JP2008238633A - Optical head, method for driving the same, and image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To correct variation in emission luminance by a simple structure. <P>SOLUTION: In an optical head 10A, light emitting elements 36 in a block B1 are driven by a demultiplexer DMP1 and unit circuits U1-U4. Four phases of selection signals SELa-SELd are supplied to a latch circuit 21 of the demultiplexer DMP1 in each sub-gradation time period. Consequently, a light-on control signals d11-d14 for designating a light-on state or a light-off state are respectively supplied to the unit circuits U1-U4 by each sub-gradation time period. The unit circuits U1-U4 supply drive currents according to correction data to the light emitting elements 36. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a technique for correcting the luminance of a plurality of light emitting elements.

A printer as an image forming apparatus uses an optical head for forming an electrostatic latent image on an image carrier such as a photosensitive drum. In the optical head, a plurality of light emitting elements are arranged in an array in the main scanning direction. A light emitting diode may be used as the light emitting element.
Such an optical head is generally configured by mounting a plurality of semiconductor chips on which light emitting diodes are formed and a separate driving IC on a printed circuit board on which a wiring pattern has been formed. For manufacturing reasons, there are variations in light emission luminance between different light emitting diodes between different semiconductor chips or within the same semiconductor chip. If the light emission luminance varies, for example, unevenness in the form of vertical stripes occurs during printing, which is not preferable in terms of printing quality. For this reason, fine adjustment of the driving conditions from the driving IC is performed in accordance with the characteristics of the light emitting diode.
The following techniques are known as correction methods used in conventional optical heads. Japanese Patent Application Laid-Open No. 2004-151561 describes a method of finely adjusting and correcting a period during which a light emission current is applied. Patent Document 2 describes a method of correcting a light emission drive current value using a DAC. In this method, a large number of memory circuits for correction are mounted by fine processing of a semiconductor process, and the current value is finely adjusted according to the memory contents.
JP-A-6-297769 JP-A-8-39862

By the way, the technique described in Patent Document 1 is based on the principle of finely adjusting the light emission period to adjust the latent image forming area on the photoconductor, but is very expensive for fine adjustment of the light emission period. It is necessary to operate the circuit at the clock frequency. However, it is difficult to apply this method to an optical head integrated with a drive circuit. This is because a drive circuit is formed on a substrate that is much larger than a conventional drive IC, so that the time constant increases due to parasitic capacitance, and high-speed drive cannot be performed.
In the technique described in Patent Document 2, the processing precision of a process for forming a circuit on a large glass substrate is rougher than that of a semiconductor process on a silicon wafer, and the correction is equivalent to that of an optical head integrated with a drive circuit. It is difficult to mount a memory circuit.
The present invention has been made in view of such circumstances, and an object thereof is to solve the problem of providing an optical head having a large correction range with a simple configuration, a control method thereof, and an image forming apparatus.

In order to solve the above-described problem, an optical head driving method according to the present invention divides one gradation period in which one gradation is engraved into a plurality of sub-gradation periods, and the plurality of light emitting elements are divided into the plurality One or a plurality of sub gradation periods are selected from the sub gradation period, and a driving signal having a level corresponding to light emission characteristics is generated for each of the plurality of light emitting elements in the selected one or more sub gradation periods, It supplies to each of a some light emitting element, It is characterized by the above-mentioned.
According to the present invention, when the light emission luminance of the light emitting element varies, two types of adjustments such as selection of the sub gradation period and adjustment of the level of the drive signal supplied to each light emitting element can be performed. The adjustment of the level of the drive signal has a range limit, but a larger range can be corrected by combining with the selection of the sub gradation period.

In the optical head driving method according to the present invention, the plurality of light emitting elements adjacent to each other are grouped and divided into a plurality of blocks, and each of the plurality of blocks includes the plurality of light emitting elements belonging to the block. The level of the drive signal supplied to the plurality of light emitting elements is adjusted so that the light emission luminances of the elements are equal, and one gradation is engraved so that the light emission luminances of the light emitting elements are equal between the plurality of blocks. One gradation period is divided into a plurality of sub gradation periods, and one or a plurality of sub gradation periods are selected from the plurality of sub gradation periods.
According to the present invention, when the light emission luminance of the light emitting element varies, two types of adjustments such as selection of the sub gradation period and adjustment of the level of the drive signal supplied to each light emitting element can be performed. In addition, since the selection of the sub gradation period is performed in units of blocks, the processing can be simplified.

