JP4817668B2 - Optical scanning device - Google Patents

Description

  The present invention relates to an optical scanning device used in an image forming apparatus such as a laser printer or a digital copying machine.

  2. Description of the Related Art Conventionally, an optical scanning device used in an image forming apparatus such as a laser beam printer or a digital copying machine guides a light beam emitted from a light source to a deflecting element by incident optical means, and the light beam deflected by the deflecting element. Are imaged in a spot shape on the surface of the photosensitive drum, which is the surface to be scanned, by the scanning optical means, and the photosensitive drum surface is optically scanned with the luminous flux.

  In such an optical scanning device, the light beam emitted from the light source is converted into substantially parallel light by a collimator lens or the like, and the light beam converted into substantially parallel light for performing tilt correction is lined near the deflection surface by a cylindrical lens. An image is formed. The light beam deflected by the deflecting surface of the deflector scans the surface of the photosensitive drum with a scanning lens at a substantially constant speed to form a spot.

  In FIG. 9, reference numerals 1a, 1b, 1c, and 1d denote light source means, for example, semiconductor lasers. Reference numerals 2a, 2b, 2c and 2d denote cylindrical lenses which have a predetermined refractive power only in the sub-scanning direction. Each element such as the light source means 1 and the cylindrical lens 2 constitutes one element of the incident optical means 51.

  Reference numeral 3 denotes an optical deflector as a deflecting element, which is composed of, for example, a rotating polygon mirror, and is rotated at a constant speed in the direction of arrow A in the figure by driving means (not shown) such as a motor. Reference numeral 11 denotes three fθ lenses having fθ characteristics, which are located between the vicinity of the deflecting surfaces 3a and 3b of the optical deflector 3 and the vicinity of the photosensitive drum surfaces 100a, 100b, 100c and 100d as the scanned surfaces in the sub-scan section. Has a tilt correction function.

In order to make the image forming apparatus compact, a folding mirror for folding the optical path is disposed on the downstream side of the deflector to achieve compactness.
JP-A-2-35413 Japanese Patent Laid-Open No. 7-294837 JP 11-119135 A

  In the conventional optical scanning device described above, since the scanning lens is made of a glass lens, the lens surface is usually provided with an antireflection coating, and surface reflection does not occur. That is, the amount of light on the surface to be scanned is uneven. Is difficult to occur.

  However, recent scanning lenses are made of plastic lenses, which are designed to be low in price and high in performance. In an optical scanning device composed of such a plastic lens, it is generally difficult to apply an antireflection coating to the lens surface, so that the Fresnel reflection on the lens surface changes depending on the scanning angle on the surface to be scanned. Uneven light intensity occurs.

  In order to solve these problems, as disclosed in Patent Document 1, the reflectivity of the folding mirror is set so that the reflectivity in the vicinity of the optical axis is different from the reflectivity in the off-axis, and on the surface to be scanned. It is known to correct the unevenness of the amount of light. Further, as disclosed in Patent Document 2, linearly polarized light is converted into elliptically polarized light before the polarizing means, and the light amount unevenness on the surface to be scanned is corrected. A technique for correcting by changing the transmittance of a material is known.

However, in the optical scanning device described in Patent Document 1,
(1) Consider only the reflection characteristics depending on the polarization direction.
(2) Only light quantity unevenness due to the transmittance inside the lens is considered.
(3) Only a technique when a single folding mirror is used in the optical path is disclosed.
From the above, there is no clear description of light amount unevenness caused by surface reflection when the scanning lens is made of plastic, or light amount unevenness correction when two or more folding mirrors are provided in the optical path. In addition, the technique disclosed in Patent Document 3 requires an optical member further, and the technique disclosed in Patent Document 3 is inexpensive because both optical materials are expected to be expensive. It is difficult to configure an optical scanning device.

  In the optical scanning device in which the scanning lens is made of plastic and has two or more folding mirrors in the optical path, at least one folding mirror can obtain a desired reflectance according to the incident angle and the polarization direction. Thus, it is possible to provide a low-cost and high-quality image forming apparatus by correcting the light amount unevenness on the surface to be scanned due to lens surface reflection. Further, it is possible to correct unevenness of the image plane illuminance due to a decrease in the amount of light at the edge of the image in the overfilled scanning device, and a high-speed image forming apparatus can be provided at a low price.

