Exposure device and image forming device

Abstract

An object of the present invention is to suppress deterioration in print quality even when an image forming apparatus has been used for a long period of time. [Solution] A print head 1 (exposure device) exposes a photosensitive drum 11 (image carrier). The print head 1 includes an LED array (light-emitting element array) 3 in which multiple LEDs 31 are arranged in an X direction (first direction), and a lens array 2 in which multiple rod lenses 21 are arranged in the X direction and are positioned opposite the LED array 3 in a Z direction (second direction) perpendicular to the X direction. If the focal length of the rod lens 21 is L0, the distance between the LED array 3 and the lens array 2 in the Z direction is L1, and the distance between the lens array 2 and the photosensitive drum 11 in the Z direction is L2, then 105 μm≦L0−L1≦140 μm and 105 μm≦L0−L2≦140 μm hold true.

Description

[Technical Field]

[0001] The present disclosure relates to an exposure device and an image forming apparatus. [Background technology]

[0002] The image forming apparatus includes an exposure device that exposes the surface of an image carrier to light to form an electrostatic latent image. The exposure device includes a light-emitting element array in which a plurality of light-emitting elements are arranged in one direction, and a lens array in which a plurality of lens elements are arranged in one direction. Light emitted from the light-emitting elements passes through the lens elements and is focused on the surface of the image carrier.

[0003] Here, if there is variation in the characteristics of each lens element of the lens array, the light image formation state on the surface of the image carrier becomes non-uniform, which may result in a deterioration in print quality. For this reason, it has been proposed to make the distance L1 from the light-emitting element array to the lens array and the distance L2 from the lens array to the image carrier both 175 μm to 250 μm shorter than the focal length of the lens elements (for example, Patent Document 1). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent Publication No. 2017-128045 (see abstract) Summary of the Invention [Problem to be solved by the invention]

[0005] When an image forming apparatus is used for a long period of time, the sensitivity of the photosensitive layer on the surface of the image carrier decreases or the thickness of the photosensitive layer decreases (film thinning). In this case, even if the distances L1 and L2 are set as described above, it is not possible to prevent a decrease in print quality.

[0006] The present disclosure has been made to solve the above-mentioned problems, and aims to suppress deterioration in print quality even when an image forming apparatus is used for a long period of time. [Means for solving the problem]

[0007] The exposure apparatus of the present disclosure is an exposure apparatus that exposes an image carrier, and includes: a light-emitting element array in which a plurality of light-emitting elements are arranged in a first direction; and a lens array in which a plurality of lens elements are arranged in the first direction and arranged to face the light-emitting element array in a second direction perpendicular to the first direction. When the focal length of the lens element is L0, the distance between the light-emitting element array and the lens array in the second direction is L1, and the distance between the lens array and the image carrier in the second direction is L2, the following relationships hold: 105 μm≦L0−L1≦140 μm, and 105 μm≦L0−L2≦140 μm.

[0008] The image forming apparatus of the present disclosure includes the above-mentioned exposure device, an image carrier arranged opposite the exposure device, a development unit that develops the latent image formed on the image carrier by the exposure device, and a transfer unit that transfers the image developed by the development unit to a medium. [Effects of the Invention]

[0009] According to the present disclosure, since the above-mentioned distances L1, L2 and focal length L0 satisfy the conditions 105 μm≦L0-L1≦140 μm and 105 μm≦L0-L2≦140 μm, deterioration in print quality can be suppressed even when the image forming device is used for a long period of time. [Brief explanation of the drawings]

[0010] [Figure 1] 1 is a diagram illustrating an overall configuration of an image forming apparatus according to an embodiment; [Figure 2] FIG. 2 is a perspective view showing an exposure head according to the embodiment. [Figure 3] FIG. 2 is a partial cross-sectional perspective view showing a part of the exposure head according to the embodiment. [Figure 4] FIG. 2 is a partial cross-sectional perspective view showing lens elements of a lens array according to an embodiment. [Figure 5]1A is a diagram showing the focal length of a lens element according to an embodiment, and FIG. 1B is a diagram showing the positional relationship between an LED array, a lens array, and a photosensitive drum. [Figure 6] 1A and 1B are schematic diagrams showing the state of light collection on the surface of a photosensitive drum. [Figure 7] 10 is a graph showing experimental results regarding the relationship between the number of printed sheets and MTF. DETAILED DESCRIPTION OF THE INVENTION

[0011] <Configuration of image forming device> Fig. 1 is a diagram showing the configuration of an image forming apparatus 100 according to an embodiment. The image forming apparatus 100 shown in Fig. 1 forms images using electrophotography, and is, for example, a color printer.

[0012] The image forming apparatus 100 has a medium supply section 60, process units 10K, 10Y, 10M, and 10C as image forming sections, print heads 1K, 1Y, 1M, and 1C as exposure devices, a transfer unit 70, a fixing unit 80, and a medium discharge section 90.

