JP2016151590A - Optical scanner and image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical scanner and an image forming apparatus that can be easily used for a plurality of screen patterns and can provide an output image with suitable image quality.SOLUTION: An optical scanner comprises: light source means that emits first and second light beams according to first and second screen patterns different from each other; a deflector that deflects the first and second light beams to optically scan first and second scanning target surfaces; and first and second imaging optical systems that collect the first and second light beams deflected by the deflector to form first and second spots respectively on the first and second scanning target surfaces. The shapes of the first and second spots are different from each other according to the first and second screen patterns.SELECTED DRAWING: Figure 2

Description

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

  2. Description of the Related Art Conventionally, an image forming apparatus such as a laser printer incorporates a laser scanner equipped with an optical system for forming an image of a laser light source on a photosensitive drum surface. When the laser beam from the laser scanner is irradiated onto the charged photosensitive drum, the potential on the surface of the photosensitive drum is lowered. By doing so, the distribution of the exposed portion and the non-exposed portion is formed on the drum surface, and an electrostatic potential distribution generally called a latent image is formed.

  However, with regard to the latent image on the photosensitive drum surface, when the spot scans on the photosensitive drum surface, the exposed portion spreads with respect to the original size, so that the potential of the non-exposed portion also spreads over the exposed portion. There has been a problem of lowering. In this case, the image quality of the printed product that is the final output product is deteriorated.

  In order to solve such a problem, the diameter of the laser spot written on the surface of the photosensitive drum is reduced, or the gap between the spots written on the photosensitive drum (space between adjacent spots) is filled. Technology is known.

  Patent Document 1 discloses that a blue semiconductor laser is used in place of a conventional red or infrared laser light source as a light source for writing in order to reduce the spot diameter. Further, in Patent Document 1, in order to improve gradation, a spot diameter is adjusted by adjusting a focus adjustment lens between a collimator lens and a cylindrical lens in an incident optical system in an optical scanning device in the optical axis direction. Is disclosed.

  Further, in Patent Document 2, a diamond-shaped opening is mounted on an incident optical system in an optical scanning device so that the spot shape on the surface of the photosensitive drum has a shape in which the intensity distribution spreads in a diagonal direction, and the spot by deflection scanning is used. It is disclosed to eliminate the gap between each other.

  Patent Document 3 discloses that various types of screen angles and the number of lines are used for intermediate gradation expression in recent color image forming apparatuses. Further, Patent Document 4 discloses that in the case of halftone image formation using screen processing, intermediate gradation is expressed by a periodic halftone dot pattern corresponding to the number of lines and the angle of the screen.

JP 2003-285466 A Japanese Patent Laid-Open No. 2002-287067 JP 2008-151847 A JP 2009-271377 A

  However, in Patent Document 1, there is a problem that the manufacturing cost of the optical scanning device itself is expensive because a motor for a blue semiconductor laser and a focus adjustment lens is used to realize a small spot. In addition, aberration may occur in the spot shape on the surface of the photosensitive drum due to the movement of the focus adjustment lens, and there is a problem that a desired spot cannot be reproduced and print image quality may be deteriorated.

  Further, in Patent Document 2, since the surface of the photosensitive drum is exposed with a spot formed by a rhombus opening shape, various screen patterns cannot be handled, and there are cases where the granularity of the image deteriorates. There is a problem.

  An object of the present invention is to provide an optical scanning device and an image forming apparatus that can easily cope with a plurality of screen patterns and can improve the quality of an output image.

  In order to achieve the above object, an optical scanning device according to the present invention includes light source means for emitting first and second light beams according to different first and second screen patterns, and the first and second light sources. Deflecting the light beam and optically scanning the first and second scanned surfaces, and condensing the first and second light beams deflected by the deflector to collect the first and second light beams. And first and second imaging optical systems that form first and second spots on the surface to be scanned, respectively, and the shapes of the first and second spots are the first and second spots, respectively. The shapes are different from each other according to the screen pattern.

  Another optical scanning device according to the present invention includes a light source unit that emits a light beam according to a screen pattern, a deflector that deflects the light beam and optically scans a surface to be scanned, and is deflected by the deflector. And an imaging optical system for condensing the luminous flux to form a spot on the scanned surface, wherein the spot has a shape corresponding to the screen pattern.

  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 that can easily cope with various screen patterns and can improve the image quality of a printed matter that is a final output product.

1 is a schematic diagram illustrating an image forming apparatus equipped with an optical scanning device according to an embodiment of the present invention. It is the schematic which shows the optical scanning device which concerns on the black station which concerns on embodiment of this invention. It is explanatory drawing of the screen pattern which concerns on embodiment of this invention. It is a figure regarding the definition of a screen plane (HV plane). It is a schematic diagram explaining the exposure expansion part by scanning of a laser beam. It is a figure explaining the optimal spot shape for a screen pattern. It is a figure explaining the difference in the entrance pupil shape when the imaging magnification of an imaging optical system differs in the main scanning direction / sub-scanning direction. It is a figure explaining the difference in the entrance pupil shape for every spot shape, and ellipticity. It is a figure explaining the rotation angle and imaging magnification ratio of the spot for every ratio of the major axis and the minor axis of the spot. It is explanatory drawing regarding the screen pattern, spot shape, and opening shape corresponding to 1st Embodiment. It is a figure explaining the difference in the spot diameter by the focus shift | offset | difference in 40 micrometers and 60 micrometers. It is a figure explaining the value of the print image quality parameter | index with respect to the magnitude | size of the spot for every number of lines. It is explanatory drawing regarding the screen pattern, spot shape, and opening shape corresponding to 2nd Embodiment. It is explanatory drawing regarding the screen pattern, spot shape, and opening shape corresponding to 3rd Embodiment. (A) and (b) are a main scanning sectional view and a sub-scanning sectional view of an optical scanning device including first and second imaging optical systems. It is a sub-scan sectional view concerning the first and second incident optical systems.

