Light source device and image forming apparatus

Abstract

To provide a light source device that enables reduction of unnecessary reflected light due to an error in mounting the device.SOLUTION: A light source device 105 comprises a first light emitting unit 209 and a second light emitting unit 210 arranged at positions different from each other in a first direction and in a second direction perpendicular to the first direction, and a lens unit 204 that condenses light emitted from the first and the second light emitting units. The first and the second light emitting units respectively include a plurality of light emitting elements arranged in the first direction. When viewed from the first direction, when a distance between a center of the first light emitting element in the first light emitting unit and a center of the second light emitting element in the second light emitting unit is defined as T, and a distance between a first straight line passing on the centers of the first and the second light emitting elements and a center of an incidence plane of the lens unit is defined as D, the light source device satisfies a condition shown by a relational expression of 0.16≤T / D≤0.31.SELECTED DRAWING: Figure 6

Description

[Technical Field]

[0001] The present invention relates to a light source device used in an exposure head of an image forming apparatus. [Background technology]

[0002] Some electrophotographic printers form a latent image by exposing a photosensitive drum, which serves as an irradiated surface, to light using an exposure head that uses LEDs, organic EL, etc. The exposure head is composed of a light source that includes multiple light-emitting units arranged in the longitudinal direction of the photosensitive drum, and a lens unit (lens array) that focuses the light from the light source onto the photosensitive drum to form an image.

[0003] Patent Document 1 discloses an exposure head that uses a light source in which a plurality of light-emitting units are arranged in two staggered rows, and a gradient index lens array. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2002-248803 Summary of the Invention [Problem to be solved by the invention]

[0005] However, when the exposure head of Patent Document 1 is attached to an image forming apparatus, the attachment angle may deviate from the expected value due to attachment error. If the attachment angle of the exposure head deviates from the expected value, reflected light (ghost light) occurs between the photosensitive drum and the light source, and the amount of light focused on the photosensitive drum differs from the expected amount of light. As a result, an image with a density different from the expected density is formed.

[0006] The present invention provides a light source device that can reduce unnecessary reflected light caused by installation errors, and an image forming apparatus using the same. [Means for solving the problem]

[0007] A light source device according to one aspect of the present invention includes a first light-emitting unit and a second light-emitting unit arranged at different positions in a first direction and a second direction perpendicular to the first direction, and a lens unit that collects light from the first and second light-emitting units. Each of the first and second light-emitting units includes a plurality of light-emitting elements arranged in the first direction. When viewed from the first direction, the light source device satisfies the condition 0.16≦T / D≦0.31, where T is the distance between the center of a first light-emitting element in the first light-emitting unit and the center of a second light-emitting element in the second light-emitting unit, and D is the distance between a first line passing through the centers of the first and second light-emitting elements and the center of the entrance surface of the lens unit. An image forming apparatus including the light source device as an exposure head also constitutes another aspect of the present invention. [Effects of the Invention]

[0008] According to the present invention, it is possible to reduce unnecessary reflected light caused by installation errors of the light source device. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 1 is a diagram showing the configuration of an image forming apparatus using an exposure head according to a first embodiment. [Figure 2] FIG. 2 is a diagram showing an exposure head and a photosensitive drum according to the first embodiment. [Figure 3] FIG. 2 is a diagram showing a light-emitting substrate in the exposure head of the first embodiment. [Figure 4] 3 is a diagram showing a light-emitting section and a gradient index lens array in the exposure head of the first embodiment. FIG. [Figure 5] FIG. 2 is a schematic diagram showing an exposure head and a photosensitive drum according to the first embodiment. [Figure 6] FIG. 2 is a cross-sectional view showing the optical path of the exposure head according to the first embodiment. [Figure 7] FIG. 2 is a cross-sectional view showing the optical path between the exposure head and the photosensitive drum according to the first embodiment. [Figure 8] 4 is a diagram showing the relationship between the rotation angle of the exposure head and the amount of light on the photosensitive drum in the first embodiment. FIG. [Figure 9]FIG. 10 is a schematic diagram showing an exposure head and a photosensitive drum according to a second embodiment. [Figure 10] FIG. 10 is a cross-sectional view showing the optical path of the exposure head according to the second embodiment. [Figure 11] FIG. 10 is a cross-sectional view showing the optical path between the exposure head and the photosensitive drum according to the second embodiment. [Figure 12] 10 is a diagram showing the relationship between the rotation angle of the exposure head and the amount of light on the photosensitive drum in the second embodiment. [Figure 13] FIG. 10 is a schematic diagram showing an exposure head and a photosensitive drum according to a third embodiment. [Figure 14] FIG. 10 is a cross-sectional view showing the optical path of the exposure head according to the third embodiment. [Figure 15] FIG. 11 is a cross-sectional view showing the optical path between the exposure head and the photosensitive drum according to the third embodiment. [Figure 16] FIG. 11 is a diagram showing the relationship between the rotation angle of the exposure head and the amount of light on the photosensitive drum in the third embodiment. [Figure 17] FIG. 10 is a schematic diagram showing an exposure head and a photosensitive drum according to a fourth embodiment. [Figure 18] FIG. 10 is a cross-sectional view showing the optical path of the exposure head according to the fourth embodiment. [Figure 19] FIG. 10 is a cross-sectional view showing the optical path between the exposure head and the photosensitive drum according to the fourth embodiment. [Figure 20] 10 is a diagram showing the relationship between the rotation angle of the exposure head and the amount of light on the photosensitive drum in the fourth embodiment. [Figure 21] FIG. 10 is a schematic diagram showing an exposure head and a photosensitive drum of a comparative lens group. [Figure 22] FIG. 10 is a cross-sectional view showing the optical path of an exposure head of a comparative example. [Figure 23] FIG. 10 is a cross-sectional view showing the optical path between the exposure head and the photosensitive drum of a comparative example. [Figure 24] FIG. 10 is a cross-sectional view showing the optical path between the exposure head and the photosensitive drum of a comparative example. [Figure 25] FIG. 10 is a cross-sectional view showing the optical path of reflected light from the lens array toward the light-emitting unit in the comparative example. [Figure 26] FIG. 10 is a cross-sectional view showing the optical path of reflected light from a light-emitting unit toward a lens array in a comparative example. [Figure 27] FIG. 10 is a diagram showing the relationship between the rotation angle of the exposure head and the amount of light on the photosensitive drum in a comparative example. DETAILED DESCRIPTION OF THE INVENTION

