Light source device and image forming apparatus

Abstract

To suppress variation in light quantities on an irradiated surface due to an error in mounting a light source device.SOLUTION: A light source device 105 comprises a first light emitting unit 206 and a second light emitting unit 207 arranged at positions different from each other in a first direction and in a second direction, and a lens unit 204 that condenses light emitted from the first and the second light emitting units onto a rotary irradiated surface 103. When viewed from the first direction, when 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 defined as T, a middle-point between the centers of the first and the second light emitting elements is defined as a first point, a rotation center of the irradiated surface is defined as a second point, a straight line passing on the first point and the second point is defined as a first straight line, centers of images of the first and the second light emitting elements formed on the irradiated surface by the lens unit are defined as a third point and a fourth point respectively, and a distance of the first straight line and a distance between the third point and the fourth point, whichever is longer, is defined as W, a relational expression of 0.6≤W / T≤2.0 is satisfied.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, the amount of light focused on the photosensitive drum will differ from the expected amount of light. As a result, an image with a density different from the expected density will be formed.

[0006] The present invention provides a light source device that can suppress variations in the amount of light on the illuminated surface due to 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 focuses light from the first and second light-emitting units onto a rotating illuminated surface. 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, let T be 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, a first point be the midpoint between the centers of the first and second light-emitting elements, a second point be the center of rotation of the illuminated surface, a first straight line be the line passing through the first and second points, a third point be the center of an image of the first light-emitting element formed on the illuminated surface by the lens unit, a fourth point be the center of an image of the second light-emitting element formed on the illuminated surface by the lens unit, and W be the longer of the distances between the first straight line and the third and fourth points. 0.6≦W / T≦2.0 The present invention is characterized by satisfying the following conditions: Note that an image forming apparatus including the above 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 suppress fluctuations in the amount of light on the illuminated surface 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. 3 is a diagram showing optical paths from first and second light-emitting units in the first embodiment. [Figure 7] FIG. 2 is a diagram showing an optical path from a gradient index lens array in the first embodiment. [Figure 8] 3 is a diagram showing the positional relationship between the first and second light-emitting units, the gradient index lens array, and the photosensitive drum in the first embodiment. FIG. [Figure 9] FIG. 4 is a diagram showing the amount of light on a photosensitive drum in the first embodiment. [Figure 10] FIG. 10 is a schematic diagram showing an exposure head and a photosensitive drum according to a second embodiment. [Figure 11] FIG. 10 is a diagram showing optical paths from the first and second light-emitting units in the second embodiment. [Figure 12] FIG. 10 is a diagram showing the optical path from a refractive index distribution lens array in the second embodiment. [Figure 13] FIG. 10 is a diagram showing the positional relationship between the first and second light-emitting units, the gradient index lens array, and the photosensitive drum in the second embodiment. [Figure 14] FIG. 10 is a diagram showing the amount of light on a photosensitive drum in the second embodiment. [Figure 15] FIG. 10 is a schematic diagram showing an exposure head and a photosensitive drum according to a third embodiment. [Figure 16] FIG. 10 is a diagram showing optical paths from the first and second light-emitting units in Example 3. [Figure 17] FIG. 10 is a diagram showing the optical path from a gradient index lens array in the third embodiment. [Figure 18] FIG. 10 is a diagram showing the positional relationship between the first and second light-emitting units, the gradient index lens array, and the photosensitive drum in the third embodiment. [Figure 19] FIG. 11 is a diagram showing the amount of light on a photosensitive drum in the third embodiment. [Figure 20] FIG. 10 is a schematic diagram showing an exposure head and a photosensitive drum according to a fourth embodiment. [Figure 21] FIG. 10 is a diagram showing optical paths from the first and second light-emitting units in Example 4. [Figure 22] FIG. 10 is a diagram showing the optical path from a gradient index lens array in the fourth embodiment. [Figure 23]FIG. 10 is a diagram showing the positional relationship between the first and second light-emitting units, the gradient index lens array, and the photosensitive drum in the fourth embodiment. [Figure 24] FIG. 10 is a diagram showing the amount of light on a photosensitive drum in the fourth embodiment. [Figure 25] FIG. 10 is a schematic diagram showing an exposure head and a photosensitive drum according to a fifth embodiment. [Figure 26] FIG. 13 is a diagram showing optical paths from the first and second light-emitting units in Example 5. [Figure 27] FIG. 13 is a diagram showing the optical path from a gradient index lens array in the fifth embodiment. [Figure 28] FIG. 10 is a diagram showing the positional relationship between the first and second light-emitting units, the gradient index lens array, and the photosensitive drum in the fifth embodiment. [Figure 29] FIG. 13 is a diagram showing the amount of light on a photosensitive drum in the fifth embodiment. [Figure 30] FIG. 13 is a schematic diagram showing an exposure head and a photosensitive drum according to a sixth embodiment. [Figure 31] FIG. 20 is a diagram showing the optical paths from the first and second light-emitting units in Example 6. [Figure 32] FIG. 20 is a diagram showing the optical path from a gradient index lens array in the sixth embodiment. [Figure 33] FIG. 20 is a diagram showing the positional relationship between the first and second light-emitting units, the gradient index lens array, and the photosensitive drum in Example 6. [Figure 34] FIG. 13 is a diagram showing the amount of light on a photosensitive drum in the sixth embodiment. [Figure 35] FIG. 13 is a schematic diagram showing an exposure head and a photosensitive drum according to a seventh embodiment. [Figure 36] FIG. 13 is a diagram showing the optical paths from the first and second light-emitting units in Example 7. [Figure 37] FIG. 13 is a diagram showing the optical path from a gradient index lens array in Example 7. [Figure 38] FIG. 13 is a diagram showing the positional relationship between the first and second light-emitting units, the gradient index lens array, and the photosensitive drum in Example 7. [Figure 39] FIG. 13 is a diagram showing the amount of light on a photosensitive drum in the seventh embodiment. [Figure 40]FIG. 13 is a schematic diagram showing an exposure head and a photosensitive drum according to an eighth embodiment. [Figure 41] FIG. 13 is a diagram showing the optical paths from the first and second light-emitting units in Example 8. [Figure 42] FIG. 20 is a diagram showing the optical path from the refractive index distribution type lens array in the eighth embodiment. [Figure 43] FIG. 13 is a diagram showing the positional relationship between the first and second light-emitting units, the gradient index lens array, and the photosensitive drum in Example 8. [Figure 44] FIG. 13 is a diagram showing the amount of light on a photosensitive drum in the eighth embodiment. [Figure 45] FIG. 13 is a schematic diagram showing an exposure head and a photosensitive drum according to a ninth embodiment. [Figure 46] FIG. 13 is a diagram showing the optical paths from the first and second light-emitting units in Example 9. [Figure 47] FIG. 13 is a diagram showing the optical path from a gradient index lens array in Example 9. [Figure 48] FIG. 13 is a diagram showing the positional relationship between the first and second light-emitting units, the gradient index lens array, and the photosensitive drum in Example 9. [Figure 49] FIG. 13 is a diagram showing the amount of light on a photosensitive drum in Example 9. [Figure 50] FIG. 22 is a schematic diagram showing an exposure head and a photosensitive drum according to a tenth embodiment. [Figure 51] FIG. 23 is a diagram showing the optical paths from the first and second light-emitting units in Example 10. [Figure 52] FIG. 23 is a diagram showing the optical path from the refractive index distribution lens array in the tenth embodiment. [Figure 53] FIG. 23 is a diagram showing the positional relationship between the first and second light-emitting units, the gradient index lens array, and the photosensitive drum in Example 10. [Figure 54] FIG. 20 is a diagram showing the amount of light on the photosensitive drum in the tenth embodiment. [Figure 55] FIG. 22 is a schematic diagram showing an exposure head and a photosensitive drum according to an eleventh embodiment. [Figure 56] FIG. 20 is a diagram showing the optical paths from the first and second light-emitting units in Example 11. [Figure 57]FIG. 23 is a diagram showing the optical path from the refractive index distribution lens array in the eleventh embodiment. [Figure 58] FIG. 20 is a diagram showing the positional relationship between the first and second light-emitting units, the gradient index lens array, and the photosensitive drum in Example 11. [Figure 59] FIG. 20 is a diagram showing the amount of light on the photosensitive drum in the eleventh embodiment. [Figure 60] FIG. 22 is a schematic diagram showing an exposure head and a photosensitive drum according to a twelfth embodiment. [Figure 61] FIG. 23 is a diagram showing the optical paths from the first and second light-emitting units in Example 12. [Figure 62] FIG. 23 is a diagram showing the optical path from the refractive index distribution type lens array in Example 12. [Figure 63] FIG. 23 is a diagram showing the positional relationship between the first and second light-emitting units, the gradient index lens array, and the photosensitive drum in Example 12. [Figure 64] FIG. 20 is a diagram showing the amount of light on the photosensitive drum in the twelfth embodiment. [Figure 65] FIG. 2 is a schematic diagram showing an exposure head and a photosensitive drum of Comparative Example 1. [Figure 66] FIG. 10 is a diagram showing the optical paths from the first and second light-emitting units in Comparative Example 1. [Figure 67] FIG. 10 is a diagram showing the optical path from a gradient index lens array in Comparative Example 1. [Figure 68] FIG. 10 is a diagram showing the optical path from the photosensitive drum in Comparative Example 1. [Figure 69] FIG. 10 is a graph showing the amount of light on a photosensitive drum in Comparative Example 1. [Figure 70] FIG. 10 is a schematic diagram showing an exposure head and a photosensitive drum of Comparative Example 2. [Figure 71] FIG. 10 is a diagram showing the optical paths from the first and second light-emitting units in Comparative Example 2. [Figure 72] FIG. 10 is a diagram showing the optical path from a gradient index lens array in Comparative Example 2. [Figure 73] FIG. 10 is a diagram showing the optical path from the photosensitive drum in Comparative Example 2. [Figure 74] FIG. 10 is a graph showing the amount of light on a photosensitive drum in Comparative Example 2. [Figure 75]FIG. 10 is a schematic diagram showing an exposure head and a photosensitive drum of Comparative Example 3. [Figure 76] FIG. 10 is a diagram showing the optical paths from the first and second light-emitting units in Comparative Example 3. [Figure 77] FIG. 10 is a diagram showing the optical path from a gradient index lens array in Comparative Example 3. [Figure 78] FIG. 10 is a diagram showing the optical path from the photosensitive drum in Comparative Example 3. [Figure 79] FIG. 10 is a graph showing the amount of light on a photosensitive drum in Comparative Example 3. DETAILED DESCRIPTION OF THE INVENTION

[0010] Examples of the present invention will be described below with reference to the drawings. Examples 1 to 3 correspond to claims 1 to 6, Examples 4 to 6 correspond to claims 1 and 7 to 11, Examples 7 to 9 correspond to claims 1 and 12 to 15, and Examples 10 and 12 correspond to claims 1 and 16 to 20. [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 perpendicular to the first direction, sub-scanning 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 shifted 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.

[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 according to the first embodiment of the present invention will be described with reference to FIG. 5, FIGS. 6(a) and 6(b), FIGS. 7(a) and 7(b), and FIGS. 8(a) and 8(b).

[0032] FIG. 5 shows a ZX cross section as viewed from the Y direction, illustrating the arrangement of the exposure head 105 (light-emitting substrate 201 and gradient index lens array 204) 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 is incident on 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 is incident on 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] 8(a) shows the positional relationship between the first and second light-emitting units 206 and 207 and the first and second gradient index lens arrays 204-1 and 204-2. FIG. 8(b) shows the positional relationship between the first and second gradient index lens arrays 204-1 and 204-2 and the photosensitive drum 103.

[0035] 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 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.

[0036] The exposure head 105 of this embodiment has a configuration that satisfies the following conditions in order to suppress variations in the amount of light on the photosensitive drum 103 due to 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. As shown in the above figures, the midpoint between the center of the light-emitting element of the first light-emitting unit 206 and the center of the light-emitting element of the second light-emitting unit 207 is defined as point p, the rotation center of the photosensitive drum 103, which is the irradiated surface, is defined as point c, and the line connecting points p and c is defined as the first line. The center of the light source image formed on the photosensitive drum 103, which is the irradiated surface, by light from the light-emitting element of the first light-emitting unit 206 is defined as point f, which is the third point. The center of the light source image formed on the photosensitive drum 103 by light from the light-emitting element of the second light-emitting unit 207 is defined as point s, which is the fourth point. The longer of the distance between the first line and point f and the distance between the first line and point s is defined as W. This distance is the shortest distance. At this time, the exposure head 105 satisfies the condition of the following equation (1).

[0037] 0.6≦W / T≦2.0 (1) The condition of formula (1) indicates the appropriate positioning of the exposure head 105 relative to the photosensitive drum 103. When W / T is within the range of formula (1), fluctuations in the amount of light on the photosensitive drum 103 due to mounting errors of the exposure head 105 on the image forming apparatus 1 can be reduced, and a good image with a small difference in actual density from the expected density can be formed. When W / T is below the lower limit of formula (1), it is not preferable because it is not possible to reduce fluctuations in the amount of light due to mounting errors and it is not possible to form a good image. When W / T is above the upper limit of formula (1), it is not preferable because it increases the size of the image forming apparatus 1.

[0038] In this embodiment, the diameter of the photosensitive drum 103 is 30 mm, and T is 0.25 mm. In this embodiment, when viewed from the Y direction, the exposure head 105 is tilted by 2.0° (θa, described later) compared to a case where there is no tilt (tilt angle is 0°) as in Comparative Example 1, described later, and W is 0.40 mm. Therefore, W / T is 1.60, which satisfies the condition of formula (1).

[0039] When viewed from the Y direction, the line connecting the center of the light-emitting portion of the first light-emitting unit 206 and the center of the light-emitting portion of the second light-emitting unit 207 is defined as the second line, and the line connecting point f and point s is defined as the third line. The exposure head 105 is tilted so that the second and third lines are non-parallel to each other. When viewed from the Y direction, the tilt angle of the second line of the present embodiment relative to the second line of Comparative Example 1 is θa = 2.0°, and the tilt angle of the third line of the present embodiment relative to the third line of Comparative Example 1 is θb = 0.7°. Therefore, the second and third lines are non-parallel to each other.

[0040] Furthermore, point a denotes the fifth point, which is the center of the entrance surface of each gradient index lens 203 included in the gradient index lens array 204 when viewed from the Y direction, and point b denotes the sixth point, which is the center of the exit surface of each gradient index lens 203. The line connecting points a and b is the fourth line, and the normal to the photosensitive drum 103 at point f or point s, whichever is closer to the first line, is the fifth line. The angle (tilt angle) between the fourth line and the fifth line is α, and the distance from the intersection of the fourth line and the second line to the intersection of the fourth line and the entrance surface of the gradient index lens 203 is D. In this case, it is preferable that the exposure head 105 satisfies the condition of the following equation (2).

[0041] 0.15≦α / tan -1 (T / D)≦0.50 (2) The condition of equation (2) indicates the appropriate tilt amount (tilt angle α) of the exposure head 105 with respect to the photosensitive drum 103 when viewed from the Y direction. -1 When (T / D) is within the range of the formula (2), it is possible to more effectively reduce the fluctuation in the amount of light due to the installation error of the exposure head 105.-1 If (T / D) is below the lower limit of the formula (2), it is not possible to effectively reduce the fluctuation in light intensity due to installation errors, which is undesirable. -1 If (T / D) exceeds the upper limit of the formula (2), the image forming apparatus 1 becomes large, which is not preferable.

[0042] In this embodiment, D = 2.74 mm. The shortest distance to the first line is shorter at point s than at point f. Therefore, the fifth line is the normal line at point s. The angle α between the fourth line and the fifth line is 2.2°. Therefore, α / tan -1 (T / D)=0.42, which satisfies the condition of equation (2).

[0043] Furthermore, when viewed from the Y direction, the exposure head 105 preferably satisfies the condition of the following formula (3).