For this purpose, in the present invention, a light source means, a deflecting means for deflecting a light beam emitted from the light source means, and an imaging optical system for focusing the light beam deflected by the deflecting means on a scanning surface in a spot shape. And a plurality of mirrors provided in an optical path between the deflecting means and the surface to be scanned,
The width of the light beam incident on the deflection surface of the deflection unit in the main scanning direction is smaller than the width of the deflection surface of the deflection unit in the main scanning direction, and the scanning lens constituting the imaging optical system is made of plastic. The light beam emitted from the light source means is incident on the incident surface of the scanning lens that constitutes the imaging optical system, and the plurality of mirrors include a first mirror and a second mirror, The reflectance of the light beam reflected by the second mirror is 90% or less, and the image plane illuminance ratio on the surface to be scanned is ±± within the effective scanning region on the basis of the image surface illuminance on the axis. The reflectance of the light beam reflected by the second mirror within 5% decreases from the on-axis to the off-axis, and the on-axis reflectance of the first mirror is the axis of the second mirror. Higher than the upper reflectivity and the reflectivity on the axis of the first mirror is It took a structure characterized by equal reflectivity outside the axis of the first mirror.

  According to the present invention, as described above, in an optical scanning device composed of two or more reflecting mirrors, the optical characteristics of each mirror are made different to correct unevenness in the amount of light on the surface to be scanned. A compact optical scanning device can be obtained.

  Further, according to the present invention, it is possible to reduce density unevenness that occurs in a color image forming apparatus that superimposes images on a plurality of different photoreceptors.

Example 1
FIG. 1 is a sub-scan sectional view of the optical scanning device of the present invention. FIG. 2 is an exploded view of the optical system main scanning section in the optical apparatus of the present invention.

  The folding mirror of the present invention is a plane mirror and has no power in the main scanning direction and the sub-scanning direction.

  In FIG. 2, 91 is a light source such as a semiconductor laser, and 92 is a collimator lens that converts divergent light from the light source into a parallel light beam. There is one light beam emitted from the light emitting portion of the light source 91.

  The width of the light beam incident on the polygon mirror 95 in the main scanning direction is UFS (underfilled optical system) smaller than the width of the deflecting surface 95A in the main scanning direction.

  However, the present invention can also be applied to a light source having two or more light emitting units. Furthermore, the present invention can be applied to a multi-beam light source having three or more light emitting units. For example, an edge-emitting monolithic multi-semiconductor laser or a surface-emitting monolithic multi-semiconductor laser can be used.

  Reference numeral 93 denotes an aperture stop that adjusts the diameter of the passing light beam.

  Reference numeral 94 denotes a cylindrical lens as a second optical system, which has a predetermined refractive power only in the sub-scanning direction. After passing through the stop 93, it is formed on the deflection surface 95A of the deflector 95 in the sub-scanning sectional view. It is formed almost as a line image.

  Reference numeral 95 denotes a polygon mirror as a deflector, which rotates at a constant speed in the direction of an arrow.

  Reference numeral 96 denotes an imaging optical system as a third optical system, and both the fθ lens 96a and the fθ lens 96b are constituted by an anamorphic lens having an aspherical shape in the main scanning section, and the light beam deflected by the deflector 95 is used. An image is formed on the photosensitive drum surface as the surface to be scanned, and the surface tilt of the deflector is corrected (surface tilt correction system).

  In the present embodiment, the shapes of the first and second scanning lenses 96a and 96b constituting the scanning lens system 6 are represented by the following function.

For example, the intersection between the first and second scanning lenses 96a and 96b and the optical axis is the origin, and the surface shape in the main scanning section on the scanning start side and the scanning end side with respect to the optical axis as shown in FIG. When the optical axis is the X axis, the direction orthogonal to the optical axis in the main scanning section is the Y axis, and the direction orthogonal to the optical axis in the sub scanning section is the Z axis,
The surface shape on the scanning start side is

  The surface shape on the scanning end side is

It is represented by

  However, R is a curvature radius and K, B4, B6, B8, and B10 are aspherical coefficients.