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

<< First Embodiment >>
(Image forming device)
As an image forming apparatus equipped with the optical scanning device according to the embodiment of the present invention, four optical scanning devices are arranged side by side, and in parallel, tandem type color image formation for recording image information on a photosensitive drum surface as an image carrier The apparatus will be described. In FIG. 1, 60 is a color image forming apparatus, 21, 22, 23, and 24 are optical scanning devices, 208a, 208b, 208c, and 208d are photosensitive drums as image carriers, and 31, 32, 33, and 34 are each. A developing device 51 is a conveyor belt.

  In the figure, R (red), G (green), and B (blue) color signals are input to the color image forming apparatus 60 from an external device 52 such as a personal computer. These color signals, which are code data, are converted into image patterns (screen patterns) of C (cyan), M (magenta), Y (yellow), and Bk (black) by a printer controller 53 in the apparatus. These image data are input to the optical scanning devices 21, 22, 23, and 24, respectively.

  From these optical scanning devices, light beams 41, 42, 43, and 44 modulated according to each image data are emitted. These light beams scan the photosensitive drum surfaces (scanned surfaces) 10a, 10b, 10c, and 10d of the photosensitive drums 208a, 208b, 208c, and 208d in the main scanning direction.

  In the color image forming apparatus according to this embodiment, four (21 to 24) optical scanning devices are arranged, and each of them corresponds to each color of C (cyan), M (magenta), Y (yellow), and Bk (black). Then, in parallel, image signals (image information) are recorded on the photosensitive drum surfaces 10a, 10b, 10c, and 10d, and a color image is printed at high speed.

  In the color image forming apparatus of this embodiment, as described above, the four optical scanning devices 21 to 24 form the latent images of the respective colors on the corresponding photosensitive drum surfaces using the light beams based on the respective image data. Yes. The process of forming a latent image on the surface of the photosensitive drum is as follows.

  That is, the photosensitive drum is charged by applying a uniform voltage before the light beam is irradiated. The light beam irradiates the uniformly charged surface of the photosensitive drum, whereby the voltage of only the irradiated portion decreases (a potential distribution is formed with the irradiation distribution). The voltage distribution resulting from the irradiation distribution formation on the surface of the photosensitive drum is referred to as an electrostatic latent image (latent image). Thereafter, the toner image developed by the developing device is transferred to a recording material as a transfer material by a transfer device (not shown). A single full-color image is formed by multiple transfer onto a recording material. The toner image transferred to the recording material is fixed by a fixing device (not shown).

  In this embodiment, as will be described in detail later, in order to realize colors and densities other than cyan, magenta, yellow, and black, screen patterns (line screens) having different angles and line numbers are superimposed on the paper surface. Expresses colors other than the above. That is, one full color image is formed by superimposing such screen patterns. Here, the pattern is changed for each color in order to prevent moire caused by the superposition of the screen patterns.

For example, a color image reading device including a CCD sensor may be used as the external device 52 described above. In this case, the color image reading apparatus and the color image forming apparatus 60 constitute a color digital copying machine.
(Optical scanning device)
FIG. 15A is a main part sectional view in the main scanning direction (main scanning sectional view) of the optical scanning apparatus according to the first embodiment of the present invention, and FIG. 15B is a main part sectional view in the sub scanning direction. (Sub-scanning sectional view). FIG. 16 is a cross-sectional view (sub-scanning cross-sectional view) of the main part of the incident optical system according to the first embodiment of the present invention. In FIG. 15A, FIG. 15B, and FIG. 16, the optical paths from the light sources 1a, 1b, 1c, and 1d toward the scanned surfaces 10a, 10b, 10c, and 10d are respectively the optical path a, the optical path b, and the optical path. c, and optical path d. The configuration and optical action of these optical paths are all the same. In FIG. 15A, FIG. 15B, and FIG. 16, the optical elements of the optical paths having the same function are represented by the same reference numerals. ing.

(Light source means and incident optical system)
Here, the optical action of each optical element shown in FIGS. 15A, 15B, and 16 will be described. In FIGS. 15A, 15B and 16, 1a and 1b are a first pair of light source means, and 1c and 1d are a second pair of light source means, each of which has a light emitting portion (light emitting point). It consists of a semiconductor laser. Reference numerals 2a and 2b denote a first pair of apertures (aperture stops), and 2c and 2d denote a second pair of apertures (aperture stops). Reference numerals 3a and 3b denote a first pair of collimator lenses, and reference numerals 3c and 3d denote a second pair of collimator lenses, which respectively form incident light beams in a desired optimum beam shape.

  The light beam that has entered the collimator lenses 3a, 3b, 3c, and 3d is converted into a parallel light beam and is incident on the cylindrical lens 4 having a positive refractive power only in the sub-scanning direction. These include a first pair of incident optical systems (first and second incident optical systems) L1a and L1b including a first pair of collimator lenses, and a second pair of incident light including a second pair of collimator lenses. The optical systems L1c and L1d are configured.

  The cylindrical lens 4 is used as both the first pair of incident optical systems and the second pair of incident optical systems, and has a refractive power only in the sub-scanning direction. The four light beams emitted from the cylindrical lens 4 are incident on the deflecting surface 6 of the rotary polygon mirror 5 as parallel light beams in the main scanning section and as convergent light beams that are collected near the deflecting surface 6 in the sub-scanning section. Then, an image is formed on the deflection surface 6 of the rotary polygon mirror 5 as a long line image in the main scanning direction.

  In this embodiment, the cylindrical lens 4 is composed of a resin-made four-lens cylindrical lens in which four incident surfaces corresponding to the respective optical paths a, b, c, and d and four emission surfaces are integrally molded. However, it may be composed of four individual cylindrical lenses.

  The rotary polygon mirror 5 including a plurality of deflection surfaces rotates at a constant speed in the direction of arrow A in the figure. The number of rotary polygon mirrors 5 in the present embodiment is equal to or less than the number of imaging optical systems described later, the number is one, and the number of imaging optical systems is four.

