Optical scanner and image forming apparatus

Abstract

To provide an optical scanner that can perform satisfactory image formation.SOLUTION: An optical scanner comprises: a light source that has a plurality of light emission points; a deflector having a deflection surface that deflects a plurality of light beams from the plurality of light emission points to perform scanning of a surface to be scanned in a main scanning direction; an image forming optical system that guides the plurality of light beams from the deflector to the surface to be scanned; and a control unit that controls the light source. The plurality of light beams from the image forming optical system are guided to positions different from each other on the surface to be scanned in a sub-scanning direction. The control unit sets, for every scanning without periodicity, the maximum number of the light emission points of the light source in the single scanning.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to an optical scanning device and an image forming apparatus.

Background Art

[0002] In recent years, the multi-beaming of optical scanning devices used in image forming apparatuses has been progressing. Patent Document 1 discloses an optical scanning device that uses a light source device having a prime number of light emitting points and aims for high definition.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] With the multi-beaming in an optical scanning device, there is a problem that an abnormal image called so-called "banding" is likely to occur. That is, in an image forming apparatus using a multi-beam optical scanning device, there is a problem that image defects such as moire are likely to be visually recognized. An object of the present invention is to provide an optical scanning device capable of performing good image formation.

Means for Solving the Problems

[0005] The optical scanning device of the present invention includes a light source having a plurality of light emitting points, a deflector having a deflection surface that deflects a plurality of light beams from the plurality of light emitting points to scan a surface to be scanned in a main scanning direction, an imaging optical system that guides the plurality of light beams from the deflector to the surface to be scanned, and a control unit that controls the light source. The plurality of light beams from the imaging optical system are guided to different positions on the surface to be scanned in a sub-scanning direction, and the control unit is characterized in that the maximum number of light emitting points of the light source in one scan is set without periodicity for each scan.

Effects of the Invention

[0006] According to the present invention, an optical scanning device capable of performing good image formation can be provided.

Brief Description of the Drawings

[0007]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Figure 10-1

Figure 10-2

Figure 11

Figure 12

Figure 13

Embodiments for Carrying Out the Invention

[0008] Conventionally, in an optical scanning device used in an image forming apparatus such as a laser beam printer, a light beam from a light source means is guided to a deflector by an incident optical system, and the light beam deflected and scanned by the deflector is focused in a spot shape on a surface to be scanned by an imaging optical system to perform optical scanning with the light beam. In such an optical scanning device, the light beam emitted from the light source is converted into a substantially parallel light beam by a collimator lens or the like, and the light beam converted into a substantially parallel light beam is focused near the deflection surface of the deflector by a cylindrical lens to form a line image in order to perform tilt correction. The light beam deflected by the deflection surface scans the surface of the photosensitive drum, which is the surface to be scanned, at a substantially constant speed by an imaging lens.

[0009] In recent years, in image forming apparatuses, in order to meet the requirements of high-speed writing and high-density writing, the multi-beaming of optical scanning devices used for image writing has been progressing. With multi-beaming, abnormal images called so-called "banding" are likely to occur. That is, in high-speed and high-definition image forming apparatuses, since multi-beams are used, image defects such as moiré are likely to be visually recognized. In particular, in order to achieve a resolution of 1200 dpi or higher, an optical scanning device using a VCSEL (Vertical Cavity Surface Emitting Laser) having a large number of light emitting points is required. In a conventional optical scanning device using a multi-beam light source, image defects (moiré) occur in the deflection surface period of the deflection surface, which has been a problem in achieving higher resolution.

[0010] An object of the present invention is to provide an optical scanning device that can reduce image defects caused by the beam number period and the deflection surface period in a light source device having a plurality of four or more light emitting points and perform good image formation.

[0011] Hereinafter, preferred embodiments of the present invention will be described in detail based on the accompanying drawings. Note that the drawings shown below may be drawn at scales different from the actual ones in order to facilitate understanding of the present embodiment. In the following description, the main scanning direction is a direction perpendicular to the rotation axis of the deflector and the optical axis of the imaging optical system, and the sub-scanning direction is a direction parallel to the rotation axis of the deflector. The main scanning cross-section is a cross-section parallel to the main scanning direction and the optical axis of the imaging optical system (perpendicular to the sub-scanning direction), and the sub-scanning cross-section is a cross-section parallel to the sub-scanning direction and the optical axis of the imaging optical system (perpendicular to the main scanning direction). Also, the direction parallel to the optical axis of the imaging optical system is defined as the X direction, the main scanning direction as the Y direction, and the sub-scanning direction as the Z direction, and the unit of angle is expressed in radians (rad).

Embodiment

[0012] FIG. 1 is a main scanning cross-sectional view of the optical scanning device according to Embodiment 1 of the present invention, and FIG. 2 is a sub-scanning cross-sectional view.

[0013] The light source unit 1 is composed of a semiconductor laser (multi-beam light source) having a plurality of light emitting points (light emitting units). Among the plurality of light emitting points of the light source unit 1, the distance of at least one light emitting point from the optical axis of the first optical means 3 to be described later is different from the distance of another light emitting point from the optical axis. The light source unit 1 of the present embodiment is composed of a surface emitting laser in which eight light emitting points are arranged in a one-dimensional manner as shown in FIG. 3. By using an 8-beam laser, high speed and good image formation can be realized.

[0014] The light beam emitted from the light source unit 1 is restricted in the light beam width in the sub-scanning direction and the beam shape is shaped by the first aperture 2. This is to arrange the first aperture 2 near the collimator lens 3 to be described later and arrange the exit pupil position in the sub-scanning direction near the second imaging lens 20b which is an fθ lens, so that the principal rays of the eight beams pass through the same position in the sub-scanning direction near the second imaging lens 20b. The collimator lens 3, which is a condenser lens made of glass as the first optical means, is composed of a so-called cemented lens in which a convex lens and a concave lens are cemented together. The collimator lens 3 reduces the spot diameter difference between the light emitting points.

[0015] The light beam emitted from the collimator lens 3 enters the convex spherical lens 4 made of glass. The convex spherical lens 4 is a lens for adjusting the spot diameter on the photosensitive drum surface 30. Further, the collimator lens 3 and the convex spherical lens 4 cooperate to convert the divergent light beam emitted from the light source unit 1 into a parallel light beam.

