Patent ID: 12259666

DESCRIPTION OF THE EMBODIMENTS

First Embodiment

<Image Forming Apparatus>

Hereinafter, an overall configuration of an image forming apparatus A according to a first embodiment of the present invention will be described together with an operation at the time of image formation with reference to the drawings. Note that the dimensions, materials, shapes, relative arrangements, and the like of components described below are not intended to limit the scope of the present invention unless otherwise specified.

The image forming apparatus A according to the present embodiment is a full-color image forming apparatus in which four color toners of yellow Y, magenta M, cyan C, and black K are transferred to a sheet to form an image. In the following description, although members using the toners of the respective colors are given suffixes of Y, M, C, and K, configurations or operations of the respective members are substantially the same as each other except that the color of the toner used is different, and thus the suffixes are omitted as appropriate unless necessary to distinguish the members from each other.

FIG.1is a schematic cross-sectional view of the image forming apparatus A. As illustrated inFIG.1, the image forming apparatus A includes an image forming portion that forms an image. The image forming portion includes photosensitive drums1(1Y,1M,1C, and1K) serving as photosensitive members, charging devices2(2Y,2M,2C, and2K), exposure heads6(6Y,6M,6C, and6K), development devices4(4Y,4M,4C, and4K), and transfer devices5(5Y,5M,5C, and5K).

Next, an image forming operation performed by the image forming apparatus A will be described. In a case of forming an image, first, a sheet S accommodated in a sheet cassette99aor a sheet cassette99bis conveyed to a registration roller96by a pickup roller91aor91b, a feeding roller92aor92b, and conveying rollers93ato93c. Thereafter, the sheet S is fed to a conveying belt11at a predetermined timing by the registration roller96.

Meanwhile, in the image forming portion, first, a surface of the photosensitive drum1Y is charged by the charging device2Y. Next, the exposure head6Y irradiates the surface of the photosensitive drum10Y with light according to image data read by an image reading portion90or image data transmitted from an external device (not illustrated), and forms an electrostatic latent image on the surface of the photosensitive drum10Y. Thereafter, yellow toner is attached to the electrostatic latent image formed on the surface of the photosensitive drum1Y by the development device4Y to form a yellow toner image on the surface of the photosensitive drum1Y. As a transfer bias is applied to the transfer device5Y, the toner image formed on the surface of the photosensitive drum1yis transferred to the sheet S that is being conveyed by the conveying belt11.

By a similar process, the photosensitive drums1M,1C, and1K are also irradiated with light by the exposure heads6M,6C, and6K to form electrostatic latent images, and toner images of magenta, cyan, and black are formed by the development devices4M,4C, and4K. Further, as a transfer bias is applied to the transfer devices5M,5C, and5K, these toner images are overlappingly transferred onto the yellow toner image on the sheet S. As a result, a full-color toner image corresponding to the image data is formed on a surface of the sheet S.

Thereafter, the sheet S carrying the toner image is conveyed to a fixing device94by a conveying belt97, and subjected to heating and pressurization processing in the fixing device94. As a result, the toner image on the sheet S is fixed to the sheet S. Then, the sheet S to which the toner image is fixed is discharged to a discharge tray95by a discharge roller98.

<Exposure Head>

Next, a configuration of the exposure head6will be described.

FIG.2Ais a perspective view of the photosensitive drum1and the exposure head6.FIG.2Bis a cross-sectional view of the photosensitive drum1and the exposure head6.FIGS.3A and3Bare views illustrating mounting surfaces on one side and the other side of a printed circuit board22included in the exposure head6.FIG.3Cis an enlarged view of a region V illustrated inFIG.3B.

As illustrated inFIGS.2A and2B, the exposure head6is fixed at a position facing the surface of the photosensitive drum1by a fixing member (not illustrated). The exposure head6includes a light emitting element array chip40that emits light and the printed circuit board22on which the light emitting element array chip40is mounted. In addition, there are provided a rod lens array23that forms an image of (collects) light emitted from the light emitting element array chip40on the photosensitive drum1, and a housing24to which the rod lens array23and the printed circuit board22are fixed.

A connector21is mounted on a surface of the printed circuit board22that is opposite to a surface on which the light emitting element array chip40is mounted. The connector21is provided to transmit a control signal for the light emitting element array chip40transmitted from an image controller portion70(FIG.8) and to connect a power line. The light emitting element array chip40is driven via the connector21.

As illustrated inFIG.3A-3C, 20 light emitting element array chips40are mounted in a staggered manner in two rows on the printed circuit board22. In each light emitting element array chip40, 748 light emitting portions50are arranged at a predetermined resolution pitch in a longitudinal direction (arrow X direction). In each light emitting element array chip40, four light emitting portions50are arranged at a predetermined pitch in a lateral direction (arrow Y direction). That is, in each light emitting element array chip40, the light emitting portions50are two-dimensionally arranged in the arrow X direction and the arrow Y direction.

In the present embodiment, the resolution pitch of the light emitting element array chip40is 1200 dpi (about 21.16 μm). In addition, a distance from one end portion to the other end portion of the light emitting portions50included in each light emitting element array chip40in the longitudinal direction is about 15.828 mm. That is, the exposure head6includes a total of 14960 light emitting portions50in the arrow X direction, which enables exposure processing corresponding to an image width of about 316 mm (≈about 15.8 mm×20 chips) in the longitudinal direction.

In the longitudinal direction of the light emitting element array chip40, an interval L1between the light emitting portions50of adjacent light emitting element array chips40is about 21.16 μm. That is, a pitch of the light emitting portions50in the longitudinal direction at a boundary portion of the respective light emitting element array chips40is a resolution pitch of 1200 dpi. In addition, in the lateral direction (arrow Y direction) of the light emitting element array chip40, an interval L2between the light emitting portions50of the adjacent light emitting element array chips40is about 127 μm (six pixels at 1200 dpi and four pixels at 800 dpi).

