 61 in the main scanning direction and the sub-scanning direction and the optical path between the polygon mirror 50 and the photosensitive drum 23, at least one of the imaging lens 61 and a mirror (not shown) is provided. It may be formed integrally.

In the optical scanning device shown in FIGS. 2A and 2B, the axis O _I along which the principal ray of the incident laser beam directed to each reflecting surface of the polygon mirror 50 and the imaging optical system 60 the optical axis O _R, the angle alpha both when projected in the main scanning plane, respectively, is α = 5 °. Further, in a state viewed the optical scanning device from the sub-scan section, the angle between the optical axis O _R of the incident laser beam and the imaging optical system is 2 °.

  FIG. 3 is a schematic block diagram showing an example of a drive circuit of a digital copying apparatus including the optical scanning device shown in FIGS. 2 (a) and 2 (b).

  The CPU 101 as the main control device includes a ROM (read only memory) 102 in which predetermined operation rules and initial data are stored, a RAM 103 in which input control data is temporarily stored, image data from the CCD sensor 14 or This is a case where the image data supplied from the external apparatus is held, and the energization to the copying apparatus 1 is interrupted by the shared (image) RAM 104 that outputs the image data to the image processing circuit shown below and the battery backup. However, the NVM (nonvolatile memory) 105 that holds the data stored so far and the image data stored in the image RAM 104 are subjected to predetermined image processing and output to the laser driver described below. A processing device 106 and the like are connected.

  The CPU 101 also includes a laser driver 121 that emits the semiconductor laser element 41 of the optical scanning device 21, a polygon motor driver 122 that drives a polygon motor 50 </ b> A that rotates the polygon mirror 50, a photosensitive drum 23, and accompanying paper (covered paper). A main motor driver 123 for driving a main motor 23A for driving a transfer material) transport mechanism and the like is connected.

  In the optical scanning device 21 shown in FIGS. 2A and 2B, the divergent laser beam L emitted from the semiconductor laser element 41 is converged by a finite focal lens or a collimating lens 42. Converted to gender or substantially parallel (rarely may be divergent).

  The laser beam L whose cross-sectional beam shape has been converted into a predetermined shape is passed through the aperture 43 so that the light beam width and the light amount are optimally set, and the cylindrical lens 44 gives a predetermined convergence only in the sub-scanning direction. . As a result, the laser beam L has a linear shape extending in the main scanning direction on each reflection surface of the polygon mirror 50.

The polygon mirror 50 is a regular dodecahedron, for example, and has an inscribed circle diameter Dp of 29 mm. The width Wp in the main scanning direction of each reflecting surface (12 surfaces) of the polygon mirror 50 is N, where N is the number of reflecting surfaces of the polygon mirror 50.
Wp = tan (π / N) × Dp
In this example,
Wp = tan (π / 12) × 29 = 7.77 mm
It is.

In contrast, the beam width D _L in the main scanning direction of the laser beam L applied to the reflecting surfaces of the polygon mirror 50 is generally 32 mm, in the main scanning direction of each of the reflecting surfaces of the polygon mirror 50 width Wp = Widely set compared to 7.77 mm.

  The laser beam L guided to each reflecting surface of the polygon mirror 50 and continuously scanned (deflected) by being continuously reflected by rotating the polygon mirror 50 is imaged by the imaging optical system 60. The lens 61 provides predetermined imaging characteristics on the photosensitive drum 23 (image plane) so that the cross-sectional beam diameter is substantially uniform at least in the main scanning direction, and the surface of the photosensitive drum 23 is substantially linear. The image is formed into a shape. Further, the imaging lens 61 corrects the rotation angle of each reflecting surface of the polygon mirror 50 and the imaging position of the light beam imaged on the photosensitive drum 23, that is, the scanning position, to have a proportional relationship. . Accordingly, the speed of the light beam scanned linearly on the photosensitive drum 23 by the imaging lens 61 is constant in the entire scanning region. Note that the imaging lens 61 also corrects the displacement of the scanning position in the sub-scanning direction due to the influence of the respective reflecting surfaces of the polygon mirror 50 being non-parallel to the sub-scanning direction, that is, the tilting of each reflecting surface. The possible curvature (sub-scanning direction curvature) is given. The imaging lens 61 further corrects field curvature in the sub-scanning direction. In order to correct the optical characteristics in the sub-scanning direction, the curvature in the sub-scanning direction is changed depending on the scanning position.

The shape of the lens surface of the imaging lens 61 is

and

Defined by

  The material of the imaging lens 61 is acrylic (PMMA), and the refractive index n is n = 1.483987 for a laser beam having a wavelength of 780 nm. The imaging lens 61 has a thickness of 24 mm in the defocus direction on the optical axis (the direction in which the laser beam travels), and a height in the sub-scanning direction of 25 mm.