[0010] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. [Example]

[0011] (Image forming device) FIG. 1 shows the configuration of an image forming apparatus 1 that uses an exposure head as a light source device of this embodiment. The image forming apparatus 1 is a color printer (MFP: Multi Function Printer) equipped with a reading device. However, the image forming apparatus may also be a copier that does not have a reading device. The image forming apparatus 1 is a so-called tandem type color image forming apparatus equipped with multiple photosensitive drums (irradiated surfaces) 103. However, the image forming apparatus may also be a color image forming apparatus equipped with a single photosensitive drum or an image forming apparatus that forms monochrome images.

[0012] The image forming apparatus 1 includes four image forming units 102Y, 102M, 102C, and 102K that form toner images of yellow (Y), magenta (M), cyan (C), and black (K). The Y, M, C, and K symbols indicate the corresponding toner colors. The image forming units 102Y, 102M, 102C, and 102K include photosensitive drums 103Y, 103M, 103C, and 103K, respectively. These photosensitive drums 103Y, 103M, 103C, and 103K are arranged at a distance from one another.

[0013] The image forming units 102Y, 102M, 102C, and 102K are also equipped with chargers 104Y, 104M, 104C, and 104K that charge the photosensitive drums 103Y, 103M, 103C, and 103K, respectively. The image forming units 102Y, 102M, 102C, and 102K are also equipped with LED exposure heads 105Y, 105M, 105C, and 105K that serve as light source devices that emit light to expose the photosensitive drums 103Y, 103M, 103C, and 103K, respectively. The image forming apparatus 1 is a so-called "bottom exposure type" image forming apparatus that exposes the photosensitive drums 103Y, 103M, 103C, and 103K from below.

[0014] The image forming units 102Y, 102M, 102C, and 102K are also equipped with developing units 106Y, 106M, 106C, and 106K that develop the electrostatic latent images on the photosensitive drums 103Y, 103M, 103C, and 103K with toner as a developer. The developing units 106Y, 106M, 106C, and 106K form toner images (developer images) of the respective colors on the photosensitive drums 103Y, 103M, 103C, and 103K.

[0015] The image forming apparatus 1 has an intermediate transfer belt 107 onto which toner images formed on the photosensitive drums 103Y, 103M, 103C, and 103K are transferred. The image forming apparatus 1 further includes primary transfer rollers 108Y, 108M, 108C, and 108K that sequentially transfer the toner images formed on the photosensitive drums 103Y, 103M, 103C, and 103K onto the intermediate transfer belt 107. The image forming apparatus 1 also includes a secondary transfer roller 109 that transfers the toner image on the intermediate transfer belt 107 onto recording paper P transported from the paper feed unit 101, and a fuser 110 that fuses the secondarily transferred image onto the recording paper P.

[0016] (Image formation process) Photosensitive drums 103Y, 103M, 103C, and 103K, which have been uniformly charged by chargers 104Y, 104M, 104C, and 104K, are exposed by LED exposure heads 105Y, 105M, 105C, and 105K to form electrostatic latent images. The electrostatic latent images are visualized as toner images of the respective colors by developers 106Y, 106M, 106C, and 106K, and transferred to intermediate transfer belt 107 at primary transfer stations Ty, Tm, Tc, and Tk.

[0017] The toner images of each color superimposed on the intermediate transfer belt 107 are transferred at once by a secondary transfer roller 109 at a secondary transfer section T2 onto a recording sheet P conveyed from a paper feed section 101. The recording sheet P onto which the toner images have been transferred is conveyed to a fixing device 110, where the toner images are fixed by heat and pressure, and then the recording sheet is discharged from a paper discharge section 111.

[0018] (Basic configuration of exposure head) The basic configuration of the exposure head 105 of the first embodiment will be described with reference to FIGS. 2(a), 2(b), 3 and 4. FIG.

[0019] Fig. 2(a) shows the arrangement of the exposure head 105 relative to the photosensitive drum 103. Fig. 2(b) shows a ZX cross section (short cross section, sub-scanning cross section), which is a plane perpendicular (orthogonal) to the Y direction when viewed from the Y direction, and shows how light emitted from the light source unit 202 is collected on the photosensitive drum 103 by a gradient index lens array 204 as a lens unit. Note that in this embodiment, a gradient index lens array is used as the lens unit, but other lens arrays may also be used.

[0020] 3 shows a YZ cross section of the light source section 202. The light source section 202 includes a plurality of first light emitting units 206 (-1 to -10) and a plurality of second light emitting units 207 (-1 to -10) as light emitting element array chips. Each light emitting unit has a light emitting element row (light emitting section) including a plurality of light emitting elements arranged in a line in the Y direction. The light emitting elements are light emitting devices such as LEDs and organic ELs.

[0021] FIG. 4 shows the positional relationship between the plurality of first light-emitting units 206 and the plurality of second light-emitting units 207 and the gradient index lens array 204 in the YZ cross section.

[0022] 2(b) and 3, the exposure head 105 has a plurality of first light-emitting units 206 (-1 to -10) and a plurality of second light-emitting units 207 (-1 to -10) mounted on a light-emitting substrate 201, a refractive index distribution lens array 204, and a housing 205. The plurality of first light-emitting units 206 and the plurality of second light-emitting units 207 are mounted on the substrate mounting surface of the light-emitting substrate 201.

