JP5397633B2 - Optical scanning apparatus and image forming apparatus - Google Patents

Description

  The present invention relates to an optical scanning apparatus and an image forming apparatus, and more particularly to an optical scanning apparatus having a polarization separation device that separates light having different polarization directions, and an image forming apparatus including the optical scanning apparatus.

  In electrophotographic image recording, an image forming apparatus using a laser is widely used. In this case, the image forming apparatus includes an optical scanning device, and scans laser light using an optical deflector (for example, a polygon mirror) in the axial direction of a photosensitive drum (hereinafter referred to as “photosensitive drum”). In general, a method of rotating the drum and forming a latent image on the surface of the drum is common.

  In recent years, color formation and speeding-up have progressed in image forming apparatuses, and tandem type image forming apparatuses having a plurality (usually four) photosensitive drums have become widespread.

  An image forming apparatus having a plurality of photosensitive drums has a light source for each photosensitive drum. For example, when there are four photosensitive drums, four light sources are provided.

  In recent years, further downsizing and cost reduction of image forming apparatuses have been demanded, and accordingly, downsizing and cost reduction have been demanded for optical scanning apparatuses.

  Therefore, an attempt has been proposed to reduce the number of light sources in an optical scanning device used in an image forming apparatus having a plurality of photosensitive drums (see, for example, Patent Document 1 or Patent Document 2).

  However, in the optical scanning devices disclosed in Patent Document 1 and Patent Document 2, if further miniaturization is attempted, there is a risk of increasing the cost or reducing the scanning accuracy.

  The present invention has been made under such circumstances, and a first object of the invention is to provide an optical scanning device that can be miniaturized without increasing cost and reducing scanning accuracy. .

  A second object of the present invention is to provide an image forming apparatus that can be reduced in size without causing an increase in cost and a decrease in image quality.

  According to a first aspect of the present invention, there is provided an optical scanning device that optically scans at least four scanned surfaces, a single light source having at least one light emitting section; and light of a light beam emitted from the light source; A polarization switching element that is arranged on the road and alternately selects a first polarization direction and a second polarization direction that are orthogonal to each other in time series by an external signal, and emits linearly polarized light in the selected polarization direction; A beam splitter that separates the light beam from the switching element into a first light beam and a second light beam while maintaining the polarization state; and light that respectively deflects the first light beam and the second light beam separated by the beam splitter. A deflector; a first polarization separation element that transmits or reflects the first light beam deflected by the optical deflector according to a polarization direction; and the second light beam deflected by the optical deflector in a polarization direction. Depending on Or a scanning optical system including a second polarization separation element that reflects, the first light flux in the first polarization direction, the second light flux in the first polarization direction, and the second polarization direction. The first light beam and the second light beam in the second polarization direction optically scan the effective scanning range of the surface to be scanned corresponding in time series.

  Accordingly, it is possible to reduce the size without increasing the cost and reducing the scanning accuracy.

  According to a second aspect of the present invention, there is provided at least four image carriers; and at least one optical scanning device according to the present invention that scans the at least four image carriers with light modulated according to image information. And an image forming apparatus.

  According to this, since the optical scanning device of the present invention is provided, as a result, it is possible to reduce the size without increasing the cost and degrading the image quality.

1 is a diagram illustrating a schematic configuration of a color printer according to an embodiment of the present invention. FIG. 2 is a diagram (part 1) for describing the optical scanning device in FIG. 1; FIG. 3 is a second diagram for explaining the optical scanning device in FIG. 1; It is a figure for demonstrating the light source unit LU in FIG. It is a figure for demonstrating the light source in light source unit LU. It is a figure for demonstrating the structure of the liquid crystal element as a polarization switching element. It is a figure for demonstrating the orientation state of the liquid crystal molecule in the liquid crystal element of FIG. FIG. 7 is a diagram (No. 1) for describing an operation of the liquid crystal element of FIG. FIG. 7 is a diagram (part 2) for explaining the operation of the liquid crystal element of FIG. It is a figure for demonstrating a beam splitter. It is a figure for demonstrating the light beam LBa1 and the light beam LBa2. It is a figure for demonstrating the light beam LBb1 and the light beam LBb2. It is a figure for demonstrating the angle which two light beams which inject into a polygon mirror make. FIG. 14A to FIG. 14D are diagrams for explaining the operation of the polarization separation element. FIG. 15A to FIG. 15C are diagrams for explaining the configuration of the polarization separation element. It is a figure for demonstrating the polarization separation surface of a polarization separation element. It is a figure for demonstrating the grating | lattice pitch and the grating | lattice depth of a polarization beam splitting element. It is FIG. (1) for demonstrating the optical path of the two light beams deflected by the reflective surface of the polygon mirror. It is FIG. (2) for demonstrating the optical path of the two light beams deflected by the reflective surface of the polygon mirror. It is FIG. (3) for demonstrating the optical path of the two light beams deflected by the reflective surface of the polygon mirror. It is a timing chart for demonstrating operation | movement of a scanning control apparatus. It is a figure for demonstrating when image formation is performed in K station. It is a figure for demonstrating when image formation is performed in Y station. It is a figure for demonstrating when image formation is performed in C station. It is a figure for demonstrating when the image formation is performed in the M station. FIG. 26A to FIG. 26C are diagrams for describing modifications of the polarization separation element. It is a figure for demonstrating the light source which has several light emission parts. It is a figure for demonstrating the scanning line space | interval on the surface of a photoconductive drum in case a pixel density is 600 dpi. It is a figure for demonstrating the modification 1 of a scanning optical system. It is a figure for demonstrating the modification 2 of a scanning optical system.

  Hereinafter, an embodiment of the present invention will be described with reference to FIGS. FIG. 1 shows a schematic configuration of a color printer 2000 as an image forming apparatus according to an embodiment.

  The color printer 2000 is a tandem multi-color printer that forms a full-color image by superimposing four colors (black, cyan, magenta, and yellow), and includes an optical scanning device 2010, four photosensitive drums (2030a, 2030b, 2030c, 2030d), four cleaning units (2031a, 2031b, 2031c, 2031d), four charging devices (2032a, 2032b, 2032c, 2032d), four developing rollers (2033a, 2033b, 2033c, 2033d), 4 Toner cartridges (2034a, 2034b, 2034c, 2034d), transfer belt 2040, transfer roller 2042, fixing roller 2050, paper feed roller 2054, registration roller pair 2056, paper discharge roller 2058, paper feed tray 060, paper ejection tray 2070 includes a communication control unit 2080, and a printer controller 2090 for totally controlling the above elements.