[0016] The cylindrical lens 5 as the first optical means has power only in the sub-scanning cross-section (sub-scanning direction), and condenses the light beam that has passed through the collimator lens 3 and the spherical lens 4 as a line image on the deflection surface (reflective surface) 10a of the deflector 10 described later in the sub-scanning cross-section. The second aperture 6 is arranged on the deflector 10 side of the collimator lens 3, and the light beam width in the main scanning direction of the light beam that has passed through the cylindrical lens 5 is limited to shape the beam shape.

[0017] A wedge prism 7 having a wedge shape in the main scanning cross-section is arranged between the second aperture 6 and the deflector 10, and is arranged so that the incident surface of the second aperture 6 and the wedge prism 7 coincide. The incident surface and the exit surface of the wedge prism 7 have an angle of 4 degrees in the main scanning cross-section. This prevents the reflected light from the exit surface of the wedge prism 7 from entering the light quantity detection sensor 9 described later.

[0018] An imaging lens 8 is provided for condensing the light beam reflected by the incident surface of the wedge prism 7 onto the light quantity detection sensor 9. The light quantity detection sensor 9 is a light quantity detection sensor for emitting each beam of the surface-emitting laser with a desired light quantity. Different from the end-face emitting type laser, the surface-emitting laser cannot have an APC (Auto Power Control) sensor inside the element, so it has an APC sensor as the light quantity detection sensor 9 outside the laser.

[0019] The collimator lens 3 and the cylindrical lens 5 may be formed as one optical element. Also, each element of the first aperture 2, the collimator lens 3, the spherical lens 4, the cylindrical lens 5, and the second aperture 6 constitutes one element of the incident optical system LA. Further, the wedge prism 7, the imaging lens (BD lens) 8, and the light quantity detection sensor 9 constitute one element of the APC optical system.

[0020] The weakly divergent light beam emitted from the collimator lens 3 is converted into a substantially parallel light beam by the spherical lens 4, and the condensing positions (focus positions) of the light beams from a plurality of light emitting points on the photosensitive drum surface 30 or the deflection surface 10a can be made substantially the same, and the spot diameters of the plurality of light beams on the photosensitive drum surface 30 can be made substantially the same. Also, the second aperture 6 disposed near the deflector 10 can limit the light beam width in the main scanning direction and bring the principal rays of the light beams from each light emitting point on the deflection surface 10a closer together, so that the vertical line fluctuation generated during multi-beam can be reduced.

[0021] The deflector 10 as the deflecting means is composed of a polygon mirror (rotating polygon mirror) having five deflection surfaces (reflective surfaces), and is rotating at a constant speed in the direction of arrow A in the figure by a motor (not shown) which is the driving means.

[0022] An imaging optical system (fθ lens system) 20 having a condensing function and fθ characteristics is disposed between the deflector 10 and the photosensitive drum surface 30 as the surface to be scanned. The imaging optical system 20 is composed of first and second imaging lenses (fθ lenses) 20a, 20b. The first imaging lens 20a is a plano-convex spherical lens made of glass, and the second imaging lens 20b is formed of an anamorphic lens having an aspherical shape in the main scanning cross section. The imaging optical system 20 condenses the light beam based on the image information reflected and deflected by the deflector 10 onto the photosensitive drum surface 30 as the surface to be scanned, and performs skew correction by making the space between the deflection surface 10a of the deflector 10 and the photosensitive drum surface 30 conjugate in the sub-scanning cross section.

[0023] Note that the optical scanning device of the present invention has a control unit (not shown) that controls the driving of the light source unit 1 and the deflector 10.

[0024] By appropriately generating the paraxial image plane curvature in the sub-scanning direction of the imaging optical system 20 of this embodiment, the sub-scanning spot position deviation on the photosensitive drum surface 30 when skewing occurs is reduced, and the pitch unevenness is reduced.

[0025] FIG. 2 is a sub-scanning cross-sectional view schematically showing the optical path from the deflector 10 to the photosensitive drum surface 30 as the surface to be scanned. Folding mirrors 11 and 12 are arranged to fold the optical path between the imaging optical system 20 and the photosensitive drum surface 30. By folding the optical path after passing through the first and second imaging lenses 20a and 20b by the reflecting surfaces 11a of the folding mirror 11 and the reflecting surface 12a of the folding mirror 12, the deterioration of the optical performance caused by the angular deviation of the deflection surface 10a is reduced.

[0026] In this embodiment, a plurality (eight in this embodiment) of light beams that are light-modulated and emitted from the light source unit 1 according to the image information have their beam widths in the sub-scanning direction limited by the first aperture 2, are converted into substantially parallel light beams by the collimator lens 3 and the spherical lens 4, and enter the cylindrical lens 5. The light beams incident on the cylindrical lens 5 are emitted as they are in the main scanning cross-section, and the beam width in the main scanning direction is limited by the second aperture 6. Also, in the sub-scanning cross-section, they converge and pass through the second aperture 6 (the beam width in the main scanning direction is limited) to form a line image (a line image elongated in the main scanning direction) near the deflection surface 10a of the deflector 10.

[0027] The plurality of light beams reflected and deflected by the deflection surface 10a of the deflector 10 each enter the imaging optical system 20 having mainly a power convex in the main scanning direction, and are focused in spots on the photosensitive drum surface 30. By rotating the deflector 10 in the direction of arrow A, the photosensitive drum surface 30 is scanned at a constant speed in the direction of arrow 30a (main scanning direction) by light. Thereby, a plurality of scanning lines are simultaneously formed on the photosensitive drum surface which is the recording medium, and image recording is performed.

[0028] (Regarding the light source unit) In recent years, the requirements for high speed and high image quality have been increasing, and the rotation speed of a polygon motor that rotates a deflector (polygon mirror) has been increasing. Also, the pixel clock that serves as the reference clock for laser modulation has been increasing in speed. However, both the increase in the speed of the polygon motor and the increase in the pixel clock speed are approaching their limits, and it has become difficult to meet the requirements for further increases in speed and high image quality with these conventional methods.

