JP2006154785A - Optical scanning apparatus and image forming apparatus using the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain an optical scanning apparatus capable of printing with a high quality and an image forming apparatus using the same. <P>SOLUTION: The optical scanning apparatus is provided with an incidence optical system 15 for guiding a light beam emitted from a light source part 1 to a deflection part 5; an imaging optical system 6 provided with a diffracting surface 14 having a power at least in a sub-scanning section that guides the light beam deflected by the deflection part on a surface to be scanned 7; and a surface inclination correction function, in which a principal ray of the light beam is always positioned outside of a main scanning section during scanning. In the optical scanning apparatus, each element is set so as to satisfy each conditional expression. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an optical scanning device and an image forming apparatus using the same.

  In particular, a light beam (laser light) emitted from a light source means is optically scanned on a surface to be scanned through an imaging optical system to record image information, for example, a laser beam printer having an electrophotographic process or digital copying. It is suitable for an image forming apparatus such as a printer.

  2. Description of the Related Art Conventionally, as an optical scanning device used in an image forming apparatus, an optical system (oblique incident light scanning device) that makes a light beam incident from an angle that is not orthogonal to the rotation axis of a deflecting unit (optical deflector) is known.

  For example, in an overfilled optical scanning device (OFS optical system), a light beam is incident on an optical deflector at an angle in the sub-scanning direction from the imaging optical system (scanning lens system) side so that the light quantity distribution does not become asymmetric on the left and right. There are many cases.

  Further, in an optical scanning device in which the number of optical deflectors is smaller than the number of scanned surfaces (photosensitive drum surfaces), different angles in the sub-scanning direction are used to separate a plurality of light beams deflected by the optical deflectors. In some cases, a plurality of light beams are incident on the optical deflector.

  In such an optical scanning device, it is difficult to keep the light beam (light beam) constantly incident on the main scanning section including the optical axis of the optical surface provided in the imaging optical system.

  This is because the light beam deflected by the optical deflector changes the traveling direction of the light beam in a conical shape. In addition, since the scanning line curve also occurs, conventionally, the scanning line curve is reduced by positively decentering the optical surface.

  In recent years, various optical scanning devices in which a diffractive surface is provided in an imaging optical system or the like have been proposed for temperature compensation. In such an optical scanning device, the power arrangement is such that the change in the positive refractive power (power) when the environmental temperature changes is canceled by the change in the positive diffraction power when the environmental temperature changes (Patent Document). 2).

  In such an optical scanning device, when the wavelength changes regardless of the environmental temperature due to mode hopping (mode hopping), the imaging position is shifted in the optical axis direction.

For this reason, it is necessary to ensure a sufficient depth width. An optical scanning device that sufficiently secures this depth width has been proposed (see Patent Document 1).
JP-A-9-54263 (no corresponding foreign country) Japanese Patent Laid-Open No. 10-333070 (US Pat. No. 6,067,106)

  By the way, in the optical scanning device in which the diffractive surface is provided in the imaging optical system, when the optical element constituting the imaging optical system is decentered in the sub-scanning direction, if the wavelength of the emitted light from the light source changes, the imaging The position is not only shifted in the optical axis direction but also shifted in the sub-scanning direction.

  For this reason, when a mode hop occurs during scanning (when an image is formed on the surface to be scanned with the light beam emitted from the light source means), the scanning line interval before and after that changes, so that the image quality deteriorates. Will occur.

  In particular, since color misregistration is a cause of color misregistration in the color image forming apparatus, it is necessary to particularly reduce the misalignment of the scanning lines.

  An object of the present invention is to provide an optical scanning apparatus capable of high-quality printing and an image forming apparatus using the same.

Therefore, in the present invention, the incident optical system that guides the light beam emitted from the light source unit to the deflecting unit, and at least the power in the sub-scan section that guides the light beam reflected by the deflecting unit onto the surface to be scanned. In an optical scanning device comprising an imaging optical system having a certain diffractive surface, and satisfying a conjugate relationship between the deflecting surface of the deflecting means and the scanned surface in the sub-scanning section,
In the sub-scan section, the principal ray of the light beam reflected by the deflecting means passes outside the optical axis of the diffraction surface,
The lateral magnification in the sub-scan section of the imaging optical system is β, the focal length in the sub-scan section of the imaging optical system is f [mm], and the wavelength of the light beam emitted from the light source means is 1 [nm]. The amount of change in the focal length in the sub-scanning section of the imaging optical system when changed is δf [mm], and the distance in the sub-scanning section between the principal ray on the diffraction surface and the optical axis of the diffraction surface is When h [mm] and the resolution is R [dot / inch],
| (1-β) · (δf / f) · h | <{25.4 / R} / 2, | h |> 0
It has a configuration that satisfies the following conditions.

  According to the present invention, there is provided an imaging optical system having a diffractive surface with power in the sub-scanning section, and the optical scanning that satisfies the conjugate relationship between the deflection surface of the deflecting means and the scanned surface in the sub-scanning section. In the apparatus, by appropriately setting each element constituting the optical scanning apparatus, it is possible to reduce the height deviation of the scanning line due to the mode hop, thereby enabling high-quality printing at high speed. And an image forming apparatus using the same can be achieved.