[0044] 0.05≦T / D≦0.20 (3) The condition of formula (3) indicates the appropriate relationship between the distance between the centers of the light-emitting portions of the first and second light-emitting units 206, 207 when viewed from the Y direction and the distance from these first and second light-emitting units 206, 207 to the incident surface of the refractive index gradient lens 203. When T / D is within the range of formula (3), it is possible to more effectively reduce fluctuations in light intensity due to installation errors of the exposure head 105. In this embodiment, T / D=0.09, which satisfies the condition of formula (3).

[0045] Furthermore, because the exposure head 105 is disposed at an angle when viewed from the Y direction, the second line and the fourth line are perpendicular to each other. Furthermore, although not shown, the distance from the intersection of the second line and the fourth line connecting points a and b of the lens 203 in the first gradient index lens array 204-1 when viewed from the Y direction to the center of the light-emitting portion of the first light-emitting unit 206 is defined as T1. Similarly, the distance from the intersection of the second line and the fourth line connecting points a and b of the lens 203 in the second gradient index lens array 204-2 when viewed from the Y direction to the center of the light-emitting portion of the second light-emitting unit 207 is defined as T2. In this case, T1 and T2 are equal to each other.

[0046] The above conditions described in this example are satisfied for 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 Examples 2 and 3 described below.

[0047] Here, using Figures 65, 66(a), (b), 67(a), (b), 68(a), (b) and 69, we will explain Comparative Example 1 and the light intensity fluctuations in Comparative Example 1.

[0048] Figure 65 shows the arrangement of the exposure head 1405 relative to the photosensitive drum 1403 in the ZX cross section when viewed from the Y direction. Figures 66(a) and (b) show enlarged views of enlarged area 1 in Figure 65. Figures 67(a) and (b) and Figures 68(a) and (b) show enlarged views of enlarged area 2 in Figure 65. The exposure head 1405 of Comparative Example 1 has the same configuration as the exposure head 105 of Example 1, except that it is arranged without being tilted relative to the photosensitive drum 1403.

[0049] Fig. 66(a) shows how a light ray A1 emitted from the first light-emitting unit 1406 is incident on the first and second gradient index lens arrays 1404-1 and 1404-2. Fig. 66(b) shows how a light ray A2 emitted from the second light-emitting unit 1407 is incident on the first and second gradient index lens arrays 1404-1 and 1404-2. Fig. 67(a) shows how a light ray A1 emitted from the first light-emitting unit 1406 and then from the first and second gradient index lens arrays 1404-1 and 1404-2 is condensed on the photosensitive drum 1403. FIG. 66(b) shows how light rays A2 emitted from the second light-emitting unit 1407 and then emitted from the first and second refractive index distribution type lens arrays 1404-1 and 1404-2 are condensed onto the photosensitive drum 1403.

[0050] Fig. 68(a) shows a light ray A1 emitted from the first light-emitting unit 1406 and focused (irradiated) on the photosensitive drum 1403 via the first and second gradient index lens arrays 1404-1 and 1404-2, and a light ray B1 generated when the light ray A1 is reflected by the photosensitive drum 1403. Fig. 68(b) shows a light ray B2 generated when a light ray A2 emitted from the second light-emitting unit 1407 and focused on the photosensitive drum 1403 via the first and second gradient index lens arrays 1404-1 and 1404-2, and a light ray B2 generated when the light ray A2 is reflected by the photosensitive drum 1403.

[0051] In Comparative Example 1, the diameter of the photosensitive drum 1403 is 30 mm, and T is 0.25 mm. The exposure head 1405 is not tilted relative to the photosensitive drum 1403, so W is 0.125 mm. Therefore, W / T is 0.50, which does not satisfy the condition of formula (1). Furthermore, the second and third lines have tilt angles (θa, θb) of 0, and are parallel to each other.

[0052] When the exposure head 1405 is attached to the image forming apparatus 1, its 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 1405 due to attachment error is set to ±0.5°. The expected value of the attachment angle of the exposure head 1405 in this embodiment is set to 0°, and the amount of light emitted from the first and second light-emitting units 1406 and 1407 and focused on the photosensitive drum 1403 at this expected value is set to 100.00, respectively.

[0053] FIG. 69 shows the relationship between the mounting error (rotation angle) of the exposure head 1405 in Comparative Example 1 and the amount of light on the photosensitive drum 1403. When the exposure head 1405 rotates by +0.5° due to a mounting error, the amount of light emitted from the first light-emitting unit 1406 and condensed on the photosensitive drum 1403 decreases to 99.89. On the other hand, the amount of light emitted from the second light-emitting unit 1407 and condensed on the photosensitive drum 1403 increases to 100.06. Conversely, when the exposure head 1405 rotates by -0.5° due to a mounting error, the amount of light emitted from the first light-emitting unit 1406 and condensed on the photosensitive drum 1403 increases to 100.06. On the other hand, the amount of light emitted from the second light-emitting unit 1407 and condensed on the photosensitive drum 1403 decreases to 99.88. In this way, if there is a variation in the amount of light due to an installation error of the exposure head 1405, the density of the image formed on the photosensitive drum 1403 changes, and an image with the expected density cannot be obtained.

[0054] The reason why the amount of light condensed on the photosensitive drum 1403 fluctuates when the exposure head 1405 rotates due to an installation error will be explained below. As shown in Fig. 66(a), light ray A1 emitted from the first light-emitting unit 1406 is incident on the first and second gradient index lens arrays 1404-1 and 1404-2. Then, light ray A1 emitted from the first and second gradient index lens arrays 1404-1 and 1404-2 is condensed in an area centered on point f on the photosensitive drum 1403 to form a light source image, as shown in Fig. 67(a).

[0055] Generally, the surface of a photosensitive drum has a reflectivity greater than 0%. Therefore, the light ray A1 condensed at point f is also reflected. In this comparative example, the reflectivity of the photosensitive drum 1403 is set to 10.0%. As shown in FIG. 68(a), a portion of the light ray B1 generated when the light ray A1 is reflected by the photosensitive drum 1403 is incident on the first and second gradient index lens arrays 1404-1 and 1404-2. Then, the light ray B1 emitted from the first and second gradient index lens arrays 1404-1 and 1404-2 is condensed on the first light-emitting unit 1406 as shown in FIG. 66(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 1406. In this comparative example, the reflectivity of the first light-emitting unit 1406 is set to 25.0%. As shown in FIG. 66(a), the light ray C1 reflected by the first light-emitting unit 1406 follows the same optical path as the light ray A1 as multiple reflection light and is collected on the photosensitive drum 1403.

[0056] 66(b), the light ray A2 emitted from the second light-emitting unit 1407 is incident on the first and second refractive index distribution type lens arrays 1404-1 and 1404-2. Then, the light ray A2 emitted from the first and second refractive index distribution type lens arrays 1404-1 and 1404-2 is condensed in an area centered on point s on the photosensitive drum 1403 to form a light source image, as shown in FIG.

[0057] As shown in FIG. 68(b), a portion of light ray B2 generated when light ray A2 is reflected by photosensitive drum 1403 is incident on first and second refractive index distribution lens arrays 1404-1 and 1404-2. Then, light ray B2 emitted from first and second refractive index distribution lens arrays 1404-1 and 1404-2 is condensed on second light-emitting unit 1407 as shown in FIG. 66(b) and reflected by second light-emitting unit 1407. The reflectance of second light-emitting unit 1407 is also 25.0%. As shown in FIG. 66(b), light ray C2 reflected by second light-emitting unit 1407 follows the same optical path as light ray A2 as multiple reflection light and is condensed on photosensitive drum 1403.

[0058] In Figures 68(a) and (b), the light rays B1 and B2 reflected by the photosensitive drum 1403 when there is no mounting error in the exposure head 1405 (mounting angle is 0°) are shown by dashed lines, and the light rays B1 and B2 reflected by the photosensitive drum 1403 when rotated (+0.5°) due to a mounting error are shown by dashed lines.

[0059] The case where the exposure head 1405 is rotated by +0.5° from 0° will be described. In this case, the proportion of light rays B1 emitted from the first light-emitting unit 1406 and reflected by the photosensitive drum 1403 that enter the first and second gradient index lens arrays 1404-1 and 1404-2 (dashed line) is reduced compared to the light rays (dashed line) when there is no installation error. Therefore, as shown in FIG. 69, the amount of light emitted from the first light-emitting unit 1406 and condensed on the photosensitive drum 1403 is reduced to 99.89. On the other hand, the proportion of light rays B2 emitted from the second light-emitting unit 1407 and reflected by the photosensitive drum 1403 that enter the first and second gradient index lens arrays 1404-1 and 1404-2 (dashed line) is increased compared to the light rays (dashed line) when there is no installation error. Therefore, the amount of light emitted from the second light-emitting unit 1407 and collected on the photosensitive drum 1403 increases to 100.06.

[0060] The case where the exposure head 1405 is rotated from 0° to -0.5° will be described. In this case, contrary to when the exposure head 1405 is rotated +0.5°, the amount of light emitted from the first light-emitting unit 1406 and collected on the photosensitive drum 1403 increases to 100.06. On the other hand, the amount of light emitted from the second light-emitting unit 1407 and collected on the photosensitive drum 1403 decreases to 99.88.

[0061] In this way, in Comparative Example 1, rotating the exposure head 1405 by ±0.5° increases or decreases the amount of multiple reflected light, and the fluctuation in the amount of light on the photosensitive drum 1403 is a maximum of 0.12% (=|99.88-100.00|).

[0062] FIG. 9 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. In this embodiment, the exposure head 105 is rotated 2.0° from 0°. As a result, as shown in FIG. 7(a), the proportion of light rays B1 generated when light rays A1 emitted from the first light-emitting unit 206 are reflected by the photosensitive drum 103 and incident on the first and second refractive index gradient lens arrays 204-1 and 204-2 is very small (e.g., 1% or less). As a result, as shown in FIG. 6(a), there is almost no multiple reflection light. Therefore, as shown in FIG. 9, even when the exposure head 105 is rotated +0.5° from 0° to a mounting angle of 2.5°, the amount of light emitted from the first light-emitting unit 206 and focused on the photosensitive drum 103 is 99.99, and the fluctuation in light amount from 100.00 is almost eliminated.

[0063] Furthermore, when the exposure head 105 is rotated by −0.5° and the mounting angle becomes 1.5°, the proportion of light rays B1 that are emitted from the first light-emitting unit 206 and reflected by the photosensitive drum 103 and that enter the first and second refractive index gradient lens arrays 204-1 and 204-2 increases. As a result, as shown in FIG. 9, the amount of light that is emitted from the first light-emitting unit 206 and focused on the photosensitive drum 103 becomes 100.06.

[0064] On the other hand, when the exposure head 105 is rotated by 2.0°, as shown in FIG. 7(b), the proportion of light rays B2 emitted from the second light-emitting unit 207 and reflected by the photosensitive drum 103 that are incident on the first and second gradient index lens arrays 204-1 and 204-2 increases. As a result, as shown in FIG. 6(b), the amount of light rays C2 that are multiple-reflected increases. Therefore, even when the exposure head 105 is rotated by ±0.5°, the increase or decrease in the amount of light rays C2 that reach the photosensitive drum 103 can be suppressed. Therefore, as shown in FIG. 9, when the exposure head 105 is rotated by +0.5°, the amount of light emitted from the second light-emitting unit 207 and focused on the photosensitive drum 103 is 99.94, and when the exposure head 105 is rotated by −0.5°, the amount of light that is focused on the photosensitive drum 103 is 100.01.

[0065] From the above, the maximum fluctuation in the amount of light on the photosensitive drum 103 in this embodiment is 0.06% (=|100.06-100.00|). That is, in this embodiment, the fluctuation in the amount of light can be reduced by 50% compared to 0.12% (|-0.12|) in Comparative Example 1.

[0066] In this example and Comparative Example 1, the reflectance of the photosensitive drum is 10.0%, and the reflectance of each light-emitting unit is 25.0%. Here, the relationship between these reflectances and the fluctuation in light intensity on the photosensitive drum will be explained. When the reflectance of the photosensitive drum is halved from 10.0% to 5.0%, the fluctuation in light intensity is halved to |0.03| in this example and |0.06| in Comparative Example 1. In this case, too, the fluctuation in light intensity in this example is reduced by 50% compared to Comparative Example 1, and is the same as when the reflectance of the photosensitive drum is 10.0%. It is desirable that the reflectance of the photosensitive drum be 5.0% or higher.

[0067] Furthermore, when the reflectance of each light-emitting unit is halved from 25.0% to 12.5%, the light intensity fluctuation is halved to |0.03| in this example and |0.06| in Comparative Example 1. In this case, the light intensity fluctuation in this example is improved by 50% compared to Comparative Example 1, and is the same as when the reflectance of each light-emitting unit is 25.0%. It is desirable that the reflectance of each light-emitting unit is 10.0% or more. The reflectances of the photosensitive drum and each light-emitting unit described above are the same in other examples described below.

[0068] In this way, in this embodiment, compared to Comparative Example 1, it is possible to suppress fluctuations in the amount of light on the photosensitive drum 103 due to installation errors in the exposure head 105, and it is possible to reduce changes in the density of the image formed on the photosensitive drum 103 from the expected density. [Example]

[0069] 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.

[0070] The exposure head 305 of this embodiment will be described using Figures 10, 11(a) and 11(b), 12(a) and 12(b), 13(a) and 13(b), and 14. Figure 10 shows a ZX cross section as viewed from the Y direction, illustrating the arrangement of the exposure head 305 (light-emitting substrate 301 and gradient index lens array 304) relative to the photosensitive drum 303. Figures 11(a) and 11(b) show an enlarged view of enlarged region 1 in Figure 10 as viewed from the Y direction. Figure 11(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 11(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.

[0071] 12(a) and (b) show enlarged views of enlarged region 2 in FIG. 10. Fig. 12(a) shows how light ray A1, which is emitted from the first light-emitting unit 306 and then from the first and second gradient index lens arrays 304-1 and 304-2, is focused at point f on the photosensitive drum 303, and how light ray B1 is generated when light ray A1 is reflected by the photosensitive drum 303. Fig. 12(b) shows how light ray A2, which is emitted from the second light-emitting unit 307 and then from the first and second gradient index lens arrays 304-1 and 304-2, is focused at point s on the photosensitive drum 303, and how light ray B2 is generated when light ray A2 is reflected by the photosensitive drum 303.

[0072] 13(a) shows the positional relationship between the first and second light-emitting units 306 and 307 and the first and second gradient index lens arrays 304-1 and 304-2, while FIG. 13(b) shows the positional relationship between the first and second gradient index lens arrays 304-1 and 304-2 and the photosensitive drum 303.

[0073] In this embodiment, the diameter of the photosensitive drum 303 is 30 mm, and T is 0.40 mm. Furthermore, in this embodiment, when viewed from the Y direction, the exposure head 305 is tilted by 2.0° (θa) compared to a case where there is no tilt (tilt angle is 0°) as in Comparative Example 2 described below, and W is 0.48 mm. Therefore, W / T is 1.20, which satisfies the condition of formula (1). Furthermore, the tilt angle of the second line is θa = 2.0°, and the tilt angle of the third line is θb = 0.7°, and the second line and the third line are not parallel to each other.

[0074] Furthermore, in this embodiment, D=2.74 mm. The shortest distance to the first line is shorter at point s than at point f. Therefore, the fifth line is the normal to the photosensitive drum 303 at point s. The angle α between the fourth line and the fifth line is 1.9°. Therefore, α / tan -1 (T / D)=0.23, which satisfies the condition of formula (2). Also, T / D=0.15, which satisfies the condition of formula (3).

[0075] In this embodiment, the exposure head 305 is also tilted when viewed from the Y direction, so the second line and the fourth line are perpendicular to each other. Furthermore, in this embodiment, the distance corresponding to the distance T1 and the distance T2 when viewed from the Y direction described in the first embodiment are equal to each other.

[0076] Here, comparative example 2 will be explained using Figures 70, 71(a), (b), 72(a), (b), 73(a), (b) and 74, and the fluctuation in light intensity in comparative example 2 will be explained.

