JP2017165023A - Optical scanner - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical scanner which has a small size and which can suppress image density unevenness.SOLUTION: An optical scanner 100 comprises: a light source 1; a deflector 4 which deflects a light flux from the light source 1 and optically scans a scanned surface 7 in a main scanning direction; an incident optical system which injects the light flux from the light source 1 to the deflector 4; and an image formation optical system which guides the light flux deflected by the deflector 4 to the scanned surface 7. When an injection side F number at an axis upper image height is represented by Fm0 and an injection F number at an axis outermost image height is represented by Fm1 in a main scanning cross section, and an injection side F number at an axis upper image height is represented by Fs0 and an injection F number at an axis outermost image height is represented by Fs1 in a sub-scanning cross section, the conditions having relations of 1.1<Fm1/Fm0<1.4 and 0.8<(Fm1×Fs1)/(Fm0×Fs0)<1.2 are satisfied.SELECTED DRAWING: Figure 1

Description

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

  In recent years, optical scanners are required to be reduced in cost and size.

Patent Document 1 discloses an optical scanning device that scans a surface to be scanned at a non-constant speed in order to achieve cost reduction and miniaturization.
Specifically, in such an inconstant speed optical scanning device, the imaging optical system can be configured with only one imaging lens that is thinner and smaller in width than the conventional one, so that the cost and size of the device can be reduced. Can be achieved.

In general, in the non-constant speed optical scanning device, the scanning speed is different between the on-axis image height and the off-axis image height, and thus the optical magnification in the main scanning direction is different, thereby causing image density unevenness. .
In the optical scanning device disclosed in Patent Literature 1, such image density unevenness is reduced by controlling the light emission timing of the light source according to each image height and electrically correcting the scanning position. .

However, in the optical scanning device disclosed in Patent Document 1, even when the light emission timing of the light source is controlled according to each image height, the main scanning direction on the surface to be scanned is at the time of intermediate gradation image formation. The spot diameter increases at the off-axis image height rather than the on-axis image height. As a result, image density unevenness occurs.
For example, if the light emission time of the light source when forming one dot is significantly shortened, while the light emission intensity of the light source is instantaneously significantly increased, the spot diameter of the latent image convoluted on the surface to be scanned is simulated. It can be made uniform. However, since the light emission intensity of the light source necessary for this is remarkably too high, a general semiconductor laser does not have a sufficient dynamic range, so that such an embodiment cannot be realized.
Therefore, the non-constant speed optical scanning device disclosed in Patent Document 1 cannot sufficiently reduce the image density unevenness when forming an intermediate gradation image.

JP2015-31824A

  Accordingly, an object of the present invention is to provide an optical scanning device that is small in size and can suppress image density unevenness.

An optical scanning device according to the present invention includes a light source, a deflector that deflects a light beam from the light source and optically scans a surface to be scanned in the main scanning direction, an incident optical system that causes the light beam from the light source to enter the deflector, And an imaging optical system for guiding the light beam deflected by the deflector to the surface to be scanned, wherein the emission side F number at the axial image height in the main scanning section is Fm0. The exit side F number at the off-axis image height is Fm1, the exit side F number at the on-axis image height in the sub-scan section is Fs0, and the exit side F number at the most off-axis image height is Fs1. When
1.1 <Fm1 / Fm0 <1.4
0.8 <(Fm1 × Fs1) / (Fm0 × Fs0) <1.2
It satisfies the following condition.

  According to the present invention, it is possible to provide an optical scanning device that is small in size and can suppress image density unevenness.

FIG. 3 is a main scanning sectional view and a partial sub-scanning sectional view of the optical scanning device according to the first embodiment. The figure which showed the relationship between the partial magnification shift and image height in the optical scanning device which concerns on 1st embodiment. The figure which showed the LSF spot diameter in each image height in the optical scanning device which concerns on 1st embodiment. The figure which showed the exit side F number in each image height in the optical scanning device which concerns on 1st embodiment. The figure which showed the opening part shape of the sub-scanning stop of the optical scanning device which concerns on 1st embodiment. The figure which showed the LSF depth center position in each image height in the optical scanning device which concerns on 1st embodiment. The figure which showed the relationship between the sub-scan magnification deviation of the imaging optical system of the optical scanning device concerning 1st embodiment, and image height. The figure which showed the spot profile in each image height on the to-be-scanned surface in the optical scanning device which concerns on 1st embodiment. FIG. 6 is a main scanning sectional view and a partial sub-scanning sectional view of an optical scanning device according to a second embodiment. The figure which showed the relationship between the partial magnification shift and image height in the optical scanning device concerning a second embodiment. The figure which showed the LSF spot diameter in each image height in the optical scanning device which concerns on 2nd embodiment. The figure which showed the exit side F number in each image height in the optical scanning device which concerns on 2nd embodiment. The figure which showed the opening part shape of the subscanning stop of the optical scanning device concerning 2nd embodiment. The figure which showed the LSF depth center position in each image height in the optical scanning device which concerns on 2nd embodiment. The figure which showed the relationship between the subscanning magnification deviation of the imaging optical system of the optical scanning device concerning 2nd embodiment, and image height. The figure which showed the spot profile in each image height on the to-be-scanned surface in the optical scanning device which concerns on 2nd embodiment. The main scanning sectional view and partial sub-scanning sectional view of the optical scanning device concerning a third embodiment. The figure which showed the sub-scanning horizontal magnification deviation in each image height of the imaging optical system of the optical scanning device concerning 3rd embodiment. The figure which showed the relationship between the partial magnification shift and image height in the optical scanning device which concerns on 3rd embodiment. The figure which showed the LSF spot diameter in each image height in the optical scanning device concerning 3rd embodiment. The figure which showed the exit side F number in each image height in the optical scanning device concerning 3rd embodiment. The figure which showed the LSF depth center position in each image height in the optical scanning device which concerns on 3rd embodiment. The figure which showed the spot profile in each image height on the to-be-scanned surface in the optical scanning device which concerns on 3rd embodiment. FIG. 6 is a sub-scan sectional view of a main part of a monochrome image forming apparatus on which the optical scanning device according to the first or third embodiment is mounted. FIG. 10 is a cross-sectional view of main parts of a color image forming apparatus on which an optical scanning device according to a second embodiment is mounted.

  Hereinafter, the optical scanning device according to the present embodiment will be described with reference to the drawings. It should be noted that the drawings shown below may be drawn at a scale different from the actual scale so that the present embodiment can be easily understood.

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

[First embodiment]
FIG. 1A shows a main scanning sectional view of the optical scanning device 100 according to the first embodiment. FIGS. 1B and 1C are partial sub-scan sectional views of the optical scanning device 100 according to the first embodiment.

  The optical scanning device 100 includes a light source 1, an anamorphic lens 2, a main scanning diaphragm 3, a deflector 4, a sub-scanning diaphragm 5, and an imaging lens (imaging optical element) 6.

As the light source 1, a semiconductor laser having a light emitting point is used. There may be a plurality of light emitting points.
The anamorphic lens 2 has positive refracting powers different from each other in the main scanning section and the sub-scanning section. The anamorphic lens 2 converts the light beam emitted from the light source 1 into a substantially parallel light beam and condenses it in the sub-scanning direction. Here, the substantially parallel light beam includes a weak divergent light beam, a weakly convergent light beam, and a parallel light beam.
The main scanning stop 3 has a rectangular opening, and restricts the diameter of the light beam that has passed through the anamorphic lens 2 in the main scanning direction. The width of the opening of the main scanning stop 3 in the sub-scanning direction is a rough aperture wider than the diameter of the light beam to be used.

In this way, the light beam emitted from the light source 1 is condensed only in the sub-scanning direction in the vicinity of the deflecting surface 4a of the deflector 4, and is formed as a long line image in the main scanning direction.
The anamorphic lens 2 and the main scanning diaphragm 3 constitute an incident optical system of the optical scanning device 100 according to the present embodiment.

The deflector 4 is reflected and deflected toward the scanned surface 7 by rotating in the direction of arrow A in the figure by a driving means such as a motor (not shown). For example, the deflector 4 is configured by a polygon mirror or the like.
The sub-scanning diaphragm 5 has an opening having a different opening width in the sub-scanning direction in accordance with each image height, and the light beam reflected and deflected by the deflector 4 is converted into a light beam in a desired sub-scanning direction at each image height. Shaped to be width.
Details of the shape of the opening of the sub-scanning diaphragm 5 will be described later.

The imaging lens 6 has two optical surfaces (lens surfaces), an incident surface (first surface) and an exit surface (second surface), and is reflected and deflected by the deflector 4 in the main scanning section. The light beam scans the surface to be scanned 7 with desired scanning characteristics. In the sub-scan section, the imaging lens 6 has a conjugate relationship between the vicinity of the deflecting surface 4a of the deflector 4 and the vicinity of the surface to be scanned 7, thereby compensating for surface tilt (the deflecting surface 4a is tilted). (Scanning position deviation in the sub-scanning direction on the surface to be scanned 7 is reduced).
The sub-scanning diaphragm 5 and the imaging lens 6 constitute an imaging optical system of the optical scanning device 100 according to this embodiment.

The light beam emitted from the light emitting point of the light source 1 is converted by the anamorphic lens 2 into a substantially parallel light beam in the main scanning section, and is substantially linear on the polarization plane of the deflector 4 in the sub-scanning section. Focused. The light beam that has passed through the anamorphic lens 2 has its light beam diameter in the main scanning direction limited by the main scanning diaphragm 3 and is incident on the deflection surface 4 a of the deflector 4.
The light beam emitted from the light source 1 and incident on the deflector 4 is reflected and deflected by the deflector 4, and then the light beam diameter in the sub-scanning direction is limited by the sub-scanning diaphragm 5. The light beam that has passed through the sub-scanning diaphragm 5 is guided and imaged on the scanned surface 7 by the imaging lens 6, and scans the scanned surface 7.
In this way, the imaging lens 6 forms a spot-like image in the vicinity of the surface to be scanned 7 in both the main scanning section and the sub-scanning section. Then, by rotating the deflector 4 at a constant speed and optically scanning the scanned surface 7 in the main scanning direction, an electrostatic latent image is formed on the scanned surface 7.

