JP7328040B2 - image forming device - Google Patents

Description

The present invention relates to image forming apparatuses such as copiers and printers.

As an image forming apparatus using electrophotographic technology, as disclosed in Patent Document 1, a color image is formed by arranging photosensitive drums corresponding to four colors of yellow, magenta, cyan, and black and developing devices in a substantially straight line. Forming devices are known. In Patent Document 1, one optical scanning unit that irradiates laser light to each photosensitive drum is arranged above a plurality of photosensitive drums arranged side by side.

In the optical scanning unit of Patent Document 1, a laser beam from a light source is deflected by a single deflector and irradiated onto each photosensitive drum via a mirror. In this optical scanning unit, the optical path length from the deflector to the photosensitive drums of the optical system for scanning the inner two photosensitive drums out of the four photosensitive drums is shorter than the optical path length to the outer two photosensitive drums. As a result, the thickness of the optical scanning unit is reduced. As a result, it is possible to reduce the height of the image forming apparatus.

Japanese Unexamined Patent Application Publication No. 2016-33598

However, in the optical scanning unit described in Patent Document 1, when one photosensitive drum is scanned with a plurality of laser beams (hereinafter referred to as "multi-beams"), the plurality of laser beams respectively scan different portions of the same lens. pass. For this reason, it has been found that the scanning magnification in the scanning direction (hereinafter referred to as "main scanning direction") differs between the plurality of laser beams.

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide an image forming apparatus in which the amount of deviation in scanning magnification between multi-beams is small.

A representative configuration of an image forming apparatus according to the present invention for achieving the above object includes a first photosensitive drum, a second photosensitive drum, a first laser beam corresponding to image information, and a second laser beam. a first light source that emits a first multi-beam having light; a second light source that emits a second multi-beam having a first laser beam and a second laser beam according to image information; a deflector for deflecting the first multi-beam emitted from the first light source and the second multi-beam emitted from the second light source ; an optical scanning unit that forms an image on a first photosensitive drum and forms an image of the second multi-beam deflected by the deflector on the second photosensitive drum; In an image forming apparatus for forming an image on a recording material, the first laser light and the second laser light of the first multi-beam have different positions in the sub-scanning direction passing through the imaging lens, The first laser light and the second laser light of the second multi-beam are different in position in the sub-scanning direction passing through the imaging lens, and the first laser light of the first multi-beam is the the radius of curvature of the lens surface in the main scanning cross section of the imaging lens through which the first laser beam passes is larger than the radius of curvature of the lens surface in the main scanning cross section of the imaging lens through which the second laser beam passes; The incident angle α1 of the first laser beam with respect to the normal to the irradiation position of the first laser beam on the first photosensitive drum is the normal to the irradiation position of the second laser beam on the first photosensitive drum . A curvature radius of a lens surface in a main scanning cross section of the imaging lens through which the first laser beam of the second multi-beam passes, which is smaller than the incident angle β1 of the second laser beam with respect to the line . is larger than the radius of curvature of the lens surface in the main scanning cross section of the imaging lens through which the second laser beam passes, and the normal to the irradiation position of the first laser beam on the second photosensitive drum. The incident angle α2 of the first laser beam is smaller than the incident angle β2 of the second laser beam with respect to a normal to the position of the second photosensitive drum irradiated with the second laser beam, and the incident angle α2 is It is characterized by being larger than the incident angle α1 .

According to the present invention, it is possible to provide an image forming apparatus with a small amount of deviation in scanning magnification of multi-beams.

1 is a cross-sectional view showing the configuration of an image forming apparatus according to a first embodiment; FIG. FIG. 4 is a cross-sectional view showing how a laser beam emitted from a light source reaches a reflecting surface of a rotating polygon mirror provided in a deflector in the optical scanning unit of the first embodiment; 4 is a cross-sectional view showing how laser light reflected by a reflecting surface of a rotating polygon mirror reaches the surface of each photosensitive drum in the optical scanning unit of the first embodiment; FIG. FIG. 2 is a diagram showing optical paths of laser light that scans a photosensitive drum in the first embodiment; 5A is a diagram showing the JJ cross section at the center of the image of the imaging lens shown in FIG. 4 and the BB cross section at the end of the image superimposed. (b) shows the CC section at the center of the imaging lens in the sub-scanning direction shown in (a), the EE section at the top of the imaging lens in the sub-scanning direction, and the sub-scanning direction of the imaging lens. is a view showing a DD cross section between the CC cross section and the EE cross section in FIG. 1 is a diagram showing the configuration of a semiconductor laser according to a first embodiment; FIG. 2 is a perspective view showing the configuration of an imaging lens according to the first embodiment; FIG. FIG. 4 is a diagram showing how the surface of a photosensitive drum is scanned with multi-beams passing through different positions in the sub-scanning direction in the first embodiment; FIG. 2 is a view of a state in which multi-beams are incident on the surface of a photosensitive drum as viewed from the main scanning direction in the first embodiment; It is the figure which looked at FIG. 9 from the arrow G direction. 3 is a cross-sectional view showing the configuration of the left half of the optical scanning unit of the first embodiment; FIG. FIG. 5 is a cross-sectional view showing the configuration of an optical scanning unit according to a second embodiment; FIG. 11 is a cross-sectional view showing the configuration of an optical scanning unit according to a third embodiment; FIG. 11 is a top plan view of a laser beam reflected by a reflecting surface of a rotating polygon mirror of a deflector according to a third embodiment; It is a figure which shows the structure of a part of imaging optical system of 3rd Embodiment. FIG. 11 is a cross-sectional view showing how multi-beams emitted from an optical scanning unit according to a third embodiment are incident on a photosensitive drum;

An embodiment of the image forming apparatus according to the present invention will be specifically described with reference to the drawings. However, the dimensions, materials, shapes, relative positions, etc. of the components described in the following embodiments should be appropriately changed according to the configuration of the apparatus to which the present invention is applied and various conditions. Accordingly, it is not intended that the scope of the invention be limited solely thereto unless specifically stated.

[First embodiment]
A configuration of a first embodiment of an image forming apparatus according to the present invention will be described with reference to FIGS. 1 to 11. FIG.

<Image forming apparatus>
First, the configuration of the image forming apparatus 100 will be described with reference to FIG. FIG. 1 is a cross-sectional view showing the configuration of an image forming apparatus 100. As shown in FIG. An image forming apparatus 100 shown in FIG. 1 is an example of a laser beam printer that forms an image on a recording material S such as paper using an electrophotographic method. The right side of FIG. 1 is the front side of the image forming apparatus 100 . The left side of FIG. 1 is the rear side of the image forming apparatus 100 . The front-rear direction of the image forming apparatus 100 is the direction from the rear side to the front side of the image forming apparatus 100 (forward direction) and the opposite direction (rear direction).

In the image forming apparatus 100, four process cartridges PY, PM, PC, and PK corresponding to yellow Y, magenta M, cyan C, and black K are arranged substantially horizontally from the rear side to the front side. be. Such a configuration is called an in-line configuration or a tandem type. Incidentally, one of the four process cartridges PY, PM, PC, and PK may be referred to as the process cartridge P in some cases.

Each process cartridge P has photosensitive drums 11a, 11b, 11c, and 11d as image carriers. One of the four photosensitive drums 11a, 11b, 11c, and 11d may be referred to as the photosensitive drum 11 in some cases. The same applies to other process means necessary for image formation.

