JP5087320B2 - 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 that scans a surface to be scanned with a light beam, and an image forming apparatus including the optical scanning apparatus.

  As an image forming apparatus that forms an image using the Carlson process, for example, a surface of a rotating photosensitive drum is scanned with a light beam to form a latent image on the surface of the photosensitive drum, and the latent image is visualized. There is known an image forming apparatus that forms an image by fixing the toner image on a sheet as a recording medium. In recent years, this type of image forming apparatus is often used for simple printing as an on-demand printing system, and demands for higher image density and faster image output are increasing.

  In general, as a method for speeding up image output, it is conceivable to increase the rotation speed of a deflector for deflecting a light beam and the rotation speed of a photosensitive drum to increase the printing speed. However, when the rotation speed of the deflector is increased, noise and vibration from the drive system increase, power consumption increases, and the durability of the apparatus decreases. Also, high-speed image output, since that is a trade-off relationship with respect to density of an image, there As you increase the rotational speed of the deflector, also inconvenience the image quality is reduced accordingly.

  Therefore, as a method for simultaneously achieving higher image density and higher image output speed, an image forming apparatus has been proposed in which a light source is made into a multi-beam and a photosensitive drum is scanned with a plurality of light beams at a time. The image forming apparatus, as a light beam source, a surface emitting laser array having a plurality of light emitting sources: the divergent light from the (VCSEL vertical cavity surface emitting laser), by deflecting the collectively deflector, photosensitive This is a device capable of simultaneously scanning a drum with a plurality of light beams.

  Recently, an optical scanning device using an overfilled optical system in which the width of a deflecting surface of a deflector that deflects a plurality of light beams is smaller than an irradiation range of the plurality of light beams has been proposed (for example, Patent Document 1). reference). With overfilled optical system, it is possible to reduce the width of the deflecting surface (around the rotation axis, the deflector having a diameter of a circle inscribed) same diameter as compared to a conventional deflector is, More deflection surfaces can be provided. For this reason, the scanning speed can be improved without increasing the rotational speed of the deflector.

  However, the optical scanning device using the overfilled optical system has a disadvantage that the light amount distribution of the light beam reflected by the deflecting surface is not uniform. Further, in the above-described surface-emitting laser array or the like, the light beam divergence angle is small, so that the light amount distribution of the light beam incident on the deflecting surface is slightly non-uniform compared to the edge-emitting laser or the like. For this reason, in order to combine both the surface-emitting light source and the overfilled optical system, a technique for making the light quantity distribution of the light beam uniform is required.

JP 2003-270777 A

  The present invention has been made under such circumstances. The first object of the present invention is to improve the non-uniformity of the light amount distribution of scanning light when applying a surface-emitting light source to an overfilled optical system. An object of the present invention is to provide an optical scanning device capable of scanning a surface to be scanned with high accuracy.

  A second object of the present invention is to provide an image forming apparatus capable of forming an image with high accuracy.

According to a first aspect of the present invention, a light source including a plurality of light emitting units, a deflector that deflects a plurality of light beams from the light source and scans in a main scanning direction, and the plurality of light beams deflected. And an imaging optical system that forms an image in the sub-scanning direction orthogonal to the main scanning direction in the vicinity of the deflecting surface of the scanner. The imaging optical system has a positive power in the main scanning direction. having first and second imaging optical element, the deflecting surface of the deflector, the main scanning direction dimension, rather smaller than the beam width of the main scanning direction, said first imaging optical element The first imaging optical element is positioned closer to the light source than the second imaging optical element, and the first imaging optical element transmits principal rays of the plurality of light beams to the first imaging optical element and the second imaging optical element. The second imaging optical element crosses the main scanning direction with respect to the imaging optical element, and the second imaging optical element In the vicinity of the principal rays of the plurality of light beams, an optical scanning device to intersect in the main scanning direction.

According to this, it becomes possible to scan the surface to be scanned with high accuracy.

  According to a third aspect of the present invention, there is provided an image forming apparatus for forming an image by fixing a toner image formed on the basis of a latent image obtained from information relating to an image to a recording medium. An optical scanning device of the invention; a photosensitive member on which a latent image is formed by the optical scanning device; a developing unit that visualizes a latent image formed on a surface to be scanned of the photosensitive member; and a visible image formed by the developing unit An image forming apparatus comprising: transfer means for fixing the converted toner image to the recording medium.

  According to this, the image forming apparatus includes the optical scanning device of the present invention. For this reason, a final image is formed based on a latent image formed by a light beam having a uniform amount of light. Therefore, it is possible to form an image on the recording medium with high accuracy.

  According to a fourth aspect of the present invention, a toner image formed based on a latent image for each color obtained from information relating to a multicolor image is superimposed and fixed on a recording medium to form a multicolor image. An optical scanning device according to the present invention; a plurality of photosensitive members on which latent images corresponding to respective colors are formed by the optical scanning device; and formed on the scanned surfaces of the plurality of photosensitive members, respectively. an image forming apparatus provided with; a developing unit that visualizes the latent image is; a transfer means for fixing overlapped toner images of each color is visualized by said developing means to said recording medium.

  According to this, the image forming apparatus includes the optical scanning device of the present invention. For this reason, a final multicolor image is formed based on the latent image formed by the light beam having a uniform amount of light. Therefore, a multicolor image can be formed on the recording medium with high accuracy.

<< First Embodiment >>

  Hereinafter, a first embodiment of the present invention will be described with reference to FIGS. 1 to 8B. FIG. 1 shows a schematic configuration of an image forming apparatus 200 according to the present embodiment.

  The image forming apparatus 200 is a printer that prints an image by transferring a toner image onto plain paper (paper) using a Carlson process. As shown in FIG. 1, the image forming apparatus 200 includes an optical scanning device 100, a photosensitive drum 201, a charging charger 202, a toner cartridge 204, a cleaning case 205, a paper feeding tray 206, a paper feeding roller 207, and a registration roller pair 208. A transfer charger 211, a fixing roller 209, a paper discharge roller 212, a paper discharge tray 210, and a housing 215 for housing them.

  The housing 215 has a substantially rectangular parallelepiped shape, and openings that communicate with the internal space are formed on the side walls on the + X side and the −X side.

  The optical scanning device 100 is disposed above the housing 215 and scans the surface of the photosensitive drum 201 by deflecting a light beam modulated based on image information in the main scanning direction (Y-axis direction in FIG. 1). To do. The configuration of the optical scanning device 100 will be described later.

