JP2005338865A - Scanning optical apparatus and laser beam printer having the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain a scanning optical apparatus which is compact and suitable for high-definition printing, by appropriately setting the shape of an fθ lens. <P>SOLUTION: In a scanning optical apparatus, a luminous flux emitted from a light source means causes to form an image linearly in a main scanning direction on the deflecting surface of a deflecting element via a first optical element and a second optical element, and a plurality of luminous fluxes deflected with deflected element causes to form the image into a spot-like shape on a surface to be scanned via a third optical element, wherein the third optical element is construted by at least two lens faces, and a principal plane position in the on-axis subscanning direction on the optical axis makes the principal plane position in the off-axis subscanning direction closer to the position of the surface to be scanned by allowing curvatures of the two lens faces in a subscanning direction to be consecutively varied from the on-axis toward the off-axis, so that the change of an f-number in the subscanning direction caused by the image height of the luminous flux made incident on the surface to be scanned is restrained. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a scanning optical device and a laser beam printer having the same, and in particular, has a fθ characteristic after a light beam modulated and emitted from a light source means is deflected and reflected by an optical deflector (deflection element) composed of a rotating polygon mirror or the like. Suitable for devices such as a laser beam printer (LBP) having an electrophotographic process, a digital copying machine, and the like, in which image information is recorded by optically scanning the surface to be scanned through an imaging optical system (fθ lens). It is a thing.

  Conventionally, in a scanning optical device such as a laser beam printer, a light beam modulated and emitted from a light source means according to an image signal is periodically deflected by, for example, an optical deflector composed of a rotating polygon mirror (polygon mirror) to obtain an fθ characteristic. Are focused in a spot shape on the surface of a photosensitive recording medium (photosensitive drum) by an image forming optical system, and image recording is performed by optically scanning the surface.

  FIG. 25 is a schematic view of a main part of a conventional scanning optical apparatus.

  In the figure, a divergent light beam emitted from a light source means 61 is made into a substantially parallel light beam by a collimator lens 62, and the light beam (light quantity) is limited by a diaphragm 63 to form a cylindrical lens 64 having a predetermined refractive power only in the sub-scanning direction. Incident. Out of the parallel light beams incident on the cylindrical lens 64, the light beams are emitted as they are in the main scanning section. In the sub-scan section, the light beam is converged and formed as a substantially line image on the deflecting surface (reflecting surface) 65a of the optical deflector 65 composed of a rotating polygon mirror (polygon mirror).

  Here, the main scanning cross section refers to a cross section of the light beam formed by the light beam deflected and reflected by the deflecting surface of the optical deflector over time. Further, the sub-scanning section refers to a section that includes the optical axis of the fθ lens and is orthogonal to the main scanning section.

  Then, the light beam deflected and reflected by the deflecting surface 65a of the optical deflector 65 is guided to the surface of the photosensitive drum 68 as the surface to be scanned through the imaging optical system (fθ lens) 66 having the fθ characteristic, and the light deflection is performed. By rotating the device 65 in the direction of arrow A, the surface of the photosensitive drum 68 is optically scanned to record image information.

  In order to perform highly accurate image information recording with this type of scanning optical apparatus, the field curvature is well corrected over the entire surface to be scanned and the spot diameter is uniform, and the angle of incident light is proportional to the image height. It is necessary to have distortion (fθ characteristics) that is relevant. Various scanning optical devices or their correction optical systems (fθ lenses) satisfying such optical characteristics have been proposed.

  On the other hand, with the downsizing and cost reduction of laser beam printers and digital copying machines, the same is required for scanning optical devices.

  In order to satisfy these demands, a scanning optical device having a single fθ lens is known (Patent Documents 1 to 5).

Among these publications, in Patent Documents 1 and 2, etc., a parallel single beam from the collimator lens is focused on the recording medium surface by using a concave single lens on the optical deflector side as the fθ lens. In Patent Document 3, a light beam converted into convergent light by a collimator lens using a single lens having a concave surface on the optical deflector side and a toroidal surface on the image surface side as an fθ lens is incident on the fθ lens. In Patent Document 4, a single lens in which a high-order aspheric surface is introduced as a lens surface is used as an fθ lens, and a light beam converted into convergent light by a collimator lens is incident on the fθ lens.
JP 61-48684 A JP 63-157122 A JP-A-4-104213 Japanese Patent Laid-Open No. 4-50908 JP-A-6-239386

  However, in the conventional scanning optical apparatus shown above, in Patent Document 1, the curvature of field in the sub-scanning direction remains and a parallel light beam is imaged on the surface to be scanned, so that from the fθ lens to the surface to be scanned. Is a long focal length f, and it is difficult to construct a compact scanning optical device. In Patent Document 2, since the fθ lens is thick, it is difficult to manufacture by molding, resulting in an increase in cost. Patent Document 3 has a problem that distortion remains. In Patent Document 4, although a high-order aspherical fθ lens is used to correct aberrations, there is a problem that jitter of a polygon surface period occurs due to an attachment error of a polygon mirror as an optical deflector.

  Furthermore, a problem common to these one-piece fθ lenses is that the spot diameter in the sub-scanning direction varies depending on the image height due to non-uniformity of the lateral magnification in the sub-scanning direction between the optical deflector and the surface to be scanned. There was a problem of doing.

  FIGS. 26A and 26B are cross-sectional views of main parts in the main scanning direction and the sub-scanning direction in the conventional single-beam scanning optical device proposed by the present inventor in Patent Document 5, and are sub-scanning according to image height. The change of the spot diameter (F number) of a direction is shown. In the figure, the same elements as those shown in FIG.

Usually, in the surface tilt correction optical system, in order to optically correct the surface tilt of the deflecting surface of the optical deflector, it is necessary to optically conjugate the deflection surface and the surface to be scanned (imaging relationship). .
Therefore, in the fθ lens having a predetermined lens shape in the main scanning plane as in the conventional example described above, the lateral magnification is high on the axis (on-axis light beam 21) as shown in (a) in FIG. In the off-axis direction (outmost-axis light beam 22), the lateral magnification is low as shown in (b) in FIG. 26B (in some cases, it is reversed depending on the lens shape in the main scanning plane).

  Thus, the lateral magnification in the sub-scanning direction can vary depending on the lens shape in the main scanning plane of the fθ lens, and the spot diameter in the sub-scanning direction varies depending on the image height.

  On the other hand, the scanning optical device used for LBP is required to be capable of high-speed scanning by increasing the speed and definition of the LBP. The rotation speed of the motor as the scanning means and the polygon as the deflection means are required. Due to limitations such as the number of mirror surfaces, there is an increasing demand for a multi-beam scanning optical apparatus that can simultaneously scan a plurality of light beams.