  The communication control device 2080 controls bidirectional communication with a host device (for example, a personal computer) via a network or the like.

  The photosensitive drum 2030a, the charging device 2032a, the developing roller 2033a, the toner cartridge 2034a, and the cleaning unit 2031a are used as a set, and form an image forming station (hereinafter also referred to as “K station” for convenience) that forms a black image. Configure.

  The photosensitive drum 2030b, the charging device 2032b, the developing roller 2033b, the toner cartridge 2034b, and the cleaning unit 2031b are used as a set and form an image forming station (hereinafter also referred to as “C station” for convenience) that forms a cyan image. Configure.

  The photosensitive drum 2030c, the charging device 2032c, the developing roller 2033c, the toner cartridge 2034c, and the cleaning unit 2031c are used as a set, and form an image forming station (hereinafter also referred to as “M station” for convenience) that forms a magenta image. Configure.

  The photosensitive drum 2030d, the charging device 2032d, the developing roller 2033d, the toner cartridge 2034d, and the cleaning unit 2031d are used as a set, and form an image forming station (hereinafter also referred to as “Y station” for convenience) that forms a yellow image. Configure.

  Each photosensitive drum has a photosensitive layer formed on the surface thereof. That is, the surface of each photoconductive drum is a surface to be scanned. Each photosensitive drum is rotated in the direction of the arrow in the plane of FIG. 1 by a rotation mechanism (not shown).

  In the following description, in the XYZ three-dimensional orthogonal coordinate system, the direction along the longitudinal direction of each photosensitive drum is defined as the Y-axis direction, and the direction along the arrangement direction of the four photosensitive drums is defined as the X-axis direction.

  Each charging device uniformly charges the surface of the corresponding photosensitive drum.

  Based on the multicolor image information (black image information, cyan image information, magenta image information, yellow image information) from the higher-level device, the optical scanning device 2010 charges the light flux modulated for each color correspondingly. Irradiate each surface of the photosensitive drum. As a result, on the surface of each photoconductive drum, the charge disappears only in the portion irradiated with light, and a latent image corresponding to the image information is formed on the surface of each photoconductive drum. The latent image formed here moves in the direction of the corresponding developing roller as the photosensitive drum rotates. The configuration of the optical scanning device 2010 will be described later.

  The toner cartridge 2034a stores black toner, and the toner is supplied to the developing roller 2033a. The toner cartridge 2034b stores cyan toner, and the toner is supplied to the developing roller 2033b. The toner cartridge 2034c stores magenta toner, and the toner is supplied to the developing roller 2033c. The toner cartridge 2034d stores yellow toner, and the toner is supplied to the developing roller 2033d.

  As each developing roller rotates, the toner from the corresponding toner cartridge is thinly and uniformly applied to the surface thereof. Then, when the toner on the surface of each developing roller comes into contact with the surface of the corresponding photosensitive drum, the toner moves only to a portion irradiated with light on the surface and adheres to the surface. In other words, each developing roller causes toner to adhere to the latent image formed on the surface of the corresponding photosensitive drum so as to be visualized. Here, the toner-attached image (toner image) moves in the direction of the transfer belt 2040 as the photosensitive drum rotates.

  The yellow, magenta, cyan, and black toner images are sequentially transferred onto the transfer belt 2040 at a predetermined timing, and are superimposed to form a color image.

  Recording paper is stored in the paper feed tray 2060. A paper feed roller 2054 is disposed in the vicinity of the paper feed tray 2060, and the paper feed roller 2054 takes out the recording paper one by one from the paper feed tray 2060 and conveys it to the registration roller pair 2056. The registration roller pair 2056 feeds the recording paper toward the gap between the transfer belt 2040 and the transfer roller 2042 at a predetermined timing. As a result, the color image on the transfer belt 2040 is transferred to the recording paper. The recording sheet transferred here is sent to the fixing roller 2050.

  In the fixing roller 2050, heat and pressure are applied to the recording paper, whereby the toner is fixed on the recording paper. The recording paper fixed here is sent to a paper discharge tray 2070 via a paper discharge roller 2058 and is sequentially stacked on the paper discharge tray 2070.

  Each cleaning unit removes toner (residual toner) remaining on the surface of the corresponding photosensitive drum. The surface of the photosensitive drum from which the residual toner has been removed returns to the position facing the corresponding charging device again.

  Next, the configuration of the optical scanning device 2010 will be described.

As shown in FIG. 2 and FIG. 3 as an example, the optical scanning device 2010 includes a light source unit LU, a polarization switching element 40, a beam splitter 30, two cylindrical lenses (12 ₁ , 12 ₂ ), polygon mirrors 14, ₂ Two fθ lenses (15 ₁ , 15 ₂ ), two polarization separation elements (16 ₁ , 16 ₂ ), four reflection mirrors (13 ₁ , 13 ₂ , 17 ₁ , 17 ₂ ), a plurality of folding mirrors (18a, 18b) ₁ , 18b ₂ , 18c ₁ , 18c ₂ , 18d), four anamorphic lenses (19a, 19b, 19c, 19d), a condensing lens 51, a light detection sensor 52, and a scanning control device (not shown). .

  In the following, for convenience, the direction corresponding to the main scanning direction is abbreviated as “main scanning corresponding direction”, and the direction corresponding to the sub scanning direction is abbreviated as “sub scanning corresponding direction”.

Further, when there is no need to distinguish between the fθ lens 15 ₁ and fθ lens 15 ₂ are collectively referred to as "fθ lens 15 '. Similarly, when there is no need to distinguish between the polarization separation element 16 ₁ and the polarization beam splitter 16 ₂ are collectively referred to as "polarization separation element 16 '.

The light source unit LU, as shown in FIG. 4 as an example, the light source 10 _1, the light source 10 driving chip 10 ₂ containing ₁ light source driving circuit for driving the light source 10 ₁ and the driving chip 10 ₂ is mounted Circuit board 10 ₃ , collimating lens 11, and the like.