  The photosensitive drum 201 is a cylindrical member in which a photosensitive layer having a property that becomes conductive when irradiated with a light beam on the surface thereof. The direction is arranged as a longitudinal direction, and is rotated clockwise in FIG. 1 (direction indicated by an arrow in FIG. 1) by a rotation mechanism (not shown). Then, its circumference, is disposed a charger 202 to the position of 12 o'clock in Fig. 1 (upper), the toner cartridge 204 is disposed at the position of 2 o'clock, a transfer charger 211 is arranged on the 6 o'clock position, 10 A cleaning case 205 is disposed at the hour position.

  The charging charger 202 is disposed with a predetermined clearance with respect to the surface of the photosensitive drum 201, and charges the surface of the photosensitive drum 201 with a predetermined voltage.

  The toner cartridge 204 includes a cartridge main body filled with toner, a developing roller charged with a voltage having a polarity opposite to that of the photosensitive drum 201, and the toner charged in the cartridge main body via the developing roller. Supply to the surface.

  The cleaning case 205 includes a rectangular cleaning blade whose longitudinal direction is the Y-axis direction, and is disposed so that one end of the cleaning blade is in contact with the surface of the photosensitive drum 201. The toner adsorbed on the surface of the photosensitive drum 201 is peeled off by the cleaning blade as the photosensitive drum 201 rotates, and is collected in the cleaning case 205.

  The transfer charger 211 is arranged with a predetermined clearance with respect to the surface of the photosensitive drum 201, and a voltage having a polarity opposite to that of the charging charger 202 is applied.

  The paper feed tray 206 is disposed in a state where the + X side end protrudes from an opening formed in the side wall on the + X side of the housing 215 and can accommodate a plurality of sheets 213 supplied from the outside. .

  The paper feed roller 207 takes out from the paper feed tray 206 paper 213 one by one, through the resist roller pair 208 consists of a pair rotating rollers, the gap formed by the transfer charger 211 and the photosensitive drum 201 To derive.

  The fixing roller 209 is composed of a pair of rotating rollers. The fixing roller 209 overheats and pressurizes the paper 61 and then guides it to the paper discharge roller 212.

  The sheet discharge roller 212 is formed from, a pair rotating rollers, with respect to the discharge tray 210 -X side end is arranged so as to protrude from an opening formed in the side wall of the -X side of the housing 215, The sheets 213 sent from the fixing roller 209 are sequentially stacked.

  Next, the configuration of the optical scanning device 100 will be described. FIG. 2 is a diagram showing a schematic configuration of the optical scanning device 100. As shown in FIG. 2, the optical scanning device 100 includes a light source 10, a coupling lens 11, an aperture member 12, and a collimator lens 13 that are sequentially arranged from the light source 10 in a 60 ° oblique direction in FIG. 2. And a polygon mirror 15, and a first scanning lens 16 and a second scanning lens 17 which are sequentially arranged on the + X side of the polygon mirror 15.

The light source 10 is a surface emitting semiconductor laser array in which, for example, VCSELs are two-dimensionally arranged as light emitting sources. As shown in FIG. 3, 40 VCSELs are formed on the light emitting surface (the surface on the −x side). They are arranged in a matrix of 8 rows and 5 columns with the direction parallel to the straight line L forming the angle θ with the y axis as the row direction and the direction parallel to the z axis as the column direction. Each VCSEL has a near-field pattern diameter of 4 μm, and a light beam having a wavelength of 780 nm is emitted with a divergence angle of 7 ± 1 degrees in the main scanning direction and the sub-scanning direction. In this embodiment, the row interval Dz is 24.0 μm, the column interval Dy is 23.9 μm, and the interval dz between adjacent VCSELs in the Z-axis direction (sub-scanning direction) of each VCSEL is 4.8 μm. (= Dz / 5). In the following description, as shown in FIG. 3, the VCSEL located in the mth row and the nth column is expressed as VCSEL _mn for convenience.

FIG. 4 is a schematic view showing a cross-sectional structure of the VCSEL, and FIG. 5 is an enlarged view around the active layer in FIG. 4 and 5, the VCSEL is a 780 nm band VCSEL, and the Al _0.12 Ga _0.88 As quantum well layer 24a is formed on the n-GaAs substrate 21 on which the n-side electrode 20 is formed. And an active layer 24 including an Al _0.3 Ga _0.7 As barrier layer 24 b is formed between the Al _0.6 Ga _0.4 As spacer layers 23 and 25. Further, the resonator region having a one-wavelength optical thickness including the active layer 24 and the spacer layers 23 and 25 has a high refractive index of 40.5 pairs of n-Al _0.3 Ga _0.7 As with an optical thickness of each layer λ / 4. The lower reflecting mirror 22 composed of a refractive index layer and a low refractive index layer of n-Al _0.9 Ga _0.1 As, 24 pairs of p-Al _0.3 Ga _0.7 As high refractive index layer and p-Al _The structure is sandwiched between the upper reflecting mirror 27 made of a _0.9 Ga _0.1 As low refractive index layer. An AlAs selectively oxidized layer 30 surrounded by the Al _x O _y current confinement layer 26 is provided on the upper reflecting mirror 27 located at a distance of λ / 4 from the resonator region. Note that a composition gradient layer (not shown) whose composition gradually changes to reduce the resistance value is included between the layers of the reflecting mirrors 22 and 27.

  Here, a method for forming a VCSEL provided in the light source 10 will be described. First, each of the above layers is formed by a crystal growth method using a metal organic chemical vapor deposition method (MOCVD method) or a molecular beam crystal growth method (MBE method).

  Next, a mesa shape is formed by forming a groove having a depth of, for example, 4.5 μm around the region to be an element region by dry etching. In general, the etching bottom surface is provided at least beyond the AlAs selective oxidation layer 30.

Next, the AlAs selectively oxidized layer 30 whose side surface is exposed by the groove forming step by etching is heat-treated in water vapor to oxidize the periphery and change it to an Al _x O _y insulator layer. As a result, a current confinement structure is formed that restricts the path of the element driving current only to the unoxidized AlAs region at the center.

Next, a SiO ₂ protective layer (not shown) having a thickness of 150 nm, for example, is formed except for the region where the upper electrode 31 is formed on each element region and the light emitting portion 32, and the etched portion is embedded with polyimide 29. Flatten with.