  The above-described non-uniformity of the lateral magnification in the sub-scanning direction causes the scanning line to be bent when the position of the light source (light source unit) deviates from the optical axis in the Z direction shown in FIG. In an optical system such as an optical system (multi-beam scanning optical device) that simultaneously scans a surface to be scanned using a plurality of light beams that deviate from the optical axis, the scanning line is bent on the surface to be scanned, resulting in an image due to pitch unevenness. There was a problem that the quality deteriorated.

  A first object of the present invention is to perform main scanning of a fθ lens when a light beam from a light source converted by a collimator lens or the like is imaged on a surface to be scanned by a single fθ lens via an optical deflector. By optimizing the lens shape (main scanning surface shape) in the surface, the curvature of field, distortion, etc. are corrected, and the lens shape in the main scanning surface is independent of the lens shape in the main scanning surface. By eliminating the non-uniformity of the lateral magnification in the sub-scanning direction between the optical deflector and the surface to be scanned using only the lens shape (sub-scanning surface shape), the change in the F-number (FNo) in the sub-scanning direction due to the image height, It is an object of the present invention to provide a scanning optical device that can suppress changes in the spot diameter and is compact and suitable for high-definition printing.

  A second object of the present invention is to form a plurality of light beams from a light source converted by a collimator lens or the like on a surface to be scanned by an fθ lens via an optical deflector, and to scan the main scanning surface of the fθ lens. Lens shape (main scanning surface shape) is optimized to correct curvature of field, distortion, etc., and the lens in the sub-scanning surface is independent of the lens shape in the main scanning surface. By eliminating the nonuniformity of the lateral magnification in the sub-scanning direction between the optical deflector and the surface to be scanned only by the shape (sub-scanning surface shape), the change in the F-number (FNo) in the sub-scanning direction due to the image height, that is, Suitable for compact and high-definition printing that suppresses changes in the spot diameter and can scan with high accuracy without causing bending of the scanning line even for a light beam from a light source deviating from the optical axis in the sub-scanning direction. To provide a scanning optical device.

In the scanning optical device according to the first aspect of the present invention, the light beam emitted from the light source means is imaged in the form of a long line in the main scanning direction on the deflection surface of the deflection element via the first optical element and the second optical element. A scanning optical device that forms a plurality of light beams deflected by the deflecting element in a spot shape on a scanned surface via a third optical element and scans the scanned surface;
The third optical element is composed of at least two lens surfaces, and the axis in the optical axis direction is changed by continuously changing the curvature of the two lens surfaces in the sub-scanning direction from on-axis to off-axis. The main plane position in the upper sub-scanning direction is a position closer to the surface to be scanned than the main plane position in the sub-scanning direction off the axis, and thereby the sub-scanning direction due to the image height of the light beam incident on the surface to be scanned It is characterized by suppressing changes in F number.

In the scanning optical device according to the second aspect of the present invention, the light beam emitted from the light source means is imaged in the form of a long line in the main scanning direction on the deflection surface of the deflection element via the first optical element and the second optical element. A scanning optical device that forms a plurality of light beams deflected by the deflecting element in a spot shape on a scanned surface via a third optical element and scans the scanned surface;
The third optical element is composed of a single lens, and by continuously changing the curvature in the sub-scanning direction of both lens surfaces of the single lens from the on-axis to the off-axis, the on-axis in the optical axis direction. The main plane position in the sub-scanning direction is a position closer to the surface to be scanned than the main plane position in the sub-scanning direction off the axis, and thereby F in the sub-scanning direction due to the image height of the light beam incident on the surface to be scanned. It is characterized by suppressing changes in numbers.

According to a third aspect of the present invention, the light beam emitted from the light source means is imaged in the form of a long line in the main scanning direction on the deflection surface of the deflection element via the first optical element and the second optical element. A scanning optical device that forms a plurality of light beams deflected by the deflecting element in a spot shape on a scanned surface via a third optical element and scans the scanned surface;
The third optical element is composed of at least two lenses, and by continuously changing the curvature in the sub-scanning direction of at least two lens surfaces of the two lenses from on-axis to off-axis, The main plane position in the sub-scanning direction on the axis in the optical axis direction makes the main plane position in the sub-scanning direction off the axis closer to the surface to be scanned, and thereby the light beam incident on the surface to be scanned The feature is that the change in the F-number in the sub-scanning direction due to the image height is suppressed.

  According to a fourth aspect of the present invention, in the first, second, or third aspect of the invention, the curvature in the sub-scanning direction of at least two lens surfaces of the third optical element is asymmetric with respect to the optical axis from the on-axis to the off-axis. It is characterized by changing to.

  According to a fifth aspect of the present invention, in the first, second, or third aspect of the present invention, a trajectory connecting the main plane position in the sub-scanning direction in the main scanning direction is a convex shape on the scanned surface side. .

According to a sixth aspect of the present invention, the light beam emitted from the light source means is imaged in the form of a long line in the main scanning direction on the deflection surface of the deflection element via the first optical element and the second optical element. A scanning optical device that forms a plurality of light beams deflected by the deflecting element in a spot shape on a scanned surface via a third optical element and scans the scanned surface;
The third optical element is constituted by at least two lens surfaces, and the curvature in the sub-scanning direction of the two lens surfaces is continuously changed asymmetrically from on-axis to off-axis to thereby change the surface to be scanned. It is characterized in that the change in the F number in the sub-scanning direction due to the image height of the incident light beam is suppressed.

  A laser beam printer according to a seventh aspect of the invention includes the scanning optical device according to any one of the first to sixth aspects and a photosensitive drum surface as the surface to be scanned.

  According to the present invention, it is possible to obtain a scanning optical device that facilitates high-definition printing.

  First, means for achieving the object of the present invention will be described before describing an embodiment of the scanning optical apparatus of the present invention. In order to achieve the above-described object in the present scanning optical apparatus, it is necessary to optimize the lens shape of the fθ lens and to align the lateral magnification in the sub-scanning direction on and off the axis. FIG. 27 is a cross-sectional view of the main part in the main scanning direction between the optical deflector (deflection element) of the scanning optical apparatus and the surface to be scanned. Here, in order to align the lateral magnification in the sub-scanning direction on the axis and off-axis, it is necessary to determine the main plane position so that the ratio of the optical path lengths on the axis and off-axis is equal.