[0077] Figure 70 shows the arrangement of the exposure head 1505 relative to the photosensitive drum 1503 in the ZX cross section when viewed from the Y direction. Figures 71(a) and (b) show enlarged views of enlarged area 1 in Figure 70. Figures 72(a) and (b) and Figures 73(a) and (b) show enlarged views of enlarged area 2 in Figure 70. The exposure head 1505 of Comparative Example 2 has the same configuration as the exposure head 305 of Example 2, except that it is arranged without being tilted relative to the photosensitive drum 1503.

[0078] Fig. 71(a) shows how a light ray A1 emitted from the first light-emitting unit 1506 is incident on the first and second gradient index lens arrays 1504-1 and 1504-2. Fig. 71(b) shows how a light ray A2 emitted from the second light-emitting unit 1507 is incident on the first and second gradient index lens arrays 1504-1 and 1504-2. Fig. 72(a) shows how a light ray A1 emitted from the first light-emitting unit 1506 and then from the first and second gradient index lens arrays 1504-1 and 1504-2 is condensed on the photosensitive drum 1503. FIG. 72(b) shows how light rays A2 emitted from the second light-emitting unit 1507 and then emitted from the first and second refractive index distribution lens arrays 1504-1 and 1504-2 are condensed onto the photosensitive drum 1503.

[0079] 73(a) shows a light ray A1 emitted from the first light-emitting unit 1506 and focused (irradiated) on the photosensitive drum 1503 via the first and second gradient index lens arrays 1504-1 and 1504-2, and a light ray B1 generated when the light ray A1 is reflected by the photosensitive drum 1503. FIG. 73(b) shows a light ray B2 generated when a light ray A2 emitted from the second light-emitting unit 1507 and focused on the photosensitive drum 1503 via the first and second gradient index lens arrays 1504-1 and 1504-2, and a light ray B2 generated when the light ray A2 is reflected by the photosensitive drum 1503.

[0080] In Comparative Example 2, the diameter of the photosensitive drum 1503 is 30 mm, and T is 0.40 mm. The exposure head 1505 is not tilted, so W is 0.20 mm. Therefore, W / T is 0.50, which does not satisfy the condition of formula (1). In addition, the second line and the third line are parallel to each other.

[0081] 74 shows the relationship between the mounting error (rotation angle) of the exposure head 1505 in Comparative Example 2 and the amount of light on the photosensitive drum 1503. Here too, the assumed value of the mounting angle of the exposure head 1505 is set to 0°, and the amount of light on the photosensitive drum 1503 at this assumed value is set to 100.00.

[0082] As in Comparative Example 1, when the exposure head 1505 rotates by +0.5° due to an installation error, the amount of light emitted from the first light-emitting unit 1506 and condensed on the photosensitive drum 1503 decreases from 100.00 to 99.99. On the other hand, the amount of light emitted from the second light-emitting unit 1507 and condensed on the photosensitive drum 1503 increases to 100.06. Conversely, when the exposure head 1505 rotates by -0.5° due to an installation error, the amount of light emitted from the first light-emitting unit 1506 and condensed on the photosensitive drum 1503 increases to 100.06. On the other hand, the amount of light emitted from the second light-emitting unit 1507 and condensed on the photosensitive drum 1503 decreases to 99.99. In this way, when there is a fluctuation in the amount of light due to an installation error of the exposure head 1505, the density of the image formed on the photosensitive drum 1503 changes, and an image with the expected density cannot be obtained.

[0083] The reason why the amount of light condensed on the photosensitive drum 1503 fluctuates when the exposure head 1505 rotates due to an installation error will be explained below. As shown in Fig. 71(a), a light ray A1 emitted from the first light-emitting unit 1506 is incident on the first and second refractive index gradient lens arrays 1504-1 and 1504-2. Then, the light ray A1 emitted from the first and second refractive index gradient lens arrays 1504-1 and 1504-2 is condensed in an area centered at point f on the photosensitive drum 1503 to form a light source image, as shown in Fig. 72(a).

[0084] As shown in Fig. 73(a), a part of a light ray B1 generated when a light ray A1 focused at point f is reflected by a photosensitive drum 1503 having a reflectance of 10.0% is incident on the first and second refractive index distribution lens arrays 1504-1 and 1504-2. Then, the light ray B1 emitted from the first and second refractive index distribution lens arrays 1504-1 and 1504-2 is focused on the first light-emitting unit 1506 as shown in Fig. 71(a). A light ray C1 reflected by the first light-emitting unit 1506 having a reflectance of 25.0% follows the same optical path as the light ray A1 as multiple reflection light and is focused on the photosensitive drum 1503.

[0085] Similarly, as shown in Fig. 71(b), light ray A2 emitted from the second light-emitting unit 1507 is incident on the first and second refractive index gradient lens arrays 1504-1 and 1504-2. Then, light ray A2 emitted from the first and second refractive index gradient lens arrays 1504-1 and 1504-2 is condensed in an area centered on point s on the photosensitive drum 1503 to form a light source image, as shown in Fig. 72(b).

[0086] As shown in Fig. 73(b), part of light ray B2 generated when light ray A2 focused at point s is reflected by photosensitive drum 1503 is incident on first and second refractive index distribution lens arrays 1504-1 and 1504-2. Then, light ray B2 emitted from first and second refractive index distribution lens arrays 1504-1 and 1504-2 is focused on second light-emitting unit 1507 as shown in Fig. 71(b). Light ray C2 reflected by second light-emitting unit 1507, which has a reflectance of 25.0%, follows the same optical path as light ray A2 as multiple reflection light and is focused on photosensitive drum 1503.

[0087] In Figures 73(a) and (b), the light rays B1 and B2 reflected by the photosensitive drum 1503 when there is no mounting error in the exposure head 1505 (mounting angle is 0°) are shown by dashed lines, and the light rays B1 and B2 reflected by the photosensitive drum 1503 when rotated (+0.5°) due to a mounting error are shown by dashed lines.

[0088] The case where the exposure head 1505 is rotated by +0.5° from 0° will be described. In this case, the proportion of light rays B1 emitted from the first light-emitting unit 1506 and reflected by the photosensitive drum 1503 that enter the first and second gradient index lens arrays 1504-1 and 1504-2 (dashed line) is reduced compared to the light rays (dashed line) when there is no installation error. Therefore, as shown in FIG. 74, the amount of light emitted from the first light-emitting unit 1506 and focused on the photosensitive drum 1503 is reduced to 99.99. On the other hand, the proportion of light rays B2 emitted from the second light-emitting unit 1507 and reflected by the photosensitive drum 1503 that enter the first and second gradient index lens arrays 1504-1 and 1504-2 (dashed line) is increased compared to the light rays (dashed line) when there is no installation error. Therefore, the amount of light emitted from the second light-emitting unit 1507 and collected on the photosensitive drum 1503 increases to 100.06.

[0089] The case where the exposure head 1505 is rotated from 0° to -0.5° will be described. In this case, contrary to when the exposure head 1505 is rotated +0.5°, the amount of light emitted from the first light-emitting unit 1506 and collected on the photosensitive drum 1503 increases to 100.06. On the other hand, the amount of light emitted from the second light-emitting unit 1507 and collected on the photosensitive drum 1503 decreases to 99.99.

[0090] In this way, in Comparative Example 2, rotating the exposure head 1505 by ±0.5° increases or decreases the amount of multiple reflected light, and the fluctuation in the amount of light on the photosensitive drum 1503 is a maximum of 0.06% (=|100.06-100.00|).

[0091] FIG. 14 shows the relationship between the mounting error (rotation angle) of the exposure head 305 and the amount of light on the photosensitive drum 303 in this embodiment. In this embodiment, the exposure head 305 is rotated 2.0° from 0°. As a result, as shown in FIG. 12(a), of the light beam B1 generated when the light beam A1 emitted from the first light-emitting unit 306 is reflected by the photosensitive drum 303, none of the light beams enter the first and second gradient index lens arrays 304-1 and 304-2. Therefore, as shown in FIG. 11(a), no multiple reflections occur. As a result, as shown in FIG. 14, even when the exposure head 305 is rotated +0.5° from 0° to a mounting angle of 2.5°, the amount of light emitted from the first light-emitting unit 306 and focused on the photosensitive drum 303 is 100.00, and there is no fluctuation in the amount of light.

[0092] Also, when the exposure head 305 is rotated by −0.5° and the mounting angle becomes 1.5°, none of the light rays B1 emitted from the first light-emitting unit 306 and reflected by the photosensitive drum 303 enter the first and second refractive index gradient lens arrays 304-1 and 304-2. As a result, as shown in Fig. 14, the amount of light emitted from the first light-emitting unit 306 and focused on the photosensitive drum 303 becomes 100.00, and there is no fluctuation in the amount of light.

[0093] On the other hand, when the exposure head 305 is rotated by 2.0°, as shown in FIG. 12(b), the proportion of light rays B2 emitted from the second light-emitting unit 307 and reflected by the photosensitive drum 303 that are incident on the first and second gradient index lens arrays 304-1 and 304-2 increases. As a result, as shown in FIG. 11(b), the amount of light rays C2 that are multiple-reflected increases. Therefore, even when the exposure head 305 is rotated by ±0.5°, the increase or decrease in the amount of light rays C2 that reach the photosensitive drum 303 can be suppressed. Therefore, as shown in FIG. 14, when the exposure head 305 is rotated by +0.5° and -0.5°, the amount of light emitted from the second light-emitting unit 307 and focused on the photosensitive drum 303 is 99.98.

[0094] From the above, in this embodiment, the light quantity fluctuation on the photosensitive drum 303 is a maximum of 0.02% (=|99.98-100.00|). That is, in this embodiment, the light quantity fluctuation can be reduced by 67% compared to 0.06% (|0.06|) in Comparative Example 2.

[0095] In this way, in this embodiment, compared to Comparative Example 2, it is possible to suppress fluctuations in the amount of light on the photosensitive drum 303 due to installation errors in the exposure head 305, and it is possible to reduce changes in the density of the image formed on the photosensitive drum 303 from the expected density. [Example]

[0096] Next, an exposure head of Example 3 will be described. The image forming apparatus in which the exposure head of Example 3 is used is similar to the image forming apparatus 1 described in Example 1. The exposure head of Example 3 differs from the exposure head 105 of Example 1 in the distance T, the diameter of the photosensitive drum, and the inclination of the exposure head, but the other configurations are similar to the exposure head 105 of Example 1.

[0097] The exposure head 405 of this embodiment will be described using Figures 15, 16(a) and 16(b), 17(a) and 17(b), 18(a) and 18(b), and 19. Figure 15 shows a ZX cross section as viewed from the Y direction, illustrating the arrangement of the exposure head 405 (light-emitting substrate 401 and gradient index lens array 404) relative to the photosensitive drum 403. Figures 16(a) and 16(b) show an enlarged view of enlarged region 1 in Figure 15 as viewed from the Y direction. Figure 16(a) shows how a light ray A1 emitted from a first light-emitting unit 406 is incident on the first and second gradient index lens arrays 404-1 and 404-2. Figure 16(b) shows how a light ray A2 emitted from a second light-emitting unit 407 is incident on the first and second gradient index lens arrays 404-1 and 404-2.

[0098] 17(a) and (b) show an enlarged view of the enlarged region 2 in FIG. 15. FIG. 17(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 point f on the photosensitive drum 403, and a light ray B1 generated when the light ray A1 is reflected by the photosensitive drum 403. FIG. 14(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 point s on the photosensitive drum 403, and a light ray B2 generated when the light ray A2 is reflected by the photosensitive drum 403.

[0099] 18(a) shows the positional relationship between the first and second light-emitting units 406 and 407 and the first and second gradient index lens arrays 404-1 and 404-2. FIG. 19(b) shows the positional relationship between the first and second gradient index lens arrays 404-1 and 404-2 and the photosensitive drum 403.

[0100] In this embodiment, the diameter of the photosensitive drum 403 is 20 mm, and T is 0.30 mm. Furthermore, in this embodiment, when viewed from the Y direction, the exposure head 405 is tilted by 1.5° (θa) compared to a case where there is no tilt (tilt angle is 0°) as in Comparative Example 3 described below, and W is 0.34 mm. Therefore, W / T is 1.13, which satisfies the condition of formula (1). Furthermore, the tilt angle of the second line is θa = 1.5°, and the tilt angle of the third line is θb = 0.7°, and the second line and the third line are not parallel to each other.

[0101] Furthermore, in this embodiment, D = 2.74 mm. The shortest distance to the first line is shorter at point s than at point f. Therefore, the fifth line is the normal line at point s. The angle α between the fourth line and the fifth line is 1.4°. Therefore, α / tan -1 (T / D)=0.22, which satisfies the condition of formula (2). Also, T / D=0.11, which satisfies the condition of formula (3).

[0102] In this embodiment, the exposure head 405 is also tilted when viewed from the Y direction, so the second line and the fourth line are perpendicular to each other. Furthermore, in this embodiment, the distance corresponding to the distance T1 and the distance T2 when viewed from the Y direction described in the first embodiment are equal to each other.

[0103] Here, comparative example 2 will be explained using Figures 75, 76(a), (b), 77(a), (b), 78(a), (b) and 79, and the fluctuation in light intensity in comparative example 2 will be explained.

[0104] Figure 75 shows the arrangement of the exposure head 1605 relative to the photosensitive drum 1603 in the ZX cross section when viewed from the Y direction. Figures 76(a) and (b) show enlarged views of enlarged area 1 in Figure 75. Figures 77(a) and (b) and Figures 78(a) and (b) show enlarged views of enlarged area 2 in Figure 75. The exposure head 1605 of Comparative Example 3 has the same configuration as the exposure head 405 of Example 3, except that it is arranged without being tilted relative to the photosensitive drum 1603.

[0105] Fig. 76(a) shows how a light ray A1 emitted from the first light-emitting unit 1606 is incident on the first and second gradient index lens arrays 1604-1 and 1604-2. Fig. 76(b) shows how a light ray A2 emitted from the second light-emitting unit 1607 is incident on the first and second gradient index lens arrays 1604-1 and 1604-2. Fig. 77(a) shows how a light ray A1 emitted from the first light-emitting unit 1606 and then from the first and second gradient index lens arrays 1604-1 and 1604-2 is condensed on the photosensitive drum 1603. FIG. 77(b) shows how light rays A2 emitted from the second light-emitting unit 1507 and then emitted from the first and second refractive index distribution lens arrays 1604-1 and 1604-2 are condensed onto the photosensitive drum 1603.

[0106] Fig. 78(a) shows a light ray A1 emitted from the first light-emitting unit 1506 and focused (irradiated) on the photosensitive drum 1503 via the first and second gradient index lens arrays 1504-1 and 1504-2, and a light ray B1 generated when the light ray A1 is reflected by the photosensitive drum 1503. Fig. 78(b) shows a light ray B2 generated when a light ray A2 emitted from the second light-emitting unit 1507 and focused on the photosensitive drum 1503 via the first and second gradient index lens arrays 1504-1 and 1504-2, and a light ray B2 generated when the light ray A2 is reflected by the photosensitive drum 1503.

[0107] In Comparative Example 3, the diameter of the photosensitive drum 1503 is 20 mm, and T is 0.30 mm. The exposure head 1605 is not tilted, so W is 0.15 mm. Therefore, W / T is 0.50, which does not satisfy the condition of formula (1). In addition, the second line and the third line are parallel to each other.

[0108] 79 shows the relationship between the mounting error (rotation angle) of the exposure head 1605 in Comparative Example 3 and the amount of light on the photosensitive drum 1603. Here too, the assumed value of the mounting angle of the exposure head 1605 is set to 0°, and the amount of light on the photosensitive drum 1603 at this assumed value is set to 100.00.

[0109] As in Comparative Examples 1 and 2, when the exposure head 1605 rotates by +0.5° due to an installation error, the amount of light emitted from the first light-emitting unit 1606 and condensed on the photosensitive drum 1603 decreases from 100.00 to 99.85. On the other hand, the amount of light emitted from the second light-emitting unit 1607 and condensed on the photosensitive drum 1603 increases to 100.14. Conversely, when the exposure head 1605 rotates by -0.5° due to an installation error, the amount of light emitted from the first light-emitting unit 1606 and condensed on the photosensitive drum 1603 increases to 100.14. On the other hand, the amount of light emitted from the second light-emitting unit 1607 and condensed on the photosensitive drum 1603 decreases to 99.85. In this way, when there is a fluctuation in the amount of light due to an installation error of the exposure head 1605, the density of the image formed on the photosensitive drum 1603 changes, and an image with the expected density cannot be obtained.