  In the present invention, the distance (light ray height) in the sub-scan section between the principal ray of the light beam on the diffraction surface and the optical axis of the diffraction surface is defined as h.

  First, the principle for achieving the object of the present invention will be described with reference to FIG.

  FIG. 9 is a sectional view (sub-scanning sectional view) of the principal part in the sub-scanning direction schematically showing the optical path after being deflected by the deflecting means (optical deflector).

  In FIG. 9, 5a is a deflecting surface provided in the optical deflector, 6 is an imaging optical system (scanning lens system) having a diffractive surface having a positive diffracting power (power by diffraction) in the sub-scan section, and 7 is The surface to be scanned, 8 is the optical axis of the imaging optical system 6, the broken line 9 is the optical path of the principal ray before the wavelength of the light beam deflected by the deflecting surface 5a is changed, and the solid line 10 is the light beam deflected by the deflecting surface 5a. It is the optical path of the chief ray after the wavelength is changed.

  In this specification, the principal ray refers to a central ray of a light beam or a ray passing through the center of a stop.

  As shown in FIG. 9, even if the light beam (principal light beam) passes through a position h away from the optical axis 8 of the imaging optical system 6, the light beam is indicated by a broken line 9 if the imaging optical system has a surface tilt correction function. It returns to the optical axis 8 again through the optical path.

  However, since the positive diffracting power (power) changes particularly when the wavelength changes, for example, an image is formed at a location away from the scanning surface 7 through the optical path as shown by the solid line 10.

  For this reason, the light beam passes through the position 7 away from the optical axis 8 on the scanned surface 7, and the scanning line position is shifted by this distance x.

  The focal length in the sub-scan section of the imaging optical system 6 before the mode hop is f [mm], and the lateral magnification in the sub-scan section of the imaging optical system 6 (hereinafter also referred to as “lateral magnification in the sub-scanning direction”). ) Is β.

  Further, the focal length in the sub-scan section of the imaging optical system 6 after the mode hop is defined as f + df [mm], and the lateral magnification in the sub-scanning direction of the imaging optical system 6 is defined as β ′.

  At this time, the distance from the image-side main plane of the imaging optical system 6 to the imaging point changes from (1−β) f → (1−β ′) (f + df) before and after the mode hop. In FIG. 9, β <0 and β ′ <0.

Since the positional relationship between the deflecting surface 5a of the optical deflector 5 and the imaging optical system 6 does not change before and after the mode hop, the light beam passage position (diffraction on the main plane on the image side of the imaging optical system 6 in the sub-scanning section. If the chief ray with respect to the optical axis of the surface is incident on the diffractive surface (the height of the ray) is h [mm], the scanning line positional deviation x is x = h (1-β / β ′)
It can be expressed as. -Β '= (f + df) / {f / (-β) -df} After substituting, the above equation is rearranged: x = hdf (1-β) / (f + df)
Approximate
x≈ (1-β) · (df / f) · h
It can be expressed as.

  The inventor has found that the scanning line misalignment | x | must be less than half of the scanning line interval because the scanning line misalignment causes the print quality to be extremely deteriorated if it deviates by more than half of the scanning line interval. I found it.

If the resolution is R [dot / inch], the scanning line interval is 25.4 / R [mm]. Further, since it is conceivable that the wavelength changes by about 1 [nm] due to the mode hop, the amount of change in the focal length in the sub-scanning section of the imaging optical system 6 when the wavelength changes by 1 [nm] is represented by δf. Then
| X | ≈ | (1-β) · (δf / f) · h | <{25.4 / R} / 2, h ≠ 0 (1)
It is necessary to configure the optical scanning device so as to satisfy the above.

  In the present invention, the value of the light beam height h preferably satisfies | h | <4.8. The reason is that if the upper limit is exceeded, the spot rotates and the printing performance deteriorates.

  Further, the lateral magnification β in the sub-scanning direction preferably satisfies 0.8 ≦ β ≦ 3.5.

  The reason is that if the lower limit is exceeded, the length of the lens in the main scanning direction becomes long, and the lens cost increases.

  If the upper limit is exceeded, the lateral magnification in the sub-scanning direction becomes too high, and a printing position shift in the sub-scanning direction due to eccentricity tends to appear remarkably.

[Temperature compensation at focus position]
In the following, a case where the temperature compensation of the focus position in the sub-scan section is performed in the imaging optical system 6 will be considered.

  In order for the imaging optical system 6 to perform temperature compensation at the focus position in the sub-scan section, the refractive power and diffraction power of the imaging optical system 6 must be in a certain ratio.

  The power of the entire system of the imaging optical system 6 is positive. The reason is that it is necessary to form an image of the light beam on the scanned surface 7.

  In order to perform temperature compensation at the focus position in the sub-scan section, the refractive power φr and the diffraction power φd need to have the same sign power. Therefore, both the refractive power φr and the diffraction power φd are positive powers. For details, see Patent Document 2.

Now, the total power in the sub-scan cross section combining the refractive power φr and the diffraction power φd is φa, the wavelength of the light beam is λ, the refractive index of the lens material of the imaging optical system 6 is n, and φa, λ with respect to the temperature T , N are dφa / dT, dλ / dT, and dn / dT, respectively. Dφa / dT = (φd / λ) · (dλ / dT) + {φr / (n−1)} · (dn / dT)
It can be expressed as.