Therefore,
Ipri: Epri = Imar: Emar
Ipri · Emar = Epri · Imar (a)
However,
Ipri ... Front main in the sub-scanning direction from the deflecting surface of the optical deflector in the axial light beam
Distance to the plane Epri... From the back main plane in the sub-scanning direction of the axial light beam to the surface to be scanned
Distance Imar ... front side in the sub-scanning direction from the deflecting surface of the optical deflector in the most off-axis light beam
Distance to main plane Emar: From the rear main plane in the sub-scanning direction in the most off-axis light beam to the scanned surface
The main plane position in the sub-scanning direction of the fθ lens is determined so as to satisfy the condition of the distance at.

In general, the off-axis light beam is refracted in the direction of the optical axis in the main scanning plane in order to satisfy the fθ characteristic. Therefore, in the main scanning plane in the main plane in the sub-scanning direction to satisfy the above formula (a). As shown in FIG. 27, the locus 71 is a surface that is off-axis and curved toward the optical deflector 5 side. Here, if the amount of bending off the most axis is dx, Emar = (Epri + dx) / cos θimg
Imar = (Ipri−dx) / cos θ por
Therefore,
Ipri (Epri + dx) / cos θimg
= Epri (Ipri-dx) / cos [theta] img
dx (Ipri · cos θ por + Epri · cos θ img)
= Ipri · Epri (cos θimg -cos θ por)

However,
θpor is the angle formed by the most off-axis light beam deflected by the optical deflector in the main scanning plane and the optical axis of the fθ lens. The angle to make.

  Therefore, in order to align the horizontal magnification in the sub-scanning direction, it is necessary to set the curvature dx of the trajectory of the main plane in the sub-scanning direction to a value derived from the above equation (b).

That is, in an actual scanning optical apparatus, the amount of curvature of the locus in the main scanning plane of the front main plane and the rear main plane in the sub-scanning direction of the fθ lens (the main plane position off the most axis and the main plane position on the axis). Xm ≦ dx ≦ xu (1) where xm and xu are the differences in the optical axis direction)
It is desirable to determine the main plane position so as to satisfy the following condition.

  If the above conditional expression (1) is deviated, the lateral magnification in the sub-scanning direction varies, and the change in the spot diameter due to the image height becomes large, causing a problem in practical use.

  Next, there is a method of changing the main plane position in the sub-scanning direction. As described above, this means that the deflection surface of the optical deflector and the surface to be scanned are optically conjugate in the sub-scanning direction of the fθ lens. Therefore, the refractive power of the fθ lens itself cannot be changed.

  Accordingly, the main plane position is moved by bending the first lens surface (R1 surface) and the second lens surface (R2 surface) in the sub-scanning direction of the fθ lens. The main plane of the lens can be moved by bending without changing the refractive power of the lens itself, so that the concentric line r is continuously changed from on-axis to off-axis to make the lens shape optimal depending on the location. Thus, the horizontal magnification in the sub-scanning direction can be made uniform.

  By optimizing the lens shape of the fθ lens in this way, the F number (FNo) in the sub-scanning direction of the light beam incident on the surface to be scanned can be made uniform, which has conventionally been a problem with a single ball fθ lens. The change in the spot diameter in the sub-scanning direction due to the image height can be suppressed small.

  In addition, it is possible to scan the surface to be scanned with high accuracy with respect to a light beam emitted from a light source (light source unit) deviated from the optical axis, and this is suitable for multi-beam scanning. A scanning optical device can be obtained.

  Next, each embodiment of the present invention will be described in order.

  1A and 1B are cross-sectional views of main parts in the main scanning direction and the sub-scanning direction, respectively, according to the first embodiment of the present invention.

  In the figure, reference numeral 1 denotes a light source means (light source unit), which is composed of, for example, a semiconductor laser.

  Reference numeral 2 denotes a collimator lens as a first optical element, which converts a divergent light beam (light beam) emitted from the light source means 1 into a convergent light beam. Reference numeral 3 denotes an aperture stop, which adjusts the diameter of a passing light beam.

  A cylindrical lens 4 as a second optical element has a predetermined refractive power only in the sub-scanning direction, and a light deflector (deflection element) to be described later in the sub-scan section of the light beam that has passed through the aperture stop 3. ) 5 is formed as a substantially linear image on the deflecting surface 5a.

  Reference numeral 5 denotes an optical deflector composed of, for example, a polygon mirror (rotating polygonal mirror) as a deflecting element, which 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 6 denotes an fθ lens (imaging optical system) composed of a single lens having an fθ characteristic as a third optical element, and the light from the middle between the optical deflector 5 and the photosensitive drum surface 8 as a surface to be scanned. Arranged on the deflector 5 side. Both lens surfaces of the fθ lens 6 in the present embodiment are both aspherical toric surfaces in the main scanning surface, and are in the sub-scanning surface (a surface that includes the optical axis of the third optical element and is orthogonal to the main scanning surface). Is continuously changed from the on-axis to the off-axis within the effective portion of the lens. Thereby, in this embodiment, the change in the F number (FNo) in the sub-scanning direction, that is, the change in the spot diameter due to the image height of the light beam incident on the surface to be scanned 8 is suppressed to be small. The fθ lens 6 forms an image of a light beam based on the image information deflected and reflected by the optical deflector 5 on the surface of the photosensitive drum 8 and corrects the tilting of the deflecting surface of the optical deflector 5.

  In the present embodiment, the fθ lens 6 may be manufactured by plastic molding, or may be manufactured by glass molding (glass molding).

  In this embodiment, the divergent light beam emitted from the semiconductor laser 1 is converted into convergent light by the collimator lens 2, and the light beam (light amount) is limited by the aperture stop 3 and is incident on the cylindrical lens 4. Out of the light beam incident on the cylindrical lens 4, the light beam exits as it is in the main scanning section. In the sub-scan section, the light beam is focused and formed on the deflecting surface 5a of the optical deflector 5 as a substantially line image (a line image that is long in the main scanning direction). The light beam deflected and reflected by the deflecting surface 5a of the optical deflector 5 is guided onto the surface of the photosensitive drum 8 through the fθ lens 6 having different refractive powers in the main scanning direction and the sub-scanning direction, and the light deflection is performed. The surface of the photosensitive drum 8 is scanned in the direction of arrow B by rotating the device 5 in the direction of arrow A. Thereby, an image is recorded on the photosensitive drum 8 which is a recording medium.

