Image forming apparatus and image forming method

Abstract

To adjust appropriately the amount of toner adhered to the photosensitive drum.SOLUTION: An image forming apparatus comprises: a photosensitive drum; an exposure head including a plurality of light-emitting unit; and a control unit that controls the photosensitive drum and the light-emitting units. The control unit can execute a first mode in which the drum is rotated at a first rotational speed and a second mode in which the photosensitive drum is rotated at a second rotational speed that is a / b times of the first rotational speed of the first mode. In the second mode in order to perform exposure for forming an image of one line on a sheet, the control unit controls a cycle of a light emission timing of the light-emitting units at the cycle of b / a times of a cycle of the first mode, sets c / d (c and d are integers) so that n×(c / d) is an integer and controls the light-emitting units to cause, from among the number of rows n, the light-emitting units of a number of rows (n×(c / d)) to emit light and to cause the light-emitting units of a number of rows (n×(d-c) / d)) to not emit light, and sets a drive current of the light-emitting units so that an amount of light of the plurality of light-emitting units matches (a×d) / (b×c) times of an amount of light in the first mode.SELECTED DRAWING: Figure 17

Description

[Technical Field]

[0001] The present invention relates to an image forming apparatus and an image forming method. [Background technology]

[0002] When forming an image using an electrophotographic image forming apparatus, an electrostatic latent image is first formed on the surface of a photosensitive drum by irradiating the surface of the photosensitive drum with light according to image data, then a developing unit attaches toner to the electrostatic latent image on the surface of the photosensitive drum to form a toner image, the toner image is transferred to a sheet, and the toner image transferred to the sheet is heated by a fixing unit to fix it to the sheet, thereby forming an image.

[0003] In order to accommodate multiple types of sheets, such as paper, such image forming apparatuses have a normal mode in which the sheet is transported at a normal speed and a low-speed mode in which the sheet is transported at a speed slower than that in the normal mode. Because the rotation speed of the photosensitive drum is related to the sheet transport speed, the rotation speed of the photosensitive drum is slower in the low-speed mode than in the normal mode. If the photosensitive drum surface is irradiated with light in the same manner when the rotation speeds of the photosensitive drum are different, the amount of light irradiation will be greater in the low-speed mode, resulting in excessive toner being deposited on the sheet.

[0004] Therefore, Patent Document 1 describes an image forming device that, when forming an image in a mode in which the rotation speed of the photosensitive drum is slow, generates a line synchronization signal at a shorter period than the line synchronization signal generated in normal mode, and adjusts the amount of exposure by controlling the number of times the exposure head emits light in synchronization with the line synchronization signal, thereby adjusting the amount of toner placed on the photosensitive drum. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2022-191734 Summary of the Invention [Problem to be solved by the invention]

[0006] However, with the above-mentioned technology, there are cases where the amount of exposure cannot be adequately controlled depending on the rotation speed of the photosensitive drum, and there is a problem in that the amount of toner placed on the photosensitive drum cannot be appropriately adjusted.

[0007] Therefore, an object of the present invention is to provide an image forming apparatus that can appropriately adjust the toner placed on the photosensitive drum even when forming an image in a mode in which the rotation speed of the photosensitive drum is different. [Means for solving the problem]

[0008] In order to solve this problem, for example, an image forming apparatus according to the present invention has the following configuration: a photosensitive drum that rotates around a rotation axis; an exposure head having a plurality of light-emitting units arranged in n columns along a direction intersecting the rotation axis direction and emitting light based on image data signals to expose the photosensitive drum; an image forming apparatus including a control unit that controls a rotation speed of the photosensitive drum and light emission of the light emitting unit, The control unit a first mode in which the photosensitive drum is rotated at a first rotation speed, and a second mode in which the photosensitive drum is rotated at a second rotation speed that is slower than the first rotation speed and is a / b times (a and b are integers) the first rotation speed; In the second mode, in order to perform exposure for forming an image of one line along the rotation axis direction on a sheet, A cycle of the timing at which the light emitting unit emits light is controlled to a cycle that is b / a times the cycle of the first mode, setting c / d (c and d are integers) such that n×(c / d) is an integer, and controlling the light-emitting units so that the number of columns (n×(c / d)) of the number of columns n emit light and the number of columns (n×((dc) / d)) of the light-emitting units does not emit light; The drive current for the light emitting units is set so that the light intensity of the plurality of light emitting units is equal to (a×d) / (b×c) times the light intensity in the first mode. [Effects of the Invention]

[0009] According to the present invention, in an image forming apparatus, the toner carried on the photosensitive drum can be appropriately adjusted regardless of the rotation speed of the photosensitive drum. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 1 is a schematic cross-sectional view of an image forming apparatus. [Figure 2] 3A and 3B are a perspective view and a cross-sectional view of a photosensitive drum and an exposure head. [Figure 3] FIG. 2 is a diagram showing the mounting surface of a printed circuit board provided in the exposure head. [Figure 4] FIG. 1 is a schematic diagram of a light-emitting element array chip. [Figure 5] FIG. [Figure 6] FIG. 3 is a schematic diagram for explaining the arrangement of light-emitting units. [Figure 7] 5A and 5B are schematic diagrams for explaining irradiation positions on a photosensitive drum of light emitted from two light-emitting elements adjacent to each other in the sub-scanning direction. [Figure 8] FIG. 2 is a block diagram showing the system configuration of an image controller unit and an exposure head. [Figure 9] FIG. 2 is a block diagram showing the configuration of a chip data conversion unit. [Figure 10] FIG. 2 is a block diagram showing a system configuration of a light-emitting element array chip. [Figure 11] FIG. 3 is a circuit diagram of an image data storage unit. [Figure 12] 10 is a timing chart showing the operation in the main scanning direction in the image data storage unit. [Figure 13] 10 is a timing chart showing the operation in the sub-scanning direction in the image data storage unit in the normal mode. [Figure 14] FIG. 2 is a block diagram showing the configuration of an analog section. [Figure 15] Circuit diagram of the drive unit. [Figure 16] 10 is a timing chart showing light intensity adjustment by thinning-out lighting control in units of one line in the main scanning direction. [Figure 17] 6 is a timing chart showing the operation in the sub-scanning direction in the low-speed mode in the image data storage unit of the first embodiment. [Figure 18] 10 is a timing chart showing the operation in the sub-scanning direction in the low-speed mode in the image data storage unit of the second embodiment. [Figure 19] 10 is a timing chart showing light intensity adjustment by lighting control using a period-divided line synchronization signal according to the third embodiment. [Figure 20] 10 is a timing chart showing the operation in the sub-scanning direction in the low-speed mode in the image data storage unit of the third embodiment. [Figure 21] 10 is a timing chart showing the operation in the sub-scanning direction in the low-speed mode in the image data storage unit of the fourth embodiment. [Figure 22] 13 is a timing chart showing the operation in the sub-scanning direction in the low-speed mode in the image data storage unit of the fifth embodiment. [Figure 23] 10A and 10B are schematic diagrams illustrating the arrangement of light-emitting units in another embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0011] Hereinafter, embodiments will be described in detail with reference to the accompanying drawings. Note that the following embodiments do not limit the scope of the claimed invention. Although multiple features are described in the embodiments, not all of these multiple features are necessarily essential to the invention, and multiple features may be combined arbitrarily. Furthermore, in the accompanying drawings, the same reference numerals are used to designate the same or similar components, and redundant explanations will be omitted.

[0012] [First embodiment] <Image forming device> The overall configuration of the image forming apparatus A according to this embodiment will be described below with reference to the drawings, along with its operation during image formation. Note that the dimensions, materials, shapes, relative positions, etc. of the components described below are not intended to limit the scope of the present invention unless otherwise specified.

[0013] Image forming apparatus A is a full-color image forming apparatus that forms an image by transferring toner of four colors, yellow Y, magenta M, cyan C, and black K, onto a sheet. In the following description, components that use toner of each color are given the suffixes Y, M, C, and K, but the configuration and operation of each component are essentially the same except for the color of the toner used, so the suffixes will be omitted as appropriate unless a distinction is required.

[0014] FIG. 1 is a schematic cross-sectional view of an image forming apparatus A. As shown in FIG. 1, the image forming apparatus A has an image forming unit that forms an image. The image forming unit includes photosensitive drums 1 (1Y, 1M, 1C, 1K), charging devices 2 (2Y, 2M, 2C, 2K), and exposure heads 6 (6Y, 6M, 6C, 6K). The image forming unit also includes developing devices 4 (4Y, 4M, 4C, 4K) as a developing unit, transfer devices 5 (5Y, 5M, 5C, 5K) as a transfer unit, and a conveyor belt 11.

[0015] Image forming apparatus A also includes a touch panel type operation unit 300 (detection unit) that a user operates to make various settings related to image formation. By operating operation unit 300, a user can specify the number of sheets to be imaged, the size of the sheets on which the images are to be formed, and the like. By operating operation unit 300, a user can also input the basis weight (e.g., cardboard, plain paper, sheet brand, etc.) of sheets S stored in sheet cassettes 99a, 99b. Note that the basis weight of sheets S may not be input from operation unit 300, but may instead be measured by a sensor 38 (detection unit) disposed within the conveyance path, for example.

[0016] The sensor 38 for measuring the basis weight may be, for example, a transmission-type optical sensor. A transmission-type optical sensor is used by combining a light-emitting element such as an LED with a light-receiving element such as a photodiode. When the sheet S blocks the light emitted from the light-emitting element and directed toward the light-receiving element, the amount of light received by the light-receiving element decreases. The light-receiving element converts the amount of light received into a voltage value and functions as a switch by setting a predetermined threshold for the voltage amount.

[0017] Generally, the thicker the sheet S on which an image is formed, the greater the basis weight. In the case of thin paper with a low basis weight, the light emitted from the LED passes through the sheet S. In the case of thick paper with a high basis weight, almost no light emitted from the LED passes through. Therefore, by focusing on the relationship between the sensor 38 and the basis weight of the sheet S, and measuring the amount of light received by the light-receiving element at the leading or trailing edge of the sheet S passing between the sensor 38 with the light-emitting element's light emission level kept constant, the basis weight of the sheet S can be predicted. This measurement position is intended to improve measurement accuracy by utilizing the leading or trailing edge of the sheet S, where images tend to be less likely to be formed compared to the center.

[0018] 8. The CPU 73 controls the rotation speed of various rollers that transport the sheet S in accordance with the basis weight of the sheet S detected by the operation unit 300 or the sensor 38, and sets the transport speed of the sheet S during image formation. The transport speed here is defined as the speed of the sheet S when it passes through the fixing device 94 (fixing unit).

[0019] For example, when executing a low-speed mode in which an image is formed on thick paper having a basis weight of a predetermined or greater, the CPU 73 forms an image by setting the conveyance speed of the sheet S to a lower speed than when executing a normal mode in which an image is formed on plain paper having a basis weight of a less than predetermined value. Furthermore, since the rotation speed of the photosensitive drum 1 is related to the conveyance speed of the sheet S, when executing the low-speed mode (second mode), the CPU 73 sets the rotation speed of the photosensitive drum 1 to a lower speed than when executing the normal mode (first mode).

[0020] This increases the time for which the fixing device 94 heats the toner image carried on the sheet S with a large basis weight, thereby applying more heat to the toner image and the sheet S, and enabling the toner image to be stably fixed even on the sheet S with a large heat capacity. The timing at which the CPU 73 switches between the low-speed mode and the normal mode is not limited to the above timing. For example, the CPU 73 may be configured to set the low-speed mode when forming an image with higher image quality than normal, and to increase the resolution in the sub-scanning direction, which is the direction of rotation of the photosensitive drum 1, compared to the normal mode.

[0021] Next, a description will be given of the image forming operation by the image forming apparatus A. When forming an image, first, a sheet S stored in a sheet cassette 99a or a sheet cassette 99b is sent to a registration roller 96 by pickup rollers 91a and 91b, feeding rollers 92a and 92b, and conveying rollers 93a to 93c. Thereafter, the sheet S is sent to the conveyor belt 11 by the registration roller 96 at a predetermined timing.

[0022] Meanwhile, in the image forming unit, the surface of the photosensitive drum 1Y is first charged by the charging device 2Y. Next, the exposure head 6Y irradiates the surface of the photosensitive drum 1Y with light in accordance with image data read by the image reading unit 90 or image data transmitted from an external device (not shown), thereby forming an electrostatic latent image on the surface of the photosensitive drum 1Y. Thereafter, the developing device 4Y causes yellow toner to adhere to the electrostatic latent image formed on the surface of the photosensitive drum 1Y, thereby forming a yellow toner image on the surface of the photosensitive drum 1Y. A transfer bias is applied to the transfer device 5Y, and the toner image formed on the surface of the photosensitive drum 1Y is transferred to the sheet S being transported by the transport belt 11.

[0023] Using a similar process, photosensitive drums 1M, 1C, and 1K are also irradiated with light from exposure heads 6M, 6C, and 6K to form electrostatic latent images, and magenta, cyan, and black toner images are then formed by developing devices 4M, 4C, and 4K. Then, by applying a transfer bias to transfer devices 5M, 5C, and 5K, these toner images are transferred and superimposed on the yellow toner image on sheet S. As a result, a full-color toner image corresponding to the image data is formed on the surface of sheet S.

[0024] Thereafter, the sheet S carrying the toner image is conveyed by a conveyor belt 97 to a fixing device 94, where it is subjected to a heat and pressure treatment. This fixes the toner image on the sheet S to the sheet S. Thereafter, the sheet S with the fixed toner image is discharged onto a discharge tray 95 by a discharge roller 98.

[0025] <Exposure head> Next, the configuration of the exposure head 6 will be described.

[0026] Fig. 2(a) is a perspective view of the photosensitive drum 1 and the exposure head 6. Fig. 2(b) is a cross-sectional view of the photosensitive drum 1 and the exposure head 6. Figs. 3(a) and 3(b) are diagrams showing the mounting surfaces on one side and the other side of the printed circuit board 22 provided in the exposure head 6. Fig. 3(c) is an enlarged view of area V shown in Fig. 3(b).

[0027] 2, the exposure head 6 is fixed by a fixing member (not shown) at a position facing the surface of the photosensitive drum 1. The exposure head 6 has a light-emitting element array chip 40 that emits light, and a printed circuit board 22 on which the light-emitting element array chip 40 is mounted. The exposure head 6 also has a rod lens array 23 that focuses (focuses) the light emitted from the light-emitting element array chip 40 on the photosensitive drum 1, and a housing 24 to which the rod lens array 23 and the printed circuit board 22 are fixed.

[0028] A connector 21 is mounted on the surface of the printed circuit board 22 opposite to the surface on which the light-emitting element array chip 40 is mounted. The connector 21 is provided for transmitting control signals for the light-emitting element array chip 40 sent from the image controller unit 70 (FIG. 8) and for connecting a power supply line. The light-emitting element array chip 40 is driven via the connector 21.

[0029] As shown in Fig. 3, 20 light-emitting element array chips 40 are mounted on the printed circuit board 22 in a staggered arrangement in two rows. Each light-emitting element array chip 40 has 748 light-emitting units 50 arranged at a predetermined resolution pitch in its longitudinal direction (arrow X direction). Each light-emitting element array chip 40 also has light-emitting units 50 arranged at a predetermined pitch in its lateral direction (arrow Y direction). That is, in each light-emitting element array chip 40, the light-emitting units 50 are two-dimensionally arranged in the arrow X direction and the arrow Y direction, which is a direction perpendicular to (intersecting) the arrow X direction.

[0030] In this embodiment, the resolution pitch of the light-emitting element array chip 40 is 1200 dpi (approximately 21.16 μm). The distance from one end to the other end of the light-emitting section 50 in the longitudinal direction of each light-emitting element array chip 40 is approximately 15.8 mm. That is, the exposure head 6 has a total of 14,960 light-emitting sections 50 in the direction of arrow X, which enables exposure processing corresponding to an image width in the longitudinal direction of approximately 316 mm (≈ approximately 15.8 mm × 20 chips).

[0031] In the longitudinal direction of the light-emitting element array chips 40, the interval L1 between the light-emitting sections 50 of adjacent light-emitting element array chips 40 is approximately 21.16 μm. In other words, the longitudinal pitch of the light-emitting sections 50 at the boundary between each light-emitting element array chip 40 is a pitch corresponding to a resolution of 1200 dpi. In addition, in the lateral direction of the light-emitting element array chips 40 (direction of arrow Y), the interval L2 between the light-emitting sections 50 of the light-emitting element array chips 40 arranged in two rows is approximately 105 μm (equivalent to five pixels at 1200 dpi). Note that this embodiment is also applicable to configurations in which the resolution pitch of the light-emitting element array chips 40 and the light-emitting sections 50 is a value other than 1200 dpi.