Suppose that λ = 780 × 10 ⁻⁶ [mm], n = 1.5242, dλ / dT = 0.255 × 10 ⁻⁶ [mm / ° C.], dn / dT = −0.85 × 10 ⁻⁴ [/ ° C. ]
dφa / dT = 3.269 × 10 ⁻⁴ φd−1.622 × 10 ⁻⁴ φr
It becomes. In order to perform temperature compensation, it is only necessary that dφa / dt = 0, so that φr≈2.016φd
Can be derived.

Further, the amount of change dφa of the total power φa after the wavelength is changed is defined as dφa = φd · dλ / λ where the diffraction power is φd.
Thus, the ratio δf / of the focal length f in the sub-scanning section of the imaging optical system 6 to the change amount δf of the focal length in the sub-scanning section of the imaging optical system 6 when the wavelength changes by 1 [nm]. f is δf / f = 1 / (1 + φd / φa · dλ / λ) −1
Approximately δf / f ≒ −φd / φa · dλ / λ
It can be expressed as. Further, from the relationship between the diffraction power φd and the refractive power φr in the temperature compensation system, φa = φd + φr = 3.016φd
Further, if λ = 780 [nm] and the wavelength change due to the mode hop is 1 [nm], δf / f = −4.251 × 10 ⁻⁴
Can guide. Therefore, the scanning line positional deviation x for preventing the print quality from being extremely deteriorated at the time of temperature compensation is | x | ≈4.251 × 10 ⁻⁴ × | (1-β) · h | <{25.4 / R} / 2, h ≠ 0 (2)
It is necessary to configure the optical scanning device so as to satisfy the above.

  Embodiments of the present invention will be described below with reference to the drawings.

  FIG. 1 is a sectional view (main scanning sectional view) of a main part in the main scanning direction of an optical scanning device (optical scanning device) according to a first embodiment of the present invention. FIG. ) It is explanatory drawing which showed the positional relationship of 1a, 1b.

  Here, the main scanning direction is a direction perpendicular to the rotation axis of the deflection means (the direction in which the light beam is reflected and deflected (deflection scanning) by the deflection means), and the sub-scanning direction is a direction parallel to the rotation axis of the deflection means. Indicates. The main scanning section is a plane parallel to the main scanning direction and including the optical axis of the diffraction surface. The sub-scanning cross section indicates a cross section perpendicular to the main scanning cross section.

  In FIG. 1, reference numeral 1 denotes a light source means, which is composed of, for example, a monolithic two-beam laser having two light emitting portions 1a and 1b.

  As shown in FIG. 2, the two light emitting units 1a and 1b are arranged apart from each other in the main scanning direction and the sub scanning direction, and the distance between the light emitting units is longer in the main scanning direction than in the sub scanning direction. It is configured to be.

  This is because the distance between the light emitting units in the sub-scanning direction can be set to a desired value by rotating the light source means 1.

  Further, in the initial state, the light source means 1 is light-exposed because the scanning line spacing (scanning line positional deviation in the sub-scanning direction) is shifted due to the difference in the wavelengths of the two light beams emitted from the light emitting units 1a and 1b. It is adjusted by rotating around the axis.

  Note that the number of light emitting units is not limited to two, and may be three or four or more as necessary. Of course, the number of light emitting units may be one.

  Reference numeral 2 denotes a collimator lens (condenser lens), which converts two light beams emitted from the light source means 1 into substantially parallel light beams. A cylindrical lens (optical system) 3 has a predetermined refractive power only in the sub-scanning direction. Reference numeral 4 denotes an aperture stop, which shapes two light beams emitted from the cylindrical lens 3 into a desired optimum beam shape.

  In the first embodiment, the aperture stop 4 is disposed close to the deflection surface 5a of the deflecting means 5 described later, so that the deflection point of the two light beams (the light beam passing through the center of the aperture stop 4 is deflected and reflected by the deflection surface 5a). The dot position shift (jitter in the main scanning direction) of the imaging spot in the main scanning direction is reduced.

  Each element such as the collimator lens 2, the cylindrical lens 3, and the aperture stop 4 constitutes an element of the incident optical system 15.

  Reference numeral 5 denotes an optical deflector serving as a deflecting unit, which is composed of, for example, a rotating polygon mirror (polygon mirror) and is rotated at a constant speed in the direction of arrow PA in the figure by a driving unit (not shown) such as a motor.

  Reference numeral 6 denotes an imaging optical system having fθ characteristics, which has first and second scanning optical elements (fθ lenses) 6a and 6b in order from the optical deflector side, and is deflected by the optical deflector 5. Two light beams are imaged in a spot shape on the photosensitive drum surface 7 as a surface to be scanned, and two scanning lines are formed simultaneously.

  In the first embodiment, the second scanning optical element 6b is given power (refractive power) in the sub-scanning section with respect to the first scanning optical element 6a, thereby reducing the magnification in the sub-scanning section and the sensitivity. Is suppressed from becoming high.

  Further, the second scanning optical element 6b has a diffractive surface 14 having a positive diffractive power in the sub-scan section on the lens surface on the scanning surface 7 side, and color compensation at the focus position in the main scan section. And temperature compensation for the focus position in the sub-scan section.