[0110] The reason why the amount of light condensed on the photosensitive drum 1603 fluctuates when the exposure head 1605 rotates due to an installation error will be explained below. As shown in Fig. 76(a), a light ray A1 emitted from the first light-emitting unit 1606 is incident on the first and second refractive index gradient lens arrays 1604-1 and 1604-2. Then, the light ray A1 emitted from the first and second refractive index gradient lens arrays 1604-1 and 1604-2 is condensed in an area centered at point f on the photosensitive drum 1603 to form a light source image, as shown in Fig. 77(a).

[0111] As shown in Fig. 78(a), a part of a light ray B1 generated when a light ray A1 focused at point f is reflected by a photosensitive drum 1603 having a reflectance of 10.0% is incident on the first and second refractive index distribution lens arrays 1604-1 and 1604-2. Then, the light ray B1 emitted from the first and second refractive index distribution lens arrays 1604-1 and 1604-2 is focused on the first light-emitting unit 1606 as shown in Fig. 76(a). A light ray C1 reflected by the first light-emitting unit 1606 having a reflectance of 25.0% follows the same optical path as the light ray A1 as multiple reflection light and is focused on the photosensitive drum 1603.

[0112] Similarly, as shown in Fig. 76(b), the light ray A2 emitted from the second light-emitting unit 1607 is incident on the first and second refractive index gradient lens arrays 1604-1 and 1604-2. Then, the light ray A2 emitted from the first and second refractive index gradient lens arrays 1604-1 and 1604-2 is condensed in an area centered on point s on the photosensitive drum 1603 to form a light source image, as shown in Fig. 77(b).

[0113] As shown in Fig. 78(b), part of light ray B2 generated when light ray A2 focused at point s is reflected by photosensitive drum 1603 is incident on first and second refractive index gradient lens arrays 1604-1 and 1604-2. Then, light ray B2 emitted from first and second refractive index gradient lens arrays 1604-1 and 1604-2 is focused on second light-emitting unit 1607 as shown in Fig. 76(b). Light ray C2 reflected by second light-emitting unit 1607, which has a reflectance of 25.0%, follows the same optical path as light ray A2 as multiple reflection light and is focused on photosensitive drum 1603.

[0114] In Figures 78(a) and (b), the light rays B1 and B2 reflected by the photosensitive drum 1603 when there is no mounting error in the exposure head 1605 (mounting angle is 0°) are shown by dashed lines, and the light rays B1 and B2 reflected by the photosensitive drum 1603 when rotated (+0.5°) due to a mounting error are shown by dashed lines.

[0115] The case where the exposure head 1605 is rotated by +0.5° from 0° will be described. In this case, the proportion of light rays B1 emitted from the first light-emitting unit 1606 and reflected by the photosensitive drum 1603 that enter the first and second gradient index lens arrays 1604-1 and 1606-2 (dashed lines) is reduced compared to the light rays (dashed lines) when there is no installation error. Therefore, as shown in FIG. 79, the amount of light emitted from the first light-emitting unit 1606 and focused on the photosensitive drum 1603 is reduced to 99.85. On the other hand, the proportion of light rays B2 emitted from the second light-emitting unit 1607 and reflected by the photosensitive drum 1603 that enter the first and second gradient index lens arrays 1604-1 and 1604-2 (dashed lines) is increased compared to the light rays (dashed lines) when there is no installation error. Therefore, the amount of light emitted from the second light-emitting unit 1607 and collected on the photosensitive drum 1603 increases to 100.14.

[0116] The case where the exposure head 1605 is rotated from 0° to -0.5° will be described. In this case, contrary to when the exposure head 1605 is rotated +0.5°, the amount of light emitted from the first light-emitting unit 1606 and collected on the photosensitive drum 1603 increases to 100.14. On the other hand, the amount of light emitted from the second light-emitting unit 1607 and collected on the photosensitive drum 1603 decreases to 99.85.

[0117] In this way, in Comparative Example 3, rotating the exposure head 1605 by ±0.5° increases or decreases the amount of multiple reflected light, and the fluctuation in the amount of light on the photosensitive drum 1603 is a maximum of 0.15% (=|99.85-100.00|).

[0118] FIG. 19 shows the relationship between the mounting error (rotation angle) of the exposure head 405 and the amount of light on the photosensitive drum 403 in this embodiment. In this embodiment, the exposure head 405 is rotated 1.5° from 0°. As a result, as shown in FIG. 17(a), of the light beam B1 generated when the light beam A1 emitted from the first light-emitting unit 406 is reflected by the photosensitive drum 403, none of the light beams enter the first and second gradient index lens arrays 404-1 and 404-2. Therefore, no multiple reflections occur, as shown in FIG. 16(a). As a result, as shown in FIG. 19, even when the exposure head 405 is rotated +0.5° from 0° to a mounting angle of 2.0°, the amount of light emitted from the first light-emitting unit 406 and focused on the photosensitive drum 403 is 100.00, and there is no fluctuation in the amount of light.

[0119] Furthermore, when the exposure head 305 is rotated by -0.5° and the mounting angle becomes 1.5°, the proportion of light rays B1 that are emitted from the first light-emitting unit 306 and reflected by the photosensitive drum 303 and that enter the first and second refractive index gradient lens arrays 304-1 and 304-2 increases slightly. As a result, as shown in Fig. 19, the amount of light that is emitted from the first light-emitting unit 406 and focused on the photosensitive drum 403 becomes 100.02.

[0120] On the other hand, when the exposure head 405 is rotated by 1.5°, as shown in FIG. 17(b), the proportion of light rays B2 emitted from the second light-emitting unit 407 and reflected by the photosensitive drum 403 that are incident on the first and second gradient index lens arrays 404-1 and 404-2 increases. As a result, as shown in FIG. 16(b), the amount of light rays C2 that are multiple-reflected increases. Therefore, even when the exposure head 405 is rotated by ±0.5°, the increase or decrease in the amount of light rays C2 that reach the photosensitive drum 403 can be suppressed. Therefore, as shown in FIG. 19, when the exposure head 405 is rotated by +0.5° and -0.5°, the amounts of light emitted from the second light-emitting unit 407 and focused on the photosensitive drum 403 are 99.98 and 99.97, respectively.

[0121] From the above, in this embodiment, the fluctuation in the amount of light on the photosensitive drum 403 is a maximum of 0.03% (=|99.97-100.00|). That is, in this embodiment, the fluctuation in the amount of light can be reduced by 80% compared to 0.15% (|0.15|) in Comparative Example 3.

[0122] In this way, in this embodiment, compared to Comparative Example 3, it is possible to suppress fluctuations in the amount of light on the photosensitive drum 403 due to installation errors in the exposure head 405, and it is possible to reduce changes in the density of the image formed on the photosensitive drum 403 from the expected density. [Example]

[0123] 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 that it does not have an inclination (i.e., similar to Comparative Example 1) and is arranged at a position shifted parallel to the photosensitive drum, but other configurations are similar to the exposure head 105 of Example 1.

[0124] The exposure head 505 of this embodiment will be described using Figures 20, 21(a) and 21(b), 22(a) and 22(b), 23(a) and 23(b), and 24. Figure 20 shows a ZX cross section as viewed from the Y direction, illustrating the arrangement of the exposure head 505 (light-emitting substrate 501 and gradient index lens array 504) relative to the photosensitive drum 503. Figures 21(a) and 21(b) show an enlarged view of the enlarged area 1 in Figure 20 as viewed from the Y direction. Figure 21(a) shows how a light ray A1 emitted from a first light-emitting unit 506 is incident on the first and second gradient index lens arrays 504-1 and 504-2. Figure 21(b) shows how a light ray A2 emitted from a second light-emitting unit 507 is incident on the first and second gradient index lens arrays 504-1 and 504-2.

[0125] 22(a) and (b) show an enlarged view of the enlarged region 2 in FIG. 20. FIG. 22(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 point f on the photosensitive drum 503, and a light ray B1 generated when the light ray A1 is reflected by the photosensitive drum 503. FIG. 22(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 point s on the photosensitive drum 503, and a light ray B2 generated when the light ray A2 is reflected by the photosensitive drum 503.

[0126] 23(a) shows the positional relationship between the first and second light-emitting units 506 and 507 and the first and second gradient index lens arrays 504-1 and 504-2. FIG. 23(b) shows the positional relationship between the first and second gradient index lens arrays 504-1 and 504-2 and the photosensitive drum 503.

[0127] In the exposure head 505 of this embodiment, T = 0.25 mm. Furthermore, the exposure head 505 of this embodiment is positioned at a position shifted in parallel by 0.7 mm in the Z direction when viewed from the Y direction relative to Comparative Example 1. W = 0.40 mm. Therefore, W / T = 1.60, which satisfies the condition of formula (1).

[0128] In this embodiment, when viewed from the Y direction, a straight line that passes through point p and intersects with the second straight line at right angles is defined as a sixth straight line. In this case, the exposure head 505 of this embodiment is disposed in a position where the sixth straight line does not pass through point c, which is the rotation center of the photosensitive drum (irradiated surface) 503, as shown in FIG.

[0129] 20 and 23(a), the distance (parallel movement amount) between the sixth line and point c when viewed from the Y direction is A, the diameter of the photosensitive drum 503 is B, and the distance from point p, which is the intersection of the sixth line and the second line, to the intersection of the sixth line and the incident surface of the refractive index gradient lens is D. In this case, it is preferable that the exposure head 505 satisfies the condition of the following formula (4).

[0130] 2≦(B×T) / (D×A)≦10 (4) The condition of equation (4) indicates an appropriate amount of parallel movement A in the Z direction of the exposure head 505 relative to the photosensitive drum 503. By setting (B×T) / (D×A) within the range of equation (4), it is possible to reduce fluctuations in the amount of light on the photosensitive drum 503 due to installation errors of the exposure head 505. If (B×T) / (D×A) is below the lower limit of equation (4), it is undesirable because the image forming apparatus 1 becomes large. If (B×T) / (D×A) is above the upper limit of equation (4), it is undesirable because it is not possible to reduce fluctuations in the amount of light due to installation errors.

[0131] Furthermore, in this embodiment, A = 0.7 mm, B = 30 mm, and D = 2.74 mm. Therefore, (B × T) / (D × A) = 3.9, which satisfies the condition of formula (4). Also, T / D = 0.09, which satisfies the condition of formula (3).

[0132] 23(a) and 23(b), when viewed from the Y direction, the fourth line is a line connecting point a, which is the center of the incident surface of the gradient index lens included in each gradient index lens array (504-1, 504-2), and point b, which is the center of the exit surface of the lens. The exposure head 505 is positioned so that the fourth line and the sixth line are parallel to each other. When viewed from the Y direction, the sixth line passes through point d, which is the midpoint (seventh point) between the center of the incident surface of the gradient index lens included in the first gradient index lens array 504-1 and the center of the incident surface of the gradient index lens included in the second gradient index lens array 504-2.

[0133] The above conditions described in this example are satisfied for all combinations of the first and second light-emitting units in the plurality of first light-emitting units 506 and the plurality of second light-emitting units 507. This also applies to Examples 5 and 6 described below.

[0134] 24 shows the relationship between the mounting error (rotation angle) of the exposure head 505 in this embodiment and the amount of light on the photosensitive drum 503. The assumed value of the mounting angle of the exposure head 505 in this embodiment is 0°, the same as in Comparative Example 1.

[0135] As shown in FIG. 22(a), of the light beams B1 generated when the light beam A1 emitted from the first light-emitting unit 506 is reflected by the photosensitive drum 503, only a small proportion of the light beams enter the first and second gradient index lens arrays 504-1 and 504-2. Therefore, as shown in FIG. 21(a), almost no multiple reflection occurs. As a result, as shown in FIG. 24, even when the exposure head 505 is rotated from 0° to +0.5°, the amount of light focused on the photosensitive drum 503 is 99.99, and there is almost no fluctuation in the amount of light on the photosensitive drum 503. Furthermore, when the exposure head 505 is rotated −0.5°, of the light beams B1 generated when the light beam A1 emitted from the first light-emitting unit 506 is reflected by the photosensitive drum 503, the proportion of the light beams that enter the first and second gradient index lens arrays 504-1 and 504-2 increases. Therefore, the amount of light condensed on the photosensitive drum 503 is 100.05.

[0136] On the other hand, as shown in FIG. 22(b), the proportion of light rays B2 generated when light ray A2 emitted from the second light-emitting unit 507 is reflected by the photosensitive drum 503 increases, resulting in light rays B2 that are incident on the first and second gradient index lens arrays 504-1 and 504-2. At this time, when the exposure head 505 is translated 0.7 mm in the Z direction, the amount of light ray C2 as multiple reflection light increases, as shown in FIG. 21(b). Therefore, even when the exposure head 505 is rotated ±0.5°, the increase or decrease in the amount of multiple reflection light can be suppressed. As a result, as shown in FIG. 24, the amount of light focused on the photosensitive drum 503 when the exposure head 505 is rotated +0.5° is 99.94, and the amount of light focused on the photosensitive drum 503 when the exposure head 505 is rotated −0.5° is 100.01.

[0137] From the above, in this embodiment, the fluctuation in the amount of light on the photosensitive drum 503 is a maximum of 0.06% (=|99.94-100.00|). That is, in this embodiment, the fluctuation in the amount of light can be reduced by 50% compared to 0.12% (|0.12|) in Comparative Example 1.

[0138] In this way, in this embodiment, compared to Comparative Example 1, it is possible to suppress fluctuations in the amount of light on the photosensitive drum 503 due to installation errors of the exposure head 505, and it is possible to reduce changes in the density of the image formed on the photosensitive drum 503 from the expected density. [Example]

[0139] Next, an exposure head of Example 5 will be described. The image forming apparatus in which the exposure head of Example 5 is used is similar to the image forming apparatus 1 described in Example 1. The exposure head of Example 5 differs from the exposure head 305 of Example 2 in that it has no inclination (i.e., similar to Comparative Example 2) and is arranged at a position shifted parallel to the photosensitive drum, but other configurations are similar to the exposure head 305 of Example 2.

[0140] The exposure head 605 of this embodiment will be described using Figures 25, 26(a) and 26(b), 27(a) and 27(b), 28(a) and 28(b), and 29. Figure 25 shows a ZX cross section as viewed from the Y direction, illustrating the arrangement of the exposure head 605 (light-emitting substrate 601 and gradient index lens array 604) relative to the photosensitive drum 603. Figures 26(a) and 26(b) show an enlarged view of the enlarged region 1 in Figure 25 as viewed from the Y direction. Figure 26(a) shows how a light ray A1 emitted from a first light-emitting unit 606 is incident on the first and second gradient index lens arrays 604-1 and 604-2. Figure 26(b) shows how a light ray A2 emitted from a second light-emitting unit 607 is incident on the first and second gradient index lens arrays 604-1 and 604-2.

[0141] Figures 27(a) and (b) show an enlarged view of enlarged region 2 in Figure 25. Figure 27(a) shows how light ray A1, which is emitted from the first light-emitting unit 606 and then emitted from the first and second gradient index lens arrays 604-1 and 604-2, is focused at point f on the photosensitive drum 603, and also shows light ray B1 generated when light ray A1 is reflected by the photosensitive drum 603. Figure 27(b) shows how light ray A2, which is emitted from the second light-emitting unit 607 and then emitted from the first and second gradient index lens arrays 604-1 and 604-2, is focused at point s on the photosensitive drum 603, and also shows light ray B2 generated when light ray A2 is reflected by the photosensitive drum 603.

[0142] 28(a) shows the positional relationship between the first and second light-emitting units 606 and 607 and the first and second gradient index lens arrays 604-1 and 604-2. FIG. 28(b) shows the positional relationship between the first and second gradient index lens arrays 604-1 and 604-2 and the photosensitive drum 603.

[0143] In the exposure head 605 of this embodiment, T = 0.40 mm. Furthermore, the exposure head 605 of this embodiment is positioned at a position shifted in parallel by 0.7 mm in the Z direction when viewed from the Y direction relative to Comparative Example 2. W = 0.40 mm. Therefore, W / T = 1.20, which satisfies the condition of formula (1).