Each process cartridge P further has developing rollers 12 a , 12 b , 12 c and 12 d as process means acting on each photosensitive drum 11 . Each photosensitive drum 11 and each developing roller 12 are assembled in each process cartridge P integrally. Of the four process cartridges P, the black K process cartridge PK has a larger toner capacity than the other three color process cartridges PY, PM, and PC, and is therefore taller.

An optical scanning unit 2 is provided above each process cartridge P. As shown in FIG. The optical scanning unit 2 irradiates each photosensitive drum 11 of each process cartridge P with laser beams L1 to L4 corresponding to each color to form an electrostatic latent image on the surface of each photosensitive drum 11 . Subsequently, the electrostatic latent image is developed by each developing roller 12 to form a toner image on the surface of each photosensitive drum 11 . Below each process cartridge P, a transfer unit 20 is provided. In the transfer unit 20, an intermediate transfer belt 21 stretched by a drive roller 22, a tension roller 23, and a driven roller 24 is provided so as to rotate clockwise in FIG.

Each photosensitive drum 11 of each process cartridge P is in contact with the outer peripheral surface of the intermediate transfer belt 21 . Four primary transfer rollers 25a, 25b, 25c, and 25d are arranged on the inner peripheral surface side of the intermediate transfer belt 21 so as to face the photosensitive drums 11 of the process cartridges P, respectively. Each photosensitive drum 11 and the intermediate transfer belt 21 form a primary transfer nip portion N1. By applying a primary transfer bias to each primary transfer roller 25, the toner images formed on the surface of each photosensitive drum 11 at each primary transfer nip portion N1 are sequentially transferred and superimposed on the outer peripheral surface of the intermediate transfer belt 21. be. A secondary transfer roller 26 is in contact with the drive roller 22 with the intermediate transfer belt 21 interposed therebetween.

A fixing unit 30 and a pair of discharge rollers 40 are provided on the rear side (left side in FIG. 1) of the image forming apparatus 100 . A discharge tray 50 is provided on the upper surface of the image forming apparatus 100 . The fixing unit 30 has a fixing film 31 and a pressure roller 32 . The discharge roller pair 40 has a discharge roller 41 and a discharge roller 42 . The discharge tray 50 has a slope portion 51 .

A recording material S such as paper stacked in a feeding tray 61 of the feeding unit 60 is fed by a feeding roller 62 rotating clockwise in FIG. At this time, the multi-fed recording materials S are separated one by one by the separation roller 63 and conveyed. Next, the recording material S is sent to the secondary transfer nip portion N2 formed by the intermediate transfer belt 21 and the secondary transfer roller 26 . At this time, a secondary transfer bias is applied to the secondary transfer roller 26, so that the toner image formed on the outer peripheral surface of the intermediate transfer belt 21 is collectively transferred onto the recording material S at the secondary transfer nip portion N2. be done.

The recording material S to which the toner image has been transferred is sent to a fixing nip portion N3 formed between the fixing film 31 and the pressure roller 32. As shown in FIG. At the fixing nip portion N3, the toner image is fixed on the recording material S by being heated and pressed. The recording material S on which the toner image is fixed is nipped and conveyed by the discharge roller 41 and the discharge roller 42 to be discharged onto the discharge tray 50 and stacked on the slope portion 51 by the weight of the recording material S itself.

Each process cartridge P is arranged substantially in a line in the image forming apparatus 100 . Of the four color toners, black K consumes more toner than the other three colors because, for example, it is used in monochrome printing. For this reason, in order to reduce the frequency of replacement of the process cartridge P and reduce the size of the image forming apparatus 100, it is effective to make the toner capacity of the black K larger than that of the others.

When replacing each process cartridge P, first, the front cover 90 is opened by rotating in the direction of arrow R1 around the rotation shaft 90a, and the handle 13a of the support unit 13 that detachably supports each process cartridge P is gripped. to pull out the support unit 13 in the direction of the arrow U. Then, each process cartridge P is pulled out from the support unit 13 . When installing a new process cartridge P, the operation is reversed.

In the image forming apparatus 100 shown in FIG. 1, a large-capacity, tall process cartridge PK is arranged on the frontmost side (downstream side in the direction of arrow U) of the apparatus main body 100a. That is, the process cartridge PK is located closest to the opening 1 when the front cover 90 is opened. Accordingly, when replacing the process cartridge PK, which is the heaviest among the four process cartridges, it is not necessary to pull out the support unit 13 in the direction of the arrow U by a large amount.

<Optical scanning unit>
Next, the configuration of the optical scanning unit 2 will be described with reference to FIGS. 2 and 3. FIG. FIG. 2 is a sectional view showing how the laser beams L1 and L2 emitted from the semiconductor lasers 3a and 3b are reflected by the reflecting surface 7 of the rotary polygon mirror 14 provided in the deflector 4. As shown in FIG. FIG. 3 is a cross-sectional view showing optical paths of the laser beams L1 to L4.

As shown in FIGS. 2 and 3, the optical scanning unit 2 is provided with semiconductor lasers 3a, 3b, 3c and 3d (3c and 3d are not shown) and imaging lenses 8, 9, 16 and 17. there is The semiconductor lasers 3a, 3b, 3c, and 3d are light sources that emit laser beams L1 to L4 corresponding to image information, respectively.

As shown in FIG. 2, in the optical scanning unit 2, an incident optical system including semiconductor lasers 3a and 3b, collimator lenses 5a and 5b, and a cylindrical lens 6 is arranged vertically. A deflector 4 having a rotating polygonal mirror 14 having a plurality of reflecting surfaces 7 is arranged ahead of it.

The rotating polygon mirror 14 is rotated around a rotating shaft 14 a by a motor provided in the deflector 4 . The incident optical system shown in FIG. 2 is a plane perpendicular to the axial direction F of the rotating shaft 14a, and the laser beams L1 and L2 are emitted obliquely to a plane Q passing through the center position of the reflecting surface 7 in the axial direction F. is incident. This is called an oblique incidence optical system.

The laser beams L1 and L2 emitted from the semiconductor lasers 3a and 3b pass through the collimator lenses 5a and 5b respectively, pass through the cylindrical lens 6, and form an image in the sub-scanning direction near the reflecting surface 7 of the rotary polygon mirror 14. connect the caustic lines. By causing the laser beams L1 and L2 to obliquely enter the reflecting surface 7, the optical path of the laser beam L1 after being reflected by the reflecting surface 7 and the optical path of the laser beam L2 are separated in the axial direction F. Incidentally, the incident optical system for the laser beams L3 and L4 shown in FIG. 3 is similarly constructed.

Next, a configuration of an optical path for guiding the laser light L deflected by the deflector 4 to each photosensitive drum 11 will be described with reference to FIG. The optical scanning unit 2 shown in FIG. 3 is provided with a deflector 4 at its center. Imaging lenses 8 and 16 are provided outside the deflector 4, respectively. Furthermore, outside the imaging lenses 8 and 16, there are provided imaging lenses 9 and 17 in front of the respective photosensitive drums 11, respectively. Furthermore, mirrors 10a, 10b, 18a and 18c are provided outside the imaging lenses 9 and 17, respectively. Mirrors 10c and 18b are provided above the imaging lenses 8 and 16, respectively.

As described with reference to FIG. 2, the laser beams L1 and L2 obliquely incident on the reflecting surface 7 are reflected at predetermined angles .theta.1 and .theta.2 with respect to the plane Q shown in FIG. 3, respectively.