[0013] The medium supply unit 60 supplies a medium M such as printing paper to the transfer unit 70. As the medium M (also referred to as a recording medium), in addition to printing paper, OHP sheets, envelopes, copy paper, special paper, etc. can be used.

[0014] The medium supply unit 60 has a medium cassette 61 that stores medium M, a paper feed roller 62 that sends the medium M from the medium cassette 61 one sheet at a time to a conveying path, and a pair of conveying rollers 63 that convey the medium M sent out by the paper feed roller 62 toward the transfer unit 70.

[0015] Process units 10K, 10Y, 10M, and 10C are arranged from upstream to downstream (here, from right to left) along the transport path of medium M, and form toner images of black (K), yellow (Y), magenta (M), and cyan (C), respectively.

[0016] The process units 10K, 10Y, 10M, and 10C include photosensitive drums 11K, 11Y, 11M, and 11C as image carriers, charging rollers 13K, 13Y, 13M, and 13C as charging members, and developing rollers 14K, 14Y, 14M, and 14C as developer carriers.

[0017] The process units 10K, 10Y, 10M, and 10C also include supply rollers 15K, 15Y, 15M, and 15C as supply members, developing blades 16K, 16Y, 16M, and 16C as regulating members, and toner cartridges 17K, 17Y, 17, and 17C as developer containers.

[0018] Each of the photoreceptor drums 11K, 11Y, 11M, and 11C has a cylindrical conductive support and a photosensitive layer covering the surface of the conductive support. The conductive support is made of a metal such as aluminum or stainless steel. The photosensitive layer has a structure in which a charge generation layer and a charge transport layer are stacked in this order.

[0019] A charging voltage is applied to the charging rollers 13K, 13Y, 13M, and 13C, respectively, and uniformly charges the surfaces of the photosensitive drums 11K, 11Y, 11M, and 11C.

[0020] A developing voltage is applied to the developing rollers 14K, 14Y, 14M, and 14C, respectively, and toner (developer) is attached to the electrostatic latent images formed on the surfaces of the photosensitive drums 11K, 11Y, 11M, and 11C to form toner images (developer images).

[0021] Supply voltages are applied to the supply rollers 15K, 15Y, 15M, and 15C, respectively, and the supply rollers 15K, 15Y, 15M, and 15C supply toner to the developing rollers 14K, 14Y, 14M, and 14C.

[0022] The developing blades 16K, 16Y, 16M, and 16C are metal blades pressed against the surfaces of the developing rollers 14K, 14Y, 14M, and 14C, and regulate the thickness of the toner layer formed on the surfaces of the developing rollers 14K, 14Y, 14M, and 14C.

[0023] The toner cartridges 17K, 17Y, 17K, and 17C are detachably attached to the main bodies of the process units 10K, 10Y, 10M, and 10C, respectively, and supply toner to the supply rollers 15K, 15Y, 15M, and 15C.

[0024] Of the process units 10K, 10Y, 10M, and 10C, the portions including the developing rollers 14K, 14Y, 14M, and 14C, the supply rollers 15K, 15Y, 15M, and 15C, and the developing blades 16K, 16Y, 16M, and 16C constitute a developing section.

[0025] Print heads 1K, 1Y, 1M, and 1C serving as exposure devices are arranged opposite the photosensitive drums 11K, 11Y, 11M, and 11C. Based on image data for each color, the print heads 1K, 1Y, 1M, and 1C expose the surfaces of the photosensitive drums 11K, 11Y, 11M, and 11C to light to form electrostatic latent images.

[0026] The transfer unit 70 is disposed below the process units 10K, 10Y, 10M, and 10C, and transfers the toner images formed by the process units 10K, 10Y, 10M, and 10C onto the medium M.

[0027] The transfer unit 70 includes a conveying belt 72 that adsorbs and runs the medium M, a drive roller 73 that drives the conveying belt 72, a tension roller 74 that applies tension to the conveying belt 72, and four transfer rollers 71 as transfer sections that face the photosensitive drums 11K, 11Y, 11M, and 11C via the conveying belt 72.

[0028] A transfer voltage is applied to the transfer rollers 71, which transfer the toner images on the photosensitive drums 11K, 11Y, 11M, and 11C onto the medium M on the conveyor belt 72. The transfer unit 70 may also include a belt cleaning member 75 that scrapes off toner adhering to the surface of the conveyor belt 72, and a waste toner storage unit 76 that stores the scraped off waste toner.

[0029] The fixing unit 80 is disposed downstream of the transfer unit 70 in the transport direction of the medium M, and applies heat and pressure to the toner image on the medium M to fix it to the medium M. The fixing unit 80 includes a fixing roller 81 incorporating a heat source, a pressure roller 82 that is pressed against the fixing roller 81 to form a fixing nip, and a temperature sensor 83 that detects the surface temperature of the fixing roller 81.