[0144] Furthermore, the exposure head 605 of this embodiment is disposed at a position where the sixth line does not pass through point c, which is the center of rotation of the photosensitive drum 603, as shown in FIG.

[0145] Furthermore, in this embodiment, A = 0.7 mm, B = 30 mm, and D = 2.74 mm. Therefore, (B × T) / (D × A) = 6.3, which satisfies the condition of formula (4). Also, T / D = 0.15, which satisfies the condition of formula (3).

[0146] 28(a) and 28(b), when viewed from the Y direction, the fourth line is a line connecting point a, which is the center of the incident surface of the gradient index lens included in each gradient index lens array (604-1, 604-2), and point b, which is the center of the exit surface of the lens. The exposure head 605 is positioned so that the fourth line and the sixth line are parallel to each other. When viewed from the Y direction, the sixth line passes through point d, which is the midpoint between the center of the incident surface of the gradient index lens included in the first gradient index lens array 604-1 and the center of the incident surface of the gradient index lens included in the second gradient index lens array 604-2.

[0147] FIG. 29 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 this embodiment. The assumed mounting angle of the exposure head 605 in this embodiment is 0°, as in Comparative Example 2. In this embodiment, as shown in FIG. 27(a), of the light beam B1 generated when the light beam A1 emitted from the first light-emitting unit 606 is reflected by the photosensitive drum 603, none of the light beams enter the first and second gradient index lens arrays 604-1 and 604-2. Therefore, as shown in FIG. 26(a), no multiple reflections occur. As a result, as shown in FIG. 29, even when the exposure head 605 rotates from 0° to +0.5°, the amount of light focused on the photosensitive drum 603 is 100.00, and there is no fluctuation in the amount of light on the photosensitive drum 603. Furthermore, when the exposure head 605 is rotated by −0.5°, none of the light rays B1 generated when the light rays A1 emitted from the first light-emitting unit 606 are reflected by the photosensitive drum 603 enters the first and second refractive index gradient lens arrays 604-1 and 604-2. Therefore, the amount of light condensed on the photosensitive drum 603 becomes 100.00.

[0148] On the other hand, as shown in FIG. 27(b), the proportion of light rays B2 generated when light ray A2 emitted from the second light-emitting unit 607 is reflected by the photosensitive drum 603 increases, and the proportion of light rays B2 that enter the first and second gradient index lens arrays 604-1 and 604-2 increases. At this time, when the exposure head 605 is translated 0.7 mm in the Z direction, the amount of light ray C2 as multiple reflection light increases, as shown in FIG. 26(b). Therefore, even when the exposure head 605 is rotated ±0.5°, the increase or decrease in the amount of multiple reflection light can be suppressed. As a result, as shown in FIG. 29, the amount of light focused on the photosensitive drum 603 when the exposure head 605 is rotated +0.5° is 99.98, and the amount of light focused on the photosensitive drum 603 when the exposure head 605 is rotated -0.5° is 99.99.

[0149] From the above, in this embodiment, the fluctuation in the amount of light on the photosensitive drum 603 is a maximum of 0.02% (=|99.98-100.00|). That is, in this embodiment, the fluctuation in the amount of light can be reduced by 67% compared to 0.06% (|0.06|) in Comparative Example 2.

[0150] In this way, in this embodiment, compared to Comparative Example 2, it is possible to suppress fluctuations in the amount of light on the photosensitive drum 603 due to installation errors of the exposure head 605, and it is possible to reduce changes in the density of the image formed on the photosensitive drum 603 from the expected density. [Example]

[0151] Next, an exposure head of Example 6 will be described. The image forming apparatus in which the exposure head of Example 6 is used is similar to the image forming apparatus 1 described in Example 1. The exposure head of Example 6 differs from exposure head 405 of Example 3 in that it has no inclination (i.e., similar to Comparative Example 3) and is arranged at a position shifted parallel to the photosensitive drum, but other configurations are similar to exposure head 405 of Example 3.

[0152] The exposure head 705 of this embodiment will be described using Figures 30, 31(a) and 31(b), 32(a) and 32(b), 33(a) and 33(b), and 34. Figure 25 shows a ZX cross section as viewed from the Y direction, illustrating the arrangement of the exposure head 705 (light-emitting substrate 701 and gradient index lens array 704) relative to the photosensitive drum 703. Figures 31(a) and 31(b) show an enlarged view of the enlarged region 1 in Figure 30 as viewed from the Y direction. Figure 31(a) shows how a light ray A1 emitted from a first light-emitting unit 706 is incident on the first and second gradient index lens arrays 704-1 and 704-2. Figure 31(b) shows how a light ray A2 emitted from a second light-emitting unit 707 is incident on the first and second gradient index lens arrays 704-1 and 704-2.

[0153] Figures 32(a) and (b) show an enlarged view of enlarged region 2 in Figure 30. Figure 32(a) shows how light ray A1, which is emitted from the first light-emitting unit 706 and then emitted from the first and second gradient index lens arrays 704-1 and 704-2, is focused at point f on the photosensitive drum 703, and also shows light ray B1 generated when light ray A1 is reflected by the photosensitive drum 703. Figure 32(b) shows how light ray A2, which is emitted from the second light-emitting unit 707 and then emitted from the first and second gradient index lens arrays 704-1 and 704-2, is focused at point s on the photosensitive drum 703, and also shows light ray B2 generated when light ray A2 is reflected by the photosensitive drum 703.

[0154] Fig. 33(a) shows the positional relationship between the first and second light-emitting units 706 and 707 and the first and second gradient index lens arrays 704-1 and 704-2. Fig. 33(b) shows the positional relationship between the first and second gradient index lens arrays 704-1 and 704-2 and the photosensitive drum 703.

[0155] In the exposure head 705 of this embodiment, T = 0.30 mm. Furthermore, the exposure head 705 of this embodiment is positioned at a position shifted in parallel by 0.4 mm in the Z direction when viewed from the Y direction relative to Comparative Example 3. W = 0.35 mm. Therefore, W / T = 1.17, which satisfies the condition of formula (1).

[0156] Furthermore, the exposure head 705 of this embodiment is disposed at a position where the sixth line does not pass through point c, which is the center of rotation of the photosensitive drum 703, as shown in FIG.

[0157] Furthermore, in this embodiment, A = 0.4 mm, B = 20 mm, and D = 2.74 mm. Therefore, (B × T) / (D × A) = 5.5, which satisfies the condition of formula (4). Furthermore, T / D = 0.11, which satisfies the condition of formula (3).

[0158] In this embodiment, as shown in Figures 33(a) and 33(b), when viewed from the Y direction, the fourth line is a line connecting point a, which is the center of the incident surface of the gradient index lens included in each gradient index lens array (704-1, 704-2), and point b, which is the center of the exit surface of the lens. In this case, the exposure head 705 is arranged so that the fourth line and the sixth line are parallel to each other. Furthermore, when viewed from the Y direction, the sixth line passes through point d, which is the midpoint between the center of the incident surface of the gradient index lens included in the first gradient index lens array 704-1 and the center of the incident surface of the gradient index lens included in the second gradient index lens array 704-2.

[0159] FIG. 34 shows the relationship between the mounting error (rotation angle) of the exposure head 705 and the amount of light on the photosensitive drum 703 in this embodiment. The assumed mounting angle of the exposure head 705 in this embodiment is 0°, as in Comparative Example 3. In this embodiment, as shown in FIG. 32(a), of the light beam B1 generated when the light beam A1 emitted from the first light-emitting unit 706 is reflected by the photosensitive drum 703, none of the light beams enter the first and second gradient index lens arrays 704-1 and 704-2. Therefore, as shown in FIG. 31(a), no multiple reflections occur. As a result, as shown in FIG. 34, even when the exposure head 705 rotates from 0° to +0.5°, the amount of light focused on the photosensitive drum 703 is 100.00, and there is no fluctuation in the amount of light on the photosensitive drum 703. Furthermore, when the exposure head 705 is rotated by −0.5°, the proportion of light rays B1 that are incident on the first and second gradient index lens arrays 704-1 and 704-2 increases among light rays B1 that are generated when light rays A1 emitted from the first light-emitting unit 706 are reflected by the photosensitive drum 703. As a result, the amount of light condensed on the photosensitive drum 703 becomes 100.02.

[0160] On the other hand, as shown in FIG. 32(b), the proportion of light rays B2 generated when light ray A2 emitted from the second light-emitting unit 707 is reflected by the photosensitive drum 703 increases, resulting in light rays B2 that are incident on the first and second gradient index lens arrays 704-1 and 704-2. At this time, when the exposure head 705 is translated 0.4 mm in the Z direction, light ray C2 as multiple reflection light increases, as shown in FIG. 31(b). Therefore, even when the exposure head 705 is rotated ±0.5°, the increase or decrease in multiple reflection light can be suppressed. As a result, as shown in FIG. 34, the amount of light focused on the photosensitive drum 703 when the exposure head 705 is rotated +0.5° is 99.97, and the amount of light focused on the photosensitive drum 703 when the exposure head 705 is rotated -0.5° is 99.98.

[0161] From the above, in this embodiment, the fluctuation in the amount of light on the photosensitive drum 703 is a maximum of 0.03% (=|99.97-100.00|). That is, in this embodiment, the fluctuation in the amount of light can be reduced by 80% compared to 0.15% (|0.15|) in Comparative Example 3.

[0162] In this way, in this embodiment, compared to Comparative Example 3, it is possible to suppress fluctuations in the amount of light on the photosensitive drum 703 due to installation errors of the exposure head 705, and it is possible to reduce changes in the density of the image formed on the photosensitive drum 703 from the expected density. [Example]

[0163] Next, an exposure head of Example 7 will be described. The image forming apparatus in which the exposure head of Example 7 is used is similar to the image forming apparatus 1 described in Example 1. The exposure head of Example 7 differs from the exposure head 105 of Example 1 in that it does not have an inclination (i.e., similar to Comparative Example 1) and the refractive index distribution lens array is arranged at an inclination with respect to the light-emitting substrate and the photosensitive drum, but other configurations are similar to the exposure head 105 of Example 1.

[0164] The exposure head 805 of this embodiment will be described using Figures 35, 36(a) and 36(b), 37(a) and 37(b), 38(a) and 38(b), and 39. Figure 35 shows a ZX cross section as viewed from the Y direction, illustrating the arrangement of the exposure head 805 (light-emitting substrate 801 and gradient index lens array 804) relative to the photosensitive drum 803. Figures 36(a) and 36(b) show an enlarged view of enlarged region 1 in Figure 35 as viewed from the Y direction. Figure 36(a) shows how a light ray A1 emitted from the first light-emitting unit 706 is incident on the first and second gradient index lens arrays 704-1 and 704-2. Figure 36(b) shows how a light ray A2 emitted from the second light-emitting unit 707 is incident on the first and second gradient index lens arrays 704-1 and 704-2.

[0165] Figures 37(a) and (b) show an enlarged view of enlarged region 2 in Figure 35. Figure 37(a) shows how light ray A1, which is emitted from the first light-emitting unit 706 and then emitted from the first and second gradient index lens arrays 704-1 and 704-2, is focused at point f on the photosensitive drum 703, and also shows light ray B1 generated when light ray A1 is reflected by the photosensitive drum 703. Figure 37(b) shows how light ray A2, which is emitted from the second light-emitting unit 707 and then emitted from the first and second gradient index lens arrays 704-1 and 704-2, is focused at point s on the photosensitive drum 703, and also shows light ray B2 generated when light ray A2 is reflected by the photosensitive drum 703.

[0166] Fig. 38(a) shows the positional relationship between the first and second light-emitting units 706 and 707 and the first and second gradient index lens arrays 704-1 and 704-2. Fig. 38(b) shows the positional relationship between the first and second gradient index lens arrays 704-1 and 704-2 and the photosensitive drum 703.

[0167] In this embodiment, the diameter of the photosensitive drum 803 is 30 mm, and T is 0.25 mm. In addition, in the exposure head 805 of this embodiment, the refractive index distribution lens array 804 is tilted by 0.8° (θc, described later) relative to Comparative Example 1 when viewed from the Y direction. W is 0.26 mm. Therefore, W / T is 1.04, which satisfies the condition of formula (1).

[0168] 37(a) and (b), the seventh line is a line that is perpendicular to the fourth line when viewed from the Y direction. In this case, in the exposure head 805 of this embodiment, the refractive index distribution lens array 804 (804-1, 804-2) is arranged at an inclination such that the third line and the seventh line are non-parallel to each other. Specifically, the inclination angle of the third line is θb = 0.5°, and the inclination angle of the seventh line is θc = 0.8°, and the third line and the seventh line are non-parallel to each other.

[0169] In this embodiment, when viewed from the Y direction, α / tan -1 It is preferable that (T / D) satisfies the condition of the following formula (5).

[0170] 0.10≦α / tan -1 (T / D)≦0.20 (5) The condition of equation (5) indicates an appropriate tilt angle α of the refractive index distribution lens array 804 with respect to the photosensitive drum 803 when viewed from the Y direction. -1 By setting (T / D) within the range of the formula (5), it is possible to reduce the fluctuation in the amount of light on the photosensitive drum 803 due to the installation error of the exposure head 805. -1 If (T / D) is below the lower limit of the formula (5), it is not possible to reduce the fluctuation in light intensity due to installation errors, which is not preferable. -1 If (T / D) exceeds the upper limit of the formula (5), the image forming apparatus 1 becomes large, which is not preferable.

[0171] In this embodiment, D=2.74 mm. The shortest distance to the first line is shorter at point s than at point f. Therefore, the fifth line is the normal to the photosensitive drum 803 at point s. The inclination angle between the fourth line and the fifth line is α=0.8°. Therefore, α / tan -1 (T / D)=0.15, which satisfies the condition of formula (5). Also, T / D=0.09, which satisfies the condition of formula (3).

[0172] Furthermore, when viewed from the Y direction, the angle formed by the second line and the fourth line is (90°-α), specifically (90°-0.8°)=89.2°. The above conditions described in this example are satisfied for all combinations of the first and second light-emitting units in the plurality of first light-emitting units 806 and the plurality of second light-emitting units 807. This also applies to Examples 8 and 9 described below.

[0173] FIG. 39 shows the relationship between the mounting error (rotation angle) of the exposure head 805 in this embodiment and the amount of light on the photosensitive drum 803. The assumed mounting angle of the exposure head 805 in this embodiment is 0°, as in Comparative Example 1. As shown in FIG. 37(a), of the light beam B1 generated when the light beam A1 emitted from the first light-emitting unit 806 is reflected by the photosensitive drum 803, only a small proportion of the light beam enters the first and second gradient index lens arrays 804-1 and 804-2. Therefore, as shown in FIG. 36(a), almost no multiple reflections occur. As a result, as shown in FIG. 39, even when the exposure head 805 rotates from 0° to +0.5°, the amount of light focused on the photosensitive drum 803 is 99.99, and almost no fluctuation in the amount of light on the photosensitive drum 803 occurs. Furthermore, when the exposure head 805 is rotated by −0.5°, the proportion of light rays B1 that are incident on the first and second gradient index lens arrays 804-1 and 804-2 increases among light rays B1 that are generated when light rays A1 emitted from the first light-emitting unit 806 are reflected by the photosensitive drum 803. As a result, the amount of light condensed on the photosensitive drum 803 becomes 100.05.

[0174] On the other hand, as shown in FIG. 37(b), the proportion of light rays B2 generated when light ray A2 emitted from the second light-emitting unit 807 is reflected by the photosensitive drum 803 increases, and the proportion of light rays B2 that enter the first and second gradient index lens arrays 804-1 and 804-2 increases. At this time, when the gradient index lens array 804 is rotated 0.8°, the amount of light ray C2 as multiple reflection light increases, as shown in FIG. 36(b). As a result, even when the exposure head 805 is rotated ±0.5°, the increase or decrease in the amount of multiple reflection light can be suppressed. Therefore, as shown in FIG. 39, the amount of light condensed on the photosensitive drum 803 when the exposure head 805 is rotated +0.5° is 99.97, and the amount of light condensed on the photosensitive drum 803 when the exposure head 805 is rotated -0.5° is 99.95.