Next, the polyimide and SiO ₂ protective layer (not shown) on the upper reflecting mirror having the p contact layer 28 and the light emitting portion in each element region are removed, and the P side other than the light emitting portion 32 on the p contact layer 28 is removed. An individual electrode 31 is formed, and an n-side electrode is formed on the lower surface of the n-GaAs substrate 21.

  In the case of this embodiment, the mesa portion formed by the dry etching method becomes each surface emitting laser element. The arrangement of the VCSEL of the light source 10 can be realized by forming a photomask according to the arrangement of the light emitting source of the present invention, then forming an etching mask by a normal photolithographic process, and performing etching. In order to electrically and spatially separate each element of the array, it is preferable that the groove between the elements is about 4 to 5 μm or more. This is because if it is too narrow, it becomes difficult to control etching. In addition to the circular shape, the mesa portion can have any shape such as an oval, a square, or a rectangle. The size (diameter, etc.) is preferably about 10 μm or more. This is because if it is too small, heat is accumulated during device operation and the light emission characteristics deteriorate.

Note that the above-described surface emitting laser in the 780 nm band can be manufactured using another material. FIG. 6 shows an enlarged view around the active layer made of another material. As shown in FIG. 6, in this surface emitting laser, the active layer is composed of three GaInPAs quantum well active layers 24c having a compressive strain composition and a band gap wavelength of 780 nm, and four layers that are lattice-matched. The clad layers 23 and 25 (spacer layers in the present embodiment), which are composed of a Ga _0.6 In _0.4 P barrier layer 24d having tensile strain and confine electrons, have a wide band gap (Al _0.7 Ga _{0.3) 0.5} in _0.5 P is used. The band gap difference between the cladding layer and the quantum well active layer can be made extremely large as compared with the case where the carrier confinement cladding layer is formed of AlGaAs.

  Table 1 below shows AlGaAs (spacer layer) / AlGaAs (quantum well active layer) 780 nm surface emitting semiconductor lasers and 850 nm surface emitting semiconductor lasers having typical material compositions, and AlGaInP (spacer layer) / GaInPAs. The band gap difference between the spacer layer and the well layer and between the barrier layer and the well layer of the (quantum well active layer) system 780 nm surface emitting semiconductor laser is shown. The spacer layer is a layer that is usually between the active layer and the reflecting mirror, and refers to a layer that functions as a cladding layer for confining carriers.

  As shown in the following Table 1, according to the AlGaInP (spacer layer) / GaInPAs (quantum well active layer) system 780 nm surface emitting semiconductor laser, the AlGaAs / AlGaAs system 780 nm surface emitting semiconductor laser, and the AlGaAs / AlGaAs system 850 nm It can be seen that the band gap difference can be made larger than that of the surface emitting semiconductor laser. Specifically, the band gap difference between the clad layer and the active layer is 767.3 meV, which is extremely large, compared with 465.9 meV (when the Al composition is 0.6) when the clad layer is formed of AlGaAs. Similarly, the band gap difference between the barrier layer and the active layer also has a dominant difference, resulting in good carrier confinement.

  In addition, since the active layer has compressive strain, the increase in gain is increased by band separation of heavy holes and light holes. Further, since the gain increases, the output is high at a low threshold. This effect cannot be obtained with an AlGaAs-based surface emitting laser having a wavelength constant of 780 nm or a wavelength of 850 nm, which has substantially the same lattice constant as the GaAs substrate. Further, by reducing the threshold value by improving carrier confinement and increasing the gain by the strained quantum well active layer, it is possible to reduce the reflectivity of the light extraction side DBR, resulting in higher output.

  The active layer and the barrier layer are made of a material that does not contain Al, and are Al-free active regions (a quantum well active layer and a layer adjacent thereto). For this reason, since the amount of oxygen taken in decreases, the formation of a non-radiative recombination center can be suppressed, and the life of the VCSEL can be extended.

  Returning to FIG. 2, the coupling lens 11 is a lens having a focal length of 24.7 mm, and shapes the light beam from the light source 10 into substantially parallel light.

  The aperture member 12 has a rectangular or elliptical opening having a size in the y-axis direction (main scanning direction) of 5.44 mm and a size in the z-axis direction (sub-scanning direction) of 2.10 mm. The center of the aperture is arranged so as to be located at or near the focal position of the coupling lens 11.

  The collimating lens 13 is a lens having a focal length of 54.0 mm and having a refractive power in the main scanning direction (y-axis direction). The light beam that has passed through the aperture member 12 is scanned in the vicinity of the reflection surface of the polygon mirror 15. The image is formed with respect to the direction.

  The polygon mirror 15 is a regular polygonal columnar member whose upper surface is a regular dodecagon with a radius of 7 mm. Deflection surfaces for deflecting the incident light beam are formed on the 12 side surfaces of the polygon mirror 15, and are rotated at a constant angular velocity around an axis parallel to the Z axis by a rotation mechanism (not shown). Thereby, the light beam incident on the polygon mirror 15 is scanned in the Y-axis direction.

   The first scanning lens 16 and the second scanning lens 17 are scanning lenses made of, for example, resin having thicknesses at the centers (on the optical axis) of 13.5 mm and 3.5 mm, respectively.

  In the optical scanning device 100 described above, the sub-scanning lateral magnification of the optical system (scanning optical system) after the polygon mirror 15 is 0.97. Further, the focal length in the main scanning direction of the scanning optical system is 237.8 mm, the focal length in the sub-scanning direction is 71.4 mm, and the width of the writing area of the photosensitive drum 201 is centered on the point O shown in FIG. In the main scanning direction (Y-axis direction), the range is ± 105.0 mm. Note that point O is a point where a straight line passing through the rotation center of the polygon mirror 15 and parallel to the X axis in FIG. 2 intersects the surface to be scanned of the photosensitive drum 201. The aim of the spot diameter of the light beam on the surface of the photosensitive drum 201 is 52 μm in the main scanning direction and 55 μm in the sub scanning direction. As shown in FIG. 2, the optical distances d1, d2, d3, d4, d5, d6, d7 between the light source 10 and each optical element, and the sizes D1, D2, D3 of each element in the optical axis direction. , D4 are as shown in Table 2 and Table 3 as an example.