[0032] In this embodiment, the arrow X direction, which is the longitudinal direction of the light-emitting element array chip 40, is the direction of the rotation axis of the photosensitive drum 1, and the arrow Y direction, which is the lateral direction of the light-emitting element array chip 40, is the rotation direction of the photosensitive drum 1. The arrow Z direction is the stacking direction in which each layer of the light-emitting section 50 having a layered structure described below overlaps, and is also the light emission direction of the light-emitting section 50. The longitudinal direction of the light-emitting element array chip 40 may be inclined by about ±1° with respect to the rotation axis direction of the photosensitive drum 1. The lateral direction of the light-emitting element array chip 40 may also be inclined by about ±1° with respect to the rotation direction of the photosensitive drum 1.

[0033] <Light-emitting element array chip> Next, the configuration of the light-emitting element array chip 40 will be described.

[0034] Fig. 4 is a schematic diagram of the light-emitting element array chip 40. Fig. 5 is a cross-sectional view taken along the MM cross section shown in Fig. 4. Fig. 6 is a schematic diagram for explaining the arrangement of the light-emitting sections 50.

[0035] 4, the light-emitting element array chip 40 has a light-emitting substrate 42 incorporating a circuit section 46 for controlling the light-emitting sections 50, a light-emitting region 44 in which a plurality of light-emitting sections 50 are regularly arranged on the light-emitting substrate 42, and wire-bonding pads 48. Signals are input and output between the outside of the light-emitting element array chip 40 and the circuit section 46, and power is supplied to the circuit section 46 through the wire-bonding pads 48. The circuit section 46 can be an analog drive circuit, a digital control circuit, or a circuit including both.

[0036] As shown in FIG. 5, the light-emitting section 50 is composed of a light-emitting substrate 42, a plurality of lower electrodes 54 two-dimensionally arranged on the light-emitting substrate 42 at regular intervals (intervals d1 and d2 shown in FIG. 6) in the directions of arrows X and Y, a light-emitting layer 56, and an upper electrode 58.

[0037] The lower electrodes 54 (first electrode layer having a plurality of electrodes) are a plurality of electrodes formed in a layered and separated manner on the light emitting substrate 42, and are electrodes provided corresponding to each pixel. In other words, each lower electrode 54 is provided to form one pixel.

[0038] The upper electrode 58 (second electrode layer) is laminated on the light-emitting layer 56 at a position opposite to the side on which the lower electrode 54 is disposed with respect to the light-emitting layer 56. The upper electrode 58 is an electrode that can transmit (be transparent to) light of the emission wavelength of the light-emitting layer 56.

[0039] The circuit unit 46 controls the potential of the selected lower electrode 54 based on a control signal generated in response to image data, thereby generating a potential difference between the selected lower electrode 54 and the upper electrode 58. When a potential difference is generated between the upper electrode 58, which is an anode, and the lower electrode 54, which is a cathode, electrons flow from the cathode into the light-emitting layer 56, and holes flow from the anode into the light-emitting layer 56. The recombination of the electrons and holes in the light-emitting layer 56 causes the light-emitting layer 56 to emit light.

[0040] When the light-emitting layer 56 emits light, the light that travels toward the upper electrode 58 passes through the upper electrode 58 and is emitted. Light that travels from the light-emitting layer 56 toward the lower electrode 54 is reflected by the lower electrode 54 toward the upper electrode 58, and this reflected light also passes through the upper electrode 58 and is emitted. In this manner, the light-emitting section 50 emits light. Note that although there is a time difference in the emission timing between the light that is emitted from the light-emitting layer 56 directly toward the upper electrode 58 and the light that is reflected by the lower electrode 54 and emitted from the upper electrode 58, the emission can be considered to be nearly simultaneous because the layer thickness of the light-emitting section 50 is extremely thin.

[0041] In this embodiment, the light emitting substrate 42 is a silicon substrate. The upper electrode 58 is preferably transparent to the emission wavelength of the light emitting layer 56. For example, by using a transparent electrode such as indium tin oxide (ITO), the aperture ratio becomes substantially 100%, and light emitted from the light emitting layer 56 is emitted directly through the upper electrode 58. In this embodiment, the upper electrode 58 is an anode provided in common to each of the lower electrodes 54, but it may also be configured so that an individual upper electrode 58 is provided for each of the lower electrodes 54, or so that one upper electrode 58 is provided for each of the plurality of lower electrodes 54.

[0042] The light-emitting layer 56 may be an organic EL film or an inorganic EL layer. When an organic EL film is used as the light-emitting layer 56, the light-emitting layer 56 may be a laminated structure including functional layers such as an electron transport layer, a hole transport layer, an electron injection layer, a hole injection layer, an electron blocking layer, and a hole blocking layer as needed. The light-emitting layer 56 may be formed continuously in the direction of arrow X, or may be divided into segments of the same size as the lower electrodes 54. Alternatively, each lower electrode 54 may be divided into multiple groups, and one light-emitting layer 56 may be laminated on top of the lower electrodes 54 belonging to each group.

[0043] When using a moisture-sensitive light-emitting material such as an organic EL layer or an inorganic EL layer as the light-emitting layer 56, it is desirable to seal the light-emitting region 44 to prevent moisture from entering. For example, a sealing film is formed by forming a thin film, either alone or in layers, of silicon oxide, silicon nitride, aluminum oxide, or the like. A method that excels in covering structures such as steps is preferred as a method for forming the sealing film, and atomic layer deposition (ALD) can be used, for example. The materials, configurations, and formation methods of the sealing film are merely examples, and are not limited to the above examples; any suitable method may be selected as appropriate.

[0044] The lower electrode 54 is preferably made of a metal having a high reflectivity at the wavelength of light emitted by the light-emitting layer 56. For example, Ag, Al, or an alloy of Ag and Al can be used. The lower electrode 54 is formed using a Si process together with the formation of the circuit section 46 and is directly connected to the drive section of the circuit section 46. Forming the lower electrode 54 using a Si process in this manner allows for high precision with a process rule of approximately 0.2 μm, allowing the lower electrodes 54 to be arranged with high precision and high density. Furthermore, since the lower electrodes 54 can be arranged with high density, most of the light-emitting region 44 can be made to emit light, thereby improving the utilization efficiency of the light-emitting region 44. The organic material of the light-emitting layer 56 is filled between each of the lower electrodes 54, and each of the lower electrodes 54 is separated by the organic material.

[0045] Here, the current flowing through the light-emitting unit 50 is approximately proportional to the emitted light intensity. Therefore, the light intensity of the light-emitting unit 50 can be controlled by controlling the current flowing through the light-emitting unit 50. The light-emitting unit 50 also has a threshold voltage. When the voltage across the light-emitting unit 50 exceeds the threshold voltage, current begins to flow through the light-emitting unit 50, and the current then flows approximately linearly. Because the threshold voltages of the light-emitting units 50 vary, the voltages at which current begins to flow for each light-emitting unit 50 vary slightly. Therefore, prior to product shipment from the factory, the light-emitting units 50 of the light-emitting element array chip 40 are individually and sequentially illuminated, and the current flowing through the light-emitting unit 50 is adjusted so that the light focused through the rod lens array 23 has a predetermined light intensity. Prior to product shipment from the factory, the exposure head 6 is not only subjected to the above-described light intensity adjustment, but also to focus adjustment, which adjusts the distance between the light-emitting element array chip 40 and the rod lens array 23.

[0046] 6, the light emitting units 50 are two-dimensionally arranged in the light emitting region 44 at predetermined intervals in the directions of the arrow X and the arrow Y. That is, in the light emitting region 44 of one light emitting element array chip 40, a plurality of light emitting units 50 are arranged along the direction of the arrow X, and a light emitting column is formed by the plurality of light emitting units 50 arranged along the direction of the arrow X. These light emitting columns are arranged in multiple rows in the direction of the arrow Y, and in this embodiment, the number of rows n of the light emitting columns is six. Note that this embodiment is also applicable to configurations in which the number of rows of the light emitting columns is a value other than six.

[0047] In this embodiment, the width W1 of the light-emitting section 50 in the arrow X direction is 20.90 μm, and the interval d1 between adjacent light-emitting sections 50 in the arrow X direction is 0.26 μm. That is, the light-emitting sections 50 are arranged at a pitch of 21.16 μm (1200 dpi) in the arrow X direction. The width W2 of the light-emitting section 50 in the arrow Y direction is also 20.90 μm, like the width W1, and the interval d2 is also 0.26 μm, like the interval d1, so the light-emitting sections 50 are also arranged at a pitch of 21.16 μm (1200 dpi) in the arrow Y direction. That is, the light-emitting section 50 in this embodiment is square-shaped with sides of 20.90 μm, and its area is 436.81 μm. 2This means that the area of ​​one pixel is 447.7456 μm 2 The organic light-emitting material emits less light than an LED. In response to this, by making the light-emitting sections 50 square and reducing the distance between adjacent light-emitting sections 50 as described above, it is possible to ensure a light-emitting area large enough to obtain a light amount sufficient to change the potential of the photosensitive drum 1.

[0048] It is desirable to ensure that the area of ​​the light-emitting unit 50 is 90% or more of the area occupied by one pixel. Therefore, for an image forming apparatus A with an output resolution of 1200 dpi, it is desirable to form the light-emitting unit 50 with a side width of approximately 20.07 μm or more. Furthermore, for an image forming apparatus A with an output resolution of 2400 dpi, it is desirable to form the light-emitting unit 50 with a side width of approximately 10.04 μm or more. Furthermore, the shape of the light-emitting unit 50 is not limited to a square, and it may be any polygonal shape greater than a square, circular, elliptical, or the like, as long as it emits light of an exposure area size corresponding to the output resolution of the image forming apparatus A and the image quality of the output image satisfies the design specifications of the image forming apparatus A. Furthermore, the distance d2 between adjacent light-emitting units 50 in the direction of the arrow Y and the number of rows of light-emitting units 50 in the direction of the arrow Y are determined based on the scanning speed of the exposure head 6, the amount of light required for exposure processing, the resolution, etc.

[0049] FIG. 7 is a schematic diagram illustrating the irradiation positions on the photosensitive drum 1 of light emitted from two light-emitting elements 50 that overlap in the direction of the arrow Y. As shown in FIG. 7(a), when two light-emitting elements 50 that overlap in the direction of the arrow Y are simultaneously turned on, the irradiation positions on the photosensitive drum 1 of the light H emitted from the two light-emitting elements 50 are shifted in the rotation direction of the photosensitive drum 1 (the direction of the arrow Y, the sub-scanning direction) in accordance with the relative positions of the two light-emitting elements 50. In contrast, as shown in FIG. 7(b), when the turn-on timing of the two light-emitting elements 50 is changed depending on the rotation speed of the photosensitive drum 1, the irradiation positions on the photosensitive drum 1 of the light H emitted from the two light-emitting elements 50 can be made substantially identical. This irradiation of substantially the same position on the photosensitive drum 1 from multiple light-emitting elements 50 arranged in the direction of the arrow Y is referred to as multiple exposure. The greater the number of light-emitting elements 50 arranged in the direction of the arrow Y used for multiple exposure, the greater the amount of light received at a portion of the photosensitive drum 1 during multiple exposure.

[0050] In order to align the irradiation positions on the photosensitive drum 1 of the light emitted from two light-emitting elements 50 that are positioned to overlap in the direction of arrow Y, it is necessary to delay the lighting timing of the light-emitting element 50 on the downstream side in the rotation direction of the photosensitive drum 1 by a delay amount T relative to the lighting timing of the light-emitting element 50 on the upstream side. Here, the delay amount T (μs) is calculated by the following formula 1, where the rotation speed of the photosensitive drum 1 is Vdr (mm / s), the width is W2 (μm), and the interval is d2 (μm). T=((W2+d2)÷1000)÷Vdr (Formula 1)

[0051] In this embodiment, the light emission signal is generated so that the maximum light emission time Tw(s) of each light-emitting element 50 is equal to one line time in the sub-scanning direction, and is expressed by the following equation 2 using the resolution of 1200 dpi and the rotation speed Vdr(mm / s) of the photosensitive drum 1. Tw=(25.4÷1200)÷Vdr (Formula 2)

[0052] <Exposure head system configuration> Next, we will explain the system configuration of the image controller unit 70 provided on the main body side of the image forming apparatus A and the exposure head 6. Although the following explains processing for a single color out of the four colors of yellow, magenta, cyan, and black, when performing image formation operations, similar processing is performed in parallel for the above four colors.

[0053] Fig. 8 is a block diagram showing the system configuration of the image controller unit 70 and the exposure head 6. As shown in Fig. 8, the image controller unit 70 includes an image data signal generation unit 71 (image data signal generation unit), a chip data conversion unit 72 (image data signal transmission unit), a CPU 73 (control unit), and a synchronization signal generation unit 74 (control signal generation unit).

[0054] CPU 73 is an abbreviation for Central Processing Unit and is a processor. Note that the image controller unit 70 may have other processors such as an MPU (Micro Processing Unit), a GPU (Graphics Processing Unit), and a QPU (Quantum Processing Unit) as control units instead of or in addition to the CPU 73. The CPU 73 executes each process described below by reading programs stored in storage such as an HDD (Hard Disk Drive) and an SSD (Solid State Drive).

[0055] The image controller unit 70 processes image data and image formation timing using the above-mentioned units, and transmits control signals to the printed circuit board 22 of the exposure head 6 to control the exposure head 6. Specifically, the control signals are image data signals, chip select signals, clock signals, line synchronization signals, and communication signals for the CPU 73, and these signals are transmitted from the image controller unit 70 to the exposure head 6 via the various signal lines described below.

[0056] The image data signal generation unit 71 receives image data of a document read by the image reading unit 90 and image data transferred from an external device via a network. The image data signal generation unit 71 performs dithering processing on the input image data at a resolution specified by the CPU 73, and generates an image data signal for outputting an image. In this embodiment, the image data signal generation unit 71 performs dithering processing at a resolution of 1200 dpi. The image data signal is 3 bits wide and represents 8 gradations with density values ​​0 to 7. Density value 0 is the minimum density, and density value 7 is the maximum density. The CPU 73 issues various instructions to the image data signal generation unit 71 by sending communication signals via a communication signal line 79.

[0057] The synchronization signal generator 74 periodically generates a line synchronization signal representing a line separation in the main scanning direction of the image data signal. In other words, the synchronization signal generator 74 periodically generates a line synchronization signal, which is a control signal that controls the timing at which the light-emitting unit 50 selected in accordance with the image data signal starts emitting light when forming an electrostatic latent image for one line in the main scanning direction. When forming an image in the normal mode, the CPU 73 defines a line period as the period during which the surface of the photosensitive drum 1 moves in the rotational direction by a pixel size corresponding to the resolution in the sub-scanning direction at a predetermined rotation speed of the photosensitive drum 1, and instructs the synchronization signal generator 74 on the period of the line synchronization signal. Specifically, when executing the normal mode, the CPU 73 instructs the synchronization signal generator 74 to generate a line synchronization signal every time the photosensitive drum 1 rotates by a pitch (approximately 21.16 μm) corresponding to a resolution of 1200 dpi. For example, when the photosensitive drum 1 rotates at 200 mm / s in the normal mode, the synchronization signal generator 74 generates a line synchronization signal at a period of 105.8 μs.

[0058] As will be described later, the chip data conversion unit 72 has a line buffer 15, and receives and stores image data signals from the image data signal generation unit 71. Then, in synchronization with a line synchronization signal generated by a synchronization signal generation unit 74 and input via a line synchronization signal line 78, the chip data conversion unit 72 divides one line of image data signals into individual light-emitting element array chips 40 in accordance with an instruction from the CPU 73 and transmits the divided signals via an image data signal line 77. The chip data conversion unit 72 also transmits a chip select signal indicating the effective range of the clock signal and the image data signal to the light-emitting element array chips 40 via a chip select signal line 75 and a clock signal line 76.

[0059] The head information storage unit 171 provided in the exposure head 6 is connected to the CPU 73 via a communication signal line 79. The head information storage unit 171 stores information on the light emission amount and mounting position of each light-emitting element array chip 40 as head information. In addition, a chip select signal line 75, a clock signal line 76, an image data signal line 77, a line synchronization signal line 78, and a communication signal line 79 are all connected to the light-emitting element array chip 40. The light-emitting element array chip 40 causes the light-emitting unit 50 to emit light based on the setting values ​​of each signal input from the image controller unit 70 via the above-mentioned signal lines. In addition, each light-emitting element array chip 40 is cascade-connected to other light-emitting element array chips 40 via the chip select signal line 75. Each light-emitting element array chip 40 generates a chip select signal used by the other light-emitting element array chips 40 and transmits it via the chip select signal line 75.