  By using such an imaging lens 61, the rotation angle θ of each reflecting surface of the polygon mirror 50 and the position of the laser beam L imaged on the photosensitive drum 23 are approximately proportional to each other. The position at which L is imaged on the photosensitive drum 23 can be corrected.

  The imaging lens 61 can also correct a deviation in the sub-scanning direction between the reflecting surfaces of the polygon mirror 50, that is, a positional deviation in the sub-scanning direction caused by variations in the amount of surface tilt. More specifically, an arbitrary optical conjugation relationship between the laser beam incident surface (polygon mirror 50 side) and the exit surface (photosensitive drum 23 side) of the imaging lens 61 is obtained. Even when the inclination defined between the reflecting surface and the rotation axis of the polygon mirror 50 is different (for each reflecting surface), the scanning position of the laser beam L guided on the photosensitive drum 23 in the sub-scanning direction is changed. The deviation can be corrected.

  The imaging lens 61 also corrects the field curvature in the sub-scanning direction and the bending of the scanning line in the sub-scanning direction. In order to correct these, the curvature of the imaging lens 61 in the sub-scanning direction is changed according to the scanning position.

  The cross-sectional beam diameter of the laser beam L depends on the wavelength of the light beam L emitted from the semiconductor laser element 41. Therefore, by setting the wavelength of the laser beam L to 650 nm, 630 nm, or a shorter wavelength, the laser beam The cross-sectional beam diameter of L can be further reduced.

  FIG. 4 illustrates a state in which the laser beam reciprocates between the individual reflecting surfaces of the polygon mirror and the imaging lens in the over-illumination type optical scanning device shown in FIGS. 2 (a) and 2 (b). FIG.

As shown in FIG. 4, when the laser beams L incident on the individual reflecting surfaces of the polygon mirror 61 are incident from substantially the front surface of the polygon mirror 61 within the main scanning plane, each reflecting surface of the polygon mirror 50 is directed to. The incident laser beam L ₁ is continuously reflected by an arbitrary reflecting surface of the polygon mirror 50 to become an image laser beam L ₂ . The image the laser beam L ₂ is reflected by the lens surface of the imaging lens 61, the reflected laser beam L _3.

Reflected laser beam L ₃ from the lens surface of the imaging lens 61 is returned to the individual reflecting surfaces of the polygon mirror 50, the reflected light of the individual reflecting surfaces of the polygon mirror 50 is reflected by the contributing surface the image area Thus, the reflected laser beam L ₄ toward the imaging lens 61 is obtained.

In this way, after reciprocating between an arbitrary reflecting surface of the polygon mirror 50 and the lens surface of the imaging lens 61, the reflected laser beam L is reflected again by the surface where the reflected light of the polygon mirror 50 contributes to the image area. _{When 4} is imaged on the image area on the photosensitive drum 23 by the imaging lens 61, the image quality of the output image is deteriorated. The light intensity of the reflected laser beam L ₃ reflected by the lens surface of the imaging lens 61, the lens surface of the imaging lens 61, when the coating to improve the light transmittance (antireflection film) is not formed In particular, it becomes prominent. Further, even if the imaging lens 61 is a glass lens that can be provided with an antireflection film, if the imaging lens 61 is formed of a material having a low glass transition temperature, the lens is formed in the step of providing the antireflection film. The surface accuracy of the surface is likely to deteriorate (change), and as a result, there is a problem that the optical characteristics deteriorate.

In order that the image quality of the image is not deteriorated by the laser beam (L ₄ ) reciprocating between an arbitrary reflecting surface of the polygon mirror 50 and the lens surface of the imaging lens 61, it is incident on each reflecting surface of the polygon mirror 50. The incident laser beam needs to be incident from an arbitrary angle direction with respect to the optical axis of the imaging lens 61 in the main scanning section (the optical axis of the imaging lens 61 and the individual reflecting surfaces of the polygon mirror 50). It is necessary to provide an inclination between the incident laser beam and the incident laser beam.

However, when there is the above-described inclination between the optical axis of the imaging lens 61 and the incident laser beam incident on each reflecting surface of the polygon mirror 50, each reflecting surface of the polygon mirror 50 is shown in FIG. If the widths in the main scanning direction of the laser beams L _a , L _b , and L _c scanned in step D are D _a , D _b, and D _c , respectively, the width relationship in the main scanning direction of each reflected laser beam (long and short) Becomes Da>Db> Dc. That is, if the overall focal length of the imaging optical system 60 f, the main scanning direction of the light beam reflected by any of the reflecting surface of the polygon mirror 50 diameter (width) and D _x, defined by f / D _x It is well known that the F number Fn is different depending on the scanning angle.