In the optical scanning device 100 according to the present embodiment, the anamorphic lens 2 is used, but a coupling lens and a cylindrical lens, which are two lenses having different functions, may be used instead.
Further, in the optical scanning device 100 according to the present embodiment, a rotary polygon mirror (polygon mirror) having four deflection surfaces is adopted as the deflector 4, but the number of deflection surfaces may be more than four. Absent.
In the optical scanning device 100 according to the present embodiment, the anamorphic lens 2 and the imaging lens 6 are plastic mold lenses formed by injection molding, but not limited thereto, glass mold lenses may be adopted. . Since the molded lens can be easily molded into an aspherical shape and is suitable for mass production, the productivity and optical performance can be improved.

Next, various characteristics of the optical scanning device 100 according to the present embodiment are shown in Table 1 below.
In Table 1, “E−x” means “× 10 ^−x ”.

  The incident surface of the anamorphic lens 2 of the optical scanning device 100 according to the present embodiment is a diffraction surface on which a diffraction grating is formed. The anamorphic lens 2 is formed by injection molding using a plastic material, and is a so-called temperature compensation optical system that compensates for changes in refractive power due to environmental fluctuations by changes in diffraction power due to wavelength changes of the semiconductor laser.

The incident diffractive surface of the anamorphic lens 2 is defined by a phase function represented by the following equation (1).
φ = 2πM / λ × (C3Z ² × C5Y ² ) (1)
Here, φ is a phase function, and M is a diffraction order. In this embodiment, first-order diffracted light (M = 1) is used. Further, λ is a design wavelength, and in this embodiment, λ = 790 nm.

The generatrix shape (the shape of the lens surface in the main scanning section) of the entrance surface and the exit surface of the imaging lens 6 of the optical scanning device 100 according to the present embodiment is up to the twelfth order as in the following equation (2). Aspherical shape that can be expressed as a function of
Here, the optical axis direction, the axis orthogonal to the optical axis in the main scanning section, and the axis orthogonal to the optical axis in the sub-scanning section when the intersection of each lens surface and the optical axis is the origin, X-axis, Y-axis, and Z-axis are used.
R is the radius of curvature of the bus, K is the eccentricity, and B _i (i = 4, 6, 8, 10, 12) is an aspheric coefficient.
Regarding y, the side on which the light source 1 of the optical scanning device 100 is disposed and the side on which the light source 1 is not disposed are defined as a plus side and a minus side, respectively. When the coefficient _Bi is different between the plus side and the minus side, as shown in Table 1, the suffix u is attached to the plus side coefficient (that is, _Biu ), and the suffix l is attached to the minus side coefficient. (Ie, B _il ). In this case, the bus bar shape is asymmetric in the main scanning direction.

Further, the sub-line shapes of the entrance surface and the exit surface of the imaging lens 6 of the optical scanning device 100 according to this embodiment (the shape of the lens surface in the sub-scanning section at an arbitrary image height) are expressed by the following formula (3 ).
Here, S is a child line shape defined in a plane perpendicular to the main scanning section including the normal line of the bus bar at each position in the bus bar direction, and m _jk is an aspheric coefficient.

Further, the sub-curvature radius of curvature r ′ continuously changes according to the y coordinate of the lens surface as shown in the following formula (4).
Here, r is a sub-wire curvature radius on the optical axis, and E _j (j = 2, 4, 6, 8, 10, 12) is a coefficient of change of the sub-wire curvature radius. If the coefficient E _j is different between the positive side and the negative side with respect to y, as shown in Table 1, the subscript u is added to the positive side coefficient (ie, E _ju ), and the subscript is added to the negative side coefficient. l is attached (ie, E _jl ). In this case, the child wire shape is asymmetric in the main scanning direction.

  In the present embodiment, the bus line shape and the child line shape of the lens surface of the imaging lens 6 are defined by the functions expressed by the equations (2) and (3), respectively. You may define it with a function.

The scanning characteristic of the imaging lens 6 of the optical scanning device 100 according to the present embodiment is expressed by the following formula (5).
Y = Kθ + αθ ³ (5)
Here, the scanning angle by the deflector 4 is θ, the condensing position (image height) in the main scanning direction on the scanned surface 7 of the light beam deflected at the scanning angle θ is Y, and the imaging coefficient at the on-axis image height. Is K.
In this embodiment, the on-axis image height corresponds to the image height on the optical axis (Y = 0), that is, corresponds to the scanning angle θ = 0. On the other hand, the most off-axis image height corresponds to the image height when the scanning angle θ is maximum (maximum scanning angle).

  Here, the imaging coefficient K is a coefficient corresponding to f at the scanning characteristic (fθ characteristic) Y = fθ when parallel light is incident on the imaging lens 6. In other words, the imaging coefficient K is a coefficient for making the condensing position Y and the scanning angle θ proportional to each other in the same manner as the fθ characteristic when a light beam other than parallel light enters the imaging lens 6. In the present embodiment, as shown in Table 1, K = 107.

  Further, α in Expression (5) is a coefficient for determining the scanning characteristic of the imaging lens 6 of the optical scanning device 100 according to the present embodiment (hereinafter referred to as a scanning characteristic coefficient). α = 12.5. Further, for example, when α is 0, Equation (5) can be expressed as Y = Kθ, and therefore corresponds to the scanning characteristic Y = fθ of the imaging lens used in the conventional optical scanning device that performs constant speed scanning. Will be.

Here, when the equation (5) is differentiated by the scanning angle θ, the scanning speed dY / dθ of the light beam on the surface to be scanned 7 with respect to the scanning angle θ is obtained as shown in the following equation (6).

Further, when the scanning speed dY / dθ = K at the axial image height is subtracted from the expression (6) and divided by the scanning speed dY / dθ = K at the axial image height, the following expression (7) is obtained.

Expression (7) is the ratio of the scanning speed shift amount at each image height to the axial image height and the scanning speed at the axial image height, that is, the portion at each image height with respect to the partial magnification at the axial image height. This represents the ratio (partial magnification deviation) between the amount of magnification deviation and the partial magnification at the on-axis image height.
In the optical scanning device 100 according to the present embodiment, α ≠ 0, so that the scanning speed of the light beam is different between the on-axis image height and the off-axis image height.
That is, since the scanning position (scanning distance per unit time) at the off-axis image height is extended depending on the partial magnification deviation, when the surface to be scanned is scanned without considering this partial magnification deviation, Deterioration of the image formed on the surface to be scanned (deterioration of printing performance) is caused.

  Therefore, in the optical scanning device 100 according to the present embodiment, a control unit (not shown) controls the modulation timing (light emission timing) of the light source 1 in accordance with the partial magnification deviation associated with α ≠ 0, and is electrically scanned. And the print width is corrected. As a result, it is possible to obtain the same printing performance as when constant speed is ensured.

  FIG. 2 shows the relationship between the partial magnification shift and the image height Y in the optical scanning device 100 according to the present embodiment. FIG. 2 shows a partial magnification deviation before electrical correction and a partial magnification deviation after electrical correction.

As shown in FIG. 2, in the optical scanning device 100 according to the present embodiment, the partial magnification deviation is electrically corrected by partially correcting the partial magnification deviation that occurs about 27% at the most off-axis image height. It can be seen that the height is 1% or less.
In addition, the light emission time when scanning an arbitrary width at the off-axis image height is shorter by the ratio of the speed than the on-axis image height, so that the image density becomes thinner at the off-axis image height. That is, density unevenness occurs.
Therefore, in the optical scanning device 100 according to the present embodiment, in order to correct density unevenness, a control unit (not shown) changes the light emission amount of the light source 1 between the speed at the off-axis image height and the speed at the on-axis image height. The amount of emitted light at the off-axis image height is controlled to be increased by the ratio. Thereby, the illuminance distribution is made uniform over the entire image height on the surface to be scanned.
In the optical scanning device 100 according to the present embodiment, by controlling the light emission timing and the light emission amount of the light source 1 as described above, the basic printing performance deterioration due to the non-constant speed is reduced.

  As described above, in the optical scanning device 100 according to this embodiment, the imaging lens 6 has scanning characteristics such that the light beam passing through the imaging lens 6 does not have constant velocity on the scanned surface 7. By providing the imaging lens 6 with such scanning characteristics, the imaging lens 6 can be disposed in the vicinity of the deflector 4, and the diameter of the imaging lens 6 can be reduced. Miniaturization is realized.

Here, although the deterioration of the printing performance due to the partial magnification deviation has been discussed, the deterioration of the printing performance due to the non-uniformity is not limited to this.
That is, as described above, even when the illuminance distribution is made substantially uniform over the entire image height on the surface to be scanned by electrical correction, the image density becomes uneven for the following reasons.

In a general image forming apparatus, when expressing a halftone, a linear pattern called a screen is formed on a recording material, and gradation is expressed by controlling the density of the pattern. .
A diagonal pattern having an angle of about 45 degrees with respect to the scanning direction is commonly used in this linear pattern screen. Since the line width of this pattern line is as narrow as 1 dot at the minimum, it depends on the size of the spot diameter for forming the latent image.
For this reason, when an inconstant speed optical scanning device is used as the optical scanning device, if an intermediate gradation image is to be formed, a main scanning spot is obtained with an on-axis image height and an off-axis image height even if electrical correction is performed. Due to the difference in the size of the diameter, the density of the image becomes uneven.

  Therefore, in the optical scanning device 100 according to the present embodiment, density unevenness of the intermediate gradation image due to the difference in the size of the main scanning spot diameter at the on-axis image height and the main scanning spot diameter at the off-axis image height is reduced. Therefore, the size of the sub-scanning spot diameter is made different between the on-axis image height and the off-axis image height.