  M2a, M2b, M2c, and M2d are four imaging optical systems each having a condensing function and an fθ characteristic, and have a positive power (refractive power) in the main scanning section and a main scanning section in the sub-scanning section. It has a positive power different from the positive power in the scanning section.

  In the present embodiment, the imaging optical system M2a (M2d) includes first and second imaging lenses 71 and 72 and a folding mirror 81. The imaging optical system M2b (M2c) includes first and second imaging lenses 71 and 72, and folding mirrors 82, 83, and 84.

  The imaging optical systems M2a, M2b (M2c, M2d) are light-sensitive drums as scanning surfaces shown in FIG. 1 (b) in the main scanning section with a light beam based on image information deflected and scanned by the rotary polygon mirror 5. Images are formed in spots on the surfaces 10a and 10b (10c and 10d). Further, the imaging optical systems M2a and M2b (M2c and M2d) optically conjugate between the deflection surface 6 of the rotary polygon mirror 5 and the photosensitive drum surfaces 10a and 10b (10c and 10d) in the sub-scan section. By doing so, surface tilt compensation of the deflection surface 6 is performed.

  Reference numerals 10a, 10b, 10c, and 10d denote photosensitive drums (photosensitive drum surfaces) for forming a plurality of color light images. In this embodiment, the plurality of color lights are yellow (Y), magenta (M), and cyan. (C) 4 colors of black (Bk).

(Black Station)
Next, an optical scanning device for one station in the image forming apparatus will be described with reference to FIG. FIG. 2 is a schematic diagram of a black station of an optical scanning device included in the image forming apparatus. Illustrations of other stations are omitted.

  Reference numeral 1 denotes a light source (semiconductor laser), 201 denotes a collimator lens, 2 denotes a diaphragm aperture, and 203 denotes a cylindrical lens. Reference numeral 205 denotes a rotary polygon mirror (polygon mirror), and 206 and 207 denote imaging scanning lenses. Reference numeral 208 denotes a photosensitive drum as an image carrier.

  Next, the role of each component in the optical scanning device will be described. The light source 1 on which the semiconductor laser is mounted has an oscillation wavelength of, for example, 680 nm, and the cross-sectional shape of light emitted from the element is circular and elliptical in a direction perpendicular to the optical axis. One or a plurality of light sources 1 may be provided for one optical scanning device. The collimator lens 201 couples light emitted from the light source 1 into substantially parallel light. The collimator lens 201 is generally an element that converts an emitted light beam into parallel light, but it may be divergent light or convergent light as substantially parallel light.

  The diaphragm aperture 2 limits the light beam emitted from the collimator lens 201 (which is a feature of the present embodiment and will be described in detail later). The cylindrical lens 203 functions as a line image imaging optical system that forms an image only in the sub-scanning direction. In general, an optical element group from the light source 1 to the cylindrical lens 203 is referred to as an incident optical system.

  The polygon mirror 204 deflects and scans the light emitted from the incident optical system onto the image carrier (photosensitive drum in this case) 208. In order to scan the laser beam in the longitudinal direction (main scanning direction) of the photosensitive drum, the polygon mirror 204 is rotated around the rotation shaft 205, and the photosensitive drum 208 is rotated according to the rotation angle of the polygon mirror 204. The irradiation position with respect to the main scanning direction changes.

  Further, this optical scanning device is equipped with imaging scanning lenses 206 and 207 for imaging a laser beam on the photosensitive drum on the exit side of the polygon mirror, and this imaging scanning lens group is used as a scanning optical system. It is called (imaging optical system). The light that has passed through the scanning lens is condensed as a spot on the surface of the photosensitive drum.

Table 1 shows the design value data of the optical members in the optical scanning device from the light source to the photosensitive drum surface. Table 1 shows numerical values of the optical scanning device according to the first, second, and third embodiments of the present invention. Here, “E−x” indicates “10 ^−x ”. The R1 surface is the surface on the optical deflector 204 side of the imaging scanning lens 206, the R2 surface is the surface on the image carrier 208 side of the imaging scanning lens 206, and the R3 surface is the optical deflector 204 side of the imaging scanning lens 207. The R4 surface is a surface of the imaging scanning lens 207 on the image carrier 208 side.

  Further, in the present embodiment, the shape in the main scanning direction is configured symmetrically with respect to the optical axis. That is, the aspherical coefficients on the scanning start side and the scanning end side are matched. The curvature in the sub-scanning surface of one surface of the second imaging lens 207 is continuously changed in the effective portion of the imaging scanning lens on the scanning start side and the scanning end side with respect to the optical axis. Yes. Thus, the main scanning is symmetric with respect to the optical axis, and the power change of the sub scanning is asymmetric with respect to the direction of the main scanning.

  Here, the collimator lens 201 and the imaging scanning lenses 206 and 207 will be described in more detail below.

(Collimator lens)
The exit surface of the collimator lens 201 of the present embodiment is configured to have a rotation target aspheric shape represented by the following mathematical formula.

(Imaging scanning lens)
The shapes of the first and second imaging scanning lenses 206 and 207 in the present embodiment are represented by the following mathematical expressions. The intersection of the imaging scanning lens and the optical axis is the origin, and the direction perpendicular to the optical axis in the main scanning section is the Y axis on the scanning start side and the scanning end side with respect to the optical axis, as shown in FIG. The direction orthogonal to the optical axis in the sub-scanning plane is the Z axis, and can be expressed by the following function.

Where R is the radius of curvature in the main scanning direction, K, B ₄ , B ₆ , B ₈ and B ₁₀ are aspherical coefficients, and the suffixes s and e of the aspherical coefficient B are on the scanning start side and scanning end side, respectively. Represent. The shape in the sub scanning direction is the X axis on the scanning start side and the scanning end side with respect to the optical axis, the Y axis in the direction perpendicular to the optical axis in the main scanning plane, and the optical axis in the sub scanning plane. The direction is the Z axis, and can be expressed by the following continuous function.