Since the cross-sectional beam diameter of the laser beam imaged at a predetermined position on the photosensitive drum 23 is substantially proportional to the F number Fn, the beam diameter on the photosensitive drum 23 is set to ω _a , ω _b , ω _c , respectively. Then, ω _a <ω _b <ω _c .

  Accordingly, when there is the above-described inclination between the optical axis of the imaging lens 61 and the incident laser beam incident on each reflecting surface of the polygon mirror 50, the imaging position (scanning position) on the photosensitive drum 23 is set. As a result, the cross-sectional beam diameter of the laser beam varies, and the image quality of the image recorded on the photosensitive drum 23 deteriorates.

  The cross-sectional beam diameter ω of the laser beam focused on the photosensitive drum 23 is ψ, where λ is the convergence angle (angle formed by the center light of the laser beam and the outer edge of the laser beam), and λ is the wavelength of the laser beam. Since it can also be expressed by ω≈λ / (πtanψ), in order to reduce the variation in the cross-sectional beam diameter ω, the variation in the convergence angle ψ when the laser beam converges (images) on the photosensitive drum 23. It needs to be reduced.

  FIGS. 6A and 6B show the variation in the cross-sectional beam diameter of the laser beam imaged on the photosensitive drum by reducing the above-described variation in the convergence angle in the entire scanning region in the main scanning direction. It is the schematic explaining the principle which can suppress a grade, ie, the characteristic of the shape of an imaging lens.

FIG. 6A is a schematic diagram for explaining the characteristics of a laser beam passing through the imaging lens 61 applicable when the cross-sectional beam diameter on each reflecting surface of the polygon mirror 50 is large, that is, when the F number Fn is small. FIG. Incidentally, this state corresponds to the relationship with any of the reflection surface of the laser beam L _a and the polygon mirror 50 shown in FIG.

6 as (a), the relates the main scanning direction of the image forming lens 61, the incident laser beam diameter (width) emitting a laser beam of diameter (width) imaging lens as D ₂ is smaller than D ₁ By setting the characteristic 61, the convergence angle ψ of the laser beam that reaches the photosensitive drum 23 (image plane) is reduced. Therefore, the width (laser beam diameter) of the scanning line (latent image) scanned on the photosensitive drum 23 (image surface) in the main scanning direction is larger than that when the imaging lens 61 is not used. .

Needless to say, as shown in FIG. 6 (a), since the diameter D ₂ of the outgoing laser beam can be made smaller than the diameter D ₁ of the incident laser beam, the characteristics of that portion of the image forming lens 61, in particular The shape of the incident surface is a convex lens shape. The shape of the exit surface is defined as a shape that allows the combined focal position provided by the imaging optical system 60 to coincide with a predetermined position (image surface) on the surface of the photosensitive drum 23 or in the vicinity thereof. .

FIG. 6B is a schematic diagram for explaining the characteristics of the laser beam passing through the imaging lens 61 applicable when the cross-sectional beam diameter on each reflecting surface of the polygon mirror 50 is large, that is, when the F number Fn is small. FIG. Incidentally, this state corresponds to the relationship with any of the reflection surface of the laser beam L _c and the polygon mirror 50 shown in FIG.

FIG 6 (b) as indicated, relates to the main scanning direction of the image forming lens 61, the incident laser beam diameter (width) emitting a laser beam of diameter (width) imaging lens as D ₂ is greater than D ₁ Setting the characteristic 61 increases the convergence angle ψ of the laser beam that reaches the photosensitive drum 23 (image plane). Accordingly, the width (laser beam diameter) of the scanning line (latent image) scanned on the photosensitive drum 23 (image surface) in the main scanning direction is smaller than that when the imaging lens 61 is not used. .

Needless to say, as shown in FIG. 6 (b), to the diameter D ₂ of the outgoing laser beam can be made larger than the diameter D ₁ of the incident laser beam, the characteristics of that portion of the image forming lens 61, in particular The shape of the incident surface is a concave lens shape. The shape of the exit surface is defined as a shape that allows the combined focal position provided by the imaging optical system 60 to coincide with a predetermined position (image surface) on the surface of the photosensitive drum 23 or in the vicinity thereof. . Even if the shape of the incident surface of the imaging lens 61 is a convex lens shape, the same effect as when the shape of the incident surface is a concave lens shape can be obtained if the curvature is small.

  In this way, by changing the ratio of the curvature of the lens shape of the entrance surface and the exit surface of the imaging lens 61 according to the position where the laser beam is scanned in the main scanning direction, the ratio is changed. Further, it is possible to reduce the variation in the convergence angle ψ of the laser beam scanned on the photosensitive drum 23. As a result, the cross-sectional beam diameter of the laser beam can be made uniform over the entire area in the length direction of the scanning line scanned by the photosensitive drum 23.