FIG. 3 shows LSF (Line Spread Function) spot diameters in the main scanning direction and the sub-scanning direction at each image height in the optical scanning device 100 according to the present embodiment.
Here, the LSF spot diameter in the main scanning direction (main scanning LSF spot diameter) is a light amount profile obtained by integrating the spot profiles in the sub-scanning direction at each image height with respect to the maximum value of 13.5. It is the width when sliced at the% position. Further, the LSF spot diameter in the sub-scanning direction (sub-scanning LSF spot diameter) is a light amount profile obtained by integrating the spot profiles in the main scanning direction at each image height, which is 13.5% of the maximum value. It is the width when sliced at the position of.

As shown above, in the optical scanning device 100 according to the present embodiment, the partial magnification deviation is set to be 27% larger from the on-axis image height to the most off-axis image height, and thus is shown in FIG. As shown, the main scanning LSF spot diameter increases from the on-axis image height to the most off-axis image height.
In the optical scanning device 100 according to this embodiment, as shown in FIG. 3, the sub-scanning LSF spot diameter is used to cancel the image density unevenness due to the enlargement of the main-scanning LSF spot diameter at the off-axis image height. Is set so as to decrease from the on-axis image height to the most off-axis image height.

  FIG. 4 shows emission side F numbers in the main scanning direction and the sub-scanning direction at each image height in the optical scanning device 100 according to the present embodiment.

  In general, the image of the aperture stop is called an exit pupil, and a value obtained by dividing the distance from the rear focal point to the exit pupil by the exit pupil diameter is called an exit side F number of the optical scanning device. Hereinafter, the emission side F number of the optical scanning device in the main scanning section is called a main scanning emission side F number, and the emission side F number of the optical scanning device in the sub scanning section is called a sub scanning emission side F number.

  In the optical scanning device 100 according to the present embodiment, in order to make the spot diameter as shown in FIG. 3, the main scanning exit side F number becomes darker from the on-axis image height to the most off-axis image height. Each F number is set so that the F number on the sub-scanning emission side becomes brighter.

Here, the main scanning emission side F number at the on-axis image height is Fm0, the main scanning emission side F number at the most off-axis image height is Fm1, the sub-scanning emission side F number at the on-axis image height is Fs0, and the maximum. The sub-scanning exit F number at the off-axis image height is Fs1.
At this time, the optical scanning device 100 according to the present embodiment is designed to satisfy the following conditional expressions (8) and (9).
1.1 <Fm1 / Fm0 <1.4 (8)
0.8 <(Fm1 × Fs1) / (Fm0 × Fs0) <1.2 (9)

Fm1 / Fm0 in the conditional expression (8) is a ratio between the main scanning emission side F number at the most off-axis image height and the main scanning emission side F number at the on-axis image height, that is, the most off-axis image height. Is equivalent to the ratio of the partial magnification in the main scanning direction to the partial magnification in the main scanning direction at the axial image height.
When the imaging optical system is a constant speed scanning system, the partial magnification deviation is zero, so the partial magnification ratio is 1, that is, Fm1 / Fm0 is 1.
Therefore, the conditional expression (8) also defines that the optical scanning device 100 according to this embodiment is a non-constant speed scanning system.

If the upper limit value of the conditional expression (8) is exceeded, the partial magnification deviation becomes excessively large. Therefore, the amount of change in the main scanning direction of the sub-scanning emission side F-number necessary for correcting the partial magnification deviation is abrupt. Become. Therefore, the spot profile has a distorted shape and a good image cannot be obtained.
On the other hand, if the lower limit value of conditional expression (8) is not reached, the partial magnification deviation is not sufficient. Therefore, in order to obtain performance, the imaging lens 6 is increased in thickness, and the imaging lens 6 is removed from the deflector 4. The imaging lens 6 and hence the optical scanning device 100 cannot be reduced in size.
Therefore, in the optical scanning device 100 according to the present embodiment, by setting the imaging optical system so as to satisfy the conditional expression (8), the problem of image density unevenness caused by the non-constant speed scanning system is solved. The imaging lens 6 and thus the optical scanning device 100 can be reduced in size.

In conditional expression (9), Fs1 / Fs0 is a ratio between the sub-scanning emission side F number Fs1 at the most off-axis image height and the sub-scanning emission side F number Fs0 at the on-axis image height.
That is, the conditional expression (9) indicates the degree of cancellation by the sub-scanning emission side F number ratio with respect to the main scanning emission side F number ratio.
If Fs1 / Fs0 is just the reciprocal of Fm1 / Fm0, ie, (Fm1 × Fs1) / (Fm0 × Fs0) = 1, a screen that represents a halftone is generally used for scanning. If the linear pattern is inclined by 45 degrees with respect to the direction, the main scanning emission side F-number ratio can be completely canceled by the sub-scanning emission side F-number ratio, and the influence on the image density unevenness can be minimized.

If the upper limit of conditional expression (9) is exceeded, the amount of change in the sub-scanning spot diameter at the most off-axis image height with respect to the sub-scanning spot diameter at the on-axis image height is insufficient, and the main scanning spot diameter is enlarged off the axis. In particular, the influence on density unevenness caused by this cannot be reduced to a level where there is no problem.
On the other hand, below the lower limit value of conditional expression (9), the amount of change in the sub-scanning spot diameter at the most off-axis image height with respect to the sub-scanning spot diameter at the on-axis image height becomes excessively large, resulting in overcorrection. The on-axis image height is too dark.
Therefore, in the optical scanning device 100 according to this embodiment, the imaging optical system is designed so as to satisfy the conditional expression (9), thereby solving the problem of image density unevenness caused by the non-constant speed scanning system. The imaging lens 6 and thus the optical scanning device 100 can be reduced in size.

  In the optical scanning device 100 according to the present embodiment, Fm0 = 63.1, Fm1 = 81.5, Fs0 = 79.2, and Fs1 = 61.3. Therefore, the optical scanning device 100 according to the present embodiment satisfies Fm1 / Fm0 = 1.29, satisfies the conditional expression (8), and (Fm1 × Fs1) / (Fm0 × Fs0) = 1.00. Conditional expression (9) is satisfied.

Further, the optical scanning device 100 according to the present embodiment is designed to satisfy the following conditional expression (10).
1.05 <Fs0 / Fs1 <1.45 (10)

If the lower limit of conditional expression (10) is not reached, the amount of change in the sub-scanning spot diameter at the most off-axis image height with respect to the sub-scanning spot diameter at the on-axis image height is insufficient, and the main scanning spot diameter is enlarged off-axis. In particular, the influence on density unevenness due to the above cannot be reduced to a level where there is no problem.
On the other hand, when the upper limit of conditional expression (10) is exceeded, the amount of change in the sub-scanning spot diameter at the most off-axis image height with respect to the sub-scanning spot diameter at the on-axis image height becomes excessively large, resulting in overcorrection. The on-axis image height is too dark.
In the optical scanning device 100 according to this embodiment, since Fs0 / Fs1 = 1.29, the conditional expression (10) is satisfied. Therefore, since the imaging optical system is designed as described above, it is possible to reduce the size of the imaging lens 6 and thus the optical scanning device 100 while solving the problem of uneven image density caused by the non-constant scanning system. it can.

In the optical scanning device 100 according to the present embodiment, the sub-scanning diaphragm 5 is provided in the optical path of the imaging optical system so that the sub-scan emission side F-number satisfies the conditional expressions (8) and (9). Yes.
The sub-scanning diaphragm 5 has an opening having a different opening width in the sub-scanning direction in accordance with each image height, and the light beam reflected and deflected by the deflector 4 is converted into a light beam in a desired sub-scanning direction at each image height. Shaped to be width.

FIG. 1C shows a partial sub-scanning sectional view of the optical scanning device 100 according to the present embodiment.
The light beam reflected and deflected by the deflector 4 is incident on the sub-scanning diaphragm 5 with a light beam width wider than the opening width of the sub-scanning diaphragm 5 in the sub-scanning direction. Diameter is limited.
That is, in the optical scanning device 100 according to this embodiment, the sub-scanning exit side F number at each image height is defined by the sub-scanning diaphragm 5.

  FIG. 5 is a diagram showing the shape of the opening of the sub-scanning diaphragm 5 of the optical scanning device 100 according to this embodiment, and the sub-scanning diaphragm 5 is viewed from the optical axis direction of the imaging optical system.

As shown in FIG. 5, in the optical scanning device 100 according to the present embodiment, the opening of the sub-scanning diaphragm 5 is sub-scanned as it goes from the optical axis of the imaging optical system to the end along the main scanning direction. The opening width (diaphragm diameter) in the direction is increased.
Accordingly, as shown in FIG. 4, the sub-scanning emission side F number becomes brighter as it goes from the on-axis image height to the off-axis image height, and the change is opposite to the change of the main scanning emission side F-number. Can do.

Here, at the opening of the sub-scanning diaphragm 5, the aperture width in the sub-scanning direction at the passage position of the light beam that scans the axial image height is W0, and the sub-scan at the passage position of the light beam that scans the most off-axis image height. The opening width in the direction is W1, and the maximum scanning angle of the optical scanning device 100 is θ.
At this time, the optical scanning device 100 according to the present embodiment is designed to satisfy the following conditional expression (11).
1.0 <W1 / W0 × cos θ (11)

  If W1 / W0 × cos θ in the conditional expression (11) is 1, that is, if the sub-scan magnification of the imaging optical system is substantially uniform at the entire image height, the sub-scan exit side F number is uniform at the entire image height. It means to become.

Next, derivation of conditional expression (11) will be described.
Here, for explanation, as a virtual optical scanning system, a sub-scanning stop is provided in the incident optical system, and the sub-scanning magnification of the imaging optical system is uniform at all image heights. Let us consider a scanning optical system having a virtual surface at a position separated from the deflection point of the deflector by L0 in the optical axis direction.
In such a scanning optical system, since the sub-scanning magnification of the imaging optical system is uniform at the entire image height, the F-number on the sub-scanning exit side is uniform at the entire image height. A beam having a certain divergence in the sub-scanning direction is deflected and scanned from the deflector regardless of the image height, and the beam width in the sub-scanning direction at each image height on the virtual plane of the deflected and scanned beam is the deflection. It is proportional to the distance from the deflection point of the vessel to the virtual plane.