Both of the two laser beams L1 and L2 pass through the imaging lens 8 and the imaging lens 9 . Of the laser light L that has passed through the imaging lens 9, the laser light L1 that passes below the plane Q reaches the surface of the photosensitive drum 11a after being reflected by the mirror 10a. Of the laser light L that has passed through the imaging lens 9, the laser light L2 that passes above the plane Q is reflected by the mirror 10b, and after being reflected by the mirror 10c, reaches the surface of the photosensitive drum 11b.

Next, optical paths of the laser beams L3 and L4 reflected to the right side of FIG. 3 by the reflecting surface 7 will be described. The laser beams L3 and L4 incident on the deflector 4 are reflected by the reflecting surface 7 of the rotary polygon mirror 14 in the same manner as the laser beams L1 and L2 on the left side of FIG. The laser beams L3 and L4 incident on the reflecting surface 7 are reflected by the reflecting surface 7 at predetermined angles .theta.3 and .theta.4 with respect to the plane Q shown in FIG.

Both the two laser beams L3 and L4 pass through the imaging lens 16 and the imaging lens 17 . Of the laser beams L that have passed through the imaging lens 17, the laser beam L3 that passes above the plane Q is reflected by the mirror 18a, and after being reflected by the mirror 18b, reaches the surface of the photosensitive drum 11c.

Of the laser light L that has passed through the imaging lens 17, the laser light L4 that passes below the plane Q reaches the surface of the photosensitive drum 11d after being reflected by the mirror 18c. As for the optical path lengths from the reflecting surface 7 to the photosensitive drums 11a to 11d, the optical path lengths of the laser beams L1 and L4 are shorter than the optical path lengths of the laser beams L2 and L3. Since the optical path lengths of the laser beams L1 and L4 are short, the positions of the mirrors 10a and 18c can be brought closer to the deflector 4. FIG. Therefore, the size of the optical scanning unit 2 in the left and right direction can be reduced in the plane of FIG. In order to establish such a relationship of optical path lengths, the shapes of the four imaging lenses 8, 9, 16, and 17 are determined such that the positions through which the laser beams L2 and L3 pass and the positions through which the laser beams L1 and L4 pass. and are different.

Although the laser beams L1 to L4 shown in FIGS. 2 and 3 are indicated by a single solid line for convenience of explanation, actually, as shown in FIG. Laser beams (multi-beams) La and Lb are emitted.

Next, the reason why scanning magnification deviation occurs when scanning the same photosensitive drum 11a with a plurality of laser beams La and Lb in an optical scanning unit employing an oblique incidence optical system will be described with reference to FIGS. 4 to 8. FIG. .

FIG. 4 is a diagram showing the optical path of the laser light L1 that scans the photosensitive drum 11a. A laser beam L1 emitted from the semiconductor laser 3a passes through a collimator lens 5a and a cylindrical lens 6. As shown in FIG. After that, the laser beam L1 passes through the diaphragm 134, is deflected by the rotary polygon mirror 14, and passes through the imaging lens 8 and the imaging lens 9. FIG. After that, an image is formed on the surface of the photosensitive drum 11a. Among the laser beams L1, the laser beam passing through the center of the image in the main scanning direction is denoted by L11, and the laser beam passing through the edge of the image in the main scanning direction is denoted by L12.

Next, the shape of the surface of the imaging lens 9 on the side from which the laser light L is emitted (hereinafter referred to as "lens surface 9a") will be described with reference to FIG. First, referring to FIG. 5A, the cross-sectional shape of the imaging lens 9 cut in the sub-scanning direction will be described. FIG. 5(a) is a view showing the JJ cross section and the BB cross section of the imaging lens 9 shown in FIG. 4 superimposed. Reference numeral 34 indicates a lens surface of the imaging lens 9 on the JJ section, and reference numeral 35 indicates a lens surface of the imaging lens 9 on the BB section.

As shown in FIG. 5A, the radius of curvature of lens surface 35 is greater than the radius of curvature of lens surface 34 . The reason is that, as shown in FIG. 4, the optical path length of the laser light L11 passing through the area of the imaging lens 9 corresponding to the image center is longer than the optical path length of the laser light L11 passing through the area of the imaging lens 9 corresponding to the image edge. This is because the optical path length of the light L12 is longer.

In the case of an optical system having a function of correcting the tilt of the reflecting surface 7 of the rotating polygon mirror 14, the shape of the lens is set so as to satisfy the conjugate relationship over the entire scanning area of the laser beam L1. In that case, the power (refractive power=reciprocal of the focal length) in the sub-scanning direction of the area of the imaging lens 9 through which the laser beam L11 passes is power becomes smaller. Therefore, as shown in FIG. 5A, the radius of curvature of the lens surface 9a in the sub-scanning direction increases from the center (surface 34) in the main scanning direction toward the end (surface 35).

Next, the cross-sectional shape of the lens surface 9a when the imaging lens 9 is cut in the main scanning direction will be described with reference to FIG. 5(b). FIG. 5(b) shows the CC cross section of the imaging lens 9 shown in FIG. 5(a), the EE cross section, and the DD cross section between the CC cross section and the EE cross section. It is the figure which overlapped and showed.

Reference numeral 36 indicates the shape of the lens surface 9a in the CC section. Reference numeral 38 indicates the shape of the lens surface 9a in the EE cross section (main scanning cross section). Reference numeral 37 indicates the shape of the lens surface 9a in the DD cross section (main scanning cross section). As shown in FIG. 5B, the radii of curvature of the lens surfaces 37 and 38 are gradually larger than the radius of curvature of the central lens surface 36 in the sub-scanning direction of the imaging lens 9 .

<Semiconductor laser>
Next, the configuration of the semiconductor laser 3 will be described with reference to FIG. As shown in FIG. 6, the semiconductor laser 3 emits a plurality of laser beams (multi-beams) La and Lb.

<Imaging lens>
Next, the configuration of the imaging lens 9 will be described with reference to FIG. As shown in FIG. 7, when a plurality of laser beams La and Lb pass through the imaging lens 9, the passing positions of the laser beams La and Lb are different in the sub-scanning direction. Further, since the oblique incidence optical system is used, the laser beams La and Lb pass through a position away from the lens center of the imaging lens 9 in the sub-scanning direction.

As described with reference to FIG. 5B, the radius of curvature of the lens surface 9a when the imaging lens 9 is cut in the main scanning direction increases with distance from the center of the imaging lens 9 in the sub-scanning direction in the sub-scanning direction. Become. Therefore, as shown in FIG. 7, the laser light La passing through a region far from the center of the imaging lens 9 in the sub-scanning direction is higher than the laser light Lb passing through a region near the center of the imaging lens 9 in the sub-scanning direction. Scanning magnification is increased.

FIG. 8 is a diagram showing how the laser beams La and Lb scan the surface of the photosensitive drum 11a. As described above, the laser beam La has a higher scanning magnification in the main scanning direction than the laser beam Lb. Therefore, when the rotating polygon mirror 14 has the same rotation angle, the scanning range of the laser light La incident on the surface of the photosensitive drum 11a is wider by ΔJ1 on both sides in the main scanning direction than the scanning range of the laser light Lb.

As described above, when the laser beams La and Lb spaced apart in the sub-scanning direction pass through the imaging lens 9, the scanning magnifications of the laser beams La and Lb are different. different. This tendency is remarkable in a lens having a strong power in the sub-scanning direction.

It should be noted that the irradiation position deviation of the laser beams La and Lb in the main scanning direction that occurs in the oblique incidence optical system described above is hereinafter referred to as "first dot deviation (dot deviation due to oblique incidence)". In this embodiment, an example in which the laser beam L1 is composed of a multi-beam including a plurality of laser beams La and Lb has been described. However, since the laser beams L2 to L4 are also multi-beams having a plurality of laser beams La and Lb, the first dot deviation occurs in the laser beams L2 to L4 as well as the laser beam L1.