  In this embodiment, the shapes in the main scanning section of the first and second scanning lenses 96a and 96b are formed substantially symmetrical with respect to the optical axis, that is, the aspheric coefficients on the scanning start side and the scanning end side are set. Match.

  In the sub-scan section, the exit surface of the second scan lens 96b (the lens closest to the scan surface) among the first and second scan lenses 96a and 96b on the scan start side and the scan end side with respect to the optical axis. Surface) The curvature of r4 is continuously changed in the effective portion of the scanning lens 96b.

  The exit surface r4 is configured to have the strongest refractive power (power, reciprocal of focal length) among the plurality of lens surfaces of the plurality of lenses constituting the scanning lens system. The first scanning lens 96a includes a non-arc surface in the main scanning section.

  The shapes of the first and second scanning lenses 96a and 96b in the sub-scanning section are as shown in FIG. 2 on the scanning start side and the scanning end side with respect to the optical axis. When the direction orthogonal to the optical axis is the Y axis and the direction orthogonal to the optical axis in the sub-scanning section is the Z axis, it can be expressed by the following continuous function.

  The surface shape on the scanning start side is

  The surface shape on the scanning end side is

It is represented by

  However, r 'is a radius of curvature in the sub-scanning direction, and D2, D4, D6, D8, and D10 are coefficients.

  The coefficient suffix s represents the scanning start side, and e represents the scanning end side. Note that the radius of curvature in the sub-scanning direction is a radius of curvature in a cross section orthogonal to the shape (bus line) in the main scanning direction.

  The optical parameters of this embodiment are shown below.

  FIG. 3 is a sub-scan sectional view of the scanning optical system in the optical scanning device of the present invention.

  91 is a light source such as a semiconductor laser, and 92 is a collimator lens that converts divergent light from the light source into a parallel light beam.

  Reference numeral 93 denotes an aperture stop that adjusts the diameter of the passing light beam.

  Reference numeral 94 denotes a cylindrical lens as a second optical system, which has a predetermined refractive power only in the sub-scanning direction. It is formed as an image.

  The light beam deflected by the deflecting surface 95A is imaged on a photosensitive drum surface as a surface to be scanned by an imaging optical system 96 as a third optical system.

  As shown in FIG. 1, the optical scanning device of the present embodiment is configured by using a plurality of optical systems shown in FIGS. 2 and 3, but the optical characteristics are all the same.

  In order to make the image forming apparatus compact, two folding mirrors 99a and 99b according to the present embodiment are arranged in the optical path after the deflector 95, and are configured to have different reflectivities.

  As shown in FIG. 4, the incident angle θ to the folding mirror varies depending on the scanning field angle α, and the incident angle of the axial light beam to the folding mirror is given by the following equation using φ.

  For example, the on-axis incident angle φ = 45 deg. , Scanning angle of view α = 40 deg. Then, the incident angle φo of the off-axis light flux with respect to the folding mirror is 57.2 deg. It becomes.

When the on-axis light beam is incident on the folding mirror as S-polarized light, the polarization direction of the off-axis light beam has P-polarized light intensity E _p ² and S-polarized light intensity E _s ² at the following ratios.

From the above equation, the on-axis incident angle φ = 45 deg. , Scanning angle of view α = 40 deg. When the on-axis light beam is incident on the folding mirror as S-polarized light, Ep ² : Es ² = 0.29: 0.71, and the P-polarized light component continuously increases with the angle of view from the on-axis to the off-axis. To increase. That is, the incident angle of the folding mirror changes continuously and the polarization direction also changes continuously.

  In this embodiment, as shown in FIG. 4, the light beam emitted from the light source is linearly polarized light, and the polarization direction is 201 directions. That is, a P-polarized light beam enters the deflector 102 and the fΘ lens 103. An S-polarized light beam is incident on the folding mirror 104.

  In the present embodiment, the polarization direction of the light source means is incident on the deflecting surface of the deflecting means 95 as P-polarized light, and the on-axis incident angle φ and scanning angle α are set as follows.