[0060] <Chip data conversion unit> Next, the configuration of the chip data conversion unit 72 will be described.

[0061] Fig. 9 is a block diagram showing the configuration of the chip data conversion unit 72. As shown in Fig. 9, the chip data conversion unit 72 includes a line buffer 15, a buffer write control unit 16, and a buffer read control unit 17. The CPU 73 controls the line buffer 15, the buffer write control unit 16, and the buffer read control unit 17 by transmitting control signals to the chip data conversion unit 72.

[0062] The buffer write control unit 16 transmits a reception enable notification signal to the image data signal generation unit 71. This causes the image data signal generation unit 71 to output the image data signal. The buffer write control unit 16 stores the image data signal output from the image data signal generation unit 71 in the line buffer 15, line by line. The line buffer 15 has four data storage areas (Buf000 to Buf003), and one line's worth of image data signal is stored in the order Buf000 → Buf001 → Buf002 → Buf003 → Buf000 → Buf001, etc. The buffer write control unit 16 transmits a write completion notification signal to the buffer read control unit 17 each time one line's worth of image data signal is stored in the line buffer 15. Furthermore, if the data storage area of ​​the line buffer 15 is full, the buffer write control unit 16 stops transmitting the reception enable notification signal to the image data signal generation unit 71 and stops the output of the image data signal. The number of data storage areas of the line buffer 15 is not limited to four, and may be any number as long as it can realize the function of the chip data conversion unit 72.

[0063] Upon receiving the write completion notification signal, the buffer read control unit 17 synchronizes with the line synchronization signal, divides one line's worth of image data signals stored in the line buffer 15 so that the signals can be assigned to each light-emitting element array chip 40, and transmits the image data signals to the light-emitting element array chip 40 along with a clock signal and a chip select signal. After completing transmission of one line's worth of image data signals to the light-emitting element array chip 40, the buffer read control unit 17 transmits a read completion notification signal to the buffer write control unit 16, thereby notifying the light-emitting element array chip 40 that one data storage area in the line buffer 15 has become available. In addition, the buffer read control unit 17 can also transmit, in response to an instruction from the CPU 73, an image data signal for turning off all of the light-emitting elements 50 of the exposure head 6, instead of the image data signal stored in the line buffer 15. The image data signal for turning off all of the light-emitting elements 50 of the exposure head 6 is an image data signal of all "0"s or "1"s.

[0064] <System configuration of light-emitting element array chip> Next, the system configuration of the light-emitting element array chip 40 will be described.

[0065] Fig. 10 is a block diagram showing the system configuration of the light-emitting element array chip 40. As shown in Fig. 10, the circuit section 46 of the light-emitting element array chip 40 is composed of a digital section 80 and an analog section 86. As will be described later, the analog section 86 generates a signal for driving the light-emitting section 50 based on the pulse signal generated in the digital section 80.

[0066] The digital unit 80 includes a communication IF unit 81, a register unit 82, a chip select signal generation unit 83, an image data storage unit 84, and a pulse signal generation unit 85. Using these units, the digital unit 80 generates a pulse signal for causing the light emitting unit 50 to emit light based on a preset setting value, a chip select signal, an image data signal, and a line synchronization signal, which are set by a communication signal in synchronization with a clock signal, and transmits the pulse signal to the analog unit 86.

[0067] The communication IF unit 81 controls writing and reading of setting values ​​to and from the register unit 82 based on a communication signal input from the CPU 73. The register unit 82 stores setting values ​​necessary for operation. These setting values ​​include width information of the pulse signal generated by the pulse signal generating unit 85, period information of the line synchronization signal, setting information of the drive current set by the analog unit 86, and information on the rotation speed of the photosensitive drum 1 in the normal mode and the low-speed mode.

[0068] The chip select signal generation unit 83 delays the input chip select signal and generates a chip select signal to be used in other light-emitting element array chips 40 connected via the chip select signal line 75. The image data storage unit 84 holds the image data signal while the input chip select signal is valid, and outputs the image data signal to the lighting control unit 88 in synchronization with the line synchronization signal.

[0069] The pulse signal generating unit 85 generates a pulse signal that controls the light emission timing of the light emitting unit 50 based on the pulse signal width information stored in the register unit 82 and the period of the line synchronization signal, and outputs the generated pulse signal to the lighting control unit 88. Based on the image data signal output from the image data storage unit 84, the lighting control unit 88 selects for each light emitting unit 50 whether or not to output the pulse signal generated by the pulse signal generating unit 85 to the analog unit 86, and outputs the pulse signal to the analog unit 86.

[0070] <Image data storage section> Next, the configuration of the image data storage unit 84 will be described.

[0071] Fig. 11 is a circuit diagram of the image data storage unit 84. In Fig. 11, the image data signal is also referred to as "data." Although the chip select signal and line synchronization signal are negative logic signals, they may also be positive logic signals.

[0072] 11, the image data storage unit 84 has a clock gate circuit 30 and flip-flop circuits 31 to 37. The flip-flop circuits 31 (31-000 to 31-747) use the image data signal data input to the image data storage unit 84 as the original input, and 748 flip-flop circuits 31 (31-000 to 31-747) are connected in series, the same number as the number of light-emitting units 50 in the direction of the arrow X of the light-emitting element array chip 40.

[0073] Similarly, the flip-flop circuits 32 to 37 are provided in the same number as the light-emitting units 50 in the direction of the arrow X of the light-emitting element array chip 40 (32-000 to 32-747, 33-000 to 33-747, 34-000 to 34-747, 35-000 to 35-747, 36-000 to 36-747, 37-000 to 37-747).

[0074] The clock gate circuit 30 outputs the logical product of the inverted chip select signal and the clock signal, and outputs the clock signal to the flip-flop circuit 31 only when the chip select signal is valid. The flip-flop circuit 31 operates on the clock signal sent from the clock gate circuit 30, and outputs an image data signal (dly_data_000).

[0075] The flip-flop circuit 32-000 receives the output of the flip-flop circuit 31-000 as an input and operates in response to a line synchronization signal. The output (buf_data_0_000) of the flip-flop circuit 32-000 is input to the flip-flop circuit 33-000 and the lighting control unit 88.

[0076] The flip-flop circuit 33-000 receives the output of the flip-flop circuit 32-000 as an input and operates in response to a line synchronization signal. The output (buf_data_1_000) of the flip-flop circuit 33-000 is input to the flip-flop circuit 34-000 and the lighting control unit 88.

[0077] The flip-flop circuit 34-000 receives the output of the flip-flop circuit 33-000 as its input and operates in response to a line synchronization signal. The output (buf_data_2_000) of the flip-flop circuit 34-000 is input to the flip-flop circuit 35-000 and the lighting control unit 88.

[0078] The flip-flop circuit 35-000 receives the output of the flip-flop circuit 34-000 as an input and operates in response to a line synchronization signal. The output (buf_data_3_000) of the flip-flop circuit 35-000 is input to the flip-flop circuit 36-000 and the lighting control unit 88.

[0079] The flip-flop circuit 36-000 receives the output of the flip-flop circuit 35-000 as an input and operates in response to a line synchronization signal. The output (buf_data_4_000) of the flip-flop circuit 36-000 is input to the flip-flop circuit 37-000 and the lighting control unit 88.

[0080] The flip-flop circuit 37-000 receives the output of the flip-flop circuit 36-000 and operates in response to a line synchronization signal. The output of the flip-flop circuit 37-000 (buf_data_5_000) is input to the lighting control unit 88.

[0081] Each of the flip-flop circuits 32-001 to 32-747, 33-001 to 33-747, 34-001 to 34-747, 35-001 to 35-747, 36-001 to 36-747, and 37-001 to 37-747 operates in the same manner as the above-described flip-flop circuits 32-000 to 37-000.

[0082] FIG. 12 is a timing chart showing the operation of the image data storage unit 84 in the main scanning direction. The symbols in FIG. 12 have the same meaning as those in FIG. 11. As shown in FIG. 12, between times T0 and T1, when the low output of the chip select signal is captured by the rising edge of the clock signal, the image data signal shifts in the order data → dly_data_000 → dly_data_001. The low output of the chip select signal is input for 748 clock signals, which is the same number as the number of light-emitting units 50 in the main scanning direction. As a result, one line's worth of image data signal is held in dly_data_000 to dly_data_747.

[0083] After time T1, the chip select signal is high and no shift operation is performed. At time T2, when the low output of the line synchronization signal is captured by the rising edge of the clock signal, the image data signals for one line are shifted simultaneously as buf_data_0_000 to buf_data_0_747, in the order of dly_data_000 → buf_data_0_000, dly_data_001 → buf_data_0_001, and are output to the lighting control unit 88.

[0084] Figure 13 is a timing chart showing the operation in the sub-scanning direction in the normal mode in the image data storage unit 84. The symbols in Figure 13 have the same meanings as those in Figure 11. Below, we will explain the outputs buf_data_0_000, buf_data_1_000, buf_data_2_000, buf_data_3_000, buf_data_4_000, and buf_data_5_000 of the flip-flop circuits 32-000, 33-000, 34-000, 35-000, 36-000, and 37-000 shown in Figure 11. Although explanation will be omitted below, the same applies to buf_data_0_001 to buf_data_0_747, buf_data_1_001 to buf_data_1_747, buf_data_2_001 to buf_data_2_747, buf_data_3_001 to buf_data_3_747, buf_data_4_001 to buf_data_4_747, and buf_data_5_001 to buf_data_5_747.

[0085] 13, each time the line synchronization signal rises from low to high, the data shifts from dly_data_000 to buf_data_0_000, and from buf_data_0_000 to buf_data_1_000, etc. Therefore, the value of B000 in dly_data_000 at time TA0 is output to the lighting control unit 88 as buf_data_0_000 at time TA1, buf_data_1_000 at time TA2, and buf_data_2_000 at time TA3.

[0086] In this way, multiple exposure is realized by connecting buf_data_0_000, buf_data_1_000, buf_data_2_000, buf_data_3_000, buf_data_4_000, and buf_data_5_000 in order from the light-emitting unit 50 that is exposed first on the photosensitive drum 1.

[0087] <Analog section> Next, we will explain the configuration of the analog unit 86. In the following explanation, we will explain two drive units 61 that drive two light-emitting units 50, but all of the light-emitting units 50 are driven in the same way by the same drive unit 61.

[0088] 14 is a block diagram showing the configuration of the analog unit 86. As shown in Fig. 14, the analog unit 86 includes a driver 61 that drives the light emitting unit 50, a DAC 62 that is a digital-to-analog converter, and a driver selection unit 67.

[0089] The DAC 62 supplies an analog voltage that determines the drive current to the drive unit 61 via a signal line 63 based on the data set in the register unit 82. A pulse signal output from the lighting control unit 88 is input to the drive unit 61 via a signal line 66. In this way, the analog voltage that determines the drive current and the pulse signal are input to the drive unit 61. Then, based on these signals, the drive unit 61 controls the drive current and light emission time of the light-emitting unit 50 using a drive circuit, which will be described later.

[0090] The driver selection unit 67 supplies driver selection signals for selecting a driver 61 to the two drivers 61 via signal lines 64 and 65 based on the data set in the register unit 82. The driver selection signal is generated so that only the signal connected to the selected driver 61 goes high. For example, when the upper driver 61 shown in FIG. 14 is selected, a high signal is supplied to only signal line 64, and a low signal is supplied to signal line 65. The two drivers 61 receive analog voltages that determine their drive currents from the DAC 62 when the driver selection signal goes high. In this way, the CPU 73 sequentially selects the drivers 61 via the register unit 82 and sets the analog voltages of the selected driver 61, thereby using a single DAC 62 to set the analog voltages of all the drivers 61.

[0091] <Drive unit> Next, the configuration of the drive unit 61 will be described.

[0092] 15 is a circuit diagram of the driving unit 61. As shown in FIG.

[0093] MOSFET 112 supplies a drive current to light-emitting unit 50 according to the value of the gate voltage, and controls the current so that the drive current is turned off (light is turned off) when the gate voltage is at a low level. A signal line 63 is connected to the gate of MOSFET 114. When a pulse signal input via signal line 66 is high, MOSFET 114 passes the voltage charged in capacitor 116 to MOSFET 112.

[0094] A driver unit select signal transmitted from the driver unit selection unit 67 via a signal line 64 is connected to the gate of the MOSFET 115. When the input driver unit select signal is high, the MOSFET 115 turns on and charges the analog voltage output from the DAC 62 and transmitted via a signal line 63 to the capacitor 116. In this embodiment, the DAC 62 sets the analog voltage in the capacitor 116 at a timing before image formation, and keeps the MOSFET 115 in an off state during the image formation operation to maintain the voltage level.

[0095] Through the above operation, MOSFET 112 supplies a drive current to light-emitting unit 50 in accordance with the set analog voltage and pulse signal. Furthermore, if the input capacitance of light-emitting unit 50 is large and the response speed when turned off is slow, MOSFET 113 can speed up the response speed when turned off. A signal obtained by logically inverting the pulse signal by inverter 117 is input to the gate of MOSFET 113. When the pulse signal is low, the gate of MOSFET 113 goes high, forcibly discharging the charge stored in the input capacitance of light-emitting unit 50.

[0096] <Issues in low-speed mode> Next, a problem that occurs in the low-speed mode will be described. The CPU 73 can execute a normal mode and a low-speed mode during image formation, and changes the rotation speed of the photosensitive drum 1 when switching between these modes. Here, if the CPU 73 is configured to control the synchronization signal generation unit 74 in the low-speed mode, as in the normal mode, by using the time it takes for the photosensitive drum 1 to rotate by an amount corresponding to the pixel size corresponding to the resolution in the sub-scanning direction as the period of the line synchronization signal, the following problem occurs.

[0097] That is, when the rotation speed of the photosensitive drum 1 decreases, the period of the line synchronization signal becomes longer, and therefore, an increase in the amount of light irradiated onto an area corresponding to one pixel on the photosensitive drum 1 occurs. For example, if the rotation speed of the photosensitive drum 1 in low-speed mode is half that of normal mode, the period of the line synchronization signal becomes twice that of normal mode, and the amount of light irradiated onto an area corresponding to one pixel on the photosensitive drum 1 becomes twice as long. If the amount of light irradiated onto an area corresponding to one pixel on the photosensitive drum 1 increases, too much toner will be placed on the photosensitive drum 1 when the electrostatic latent image is developed.

[0098] <Light intensity adjustment method by thinning out lighting control in units of one line in the main scanning direction> Next, a method of adjusting the amount of light by thinning out and lighting control in units of one line in the main scanning direction will be described.

[0099] 16 is a timing chart showing light intensity adjustment by thinning-out lighting control in units of one line in the main scanning direction. FIG. 16 shows the operation in the sub-scanning direction in the low-speed mode in the image data storage unit 84. Here, a case will be described in which the rotation speed of the photosensitive drum 1 in the low-speed mode is set to half the rotation speed of the photosensitive drum 1 in the normal mode. Note that the meanings of the symbols shown in FIG. 16 are the same as those shown in FIG. 11. Furthermore, this embodiment is not limited to the case in which the rotation speed of the photosensitive drum 1 in the low-speed mode is set to half the rotation speed of the photosensitive drum 1 in the normal mode, and other speed differences are also applicable.

[0100] Assume that the rotational speed of the photosensitive drum 1 in the low-speed mode is a / b times (a and b are integers) the rotational speed of the photosensitive drum 1 in the normal mode. Since the rotational speed of the photosensitive drum 1 is slower in the low-speed mode, a < b. The CPU 73 first calculates a value that is b / a (the reciprocal of a / b) times the period of the line synchronization signal in the normal mode. The CPU 73 then instructs the synchronization signal generation unit 74 to use the calculated value as the period of the line synchronization signal in the low-speed mode. Note that the calculated period of the line synchronization signal is the same as the time it takes for the photosensitive drum 1 to rotate by a pixel size corresponding to the resolution in the sub-scanning direction in the low-speed mode. In the example shown in Fig. 16, first, the CPU 73 instructs the synchronization signal generation unit 74 to set the period of the line synchronization signal in the low-speed mode to be twice the period of the line synchronization signal in the normal mode. For example, the time from time TB0 to time TB1 shown in Fig. 16 is twice the time from time TA0 to time TA1 shown in Fig. 13.