  In the present embodiment, the lens shape of the fθ lens 6 is an aspherical shape in which the main scanning direction can be expressed by a function up to the 10th order, and the sub-scanning direction is constituted by a spherical surface that continuously changes in the image height direction. The lens shape is, for example, the intersection of the fθ lens 6 and the optical axis as the origin, the optical axis direction is the X axis, the axis orthogonal to the optical axis in the main scanning plane is the Y axis, and the optical axis is orthogonal to the sub scanning plane. When the axis to be operated is the Z axis, the bus direction corresponding to the main scanning direction is

  (Where R is the radius of curvature, and K, B4, B6, B8, and B10 are aspherical coefficients), and the sub-scanning direction (the direction perpendicular to the main scanning direction including the optical axis). The corresponding line direction is

(Where r ′ = r (1 + D2Y2 + D4Y4 + D6Y6 + D8Y8 + D10Y10)
It can be expressed by the following formula.

  Table 1 shows the optical arrangement and the aspherical coefficient of the fθ lens 6 in this embodiment.

  FIG. 4 is an explanatory diagram showing the change in the curvature in the sub-scanning direction with respect to the longitudinal position of the fθ lens 6. As shown in the figure, the meniscus curvature is tight on the axis, and as it goes from on-axis to off-axis. It turns out that it becomes plano-convex. FIG. 7 is an explanatory diagram showing the aspherical shape of the fθ lens 6. The thick solid line is the lens surface shape in the main scanning direction, and the thin solid line is the locus of the main plane in the sub-scanning direction. Is shown.

In this embodiment, the curvature dx of the trajectory of the main plane for suppressing the change in the lateral magnification in the sub-scanning direction due to the image height is Ipri = 48.73 Epi = 108.77
θ por = 44.4 ° θ img = 29.10 °
Than
dx = 6.50
It becomes. The curvature amount xm of the trajectory of the front main plane in the sub-scanning direction of the fθ lens 6 and the curvature amount xu of the trajectory of the rear main plane are xm = 3.24 xu = 7.48.
These values satisfy the above-described conditional expression (1) (xm ≦ dx ≦ xu).

  As a result, in the present embodiment, the lateral magnification in the sub-scanning direction between the optical deflector 5 and the surface to be scanned 8 can be aligned to a level that does not cause any practical problem on the axis and off-axis, as shown in FIG. A change in the spot diameter in the sub-scanning direction due to the height can be suppressed small. This achieves an inexpensive scanning optical device suitable for high-definition printing.

  2A and 2B are cross-sectional views of main parts in the main scanning direction and the sub-scanning direction, respectively, according to the second embodiment of the present invention. In the figure, the same elements as those shown in FIG.

  This embodiment is different from the first embodiment described above in that a divergent light beam emitted from a semiconductor laser (light source unit) is converted into a parallel light beam instead of a convergent light beam by a collimator lens, and accordingly, an fθ lens. The lens shape is different. Other configurations and optical actions are substantially the same as those of the first embodiment described above, thereby obtaining the same effects.

  Table 2 shows the optical arrangement and the aspherical coefficient of the fθ lens 26 in this embodiment.

  FIG. 5 is an explanatory diagram showing the change in curvature in the sub-scanning direction with respect to the longitudinal position of the fθ lens 26, and the meniscus curvature becomes stiffer from the on-axis to the off-axis as shown in FIG. I understand. FIG. 8 is an explanatory view showing the aspherical shape of the fθ lens 26. The thick solid line is the lens surface shape in the main scanning direction, and the thin solid line is the locus of the main plane in the sub-scanning direction. Is shown.

In the present embodiment, the curvature dx of the trajectory of the main plane for suppressing the change in the lateral magnification in the sub-scanning direction due to the image height is Ipri = 53.94 Epi = 147.51.
θ por = 42.0 ° θ img = 24.57 °
Than
dx = 7.60
It becomes. The curvature amount xm of the trajectory of the front main plane in the sub-scanning direction of the fθ lens 26 and the curvature amount xu of the trajectory of the rear main plane are xm = 7.34 xu = 12.31.
These values satisfy the above-described conditional expression (1) (xm ≦ dx ≦ xu).

  As a result, in this embodiment, as in the first embodiment, the lateral magnification in the sub-scanning direction between the optical deflector 5 and the surface to be scanned 8 can be aligned to a level where there is no practical problem on and off the axis. As shown in FIG. 11, the change in the spot diameter in the sub-scanning direction due to the image height can be suppressed small. This achieves an inexpensive scanning optical device suitable for high-definition printing.

  In this embodiment, the divergent light beam emitted from the semiconductor laser 1 is converted into a parallel light beam by the collimator lens 2 as described above, so that there is no jitter due to the optical deflector and power in the sub-scanning direction is generated with priority. Since the lens shape of the lens surface R2 in the main scanning direction is similar to the shape of the trajectory of the main plane for aligning the lateral magnification, the lateral magnification can be aligned even if there is little change in the curvature in the child line direction due to the image height. This makes it possible to achieve a scanning optical device suitable for further high-definition printing.

  3A and 3B are cross-sectional views of main parts in the main scanning direction and the sub-scanning direction, respectively, according to the third embodiment of the present invention. In the figure, the same elements as those shown in FIG.

  In this embodiment, the difference from the first embodiment is that a plurality of light beams emitted from the light source means 11 having a plurality of (two in this embodiment) light source units that can be independently modulated are constant on the surface to be scanned. This is because the lens shape of the fθ lens in the sub-line direction (sub-scanning direction) is made different from the point constituted by the multi-beam scanning optical system that simultaneously scans so as to be the interval. Other configurations and optical actions are substantially the same as those of the first embodiment described above, thereby obtaining the same effects. The plurality of light source units are arranged at predetermined intervals in the sub-scanning direction.

  Table 3 shows the optical arrangement and the aspherical coefficient of the fθ lens 36 in this embodiment.

  In the present embodiment, the lens shape in the sub-line direction of at least one lens surface of the fθ lens 36 is set so that the sign of curvature is reversed from on-axis to off-axis. Therefore, the direction of the sub-line corresponding to the sub-scanning direction of the fθ lens 36 is

Here, r ′ = r + d2 Y2 + d4 Y4 + d6 Y6 + d8 Y8
+ D10Y10
It is expressed by the following formula. The bus direction corresponding to the main scanning direction is expressed by the same equation (c) as in the first embodiment.

  FIG. 6 is an explanatory diagram showing a change in curvature in the sub-scanning direction with respect to the longitudinal position of the fθ lens 36 in the present embodiment. As shown in FIG. 6, the lens surface R1 is sub-scanned from the on-axis toward the off-axis. It can be seen that the sign of the curvature of the direction is reversed, and the meniscus shape on the axis changes to a biconvex shape off the axis. FIG. 9 is an explanatory view showing the aspherical shape of the fθ lens 36, where the thick solid line is the lens surface shape in the main scanning direction, and the thin solid line is the locus of the main plane in the sub-scanning direction, the front main plane and the rear main plane. Is shown.