In the present embodiment, the arrow X direction which is the longitudinal direction of the light emitting element array chip40is a rotational axis direction of the photosensitive drum1and is also a main scanning direction. The arrow Y direction, which is the lateral direction of the light emitting element array chip40, is a rotation direction of the photosensitive drum1, and is also a sub-scanning direction. The rotation direction of the photosensitive drum1is a tangential direction of the photosensitive drum1at an exposure position on the photosensitive drum1where light is collected by the exposure head6. In addition, an arrow Z direction is a stacking direction in which layers of the light emitting portion50having a layer structure described below overlap each other. Note that the longitudinal direction of the light emitting element array chip40may be inclined by about ±1° with respect to the rotational axis direction of the photosensitive drum1. The lateral direction of the light emitting element array chip40may also be inclined by about ±1° with respect to the rotation direction of the photosensitive drum1.

FIG.4is a view illustrating a positional relationship between the rod lens arrays23and the light emitting portions50of the light emitting element array chip40. As illustrated inFIG.4, a predetermined number of rod lens arrays23are arranged in the arrow X direction, and the rod lens arrays23are arranged in a staggered manner in two rows in the arrow Y direction in such a way as to cover the light emitting portions50of the light emitting element array chip40. Further, a diameter of the rod lens array23is set to 290 um, and light emitted from the plurality of light emitting portions50is collected by one rod lens array23.

<Light Emitting Element Array Chip>

Next, a configuration of the light emitting element array chip40will be described.

FIG.5is a schematic view of the light emitting element array chip40.FIG.6is a cross-sectional view of the light emitting element array chip40taken along line M-M ofFIG.5.FIG.7is a schematic view for explaining arrangement of the light emitting portions50of the light emitting element array chip40.

As illustrated inFIG.5, the light emitting element array chip40includes a light emitting substrate42(substrate) incorporating a circuit portion46for controlling the light emitting portions50, a light emitting region44in which the plurality of light emitting portions50are regularly arranged on the light emitting substrate42, and a wire bonding pad48. Input and output of a signal between the outside of the light emitting element array chip40and the circuit portion46and power supply to the circuit portion46are performed through the wire bonding pad48. Note that the circuit portion46can use an analog drive circuit, a digital control circuit, or a circuit including both of them.

As illustrated inFIG.6, the light emitting portion50includes the light emitting substrate42, a plurality of lower electrodes54two-dimensionally arranged at regular intervals (intervals d1and d2illustrated inFIG.7) in the arrow X direction and the arrow Y direction on the light emitting substrate42, a light emitting layer56, and an upper electrode58.

The lower electrodes54(a first electrode layer including a plurality of electrodes) are a plurality of electrodes formed in a layer form at intervals on the light emitting substrate42, and are electrodes provided corresponding to pixels, respectively. That is, each lower electrode54is provided to form one pixel.

The upper electrode58(second electrode layer) is stacked on the light emitting layer56at a position on a side opposite to a side where the lower electrode54is arranged with respect to the light emitting layer56. The upper electrode58is an electrode through which light having a light emission wavelength of the light emitting layer56can be transmitted (transmissible).

The circuit portion46controls a potential of a selected lower electrode54based on the control signal generated according to the image data, and generates a potential difference between the selected lower electrode54and the upper electrode58. When the potential difference is generated between the upper electrode58as a positive electrode and the lower electrode54as a negative electrode, electrons flow into the light emitting layer56from the negative electrode, and holes flow into the light emitting layer56from the positive electrode. The light emitting layer56emits light by recombination of the electrons and the holes in the light emitting layer56.

Light directed to the upper electrode58by light emission of the light emitting layer56is transmitted through the upper electrode58and emitted. Further, the light directed from the light emitting layer56toward the lower electrode54is reflected from the lower electrode54toward the upper electrode58, and the reflected light is also transmitted through the upper electrode58and emitted. In this manner, the light emitting portion50emits light. Note that, although there is a time difference between an emission timing of the light emitted directly from the light emitting layer56toward the upper electrode58and an emission timing of the light reflected by the lower electrode54and emitted from the upper electrode58, since a layer thickness of the light emitting portion50is extremely small, the emission timings can be regarded as almost the same.

Note that, in the present embodiment, the light emitting substrate42is a silicon substrate. The upper electrode58is preferably transparent to the light emission wavelength of the light emitting layer56. For example, by using a transparent electrode formed of indium tin oxide (ITO), an opening ratio becomes substantially 100%, and light emitted from the light emitting layer56passes through the upper electrode58and is emitted as it is. In the present embodiment, the upper electrode58is a positive electrode provided in common for the respective lower electrodes54, but the upper electrode58may also be provided individually for each of the lower electrodes54, or one upper electrode58may be provided for a plurality of lower electrodes54. In a case where a transparent electrode is used as the upper electrode58, the whole electrode is not necessarily a transparent electrode, and only an opening through which light is emitted may be a transparent electrode, and a portion other than the opening may be an electrode other than the transparent electrode, such as a metal wire.

As the light emitting layer56, an organic EL film, an inorganic EL layer, or the like is used. In a case where an organic EL film is used as the light emitting layer56, the light emitting layer56may be a stacked 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 necessary. Further, the light emitting layer56may be continuously formed in the arrow X direction, or may be divided in the same size as the lower electrode54. In addition, each of the lower electrodes54may be divided into a plurality of groups, and one light emitting layer56may be stacked on the lower electrodes54belonging to each divided group.

Note that when a light emitting material vulnerable to moisture such as an organic EL layer (organic light emitting layer) or an inorganic EL layer is used as the light emitting layer56, it is desirable to perform sealing in order to prevent moisture from entering the light emitting region44. As a sealing method, for example, a single thin film of silicon oxide, silicon nitride, aluminum oxide, or the like or a sealing film in which the thin films are stacked is formed. As a method for forming the sealing film, a method excellent in covering performance for a structure such as a step is preferable, and for example, an atomic layer deposition method (ALD method) or the like can be used. Note that the material, configuration, forming method, and the like of the sealing film are merely examples, and are not limited to the examples described above, and it is sufficient if a suitable material, configuration, forming method, and the like are appropriately selected.