[0101] Next, the CPU 73 instructs the light-emitting element array chip 40 to light up 6×a / b columns and to turn off 6×(ba) / b columns of the six light-emitting columns used for multiple exposure. Turning on some of the six light-emitting columns used for multiple exposure and turning off the remaining columns is called thinning-out control in units of one line in the main scanning direction. The light-emitting column lighting / non-lighting settings sent by the CPU 73 to the light-emitting element array chip 40 are stored in the register unit 82 via the communication IF unit 81. Based on the light-emitting column lighting / non-lighting settings stored in the register unit 82, the lighting control unit 88 controls the lighting / non-lighting of the light-emitting columns by inputting a pulse signal to the driver unit 61 via the signal line 66. Alternatively, the driver selector 67 may control the lighting / non-lighting of the light-emitting columns by inputting a driver select signal to the driver unit 61 via the signal line 64 based on the light-emitting column lighting / non-lighting settings stored in the register unit 82. 16, the CPU 73 instructs the light-emitting element array chip 40 to light up three of the six light-emitting columns used for multiple exposure (light-emitting units connected to buf_data_0_000, buf_data_2_000, and buf_data_4_000) and to turn off three of the other columns (light-emitting units connected to buf_data_1_000, buf_data_3_000, and buf_data_5_000). The combination of the three columns to be lit and the three columns to be turned off is not limited to the above, and another combination may be selected from the six light-emitting columns.

[0102] Here, the amount of light irradiated onto an area corresponding to one pixel on the photosensitive drum 1 in normal mode and low-speed mode will be compared. The cycle of the line synchronization signal in low-speed mode is changed by b / a times compared to normal mode, so the amount of light is changed by b / a times. Also, the number of light-emitting rows that are turned on in low-speed mode is changed by a / b times compared to normal mode, so the amount of light is changed by a / b times. Therefore, as shown in the following equation 3, the ratio of the amount of light in low-speed mode to that in normal mode is equal, so it is possible to make the amount of light irradiated onto an area corresponding to one pixel on the photosensitive drum 1 equal in normal mode and low-speed mode.

[0103] (Change in light intensity due to change in the cycle of the line synchronization signal: b / a times) × (Change in light intensity due to change in the lit light-emitting row: a / b times) = (ratio of light intensity in slow mode to normal mode: 1 times) (Equation 3)

[0104] To summarize the above, when executing the low-speed mode in which the photosensitive drum 1 rotates at a speed slower than the normal mode rotation speed and a / b times the normal mode rotation speed, the CPU 73 performs the following control. That is, in the low-speed mode, the CPU 73 controls the synchronization signal generator 74 to generate a line synchronization signal at a period b / a times the period of the line synchronization signal generated in the normal mode. The CPU 73 also controls the light-emitting element array chip 40 to light up 6×a / b columns and turn off 6×(ba) / b columns out of the six light-emitting columns used for multiple exposure. This allows the amount of light irradiated onto an area corresponding to one pixel on the photosensitive drum 1 to be equal in both the normal mode and the low-speed mode. Therefore, excessive toner deposition on the photosensitive drum 1 can be prevented in the low-speed mode.

[0105] <Light intensity adjustment method by controlling the drive current of the light-emitting unit> Next, a method for adjusting the amount of light by controlling the drive current of the light emitting unit will be described.

[0106] Because the drive current flowing through the light-emitting unit 50 and the light intensity are approximately proportional to each other, the light intensity of the light-emitting unit 50 can be adjusted to match a target value by controlling the drive current flowing through the light-emitting unit 50. A head information storage unit 171 provided in the exposure head 6 stores a table defining the relationship between the drive current flowing through the light-emitting unit 50 and the light intensity. First, the CPU 73 reads the table of drive current and light intensity from the head information storage unit 171 and calculates the drive current value required to match the light intensity of the light-emitting unit 50 to the target value. Next, the CPU 73 instructs the light-emitting element array chip 40 to set the drive current to be flowed through the light-emitting unit 50. The setting of the drive current to be flowed through the light-emitting unit 50 transmitted by the CPU 73 to the light-emitting element array chip 40 is stored in the register unit 82 via the communication IF unit 81. The DAC 62 provided in the analog unit 86 then supplies an analog voltage that determines the drive current to the driver 61 via the signal line 63, based on the setting of the drive current to be flowed through the light-emitting unit 50 stored in the register unit 82. As a result, a set drive current flows through the light-emitting unit 50, and the light intensity of the light-emitting unit 50 is adjusted to match the target value.

[0107] In the low-speed mode, the light intensity can be adjusted by controlling the drive current of the light-emitting unit as described above. That is, the CPU 73 sets the drive current flowing through the light-emitting unit 50 in the normal mode to a value between the upper and lower limits, but not at either the upper or lower limit. This leaves room for changing the drive current in the low-speed mode. In the low-speed mode, this embodiment can reduce the light intensity by reducing the drive current flowing through the light-emitting unit 50, and can increase the light intensity by increasing the drive current flowing through the light-emitting unit 50. This allows the light intensity irradiated onto an area corresponding to one pixel on the photosensitive drum 1 to be equal in both the normal mode and the low-speed mode. Therefore, this embodiment can prevent excessive toner from being deposited on the photosensitive drum 1 in the low-speed mode.

[0108] <Control of the Light Emitting Unit in the Low-Speed ​​Mode of the First Embodiment> Next, we will explain the control of the light-emitting units in the low-speed mode of the first embodiment. In the first embodiment, the light-emitting units are controlled using a light intensity adjustment method that thins out the lighting in units of one line in the main scanning direction and a light intensity adjustment method that controls the drive current of the light-emitting units.

[0109] First, we will discuss the advantages and limitations of the light intensity adjustment method using line-by-line thinning control in the main scanning direction. The advantage of this method is that, because multiple light-emitting arrays used for multiple exposure are turned off, the light intensity can be adjusted over a wide range, allowing for significant increases and decreases. On the other hand, the limitation of this method is that if the rotation speed of the photosensitive drum 1 in the low-speed mode is a / b times faster than the rotation speed of the photosensitive drum 1 in the normal mode, the number of light-emitting arrays that are turned on (total number of light-emitting arrays × a / b) must be an integer. For example, if the rotation speed of the photosensitive drum 1 in the low-speed mode is 1 / 4 times faster than the rotation speed of the photosensitive drum 1 in the normal mode, the number of light-emitting arrays that are turned on (total number of light-emitting arrays × 1 / 4) is not an integer. In this case, the light intensity adjustment method using line-by-line thinning control in the main scanning direction cannot be used to equalize the amount of light irradiated onto an area corresponding to one pixel on the photosensitive drum 1 in the normal mode and the low-speed mode.

[0110] Next, advantages and limitations of the light intensity adjustment method based on drive current control of the light-emitting unit will be described. The advantage of this method is that the light intensity can be changed linearly with respect to the drive current flowing through the light-emitting unit 50, allowing for fine adjustment of the light intensity. On the other hand, a limitation of this method is that the upper and lower limits of the drive current that can be flowed are determined by the characteristics of the light-emitting unit 50, resulting in a narrower range of adjustment of the light intensity compared to the light intensity adjustment method based on thinning-out lighting control in units of one line in the main scanning direction and the light intensity adjustment method based on lighting control using a period-divided line synchronization signal, which will be described in the third embodiment.

[0111] As described above, there are limitations to both the light intensity adjustment method using thinning-out lighting control in units of one line in the main scanning direction and the light intensity adjustment method using drive current control for the light-emitting unit. Therefore, there are cases where it is not possible to adjust the light intensity so that the light intensity irradiated onto an area corresponding to one pixel on the photosensitive drum 1 is equal in normal mode and low-speed mode. Even in such cases, appropriate light intensity adjustment is possible by controlling the light-emitting unit in the low-speed mode of the first embodiment. Below, the control of the light-emitting unit in the low-speed mode of the first embodiment will be described with reference to FIG. 17 .

[0112] 17 is a timing chart showing the operation in the sub-scanning direction in the low-speed mode in the image data storage unit 84 of the first embodiment. Here, a case will be described as an example in which the rotation speed of the photosensitive drum 1 in the low-speed mode is set to 1 / 4 of the rotation speed of the photosensitive drum 1 in the normal mode. Note that the symbols shown in FIG. 17 have the same meanings as the symbols shown in FIG. 11. Furthermore, this embodiment is not limited to the case in which the rotation speed of the photosensitive drum 1 in the low-speed mode is set to 1 / 4 of the rotation speed of the photosensitive drum 1 in the normal mode, and can be applied to other speed differences.

[0113] If the rotation speed of the photosensitive drum 1 in the low-speed mode is a / b times faster than the rotation speed of the photosensitive drum 1 in the normal mode, the CPU 73 first calculates a value that multiplies the period of the line synchronization signal in the normal mode by b / a. The CPU 73 then instructs the synchronization signal generator 74 to use the calculated value as the period of the line synchronization signal in the low-speed mode. The period of the line synchronization signal calculated here is the same as the time it takes for the photosensitive drum 1 to rotate an amount corresponding to the pixel size corresponding to the resolution in the sub-scanning direction in the low-speed mode. In the example shown in FIG. 17, the CPU 73 first instructs the synchronization signal generator 74 to set the period of the line synchronization signal in the low-speed mode to four times the period of the line synchronization signal in the normal mode. For example, the time from time TC0 to TC1 shown in FIG. 17 is four times the time from time TA0 to time TA1 shown in FIG. 13.

[0114] Next, the CPU 73 selects, from the magnification candidates such that the number of light-emitting rows to be lit becomes an integer, the magnification c / d (c and d are integers, c < d) that is greater than or equal to a / b and is the closest. Then, the CPU 73 instructs the light-emitting element array chip 40 to light 6 × c / d rows out of the 6 rows of light-emitting rows used for multi-exposure and turn off 6 × (d - c) / d rows. In the example shown in FIG. 17, the value of 6 × 1 / 4 times the total number of light-emitting rows does not become an integer. Therefore, the CPU 73 selects 2 / 6, which is the magnification that is greater than or equal to 1 / 4 times and is the closest, from the magnification candidates 1 / 6, 2 / 6, 3 / 6, 4 / 6, 5 / 6 where the number of light-emitting rows to be lit becomes an integer. The CPU 73 instructs the light-emitting element array chip 40 to light 2 rows (= 6 × 2 / 6) out of the 6 rows of light-emitting rows used for multi-exposure and turn off 4 rows (6 × (6 - 2) / 6). In FIG. 17, the 2 rows to be lit are the light-emitting parts to which buf_data_0_000 and buf_data_2_000 are connected. Also, the 4 rows to be turned off are the light-emitting parts to which buf_data_1_000, buf_data_3_000, buf_data_4_000, and buf_data_5_000 are connected. Note that the 2 rows to be lit and the 4 rows to be turned off are not limited to the above combination, and other combinations may be selected from the 6 rows of light-emitting rows.

[0115] Next, the CPU 73 reads a table of drive current and light amount from the head information storage unit 171 and calculates the value of the drive current required to make the light amount of the light-emitting unit 50 match the target value (the (a × d) / (b × c) times the light amount in the normal mode). Then, the CPU 73 instructs the setting of the drive current to be passed through the light-emitting unit 50 of the light-emitting element array chip 40. As a result, the drive current as set flows through the light-emitting unit 50, and the light amount of the light-emitting unit 50 is adjusted to match the target value (the (a × d) / (b × c) times the light amount in the normal mode). In the example shown in FIG. 17, the target value is 3 / 4 times.

[0116] Here, the amount of light irradiated onto an area corresponding to one pixel on the photosensitive drum 1 in normal mode and low-speed mode will be compared. The cycle of the line synchronization signal in low-speed mode is changed by b / a times compared to normal mode, so the amount of light is changed by b / a times. Furthermore, the number of light-emitting rows that are turned on in low-speed mode is changed by c / d times compared to normal mode, so the amount of light is changed by c / d times. The amount of light of the light-emitting element in low-speed mode is changed by (a×d) / (b×c) times compared to normal mode, so the amount of light is changed by (a×d) / (b×c) times. Therefore, as shown in the following equation 4, the light amount ratio in low-speed mode to normal mode is equal, so the amount of light irradiated onto an area corresponding to one pixel on the photosensitive drum 1 can be made equal in normal mode and low-speed mode.

[0117] (Change in light intensity due to change in the line synchronization signal cycle: b / a times) × (Change in light intensity due to change in the light-emitting row to be lit: c / d times) × (Change in light intensity due to drive current control: (a × d) / (b × c) times) = (ratio of light intensity in slow mode to normal mode: 1 times) (Equation 4)

[0118] To summarize the above, according to the first embodiment, the number and light intensity of the lit light-emitting columns are adjusted so as not to be excessive by thinning-out control in units of one line in the main scanning direction, and the drive current of the light-emitting unit is further controlled to reduce the drive current and eliminate the excess light intensity. That is, when executing the low-speed mode in which the photosensitive drum 1 rotates at a speed (second rotation speed) that is slower than the rotation speed in the normal mode (first rotation speed) and a / b times the rotation speed in the normal mode, the CPU 73 performs the following control. In the low-speed mode, the CPU 73 controls the synchronization signal generator 74 to generate a line synchronization signal at a period b / a times the period of the line synchronization signal generated in the normal mode. Furthermore, the CPU 73 selects a magnification c / d that is equal to or greater than a / b and closest to the period from among the candidate magnifications that result in an integer number of lit light-emitting columns, and controls the light-emitting element array chip 40 to light 6×c / d columns and turn off 6×(dc) / d columns out of the six light-emitting columns used for multiple exposure. Furthermore, the CPU 73 instructs the light-emitting element array chip 40 to set the drive current to be passed through the light-emitting unit 50 so that the light intensity of the light-emitting unit 50 matches the target value ((a×d) / (b×c) times the light intensity in normal mode). The above control makes it possible to equalize the light intensity of light irradiated onto an area corresponding to one pixel on the photosensitive drum 1 in normal mode and low-speed mode. Therefore, in low-speed mode, it is possible to prevent too much toner from being deposited on the photosensitive drum 1 and appropriately adjust the amount of toner deposited on the photosensitive drum 1.

[0119] Furthermore, according to the first embodiment, in the light intensity adjustment method using thinning-out control in units of one line in the main scanning direction, there are cases where the number of lit light-emitting rows (total number of light-emitting rows × a / b times) is not an integer and the light intensity cannot be adjusted appropriately. Even in such cases, by controlling the light-emitting units in the low-speed mode of the first embodiment, it is possible to make the light intensity of light irradiated onto an area corresponding to one pixel on the photosensitive drum 1 equal in normal mode and low-speed mode.

[0120] Furthermore, according to the first embodiment, in the method of adjusting the light intensity by controlling the drive current of the light-emitting unit, there are cases where the light intensity cannot be adjusted appropriately because the light intensity adjustment range is narrow. Even in such cases, the light intensity adjustment range can be widened by controlling the light-emitting unit in the low-speed mode of the first embodiment, and the light intensity of light irradiated onto an area corresponding to one pixel on the photosensitive drum 1 can be made equal in both normal mode and low-speed mode.

[0121] Furthermore, according to the first embodiment, the light intensity of the light emitting unit is controlled to be lower in the low speed mode compared to the normal mode, so that wear of the light emitting unit can be reduced.

[0122] [Second embodiment] Since the image forming apparatus of the second embodiment has the same configuration as that of the first embodiment, a description of the configuration of the image forming apparatus of the second embodiment will be omitted. Since the control of the light emitting unit in the low-speed mode of the second embodiment is different from that of the first embodiment, the control of the light emitting unit in the low-speed mode of the second embodiment will be described.

[0123] <Control of the Light Emitting Unit in the Low-Speed ​​Mode of the Second Embodiment> The control of the light-emitting units in the low-speed mode of the second embodiment will be described. In the second embodiment, as in the first embodiment, the light-emitting units are controlled using a light intensity adjustment method that thins out lighting control in units of one line in the main scanning direction and a light intensity adjustment method that controls the drive current of the light-emitting units. The difference between the second embodiment and the first embodiment is the process of selecting the light-emitting units that are turned on and the light-emitting units that are not turned on.

[0124] 18 is a timing chart showing the operation in the sub-scanning direction in the low-speed mode in the image data storage unit 84 of the second embodiment. Here, a case will be described in which the rotation speed of the photosensitive drum 1 in the low-speed mode is set to 1 / 5 of the rotation speed of the photosensitive drum 1 in the normal mode. Note that the symbols shown in FIG. 18 have the same meanings as those shown in FIG. 11. Furthermore, this embodiment is not limited to the case in which the rotation speed of the photosensitive drum 1 in the low-speed mode is set to 1 / 5 of the rotation speed of the photosensitive drum 1 in the normal mode, and other speed differences are also applicable.