Light source 10 _1, as shown in FIG. 5, includes one of the semiconductor laser 101. Linearly polarized light is emitted from the light emitting portion of the semiconductor laser 101. Here, it is assumed that linearly polarized light whose polarization direction (vibration plane of the electric field vector) is parallel to the Z-axis direction is emitted. Hereinafter, the light beam emitted from the semiconductor laser 101 is also referred to as “light beam LB”. Further, linearly polarized light whose polarization direction is parallel to the Z-axis direction is referred to as “longitudinal polarized light”, and linearly polarized light in a direction orthogonal thereto is referred to as “laterally polarized light”.

Collimator lens 11 is disposed on the light beam LB on the optical path from the light source 10 _1, substantially parallel light the light beam LB. A light beam LB that has passed through the collimator lens 11 is a light beam emitted from the light source unit LU.

  Returning to FIG. 2, the polarization switching element 40 is disposed on the optical path of the light beam LB emitted from the light source unit LU. The polarization switching element 40 changes the polarization direction of the emitted light beam to either “vertical polarization” or “lateral polarization” in accordance with a signal (switching signal) from the scanning control device.

  The scanning control device sends the switching signal to the polarization switching element so that the polarization direction of the light beam LB emitted from the polarization switching element 40 changes between “vertical polarization” and “lateral polarization” every predetermined time. 40.

  Therefore, the light beam LB emitted from the polarization switching element 40 becomes “vertical polarization” → “horizontal polarization” → “vertical polarization” → “transverse polarization” →... At predetermined time intervals.

  Here, a liquid crystal element (SSFLC element) including a surface-stabilized ferroelectric liquid crystal is used as the polarization switching element 40.

The liquid crystal device (SSFLC device), as shown in FIG. 6 as an example, between two transparent glass plates 40 ₁ through the transparent electrode 40 _3, consisting of a chiral smectic C phase forming a homogeneous orientation ferroelectric liquid crystal 40 ₂ are configurations sealed. Incidentally, the orientation film between the transparent electrode 40 ₃ and the ferroelectric liquid crystal 40 ₂ is omitted.

When a voltage is applied between the two transparent electrodes 40 _3, an electric field is generated in a direction perpendicular to the surface of the glass plate 40 _1.

  By the way, nematic liquid crystal is often used for a general liquid crystal element (for example, a liquid crystal display device). The response of this nematic liquid crystal is generally several ms to several tens ms. On the other hand, the response of a ferroelectric liquid crystal composed of a homogeneously oriented chiral smectic C phase is from several tens of microseconds to several hundreds of microseconds, and a high-speed response is possible.

As the alignment film, a normal alignment film such as polyimide used for TN liquid crystal, STN liquid crystal, or the like, or SiO, SiO ₂ , or polysiloxane-based inorganic high distribution film having high durability can be used. In order to strongly restrict the liquid crystal director, it is preferable to separately perform a rubbing process or a photo-alignment process. Moreover, ITO etc. can be used for each transparent electrode.

  A ferroelectric liquid crystal composed of a chiral smectic C phase generally has a helical structure. When the ferroelectric liquid crystal is sandwiched between cell gaps (“d” in FIG. 6) thinner than the helical pitch, the helical structure is unwound and becomes a surface stabilized ferroelectric liquid crystal layer (SSFLC).

  As shown in FIG. 7 as an example, the surface-stabilized ferroelectric liquid crystal layer (SSFLC) has an inclination angle −θ (here, θ = 22.5) with respect to the normal W of the layer formed by the smectic phase. An orientation state (hereinafter referred to as a “first orientation state”) that is tilted by only (°) and a stable orientation state (hereinafter referred to as a “second orientation state”) that is tilted and stabilized by θ in the opposite direction. A mixed state can be realized. In FIG. 7, the symbol W is a normal line of the smectic phase, and the symbol n is a major axis direction (director) of liquid crystal molecules.

  By generating an electric field in a direction perpendicular to the paper surface in FIG. 7, the direction of spontaneous polarization of liquid crystal molecules can be made uniform, and that state can be maintained. Then, switching between two alignment states can be performed by switching the polarity of the generated electric field.

  In FIG. 7, the first alignment state can be stabilized by generating a −E electric field, and the second alignment state can be stabilized by generating a + E electric field. When θ = 22.5 °, the angle formed between the major axis direction of the liquid crystal molecules in the first alignment state and the major axis direction of the liquid crystal molecules in the second alignment state is 45 °.

  When the scanning control device is stabilized in the first alignment state, the scanning control device supplies a switching signal that generates an electric field of −E to the polarization switching element 40 and stabilizes it in the second alignment state. , A switching signal that generates an electric field of + E is supplied to the polarization switching element 40.

  The thickness (cell gap) d of the liquid crystal layer is determined by the wavelength λ of the incident light and the refractive index anisotropy Δn of the liquid crystal material at the wavelength λ, so that Δn × d = λ / 2 is satisfied, that is, It is determined so that the condition of the half-wave plate is satisfied.

  Here, the polarization switching element 40 is arranged so that the major axis direction of the liquid crystal molecules coincides with the polarization direction of the vertically polarized light when in the first alignment state. At this time, the light beam emitted from the light source unit LU is parallel to the polarization direction of the light beam and the major axis direction of the liquid crystal molecules, so that the polarization direction of the light beam remains unchanged and the polarization state is maintained. The light is emitted from the polarization switching element 40 (see FIG. 8).

  When the polarization switching element 40 is in the second orientation state, the light beam emitted from the light source unit LU is inclined by 45 ° in the major axis direction of the liquid crystal molecules with respect to the polarization direction of the light beam. Is rotated by 90 ° and emitted from the polarization switching element 40 (see FIG. 9).

  By the way, it is conceivable to use an electro-optic crystal such as PLZT or LN to switch the polarization direction. However, the electro-optic crystal generally has a high driving voltage (several hundred to several thousand volts), and switching on the power source side. Since the speed is limited, it is difficult to ensure the response speed required in this embodiment. In addition, it is conceivable to use a liquid crystal as one that can be driven at a low voltage, but it is difficult to obtain a high-speed response corresponding to a scanning cycle in a nematic liquid crystal widely used for display applications and wavefront control applications. In this embodiment, since ferroelectric liquid crystal is used, low voltage driving is possible and high-speed response can be ensured. Furthermore, there are advantages of low power consumption, low heat generation, and low cost.

  The beam splitter 30 is a beam splitter that has the same transmittance and reflectance with respect to incident longitudinally polarized light and transversely polarized light, and can emit while maintaining the polarization state of incident light. The beam splitter 30 is arranged on the optical path of the light beam LB from the polarization switching element 40, reflects substantially half of the light beam LB without changing the polarization state of the light beam LB, and transmits the rest.