The lower electrode54is preferably formed of a metal having a high reflectance with respect to the light emission wavelength of the light emitting layer56. For example, Ag, Al, or an alloy of Ag and Al is used. The lower electrode54is formed by using a Si integrated circuit processing technology together with the formation of the circuit portion46, and is directly connected to a drive portion of the circuit portion46. As described above, as the lower electrode54is formed by the Si integrated circuit processing technology, the process rule is about 0.2 μm, and high accuracy is obtained, so that the lower electrodes54can be accurately and densely arranged. Furthermore, since the lower electrodes54can be densely arranged, most of the light emitting region44can be caused to emit light, and utilization efficiency of the light emitting region44can be enhanced. An organic material of the light emitting layer56fills a space between the respective lower electrodes54, and the respective lower electrodes54are partitioned by the organic material.

In addition, when a voltage applied across the light emitting portion50becomes a predetermined value or more, a current starts to flow, and thereafter, a value of the current increases substantially in proportion to the value of the voltage. The voltage at which the current starts to flow in each light emitting portion50varies. Therefore, before product shipment from the factory, the light emitting portions50of the light emitting element array chip40are caused to individually and sequentially emit light, and the current flowing through the light emitting portion50is adjusted in such a way that light collected through the rod lens array23has a predetermined light quantity. Note that the exposure head6performs not only the above-described light quantity adjustment but also focus adjustment for adjusting an interval between the light emitting element array chip40and the rod lens array23before product shipment from the factory.

As illustrated inFIG.7, the light emitting portions50are arranged in a matrix form at predetermined intervals in the arrow X direction and the arrow Y direction in the light emitting region44. In the present embodiment, a width W1of the light emitting portion50in the arrow X direction is 19.80 μm, and the interval d1between the light emitting portions50adjacent to each other in the arrow X direction is 0.68 μm. That is, the light emitting portions50are arranged at a pitch of 21.16 μm (1200 dpi) in the arrow X direction. Note that the pitch of the light emitting portions50in the arrow X direction may have a deviation within a tolerance range. The tolerance of the pitch of the light emitting portions50in the arrow X direction is ±1% with respect to a design nominal pitch of the light emitting portions50in the arrow X direction. That is, the tolerance of the pitch of the light emitting portions50in the arrow X direction according to the present embodiment is ±0.21 μm. In addition, the width, shape, arrangement, and the like of the light emitting portion50are substantially determined by the width, shape, and arrangement of the lower electrode54in the present embodiment, and thus can also be referred to as the width, shape, and arrangement of the lower electrode54.

A width W2of the light emitting portion50in the arrow Y direction is also 19.80 μm similarly to the width W1. That is, the light emitting portion50of the present embodiment has a square shape having one side of 19.80 μm. Although the light emitting portion50has a square shape because the width W1and the width W2are equal to each other, the widths W1and W2may have deviations within a tolerance range. In the present embodiment, the tolerances of the widths W1and W2are both ±0.2 μm.

In addition, the interval d2between the light emitting portions50adjacent to each other in the arrow Y direction is also 0.68 μm similarly to the interval d1, and the light emitting portions50are arranged at a pitch of 21.16 μm (1200 dpi) also in the arrow Y direction. Note that the pitch of the light emitting portions50in the arrow Y direction may have a deviation within a tolerance range. The tolerance of the pitch of the light emitting portions50in the arrow Y direction is ±1% with respect to a design nominal pitch of the light emitting portions50in the arrow Y direction. That is, the tolerance of the pitch of the light emitting portions50in the arrow Y direction according to the present embodiment is ±0.21 μm. Here, the intervals d1and d2between the light emitting portions50are set to be larger than an interval dz (FIG.6) between the upper electrode58and the lower electrode54. With such a configuration, a leakage current between the lower electrodes54adjacent to each other in the arrow X direction and the arrow Y direction can be suppressed, and erroneous light emission of the light emitting portion50can be suppressed.

In the present invention, the shape of the light emitting portion50is not limited to a square, and may be a polygon with more sides than a quadrangle, a circle, an ellipse, or the like as long as light having an exposure region size corresponding to an output resolution of the image forming apparatus A is emitted and image quality of an output image satisfies a design specification of the image forming apparatus A. However, since a light quantity of an organic light emitting material is smaller than that of an LED, it is preferable to reduce a distance between adjacent light emitting portions50having a square shape because it is possible to secure a light emitting area for obtaining a light quantity enough to change the potential of the photosensitive drum1. In addition, the number of light emitting portions50arranged in parallel in the arrow Y direction is not limited to four as long as two or more light emitting portions50are provided, and is determined based on the light quantity necessary for the exposure processing by the exposure head6, the resolution, or the like.

In addition, the distance between the light emitting portions50, that is, the distance between the lower electrodes54is defined based on design nominal centroid positions of the lower electrodes54. That is, in a case where the shape of the lower electrode54is a regular polygon, the distance between the lower electrodes54is set based on intersections of the diagonal lines, in a case where the shape of the lower electrode54is a perfect circle, the distance between the lower electrodes54is set based on the centers of the circles, and in a case where the shape of the lower electrode54is an ellipse, the distance between the lower electrodes54is set based on intersections of the major axes and the minor axes. In a case where the shape of the lower electrode54is a regular polygon, the corner does not have to be a perfect corner and may be rounded.

<System Configuration of Exposure Head>

Next, a configuration of the exposure head6and the image controller portion70(controller) that controls the exposure head6will be described. The image controller portion70is provided on a main body side of the image forming apparatus A. Although control performed when processing one piece of image data (single color) will be described below, similar processing is executed in parallel for four pieces of image data corresponding to yellow, magenta, cyan, and black when the image forming operation is performed.

FIG.8is a block diagram illustrating a system configuration of the image controller portion70and the exposure head6. As illustrated inFIG.8, the image controller portion70includes an image data generation portion71, a chip data conversion portion72, a CPU73, and a synchronization signal generation portion74. The image controller portion70executes image data processing and image forming timing processing by these parts, and transmits a control signal for controlling the exposure head6to the printed circuit board22of the exposure head6.

Image data of an original read by the image reading portion90and image data transferred from an external device via a network are input to the image data generation portion71. The image data generation portion71executes dithering processing on the input image data at a resolution indicated by the CPU73, and generates image data for outputting an image. In the present embodiment, the dithering processing is executed at a resolution of 2400 dpi in both the main scanning direction and the sub-scanning direction.