[0029] Therefore, by adopting a multi-beam light source unit 1 having a plurality of light emitting units, it is possible to cope with the increase in speed. In the light scanning method using multi-beams, the number of light beams that can be scanned simultaneously by the deflection of the deflector increases, so that it is possible to reduce the rotation speed of the polygon motor and the pixel clock frequency, and it is possible to perform high-speed and stable light scanning and image formation. Generally, as the light source unit 1 constituting the multi-beams, there are a method using a plurality of single-beam laser chips and a method using a multi-beam laser. In this embodiment, a surface-emitting laser (VCSEL: vertical-cavity surface-emitting laser) in which a plurality of light emitting units are formed on the same chip is used, and the number of light sources is significantly increased.

[0030] FIG. 3 shows the arrangement of the light emitting points of the light source unit 1 of Example 1. In this embodiment, FIG. 3 shows the light source unit 1 for an optical scanning device used in an image forming apparatus having a resolution of 1200 dpi in the sub-scanning direction on the surface to be scanned. In FIG. 3, the light source unit 1 of this embodiment is composed of a surface-emitting laser (VCSEL) and is composed of 8 light emitting points. In the figure, LD1 and LD8 indicate the light emitting points, (the description of the reference symbols of LD2 to LD7 is omitted in FIG. 3) and the light emitting points are arranged in a one-dimensional straight line with a pitch of 50 μm. Also, the arrangement line of the light source unit 1 is rotated by α = 13.4° around the optical axis so that the interval between adjacent light emitting points when the light emitting points are projected in the sub-scanning direction becomes 1200 dpi (21.2 μm) on the surface to be scanned. It is not good because image degradation such as moire and pitch unevenness occurs when the scanning line interval becomes uneven.

[0031] Table 1 to 5 show the design parameters of the optical elements arranged in the optical path from the light source unit 1 of this embodiment to the photosensitive drum surface 30 which is the surface to be scanned. In the table, "E-x" means "×10 -x ".

Table 1

Table 2

Table 3

Table 4

Table 5

[0032] In this embodiment, the shapes of the first and second imaging lenses 20a and 20b are expressed by the following formulas (1) and (2) with the intersection points with their respective optical axes as the origin, the optical axis as the X-axis, the direction orthogonal to the optical axis in the main scanning cross-section as the Y-axis, and the direction orthogonal to the optical axis in the sub-scanning cross-section as the Z-axis.

[0033] Scanning start side:

Number

Number

[0034] In this embodiment, the shape of the first imaging lens 20a is configured symmetrically with respect to the optical axis in the main scanning direction. That is, the aspherical coefficients on the scanning start side and the scanning end side are made the same. In addition, the shapes of the incident surface and the exit surface of the second imaging lens 20b in the sub-scanning cross-section are arc-shaped. Further, regarding the shape of the second imaging lens 20b in the sub-scanning direction, the curvature 1 / r in the sub-scanning cross-section of the incident surface on the scanning start side and the scanning end side with respect to the optical axis is taken as a function of the position y in the Y-axis direction, and is continuously changed within the effective portion of the lens.

[0035] The shapes of the incident surface and the exit surface of the second imaging lens 20b in the sub-scanning direction are represented by the following formulas (3) and (4).

[0036] Scanning start side:

Number

Number

[0037] Here, r' is the radius of curvature in the sub-scanning direction, and Dj is the curvature change coefficient. When the coefficients are different on the plus side and the minus side in the Y-axis direction, the subscript s represents the scanning start side and e represents the scanning end side in the main scanning direction. In addition, the radius of curvature in the sub-scanning direction is the radius of curvature in the cross-section (sub-scanning cross-section) orthogonal to the shape (generatrix) in the main scanning direction.

[0038] (Regarding the method of changing the number of beams) FIG. 4 shows the relationship between the light-emitting possible points of the light source unit 1 in the optical scanning device of Example 1 and the maximum number of light-emitting points for each scan using each deflection surface 10a. In this specification, one scan is defined as scanning from one end side (scanning start side) to the other end side (scanning end side) in the main scanning direction of the photosensitive drum surface 30, which is the surface to be scanned, by one deflection surface 10a.

[0039] In FIG. 4, the white circles indicate non-emitting light-emitting points, and the black circles indicate light-emitting points that can emit light. Scanning 1 shown in FIG. 4 indicates that deflection plane 1 is used, light-emitting point LD1 corresponds to scanning line 1, light-emitting point LD2 corresponds to scanning line 2, and similarly, light-emitting point LD8 corresponds to scanning line 8. The numbers of the deflection planes correspond to the respective deflection planes 10a of the deflector 10 (polygon mirror) shown in FIG. 5.

[0040] In FIG. 4, in scanning 1, deflection plane 1 is used, and five light-emitting points LD4 to LD8 are used as light-emitting ones, and the light-emitting points LD1 to LD3 are controlled to be non-emitting. In the next scanning 2, deflection plane 2 is used, and two light-emitting points LD4 and LD5 are made light-emitting, and the other light-emitting points are controlled to be non-emitting. Similarly hereinafter, in scanning 3, deflection plane 3 is used, and five light-emitting points LD1 to LD5 are made light-emitting, and the other three light-emitting points are controlled to be non-emitting.

[0041] As shown in FIG. 4, in each scanning, the moving speed of the surface to be scanned in the sub-scanning direction and the rotation speed of the deflector 10 are set so that the light-emitting points LD1 to LD3 always overlap the scanning lines of the light-emitting points LD6 to LD8 scanned by the deflection plane in the immediately preceding scanning. In this embodiment, three of the scanning lines by the beam laser from the eight light-emitting points are overlapped, and the average exposure beam number (the average of the maximum number of light-emitting points that can emit light among a plurality of light-emitting points set for each deflection plane) is configured to be 5.

[0042] FIG. 6 shows the relationship between the deflection plane used in each scanning and the maximum number of light-emitting points. In scans 1 to 5 shown in FIGS. 4 and 6, the maximum numbers of light-emitting points are 5, 2, 5, 5, and 8, and the maximum number of light-emitting points is changed for each scan without keeping the maximum number of light-emitting points constant.

[0043] Also, in the second rotation (scans 6 to 10) after the deflector 10 has made one rotation (scans 1 to 5), the maximum number of light emission points for deflection surfaces 1 to 5 are 5, 3, 4, 5, and 7, respectively, and the maximum number of light emission points is changed in a pattern different from that in the first rotation. Hereinafter, starting from the third rotation (scan 6) onwards, the maximum number of light emission points is changed for each scan, and the pattern of this change is also changed for each rotation. This reduces image defects such as moiré of the aforementioned beam period and deflection surface number period.