First folding mirror 99a φ: 21 deg. α: 33 deg.
Second folding mirror 99b φ: 22 deg. α: 29 deg.
In order to correct unevenness in the amount of light on the drum surface (unevenness in image plane illuminance), the reflection angle of the folding mirror is determined by continuously changing the incident angle and polarization direction of the light beam from the axis to the axis. The rate is set to change continuously according to the incident angle and the polarization direction.

  FIG. 5 shows the image plane illuminance unevenness in the present embodiment. Due to the surface reflection (Fresnel reflection) of the fθ lenses 96a and 96b and the dustproof glass 98, the amount of light on the off-axis (image end) is increased by about 6% from the amount of light on the axis (center of the image) (before correction). . The unevenness in the amount of light generated by the fθ lens is corrected by continuously changing the incident angle of the deflected light beam to the folding mirror and the reflectance with respect to the polarization direction. In the present embodiment, as shown in FIG. 5, the reflectivity of the first folding mirror with respect to the on-axis light beam is 95%, and there is no difference in reflectivity between the on-axis light beam and the off-axis light beam. The reflection film of the folding mirror 99a is optimized in consideration of the angle of the incident light beam and the polarization direction so that the reflectance is also 95%. Further, the reflectance of the second folding mirror 99b with respect to the axial light beam is 80%, and the angle and polarization direction of the incident light beam are set so that the reflectance of the off-axis light beam is 3% lower than the reflectance of the axial light beam. The reflective film is optimized in consideration. Therefore, the image plane illuminance unevenness is corrected to 3% (after correction). This makes the illuminance unevenness in the image plane equal to the optical path having only one folding mirror in the optical path (the optical path passing through the mirror 100).

  When the reflectivity of the folding mirror 99 is small (in this embodiment, the second folding mirror 99b), it is easy to form a thin film having different reflectivities for the on-axis rays and the off-axis rays. The present inventor found out.

  Therefore, in this embodiment, the on-axis reflectance and the off-axis reflectance of the second folding mirror 99b are made different.

  The present inventor has found that the reflection film that can easily vary the on-axis reflectance and the off-axis reflectance is 90% or less on the optical axis of the imaging optical system 96. .

  In the present embodiment, a folding mirror having a reflectance of 90% or less on the optical axis of the imaging optical system 96 is defined as a mirror having a low reflectance.

  In this embodiment, only the fθ lens and the Fresnel reflection component of the dust-proof glass are considered, but the incident angle characteristic of the reflectance of the polygon mirror and the image surface illuminance unevenness caused by the difference in the diffraction efficiency of the diffractive optical element, fθ Obviously, it is possible to correct unevenness of the image plane illuminance due to internal absorption of the lens, a decrease in the amount of light of the OFS (overfilled optical system), and the like.

  The OFS (overfilled optical system) applied to the present invention means an optical system in which the width of the light beam incident on the deflection surface of the deflecting means is larger than the width of the deflection surface in the main scanning direction.

  In the present invention, the image plane illuminance ratio on the surface to be scanned is preferably within ± 5% within the effective scanning region with reference to the on-axis image plane illuminance.

  In this embodiment, only two folding mirrors are arranged in the optical path. However, the mirror may be composed of three or more, and is used for a reflective optical element (curved mirror) having power like a cylindrical mirror. Also good.

  As described above, in this embodiment, the reflectance of the folding mirror is continuously changed according to the incident angle and the polarization direction, thereby correcting the image surface illuminance unevenness, and the image surface illuminance unevenness between the colors in the in-line scanning system is uniform. Therefore, a high-definition and compact optical scanning device can be provided.

  As shown in FIG. 5, in this embodiment, the image plane illuminance ratio on the surface to be scanned is compensated within ± 5% within the effective scanning region with reference to the on-axis image plane illuminance.

  In the present invention, a curved mirror having power in the main scanning direction and / or the sub-scanning direction may be used instead of the folding mirror (plane mirror). In other words, in order to compensate for unevenness of image plane illuminance on the surface to be scanned, the on-axis reflectance and off-axis reflectance of the curved mirror may be different.

(Example 2)
FIG. 6 shows a sub-scan sectional view of the optical scanning device of this embodiment.