Here, if the distance along the optical axis direction from the deflection point of the deflector to the virtual plane is L0, and an arbitrary scanning angle is θi, the deflection point from the deflector to a certain image height on the virtual plane. The distance Li can be expressed by the following formula (12).
Li = L0 / cos θi (12)

Next, assuming that the on-axis image height of the light beam reaching the virtual surface and the light beam width in the sub-scanning direction at a certain image height are w0 and wi, respectively, the light beam width in the sub-scanning direction on the virtual surface is deflected. Since it is proportional to the distance from the deflection point of the device to the image height on the virtual plane, the following equation (13) is obtained.
wi = Li / L0 × w0 (13)

Here, when the equation (12) is substituted into the equation (13), the following equation (14) is obtained from wi = Li / L0 × w0 = (L0 / cos θi) / L0 × w0 = w0 / cos θi.
wi / w0 = 1 / cos θi (14)

  wi / w0 represents the ratio of the light beam width in the sub-scanning direction at a certain image height to the light beam width in the sub-scanning direction at the axial image height. That is, if the light beam width wi in the sub-scanning direction at the position of the scanning angle θi on the virtual surface is set to 1 / cos θi times the light beam width w0 in the sub-scanning direction at the axial image height on the virtual surface, This means that the F-number on the sub-scanning exit side is uniform at all image heights.

Here, the above virtual scanning optical system is replaced with the scanning optical system of the optical scanning device 100 according to the present embodiment.
In that case, the virtual plane is replaced with the sub-scanning diaphragm 5 of the optical scanning device 100 according to the present embodiment, and the beam width w0 in the sub-scanning direction at the axial image height is the axial image at the opening of the sub-scanning diaphragm 5. It is replaced with the opening width W0 in the sub-scanning direction at high. Further, the light beam width w1 in the sub-scanning direction at the most off-axis image height (that is, the position of the maximum scanning angle θ1) is the opening width W1 in the sub-scanning direction at the most off-axis image height at the opening of the sub-scanning diaphragm 5. Replaced.

At this time, Expression (14) can be replaced with the following Expression (15).
W1 / W0 = 1 / cos θ (15)

  Therefore, when the sub-scanning diaphragm 5 is provided in the imaging optical system, if the opening of the sub-scanning diaphragm 5 is designed so as to satisfy the expression (15), the sub-scanning emission side F-number is approximately the total image height. It becomes uniform.

On the other hand, as shown in FIG. 4, the optical scanning device 100 according to the present embodiment sets the sub-scanning emission side F number so that it becomes brighter from the on-axis image height toward the off-axis image height.
Therefore, the optical scanning device 100 according to the present embodiment is designed so as to satisfy the conditional expression (11).

Note that the optical scanning device 100 according to the present embodiment more preferably satisfies the following conditional expression (16) instead of the conditional expression (11).
1.05 <W1 / W0 × cos θ <1.4 (16)

If the lower limit of conditional expression (16) is not reached, the density unevenness caused by the enlargement of the main scanning spot diameter corresponding to the partial magnification deviation in the main scanning direction cannot be sufficiently canceled out, and a good image is obtained. Can not form.
On the other hand, if the upper limit of conditional expression (16) is exceeded, the light beam shape of the off-axis image height becomes excessively asymmetrical on the left and right, and the spot shape on the surface to be scanned becomes distorted to form a good image. I can't.
Therefore, the optical scanning device 100 according to the present embodiment can be designed to satisfy the conditional expression (16), thereby sufficiently reducing the density unevenness and forming a good image.

  Next, numerical values relating to the shape of the opening of the sub-scanning diaphragm 5 of the optical scanning device 100 according to the present embodiment shown in FIG.

As shown in Table 2, the shape of the opening of the sub-scanning diaphragm 5 of the optical scanning device 100 according to this embodiment is W0 = 0.75 mm when the on-axis image height Y = 0 mm, and the most off-axis image height. When Y = ± 107 mm (scanning angle θ = ± 52.23 degrees), W1 = 1.45 mm is set.
Therefore, W1 / W0 × cos θ = 1.17, which indicates that the optical scanning device 100 according to the present embodiment satisfies the conditional expressions (11) and (16).

Further, in the optical scanning device 100 according to the present embodiment, the position dependency of the upper and lower end portions in the sub-scanning direction of the opening of the sub-scanning diaphragm 5 in the main scanning direction can be expressed by the following fourth-order polynomial (17). it can.
Here, P is a sub-scanning direction end (upper limit end, lower limit end) of the opening of the sub-scanning stop 5 at each position Y in the main scanning direction when the optical axis of the imaging optical system is Y = 0. ) In the sub-scanning direction, and A ₀ , A ₁ , A ₂ , A ₃ , and A ₄ are coefficients.

Table 3 below shows the coefficients A _{0 to} A ₄ of Expression (17) in the optical scanning device 100 according to the present embodiment. In Table 3, “E−x” means “× 10 ^−x ”.

As can be seen from Table 3, in the optical scanning device 100 according to the present embodiment, the coefficients A1 and A3 of the odd-order terms of Y are not zero, that is, the shape of the opening of the sub-scanning diaphragm 5 is that of the imaging optical system. It is designed asymmetrically on the left and right in the main scanning direction with respect to the optical axis Y = 0.
In the optical scanning device 100 according to this embodiment, the light beam emitted from the light source 1 is incident on the deflector 4 at an angle with respect to the optical axis of the imaging optical system in the main scanning section. For this reason, the position of the deflection point at the left and right scanning angles is asymmetric.
That is, in the optical scanning device 100 according to the present embodiment, in order to cancel the influence of the asymmetry of the position of the deflection point, the shape of the opening of the sub-scanning diaphragm 5 is designed asymmetrically so that the left and right of the image height can be changed. The sub-scan emission side F-numbers are made substantially equal.
Therefore, in the optical scanning device 100 according to the present embodiment, a density difference does not appear on the left and right of the image height, and a good image can be formed.

FIG. 6 shows the main scanning LSF depth center position and the sub-scanning LSF depth center position at each image height in the optical scanning device 100 according to the present embodiment.
Here, the main scanning LSF depth center position means the central position of a region where the main scanning LSF spot diameter is 95 μm or less when defocused in the optical axis direction of the imaging optical system in the vicinity of the surface to be scanned. Yes. The sub-scanning LSF depth center position means the center position of a region where the sub-scanning LSF spot diameter is 100 μm or less when defocused in the optical axis direction of the imaging optical system in the vicinity of the surface to be scanned. .

  As can be seen from FIG. 6, at any image height, the main scanning LSF depth center position and the sub-scanning LSF depth center position are both within a range of ± 2 mm, and the optical scanning device 100 according to the present embodiment is good. It can be seen that the image surface performance is excellent.

FIG. 7 shows the relationship between the sub-scanning magnification shift of the imaging optical system of the optical scanning apparatus 100 according to this embodiment and the image height.
Here, the sub-scanning magnification deviation is defined as a ratio between the sub-scan magnification deviation amount of each image height with respect to the sub-scan magnification at the axial image height and the sub-scan magnification at the axial image height.

As shown in FIG. 7, it can be seen that the sub-scanning magnification deviation is sufficiently small within a range of ± 2% at any image height.
Therefore, in the optical scanning device 100 according to the present embodiment, the sub-scanning magnification deviation is suppressed to be sufficiently small over the entire image height, so that when the multi-beam is formed, the sub-scan between the multi-beams on the scanning surface 7 is performed. It is possible to keep the direction interval at a level where there is no problem with the total image height.

FIG. 8 shows spot profiles at image heights Y = 0 mm, ± 50 mm, and ± 107 mm on the scanned surface 7 in the optical scanning device 100 according to the present embodiment.
FIG. 8 shows a spot profile when defocused by X = 0 mm and ± 1 mm from the scanned surface 7 in the optical axis direction of the imaging optical system.
Here, the contour lines of each spot profile are set at respective positions of 50%, 13.5%, 5%, and 2% with respect to the peak peak light quantity maximum value.

  In general, there is a problem when side lobes are generated at 13.5% or more. However, in the optical scanning device 100 according to the present embodiment, as shown in FIG. It can be seen that a spot profile is obtained.

If the exit F number changes rapidly as it goes from the on-axis image height to the off-axis image height, the spot profile at the off-axis image height becomes a distorted shape (for example, a triangle), and a good image can be formed. Can not.
Therefore, the optical scanning device 100 according to the present embodiment is designed to satisfy the conditional expressions (8), (9), (10), and (11), thereby reducing the size of the imaging lens. However, image density unevenness can be suppressed and a good spot profile can be obtained.

Referring to FIG. 2 again, when the value of the partial magnification deviation in the main scanning direction that is the maximum at the entire image height is defined as ΔY (%), the optical scanning device 100 according to the present embodiment has the following conditional expression (18 ) Is designed to meet.
10% <ΔY <40% (18)

If the upper limit value of the conditional expression (18) is exceeded, the partial magnification deviation in the main scanning direction is too large, so that the amount of change in the sub-scanning emission side F number necessary to cancel it becomes excessively large, and the spot profile becomes large. It becomes a distorted shape and a good image cannot be obtained.
On the other hand, if the lower limit value of conditional expression (18) is not reached, the partial magnification deviation in the main scanning direction is not sufficient, so that the thickness of the imaging lens 6 is increased or the imaging lens 6 is deflected for performance. Therefore, it is difficult to reduce the size of the imaging lens 6 and thus the optical scanning device 100.