As described with reference to FIG. 3, in the image forming apparatus 100 of this example, the optical path lengths of the laser beams L2 and L3 are longer than the optical path lengths of the laser beams L1 and L4. At this time, the longer the optical path length from the lens surfaces 9a and 17a of the imaging lenses 9 and 17 to the surface of each photosensitive drum 11, the larger the first dot deviation. This is because, as shown in FIG. 8, the laser beams La and Lb emitted from the lens surface 9a of the imaging lens 9 are not parallel in the main scanning direction but have a predetermined angle. Therefore, the laser beams L2 and L3 with longer optical path lengths have a larger first dot deviation than the laser beams L1 and L4 with shorter optical path lengths.

Next, with reference to FIGS. 9 and 10, the irradiation position deviation in the main scanning direction that occurs when a plurality of laser beams La and Lb forming a multi-beam are obliquely incident on the surface of the photosensitive drum 11 will be described. do. FIG. 9 is a view of the laser beams La and Lb incident on the surface of the photosensitive drum 11 as viewed in the main scanning direction from one end of the photosensitive drum 11 in the rotation axis direction. A laser beam La emitted from the light emitting portion of the semiconductor laser 3 is incident on the surface of the photosensitive drum 11 at an incident angle α with respect to the normal line H1 at the irradiation position Ra on the surface of the photosensitive drum 11 . Here, the incident angle α is the angle formed by the normal line H1 at the irradiation position Ra and the laser beam La. The normal line H1 is a straight line passing through the rotation center 15 of the photosensitive drum 11 and the irradiation position Ra.

On the other hand, laser light Lb emitted from a different light emitting portion of the same semiconductor laser 3 follows substantially the same trajectory as laser light La and is incident on irradiation position Rb on the surface of photosensitive drum 11 . A pitch p2 in the sub-scanning direction between the irradiation position Ra of the laser beam La and the irradiation position Rb of the laser beam Lb is adjusted according to the resolution.

Let β be the incident angle of the laser light Lb in the sub-scanning direction with respect to the normal H2 at the irradiation position Rb. The normal line H2 is a straight line passing through the rotation center 15 of the photosensitive drum 11 and the irradiation position Rb. As shown in FIG. 9, the optical path length of the laser beam Lb is longer by ΔL than the optical path length of the laser beam La. At this time, in the vicinity of the surface of the photosensitive drum 11, the laser beams La and Lb are incident on the surface of the photosensitive drum 11 substantially in parallel.

FIG. 10 is a view of FIG. 9 viewed in the direction of arrow G, and shows how the laser beams La and Lb are incident on the surface of the photosensitive drum 11 . As shown in FIG. 10, the laser beams La and Lb are projected onto the surface of the photosensitive drum 11 at an angle γ with respect to the direction orthogonal to the main scanning direction in the area corresponding to the edge of the image on the surface of the photosensitive drum 11 . Incident. The image of the laser light Lb incident on the surface of the photosensitive drum 11 whose optical path length is increased by ΔL with respect to the optical path length of the laser light La is shifted outward by ΔJ2 (=ΔL×tan γ) in the main scanning direction. will occur.

As shown in FIG. 9, the optical path length difference ΔL between the laser light La and the laser light Lb can be expressed by the following formula (1). The symbol r is the radius of the photosensitive drum 11, the symbol α is the incident angle of the laser beam La, and the symbol β is the incident angle of the laser beam Lb.

[Number 1]
ΔL=r×(cosβ−cosα)

Also, the irradiation position deviation ΔJ2 can be expressed by the following equation (2). The symbol ΔL is the optical path length difference between the laser beams La and Lb, and the symbol γ is the angle of the laser beams La and Lb with respect to the direction orthogonal to the main scanning direction.

[Number 2]
ΔJ2=ΔL×tanγ

Substituting Equation 1 into Equation 2, the irradiation position deviation ΔJ2 can be expressed by Equation 3 below.

[Number 3]
ΔJ2=r×(cosβ-cosα)×tanγ

Assuming that the difference between the angles α and β shown in FIG.

[Number 4]
p2=2πr×δ/360

When .delta. is obtained from Equation 4 above, it is represented by Equation 5 below.

[Number 5]
δ=(p2)×180/πr

On the other hand, from β=α+δ, β is expressed by the following Equation 6 using Equation 5 above.

[Number 6]
β=α+(p2)×180/πr

Here, assuming that the resolution (pixel density) is D (dot/inch), 1 inch is 25.4 mm. be

[Number 7]
p2 = 25.4/D [mm]

By substituting Equation 7 into Equation 6, the incident angle β is given by Equation 8 below.

[Number 8]
β=α+4572/(π×r×D)

As described above, the first dot misalignment is determined by the optical system. On the other hand, the irradiation position deviation ΔJ2 in the main scanning direction caused by the photosensitive drum 11 shown in FIG. It can be changed by changing the incident angle α of La. In this case, the radius r of the photosensitive drum 11 and the resolution D are constant in the above equation (8).

Therefore, by optimizing the angles and positions at which the laser beams La and Lb are incident on the surface of the photosensitive drum 11, the first dot deviation can be canceled out by the second dot deviation.

Here, the first dot deviation is ΔJ1 [mm]. At this time, if the sum of ΔJ1 and the second dot displacement ΔJ2 [mm] is 1/10 or less of the pitch p2 in the sub-scanning direction, it is desirable because the amount of dot displacement is below the amount that can be discerned by the human eye. That is, since the pitch p2 in the sub-scanning direction is 25.4/D [mm] with respect to the resolution D, it is sufficient that the range satisfies the following formula (9).

[Number 9]
|ΔJ1+ΔJ2|<(1/10)×p2

Substituting Equation 3 into Equation 9 gives the following Equation 10.

[Number 10]
|ΔJ1+r×(cosβ−cosα)×tanγ|<(1/10)×p2

Substituting Equation 8 and p2=25.4/D [mm] into Equation 10 yields Equation 11 below.

[Number 11]
|ΔJ1+r×(cos(α+4572/(π×r×D))−cosα)×tan γ|<2.54/D

Therefore, the incident angle α may be set so as to satisfy the relationship of Equation 11 according to the values of ΔJ1, r, D, and γ which are known in advance.

Next, it will be described with reference to FIG. 11 how the first dot deviation ΔJ1 and the second dot deviation ΔJ2 cancel each other out. FIG. 11 is a sectional view showing the configuration of the left half of the optical scanning unit 2. As shown in FIG. FIG. 11 shows optical paths of laser beams L1a (solid line) and laser beams L1b (dotted line), and optical paths of laser beams L2a (solid line) and laser beams L2b (dotted line).

Let R1a be the irradiation position of the laser beam L1a on the surface of the photosensitive drum 11a, and R1b be the irradiation position of the laser beam L1b on the surface of the photosensitive drum 11a. Further, let R2a be the irradiation position on the surface of the photosensitive drum 11b of the laser beam L2a, and R2b be the irradiation position of the laser beam L2b on the surface of the photosensitive drum 11b.

Next, laser beams L1a and L1b will be described. In FIG. 11, when the laser beams L1a and L1b pass through the imaging lens 9 having a strong power in the sub-scanning direction, the laser beam L1a moves to a position farther from the center of the imaging lens 9 in the sub-scanning direction than the laser beam L1b. pass. At this time, as described above with reference to FIGS. 5B, 7 and 8, the scanning magnification of the laser beam L1a becomes larger than that of the laser beam L1b.