  Next, the operation of the image forming apparatus 200 configured as described above will be described. When image information is received from the host device, the optical scanning device 100 is driven by modulation data based on the image information, and 40 light beams modulated based on the image information are emitted from the light source 10. After these light beams are coupled by the coupling lens 11, the spot diameters are adjusted by passing through the aperture member 12. Then, each light beam that has passed through the aperture member 12 is condensed on the deflection surface of the polygon mirror 15 by the collimating lens 13.

  As shown in FIG. 7, the principal rays of the plurality of light beams emitted from the light source 10 are in the vicinity of a certain point in the deflection surface formed on the polygon mirror 15 by the collimating lens 13 having refractive power in the main scanning direction. It is focused on. As a result, the light beam incident on the deflecting surface is deflected without the amount of light becoming nonuniform between the light beams. Hereinafter, description will be made with reference to FIGS. 8A and 8B.

8A is located at the center of the first row of the VCSELs formed on the light source 10 assuming a plane parallel to the deflection surface of the polygon mirror 15 (hereinafter simply referred to as the incident surface). FIG. 6 is a diagram showing the intensity of a light beam emitted from the VCSEL ₁₃ and VCSELs ₁₁ and VCSELs ₁₅ located at both ends of the main scanning direction on one line and entering the incident surface while being separated from each other by a predetermined distance in the main scanning direction. is there. Note that the horizontal axis in FIG. 8A is the position coordinate in the main scanning direction on the incident surface, and the vertical axis is the intensity of the light beam. Y2 is a position coordinate in the center of the deflection surface of the polygon mirror 15 in the main scanning direction, and y1 and y3 are position coordinates of both ends of the deflection surface in the main scanning direction. For convenience, the light beams from VCSEL ₁₁ , VCSEL ₁₃ , and VCSEL ₁₅ are expressed as light beams LB ₁₁ , LB ₁₃ , and LB ₁₅ , respectively.

When the light beams LB ₁₁ , LB ₁₃ , and LB ₁₅ having the same intensity are scanned near the center point O of the writing area of the photosensitive drum 201, the light beams LB ₁₁ , LB ₁₃ , and LB ₁₅ are parallel to the Z axis. The intensity distribution on the incident surface of each of the light beams LB ₁₁ , LB ₁₃ , and LB ₁₅ that is deflected by the deflecting surface is indicated by curves L 1, L 2, and L 3 shown in FIG. Of the three light beams LB ₁₁ , LB ₁₃ , and LB ₁₅ , only two of the two light beams LB ₁₁ and LB ₁₅ are incident on the deflection surface, and therefore the amount of light after being reflected on the deflection surface is a curve. Proportional to the area of areas A1 and A3 (areas colored in FIG. 8A) defined by L1 or L3, a straight line passing through position y1 or position y2 and orthogonal to the Y axis, and the Y axis It becomes the light quantity. On the other hand, since all of the light beam LB ₁₃ is incident on the deflection surface, the amount of light after being reflected by the deflection surface becomes a light amount proportional to the area of the area A2 defined by the curve L2 and the Y axis. . That is, the light quantity ratio of each of the light beams LB ₁₁ , LB ₁₃ , and LB ₁₅ reflected by the deflection surface is equal to the area ratio of the regions A1, A2, and A3, and the light beam incident on the center of the deflection surface increases. The light beam incident near the edges at both ends of the main scanning direction becomes smaller.

  On the other hand, as shown in FIG. 7, the principal rays of the plurality of light beams emitted from the light source 10 are within the deflection surface formed on the polygon mirror 15 by the collimating lens 13 having refractive power in the main scanning direction. In the case of focusing near one point, the curves indicating the intensity distribution of each light beam almost overlap as shown in FIG. 8B as an example, so that a plurality of light beams with uniform intensity are emitted from the light source 10. When emitted, the amount of light between the plurality of light beams reflected by the deflecting surface becomes uniform.

  As described above, the light beam deflected by the deflecting surface of the polygon mirror 15 to rotate, the moving speed of the main scanning direction of the light beam spot is adjusted by the first scanning lens 16 and the second scanning lens 17 In this state, the light is condensed on the surface of the photosensitive drum 201.

  On the other hand, the surface of the photosensitive drum 201 is charged with a predetermined voltage by the charging charger 202, so that charges are distributed with a constant charge density. Then, when the photosensitive drum 201 is scanned by the light beam deflected by the polygon mirror 15, carriers (charges) are generated in the photosensitive layer where the light beam is incident. To do. Accordingly, the photosensitive drum 201 rotating in the direction of the arrow in FIG. 1 is scanned with a light beam modulated based on image information, thereby forming an electrostatic latent image defined by the charge distribution on the surface. Is done.

  When an electrostatic latent image is formed on the surface of the photosensitive drum 201, toner is supplied to the surface of the photosensitive drum 201 by the developing roller of the toner cartridge 204. At this time, since the developing roller of the toner cartridge 204 is charged with a voltage having a polarity opposite to that of the photosensitive drum 201, the toner attached to the developing roller is charged with the same polarity as that of the photosensitive drum 201. Therefore, no toner adheres to the portion of the surface of the photosensitive drum 201 where the electric charges are distributed, and the toner adheres only to the scanned portion, so that the electrostatic latent image is visualized on the surface of the photosensitive drum 201. A toner image is formed. Then, after the toner image is attached to the sheet 213 by the transfer charger 211, the toner image is fixed by the fixing roller 209, thereby forming an image on the sheet 213. The paper 213 on which the image is formed in this manner is discharged by the paper discharge roller 212 and sequentially stacked on the paper discharge tray 210.

  As described above, in the optical scanning device 100 according to the present embodiment, the amount of light of each light beam deflected on the deflection surface is substantially uniform. This is done with a light beam. Therefore, it is possible to scan the entire writing area without unevenness.

  As an example, as shown in FIG. 3, the 40 VCSELs formed in the light source 10 have a distance (= 148.0 μm) between the VCSELs farthest apart in the sub-scanning direction (Z-axis direction). It is two-dimensionally arranged on a plane parallel to the zy plane so as to be larger than the distance (= 95.6 μm) between the most distant VCSELs in the (Y-axis direction). Therefore, it is possible to avoid the light quantity distribution of the plurality of light beams deflected to the deflection surface of the polygon mirror 15 from being uneven, and it is possible to scan the surface to be scanned with high accuracy.