  The light beam from the light source is incident on the deflecting surface of the deflector 85 by an incident optical system (not shown). Reference numeral 85 denotes a polygon mirror as a deflector, which rotates at a constant speed. The imaging optical system 86 includes fθ lenses 86a and 86b. The fθ lenses 86a and 86b are both constituted by an anamorphic lens having an aspherical shape in the main scanning section, and the light beam deflected by the deflector 85 is imaged on the photosensitive drum surface 87 as a scanned surface, and The tilting of the deflector is corrected (the tilting correction system).

  In order to make the image forming apparatus compact, two folding mirrors 89a and 89b of this embodiment are arranged in the optical path after the deflector 85, and are configured to have different reflectivities.

  The difference of this embodiment from Embodiment 1 is that the polarization direction of the light source with respect to the deflecting surface is S-polarized light (202 direction in FIG. 4), and the reflectance of the plurality of folding mirrors in the optical path is set to be low. This is a point where the surface illuminance unevenness is corrected to approximately 0%. Other configurations and effects are the same as those of the first embodiment.

  FIG. 7 shows image plane illuminance unevenness correction values of the present embodiment. Also in this embodiment, the amount of light on the off-axis (image end) is increased by about 6% from the amount on the axis (the center of the image) due to the surface reflection (Fresnel reflection) of the fθ lens and dust-proof glass as in the first embodiment. is doing. The unevenness in the amount of light generated by the fθ lens is corrected by continuously changing the incident angle of the deflected light beam to the folding mirror and the reflectance with respect to the polarization direction.

  As shown in FIG. 6, the optical scanning device of this embodiment has two folding mirrors 89a and 89b in the same optical path, and the reflectance of the axial light beam is the first folding mirror from the deflecting means side. 89a is set to 70%, and the second folding mirror 89b is set to 60%. Further, as shown in FIG. 7, the light amount unevenness on the drum surface is almost 0% (substantially uniform) by correcting the light amount unevenness by 2% with the first folding mirror and 4% with the second folding mirror. ).

  The present inventor has found that when the reflectivity of the folding mirror 89 is small, it is easy to form a thin film having different on-axis reflectivity and off-axis reflectivity.

  Therefore, in this embodiment, the reflectivity for the on-axis light beam and the reflectivity for the off-axis light beam of the folding mirrors 89a and 89b are different.

  The present inventor has found that a reflection film that can easily change the reflectance for the on-axis light beam and the reflectance for the off-axis light beam is 90% or less on the optical axis of the imaging optical system. It was.

  In this embodiment, a folding mirror having a reflectance of 90% or less on the optical axis of the imaging optical system is defined as a mirror having a low reflectance.

  In the present embodiment, the polarization direction of the laser diode as the light source means is set so as to be reflected by the S-polarized light with respect to the deflector. This is because the narrow direction of the laser diode emission angle is used for the main scanning direction, and the wide direction of the laser diode emission angle is used for the sub-scanning direction. This is to ensure the necessary amount of light. In addition, since the focal length of the cylindrical lens can be shortened compared to the case where the deflecting surface of the deflector is configured by P-polarized light, the distance from the laser diode to the deflecting surface can be shortened.

  In the present embodiment, by using the direction in which the radiation angle of the laser diode as the light source means is narrow in the main scanning direction (using S-polarized light with respect to the deflection surface), the amount of exposure light is increased compared to using P-polarized light. Therefore, since the reflectance of the folding mirror can be set low, there is an advantage that the incident angle characteristic and polarization characteristic of the folding mirror can be increased. In addition, since correction is performed by two mirrors, the correction amount can be increased.

  In this embodiment, only the fθ lens and the Fresnel reflection component of the dust-proof glass are considered, but the incident angle characteristic of the reflectance of the polygon mirror and the image surface illuminance unevenness caused by the difference in the diffraction efficiency of the diffractive optical element, fθ Obviously, it is possible to correct unevenness of the image plane illuminance due to internal absorption of the lens, a decrease in the amount of light of the OFS (overfilled optical system), and the like.

  In this embodiment, only two folding mirrors are arranged in the optical path. However, the mirror may be composed of three or more, and may be used for a reflective optical element having a power like a cylindrical mirror.

  As described above, in this embodiment, the polarization direction of the incident means is optimized, and the reflectivity of the folding mirror is set to be low, so that the unevenness in the amount of light on the surface to be scanned is made substantially uniform, and a high-definition and compact optical scanning device. Can be obtained.