Here, r ′ is the radius of curvature in the sub-scanning direction, D ₄ , D ₆ , D ₈ and D ₁₀ are coefficients, and the suffixes s and e represent the scanning start side and the scanning end side, respectively.

(Screen pattern))
Next, a screen pattern in an image forming apparatus (color machine) equipped with a plurality of photosensitive drums will be described. In laser printers, in order to realize colors and densities other than cyan, magenta, yellow, and black, screen patterns (line patterns) with different angles (rotation angles) and line numbers on the paper surface are overlaid and other than the above Expresses color.

  Here, in this embodiment, a line pattern (line screen pattern) is used. The line screen pattern refers to a screen in which periodically exposed exposure portions are linearly arranged at an arbitrary angle as indicated by reference numeral 301 in FIG.

  Next, definitions of the screen plane and the angle and number of lines of the screen pattern will be described. On the screen plane, as shown in FIG. 4, the direction in which the laser beam scans in the main scanning direction is the main scanning direction (H), and the direction in which the photosensitive drum rotates about the rotation axis 401a-401b is the sub-scanning direction. A plane (screen plane: HV plane) is defined as (V).

  Further, the angle θ of the screen pattern is equal to the arrangement direction of the periods on the HV plane, and the rotation angle with respect to the screen pattern indicated by reference numeral 301 in FIG. For example, the line screen 301 is orthogonal to the axis of 302 inclined by 30 degrees and represents a screen pattern of 30 degrees.

  The number of lines of the screen pattern represents how many lines are included in one inch in the direction in which the screen patterns are periodically arranged, and is preferably 150 to 300 LPI (Line Per Inch).

  Note that screen patterns having different angles and numbers of lines are used to suppress moire patterns caused by overlapping periodic patterns when the screen pattern images at each station are superimposed.

  Next, the screen before and after exposure on the photosensitive drum surface will be described. In the screen of <5A> in FIG. 5, black portions 501a and white portions 501b represent respective exposed portions and non-exposed portions on the surface of the photosensitive drum. For example, in the case of FIG. 4, the laser beam exposes only the portion 501a in FIG. 5 as the exposure unit while scanning in the direction of reference numerals 401a to 401b.

  When the laser beam reaches the scanning end, the photosensitive drum is displaced in the V direction by an amount corresponding to one pixel (one interval of the dotted line in the V direction), and the laser beam is scanned again from 501a to 501b. The photosensitive drum is exposed. Here, as described above, the potential of only the exposed portion is ideally lowered, and the contrast between the exposed portion and the non-exposed portion is formed on the surface of the photosensitive drum to become a latent image.

  However, since the laser beam having a finite size scans the drum surface, it is known that the exposed portion is wider than the original exposure portion 501a as shown in <5B> in FIG. Enlarged portion 502b). Since the exposure enlargement unit 502b is not an area where the potential is ideally lowered with respect to the original exposure unit 501a, the latent image contrast in the exposure enlargement unit 502b is lower than ideal.

(Spot shape)
Therefore, a description will be given of a spot shape that suppresses the decrease in the latent image contrast and is insensitive to tolerance and low in design difficulty when forming an image with a screen pattern inclined in an arbitrary direction. 601 shown in << 6A >> in FIG. 6 is a screen pattern inclined by 135 ° on the HV plane, and << 6C >> in FIG. 6 is an example of a spot shape suitable for the screen pattern 601. The spot shape is a substantially elliptical shape (or a substantially rectangular shape), and the rotation angle of the short axis (short side) is substantially equal to the rotation angle 135 ° of the screen pattern.

(Spot rotation angle)
The reason why the image quality is best when the spot shape shown in << 6C >> in FIG. 6 is used for the screen pattern 601 shown in << 6A >> in FIG. 6 will be described below. In FIG. 6, << 6B >> indicates a print image quality index in which the horizontal axis indicates the rotation angle of the spot and the vertical axis indicates the print image quality. This print image quality index is substantially equal to the ratio of the pixels of the exposure enlargement portion included in the pixels of the entire image, and indicates that the print image quality is low when the value is high and the print image quality is high when the value is low.

  The solid line 602 of << 6B >> in FIG. 6 indicates the print image quality when an elliptical spot having a major axis of 80 μm and a minor axis of 40 μm shown in << 6C >> of FIG. 6 is rotated at an arbitrary angle on the HV plane. Indicates an indicator. A dotted line 603 of << 6B >> is a print image quality index when a circular (perfect circle) spot having the same area as the elliptical spot shown in << 6C >> is rotated at an arbitrary angle on the HV plane. Show.

  From the graph of <6B> in FIG. 6, the rotation angle of the spot with the lowest print image quality index and the highest print image quality is 135 degrees where the short axis rotation angle of the elliptical spot matches the rotation angle of the screen pattern. I understand that there is. This is because the area of the exposure enlargement unit 605 is minimized and the print image quality index is the lowest when exposure is performed in the area where the elliptical spot is most narrowed, as in <6d> of FIG. .

  On the other hand, compared with the case where the rotation angle of the short axis of the elliptical spot and the rotation angle of the screen pattern are substantially the same, the print image quality index is the same when the rotation angle of the long axis of the elliptical spot and the rotation angle of the screen pattern are the same. Is the highest and the print quality is the lowest. The graph of <6B> in FIG. 6 corresponds to the case where the rotation angle is 45 °. This is because the area of the exposure enlargement unit 605 is maximized and the print image quality index is the highest when exposure is performed in an area where the elliptical spot is not most narrowed as shown in << 6e >> of FIG. .

  Therefore, it can be seen that in order to improve the print image quality, the spot may be narrowed down in the direction of the rotation angle of the screen pattern on the HV plane.