In this embodiment, the curvature dx of the trajectory of the main plane for suppressing the change in the lateral magnification in the sub-scanning direction due to the image height is Ppri = 48.73 Epi = 108.77
θ por = 44.4 ° θ img = 29.10 °
Than
dx = 6.50
It becomes. The curvature amount xm of the trajectory of the front main plane in the sub-scanning direction of the fθ lens 36 and the curvature amount xu of the trajectory of the rear main plane are xm = 4.93 xu = 9.10.
These values satisfy the above-described conditional expression (1) (xm ≦ dx ≦ xu).

  As a result, in the present embodiment, the lateral magnification in the sub-scanning direction between the optical deflector 5 and the surface to be scanned 8 is aligned to a level that causes no practical problem on the axis and off-axis, as in the first and second embodiments. Thus, the change in the spot diameter in the sub-scanning direction due to the image height can be suppressed to be small. This achieves an inexpensive scanning optical device suitable for high-definition printing.

  Further, in the present embodiment, since it is a multi-beam scanning optical device that simultaneously scans the surface to be scanned using a plurality of light beams, the curve of the scanning line on the surface to be scanned becomes uneven in pitch on the image, which is not good. .

  Therefore, in this embodiment, the curvature of the scanning line on the surface to be scanned can be eliminated as shown in FIG. 12 by continuously changing the curvature radius in the sub-scanning direction according to the image height in the effective portion of the lens. A high-quality scanning optical device (multi-beam scanning optical device) without pitch unevenness is achieved.

  FIGS. 13A and 13B are cross-sectional views of main parts in the main scanning direction and the sub-scanning direction, respectively, according to the fourth embodiment of the present invention. In the figure, the same elements as those shown in FIG.

  In the figure, reference numeral 46 denotes an fθ lens (imaging optical system) composed of a single lens having an fθ characteristic as a third optical element, which is intermediate between the optical deflector 5 and the photosensitive drum surface 8 as a scanned surface. Further, it is arranged on the optical deflector 5 side.

  Both the lens surfaces of the fθ lens 46 in the present embodiment continuously change the curvature in the sub-scanning direction from on-axis to off-axis. Thereby, in the present embodiment, the change in the F number in the sub-scanning direction due to the image height of the light beam incident on the surface to be scanned, that is, the change in the spot diameter, is suppressed to be small. Further, the sign of the curvature in the sub-scanning direction of at least one lens surface (first surface) R1 of both lens surfaces of the fθ lens 46 is reversed from the on-axis to the off-axis. Further, the curvature in the sub-scanning direction of both lens surfaces of the fθ lens 46 is changed from on-axis to off-axis so as to be asymmetric with respect to the optical axis. The fθ lens 46 forms a plurality of light beams based on the image information deflected and reflected by the optical deflector 5 on the surface of the photosensitive drum 8 and corrects the surface tilt of the deflecting surface of the optical deflector 5.

  In the present embodiment, the fθ lens 46 may be manufactured by plastic molding, or may be manufactured by glass molding (glass molding).

  In the present embodiment, the two independently modulated divergent light beams emitted from the semiconductor laser 11 are converted into a convergent light beam by the collimator lens 2, and the light beam (light quantity) is limited by the aperture stop 3 to be incident on the cylindrical lens 4. ing. Out of the light beam incident on the cylindrical lens 4, the light beam exits as it is in the main scanning section. In the sub-scan section, the light beam converges and forms a substantially line image (line image long in the main scanning direction) on the deflecting surface 5a of the optical deflector 5. The two light beams deflected and reflected by the deflecting surface 5a of the optical deflector 5 form two spots on the surface of the photosensitive drum 8 through the fθ lens 46 having different refractive powers in the main scanning direction and the sub-scanning direction. Then, the surface of the photosensitive drum 8 is scanned in the direction of arrow B by rotating the optical deflector 5 in the direction of arrow A. Thus, image recording is performed.

  In the present embodiment, the lens shape of the fθ lens 46 is an aspherical shape in which the main scanning direction can be expressed by a function up to the tenth order, and the sub-scanning direction is constituted by a spherical surface that continuously changes in the image height direction. The lens shape is such that the generatrix direction corresponding to the main scanning direction is shown in the above equation (c), and the sub-scanning direction (the direction orthogonal to the main scanning direction including the optical axis of the fθ lens). The corresponding line direction is

It can be expressed by the following formula.

  In general, in order to make the uneven pitch visually inconspicuous in a multi-beam scanning optical apparatus, it is desirable that the uneven pitch due to the scanning line bend is 1/10 or less of the beam pitch in the sub-scanning direction. For example, in the case of a scanning optical apparatus having a resolution of 600 dpi in the sub-scanning direction, the beam pitch in the sub-scanning direction is 42 μm, and therefore allowable pitch unevenness is about 4 μm or less.

Therefore, in this embodiment, when the maximum value of the F number in the sub-scanning direction of the light beam incident on the surface to be scanned is Fmax and the minimum value is Fmin,
Fmin / Fmax ≧ 0.9 (2)
By continuously changing the curvature in the sub-scanning direction of both lens surfaces of the fθ lens 46 from the axial direction to the off-axis direction so as to satisfy the following condition, it is possible to eliminate scanning line bending on the scanned surface, This achieves a compact, multi-beam scanning optical device with high image quality with little pitch unevenness.

  If the above conditional expression (2) is deviated, the pitch unevenness is visually noticeable and becomes a practical problem due to the scanning line bending.

  Table 4 shows the optical arrangement and the aspherical coefficient of the fθ lens 46 in the fourth embodiment.

FIG. 16 is an explanatory diagram showing changes in the F-number in the sub-scanning direction on the surface to be scanned according to the fourth embodiment. In this embodiment, the curvature of the fθ lens 46 in the sub-scanning direction is continuously changed from on-axis to off-axis on both lens surfaces as shown in FIG.
Fmin / Fmax = 64.52 / 66631 = 0.993
It is suppressed to 0.9 or more.

  FIG. 22 is an explanatory diagram showing scanning line bending when the multi-beam scanning optical apparatus of the present embodiment is used at a resolution of 600 dpi (scanning line interval 42.3 μm). As described above, by suppressing the change in the F-number due to the image height, the scanning line curve can be reduced to a level that is practically no problem at about 0.2 μm (pitch unevenness is about 0.4 μm).