Therefore, the optical scanning device 100 according to the present embodiment is designed so as to satisfy the conditional expression (18), thereby solving the problem of uneven image density due to the non-constant speed scanning system, and thus the imaging lens 6. The optical scanning device 100 can be reduced in size.
In the optical scanning device 100 according to the present embodiment, as shown in FIG. 2, the partial magnification deviation in the main scanning direction at the most off-axis image height is the largest at 27% (that is, ΔY = 27%). It can be seen that equation (18) is satisfied.

In the optical scanning device 100 according to the present embodiment, the partial magnification deviation increases as a quadratic function from the on-axis image height to the off-axis image height. By setting the partial magnification deviation to such a simple characteristic, a coefficient necessary for electrical correction is simplified, and the cost of the electrical correction circuit is reduced.
However, the present embodiment is not limited to this, and even if the partial magnification deviation characteristic is a linear function or a function of cubic or higher, the effect according to the present embodiment can be sufficiently obtained.

  In the optical scanning device 100 according to the present embodiment, the shape of the opening end portion of the sub-scanning diaphragm 5 is a continuous shape expressed by a fourth-order polynomial. The effect of this embodiment can be obtained even when the shape of the end portion is discontinuous.

If the optical scanning device adopts an OFS (Over Filled Scan) system in which the main scanning direction width of the incident light beam is wider than the main scanning direction width of the deflecting surface of the deflector, such an optical scanning device is used. Even when this embodiment is adopted, density unevenness is further deteriorated due to a light amount drop at the most off-axis image height unique to the OFS system. In addition to this, even if electrical correction is performed, the amount of laser light emission is insufficient.
Therefore, the optical scanning device 100 according to the present embodiment employs a UFS (Under Filled Scan) system in which the width of the incident light beam in the main scanning direction is narrower than the width of the deflecting surface 4a of the deflector 4 in the main scanning direction. Therefore, a good image can be obtained by setting only the non-constant scanning property as a factor affecting density unevenness.

  In the optical scanning device 100 according to the present embodiment, the sub-scanning diaphragm 5 is disposed between the imaging lens 6 and the deflector 4 in order to simplify the shape of the opening of the sub-scanning diaphragm 5. However, the present embodiment is not limited to this, and the effect of the present embodiment can be achieved even if the sub-scanning diaphragm 5 is disposed behind the imaging lens 6 as long as the sub-scanning emission side F-number satisfies the above-described conditions. Can be obtained.

  Further, in the optical scanning device 100 according to the present embodiment, the imaging lens of the imaging optical system is configured by only one imaging lens 6 in order to achieve cost reduction of the imaging optical system. . However, the present embodiment is not limited to this, and the effect of the present embodiment can be obtained even if the imaging optical system is configured by a plurality of imaging lenses.

  Further, in the optical scanning device 100 according to the present embodiment, the sub-scanning diaphragm 5 is provided in the imaging optical system, and the on-axis image height and off-axis are adjusted by adjusting the shape of the opening of the sub-scanning diaphragm 5. The F-number on the sub-scanning exit side is varied depending on the image height. However, the present embodiment is not limited to this. If the exit-side F number at each image height is set as described above, the effect of the present embodiment can be obtained even if the sub-scanning diaphragm 5 is provided in the incident optical system. Can be obtained.

  Further, in the optical scanning device 100 according to the present embodiment, the main scanning diaphragm 3 and the sub scanning diaphragm 5 are provided, but the present invention is not limited thereto, and the light beam diameters in the main scanning direction and the sub scanning direction may be limited. Only one possible aperture may be provided.

  As described above, according to the present embodiment, it is possible to obtain an optical scanning device that can achieve cost reduction and downsizing while ensuring good imaging performance and printing performance.

[Second Embodiment]
FIG. 9A shows a main scanning sectional view of the optical scanning device 200 according to the second embodiment. FIGS. 9B and 9C are partial sub-scan sectional views of the optical scanning device 200 according to the second embodiment.

The optical scanning device 200 includes light sources 1a and 1b, anamorphic lenses 2a and 2b, main scanning diaphragms 3a and 3b, a deflector 4, sub-scanning diaphragms 5a and 5b, and imaging lenses 6a and 6b.
Since each member is the same as the member used in the optical scanning device 100 according to the first embodiment, the same reference numeral is assigned and description thereof is omitted.
Unlike the optical scanning device 100 according to the first embodiment, the optical scanning device 200 employs a double-sided scanning optical system that optically scans the scanned surfaces 7a and 7b disposed on both sides of the deflector 4. ing.

The anamorphic lens 2a and the main scanning stop 3a constitute a first incident optical system of the optical scanning device 200 according to the present embodiment.
The anamorphic lens 2b and the main scanning stop 3b constitute a second incident optical system of the optical scanning device 200 according to the present embodiment.
Further, the sub-scanning diaphragm 5a and the imaging lens 6a constitute a first imaging optical system of the optical scanning device 200 according to the present embodiment.
Further, the sub-scanning diaphragm 5b and the imaging lens 6b constitute a second imaging optical system of the optical scanning device 200 according to the present embodiment.
The light source 1a, the first incident optical system, the deflector 4, and the first imaging optical system constitute the first scanning optical system 200a of the optical scanning device 200 according to the present embodiment.
Further, the light source 1b, the second incident optical system, the deflector 4, and the second imaging optical system constitute a second scanning optical system 200b of the optical scanning device 200 according to the present embodiment.

The light beam (first light beam) emitted from the light emitting point of the light source 1a is converted into a substantially parallel light beam in the main scanning section by the anamorphic lens 2a, and the deflecting surface 4a (first) of the deflector 4 in the sub-scanning section. (1 deflection surface) is condensed so as to be substantially linear. The light beam that has passed through the anamorphic lens 2 a is incident on the deflection surface 4 a of the deflector 4 with the light beam diameter in the main scanning direction being limited by the main scanning diaphragm 3 a.
The light beam emitted from the light source 1a and incident on the deflecting surface 4a of the deflector 4 is reflected and deflected by the deflecting surface 4a of the deflector 4, and then the light beam diameter in the sub-scanning direction is limited by the sub-scanning diaphragm 5a. . The light beam that has passed through the sub-scanning diaphragm 5a is guided and imaged onto the surface to be scanned (first surface to be scanned) 7a by the imaging lens 6a, and scans the surface to be scanned 7a.
In this manner, a spot-like image is formed in the vicinity of the scanned surface 7a by the imaging lens 6a in both the main scanning section and the sub-scanning section. Then, by rotating the deflector 4 at a constant speed and optically scanning the scanned surface 7a in the main scanning direction, an electrostatic latent image is formed on the scanned surface 7a.

The light beam (second light beam) emitted from the light emitting point of the light source 1b is converted into a substantially parallel light beam in the main scanning section by the anamorphic lens 2b, and the deflecting surface 4b (first) of the deflector 4 in the sub-scanning section. 2), the light is condensed so as to be substantially linear. The light beam that has passed through the anamorphic lens 2 b has its light beam diameter in the main scanning direction limited by the main scanning stop 3 b and is incident on the deflection surface 4 b of the deflector 4.
The light beam emitted from the light source 1b and incident on the deflecting surface 4b of the deflector 4 is reflected and deflected by the deflecting surface 4b of the deflector 4, and then the light beam diameter in the sub-scanning direction is limited by the sub-scanning diaphragm 5b. . The light beam that has passed through the sub-scanning diaphragm 5b is guided and imaged on the surface to be scanned (second surface to be scanned) 7b by the imaging lens 6b, and scans the surface to be scanned 7b.
In this manner, a spot-like image is formed in the vicinity of the scanned surface 7b in both the main scanning section and the sub-scanning section by the imaging lens 6b. Then, by rotating the deflector 4 at a constant speed and optically scanning the scanned surface 7b in the main scanning direction, an electrostatic latent image is formed on the scanned surface 7b.

Table 4 shows various characteristics of the scanning optical system 200a of the optical scanning device 200 according to the present embodiment.
Table 5 shows numerical values related to the shape of the opening of the sub-scanning diaphragm 5a of the scanning optical system 200a of the optical scanning device 200 according to the present embodiment.
Table 6 shows coefficients A _{0 to} A ₄ of Expression (17) in the sub-scanning diaphragm 5a of the scanning optical system 200a of the optical scanning device 200 according to the present embodiment.
Table 7 shows various characteristics of the scanning optical system 200b of the optical scanning device 200 according to the present embodiment.
Table 8 shows numerical values related to the shape of the opening of the sub-scanning diaphragm 5b of the scanning optical system 200b of the optical scanning device 200 according to the present embodiment.
Table 9 shows coefficients A _{0 to} A ₄ of Expression (17) in the sub-scanning diaphragm 5b of the scanning optical system 200b of the optical scanning device 200 according to the present embodiment.
In Tables 4, 6, 7, and 9, “E-x” means “× 10 ^−x ”.

FIG. 10A shows the relationship between the partial magnification deviation and the image height Y in the scanning optical system 200a of the optical scanning device 200 according to this embodiment. FIG. 10A shows a partial magnification deviation before electrical correction and a partial magnification deviation after electrical correction.
FIG. 10B shows the relationship between the partial magnification deviation and the image height Y in the scanning optical system 200b of the optical scanning device 200 according to this embodiment. FIG. 10B shows the partial magnification deviation before electrical correction and the partial magnification deviation after electrical correction.

As shown in FIG. 10A, in the scanning optical system 200a of the optical scanning apparatus 200 according to the present embodiment, the partial magnification deviation that occurs about 27% at the most off-axis image height is electrically corrected. Thus, it can be seen that the partial magnification deviation is 1% or less in the total image height.
Further, as shown in FIG. 10B, in the scanning optical system 200b of the optical scanning apparatus 200 according to the present embodiment, the partial magnification deviation that occurs about 27% at the most off-axis image height is electrically corrected. By doing so, it can be seen that the partial magnification deviation is 1% or less in the total image height.

FIG. 11A shows LSF spot diameters in the main scanning direction and the sub-scanning direction at each image height in the scanning optical system 200a of the optical scanning device 200 according to this embodiment.
FIG. 11B shows LSF spot diameters in the main scanning direction and the sub-scanning direction at each image height in the scanning optical system 200b of the optical scanning device 200 according to this embodiment.