  Next, an optical head according to the present invention includes a plurality of light emitting elements, and each of the plurality of light emitting elements includes a first adjustment unit that adjusts a drive signal level, and the plurality of light emitting elements. A second adjustment unit that divides one gradation period in which each gradation is engraved into a plurality of sub gradation periods and selects one or a plurality of sub gradation periods from the plurality of sub gradation periods; Prepare. According to the present invention, when the light emission luminance of the light emitting element varies, two types of adjustments such as selection of the sub gradation period and adjustment of the level of the drive signal supplied to each light emitting element can be performed.

  As a more specific aspect, the first adjusting means has a plurality of unit circuits provided for the plurality of light emitting elements, and each of the plurality of unit circuits turns on or off the light emitting elements. In a period in which the designated lighting control signal designates lighting, a driving signal having a level that becomes a target luminance corresponding to a duty ratio set for each of the plurality of light emitting elements is generated, and the second adjustment unit is configured to A plurality of latch circuits corresponding to each of the unit circuits, and each of the plurality of latch circuits latches the data signal obtained by time-division-multiplexing the lighting control signal according to a selection signal that specifies the timing for latching, and It is preferable to generate the lighting control signal having a duty ratio set for each of the plurality of light emitting elements. According to the present invention, since the level of the drive signal is adjusted so that the target luminance according to the duty ratio is obtained, the emission luminance of the plurality of light emitting elements can be corrected and made uniform.

  An optical head according to the present invention includes a plurality of light emitting elements, and groups the light emitting elements adjacent to each other, groups them into a plurality of blocks, and each of the plurality of blocks has a corresponding block. First adjustment means for adjusting the level of a drive signal supplied to the plurality of light emitting elements so that the light emission luminances of the plurality of light emitting elements belonging to the same are equal, and the light emission luminances of the light emitting elements are equal between the plurality of blocks And a second adjustment unit that divides one gradation period for engraving one gradation into a plurality of sub gradation periods and selects one or a plurality of sub gradation periods from the plurality of sub gradation periods. Is provided. According to this invention, the light emission luminance is adjusted so as to be the target luminance within the block, and further, the light emission luminance is adjusted to be equal between the blocks. Since the variation of the light emitting elements in the block is relatively small, fine adjustment can be made by adjusting the level of the drive signal, and the large variation between the blocks can be corrected by selecting the sub gradation period.

  As a more specific aspect, the first adjustment unit includes a plurality of unit circuits provided for each light emitting element belonging to the block in each of the plurality of blocks, and each of the plurality of unit circuits includes: Generating a driving signal at a level determined so that the luminance of the plurality of light emitting elements belonging to the block becomes a target luminance in a period in which a lighting control signal designating lighting or extinguishing of the light emitting elements designates lighting; The second adjustment unit includes a plurality of latch circuits corresponding to each of the plurality of unit circuits belonging to the block in each of the plurality of blocks, and each of the plurality of latch circuits receives the lighting control signal. The data signal time-division multiplexed in block units is converted into a plurality of selection signals that specify the timing for latching the data signal for each of the plurality of unit circuits. Latches I, it is preferable to generate the lighting control signals. Further, the plurality of selection signals are set for each of the plurality of blocks, and a plurality of sub-gradations constituting the one gradation period so that the light emission luminance of the light emitting element is equal between the plurality of blocks. It is preferable to designate one or a plurality of sub-gradation periods from the period. According to the present invention, since the number of inversions of the logic level of the selection signal can be reduced, power consumption can be reduced.

  Next, an image forming apparatus according to the present invention includes the above-described optical head and an image carrier that carries an image formed by light from the optical head. According to the image forming apparatus of the present invention, any of the effects described above can be achieved.

Various embodiments of the present invention will be described with reference to the drawings. In addition, the same code | symbol is attached | subjected to the common part in each figure.
<1. First Embodiment>
FIG. 1 is a perspective view showing a partial configuration of an image forming apparatus using an optical head according to the present embodiment. As shown in the figure, the image forming apparatus includes an optical head 10A, a condensing lens array 15, and a photosensitive drum (image carrier) 110. The optical head 10A has a large number of light emitting elements arranged in an array. These light emitting elements selectively emit light according to an image to be printed on a recording material such as paper. The light emitting element may be any element as long as it can form a latent image on the photosensitive drum 110, but in this example, an OLED (Organic Light Emitting Diode) element is used. The condensing lens array 15 is disposed between the optical head 10 </ b> A and the photosensitive drum 110. The condensing lens array 15 includes a large number of gradient index lenses arranged in an array with each optical axis directed to the optical head 10A. Light emitted from each light emitting element of the optical head 10 </ b> A passes through each refractive index distribution type lens of the condensing lens array 15 and forms an image on the surface of the photosensitive drum 110. The photosensitive drum 110 rotates, and a latent image corresponding to a desired image is formed at a predetermined exposure position on the surface of the photosensitive drum 110.