[0030] The medium discharge section 90 is disposed downstream of the fixing unit 80 and discharges the medium M from a discharge port after fixing has been completed. The medium discharge section 90 has a pair of discharge rollers 91, 92 that transport the medium M discharged from the fixing unit 80 and discharge it from the discharge port. A stacker section 93 is provided at the top of the image forming apparatus 100 to place the medium M discharged by the pair of discharge rollers 91, 92.

[0031] The image forming apparatus 100 has a housing 101 that houses these components, and an openable top cover 102 that covers the top of the housing 101. The print heads 1K, 1Y, 1M, and 1C are suspended and supported by this top cover 102. Also, inside the housing 101, passage sensors S1, S2, S3, and S4 that detect the passage of the medium M are arranged along the transport path of the medium M.

[0032] If the image forming apparatus 100 has a double-sided printing function, a re-conveying mechanism may be provided that turns over the medium M having the toner image fixed on its front surface and conveys it to the transfer unit 70.

[0033] Since process units 10K, 10Y, 10M, and 10C have a common configuration except for the toner they use, they will be referred to as "process unit 10" unless there is a need to distinguish them. The components of process units 10K, 10Y, 10M, and 10C will also be described without the letters K, Y, M, and C unless there is a need to distinguish them. Since print heads 1K, 1Y, 1M, and 1C also have a common configuration, they will be referred to as "print head 1" unless there is a need to distinguish them.

[0034] In the above configuration, the direction of the rotation axis of each photosensitive drum 11 of process units 10K, 10Y, 10M, and 10C is defined as the X direction. The direction of movement of medium M as it passes through process units 10K, 10Y, 10M, and 10C is defined as the Y direction (more specifically, the +Y direction).

[0035] The direction perpendicular to both the X and Y directions is defined as the Z direction. Here, the direction from the print head 1 toward the photosensitive drum 11 is defined as the +Z direction, and the opposite direction is defined as the -Z direction. The Z direction is, for example, the vertical direction, and the XY plane is, for example, the horizontal plane, but is not limited to these.

[0036] <Print head configuration> Next, the configuration of the print head 1 as an exposure device will be described. Figure 2 is a perspective view showing the appearance of the print head 1. Figure 3 is an enlarged perspective view of a portion of the print head 1, with a portion shown in cross section. The cross section in Figure 3 corresponds to the cross section indicated by reference symbol III in Figure 2.

[0037] 2, print head 1 is elongated, with its length in the X direction, its width in the Y direction, and its height in the Z direction. Print head 1 is attached to top cover 102 so as to maintain a fixed distance from the surface of photosensitive drum 11 (FIG. 1). The X direction is also referred to as the first direction (or main scanning direction), and the Z direction is also referred to as the second direction.

[0038] As shown in FIG. 3, the print head 1 includes an LED array 3 as a light-emitting element array, a mounting substrate 4 as a substrate, a lens array 2, and a holder 5 that holds these.

[0039] The LED array 3 is an array of a plurality of LEDs (light emitting diodes) 31 as light emitting elements arranged in the X direction. Each LED 31 in the LED array 3 is controlled by a drive circuit mounted on a mounting board 4, and emits light toward the photosensitive drum 11. Although LEDs are used as light emitting elements here, light emitting elements other than LEDs can also be used.

[0040] The LED array 3 is configured to support, for example, a resolution of 600 dpi or 1200 dpi. When the resolution is 600 dpi, the LEDs 31 are arranged at an arrangement pitch of 0.04233 mm in the X direction. When the resolution is 1200 dpi, the LEDs 31 are arranged at an arrangement pitch of 0.021167 mm in the X direction. The central wavelength of the light emitted from the LEDs 31 is, for example, 740 nm to 780 nm.

[0041] The mounting substrate 4 is a substrate made of, for example, glass epoxy resin. The LED array 3 is mounted on the surface of the mounting substrate 4. A drive circuit (driver IC) (not shown) for driving each LED 31 of the LED array 3 is also mounted on the mounting substrate 4. The drive circuit and each LED 31 are electrically connected by metal wires or the like.

[0042] The lens array 2 has a plurality of rod lenses 21 as lens elements that focus the light emitted from each LED 31 of the LED array 3 onto the surface of the photosensitive drum 11 (FIG. 1). The rod lenses 21 are arranged in one row or multiple rows (two rows in this example) in the X direction.

[0043] The rod lenses 21 of the lens array 2 are generally cylindrical transparent members with their axial direction aligned in the Z direction, and have an incident surface 21a facing the LED array 3 and an exit surface 21b facing the photosensitive drum 11. The rod lenses 21 are made of, for example, an acrylic resin that has high transparency and relatively high weather resistance.