[0044] Fig. 6 shows the average of the maximum number of light emission points for every 5 scans in 13 consecutive scans in this embodiment. In the figure, the average value of the maximum number of light emission points 5, 2, 5, 5, and 8 for scans 1 to 5 indicates that it is 5. Also, the average value of the maximum number of light emission points 2, 5, 5, 8, and 5 for scans 2 to 6 is also made to be 5. Thus, for example, image data for 13 scans is stored in the memory, and the maximum number of light emission points is set so that the average value of 5 consecutive surfaces in 13 scans becomes 5. This makes the amount of light on the surface to be scanned constant and reduces the change in image density.

[0045] In this embodiment, it is set so that the average value of 5 consecutive surfaces in 13 scans becomes the average maximum number of light emissions, but the same effect can be obtained even under other conditions. Specifically, the conditions to be satisfied in the present invention will be described in detail below.

[0046] This embodiment is configured to satisfy the following conditional expression (5). 3G - N ≤ W ≤ N + G ··· (5) Here, W is the sum of the maximum number of light emission points in two consecutive scans. G is the average value of the maximum number of light emission points in Rn consecutive scans (the first number of scan times), where Rn is the number of deflection surfaces 10a provided in the deflector 10. In this embodiment, as described above, G is set to be 5. N indicates the number of light emission points provided in the light source unit 1, and in this embodiment, N = 8.

[0047] In this embodiment, The value of the left side of conditional expression (5) = 3×5 - 8 = 7 The value of the right side of conditional expression (5) = 8 + 5 = 13 Furthermore, the sum W of the maximum number of light emission points in two consecutive scans is configured to be 7 or more and 13 or less. As shown in FIG. 6, Example 1 satisfies the conditional expression (5).

[0048] In this embodiment, when the difference between the maximum number of light emission points in two consecutive scans is S, it is configured to satisfy the following conditional expression (6). S ≦ 2(N - G) ··· (6) In this embodiment, The value of the left side of the conditional expression (6) = 2×(8 - 5) = 6 That is, the difference S between the maximum number of light emission points in two consecutive scans is configured to be 6 or less. As shown in FIG. 6, Example 1 satisfies the conditional expression (6).

[0049] Also, in this embodiment, when the number of deflection surfaces 10a provided in the deflector 10 is Rn, and the minimum value of the maximum number of light emission points in Rn consecutive scans is H, it is configured to satisfy the following conditional expression (7). H ≧ 2G - N ··· (7) In this embodiment, The value of the right side of the conditional expression (7) = 2×5 - 8 = 2 Therefore, the minimum value of the maximum number of light emission points in Rn consecutive scans is configured to be 2 or more. As shown in FIG. 6, Example 1 satisfies the conditional expression (7).

[0050] By satisfying the conditional expressions (5) to (7), the optical scanning device of the present invention enables drawing at a resolution of 1200 dpi in the design without creating a gap between the scanning lines. If the conditional expressions (5) to (7) are not satisfied, the resolution becomes lower than 1200 dpi depending on the position in the sub-scanning direction, or the same scanning line is repeatedly exposed on different deflection surfaces (in the scanning cycle), resulting in density unevenness in the drawn image, which is not good.

[0051] Here, in this embodiment, in conditional expressions (5) to (7), the number of deflection surfaces 10a provided in the deflector 10 is used as the population for determining the minimum value H and the average value G of the maximum number of light emission points. That is, five scans in one rotation (five deflection surfaces 10a) of the deflector 10 are used as the population. If the population for obtaining the minimum value H and the average value G of the maximum number of light emission points is set to a smaller number of scans, periodicity will appear in the exposure beam, which is not good. Also, if the population is set to a large number of scans, such as 100 or more, density unevenness in the image due to light quantity changes will be prominent, which is not good. Therefore, in this embodiment, the minimum value and the average value are calculated using one rotation of scans of the deflector 10 as the population, and by setting to satisfy conditional expressions (5) to (7), a high-quality image with reduced periodicity and density unevenness is obtained.

[0052] In this embodiment, it is more preferable to determine the maximum number of light emission points using a random number so as to satisfy conditional expressions (5) to (7). By using a random number, the spatial frequency of the exposure beam can be minimized, and the density unevenness in the image can be made less visible. Also, in the present invention, if the spatial frequency that is difficult to visually recognize can be set without using a random number to determine the maximum number of light emission points, the same effects as in this embodiment can be obtained.

[0053] In this embodiment, a control unit (not shown) that controls the light source unit 1 and the deflector 10 sets the maximum number of light emission points in one scan in the main scanning direction without periodicity for each scan. Here, periodicity means, for example, the time for scanning the range for drawing one piece of image information when drawing an image on the photosensitive drum 30 surface, which is the surface to be scanned, using the optical scanning device of the present invention, the number of scans, etc. as the unit of the period. More specifically, within the drawing range such as one photo or image, one page of document, the maximum number of light emission points in one scan in the main scanning direction is set without periodicity for each scan.

[0054] As described above, in the optical scanning device having the features of the present invention, good image formation can be achieved by changing the number of scan lines (maximum number of light emission points) for each deflection surface in each scan.

Embodiment

[0055] With reference to FIGS. 7 and 8, the optical scanning device according to Embodiment 2 will be described. FIG. 7 shows the relationship between the light-emitting points of the light source unit 1 in the optical scanning device of Embodiment 2 and the maximum number of light-emitting points for each scan using each deflection surface 10a. The difference between Embodiment 2 and Embodiment 1 is that the average of the maximum number of light-emitting points per rotation on the deflection surfaces 1 to 5 is set to be constant. That is, the average printing beam per rotation of the deflector 10 starting from the deflection surface 1 is set to 5 beams.

[0056] In FIG. 7, the circles indicate the light-emitting points, the white circles indicate the non-light-emitting light-emitting points, and the black circles indicate the light-emittable light-emitting points. It shows that the light-emitting points LD1, LD2, LD3, LD4, LD5, LD6, LD7, and LD8 correspond to the scanning lines 1, 2, 3, 4, 5, 6, 7, and 8, respectively. The numbers of the deflection surfaces correspond to the respective deflection surfaces of the deflector 10 shown in FIG. 5. As shown in FIG. 7, in the scan 1 scanned on the deflection surface 1, five light-emitting points LD3 to LD7 are made light-emittable, and the light-emitting points LD1, LD2, and LD8 are made non-light-emitting.