[0175] From the above, in this embodiment, the fluctuation in the amount of light on the photosensitive drum 803 is a maximum of 0.05% (=|99.95-100.00|). That is, in this embodiment, the fluctuation in the amount of light can be reduced by 58% compared to 0.12% (|0.12|) in Comparative Example 1.

[0176] In this way, in this embodiment, compared to Comparative Example 1, it is possible to suppress fluctuations in the amount of light on the photosensitive drum 803 due to installation errors in the exposure head 805, and it is possible to reduce changes in the density of the image formed on the photosensitive drum 803 from the expected density. [Example]

[0177] Next, an exposure head of Example 8 will be described. The image forming apparatus in which the exposure head of Example 8 is used is similar to the image forming apparatus 1 described in Example 1. The exposure head of Example 8 differs from the exposure head 305 of Example 2 in that it does not have an inclination (i.e., similar to Comparative Example 2) and the refractive index distribution lens array is arranged at an inclination with respect to the light-emitting substrate and the photosensitive drum, but other configurations are similar to the exposure head 305 of Example 2.

[0178] The exposure head 905 of this embodiment will be described using Figures 40, 41(a) and 41(b), 42(a) and 42(b), 43(a) and 43(b), and 45. Figure 40 shows a ZX cross section as viewed from the Y direction, illustrating the arrangement of the exposure head 905 (light-emitting substrate 901 and gradient index lens array 904) relative to the photosensitive drum 903. Figures 41(a) and 41(b) show an enlarged view of enlarged region 1 in Figure 40 as viewed from the Y direction. Figure 41(a) shows how a light ray A1 emitted from a first light-emitting unit 906 enters the first and second gradient index lens arrays 904-1 and 904-2. Figure 41(b) shows how a light ray A2 emitted from a second light-emitting unit 907 enters the first and second gradient index lens arrays 904-1 and 904-2.

[0179] Figures 42(a) and (b) show an enlarged view of enlarged region 2 in Figure 40. Figure 42(a) shows how light ray A1, which is emitted from the first light-emitting unit 906 and then emitted from the first and second gradient index lens arrays 904-1 and 904-2, is focused at point f on the photosensitive drum 903, and also shows light ray B1 generated when light ray A1 is reflected by the photosensitive drum 903. Figure 42(b) shows how light ray A2, which is emitted from the second light-emitting unit 907 and then emitted from the first and second gradient index lens arrays 904-1 and 904-2, is focused at point s on the photosensitive drum 903, and also shows light ray B2 generated when light ray A2 is reflected by the photosensitive drum 903.

[0180] 43(a) shows the positional relationship between the first and second light-emitting units 906 and 907 and the first and second gradient index lens arrays 904-1 and 904-2. FIG. 43(b) shows the positional relationship between the first and second gradient index lens arrays 904-1 and 904-2 and the photosensitive drum 903.

[0181] In this embodiment, the diameter of the photosensitive drum 903 is 30 mm, and T is 0.40 mm. In addition, in the exposure head 905 of this embodiment, the refractive index distribution lens array 904 is tilted by 1.1° (θc) compared to Comparative Example 2 when viewed from the Y direction. W is 0.39 mm. Therefore, W / T is 0.98, which satisfies the condition of formula (1).

[0182] 42(a) and 42(b), the seventh line is a line that is perpendicular to the fourth line when viewed from the Y direction. In this case, in the exposure head 905 of this embodiment, the gradient index lens array 904 (904-1, 904-2) is arranged at an inclination such that the third line and the seventh line are non-parallel to each other. Specifically, the inclination angle of the third line is θb = 0.7°, and the inclination angle of the seventh line is θc = 1.1°, and the third line and the seventh line are non-parallel to each other.

[0183] In this embodiment, D=2.74 mm. The shortest distance to the first line is shorter at point s than at point f. Therefore, the fifth line is the normal to the photosensitive drum 903 at point s. The inclination angle between the fourth line and the fifth line is α=1.1°. Therefore, α / tan -1 (T / D)=0.13, which satisfies the condition of formula (5). Also, T / D=0.15, which satisfies the condition of formula (3).

[0184] Furthermore, when viewed from the Y direction, the angle (90°-α) formed by the second line and the fourth line is (90°-1.1°)=88.9°.

[0185] FIG. 44 shows the relationship between the mounting error (rotation angle) of the exposure head 905 and the amount of light on the photosensitive drum 903 in this embodiment. The assumed mounting angle of the exposure head 905 in this embodiment is 0°, as in Comparative Example 2. In this embodiment, as shown in FIG. 42(a), of the light beam B1 generated when the light beam A1 emitted from the first light-emitting unit 906 is reflected by the photosensitive drum 903, none of the light beams enter the first and second gradient index lens arrays 904-1 and 904-2. Therefore, as shown in FIG. 41(a), no multiple reflections occur. As a result, as shown in FIG. 44, even when the exposure head 905 rotates from 0° to +0.5°, the amount of light focused on the photosensitive drum 903 is 100.00, and no fluctuations in the amount of light on the photosensitive drum 903 occur. Furthermore, when the exposure head 905 is rotated by −0.5°, none of the light rays B1 generated when the light rays A1 emitted from the first light-emitting unit 906 are reflected by the photosensitive drum 903 enters the first and second refractive index gradient lens arrays 904-1 and 904-2. Therefore, the amount of light condensed on the photosensitive drum 903 becomes 100.00.

[0186] On the other hand, as shown in FIG. 42(b), the proportion of light rays B2 generated when light ray A2 emitted from the second light-emitting unit 907 is reflected by the photosensitive drum 903 and incident on the first and second gradient index lens arrays 904-1 and 904-2 increases. At this time, when the gradient index lens array 904 is rotated by 1.1°, the amount of light ray C2 as multiple reflection light increases, as shown in FIG. 41(b). As a result, even when the exposure head 905 is rotated by ±0.5°, the increase or decrease in the amount of multiple reflection light can be suppressed. Therefore, as shown in FIG. 44, the amount of light focused on the photosensitive drum 903 when the exposure head 905 is rotated by +0.5° is 99.97, and the amount of light focused on the photosensitive drum 903 when the exposure head 905 is rotated by -0.5° is 100.01.

[0187] From the above, in this embodiment, the fluctuation in the amount of light on the photosensitive drum 903 is a maximum of 0.03% (=|99.97-100.00|). That is, in this embodiment, the fluctuation in the amount of light can be reduced by 50% compared to 0.06% (|0.12|) in Comparative Example 2.

[0188] In this way, in this embodiment, compared to Comparative Example 2, it is possible to suppress fluctuations in the amount of light on the photosensitive drum 903 due to installation errors of the exposure head 905, and it is possible to reduce changes in the density of the image formed on the photosensitive drum 903 from the expected density. [Example]

[0189] Next, an exposure head of Example 9 will be described. The image forming apparatus in which the exposure head of Example 9 is used is similar to the image forming apparatus 1 described in Example 1. The exposure head of Example 9 differs from exposure head 405 of Example 3 in that it does not have an inclination (i.e., similar to Comparative Example 3) and the refractive index distribution lens array is arranged at an inclination with respect to the light-emitting substrate and the photosensitive drum, but other configurations are similar to exposure head 405 of Example 3.

[0190] The exposure head 1005 of this embodiment will be described using Figures 45, 46(a) and 46(b), 47(a) and 47(b), 48(a) and 48(b), and 49. Figure 45 shows a ZX cross section as viewed from the Y direction, illustrating the arrangement of the exposure head 1005 (light-emitting substrate 1001 and gradient index lens array 1004) relative to the photosensitive drum 1003. Figures 46(a) and 46(b) show an enlarged view of the enlarged area 1 in Figure 45 as viewed from the Y direction. Figure 46(a) shows how a light ray A1 emitted from a first light-emitting unit 1006 is incident on the first and second gradient index lens arrays 1004-1 and 1004-2. Figure 46(b) shows how a light ray A2 emitted from a second light-emitting unit 1007 is incident on the first and second gradient index lens arrays 1004-1 and 1004-2.

[0191] Figures 47(a) and (b) show an enlarged view of enlarged region 2 in Figure 45. Figure 47(a) shows how light ray A1, which is emitted from the first light-emitting unit 1006 and then from the first and second gradient index lens arrays 1004-1 and 1004-2, is focused at point f on the photosensitive drum 1003, and also shows light ray B1 generated when light ray A1 is reflected by the photosensitive drum 1003. Figure 47(b) shows how light ray A2, which is emitted from the second light-emitting unit 1007 and then from the first and second gradient index lens arrays 1004-1 and 1004-2, is focused at point s on the photosensitive drum 1003, and also shows light ray B2 generated when light ray A2 is reflected by the photosensitive drum 1003.

[0192] Fig. 48(a) shows the positional relationship between the first and second light-emitting units 1006 and 1007 and the first and second gradient index lens arrays 1004-1 and 1004-2. Fig. 48(b) shows the positional relationship between the first and second gradient index lens arrays 1004-1 and 1004-2 and the photosensitive drum 1003.

[0193] In this embodiment, the diameter of the photosensitive drum 903 is 20 mm, and T is 0.30 mm. In addition, in the exposure head 1005 of this embodiment, the refractive index distribution lens array 1004 is tilted by 0.9° (θc) compared to Comparative Example 3 when viewed from the Y direction. W is 0.31 mm. Therefore, W / T is 1.03, which satisfies the condition of formula (1).

[0194] As shown in Figures 47(a) and (b), the seventh line is a line that is perpendicular to the fourth line when viewed from the Y direction. In this case, in the exposure head 1005 of this embodiment, the gradient index lens array 1004 (1004-1, 1004-2) is arranged at an inclination so that the third line and the seventh line are non-parallel to each other. Specifically, the inclination angle of the third line is θb = 0.8°, and the inclination angle of the seventh line is θc = 0.9°, and the third line and the seventh line are non-parallel to each other.

[0195] In this embodiment, D=2.74 mm. The shortest distance to the first line is shorter at point s than at point f. Therefore, the fifth line is the normal to the photosensitive drum 1003 at point s. The inclination angle between the fourth line and the fifth line is α=0.9°. Therefore, α / tan -1 (T / D)=0.14, which satisfies the condition of formula (5). Also, T / D=0.11, which satisfies the condition of formula (3).

[0196] Furthermore, when viewed from the Y direction, the angle (90°-α) formed by the second line and the fourth line is (90°-0.9°)=89.1°.

[0197] FIG. 49 shows the relationship between the mounting error (rotation angle) of the exposure head 1005 and the amount of light on the photosensitive drum 1003 in this embodiment. The assumed mounting angle of the exposure head 1005 in this embodiment is 0°, as in Comparative Example 3. In this embodiment, as shown in FIG. 47(a), of the light beam B1 generated when the light beam A1 emitted from the first light-emitting unit 1006 is reflected by the photosensitive drum 1003, none of the light beams enter the first and second gradient index lens arrays 1004-1 and 1004-2. Therefore, as shown in FIG. 46(a), no multiple reflections occur. As a result, as shown in FIG. 49, even when the exposure head 1005 rotates from 0° to +0.5°, the amount of light focused on the photosensitive drum 1003 is 100.00, and no fluctuations in the amount of light on the photosensitive drum 1003 occur. Furthermore, when the exposure head 1005 is rotated by −0.5°, none of the light rays B1 generated when the light rays A1 emitted from the first light-emitting unit 1006 are reflected by the photosensitive drum 1003 enters the first and second refractive index distribution lens arrays 1004-1 and 1004-2. As a result, the amount of light focused on the photosensitive drum 1003 becomes 100.00.

[0198] On the other hand, as shown in FIG. 47(b), the proportion of light rays B2 generated when light ray A2 emitted from the second light-emitting unit 1007 is reflected by the photosensitive drum 1003 increases, and the proportion of light rays B2 that enter the first and second gradient index lens arrays 1004-1 and 1004-2 increases. At this time, when the gradient index lens array 904 is rotated 0.9°, the amount of light ray C2 as multiple reflection light increases, as shown in FIG. 46(b). As a result, even when the exposure head 1005 is rotated ±0.5°, the increase or decrease in the amount of multiple reflection light can be suppressed. Therefore, as shown in FIG. 49, the amount of light condensed on the photosensitive drum 1003 when the exposure head 1005 is rotated +0.5° is 99.97, and the amount of light condensed on the photosensitive drum 1003 when the exposure head 1005 is rotated -0.5° is 100.00.

[0199] From the above, the light quantity fluctuation on the photosensitive drum 1003 in this embodiment is a maximum of 0.03% (=|99.97-100.00|). That is, in this embodiment, the light quantity fluctuation can be reduced by 80% compared to 0.15% (|0.15|) in Comparative Example 3.

[0200] In this way, in this embodiment, compared to comparison example 3, it is possible to suppress fluctuations in the amount of light on the photosensitive drum 1003 due to installation errors in the exposure head 1005, and it is possible to reduce changes in the density of the image formed on the photosensitive drum 1003 from the expected density. [Example]

[0201] Next, an exposure head of Example 10 will be described. The image forming apparatus in which the exposure head of Example 10 is used is similar to the image forming apparatus 1 described in Example 1. The exposure head of Example 10 differs from the exposure head 105 of Example 1 in that it has no inclination (i.e., similar to Comparative Example 1) and the first and second light-emitting units are arranged by moving parallel to the photosensitive drum, but other configurations are similar to the exposure head 105 of Example 1.

[0202] The exposure head 1105 of this embodiment will be described using Figures 50, 51(a) and 51(b), 52(a) and 52(b), 53(a) and 53(b), and 54. Figure 50 shows a ZX cross section as viewed from the Y direction, illustrating the arrangement of the exposure head 1105 (light-emitting substrate 1101 and gradient index lens array 1104) relative to the photosensitive drum 1103. Figures 51(a) and 51(b) show an enlarged view of enlarged region 1 in Figure 50 as viewed from the Y direction. Figure 51(a) shows how a light ray A1 emitted from a first light-emitting unit 1106 is incident on the first and second gradient index lens arrays 1104-1 and 1104-2. Figure 51(b) shows how a light ray A2 emitted from a second light-emitting unit 1107 is incident on the first and second gradient index lens arrays 1104-1 and 1104-2.

[0203] Figures 52(a) and (b) show an enlarged view of enlarged region 2 in Figure 50. Figure 52(a) shows how light ray A1, which is emitted from the first light-emitting unit 1106 and then from the first and second gradient index lens arrays 1104-1 and 1104-2, is focused at point f on the photosensitive drum 1103, and also shows light ray B1 generated when light ray A1 is reflected by the photosensitive drum 1103. Figure 52(b) shows how light ray A2, which is emitted from the second light-emitting unit 1107 and then from the first and second gradient index lens arrays 1104-1 and 1104-2, is focused at point s on the photosensitive drum 1103, and also shows light ray B2 generated when light ray A2 is reflected by the photosensitive drum 1103.

[0204] Fig. 53(a) shows the positional relationship between the first and second light-emitting units 1106 and 1107 and the first and second gradient index lens arrays 1104-1 and 1104-2. Fig. 53(b) shows the positional relationship between the first and second gradient index lens arrays 1104-1 and 1104-2 and the photosensitive drum 1103.

[0205] In the exposure head 1105 of this embodiment, T=0.25 mm. Furthermore, in the exposure head 1105 of this embodiment, the first and second light-emitting units 1106, 1107 on the light-emitting substrate 1101 are positioned 0.125 mm parallel to the Z direction relative to Comparative Example 1 when viewed from the Y direction. W=0.17 mm. Therefore, W / T=0.68, which satisfies the condition of formula (1).

[0206] In this embodiment, as shown in Figures 50 and 53(a) and (b), the first and second light-emitting units 1106 and 1107 are positioned so that a sixth line that passes through point p and is perpendicular to the second line does not pass through point c, which is the center of rotation of the photosensitive drum 1103.