[0044] 4 is a partially cutaway perspective view of rod lens 21. The central axis of rod lens 21 defines optical axis Ax. Rod lens 21 has incident surface 21a at its end face in the -Z direction and exit surface 21b at its end face in the +Z direction. Rod lens 21 also has an outer peripheral surface 21c facing radially outward.

[0045] The rod lens 21 has a light absorbing layer 24 on its outer circumferential portion (i.e., a portion of a predetermined thickness including the outer circumferential surface 21c), and a cylindrical lens portion 23 on the inner circumferential side of the light absorbing layer 24. The lens portion 23 has a refractive index distribution in which the refractive index decreases radially inward. For this reason, the rod lens 21 is also called a radial type refractive index distribution lens.

[0046] The light absorbing layer 24 is, for example, a medium having a refractive index substantially equal to that of the outermost periphery of the lens portion 23, in which a component that absorbs light (dye, pigment, etc.) is dispersed.

[0047] The focal length L0 of the rod lens 21 is, for example, 2.2 mm to 2.5 mm. The aperture angle of the rod lens 21 is, for example, 10° to 15°. The length Z1 of the rod lens 21 in the Z direction is, for example, 4.2 mm to 4.4 mm. The radius R1 of the rod lens 21 is, for example, 0.14 mm to 0.16 mm.

[0048] For example, a SELFOC (registered trademark) lens array "SLA-12E" (aperture angle 12°) can be used as the lens array 2 having such rod lenses 21. However, the rod lenses 21 are not limited to this.

[0049] By using a radial gradient index lens as the rod lens 21, light that is incident on the incident surface 21a of the rod lens 21 in a direction inclined with respect to the optical axis Ax can be emitted from the exit surface 21b without divergence and can be concentrated. In other words, the light emitted from the LED 31 can be used without waste.

[0050] 3, the rod lenses 21 are arranged, for example, in two rows to form a lens group 20. The lens group 20 has a first lens row 201 in which the rod lenses 21 are arranged at a predetermined pitch P in the X direction, and a second lens row 202 in which the rod lenses 21 are arranged at the same pitch P in the X direction.

[0051] The lens rows 201 and 202 are arranged adjacent to each other in the Y direction, and the rod lenses 21 of the second lens row 202 are arranged offset in the X direction by half (P / 2) of the pitch P relative to the rod lenses 21 of the first lens row 201. However, the rod lenses 21 are not limited to being arranged in this manner, and may be arranged in one row or three or more rows.

[0052] The lens group 20 is held between a pair of side plates 25, 26 on both sides in the Y direction. Both side plates 25, 26 are approximately rectangular plate members having the same dimension (height) in the Z direction as the length Z1 of the rod lens 21. An adhesive is filled in the gaps between adjacent rod lenses 21 and between the rod lenses 21 and the side plates 25, 26.

[0053] The holder 5 is made of, for example, structural resin. The holder 5 has a groove 51 extending in the X direction on the side (+Z side) facing the photosensitive drum 11. The lens array 2 is fixed inside the groove 51.

[0054] The gap between the groove 51 of the holder 5 and the lens array 2 is sealed with a sealing member (not shown) to prevent light or foreign matter from entering the print head 1. The holder 5 is formed with abutment surfaces 53 that abut against the -Z direction end surfaces of the side plates 25, 26 of the lens array 2 inserted into the groove 51, and position the lens array 2 in the Z direction.

[0055] The holder 5 has an accommodating recess 52 for accommodating the mounting board 4 on the side (+Z side) opposite the photosensitive drum 11. The holder 5 is formed with an abutment surface 54 that abuts against the surface in the +Z direction of the mounting board 4 accommodated in the accommodating recess 52 and positions the mounting board 4 in the Z direction. The holder 5 also has a hollow portion 55 between the groove portion 51 and the accommodating recess 52 in the Z direction.

[0056] The mounting substrate 4 is fixed to the accommodation recess 52 of the holder 5 so that the LED array 3 faces the +Z direction. A shield plate may be attached to the back side (+Z side) of the mounting substrate 4 to seal it.

[0057] The holder 5 is not limited to the configuration shown in Fig. 3. The holder 5 may be any holder that holds the lens array 2 and the LED array 3 at a predetermined distance in the Z direction (a distance L1 described later) and holds the lens array 2 at a predetermined distance in the Z direction from the photosensitive drum 11 (a distance L2 described later).

[0058] <Optical distance> Next, we will explain the arrangement of the LED 31 and the rod lens 21. Fig. 5(A) is a diagram for explaining the focal length L0 of the rod lens 21. Fig. 5(B) is a diagram for explaining the positional relationship between the LED 31, the rod lens 21, and the photosensitive drum 11.

[0059] 5(B), the rod lenses 21 of the lens array 2 are illustrated as being arranged in a single row. As described above, the rod lenses 21 are not limited to being arranged in a single row, but may be arranged in two or more rows. Also, in FIG. 5(B), the shape of the holder 5 is shown in a simplified form.