In the scanning optical systems 200a and 200b of the optical scanning device 200 according to the present embodiment, the partial magnification deviation is set to be 27% larger from the on-axis image height to the most off-axis image height. Therefore, as shown in FIGS. 11A and 11B, the main scanning LSF spot diameter also increases from the on-axis image height to the most off-axis image height.
In the scanning optical systems 200a and 200b of the optical scanning device 200 according to the present embodiment, in order to cancel out the image density unevenness due to the enlargement of the main scanning LSF spot diameter at the off-axis image height, FIG. ), The sub-scanning LSF spot diameter is set to decrease from the on-axis image height to the most off-axis image height.
However, it should be noted that the amount of change in the sub-scanning LSF spot diameter differs between the scanning optical systems 200a and 200b.
The reason for this will be described below.

In the present embodiment, a photosensitive drum surface corresponding to cyan is disposed as the scanned surface 7a, and a photosensitive drum surface corresponding to black is disposed as the scanned surface 7b.
In the optical scanning device 200 according to the present embodiment, a scanning optical system and a surface to be scanned (not shown) corresponding to yellow and magenta are also provided in order to form a color image, but description thereof is omitted. To do.

In the optical scanning device 200 according to the present embodiment, the screen pattern used for intermediate gradation expression is different for each color.
Specifically, in monochrome, the angle of the screen line expressing the intermediate gradation is about 45 degrees, whereas in other colors such as cyan, the screen line angle is small.
The smaller the screen line angle, the less the influence on the image density unevenness due to the difference in the size of the main scanning spot diameter between the on-axis image height and the off-axis image height. This tends to affect the image density unevenness.
Therefore, the optical scanning device 200 according to the present embodiment has a scanning optical system that optically scans the photosensitive drum surface 7b corresponding to black, as compared with the scanning optical system 200a that optically scans the photosensitive drum surface 7a corresponding to cyan. The correction effect at 200b is set to be strong.

Therefore, as shown in FIG. 11A, in the scanning optical system 200a that optically scans the photosensitive drum surface 7a corresponding to cyan, the most off-axis image is obtained from the on-axis image height of the sub-scanning LSF spot diameter. The change over high can be made relatively small.
On the other hand, as shown in FIG. 11B, in the scanning optical system 200b that optically scans the photosensitive drum surface 7b corresponding to black, the most off-axis from the on-axis image height of the sub-scanning LSF spot diameter. The change over image height is relatively large.

FIG. 12A shows emission side F numbers in the main scanning direction and the sub-scanning direction at each image height in the scanning optical system 200a of the optical scanning device 200 according to the present embodiment.
FIG. 12B shows emission side F numbers in the main scanning direction and the sub-scanning direction at each image height in the scanning optical system 200b of the optical scanning device 200 according to the present embodiment.

In the scanning optical systems 200a and 200b of the optical scanning device 200 according to the present embodiment, in order to make the spot diameter have the relationship as shown in FIGS. Each F number is set so that the sub-scanning emission side F number becomes brighter as the main scanning emission side F-number becomes darker toward the image height.
As shown in FIG. 12A, in the scanning optical system 200a that optically scans the photosensitive drum surface 7a corresponding to cyan, the most off-axis from the on-axis image height of the F-number on the sub-scanning emission side. It can be seen that the change over the image height is relatively small.
On the other hand, as shown in FIG. 12 (b), in the scanning optical system 200b that optically scans the photosensitive drum surface 7b corresponding to black, the highest axis from the on-axis image height of the F-number on the sub-scanning emission side. It can be seen that the change toward the external image height is relatively large.

FIG. 13A is a diagram showing the shape of the opening of the sub-scanning diaphragm 5a of the scanning optical system 200a of the optical scanning device 200 according to the present embodiment, and the sub-scanning diaphragm 5a from the optical axis direction of the imaging optical system. It is what saw.
FIG. 13B is a diagram showing the shape of the opening of the sub-scanning diaphragm 5b of the scanning optical system 200b of the optical scanning device 200 according to this embodiment, and the sub-scanning is performed from the optical axis direction of the imaging optical system. The diaphragm 5b is seen.

As shown in FIGS. 13A and 13B, in the scanning optical systems 200a and 200b of the optical scanning device 200 according to this embodiment, the openings of the sub-scanning apertures 5a and 5b are light beams of the imaging optical system. The aperture width (diaphragm diameter) in the sub-scanning direction increases as it goes from the axis to the end along the main scanning direction.
Thereby, as shown in FIGS. 12A and 12B, the sub-scanning emission side F number becomes brighter as it goes from the on-axis image height to the off-axis image height. Can be the opposite change.
Further, as described above, in order to make the sub-scanning emission side F number change different between the scanning optical system 200a and the scanning optical system 200b, FIGS. 13 (a) and 13 (b) and Tables 5 and 8 show. Thus, the opening shapes of the sub-scanning diaphragms 5a and 5b are not the same.

FIG. 14A shows the main scanning LSF depth center position and the sub-scanning LSF depth center position at each image height in the scanning optical system 200a of the optical scanning apparatus 200 according to the present embodiment.
FIG. 14B shows the main scanning LSF depth center position and the sub-scanning LSF depth center position at each image height in the scanning optical system 200b of the optical scanning device 200 according to the present embodiment.

  As can be seen from FIGS. 14A and 14B, at any image height, the main scanning LSF depth center position and the sub-scanning LSF depth center position are both within a range of ± 2 mm. It can be seen that the optical scanning device 200 has good image surface performance.

FIG. 15A shows the relationship between the sub-scanning magnification shift of the imaging optical system of the scanning optical system 200a of the optical scanning device 200 according to this embodiment and the image height.
FIG. 15B shows the relationship between the sub-scanning magnification deviation of the imaging optical system of the scanning optical system 200b of the optical scanning device 200 according to this embodiment and the image height.

As shown in FIGS. 15 (a) and 15 (b), it can be seen that the sub-scan magnification deviation is sufficiently small within a range of ± 2% at any image height.
Therefore, in the optical scanning device 200 according to the present embodiment, the sub-scanning magnification deviation is suppressed to be sufficiently small in the entire image height, and therefore when the multi-beam is formed, the sub-scanning direction between the multi-beams on the surface to be scanned. The interval can be kept at a level where there is no problem with the total image height.

FIG. 16A shows spot profiles at image heights Y = 0 mm, ± 50 mm, and ± 107 mm on the scanned surface 7 a in the scanning optical system 200 a of the optical scanning device 200 according to the present embodiment. FIG. 16A shows a spot profile when defocusing is performed by X = 0 mm and ± 1 mm from the scanned surface 7 a in the optical axis direction of the imaging optical system.
FIG. 16B shows spot profiles at image heights Y = 0 mm, ± 50 mm, and ± 107 mm on the scanned surface 7 b in the scanning optical system 200 b of the optical scanning device 200 according to the present embodiment. . FIG. 16B shows a spot profile when defocusing is performed by X = 0 mm and ± 1 mm from the scanned surface 7 b in the optical axis direction of the imaging optical system.
Here, the contour lines of each spot profile are set at respective positions of 50%, 13.5%, 5%, and 2% with respect to the peak peak light quantity maximum value.

  As shown in FIGS. 16A and 16B, it can be seen that in the optical scanning device 200 according to the present embodiment, a good spot profile with few side lobes is obtained at each image height.

  Next, the relationship between various numerical values and the conditional expression in the optical scanning device 200 according to the present embodiment will be described.

In the scanning optical system 200a of the optical scanning device 200 according to the present embodiment, Fm0 = 78.1, Fm1 = 100.9, Fs0 = 73.8, and Fs1 = 61.3.
Therefore, Fm1 / Fm0 = 1.29, and the scanning optical system 200a of the optical scanning device 200 according to this embodiment satisfies the conditional expression (8).
Further, (Fm1 × Fs1) / (Fm0 × Fs0) = 1.07 and Fs0 / Fs1 = 1.20, and the scanning optical system 200a of the optical scanning device 200 according to the present embodiment has conditional expressions (9) and (9) 10) is satisfied.

On the other hand, in the scanning optical system 200b of the optical scanning device 200 according to the present embodiment, Fm0 = 78.1, Fm1 = 100.9, Fs0 = 88.1, and Fs1 = 61.3.
Therefore, Fm1 / Fm0 = 1.29, and the scanning optical system 200b of the optical scanning device 200 according to this embodiment satisfies the conditional expression (18).
Further, (Fm1 × Fs1) / (Fm0 × Fs0) = 0.899 and Fs0 / Fs1 = 1.44, and the scanning optical system 200b of the optical scanning device 200 according to this embodiment has conditional expressions (9) and (9) 10) is satisfied.

  As described above, the optical scanning device 200 according to this embodiment satisfies the conditional expressions (8), (9), and (10). Further, in the scanning optical systems 200a and 200b, the ratio between the sub-scanning emission side F-number with the on-axis image height and the sub-scanning emission side F-number with the most off-axis image height is different from each other, so that each can be optimally downsized. The printing performance is compatible.

Further, in the scanning optical system 200a of the optical scanning apparatus 200 according to the present embodiment, when the on-axis image height Y = 0 mm, W0 = 0.81 mm, the most off-axis image height Y = ± 107 mm (scanning angle θ = ± 52.23). Degree), W1 = 1.45 mm.
Therefore, W1 / W0 × cos θ = 1.10, and the scanning optical system 200a of the optical scanning device 200 according to this embodiment satisfies the conditional expressions (11) and (16).

Further, in the scanning optical system 200b of the optical scanning device 200 according to the present embodiment, when the on-axis image height Y = 0 mm, W0 = 0.68 mm, the most off-axis image height Y = ± 107 mm (scanning angle θ = ± 52.23). Degree), W1 = 1.45 mm.
Accordingly, W1 / W0 × cos θ = 1.31, and the scanning optical system 200b of the optical scanning device 200 according to this embodiment satisfies the conditional expressions (11) and (16).