As described above, in order to avoid the degradation of image quality caused by the reflected laser beam reflected by the lens surface of the imaging lens reciprocating between each reflection surface of the polygon mirror and entering the imaging lens. In addition, the cross-sectional beam in the main scanning direction of the laser beam generated when the laser beam is incident on each reflecting surface of the polygon mirror at a predetermined angle with respect to the optical axis of the imaging optical system in the main scanning plane. The change in the diameter sets each lens surface shape of the imaging lens in the main scanning direction asymmetrically with respect to the main scanning direction (the lens shape and the emission surface of the imaging lens 61 according to the position where the laser beam is scanned). By changing the curvature of each lens shape of the surface and changing the ratio thereof, the surface is generally made uniform. The shape of each lens surface described above, (2) a non-spherical term expression and y ^m, by including an odd term in m, is achieved.

  As described above, the respective shapes of the entrance surface and the exit surface of the imaging lens are changed by changing the respective curvatures according to the position where the laser beam is scanned in the entire scanning region in the main scanning direction. By changing the ratio, variation in the F number Fn can be corrected, and the cross-sectional beam diameter of the laser beam can be made uniform in all scanning regions. Further, in the conventional optical scanning device, in order to maintain the convergence angle constant with respect to the main scanning direction, many optical components having power are required. The cross-sectional beam diameter of the laser beam imaged at a predetermined position of the body drum 23 can be set substantially uniformly over the entire length in the main scanning direction.

  The above-described optical component in which a surface having an asymmetric shape in the main scanning direction is defined is not limited to a lens, and the same effect can be obtained by an optical component such as a power mirror surface. Further, if the optical component having a surface having an asymmetric shape in the main scanning direction is a lens and made of glass, the refractive index is high and various characteristics can be easily corrected. It is more effective to make the optical component made of glass in that it is less susceptible to moisture absorption and changes in ambient temperature. In this case, if the lens surface shape becomes complicated, the processing time required for the cutting process and the polishing process becomes longer, and the cost is greatly increased. Therefore, the molding process is preferable.

FIG. 7 shows variations in the convergence angle ψ with respect to the position in the main scanning direction among the optical characteristics obtained by using the optical scanning device described with reference to FIGS. 1, 2A, 2B and 3. 6 is a graph showing a result of comparing the above state with a known optical scanning device on the photosensitive drum as an image region in a range from −160 mm to 160 mm. In FIG. 7, a curve a represents characteristics obtained by the optical scanning device to which the embodiment of the present invention is applied, and a curve (dashed line) b represents a known optical scanning device using a symmetrical imaging lens. The characteristics obtained by are shown respectively. Incidentally, the imaging lens of the symmetrical shape used in known optical scanning device, corresponds to the case where (2) the coefficient of the odd terms of y ^m all type "0". As shown in FIG. 7, in the optical scanning device of the present invention, the scanning end (in this case, the “+” direction) opposite to the incident angle of the laser beam incident on each reflecting surface of the polygon mirror 50 in the main scanning direction. ), As described above, the F number Fn increases and the convergence angle decreases.

  From FIG. 7, it can be seen that by using the imaging lens 61 of the present invention in which a surface having an asymmetric shape in the main scanning direction is used, the fluctuation width of the convergence angle ψ is reduced as shown by the curve a. . In particular, it is recognized that the effect is high at the scanning end.

  FIG. 8 shows a laser beam focused on the photosensitive drum among the optical characteristics obtained by using the optical scanning device described with reference to FIGS. 1, 2A, 2B and 3. 6 is a graph showing the result of comparison of the variation in the cross-sectional beam diameter with a known optical scanning device when the range from −160 mm to 160 mm is set as an image area on the photosensitive drum. In FIG. 8, a curve a is a characteristic obtained by the optical scanning device to which the embodiment of the present invention is applied, and a curve (dashed line) b is a known optical scanning device using symmetrical imaging lenses. The characteristics obtained by are shown respectively.

  From FIG. 8, by using the imaging lens 61 of the present invention in which a surface having an asymmetric shape in the main scanning direction is defined, the degree of variation in cross-sectional beam diameter in the main scanning direction is suppressed as indicated by the curve a. You can see that In particular, the effect is large at the scanning end.

  FIG. 9 shows a laser imaged on the photosensitive drum among the optical characteristics obtained by using the optical scanning device described with reference to FIGS. 1, 2A, 2B and 3. Deviation of linearity between beam position and polygon mirror rotation angle, that is, deviation between scan position (target position) and design value when there is linearity between polygon mirror rotation angle and scan position (Difference in distance (image expansion and contraction) between the optical axis position of the imaging optical system, the theoretical main scanning direction position (length), and the actual laser beam scanning position) It is a graph which shows f (theta) characteristic which is. In FIG. 9, a curve a is a characteristic obtained by the optical scanning device to which the embodiment of the present invention is applied, and a curve (dashed line) b is a known optical scanning device using a symmetrical imaging lens. The characteristics obtained by are shown respectively.