[0060] As shown in FIG. 5(A), the print head 1 is disposed so that light emitted from the LEDs 31 of the LED array 3 passes through the rod lenses 21 of the lens array 2 and is collected on the surface 11a of the photosensitive drum 11.

[0061] 5(A), OP denotes the object plane of the rod lens 21, and IP denotes the image plane of the rod lens 21. The distance from the entrance surface 21a of the rod lens 21 to the object plane OP is the focal length L0. The distance from the exit surface 21b of the rod lens 21 to the image plane IP is also the focal length L0.

[0062] 5(B), the distance L1 from the LED 31 to the incident surface 21a of the rod lens 21 is shorter than the focal length L0 by ΔL1 (>0). In other words, the distance L1 from the LED 31 to the incident surface 21a of the rod lens 21 is set to satisfy L1=L0-ΔL1.

[0063] Similarly, the distance L2 from the exit surface 21b of the rod lens 21 to the surface 11a of the photosensitive drum 11 is shorter than the focal length L0 by ΔL2 (>0). In other words, the distance L2 from the exit surface 21b of the rod lens 21 to the surface 11a of the photosensitive drum 11 is set to satisfy L2 = L0 - ΔL2.

[0064] It is desirable that both the distances ΔL1 and ΔL2 be 105 μm or more and 140 μm or less. In other words, it is desirable that both the distances L1 and L2 be 105 μm to 140 μm shorter than the focal length L0. The reason for this will be described later.

[0065] The distance L1 from the LED 31 to the incident surface 21a of the rod lens 21 can also be referred to as the distance L1 between the lens array 2 and the LED array 3. Similarly, the distance L2 from the exit surface 21b of the rod lens 21 to the surface 11a of the photosensitive drum 11 can also be referred to as the distance L2 between the lens array 2 and the photosensitive drum 11.

[0066] <Printing operation by image forming device> Next, a printing operation by the image forming apparatus 100 will be described with reference to Figures 1 and 3. The control unit of the image forming apparatus 100 starts an image forming operation based on a print command sent from a host device.

[0067] First, the paper feed roller 62 rotates, sending the media M in the media cassette 61 one by one onto the transport path. Then, the transport roller pair 63 rotates at a predetermined timing, transporting the media M sent onto the transport path to the transport belt 72. The transport belt 72 travels in the direction indicated by arrow B by the rotation of the drive roller 73, and adsorbs and holds the media M to transport them.

[0068] On the other hand, in the process units 10K, 10Y, 10M, and 10C, the surfaces of the photosensitive drums 11K, 11Y, 11M, and 11C are uniformly charged by the charging rollers 13K, 13Y, 13M, and 13C, respectively.

[0069] Furthermore, print heads 1K, 1Y, 1M, and 1C irradiate photoconductor drums 11K, 11Y, 11M, and 11C with light in accordance with image data for each color. The light emitted from print heads 1K, 1Y, 1M, and 1C irradiates the surfaces of photoconductor drums 11K, 11Y, 11M, and 11C, forming electrostatic latent images of the respective colors.

[0070] The electrostatic latent images formed on the surfaces of the photosensitive drums 11K, 11Y, 11M, and 11C are developed with toner by the developing rollers 14K, 14Y, 14M, and 14C to become toner images. As the conveyor belt 72 travels, the medium M passes between the process units 10K, 10Y, 10M, and 10C and the transfer roller 71, and at that time, the toner images formed on the surfaces of the photosensitive drums 11K, 11Y, 11M, and 11C are sequentially transferred to the medium M on the conveyor belt 72.

[0071] The medium M onto which the toner image has been transferred is sent to a fixing unit 80. In the fixing unit 80, the toner image is heated and pressurized by a fixing roller 81 and a pressure roller 82, causing the toner image to melt and be fixed to the medium M. The medium M onto which the toner image has been fixed is discharged to the outside of the image forming apparatus 100 by a pair of discharge rollers 91, 92, and is stacked on a stacker unit 93 provided on the top of the image forming apparatus 100.

[0072] <Print head operation> 5(B), in the print head 1, a voltage is applied to the LEDs 31 of the LED array 3 in accordance with image data by a drive circuit on the mounting board 4. When a voltage is applied, the LEDs 31 emit light 31L of a predetermined intensity.

[0073] Light 31L emitted from the LED 31 is incident on the incident surface 21a of the rod lens 21, travels through the rod lens 21, and is emitted from the exit surface 21b. Light 21L emitted from the exit surface 21b of the rod lens 21 is focused on the surface 11a of the photosensitive drum 11. The surface potential of the photosensitive layer on the surface 11a of the photosensitive drum 11 attenuates in the area irradiated with light, thereby forming an electrostatic latent image.