[0207] In this embodiment, when viewed from the Y direction, the midpoint between the center of the incident surface of the gradient index lens included in the first gradient index lens array 1104-1 and the center of the incident surface of the gradient index lens included in the second gradient index lens array 1104-2 is defined as point d (seventh point). The midpoint between the center of the exit surface of the gradient index lens of the first gradient index lens array 1104-1 and the center of the exit surface of the gradient index lens of the second gradient index lens array 1104-2 is defined as point e (eighth point). The line passing through points d and e is defined as the eighth line, and the distance from the intersection of the eighth line and the second line to the intersection of the eighth line and the incident surface of each gradient index lens is defined as D. In this case, it is preferable to satisfy the following equation (6):

[0208] 10≦(B×T) / (D×A)≦30 (6) The condition of formula (6) indicates a preferable amount of parallel movement A in the Z direction of the first and second light-emitting units 1106 and 1107 relative to the photosensitive drum 1103 when viewed from the Y direction (the distance between the sixth line and point c shown in Figures 50 and 53(a)). By setting (B x T) / (D x A) within the range of formula (6), it is possible to reduce fluctuations in the amount of light on the photosensitive drum 1103 due to installation errors of the exposure head 1105. If (B x T) / (D x A) is below the lower limit of formula (6), it is undesirable because the image forming apparatus 1 becomes large. If (B x T) / (D x A) is above the upper limit of formula (6), it is undesirable because it is not possible to reduce fluctuations in the amount of light on the photosensitive drum 1103 due to installation errors of the exposure head 1105.

[0209] In this example, A = 0.125 mm, B = 30 mm, and D = 2.74 mm. Therefore, (B × T) / (D × A) = 21.9, which satisfies the condition of formula (6). Furthermore, T / D = 0.09, which satisfies the condition of formula (3).

[0210] Furthermore, in this embodiment, the eighth line passes through point c when viewed from the Y direction. As shown in Figure 53(a), the sixth line and the eighth line are parallel to each other.

[0211] The above conditions described in this example are satisfied for all combinations of the first and second light-emitting units in the plurality of first light-emitting units 1106 and the plurality of second light-emitting units 1107. This also applies to Examples 11 and 12 described below.

[0212] FIG. 54 shows the relationship between the mounting error (rotation angle) of the exposure head 1105 and the amount of light on the photosensitive drum 1103 in this embodiment. The assumed mounting angle of the exposure head 1105 in this embodiment is 0°, as in Comparative Example 1. In this embodiment, as shown in FIG. 52(a), of the light beam B1 generated when the light beam A1 emitted from the first light-emitting unit 1106 is reflected by the photosensitive drum 1103, none of the light beams enter the first and second gradient index lens arrays 1104-1 and 1104-2. Therefore, as shown in FIG. 51(a), there is no multiple reflection light. As a result, as shown in FIG. 54, even when the exposure head 1105 rotates from 0° to +0.5°, the amount of light focused on the photosensitive drum 1103 is 100.00, and there is no fluctuation in the amount of light on the photosensitive drum 1103. Furthermore, when the exposure head 1105 is rotated by −0.5°, none of the light rays B1 generated when the light rays A1 emitted from the first light-emitting unit 1106 are reflected by the photosensitive drum 1103 enters the first and second refractive index gradient lens arrays 1104-1 and 1104-2. As a result, the amount of light focused on the photosensitive drum 1103 becomes 100.00.

[0213] On the other hand, as shown in FIG. 52(b), the proportion of light rays B2 generated when light ray A2 emitted from the second light-emitting unit 1107 is reflected by the photosensitive drum 1103 and incident on the first and second gradient index lens arrays 1104-1 and 1104-2 increases. At this time, when the first and second light-emitting units 1106 and 1107 are translated 0.7 mm in the Z direction, the amount of light ray C2 as multiple reflection light increases, as shown in FIG. 51(b). Therefore, even if the exposure head 1105 is rotated ±0.5°, the increase or decrease in the multiple reflection light can be suppressed. As a result, as shown in FIG. 54, the amount of light condensed on the photosensitive drum 1103 when the exposure head 1105 is rotated +0.5° and -0.5° is 99.96.

[0214] From the above, in this embodiment, the light quantity fluctuation on the photosensitive drum 1103 is a maximum of 0.04% (=|99.96-100.00|). That is, in this embodiment, the light quantity fluctuation can be reduced by 67% compared to 0.12% (|0.12|) in Comparative Example 1.

[0215] In this way, in this embodiment, compared to Comparative Example 1, it is possible to suppress fluctuations in the amount of light on the photosensitive drum 603 due to installation errors of the exposure head 605, and it is possible to reduce changes in the density of the image formed on the photosensitive drum 603 from the expected density. [Example]

[0216] Next, an exposure head of Example 11 will be described. The image forming apparatus in which the exposure head of Example 11 is used is similar to the image forming apparatus 1 described in Example 1. The exposure head of Example 11 differs from the exposure head 305 of Example 2 in that it has no inclination (i.e., similar to Comparative Example 2) and the first and second light-emitting units are arranged by moving parallel to the photosensitive drum, but other configurations are similar to the exposure head 305 of Example 2.

[0217] The exposure head 1205 of this embodiment will be described using Figures 55, 56(a) and 56(b), 57(a) and 57(b), 58(a) and 58(b), and 59. Figure 55 shows a ZX cross section as viewed from the Y direction, illustrating the arrangement of the exposure head 1205 (light-emitting substrate 1201 and gradient index lens array 1204) relative to the photosensitive drum 1203. Figures 56(a) and 56(b) show an enlarged view of enlarged region 1 in Figure 55 as viewed from the Y direction. Figure 56(a) shows how a light ray A1 emitted from a first light-emitting unit 1206 is incident on the first and second gradient index lens arrays 1204-1 and 1204-2. Figure 56(b) shows how a light ray A2 emitted from a second light-emitting unit 1207 is incident on the first and second gradient index lens arrays 1204-1 and 1204-2.

[0218] Figures 57(a) and (b) show an enlarged view of enlarged region 2 in Figure 55. Figure 57(a) shows how light ray A1, which is emitted from the first light-emitting unit 1206 and then from the first and second gradient index lens arrays 1204-1 and 1204-2, is focused at point f on the photosensitive drum 1203, and also shows light ray B1 generated when light ray A1 is reflected by the photosensitive drum 1203. Figure 57(b) shows how light ray A2, which is emitted from the second light-emitting unit 1207 and then from the first and second gradient index lens arrays 1204-1 and 1204-2, is focused at point s on the photosensitive drum 1203, and also shows light ray B2 generated when light ray A2 is reflected by the photosensitive drum 1203.

[0219] Fig. 58(a) shows the positional relationship between the first and second light-emitting units 1206 and 1207 and the first and second gradient index lens arrays 1204-1 and 1204-2. Fig. 58(b) shows the positional relationship between the first and second gradient index lens arrays 1204-1 and 1204-2 and the photosensitive drum 1203.

[0220] In the exposure head 1205 of this embodiment, T=0.40 mm. Furthermore, in the exposure head 1205 of this embodiment, the first and second light-emitting units 1206, 1207 on the light-emitting substrate 1201 are positioned 0.2 mm parallel to the Z direction relative to Comparative Example 2 when viewed from the Y direction. W=0.28 mm. Therefore, W / T=0.70, which satisfies the condition of formula (1).

[0221] In this embodiment, as shown in Figures 55 and 58(a) and (b), the first and second light-emitting units 1206 and 1207 are positioned so that the sixth line does not pass through point c, which is the center of rotation of the photosensitive drum 1203.

[0222] In this embodiment, A = 0.2 mm, B = 30 mm, and D = 2.74 mm. Therefore, (B × T) / (D × A) = 21.9, which satisfies the condition of formula (6). Furthermore, T / D = 0.15, which satisfies the condition of formula (3).

[0223] Furthermore, in this embodiment, the eighth line passes through point c when viewed from the Y direction. As shown in Figure 58(a), the sixth line and the eighth line are parallel to each other.

[0224] FIG. 59 shows the relationship between the mounting error (rotation angle) of the exposure head 1205 and the amount of light on the photosensitive drum 1203 in this embodiment. The assumed mounting angle of the exposure head 1205 in this embodiment is 0°, as in Comparative Example 2. In this embodiment, as shown in FIG. 57(a), of the light beam B1 generated when the light beam A1 emitted from the first light-emitting unit 1206 is reflected by the photosensitive drum 1203, none of the light beams enter the first and second gradient index lens arrays 1204-1 and 1204-2. Therefore, as shown in FIG. 56(a), there is no multiple reflection light. As a result, as shown in FIG. 59, even when the exposure head 1205 rotates from 0° to +0.5°, the amount of light focused on the photosensitive drum 1203 is 100.00, and there is no fluctuation in the amount of light on the photosensitive drum 1203. Furthermore, when the exposure head 1205 is rotated by −0.5°, none of the light rays B1 generated when the light rays A1 emitted from the first light-emitting unit 1206 are reflected by the photosensitive drum 1203 enters the first and second refractive index gradient lens arrays 1204-1 and 1204-2. As a result, the amount of light condensed on the photosensitive drum 1203 becomes 100.00.

[0225] On the other hand, as shown in FIG. 57(b), the proportion of light rays B2 generated when light ray A2 emitted from the second light-emitting unit 1207 is reflected by the photosensitive drum 1203 increases, and the proportion of light rays B2 that enter the first and second gradient index lens arrays 1204-1 and 1204-2 increases. At this time, when the first and second light-emitting units 1206 and 1207 are translated 0.2 mm in the Z direction, the amount of light ray C2 as multiple reflection light increases, as shown in FIG. 56(b). Therefore, even if the exposure head 1205 is rotated ±0.5°, the increase or decrease in the multiple reflection light can be suppressed. As a result, as shown in FIG. 59, the amount of light condensed on the photosensitive drum 1203 when the exposure head 1205 is rotated +0.5° and -0.5° is 99.95.

[0226] From the above, in this embodiment, the fluctuation in the amount of light on the photosensitive drum 1203 is a maximum of 0.05% (=|99.95-100.00|). That is, in this embodiment, the fluctuation in the amount of light can be reduced by 17% compared to 0.06% (|0.06|) in Comparative Example 2.

[0227] In this way, in this embodiment, compared to Comparative Example 2, it is possible to suppress fluctuations in the amount of light on the photosensitive drum 1203 due to installation errors in the exposure head 1205, and it is possible to reduce changes in the density of the image formed on the photosensitive drum 1203 from the expected density. [Example]

[0228] Next, an exposure head of Example 12 will be described. The image forming apparatus in which the exposure head of Example 12 is used is similar to the image forming apparatus 1 described in Example 1. The exposure head of Example 12 differs from exposure head 405 of Example 3 in that it has no inclination (i.e., similar to Comparative Example 3) and the first and second light-emitting units are arranged by moving parallel to the photosensitive drum, but other configurations are similar to exposure head 405 of Example 3.

[0229] The exposure head 1305 of this embodiment will be described using Figures 60, 61(a) and 61(b), 62(a) and 62(b), 63(a) and 63(b), and 64. Figure 60 shows a ZX cross section as viewed from the Y direction, illustrating the arrangement of the exposure head 1305 (light-emitting substrate 1301 and gradient index lens array 1304) relative to the photosensitive drum 1303. Figures 61(a) and 61(b) show an enlarged view of the enlarged area 1 in Figure 60 as viewed from the Y direction. Figure 61(a) shows how a light ray A1 emitted from a first light-emitting unit 1306 is incident on the first and second gradient index lens arrays 1304-1 and 1304-2. Figure 61(b) shows how a light ray A2 emitted from a second light-emitting unit 1307 is incident on the first and second gradient index lens arrays 1304-1 and 1304-2.

[0230] Figures 62(a) and (b) show an enlarged view of enlarged region 2 in Figure 60. Figure 62(a) shows how light ray A1, which is emitted from the first light-emitting unit 1306 and then from the first and second gradient index lens arrays 1304-1 and 1304-2, is focused at point f on the photosensitive drum 1303, and also shows light ray B1 generated when light ray A1 is reflected by the photosensitive drum 1303. Figure 62(b) shows how light ray A2, which is emitted from the second light-emitting unit 1307 and then from the first and second gradient index lens arrays 1304-1 and 1304-2, is focused at point s on the photosensitive drum 1303, and also shows light ray B2 generated when light ray A2 is reflected by the photosensitive drum 1303.

[0231] Fig. 63(a) shows the positional relationship between the first and second light-emitting units 1306 and 1307 and the first and second gradient index lens arrays 1304-1 and 1304-2. Fig. 63(b) shows the positional relationship between the first and second gradient index lens arrays 1304-1 and 1304-2 and the photosensitive drum 1303.

[0232] In the exposure head 1305 of this embodiment, T=0.30 mm. Furthermore, in the exposure head 1305 of this embodiment, the first and second light-emitting units 1306, 1307 on the light-emitting substrate 1301 are positioned 0.15 mm parallel to the Z direction when viewed from the Y direction relative to Comparative Example 3. W=0.22 mm. Therefore, W / T=0.73, which satisfies the condition of formula (1).

[0233] In this embodiment, as shown in Figures 60 and 63(a) and (b), the first and second light-emitting units 1306 and 1307 are positioned so that the sixth line does not pass through point c, which is the center of rotation of the photosensitive drum 1303.

[0234] In this embodiment, A = 0.15 mm, B = 20 mm, and D = 2.74 mm. Therefore, (B × T) / (D × A) = 14.6, which satisfies the condition of formula (6). Furthermore, T / D = 0.11, which satisfies the condition of formula (3).

[0235] Furthermore, in this embodiment, the eighth line passes through point c when viewed from the Y direction. As shown in Figure 63(a), the sixth line and the eighth line are parallel to each other.

[0236] FIG. 64 shows the relationship between the mounting error (rotation angle) of the exposure head 1305 and the amount of light on the photosensitive drum 1303 in this embodiment. The assumed mounting angle of the exposure head 1305 in this embodiment is 0°, as in Comparative Example 3. In this embodiment, as shown in FIG. 62(a), of the light beam B1 generated when the light beam A1 emitted from the first light-emitting unit 1306 is reflected by the photosensitive drum 1303, none of the light beams enter the first and second gradient index lens arrays 1304-1 and 1304-2. Therefore, as shown in FIG. 61(a), there is no multiple reflection light. As a result, as shown in FIG. 64, even when the exposure head 1305 rotates from 0° to +0.5°, the amount of light focused on the photosensitive drum 1303 is 100.00, and there is no fluctuation in the amount of light on the photosensitive drum 1303. Furthermore, when the exposure head 1305 is rotated by −0.5°, none of the light rays B1 generated when the light rays A1 emitted from the first light-emitting unit 1306 are reflected by the photosensitive drum 1303 enters the first and second refractive index gradient lens arrays 1304-1 and 1304-2. As a result, the amount of light condensed on the photosensitive drum 1303 becomes 100.00.

[0237] On the other hand, as shown in FIG. 62(b), the proportion of light rays B2 generated when light ray A2 emitted from the second light-emitting unit 1307 is reflected by the photosensitive drum 1303 and incident on the first and second gradient index lens arrays 1304-1 and 1304-2 increases. At this time, when the first and second light-emitting units 1306 and 1307 are translated 0.15 mm in the Z direction, the amount of light ray C2 as multiple reflection light increases, as shown in FIG. 61(b). Therefore, even if the exposure head 1305 is rotated ±0.5°, the increase or decrease in the amount of multiple reflection light can be suppressed. As a result, as shown in FIG. 64, the amount of light condensed on the photosensitive drum 1303 when the exposure head 1305 is rotated +0.5° and -0.5° is 99.95.

[0238] From the above, in this embodiment, the fluctuation in the amount of light on the photosensitive drum 1303 is a maximum of 0.05% (=|99.95-100.00|). That is, in this embodiment, the fluctuation in the amount of light can be reduced by 67% compared to 0.15% (|0.15|) in Comparative Example 3.

[0239] In this way, in this embodiment, compared to Comparative Example 3, it is possible to suppress fluctuations in the amount of light on the photosensitive drum 1303 due to installation errors in the exposure head 1305, and it is possible to reduce changes in the density of the image formed on the photosensitive drum 1303 from the expected density.

[0240] The above embodiment includes the following configurations.