[0023] The plurality of first light-emitting units 206 (-1 to -10) are arranged in a row in the Y direction (first direction, main scanning direction), which is the longitudinal direction of each light-emitting unit. The plurality of second light-emitting units 207 (-1 to -10) are arranged in a row in the Y direction at different positions from each other in the Z direction (second direction, sub-scanning direction perpendicular to the first direction), which is the short direction of each light-emitting unit, relative to the arrangement positions of the first light-emitting units 206. Furthermore, the plurality of first light-emitting units 206 and the plurality of second light-emitting units 207 are arranged at positions offset from each other in the Y direction. In this way, the plurality of first light-emitting units 206 and the plurality of second light-emitting units 207 are arranged in two staggered rows. Note that the plurality of first light-emitting units 206 and the plurality of second light-emitting units 207 may be arranged in two rows without any offset in the Y direction, rather than in a staggered pattern.

[0024] In each of the first light-emitting unit 206 and the second light-emitting unit 207, a light-emitting element row including a plurality of light-emitting elements is mounted on the unit mounting surface. In this embodiment, n=748 light-emitting elements are arranged in the Y direction at a predetermined image resolution pitch in the light-emitting element row of each light-emitting unit. The image resolution pitch is, for example, 1200 dpi (approximately 21.16 μm). The length from the -Y end to the +Y end of the light-emitting element row including 748 light-emitting elements is approximately 15.8 mm.

[0025] Each of the plurality of first light-emitting units 206 and the plurality of second light-emitting units 207 includes 10 light-emitting units. That is, the total number of first light-emitting units 206 and second light-emitting units 207 is 20. This brings the total number of light-emitting elements to 14,960, enabling image formation corresponding to an image width of approximately 316 mm.

[0026] In this embodiment, a light-emitting element having Lambertian emission characteristics is used. However, the emission characteristics of the light-emitting element are not limited to this. In addition, although the emission spectrum of the light-emitting element in this embodiment has a peak at 600 nm, the emission spectrum is not limited to this. For example, a light-emitting element that emits near-infrared light with a peak at 780 nm may be used.

[0027] 4 includes a first gradient index lens array (first lens array) 204-1 extending in the Y direction and a second gradient index lens array (second lens array) 204-2 extending in the Y direction at a position shifted in the Z direction from the first gradient index lens array 204-1. Each of the first and second gradient index lens arrays 204-1 and 204-2 includes a plurality of gradient index lenses 203 arranged at a predetermined pitch in the Y direction. For example, the diameter of each cylindrical gradient index lens 203 is 290 μm.

[0028] 2(b), the gradient index lens array 204 is disposed so that the distance from the light emitting element row of the light source unit 202 to each lens 203 is a first predetermined distance, and the distance from the light emitting surface of each lens 203 to the surface of the photosensitive drum 103 is a second predetermined distance. The first predetermined distance and the second predetermined distance are substantially equal. The gradient index lens array 204 focuses light emitted from the light emitting element row so that an erect image of equal size is formed on the photosensitive drum 103.

[0029] The refractive index distribution type lens array 204 and the light emitting substrate 201 are fixed to the housing 205 with an adhesive.

[0030] The exposure head 105 having the above configuration is assembled individually in a factory, and is then completed by performing focus adjustment and light intensity adjustment to adjust the spot at the condensed position to a predetermined size. In focus adjustment, the mounting position of the refractive index distribution lens array 204 is adjusted so that the distance between the refractive index distribution lens array 204 and the light emitting element row is a first predetermined distance. In light intensity adjustment, each of the multiple light emitting elements in the light emitting element row is sequentially made to emit light, and the drive current of each light emitting element is adjusted so that the light condensed on the photosensitive drum 103 via the refractive index distribution lens array 204 has a predetermined light intensity.

[0031] (Detailed configuration of exposure head) The detailed configuration of the exposure head 105 of this embodiment will be described with reference to FIG. 5, FIGS. 6(a) and 6(b), and FIGS. 7(a) and 7(b).

[0032] FIG. 5 shows a ZX cross section as viewed from the Y direction, illustrating the arrangement of the exposure head 105 relative to the photosensitive drum 103. FIGS. 6(a) and 6(b) show an enlarged view of enlarged area 1 in FIG. 5 as viewed from the Y direction. FIG. 6(a) shows how a light ray A1 emitted from the first light-emitting unit 206 enters the first and second gradient index lens arrays 204-1 and 204-2. FIG. 6(b) shows how a light ray A2 emitted from the second light-emitting unit 207 enters the first and second gradient index lens arrays 204-1 and 204-2.

[0033] 7(a) and (b) show an enlarged view of the enlarged region 2 in FIG. 5. Fig. 7(a) shows how a light ray A1 emitted from the first light-emitting unit 206 and then emitted from the first and second gradient index lens arrays 204-1 and 204-2 is focused (irradiated) on the photosensitive drum 103, and a light ray B1 generated when the light ray A1 is reflected by the photosensitive drum 103. Fig. 7(b) shows how a light ray A2 emitted from the second light-emitting unit 207 and then emitted from the first and second gradient index lens arrays 204-1 and 204-2 is focused on the photosensitive drum 103, and a light ray B2 generated when the light ray A2 is reflected by the photosensitive drum 103.

[0034] When the exposure head 105 is attached to the main body (chassis) of the image forming apparatus 1, even if the attachment angle deviates from the expected value due to an attachment error, it is desirable that the amount of light emitted from the exposure head 105 and focused on the photosensitive drum 103 deviates little from the expected amount of light. The variation in the amount of light focused on the photosensitive drum 103 is caused by unwanted reflected light (ghost light) between the exposure head 105 and the photosensitive drum 103 when there is an attachment error, which reaches the photosensitive drum 103. For this reason, it is necessary to reduce such unwanted reflected light.

[0035] The exposure head 105 of this embodiment has a configuration that satisfies the following condition in order to reduce unnecessary reflected light caused by installation errors. When the exposure head 105 is viewed from the Y direction, the distance between the center of the light-emitting portion (first light-emitting element) of the first light-emitting unit 206 and the center of the light-emitting portion (second light-emitting element) of the second light-emitting unit 207 is defined as T, and the line connecting the centers of these light-emitting portions (first and second light-emitting elements) is defined as the first line. Furthermore, when viewed from the Y direction, the center of the incident surface of the gradient index lens 203 included in the gradient index lens array 204 is defined as the first point, and the distance (shortest distance) between the first line and the first point is defined as D. In this case, the condition of the following formula (1) is satisfied.