  FIG. 2 shows a schematic mechanical configuration of the optical head 10A. As shown in this figure, the optical head 10 </ b> A is configured by forming a plurality of light emitting elements 36 and a drive circuit 60 on a glass substrate 70. The plurality of light emitting elements 36 are arranged in the main scanning direction and are divided into n (n is an integer of 2 or more) blocks. In this example, the number of light emitting elements 36 belonging to each of the blocks B1 to Bn is four. That is, the optical head 10A of this embodiment is configured by arranging 4n (n is a natural number) light emitting elements in the main scanning direction.

  FIG. 3 shows a block diagram of an exposure apparatus A using the optical head 10A. As shown in this figure, the exposure apparatus A includes a control circuit 50A and an optical head 10A. The control circuit 50A generates data signals D1 to Dn and selection signals SELa to SELd corresponding to the blocks B1 to Bn based on the input image data Din supplied from the host device. When i is an arbitrary integer of 1 ≦ i ≦ n, the data signal Di is a time-division multiplexed lighting control signal for designating lighting or extinguishing of the four light emitting elements 36 belonging to the i-th block Bi. .

  The selection signals SELa to SELd are signals used for demultiplex processing of the data signal Di, and the data signals Di are latched using the selection signals SELa to SELd, so that each of the four light emitting elements 36 has a latch. A corresponding lighting control signal is obtained. Although details will be described later, in the present embodiment, a duty ratio is set for each light emitting element 36, and the level of the drive current of the light emitting element 36 is adjusted so as to match the target luminance determined according to the duty ratio.

  FIG. 4 shows a block diagram of the optical head. The optical head 10A includes n demultiplexers DMP1 to DMPn corresponding to n blocks B1 to Bn, and four unit circuits U1 to U4 for each block. FIG. 4 shows details of the demultiplexer DMP1 and the unit circuits U1 to U4 regarding the block B1, but the same applies to the other blocks B2 to Bn.

  The demultiplexer DMP1 includes four latch circuits 21. Each latch circuit 21 latches the data signal D1 by the selection signals SELa to SELd. As shown in FIG. 5, one gradation period in which gradation is engraved at a time is composed of a plurality of sub gradation periods. In this example, one gradation period is composed of first to fourth sub gradation periods. In this embodiment, the duty ratio is selected by selecting one or a plurality of sub gradation periods from the first to fourth sub gradation periods. For example, when the duty ratio is 100%, the first to fourth sub gradation periods are selected. When the duty ratio is 75%, the first to third sub gradation periods are selected. When the duty ratio is 50%, the first and fourth sub gradation periods are selected. When the duty ratio is 25%, the first sub gradation period is selected.

  A data signal D1 is supplied to the optical head 10A, and selection signals SELa to SELd that are sequentially activated are supplied in synchronization with the data signal D1. Each latch circuit 21 latches the data signal D1 during a period in which the selection signals SELa to SELd are active. As a result, the lighting control signals d11 to d14 are obtained.

  As shown in FIG. 4, the unit circuit U1 includes a drive transistor 35, a light emitting element 36, and a current digital-to-analog converter (current DAC). On / off of the driving transistor 35 is controlled by lighting control signals d11 to d14. The effective light emission luminance of the light emitting element 36 is determined by the product of the duty ratio and the magnitude of the drive current Iel supplied from the drive transistor 35. The magnitude of the drive current Iel is determined by the current DAC.

  The current DAC includes transistors 31 to 34 and memories M1 to M4. In this example, the transistor sizes of the transistors 31 to 34 are set to 1: 2: 4: 8. In other words, the ratio of the current that flows when the transistors 31 to 34 are on is 1: 2: 4: 8. In terms of design, this means that the channel width of the transistor is set to 4 μm: 8 μm: 16 μm: 32 μm. The memories M1 to M4 store a potential for turning off the transistors 31 to 34 and a potential for turning them on. Among these, the potential to be turned on is common to the memories M1 to M4. Therefore, the magnitude of the drive current Iel is controlled by 4-bit correction data stored in the memories M1 to M4. In the present embodiment, correction data is stored in the memories M1 to M4 so as to coincide with the target luminance. The transistor 30 is a basic current source, and an on / off state is defined by the memory M0. Since the basic current source may always be in the on state, the memory M0 may be omitted, and the gate potential of the transistor 30 may be directly connected to the on state potential. The magnitude of the drive current Iel when the drive transistor 35 is on is set by the current output from the current DAC.