[0074] <Configuration for improving print quality> Here, the rod lenses 21 have a relatively narrow aperture angle (for example, 10 to 15 degrees) and a relatively high resolution. Therefore, due to variations in the characteristics of the rod lenses 21 of the lens array 2, the size of the light spot formed on the surface 11a of the photosensitive drum 11 is likely to vary depending on the position in the X direction.

[0075] In this case, the amount of toner adhering to the photosensitive drum 11 varies depending on the position in the X direction, which causes printing defects such as vertical streaks (streak-like unevenness extending circumferentially around the photosensitive drum 11) in the printed image.

[0076] In particular, when the distance L1 from the LED 31 to the incident surface 21a of the rod lens 21 and the distance L2 from the exit surface 21b of the rod lens 21 to the surface 11a of the photosensitive drum 11 are both equal to the focal length L0 (when L1 = L2 = L0), the light intensity distribution on the surface 11a of the photosensitive drum 11 becomes sharp, and changes in the size of the light spot are more likely to occur if there are variations in the characteristics of the rod lens 21.

[0077] Therefore, in this embodiment, as described above, the distance L1 from the LED 31 to the incident surface 21a of the rod lens 21 is set to be shorter than the focal length L0 by ΔL1 (L1=L0-ΔL1). Similarly, the distance L2 from the exit surface 21b of the rod lens 21 to the surface 11a of the photosensitive drum 11 is set to be shorter than the focal length L0 by ΔL2 (L2=L0-ΔL2).

[0078] In other words, the LED 31 is offset with respect to the object plane OP, and the surface 11a of the photosensitive drum 11 is offset with respect to the image plane IP.

[0079] This makes the light intensity distribution on the surface 11a of the photosensitive drum 11 gentler than when the distances L1 and L2 are equal to the focal length L0. As a result, even if there is variation in the characteristics of the rod lens 21, changes in the size of the light spot are less likely to appear.

[0080] In recent years, there has been a demand for maintaining high print quality over a long period of time in order to extend the life of the photosensitive drum 11. For example, there is a demand for maintaining high print quality even after the number of printed sheets reaches 50,000.

[0081] However, when the photosensitive drum 11 is used for a long period of time, the thickness of the photosensitive layer of the photosensitive drum 11 decreases (film thinning) or the sensitivity of the photosensitive layer decreases, and the imaging state of light on the surface 11a of the photosensitive drum 11 changes.

[0082] 6(A) and (B) are schematic diagrams showing the light intensity distribution corresponding to one light spot formed on the surface 11a of the photosensitive drum 11. Fig. 6(A) shows the light intensity distribution when the distances ΔL1 and ΔL2 are relatively small, and Fig. 6(B) shows the light intensity distribution when the distances ΔL1 and ΔL2 are relatively large.

[0083] As shown in FIGS. 6A and 6B, the smaller the distances ΔL1 and ΔL2, the sharper the light intensity distribution becomes, and the larger the distances ΔL1 and ΔL2, the gentler the light intensity distribution becomes.

[0084] Here, when the number of printed sheets of the image forming apparatus 100 reaches 50,000, the light imaging state changes from the initial state (number of printed sheets: 0), and the size of the light spot changes from D0 to D 50K changes to.

[0085] Comparing Figures 6(A) and (B), in Figure 6(B), the change in the size of the light spot from the initial state to the number of printed sheets of 50,000 is large, and the size of the light spot after printing 50,000 sheets is D 50K The size of the light spot D is large. 50K When the value of the square root of the light spot is large, adjacent light spots overlap each other, causing vertical stripes.

[0086] Therefore, in order to maintain high print quality despite changes such as film wear on the photosensitive drum 11, it is better that the values ​​of the distances ΔL1 and ΔL2 are not too large.

[0087] <Experimental Results> Here, we will explain an experiment to investigate the relationship between the values ​​of distances ΔL1 and ΔL2 and print quality, and the results. In the experiment, distances ΔL1 and ΔL2 were changed in several ways, and the change in print quality when image forming apparatus 100 printed 50,000 sheets was examined. Note that because distances ΔL1 and ΔL2 have the same value, the following explanation will be given as ΔL.

[0088] Print quality was evaluated using MTF (Modulation Transfer Function). MTF is an index that indicates the imaging performance of a printed image. The higher the MTF value, the better the imaging performance, resulting in a printed image with high contrast.

[0089] It has been confirmed that if the MTF is less than 79%, the overlap of adjacent light spots on the photosensitive drum 11 increases, making vertical stripes more likely to occur in printed images. Also, if the MTF exceeds 90%, the size of the light spot becomes very small, and the exposure time required to provide the energy necessary to form an electrostatic latent image exceeds the specified time.

[0090] Therefore, print quality was judged as "good" when the MTF was in the range of 79% to 90%, and as "poor" when the MTF was less than 79% or more than 90%.