  Among the first scanning optical element 6a and the second scanning optical element 6b, the environmental temperature is changed by providing the diffraction surface 14 on the surface of the second scanning optical element 6b having the strongest power in the sub-scanning section. Sometimes the surface where the power is most likely to change and the surface where the diffraction force is most likely to change can be arranged in the vicinity.

  For this reason, the change of the passage position of the light beam due to the change of the environmental temperature in the sub-scanning section is reduced, and the deterioration of new optical performance is reduced.

  Further, the imaging optical system 6 has a surface tilt correction function by making a conjugate relationship between the deflection surface 5a of the optical deflector 5 and the photosensitive drum surface 7 in the sub-scan section.

  Reference numeral 7 denotes a photosensitive drum surface as a surface to be scanned.

  In the first embodiment, two light beams that are light-modulated and emitted from the light source unit 1 according to image information are converted into parallel light beams by the collimator lens 2 and are incident on the cylindrical lens 3.

  Out of the light beam incident on the cylindrical lens 3, it exits as it is in the main scanning section and passes through the aperture stop 4 (partially shielded).

  In the sub-scan section, the light beam converges and passes through the aperture stop 4 (partially shielded) to form a line image (a line image elongated in the main scanning direction) on the deflection surface 5a of the optical deflector 5.

  The two light beams reflected and deflected by the deflecting surface 5a of the optical deflector 5 are each imaged in a spot shape on the photosensitive drum surface 7 by the imaging optical system 6, and the optical deflector 5 is moved in the direction of the arrow PA. By rotating, optical scanning is performed on the photosensitive drum surface 7 in the direction of arrow PB (main scanning direction) at a constant speed.

  As a result, two scanning lines are simultaneously formed on the photosensitive drum surface 7 as a recording medium to perform image recording.

  In the optical scanning apparatus of the first embodiment, during scanning (when an image is formed on the surface to be scanned 7 with the light beam emitted from the light source means 1), the main light beams from the light emitting points 1a and 1b in the sub-scan section are shown. The light beam Lp is always located outside the main scanning section (outside the optical axis of the diffraction surface).

  Here, FIG. 3 is an explanatory diagram showing the positional relationship between the diffractive surface 14 and the light beams a, b, and c when the second scanning optical element 6b is viewed from the scanned surface 7.

  The second scanning optical element 6b is a plastic molded lens having a diffractive surface integrally formed with a lens surface.

  The shapes of the buses of the first imaging optical element 6a and the second imaging optical element 6b, which are imaging lenses, are expressed by the following equations. Y is a coordinate in the main scanning direction, 0 on the lens optical axis, positive on the light source means 1 side, and negative on the counter light source means side. X is a coordinate in the optical axis direction, with 0 on the axis, positive on the scanned surface 7 side, and negative on the deflection means 5 side.

The curvature radius r of the cross section (sub-scanning section) of the imaging lens in the present embodiment changes according to the coordinate Y in the main scanning direction and is expressed by the following equation.
r = R {1 + Σ (ai * Y ⁱ )}
k and ai are aspheric coefficients.

  The definition formula of the definition formula of the diffraction surface 14 is shown below.

  φ is the phase difference, λ is the wavelength, Cn is the phase polynomial coefficient, y is the coordinate in the main scanning direction viewed from the optical axis, z is the coordinate in the sub-scanning direction viewed from the optical axis, and the above equation is The phase at the location of coordinates (y, z) when the phase is 0 is shown.

  In FIG. 3, each ellipse indicates an annular zone of the diffractive surface 14, Oa is the optical axis of the diffractive surface 14, and 13 is the 0th annular zone of the diffractive surface 14 (having a relatively wide area near the center of the mirror surface). Ellipse). Three circles a, b, and c each indicate a light beam during scanning. Lp represents the chief ray of the luminous flux. In the figure, the light beams a, b, and c being scanned are scanned in the direction of the arrow A in the left-right direction.

  In this embodiment, as shown in FIG. 3, the principal ray Lp of the light beam passing through the second imaging optical element 6b passes through the sub-scan section other than the 0th annular zone 13 of the diffractive surface 14. ing.

  This is because the change in the lattice height with respect to the radial direction in the other annular zone is almost straight and easy to process, whereas the 0th annular zone 13 needs to be processed in a curved surface, so that the degree of difficulty is higher. This is because high processing is required, and when the light beam passes through the 0th annular zone 13, unintended power and wavefront aberration are not given to the light beam.

  In such a case, the light beam passes through the diffractive surface 14 away from the optical axis Oa in the sub-scanning direction.

In the first embodiment, the width of the 0th annular zone 13 in the sub-scanning direction is 1.4 (mm), the light flux width on the diffractive surface 14 is 4 (mm), and the amount of fluctuation of the passage position of the light flux due to assembly tolerances is ± 1 (mm), the height of the light ray when the principal ray Lp is incident on the diffractive surface 14 with respect to the optical axis of the diffractive surface 14 is h = 3.7 (mm)
It is said.