In the present embodiment, the corrected luminance is given by the following equation.
Brightness after correction (effective luminance) = (1 + DAC gradation current (%) * DAC gradation)) * Duty ratio * Luminance before correction Here, DAC gradation current (%) is the first floor of DAC with respect to the basic current source. It means the ratio of current regulation. In other words, it is determined how much the current is added to the basic current value. The DAC gradation determines how much current is added to the basic current. In this example, since a 4-bit current DAC is configured, a current up to 15 times the DAC gradation current can be added to the basic current.
FIG. 7 shows luminance (pre-correction luminance) that can be corrected by the unit circuit U1 with the duty ratio fixed at 100%. Here, the luminance before correction is indicated by a relative ratio of 1 to 100, and the target luminance is a luminance having a relative ratio of 100. The reason why the duty ratio is fixed to 100% is to explain the case of correction using only the DAC. As shown in this figure, when a 4-bit current DAC is used for a light emitting element that emits light with a relative brightness of 1 to 100, the pre-correction brightness “66” no matter how the gradation of the current DAC is set. Can only be corrected. Whether correction is possible or not is determined by whether or not the corrected luminance falls within ± 2% of the target luminance. The numerical value of 2% is a numerical value set based on the visual recognition determination of unevenness in actual printing. The light emitting element having the luminance before correction “66” has the luminance after correction “99” and is within ± 2% of the target luminance “100”, and is “correctable”. However, the light-emitting element with the luminance before correction “65” has the luminance after correction “97.5”, which is not within ± 2% of the target luminance “100”, and is “uncorrectable”.

On the other hand, when the duty ratio can be selected as in this embodiment, correction can be performed as shown in FIG. Here, a current DAC having a current step of 3.3% with respect to the basic current source is assumed. This is a numerical value obtained when, for example, the channel width of the transistor of the basic current source is 120 μm and the basic channel width of the current DAC is 4 μm. Other channel lengths, operating point voltages, etc. are set to the same value.
In the correction, first, the duty ratio = 50% / DAC gradation 0 (addition current by DAC = 0) is set at the maximum value of the luminance variation before correction. Second, the voltage of the current source is adjusted so that the maximum value of the pre-correction luminance variation becomes the target luminance. Thirdly, the correction data of the duty ratio and the current DAC is adjusted with respect to the light emitting element having the relative luminance of 1 to 100 so as to match the target luminance = the maximum value of the luminance variation before correction. In other words, the duty ratio is set for each light emitting element 36, and the level of the drive current of the light emitting element 36 is adjusted so as to match the target luminance determined according to the duty ratio. For example, the target luminance of a light emitting element with a duty ratio of 50% is twice the target luminance of a light emitting element with a duty ratio of 100%.

  When the pre-correction luminance is in the range of 100 to 67, the post-correction luminance can be corrected to a range of 100 ± 2% by setting the duty ratio to 50% and the DAC gradations 0 to 15 (correction data values). Further, when the pre-correction luminance is in the range of 66 to 46, the post-correction luminance can be corrected to a range of 100 ± 2% by setting the duty ratio to 75% and the DAC gradations 0 to 15 (correction data value). In addition, when the luminance before correction is in the range of 45 to 33, the luminance after correction can be corrected to the range of 100 ± 2% by setting the duty ratio to 100% and the DAC gradations 0 to 15 (correction data value). It should be noted that correction is not possible when the pre-correction luminance is in the range of 32-1. In this way, even if the configuration of the optical head 10A is not changed, the duty ratio can be set to 50%, 75%, and 100% by performing quadruple speed driving, that is, one gradation period is first to first. The range that can be corrected can be expanded by scanning in the fourth sub-gradation period.

<2. Second Embodiment>
In the first embodiment described above, the control circuit 50A supplies the selection signals SELa to SELd common to the blocks B1 to Bn. For this reason, sampling is performed even when the value of the lighting control signal is the same in the adjacent sub-gradation periods. On the other hand, in the second embodiment, the duty ratio is set for each block, and the sampling operation corresponding to this is executed.
FIG. 8 shows a block diagram of an exposure apparatus B according to the second embodiment. The control circuit 50B generates selection signals SELa1 to SELd1, SELa2 to SELd2,... SELan to SELdn for each of the blocks B1 to Bn.

FIG. 9 shows a block diagram of the optical head 10B of the second embodiment, and FIG. 10 shows a timing chart thereof. The optical head 10B is configured in the same manner as the optical head 10A of the first embodiment shown in FIG. 4 except that a selection signal is supplied independently to the demultiplexers DMP1 to DMPn.
In the present embodiment, as shown in FIG. 10, when the duty ratio is 100%, the selection signals SELa1 to SELd1 that are sequentially activated in the first sub gradation period are generated. In this case, the sampling of the data signal D1 is executed only in the first sub gradation period, and the result is held in the latch circuit 21. Therefore, the duty ratio is 100%.
Next, when the duty ratio is 75%, selection signals SELa1 to SELd1 that are sequentially active in the first sub-gradation period and the fourth sub-gradation period are generated. In this case, the data signal D1 in the fourth sub gradation period is OFF (0). Therefore, the lighting control signals d11 to d14 in the fourth sub gradation period are forcibly set to “0”. As a result, since the period contributing to light emission is the first to third sub gradation periods, the duty ratio is 75%.