[0091] To investigate the change in MTF with respect to the number of printed sheets, 50,000 sheets (i.e., 50K sheets) of A4-sized plain paper were printed using the image forming apparatus 100 shown in Figure 1 as the medium M. A black dot pattern with a print density (print duty) of 20% was printed as the print pattern. The MTF was measured every time 5,000 sheets (5K sheets) of printing was completed.

[0092] The MTF was measured using an MTF measuring device equipped with a CCD camera, etc. Each LED 31 of the LED array 3 was illuminated, and the light intensity distribution on the surface 11a of the photosensitive drum 11 was measured with the MTF measuring device to determine the maximum value Imax and minimum value Imin of the light intensity, and the MTF was calculated using the following formula (1). MTF=(Imax-Imin) / (Imax+Imin)×100 … (1)

[0093] 7 shows the relationship between the number of prints and MTF. The vertical axis represents MTF, and the horizontal axis represents the number of prints. The number of prints is a value obtained by the control unit of image forming apparatus 100 based on the drum count (the number of rotations of photosensitive drum 11).

[0094] The distance ΔL was changed to four values: 95 μm, 105 μm, 140 μm, and 150 μm. The life of the photosensitive drum 11 generally corresponds to 30,000 printed sheets, but here the maximum number of printed sheets was set to 50,000.

[0095] As shown in FIG. 7, when the distance ΔL is 95 μm, the MTF exceeds the good range (79% to 90%) when the number of printed sheets is 0.

[0096] In contrast, when the distance ΔL is 105 μm and 140 μm, the MTF is within the good range (79% to 90%) over the entire range from 0 to 50,000 prints (the range indicated by arrow A1 in FIG. 7).

[0097] On the other hand, when the distance ΔL is 150 μm, the MTF falls within the good range when the number of printed sheets is between 0 and 30,000 (the range indicated by arrow A2 in Figure 7), but falls below the good range when the number of printed sheets exceeds 30,000.

[0098] That is, when the distance ΔL is 150 μm, high print quality is maintained for the life of a typical photosensitive drum 11 (30,000 printed sheets), but it becomes difficult to maintain print quality once the number of printed sheets reaches 50,000.

[0099] In contrast, when the distance ΔL is 105 μm to 140 μm, the light intensity distribution is sharper than when the distance ΔL is 150 μm or more (see FIG. 6(A)), so even in situations where film wear on the photosensitive drum 11 occurs, the change in the size of the light spot is suppressed, and high print quality is maintained.

[0100] From the above results, it can be seen that when the distance ΔL is in the range of 105 μm or more and 140 μm or less, high print quality is maintained even when the photosensitive drum 11 is used for a long period of time.

[0101] In other words, by making the focal length L0 of the rod lens 21, the distance L1 between the LED array 3 and the lens array 2, and the distance L2 between the lens array 2 and the photosensitive drum 11 satisfy 105 μm≦L0-L1≦140 μm and 105 μm≦L0-L2≦140 μm, high printing quality can be maintained even if the photosensitive drum 11 is used for a long period of time.

[0102] <Effects of the embodiment> As described above, the print head 1 serving as an exposure device of this embodiment includes an LED array (light-emitting element array) 3 in which a plurality of LEDs (light-emitting elements) 31 are arranged in the X direction (first direction), and a lens array 2 in which a plurality of rod lenses (lens elements) 21 are arranged in the X direction and are disposed opposite the LED array 3 in the Z direction (second direction) perpendicular to the X direction. The focal length L0 of the rod lens 21, the distance L1 between the LED array 3 and the lens array 2 in the Z direction, and the distance L2 between the lens array 2 and the photosensitive drum 11 in the Z direction satisfy 105 μm≦L0−L1≦140 μm and 105 μm≦L0−L2≦140 μm.

[0103] This configuration makes it possible to suppress changes in the size of the light spot on the surface 11a of the photosensitive drum 11 in response to changes such as film wear that occur over long periods of use of the photosensitive drum 11. As a result, it is possible to suppress the occurrence of printing defects such as vertical streaks, and maintain high print quality over long periods of time.

[0104] Furthermore, since the opening angle of the rod lens 21 is greater than or equal to 10° and less than or equal to 15° and has a refractive index distribution in the radial direction, high resolution can be obtained and the light incident on the rod lens 21 from the LED 31 can be emitted without waste.

[0105] Furthermore, since the radius R1 of the rod lens 21 is 0.14 mm or more and 0.16 mm or less, the length Z1 in the Z direction is 4.2 mm or more and 4.4 mm or less, and the focal length L0 is 2.2 mm or more and 2.5 mm or less, high printing quality can be maintained while suppressing increases in manufacturing costs.

[0106] Furthermore, since the rod lens 21 has the light absorption layer 24 on the outer periphery, it is possible to prevent light from being emitted from the outer periphery surface 21c of the rod lens 21 and being incident on an adjacent rod lens 21.