  The beam splitter 30 includes a light beam having a polarization direction parallel to the incident surface (see FIG. 10) including the normal direction of the beam separation surface and the incident direction of incident light (laterally polarized light), and a vertical light beam ( Longitudinal polarization) is incident. In this case, the dielectric multilayer film formed on the beam separation surface of the beam splitter 30 can reduce the types of film materials and the number of films as compared with the multilayer film structure in which the phase difference is eliminated.

  Further, each light beam emitted from the beam splitter 30 becomes a light beam having a polarization direction parallel to the deflection surface of the polygon mirror 14 (horizontal polarization) and a vertical light beam (longitudinal polarization). In this case, the polarization direction of the light beam deflected by the polygon mirror 14 is prevented from being different depending on the deflection angle, and the adverse effect on the polarization separation in the subsequent stage can be suppressed.

  The light beam surface formed by the light beam deflected by the reflection surface of the polygon mirror 14 with time is called a “deflection surface” (see Japanese Patent Application Laid-Open No. 11-202252). Here, the deflection surface is a plane orthogonal to the Z axis.

  Hereinafter, for convenience, the light beam reflected by the beam splitter 30 is also referred to as a “first light beam”, and the light beam transmitted through the beam splitter 30 is also referred to as a “second light beam”. Therefore, the light intensity of the first light beam emitted from the beam splitter 30 is substantially equal to the light intensity of the second light beam.

  The polarization directions of the first light beam and the second light beam change at a predetermined time interval from “longitudinal polarization” → “lateral polarization” → “vertical polarization” → “transverse polarization” →. . Therefore, the longitudinally polarized first light beam is “LBa1” and the laterally polarized first light beam is “LBb1”. Further, the vertically polarized second light beam is “LBa2”, and the horizontally polarized second light beam is “LBb2”. Therefore, the light beam LBa1 and the light beam LBa2 are obtained by separating one light beam by the beam splitter 30 (see FIG. 11), and the light beam LBb1 and the light beam LBb2 are obtained by separating one light beam by the beam splitter 30. (See FIG. 12).

  The light beam LBa1, the light beam LBa2, the light beam LBb1, and the light beam LBb2 emitted from the beam splitter 30 are all in the same plane orthogonal to the Z axis.

The cylindrical lens 12 _1, the first light flux (light flux LBA1, light flux LBb1) reflected by the beam splitter 30 is disposed on the optical path of said first light flux (light flux LBA1, light flux LBb1) and via the reflecting mirror 13 ₁ An image is formed in the vicinity of the reflection surface of the polygon mirror 14 in the Z-axis direction.

The cylindrical lens 12 _2, the second light flux (light flux LBA 2, the light flux LBb2) transmitted through the beam splitter 30 is disposed on the optical path of the second light flux (light flux LBA 2, the light flux LBb2) and via the reflecting mirror 13 ₂ polygons An image is formed in the vicinity of the reflecting surface of the mirror 14 in the Z-axis direction.

  The polygon mirror 14 has a four-sided mirror as an example, and each mirror serves as a reflecting surface. The polygon mirror 14 rotates at a constant speed around an axis parallel to the Z-axis direction, and the first light beam (light beam LBa1, light beam LBb1) and the second light beam (light beam LBa2, light beam LBb2) are within a plane orthogonal to the Z axis. Deflection at a constant angular velocity.

  Here, the first light beam (light beam LBa1, light beam LBb1) is incident on the reflecting surface located on the −X side of the rotation axis of the polygon mirror 14, and the second light beam (light beam LBa2, light beam LBb2) is incident on the rotation axis. Incident on the reflecting surface located on the + X side.

  The angle formed between the first light beam (light beam LBa1, light beam LBb1) and the second light beam (light beam LBa2, light beam LBb2) incident on the polygon mirror 14 is approximately 90 ° in a plane orthogonal to the Z axis ( (See FIG. 13).

  Incidentally, since the first light beam (light beam LBa1, light beam LBb1) and the second light beam (light beam LBa2, light beam LBb2) incident on the polygon mirror 14 are in the same plane orthogonal to the Z axis, the reflection surface of the polygon mirror 14 The length (height) in the Z-axis direction can be reduced, and the cost can be reduced. Further, as compared with the case where each light beam is obliquely incident on the polygon mirror 14, it is possible to reduce scanning line bending, wavefront aberration, and the like on the surface of the photosensitive drum.

  Here, the first light beam (light beam LBa1, light beam LBb1) is deflected to the −X side of the polygon mirror 14, and the second light beam (light beam LBa2, light beam LBb2) is deflected to the + X side of the polygon mirror 14.

Returning to Figure 3, f [theta] lens 15 ₁ is a -X side of the polygon mirror 14, the first light beam deflected by the polygon mirror 14 (light beam LBA1, light flux LBb1) is disposed on the optical path of the.

fθ lens 15 _2, a + X side of the polygon mirror 14, the second light beam deflected by the polygon mirror 14 (light beam LBA 2, the light beam LBb2) is disposed on the optical path of the.

Polarization separation element 16 ₁ is an fθ lens 15 ₁ on the -X side, the first light flux (light flux LBA1, light flux LBb1) through the fθ lens 15 ₁ is disposed on the optical path of the.

Polarization separating element 16 ₂ is an fθ lens 15 ₂ + X side, the second light flux through the fθ lens 15 ₂ (light beam LBA 2, the light beam LBb2) is disposed on the optical path of the.

Each polarization separation element is a polarization separation element that transmits vertically polarized light and reflects horizontally polarized light. Therefore, the light beam LBa1 is transmitted through the polarization separation element 16 ₁ (see FIG. 14 (A)), the light flux LBb1 is reflected in the -Z direction by the polarization separation element 16 ₁ (see FIG. 14 (B)). Further, the light flux LBa2 is transmitted through the polarization separating element 16 ₂ (see FIG. 14 (C)), the light flux LBb2 is reflected in the -Z direction by the polarization separating element 16 ₂ (see FIG. 14 (D)).

  Here, as each polarization separation element, as shown in FIGS. 15A to 15C, a wire grid is formed on a plate-like substrate as a fine structure grating whose grating pitch is smaller than the wavelength of incident light. The wire grid element in which is formed is used. 15B is a cross-sectional view taken along the line AA in FIG. 15A, and FIG. 15C is a cross-sectional view taken along the line BB in FIG. 15A.