  Next, when the duty ratio is 50%, selection signals SELa1 to SELd1 that are sequentially active in the first sub-gradation period and the third sub-gradation period are generated. In this case, the data signal D1 in the third sub gradation period is OFF (0). Accordingly, the lighting control signals d11 to d14 in the third sub gradation period are forcibly set to “0”. As a result, since the period contributing to light emission is the first and second sub-gradation periods, the duty ratio is 50%.

  Next, when the duty ratio is 25%, the selection signals SELa1 to SELd1 that are sequentially activated in the first sub gradation period and the second sub gradation period are generated. In this case, the data signal D1 in the second sub gradation period is OFF (0). Accordingly, the lighting control signals d11 to d14 in the second sub gradation period are forced to be “0”. As a result, since the period contributing to light emission is the first and second sub-gradation periods, the duty ratio is 25%.

  As described above, in this embodiment, the duty ratio is set for each block, and the selection signal is activated in the start of the effective sub gradation period and the sub gradation period next to the sub gradation period to be ended. While the data signal D1 is enabled in the sub gradation period, the data signal D1 is set to invalid (0) in the sub gradation period subsequent to the sub gradation period to be ended. Power consumption can be reduced.

  Note that the control circuit 50B may always output the high level “1” or the low level “0” as the data signal D1 as in the first embodiment described above, or the sub gray level in which the selection signal is valid. The data signal D1 may be output only during the period, and the output of the data signal D1 may be stopped during the other periods to enter a high impedance state.

  The reason why the two-step correction such as the flattening of the light emission luminance within the block and the flattening of the light emission luminance between the blocks is executed is as follows. That is, even if the variation in the light emission luminance of the light emitting element 36 is large in the entire optical head 10A, the possibility that the maximum value and the minimum value of the variation are locally concentrated is extremely small. In other words, even if the variation of the optical head 10A is large, the variation in the divided blocks is small. Therefore, the variation in the block is corrected using the current DAC having a relatively small number of bits, and the variation in the light emission luminance between the blocks is corrected by adjusting the light emission period (= duty ratio). That is, relatively small variations in the blocks are corrected by adjusting the magnitude of the drive current, and relatively large variations between the blocks are corrected by adjusting the light emission period.

  If the light emission luminance of the entire optical head 10A is corrected uniformly only by adjusting the magnitude of the drive current, the number of bits of the current DAC needs to be significantly increased. Necessary. On the other hand, since the optical head 10A according to the present embodiment performs two kinds of corrections such as within a block and between blocks, the light emission luminance of the entire optical head 10A can be made uniform with a simple configuration.

<3. Modification>
The present invention is not limited to the above-described embodiments, and for example, the following modifications are possible.
In each of the embodiments described above, an OLED element is taken as an example of the light emitting element 36, but an inorganic light emitting diode may be used. In short, any element may be used as long as it emits light with light emission luminance corresponding to the level of the drive signal. For example, the light emitting element 36 includes a field emission device (FED), a surface conduction electron emission device (SED), a ballistic electron surface emitting device (BSD), and the like. Can be included.
In each embodiment described above, the number of the light emitting elements 36 included in each of the blocks B1 to Bn has been described as four. However, the number is arbitrary as long as the number is two or more. In the first embodiment, the drive current level of the light emitting element 36 is adjusted so as to match the target luminance determined for each block, and light emission and non-light emission are performed for each block so that the light emission luminance matches between the blocks. The duty ratio may be controlled.

<4. Image forming apparatus>
The optical heads 10A and 10B according to the above embodiments can be used as a line-type optical head for writing a latent image on an image carrier in an image forming apparatus using an electrophotographic system. Examples of the image forming apparatus include a printer, a printing part of a copying machine, and a printing part of a facsimile. FIG. 11 is a longitudinal sectional view showing an example of an image forming apparatus using the optical heads 10A and 10B as line type optical heads. This image forming apparatus is a tandem type full color image forming apparatus using a belt intermediate transfer body system.