[0036] 0.16≦T / D≦0.31 (1) The condition of formula (1) indicates the appropriate positional relationship between the gradient index lens array 204 and the first and second light-emitting units 206 and 207 when viewed from the Y direction. By keeping T / D within the range of formula (1), unnecessary reflected light (ghost light) generated by installation errors of the exposure head 105 can be reduced, resulting in the formation of a good image with minimal impact of reflected light on density. If T / D is smaller than the lower limit of formula (1), it is not possible to reduce reflected light due to installation errors, and the impact of reflected light on image density cannot be suppressed, which is undesirable. If T / D is larger than the upper limit of formula (1), the amount of light emitted from the first and second light-emitting units 206 and 207 that cannot enter the gradient index lens array 204 (i.e., does not reach the photosensitive drum 103) increases, resulting in a decrease in light utilization efficiency, which is undesirable. It is more preferable to set the upper limit of formula (1) to 0.26.

[0037] In this embodiment, the diameter of the photosensitive drum 103 is 30 mm, D is 2.74 mm, and T is 0.66 mm. Therefore, T / D is 0.24, which satisfies the condition of formula (1).

[0038] Furthermore, the center of the light source image formed on the photosensitive drum 103 (on the irradiated surface) by light from the light-emitting section of the first light-emitting unit 206 when viewed from the Y direction is defined as a second point, and the center of the light source image formed on the photosensitive drum 103 by light from the light-emitting section of the second light-emitting unit 307 is defined as a third point. The line connecting the second point and the third point is defined as a second line, and the angle between the first line and the second line is defined as θ. In this case, it is preferable that the exposure head 105 satisfies the condition of the following equation (2).

[0039] -2°≦θ≦2° (2) The condition of formula (2) indicates the appropriate positional relationship between the exposure head 105 and the photosensitive drum 103. By keeping θ within the range of formula (2), it is possible to more effectively reduce ghost light generated due to installation errors of the exposure head 105. If θ is smaller than the lower limit value or larger than the upper limit value of formula (2), it becomes difficult to reduce ghost light generated by reflection of light from one of the light-emitting elements of the first and second light-emitting units 206, 207, which is not preferable.

[0040] In this embodiment, the first line and the second line are parallel, that is, θ=0°, and the condition of formula (2) is satisfied.

[0041] The conditions of formula (1) and formula (2) are satisfied in all combinations of the first and second light-emitting units in the plurality of first light-emitting units 206 and the plurality of second light-emitting units 207. This also applies to other examples described later.

[0042] Here, we will explain a comparative example using Figures 21, 22(a), (b), 23(a), (b), 24(a), (b), 25(a), (b), 26(a), (b) and 27, and also explain the occurrence of ghost light in the comparative example.

[0043] Figure 21 shows the arrangement of an exposure head 605 relative to a photosensitive drum 603 in a ZX cross section when viewed from the Y direction. The exposure head 605 of the comparative example has a configuration similar to the exposure head 105 of Example 1, except that the distance T between the centers of the light-emitting portions of the first light-emitting unit 606 and the second light-emitting unit 607 is different. Figures 22(a) and (b), Figures 25(a) and (b), and Figures 26(a) and (b) show enlarged views of enlarged region 1 in Figure 21. Figures 23(a) and (b) and Figures 24(a) and (b) show enlarged views of enlarged region 2 in Figure 21.

[0044] Fig. 22(a) shows how a light ray A1 emitted from the first light-emitting unit 606 is incident on the first and second gradient index lens arrays 604-1 and 604-2. Fig. 22(b) shows how a light ray A2 emitted from the second light-emitting unit 607 is incident on the first and second gradient index lens arrays 604-1 and 604-2. Fig. 23(a) shows how a light ray A1 emitted from the first light-emitting unit 606 and then from the first and second gradient index lens arrays 604-1 and 604-2 is focused on the photosensitive drum 603. Fig. 23(b) shows how a light ray A2 emitted from the second light-emitting unit 607 and then from the first and second gradient index lens arrays 604-1 and 604-2 is focused on the photosensitive drum 603.

[0045] 24(a) shows a light ray B1 generated when a light ray A1 emitted from the first light-emitting unit 606 and converged (irradiated) on the photosensitive drum 603 via the first and second gradient index lens arrays 604-1 and 604-2 is reflected by the photosensitive drum 603. FIG. 24(b) shows a light ray B2 generated when a light ray A2 emitted from the second light-emitting unit 607 and converged on the photosensitive drum 603 via the first and second gradient index lens arrays 604-1 and 604-2 is reflected by the photosensitive drum 603.

[0046] Fig. 25(a) shows how, of the light rays B1 shown in Fig. 24(a), the light rays that have passed through the first gradient index lens array 604-1 are condensed while returning to the first light-emitting unit 606. Fig. 25(b) shows how, of the light rays B2 shown in Fig. 24(b), the light rays that have passed through the second gradient index lens array 604-2 are condensed while returning to the second light-emitting unit 607.

[0047] Fig. 26(a) shows how the light beam returning to the first light-emitting unit 606 in Fig. 25(a) is reflected by the first light-emitting unit 606 and again enters the first gradient index lens array 604-1 (re-irradiates the photosensitive drum 103). Fig. 26(b) shows how the light beam returning to the second light-emitting unit 607 in Fig. 25(b) is reflected by the second light-emitting unit 607 and again enters the second gradient index lens array 604-2 (re-irradiates the photosensitive drum 103).

[0048] In the comparative example, the diameter of the photosensitive drum 603 is 30 mm, D is 2.74 mm, T is 0.40 mm, and θ is 0°. Therefore, T / D is 0.15, which does not satisfy the condition of formula (1). However, θ is 0°, and the condition of formula (2) is satisfied.

[0049] When the exposure head 605 is attached to the image forming apparatus 1, the attachment angle may deviate (rotate) from the expected value due to attachment error, and here the range of variation in the attachment angle of the exposure head 605 due to attachment error is set to ±0.5°. The expected value of the attachment angle of the exposure head 605 is set to 0°, and the amount of light emitted from the first and second light-emitting units 606 and 607 and focused on the photosensitive drum 603 at this expected value is set to 100.00, respectively.