[0057] Similarly, in the subsequent scan 2 on the deflection surface 2, four light-emitting points LD3 to LD6 are set as the light-emittable light-emitting points, and the other four light-emitting points are set as the non-light-emitting light-emitting points. Hereinafter, similarly, in the scan 3 on the deflection surface 3, the light-emitting points used for the deflection scan on the deflection surface 3 are shown. In FIG. 7, LD1 to LD3 always overlap with the scanning line scanned one surface ahead, and the moving speed of the photosensitive drum surface 30, which is the surface to be scanned, in the sub-scanning direction and the rotation speed of the deflector 10 are set. In this embodiment, by overlapping three 8-beam lasers, the average number of exposure beams is configured to be 5.

[0058] Fig. 8 shows the relationship between the deflection plane and the maximum number of light emission points for each scan. In scans 1 to 5 shown in Figs. 7 and 8, the maximum number of light emission points is set to 5, 4, 6, 4, and 6, and the maximum number of light emission points is changed for each scan (deflection plane). Also, in the second rotation (scans 6 to 10) after the deflector 10 makes one rotation, the maximum number of light emission points is set to 4, 7, 2, 7, and 5 for deflection planes 1 to 5, creating a pattern of maximum number of light emission points different from that in the first rotation (scans 1 to 5). Hereinafter, after the third rotation (scans 11 and later), the maximum number of light emission points is changed for each scan, and the pattern of this change is changed for each rotation of the deflector 10. This reduces image defects such as moiré of the aforementioned beam period and deflection plane number period.

[0059] Also, under the condition that the average value of the maximum number of light emission points for every 5 scans in one rotation of the deflector 10 is constant, the maximum number of light emission points in each scan is set using a random number, so the amount of image data stored in the memory can be reduced compared to Example 1. In Example 2, the average of the maximum number of light emission points is made constant for each rotation of the deflector 10 (for each of deflection planes 1 to 5), but the present invention is not limited to this. As long as the amount of image data stored in the memory can be reduced, the same effect can be obtained even for rotations other than one rotation.

[0060] The maximum number of light emission points for scans on 20 consecutive deflection planes of this example is shown in Fig. 8. In the figure, it shows that the average value of the maximum number of light emission points 5, 4, 6, 4, and 6 for scans 1 to 5 is 5. Also, the average value of the maximum number of light emission points 4, 7, 2, 7, and 5 for scans 6 to 10 corresponding to the second rotation of the deflector 10 is also 5. That is, for scans 1 to 5 in the first rotation, scans 6 to 10 in the second rotation, and scans 11 and later (not shown) in the third rotation and later, the average value of the maximum number of light emission points for deflection planes 1 to 5 for each rotation is set to 5. This reduces image defects, makes the amount of light on the scanned surface constant, and reduces changes in image density.

[0061] (Regarding the values of conditional expressions (5) to (7)) Verify the values for conditional expressions (5) to (7) for the optical scanning device of Example 2. In Example 2, the number Rn of deflection surfaces 10a provided in the deflector 10 is 5, the number N of light emitting points provided in the light source unit 1 is 8, and the average value G of the maximum number of light emitting points in Rn consecutive scans (5 scans) is 5.

[0062] Regarding the conditional expression (5) for Example 2, The value on the left side of the conditional expression (5) = 3×5 - 8 = 7 The value on the right side of the conditional expression (5) = 8 + 5 = 13 Therefore, the maximum value of the sum W of the maximum number of light emitting points in two consecutive scans is configured to be 7 or more and 13 or less. As shown in FIG. 8, the optical scanning device of Example 2 satisfies the conditional expression (5).

[0063] In Example 2, the number N of light emitting points provided in the light source unit 1 is 8, and the average value G of the maximum number of light emitting points in Rn consecutive scans (5 scans) is 5. The value of the conditional expression (6) for this example is The left side of the conditional expression (6) = 2×(8 - 5) = 6 and the difference S between the maximum number of light emitting points in two consecutive scans is configured to be 6 or less. As shown in FIG. 8, the optical scanning device of Example 2 satisfies the conditional expression (6).

[0064] Regarding the conditional expression (7) for Example 2, since the number N of light emitting points provided in the light source unit 1 is 8 and the average value G of the maximum number of light emitting points in Rn consecutive scans (5 scans) is 5, The value on the right side of the conditional expression (7) = 2×5 - 8 = 2 From this, in Example 3, the minimum value of the maximum number of light emitting points in Rn consecutive scans is configured to be 2 or more. As shown in FIG. 8, the optical scanning device of Example 2 satisfies the conditional expression (7).

[0065] By satisfying the conditional expressions (5) to (7), it is possible to print at a resolution of 1200 dpi without gaps between the scanning lines. If the conditional expressions (5) to (7) are not satisfied, the resolution becomes lower than 1200 dpi or the same scanning line is exposed on different deflection surfaces, which is not good.

[0066] In this embodiment, the maximum number of light-emitting points for each scan is determined using random numbers that satisfy conditional expressions (5) to (7). By using random numbers, the spatial frequency of the maximum number of light-emitting points is minimized, making it difficult to visually recognize on the image. If it is possible to set the spatial frequency to be difficult to visually recognize without using random numbers, the same effect as this embodiment can be obtained.

[0067] In Example 2, as described above, the average value of the maximum number of light-emitting points in five scans is made constant (5). That is, it is configured such that the amount of light is constant when exposing 25 scan lines in five scans (one rotation). If the average value of the maximum number of light-emitting points in five scans (one rotation) is not made constant, the exposure amount on the scanned surface changes, and density unevenness of the image occurs, which is not good.

[0068] As described above, in the light source unit 1 having eight light-emitting points, by changing the maximum number of light-emitting points for each scan, good image formation can be performed. Also, the memory capacity can be reduced compared to Example 1, and a more inexpensive optical scanning device and an image forming apparatus using the same can be provided.

Example

[0069] The optical scanning device of Example 3 will be described with reference to FIGS. 9 to 11. FIGS. 10-1 and 10-2 show the relationship between the light-emitting possible points of the light source unit 1 in the optical scanning device of Example 3 and the maximum number of light-emitting points for each scan using each deflection surface 10a. The differences from Example 2 of Example 3 are that the number of light-emitting points (beam number) provided in the light source unit 1 is 32 beams and the resolution is 2400 dpi. Other configurations are the same as those in Example 2.