[0125] First, the period of the line synchronization signal in the low-speed mode is set by the same process as in the first embodiment. In the example shown in Fig. 18, the CPU 73 instructs the synchronization signal generation unit 74 to set the period of the line synchronization signal in the low-speed mode to five times the period of the line synchronization signal in the normal mode. For example, the time from time TD0 to TD1 shown in Fig. 18 is five times the time from time TA0 to time TA1 shown in Fig. 13.

[0126] Next, regarding the selection process of the lit light-emitting columns and the non-lit light-emitting columns, different processing is performed in the second embodiment from the first embodiment. The CPU 73 selects the magnification c / d (c and d are integers, c < d) that is less than or equal to a / b times and is the closest among the magnification candidates such that the number of lit light-emitting columns becomes an integer. Then, the CPU 73 instructs the light-emitting element array chip 40 to light 6×c / d columns and turn off 6×(d - c) / d columns among the six light-emitting columns used for multi-exposure. In the example shown in FIG. 18, the value of 6×1 / 5 times the total number of light-emitting columns does not become an integer. Therefore, the CPU 73 selects 1 / 6, which is less than 1 / 5 times and is the closest magnification among the magnification candidates 1 / 6, 2 / 6, 3 / 6, 4 / 6, 5 / 6 for which the number of lit light-emitting columns becomes an integer. The CPU 73 instructs the light-emitting element array chip 40 to light one column (= 6×1 / 6) and turn off five columns (6×(6 - 1) / 6) among the six light-emitting columns used for multi-exposure. In FIG. 18, the one lit column is the light-emitting portion to which buf_data_0_000 is connected. Also, the five non-lit columns are the light-emitting portions to which buf_data_1_000, buf_data_2_0_000, buf_data_3_0_000, buf_data_4_0_000, and buf_data_5_0_000 are connected. Note that the one lit column and the five non-lit columns are not limited to the above combination, and other combinations may be selected from among the six light-emitting columns.

[0127] Next, the drive current flowing through the light-emitting portion 50 in the low-speed mode is set by the same processing as in the first embodiment. The CPU 73 reads the table of drive current and light amount from the head information storage unit 171, and calculates the value of the drive current required to make the light amount of the light-emitting portion 50 match the target value (the (a×d) / (b×c) times the light amount in the normal mode). Then, the CPU 73 instructs the setting of the drive current flowing through the light-emitting portion 50 of the light-emitting element array chip 40. In the example shown in FIG. 18, the target value is 6 / 5 times.

[0128] Here, a comparison of the amount of light irradiated onto an area corresponding to one pixel on the photosensitive drum 1 in the normal mode and the low-speed mode results in the same equation as equation 4 in the first embodiment. That is, the ratio of the amount of light in the low-speed mode to the amount of light in the normal mode is the same, so that the amount of light irradiated onto an area corresponding to one pixel on the photosensitive drum 1 can be made equal in the normal mode and the low-speed mode.

[0129] To summarize the above, according to the second embodiment, the number of lit light-emitting columns and the amount of light are adjusted to compensate for a deficiency by controlling the thinning-out of the light by one line in the main scanning direction, and the drive current of the light-emitting unit is increased to compensate for the deficiency in the amount of light. That is, when executing the low-speed mode in which the photosensitive drum 1 rotates at a speed (second rotation speed) slower than the rotation speed in the normal mode (first rotation speed) and a / b times the rotation speed in the normal mode, the CPU 73 performs the following control. In the low-speed mode, the CPU 73 controls the synchronization signal generator 74 to generate a line synchronization signal at a period b / a times the period of the line synchronization signal generated in the normal mode. Furthermore, the CPU 73 selects a magnification c / d that is equal to or smaller than a / b from among the candidate magnifications that result in an integer number of lit light-emitting columns, and controls the light-emitting element array chip 40 to light up 6×c / d columns and turn off 6×(dc) / d columns out of the six light-emitting columns used for multiple exposure. Furthermore, the CPU 73 instructs the light-emitting element array chip 40 to set the drive current to be passed through the light-emitting unit 50 so that the light intensity of the light-emitting unit 50 matches the target value ((a×d) / (b×c) times the light intensity in normal mode). The above control makes it possible to equalize the light intensity of light irradiated onto an area corresponding to one pixel on the photosensitive drum 1 in normal mode and low-speed mode. Therefore, in low-speed mode, it is possible to prevent too much toner from being deposited on the photosensitive drum 1 and appropriately adjust the amount of toner deposited on the photosensitive drum 1.

[0130] In the first embodiment, the magnification c / d that is equal to or greater than a / b and closest to the magnification is selected from among the magnification candidates that result in an integer number of lit light-emitting columns. In the second embodiment, the magnification c / d that is equal to or less than a / b and closest to the magnification is selected from among the magnification candidates that result in an integer number of lit light-emitting columns. As another alternative, the magnification c / d that is closest to a / b may be selected from among the magnification candidates that result in an integer number of lit light-emitting columns. This configuration minimizes changes in light intensity due to drive current control, and allows the present embodiment to be applied to adjust the light intensity even when the range of drive current that can be applied due to the characteristics of the light-emitting unit 50 is narrow.

[0131] [Third embodiment] Since the image forming apparatus of the third embodiment has the same configuration as that of the first embodiment, a description of the configuration of the image forming apparatus of the third embodiment will be omitted. Since the control of the light-emitting unit in the low-speed mode of the third embodiment is different from that of the first embodiment, the control of the light-emitting unit in the low-speed mode of the third embodiment will be described. In the third embodiment, the light-emitting unit is controlled using a light intensity adjustment method by lighting control using a period-divided line synchronization signal and a light intensity adjustment method by controlling the drive current of the light-emitting unit.

[0132] <Light intensity adjustment method using lighting control with period-divided line synchronization signals> First, a method for adjusting the amount of light by lighting control using a period-divided line synchronization signal will be described.

[0133] Fig. 19 is a timing chart showing light intensity adjustment by lighting control using a period-divided line synchronization signal. Fig. 19 shows operation in the sub-scanning direction in the low-speed mode in the image data storage unit 84. Here, a case will be described in which the rotation speed of the photosensitive drum 1 in the low-speed mode is set to 2 / 3 times the rotation speed of the photosensitive drum 1 in the normal mode. Note that the meanings of the symbols shown in Fig. 19 are the same as those shown in Fig. 11. Furthermore, this embodiment is not limited to the case in which the rotation speed of the photosensitive drum 1 in the low-speed mode is set to 2 / 3 times the rotation speed of the photosensitive drum 1 in the normal mode, and other speed differences are also applicable.

[0134] If the rotation speed of the photosensitive drum 1 in the low-speed mode is a / b times faster than the rotation speed of the photosensitive drum 1 in the normal mode, the CPU 73 first calculates a value that multiplies the period of the line synchronization signal in the normal mode by 1 / a. The CPU 73 then instructs the synchronization signal generator 74 to use the calculated value as the period of the line synchronization signal in the low-speed mode. Note that the value that multiplies the period of the line synchronization signal in the normal mode by b / a corresponds to the time it takes for the photosensitive drum 1 to rotate an amount of pixels corresponding to the resolution in the sub-scanning direction in the low-speed mode. Therefore, the value that multiplies the period of the line synchronization signal in the normal mode by 1 / a corresponds to the time it takes for the photosensitive drum 1 to rotate an amount of pixels corresponding to the resolution in the sub-scanning direction in the low-speed mode divided by b. In the example shown in FIG. 19, the CPU 73 first instructs the synchronization signal generator 74 to set the period of the line synchronization signal in the low-speed mode to 1 / 2 the period of the line synchronization signal in the normal mode. For example, the time from time TE0_0 to time TE0_1 shown in FIG. 19 is half the time from time TA0 to time TA1 shown in FIG.

[0135] In the low-speed mode, within the time it takes for the photosensitive drum 1 to rotate an amount corresponding to the pixel size of the resolution in the sub-scanning direction, the synchronization signal generation unit 74 generates a line synchronization signal b times. The CPU 73 controls the buffer read control unit 17 to transmit the same image data signal for one line a times in synchronization with each of the b line synchronization signals to cause the light-emitting unit 50 to emit light, and to transmit an image data signal for not causing the light-emitting unit 50 to emit light (to be unlit) ba times. In the example shown in FIG. 19 , within the time it takes for the photosensitive drum 1 to rotate an amount corresponding to the pixel size of the resolution in the sub-scanning direction in the low-speed mode, the synchronization signal generation unit 74 generates a line synchronization signal three times. The CPU 73 controls the buffer read control unit 17 to transmit the same image data signal for one line twice in synchronization with each of the three line synchronization signals, and to transmit an image data signal for not causing the light-emitting unit 50 to emit light (to be unlit) once. Upon receiving an instruction from the CPU 73, the buffer read control unit 17 reads one line of the same image data signal from the line buffer 15 in synchronization with the first line synchronization signal generated at time TE0_0 and the second line synchronization signal generated at time TE0_1, and transmits the same image data signal for one line to the light-emitting element array chip 40. Furthermore, the buffer read control unit 17 transmits an image data signal that causes the light-emitting unit 50 to not emit light (to be unlit) to the light-emitting element array chip 40 in synchronization with the third line synchronization signal generated at time TE0_2. The CPU 73 repeats the above control from time TE1_0 onwards. Note that the order of transmission is not important as long as it is ensured that the same image data signal for one line is transmitted a times in synchronization with b line synchronization signals, and that the image data signal that causes the light-emitting unit 50 not to emit light (to be unlit) is transmitted ba times. In the example shown in FIG. 19, the CPU 73 may be configured to transmit an image data signal that causes the light-emitting unit 50 to not emit light (to be unlit) in synchronization with the first line synchronization signal, and transmit one line of the same image data signal in synchronization with the second and third line synchronization signals.

[0136] Here, the amount of light irradiated onto an area on the photosensitive drum 1 corresponding to one pixel in normal mode and low-speed mode is compared. The cycle of the line synchronization signal in low-speed mode is changed to 1 / a times that in normal mode, so the amount of light is changed to 1 / a times. Furthermore, in low-speed mode, within the time it takes for the photosensitive drum 1 to rotate an amount corresponding to the pixel size corresponding to the resolution in the sub-scanning direction, the same image data signal for one line is transmitted a times in synchronization with the b line synchronization signals generated, and an image data signal that does not cause the light-emitting element 50 to emit light (to be turned off) is transmitted ba times. Therefore, as shown in the following equation 5, the light intensity ratio in low-speed mode relative to normal mode is equal, so the amount of light irradiated onto an area on the photosensitive drum 1 corresponding to one pixel can be made equal in normal mode and low-speed mode.

[0137] (Change in light intensity due to change in the line synchronization signal cycle: 1 / a times) × (number of times a light is turned on due to the same image data for one line) = (ratio of light intensity in slow mode to normal mode: same) (Equation 5)

[0138] To summarize the above, when executing the low-speed mode in which the photosensitive drum 1 rotates at a speed slower than the normal mode rotation speed and a / b times the normal mode rotation speed, the CPU 73 performs the following control. That is, in the low-speed mode, the CPU 73 controls the synchronization signal generator 74 to generate a line synchronization signal at a period 1 / a times the period of the line synchronization signal generated in the normal mode. Furthermore, the CPU 73 controls the buffer read control unit 17 to transmit the same image data signal for one line a times and transmit an image data signal that does not cause the light-emitting unit 50 to emit light (to be unlit ba times) in synchronization with b line synchronization signals generated by the synchronization signal generator 74 during the time it takes the photosensitive drum 1 to rotate an amount corresponding to the pixel size corresponding to the resolution in the sub-scanning direction in the low-speed mode. This allows the amount of light irradiated onto an area corresponding to one pixel on the photosensitive drum 1 to be the same in both the normal mode and the low-speed mode. Therefore, excessive toner deposition on the photosensitive drum 1 can be prevented in the low-speed mode.

[0139] <Control of the light emitting unit in the slow mode of the third embodiment> Next, we will explain the control of the light-emitting unit in the low-speed mode of the third embodiment. In the third embodiment, the light-emitting unit is controlled using a light intensity adjustment method based on lighting control using a period-divided line synchronization signal and a light intensity adjustment method based on drive current control for the light-emitting unit.

[0140] First, we will explain the advantages and limitations of the light intensity adjustment method using lighting control using a period-divided line synchronization signal. The advantage of this method is that the ratio of the number of times the light is turned on to the number of times it is not turned on is adjusted by dividing the period of the line synchronization signal, allowing for a wide range of light intensity adjustment and the ability to significantly increase or decrease the light intensity. On the other hand, the limitation of this method is that if the period of the line synchronization signal in low-speed mode is set too short, the transmission of one line of image data signals will not be completed in time. As shown in Figure 12, the time T1 is the time when the chip data conversion unit 72 completes transmission of one line of image data signals to the light-emitting element array chip 40. Time T1 is determined by the number of pixels in one line and the period of the clock signal. Furthermore, at time T2, one line of image data signals is simultaneously captured into buf_data_0_000 to buf_data_0_747 in synchronization with the line synchronization signal. Time T2 is determined by the period of the line synchronization signal. When the period of the line synchronization signal in low-speed mode is set to 1 / a times the period of the line synchronization signal in normal mode, if the value of the denominator a is increased to shorten the period of the line synchronization signal in low-speed mode too much, time T2 may occur earlier than time T1, and the transmission of one line's worth of image data signal may not be in time.

[0141] Next, the advantages and limitations of the light intensity adjustment method by controlling the drive current of the light emitting unit are as explained in the control of the light emitting unit in the low speed mode in the first embodiment.

[0142] As described above, there are limitations to both the light intensity adjustment method using lighting control using a period-divided line synchronization signal and the light intensity adjustment method using drive current control for the light-emitting unit. Therefore, there are cases where it is not possible to adjust the light intensity so that the light intensity irradiated onto an area corresponding to one pixel on the photosensitive drum 1 is equal in normal mode and low-speed mode. Even in such cases, appropriate light intensity adjustment is possible by controlling the light-emitting unit in low-speed mode according to the third embodiment. Below, the control of the light-emitting unit in low-speed mode according to the third embodiment will be described with reference to FIG. 20.

[0143] 20 is a timing chart showing the operation in the sub-scanning direction in the low-speed mode in the image data storage unit 84 of the third embodiment. Here, a case will be described as an example in which the rotation speed of the photosensitive drum 1 in the low-speed mode is set to 5 / 7 times the rotation speed of the photosensitive drum 1 in the normal mode. Note that the meanings of the symbols shown in FIG. 20 are the same as those shown in FIG. 11. Furthermore, this embodiment is not limited to the case in which the rotation speed of the photosensitive drum 1 in the low-speed mode is set to 5 / 7 times the rotation speed of the photosensitive drum 1 in the normal mode, and other speed differences are also applicable.

[0144] If the rotation speed of the photosensitive drum 1 in the low-speed mode is a / b times faster than the rotation speed of the photosensitive drum 1 in the normal mode, the CPU 73 first calculates a value that multiplies the period of the line synchronization signal in the normal mode by b / (a×e) (e is an integer). The CPU 73 then instructs the synchronization signal generator 74 to use the calculated value as the period of the line synchronization signal in the low-speed mode. Note that the value of b / a multiplied by the period of the line synchronization signal in the normal mode corresponds to the time it takes for the photosensitive drum 1 to rotate an amount of pixels corresponding to the resolution in the sub-scanning direction in the low-speed mode. Therefore, the value of b / (a×e) multiplied by the period of the line synchronization signal in the normal mode corresponds to the time it takes for the photosensitive drum 1 to rotate an amount of pixels corresponding to the resolution in the sub-scanning direction in the low-speed mode divided by e. Here, the method for calculating the division coefficient e will be described. The lower limit of the multiplication factor of the period of the line synchronization signal in the low-speed mode to the period of the line synchronization signal in the normal mode is set to Tmin. The CPU 73 calculates Tmin from the number of pixels per line and the period of the clock signal. To ensure that one line's worth of image data signal is transmitted in time, the magnification b / (a×e) of the period of the line synchronization signal in low-speed mode must be equal to or greater than the lower limit value Tmin, and so Equation 6 holds. Equation 7 is obtained by transforming Equation 6 to find the division coefficient e. The CPU 73 finds the value of b / (a×Tmin) and selects the nearest integer that is equal to or less than the value of b / (a×Tmin) as the division coefficient e.