The synchronization signal generation portion74periodically generates a line synchronization signal (control signal) indicating start of taking-in of image data, and transmits the line synchronization signal to the chip data conversion portion72. The CPU73sets, as one line cycle, a cycle in which the surface of the photosensitive drum1moves by a pixel size in the rotation direction at a preset rotation speed of the photosensitive drum1according to the resolution of the image formed by the image forming apparatus A in the sub-scanning direction, and indicates, to the synchronization signal generation portion74, a time interval of a signal cycle.

In the present embodiment, the resolution of the image formed by the image forming apparatus A in the sub-scanning direction is 2400 dpi, and the photosensitive drum1rotates at 200 mm/s. Therefore, a time for which the photosensitive drum1moves by a distance (about 10.58 μm) of a pixel size of 2400 dpi is 52.92 us, and the cycle of the line synchronization signal is 52.92 us. Note that the rotation speed of the photosensitive drum1is calculated by the CPU73based on a set value stored in a storage portion (not illustrated).

The chip data conversion portion72divides image data of one line×four rows (the number of light emitting portions50in the arrow Y direction) into the respective light emitting element array chips40in synchronization with the line synchronization signal generated and input by the synchronization signal generation portion74. Then, the chip data conversion portion72transmits the image data together with a clock signal and the line synchronization signal to each light emitting element array chip40via a line synchronization signal line75, a clock signal line76, and an image data signal line77. Note that the number of image data signal lines77is four, which is the same as the number of light emitting portions50in the arrow Y direction.

A head information storage portion171included in the exposure head6is connected to the CPU73via a communication signal line79. The head information storage portion171stores a light emission quantity and mounting position information of each light emitting element array chip40as head information. The light emitting element array chip40causes the light emitting portion50to emit light based on a set value of each of the above-described signals input from the image controller portion70. In addition, the light emitting element array chip40generates a line synchronization signal to be used in another light emitting element array chip40connected via the line synchronization signal line75.

<System Configuration of Light Emitting Element Array Chip>

Next, a system configuration of the light emitting element array chip40will be described.

FIG.9is a block diagram illustrating a system configuration of the light emitting element array chip40. InFIG.9, since the clock signal is input to all blocks of a digital portion80, the connection is omitted. As illustrated inFIG.9, the circuit portion46of the light emitting element array chip40includes the digital portion80and an analog portion86.

The digital portion80includes a communication IF portion81, a register portion82, a taking-in signal generation portion83, a line synchronization signal generation portion84, and a data holding portion85. The digital portion80generates a pulse signal for causing the light emitting portion50to emit light based on the set value set in advance by a communication signal in synchronization with the clock signal, an image data signal, and the line synchronization signal by these parts, and transmits the pulse signal to the analog portion86. Note that 748 light emitting portions50are provided as the data holding portions85, 748 (85-001to85-748) being the number of light emitting portions50included in one light emitting element array chip40in the arrow X direction.

The line synchronization signal generation portion84delays the input line synchronization signal by a predetermined time, and generates a line synchronization signal to be used in another light emitting element array chip40connected via the line synchronization signal line75. The taking-in signal generation portion83outputs a data latch signal we001to the data holding portion85-001at a timing delayed from the input line synchronization signal by a predetermined set time input from the register portion82.

The register portion82stores information regarding the delay time of the taking-in signal generation portion83described above, setting information of a drive current set by the analog portion86, and the like. The communication IF portion81controls writing and reading of the set value to and from the register portion82based on the communication signal input from the CPU73.

<Data Holding Portion>

Next, a configuration of the data holding portion85will be described.

FIG.10is a circuit diagram of the data holding portion85. As illustrated inFIG.10, pieces of image data (image data1to4) for four lines, a clock signal, and a data latch signal wen (n=1 to 748) are input to the data holding portion85. Each data holding portion85includes four flip-flop circuits and four gate circuits for latching the pieces of image for four lines simultaneously input at a timing when the data latch signal is input. Each data holding portion85includes one flip-flop circuit for delaying by one clock pulse and outputting the data latch signal.

FIG.11is an operation timing chart of the data holding portion85. As illustrated inFIG.11, pieces of image data (D1[1] to D1[4]) for four lines are simultaneously input to the data holding portion85-001. The data holding portion85-001latches the pieces of image data at a timing when a data latch signal we001is input from the taking-in signal generation portion83, and generates drive signals (P001[1] to P001[4]). In addition, the data holding portion85-001delays the input data latch signal we001by one clock pulse and transmits the delayed data latch signal to the next data holding portion85-002as a data latch signal we002.

Pieces of image data (D2[1] to D2[4]) for four lines are simultaneously input to the data holding portion85-002. The data holding portion85-002latches the pieces of image data at a timing when the data latch signal we002is input from the data holding portion85-001, and generates drive signals (P002[1] to P002[4]). In addition, the data holding portion85-002delays the data latch signal we002by one clock pulse and transmits the delayed data latch signal to the data holding portion85-003as a data latch signal we003.

In this manner, the data holding portion85(-001to748) sequentially latches the pieces of image data while transmitting the data latch signal up to the 748-th data holding portion85. Then, once the image data is latched, the data holding portion85(-001to748) transmits the latched signal to the analog portion86as the drive signal. In the present embodiment, since pieces of image data for four lines are latched by one data latch signal, drive signals for four lines (four pixels) are simultaneously output.

<Analog Portion>

Next, a configuration of the analog portion86will be described. The analog portion86includes a drive circuit connected to each of the light emitting portions50on a one-to-one basis. Hereinafter, for convenience of description, one drive circuit will be described, but it is assumed that the same number of drive circuits as the number of light emitting portions50, that is, 2992 drive circuits (748×4 rows) exist.

FIG.12is a circuit diagram of the analog portion86. As illustrated inFIG.12, the analog portion86includes a current setting DAC61, a current control MOSFET62, and a switching MOSFET63. The DAC61receives a set value of a current flowing from the register portion82of the digital portion80to the light emitting portion50as a digital value, converts the set value of the current into an analog voltage, and outputs the analog voltage.