  From FIG. 9, by using the imaging lens 61 of the present invention in which a surface having an asymmetric shape in the main scanning direction is defined, the left-right symmetry is improved with respect to the fθ characteristic in the main scanning direction as shown by the curve a. And the overall balance is also improved. In addition, the maximum value of the amount of change is improved small.

  FIGS. 10A and 10B are schematic views for explaining another example of the optical scanning device shown in FIGS. 2A and 2B. In addition, the same code | symbol is attached | subjected to the structure same as the structure already demonstrated using Fig.2 (a) and FIG.2 (b), and detailed description is abbreviate | omitted.

  An optical scanning device 71 shown in FIGS. 10A and 10B includes a semiconductor laser element 41, a finite focal lens or collimating lens 42, an aperture 43, a cylindrical lens 44, etc., a pre-deflection optical system 70, an inscribed circle. The image forming optical system 90 includes a polygon mirror 80 having a diameter of 25 mm and a regular dodecahedron, an image forming lens 91, a dustproof glass 62, and the like.

FIG. 10 (a) and the optical scanning apparatus 71 shown in FIG. 10 (b), the laser beam from the semiconductor laser element 41, in the main scanning plane with respect to the optical axis O _R of the system of the imaging optical system 90 The light is incident on each reflecting surface of the polygon mirror 80 from outside the scanning area having a predetermined angle. The angle α between the optical axis O _R of the system optical axis O ₁ and the imaging optical system 90 on the entrance side laser beam directed to the polygon mirror 80 should pass, for example at 46.42 °, both the angle between the incident laser beam L ₀ when viewed from the sub-scanning section (including the optical axis O _R) scanning plane is 0 °.

The imaging lens 91 is made of acrylic having a refractive index n of n = 1.483987, and the thickness in the defocus direction on the optical axis is 15 mm. In addition, the shape of the lens surface is shown in the following formula (2) as follows:

It is defined by applying the data.

  FIGS. 11 to 13 are graphs for explaining various optical characteristics obtained by using the optical scanning device described with reference to FIGS. 10 (a) and 10 (b).

  FIG. 11 is a graph showing the result of comparing the state of variation of the convergence angle ψ with respect to the position in the main scanning direction with a known optical scanning device on the photosensitive drum, with the range from −160 mm to 160 mm as an image area. . In FIG. 11, a curve a represents characteristics obtained by the optical scanning device to which the embodiment of the present invention is applied, and a curve (dashed line) b represents a known optical scanning device using a symmetrical imaging lens. The characteristics obtained by are shown respectively.

  From FIG. 11, it can be seen that by using the imaging lens 91 of the present invention in which a surface having an asymmetric shape in the main scanning direction is used, the fluctuation width of the convergence angle ψ is reduced as shown by the curve a. . In particular, it is recognized that the effect is large at the scanning end.

  FIG. 12 compares the variation of the cross-sectional beam diameter of the laser beam imaged on the photosensitive drum with a known optical scanning device on the photosensitive drum when the range from −160 mm to 160 mm is an image area. It is a graph which shows a result. In FIG. 12, a curve a is a characteristic obtained by the optical scanning device to which the embodiment of the present invention is applied, and a curve (dashed line) b is a known optical scanning device using symmetrical imaging lenses. The characteristics obtained by are shown respectively.

  From FIG. 12, by using the imaging lens 91 of the present invention in which a surface having an asymmetric shape in the main scanning direction is used, the degree of variation in cross-sectional beam diameter in the main scanning direction is suppressed as indicated by the curve a. You can see that

  FIG. 13 shows a linearity deviation between the position of the laser beam imaged on the photosensitive drum and the rotation angle of the polygon mirror, that is, when there is linearity between the rotation angle of the polygon mirror and the scanning position. Deviation between the scanning position (target position) and the design value (between the optical axis position of the imaging optical system, the theoretical main scanning direction position (length), and the actual laser beam scanned position 5 is a graph showing an fθ characteristic which is a difference in distance (deviation of image expansion and contraction). In FIG. 13, a curve a represents characteristics obtained by the optical scanning device to which the embodiment of the present invention is applied, and a curve (dashed line) b represents a well-known optical scanning device using symmetrical imaging lenses. The characteristics obtained by are shown respectively.

  From FIG. 13, by using the imaging lens 91 of the present invention in which a surface having an asymmetric shape in the main scanning direction is defined, the left-right symmetry is improved with respect to the fθ characteristic in the main scanning direction as shown by the curve a. It is recognized that the overall balance is also improved.