[0145] b / (a×e)≧Tmin (Formula 6) b / (a×Tmin)≧e (Formula 7)

[0146] In the example shown in FIG. 20, Tmin=1 / 4. The CPU 73 calculates Equation 7 and selects 5 as the division coefficient e so that 28 / 5≧e is satisfied. The CPU 73 instructs the synchronization signal generation unit 74 to set the period of the line synchronization signal in low-speed mode to 7 / 25 times the period of the line synchronization signal in normal mode. For example, the time from time TF0_0 to TF0_1 shown in FIG. 20 is 7 / 25 times the time from time TA0 to time TA1 shown in FIG. 13.

[0147] In the low-speed mode, within the time it takes for the photosensitive drum 1 to rotate an amount corresponding to the pixel size corresponding to the resolution in the sub-scanning direction, the synchronization signal generation unit 74 generates and transmits a line synchronization signal e times. The CPU 73 synchronizes with each of the e line synchronization signals to transmit the same image data signal for one line f times (f is an integer) to cause the light-emitting unit 50 to emit light f times, and controls the buffer read control unit 17 to transmit an image data signal ef times to prevent the light-emitting unit 50 from emitting light (turning it off) f times. Here, the method for calculating the number of times f that the same image data for one line is turned on will be described. The CPU 73 calculates the value of (a×e) / b and selects an integer that is greater than or equal to (a×e) / b and closest to (a×e) / b as the number of times f that the same image data for one line is turned on. In the example shown in FIG. 20, the CPU 73 calculates (a×e) / b=25 / 7 and selects 4, which is the integer that is greater than or equal to 25 / 7 and closest to 25 / 7, as the number of times f that the same image data for one line is turned on. In the low-speed mode, the synchronization signal generating unit 74 generates the line synchronization signal five times within the time it takes for the photosensitive drum 1 to rotate by an amount corresponding to the pixel size corresponding to the resolution in the sub-scanning direction.

[0148] The CPU 73 controls the buffer read control unit 17 to transmit the same image data signal for one line four times in synchronization with each of the five line synchronization signals, and to transmit an image data signal that does not cause the light-emitting unit 50 to emit light (to be unlit) once. Upon receiving instructions from the CPU 73, the buffer read control unit 17 reads the same image data signal for one line from the line buffer 15 in synchronization with the first, second, third, and fourth line synchronization signals generated at times TF0_0, TF0_1, TF0_2, and TF0_3, respectively, and transmits the same image data signal for one line to the light-emitting element array chip 40. Furthermore, the buffer read control unit 17 transmits an image data signal that does not cause the light-emitting unit 50 to emit light (to be unlit) to the light-emitting element array chip 40 in synchronization with the fifth line synchronization signal generated at time TF0_4. The CPU 73 repeats the above control from time TF1_0 onwards. Note that the order of transmission does not matter as long as the CPU 73 transmits the same image data signal for one line f times in synchronization with each of the e line synchronization signals to cause the light-emitting unit 50 to emit light, and transmits the image data signal ef times to prevent the light-emitting unit 50 from emitting light (making it unlit). In the example shown in Fig. 20, the configuration may be such that the image data signal for preventing the light-emitting unit 50 from emitting light (making it unlit) is transmitted in synchronization with the first line synchronization signal, and the same image data signal for one line is transmitted in synchronization with each of the second, third, fourth, and fifth line synchronization signals.

[0149] Next, the CPU 73 reads the table of drive current and light intensity from the head information storage unit 171, and calculates the value of the drive current required to make the light intensity of the light-emitting unit 50 match the target value ((a×e) / (b×f) times the light intensity in normal mode). The CPU 73 then instructs the setting of the drive current to be passed through the light-emitting unit 50 of the light-emitting element array chip 40. As a result, the drive current flows as set in the light-emitting unit 50, and the light intensity of the light-emitting unit 50 is adjusted to match the target value ((a×e) / (b×f) times the light intensity in normal mode). In the example shown in FIG. 20, the target value is 25 / 28 times.

[0150] Here, the amount of light irradiated onto an area on the photosensitive drum 1 corresponding to one pixel in normal mode and low-speed mode is compared. The cycle of the line synchronization signal in low-speed mode is changed by b / (a×e) times compared to normal mode, so the amount of light is changed by b / (a×e) times. Furthermore, in low-speed mode, within the time it takes for the photosensitive drum 1 to rotate an amount corresponding to the pixel size corresponding to the resolution in the sub-scanning direction, the same image data signal for one line is transmitted f times in synchronization with each of the e line synchronization signals generated, and an image data signal that does not emit light (turns off the light-emitting element 50) is transmitted ef times. Furthermore, the amount of light emitted from the light-emitting element in low-speed mode is changed by (a×e) / (b×f) times compared to normal mode. Therefore, as shown in the following equation (8), the light intensity ratio in low-speed mode relative to normal mode is equal, so the amount of light irradiated onto an area on the photosensitive drum 1 corresponding to one pixel can be made equal in normal mode and low-speed mode.

[0151] (Change in light intensity due to change in the line synchronization signal cycle: b / (a×e) times) x (number of times f times lighting is performed for the same image data for one line) x (change in light intensity due to drive current control: (a×e) / (b×f) times) = (ratio of light intensity in low-speed mode to normal mode: 1x) ...(Formula 8)

[0152] To summarize the above, according to the third embodiment, the CPU 73 adjusts the light intensity so that it is excessive by lengthening the period of the line synchronization signal through lighting control using a period-divided line synchronization signal, and further adjusts the light intensity so that it is excessive by reducing the drive current through control of the drive current of the light-emitting unit. That is, when executing the low-speed mode in which the photosensitive drum 1 is rotated at a speed (second rotation speed) that is slower than the rotation speed in the normal mode (first rotation speed) and is a / b times the rotation speed in the normal mode, the CPU 73 performs the following control: In the low-speed mode, the CPU 73 controls the synchronization signal generation unit 74 to generate a line synchronization signal with a period that is b / (a×e) times the period of the line synchronization signal generated in the normal mode. Furthermore, the CPU 73 controls the buffer read control unit 17 to transmit the same image data signal for one line f times and transmit an image data signal that does not cause the light-emitting unit 50 to emit light (become unlit) ef times in synchronization with e line synchronization signals generated by the synchronization signal generation unit 74 during the time it takes for the photosensitive drum 1 to rotate an amount corresponding to the pixel size in the sub-scanning direction in the low-speed mode. The CPU 73 selects an integer equal to or greater than (a×e) / b and closest to (a×e) / b as the number of times f the light is turned on by the same image data for one line. The CPU 73 also instructs the light-emitting element array chip 40 to set the drive current to be passed through the light-emitting unit 50 so that the light intensity of the light-emitting unit 50 matches a target value ((a×e) / (b×f) times the light intensity in the normal mode). This control allows the amount of light irradiated onto an area corresponding to one pixel on the photosensitive drum 1 to be the same in both the normal mode and the low-speed mode. Therefore, in the low speed mode, it is possible to prevent the toner from being deposited on the photosensitive drum 1 too much, and toner deposited on the photosensitive drum 1 can be adjusted appropriately.

[0153] Furthermore, according to the third embodiment, in the light intensity adjustment method by lighting control using a period-divided line synchronization signal, if the period of the line synchronization signal in low-speed mode is made too short, there are cases where the transmission of one line's worth of image data signal is not in time. Even in such cases, by controlling the light-emitting unit in low-speed mode according to the third embodiment, it is possible to make the amount of light irradiated onto an area corresponding to one pixel on the photosensitive drum 1 equal in normal mode and low-speed mode.

[0154] Furthermore, according to the third embodiment, in the method of adjusting the light intensity by controlling the drive current of the light-emitting unit, there are cases where the light intensity cannot be adjusted appropriately because the light intensity adjustment range is narrow. Even in such cases, the light intensity adjustment range can be widened by controlling the light-emitting unit in the low-speed mode of the third embodiment, and the light intensity of light irradiated onto an area corresponding to one pixel on the photosensitive drum 1 can be made equal in both normal mode and low-speed mode.

[0155] Furthermore, according to the third embodiment, the light intensity of the light emitting unit is controlled to be lower in the low speed mode compared to the normal mode, so that wear of the light emitting unit can be suppressed.

[0156] [Fourth embodiment] Since the image forming apparatus of the fourth embodiment has the same configuration as that of the first embodiment, a description of the configuration of the image forming apparatus of the fourth embodiment will be omitted. Since the control of the light emitting unit in the low-speed mode of the fourth embodiment is different from that of the first embodiment, the control of the light emitting unit in the low-speed mode of the fourth embodiment will be described.

[0157] <Control of the Light Emitting Unit in the Low-Speed ​​Mode of the Fourth Embodiment> The control of the light-emitting unit in the low-speed mode of the fourth embodiment will be described. In the fourth embodiment, as in the third embodiment, the light-emitting unit is controlled using a light intensity adjustment method based on lighting control using a period-divided line synchronization signal and a light intensity adjustment method based on driving current control for the light-emitting unit. The fourth embodiment differs from the third embodiment in the calculation process for the lighting count f based on the same image data for one line.

[0158] 21 is a timing chart showing the operation in the sub-scanning direction in the low-speed mode in the image data storage unit 84 of the fourth embodiment. Here, a case will be described as an example in which the rotation speed of the photosensitive drum 1 in the low-speed mode is set to 5 / 7 times the rotation speed of the photosensitive drum 1 in the normal mode. Note that the meanings of the symbols shown in FIG. 21 are the same as those shown in FIG. 11. Furthermore, this embodiment is not limited to the case in which the rotation speed of the photosensitive drum 1 in the low-speed mode is set to 5 / 7 times the rotation speed of the photosensitive drum 1 in the normal mode, and other speed differences are also applicable.

[0159] First, the period of the line synchronization signal in low-speed mode is set by processing similar to that in the third embodiment. In the example shown in FIG. 21, Tmin=1 / 4. The CPU 73 calculates Equation 7 and selects 5 as the division coefficient e so that 28 / 5≧e is satisfied. The CPU 73 instructs the synchronization signal generation unit 74 to set the period of the line synchronization signal in low-speed mode to 7 / 25 times the period of the line synchronization signal in normal mode. For example, the time from time TG0_0 to TG0_1 shown in FIG. 21 is 7 / 25 times the time from time TA0 to time TA1 shown in FIG. 13.

[0160] Next, the fourth embodiment performs a process different from that of the third embodiment regarding the calculation of the number of times f the light is turned on for one line of the same image data. In low-speed mode, within the time it takes for the photosensitive drum 1 to rotate an amount corresponding to the pixel size corresponding to the resolution in the sub-scanning direction, the synchronization signal generation unit 74 generates e line synchronization signals. The CPU 73 controls the buffer read control unit 17 to transmit f (f is an integer) image data signals for one line of the same image data to cause the light-emitting unit 50 to emit light, and ef (ef) image data signals to prevent the light-emitting unit 50 from emitting light. Here, the method for calculating the number of times f the light is turned on for one line of the same image data will be described. The CPU 73 calculates the value of (a×e) / b and selects an integer that is equal to or smaller than (a×e) / b and closest to (a×e) / b as the number of times f the light is turned on for one line of the same image data. In the example shown in FIG. 21, the CPU 73 calculates (a×e) / b=25 / 7 and selects 3, which is the nearest integer equal to or smaller than 25 / 7, as the number of times f the light is turned on for one line of the same image data.

[0161] In the low-speed mode, the synchronization signal generator 74 generates five line synchronization signals during the time it takes for the photosensitive drum 1 to rotate by a pixel size corresponding to the resolution in the sub-scanning direction. The CPU 73 controls the buffer read controller 17 to transmit the same image data signal for one line three times in synchronization with each of the five line synchronization signals and to transmit an image data signal that does not cause the light-emitting unit 50 to emit light (to be unlit) twice. Upon receiving instructions from the CPU 73, the buffer read controller 17 reads the same image data signal for one line from the line buffer 15 and transmits it to the light-emitting element array chip 40 in synchronization with the first, second, and third line synchronization signals generated at times TG0_0, TG0_1, and TG0_2, respectively. Furthermore, the buffer read controller 17 transmits an image data signal that does not cause the light-emitting unit 50 to emit light (to be unlit) to the light-emitting element array chip 40 in synchronization with the fourth and fifth line synchronization signals generated at times TG0_3 and TG0_4. The CPU 73 repeatedly performs the above control after time TG1_0. The order of transmission does not matter as long as the same image data signal for one line is transmitted f times in synchronization with e line synchronization signals, and an image data signal that does not cause the light-emitting unit 50 to emit light (becomes unlit) is transmitted ef times. In the example shown in Fig. 21, the CPU 73 may be configured to transmit an image data signal that does not cause the light-emitting unit 50 to emit light (becomes unlit) in synchronization with the first and second line synchronization signals, and to transmit the same image data signal for one line in synchronization with the third, fourth, and fifth line synchronization signals, respectively.

[0162] Next, the drive current to be passed through the light-emitting unit 50 in the low-speed mode is set by processing similar to that in the third embodiment. The CPU 73 reads a table of drive current and light intensity from the head information storage unit 171, and calculates the value of the drive current required to make the light intensity of the light-emitting unit 50 match the target value ((a×e) / (b×f) times the light intensity in the normal mode). The CPU 73 then instructs the setting of the drive current to be passed through the light-emitting unit 50 of the light-emitting element array chip 40. In the example shown in FIG. 21, the target value is 25 / 21 times.

[0163] Here, the result of comparing the amount of light irradiated onto an area corresponding to one pixel on the photosensitive drum 1 in the normal mode and the low-speed mode is the same as equation 8 in the third embodiment. That is, since the ratio of the amount of light in the low-speed mode to the amount in the normal mode is the same, it is possible to make the amount of light irradiated onto an area corresponding to one pixel on the photosensitive drum 1 equal in the normal mode and the low-speed mode.

[0164] To summarize the above, according to the fourth embodiment, the CPU 73 adjusts the light intensity by shortening the period of the line synchronization signal through lighting control using a period-divided line synchronization signal, and further adjusts the light intensity by increasing the drive current through drive current control of the light-emitting unit to compensate for the lack of light intensity. That is, when executing the low-speed mode in which the photosensitive drum 1 is rotated at a speed (second rotation speed) that is slower than the rotation speed in the normal mode (first rotation speed) and is a / b times the rotation speed in the normal mode, the CPU 73 performs the following control. In the low-speed mode, the CPU 73 controls the synchronization signal generation unit 74 to generate a line synchronization signal with a period that is b / (a×e) times the period of the line synchronization signal generated in the normal mode. Furthermore, the CPU 73 controls the buffer read control unit 17 to transmit the same image data signal for one line f times and transmit an image data signal that does not cause the light-emitting unit 50 to emit light (becomes unlit) ef times in synchronization with e line synchronization signals generated by the synchronization signal generation unit 74 during the time it takes for the photosensitive drum 1 to rotate an amount corresponding to the pixel size in the sub-scanning direction in the low-speed mode. The CPU 73 selects an integer that is equal to or smaller than (a×e) / b and closest to (a×e) / b as the number of times f the light is turned on for the same image data for one line. The CPU 73 also instructs the light-emitting element array chip 40 to set the drive current to be passed through the light-emitting unit 50 so that the light intensity of the light-emitting unit 50 matches a target value ((a×e) / (b×f) times the light intensity in the normal mode). This control allows the amount of light irradiated onto an area corresponding to one pixel on the photosensitive drum 1 to be the same in both the normal mode and the low-speed mode. Therefore, in the low speed mode, it is possible to prevent the toner from being deposited on the photosensitive drum 1 too much, and toner deposited on the photosensitive drum 1 can be adjusted appropriately.

[0165] In the third embodiment, an integer equal to or greater than the value of (a×e) / b and closest thereto is selected as the number of times f that one line of light will be turned on with the same image data. On the other hand, in the fourth embodiment, an integer equal to or less than the value of (a×e) / b and closest thereto is selected as the number of times f that one line of light will be turned on with the same image data. As yet another configuration, the CPU 73 may be configured to select an integer closest to the value of (a×e) / b as the number of times f that one line of light will be turned on with the same image data. By adopting such a configuration, it is possible to minimize changes in the light intensity due to drive current control, and even when the range of drive current that can be passed due to the characteristics of the light-emitting unit 50 is narrow, the light intensity can be adjusted by applying this embodiment.

[0166] [Fifth embodiment] Since the image forming apparatus of the fifth embodiment has the same configuration as that of the first embodiment, a description of the configuration of the image forming apparatus of the fifth embodiment will be omitted. Since the control of the light-emitting unit in the low-speed mode of the fifth embodiment is different from that of the first embodiment, the control of the light-emitting unit in the low-speed mode of the fifth embodiment will be described. In the fifth embodiment, the light-emitting unit is controlled using a light intensity adjustment method by lighting control using a period-divided line synchronization signal, a light intensity adjustment method by thinning lighting control in units of one line in the main scanning direction, and a light intensity adjustment method by controlling the drive current of the light-emitting unit.