  Note that the spot diameter in the direction to be evaluated corresponds to a value of 13.5% of the integrated value when the light amount is integrated in a direction perpendicular to the direction to be evaluated in the light amount distribution of the spot on the HV plane. Use the diameter.

  Next, the allowable rotation angle in the rotation of the spot will be described. In our study, it was found that in this example (screen pattern rotation angle is 135 degrees), if the difference in print image quality index is within 0.035, there is almost no difference when the printed matter is viewed. Accordingly, in the spot indicated by << 6C >> in FIG. 6 where the major axis is 80 μm and the minor axis is 40 μm, the error of the rotation angle corresponding to the print image quality index difference of 0.035 can be within 25 degrees (± 25 degrees). It can be said that the allowable rotation angle of the spot.

  However, since the print image quality index value varies depending on the spot diameters of the long axis and the short axis, the allowable rotation angle of the spot where the print image quality index value falls within 0.035 varies depending on the spot. Taking these into consideration, the allowable rotation angle range including the ideal rotation angle is considered to be ± 25 degrees, and further considering the spot having a large ratio of the major axis to the minor axis, the tolerance including the ideal rotation angle is more preferable. The rotation angle range is considered to be ± 10 degrees.

(Opening shape)
Next, the aperture shape of the diaphragm aperture 2 (FIG. 2) in the incident optical system for specifically forming substantially elliptical and substantially rectangular spots on the photosensitive drum will be described. Here, the ratio of the major axis (long side) to the minor axis (short side) of the spot of the substantially elliptical shape (substantially rectangular shape) is 2 with respect to the screen pattern in which the minor axis (short side) is inclined in the direction of 135 degrees. : 1.

  In this case, the aperture shape of the aperture 2 (FIG. 2) is made substantially elliptical (substantially rectangular), and the ratio of the major axis (long side) to the minor axis (short side) of the aperture is opposite to the spot ratio. The ratio may be 1: 2. The long axis (long side) of the aperture shape corresponds to the short axis (short side) of the spot on the surface of the photosensitive drum, and the short axis (short side) of the aperture shape is the long axis of the spot on the surface of the photosensitive drum. Corresponds to (long side).

(Imaging magnification (optical magnification))
Here, if the imaging magnification (optical magnification) of the imaging optical system in the optical scanning device is approximately equal in the main scanning direction and the sub-scanning direction, the ratio of the long and short axes (sides) of the aperture shape and the photoconductor The ratio of the long and short axes (sides) of the spot on the drum surface can be made the same, which is advantageous in processing the opening. The reason for this will be described below.

  FIG. 7 shows the shape of the entrance pupil for a desired spot when the imaging magnification ratio β main / β sub (main scanning: β main, sub scanning: β sub) of the main and sub optical systems deviate from the same magnification. ing. Here, the desired spot refers to an elliptical spot that is inclined 35 ° on the HV plane in FIG. 7 and the ratio of the major axis to minor axis spot diameter is 3: 2.

  Further, the numbers in the spots in FIG. 7 indicate the spot diameters in the main scanning / sub-scanning directions. FIG. 7 also shows a schematic diagram of the imaging magnification in the HV plane, in which the primary and secondary imaging magnification ratio changes from equal magnification to non-equal magnification (changes from Type A to Type B, Type C). Then, the imaging magnification in an arbitrary angle direction α changes, and the locus changes from a circle to an ellipse.

Next, an aperture shape is considered in order to realize a desired spot, but a spot shape (herein referred to as an entrance pupil) immediately after passing through the aperture is considered as a previous step. In this case, the diameter of the entrance pupil in an arbitrary angle direction α can be expressed by the following expression.
P (α) = σ (α) / β (α) …… (e)
However, α represents an arbitrary angle, and P (α), σ (α), and β (α) represent the entrance pupil diameter, the spot diameter, and the imaging magnification of the optical system at arbitrary angles, respectively. The number in the drawing of the entrance pupil typically represents the diameter of the entrance pupil in the main scanning direction and the sub-scanning direction, and was calculated from the above formula. As the imaging magnification changes from equal magnification to non-equal magnification, the magnification in the main scanning direction becomes larger than the imaging magnification in the sub-scanning direction, so that the diameter of the entrance pupil in the main scanning direction is reduced. At this time, the shape of the entrance pupil changes to a lazy shape as compared with the spot on the original photosensitive drum surface.

  FIG. 7 shows the diameter of the entrance pupil at an arbitrary angle together with the spot diameter and the imaging magnification (optical magnification). In this way, the shape of the aperture with respect to the slender entrance pupil is similarly sluggish, which increases the difficulty of processing the aperture, and the imaging magnification ratio in the main scanning direction and the sub-scanning direction is unequal. This condition is not preferable. Therefore, when imaging an elliptical spot, if the imaging magnification in the main scanning direction and the sub-scanning direction of the imaging optical system is approximately equal, the shape is difficult to be processed because the shape is not distorted. Will be convenient.

  Next, the range of the imaging magnification will be described. The ratio of the imaging magnification in the main scanning direction and the sub-scanning direction of the entire imaging optical system is preferably in the range of 0.8 to 1.2.

That is, in the first and second imaging optical systems, when the imaging magnification in the main scanning direction is β1, γ1, and the imaging magnification in the sub scanning is β2, γ2,
0.8 ≦ (β1 / β2) ≦ 1.2
0.8 ≦ (γ1 / γ2) ≦ 1.2
It is preferable to satisfy.

  In this range, it has been found that the imaging magnification ratio is such that the entrance pupil is not distorted.

  The details will be described below. FIG. 8 shows the spots on the photosensitive drum surface and the entrance pupil shape for each ellipticity which is an index unique to the present inventor. << 8a >> indicates that the spot has a major axis of 60 μm, a minor axis of 40 μm, and << 8b. >> is a diagram with a major axis of 80 μm and a minor axis of 40 μm. Further, the ellipticity ε here is expressed by the following equation.