[0050] 27 shows the relationship between the mounting error (rotation angle) of the exposure head 605 and the amount of light on the photosensitive drum 603 in the comparative example. When the exposure head 605 rotates by +0.5° due to a mounting error, the amount of light emitted from the first light-emitting unit 606 and condensed on the photosensitive drum 603 decreases to 99.99. On the other hand, the amount of light emitted from the second light-emitting unit 607 and condensed on the photosensitive drum 603 increases to 100.06. Conversely, when the exposure head 605 rotates by -0.5° due to a mounting error, the amount of light emitted from the first light-emitting unit 606 and condensed on the photosensitive drum 603 increases to 100.06. On the other hand, the amount of light emitted from the second light-emitting unit 607 and condensed on the photosensitive drum 603 decreases to 99.99. In this way, if there is a variation in the amount of light due to an installation error of the exposure head 605, the density of the image formed on the photosensitive drum 603 changes, and an image with the expected density cannot be obtained.

[0051] Here, we will explain why the amount of light condensed on the photosensitive drum 603 fluctuates when the exposure head 605 rotates due to an installation error. As shown in Fig. 22(a), a light ray A1 emitted from the light-emitting section (first light-emitting element) of the first light-emitting unit 606 is incident on the first and second gradient index lens arrays 604-1 and 604-2. Then, the light ray A1 emitted from the first and second gradient index lens arrays 604-1 and 604-2 is condensed in an area centered on a second point on the photosensitive drum 603 to form a light source image, as shown in Fig. 23(a).

[0052] Generally, the surface of a photosensitive drum has a reflectivity greater than 0%. Therefore, the light ray A1 condensed at the second point is also reflected. In this comparative example, the reflectivity of the photosensitive drum 603 is set to 10.0%. As shown in FIG. 24(a), a portion of the light ray B1 generated when the light ray A1 is reflected by the photosensitive drum 603 is incident on the first refractive index distribution lens array 604-1. The light ray B1 emitted from the first refractive index distribution lens array 604-1 is condensed on the first light-emitting unit 606 as shown in FIG. 25(a). Generally, the surface of a light-emitting unit also has a reflectivity greater than 0%. Therefore, the light ray B1 is also reflected by the first light-emitting unit 606. In this comparative example, the reflectivity of the first light-emitting unit 606 is set to 25.0%.

[0053] As shown in FIG. 26(a), the light ray C1 reflected by the first light-emitting unit 606 follows the same optical path as the light ray A1 as multiple reflection light (ghost light) and is collected on the photosensitive drum 603.

[0054] As shown in FIG. 22(b), light ray A2 emitted from the light-emitting section (second light-emitting element) of the second light-emitting unit 607 is incident on the first and second gradient index lens arrays 604-1 and 604-2. Then, light ray A2 emitted from the first and second gradient index lens arrays 604-1 and 604-2 is condensed in an area centered on a third point on the photosensitive drum 603 to form a light source image, as shown in FIG. 23(b). Then, as shown in FIG. 24(b), a part of light ray B2 generated by reflection of light ray A2 on the photosensitive drum 603 is incident on the second gradient index lens array 604-2. Light ray B2 emitted from the second gradient index lens array 604-2 is condensed and reflected on the second light-emitting unit 607, as shown in FIG. 25(b). Furthermore, as shown in FIG. 26(b), the light ray C2 reflected by the second light emitting unit 607 follows the same optical path as the light ray A2 as multiple reflection light and is collected on the photosensitive drum 603.

[0055] In Figures 24(a) and (b), the light rays B1 and B2 reflected by the photosensitive drum 603 when there is no mounting error of the exposure head 605 (mounting angle is 0°) are shown by dashed lines, and the light rays B1 and B2 reflected by the photosensitive drum 603 when rotated due to a mounting error (+0.5°) are shown by dashed lines.

[0056] When the exposure head 605 is rotated from 0° to +0.5°, the proportion of light rays B1 emitted from the first light-emitting unit 606 and reflected by the photosensitive drum 603 that enter the first gradient index lens array 604-1 (dashed line) decreases compared to the light rays (dash-dotted line) when there is no installation error. Therefore, as shown in FIG. 27, the amount of light emitted from the first light-emitting unit 606 and focused on the photosensitive drum 603 decreases to 99.99. On the other hand, the proportion of light rays B2 emitted from the second light-emitting unit 607 and reflected by the photosensitive drum 603 that enter the second gradient index lens array 604-2 (dashed line) increases compared to the light rays (dash-dotted line) when there is no installation error. Therefore, as shown in FIG. 27, the amount of light emitted from the second light-emitting unit 607 and focused on the photosensitive drum 603 increases to 100.06.

[0057] When the exposure head 605 is rotated from 0° by −0.5°, the amount of light emitted from the first light-emitting unit 606 and focused on the photosensitive drum 603 increases to 100.06, in contrast to when the exposure head 605 is rotated by +0.5°. On the other hand, the amount of light emitted from the second light-emitting unit 607 and focused on the photosensitive drum 603 decreases to 99.99.

[0058] In this way, in the comparative example, when the exposure head 605 is rotated by ±0.5°, the fluctuation in the amount of light on the photosensitive drum 603 due to multiple reflections is a maximum of 0.06%.

[0059] In this embodiment, the distance T (=0.66 mm) between the light-emitting portions of the first light-emitting unit 206 and the second light-emitting unit 207 is set longer than the distance T (=0.40 mm) in the comparative example. Therefore, as shown in FIG. 6A, the angle α of the traveling direction of the light ray A1 emitted from the first light-emitting unit 206 and incident on the first and second gradient index lens arrays 204-1 and 204-2 is larger than that in the comparative example. As a result, as shown in FIG. 7A, the light ray B1 emitted from the first light-emitting unit 206 and reflected by the photosensitive drum 103 does not enter the first and second gradient index lens arrays 204-1 and 204-2. As shown in FIGS. 6B and 7B, the light ray B2 emitted from the second light-emitting unit 207 and reflected by the photosensitive drum 103 also does not enter the first and second gradient index lens arrays 204-1 and 204-2.