Therefore, the laser beam L1a causes a first dot deviation ΔJ11 (not shown) corresponding to the first dot deviation ΔJ1 in FIG. generate. The laser beam L1a is incident at an incident angle α1 in the sub-scanning direction with respect to the normal line H1a at the irradiation position R1a. Similarly, the laser beam L1b is incident at an incident angle β1 with respect to the normal line H1b at the irradiation position R1b.

By the way, if each member is arranged so that the optical path length of the first laser beam La is shorter than the optical path length of the second laser beam Lb, ΔJ22 ( (not shown) can cancel the first dot deviation ΔJ11. As a result, dot misalignment that occurs on the surface of the photosensitive drum 11a is reduced.

Next, the laser light L2a and the laser light L2b forming the multi-beam incident on the second photosensitive drum 11b will be described. The laser beam L2a (solid line) passes through a position farther from the center of the imaging lens 9 than the laser beam L2b (dotted line) in the sub-scanning direction.

At this time, the scanning magnification of the laser beam L2a becomes larger than the scanning magnification of the laser beam L2b for the reason described above. Therefore, the irradiation position R2a of the laser light L2a generates a first dot shift ΔJ12 corresponding to the first dot shift ΔJ1 in FIG. 8 outward in the main scanning direction with respect to the irradiation position R2b of the laser light L2b. Also, as shown in FIG. 3, the optical path length of the laser beam L2 is longer than the optical path length of the laser beam L1, so the first dot deviation ΔJ12 is larger than the first dot deviation ΔJ11.

The laser beam L2a is incident at an incident angle α2 in the sub-scanning direction with respect to the normal line H2a at the irradiation position R2a on the surface of the photosensitive drum 11b. The laser beam L2b is incident at an incident angle β2 in the sub-scanning direction with respect to the normal line H2b at the irradiation position R2b on the surface of the photosensitive drum 11b.

By arranging the incident angle α2 to be smaller than the incident angle β2, the irradiation position R2a of the laser light L2a generates a second dot shift ΔJ22 inward in the main scanning direction with respect to the irradiation position R2b of the laser light L2b. . As a result, the two types of first dot deviation ΔJ12 and second dot deviation ΔJ22 cancel each other out, so dot deviation occurring on the surface of the photosensitive drum 11b is also reduced. Here, the incident angle α2 is arranged to be larger than the incident angle α1.

Since the first dot deviation ΔJ12 is larger than the first dot deviation ΔJ11, the value of the second dot deviation ΔJ22 that cancels the first dot deviation ΔJ12 needs to be larger than the second dot deviation ΔJ21 that cancels the first dot deviation ΔJ11. Because there is As a result, it is possible to reduce the final main scanning dot deviation difference between the photosensitive drums 11a and 11b and bring the dot deviations of both closer to "0".

Next, a description will be given using specific values. When the resolution D=600, the radius r of the photosensitive drums 11a and 11b=10 mm, the first dot deviation ΔJ11=4.4 μm, and the first dot deviation ΔJ12=6 μm, α1=10.28° and β1=10.52. °, α2=13.58°, β2=13.82°. As a result, the first dot deviations ΔJ11 and ΔJ12 cancel out the second dot deviations ΔJ21 and ΔJ22, respectively. Then, the sum of each of the first dot misalignment and the second dot misalignment is approximately "0". The optical scanning unit 2 is substantially bilaterally symmetrical, and the right half shown in FIG. 3 (the station emitting the laser beam L3 and the station emitting the laser beam L4) has substantially the same configuration as the left half shown in FIG. is.

The direction of the normal line H1a and the direction of the normal line H2a are the same. As a result, the irradiation position R1a on the surface of the first photosensitive drum 11a and the irradiation position R2a on the surface of the second photosensitive drum 11b are in the same phase with respect to the rotational phases of the respective photosensitive drums 11a and 11b.

This also applies to the photosensitive drums 11c and 11d shown in FIG. In FIG. 3, the incident angles of the laser beams L3 and L4 in the sub-scanning direction on the surfaces of the photosensitive drums 11c and 11d differ greatly from the incident angles of the laser beams L1 and L2 on the surfaces of the photosensitive drums 11a and 11b. . However, since the irradiation positions of the laser beams L1 to L4 are in the same phase in the rotational phases of the photosensitive drums 11a to 11d, it is possible to simplify the image forming process after the exposure process.

In the present embodiment, the curvature in the sub-scanning section of the lens surfaces 9a and 17a of the imaging lenses 9 and 17 is continuous from the center to the end in the longitudinal direction of the imaging lenses 9 and 17 within the effective portion of the lens. is changing to However, the surfaces whose curvature continuously changes in the scanning section are not limited to the lens surfaces 9a and 17a of the imaging lenses 9 and 17, and may be other surfaces or a plurality of surfaces.

Furthermore, in the present embodiment, the imaging optical system is composed of two lenses each of the imaging lens 8 and the imaging lens 9, or the imaging lens 16 and the imaging lens 17. However, the imaging optical system is not limited to this. Each lens may be composed of a single lens or three or more lenses. Also, the imaging optical system may include a diffractive optical element.

The optical scanning unit 2 of this embodiment optimizes the oblique incidence angle of the laser light L with respect to the surface of each photosensitive drum 11 for each of a plurality of different optical systems. As a result, it is possible to provide the image forming apparatus 100 with high image quality by canceling out the oblique incident optical system dot deviation and reducing the deviation of the scanning magnification in the main scanning direction of the multi-beams. In addition, since the optical scanning unit 2 has a high degree of freedom in shape, the optical scanning unit 2 can be made thinner, and the size of the image forming apparatus 100 can be reduced.

Further, when a plurality of optical systems having scanning lenses with different shapes are used, the amount of scanning magnification deviation in the main scanning direction that occurs in each optical system also differs. However, in this embodiment, the first dot misalignment and the second dot misalignment cancel each other out so that the scanning magnification of each optical system becomes substantially the same, so a high-quality image can be output.

[Second embodiment]
Next, the configuration of the image forming apparatus according to the second embodiment of the present invention will be described with reference to FIG. It should be noted that components configured in the same manner as in the first embodiment are denoted by the same reference numerals, or even if the reference numerals are different, the same member names are given, and the description thereof is omitted. The optical scanning unit 102 of this embodiment is an optical scanning unit 102 that uses a plurality of different optical systems as in the first embodiment.

This embodiment differs from the first embodiment in that each laser beam L21 to L24 emitted from each semiconductor laser 3 (not shown) reaches the surface of each photosensitive drum 111a to 111d. This is the point where the optical path lengths are equal. Another point is that the scanning speeds of the laser beams L21 to L24 reflected by the reflecting surface 107 of the rotary polygon mirror 14 are the same. It is assumed that each of the laser beams L21 to L24 is configured as a multi-beam having laser beams La and Lb.

Next, the configuration of the optical scanning unit 102 of this embodiment will be described with reference to FIG. FIG. 12 is a cross-sectional view showing the configuration of the optical scanning unit 102 of this embodiment. The optical scanning unit 102 is provided with a plurality of pairs of semiconductor lasers 3 and imaging lenses 109 and 117 (not shown). A plurality of sets of photosensitive drums 111a to 111d are provided.