  As described above, in the optical scanning device 200 according to the present embodiment, the scanning optical systems 200a and 200b both satisfy the conditional expressions (11) and (16). Further, by making the aperture width and the aperture shape different from each other in the sub-scanning direction, it is possible to optimally reduce the size of the imaging lens, respectively, and achieve both good image formation without image density unevenness.

  Further, in the scanning optical systems 200a and 200b of the optical scanning device 200 according to the present embodiment, the sub-scanning direction opening width W0 at the axial image height of the sub-scanning diaphragms 5a and 5b is relatively narrow. As a result, it is also possible to obtain an effect of further blocking the ghost light that passes through the vicinity of the optical axis that is a particular problem and enters the other scanning optical system from one scanning optical system.

  In the optical scanning device 200 according to the present embodiment, a double-sided scanning optical system sharing a deflector is adopted for cost reduction, but the present embodiment is not limited to this. That is, if a plurality of optical scanning devices each having a sub-scanning aperture having different opening shapes are used, the effect of this embodiment can be sufficiently obtained.

[Third embodiment]
FIGS. 17A and 17B respectively show a main scanning sectional view and a partial sub-scanning sectional view of an optical scanning device 300 according to the third embodiment.

  In the optical scanning device 300 according to the third embodiment, only the stop 30 is provided in the incident optical system instead of the main scanning stop 3 and the sub-scanning stop 5, and the coupling lens 8 is used instead of the anamorphic lens 2. And the anamorphic lens 9 is different from the optical scanning device 100 according to the first embodiment. Other members are designated by the same reference numerals as those of the optical scanning device 100 according to the first embodiment, and description thereof is omitted.

  The optical scanning device 300 includes a light source 1, a diaphragm 30, a coupling lens 8, an anamorphic lens 9, a deflector 4, and an imaging lens 6.

The diaphragm 30 limits the diameter of the light beam emitted from the light source 1 in the main scanning direction and the sub-scanning direction.
The coupling lens 8 is made of glass, and converts the light beam that has passed through the diaphragm 30 into a substantially parallel light beam. Here, the substantially parallel light beam includes a weak divergent light beam, a weakly convergent light beam, and a parallel light beam.
The anamorphic lens 9 is made of glass and condenses the light beam that has passed through the coupling lens 8 in the sub-scanning direction.
The diaphragm 30, the coupling lens 8, and the anamorphic lens 9 constitute an incident optical system of the optical scanning device 300 according to the present embodiment.
Further, the imaging lens 6 constitutes an imaging optical system of the optical scanning device 300 according to the present embodiment.

The luminous flux emitted from the light emitting point of the light source 1 is limited by the diaphragm 30 in the main scanning direction and the sub-scanning direction. The light beam that has passed through the diaphragm 30 is converted into a substantially parallel light beam by the coupling lens 8, is condensed in the sub-scanning direction by the anamorphic lens 9, and is incident on the deflection surface 4 a of the deflector 4.
Then, the light beam emitted from the light source 1 and incident on the deflection surface 4 a of the deflector 4 is reflected and deflected by the deflection surface 4 a of the deflector 4, and then guided and connected to the scanned surface 7 by the imaging lens 6. The scanned surface 7a is scanned.
In this way, the imaging lens 6 forms a spot-like image in the vicinity of the surface to be scanned 7 in both the main scanning section and the sub-scanning section. Then, by rotating the deflector 4 at a constant speed and optically scanning the scanned surface 7 in the main scanning direction, an electrostatic latent image is formed on the scanned surface 7.

Table 10 shows various characteristics of the optical scanning device 300 according to the present embodiment.
In Table 10, “E−x” means “× 10 ^−x ”.

In order to change the sub-scanning emission side F number at each image height, in the optical scanning devices 100 and 200 according to the first and second embodiments, the shape of the opening of the sub-scanning diaphragm 5 is determined in the sub-scanning direction at each image height. The opening width of was changed.
On the other hand, in the optical scanning device 300 according to the present embodiment, the sub-scanning magnification of the imaging lens 6 is changed at each image height in order to change the sub-scan emission side F number at each image height.

FIG. 18 shows a sub-scanning lateral magnification shift at each image height of the imaging optical system of the optical scanning device 300 according to the present embodiment.
Here, the sub-scanning lateral magnification deviation is the ratio of the amount of deviation of the sub-scanning lateral magnification at each image height to the sub-scanning lateral magnification at the on-axis image height and the sub-scanning lateral magnification at the on-axis image height. I mean.

  As shown in FIG. 18, the optical scanning device 300 according to the present embodiment is set so that the sub-scanning lateral magnification decreases as the axial image height increases from the on-axis image height.

Here, when the sub-scanning lateral magnification of the imaging optical system at the on-axis image height is βs0, and the sub-scanning lateral magnification of the imaging optical system at the most off-axis image height is βs1, the optical scanning device 300 according to the present embodiment. Is set so as to satisfy the following conditional expression (19).
1.05 <| βs0 / βs1 | <1.4 (19)

If the upper limit value of the conditional expression (19) is exceeded, the non-uniformity of the sub-scanning lateral magnification is too large, and the change in the image height of the sub-scanning exit side F-number becomes too abrupt and the spot profile is distorted. Thus, a good image cannot be obtained. In addition, another problem of image quality deterioration that the interval unevenness of the scanning lines on the surface to be scanned 7 becomes large also occurs.
On the other hand, if the lower limit of conditional expression (19) is not reached, the partial magnification deviation is not sufficient, so that in order to obtain performance, the imaging lens 6 becomes thicker or the imaging lens 6 is removed from the deflector. The imaging lens 6 and thus the optical scanning device 300 cannot be reduced in size.

In the optical scanning device 300 according to the present embodiment, βs0 = −3.5 and βs1 = −3.3, and thus | βs0 / βs1 | = 1.06, and the optical scanning device 300 according to the present embodiment It is set to satisfy the conditional expression (19).
Therefore, the optical scanning device 300 according to the present embodiment is designed to satisfy the conditional expression (19), thereby solving the problem of image density unevenness due to the non-constant scanning system, and the imaging lens 6. As a result, the optical scanning device 300 can be reduced in size.

  FIG. 19 shows the relationship between the partial magnification shift and the image height Y in the optical scanning device 300 according to this embodiment. FIG. 19 shows a partial magnification deviation before electrical correction and a partial magnification deviation after electrical correction.

  As shown in FIG. 19, in the optical scanning device 300 according to the present embodiment, the partial magnification deviation is electrically corrected by partially correcting the partial magnification deviation that occurs about 11% at the most off-axis image height. It can be seen that the height is 1% or less.

  FIG. 20 shows LSF spot diameters in the main scanning direction and the sub-scanning direction at each image height in the optical scanning device 300 according to the present embodiment.

In the optical scanning device 300 according to the present embodiment, since the partial magnification deviation is set to be 11% larger from the on-axis image height to the most off-axis image height, as shown in FIG. The LSF spot diameter also increases from the on-axis image height to the most off-axis image height.
In the optical scanning device 300 according to the present embodiment, in order to cancel out the uneven image density due to the enlargement of the main scanning LSF spot diameter at the off-axis image height, as shown in FIG. Is set so as to decrease from the on-axis image height to the most off-axis image height.

  FIG. 21 shows emission side F numbers in the main scanning direction and the sub-scanning direction at each image height in the optical scanning device 300 according to the present embodiment.

  In the optical scanning device 200 according to the present embodiment, in order to make the spot diameter as shown in FIG. 20, the main scanning exit side F number becomes darker from the on-axis image height to the most off-axis image height. Each F-number is set so that the sub-scan emission side F-number becomes brighter.

  FIG. 22 shows the main scanning LSF depth center position and the sub-scanning LSF depth center position at each image height in the optical scanning device 300 according to the present embodiment.

  As can be seen from FIG. 22, at any image height, the main scanning LSF depth center position and the sub-scanning LSF depth center position are both within a range of ± 3 mm, and the optical scanning device 300 according to this embodiment is good. It can be seen that the image surface performance is excellent.

FIG. 23 shows spot profiles at image heights Y = 0 mm, ± 100 mm, and ± 156 mm on the scanned surface 7 in the optical scanning device 300 according to the present embodiment. FIG. 23 shows a spot profile when defocused by X = 0 mm and ± 1 mm from the scanned surface 7 in the optical axis direction of the imaging optical system.
Here, the contour lines of each spot profile are set at respective positions of 50%, 13.5%, 5%, and 2% with respect to the peak peak light quantity maximum value.

  As can be seen from FIG. 23, in the optical scanning device 300 according to the present embodiment, a good spot profile with few side lobes is obtained at each image height.

  Next, the relationship between various numerical values and the conditional expression in the optical scanning device 300 according to the present embodiment will be described.

In the optical scanning device 300 according to the present embodiment, Fm0 = 48.7, Fm1 = 54.2, Fs0 = 60.7, and Fs1 = 55.1.
Therefore, Fm1 / Fm0 = 1.11, and the optical scanning device 300 according to this embodiment satisfies the conditional expression (8).
Further, (Fm1 × Fs1) / (Fm0 × Fs0) = 1.01 and Fs0 / Fs1 = 1.10, and the optical scanning device 300 according to this embodiment satisfies the conditional expressions (9) and (10).

  As described above, the optical scanning device 300 according to the present embodiment satisfies both the conditional expressions (8), (9), and (10), thereby achieving both downsizing and good printing performance.

  Further, as shown in FIG. 19, in the optical scanning device 300 according to the present embodiment, the partial magnification deviation ΔY in the main scanning direction that is the maximum at the entire image height is 11.3%. The optical scanning device 300 according to the above is designed to satisfy the conditional expression (18).