In addition, since the image forming apparatus 200 according to the present embodiment includes the optical scanning device 100, a final image is formed based on a latent image formed on the photosensitive drum 201 by a light beam having a uniform light amount. Is done. Accordingly, it is possible to form an image on the paper with high accuracy.
<< Second Embodiment >>
Next, a second embodiment of the present invention will be described with reference to FIGS. In addition, the same code | symbol is used for the component which is the same as that of 1st Embodiment mentioned above, or is equivalent, and the description shall be abbreviate | omitted or simplified.

  FIG. 9 shows a schematic configuration of the optical scanning device 100 included in the image forming apparatus of the second embodiment. As shown in FIG. 9, the optical scanning device 100 includes a light source 10, a coupling lens 11, an aperture member 12, a collimating lens 13, and a light source 10 that are sequentially arranged from the light source 10 in the direction of 60 degrees diagonally downward to the left in FIG. And a polygon mirror 15 and a first scanning lens 16 and a second scanning lens 17 which are sequentially arranged on the + X side of the polygon mirror 15.

  The coupling lens 11 is a lens having a focal length of 47.7 mm, and shapes the light beam from the light source 10 into substantially parallel light.

  Ray imaging 13 is a focal length 107.0Mm, a lens having a refractive power in the sub-scanning direction (Z axis direction), the light beam passing through the aperture member 12, the main in the vicinity of the reflecting surface of the polygon mirror 15 An image is formed in the scanning direction.

  In the optical scanning device 100 described above, the sub-scanning lateral magnification of the optical scanning device 100 as a whole is 2.18 times, and the sub-scanning lateral magnification of the optical system (scanning optical system) after the polygon mirror 15 is 0.97 times. ing. Further, the focal length in the main scanning direction of the scanning optical system is 237.8 mm, the focal length in the sub-scanning direction is 71.4 mm, and the width of the writing area of the photosensitive drum 201 is centered on the point O shown in FIG. In the main scanning direction (Y-axis direction), the range is ± 105.0 mm. A point O is a point where a straight line passing through the rotation center of the polygon mirror 15 and parallel to the X axis in FIG. 9 intersects the scanned surface of the photosensitive drum 201. The aim of the spot diameter of the light beam on the surface of the photosensitive drum 201 is 52 μm in the main scanning direction and 55 μm in the sub scanning direction. Further, as shown in FIG. 9, the optical distances d1, d2, d3, d4, d5, d6, d7, d8 between the light source 10 and each optical element, and the sizes D1, D2 of each element in the optical axis direction. , D3, and D4 are as shown in Table 4 and Table 5 as an example.

  Next, the relationship among the magnification of the optical system, the near field pattern of the light emitting portion, and the beam spot diameter will be described.

  As in the second embodiment, an optical system having a coupling lens focal length of 47.7 mm (lateral magnification in the main scanning direction is about 5.0 times and lateral magnification in the sub scanning direction is 2.0 times). Simulation results of the relationship between the beam spot diameter and the defocus amount when used are shown in FIGS. 10 (A) to 13 (B). FIG. 10A shows the beam spot diameter in the main scanning direction when the near field pattern (Am) in the main scanning direction is infinitesimal (= 0), and FIG. 10B shows the sub-scanning. The beam spot diameter in the sub-scanning direction when the near field pattern (As) in the direction is infinitesimal (= 0) is shown. 11A shows the beam spot diameter in the main scanning direction when Am is 2 μm, and FIG. 11B shows the beam spot diameter in the sub-scanning direction when As is 2 μm. 12A shows the beam spot diameter in the main scanning direction when Am is 4 μm, and FIG. 12B shows the beam spot diameter in the sub-scanning direction when As is 4 μm. 13A shows the beam spot diameter in the main scanning direction when Am is 6 μm, and FIG. 13B shows the beam spot diameter in the sub-scanning direction when As is 6 μm.

  Beam spot diameter when using a conventional optical system with a coupling lens focal length of 26.8 mm (horizontal magnification in the main scanning direction is about 8.9 times and lateral magnification in the sub-scanning direction is 4.5 times) FIGS. 14A to 17B show simulation results of the relationship between and the defocus amount. FIG. 14A shows the beam spot diameter in the main scanning direction when Am is infinitesimal (= 0), and FIG. 14B shows the sub-scanning direction in the case of As being infinitesimal (= 0). The beam spot diameter is shown. FIG. 15A shows the beam spot diameter in the main scanning direction when Am is 2 μm, and FIG. 15B shows the beam spot diameter in the sub-scanning direction when As is 2 μm. 16A shows the beam spot diameter in the main scanning direction when Am is 4 μm, and FIG. 16B shows the beam spot diameter in the sub-scanning direction when As is 4 μm. 17A shows the beam spot diameter in the main scanning direction when Am is 6 μm, and FIG. 17B shows the beam spot diameter in the sub-scanning direction when As is 6 μm.

  In these optical systems, the target value of the beam spot diameter in the main scanning direction is 52 ± 5 μm, the target value of the beam spot diameter in the sub-scanning direction is 55 ± 5 μm, and each of FIGS. 10 (A) to 17 (B). The boundary line is indicated by a broken line in the figure.

  In the conventional optical system, the depth of focus is sufficiently secured until Am is 2 μm, and the variation of the beam spot diameter is not so large. However, if Am = 4 μm or more, the depth of focus is drastically decreased in the main scanning direction, resulting in a device in which variation tends to increase. On the other hand, in the optical system similar to the present embodiment, the depth of focus is ensured up to Am = 6, and the increase of the beam spot diameter is allowed.

  This shows that a low lateral magnification optical system is required for a light source having a near field pattern of several μm or more. Since the VCSEL is such a light source, the lateral magnification of the entire optical system including the near field pattern A and the scanning optical system in at least one of the direction corresponding to the sub-scanning direction and the direction corresponding to the main scanning direction. It is desirable that the following expression (1) is satisfied using β and the beam spot diameter ω on the surface to be scanned.

{(Ω / β · A) ² −1/2} ⁻² <0.7 (1)

This will be described with reference to FIG. FIG. 18 shows the difference between the beam spot diameter when (β × A) = 0 (that is, A = 0) and the beam spot diameter when (β × A)> 0 when the defocus amount is zero. FIG. 10 (A) to FIG. 17 (B) are obtained and plotted. The curve in FIG. 18 is a curve showing the following equation (2). Here, ω ₀ is the beam spot diameter when A = 0, and it is assumed that the relationship with the beam spot diameter ω when A ≠ 0 is expressed by the following equation (3).