In this equation, the numerator represents the spot area, and the denominator represents the area of the entrance pupil. The ellipticity here indicates how much the entrance pupil is the same area as the spot area. FIG. 8 shows how the shape of the entrance pupil collapses from the spot shape as the ellipticity ε departs from 1, and shows the shape of the entrance pupil for each spot shape and ellipticity.

  From these facts, it can be seen that as the ellipticity ε moves away from 1, the entrance pupil shape collapses with respect to the spot shape. In particular, when the ratio between the major axis and the minor axis of the spot is between 1.11 and 2.0, it has been found that at least the shape of the entrance pupil is not frustrated if the ellipticity is 0.85 or more.

  Next, the derivation of the imaging magnification (optical magnification) of a suitable imaging optical system will be described with reference to FIG. FIG. 9 is a graph in which the horizontal axis represents the rotation angle of the spot and the vertical axis represents the imaging magnification ratio at the above-described ellipticity ε = 0.85 for each ratio of the major axis and the minor axis of the spot. This graph shows the following two points.

  First, when the rotation angle α of the spot is between 0 ° and 45 °, the ratio of the main scanning to the sub scanning of the entire imaging optical system is 1.2 or more, and the entrance pupil is obscured. On the other hand, between 45 degrees and 90 degrees represents that the ratio of the imaging magnification of the main scanning and the sub scanning of the entire imaging optical system is allowed to be 1.2 or more.

  Second, the higher the ratio of the major and minor axes of the spot on the photosensitive drum surface, the lower the allowable optical magnification from 0 to 45 °, and the allowable optical magnification from 45 to 90 °. It is expensive. Therefore, when the screen angle is 45 degrees or less, the ratio of the imaging magnification of the imaging optical system is preferably in the range of 0.8 to 1.1 in consideration of the negative rotation direction, and the screen angle is from 45 degrees to 90 degrees. In the case of a degree, the ratio of the imaging magnification of the imaging optical system is preferably in the range of 0.9 to 1.2.

  Here, considering that the same optical system is used at each station, it can be said that the ratio of the imaging magnification of the entire imaging optical system is preferably in the range of 0.9 to 1.1. Further, in the case of a substantially elliptical spot, if the ratio of the major axis to the minor axis is about 1.5 times, it can be said that the versatility is high considering screens of various angles.

(Best spot shape and aperture shape for screen pattern)
Based on these, the best spot shape for an arbitrary screen pattern and the corresponding opening shape will be described. In the tandem type color image forming apparatus, as described above, the printed matter is output by superimposing screen patterns at different angles at each station. Accordingly, if the substantially elliptical (substantially rectangular) opening is rotated corresponding to the screen pattern used at each station, the print image quality is improved.

  FIG. 10 shows the screen pattern, spot shape, and aperture shape of the yellow station (Yst), magenta station (Mst), cyan station (Cst), and black station (Bkst) on the HV plane. FIG. 10 shows the angle and number of lines of the screen, the length and angle of the major axis and minor axis of the spot, and the opening width and angle of the main and sub axes.

  In FIG. 10, the spot has a substantially elliptical shape (long axis is 60 μm, short axis is 40 μm), and the short axis of the spot is overlapped in a direction (pattern period direction) orthogonal to the inclination direction (longitudinal direction) of the screen pattern. . The rotation angles of the spots on the photosensitive drum surfaces of Y, M, C, and K are 0 degree, 30 degrees, 70 degrees, and 40 degrees on the HV plane, respectively, and the rotation angles of the spots are ± 10 degrees. Is acceptable.

  As described above, the ratio of the imaging magnification in the main scanning direction and the sub-scanning direction of the entire imaging optical system is preferably 0.9 to 1.1. In this embodiment, 1.05 times is adopted. Yes. The opening has a rectangular shape or a substantially rectangular shape, and the angle of the long side of the opening is 0 degree, 30 degrees, 70 degrees, and 40 degrees for Y, M, C, and K (allowable rotation angle range is ± 10 degrees). .

  As described above, as shown in FIG. 6, it is possible to provide the best image quality with the screen pattern only by rotating the opening shape according to the inclination direction of the screen pattern. In other words, since it is only necessary to narrow the spot diameter only in the direction in which the screen pattern is periodically arranged (pattern period direction), even a spot that is thick in the orthogonal direction can be allowed. Therefore, it is not necessary to narrow down to all directions unlike the conventional case.

  FIG. 11 shows a change in spot diameter when the spots of perfect circles of 40 μm and 60 μm are out of focus. It has been found that the image quality index value degrades as the spot diameter increases. For example, if a focus shift of 3 mm occurs, the spot is enlarged to 78 μm at 40 μm and 63 μm at 60 μm.

  Considering from the graph of FIG. 6, the image quality index value at these enlarged spots corresponds to a rotation angle of 45 °, and the image quality index value is 0.59. On the other hand, it corresponds to a spot rotation angle of 105 ° of 63 μm, and the image quality index value is 0.5. That is, there is a difference of 0.1 in the image quality index value, and the difference in image quality can be visually recognized with the actual product.

  Therefore, narrowing the spot only in the periodic arrangement direction (pattern period direction) of the screen is very advantageous for defocusing and can be realized at low cost. In the present embodiment, an example in which the spot shape is an ellipse having a major axis of 60 μm and a minor axis of 40 μm or an approximately ellipse is shown, but the combination of the major axis and minor axis length is not limited to this example, and the spot shape is rectangular. Alternatively, it may be substantially rectangular, and the combination of the lengths of the long side and the short side is not limited to this example.

  Further, the rotation angle of the spot on the surface of the photosensitive drum does not have to be different in all stations, and the rotation angle of the spot on the surface of the photosensitive drum is different from that of other stations in at least one station. Further, the number of stations is not limited to four, and there may be a single station and a plurality of stations (excluding four).