Further, in the optical scanning device of the first embodiment, the wavelength λ of the light beam is λ = 780 × 10 ⁻⁶ [mm],
The refractive index n of the material of the lenses of the first and second imaging optical elements 6a and 6b constituting the imaging optical system 6 is n = 1.5242,
Sensitivity of λ and n to temperature, dλ / dT and dn / dT, respectively, dλ / dT = 0.255 × 10 ⁻⁶ [mm / ° C.],
dn / dT = −0.85 × 10 ⁻⁴ [/ ° C.],
The lateral magnification β in the sub-scanning direction of the imaging optical system 6 is β = −3,
Resolution R is R = 600 [dot / inch]
It is said.

In the first embodiment, temperature compensation is performed in the sub-scan section, and the total power in the sub-scan section of the imaging optical system 6 is φa [1 / mm], and the diffraction power in the sub-scan section is set. When φd [1 / mm],
2.0 <φa / φd <4.0 (3)
Each element is set to satisfy the following conditions. If the conditional expression (3) is deviated, the temperature compensation is not valid, which is not good. Incidentally, in this embodiment, φa / φd = 3.016.
This satisfies the conditional expression (3).

  In this embodiment, temperature compensation is performed not only in the sub-scanning section but also in the main scanning section.

In the first embodiment, the ratio δf / f between the focal length f in the sub-scan section of the imaging optical system 6 and the change amount δf of f when the wavelength is changed by +1 [nm] is δf / f = -4.251 × 10 ^-4
It is.

In the first embodiment, when the wavelength fluctuates by 1 [nm] due to the mode hop, the deviation x of the scanning line height is x = (1-β) · (δf / f) · h = −6.29 × 10 ⁻³ [Mm]
It becomes. That is,
| −6.29 × 10 ⁻³ [mm] | <{25.4 / R} /2=21.1×10 ⁻³ [mm]
This satisfies the conditional expression (1) and the conditional expression (2).

  In the case of using a plurality of light beams as in the first embodiment, if a mode hop is caused by only a part of the light beams, the scanning line interval (scan line misalignment in the sub-scanning direction) varies. Image quality is more likely to deteriorate than when it is used. For this reason, when a plurality of light beams are used, it is necessary to strictly manage the deviation of the scanning line height more than when using a single light beam.

Therefore, in the first embodiment, each element is set so as to satisfy the following conditional expressions (4) and (5) so that the scanning line height deviation x due to the mode hop is ¼ or less of the scanning line interval. is doing.
| (1-β) · (δf / f) · h | <{25.4 / R} / 4 (4)
4.251 × 10 ⁻⁴ × | (1-β) · h | <{25.4 / R} / 4 (5)
Specifically, the scanning line interval d is
From R = 600 [dot / inch]
d = 25.4 / R = 42.3 × 10 ⁻³ [mm]
It is. In the first embodiment, the deviation x of the scanning line height when the wavelength fluctuates by 1 (nm) due to the mode hop is as described above.
x = (1-β) · (δf / f) · h = −6.29 × 10 ⁻³ [mm]
It becomes. That is,
| −6.29 × 10 ⁻³ [mm] | <{25.4 / R} /4=10.6×10 ⁻³ [mm]
This satisfies the conditional expressions (4) and (5).

  Thus, it can be seen that in the first embodiment, the image quality is hardly deteriorated even if the scanning line height is changed by the mode hop.

  In the first embodiment, one diffractive surface is provided. However, the present invention is not limited to this, and a plurality of diffractive surfaces may be provided. In this case, the principal ray may be positioned outside the main scanning section of the at least one diffraction surface (outside the optical axis of the diffraction surface) in the sub-scanning section.

  In the first embodiment, the imaging optical system 6 is composed of two lenses. However, the present invention is not limited to this. For example, the imaging optical system 6 may be composed of a single lens or three or more lenses.

  FIG. 4 is a sub-scan sectional view of the optical system of the image forming apparatus according to the second embodiment of the present invention.

  FIG. 5 shows a state in which the light beam separating unit 11 and the folding mirror 12 are omitted in FIG. 4, and in particular, four light beams are incident on the optical deflector 5. A single light beam is taken as an example.

  An arbitrary one light beam emitted from the light source means 1 sequentially passes through a collimator lens 2, a cylindrical lens 3 having power only in the sub-scanning direction, and a diaphragm 4, and an optical deflector (polygon mirror) 5 having six deflection surfaces. 2 is a main scanning sectional view illustrating an optical scanning device that is deflected and reflected by the light beam and passes through the imaging optical system 6 and then reaches the scanned surface 7. FIG.

  4 and 5, the same elements as those shown in FIG. 1 are denoted by the same reference numerals.

  The second embodiment is particularly different from the first embodiment described above in that four scanning surfaces (photosensitive drum surfaces) 7 are provided (that is, a scanning unit including four optical scanning devices shown in FIG. 5). is there. However, the optical deflector 5 is shared.

  That is, the second embodiment is a four station type.

  Further, as shown in FIG. 4, each light beam is incident on the same light deflector 5 at an angle of ± 2 ° in the vertical direction on the drawing, and the light beam deflected by the light deflector 5 is separated into light beams. After being separated by the means (folding mirror) 11, the respective light beams are respectively guided to the corresponding scanned surface 7 via the corresponding folding mirror 12.

  In the second embodiment, as shown in FIG. 6, the principal ray Lp of the light beam passing through the second imaging optical element 6b passes through the 0th annular zone 13 of the diffractive surface 14 in the sub-scan section. It is configured.