  As described above, in this embodiment, the curvature of the fθ lens 46 in the sub-scanning direction (the sub-line direction) is continuously changed from on-axis to off-axis on both lens surfaces while satisfying the conditional expression (2) as described above. Therefore, the variation in the F-number in the sub-scanning direction due to the image height, that is, the change in the spot diameter, is suppressed to a predetermined amount or less (within the allowable value of the present apparatus), and pitch unevenness due to scanning line bending, which is a problem in the multi-beam scanning optical apparatus. Is missing.

  In the present embodiment, since the third optical element (fθ lens) 46 is constituted by a single lens, a compact and low-cost multi-beam scanning optical apparatus can be achieved.

  14A and 14B are cross-sectional views of main parts in the main scanning direction and the sub-scanning direction, respectively, according to the fifth embodiment of the present invention. In the figure, the same elements as those shown in FIG.

  In this embodiment, the difference from the above-described fourth embodiment is that the curvature in the generatrix direction of both lens surfaces of the fθ lens 56 is used as the optical axis in order to reduce the curvature of field in the main scanning direction so as to cope with higher-definition printing. On the other hand, it is set so as to be asymmetrical, and the number of polygon surfaces of the polygon mirror 15 is changed from 4 to 6 to support high-speed printing. Other configurations and optical actions are substantially the same as those of the above-described fourth embodiment, thereby obtaining the same effects.

  Table 5 shows the optical arrangement and the aspherical coefficient of the fθ lens 56 in the fifth embodiment.

FIG. 17 is an explanatory diagram showing changes in the F-number in the sub-scanning direction on the surface to be scanned according to the fifth embodiment. In the present embodiment, the curvature of the fθ lens 56 in the sub-scanning direction is continuously changed from the on-axis to the off-axis on both lens surfaces as shown in FIG.
Fmin / Fmax = 49.75 / 53.08 = 0.937
It is suppressed to 0.9 or more.

  FIG. 23 is an explanatory diagram showing scanning line bending when the multi-beam scanning optical apparatus of this embodiment is used at a resolution of 600 dpi (scanning line interval 42.3 μm). As described above, by suppressing the change in the F-number due to the image height, the scanning line curve can be reduced to about 1.2 μm (pitch unevenness is about 2.4 μm), which is a practically no problem level.

  As described above, in the present embodiment as well, the curvature in the sub-scanning direction (child-line direction) of the fθ lens 56 is changed from on-axis to off-axis on both lens surfaces while satisfying the conditional expression (2) as in the above-described fourth embodiment. By changing continuously, the change in the F-number in the sub-scanning direction due to the image height, that is, the change in the spot diameter is suppressed to a predetermined amount or less, and the pitch unevenness due to the scanning line bending which is a problem in the multi-beam scanning optical apparatus is eliminated. ing.

  In this embodiment, by setting the curvature in the generatrix direction of both lens surfaces of the fθ lens (third optical element) 56 to be asymmetric with respect to the optical axis, the curvature of field in the main scanning direction is suppressed, A multi-beam scanning optical apparatus suitable for further high-definition printing is achieved.

  15A and 15B are cross-sectional views of main parts in the main scanning direction and the sub-scanning direction, respectively, according to the sixth embodiment of the present invention. In the figure, the same elements as those shown in FIG.

  This embodiment is different from the above-described fourth embodiment in that the fθ lens (third optical element) 76 is composed of two lenses, and the pitch unevenness due to the scanning line bending is reduced with further accuracy. The collimator lens 2 converts the light beam from the semiconductor laser 11 having a plurality of light-emitting portions that can be modulated into a substantially parallel light beam, and the polygon mirror 15 has four to six polygon surfaces for high-speed printing. It is a corresponding point. Other configurations and optical actions are substantially the same as those of the above-described fourth embodiment, thereby obtaining the same effects.

  That is, in the figure, reference numeral 76 denotes an fθ lens as a third optical element, a spherical lens (glass spherical lens) 76a as a first fθ lens made of a glass material, and an aspherical second fθ lens made of a plastic material. And a toric lens (aspheric plastic toric lens) 76b. The glass spherical lens 76a is disposed on the optical deflector 15 side from the middle between the optical deflector 15 and the photosensitive drum surface 8, and mainly has a role of correcting the fθ characteristic. The aspheric plastic toric lens 76b mainly corrects curvature of field and lateral magnification in the sub-scanning direction.

  In this embodiment, the curvature in the sub-line direction (sub-scanning direction) of both lens surfaces of the aspheric plastic toric lens 76b, which bears most of the refractive power in the sub-scanning direction, is continuously increased from on-axis to off-axis. By changing, the change of the F number in the sub-scanning direction on the surface to be scanned, that is, the change of the spot diameter is suppressed.

  Table 6 shows the optical arrangement and the aspherical coefficient of the fθ lens (spherical lens 76a and toric lens 76b) 76 in the sixth embodiment.

FIG. 18 is an explanatory diagram showing changes in the F-number in the sub-scanning direction on the surface to be scanned according to the sixth embodiment. In this embodiment, the curvature of the toric lens 76b in the sub-scanning direction is continuously changed from on-axis to off-axis on both lens surfaces as shown in FIG.
Fmin / Fmax = 72.67 / 73.75 = 0.985
It is suppressed to 0.9 or more.

  FIG. 24 is an explanatory diagram showing scanning line bending when the multi-beam scanning optical apparatus of this embodiment is used at a resolution of 600 dpi (scanning line interval 42.3 μm). As described above, by suppressing the change in the F-number due to the image height, the scanning line curve can be reduced to a practically no problem level of about 0.1 μm (pitch unevenness is about 0.2 μm).

  As described above, in this embodiment as well, the curvature in the sub-scanning direction (the sub-line direction) of the toric lens 76b constituting the fθ lens 76 is satisfied for both lens surfaces while satisfying the conditional expression (2) as in the fourth embodiment. By continuously changing from the on-axis to the off-axis, the change in the F-number in the sub-scanning direction due to the image height, i.e., the change in spot diameter, is suppressed to a predetermined amount or less, and scanning that poses a problem in the multi-beam scanning optical apparatus Eliminates pitch unevenness due to line bending. In this embodiment, the fθ lens (third optical element) 76 is composed of two lenses, so that the scanning line bending can be corrected with higher accuracy, and multi-beam scanning suitable for higher-definition printing. Has achieved an optical device.

  It should be noted that the sign of the curvature in the sub-scanning direction of at least one lens surface of the two lenses constituting the third optical element may be reversed from the on-axis to the off-axis, The curvatures in the sub-scanning direction of at least two lens surfaces of the lenses may be formed to change asymmetrically with respect to the optical axis from the on-axis to the off-axis. As a result, it is possible to achieve a multi-beam scanning optical apparatus more suitable for high-definition printing.