The optical scanning unit 102 reflects and deflects the laser beams L21 to L24 respectively emitted from a plurality of semiconductor lasers (not shown) by the rotary polygon mirror 14 of the deflector 104 arranged in the center of the optical scanning unit 102 . Then, each of the laser beams L21 to L24 is irradiated onto the plurality of photosensitive drums 111a to 111d via each optical member. Each of the photosensitive drums 111a and 111b is configured as a first photosensitive drum that is irradiated with a plurality of laser beams L21 and L22 that have passed through each first imaging lens 109, respectively. Each of the photosensitive drums 111c and 111d is configured as a second photosensitive drum that is irradiated with a plurality of laser beams L23 and L24 that have passed through each second imaging lens 117, respectively.

The optical members are imaging lenses 108 and 116, imaging lenses 109 and 117, and mirrors 110a-110c and 118a-118c. The imaging lens 109 is configured as a first imaging lens that forms images of the plurality of laser beams L21 and L22 deflected by the deflector 104 on the respective photosensitive drums 111a and 111b. The imaging lens 117 is configured as a second imaging lens that forms images of the plurality of laser beams L23 and L24 deflected by the deflector 104 on the photosensitive drums 111c and 111d, respectively.

In FIG. 12, the left side of the optical scanning unit 102 is the scanning area A1, and the right side is the scanning area A2. The laser beams L21 and L22 distributed to the scanning area A1 and the laser beams L23 and L24 distributed to the scanning area A2 by the deflector 104 will be described in order.

On the scanning area A1 side, the two laser beams L21 and L22 reflected by the reflecting surface 107 of the rotating rotary polygon mirror 14 are incident on a common imaging lens 108 . Of the laser light L that has passed through the imaging lens 108, the laser light L21 passes under the mirror 110a, is reflected by the mirror 110c, passes through the imaging lens 109, and reaches the surface of the photosensitive drum 111a. On the other hand, the laser beam L22 is reflected by the mirror 110a after passing through the imaging lens 108. FIG. After that, the light is reflected again by the mirror 110b, passes through the imaging lens 109, and reaches the surface of the photosensitive drum 111b.

On the scanning area A2 side, on the other hand, the two laser beams L23 and L24 reflected by the reflecting surface 107 of the rotating rotary polygon mirror 14 enter a common imaging lens 116. FIG. Of the laser beams L that have passed through the imaging lens 116, the laser beam L23 is reflected by the mirror 118a. After that, the laser beam L23 passes through the imaging lens 117, is reflected again by the mirror 118b, and reaches the surface of the photosensitive drum 111c.

The laser beam L24 passes over the mirror 118a, is reflected by the mirror 118c, passes through the imaging lens 117, and reaches the surface of the photosensitive drum 111d. On the scanning area A2 side, similarly to the scanning area A1 side, the imaging lens 116 is shared by the laser beams L23 and L24, and the imaging lens 117 is provided for each of the laser beams L23 and L24.

In the optical scanning unit 102 of this embodiment, the laser lights L21 to L24 emitted from the semiconductor lasers (not shown) have the same optical path length until they reach the surfaces of the photosensitive drums 111a to 111d.

However, on the scanning area A1 side and the scanning area A2 side, the optical path lengths of the laser beams L21 to L24 that are emitted from the imaging lenses 109 and 117 and reach the surfaces of the photosensitive drums 111a to 111d are different. there is In order to make the capacity of the process cartridge PK shown in FIG. This is because the

The imaging lens 116 is common to the laser beams L23 and L24. Therefore, each imaging lens 117 is arranged so that the optical path length of the laser beam L23 from the imaging lens 117 to the photosensitive drum 111c and the optical path length of the laser beam L24 from the imaging lens 117 to the photosensitive drum 111d are the same. There is a need to.

Further, each imaging lens 109 on the scanning area A1 side has mirrors 110b and 110c arranged upstream of each imaging lens 109 in the traveling direction of the laser beams L21 and L22, respectively, so that the mirrors 110b and 110c are arranged at the same position as the imaging lens 117. cannot be placed in The arrangement of each optical member differs between the scanning area A1 side and the scanning area A2 side. Therefore, dot misalignment occurring on the surface of each of the photosensitive drums 111a to 111d is also different.

Each imaging lens 117 in the scanning area A2 is farther from the surfaces of the photosensitive drums 111c and 111d than each imaging lens 109 in the scanning area A1. Therefore, the first dot deviation ΔJ14 occurring in the scanning area A2 is larger than the first dot deviation ΔJ13 occurring in the scanning area A1.

As shown in FIG. 12, let α3 be the incident angle of each laser beam L21, L22 with respect to the normal line H21, H22 at each irradiation position R21, R22 of each laser beam L21, L22.

On the other hand, let α4 be the incident angle of the laser beams L23 and L24 with respect to the normal lines H23 and H24 at the irradiation positions R23 and R24 of the laser beams L23 and L24.

In order to cancel the first dot misalignment with the second dot misalignment and to bring the dot misalignment on the surface of each of the photosensitive drums 111a to 111d closer to "0", the relationship between the incident angles α3 and α4 should be α3<α4. good. As in the example of FIG. 12, even in the case of "an optical system having the same optical path length and the same scanning speed but different positions of the lenses in the scanning system", the first dot deviation amount is different. However, by setting the drum incident angle to an optimum value for each optical system, it is possible to cancel out the first dot deviation and the second dot deviation. In this example, since the scanning speed of the laser beam on each drum is substantially the same, it is possible to unify the frequency of the image data, and the optical scanning unit can be provided with an electrically simple configuration.

[Third embodiment]
Next, the configuration of the third embodiment of the image forming apparatus according to the present invention will be described with reference to FIGS. 13 to 16. FIG. It should be noted that components configured in the same manner as those of the above-described embodiments are denoted by the same reference numerals, or even if the reference numerals are different, the same member names are given, and description thereof is omitted. FIG. 13 is a cross-sectional view showing the configuration of the optical scanning unit 121 of this embodiment. It is assumed that each of the laser beams L31 to L34 is composed of a multi-beam consisting of each of the laser beams L31a and L31b as described above in the first embodiment.

The optical scanning unit 121 of this embodiment is configured using two optical scanning units 122 . One optical scanning unit 122 causes laser beams L31 and L32 or laser beams L33 and L34 to enter two photosensitive drums 131a and 131b or two photosensitive drums 131c and 131d, respectively. In order to shorten the length of each optical path from each imaging lens 129, 137 to the surface of each photosensitive drum 131a to 131d, as shown in FIG. It is a converging optical system that is incident on the device 124 .

First, the configuration of the optical scanning unit 122 in which the laser beams L31 and L32 are incident on the surfaces of the photosensitive drums 131a and 131b on the left side of FIG. 13 will be described. A deflector 124 is arranged substantially in the center of the optical scanning unit 122 . A plurality of reflecting surfaces 127 are provided on the rotating polygon mirror 14 of the deflector 124 . As shown in FIGS. 15 and 16, the photosensitive drum 131a is a first photosensitive drum that is irradiated with a plurality of laser beams L31a and L31b that have passed through the imaging lens 129. FIG. The photosensitive drum 131b is a second photosensitive drum that is irradiated with a plurality of laser beams L32a and L32b that have passed through the imaging lens 137. As shown in FIG.

An imaging lens 128 , a mirror 130 and an imaging lens 129 are arranged on the left side of the deflector 124 . A mirror 138 , an imaging lens 136 and an imaging lens 137 are arranged on the right side of the deflector 124 . The imaging lens 129 is a first imaging lens that forms an image of the plurality of laser beams L31a and L31b deflected by the deflector 124 on the photosensitive drum 131a. The imaging lens 137 is a second imaging lens that forms an image of the plurality of laser beams L32a and L32b deflected by the deflector 124 on the photosensitive drum 131b.