[0241] (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 focuses light from the first and second light-emitting units onto a rotating illuminated surface; 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, the midpoint between the centers of the first and second light-emitting elements is a first point, the center of rotation of the irradiated surface is a second point, a straight line passing through the first point and the second point is a first straight line, the center of an image of the first light-emitting element formed on the irradiated surface by the lens unit is a third point, the center of an image of the second light-emitting element formed on the irradiated surface by the lens unit is a fourth point, and the longer of the distances between the first straight line and the third and fourth points is W, 0.6≦W / T≦2.0 A light source device characterized by satisfying the following conditions. (Configuration 2) The light source device described in configuration 1, characterized in that when viewed from the first direction, a line passing through the centers of the first and second light-emitting units is defined as a second line, and a line passing through the third point and the fourth point is defined as a third line, the second line and the third line are non-parallel to each other. (Configuration 3) When viewed from the first direction, a fourth line is a line passing through a fifth point that is the center of the entrance surface of the lens included in the lens unit and a sixth point that is the center of the exit surface of the lens, a fifth line is a normal to the illuminated surface at a point between the third point and the fourth point that is closer to the first line, an angle formed by the fourth line and the fifth line is α, and a distance from the intersection of the fourth line and the second line to the intersection of the fourth line and the entrance surface of the lens is D, 0.15≦α / tan -1 (T / D)≦0.50 3. The light source device according to configuration 2, which satisfies the following conditions: (Configuration 4) 0.05≦T / D≦0.20 4. The light source device according to configuration 3, which satisfies the following conditions: (Configuration 5) 4. The light source device according to configuration 2 or 3, wherein when viewed from the first direction, a fourth line is defined as a line passing through a fifth point, which is the center of the entrance surface of the lens included in the lens unit, and a sixth point, which is the center of the exit surface of the lens, and the second line and the fourth line are perpendicular to each other. (Configuration 6) the lens unit includes first and second lens arrays each including a plurality of lenses arranged in the first direction; 6. The light source device according to any one of configurations 2 to 5, wherein, when viewed from the first direction, a line passing through the center of each of the entrance surface and exit surface of the lens included in the first lens array is defined as a fourth line of the first lens array, a line passing through the center of each of the entrance surface and exit surface of the lens included in the second lens array is defined as a fourth line of the second lens array, a distance from the intersection of the fourth line and the second line of the first lens array to the center of the first light-emitting unit is defined as T1, and a distance from the intersection of the fourth line and the second line of the second lens array to the center of the second light-emitting unit is defined as T2, T1 and T2 are equal to each other. (Configuration 7) When viewed from the first direction, a line passing through the center of the first light-emitting unit and the center of the second light-emitting unit is defined as a second line, and a line passing through the first point and perpendicular to the second line is defined as a sixth line. 2. The light source device according to configuration 1, wherein the sixth straight line is disposed so as not to pass through the second point relative to the illuminated surface. (Configuration 8) When viewed from the first direction, when the distance between the sixth straight line and the second point is A, the diameter of the irradiated surface is B, and the distance from the first point, which is the intersection point of the sixth straight line and the second straight line, to the intersection point of the sixth straight line and the incident surface of the lens included in the lens unit is D, 2≦(B×T) / (D×A)≦10 8. The light source device according to configuration 7, which satisfies the following conditions: (Configuration 9) 0.05≦T / D≦0.20 9. The light source device according to configuration 8, which satisfies the following conditions: (Configuration 10) When viewed from the first direction, a line passing through a fifth point that is the center of the entrance surface of the lens included in the lens unit and a sixth point that is the center of the exit surface of the lens is defined as a fourth line, 10. The light source device of any one of configurations 7 to 9, wherein the fourth straight line and the sixth straight line are parallel to each other. (Configuration 11) the lens unit includes first and second lens arrays each including a plurality of lenses arranged in the first direction; 11. The light source device of any one of configurations 7 to 10, wherein when viewed from the first direction, the sixth straight line passes through a seventh point that is the midpoint between the center of the incident surface of the lens included in the first lens array and the center of the incident surface of the lens included in the second lens array. (Configuration 12) When viewed from the first direction, a line passing through a fifth point that is the center of the entrance surface of the lens included in the lens unit and a sixth point that is the center of the exit surface of the lens is defined as a fourth line, a line perpendicular to the fourth line is defined as a seventh line, and a line passing through the third point and the fourth point is defined as a third line, 2. The light source device according to configuration 1, wherein the lens unit is disposed so that the third straight line and the seventh straight line are not parallel to each other. (Configuration 13) When viewed from the first direction, a line passing through the center of the first light-emitting unit and the center of the second light-emitting unit is a second line, a normal to the illuminated surface at a point between the third point and the fourth point that is closer to the first line is a fifth line, an angle formed by the fourth line and the fifth line is α, and a distance from an intersection point of the fourth line and the second line to an intersection point of the fourth line and the incident surface of the lens is D, 0.10≦α / tan -1 (T / D)≦0.20 13. The light source device according to configuration 12, which satisfies the following conditions: (Configuration 14) 0.05≦T / D≦0.20 14. The light source device according to configuration 13, which satisfies the following conditions: (Configuration 15) When viewed from the first direction, the angle between the second line and the fourth line is (90°-α) 15. The light source device according to configuration 13 or 14, wherein: (Configuration 16) When viewed from the first direction, a line passing through the center of the first light-emitting unit and the center of the second light-emitting unit is defined as a second line, and a line passing through the first point and perpendicular to the second line is defined as a sixth line. 2. The light source device according to configuration 1, wherein the first and second light emitting units are arranged so that the sixth line does not pass through the second point. (Configuration 17) the lens unit includes first and second lens arrays each including a plurality of lenses arranged in the first direction; When viewed from the first direction, when the distance between the sixth straight line and the second point is A, the diameter of the irradiated surface is B, the midpoint between the center of the incident surface of the lens included in the first lens array and the center of the incident surface of the lens included in the second lens array is a seventh point, the midpoint between the center of the exit surface of the first lens array and the center of the exit surface of the second lens array is an eighth point, a line passing through the seventh point and the eighth point is an eighth straight line, and a distance from the intersection of the eighth straight line and the second straight line to the intersection of the eighth straight line and the incident surface is D, 10≦(B×T) / (D×A)≦30 17. The light source device according to configuration 16, which satisfies the following conditions: (Configuration 18) 0.05≦T / D≦0.20 18. The light source device according to configuration 17, which satisfies the following conditions: (Configuration 19) the lens unit includes first and second lens arrays each including a plurality of lenses arranged in the first direction; The light source device described in configuration 16 or 18, characterized in that when viewed from the first direction, the midpoint between the center of the incident surface of the lens included in the first lens array and the center of the incident surface of the lens included in the second lens array is defined as a seventh point, the midpoint between the center of the exit surface of the first lens array and the center of the exit surface of the second lens array is defined as an eighth point, and a line passing through the seventh point and the eighth point is defined as an eighth line. (Configuration 20) the lens unit includes first and second lens arrays each including a plurality of lenses arranged in the first direction; 20. The light source device of any one of configurations 16 to 19, characterized in that when viewed from the first direction, the midpoint between the center of the incident surface of the lens included in the first lens array and the center of the incident surface of the lens included in the second lens array is defined as a seventh point, the midpoint between the center of the exit surface of the first lens array and the center of the exit surface of the second lens array is defined as an eighth point, and a line passing through the seventh point and the eighth point is defined as an eighth line, the sixth line and the eighth line are parallel to each other. (Configuration 21) 21. The light source device according to any one of configurations 1 to 20, wherein the lens unit is a gradient index lens unit. (Configuration 22) 22. The light source device according to any one of configurations 1 to 21, wherein the light emitted from the lens unit is irradiated onto the illuminated surface having a reflectance of 5% or more. (Configuration 23) 23. The light source device according to any one of configurations 1 to 22, wherein the surfaces on which the plurality of light emitting elements are provided in the first and second light emitting units have a reflectance of 10% or more. (Configuration 24) a light source device according to any one of configurations 1 to 23; and and a developing device for developing the electrostatic latent image formed on the surface to be irradiated by the light source device. (Other Examples) The present invention can also be realized by supplying a program that realizes one or more functions of the above-described embodiments to a system or device via a network or a storage medium, and having one or more processors in the computer of the system or device read and execute the program.The present invention can also be realized by a circuit (e.g., ASIC) that realizes one or more functions.

[0242] 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]

[0243] 1. Image forming device 103,303,403,503,603,703,803,903,1003,1103,1203,1303 Photosensitive drum 105,305,405,505,605,705,805,905,1005,1105,1205,1305 exposure head 204,304,404,504,604,704,804,904,1004,1104,1204,1304(-1,-2) Gradient index lens array 206,306,406,506,606,706,806,906,1006,1106,1206,1306 First light-emitting unit 207,307,407,507,607,707,807,907,1007,1107,1207,1307 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 focuses light from the first and second light-emitting units onto a rotating illuminated surface; 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, the midpoint between the centers of the first and second light-emitting elements is a first point, the center of rotation of the irradiated surface is a second point, a line passing through the first point and the second point is a first line, the center of an image of the first light-emitting element formed on the irradiated surface by the lens unit is a third point, the center of an image of the second light-emitting element formed on the irradiated surface by the lens unit is a fourth point, and the longer of the distances between the first line and the third and fourth points is W, 0.6≦W / T≦2.0 A light source device characterized by satisfying the following conditions.

2. 2. The light source device according to claim 1, wherein when viewed from the first direction, a line passing through the centers of the first and second light-emitting units is defined as a second line, and a line passing through the third point and the fourth point is defined as a third line, the second line and the third line are non-parallel to each other.

3. When viewed from the first direction, a fourth line is a line passing through a fifth point that is the center of the entrance surface of the lens included in the lens unit and a sixth point that is the center of the exit surface of the lens, a fifth line is a normal to the illuminated surface at a point between the third point and the fourth point that is closer to the first line, an angle formed by the fourth line and the fifth line is α, and a distance from the intersection of the fourth line and the second line to the intersection of the fourth line and the entrance surface of the lens is D, 0.15≦α / tan －１ (44)≦00.50 3. The light source device according to claim 2, wherein the following conditions are satisfied:

4. 0.05≦T / D≦0.20 4. The light source device according to claim 3, wherein the following conditions are satisfied:

5. 3. The light source device according to claim 2, wherein when viewed from the first direction, a fourth line is defined as a line passing through a fifth point, which is the center of the incident surface of the lens included in the lens unit, and a sixth point, which is the center of the exit surface of the lens, and the second line and the fourth line are perpendicular to each other.

6. the lens unit includes first and second lens arrays each including a plurality of lenses arranged in the first direction; 3. The light source device according to claim 2, wherein, when viewed from the first direction, a line passing through the center of each of the incident surface and the exit surface of the lens included in the first lens array is defined as a fourth line of the first lens array, a line passing through the center of each of the incident surface and the exit surface of the lens included in the second lens array is defined as a fourth line of the second lens array, a distance from the intersection of the fourth line and the second line of the first lens array to the center of the first light-emitting unit is defined as T1, and a distance from the intersection of the fourth line and the second line of the second lens array to the center of the second light-emitting unit is defined as T2, and T1 and T2 are equal to each other.

7. When viewed from the first direction, a line passing through the center of the first light-emitting unit and the center of the second light-emitting unit is defined as a second line, and a line passing through the first point and perpendicular to the second line is defined as a sixth line. The light source device according to claim 1 , wherein the sixth straight line is disposed so as not to pass through the second point with respect to the illuminated surface.

8. When viewed from the first direction, the distance between the sixth straight line and the second point is A, the diameter of the illuminated surface is B, and the distance from the first point, which is the intersection of the sixth straight line and the second straight line, to the intersection of the sixth straight line and the incident surface of the lens included in the lens unit is D. 2≦(B×T) / (D×A)≦10 8. The light source device according to claim 7, wherein the following condition is satisfied:

9. 0.05≦T / D≦0.20 9. The light source device according to claim 8, wherein the following condition is satisfied:

10. When viewed from the first direction, a line passing through a fifth point that is the center of the entrance surface of the lens included in the lens unit and a sixth point that is the center of the exit surface of the lens is defined as a fourth line, 8. The light source device according to claim 7, wherein the fourth straight line and the sixth straight line are parallel to each other.

11. the lens unit includes first and second lens arrays each including a plurality of lenses arranged in the first direction; 8. The light source device according to claim 7, wherein, when viewed from the first direction, the sixth straight line passes through a seventh point which is the midpoint between the center of the incident surface of the lens included in the first lens array and the center of the incident surface of the lens included in the second lens array.

12. When viewed from the first direction, a line passing through a fifth point that is the center of the incident surface of the lens included in the lens unit and a sixth point that is the center of the exit surface of the lens is defined as a fourth line, a line perpendicular to the fourth line is defined as a seventh line, and a line passing through the third point and the fourth point is defined as a third line, 2. The light source device according to claim 1, wherein the lens unit is disposed so that the third straight line and the seventh straight line are not parallel to each other.

13. When viewed from the first direction, a line passing through the center of the first light-emitting unit and the center of the second light-emitting unit is defined as a second line, a normal to the illuminated surface at one of the third point and the fourth point that is closer to the first line is defined as a fifth line, an angle formed by the fourth line and the fifth line is defined as α, and a distance from an intersection point of the fourth line and the second line to an intersection point of the fourth line and the incident surface of the lens is defined as D, 0.10≦α / tan －１ (44)≦00.200 13. The light source device according to claim 12, wherein the following condition is satisfied:

14. 0.05≦T / D≦0.20 14. The light source device according to claim 13, wherein the following condition is satisfied:

15. When viewed from the first direction, the angle between the second line and the fourth line is （９０°－α） 14. The light source device according to claim 13, wherein:

16. When viewed from the first direction, a line passing through the center of the first light-emitting unit and the center of the second light-emitting unit is defined as a second line, and a line passing through the first point and perpendicular to the second line is defined as a sixth line.

2. The light source device according to claim 1, wherein the first and second light emitting units are arranged so that the sixth line does not pass through the second point.

17. the lens unit includes first and second lens arrays each including a plurality of lenses arranged in the first direction; When viewed from the first direction, when the distance between the sixth straight line and the second point is A, the diameter of the irradiated surface is B, the midpoint between the center of the incident surface of the lens included in the first lens array and the center of the incident surface of the lens included in the second lens array is a seventh point, the midpoint between the center of the exit surface of the first lens array and the center of the exit surface of the second lens array is an eighth point, a line passing through the seventh point and the eighth point is an eighth line, and a distance from the intersection of the eighth line and the second line to the intersection of the eighth line and the incident surface is D, 10≦(B×T) / (D×A)≦30 17. The light source device according to claim 16, wherein the following condition is satisfied:

18. 0.05≦T / D≦0.20 18. The light source device according to claim 17, wherein the following condition is satisfied:

19. the lens unit includes first and second lens arrays each including a plurality of lenses arranged in the first direction; 17. The light source device of claim 16, wherein when viewed from the first direction, the midpoint between the center of the incident surface of the lens included in the first lens array and the center of the incident surface of the lens included in the second lens array is defined as a seventh point, the midpoint between the center of the exit surface of the first lens array and the center of the exit surface of the second lens array is defined as an eighth point, and a line passing through the seventh point and the eighth point is defined as an eighth line.

20. the lens unit includes first and second lens arrays each including a plurality of lenses arranged in the first direction; 17. The light source device of claim 16, wherein when viewed from the first direction, the midpoint between the center of the incident surface of the lens included in the first lens array and the center of the incident surface of the lens included in the second lens array is defined as a seventh point, the midpoint between the center of the exit surface of the first lens array and the center of the exit surface of the second lens array is defined as an eighth point, and a line passing through the seventh point and the eighth point is defined as an eighth line, the sixth line and the eighth line are parallel to each other.

21. 2. The light source device according to claim 1, wherein the lens unit is a gradient index lens unit.

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

23. 2. The light source device according to claim 1, wherein the surfaces of the first and second light emitting units on which the plurality of light emitting elements are provided have a reflectance of 10% or more.

24. A light source device according to any one of claims 1 to 23; and a developing device for developing the electrostatic latent image formed on the surface to be irradiated by the light source device.

Images