[0060] 8 shows the relationship between the mounting error (rotation angle) of the exposure head 105 and the amount of light on the photosensitive drum 103 in this embodiment. As described above, the reflected light rays B1 and B do not enter the first and second refractive index gradient lens arrays 204-1 and 204-2. As a result, as shown in FIG. 8, even if the exposure head 105 rotates ±0.5° from 0°, the amount of light emitted from the first and second light-emitting units 206 and 207 and focused on the photosensitive drum 103 is 100.00. In other words, there is no fluctuation in the amount of light on the photosensitive drum 103 due to the mounting error of the exposure head 105.

[0061] As shown in FIG. 8, even if the exposure head 105 rotates from 0° by a maximum of ±1.5°, fluctuations in the amount of light on the photosensitive drum 103 can be eliminated.

[0062] In this way, in this embodiment, compared to the comparative example, it is possible to suppress fluctuations in the amount of light on the photosensitive drum 103 caused by ghost light as multiple reflected light when there is an installation error in the exposure head 105, and it is possible to reduce changes in image density caused by ghost light.

[0063] Note that the greater the reflectivity of the surfaces of the photosensitive drum 103 and the first and second light-emitting units 206 and 207, the more effective it is in suppressing fluctuations in the amount of light on the photosensitive drum 103 due to multiple reflections. For this reason, the reflectivity of the photosensitive drum 103 is desirably 5.0% or more, and the reflectivity of the first and second light-emitting units 206 and 207 is desirably 25.0% or more. This also applies to other embodiments described below. [Example]

[0064] Next, an exposure head of Example 2 will be described. The image forming apparatus in which the exposure head of Example 2 is used is similar to the image forming apparatus 1 described in Example 1. The exposure head of Example 2 differs from the exposure head 105 of Example 1 in the distance T, but other configurations are similar to the exposure head 105 of Example 1.

[0065] The exposure head 305 of this embodiment will be described using Figures 9, 10(a) and 10(b), 11(a) and 11(b), and 12. Figure 9 shows a ZX cross section as viewed from the Y direction, illustrating the arrangement of the exposure head 305 relative to the photosensitive drum 303. Figures 10(a) and 10(b) show an enlarged view of the enlarged area 1 in Figure 9 as viewed from the Y direction. Figure 10(a) shows how a light ray A1 emitted from the first light-emitting unit 306 is incident on the first and second gradient index lens arrays 304-1 and 304-2. Figure 10(b) shows how a light ray A2 emitted from the second light-emitting unit 307 is incident on the first and second gradient index lens arrays 304-1 and 304-2.

[0066] 11(a) and (b) show an enlarged view of the enlarged region 2 in FIG. 9. Fig. 11(a) shows how a light ray A1 emitted from the first light-emitting unit 306 and then emitted from the first and second gradient index lens arrays 304-1 and 304-2 is focused at a second point on the photosensitive drum 303, and how a light ray B1 is generated when the light ray A1 is reflected by the photosensitive drum 303. Fig. 11(b) shows how a light ray A2 emitted from the second light-emitting unit 307 and then emitted from the first and second gradient index lens arrays 304-1 and 304-2 is focused at a third point on the photosensitive drum 303, and how a light ray B2 is generated when the light ray A2 is reflected by the photosensitive drum 303.

[0067] In this embodiment, the diameter of the photosensitive drum 303 is 30 mm, D is 2.74 mm, T is 0.86 mm, and θ is 0°. Therefore, T / D is 0.31, which satisfies the condition of formula (1). Also, θ is 0°, which satisfies the condition of formula (2).

[0068] In this embodiment, the distance T (=0.86 mm) between the light emitting portion of the first light emitting unit 306 and the light emitting portion of the second light emitting unit 307 is set to be longer than T (=0.40 mm) in the comparative example. Therefore, similar to the first embodiment, the light rays B1 and B2 generated by the reflection of the light rays A1 and A2 on the photosensitive drum 303 do not enter the first and second gradient index lens arrays 304-1 and 304-2.

[0069] FIG. 12 shows the relationship between the mounting error (rotation angle) of the exposure head 305 and the light intensity on the photosensitive drum 303 in this embodiment. As described above, even if the exposure head 305 rotates ±0.5° from 0°, the reflected light beams B1 and B2 do not enter the first and second gradient index lens arrays 304-1 and 304-2. As a result, as shown in FIG. 12, the amount of light emitted from the first and second light-emitting units 306 and 307 and focused on the photosensitive drum 303 is 100.00. In other words, there is no fluctuation in the light intensity on the photosensitive drum 303 due to the mounting error of the exposure head 305. Note that, as shown in FIG. 12, there is no fluctuation in the light intensity on the photosensitive drum 303 even if the exposure head 305 rotates a maximum of ±1.5° from 0°.

[0070] In this way, in this embodiment, as compared with the comparative example, it is possible to suppress fluctuations in the amount of light on the photosensitive drum 303 due to ghost light as multiple reflected light, and it is possible to reduce changes in image density due to ghost light. [Example]

[0071] Next, an exposure head according to Example 3 will be described. The image forming apparatus in which the exposure head according to Example 3 is used is similar to the image forming apparatus 1 described in Example 1. The exposure head according to Example 3 differs from the exposure head 105 according to Example 1 in the distance T, but other configurations are similar to those of the exposure head 105 according to Example 1.

[0072] The exposure head 405 of this embodiment will be described using Figures 13, 14(a) and 14(b), 15(a) and 15(b), and 16. Figure 13 shows a ZX cross section as viewed from the Y direction, illustrating the arrangement of the exposure head 405 of this embodiment relative to the photosensitive drum 403. Figures 14(a) and 14(b) show an enlarged view of the enlarged area 1 in Figure 13 as viewed from the Y direction. Figure 14(a) shows how a light ray A1 emitted from the first light-emitting unit 406 is incident on the first and second gradient index lens arrays 404-1 and 404-2. Figure 14(b) shows how a light ray A2 emitted from the second light-emitting unit 407 is incident on the first and second gradient index lens arrays 404-1 and 404-2.