  In this image forming apparatus, four organic EL arrays 10K, 10C, 10M, and 10Y having the same configuration are exposed to four photosensitive drums (image carriers) 110K, 110C, 110M, and 110Y having the same configuration. It is arranged at each position. The organic EL arrays 10K, 10C, 10M, and 10Y are the optical heads 10A and 10B according to any one of the embodiments exemplified above.

  As shown in FIG. 11, the image forming apparatus is provided with a driving roller 121 and a driven roller 122. An endless intermediate transfer belt 120 is wound around these rollers 121 and 122, and an arrow indicates As shown, the periphery of the rollers 121 and 122 is rotated. Although not shown, tension applying means such as a tension roller that applies tension to the intermediate transfer belt 120 may be provided.

  Around the intermediate transfer belt 120, four photosensitive drums 110K, 110C, 110M, and 110Y each having a photosensitive layer on the outer peripheral surface are arranged at a predetermined interval. The subscripts K, C, M, and Y mean that they are used to form black, cyan, magenta, and yellow visible images, respectively. The same applies to other members. The photosensitive drums 110K, 110C, 110M, and 110Y are rotationally driven in synchronization with the driving of the intermediate transfer belt 120.

  Around each photosensitive drum 110 (K, C, M, Y), there is a corona charger 111 (K, C, M, Y), an organic EL array 10 (K, C, M, Y), and development. A device 114 (K, C, M, Y) is arranged. The corona charger 111 (K, C, M, Y) uniformly charges the outer peripheral surface of the corresponding photosensitive drum 110 (K, C, M, Y). The organic EL array 10 (K, C, M, Y) writes an electrostatic latent image on the charged outer peripheral surface of the photosensitive drum. Each organic EL array 10 (K, C, M, Y) is installed such that the arrangement direction of the plurality of light emitting elements P is along the bus (main scanning direction) of the photosensitive drum 110 (K, C, M, Y). Is done. The electrostatic latent image is written by irradiating the photosensitive drum with light by the plurality of light emitting elements P described above. The developing device 114 (K, C, M, Y) forms a visible image, that is, a visible image on the photosensitive drum by attaching toner as a developer to the electrostatic latent image.

  The black, cyan, magenta, and yellow developed images formed by the four-color single-color image forming station are sequentially transferred onto the intermediate transfer belt 120 to be superimposed on the intermediate transfer belt 120. As a result, a full-color image is obtained. Four primary transfer corotrons (transfer devices) 112 (K, C, M, Y) are arranged inside the intermediate transfer belt 120. The primary transfer corotron 112 (K, C, M, Y) is disposed in the vicinity of the photosensitive drum 110 (K, C, M, Y), and the photosensitive drum 110 (K, C, M, Y). The electrostatic image is electrostatically attracted from the toner image to transfer the visible image to the intermediate transfer belt 120 passing between the photosensitive drum and the primary transfer corotron.

  A sheet 102 as an object on which an image is to be finally formed is fed one by one from the sheet feeding cassette 101 by the pickup roller 103, and between the intermediate transfer belt 120 and the secondary transfer roller 126 in contact with the driving roller 121. Sent to the nip. The full-color visible image on the intermediate transfer belt 120 is secondarily transferred to one side of the sheet 102 by the secondary transfer roller 126 and fixed on the sheet 102 through the fixing roller pair 127 as a fixing unit. . Thereafter, the sheet 102 is discharged onto a paper discharge cassette formed in the upper part of the apparatus by a paper discharge roller pair 128.

Next, another embodiment of the image forming apparatus according to the present invention will be described.
FIG. 12 is a longitudinal sectional view of another image forming apparatus using the optical heads 10A and 10B as line type optical heads. This image forming apparatus is a rotary developing type full-color image forming apparatus using a belt intermediate transfer body system. In the image forming apparatus shown in FIG. 13, a corona charger 168, a rotary developing unit 161, an organic EL array 167, and an intermediate transfer belt 169 are provided around the photosensitive drum 165.

  The corona charger 168 uniformly charges the outer peripheral surface of the photosensitive drum 165. The organic EL array 167 writes an electrostatic latent image on the charged outer peripheral surface of the photosensitive drum 165. The organic EL array 167 is the optical heads 10 </ b> A and 10 </ b> B of each aspect exemplified above, and is installed so that the arrangement direction of the plurality of light emitting elements P is along the bus line (main scanning direction) of the photosensitive drum 165. The electrostatic latent image is written by irradiating the photosensitive drum 165 with light from these light emitting elements P.

  The developing unit 161 is a drum in which four developing units 163Y, 163C, 163M, and 163K are arranged at an angular interval of 90 °, and can rotate counterclockwise about the shaft 161a. The developing units 163Y, 163C, 163M, and 163K supply yellow, cyan, magenta, and black toners to the photosensitive drum 165, respectively, and attach the toner as a developer to the electrostatic latent image, thereby the photosensitive drum 165. A visible image, that is, a visible image is formed.