[0070] FIG. 9 shows the arrangement of the light-emitting points provided in the light source unit 1 of the optical scanning device according to Embodiment 3. The light source unit 1 is a light source for an optical scanning device used in an image forming apparatus having a resolution of 2400 dpi in the sub-scanning direction on the surface to be scanned. In FIG. 9, the light-emitting points LD1 and LD32 indicate the light-emitting points, (the description of the light-emitting points LD2 to LD31 is omitted) and the light-emitting points are arranged in a one-dimensional straight line at a pitch of 50 μm. Further, the direction in which the light-emitting points are arranged so that the interval between adjacent light-emitting points when projected in the sub-scanning direction is 2400 dpi (10.6 μm) on the surface to be scanned is rotated by α = 6.7° around the optical axis with respect to the main scanning plane.

[0071] In FIGS. 10-1 and 10-2, the circles indicate the light-emitting points, the white circles indicate the non-light-emitting light-emitting points, and the black circles indicate the light-emitting points capable of emitting light. It is shown that the light-emitting point LD1 corresponds to the scanning line 1, the light-emitting point LD2 corresponds to the scanning line 2, ··· the light-emitting point LD32 corresponds to the scanning line 32. The numbers on the deflection surfaces correspond to the respective deflection surfaces of the deflector 10 shown in FIG. 5. As shown in FIGS. 10-1 and 10-2, in the scan 1 scanned by the deflection surface 1, 28 light-emitting points from LD3 to LD30 are set to be capable of emitting light, and the four light-emitting points LD1, 2, 31, and 32 are set to be non-light-emitting.

[0072] Similarly, in the scan 2 scanned by the deflection surface 2, 30 light-emitting points from LD1 to LD30 are set to be capable of emitting light, and the light-emitting points LD31 and 32 are set to be non-light-emitting. Hereinafter, similarly, in the scan 3 in which the deflection surface 3 is used, 31 light-emitting points from LD1 to LD31 are set to be capable of emitting light, and the light-emitting point LD32 is set to be non-light-emitting.

[0073] As shown in FIGS. 10-1 and 10-2, the moving speed in the sub-scanning direction of the photosensitive drum surface 30, which is the surface to be scanned, and the rotation speed of the deflector 10 are set so that the light-emitting points LD1 and LD2 always overlap with the scanning line scanned one surface ahead. In this embodiment, by overlapping two 32-beam lasers, the average number of exposure beams is configured to be 30.

[0074] FIG. 11 shows the relationship between the deflection plane and the maximum number of light-emitting points for each scan. In scans 1 to 5 shown in FIGS. 10-1, 10-2, and 11, the maximum number of light-emitting points is set to 28, 30, 31, 31, and 30, and the maximum number of light-emitting points is changed for each scan (deflection plane). Also, in the second rotation (scans 6 to 10), the maximum number of light-emitting points for deflection planes 1 to 5 is set to 29, 29, 31, 30, and 31, which is different from that in the first rotation (scans 1 to 5). Hereinafter, in the third rotation and subsequent (scans 11 and later), the maximum number of light-emitting points is changed for each scan, and the method of changing it is changed for each rotation of the deflector 10. This reduces image defects such as moiré of the aforementioned beam period and deflection plane number period.

[0075] In this embodiment, the average exposure beam is made constant for each rotation, but the same effect can be obtained even in rotations other than the first rotation if the amount of image data stored in the memory can be reduced. As shown in FIG. 11, the average of the maximum number of light-emitting points in 20 consecutive scans of this embodiment indicates that the average value of the maximum number of light-emitting points 28, 30, 31, 31, and 30 in scans 1 to 5 is 30. Also, the average value of the maximum number of light-emitting points 29, 29, 31, 30, and 31 in scans 1 to 5 of the second rotation is also 30. That is, the average value of one rotation (5 planes) is set to 30 for scans 1 to 5 of the first rotation, scans 6 to 10 of the second rotation, scans 11 to 15 of the third rotation, scans 16 to 20 of the fourth rotation, and the fifth rotation and subsequent. This makes the amount of light on the surface to be scanned constant and reduces the change in image density.

[0076] (Regarding the values of conditional expressions (5) to (7)) Verify the values of conditional expressions (5) to (7) for the optical scanning device of Example 3. In Example 3, the number Rn of deflection planes 10a provided in the deflector 10 is 5, the number N of light-emitting points provided in the light source unit 1 is 32, and the average value G of the maximum number of light-emitting points in Rn consecutive scans (5 scans) is 30.

[0077] Regarding conditional expression (5) for Example 3, The value on the left side of conditional expression (5) = 3×30 - 32 = 58 Value on the right side of conditional expression (5) = 32 + 30 = 62 Therefore, the maximum value of the sum W of the maximum number of emission points in two consecutive scans is configured to be 58 or more and 62 or less. As shown in FIG. 11, the optical scanning device of Example 3 satisfies conditional expression (5). Thereby, image defects such as moiré of the beam period and the deflection plane number period are reduced, and printing can be performed at a resolution of 2400 dpi without gaps.

[0078] In Example 3, the number of emission points N provided in the light source unit 1 is 32, and the average value G of the maximum number of emission points in Rn consecutive scans (5 times) is 30. The value of conditional expression (6) for this example is, Left side of conditional expression (6) = 2×(32 - 30) = 4 and the difference S between the maximum number of emission points in two consecutive scans is configured to be 4 or less. As shown in FIG. 11, the optical scanning device of Example 3 satisfies conditional expression (6).

[0079] Regarding conditional expression (7) for Example 3, since the number of emission points N provided in the light source unit 1 is 32 and the average value G of the maximum number of emission points in Rn consecutive scans (5 times) is 30, Value on the right side of conditional expression (7) = 2×30 - 32 = 28 From this, in Example 3, the minimum value of the maximum number of emission points in Rn consecutive scans is configured to be 28 or more. As shown in FIG. 11, the optical scanning device of Example 3 satisfies conditional expression (7).