  As shown in FIG. 7, in this embodiment, the image plane illuminance ratio on the surface to be scanned is compensated within ± 5% within the effective scanning region with reference to the on-axis image plane illuminance.

  In the present invention, a curved mirror having power in the main scanning direction and / or the sub-scanning direction may be used instead of the folding mirror (plane mirror). That is, in order to compensate for unevenness of the image plane illuminance on the surface to be scanned, the reflectance of the curved mirror for the on-axis ray and the reflectance for the off-axis ray may be different.

(Example 3)
The difference of the present embodiment from the first embodiment is that the image plane illuminance unevenness correction amounts of the plurality of mirrors are substantially the same. Other configurations and effects are the same as those of the first embodiment. The configuration of the optical scanning device is the same as that shown in FIG.

  FIG. 8 shows image surface illuminance unevenness correction values of the present embodiment. Also in this embodiment, the amount of light on the off-axis (image end) is increased by about 6% from the amount on the axis (the center of the image) due to the surface reflection (Fresnel reflection) of the fθ lens and dust-proof glass as in the first embodiment. is doing. The unevenness in the amount of light generated by the fθ lens is corrected by continuously changing the incident angle of the deflected light beam to the folding mirror and the reflectance with respect to the polarization direction.

  As shown in FIG. 6, the optical scanning device of this embodiment has two folding mirrors 87a and 87b in the same optical path, and the reflectivity of the axial light beam is first and second from the deflection means side. Both folding mirrors are set to 90%. Further, as shown in FIG. 8, the light amount unevenness on the drum surface is almost 0% (by correcting the light amount unevenness by 3% by the first folding mirror 87a and by correcting 3% by the second folding mirror 87b. (Substantially uniform).

  The first folding mirror and the second folding mirror of this embodiment have the same reflectivity with respect to the axial light beam, but the structures of the reflective films 87a and 87b are different. This is because the incident angle φ of the light beam incident on each mirror is different from the scanning angle α.

  In the present embodiment, the image plane illuminance unevenness is evenly corrected by using the first folding mirror and the second folding mirror, and even when 80% or more of the reflection mirror is used, the image plane illuminance unevenness is substantially zero. % (Substantially uniform) can be corrected. In particular, in systems where the image surface illuminance unevenness is as large as 10% or more, it is possible to correct with a plurality of mirrors without significantly reducing the reflectivity of the folding mirror. In addition, there is an effect of reducing power consumption.

  In this embodiment, only the fθ lens and the Fresnel reflection component of the dust-proof glass are considered, but the incident angle characteristic of the reflectance of the polygon mirror and the image surface illuminance unevenness caused by the difference in the diffraction efficiency of the diffractive optical element, fθ Obviously, it is possible to correct unevenness of the image plane illuminance due to internal absorption of the lens, a decrease in the amount of light of the OFS (overfilled optical system), and the like.

  In this embodiment, only two folding mirrors are arranged in the optical path. However, the mirror may be composed of three or more, and may be used for a reflective optical element having a power like a cylindrical mirror.

  As described above, in this embodiment, in the optical scanning apparatus having a plurality of high-reflectance folding mirrors, the reflection film is configured so that the correction amount of the unevenness of the image plane illuminance is approximately equal for each mirror, thereby forming the surface to be scanned. It is possible to obtain a high-definition and compact optical scanning device by making the above light amount unevenness substantially uniform.

  As shown in FIG. 8, in this embodiment, the image plane illuminance ratio on the surface to be scanned is compensated within ± 5% within the effective scanning region with reference to the on-axis image plane illuminance.

Example 4
FIG. 11 shows an optical system main scanning sectional view of this example.

  In the fourth embodiment, the UFS optical system is used in the first embodiment, but an OFS optical system is applied instead of the UFS optical system. The optical scanning device according to the fourth embodiment is the same as that shown in FIGS.

  Reference numeral 71 denotes a light source such as a semiconductor laser, which emits a P-polarized light beam with respect to the polarization plane.

  Reference numeral 72 denotes a collimator lens that includes two lenses 72a and 72b and converts divergent light from the light source into parallel light fluxes.