The current control MOSFET62is a Pch MOSFET, has a source terminal connected to a power supply voltage VDD, and has a gate terminal connected to an output of the DAC61. Further, a current flowing from the source to a drain increases as the analog voltage input from the DAC61increases.

The switching MOSFET63is a Pch MOSFET, has a source terminal connected to a drain terminal of the current control MOSFET62, and has a gate terminal to which the drive signal output from the data holding portion85is input. The drive signal is a binary signal indicating a high level and a low level, and when the high level is input, the MOSFET63is turned on, and a current controlled by the current control MOSFET62flows from the source to the drain. Since the drain terminal is connected to an anode terminal of the light emitting portion50, the current becomes a drive current for the light emitting portion50. In the present embodiment, since drive currents for four lines (for four pixels) are simultaneously output, the light emitting portions50for four lines (for four pixels) simultaneously emit light.

<Lighting Control of Light Emitting Portion at Time of Image Formation>

Next, lighting control of the light emitting portion50at the time of image formation will be described. In the following description, the light emission of the light emitting portion50means that the light emitting portion50emits light of a light quantity enough to change a charging potential of the photosensitive drum1to the extent of being developed by toner. That is, the light emission of the light emitting portion50does not include a case where the light emitting portion50emits light of a light quantity enough to change the charging potential of the photosensitive drum1to such an extent that a toner image is not developed as a visible image.

FIG.13is a diagram illustrating an exposure image of the photosensitive drum1. InFIG.13, a rectangle on the photosensitive drum1indicates a pixel on the photosensitive drum1, and a number (1-1to16-4) in the pixel indicates a type of image data to be written in each pixel. Although the number of pixels in the arrow X direction is 748 pixels×20 chips=14960 pixels, only four pixels are illustrated as the pixels in the arrow X direction inFIG.13for convenience of description.

As illustrated inFIG.13, first, at time T1, pieces of image data for four lines are transmitted from the image controller portion70to the exposure head6. As a result, four light emitting portions50arranged in parallel in the arrow Y direction simultaneously emit light, and pixels (1-1,3-1,5-1, and7-1) for four lines are simultaneously exposed on the photosensitive drum1.

One clock pulse later, pieces of image data for the next four lines are transmitted from the image controller portion70to the exposure head6. As a result, four light emitting portions50adjacent to the four light emitting portions50that initially emit light in the arrow X direction simultaneously emit light, and pixels (1-2,3-2,5-2, and7-2) for four lines adjacent to each other in the arrow X direction are simultaneously exposed on the photosensitive drum1.

By repeating this operation every clock pulse, pixels for 2400 dpi×4 lines are exposed on the photosensitive drum1during 52.92 us which is the cycle of the line synchronization signal. Here, at time T1, pieces of image data (1-1to1-4) for exposing the first line of the photosensitive drum1are transmitted to the light emitting portions50positioned most downstream in the rotation direction of the photosensitive drum1. On the other hand, in the present embodiment, since the resolution pitch of the light emitting element array chip40is 1200 dpi (about 21.16 μm) with respect to the image resolution of 2400 dpi in the sub-scanning direction, pieces of image data spaced by one line of 2400 dpi are transmitted to the light emitting portions50positioned upstream of the light emitting portions50to which the pieces of image data for exposing the first line are transmitted, in the rotation direction of the photosensitive drum1. For example, pieces of image data (3-1to3-4) for exposing the third line of the photosensitive drum1are transmitted to the light emitting portions50positioned upstream of the light emitting portion50for exposing the first line. Similarly, pieces of image data (5-1to5-4) and pieces of image data (7-1to7-4) are transmitted to the light emitting portions50positioned further upstream. That is, images formed on the photosensitive drum1at time T1are spaced by one line of 2400 dpi as illustrated inFIG.1.

Next, at time T2when the photosensitive drum1is rotated for one line (10.58 um) of 2400 dpi in the sub-scanning direction (arrow Y direction) with respect to time T1, pieces of image data for four lines are transmitted from the image controller portion70to the exposure head6in the same manner as time T1. Here, the pieces of image data transmitted to each line of the light emitting portions50at time T2are transmitted while being shifted by one line with respect to time T1.

That is, at time T2, pieces of image data (2-1to2-4) for exposing the second line of the photosensitive drum1are transmitted to the light emitting portions50positioned most downstream in the rotation direction of the photosensitive drum1. Further, pieces of image data (4-1to4-4) for exposing the fourth line of the photosensitive drum1spaced by one line of 2400 dpi are transmitted to the light emitting portions50positioned upstream of the light emitting portions50to which the pieces of image data for exposing the second line are transmitted, in the rotation direction of the photosensitive drum1. Similarly, each of pieces of image data (6-1to6-4) and pieces of image data (8-1to8-4) are transmitted to the light emitting portions50adjacent in the rotation direction of the photosensitive drum1for each line.

Further, at time T3when the photosensitive drum1is rotated for one line (10.58 um) of 2400 dpi in the sub-scanning direction (arrow Y direction) with respect to time T2, pieces of image data for four lines are transmitted from the image controller portion70to the exposure head6in the same manner as times T1and T2. Here, the pieces of image data transmitted to each line of the light emitting portions50at time T3are transmitted while being shifted by one line with respect to time T2.

That is, at time T3, pieces of image data (3-1to3-4) for exposing the third line of the photosensitive drum1are transmitted to the light emitting portions50positioned most downstream in the rotation direction of the photosensitive drum1. Further, pieces of image data (5-1to5-4) for exposing the fifth line of the photosensitive drum1spaced by one line of 2400 dpi are transmitted to the light emitting portions50positioned upstream of the light emitting portions50to which the pieces of image data for exposing the third line are transmitted, in the rotation direction of the photosensitive drum1. Similarly, each of pieces of image data (7-1to7-4) and pieces of image data (9-1to9-4) are transmitted to the light emitting portions50adjacent in the rotation direction of the photosensitive drum1for each line.