  The endless intermediate transfer belt 169 is wound around a driving roller 170a, a driven roller 170b, a primary transfer roller 166, and a tension roller, and is rotated around these rollers in a direction indicated by an arrow. The primary transfer roller 166 transfers the visible image to the intermediate transfer belt 169 that passes between the photosensitive drum and the primary transfer roller 166 by electrostatically attracting the visible image from the photosensitive drum 165.

  Specifically, in the first rotation of the photosensitive drum 165, an electrostatic latent image for a yellow (Y) image is written by the organic array 167, and a developed image of the same color is formed by the developing unit 163Y. The image is transferred to the transfer belt 169. Further, in the next rotation, an electrostatic latent image for a cyan (C) image is written by the organic array 167, a developed image of the same color is formed by the developing device 163C, and an intermediate transfer is performed so as to overlap the yellow developed image. Transferred to the belt 169. Then, during the four rotations of the photosensitive drum 165, yellow, cyan, magenta, and black visible images are sequentially superimposed on the intermediate transfer belt 169. As a result, a full-color visible image is formed on the transfer belt 169. It is formed. When images are finally formed on both sides of a sheet as an object on which an image is to be formed, the same color images of the front and back surfaces are transferred to the intermediate transfer belt 169, and then the front and back surfaces are transferred to the intermediate transfer belt 169. A full-color visible image is obtained on the intermediate transfer belt 169 by transferring the visible image of the next color.

  The image forming apparatus is provided with a sheet conveyance path 174 through which a sheet passes. The sheets are picked up one by one from the paper feed cassette 178 by the pick-up roller 179, advanced through the sheet transport path 174 by the transport roller, and between the intermediate transfer belt 169 and the secondary transfer roller 171 in contact with the drive roller 170a. Pass through the nip. The secondary transfer roller 171 transfers the developed image to one side of the sheet by electrostatically attracting a full-color developed image from the intermediate transfer belt 169 collectively. The secondary transfer roller 171 can be moved closer to and away from the intermediate transfer belt 169 by a clutch (not shown). The secondary transfer roller 171 is brought into contact with the intermediate transfer belt 169 when a full-color visible image is transferred onto the sheet, and is separated from the secondary transfer roller 171 while the visible image is superimposed on the intermediate transfer belt 169.

  The sheet on which the image has been transferred as described above is conveyed to the fixing device 172 and is passed between the heating roller 172a and the pressure roller 172b of the fixing device 172, whereby the visible image on the sheet is fixed. The sheet after the fixing process is drawn into the discharge roller pair 176 and proceeds in the direction of arrow F. In the case of double-sided printing, after most of the sheet passes through the paper discharge roller pair 176, the paper discharge roller pair 176 is rotated in the reverse direction and introduced into the double-sided printing conveyance path 175 as indicated by an arrow G. The Then, the visible image is transferred to the other surface of the sheet by the secondary transfer roller 171, the fixing process is performed again by the fixing device 172, and then the sheet is discharged by the discharge roller pair 176.

  Since the image forming apparatus illustrated in FIGS. 11 and 12 uses the light emitting element 36 as an exposure unit, the apparatus can be made smaller than when a laser scanning optical system is used. It should be noted that the optical head of the present invention can also be used in electrophotographic image forming apparatuses other than those exemplified above. For example, the optical head according to the present invention can be applied to an image forming apparatus that directly transfers a visible image from a photosensitive drum to a sheet without using an intermediate transfer belt, and an image forming apparatus that forms a monochrome image. Is possible.

  The image forming apparatus to which the optical head according to the present invention is applied is not limited to the image forming apparatus. For example, the optical head of the present invention is also used as a lighting device in various electronic devices. Examples of such electronic devices include facsimile machines, copiers, multifunction machines, and printers. In these electronic devices, an optical head in which a plurality of light emitting elements are arranged in a planar shape is suitably employed.

1 is a perspective view showing a partial configuration of an image forming apparatus using an optical head 10A according to a first embodiment of the present invention. It is a top view which shows the structure of 10 A of optical heads. 2 is a block diagram showing a configuration of an exposure apparatus A. FIG. It is a circuit diagram which shows the structure of 10 A of optical heads. 3 is a timing chart showing the operation of the optical head 10A. It is explanatory drawing which shows the brightness | luminance which can be correct | amended by the unit circuit U1 in this embodiment. It is explanatory drawing which shows the brightness | luminance which can be correct | amended by unit circuit U1 by making a duty ratio fixed to 100%. It is a block diagram which shows the structure of the exposure apparatus B which concerns on 2nd Embodiment of this invention. It is a circuit diagram which shows the structure of the optical head 10B. 6 is a timing chart showing the operation of the optical head 10B. 1 is a longitudinal sectional view showing a configuration of an image forming apparatus using an optical head according to the present invention. It is a longitudinal cross-sectional view which shows the structure of the other image forming apparatus using the optical head which concerns on this invention.