  A cylindrical lens 73 has a predetermined refractive power only in the sub-scanning direction, and forms a line image near the deflection surface of the deflector 75 in the sub-scanning section.

  74 is a plane reflection mirror which reflects the light beam from the cylindrical lens to the deflector side.

  Reference numeral 76 denotes an imaging optical system, which includes fθ lenses 76a, 76b, and 76c, and includes a spherical surface and a cylindrical lens having the curvatures shown in the table below. The light beam deflected by the deflector 75 is imaged on the photosensitive drum surface 77 as the surface to be scanned, and the surface tilt of the deflector is corrected (surface tilt correction system).

  In this embodiment, as shown in FIG. 12, the illuminance distribution of the light beam emitted from the light source means 71 made of a semiconductor laser has a Gaussian distribution, with a large amount of light at the center of the light beam and a small amount of light at the end of the light beam. It becomes a luminous flux. Therefore, when the light beam incident on the deflection surface 75A is incident on the deflection surface 75A at an angle having a predetermined angle with the optical axis of the imaging optical system 76 in the main scanning section, the light beam deflected and reflected by the deflection surface 75A is imaged. Since the light amount distribution is asymmetric across the optical axis of the optical system, and there is a problem of compensating for the asymmetry of the image surface illuminance distribution on the surface to be scanned, the same as the optical axis of the imaging optical system 76 in the main scanning section. It is preferable to take a front incidence system that enters the deflection surface 75A of the deflection means 75 from the direction of.

  In this case, a front incidence system that enters the deflecting surface 75A of the deflecting means 75 from the same direction as the optical axis of the imaging optical system 76 is taken in the main scanning section, and in the case of OFS, it is deflected and reflected by the deflecting surface 75A. The amount of light emitted from the off-axis light beam is smaller than that from the up-axis light beam.

  In OFS, the width of the light beam incident on the deflecting surface of the deflecting unit in the main scanning direction is larger than the width of the deflecting surface in the main scanning direction, and therefore, unlike the UFS (underfilled optical system), the rotation of the deflecting unit 75, which is a polygon mirror. The light quantity of the light beam deflected and reflected by the deflecting surface 75A varies depending on the angle. And since it is front incidence, the light quantity which goes off-axis becomes smaller than the light beam which goes on an axis.

  Therefore, in the present invention, as shown in FIG. 13, in order to increase the light amount of the light beam going off-axis, the reflectance with respect to the on-axis light beam of the second folding mirror 79b is different from the reflectance with respect to the off-axis light beam. . That is, the reflectance for the off-axis light beam of the second folding mirror 79b is made larger than the reflectance for the on-axis light beam. In this embodiment, the reflectivity with respect to the first folding mirror is 95%, and there is no difference in reflectivity between the on-axis beam and the off-axis beam, that is, the off-axis beam has a reflectivity of 95%. The reflection film of the folding mirror 79a is optimized in consideration of the angle and the polarization direction. Further, the reflectance of the second folding mirror 79b with respect to the axial light beam is 80%, and the angle and polarization direction of the incident light beam are set so that the reflectance of the off-axis light beam is 5% higher than the reflectance of the axial light beam. The reflective film is optimized in consideration. Therefore, the image plane illuminance unevenness is corrected to 3% (after correction).

  As described above, in this embodiment, the reflectance of the folding mirror is continuously changed according to the incident angle and the polarization direction, thereby correcting the image surface illuminance unevenness, and the image surface illuminance unevenness between the colors in the in-line scanning system is uniform. Therefore, a high-definition and compact optical scanning device can be provided.

  As shown in FIG. 13, in this embodiment, the image plane illuminance ratio on the surface to be scanned is compensated within ± 5% within the effective scanning region with reference to the axial image plane illuminance.

  In the present invention, a curved mirror having power in the main scanning direction and / or the sub-scanning direction may be used instead of the folding mirror (plane mirror). In other words, in order to compensate for unevenness of image plane illuminance on the surface to be scanned, the on-axis reflectance and off-axis reflectance of the curved mirror may be different.