  In the optical scanning device 300 according to the present embodiment, the glass coupling lens 8 and the glass anamorphic lens 9 are used in the incident optical system in order to suppress the focus fluctuation at the time of temperature rise of the optical scanning device 300 and ease of adjustment. Is used. However, the present embodiment is not limited to this. For example, the effects of the present embodiment can be obtained even when one anamorphic lens is used, as in the first and second embodiments.

  As described above, in the optical scanning device 300 according to the present embodiment, it is possible to achieve cost reduction and downsizing while ensuring good imaging performance and printing performance.

  According to the present embodiment, in an optical scanning device configured with an inconstant speed scanning system, light capable of forming a good image quality with reduced density unevenness at intermediate gradation while achieving cost reduction and downsizing. A scanning device can be realized.

[Monochrome image forming apparatus]
FIG. 24 is a cross-sectional view of the main part of the monochrome image forming apparatus 104 on which the optical scanning unit (optical scanning apparatus) 400 according to the first or third embodiment is mounted.

FIG. 24 is a cross-sectional view of the main part of the monochrome image forming apparatus 104 on which the optical scanning unit (optical scanning apparatus) 400 according to the first or third embodiment is mounted.
The image forming apparatus 104 includes the optical scanning device (optical scanning unit) 100 in any of the above-described embodiments.
The monochrome image forming apparatus 104 receives code data Dc output from an external device 117 such as a personal computer. The code data Dc is converted into image data (dot data) by the printer controller 111 in the image forming apparatus 104. This image data is input to the optical scanning unit 400 according to the first or third embodiment. The light scanning unit 400 emits a light beam 103 modulated according to image data, and the light beam 103 scans the photosensitive surface of the photosensitive drum 101 in the main scanning direction.
The monochrome image forming apparatus 104 receives code data Dc output from an external device 117 such as a personal computer. The code data Dc is converted into image data (dot data) by the printer controller 111 in the image forming apparatus 104. This image data is input to the optical scanning unit 400 according to the first or third embodiment. The light scanning unit 400 emits a light beam 103 modulated according to image data, 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 400.

  As described above, the light beam 103 is modulated based on the image data. By irradiating the light beam 103, an electrostatic latent image is formed on the surface of the photosensitive drum 101. This electrostatic latent image is developed as a toner image by a developing device 107 disposed so as to abut on 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 (transfer unit) 108 disposed below the photosensitive drum 101 so as to face the photosensitive drum 101. The paper 112 is stored in the paper cassette 109 in front of the photosensitive drum 101 (on the right side in FIG. 24), but can be fed manually. A paper feed roller 110 is disposed 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 is transferred is further conveyed to the fixing device 150 behind the photosensitive drum 101 (left side in FIG. 24). The fixing device 150 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 discharge roller 116 is disposed behind the fixing device 150, and the fixed sheet 112 is discharged outside the monochrome image forming apparatus 104.

  The printer controller 111 not only converts the data but also controls each part in the monochrome image forming apparatus 104 including the motor 115 and the polygon motor in the optical scanning unit 400.

[Color image forming apparatus]
FIG. 25 shows a sub-scan sectional view of a main part of a color image forming apparatus 90 on which the optical scanning device 11 according to the second embodiment is mounted.

The image forming apparatus 90 is a tandem type color image forming apparatus that records image information on each photosensitive drum surface as an image carrier by the optical scanning device 11.
The image forming apparatus 90 includes an optical scanning device 11 according to the second embodiment, photosensitive drums 23, 24, 25, and 26 as image carriers, developing units 15, 16, 17, and 18, a conveyance belt 91, a printer controller 93, and A fixing device 94 is provided.

  The image forming apparatus 90 receives R (red), G (green), and B (blue) color signals from an external device 92 such as a personal computer. These color signals are converted into image data (dot data) of C (cyan), M (magenta), Y (yellow), and K (black) by a printer controller 93 in the image forming apparatus 90. These image data are input to the optical scanning device 11. The light scanning device 11 emits light beams 59, 60, 61, 62 modulated according to each image data, and the light beams 59, 60, 61, 62 emit the photosensitive drums 23, 24, The photosensitive surfaces 25 and 26 are scanned in the main scanning direction.

  Then, latent images of the respective colors are formed on the photosensitive surfaces of the corresponding photosensitive drums 23, 24, 25, and 26 by the light beams 59, 60, 61, and 62 emitted based on the respective image data by the optical scanning device 11. Is done. Thereafter, the latent images of the respective colors are developed into the respective color toner images by the developing units 15 to 18, the developed respective color toner images are multiplex-transferred onto the recording material by the transfer unit, and the transferred toner image is fixed by the fixing unit. A full color image is formed.

  Therefore, in the image forming apparatus 90, the optical scanning device 11 performs the photosensitive drums 23, 24, 25, and 26 corresponding to the respective colors of C (cyan), M (magenta), Y (yellow), and K (black). Image signals (image information) can be recorded on the surface, and color images can be printed at high speed.

  As the external device 92, 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 90 constitute a color digital copying machine.

  Further, the recording density of the image forming apparatus according to the present embodiment is not particularly limited. However, considering that the higher the recording density is, the higher the image quality is required, the effect of the present invention is obtained when the optical scanning device according to the first to third embodiments is mounted in an image forming apparatus of 1200 dpi or more. Is more effective.

1 Light source 2 Anamorphic lens (incident optical system)
3 Main scanning stop (incident optical system)
4 Deflector 5 Sub-scanning stop (imaging optical system)
6 Imaging lens (imaging optical system)
7 Scanned surface 100 Optical scanning device

Claims (17)

A light source, a deflector that deflects the light beam from the light source and optically scans the surface to be scanned in the main scanning direction, an incident optical system that causes the light beam from the light source to enter the deflector, and the deflector. And an imaging optical system that guides the light flux to the surface to be scanned,
In the main scanning section, the exit side F number at the on-axis image height is Fm0, the exit side F number at the most off-axis image height is Fm1, and the exit side F at the on-axis image height in the sub-scan section. When the number is Fs0 and the exit side F-number at the most off-axis image height is Fs1,
1.1 <Fm1 / Fm0 <1.4
0.8 <(Fm1 × Fs1) / (Fm0 × Fs0) <1.2
An optical scanning device characterized by satisfying the following condition. 1.05 <Fs0 / Fs1 <1.45
The optical scanning device according to claim 1, wherein the following condition is satisfied.   3. The optical scanning device according to claim 1, wherein the imaging optical system includes a sub-scanning aperture that limits a diameter of a light beam deflected by the deflector in a sub-scanning direction.   4. The optical scanning device according to claim 3, wherein an aperture width in the sub-scanning direction of the sub-scanning aperture increases as the distance from the optical axis of the imaging optical system increases in the main scanning direction. With respect to the sub-scanning aperture, the aperture widths in the sub-scanning direction at the passage position of the light beam incident on the on-axis image height and the passage position of the light beam incident on the most off-axis image height are W0 and W1, respectively. When the scanning angle is θ,
1.0 <W1 / W0 × cosθ
The optical scanning device according to claim 3, wherein the following condition is satisfied. 1.05 <W1 / W0 × cos θ <1.4
The optical scanning device according to claim 5, wherein the following condition is satisfied.   The optical scanning device according to claim 1, wherein a sub-scanning lateral magnification of the imaging optical system decreases as the distance from the optical axis of the imaging optical system decreases in the main scanning direction. . When the sub-scanning lateral magnification of the imaging optical system at each of the on-axis image height and the most off-axis image height is βs0 and βs1,
1.05 <| βs0 / βs1 | <1.4
The optical scanning device according to claim 1, wherein: ΔY is the maximum value of the ratio of the deviation of the partial magnification in the main scanning direction at each off-axis image height to the partial magnification in the main scanning direction at each off-axis image height with respect to the partial magnification in the main scanning direction at the on-axis image height. (%)
10% <ΔY <40%
The optical scanning device according to claim 1, wherein:   The optical scanning device according to claim 1, wherein the imaging optical system includes one imaging optical element.   The optical scanning device according to claim 1, further comprising a control unit that controls a light emission timing and a light emission amount of the light source. The optical scanning device includes a plurality of scanning optical systems each including the incident optical system and the imaging optical system,
In the sub-scan section, the exit side F number Fs0 at the on-axis image height and the exit side F number Fs1 at the most off-axis image height of at least two of the plurality of scanning optical systems are mutually The optical scanning device according to claim 1, wherein the optical scanning device is different. Each of the imaging optical systems of the plurality of scanning optical systems includes a sub-scanning diaphragm that limits a light beam diameter in the sub-scanning direction of the light beam deflected by the deflector,
The aperture of the sub-scanning aperture has a shape in which the aperture width in the sub-scanning direction increases as the distance from the optical axis of the imaging optical system increases in the main scanning direction.
The optical scanning device according to claim 12, wherein the shapes of the openings of the sub-scanning diaphragms of the plurality of imaging optical systems are different from each other. The incident optical system of the first scanning optical system among the plurality of scanning optical systems causes the first light beam to enter the first deflecting surface of the deflector,
The incident optical system of the second scanning optical system among the plurality of scanning optical systems causes the second light beam to enter the second deflecting surface of the deflector,
The first deflection surface deflects the first light beam to optically scan the first scanned surface in the main scanning direction,
The second deflecting surface deflects the second light flux to optically scan the second scanned surface in the main scanning direction,
The imaging optical system of the first scanning optical system among the plurality of scanning optical systems guides the first light beam deflected by the first deflection surface to the first scanned surface. ,
The imaging optical system of the second scanning optical system among the plurality of scanning optical systems guides the second light beam deflected by the second deflection surface to the second scanned surface. The optical scanning device according to claim 12, wherein the optical scanning device is characterized in that:   The optical scanning device according to any one of claims 1 to 14, wherein a width of a light beam from the light source in a main scanning direction is smaller than a width of a deflecting surface of the deflector in a main scanning direction. .   16. The optical scanning device according to claim 1, a developing device that develops an electrostatic latent image formed on the surface to be scanned by the optical scanning device as a toner image, and a developed toner image An image forming apparatus comprising: a transfer device that transfers the toner 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.