  However, as shown in FIG. 6, the principal ray Lp of the light beam passing through the second imaging optical element 6 b passes outside the optical axis of the diffractive surface 14 in the sub-scan section.

  Other configurations and optical actions are substantially the same as those in the first embodiment, and the same effects are obtained.

  That is, in this embodiment, the diffractive surface 14 is provided by injection molding on the lens surface on the scanning surface 7 side of the second imaging optical element 6b constituting the imaging optical system 6 as in the first embodiment. .

  The second imaging optical element 6b is a molded lens.

  FIG. 6 is an explanatory diagram showing the positional relationship between the diffractive surface 14 and the light beam when the second imaging optical element 6b is viewed from the scanned surface 7 as in FIG. In FIG. 6, the same elements as those shown in FIG.

  In the second embodiment, as shown in FIG. 6, the light beams on the diffractive surface 14 are shifted ± 2.1 (mm) in the vertical direction in the drawing.

  That is, the light beam height h is set to h = ± 2.1 (mm). This is for correcting the scanning line curvature or the like caused by the incident at an angle of ± 2 ° with respect to the deflecting surface 5a of the optical deflector 5.

  FIG. 6 shows a case where h = 2.1 (mm). The principal ray Lp is allowed to pass through other than the 0th annular zone 13 of the diffractive surface 14. The second imaging optical element 6b (6b2, 6b3) is h = 2.1 (mm), and the second imaging optical element 6b (6b1, 6b4) is h = −2.1 (mm). Through the principal ray Lp.

In the optical scanning device of the first embodiment, the wavelength λ of the light flux is λ = 780 × 10 ⁻⁶ [mm],
The refractive index n of the material of the lenses of the first and second imaging optical elements 6a and 6b constituting the imaging optical system is n = 1.5242,
Sensitivity of λ and n to temperature, dλ / dT and dn / dT, respectively, dλ / dT = 0.255 × 10 ⁻⁶ [mm / ° C.],
dn / dT = −0.85 × 10 ⁻⁴ [/ ° C.],
The lateral magnification β in the sub-scanning direction of the imaging optical system 6 is β = −1.8,
Resolution R is R = 1200 [dot / inch]
It is said.

In the present embodiment, temperature compensation is performed in the sub-scan section as in the first embodiment. That is, in this embodiment, each element is set so as to satisfy the conditional expression (3) described above. Incidentally, in this embodiment, φa / φd = 3.016.
This satisfies the conditional expression (3).

In the second embodiment, the ratio δf / f between the focal length f in the sub-scan section of the imaging optical system 6 and the change amount δf of f when the wavelength changes by +1 (nm) is δf / f = -4.251 × 10 ^-4
It is. In the second embodiment, when the wavelength fluctuates by 1 [nm] due to the mode hop, the deviation x of the scanning line height is x = (1-β) · (δf / f) · h = −2.50 × 10 ⁻³ [Mm]
It becomes. That is,
| -2.50 × 10 ⁻³ [mm] | <{25.4 / R} /2=21.1×10 ⁻³ [mm]
This satisfies the conditional expressions (1) and (2).

  When scanning a plurality of surfaces to be scanned as in the second embodiment, a color shift (registration shift) occurs instead of a scanning line interval error, but in any case, when scanning a plurality of light beams simultaneously, a single light beam is used. It is necessary to strictly manage the deviation of the scanning line height more than when scanning.

  Therefore, in this embodiment, each element is set so as to satisfy the above conditional expression (4) and conditional expression (5) so that the deviation x of the scanning line height due to the mode hop is ¼ or less of the scanning line interval. is doing.

Specifically, the scanning line interval d is
From R = 1200 [dot / inch]
d = 25.4 / R = 21.2 × 10 ⁻³ [mm]
It is. In this embodiment, the deviation x of the scanning line height when the wavelength fluctuates by 1 [nm] due to the mode hop is as described above.
x = (1-β) · (δf / f) · h = −2.50 × 10 ⁻³ [mm]
It becomes. That is,
| -2.50 × 10 ⁻³ [mm] | <{25.4 / R} /4=5.3×10 ⁻³ [mm]
This satisfies the conditional expressions (4) and (5).

  As a result, it can be seen that in this embodiment, even if the scanning line height changes due to the mode hop, image quality deterioration due to color misregistration or the like hardly occurs.

  The resolution of the first embodiment is 600 dpi, and the resolution of the second embodiment is 1200 dpi. However, if the resolution is increased, the problem of the present invention becomes remarkable. Therefore, the present invention is more effective in an optical scanning device having a resolution of 1200 dpi or higher. Conditional expressions (1) to (5) of the present invention are more effective when the resolution is 1200 dpi or more.

[Image forming apparatus]
FIG. 7 is a cross-sectional view of the main part in the sub-scanning direction showing an embodiment of the image forming apparatus of the present invention. In the figure, reference numeral 104 denotes an image forming apparatus.

  Code data Dc is input to the image forming apparatus 104 from an external device 117 such as a personal computer. The code data Dc is converted into image data (dot data) Di by a printer controller 111 in the apparatus.

  The image data Di is input to the optical scanning unit 100 having the configuration shown in the first embodiment.

  The light scanning unit 100 emits a light beam 103 modulated according to the image data Di, and the light beam 103 scans the photosensitive surface of the photosensitive drum 101 in the main scanning direction.