It can be seen from FIG. 18 that the increase in the beam spot diameter in the actual writing optical system is in good agreement with the above assumption. An increase rate δ of the beam spot diameter generated because A has a magnitude is expressed by the following equation (4). However, k = βA / ω ₀ .

In the present embodiment, k = 5.0 × 4 ÷ 50 = 0.40 with respect to the main scanning direction. At this time, δ is suppressed to about 4%, and the beam spot diameter in the main scanning direction is almost 52 μm. It does not change. Since k = 0.14 in the sub-scanning direction, the change is further small, and substantially no change with respect to ω ₀ = 55 μm, and becomes 55.5 μm.

In the conventional example, since the lateral magnification is 8.9 times in the main scanning direction, k = 0.712, and ω = 56 μm, which is an increase of 10% or more. When the near field pattern is increased, for example, when A = 7 μm, when the lateral magnification is 5 and the target beam spot diameter is 50 μm, k = 0.7 and ω = 56 μm. Therefore, this is also not desirable for improving the image quality. If k = 0.65 (for example, β = 5, A = 6.5, ω ₀ = 50), the change in image quality is within an allowable range because ω = 55 μm and the change is about 10%.

Therefore, it is desirable that at least k = βA / ω ₀ <0.7. At this time, the relationship with the actual beam spot diameter ω is expressed by the following equation (5) from the above equation (4).

  The VCSEL has only about 1 to 2 mW, and there is an improvement in light output as a problem. However, it is known that a larger near field pattern is advantageous for this. Even if the diameter can be increased and the light output can be increased in a region where no multimode oscillation occurs, the image quality cannot be improved unless the beam spot diameter can be reduced. Therefore, by setting so that the above expression (5) is satisfied, it is possible to reduce the beam spot diameter while using a VCSEL advantageous in light quantity.

  In the image forming apparatus 200 configured as described above, when image information is received from the host apparatus, the optical scanning device 100 is driven by the modulation data based on the image information, and the light source 10 is modulated based on the image information. A light beam is emitted. After these light beams are coupled by the coupling lens 11, the spot diameters are adjusted by passing through the aperture member 12. Then, each light beam that has passed through the aperture member 12 is condensed on the deflection surface of the polygon mirror 15 by the collimating lens 13.

FIG. 19A is located at the center of the first row of the VCSELs formed on the light source 10 assuming a plane parallel to the deflection surface of the polygon mirror 15 (hereinafter simply referred to as the incident surface). a VCSEL _13, emitted from each _VCSEL 11, VCSEL ₁₅ is located in the main scanning direction both ends of the row faces a diagram showing the intensity of the light beam incident on the incident surface. Note that the horizontal axis in FIG. 19A is the position coordinate in the main scanning direction on the incident surface, and the vertical axis is the intensity of the light beam. Y2 is a position coordinate in the center of the deflection surface of the polygon mirror 15 in the main scanning direction, and y1 and y3 are position coordinates of both ends of the deflection surface in the main scanning direction. For convenience, the light beams from VCSEL ₁₁ , VCSEL ₁₃ , and VCSEL ₁₅ are expressed as light beams LB ₁₁ , LB ₁₃ , and LB ₁₅ , respectively.

When the light beams LB ₁₁ , LB ₁₃ , and LB ₁₅ having the same intensity are scanned near the center point O of the writing area of the photosensitive drum 201, the light beams LB ₁₁ , LB ₁₃ , and LB ₁₅ are substantially parallel to the ZY plane. Since the light beams are deflected to the respective deflection surfaces, the intensity distributions on the respective incident surfaces of the light beams LB ₁₁ , LB ₁₃ and LB ₁₅ are indicated by curves L 1, L 2 and L 3 shown in FIG. Of the three light beams LB ₁₁ , LB ₁₃ , and LB ₁₅ , only two of the two light beams LB ₁₁ and LB ₁₅ are incident on the deflection surface, and therefore the amount of light after being reflected on the deflection surface is a curve. Proportional to the area of regions A1 and A3 (regions colored in FIG. 19A) defined by L1 or L3, a straight line passing through position y1 or position y2 and orthogonal to the Y axis, and the Y axis It becomes the light quantity. On the other hand, since all of the light beam LB ₁₃ is incident on the deflection surface, the amount of light after being reflected by the deflection surface becomes a light amount proportional to the area of the area A2 defined by the curve L2 and the Y axis. .

Therefore, in the optical scanning device 100 according to this embodiment, the VCSEL ₁₁ , the VCSEL ₁₃ , and the VCSEL ₁₅ are driven so that the intensity of the two light beams LB ₁₁ and LB ₁₅ is higher than the intensity of the light beam LB ₁₃ . For example, as shown in FIG. 19B, the areas of the regions A1 and A3 and the region A2 are almost equal so that the peak values of the two curves L1 and L3 are larger than the peak values of the curve L2. The VCSEL ₁₁ , the VCSEL ₁₃ , and the VCSEL ₁₅ are driven so as to be equal. As a result, the light amounts of the three light beams LB ₁₁ , LB ₁₃ , and LB ₁₅ reflected by the deflection surface are equalized. In the present embodiment, the light beams LB ₁₁ and LB ₁₅ have a light amount of 120% with respect to the light beam LB ₁₃ , so that the amount of reflected light is substantially matched.

For the VCSEL ₁₂ and the VCSEL ₁₄ arranged in the first row of the light source 10 as well, for example, as indicated by the curves L4 and L5 in FIG. 19C, the intensity of the emitted light beam is the light beam LB. ₁₃ so that the light intensity of each of the light beams reflected by the deflection surface is equal to the light intensity of the light beam LB ₁₃ reflected by the deflection surface. Similarly to the VCSEL _{21 in the} first row, the VCSELs ₂₁ to VCSEL ₈₅ up to the rows are arranged toward both ends in the main scanning direction rather than the intensity of the light beam emitted from the VCSEL _m3 arranged in the center of the main scanning direction. Each VCSEL is controlled so that the intensity of the light beam emitted from the VCSEL increases. Thereby, the light quantity of each light beam emitted from each VCSEL of the light source 10 and deflected by the deflecting surface of the polygon mirror 15 becomes substantially equal.