  The surface on which the wire grid is formed is a polarization separation surface, which transmits vertically polarized light and reflects horizontally polarized light (see FIG. 16).

  Here, as an example, the grid pitch of the wire grid is 0.15 μm, the “grid width / lattice pitch” duty ratio is 50%, and the grid depth is 0.05 μm (see FIG. 17). The material of the wire is aluminum. Further, a transparent material such as glass or hard plastic is selected as the substrate.

  The polarization separation element can be a dielectric multilayer film, but using a wire grid has higher light utilization efficiency and higher separation performance.

  A polarizer whose transmission axis coincides with the polarization direction of the light beam may be additionally disposed on at least one of the light paths of the light beam transmitted through the polarization separation element and the reflected light beam. Ghost light can be suppressed by the polarizer.

Returning to Figure 3, the folding mirror 18a is arranged on an optical path of the light beam LBa1 transmitted through the polarization separation element 16 _1, bent in a direction toward the optical path of the light beam LBa1 the photosensitive drum 2030 a.

  The anamorphic lens 19a is disposed on the optical path of the light beam LBa1 via the folding mirror 18a.

  The light beam LBa1 that passes through the anamorphic lens 19a is irradiated onto the surface of the photosensitive drum 2030a to form a light spot. This light spot moves in the longitudinal direction of the photosensitive drum 2030a as the polygon mirror 14 rotates. That is, the photosensitive drum 2030a is scanned. The moving direction of the light spot at this time is the “main scanning direction” on the photosensitive drum 2030a, and the rotational direction of the photosensitive drum 2030a is the “sub-scanning direction” on the photosensitive drum 2030a.

Thus, the mirror 18a and the anamorphic lens 19a folded fθ lens 15 ₁ and the polarization separation element 16 ₁ is a scanning optical system of the "K station".

Reflecting mirror 17 ₁ is disposed on the optical path of the polarization separation element 16 ₁ light flux LBb1 reflected in the -Z direction, bending in a direction toward the optical path of the light beam LBb1 in the -X direction. Accordingly, the optical path of the light beam LBb1 becomes parallel to the optical path of the light beam LBa1 transmitted through the polarization separation element 16 _1.

Folding mirrors 18b ₁ is disposed on the optical path of the light beam LBb1 through the reflection mirror 17 _1, bend the optical path of the light beam LBb1 the + Z side.

Folding mirror 18b ₂ is disposed on the optical path of the light beam LBb1 through the folding mirror 18b _1, bent in a direction toward the optical path of the light beam LBb1 the photosensitive drum 2030 b.

Anamorphic lens 19b is disposed on an optical path of the light beam LBb1 through the folding mirror 18b _2.

  The light beam LBb1 that has passed through the anamorphic lens 19b is irradiated onto the surface of the photosensitive drum 2030b to form a light spot. This light spot moves in the longitudinal direction of the photosensitive drum 2030b as the polygon mirror 14 rotates. That is, the photosensitive drum 2030b is scanned. The moving direction of the light spot at this time is the “main scanning direction” on the photosensitive drum 2030b, and the rotational direction of the photosensitive drum 2030b is the “sub-scanning direction” on the photosensitive drum 2030b.

Thus, the mirror 18b ₂ and the anamorphic lens 19b folding mirror 18b ₁ folded fθ lens 15 ₁ and the polarization separation element 16 ₁ and the reflection mirror 17 ₁ is a scanning optical system of the "C station".

That, f [theta] lens 15 ₁ and the polarization beam splitter 16 ₁ is shared by two image forming stations.

Reflecting mirror 17 ₂ is disposed on the optical path of the polarization separation element 16 _two beams LBb2 reflected in the -Z direction, bending in a direction toward the optical path of the light beam LBb2 in the + X direction. Accordingly, the optical path of the light beam LBb2 becomes parallel to the optical path of the light beam LBa2 transmitted through the polarization separating element 16 _2.

Folding mirror 18c ₁ is disposed on the optical path of the light beam LBb2 through the reflection mirror 17 _2, bending the optical path of the light beam LBb2 the + Z side.

Folding mirror 18c ₂ is disposed on the optical path of the light beam LBb2 through the folding mirror 18c _1, bent in a direction toward the optical path of the light beam LBb2 the photosensitive drum 2030 c.

Anamorphic lens 19c is disposed on an optical path of the light beam LBb2 through the folding mirror 18c _2.

  The light beam LBb2 that passes through the anamorphic lens 19c is irradiated onto the surface of the photosensitive drum 2030c to form a light spot. This light spot moves in the longitudinal direction of the photosensitive drum 2030c as the polygon mirror 14 rotates. That is, the photosensitive drum 2030c is scanned. The moving direction of the light spot at this time is the “main scanning direction” on the photosensitive drum 2030c, and the rotational direction of the photosensitive drum 2030c is the “sub-scanning direction” on the photosensitive drum 2030c.

Thus, the mirror 18c ₂ and the anamorphic lens 19c folding mirror 18c ₁ folded and fθ lens 15 ₂ and the polarization separating element 16 ₂ and the reflecting mirror 17 ₂ is a scanning optical system of the "M station".

Folding mirror 18d is arranged on an optical path of the light beam LBa2 transmitted through the polarization separating element 16 _2, bending in a direction toward the optical path of the light beam LBa2 the photosensitive drum 2030 d.

  The anamorphic lens 19d is disposed on the optical path of the light beam LBa2 via the folding mirror 18d.

  The light beam LBa2 via the anamorphic lens 19d is irradiated onto the surface of the photosensitive drum 2030d, and a light spot is formed. This light spot moves in the longitudinal direction of the photosensitive drum 2030d as the polygon mirror 14 rotates. That is, the photosensitive drum 2030d is scanned. The moving direction of the light spot at this time is the “main scanning direction” on the photosensitive drum 2030d, and the rotational direction of the photosensitive drum 2030d is the “sub-scanning direction” on the photosensitive drum 2030d.

Thus, mirror 18d and the anamorphic lens 19d folded and fθ lens 15 ₂ and the polarization separating element 16 ₂ is a scanning optical system of the "Y station".

That, f [theta] lens 15 ₂ and the polarization separating element 16 ₂ is shared by the two image forming stations.

  Incidentally, a scanning area in the main scanning direction in which image information is written on each photosensitive drum is called an “effective scanning area”, an “image forming area”, or an “effective image area”.