  The photosensitive drum 101 serving as an electrostatic latent image carrier (photoconductor) is rotated clockwise by a motor 115.

  With this rotation, the photosensitive surface of the photosensitive drum 101 moves in the sub-scanning direction perpendicular to the main scanning direction with respect to the light beam 103.

  Above the photosensitive drum 101, a charging roller 102 for uniformly charging the surface of the photosensitive drum 101 is provided so as to contact the surface.

  The surface of the photosensitive drum 101 charged by the charging roller 102 is irradiated with the light beam 103 scanned by the optical scanning unit 100.

  As described above, the light beam 103 is modulated based on the image data Di, and by irradiating the light beam 103, an electrostatic latent image is formed on the surface of the photosensitive drum 101.

  The electrostatic latent image is developed as a toner image by a developing unit 107 disposed so as to contact the photosensitive drum 101 further downstream in the rotation direction of the photosensitive drum 101 than the irradiation position of the light beam 103.

  The toner image developed by the developing unit 107 is transferred onto a sheet 112 as a transfer material by a transfer roller 108 disposed below the photosensitive drum 101 so as to face the photosensitive drum 101.

  The paper 112 is stored in a paper cassette 109 in front of the photosensitive drum 101 (on the right side in FIG. 7), but can be fed manually.

  A paper feed roller 110 is provided at the end of the paper cassette 109, and feeds the paper 112 in the paper cassette 109 into the transport path.

  As described above, the sheet 112 on which the unfixed toner image has been transferred is further conveyed to a fixing device behind the photosensitive drum 101 (left side in FIG. 7).

  The fixing device includes a fixing roller 113 having a fixing heater (not shown) therein and a pressure roller 114 disposed so as to be in pressure contact with the fixing roller 113.

  The unfixed toner image on the sheet 112 is fixed by heating the sheet 112 conveyed from the transfer unit while being pressed by the pressure contact portion between the fixing roller 113 and the pressure roller 114.

  Further, a paper discharge roller 116 is disposed behind the fixing roller 113, and the fixed paper 112 is discharged out of the image forming apparatus.

  Although not shown in FIG. 7, the print controller 111 controls not only the data conversion described above, but also controls each part in the image forming apparatus including the motor 115 and a polygon motor in the optical scanning unit described later. I do.

  The recording density of the image forming apparatus used in the present invention is not particularly limited. However, considering that the higher the recording density is, the higher the image quality is required, the configuration of the first embodiment of the present invention is more effective in an image forming apparatus of 1200 dpi or more.

[Color image forming apparatus]
FIG. 8 is a schematic view of a main part of a color image forming apparatus according to an embodiment of the present invention. This embodiment is a tandem type color image forming apparatus in which four optical scanning devices (optical scanning devices) are arranged in parallel and image information is recorded on a photosensitive drum surface as an image carrier.

  In FIG. 8, 60 is a color image forming apparatus, 61, 62, 63 and 64 are optical scanning devices having the structure shown in the first embodiment, 21, 22, 23 and 24 are photosensitive drums as image carriers, respectively. Reference numerals 31, 32, 33, and 34 denote developing units, and 51 denotes a conveyance belt.

  In FIG. 8, the color image forming apparatus 60 receives R (red), G (green), and B (blue) color signals from an external device 52 such as a personal computer.

  These color signals are converted into C (cyan), M (magenta), Y (yellow), and B (black) image data (dot data) by a printer controller 53 in the apparatus.

  These image data are input to the optical scanning devices 61, 62, 63 and 64, respectively.

  From these optical scanning devices, light beams 41, 42, 43, and 44 modulated according to each image data are emitted, and the photosensitive surfaces of the photosensitive drums 21, 22, 23, and 24 are caused by these light beams. Scanned in the main scanning direction.

  The color image forming apparatus according to the present embodiment has four optical scanning devices (61, 62, 63, 64) arranged in each color of C (cyan), M (magenta), Y (yellow), and B (black). Correspondingly, image signals (image information) are recorded on the photosensitive drums 21, 22, 23, and 24 in parallel, and a color image is printed at high speed.

  As described above, the color image forming apparatus in this embodiment uses the light beams based on the respective image data by the four optical scanning devices 61, 62, 63, and 64, and the corresponding photosensitive drums 21 and 22 respectively corresponding to the latent images of the respective colors. , 23, 24 on the surface. Thereafter, a single full color image is formed by multiple transfer onto a recording material.

  As the external device 52, for example, a color image reading device including a CCD sensor may be used. In this case, the color image reading apparatus and the color image forming apparatus 60 constitute a color digital copying machine.