  In the optical scanning devices shown in FIGS. 10A and 10B, the coefficient when m in Equation (2) is an odd number is 0, and the imaging lens 91 is 0.32 mm in the main scanning direction. As a result, the variation in the cross-sectional beam diameter of the laser beam formed on the photosensitive drum 23 is reduced, and various optical characteristics including the fθ characteristic are also improved. That is, it is possible to reduce the variation in the cross-sectional beam diameter of the laser beam formed on the photosensitive drum 23 only by shifting the optical component having power in the main scanning direction. Although not shown, the same effect can be obtained by decentering only one surface of the imaging lens 91.

  Also, in the optical scanning device shown in FIGS. 10A and 10B, the coefficient when m in Equation (2) is an odd number is 0, and the imaging lens 91 is moved around the axis in the sub-scanning direction. When tilted by 0.23 °, it was confirmed that variation in the beam diameter of the laser beam focused on the photosensitive drum 23 was reduced and various optical characteristics including the fθ characteristic were improved. In this way, by rotating the optical component having power around the axis in the sub-scanning direction, the variation in the cross-sectional beam diameter of the laser beam imaged on the photosensitive drum 23 can be reduced. The same effect can be obtained by rotating only one surface of the imaging lens 91.

  Furthermore, by subjecting the imaging lens 91 to the above-described eccentricity and rotation at the same time, it is possible to further reduce the variation in the cross-sectional beam diameter of the laser beam imaged on the photosensitive drum 23. Further, not only the rotation around the axis in the sub-scanning direction but also the rotation around the axis in the main scanning direction or the rotation around the optical axis can be further effective.

In each of the optical scanning devices shown in FIG. 2A and FIG. 2B or FIG. 10A and FIG. 10B, a multi-beam in which a plurality of laser beams are incident on the imaging lens 61 is provided. In the case of a beam system, an odd term is included in the order of z of the aspheric term in equation (2).

Thus, it is desirable to define the lens surface of the imaging lens 91.

  As described above, in the over-illumination type optical scanning device, when the laser beam reflected and scanned by the respective reflecting surfaces of the polygon mirror is imaged on the photosensitive drum, the main scanning plane or the sub-scanning cross-sectional direction By using an imaging lens whose angle is different from that of the optical axis, the laser beam reflected by the first surface (or second surface) of the imaging lens is prevented from returning to the individual reflecting surfaces of the polygon mirror. it can. As a result, the reflected laser beam returned from the imaging lens is not scanned once again toward the photosensitive drum by the individual reflecting surfaces of the polygon mirror, and the possibility that the image quality of the image is deteriorated is eliminated.

  However, if a laser beam is incident on the imaging lens at an angle in the main scanning direction, the difference in F number between the scanning start position and the scanning end position becomes large, and the laser beam imaged on the photosensitive drum. The variation in the cross-sectional beam diameter becomes large. Therefore, by making the shape of the entrance surface or exit surface in the main scanning direction of the imaging lens or the both surfaces thereof asymmetrical along the main scanning direction, the incident laser beam incident on each reflecting surface of the polygon mirror Even when the angle between each reflecting surface of the polygon mirror and the optical axis of the system between the photosensitive drums is large and the incident laser beam is incident on each reflecting surface of the polygon mirror from the outside of the image area, Variations in the cross-sectional beam diameter of the laser beam imaged on the body drum can be suppressed, the cross-sectional beam diameter can be made uniform, and the image quality of the image is improved.

  The invention of the present application can also be realized with the following configuration.

[A] A pre-deflection optical system that shapes a light beam from a light source to form a long line image in a first direction, an optical scanning unit that scans the light beam to a predetermined position of a scanning object, and the optical scanning An imaging optical system that forms an image of the light beam scanned by the means on a scanning object, and the light beam incident on the optical scanning means is in the first direction of the single reflecting surface of the optical scanning means. In an optical scanning device wider than the width,
The imaging optical system includes an optical component having a surface having at least one power, and the first direction of the shape of the surface having the power of at least one of the optical components is a y-axis, When the second direction orthogonal to the direction 1 is the z-axis and the direction in which the light beam travels is the x-axis, the aspheric term of the shape of the surface having the power of at least one surface is
A _m Σy ^m (m = 0, 1, 2, 3,...) Or
If A _mn Σy ^m z ⁿ (m = 0, 1, 2, 3,..., N = 0, 1, 2, 3,...),
including terms where A _m ≠ 0 or A _mn ≠ 0 when m = 2 × k + 1 (k = 0, 1, 2, 3...)
The optical scanning device, wherein the optical component including at least one surface having power is a plastic lens.

  This description corresponds to claims 12 and 14 of the original application, and it goes without saying that new technical matters are not introduced into the original specification of the original application.