Therefore, for the third line, the fifth line, and the seventh line of the photosensitive drum1, the light emitting portions50perform multiple exposure twice at time T1and time T3. That is, one pixel is formed by the plurality of light emitting portions50that perform multiple exposure. Thereafter, even after time T4, the same processing as time T1, time T2, and time T3is executed. As a result, at a time point of time T7, since the exposure processing is executed at each of time T1to time T7for the seventh line on the photosensitive drum1, multiple exposure is performed four times in total. By repeating this operation for one image page, an electrostatic latent image subjected to multiple exposure four times is formed over the entire region of the photosensitive drum1except for the first to sixth lines.

As described above, in the present embodiment, the pitch of the light emitting portions50of the exposure head6in the sub-scanning direction is an integer multiple of the resolution pitch in the sub-scanning direction (the rotation direction of the photosensitive drum1or the arrow Y direction) of the image formed by the image forming apparatus A. With such a configuration, it is possible to perform multiple exposure of the photosensitive drum1only by shifting the image data exposed by the light emitting portions50arranged in parallel in the arrow Y direction without shifting the light emission timings of the light emitting portions50arranged in parallel in the arrow Y direction by providing a delay circuit in the exposure head6. Therefore, an increase in circuit scale of the exposure head6can be suppressed, and the manufacturing cost can be reduced.

In the present embodiment, a configuration in which the photosensitive drum1is driven at a rotation speed of 200 mm/s has been described, but the present invention is not limited thereto. An optimum image forming condition varies depending on the type of the sheet S and the like. For example, in a case where the toner image is fixed to thick paper or coated paper in the fixing device94, a larger amount of heat is required than in a case where the toner image is fixed to plain paper. Therefore, it is preferable to decrease a conveyance speed for the sheet S to increase the fixing time. Therefore, in the following, a case where the photosensitive drum1is driven at a rotation speed of 100 mm/s in order to decrease the conveyance speed for the sheet S will be considered.

In a case where the photosensitive drum1is driven at a rotation speed of 100 mm/s, a time taken to perform exposure at a resolution of 2400 dpi (10.58 um) is 211.66 us. Therefore, as illustrated inFIG.14, the photosensitive drum1is driven at a rotation speed of 100 mm/s, and the cycle of the line synchronization signal is set to 105.83 us. The light emission order of the light emitting portions50of the exposure head6and image data to be written for each line are controlled similarly to the control described above with reference toFIG.13.

In this configuration, since the rotation speed of the photosensitive drum1is 100 mm/s, which is a half of that in the configuration described with reference toFIG.13, an exposure time for 2400 dpi (10.58 um) is doubled. Therefore, in a case where the light emitting portion50is driven with the same drive current as that in the configuration in which the rotation speed of the photosensitive drum1is 200 mm/s, the photosensitive drum1is exposed with twice the intensity. Therefore, it is preferable to adjust the exposure intensity by changing the set value of the current setting DAC61according to the rotation speed of the photosensitive drum1. For example, in a configuration in which the rotation speed of the photosensitive drum1is 100 mm/s, it is preferable that the set value of the current of the DAC61is set to a half of that in a configuration in which the rotation speed of the photosensitive drum1is 200 mm/s, and the exposure intensities are equivalent to each other.

Furthermore, in a configuration in which the photosensitive drum1is driven at a rotation speed of 100 mm/s, the following configuration can be considered as a configuration in which the exposure intensity is equivalent to that in a configuration in which the photosensitive drum1is driven at a rotation speed of 200 mm/s without changing the set value of the current of the DAC61. As illustrated inFIG.15, first, the photosensitive drum1is driven at a rotation speed of 100 mm/s, and the cycle of the line synchronization signal is set to 52.92 us without being changed from that in a configuration in which the photosensitive drum1is driven at a rotation speed of 200 mm/s.

Then, at time T1, the light emitting portions50are controlled by the same control as the control of the light emitting portions50at time T1described with reference toFIG.13, and the pixels for 2400 dpi×4 lines spaced by one line are exposed on the photosensitive drum1during 52.92 us which is the cycle of the line synchronization signal. Here, although the photosensitive drum1rotates at 100 mm/s, since the cycle of the line synchronization signal is 52.92 us corresponding to 200 mm/s, a length of a region exposed on the photosensitive drum1in the arrow Y direction at time T1is 5.29 um, which is half of 10.58 um.

Next, at time T2when the photosensitive drum1rotates by 5.29 um with respect to time T1, image data transmission from the image controller portion70to the exposure head6is not performed, and exposure of the photosensitive drum1is not performed. Next, at time T3when the photosensitive drum1rotates by 10.58 um (for one line of 2400 dpi) with respect to time T1, the light emitting portions50are caused to emit light under the same control as the control of the light emitting portions50at time T2described with reference toFIG.13.

Therefore, for the seventh line of the photosensitive drum1, the light emitting portions50perform multiple exposure twice at time T1and time T5. Thereafter, the same control is performed until time T13. That is, while the line synchronization signal is output twice, the image data is not transmitted once (the image data is thinned out), and the light emitting portions50are turned off (does not emit light). As a result, at a time point of time T13, since the exposure processing is executed at each of times T1, T5, T9, and T13for the seventh line on the photosensitive drum1, multiple exposure is performed four times in total. By repeating this operation, an electrostatic latent image subjected to multiple exposure four times is formed over the entire region of the photosensitive drum1except for the first to third lines.

By performing such control, in a configuration in which the photosensitive drum1is driven at a rotation speed of 100 mm/s, the exposure time for each line can be made the same as that in a configuration in which the photosensitive drum1is driven at a rotation speed of 200 mm/s. Accordingly, the exposure intensity can be made equivalent without changing the set value of the current of the DAC61. In this configuration, the length of the exposure region for each line on the photosensitive drum1in the sub-scanning direction (arrow Y direction) is halved. However, since the resolution of the image is not halved, and only a spot diameter in the sub-scanning direction is reduced, there is no adverse effect on the image, and the sharpness of the image is improved.

Second Embodiment

Next, a second embodiment of an image forming apparatus A according to the present invention will be described. The same portions as those in the first embodiment will be denoted by the same reference signs with reference to the same drawings, and a description thereof is omitted.