The light detection sensor 52 has a light receiving element, and is arranged so that a part of the second light flux before the start of writing deflected by the polygon mirror 14 enters the light receiving element through the condenser lens 51. Yes. The output signal of this light receiving element is amplified and inverted and supplied to the scanning control device as a light detection signal. The scan control device, based on the light detection signal, the polarization direction of the switching control of the light flux emitted from the polarization switching element 40, the intensity control of the light beam emitted from the light source _{10 1 (Auto Power Control; APC} ), performing such modulation control of the light beam emitted from the light source 10 _1. A part of the second light flux having passed through the fθ lens 15 _2, as received by the light receiving element, the light detecting sensor 52 and the condenser lens 51 may be disposed.

  Here, the number of reflection surfaces in the polygon mirror 14 is four, and the first light beam (light beam LBa1, light beam LBb1) and the second light beam (light beam LBa2, light beam LBb2) are incident on different reflection surfaces. The angle between the first light beam (light beam LBa1, light beam LBb1) and the second light beam (light beam LBa2, light beam LBb2) incident on the polygon mirror 14 is approximately 90 ° in a plane orthogonal to the Z axis. Is set to Therefore, the first light beam and the second light beam do not simultaneously scan the effective scanning area on the corresponding photosensitive drum.

  For example, as shown in FIG. 18, when the first light beam reflected by the reflecting surface of the polygon mirror 14 is LB1, and the second light beam is LB2, the writing start position on the photosensitive drum (2030c or 2030d) to which LB2 corresponds. LB1 reflected by the reflecting surface of the polygon mirror 14 is directed to a position on the + Y side of the writing end position on the corresponding photosensitive drum (2030a or 2030b).

  Further, as shown in FIG. 19, when LB2 reflected by the reflecting surface of the polygon mirror 14 goes to the center (image height 0) position of the effective scanning area in the corresponding photosensitive drum (2030c or 2030d), the polygon LB1 reflected by the reflecting surface of the mirror 14 goes in the + Y direction.

  In addition, as shown in FIG. 20, when the LB2 reflected by the reflecting surface of the polygon mirror 14 is directed to the writing end position of the effective scanning area on the corresponding photosensitive drum (2030c or 2030d), the reflection of the polygon mirror 14 is performed. LB1 reflected by the surface is directed to a position on the −Y side of the writing start position on the corresponding photosensitive drum (2030a or 2030b).

  As described above, when the LB2 reflected by the reflecting surface of the polygon mirror 14 scans the effective scanning area of the corresponding photosensitive drum (2030c or 2030d), LB1 reflected by the reflecting surface of the polygon mirror 14 is , It does not go into the effective scanning area of the corresponding photosensitive drum (2030a or 2030b).

  Conversely, when LB1 reflected by the reflecting surface of the polygon mirror 14 scans the effective scanning area of the corresponding photosensitive drum (2030a or 2030b), LB2 reflected by the reflecting surface of the polygon mirror 14 is It does not go into the effective scanning area of the corresponding photosensitive drum (2030c or 2030d).

  The angle formed between the first light beam (light beam LBa1, light beam LBb1) and the second light beam (light beam LBa2, light beam LBb2) incident on the polygon mirror 14 is slightly shifted from 90 ° in the plane orthogonal to the Z axis. May be.

Therefore, the light beam LBa1 and the light beam LBa2, in the time it is emitted from the light source 10 _1, the same modulation is performed as a single light beam, when the light beam LBa1 scans the effective scanning region on the photosensitive drum 2030a are scanning control apparatus, modulated light flux drives the light source 10 ₁ to be emitted in response to image information of black, when the light beam LBa2 scans the effective scanning region on the photosensitive drum 2030d, the scanning controller, yellow modulated light flux drives the light source 10 ₁ to be emitted in accordance with image information.

Similarly, the light flux LBb1 and the light beam LBb2, in the time it is emitted from the light source 10 _1, the same modulation is performed as a single light beam, when the light beam LBb1 scans the effective scanning region on the photosensitive drum 2030b is scanned controller, when the light flux modulated according to image information of cyan light source 10 ₁ is driven so as to be emitted, the light flux LBa2 scans the effective scanning region on the photosensitive drum 2030c, the scanning control device, modulated light flux drives the light source 10 ₁ to be emitted in accordance with image information of magenta.

  Next, the operation of the scanning control apparatus when forming a latent image on each photosensitive drum will be described with reference to the timing chart of FIG. Here, t2> t1> t3.

(1) When the light detection signal from the light detection sensor 52 changes from a high level to a low level, the count value of the timer is reset to zero.

(2) APC is performed based on the light detection signal from the light detection sensor 52.

(3) When the count value of the timer reaches t1, a switching signal is supplied to the polarization switching element 40 so that the polarization direction of the light beam LB emitted from the polarization switching element 40 becomes “longitudinal polarization”.

(4) When the count value of the timer reaches t2, so that the light flux modulated according to image information of black is emitted from the light source 10 _1, controls the light source driving circuit of the light source 10 _1. As a result, the effective scanning area on the photosensitive drum 2030a is scanned with the light beam LBa1 (see FIG. 22).

(5) When the light detection signal from the light detection sensor 52 changes from the high level to the low level, the count value of the timer is reset to zero.

(6) APC is performed based on the light detection signal from the light detection sensor 52.

(7) When the count value of the timer reaches t3, so that the light flux modulated according to image information of yellow is emitted from the light source 10 _1, controls the light source driving circuit of the light source 10 _1. As a result, the effective scanning area on the photosensitive drum 2030d is scanned with the light beam LBa2 (see FIG. 23).

(8) When the count value of the timer reaches t1, a switching signal is supplied to the polarization switching element 40 so that the polarization direction of the light beam LB emitted from the polarization switching element 40 becomes “horizontal polarization”.

(9) When the count value of the timer reaches t2, so that the light flux modulated according to image information of cyan is emitted from the light source 10 _1, controls the light source driving circuit of the light source 10 _1. As a result, the effective scanning area on the photosensitive drum 2030b is scanned with the light beam LBb1 (see FIG. 24).

(10) When the light detection signal from the light detection sensor 52 changes from the high level to the low level, the count value of the timer is reset to zero.

(11) APC is performed based on the light detection signal from the light detection sensor 52.