[0073] 15(a) and (b) show an enlarged view of the enlarged region 2 in FIG. 13. Fig. 15(a) shows how a light ray A1 emitted from the first light-emitting unit 406 and then emitted from the first and second gradient index lens arrays 404-1 and 404-2 is focused at a second point on the photosensitive drum 403, and how a light ray B1 is generated when the light ray A1 is reflected by the photosensitive drum 403. Fig. 15(b) shows how a light ray A2 emitted from the second light-emitting unit 407 and then emitted from the first and second gradient index lens arrays 404-1 and 404-2 is focused at a third point on the photosensitive drum 403, and how a light ray B2 is generated when the light ray A2 is reflected by the photosensitive drum 403.

[0074] In this embodiment, the diameter of the photosensitive drum 403 is 30 mm, D is 2.74 mm, T is 0.46 mm, and θ is 0°. Therefore, T / D is 0.17, which satisfies the condition of formula (1). Also, θ is 0°, which satisfies the condition of formula (2).

[0075] In this embodiment, the distance T (=0.46 mm) between the light emitting portion of the first light emitting unit 406 and the light emitting portion of the second light emitting unit 407 is set longer than T (=0.40 mm) in the comparative example. Therefore, similar to the first embodiment, the light rays B1 and B2 generated by the reflection of the light rays A1 and A2 on the photosensitive drum 403 do not enter the first and second gradient index lens arrays 404-1 and 404-2.

[0076] FIG. 16 shows the relationship between the mounting error (rotation angle) of the exposure head 405 and the light intensity on the photosensitive drum 403 in this embodiment. As described above, even if the exposure head 405 rotates ±0.5° from 0°, the reflected light beams B1 and B2 do not enter the first and second gradient index lens arrays 404-1 and 404-2. As a result, as shown in FIG. 16, the amount of light emitted from the first and second light-emitting units 406 and 407 and focused on the photosensitive drum 403 is 100.00. In other words, there is no fluctuation in the light intensity on the photosensitive drum 403 due to the mounting error of the exposure head 405. Note that, as shown in FIG. 16, even if the exposure head 405 rotates a maximum of ±1.0° from 0°, there is almost no fluctuation in the light intensity on the photosensitive drum 403.

[0077] In this way, in this embodiment, as compared with the comparative example, fluctuations in the amount of light on the photosensitive drum 403 due to ghost light as multiple reflected light can be suppressed, and changes in image density due to ghost light can be reduced. [Example]

[0078] Next, an exposure head of Example 4 will be described. The image forming apparatus in which the exposure head of Example 4 is used is similar to the image forming apparatus 1 described in Example 1. The exposure head of Example 4 differs from the exposure head 105 of Example 1 in the distance T, but other configurations are similar to the exposure head 105 of Example 1.

[0079] The exposure head 505 of this embodiment will be described using Figures 17, 18(a) and 18(b), 19(a) and 19(b), and 20. Figure 17 shows a ZX cross section as viewed from the Y direction, illustrating the arrangement of the exposure head 505 of this embodiment relative to the photosensitive drum 503. Figures 18(a) and 18(b) show an enlarged view of the enlarged area 1 in Figure 17 as viewed from the Y direction. Figure 18(a) shows how a light ray A1 emitted from the first light-emitting unit 506 is incident on the first and second gradient index lens arrays 504-1 and 504-2. Figure 18(b) shows how a light ray A2 emitted from the second light-emitting unit 507 is incident on the first and second gradient index lens arrays 504-1 and 504-2.

[0080] 19(a) and (b) show an enlarged view of the enlarged region 2 in FIG. 17. FIG. 19(a) shows how a light ray A1 emitted from the first light-emitting unit 506 and then emitted from the first and second gradient index lens arrays 504-1 and 504-2 is focused at a second point on the photosensitive drum 503, and a light ray B1 is generated when the light ray A1 is reflected by the photosensitive drum 503. FIG. 19(b) shows how a light ray A2 emitted from the second light-emitting unit 507 and then emitted from the first and second gradient index lens arrays 504-1 and 504-2 is focused at a third point on the photosensitive drum 503, and a light ray B2 is generated when the light ray A2 is reflected by the photosensitive drum 503.

[0081] In this embodiment, the diameter of the photosensitive drum 503 is 20 mm, D is 2.74 mm, T is 0.44 mm, and θ is 0°. Therefore, T / D is 0.16, which satisfies the condition of formula (1). Also, θ is 0°, which satisfies the condition of formula (2).

[0082] In this embodiment, the distance T (=0.44 mm) between the light emitting portion of the first light emitting unit 506 and the light emitting portion of the second light emitting unit 507 is set to be longer than T (=0.40 mm) in the comparative example. Therefore, similar to the first embodiment, the light rays B1 and B2 generated by the reflection of the light rays A1 and A2 on the photosensitive drum 503 do not enter the first and second gradient index lens arrays 504-1 and 504-2.

[0083] FIG. 20 shows the relationship between the mounting error (rotation angle) of the exposure head 505 and the light intensity on the photosensitive drum 503 in this embodiment. As described above, even if the exposure head 505 rotates ±0.5° from 0°, the reflected light beams B1 and B2 do not enter the first and second gradient index lens arrays 504-1 and 504-2. As a result, as shown in FIG. 20, the amount of light emitted from the first and second light-emitting units 506 and 507 and focused on the photosensitive drum 503 is 100.00. In other words, it is possible to eliminate fluctuations in the light intensity on the photosensitive drum 503 due to mounting error of the exposure head 505. Note that, as shown in FIG. 20, even if the exposure head 505 rotates a maximum of ±0.75° from 0°, it is possible to almost completely eliminate fluctuations in the light intensity on the photosensitive drum 503.

[0084] In this way, in this embodiment, as compared with the comparative example, fluctuations in the amount of light on the photosensitive drum 503 due to ghost light as multiple reflected light can be suppressed, and changes in image density due to ghost light can be reduced.