  Finally, for comparison with the scanning optical apparatus of the present invention, the state when multi-beam scanning is performed with the conventional single beam scanning optical apparatus shown in FIGS. 26A and 26B will be described. To do.

  FIGS. 28A and 28B show the main scanning direction and the sub-scanning direction when multi-beam scanning is performed using the conventional single beam scanning optical device shown in FIGS. 26A and 26B, respectively. It is principal part sectional drawing, and shows the change of the angular magnification of the subscanning direction by the image height, and the spot diameter (F number) of the subscanning direction in a to-be-scanned surface. Table 7 shows the aspherical coefficients of the optical arrangement and the fθ lens 86 shown in FIGS.

  In the figure, two independently modulated divergent light beams emitted from a light source means 81 having two light emitting portions arranged at a predetermined interval in the sub-scanning direction are converged by a collimator lens 82, and the light beams are converged by a diaphragm 83. The light quantity is limited and is incident on the cylindrical lens 84 having a predetermined refractive power only in the sub-scanning direction. Out of the light beam incident on the cylindrical lens 84, it exits as it is in the main scanning plane. In the sub-scan section, the light beam is converged and formed as a substantially linear image on the deflecting surface (reflecting surface) of the optical deflector 85 including a rotating polygon mirror (polygon mirror). Then, the two light beams deflected and reflected by the deflecting surface 85a of the optical deflector 85 are guided to the photosensitive drum surface as the scanned surface 88 via the imaging optical system (fθ lens) 86 having the fθ characteristic, By rotating the optical deflector 85 in the direction of arrow A, the surface of the photosensitive drum 88 is optically scanned to record image information.

  Usually, in the surface tilt correction optical system, as described above, in order to optically correct the surface tilt of the deflecting surface of the optical deflector, the deflecting surface and the surface to be scanned are optically conjugate (imaging relationship) and There is a need to. In the comparative example shown in FIG. 28 described above, the curvature of the lens surface (first surface) R1 of the fθ lens 86 on the optical deflector 85 side in the sub-scanning direction (child-line curvature) is constant, and the lens surface on the scanned surface side. (Second surface) By continuously changing the curvature in the sub-scanning direction (child-line curvature) of R2 from the on-axis to the off-axis, the conjugate relation is set at any image height.

  However, since the fθ lens 86 of the above comparative example has a single-sided (R1 surface) sub-curvature curvature as shown in FIG. 30, the F-number (FNo) depends on the image height depending on the generatrix shape as shown in FIG. It varies. That is, on the axis (on-axis light beam), as shown in (A) in FIG. 28B, since the F-number in the sub-scanning direction on the surface to be scanned is large, the angular magnification in the sub-scanning direction is small and off-axis ( In the off-axis light beam, as shown in (B) of FIG. 28B, the F-number in the sub-scanning direction is small, so the angular magnification is large (they may be reversed depending on the main scanning surface shape).

In general, the angular magnification γ and lateral magnification β are
βγ = -1
Therefore, in the above comparative example, the lateral magnification is large on the axis and the lateral magnification is small outside the axis. For this reason, the horizontal magnification in the sub-scanning direction can vary depending on the image height, and in an optical system that scans using a plurality of laser beams that deviate from the optical axis, such as a multi-beam scanning optical device, the scanning line is on the scanned surface. Causes a bend.

  FIG. 31 is an explanatory diagram showing scanning line bending when the multi-beam scanning optical apparatus of the comparative example is used at a resolution of 600 dpi (scanning line interval 42.3 μm). In the figure, since the scanning line in the peripheral portion is bent by 2.4 μm with respect to the central portion, there is a problem that the pitch unevenness becomes 4.8 μm and the image quality is deteriorated.

  In the scanning optical apparatus of the present invention, the above-mentioned problems do not occur, and according to the first invention, the light beam from the light source converted by the collimator lens or the like as described above is used as a single fθ lens via the optical deflector. When forming an image on the surface to be scanned by the above, by optimizing the lens shape of the fθ lens, the field curvature, distortion, etc. are corrected well, and the sub-scanning between the optical deflector and the surface to be scanned is performed. The fθ lens suitable for compact and high-definition printing, which can suppress the change in the F-number in the sub-scanning direction due to the image height, that is, the change in the spot diameter, by eliminating the nonuniformity of the horizontal magnification in the direction, and the A scanning optical device can be achieved.

  According to the second invention, when the plurality of light beams from the light source converted by the collimator lens or the like as described above are imaged on the surface to be scanned by the fθ lens via the optical deflector, the fθ lens By optimizing the lens shape, the image height can be corrected by correcting the curvature of field, distortion, etc., and eliminating the non-uniformity of lateral magnification in the sub-scanning direction between the optical deflector and the scanned surface. Therefore, it is possible to achieve an fθ lens and a scanning optical apparatus having the fθ lens that can suppress a change in the F number in the sub-scanning direction, that is, a change in spot diameter, and can reduce pitch unevenness due to scanning line bending.

  Furthermore, by determining the curvature of the fθ lens in the sub-scanning direction so as to satisfy the conditional expression (2), it is possible to achieve a scanning optical device that can reduce pitch unevenness to a level that causes no visual problem. .