[0107] In the above-described embodiment, a color printer (FIG. 1) has been described as an example of an image forming apparatus. However, the image forming apparatus of the present disclosure is not limited to a color printer and may be, for example, a single-color (monochrome) printer. Furthermore, the image forming apparatus is not limited to a printer and may be, for example, a copier, a facsimile machine, a multifunction machine, etc.

[0108] The above describes in detail the preferred embodiments of the present invention, but the present invention is not limited to the above embodiments, and various improvements and modifications can be made within the scope that does not deviate from the gist of the present invention.

[0109] Various aspects of the present disclosure are summarized below as appendices. (Appendix 1) An exposure device that exposes an image carrier, a light-emitting element array in which a plurality of light-emitting elements are arranged in a first direction; a lens array arranged to face the light-emitting element array in a second direction perpendicular to the first direction, and having a plurality of lens elements arranged in the first direction; Equipped with The focal length of the lens element is L0, a distance between the light-emitting element array and the lens array in the second direction is L1; When the distance between the lens array and the image carrier in the second direction is L2, 105 μm ≦ L0-L1 ≦ 140 μm, and 105μm≦L0-L2≦140μm An exposure apparatus characterized in that: (Appendix 2) The lens element has an aperture angle of 10° or more and 15° or less, and has a refractive index distribution in the radial direction. 2. An exposure apparatus according to claim 1, (Appendix 3) The aperture angle of the lens element is 12° 3. The exposure apparatus according to claim 1 or 2, (Appendix 4) The lens element comprises: The radius is 0.14 mm or more and 0.16 mm or less, The length in the second direction is 4.2 mm or more and 4.4 mm or less, The focal length is between 2.2mm and 2.5mm. 4. The exposure apparatus according to claim 1, wherein the exposure apparatus is a (Appendix 5) The lens element has a light absorbing layer on its periphery. 5. The exposure apparatus according to claim 1, wherein the exposure apparatus is a (Appendix 6) An exposure apparatus according to any one of appendices 1 to 5; an image carrier disposed opposite the exposure device; a developing unit that develops the latent image formed on the image carrier by the exposure device; a transfer section that transfers the image developed by the development section onto a medium; An image forming apparatus comprising: [Explanation of symbols]

[0110] 1, 1K, 1Y, 1M, 1C Print head (exposure device), 2 Lens array, 3 LED array (light-emitting element array), 4 Substrate, 5 Holder, 10, 10K, 10Y, 10M, 10C Process unit (image forming unit), 11, 11K, 11Y, 11M, 11C Photosensitive drum (image carrier), 11a Surface, 13, 13K, 13Y, 13M, 13C Charging roller (charging member), 14, 14K, 14Y, 14M, 14C Development roller (developer carrier), 15, 15K, 15Y, 15M, 15C Supply roller (developer supply member), 16, 16K, 16Y, 16M, 16C Development blade (developer regulating member), 20 Lens group, 21 Rod lens (lens element), 21a incident surface, 21b exit surface, 23 lens portion, 24 surface layer, 25, 26 substrate, 31 LED (light-emitting element), 60 medium supply portion, 70 transfer unit, 80 fixing device, 90 medium discharge portion, 100 printer (image forming apparatus), L1 distance between LED array (light-emitting element array) and lens array, L2 distance between lens array and photosensitive drum (image carrier), L0 focal length of rod lens (lens element).

Claims

1. An exposure device that exposes an image carrier, a light-emitting element array in which a plurality of light-emitting elements are arranged in a first direction; a lens array arranged to face the light-emitting element array in a second direction perpendicular to the first direction, and having a plurality of lens elements arranged in the first direction; Equipped with The focal length of the lens element is L0, a distance between the light-emitting element array and the lens array in the second direction is defined as L1; When the distance between the lens array and the image carrier in the second direction is L2, 105 μm≦L0−L1≦140 μm, and 105 μm≦L0−L2≦140 μm An exposure apparatus characterized in that:

2. The lens element has an aperture angle of 10° or more and 15° or less, and has a refractive index distribution in the radial direction.

2. An exposure apparatus according to claim 1.

3. The aperture angle of the lens element is 12° 3. The exposure apparatus according to claim 2.

4. The lens element comprises: A radius of 0.14 mm or more and 0.16 mm or less, a length in the second direction of 4.2 mm or more and 4.4 mm or less; A focal length of 2.2 mm or more and 2.5 mm or less 4. The exposure apparatus according to claim 1, wherein the exposure apparatus is a laser beam source.

5. The lens element has a light absorbing layer on its periphery.

4. The exposure apparatus according to claim 1, wherein the exposure apparatus is a laser beam source.

6. an exposure apparatus according to any one of claims 1 to 3; the image carrier disposed opposite the exposure device; a developing unit that develops the latent image formed on the image carrier by the exposure device; a transfer section that transfers the image developed by the development section onto a medium; An image forming apparatus comprising:

Images