Main scanning sectional view of Embodiment 1 of the present invention The figure which shows the positional relationship of the light emission part of the light source means shown in FIG. The figure which shows the positional relationship of the diffraction surface of Example 1 of this invention, and a light beam. Sub-scan sectional view of Embodiment 2 of the present invention Main scanning sectional view of Embodiment 2 of the present invention The figure which shows the positional relationship of the diffraction surface of Example 2 of this invention, and a light beam. FIG. 3 is a sub-scan sectional view showing an embodiment of the image forming apparatus of the present invention. 1 is a schematic view of a main part of a color image forming apparatus according to an embodiment of the present invention. Sub-scan sectional view of an optical scanning device for explaining the principle of the present invention

Explanation of symbols

1 Light source means 2 Condensing lens (collimator lens)
3 Cylindrical lens 4 Aperture stop 5 Deflection means (polygon mirror)
6 Imaging optical system 6a First optical element (fθ lens)
6b Second optical element (fθ lens)
7 Scanned surface (photosensitive drum surface)
DESCRIPTION OF SYMBOLS 14 Diffraction surface 15 Incident optical system 100 Optical scanning device 101 Photosensitive drum 102 Charging roller 103 Light beam 104 Image forming apparatus 107 Developing device 108 Transfer roller 109 Paper cassette 110 Paper feed roller 111 Printer controller 112 Transfer material (paper)
Reference Signs List 113 Fixing roller 114 Pressure roller 115 Motor 116 Paper discharge roller 117 External device 61, 62, 63, 64 Optical scanning device (optical scanning device)
21, 22, 23, 24 Image carrier (photosensitive drum)
31, 32, 33, 34 Developer 41, 42, 43, 44 Light beam 51 Conveying belt 52 External device 53 Printer controller 60 Color image forming apparatus

Claims (10)

An incident optical system for guiding the light beam emitted from the light source unit to the deflecting unit, and a diffractive surface having power in at least the sub-scan section for guiding the light beam reflected by the deflecting unit onto the surface to be scanned. In an optical scanning device comprising an image optical system and satisfying a conjugate relationship between the deflecting surface of the deflecting means and the scanned surface in a sub-scanning section,
In the sub-scan section, the principal ray of the light beam reflected by the deflecting means passes outside the optical axis of the diffraction surface,
The lateral magnification in the sub-scan section of the imaging optical system is β, the focal length in the sub-scan section of the imaging optical system is f [mm], and the wavelength of the light beam emitted from the light source means is 1 [nm]. The amount of change in the focal length in the sub-scanning section of the imaging optical system when changed is δf [mm], and the distance in the sub-scanning section between the principal ray on the diffraction surface and the optical axis of the diffraction surface is When h [mm] and the resolution is R [dot / inch],
| (1-β) · (δf / f) · h | <{25.4 / R} / 2, | h |> 0
An optical scanning device characterized by satisfying the following conditions. When the total power in the sub-scan section of the imaging optical system is φa [1 / mm] and the power due to diffraction in the sub-scan section of the diffraction surface is φd [1 / mm],
2.0 <φa / φd <4.0
The optical scanning device according to claim 1, wherein the following condition is satisfied. The light source means has a plurality of light emitting units, and when scanning the surface to be scanned with a plurality of light beams emitted from the plurality of light emitting units,
| (1-β) · (δf / f) · h | <{25.4 / R} / 4
The optical scanning device according to claim 1, wherein the following condition is satisfied. An incident optical system for guiding the light beam emitted from the light source means to the deflecting means; and an imaging optical system having one or more imaging optical elements for guiding the light beam deflected by the deflecting means onto the surface to be scanned; In the optical scanning device satisfying the conjugate relationship between the deflection surface of the deflecting means and the surface to be scanned in the sub-scan section,
Among the one or more imaging optical elements, the imaging optical element having the strongest power in the sub-scanning section has a diffractive surface having power in the sub-scanning section,
In the sub-scan section, the principal ray of the light beam reflected by the deflecting means passes outside the optical axis of the diffraction surface,
The lateral magnification in the sub-scan section of the imaging optical system is β, the distance between the principal ray on the diffraction surface and the optical axis of the diffraction surface is h [mm], and the resolution is R [dot]. / Inch], when the total power in the sub-scan section of the imaging optical system is φa [1 / mm] and the power due to diffraction in the sub-scan section of the diffraction surface is φd [1 / mm],
^{4.251 × 10- 4 × | (1} -β) · h | <{25.4 / R} / 2, | h |> 0
2.0 <φa / φd <4.0
An optical scanning device characterized by satisfying the following conditions. The light source means has a plurality of light emitting units, and when scanning the surface to be scanned with a plurality of light beams emitted from the plurality of light emitting units,
^{4.251 × 10- 4 × | (1} -β) · h | <{25.4 / R} / 2, | h |> 0
The optical scanning device according to claim 4, wherein the following condition is satisfied.   6. The optical scanning device according to claim 1, wherein the chief ray passing through the imaging optical system passes through a part other than the 0th annular zone of the diffractive surface.   The optical scanning device according to any one of claims 1 to 6, a photosensitive member disposed on the surface to be scanned, and a static beam formed on the photosensitive member by a light beam scanned by the optical scanning device. A developing device that develops an electrostatic latent image as a toner image, a transfer device that transfers the developed toner image onto a transfer material, and a fixing device that fixes the transferred toner image onto the transfer material. Image forming apparatus.   8. An image forming apparatus comprising: the optical scanning device according to claim 7; and a printer controller that converts code data input from an external device into an image signal and inputs the image signal to the optical scanning device.   A color image comprising: a plurality of image carriers each of which is disposed on a surface to be scanned of the optical scanning device according to any one of claims 1 to 6 and forms images of different colors. Forming equipment.   10. The color image forming apparatus according to claim 9, further comprising a printer controller that converts color signals input from an external device into image data of different colors and inputs the image data to each optical scanning device.

Images