[0167] <Control of the Light Emitting Unit in the Low-Speed ​​Mode of the Fifth Embodiment> Control of the light emitting unit in the slow mode of the fifth embodiment will be described.

[0168] 22 is a timing chart showing the operation in the sub-scanning direction in the low-speed mode in the image data storage unit 84 of the fifth embodiment. Here, a case will be described as an example in which the rotation speed of the photosensitive drum 1 in the low-speed mode is set to 5 / 9 times the rotation speed of the photosensitive drum 1 in the normal mode. Note that the meanings of the symbols shown in FIG. 22 are the same as those shown in FIG. 11. Furthermore, this embodiment is not limited to the case in which the rotation speed of the photosensitive drum 1 in the low-speed mode is set to 5 / 9 times the rotation speed of the photosensitive drum 1 in the normal mode, and other speed differences are also applicable.

[0169] If the rotation speed of the photosensitive drum 1 in the low-speed mode is a / b times faster than the rotation speed of the photosensitive drum 1 in the normal mode, the CPU 73 first calculates a value that multiplies the period of the line synchronization signal in the normal mode by b / (a×e). The CPU 73 then instructs the synchronization signal generator 74 to use the calculated value as the period of the line synchronization signal in the low-speed mode. Regarding the calculation of the division coefficient e, the fifth embodiment performs a process different from those of the third and fourth embodiments. To ensure that one line's worth of image data signal is transmitted in time, the division coefficient e is selected to satisfy Equation 7. The CPU 73 calculates the value of b / (a×Tmin) and selects the division coefficient e from integers equal to or less than b / (a×Tmin) based on a predetermined selection criterion. Various selection criteria can be applied as the predetermined selection criterion as long as they are consistent with the condition that the division coefficient e is an integer equal to or less than b / (a×Tmin). For example, the CPU 73 may refer to information about the prohibited frequency band of the line synchronization signal in the low-speed mode and select the division coefficient e so as to avoid this prohibited frequency band. This configuration can suppress the generation of switching noise, power supply noise, and radiation noise synchronized with the line synchronization signal in the prohibited frequency band. As in the third and fourth embodiments, the CPU 73 may set the division coefficient e to the nearest integer less than or equal to b / (a × Tmin). This configuration can reduce the amount of calculation and time required to calculate the division coefficient e. In the example shown in FIG. 22, Tmin = 1 / 4. The predetermined selection criterion is to avoid the prohibited frequency band based on information about the prohibited frequency band of the line synchronization signal in low-speed mode, which is prepared in advance. The CPU 73 calculates Equation 7 and selects 5 as the division coefficient e so that 36 / 5≧e and the predetermined selection criterion are satisfied. The CPU 73 instructs the synchronization signal generation unit 74 to set the period of the line synchronization signal in low-speed mode to 9 / 25 times the period of the line synchronization signal in normal mode. For example, the time from TH0_0 to TH0_1 shown in FIG. 22 is 9 / 25 times the time from TA0 to TA1 shown in FIG. 13.

[0170] Next, the CPU 73 controls the buffer read control unit 17 to transmit the same image data signal for one line f times to cause the light emitting unit 50 to emit light, and transmit the image data signal ef times to prevent the light emitting unit 50 from emitting light (turning it off), in synchronization with e line synchronization signals generated by the synchronization signal generation unit 74, within the time it takes for the photosensitive drum 1 to rotate an amount equivalent to the pixel size corresponding to the resolution in the sub-scanning direction in the low-speed mode. The CPU 73 also instructs the light emitting element array chip 40 to turn on 6×g columns and turn off 6×(1-g) columns out of the six light emitting columns used for multiple exposure. Here, a method for calculating the magnification g of the number of light emitting columns to be lit and the number of times f to light up using the same image data for one line will be described.

[0171] The CPU 73 calculates a combination of g and f that satisfies the following three selection criteria. The first selection criterion is that g≦1 (i.e., g is 1 or less) and the number of lit light-emitting columns, 6×g, is an integer. The second selection criterion is that f≦e is satisfied. The third selection criterion is to select a combination of g and f for which the value of (b×f×g) / (a×e) is closest to 1. By providing the third selection criterion, it is possible to minimize the amount of change in drive current when adjusting the light intensity by controlling the drive current of the light-emitting unit, as described below. Note that the third selection criterion is not limited to the above criteria, and various selection criteria can be applied as long as they are consistent with the first and second selection criteria. For example, a configuration may be adopted in which the magnification g of the number of lit light-emitting columns is set to a fixed value, and then f for which the value of (b×f×g) / (a×e) is closest to 1 is selected.

[0172] 22, the CPU 73 calculates g and f based on the first, second, and third selection criteria, and selects 4 / 6 as the magnification g of the number of light-emitting columns to be turned on and 4 as the number f of times the light is turned on for one line of the same image data. Then, in the low-speed mode, the CPU 73 controls the buffer read control unit 17 to transmit the same image data signal for one line four times in synchronization with five line synchronization signals generated by the synchronization signal generation unit 74, and to transmit an image data signal that does not cause the light-emitting unit 50 to emit light (to be turned off) once, within the time it takes for the photosensitive drum 1 to rotate an amount corresponding to the pixel size corresponding to the resolution in the sub-scanning direction. Upon receiving an instruction from the CPU 73, the buffer read control unit 17 reads one line of the same image data signal from the line buffer 15 in synchronization with the first, second, third, and fourth line synchronization signals generated at times TH0_0, TH0_1, TG0_2, and TH0_3, respectively, and transmits the same image data signal for one line to the light-emitting element array chip 40. Furthermore, the buffer read control unit 17 transmits an image data signal that does not cause the light-emitting unit 50 to emit light (to be unlit) to the light-emitting element array chip 40 in synchronization with the fifth line synchronization signal generated at time TH0_4. The CPU 73 repeats the above control from time TH1_0 onwards. Note that the order of transmission does not matter as long as the same image data signal for one line is transmitted four times in synchronization with each of the five line synchronization signals, and an image data signal that does not cause the light-emitting unit 50 to emit light (to be unlit) is transmitted once. 22, the CPU 73 may be configured to transmit an image data signal that causes the light-emitting unit 50 not to emit light (to be turned off) in synchronization with the first line synchronization signal, and to transmit the same image data signal for one line in synchronization with each of the second, third, fourth, and fifth line synchronization signals. Also, the CPU 73 instructs the light-emitting element array chip 40 to turn on four of the six light-emitting columns used for multiple exposure and turn off two of the remaining columns. In FIG. 22, the four lit columns are the light-emitting units to which buf_data_0_000, buf_data_2_000, buf_data_4_000, and buf_data_5_000 are connected.The two columns that are not lit are the light-emitting units connected to buf_data_1_000 and buf_data_3_000. The combination of the four columns that are lit and the two columns that are not lit is not limited to the above, and other combinations may be selected from the six lit columns.

[0173] Next, the CPU 73 reads the table of drive current and light intensity from the head information storage unit 171, and calculates the value of the drive current required to make the light intensity of the light-emitting unit 50 match the target value ((a×e) / (b×f×g) times the light intensity in normal mode). The CPU 73 then instructs the setting of the drive current to be passed through the light-emitting unit 50 of the light-emitting element array chip 40. As a result, the drive current flows as set in the light-emitting unit 50, and the light intensity of the light-emitting unit 50 is adjusted to match the target value ((a×e) / (b×f×g) times the light intensity in normal mode). In the example shown in FIG. 22, the target value is 25 / 24 times.

[0174] Here, the amount of light irradiated onto an area corresponding to one pixel on the photosensitive drum 1 in normal mode and low-speed mode is compared. The cycle of the line synchronization signal in low-speed mode is changed by b / (a×e) times compared to normal mode, so the amount of light is changed by b / (a×e) times. Furthermore, in low-speed mode, during the time it takes for the photosensitive drum 1 to rotate an amount corresponding to the pixel size corresponding to the resolution in the sub-scanning direction, the same image data signal for one line is transmitted f times in synchronization with each of the e line synchronization signals generated, and an image data signal that does not cause the light-emitting element 50 to emit light (to be turned off) is transmitted ef times. Furthermore, the number of light-emitting columns that are lit in low-speed mode is changed by g times compared to normal mode, so the amount of light is changed by g times. Furthermore, the light amount of the light-emitting element in low-speed mode is changed by (a×e) / (b×f×g) times compared to normal mode, so the amount of light is changed by (a×e) / (b×f×g) times. Therefore, since the light intensity ratio in the low-speed mode to the normal mode is equal, as shown in the following equation 9, the amount of light irradiated onto an area corresponding to one pixel on the photosensitive drum 1 can be made equal in both the normal mode and the low-speed mode.

[0175] (Change in light intensity due to change in the cycle of the line synchronization signal: b / (a×e) times) × (number of times f times lighting is performed using the same image data for one line) × (change in light intensity due to change in the light-emitting row that is lit: g times) × (change in light intensity due to drive current control: (a×e) / (b×f×g) times) = (ratio of light intensity in low-speed mode to normal mode: same) (Equation 9)

[0176] To summarize the above, according to the fifth embodiment, the CPU 73 controls the light-emitting units using a light intensity adjustment method based on lighting control using a period-divided line synchronization signal, a light intensity adjustment method based on thinning-out lighting control in units of one line in the main scanning direction, and a light intensity adjustment method based on controlling the drive current of the light-emitting units. That is, when executing the low-speed mode in which the photosensitive drum 1 is rotated at a speed (second rotation speed) that is slower than the rotation speed in the normal mode (first rotation speed) and is a / b times the rotation speed in the normal mode, the CPU 73 performs the following control. In the low-speed mode, the CPU 73 controls the synchronization signal generation unit 74 to generate a line synchronization signal at a period that is b / (a×e) times the period of the line synchronization signal generated in the normal mode. Furthermore, the CPU 73 controls the buffer read control unit 17 to transmit the same image data signal for one line f times in synchronization with e line synchronization signals generated by the synchronization signal generation unit 74 and transmit an image data signal ef times that does not cause the light-emitting unit 50 to emit light, within the time it takes for the photosensitive drum 1 to rotate an amount corresponding to the pixel size of the resolution in the sub-scanning direction in the low-speed mode. The CPU 73 also instructs the light-emitting element array chip 40 to light up 6×g columns and to turn off 6×(1−g) columns of the six light-emitting columns used for multiple exposure. The CPU 73 also instructs the light-emitting element array chip 40 to set the drive current to be passed through the light-emitting unit 50 so that the light intensity of the light-emitting unit 50 matches a target value ((a×e) / (b×f×g) times the light intensity in the normal mode). This control allows the amount of light irradiated onto an area corresponding to one pixel on the photosensitive drum 1 to be equal in both the normal mode and the low-speed mode. Therefore, in the low speed mode, it is possible to prevent the toner from being deposited on the photosensitive drum 1 too much, and toner deposited on the photosensitive drum 1 can be adjusted appropriately.

[0177] Furthermore, according to the fifth embodiment, the CPU 73 selects a combination of the division coefficient e, the magnification g of the number of light-emitting columns to be lit, and the number of times f the light is turned on for one line of the same image data from multiple options based on various selection criteria, thereby enabling optimal control tailored to the purpose. For example, as described above, the CPU 73 references information about the prohibited frequency band of the line synchronization signal in low-speed mode, which is prepared in advance, and selects the division coefficient e so as to avoid this prohibited frequency band. This prevents switching noise, power supply noise, and radiation noise synchronized with the line synchronization signal from occurring in the prohibited frequency band. Furthermore, the CPU 73 selects an integer equal to or less than and closest to b / (a×Tmin) as the division coefficient e, thereby reducing the amount of calculation and calculation time required to calculate the division coefficient e. Furthermore, the CPU 73 selects a combination of g and f such that the value of (b×f×g) / (a×e) is closest to 1, thereby minimizing the amount of change in drive current when adjusting the light intensity by controlling the drive current of the light-emitting unit. Furthermore, the CPU 73 sets the magnification factor g of the number of light-emitting columns to be lit to a fixed value, and then selects f such that the value of (b×f×g) / (a×e) is closest to 1, thereby reducing the amount of calculation and calculation time required to calculate the magnification factor g of the number of light-emitting columns to be lit and the number of times f is lit using one line of the same image data.

[0178] In this embodiment, as shown in FIG. 6, a configuration in which multiple light-emitting units 50 used for multiple exposure are arranged in a straight line in the sub-scanning direction (arrow Y direction) has been described. However, this embodiment is not limited to this configuration. That is, this embodiment can also be applied to a configuration in which multiple light-emitting units 50 are arranged in the sub-scanning direction (arrow Y direction) while being offset from each other in the main scanning direction (arrow X direction). FIG. 23 is a schematic diagram illustrating the arrangement of light-emitting units in another embodiment. In the example shown in FIG. 23, four light-emitting units 50 are arranged in the sub-scanning direction (arrow Y direction) while being offset by d3 from each other in the main scanning direction (arrow X direction). The widths W1 and W2 of the light-emitting units 50 are 20.90 μm, and the intervals d1 and d2 between adjacent light-emitting units 50 are 0.26 μm. That is, the light-emitting units 50 are arranged at a pitch of 21.16 μm (1200 dpi) in the directions of arrows X and Y. The deviation d3 of the four light emitting elements 50 in the main scanning direction (arrow X direction) is 5.29 μm (4800 dpi).

[0179] (Other embodiments) 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.