[0085] In the above Examples 1 to 4, it is desirable that all of the plurality of first light-emitting units and the plurality of second light-emitting units satisfy the condition of formula (1), but it is not necessary that all of the plurality of first light-emitting units and the plurality of second light-emitting units satisfy the condition of formula (1). That is, when the total number of the plurality of first light-emitting units and the plurality of second light-emitting units is N, it is desirable that the number of first and second light-emitting units that satisfy the condition of formula (1) is 1.0×N, but it is sufficient that it is 0.7×N or more. Furthermore, it is desirable that the number of first and second light-emitting units that satisfy both the conditions of formulas (1) and (2) is also 1.0×N, but it is sufficient that it is 0.7×N or more.

[0086] The above embodiment includes the following configurations. (Configuration 1) a first light-emitting unit and a second light-emitting unit arranged at different positions in a first direction and a second direction perpendicular to the first direction; a lens unit that collects light from the first and second light-emitting units; each of the first and second light emitting units includes a plurality of light emitting elements arranged in the first direction; When viewed from the first direction, when the distance between the center of the first light-emitting element in the first light-emitting unit and the center of the second light-emitting element in the second light-emitting unit is T, and the distance between a first line passing through the centers of the first and second light-emitting elements and the center of the incident surface of the lens unit is D, 0.16≦T / D≦0.31 A light source device characterized by satisfying the following conditions. (Configuration 2) The light source device described in configuration 1, characterized in that the distance T is a distance at which light emitted from the first and second light-emitting units and irradiated onto the irradiated surface through the lens unit does not enter the lens unit when reflected from the irradiated surface. (Configuration 3) When viewed from the first direction, a line passing through the center of an image of the first light-emitting element formed on the illuminated surface by the lens unit and the center of an image of the second light-emitting element formed on the illuminated surface by the lens unit is defined as a second line, and an angle formed between the first line and the second line is defined as θ. -2°≦θ≦2° 3. The light source device according to configuration 1 or 2, which satisfies the following conditions: (Configuration 4) The light source device of any one of configurations 1 to 3, characterized in that the lens unit comprises first and second lens arrays each including a plurality of lenses arranged in the first direction, and the first and second lens arrays are arranged at different positions in the second direction. (Configuration 5) 5. The light source device according to configuration 4, wherein the lens unit is a gradient index lens unit. (Configuration 6) 6. The light source device according to any one of configurations 1 to 5, wherein the first light-emitting unit and the second light-emitting unit are arranged at positions shifted from each other in the first direction. (Configuration 7) 7. The light source device according to any one of configurations 1 to 6, wherein the light emitted from the lens unit is irradiated onto an irradiated surface having a reflectance of 5% or more. (Configuration 8) 8. The light source device according to any one of configurations 1 to 7, wherein the surfaces on which the first and second light-emitting elements of the first and second light-emitting units are provided have a reflectance of 25% or more. (Configuration 9) a plurality of the first and second light-emitting units are arranged in the first direction; The light source device according to any one of configurations 1 to 7, characterized in that when the total number of the first light-emitting units and the second light-emitting units is N, the number of the first and second light-emitting units that satisfy the condition is 0.7×N or more. (Configuration 10) 10. The light source device according to configuration 9, wherein the number of the first and second light-emitting units that satisfy the condition is 1.0×N. (Configuration 11) 11. An image forming apparatus comprising: a light source device according to any one of configurations 1 to 10; and a developing device that develops an electrostatic latent image formed on an irradiated surface by the light source device.

[0087] The embodiments described above are merely representative examples, and various modifications and alterations are possible to each embodiment when implementing the present invention. [Explanation of symbols]

[0088] 1. Image forming device 103,303,403,503 Photosensitive drum 105,305,405,505 exposure head 204,304,404,504(-1,-2) gradient index lens array 206, 306, 406, 506 First light-emitting unit 207,307,407,507 Second light-emitting unit

Claims

1. a first light-emitting unit and a second light-emitting unit arranged at different positions in a first direction and a second direction perpendicular to the first direction; a lens unit that collects light from the first and second light-emitting units; each of the first and second light-emitting units includes a plurality of light-emitting elements arranged in the first direction; When viewed from the first direction, a distance between a center of a first light-emitting element in the first light-emitting unit and a center of a second light-emitting element in the second light-emitting unit is T, and a distance between a first line passing through the centers of the first and second light-emitting elements and a center of an incident surface of the lens unit is D. 0.16≦T / D≦0.31 A light source device characterized by satisfying the following conditions.

2. 2. The light source device according to claim 1, wherein the distance T is a distance at which light emitted from the first and second light-emitting units and irradiated onto the irradiated surface through the lens unit does not enter the lens unit when reflected by the irradiated surface.

3. When viewed from the first direction, a line passing through the center of an image of the first light-emitting element formed on the illuminated surface by the lens unit and the center of an image of the second light-emitting element formed on the illuminated surface by the lens unit is defined as a second line, and an angle formed between the first line and the second line is defined as θ. -2°≦θ≦2° 2. The light source device according to claim 1, wherein the following conditions are satisfied:

4. 2. The light source device according to claim 1, wherein the lens unit comprises first and second lens arrays each including a plurality of lenses arranged in the first direction, and the first and second lens arrays are arranged at different positions in the second direction.

5. 5. The light source device according to claim 4, wherein the lens unit is a gradient index lens unit.

6. 2. The light source device according to claim 1, wherein the light emitted from the lens unit is irradiated onto an illumination surface having a reflectance of 5% or more.

7. 2. The light source device according to claim 1, wherein the surfaces of the first and second light emitting units on which the first and second light emitting elements are provided have a reflectance of 25% or more.

8. a plurality of the first light-emitting units and a plurality of the second light-emitting units are arranged in the first direction; 2. The light source device according to claim 1, wherein when the total number of the first light-emitting units and the second light-emitting units is N, the number of the first and second light-emitting units that satisfy the condition is 0.7×N or more.

9. 9. The light source device according to claim 8, wherein the number of the first and second light emitting units that satisfy the condition is 1.0*N.

10. 10. An image forming apparatus comprising: a light source device according to claim 1; and a developing device that develops an electrostatic latent image formed on an illuminated surface by the light source device.

Images