(12) When the count value of the timer reaches t3, so that the light flux modulated according to image information of magenta is emitted from the light source 10 _1, controls the light source driving circuit of the light source 10 _1. As a result, the effective scanning area on the photosensitive drum 2030c is scanned with the light beam LBb2 (see FIG. 25).

  Thereafter, the operations (3) to (12) are repeated.

  As a result, writing to the four photosensitive drums can be performed with a single light source. By the way, for t1, t2, and t3, appropriate values are obtained in advance for each apparatus and stored in the memory of the scanning control apparatus.

  In FIG. 21, the light amount of the light beam emitted from the light source (hereinafter, abbreviated as “emitted light amount”) is constant, but actually, the transmittance and reflectance of each optical element are relatively different. For this reason, the amount of light flux reaching each photosensitive drum may be different. In this case, the amount of emitted light may be adjusted for each photosensitive drum to be scanned so that the amount of light beams reaching each photosensitive drum is substantially the same.

  As is clear from the above description, in the optical scanning device 2010 according to this embodiment, the adjustment device and the control device in the optical scanning device of the present invention are configured by the scanning control device.

As described above, according to the optical scanning device 2010 according to the present embodiment, the single light source 10 ₁ having one light emitting unit is disposed on the optical path of the light beam emitted from the light source 10 ₁ and is orthogonal to each other. The longitudinally polarized light and the laterally polarized light are alternately selected in time series by a switching signal (external signal) from the scanning control device, and the polarization switching element 40 that emits the selected linearly polarized light, the light flux from the polarization switching element 40, A beam splitter 30 that separates the first light beam and the second light beam while maintaining the polarization state, a polygon mirror 14 that deflects the first light beam and the second light beam separated by the beam splitter 30, and deflection by the polygon mirror 14. a polarization separation element 16 ₁ of the first light flux transmitted through or reflected according to the polarization direction that is, transmitting if the second light beam deflected by the polygon mirror 14 in accordance with the polarization direction Ku has a like scanning optical system including a polarization separation element 16 ₂ to reflect.

  The polygon mirror 14 has four reflecting surfaces that rotate around the rotation axis, and the angle formed by the first light beam and the second light beam incident on the polygon mirror 14 in a plane orthogonal to the rotation axis direction is It is approximately 90 °.

  The effective scanning range of the photosensitive drum corresponding in time series is optically scanned with the first vertically polarized light beam, the second vertically polarized light beam, the first horizontally polarized light beam, and the second horizontally polarized light beam.

  The polarization direction of the linearly polarized light emitted from the polarization switching element is switched every time the effective scanning range of the two photosensitive drums is optically scanned. In this case, it is possible to suppress the generation of a ghost image due to image signal interference between the photosensitive drums.

Further, the light source 10 _1, a light source for emitting vertically polarized light, as the polarization switching element 40, the liquid crystal element is used comprising a surface stabilized ferroelectric liquid crystal. In this case, compared with the case of using other elements (Faraday element, electro-optical element), it has high-speed response, low power consumption, low heat generation, and low cost. In this case, the size can be reduced as compared with the case of using other elements.

  The fθ lens is provided between the polygon mirror 14 and the polarization separation element. Since the optical path of longitudinal polarization and the optical path of lateral polarization overlap with each other in the Z-axis direction, the size (thickness) of the fθ lens in the Z-axis direction can be reduced.

  Further, since the fθ lens and the polarization separation element are shared by the two image forming stations, the size can be reduced.

  In addition, the scanning control device detects the timing between the optical scanning in the effective scanning range of the photosensitive drum 2030a by the light beam LBa1 and the optical scanning in the effective scanning range of the photosensitive drum 2030d by the light beam LBa2, and the photosensitive drum by the light beam LBb1. The light detection signal is captured at the timing between the optical scanning in the effective scanning range 2030b and the optical scanning in the effective scanning range of the photosensitive drum 2030c by the light beam LBb2. Thereby, synchronization detection and APC can be performed with high accuracy. Note that if the polarization direction is switched by the polarization switching element 40 while the light detection signal is being taken in, an error may occur in synchronization detection and APC.

  Therefore, it is possible to reduce the size (thinner) without increasing the cost and reducing the scanning accuracy.

  The color printer 2000 according to the present embodiment includes the optical scanning device 2010. As a result, it is possible to reduce the size without increasing the cost and reducing the image quality.

  In the above-described embodiment, as a modification of the polarization separation element, a wire grid may be formed as shown in FIGS. 26 (A) to 26 (C). In this case, each polarization separation element transmits laterally polarized light and reflects longitudinally polarized light. Therefore, the scanning control device needs to reverse the switching signal as described above. FIG. 26B is a cross-sectional view taken along the line AA in FIG. 26A, and FIG. 26C is a cross-sectional view taken along the line BB in FIG.

  Moreover, although the said embodiment demonstrated the case where a light source had one light emission part, it is not limited to this. For example, as shown in FIG. 27, the light source may have four light emitting units.

  When a light source having a plurality of light emitting units (multi-beam light source) is used, the interval between the light emitting units in the sub-scanning corresponding direction is set so that the interval in the sub-scanning direction between the beams scanning each photosensitive drum becomes a predetermined value. It needs to be adjusted. For example, when writing with a pixel density of 600 dpi in the sub-scanning direction, the interval between the light emitting portions is adjusted so that the interval in the sub-scanning direction between the beams on the photosensitive drum is 42 μm (see FIG. 28).

  By the way, in the multi-beam light source, since a plurality of light emitting units are arranged at equal intervals, when the plurality of light emitting units are arranged one-dimensionally, it is general to use the light source by rotating it. When the light source is rotated, the polarization direction is also rotated, so that it is difficult to emit longitudinally polarized light by itself as in the above embodiment.

  In this case, the molecular direction of the liquid crystal element that is the polarization switching element is adjusted, and the polarization direction of the light beam emitted from the polarization switching element is parallel to the incident surface of the beam splitter and one is perpendicular to the other. You can do that.

  Specifically, when the rotation angle of the polarization direction of the light beam emitted from the multi-beam light source from the Z-axis direction is α, the liquid crystal molecule direction in the first alignment state of the liquid crystal molecules is α / The rotation angle is set to 2. At this time, the polarization direction of the light beam emitted from the liquid crystal element in the first alignment state is parallel to the Z axis. Further, when the liquid crystal element is in the second alignment state, the polarization direction of the light beam emitted from the liquid crystal element is a direction orthogonal to the Z axis. In this case, it is not necessary to provide a half-wave plate in front of the liquid crystal element.