<< Second Embodiment >>
Hereinafter, an optical scanning device according to a second embodiment of the present invention will be described. In the first embodiment, it has been shown that the print image quality is improved by changing the spot shape on the surface of the photosensitive drum (rotating the spot) in accordance with the rotation angle of the screen pattern that is different for each station. On the other hand, in this embodiment, the angle (rotation angle) of the screen pattern is the same for each station, and the number of lines of the screen pattern differs. In the present embodiment, the print quality is improved by changing the spot shape in accordance with the number of lines of the screen pattern.

  In addition, about the structure and effect | action of the same member as 1st Embodiment, it is the same as that of 1st Embodiment, and omits description.

  In general, as a cause of deterioration of the print image quality, not only the number of pixels in the exposure enlargement part but also the amount of voltage decrease in the exposure enlargement part affects. It has been found that the print image quality deteriorates when the amount of decrease in voltage at the exposure enlargement portion is about ½ of the amount of decrease in voltage when the voltage is lowered after being exposed from the time of charging before exposure. .

  In the case of a screen pattern with a large number of lines, the same spot shape is used as in the case of a screen pattern with a small number of lines because the effect of deterioration of the print image quality due to a decrease in voltage at the enlarged exposure portion due to the short interval between adjacent dots is large. It becomes a problem.

  FIG. 12 is a plot of the print image quality index value with respect to the number of lines for each spot size (spot diameter). Even with the same spot diameter, it can be seen that the value of the print image quality index varies depending on the number of lines of the screen pattern. The graph also shows that it is not necessary to squeeze too much on a low line number screen than on a spot with a high line number screen.

  In FIG. 12, for example, the combinations of the number of lines and the spot diameter satisfying the print image quality index of 0.5 are Y (212 LPI, 20 μm), M (189 LPI, 30 μm), C (166 LPI, 40 μm), K (150 LPI, 60 μm). It becomes. In order to make the print print image quality with such a low line number screen pattern (for example, K) and the print print image quality with a high line number screen pattern (for example, Y) equal, it depends on the number of lines of the screen pattern. It turns out that the spot diameter must be changed.

  However, when the spot diameter is as small as 20 μm, when the spot is imaged on the surface of the photosensitive drum, the depth of the spot becomes very narrow, and the optical system in the optical scanning device has some manufacturing error. Small spots cannot be realized when they occur. The spot diameter of a conventional product called high image quality is about 50 μm. Therefore, in the present embodiment, the print diameter index value that is 50 μm at the station with the highest number of lines is used as a reference, and the spot diameter at each number of lines relative to this value is determined.

  Hereinafter, the size of the spot on the surface of the photosensitive drum corresponding to the number of lines of the screen pattern will be described. FIG. 13 shows the number of lines of the screen pattern, the spot size on the surface of the photosensitive drum, and the opening shape for realizing the spot.

  Here, the rotation angles of the Y, M, C, and K screen patterns are each 0 degrees in the HV plane. The number of lines of the screen patterns of Y, M, C, and K is 212 LPI, 189 LPI, 166 LPI, and 150 LPI, respectively, and the spot shape at that time is almost circular (substantially perfect circle).

  When considering the condition that the yellow station Yst and the other stations have the same print image quality (condition in which the position of the vertical axis in FIG. 12 is constant), it is not necessary to narrow down the spots at all stations as described above. Specifically, as shown in FIG. 13, the range of the spot size that can be taken between 45 μm and 80 μm in four stations from the number of high lines to the number of low lines. Accordingly, at least a station using a screen pattern with a low number of lines, the spot size may be large, the collapse of the spot on the surface of the photosensitive drum due to a manufacturing error can be reduced, and good image quality can be maintained. it can.

  The aperture shape of the diaphragm aperture 2 in FIG. 2 is inversely related to the spot shape on the photosensitive drum surface (that is, a small circle when the spot on the photosensitive drum surface is large, and a large circle when the spot is small). ) As described above, by appropriately changing the spot diameter according to the number of lines of the screen pattern, it is not necessary to reduce the spot diameter at all stations, thereby realizing an optical scanning device having a system insensitive to focus deviation. Is possible. That is, the size of the spot on the surface of the photosensitive drum does not have to be different at all stations, and the size of the spot on the surface of the photosensitive drum is different from that of the other stations in at least one station.

<< Third Embodiment >>
Next, a third embodiment of the present invention will be described. In the present embodiment, the print image quality is improved by changing the spot shape on the surface of the photosensitive drum according to the angle (rotation angle) of the screen pattern and the number of lines. That is, in this embodiment, as shown in FIG. 14, the angle (rotation angle) and the number of lines of the screen pattern are different at each station.

  In the present embodiment, the screen pattern angle (rotation angle) is the same as in the first embodiment, and the number of lines is the same as in the second embodiment. The spot shape on the surface of the photosensitive drum is a substantially elliptical shape or a substantially rectangular shape. The ratio of the length of the long axis (long side) to the short axis (short side) is 3: 2, and the short axis of the spot overlaps the direction perpendicular to the longitudinal direction of the screen pattern (pattern period direction). is there.

  Furthermore, the length of the short axis of the spot varies depending on the number of lines of the screen pattern. The smaller the number of lines of the screen pattern, the longer the short axis of the spot. The combination of the number of lines and the short axis length of the spot is Y (212 LPI, 48 μm), M (189 LPI, 60 μm), C (166 LPI, 65 μm), and K (150 LPI, 80 μm). On the other hand, the length of the major axis of the spot is increased to 1.5 times the length of the minor axis at each station as described above. That is, Y (72 μm), M (90 μm), C (98 μm), and K (120 μm) are set.

  In order to realize these spots, the major axis and minor axis length (mm × mm) of the aperture shape of the aperture stop 2 in the incident optical system are preferably set to the dimensions shown in FIG. That is, Y (2.08 mm × 1.44 mm), M (1.7 mm × 1.12 mm), C (1.6 mm × 0.965 mm), and K (1.3 mm × 0.82 mm) may be used.