FIG.16is a schematic diagram for explaining a configuration of a light emitting portion50of a light emitting element array chip40according to the present embodiment. As illustrated inFIG.16, the configuration of the present embodiment is a configuration in which the light emitting portions50adjacent to each other in the arrow Y direction are arranged to be shifted in position by an interval d3in the arrow X direction. In the present embodiment, the interval d3is set to 5.29 μm (4800 dpi).

Widths W1and W2and intervals d1and d2are W1=W2=19.8 um and d1=d2=0.68 um as in the first embodiment. That is, a pitch of the light emitting portions50in the arrow Y direction is set to 21.16 um (1200 dpi) as in the first embodiment. The image forming apparatus A according to the present embodiment forms an image with a resolution of 2400 dpi in the sub-scanning direction, and the rotation speed of a photosensitive drum1is 200 mm/s. Therefore, a time taken to perform exposure at a resolution of 2400 dpi (10.58 um) is 52.92 us, and a cycle of a line synchronization signal is also 52.92 us. Other configurations of the image forming apparatus A of the present embodiment are similar to those of the first embodiment except for control described below.

FIG.17is a diagram illustrating an exposure image of the photosensitive drum1. InFIG.17, a rectangle on the photosensitive drum1indicates a pixel on the photosensitive drum1, and a number (1-1to16-4) in the pixel indicates a type of image data to be written in each pixel. Although the number of pixels in the arrow X direction is 748 pixels×20 chips=14960 pixels, only four pixels are illustrated as the pixels in the arrow X direction inFIG.17for convenience of description.

As illustrated inFIG.17, first, at time T1, the light emitting portions50are controlled by the same control as the control of the light emitting portions50at time T1described with reference toFIG.13, and the pixels for 2400 dpi×4 lines spaced by one line are exposed on the photosensitive drum1during 52.92 us which is the cycle of the line synchronization signal.

Next, at time T2when the photosensitive drum1is rotated for one line (10.58 um) of 2400 dpi in the sub-scanning direction (arrow Y direction) with respect to time T1, pieces of image data for four lines are transmitted from the image controller portion70to the exposure head6in the same manner as time T1. Here, the pieces of image data transmitted to each line of the light emitting portions50at time T2are transmitted while being shifted by one line with respect to time T1.

That is, at time T2, pieces of image data (2-1to2-4) for exposing the second line of the photosensitive drum1are transmitted to the light emitting portions50positioned most downstream in the rotation direction of the photosensitive drum1. Further, pieces of image data (4-1to4-4) for exposing the fourth line spaced by one line of 2400 dpi in the photosensitive drum1are transmitted to the light emitting portions50positioned upstream of the light emitting portions50to which the pieces of image data for exposing the second line are transmitted, in the rotation direction of the photosensitive drum1. Similarly, pieces of image data (5-1to5-4) and pieces of image data (7-1to7-4) are transmitted to the light emitting portions50positioned further upstream. That is, images formed on the photosensitive drum1at time T1are spaced by one line of 2400 dpi as illustrated inFIG.1.

Further, at time T3when the photosensitive drum1is rotated for one line (10.58 um) of 2400 dpi in the sub-scanning direction (arrow Y direction) with respect to time T2, pieces of image data for four lines are transmitted from the image controller portion70to the exposure head6in the same manner as times T1and T2. Here, the pieces of image data transmitted to each line of the light emitting portions50at time T3are transmitted while being shifted by one line with respect to time T2.

That is, at time T3, pieces of image data (3-1to3-4) for exposing the third line of the photosensitive drum1are transmitted to the light emitting portions50positioned most downstream in the rotation direction of the photosensitive drum1. Further, pieces of image data (5-1to5-4) for exposing the fifth line of the photosensitive drum1spaced by one line of 2400 dpi are transmitted to the light emitting portions50positioned upstream of the light emitting portions50to which the pieces of image data for exposing the third line are transmitted, in the rotation direction of the photosensitive drum1. Similarly, each of pieces of image data (7-1to7-4) and pieces of image data (9-1to9-4) are transmitted to the light emitting portions50adjacent in the rotation direction of the photosensitive drum1for each line.

Therefore, for the third line, the fifth line, and the seventh line of the photosensitive drum1, the light emitting portions50perform multiple exposure twice at time T1and time T3. Thereafter, even after time T4, the same processing as time T1, time T2, and time T3is executed. As a result, at a time point of time T7, since the exposure processing is executed at each of time T1to time T7for the seventh line on the photosensitive drum1, multiple exposure is performed four times in total. By repeating this operation for one image page, an electrostatic latent image subjected to multiple exposure four times is formed over the entire region of the photosensitive drum1except for the first to sixth lines.

Also in the present embodiment, the pitch of the light emitting portions50of the exposure head6in the sub-scanning direction is an integer multiple of a resolution pitch in the sub-scanning direction (the rotation direction of the photosensitive drum1or the arrow Y direction) of the image formed by the image forming apparatus A. Therefore, similarly to the first embodiment, the photosensitive drum1can be subjected to multiple exposure without providing a delay circuit in the exposure head6and shifting light emission timings of the light emitting portions50arranged in parallel in the arrow Y direction. Therefore, an increase in circuit scale of the exposure head6can be suppressed, and the manufacturing cost can be reduced.

In the present embodiment, the light emitting portions50adjacent to each other in the arrow Y direction are arranged to be shifted in position by 5.29 μm (4800 dpi) in the arrow X direction. Therefore, exposure positions of the light emitting portions50adjacent to each other in the arrow Y direction on the photosensitive drum1are shifted by 5.29 um in the main scanning direction (the arrow X direction), and the resolution of the exposure in the main scanning direction is 4800 dpi. Therefore, with the configuration of the present embodiment, the resolution of exposure can be improved as compared with the configuration of the first embodiment, and the image quality can be improved.

Third Embodiment

Next, a third embodiment of an image forming apparatus A according to the present invention will be described. The same portions as those in the first and second embodiments will be denoted by the same reference signs with reference to the same drawings, and a description thereof is omitted.

FIG.18is a schematic diagram for explaining a configuration of a light emitting portion50of a light emitting element array chip40according to the present embodiment. As illustrated inFIG.18, in the present embodiment, in order to increase a light intensity of the light emitting portion50, a width W2of the light emitting portion50is set to W2=31.07 μm, which is larger than the width W2in the first embodiment. A width W1and intervals d1and d2are W1=19.8 um and d1=d2=0.68 um as in the first embodiment. That is, in the present embodiment, a pitch of the light emitting portions50in the arrow Y direction is set to 31.75 um (800 dpi).