Explanation of symbols

  A, B ... Exposure apparatus, 10A, 10B ... Optical head, DMP1-DMPn ... Demultiplexer, 31-34 ... Transistor, 35 ... Drive transistor, M1-M4 ... Memory, SELa-SELd ... Light emission control Signals, d11 to d14... Lighting control signal, 36. B1-Bn: Block.

Claims (8)

A method of driving an optical head having a plurality of light emitting elements,
One gradation period for engraving one gradation is divided into a plurality of sub gradation periods,
Selecting one or a plurality of sub gradation periods from the plurality of sub gradation periods for each of the plurality of light emitting elements;
In one or more selected sub-gradation periods, a driving signal having a level corresponding to light emission characteristics is generated for each of the plurality of light emitting elements, and is supplied to each of the plurality of light emitting elements
An optical head driving method characterized by the above. A method of driving an optical head having a plurality of light emitting elements,
Grouping the plurality of light emitting elements adjacent to each other and dividing them into a plurality of blocks,
For each of the plurality of blocks, the level of the drive signal supplied to the plurality of light emitting elements is adjusted so that the light emission luminance of the plurality of light emitting elements belonging to the block is equal,
One gradation period in which one gradation is engraved is divided into a plurality of sub gradation periods so that the light emission luminances of the light emitting elements are equal between the plurality of blocks, and one or more of the plurality of sub gradation periods are separated from each other. Select multiple sub-gradation periods;
An optical head driving method characterized by the above. An optical head having a plurality of light emitting elements,
First adjustment means for adjusting the level of a drive signal for each of the plurality of light emitting elements;
For each of the plurality of light emitting elements, one gradation period for engraving one gradation is divided into a plurality of sub gradation periods, and one or a plurality of sub gradation periods are selected from the plurality of sub gradation periods. A second adjusting means;
An optical head comprising: The first adjusting means includes
A plurality of unit circuits provided for each of the plurality of light emitting elements;
Each of the plurality of unit circuits has a level that becomes a target luminance according to a duty ratio set for each of the plurality of light emitting elements in a period in which a lighting control signal that specifies lighting or extinguishing of the light emitting elements specifies lighting. Drive signal
The second adjusting means includes
A plurality of latch circuits corresponding to each of the plurality of unit circuits;
Each of the plurality of latch circuits latches a data signal obtained by time-division-multiplexing the lighting control signal according to a selection signal that specifies a latching timing, and has a duty ratio set for each of the plurality of light-emitting elements. Generate lighting control signal,
The optical head according to claim 3. An optical head having a plurality of light emitting elements,
Grouping the plurality of light emitting elements adjacent to each other and dividing them into a plurality of blocks,
First adjustment means for adjusting the level of a drive signal supplied to the plurality of light emitting elements so that the light emission luminances of the plurality of light emitting elements belonging to the block are equal for each of the plurality of blocks;
One gradation period in which one gradation is engraved is divided into a plurality of sub gradation periods so that the light emission luminance of the light emitting element is equal between the plurality of blocks, and one or Second adjusting means for selecting a plurality of sub-gradation periods;
An optical head comprising: The first adjusting means includes
Each of the plurality of blocks has a plurality of unit circuits provided for each light emitting element belonging to the block,
Each of the plurality of unit circuits is determined such that the luminance of the plurality of light emitting elements belonging to the block becomes a target luminance during a period in which a lighting control signal designating lighting or extinguishing of the light emitting elements specifies lighting. Level drive signal,
The second adjusting means includes
Each of the plurality of blocks includes a plurality of latch circuits corresponding to each of the plurality of unit circuits belonging to the block,
Each of the plurality of latch circuits latches a data signal obtained by time-division-multiplexing the lighting control signal in the block unit according to a plurality of selection signals that specify timing for latching the data signal for each of the plurality of unit circuits. Generating the lighting control signal,
The optical head according to claim 5.   The plurality of selection signals are set for each of the plurality of blocks, and from the plurality of sub-gradation periods constituting the one gradation period so that the light emission luminance of the light emitting element is equal between the plurality of blocks. 7. The optical head according to claim 6, wherein one or a plurality of sub gray scale periods are designated. An optical head according to any one of claims 1 to 7,
An image carrier that carries an image formed by light from the optical head;
An image forming apparatus.

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