[Image forming apparatus]
FIG. 10 is a cross-sectional view of the main part in the sub-scanning direction showing the embodiment of the image forming apparatus of the present invention. 10, in FIG. 10, 60 is a color image forming apparatus, 11 is a scanning optical apparatus having any one of the configurations shown in the first to third embodiments, and 21, 22, 23, and 24 are each an image carrier. Each of the photosensitive drums 31, 32, 33, and 34 is a developing unit, and 51 is a conveyance belt.

  In the figure, R (red), G (green), and B (blue) color signals are input to the color image forming apparatus 60 from an external device 52 such as a personal computer. These color signals are converted into C (cyan), M (magenta), Y (yellow), and B (black) image data (dot data) by a printer controller 53 in the apparatus. These image data are respectively input to the scanning optical device 11. From these scanning optical devices, light beams 41, 42, 43, and 44 modulated according to each image data are emitted, and the photosensitive surfaces of the photosensitive drums 21, 22, 23, and 24 are caused by these light beams. Scanned in the main scanning direction.

  In this embodiment, the color image forming apparatus emits light beams corresponding to the respective colors of C (cyan), M (magenta), Y (yellow), and B (black) from one scanning optical device (11). Image signals (image information) are recorded on the drums 21, 22, 23, and 24, and color images are printed at high speed.

  In the color image forming apparatus according to the present embodiment, as described above, the single scanning optical device 11 uses the light beam based on the respective image data to convert the latent images of the respective colors onto the corresponding photosensitive drums 21, 22, 23, and 24. Is formed. Thereafter, a single full color image is formed by multiple transfer onto a recording material.

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

Sub-scan sectional view of the optical scanning device according to the first embodiment. Optical system main-scan sectional development view of the first embodiment Optical system sub-scan sectional development view of the first embodiment The figure which shows the incident angle and scanning angle of a present Example The figure which shows the image surface illumination intensity nonuniformity correction amount of the present Example 1. Sub-scan sectional view of the second embodiment The figure which shows the image surface illumination intensity nonuniformity correction amount of the present Example 2. The figure which shows the image surface illumination intensity nonuniformity correction amount of the present Example 3. Conventional optical scanning device Image forming apparatus of the present invention Main scanning sectional view of the fourth embodiment Schematic diagram of overfilled scanning system The figure which shows the image surface illumination intensity nonuniformity correction amount of the present Example 4.

Explanation of symbols

91 Light source such as semiconductor laser 92 Collimator lens 93 Aperture stop 94 Cylindrical lens 95 Polygon mirror 96 fθ lens

Claims (3)

A light source means, a deflecting means for deflecting a light beam emitted from the light source means, an imaging optical system for focusing the light beam deflected by the deflecting means on a scanning surface in a spot shape, the deflecting means, and the In an optical scanning device having a plurality of mirrors provided in the optical path between the scanned surfaces,
The width of the light beam incident on the deflection surface of the deflection unit in the main scanning direction is smaller than the width of the deflection surface of the deflection unit in the main scanning direction,
The scanning lens constituting the imaging optical system is made of plastic,
The light beam emitted from the light source means is P-polarized incident on the incident surface of the scanning lens constituting the imaging optical system,
The plurality of mirrors comprises a first mirror and a second mirror,
The reflectance of the light beam reflected by the second mirror is 90% or less, and the image plane illuminance ratio on the surface to be scanned is ±± within the effective scanning region on the basis of the image surface illuminance on the axis. In order to compensate within 5%, the reflectance of the light beam reflected by the second mirror decreases from on-axis to off-axis,
The reflectivity on the axis of the first mirror is higher than the reflectivity on the axis of the second mirror, and the reflectivity on the axis of the first mirror is off axis of the first mirror. An optical scanning device characterized by having a reflectance equal to that of the optical scanning device. An optical latent image formed on the photosensitive member by a light beam scanned by the optical scanning device according to claim 1, the photosensitive surface disposed on the surface to be scanned, and the optical scanning device as a toner image. An image forming apparatus comprising: a developing device that develops; a transfer unit that transfers the developed toner image onto a transfer material; and a fixing device that fixes the transferred toner image onto the transfer material. An image forming apparatus comprising: the optical scanning device according to claim 1; and a printer controller that converts code data input from an external device into an image signal and inputs the image signal to the optical scanning device.

Images