As shown in FIG. 14, the laser beams L31 and L32 incident on the reflecting surface 127 are deflected and scanned left and right in FIG. The laser beam L31 deflected to the left in FIG. 16 by the deflector 124 passes through the imaging lens 128, is reflected by the mirror 130, passes through the imaging lens 129, and reaches the surface of the photosensitive drum 131a. On the other hand, the laser beam L32 deflected to the right side in FIG. 16 by the deflector 124 is reflected by the mirror 138, passes through the imaging lens 136 and the imaging lens 137, and reaches the surface of the photosensitive drum 131b.

Here, T1 is the distance from the lens surface 129a of the imaging lens 129 on the side from which the laser beam L31 is emitted to the surface of the photosensitive drum 131a. Further, let T2 be the distance from the lens surface 137a of the imaging lens 137 on the side from which the laser beam L32 is emitted to the surface of the photosensitive drum 131b. At this time, T1>T2.

Note that the optical scanning unit 122 on the right side of FIG. 13 is configured in the same manner as the optical scanning unit 122 on the left side of FIG. 13, and redundant description will be omitted. Also in the optical scanning unit 122 on the right side of FIG. 13, the distance from the lens surface 129a of the imaging lens 129 on the side from which the laser beam L33 is emitted to the surface of the photosensitive drum 131c is assumed to be T3. Further, let T4 be the distance from the lens surface 137a of the imaging lens 137 on the side from which the laser beam L34 is emitted to the surface of the photosensitive drum 131d. At this time, T3>T4.

Next, the laser beams L31 and L32 incident on the deflector 124 of this embodiment will be described with reference to FIG. FIG. 14 is a top plan view of the laser beams L31 and L32 reflected by the reflecting surface 127 of the rotary polygon mirror 14. FIG.

This embodiment is a converging optical system in which the laser beams L31 and L32 enter the reflecting surface 127 of the rotating polygon mirror 14 while converging in the main scanning direction. The laser beams L31 and L32 are incident on the reflecting surface 127 while converging at the convergence angle v and the convergence angle w in the main scanning direction, respectively, and are deflected and scanned by the deflector 124 .

As the convergence angles v and w are increased, the focal length after passing through the imaging lenses 129 and 137 can be shortened. Therefore, the convergence angles v and w at which the laser beams L31 to L34 converge in the main scanning direction are set so that v<w.

That is, as shown in FIGS. 13 and 14, the convergence angle v of the laser light L31 in the main scanning direction is different from the convergence angle w of the laser light L32 in the main scanning direction. The same applies to the laser beams L33 and L34. As a result, as shown in FIG. 13, the distances from the lens surfaces 129a and 137a of the imaging lenses 129 and 137 to the surfaces of the photosensitive drums 131a to 131d are T1>T2 and T3>T4. be able to.

However, also in the case of the converging optical system, dot displacement in the main scanning direction occurs in the multi-beam L31 composed of the plurality of laser beams L31a and L31b shown in FIG. 15, as in the case of the oblique incident optical system. The reason will be explained below. FIG. 15 is a schematic diagram of the scanning optical system in this embodiment. FIG. 15 shows how the laser beams L31a and L31b of the multi-beam L31 scan the surface of the photosensitive drum 131a.

The laser beams L31a and L31b intersect at the diaphragm 134 in the main scanning direction and enter the reflecting surface 127 of the rotary polygon mirror 14. As shown in FIG. The laser beams L31a and L31b deflected by the rotary polygon mirror 14 rotating counterclockwise in FIG. 15 enter the imaging lens 128 at the main scanning interval e1 on the image writing side. A dot shift ΔJS occurs in the main scanning direction on the surface of the photosensitive drum 131a. Also, on the image end side, the laser beams L31a and L31b are incident on the imaging lens 128 at the main scanning interval e2, and dot displacement ΔJE occurs in the main scanning direction on the surface of the photosensitive drum 131a.

Here, the larger the incident angle of the laser beams L31a and L31b with respect to the reflecting surface 127, the wider the main scanning interval. Therefore, the relationship between the main scanning interval e1 on the image writing side and the main scanning interval e2 on the image ending side is expressed by the following equation (12).

[number 12]
e2 > e1

The relationship between the dot deviation .DELTA.JS in the main scanning direction on the image writing side on the surface of the photosensitive drum 131a and the dot deviation .DELTA.JE in the main scanning direction on the image end side on the surface of the photosensitive drum 131a is expressed by the following equation (13). be done.

[Number 13]
ΔJE > ΔJS

In other words, the laser beam L31a that scans first in the main scanning direction (hereinafter referred to as the "preceding laser beam") is the laser beam that scans later in the main scanning direction (hereinafter referred to as the "tracking laser beam"). The overall magnification is larger than that of the laser light L31b, which is referred to as "laser light").

Also, the dot deviation in the main scanning direction (hereinafter referred to as "third dot deviation") ΔJ3 due to the converging optical system can be expressed by the following equation (14).

[Number 14]
ΔJ3=(ΔJE−ΔJS)/2

FIG. 16 shows the optical path along which the laser beams L31a and L31b of the multi-beam L31 of the optical scanning unit 122 are incident on the surface of the photosensitive drum 131a, and the laser beams L32a and L32b of the multi-beam L32 are incident on the surface of the photosensitive drum 131b. It is a figure showing an optical path.

Here, the laser beams L31a and L32a are preceding laser beams, and the laser beams L31b and L32b are tracking laser beams.

In this embodiment, the laser beams L31a and L31b are directed to the photosensitive drum 131a such that the incident angle α31 of the preceding laser L31a (solid line) with respect to the normal line H31a of the photosensitive drum 131a is smaller than the incident angle β31 of the tracking laser L31b (dotted line). 131a. The normal line H31a is the normal line at the irradiation position R31a on which the preceding laser beam L31a is incident. Also, the normal to the irradiation position R31b where the tracking laser beam L31b is incident is H31b, and the incident angle β31 is the angle of the tracking laser beam L31b with respect to the normal H31b.

Therefore, as in the first embodiment described above, a second dot shift occurs such that the overall magnification of the laser beam L31a is smaller than the overall magnification of the laser beam L31b. Therefore, since the third dot misalignment and the second dot misalignment cancel each other out, it is possible to reduce the dot misalignment occurring on the surface of the photosensitive drum 131a.

Similarly for the photosensitive drum 131b, the relationship between the incident angle α32 of the preceding laser beam L32a and the incident angle β32 of the tracking laser beam L32b is α32<β32.

As shown in FIG. 14 and described above, the convergence angles v and w of the laser beams L31 and L32 incident on the reflecting surface 127 of the rotary polygon mirror 14 in the main scanning direction are v<w. Therefore, by setting the relationship between the incident angle α31 and the incident angle α32 to satisfy α31<α32, it is possible to bring the final main scanning dot deviation closer to "0".

In this embodiment, as shown in FIG. 13, by using two optical scanning units 122 in the image forming apparatus 100, an optical system other than the oblique incidence optical system can be adopted. As a result, the optical scanning unit 121 can be thinned, and as a result, the height of the image forming apparatus 100 can be reduced.

In each of the above-described embodiments, for convenience of explanation, the multi-beams emitted from the semiconductor laser 3 (not shown) are assumed to be two. However, the configuration of the above-described embodiment can also be adopted for a device that uses three or more multi-beams.