In the above-described embodiment, as shown in FIG. 29 as an example, the anamorphic lenses (19 ₁ , 19 ₂ ) are arranged at the subsequent stage of the fθ lenses (15 ₁ , 15 ₂ ), and the polarization separation element (16 ₁ , 16 ₂ ) may be arranged. In this case, since the anamorphic lens can be shared by the two image forming stations, the thickness can be further reduced.

Moreover, in the said embodiment, as FIG. 30 shows as an example, you may arrange | position a polarization splitting element (16 ₁ , 16 ₂ ) in the front stage of the fθ lens (15 ₁ , 15 ₂ ). For example, this configuration is useful when the polarization state of the light beam changes when passing through the fθ lens (15 ₁ , 15 ₂ ).

  Moreover, although the said embodiment demonstrated the case where there was one optical detection sensor, it is not limited to this. For example, a light detection sensor that receives the first light flux may be added. Further, a light detection sensor may be provided for each photosensitive drum.

  Moreover, in the said embodiment, you may provide the optical detection sensor for synchronous detection, and the optical detection sensor for APC separately.

  As described above, the optical scanning device of the present invention is suitable for downsizing without increasing the cost and reducing the scanning accuracy. Further, the image forming apparatus of the present invention is suitable for downsizing without increasing the cost and degrading the image quality.

10 1 _... light source, 11 ... coupling lens ₁₂ 1, 12 2 _... cylindrical lenses ₁₃ 1, 13 2 _... reflecting mirror, 14 ... polygon mirror (optical _deflector), 15 1, 15 ₂ ... f [theta] lens (scanning optical Part of the system), 16 ₁ ... polarization separation element (first polarization separation element), 16 ₂ ... polarization separation element (second polarization separation element), 17 ₁ , 17 ₂ ... reflection mirror (one of the scanning optical system) _{parts), 18a, 18b 1, 18b} 2, 18c 1, 18c 2, 18d ... part of the reflecting mirror (scanning optical system), 19 a to 19 d ... part of the anamorphic lens (scanning optical system), 30 ... beam splitter, DESCRIPTION OF SYMBOLS 40 ... Polarization switching element, 2000 ... Color printer (image forming apparatus), 2030a-2030d ... Photosensitive drum (image carrier), 2010 ... Optical scanning device, LU ... Light source unit Door.

JP 2009-139039 A JP 2006-284822 A

Claims (10)

An optical scanning device that optically scans at least four surfaces to be scanned,
A single light source having at least one light emitter;
A first polarization direction and a second polarization direction that are arranged on the optical path of the light beam emitted from the light source and are orthogonal to each other are alternately selected in time series by an external signal, and linearly polarized light in the selected polarization direction is obtained. An exiting polarization switching element;
A beam splitter for separating the light beam from the polarization switching element into a first light beam and a second light beam while maintaining the polarization state;
An optical deflector for deflecting each of the first light flux and the second light flux separated by the beam splitter;
A first polarization separation element that transmits or reflects the first light beam deflected by the optical deflector according to a polarization direction, and transmits or reflects the second light beam deflected by the optical deflector according to a polarization direction. A scanning optical system including a second polarization separation element that reflects, and
The first light flux in the first polarization direction, the second light flux in the first polarization direction, the first light flux in the second polarization direction, and the second light flux in the second polarization direction, An optical scanning device that optically scans an effective scanning range of a scanned surface corresponding in time series. The optical deflector has four reflecting surfaces that rotate around a rotation axis;
2. The light according to claim 1, wherein an angle formed by the first light flux and the second light flux incident on the optical deflector is approximately 90 ° in a plane orthogonal to the rotation axis direction. Scanning device.   3. The optical scanning according to claim 1, wherein the polarization direction of the linearly polarized light emitted from the polarization switching element is switched every time the effective scanning range of the two scanned surfaces is optically scanned. apparatus. The light source is a light source that emits linearly polarized light,
The optical scanning device according to claim 1, wherein the polarization switching element is a liquid crystal element including a surface-stabilized ferroelectric liquid crystal.   In the beam splitter, an incident surface including a normal direction of a beam separation surface and an incident direction of incident light in the beam splitter is parallel to one of the first polarization direction and the second polarization direction, and the other 5. The optical scanning device according to claim 1, wherein the optical scanning device is arranged so as to be orthogonal to each other.   The light beam in the first polarization direction and the light beam in the second polarization direction that pass through the beam splitter are parallel to the deflection surface of the optical deflector, and the polarization direction of one light beam is parallel to the other light beam. The optical scanning device according to claim 1, wherein the polarization directions are orthogonal.   The optical scanning device according to claim 1, wherein the polarization separation element is a wire grid polarizer. A light receiving element for monitoring in which a part of the light beam emitted from the light source is incident as a light beam for light amount monitoring, and an adjustment for adjusting the light intensity of the light beam emitted from the light source according to an output signal of the light receiving element for monitoring With the device,
The adjustment device performs optical scanning of an effective scanning range of the corresponding scanned surface by the first light beam in the first polarization direction and effective scanning of the corresponding scanned surface by the second light beam in the first polarization direction. The timing between the optical scanning of the scanning range and the optical scanning of the effective scanning range of the corresponding scanned surface by the first light flux in the second polarization direction and the second light flux in the second polarization direction 8. The output signal of the monitor light receiving element is captured at at least one of timings corresponding to optical scanning of an effective scanning range of a corresponding scanned surface. The optical scanning device according to 1. A synchronization detection light-receiving element on which the synchronization detection light beam deflected by the optical deflector is incident; and a control device that controls the polarization switching element and the light source in accordance with an output signal of the synchronization detection light-receiving element. Prepared,
The control device performs optical scanning of an effective scanning range of the corresponding scanned surface by the first light beam in the first polarization direction and effective scanning of the corresponding scanned surface by the second light beam in the first polarization direction. The timing between the optical scanning of the scanning range and the optical scanning of the effective scanning range of the corresponding scanned surface by the first light flux in the second polarization direction and the second light flux in the second polarization direction 9. The output signal of the light receiving element for synchronization detection is captured at at least one of timings corresponding to optical scanning in the effective scanning range of the corresponding scanned surface. The optical scanning device according to Item. At least four image carriers;
An image forming apparatus comprising: at least one optical scanning device according to any one of claims 1 to 9 that scans the at least four image carriers with light modulated according to image information.

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