The image forming apparatus A according to the present embodiment forms an image with a resolution of 2400 dpi in the sub-scanning direction, and the rotation speed of a photosensitive drum1is 200 mm/s as in the first embodiment. Therefore, a time taken to perform exposure at a resolution of 2400 dpi (10.58 um) is 52.92 us, and a cycle of a line synchronization signal is also 52.92 us. Other configurations of the image forming apparatus A of the present embodiment are similar to those of the first embodiment except for control described below.

FIG.19is a diagram illustrating an exposure image of the photosensitive drum1. InFIG.17, a rectangle on the photosensitive drum1indicates a pixel on the photosensitive drum1, and a number (1-1to7-4) in the pixel indicates a type of image data to be written in each pixel. Although the number of pixels in the arrow X direction is 748 pixels×20 chips=14960 pixels, only four pixels are illustrated as the pixels in the arrow X direction inFIG.19for convenience of description.

As illustrated inFIG.19, first, at time T1, the light emitting portions50are controlled by the same control as the control of the light emitting portions50at time T1described with reference toFIG.13, and the pixels for 1200 dpi×4 lines are exposed on the photosensitive drum1during 52.92 us which is the cycle of the line synchronization signal. In the present embodiment, since an image with a resolution of 2400 dpi in the sub-scanning direction (arrow Y direction) is formed by the image forming apparatus A and an interval between the light emitting portions50in the sub-scanning direction is 800 dpi, electrostatic latent images spaced by two lines of 2400 dpi at time T1are formed on the photosensitive drum1.

Next, at time T2when the photosensitive drum1is rotated for one line (10.58 um) of 2400 dpi in the sub-scanning direction (arrow Y direction) with respect to time T1, pieces of image data for four lines are transmitted from the image controller portion70to the exposure head6in the same manner as time T1. Here, the pieces of image data transmitted to each line of the light emitting portions50at time T2are transmitted while being shifted by one line with respect to time T1.

That is, at time T2, pieces of image data (2-1to2-4) for exposing the second line of the photosensitive drum1are transmitted to the light emitting portions50positioned most downstream in the rotation direction of the photosensitive drum1. Further, pieces of image data (5-1to5-4) for exposing the fifth line of the photosensitive drum1are transmitted to the light emitting portions50positioned upstream of the light emitting portions50to which the pieces of image data for exposing the second line are transmitted, in the rotation direction of the photosensitive drum1. Each of pieces of image data (8-1to8-4) and pieces of image data (11-1to11-4) are transmitted to the light emitting portions50adjacent in the rotation direction of the photosensitive drum1. As described above, at time T2, each line on the photosensitive drum1is not subjected to multiple exposure.

Next, at times T3and T4, control similar to that at times T1and T2is performed. Therefore, at a time point of time T4, for the fourth line, the seventh line, and the tenth line of the photosensitive drum1, the light emitting portions50perform multiple exposure twice at time T1and time T4. At times T5and T6, control similar to that at times T3and T4is performed. Therefore, at a time point of time T5, for the fifth line, the eighth line, and the eleventh line of the photosensitive drum1, the light emitting portions50perform multiple exposure twice at time T2and time T5. At a time point of time T6, for the sixth line, the ninth line, and the twelfth line of the photosensitive drum1, the light emitting portions50perform multiple exposure twice at time T3and time T6.

Thereafter, at time T7, pieces of image data for four lines are transmitted from the image controller portion70to the exposure head6in the same manner as at time T1. Here, the pieces of image data transmitted to each line of the light emitting portions50at time T7are transmitted while being shifted by one line with respect to time T6. As a result, at a time point of time T7, since the exposure processing is executed at each of times T1, T4, T7, and T10for the tenth line on the photosensitive drum1, multiple exposure is performed four times in total. By repeating this operation for one image page, an electrostatic latent image subjected to multiple exposure four times is formed over the entire region of the photosensitive drum1except for the first to ninth lines. Although multiple exposure is not performed for the first to third lines on the photosensitive drum1, multiple exposure is performed at least twice or more for the fourth to ninth lines.

As described above, in the present embodiment, the pitch of the light emitting portions50of the exposure head6in the sub-scanning direction is an integer multiple of the resolution in the sub-scanning direction (the rotation direction of the photosensitive drum1or the arrow Y direction) of the image formed by the image forming apparatus A. Also with such a configuration, it is possible to perform multiple exposure of the photosensitive drum1only by shifting the image data exposed by the light emitting portions50arranged in parallel in the arrow Y direction without shifting the light emission timings of the light emitting portions50arranged in parallel in the arrow Y direction by providing a delay circuit in the exposure head6. As a result, an increase in circuit scale of the exposure head6can be suppressed, and the manufacturing cost can be reduced.

In the first embodiment and the second embodiment, a configuration in which the resolution of the image formed by the image forming apparatus A in the sub-scanning direction (arrow Y direction) is 2400 dpi, and the pitch of the light emitting portions50in the sub-scanning direction (arrow Y direction) is 1200 dpi has been described. Further, in the third embodiment, a configuration in which the resolution of the image formed by the image forming apparatus A in the sub-scanning direction (arrow Y direction) is 2400 dpi, and the pitch of the light emitting portions50in the sub-scanning direction (arrow Y direction) is 800 dpi has been described. However, the present invention is not limited thereto. That is, if the pitch of the light emitting portions50of the exposure head6in the sub-scanning direction is an integer multiple of the resolution of the image formed by the image forming apparatus A in the sub-scanning direction (the rotation direction of the photosensitive drum1and the arrow Y direction), excluding an equal multiple, the resolution of the image in the sub-scanning direction and the pitch of the light emitting portions50in the sub-scanning direction may be freely set.

Although the present invention has been described with reference to the exemplary embodiments, the present invention is not limited to the disclosed exemplary embodiments. The following claims are given the broadest interpretation to encompass all modifications, equivalent structures and functions.