  As described above, the light beam deflected by the deflecting surface of the polygon mirror 15 to rotate, the moving speed of the main scanning direction of the light beam spot is adjusted by the first scanning lens 16 and the second scanning lens 17 In this state, the light is condensed on the surface of the photosensitive drum 201.

  As described above, in the optical scanning apparatus 100 according to the present embodiment, since the light amount of each light beam deflected on the deflecting surface is substantially uniform, the scanning of the writing area of the photosensitive drum 201, the light amount is more equal This is done with a light beam. Therefore, it is possible to scan the entire writing area without unevenness.

  In this embodiment, the distance L from the coupling lens 11 to the polygon mirror 15 is smaller than that of the conventional 122 so that the light beam from the outermost VCSEL comes close to the deflection surface of the polygon mirror 15. .82 mm.

  Since the focal length f of the coupling lens 11 is 47.7 mm, L / f is larger than 1 and smaller than 4. Regarding the light amount distribution, it is better that the light beam does not leave in the main scanning direction on the deflection surface of the polygon mirror 15, and considering only this point, the condition that a plurality of light beams overlap each other on the deflection surface, that is, L / f. Is preferably 1. However, if the distance between the coupling lens 11 and the polygon mirror 15 is too short, the focal length of the collimating lens 13 is shortened, and aberrations are difficult to remove. Therefore, the conditions farthest light beam in the main scanning direction is not too far, in view of the conditions under which the balance of the focal length of the collimating lens 13 take, it is desirable that the value of L / f is smaller than greater than 1 4 .

  In the above embodiment, although the optical scanning apparatus 100 has been described for use in the monochrome image forming apparatus 200, the image forming apparatus corresponds to a color image, a tandem color machine having a plurality of photosensitive drums Also good.

  Hereinafter, a multicolor image forming apparatus 300 corresponding to a color image and including a plurality of photosensitive drums will be described with reference to FIGS. The multicolor image forming apparatus 300 shown in FIG. 20 includes a photosensitive drum K1 for black (K), a charger K2, a developing device K4, a cleaning unit K5, a transfer charging unit K6, and cyan (C). Photosensitive drum C1, charging device C2, developing device C4, cleaning means C5, transfer charging means C6, magenta (M) photosensitive drum M1, charging device M2, developing device M4, cleaning means M5, and transfer Charging unit M6, yellow (Y) photosensitive drum Y1, charger Y2, developing unit Y4, cleaning unit Y5, transfer charging unit Y6, optical scanning device 900, transfer belt 901, fixing unit 902 etc. are provided.

  Each photosensitive drum rotates in the direction of the arrow in FIG. 20, and a charger, a developer, a transfer charging unit, and a cleaning unit are arranged in the order of rotation. Each charger uniformly charges the surface of the corresponding photosensitive drum. The surface of the photosensitive drum charged by the charger is irradiated with a beam by the optical scanning device 900, and an electrostatic latent image is formed on the photosensitive drum. Then, a toner image is formed on the surface of the photosensitive drum by the corresponding developing device. Further, the toner images of the respective colors are transferred onto the recording paper by the corresponding transfer charging means, and finally the image is fixed on the recording paper by the fixing means 902.

  Next, the optical scanning device 900 will be described with reference to FIGS.

  The optical scanning device 900 includes four light source units 10K, 10C, 10M, and 10Y, and the coupling lens 11 and the aperture member 12 in the same manner as the optical scanning device 100 described above. An optical system (not shown) that guides the light beams from 10C, 10M, and 10Y to the polygon mirror 230, the polygon mirror 230, the four first scanning lenses 218a, 218b, 218c, and 218d, and the eight folding mirrors 224a, 224b, 224c, 224d, 227a, 227b, 227c, 227d, four second scanning lenses 220a, 220b, 220c, 220d, and the like. 21 and 22, only a part of the optical scanning device 900 is shown for convenience.

  The four light source units 10K, 10C, 10M, and 10Y are all light source units configured to include the light source 10.

  The light source unit 10K emits a laser beam (hereinafter also referred to as a black beam) modulated according to black image information. The light source unit 10C emits a laser beam (hereinafter also referred to as a cyan beam) modulated according to cyan image information. The light source unit 10M emits a laser beam (hereinafter also referred to as a magenta beam) modulated according to magenta image information. The light source unit 10Y emits a laser beam (hereinafter also referred to as a yellow beam) modulated according to yellow image information.

  The first scanning lens 218a, the folding mirror 224a, the second scanning lens 220a, and the folding mirror 227a each correspond to a black beam.

  The first scanning lens 218b, the folding mirror 224b, the second scanning lens 220b, and the folding mirror 227b each correspond to a cyan beam.

  The first scanning lens 218c, the folding mirror 224c, the second scanning lens 220c, and the folding mirror 227c each correspond to a magenta beam.

  The first scanning lens 218d, the folding mirror 224d, the second scanning lens 220d, and the folding mirror 227d each correspond to a yellow beam.

  The laser beam emitted from each light source unit is converged in the sub-scanning direction so as to be linear on the deflection surface of the polygon mirror 230, and is focused on the deflection point of the polygon mirror 230 and the surface of the corresponding photosensitive drum. The point is conjugate with the sub-scanning direction.

  The polygon mirror 230 is composed of a six-sided mirror having a two-stage structure. A black beam from the light source unit 10K and a yellow beam from the light source unit 10Y are deflected in the first-stage six-sided mirror, respectively, and a cyan beam from the light source unit 10M and magenta from the light source unit 10C in the second-stage six-sided mirror. Each beam is deflected. That is, all the laser beams are deflected by the single polygon mirror 230.

  The first scanning lens 218a and the first scanning lens 218b are arranged on one side (here, + X side) of the polygon mirror 230, and the first scanning lens 218c and the first scanning lens 218d are on the other side of the polygon mirror 230 ( Here, it is arranged on the −X side). The first scanning lens 218a and the first scanning lens 218b, and the first scanning lens 218c and the first scanning lens 218d are stacked in a direction corresponding to the sub-scanning direction (here, the Z-axis direction).

  The black beam from the first scanning lens 218a forms a spot image on the photosensitive drum K1 via the folding mirror 224a, the second scanning lens 220a, and the folding mirror 227a.

  The cyan beam from the first scanning lens 218b forms a spot image on the photosensitive drum C1 via the folding mirror 224b, the second scanning lens 220b, and the folding mirror 227b.