Sectional drawing of the principal part of the main scanning direction of Embodiment 1 of this invention and a subscanning direction Sectional drawing of the principal part of the main scanning direction and sub-scanning direction of Embodiment 2 of this invention Sectional drawing of the principal part of the main scanning direction and subscanning direction of Embodiment 3 of this invention Explanatory drawing which shows the aspherical shape of ftheta lens in Embodiment 1 of this invention Explanatory drawing which shows the aspherical shape of ftheta lens in Embodiment 2 of this invention Explanatory drawing which shows the aspherical shape of ftheta lens in Embodiment 3 of this invention Explanatory drawing which shows the shape of the main scanning direction of the fθ lens in Embodiment 1 of the present invention. Explanatory drawing which shows the shape of the main scanning direction of the fθ lens in Embodiment 2 of the present invention. Explanatory drawing which shows the shape of the fscan lens in the main scanning direction in Embodiment 3 of the present invention. Explanatory drawing which shows the defocus characteristic of the spot diameter of the subscanning direction in the to-be-scanned surface in Embodiment 1 of this invention. Explanatory drawing which shows the defocus characteristic of the spot diameter of the subscanning direction in the to-be-scanned surface in Embodiment 2 of this invention. Explanatory drawing which shows the curve of the scanning line in Embodiment 3 of this invention. Sectional drawing of the principal part of the main scanning direction and subscanning direction of Embodiment 4 of this invention Sectional drawing of the principal part of the main scanning direction and subscanning direction of Embodiment 5 of this invention Sectional drawing of the principal part of the main scanning direction and subscanning direction of Embodiment 6 of this invention Explanatory drawing which shows the change of the F number of the subscanning direction on the to-be-scanned surface with respect to the image height in Embodiment 4 of this invention. Explanatory drawing which shows the change of the F number of the subscanning direction on the to-be-scanned surface with respect to the image height in Embodiment 5 of this invention. Explanatory drawing which shows the change of the F number of the subscanning direction on the to-be-scanned surface with respect to the image height in Embodiment 6 of this invention. Explanatory drawing which shows the curvature of the sub line direction of f (theta) lens with respect to the image height in Embodiment 4 of this invention. Explanatory drawing which shows the curvature of the child line direction of ftheta lens with respect to the image height in Embodiment 5 of this invention Explanatory drawing which shows the curvature of the child line direction of ftheta lens with respect to the image height in Embodiment 6 of this invention. Explanatory drawing which shows the curve of the scanning line at the time of the multi-beam scanning by the resolution 600dpi (scanning line space | interval 42.3micrometer) in Embodiment 4 of this invention. Explanatory drawing which shows the curve of the scanning line at the time of the multi-beam scanning by the resolution 600 dpi (scanning line space | interval 42.3 micrometers) in Embodiment 5 of this invention. Explanatory drawing which shows the curve of the scanning line at the time of the multi-beam scanning by the resolution 600dpi (scanning line space | interval 42.3micrometer) in Embodiment 6 of this invention. Schematic diagram of the main part of an optical system of a conventional scanning optical device Cross-sectional view of main parts in a main scanning direction and a sub-scanning direction of a conventional scanning optical device Cross-sectional view of the principal part in the main scanning direction between the deflection element of the scanning optical device and the surface to be scanned FIG. 26 is a cross-sectional view of the main part in the main scanning direction and the sub-scanning direction when multi-beam scanning is performed using the conventional single beam scanning optical apparatus shown in FIG. Explanatory drawing which shows the change of F number of the subscanning direction on the to-be-scanned surface with respect to the image height of the multi-beam scanning optical apparatus shown in FIG. Explanatory drawing which shows the curvature of the sub line direction of the fθ lens with respect to the image height of the multi-beam scanning optical apparatus shown in FIG. Explanatory drawing showing the bending of the scanning line at the time of multi-beam scanning at a resolution of 600 dpi (scanning line interval 42.3 μm) of the multi-beam scanning optical apparatus shown in FIG.

Explanation of symbols

1,11 Light source means
2 First optical element (collimator lens)
3 Aperture
4 Second optical element (cylindrical lens)
5,15 Deflection element (optical deflector)
6, 26, 36 Third optical element (fθ lens)
46, 56, 76 Third optical element (fθ lens)
76a Spherical lens
76b toric lens
8 Scanned surface (photosensitive drum)
21 On-axis luminous flux
22 Most off-axis luminous flux

Claims (7)

A light beam emitted from the light source means is imaged in a linear shape in the main scanning direction on the deflection surface of the deflection element via the first optical element and the second optical element, and a plurality of beams deflected by the deflection element are formed. In a scanning optical device that scans the scanned surface by forming a light beam in a spot shape on the scanned surface via a third optical element,
The third optical element is composed of at least two lens surfaces, and the axis in the optical axis direction is changed by continuously changing the curvature of the two lens surfaces in the sub-scanning direction from on-axis to off-axis. The main plane position in the upper sub-scanning direction is a position closer to the surface to be scanned than the main plane position in the sub-scanning direction off the axis, and thereby the sub-scanning direction due to the image height of the light beam incident on the surface to be scanned A scanning optical device characterized in that the change in the F number of the optical system is suppressed. A light beam emitted from the light source means is imaged in a linear shape in the main scanning direction on the deflection surface of the deflection element via the first optical element and the second optical element, and a plurality of beams deflected by the deflection element are formed. In a scanning optical device that scans the scanned surface by forming a light beam in a spot shape on the scanned surface via a third optical element,
The third optical element is composed of a single lens, and by continuously changing the curvature in the sub-scanning direction of both lens surfaces of the single lens from the on-axis to the off-axis, the on-axis in the optical axis direction. The main plane position in the sub-scanning direction is a position closer to the surface to be scanned than the main plane position in the sub-scanning direction off the axis, and thereby F in the sub-scanning direction due to the image height of the light beam incident on the surface to be scanned. A scanning optical device characterized by suppressing a change in number. A light beam emitted from the light source means is imaged in a linear shape in the main scanning direction on the deflection surface of the deflection element via the first optical element and the second optical element, and a plurality of beams deflected by the deflection element are formed. In a scanning optical device that scans the scanned surface by forming a light beam in a spot shape on the scanned surface via a third optical element,
The third optical element is composed of at least two lenses, and by continuously changing the curvature in the sub-scanning direction of at least two lens surfaces of the two lenses from on-axis to off-axis, The main plane position in the sub-scanning direction on the axis in the optical axis direction makes the main plane position in the sub-scanning direction off the axis closer to the surface to be scanned, and thereby the light beam incident on the surface to be scanned A scanning optical device characterized in that a change in F number in the sub-scanning direction due to the image height of the image is suppressed.   4. The curvature in the sub-scanning direction of at least two lens surfaces of the third optical element changes from an on-axis to an off-axis and asymmetrically with respect to the optical axis. The scanning optical device described.   The scanning optical apparatus according to claim 1, 2, or 3, wherein a trajectory connecting the main plane position in the sub-scanning direction in the main scanning direction has a convex shape on the scanned surface side. A light beam emitted from the light source means is imaged in a linear shape in the main scanning direction on the deflection surface of the deflection element via the first optical element and the second optical element, and a plurality of beams deflected by the deflection element are formed. In a scanning optical device that scans the scanned surface by forming a light beam in a spot shape on the scanned surface via a third optical element,
The third optical element is constituted by at least two lens surfaces, and the curvature in the sub-scanning direction of the two lens surfaces is continuously changed asymmetrically from on-axis to off-axis to thereby change the surface to be scanned. A scanning optical device characterized in that a change in F number in the sub-scanning direction due to an image height of an incident light beam is suppressed.   7. A laser beam printer comprising: the scanning optical apparatus according to claim 1; and a photosensitive drum surface as the scanned surface.