(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, a color image forming apparatus having four scanned surfaces is shown, but a color image forming apparatus having two, three, or five or more scanned surfaces may be used. The present invention can also be applied to a monochrome image forming apparatus having a black station having a single (one) surface to be scanned. In this case, the stop aperture of the incident optical system has a shape (inclination direction or number of lines) determined according to the screen pattern.

  In this case, when the tilt direction of the screen pattern can be changed by the aperture opening, the spot shape is an elliptical shape or a rectangular shape whose major axis direction or longitudinal direction is the tilt direction. Further, when the number of lines of the screen pattern can be changed, due to the aperture opening, the spot shape becomes an approximate circular shape having a larger diameter when the number of lines is smaller.

  When the tilt direction and the number of lines of the screen pattern can be changed, the shape of the spot becomes an ellipse having a larger diameter when the number of lines is smaller and a major axis of the tilt direction due to the aperture opening.

(Modification 2)
In the above-described embodiment, an example in which the number of lines and angles of the Y, M, C, and K screens are different has been described, but the present invention is not limited to this. It suffices that at least one spot shape (outer shape, angle) among a plurality of stations is different from the spot shape (outer shape, angle) at other stations.

(Modification 3)
In the above-described embodiment, the imaging optical system (scanning optical system) is configured by two imaging scanning lenses. However, the present invention is not limited to this, and the same effects as in the present embodiment can be achieved by configuring one or three or more lenses. Can be obtained.

  In addition, the imaging scanning lenses 206 and 207 are made of a plastic lens having light-transmitting power to achieve a reduction in weight of the imaging scanning lens and to improve design freedom by using an aspherical surface. It is also possible. The imaging scanning lens may be made of glass, and may be an optical element having diffraction power. When configured with a glass material or a diffractive surface, it is possible to provide an optical scanning device with excellent environmental characteristics, and a folding mirror may be provided in the scanning optical system.

2, 2a, 2b,... Aperture aperture, 5. .. Rotating polygon mirror, L1a, Lib,... First and second incident optical systems, M1a, Mib,.

Claims (19)

Light source means for emitting first and second light beams in accordance with different first and second screen patterns;
A deflector for deflecting the first and second light beams to optically scan the first and second scanned surfaces;
The first and second connections for condensing the first and second light beams deflected by the deflector to form first and second spots on the first and second scanned surfaces, respectively. An image optical system;
With
The optical scanning device according to claim 1, wherein the first and second spots have different shapes according to the first and second screen patterns. Comprising first and second aperture stops through which each of the first and second light beams emitted from the light source means passes,
2. The optical scanning device according to claim 1, wherein each of the first and second aperture stops is provided with openings having different shapes corresponding to the first and second screen patterns. .   The optical scanning device according to claim 1, wherein the first and second screen patterns correspond to different colors.   4. The optical scanning device according to claim 1, wherein the first and second screen patterns have shapes having different inclination directions or different numbers of lines. 5. The first and second screen patterns have the same number of lines and different inclination directions,
5. The optical scanning device according to claim 4, wherein the apertures of the first and second aperture stops are narrowed in the tilt direction.   6. The optical scanning device according to claim 5, wherein the spots on the first and second scanned surfaces have an elliptical shape having the major axis in the tilt direction. The first and second screen patterns have the same inclination direction and different numbers of lines,
The optical scanning according to any one of claims 4 to 6, wherein the apertures of the first and second aperture stops are narrowed in the tilt direction, and the aperture is narrowed more as the number of lines is smaller. apparatus.   8. The optical scanning device according to claim 7, wherein the spots on the first and second scanned surfaces have an approximate circular shape having a larger diameter when the number of lines is smaller. The first and second screen patterns have different numbers of lines and different inclination directions,
7. The optical scanning device according to claim 4, wherein the apertures of the first aperture stop and the second aperture stop have a shape narrowed in the tilt direction.   10. The light according to claim 9, wherein the spots on the first and second scanned surfaces have an elliptical shape having a larger diameter as the number of lines is smaller and a major axis in the tilt direction. Scanning device.   The angle of the major axis or the longitudinal direction of the elliptical or rectangular shape of each spot formed on the first and second scanned surfaces with respect to the tilt direction of the first and second screen patterns is within 25 degrees. The optical scanning device according to claim 5, wherein the optical scanning device is an optical scanning device. In the first and second imaging optical systems, when the imaging magnification in the main scanning direction is β1, γ1, and the imaging magnification in the sub scanning is β2, γ2,
0.8 ≦ (β1 / β2) ≦ 1.2
0.8 ≦ (γ1 / γ2) ≦ 1.2
The optical scanning device according to claim 1, wherein:   The optical scanning device according to claim 1, wherein the first and second screen patterns are line patterns. Light source means for emitting light according to the screen pattern;
A deflector for deflecting the light beam and optically scanning the surface to be scanned;
An imaging optical system for condensing the light beam deflected by the deflector to form a spot on the scanned surface;
With
The optical scanning device according to claim 1, wherein the spot has a shape corresponding to the screen pattern.   The optical scanning device according to claim 14, wherein the screen pattern can change an inclination, and the shape of the spot is an elliptical shape or a rectangular shape in which the inclination direction is a major axis direction or a longitudinal direction.   15. The optical scanning device according to claim 14, wherein the number of lines of the screen pattern can be changed, and the shape of the spot is an approximate circular shape having a larger diameter when the number of lines is smaller.   The screen pattern can be changed in inclination direction and the number of lines, and the shape of the spot is an ellipse shape having a larger diameter when the number of lines is smaller and a major axis of the inclination direction. 14. An optical scanning device according to 14.   18. The optical scanning device according to claim 1, a developing device that develops an electrostatic latent image formed on a 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 claim 1, and a printer controller that converts code data output from an external device into an image signal and inputs the image signal to the optical scanning device. Image forming apparatus.