H1a Normal line at irradiation position R1a H1b Normal line at irradiation position R1b L, L1a, L1b Laser beam R1a Laser beam L1a irradiation position on the surface of photosensitive drum 11a R1b Laser beam L1b surface of photosensitive drum 11a Upper irradiation position α1... Incident angle of laser beam L1a β1... Incident angle of laser beam L1b 3, 3a, 3b... Semiconductor laser (light source)
9 Imaging lens 9a Lens surface 11, 11a to 11d Photosensitive drum

Claims (5)

a first photosensitive drum;
a second photosensitive drum;
A first light source for emitting a first multi-beam having a first laser beam and a second laser beam according to image information, and a first laser beam and a second laser beam according to image information. a second light source that emits a second multibeam; a deflector that deflects the first multibeam emitted from the first light source and the second multibeam emitted from the second light source; The first multi-beam deflected by the deflector is imaged on the first photosensitive drum , and the second multi-beam deflected by the deflector is imaged on the second photosensitive drum. an optical scanning unit having an image lens;
In an image forming apparatus that forms an image on a recording material,
the first laser light and the second laser light of the first multi-beam are different in position in the sub-scanning direction passing through the imaging lens,
the first laser light and the second laser light of the second multi-beam are different in position in the sub-scanning direction passing through the imaging lens,
In the first multi-beam , the curvature radius of the lens surface in the main scanning cross section of the imaging lens through which the first laser beam passes is the main curvature radius of the imaging lens through which the second laser beam passes. An incident angle α1 of the first laser beam with respect to a normal line of the irradiation position of the first laser beam on the first photosensitive drum is greater than the radius of curvature of the lens surface in the scanning cross section of the first photosensitive drum. is smaller than the incident angle β1 of the second laser beam with respect to the normal to the irradiation position of the second laser beam of
In the second multi-beam, the radius of curvature of the lens surface in the main scanning cross section of the imaging lens through which the first laser beam passes is the main curvature radius of the imaging lens through which the second laser beam passes. An incident angle α2 of the first laser beam with respect to a normal line of the irradiation position of the first laser beam on the second photosensitive drum is greater than the radius of curvature of the lens surface in the scanning cross section of the second photosensitive drum. is smaller than the incident angle β2 of the second laser beam with respect to the normal to the irradiation position of the second laser beam of
The image forming apparatus , wherein the incident angle α2 is greater than the incident angle α1 . A direction of a first normal line at a position where the first laser beam of the first multi-beam is incident on the first photosensitive drum and the first laser beam of the second multi-beam 2. The image forming apparatus according to claim 1 , wherein the direction of the second normal line at the position where the light is incident on the second photosensitive drum is the same direction. The optical path length of the second multi-beam from the second light source through the imaging lens to the surface of the second photosensitive drum is determined by passing through the imaging lens from the first light source and reaching the surface of the second photosensitive drum . 3. The image forming apparatus according to claim 1 , wherein the optical path length is longer than the optical path length of said first multi-beam reaching the surface of said first photosensitive drum. a first photosensitive drum;
a second photosensitive drum;
A first light source for emitting a first multi-beam having a first laser beam and a second laser beam according to image information, and a first laser beam and a second laser beam according to image information. a second light source that emits a second multibeam; a deflector that deflects the first multibeam emitted from the first light source and the second multibeam emitted from the second light source; a first imaging lens that forms an image of the first multi-beam deflected by the deflector on the first photosensitive drum; an optical scanning unit having a second imaging lens that forms an image on the photosensitive drum;
In an image forming apparatus that forms an image on a recording material,
The optical path length of the first multi-beam from the point reflected by the deflector to the surface of the first photosensitive drum, and the second optical path length from the point reflected by the deflector to the surface of the second photosensitive drum. is equal to the optical path length of the multibeam of
The optical path length of the first multi-beam from the lens surface of the first imaging lens to the surface of the first photosensitive drum is the length of the optical path from the lens surface of the second imaging lens to the surface of the second photosensitive drum. shorter than the optical path length of the second multi-beam reaching the surface,
the first laser light and the second laser light of the first multi-beam are different in position in the sub-scanning direction passing through the first imaging lens,
the first laser light and the second laser light of the second multi-beam are different in position in the sub-scanning direction passing through the second imaging lens,
In the first multi-beam, the radius of curvature of the lens surface in the main scanning cross section of the first imaging lens through which the first laser light passes is the first multi-beam through which the second laser light passes. is larger than the radius of curvature of the lens surface in the main scanning cross section of the imaging lens, and the incident angle α3 of the first laser beam with respect to the normal to the irradiation position of the first laser beam on the first photosensitive drum is smaller than the angle of incidence of the second laser beam with respect to a normal line at the irradiation position of the second laser beam on the first photosensitive drum;
In the second multi-beam, the radius of curvature of the lens surface in the main scanning cross section of the second imaging lens through which the first laser beam passes is the second multi-beam through which the second laser beam passes. is larger than the radius of curvature of the lens surface in the main scanning cross section of the imaging lens, and the incident angle α4 of the first laser beam with respect to the normal to the irradiation position of the first laser beam on the second photosensitive drum is smaller than the angle of incidence of the second laser beam with respect to a normal line at the irradiation position of the second laser beam on the second photosensitive drum;
The image forming apparatus , wherein the incident angle α4 is larger than the incident angle α3 . a first photosensitive drum;
a second photosensitive drum;
A first light source for emitting a first multi-beam having a first laser beam and a second laser beam according to image information, and a first laser beam and a second laser beam according to image information. a second light source that emits a second multibeam; a deflector that deflects the first multibeam emitted from the first light source and the second multibeam emitted from the second light source; a first imaging lens that forms an image of the first multi-beam deflected by the deflector on the first photosensitive drum; an optical scanning unit having a second imaging lens that forms an image on the photosensitive drum;
In an image forming apparatus that forms an image on a recording material,
The optical path length of the first multi-beam from the lens surface of the first imaging lens to the surface of the first photosensitive drum is the length of the optical path from the lens surface of the second imaging lens to the surface of the second photosensitive drum. longer than the optical path length of the second multi-beam reaching the surface,
a convergence angle v in the main scanning direction of the first multi-beams incident on the deflector is smaller than a convergence angle w in the main scanning direction of the second multi-beams incident on the deflector;
the first laser light and the second laser light of the first multi-beam are different in position in the sub-scanning direction passing through the first imaging lens,
the first laser light and the second laser light of the second multi-beam are different in position in the sub-scanning direction passing through the second imaging lens,
In the first multi-beam, the radius of curvature of the lens surface in the main scanning cross section of the first imaging lens through which the first laser light passes is the first multi-beam through which the second laser light passes. is larger than the radius of curvature of the lens surface in the main scanning cross section of the imaging lens, and the incident angle α31 of the first laser beam with respect to the normal to the irradiation position of the first laser beam on the first photosensitive drum is smaller than the incident angle β31 of the second laser beam with respect to a normal line at the irradiation position of the second laser beam on the first photosensitive drum,
In the second multi-beam, the radius of curvature of the lens surface in the main scanning cross section of the second imaging lens through which the first laser beam passes is the second multi-beam through which the second laser beam passes. is larger than the radius of curvature of the lens surface in the main scanning cross section of the imaging lens, and the incident angle α32 of the first laser beam with respect to the normal to the irradiation position of the first laser beam on the second photosensitive drum is smaller than the incident angle β32 of the second laser beam with respect to a normal line at the irradiation position of the second laser beam on the second photosensitive drum,
The image forming apparatus , wherein the incident angle α32 is greater than the incident angle α31 .

Images