  The magenta beam from the first scanning lens 218c, folding mirror 224c, a second scanning lens 220c, and via the folding mirror 227c, imaged in a spot shape on the photosensitive drum M1.

  The yellow beam from the first scanning lens 218d, folding mirror 224d, the second scanning lens 220d, and via the folding mirror 227d, is imaged in a spot shape on the photosensitive drum Y1.

  The folding mirrors are arranged so that the optical path lengths from the polygon mirror 230 to the photosensitive drums coincide with each other, and the incident position and the incident angle of the laser beam on each photosensitive drum are equal to each other. Has been.

  In the multicolor image forming apparatus 300 configured as described above, a line image is formed on each of the photosensitive drums K1, C1, M1, and Y1 by a light beam whose light amount is uniformly adjusted. Therefore, it is possible to form a high-definition multicolor image with high accuracy on the recording medium.

  In each of the above embodiments, the case where the optical scanning device of the present invention is used in a printer has been described. However, the image forming device other than the printer, for example, a copier, a facsimile, or a multifunction machine in which these are integrated. Is preferred.

1 is a diagram illustrating a schematic configuration of an image forming apparatus 200 according to a first embodiment. 1 is a diagram illustrating a schematic configuration of an optical scanning device 100 according to a first embodiment. It is a figure which shows the light source. 2 is a cross-sectional view of a VCSEL formed in a light source 10. FIG. It is an enlarged view (the 1) of the active layer 24 of VCSEL. It is an enlarged view (the 2) of the active layer 24 of VCSEL. It is FIG. (1) for demonstrating the effect of a relay lens system. It is FIG. (2) for demonstrating the effect of a relay lens system. It is a figure which shows schematic structure of the optical scanning device 100 which concerns on 2nd Embodiment. FIGS. 10A and 10B are views (No. 1) for explaining the relationship between the beam spot diameter and the defocus amount when the same optical system as that of the present embodiment is used. FIGS. 11A and 11B are views (No. 2) for explaining the relationship between the beam spot diameter and the defocus amount when the same optical system as that of the present embodiment is used. FIGS. 12A and 12B are views (No. 3) for explaining the relationship between the beam spot diameter and the defocus amount when the same optical system as that of the present embodiment is used. FIGS. 13A and 13B are diagrams (No. 4) for explaining the relationship between the beam spot diameter and the defocus amount when the same optical system as that of the present embodiment is used. FIGS. 14A and 14B are views (No. 1) for explaining the relationship between the beam spot diameter and the defocus amount when a conventional optical system is used, respectively. FIGS. 15A and 15B are views (No. 2) for explaining the relationship between the beam spot diameter and the defocus amount when a conventional optical system is used, respectively. FIGS. 16A and 16B are views (No. 3) for explaining the relationship between the beam spot diameter and the defocus amount when a conventional optical system is used, respectively. FIGS. 17A and 17B are diagrams (No. 4) for explaining the relationship between the beam spot diameter and the defocus amount when a conventional optical system is used, respectively. It is a figure for demonstrating the difference of the beam spot diameter when a defocus amount is 0, and the beam spot diameter when ((beta) * A) = 0 and (beta * A)> 0. FIGS. 19A to 19C are views (No. 1 to No. 3) for explaining a method of equalizing the light amount of the light beam incident on the deflection surface. 1 is a diagram illustrating a schematic configuration of a multicolor image forming apparatus 300. FIG. 1 is a perspective view showing a schematic configuration of an optical scanning device 900. FIG. 2 is a side view showing a schematic configuration of an optical scanning device 900. FIG.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 ... Light source, 11 ... Coupling lens, 12 ... Aperture member, 13 ... Collimating lens, 15 ... Polygon mirror, 16 ... 1st scanning lens, 17 ... 2nd scanning lens, 20 ... P side electrode, 21 ... n-GaAs substrate, 22 ... lower reflecting mirror, 23, 25 ... spacer layer, the lower reflector 24 ... active layer, 26 ... _Al x _{O y} current blocking layer, 27 ... upper reflector, 28 ... p-contact layer, 29 ... polyimide, 30 DESCRIPTION OF SYMBOLS ... AlAs selective oxidation layer, 31 ... Upper electrode, 32 ... Light emission part, 100 ... Optical scanning device, 200 ... Image forming apparatus, 201 ... Photosensitive drum, 202 ... Charger charger, 204 ... Toner cartridge, 205 ... Cleaning case, 206: paper feed tray, 207 ... paper feed roller, 208 ... registration roller pair, 209 ... fixing roller, 210 ... discharge tray, 211 ... transfer char J, 212... Paper discharge roller, 213... Paper, 215... Housing, 300 .. multicolor image forming apparatus, 900.

Claims (3)

A light source including a plurality of light emitting units, a deflector that deflects a plurality of light beams from the light source and scans in a main scanning direction, and the main scanning in the vicinity of a deflection surface of the deflector In an optical scanning device comprising an imaging optical system that forms an image in a sub-scanning direction orthogonal to the direction,
The imaging optical system includes first and second imaging optical elements having positive power in the main scanning direction,
Deflecting surface of the deflector, the main scanning direction dimension, rather smaller than the beam width of the main scanning direction,
The first imaging optical element is located closer to the light source than the second imaging optical element,
The first imaging optical element crosses the principal rays of the plurality of light beams in the main scanning direction between the first imaging optical element and the second imaging optical element,
The second imaging optical element is an optical scanning device that crosses principal rays of the plurality of light beams in the main scanning direction in the vicinity of the deflection surface . An image forming apparatus that forms an image by fixing a toner image formed based on a latent image obtained from information about an image to a recording medium,
An optical scanning device according to claim 1 ;
A photoreceptor on which a latent image is formed by the optical scanning device;
Developing means for visualizing a latent image formed on the surface to be scanned of the photoreceptor;
An image forming apparatus comprising: a transfer unit that fixes the toner image visualized by the developing unit to the recording medium. An image forming apparatus for forming a multicolor image by superimposing and fixing a toner image formed on the basis of a latent image for each color obtained from information on a multicolor image on a recording medium,
An optical scanning device according to claim 1 ;
A plurality of photosensitive members on which latent images corresponding to the respective colors are formed by the optical scanning device;
Developing means for visualizing latent images formed on the scanned surfaces of the plurality of photoconductors;
An image forming apparatus comprising: a transfer unit configured to superimpose and fix the toner image of each color visualized by the developing unit on the recording medium.