[B] A pre-deflection optical system that shapes a light beam from a light source to form a long line image in a first direction, an optical scanning unit that scans the light beam to a predetermined position of a scanning object, and the optical scanning An imaging optical system that forms an image of the light beam scanned by the means on a scanning object, and the light beam incident on the optical scanning means is in the first direction of the single reflecting surface of the optical scanning means. In an optical scanning device wider than the width,
The imaging optical system includes an optical component having a surface having at least one power, and the first direction of the shape of the surface having the power of at least one of the optical components is a y-axis, When the second direction orthogonal to the direction 1 is the z-axis and the direction in which the light beam travels is the x-axis, the aspheric term of the shape of the surface having the power of at least one surface is
A _m Σy ^m (m = 0, 1, 2, 3,...) Or
If A _mn Σy ^m z ⁿ (m = 0, 1, 2, 3,..., N = 0, 1, 2, 3,...),
including terms where A _m ≠ 0 or A _mn ≠ 0 when m = 2 × k + 1 (k = 0, 1, 2, 3...)
The optical scanning device characterized in that the optical component including the surface having at least one surface has a mirror.

  This description corresponds to claims 12 and 15 of the original application, and it goes without saying that new technical matters are not introduced into the original specification of the original application.

[C] A pre-deflection optical system that shapes a light beam from a light source to form a long line image in a first direction, an optical scanning unit that scans the light beam to a predetermined position of a scanning object, and the optical scanning An imaging optical system that forms an image of the light beam scanned by the means on a scanning object, and the light beam incident on the optical scanning means is in the first direction of the single reflecting surface of the optical scanning means. In an optical scanning device wider than the width,
The imaging optical system includes an optical component having a surface having at least one power, and the first direction of the shape of the surface having the power of at least one of the optical components is a y-axis, When the second direction orthogonal to the direction 1 is the z-axis and the direction in which the light beam travels is the x-axis, the aspheric term of the shape of the surface having the power of at least one surface is
A _m Σy ^m (m = 0, 1, 2, 3,...) Or
If A _mn Σy ^m z ⁿ (m = 0, 1, 2, 3,..., N = 0, 1, 2, 3,...),
including terms where A _m ≠ 0 or A _mn ≠ 0 when m = 2 × k + 1 (k = 0, 1, 2, 3...)
The optical scanning device characterized in that the optical component including at least one surface having power is a glass lens.

  This description corresponds to claims 12 and 16 of the original application, and it goes without saying that new technical matters are not introduced into the original specification of the original application.

[D] A pre-deflection optical system that shapes a light beam from a light source to form a long line image in a first direction, an optical scanning unit that scans the light beam to a predetermined position of a scanning object, and the optical scanning An imaging optical system that forms an image of the light beam scanned by the means on a scanning object, and the light beam incident on the optical scanning means is in the first direction of the single reflecting surface of the optical scanning means. In an optical scanning device wider than the width,
The imaging optical system includes a lens having power, and the lens having power has at least one surface having no surface treatment for suppressing reflection when the light beam is incident on the imaging optical system, to the optical scanning unit. An optical component having at least one surface that is asymmetric in the first direction with respect to the center of the optical axis. An optical scanning device comprising:

  This description corresponds to claims 17 and 19 of the original application, and it goes without saying that no new technical matters are introduced into the original specification of the original application.

[E] A pre-deflection optical system that shapes a light beam from a light source to form a long line image in a first direction, an optical scanning unit that scans the light beam to a predetermined position of a scanning object, and the optical scanning An imaging optical system that forms an image of the light beam scanned by the means on a scanning object, and the light beam incident on the optical scanning means is in the first direction of the single reflecting surface of the optical scanning means. In an optical scanning device wider than the width,
The imaging optical system includes a lens having power, and the lens having power has at least one surface having no surface treatment for suppressing reflection when the light beam is incident on the imaging optical system, to the optical scanning unit. Incident light and the optical axis of the imaging optical system have an angle in the main scanning plane or the sub-scanning plane, and the imaging optical system has its own direction in the first direction with respect to the optical axis center. With respect to the rotation center of the surface rotated about the axis in the second direction orthogonal to the first direction at a position where the center is eccentric in the longitudinal direction or intersects the optical axis, An optical scanning device comprising an optical component having at least one surface rotated by a predetermined amount.

  This description corresponds to claims 17 and 20 of the original application, and it goes without saying that no new technical matters are introduced into the original specification of the original application.

  DESCRIPTION OF SYMBOLS 1 ... Digital copying apparatus, 10 ... Scanner part, 20 ... Printer part, 21 ... Optical scanning device, 22 ... Image formation part, 23 ... Photosensitive drum (Image surface, to-be-photographed object) Scanning surface, scanning object), 40 ... pre-deflection optical system, 41 ... semiconductor laser element (light source), 42 ... finite lens or collimating lens, 43 ... aperture, 44 ... cylinder lens 45 ... mirror, 50 ... polygon mirror (reflection surface), 50A ... polygon mirror motor, 60 ... post-deflection optical system, 61 ... polygon mirror, 62 ... dust-proof glass.