[0180] The disclosure of the present specification includes the following image forming apparatus and image forming method. (Item 1) a photosensitive drum that rotates around a rotation axis; an exposure head having a plurality of light-emitting units arranged in n columns along a direction intersecting the rotation axis direction and emitting light based on image data signals to expose the photosensitive drum; an image forming apparatus including a control unit that controls a rotation speed of the photosensitive drum and light emission of the light emitting unit, The control unit a first mode in which the photosensitive drum is rotated at a first rotation speed, and a second mode in which the photosensitive drum is rotated at a second rotation speed that is slower than the first rotation speed and is a / b times (a and b are integers) the first rotation speed; In the second mode, in order to perform exposure for forming an image of one line along the rotation axis direction on a sheet, A cycle of the timing at which the light emitting unit emits light is controlled to a cycle that is b / a times the cycle of the first mode, setting c / d (c and d are integers) such that n×(c / d) is an integer, and controlling the light-emitting units so that the number of columns (n×(c / d)) of the number of columns n emit light and the number of columns (n×((dc) / d)) of the light-emitting units does not emit light; Setting the drive current of the light-emitting units so that the light intensity of the plurality of light-emitting units is equal to (a×d) / (b×c) times the light intensity of the first mode An image forming apparatus characterized by: (Item 2) The control unit sets a value that is equal to or greater than a / b and closest to a / b among values ​​where n×(c / d) is an integer as c / d. 2. The image forming apparatus according to item 1, (Item 3) The control unit sets as c / d a value that is equal to or smaller than a / b times and closest to a / b, among values ​​where n×(c / d) is an integer. 2. The image forming apparatus according to item 1, (Item 4) The control unit sets the value closest to a / b as c / d among values ​​where n×(c / d) is an integer. 2. The image forming apparatus according to item 1, (Item 5) a photosensitive drum that rotates around a rotation axis; an exposure head having a plurality of light-emitting units that are arranged along the rotation axis direction and in a plurality of rows along a direction intersecting the rotation axis direction, and that emit light based on image data signals to expose the photosensitive drum; an image forming apparatus including a control unit that controls a rotation speed of the photosensitive drum and light emission of the light emitting unit, The control unit a first mode in which the photosensitive drum is rotated at a first rotation speed, and a second mode in which the photosensitive drum is rotated at a second rotation speed that is slower than the first rotation speed and is a / b times (a and b are integers) the first rotation speed; In the second mode, in order to perform exposure for forming an image of one line along the rotation axis direction on a sheet, calculating a division coefficient e, which is an integer that satisfies b / (a×e)≧Tmin, where Tmin is a lower limit value of the time that allows transmission of one line of image data signals, and controlling the cycle of the timing at which the light emitting unit emits light to a cycle that is b / (a×e) times the cycle of the first mode; controlling the plurality of light-emitting units so that the plurality of light-emitting units emit light f times and do not emit light (ef) times while transmitting one line of image data signals e times; Setting a drive current for the light-emitting units so that the light intensity of the plurality of light-emitting units is equal to (a×e) / (b×f) times the light intensity of the first mode An image forming apparatus characterized by: (Item 6) The control unit sets the f to an integer that is equal to or greater than the value of (a×e) / b and is closest to the value of (a×e) / b. 6. The image forming apparatus according to item 5, (Item 7) The control unit sets the f to an integer that is equal to or smaller than the value of (a×e) / b and is closest to the value of (a×e) / b. 6. The image forming apparatus according to item 5, (Item 8) The control unit sets the f to an integer closest to the value of (a×e) / b. 6. The image forming apparatus according to item 5, (Item 9) a photosensitive drum that rotates around a rotation axis; an exposure head having a plurality of light-emitting units that are arranged along the rotation axis direction and in n columns along a direction intersecting the rotation axis direction, and that emit light based on image data signals to expose the photosensitive drum; an image forming apparatus including a control unit that controls a rotation speed of the photosensitive drum and light emission of the light emitting unit, The control unit a first mode in which the photosensitive drum is rotated at a first rotation speed, and a second mode in which the photosensitive drum is rotated at a second rotation speed that is slower than the first rotation speed and is a / b times (a and b are integers) the first rotation speed; In the second mode, in order to perform exposure for forming an image of one line along the rotation axis direction on a sheet, calculating a division coefficient e, which is an integer that satisfies b / (a×e)≧Tmin, where Tmin is a lower limit value of the time that allows transmission of one line of image data signals, and controlling the cycle of the timing at which the light emitting unit emits light to a cycle that is b / (a×e) times the cycle of the first mode; controlling the plurality of light-emitting units so that the plurality of light-emitting units emit light f times and do not emit light (ef) times while transmitting one line of image data signals e times; setting a magnification g equal to or less than 1, where n×g is an integer, and controlling the light-emitting units so that the number of columns (n×g) of the number of columns n emit light and the number of columns (n×(1−g)) of the number of columns do not emit light; Setting the drive current of the light-emitting units so that the light intensity of the plurality of light-emitting units coincides with (a×e) / (b×f×g) times the light intensity of the first mode An image forming apparatus characterized by: (Item 10) The control unit sets the division coefficient e so that the frequency of light emitted by the light emitting unit in the second mode is different from a predetermined prohibited frequency band. 10. The image forming apparatus according to item 9, (Item 11) The control unit sets a combination of the g and the f so that a value of (b×f×g) / (a×e) is closest to 1. 11. The image forming apparatus according to item 9 or 10, (Item 12) The control unit In the first mode, the drive current of the plurality of light-emitting units is set to a value that is in a range between an upper limit value and a lower limit value and that is different from the upper limit value and the lower limit value; In the second mode, the drive current is set within a range between the upper limit and the lower limit. 12. The image forming apparatus according to any one of items 1 to 11, wherein: (Item 13) a photosensitive drum that rotates around a rotation axis; an exposure head having a plurality of light-emitting units arranged in n columns along a direction intersecting the rotation axis direction and emitting light based on image data signals to expose the photosensitive drum; an image forming method in an image forming apparatus including a control unit that controls a rotation speed of the photosensitive drum and light emission of the light emitting unit, The control unit a first mode in which the photosensitive drum is rotated at a first rotation speed, and a second mode in which the photosensitive drum is rotated at a second rotation speed that is slower than the first rotation speed and is a / b times (a and b are integers) the first rotation speed; In the second mode, in order to perform exposure for forming an image of one line along the rotation axis direction on a sheet, A cycle of the timing at which the light emitting unit emits light is controlled to a cycle that is b / a times the cycle of the first mode, setting c / d (c and d are integers) such that n×(c / d) is an integer, and controlling the light-emitting units so that the number of columns (n×(c / d)) of the number of columns n emit light and the number of columns (n×((dc) / d)) of the light-emitting units does not emit light; Setting the drive current of the light-emitting units so that the light intensity of the plurality of light-emitting units is equal to (a×d) / (b×c) times the light intensity of the first mode An image forming method comprising: (Item 14) a photosensitive drum that rotates around a rotation axis; an exposure head having a plurality of light-emitting units that are arranged along the rotation axis direction and in a plurality of rows along a direction intersecting the rotation axis direction, and that emit light based on image data signals to expose the photosensitive drum; an image forming method in an image forming apparatus including a control unit that controls a rotation speed of the photosensitive drum and light emission of the light emitting unit, The control unit a first mode in which the photosensitive drum is rotated at a first rotation speed, and a second mode in which the photosensitive drum is rotated at a second rotation speed that is slower than the first rotation speed and is a / b times (a and b are integers) the first rotation speed; In the second mode, in order to perform exposure for forming an image of one line along the rotation axis direction on a sheet, calculating a division coefficient e, which is an integer that satisfies b / (a×e)≧Tmin, where Tmin is a lower limit value of the time that allows transmission of one line of image data signals, and controlling the cycle of the timing at which the light emitting unit emits light to a cycle that is b / (a×e) times the cycle of the first mode; controlling the plurality of light-emitting units so that the plurality of light-emitting units emit light f times and do not emit light (ef) times while transmitting one line of image data signals e times; Setting a drive current for the light-emitting units so that the light intensity of the plurality of light-emitting units is equal to (a×e) / (b×f) times the light intensity of the first mode An image forming method comprising: (Item 15) a photosensitive drum that rotates around a rotation axis; an exposure head having a plurality of light-emitting units that are arranged along the rotation axis direction and in n columns along a direction intersecting the rotation axis direction, and that emit light based on image data signals to expose the photosensitive drum; an image forming method in an image forming apparatus including a control unit that controls a rotation speed of the photosensitive drum and light emission of the light emitting unit, The control unit a first mode in which the photosensitive drum is rotated at a first rotation speed, and a second mode in which the photosensitive drum is rotated at a second rotation speed that is slower than the first rotation speed and is a / b times (a and b are integers) the first rotation speed; In the second mode, in order to perform exposure for forming an image of one line along the rotation axis direction on a sheet, calculating a division coefficient e, which is an integer that satisfies b / (a×e)≧Tmin, where Tmin is a lower limit value of the time that allows transmission of one line of image data signals, and controlling the cycle of the timing at which the light emitting unit emits light to a cycle that is b / (a×e) times the cycle of the first mode; controlling the plurality of light-emitting units so that the plurality of light-emitting units emit light f times and do not emit light (ef) times while transmitting one line of image data signals e times; setting a magnification g equal to or less than 1, where n×g is an integer, and controlling the light-emitting units so that the number of columns (n×g) of the number of columns n emit light and the number of columns (n×(1−g)) of the number of columns do not emit light; Setting the drive current of the light-emitting units so that the light intensity of the plurality of light-emitting units coincides with (a×e) / (b×f×g) times the light intensity of the first mode An image forming method comprising:

[0181] The invention is not limited to the above-described embodiments, and various changes and modifications can be made without departing from the spirit and scope of the invention. Accordingly, the following claims are appended to apprise the public of the scope of the invention. [Explanation of symbols]

[0182] 1···Photosensitive drum, 6···Exposure head, 50···Light emitting unit, 73···CPU.

Claims

1. a photosensitive drum that rotates around a rotation axis; an exposure head having a plurality of light-emitting units arranged in n columns along a direction intersecting the rotation axis direction and emitting light based on image data signals to expose the photosensitive drum; an image forming apparatus including a control unit that controls a rotation speed of the photosensitive drum and light emission of the light emitting unit, The control unit a first mode in which the photosensitive drum is rotated at a first rotation speed, and a second mode in which the photosensitive drum is rotated at a second rotation speed that is slower than the first rotation speed and is a / b times (a and b are integers) the first rotation speed; In the second mode, in order to perform exposure for forming an image of one line along the rotation axis direction on a sheet, A cycle of the timing at which the light emitting unit emits light is controlled to a cycle that is b / a times the cycle of the first mode, setting c / d (c and d are integers) such that n×(c / d) is an integer, and controlling the light-emitting units so that the number of columns (n×(c / d)) of the number of columns n emits light and the number of columns (n×((d−c) / d)) of the light-emitting units does not emit light; Setting the drive current of the light-emitting units so that the light amount of the plurality of light-emitting units is equal to (a×d) / (b×c) times the light amount of the first mode An image forming apparatus characterized by:

2. The control unit sets a value that is equal to or greater than a / b and closest to a / b among values ​​where n×(c / d) is an integer as c / d.

2. The image forming apparatus according to claim 1, wherein the image forming apparatus is a recording medium.

3. The control unit sets a value that is equal to or smaller than a / b and closest to a / b among values ​​where n×(c / d) is an integer as c / d.

2. The image forming apparatus according to claim 1, wherein the image forming apparatus is a recording medium.

4. The control unit sets the value closest to a / b as c / d among values ​​where n×(c / d) is an integer.

2. The image forming apparatus according to claim 1, wherein the image forming apparatus is a recording medium.

5. a photosensitive drum that rotates around a rotation axis; an exposure head having a plurality of light-emitting units that are arranged along the rotation axis direction and in a plurality of rows along a direction intersecting the rotation axis direction, and that emit light based on image data signals to expose the photosensitive drum; an image forming apparatus including a control unit that controls a rotation speed of the photosensitive drum and light emission of the light emitting unit, The control unit a first mode in which the photosensitive drum is rotated at a first rotation speed, and a second mode in which the photosensitive drum is rotated at a second rotation speed that is slower than the first rotation speed and is a / b times (a and b are integers) the first rotation speed; In the second mode, in order to perform exposure for forming an image of one line along the rotation axis direction on a sheet, a division coefficient e is calculated, which is an integer that satisfies b / (a×e)≧Tmin, where Tmin is a lower limit value of the time that allows transmission of one line of image data signals, and the cycle of the timing at which the light-emitting unit emits light is controlled to a cycle that is b / (a×e) times the cycle of the first mode; controlling the plurality of light-emitting units so that the plurality of light-emitting units emit light f times and do not emit light (ef) times while transmitting one line of image data signals e times; The drive current of the light-emitting units is set so that the light amount of the plurality of light-emitting units is equal to (a×e) / (b×f) times the light amount of the first mode. An image forming apparatus characterized by:

6. The control unit sets the f to an integer that is equal to or greater than the value of (a×e) / b and is closest to the value of (a×e) / b.

6. The image forming apparatus according to claim 5,

7. The control unit sets f to an integer that is equal to or smaller than the value of (a×e) / b and is closest to the value of (a×e) / b.

6. The image forming apparatus according to claim 5,

8. The control unit sets the f to an integer closest to the value of (a×e) / b.

6. The image forming apparatus according to claim 5,

9. a photosensitive drum that rotates around a rotation axis; an exposure head having a plurality of light-emitting units that are arranged along the rotation axis direction and in n columns along a direction intersecting the rotation axis direction, and that emit light based on image data signals to expose the photosensitive drum; an image forming apparatus including a control unit that controls a rotation speed of the photosensitive drum and light emission of the light emitting unit, The control unit a first mode in which the photosensitive drum is rotated at a first rotation speed, and a second mode in which the photosensitive drum is rotated at a second rotation speed that is slower than the first rotation speed and is a / b times (a and b are integers) the first rotation speed; In the second mode, in order to perform exposure for forming an image of one line along the rotation axis direction on a sheet, a division coefficient e is calculated, which is an integer that satisfies b / (a×e)≧Tmin, where Tmin is a lower limit value of the time that allows transmission of one line of image data signals, and the cycle of the timing at which the light-emitting unit emits light is controlled to a cycle that is b / (a×e) times the cycle of the first mode; controlling the plurality of light-emitting units so that the plurality of light-emitting units emit light f times and do not emit light (ef) times while transmitting one line of image data signals e times; setting a magnification g equal to or less than 1, where n×g is an integer, and controlling the light-emitting units so that the number of columns (n×g) of the number of columns n emit light and the number of columns (n×(1−g)) of the number of columns do not emit light; Setting the drive current of the light-emitting units so that the light amount of the plurality of light-emitting units is equal to (a×e) / (b×f×g) times the light amount of the first mode. An image forming apparatus characterized by:

10. The control unit sets the division coefficient e so that the frequency of light emitted by the light emitting unit in the second mode is different from a predetermined prohibited frequency band.

10. The image forming apparatus according to claim 9,

11. The control unit sets a combination of g and f so that a value of (b×f×g) / (a×e) is closest to 1.

10. The image forming apparatus according to claim 9,

12. The control unit In the first mode, the drive current of the plurality of light-emitting units is set to a value that is in a range between an upper limit value and a lower limit value and that is different from the upper limit value and the lower limit value; In the second mode, the drive current is set within a range between the upper limit and the lower limit.

12. The image forming apparatus according to claim 1, wherein the first and second ink cartridges are arranged in a row.

13. a photosensitive drum that rotates around a rotation axis; an exposure head having a plurality of light-emitting units arranged in n columns along a direction intersecting the rotation axis direction and emitting light based on image data signals to expose the photosensitive drum; an image forming method in an image forming apparatus including a control unit that controls a rotation speed of the photosensitive drum and light emission of the light emitting unit, The control unit a first mode in which the photosensitive drum is rotated at a first rotation speed, and a second mode in which the photosensitive drum is rotated at a second rotation speed that is slower than the first rotation speed and is a / b times (a and b are integers) the first rotation speed; In the second mode, in order to perform exposure for forming an image of one line along the rotation axis direction on a sheet, A cycle of the timing at which the light emitting unit emits light is controlled to a cycle that is b / a times the cycle of the first mode, setting c / d (c and d are integers) such that n×(c / d) is an integer, and controlling the light-emitting units so that the number of columns (n×(c / d)) of the number of columns n emits light and the number of columns (n×((d−c) / d)) of the light-emitting units does not emit light; Setting the drive current of the light-emitting units so that the light amount of the plurality of light-emitting units is equal to (a×d) / (b×c) times the light amount of the first mode An image forming method comprising:

14. a photosensitive drum that rotates around a rotation axis; an exposure head having a plurality of light-emitting units that are arranged along the rotation axis direction and in a plurality of rows along a direction intersecting the rotation axis direction, and that emit light based on image data signals to expose the photosensitive drum; an image forming method in an image forming apparatus including a control unit that controls a rotation speed of the photosensitive drum and light emission of the light emitting unit, The control unit a first mode in which the photosensitive drum is rotated at a first rotation speed, and a second mode in which the photosensitive drum is rotated at a second rotation speed that is slower than the first rotation speed and is a / b times (a and b are integers) the first rotation speed; In the second mode, in order to perform exposure for forming an image of one line along the rotation axis direction on a sheet, a division coefficient e is calculated, which is an integer that satisfies b / (a×e)≧Tmin, where Tmin is a lower limit value of the time that allows transmission of one line of image data signals, and the cycle of the timing at which the light-emitting unit emits light is controlled to a cycle that is b / (a×e) times the cycle of the first mode; controlling the plurality of light-emitting units so that the plurality of light-emitting units emit light f times and do not emit light (ef) times while transmitting one line of image data signals e times; The drive current of the light-emitting units is set so that the light amount of the plurality of light-emitting units is equal to (a×e) / (b×f) times the light amount of the first mode. An image forming method comprising:

15. a photosensitive drum that rotates around a rotation axis; an exposure head having a plurality of light-emitting units that are arranged along the rotation axis direction and in n columns along a direction intersecting the rotation axis direction, and that emit light based on image data signals to expose the photosensitive drum; an image forming method in an image forming apparatus including a control unit that controls a rotation speed of the photosensitive drum and light emission of the light emitting unit, The control unit a first mode in which the photosensitive drum is rotated at a first rotation speed, and a second mode in which the photosensitive drum is rotated at a second rotation speed that is slower than the first rotation speed and is a / b times (a and b are integers) the first rotation speed; In the second mode, in order to perform exposure for forming an image of one line along the rotation axis direction on a sheet, a division coefficient e is calculated, which is an integer that satisfies b / (a×e)≧Tmin, where Tmin is a lower limit value of the time that allows transmission of one line of image data signals, and the cycle of the timing at which the light-emitting unit emits light is controlled to a cycle that is b / (a×e) times the cycle of the first mode; controlling the plurality of light-emitting units so that the plurality of light-emitting units emit light f times and do not emit light (ef) times while transmitting one line of image data signals e times; setting a magnification g equal to or less than 1, where n×g is an integer, and controlling the light-emitting units so that the number of columns (n×g) of the number of columns n emit light and the number of columns (n×(1−g)) of the number of columns do not emit light; Setting the drive current of the light-emitting units so that the light amount of the plurality of light-emitting units is equal to (a×e) / (b×f×g) times the light amount of the first mode. An image forming method comprising:

Images