[0080] If conditional expressions (5) to (7) are not satisfied, the resolution becomes lower than 2400 dpi, or the same scan line is exposed on different deflection planes, which is not preferable. In Example 3, as described above, the average value of the maximum number of emission points in 5 scans is made constant (30). That is, when exposing 150 scan lines in 5 scans (one rotation), the light amount is configured to be constant. If the average value of the maximum number of emission points in 5 scans (one rotation) is not constant, the exposure amount on the scanned surface changes, and density unevenness of the image occurs, which is not good.

[0081] In this embodiment, the number of beams is determined using random numbers that satisfy conditional expressions (5) to (7). By using random numbers, the spatial frequency of the maximum number of emission points is minimized, making it difficult to visually recognize on the image. If it is possible to set the spatial frequency to be difficult to visually recognize without using random numbers, the same effect as this embodiment can be obtained.

[0082] As described above, in the light source unit 1 having 32 emission points, by changing the maximum number of emission points for each scan, good image formation can be performed. In addition, it is possible to provide an optical scanning device with higher resolution than that of Example 2 and an image forming apparatus using the same.

[0083] In the illustrated Examples 1 to 3, a deflector (polygon mirror) having a plurality of deflection surfaces is described as the light deflecting means, but the present invention is not limited thereto. The present invention is also applicable to an optical deflection device using a MEMS mirror that swings one deflection reflecting surface to deflect a light beam from a light source means, for example, in a reciprocating motion, to scan a surface to be scanned. Also in that case, by setting the maximum number of emission points in one scan in the main scanning direction by the deflector including the MEMS mirror without periodicity for each scan, the effect of the present invention of reducing image defects such as moiré of the beam period while maintaining high resolution can be enjoyed.

[0084] [Monochrome Image Forming Apparatus] FIG. 12 shows a main part sub-scanning cross-sectional view of an image forming apparatus (electrophotographic printer) 104 including the optical scanning device according to any one of Examples 1 to 3.

[0085] As shown in FIG. 12, a signal output from an external device 117 such as a personal computer, specifically, code data Dc, is input to the image forming apparatus 104. Then, the input code data Dc is converted into image data (dot data) Di by a printer controller 111.

[0086] The converted image data Di is input into an optical scanning unit 100 which is an optical scanning device according to any one of the first to third embodiments. From the optical scanning unit 100, an optical beam (light beam) 103 modulated according to the image data Di is emitted, and the photosensitive surface of the photosensitive drum 101 is scanned in the main scanning direction by the optical beam 103.

[0087] The photosensitive drum 101, which is an electrostatic latent image carrier (photoconductor), is rotated clockwise by a motor 115. Along with this rotation, the photosensitive surface of the photosensitive drum 101 moves in the sub-scanning direction orthogonal to the main scanning direction with respect to the optical beam 103.

[0088] Above the photosensitive drum 101, a charging roller 102 for uniformly charging the surface of the photosensitive drum 101 is provided so as to contact the surface. And the surface of the photosensitive drum 101 charged by the charging roller 102 is irradiated with the optical beam 103 scanned by the optical scanning unit 100.

[0089] As described above, the optical beam 103 is modulated based on the image data Di, and an electrostatic latent image is formed on the surface of the photosensitive drum 101 by irradiating the optical beam 103. Then, the formed electrostatic latent image is developed as a toner image by a developing device 107 disposed so as to contact the photosensitive drum 101 on the downstream side in the rotational cross-section of the photosensitive drum 101 with respect to the irradiation position of the optical beam 103.

[0090] The toner image developed by the developing device 107 is transferred onto a sheet 112, which is a material to be transferred, by a transfer roller (transfer device) 108 disposed so as to face the photosensitive drum 101 below the photosensitive drum 101. The sheet 112 is stored in a sheet cassette 109 in front of (right side in FIG. 12) the photosensitive drum 101, but it is also possible to feed the sheet manually. And the sheet 112 in the sheet cassette 109 is fed into the conveyance path by a sheet feeding roller 110 disposed at the end of the sheet cassette 109.

[0091] The sheet 112 on which the unfixed toner image has been transferred as described above is further conveyed to a fixing device behind the photosensitive drum 101 (the left side in FIG. 12). The fixing device is composed of a fixing roller 113 having a fixing heater (not shown) inside and a pressure roller 114 disposed so as to be in pressure contact with the fixing roller 113.

[0092] Then, the sheet 112 conveyed from the transfer roller 108 is heated while being pressed by the pressure contact portion between the fixing roller 113 and the pressure roller 114, whereby the unfixed toner image on the sheet 112 is fixed. Further, a paper discharge roller 116 is disposed behind the fixing device, and the fixed sheet 112 is discharged to the outside of the image forming apparatus 104.

[0093] Although not shown in FIG. 12, in addition to the above-described data conversion, the printer controller 111 also controls each member in the image forming apparatus 104 such as the motor 115 and members such as the polygon motor in the optical scanning unit 100.

[0094] [Color Image Forming Apparatus] FIG. 13 shows a partial sub-scanning cross-sectional view of a color image forming apparatus (electrophotographic printer) 260 including an optical scanning device according to any one of Embodiments 1 to 3. In FIG. 13, 260 is a color image forming apparatus, 211, 212, 213, 214 are optical scanning devices having any one of the configurations of Embodiments 1 to 3, 221, 222, 223, 224 are photosensitive drums as image carriers, and 231, 232, 233, 234 are developing devices, and 251 is a conveying belt.

[0095] In FIG. 13, color signals of R (red), G (green), and B (blue) are input to a color image forming apparatus 260 from an external device 252 such as a personal computer. These color signals are converted into image data (dot data) of C (cyan), M (magenta), Y (yellow), and B (black) by a printer controller 253 in the apparatus. These image data are input to optical scanners 211, 212, 213, and 214, respectively. Then, light beams 241, 242, 243, and 244 modulated according to the respective image data are emitted from these optical scanners, and the photosensitive surfaces of photosensitive drums 221, 222, 223, and 224 are scanned in the main scanning direction by these light beams.

[0096] The color image forming apparatus 260 in the present embodiment emits light rays corresponding to the respective colors of C (cyan), M (magenta), Y (yellow), and B (black) from the optical scanners 211, 212, 213, and 214, records image signals (image information) on the surfaces of the photosensitive drums 221, 222, 223, and 224, and prints color images at high speed. Thereafter, multiple transfers are made onto a recording material to form a single full-color image.