Claims (10)

A pre-deflection optical system that shapes a light beam from a light source to form a line image that is long in the first direction, an optical scanning unit that scans the light beam to a predetermined position of a scanning object, and a scanning by the optical scanning unit An imaging optical system that forms an image on the scanning object, and the light beam incident on the optical scanning unit is wider than the width of the single reflecting surface of the optical scanning unit in the first direction. In an optical scanning device,
The imaging optical system has the first direction at a position where the center of the image forming optical system is decentered in the first direction with respect to the optical axis or the center of the longitudinal direction is decentered. An optical scanning device having at least one surface rotated by a predetermined amount with respect to a rotation center of a surface rotated around an axis in a second direction orthogonal to each other.   The optical scanning apparatus according to claim 1, wherein the imaging optical system includes an optical component having a plurality of powers.   A second direction perpendicular to the first direction at a position where the center of the longitudinal direction is decentered in the first direction with respect to the optical axis, or at a position intersecting the optical axis. 3. The optical scanning device according to claim 1, wherein the surface rotated by a predetermined amount with respect to the rotation center is a surface that transmits the light flux.   A second direction perpendicular to the first direction at a position where the center of the longitudinal direction is decentered in the first direction with respect to the optical axis, or at a position intersecting the optical axis. 3. The optical scanning device according to claim 1, wherein the surface rotated by a predetermined amount with respect to the rotation center is a surface that reflects the light flux. A pre-deflection optical system that shapes a light beam from a light source to form a line image that is long in the first direction, an optical scanning unit that scans the light beam to a predetermined position of a scanning object, and a scanning by the optical scanning unit An imaging optical system that forms an image on the scanning object, and the light beam incident on the optical scanning unit is wider than the width of the single reflecting surface of the optical scanning unit in the first direction. In an optical scanning device,
The imaging optical system includes an optical component having a surface having at least one power, and the first direction of the shape of the surface having the power of at least one of the optical components is a y-axis, When the second direction orthogonal to the direction 1 is the z-axis and the direction in which the light beam travels is the x-axis,
The aspheric term of the shape of the surface having the power of at least one surface is:
A _m Σy ^m (m = 0, 1, 2, 3,...) Or
If A _mn Σy ^m z ⁿ (m = 0, 1, 2, 3,..., N = 0, 1, 2, 3,...),
An optical scanning device including a term in which A _m ≠ 0 or A _mn ≠ 0 when m = 2 × k + 1 (k = 0, 1, 2, 3...). A pre-deflection optical system that shapes a light beam from a light source to form a line image that is long in the first direction, an optical scanning unit that scans the light beam to a predetermined position of a scanning object, and a scanning by the optical scanning unit An imaging optical system that forms an image on the scanning object, and the light beam incident on the optical scanning unit is wider than the width of the single reflecting surface of the optical scanning unit in the first direction. In an optical scanning device,
The imaging optical system includes a lens having power, and the lens having power has at least one surface having no surface treatment for suppressing reflection when the light beam is incident on the imaging optical system, to the optical scanning unit. And an optical axis of the imaging optical system has an angle in a main scanning plane or a sub-scanning plane. A pre-deflection optical system that shapes a light beam from a light source to form a line image that is long in the first direction, an optical scanning unit that scans the light beam to a predetermined position of a scanning object, and a scanning by the optical scanning unit An imaging optical system that forms an image on the scanning object, and the light beam incident on the optical scanning unit is wider than the width of the single reflecting surface of the optical scanning unit in the first direction. In an optical scanning device,
The imaging optical system includes a plurality of optical components, and the number of optical components having power among the plurality of optical components is 2 or less.   8. The optical scanning according to claim 5, wherein the pre-deflection optical system includes lenses having different powers in the first direction and a second direction orthogonal to the first direction. apparatus. A photoreceptor that receives a light beam to generate an image;
A pre-deflection optical system that shapes a light beam from a light source to form a long line image in a first direction, an optical scanning unit that scans the light beam to a predetermined position of the photosensitive member, and a scanning by the optical scanning unit An imaging optical system that forms an image on the photosensitive member, and the light beam incident on the optical scanning unit is wider than the width of the single reflecting surface of the optical scanning unit in the first direction. In the optical scanning device, the imaging optical system is located at a position where the center of the longitudinal direction of the imaging optical system is decentered in the first direction with respect to the optical axis or intersects the optical axis. An optical scanning device having at least one surface rotated by a predetermined amount with respect to a rotation center of a surface rotated around an axis in a second direction orthogonal to the first direction; ,
A developing device for developing an image formed on the photosensitive member by the optical scanning device;
An image forming apparatus comprising:   The image forming apparatus according to claim 9, wherein the optical component having an asymmetric surface in the first direction with the passing position of the optical axis as a boundary point is a plastic lens.

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