[0097] As the external device 252, for example, a color image reading device equipped with a CCD sensor may be used. In this case, a color digital copying machine is configured by this color image reading device and the color image forming apparatus 260.

[0098] Although the preferred embodiments have been described above, the present invention is not limited to these embodiments, and various modifications and changes are possible within the scope of the gist thereof.

[0099] The disclosure of the present embodiment includes the following configurations. (Configuration 1) A light source having a plurality of light emitting points, A deflector having a deflection surface that deflects a plurality of light beams from the plurality of light emitting points to scan a surface to be scanned in the main scanning direction, An imaging optical system that guides the plurality of light beams from the deflector to the surface to be scanned, and a control unit that controls the light source, a plurality of light beams from the imaging optical system are guided to different positions on the scanned surface in the sub-scanning direction, wherein the control unit sets, for each scan, the maximum number of light-emitting points of the light source in one scan without periodicity, characterized by an optical scanning device. (Configuration 2) In scans of a continuous first number of scan times, when the sum and difference of the maximum number of light-emitting points in two consecutive scans are W and S, respectively, the minimum value and average value of the maximum number of light-emitting points in the scans of the first number of scan times are H and G, respectively, and the number of the plurality of light-emitting points of the light source is N, 3G - N ≤ W ≤ N + G S ≤ 2(N - G) H ≥ 2G - N The optical scanning device according to Configuration 1, characterized by satisfying the conditions. (Configuration 3) The deflector is a rotating polygon mirror having a plurality of deflection surfaces, The optical scanning device according to Configuration 2, characterized in that the first number of scan times is the number of the plurality of deflection surfaces. (Configuration 4) The optical scanning device according to Configuration 3, characterized in that the plurality of scans on the plurality of deflection surfaces in one rotation of the rotating polygon mirror include two different maximum numbers of light-emitting points. (Configuration 5) The optical scanning device according to Configuration 3 or 4, characterized in that the average value of the maximum number of light-emitting points on the plurality of deflection surfaces for each rotation of the rotating polygon mirror is constant. (Configuration 6) The optical scanning device according to any one of Configurations 1 to 5, characterized in that the maximum number of light-emitting points is set based on a random number. (Configuration 7) The optical scanning device according to any one of Configurations 1 to 6, characterized in that the light-emitting points that can emit light among the plurality of light-emitting points in one scan in the main scanning direction are arranged adjacent to each other. (Configuration 8) The optical scanning device according to any one of Configurations 1 to 7, wherein the light source includes four or more of the light emitting points. (Configuration 9) The optical scanning device according to Configuration 1, wherein the deflector is composed of a MEMS mirror. (Configuration 10) The optical scanning device according to any one of Configurations 1 to 9, wherein the maximum number of light emitting points in two consecutive scans are different from each other. (Configuration 11) The optical scanning device according to any one of Configurations 1 to 10, wherein a plurality of scans on the same deflection plane include different maximum numbers of light emitting points. (Configuration 12) An image forming apparatus comprising: the optical scanning device according to any one of Configurations 1 to 11; a developing device that develops an electrostatic latent image formed on the scanned surface by the optical scanning device into a toner image; a transfer device that transfers the developed toner image onto a transfer material; and a fixing device that fixes the transferred toner image onto the transfer material. (Configuration 13) An image forming apparatus comprising: the optical scanning device according to any one of Configurations 1 to 11; and a printer controller that converts a signal output from an external device into image data and inputs the image data to the optical scanning device.

Explanation of Reference Numerals

[0100] 1 Light source unit (light source) 10 Deflector 10a Deflection plane 20 Imaging optical system 30 Scanned surface

Claims

1. A light source having a plurality of light emitting points, A deflector having a deflection surface that deflects a plurality of light beams from the plurality of light emitting points to scan a surface to be scanned in a main scanning direction, An imaging optical system that guides a plurality of light beams from the deflector to the surface to be scanned, A control unit that controls the light source, and The plurality of light beams from the imaging optical system are guided to different positions on the surface to be scanned in a sub-scanning direction, The control unit is characterized in that the maximum number of light emitting points of the light source in one scan is set without periodicity for each scan, and the optical scanning device.

2. In scans of a continuous first number of scan times, when the sum and difference of the maximum number of light emitting points in two consecutive scans are W and S, respectively, the minimum value and average value of the maximum number of light emitting points in the scans of the first number of scan times are H and G, respectively, and the number of the plurality of light emitting points of the light source is N, 3G - N ≤ W ≤ N + G S ≤ 2(N - G) H ≥ 2G - N The optical scanning device according to claim 1, characterized by satisfying the conditions.

3. The deflector is a rotating polygon mirror having a plurality of deflection surfaces, The optical scanning device according to claim 2, characterized in that the first number of scan times is the number of the plurality of deflection surfaces.

4. The plurality of scans on the plurality of deflection surfaces in one rotation of the rotating polygon mirror include two different maximum numbers of light emitting points, and the optical scanning device according to claim 3.

5. The average value of the maximum number of light emitting points on the plurality of deflection surfaces for each rotation of the rotating polygon mirror is constant, and the optical scanning device according to claim 3.

6. The optical scanning device according to claim 1, characterized in that the maximum number of light emitting points is set based on a random number.

7. Among the plurality of light emitting points in one scan in the main scanning direction, the light emitting points that can emit light are arranged adjacent to each other, and the optical scanning device according to claim 1.

8. The light source includes 4 or more of the light emitting points, and the optical scanning device according to claim 1.

9. The deflector is composed of a MEMS mirror, and the optical scanning device according to claim 1.

10. The maximum number of light emitting points in two consecutive scans is different from each other, and the optical scanning device according to claim 1.

11. The plurality of scans on the same deflection surface include different maximum numbers of light emitting points, and the optical scanning device according to claim 1.

12. An image forming apparatus comprising: an optical scanning device according to any one of claims 1 to 11; a developing device that develops an electrostatic latent image formed on the surface to be scanned by the optical scanning device into a toner image; a transfer device that transfers the developed toner image onto a transfer material; and a fixing device that fixes the transferred toner image onto the transfer material.

13. An image forming apparatus comprising: an optical scanning device according to any one of claims 1 to 11; and a printer controller that converts a signal output from an external device into image data and inputs the image data to the optical scanning device.

Images