Optical writing device and image forming apparatus

An optical writing device includes a plurality of current driven light emitting elements, first and second power source lines, a designation circuit that outputs a designation potential, first driving circuits provided for each of the light emitting elements to supply driving current to the corresponding light emitting element, second driving circuits provided for each of the light emitting elements to supply driving current to the corresponding light emitting element, and a switching control unit that alternately switches respective states of the first and second driving circuits between a state where one of the first and second driving circuits receives the designation potential while the other driving circuit supplies the driving current, and a state where the other driving circuit receives the designation potential while the one driving circuit supplies the driving current.

The entire disclosure of Japanese Patent Application No. 2014-094571 filed on May 1, 2014 including description, claims, drawings, and abstract are incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to an optical writing device and an image forming apparatus, and particularly to a technology for preventing non-uniformity of light intensity of an optical writing device which uses an organic LED.

Description of the Related Art

In recent years, an optical writing device (PH: Print Head) including organic LEDs (OLEDs: Organic Light Emitting Diodes) has been proposed as a component equipped on an image forming apparatus with an aim of miniaturization and cost reduction of the image forming apparatus. OLEDs are disposed on a TFT (Thin Film Transistor) substrate and arranged in lines in a horizontal scanning direction, and electrically connected in parallel via power source wiring similarly arranged in the horizontal scanning direction (FIG. 10).

An OLED is called an organic EL (Organic Electro-Luminescence) element as well, and provided as a current driven light emitting element. When driving current is supplied to an OLED via power source wiring, a voltage drop occurs along the power source wiring due to wiring resistance.

On the other hand, a driving circuit which generates driving current for an OLED is provided for each OLED at a position adjacent to the corresponding OLED, and generates driving current in reference to an electric potential at a junction point between the driving circuit and the power source wiring. Accordingly, the voltage drop at the power source wiring produces a drop of the reference potential, in which condition the amount of driving current to be supplied to the OLED is variable. In this case, the light emission luminance becomes variable, and non-uniformity of images may be caused (FIGS. 11A and 11B).

For overcoming this problem, reduction of impedance of power source wiring has been proposed, for example (JP 2005-144685 A, JP 2005-144686 A, JP 2005-144687 A, and JP 2010-076184 A). According to this method, the voltage drop produced by driving current is avoidable, wherefore the non-uniformity of images can decrease.

According to the foregoing conventional technology, power source wiring is further formed on sealing glass provided for sealing the TFT substrate, and the power source wiring on the TFT substrate and the power source wiring on the sealing glass are electrically connected by connecting parts at respective power supply points of the driving circuit, for the purpose of reduction of impedance of the power source wiring. In this case, there is a problem that the unit cost rises. Moreover, the auxiliary power source wiring thus formed is thin-film wiring, wherefore reduction of impedance of the power source wiring is limited.

According to another conventional technology, one line cycle is divided into a sample period and a hold period. During the sample period, OLEDs are turned off, and a luminance signal output from a DAC (Digital to Analogue Converter) circuit is temporarily held in a sample hold circuit (hereinafter referred to as “S/H circuit”) provided for each OLED. During the hold period, driving current in correspondence with the luminance signal held in the S/H circuit is supplied to each OLED to allow light emission therefrom.

According to this structure, no driving current flows during the sample period, in which condition no voltage drop occurs. Accordingly, the luminance signal is appropriately sampled, wherefore non-uniformity of luminance caused by a voltage drop is avoidable.

However, while a method so-called rolling driving turns off an OLED only when the luminance signal is input to the corresponding S/H circuit, this conventional technology turns off OLEDs throughout the sample period in which luminance signals are sequentially input to a number of S/H circuits, and only turns on the OLEDs during the hold period. In this case, light emission duty corresponding to a proportion of a light emission period in a horizontal scanning period (Hsync) lowers, wherefore the light emission period becomes short.

When the light emission amount from the OLEDs is raised by increasing the amount of driving current supplied to the OLEDs so as to obtain sufficient exposure during the short light emission period, the life of the OLEDs may decrease.

SUMMARY OF THE INVENTION

The present invention has been developed to solve the aforementioned problems. It is an object of the present invention to provide an optical writing device and an image forming apparatus, capable of improving image quality and prolonging lives of the optical writing device and the image forming apparatus, by preventing non-uniformity of light intensity of OLEDs, and increasing light emission duty.

To achieve the abovementioned object, according to an aspect, an optical writing device that exposes a photosensitive body to form an electrostatic latent image line by line for each horizontal scanning period, reflecting one aspect of the present invention, comprises: a plurality of current driven light emitting elements arranged in lines; first and second power source lines extending along the plurality of light emitting elements, and connected with a constant voltage source; a designation circuit that outputs a designation potential designating a light emission amount for each of the light emitting elements; first driving circuits provided for each of the light emitting elements, each of the first driving circuits including a first holding circuit that receives and holds the designation potential output from the designation circuit, and connected with the first power source line to supply driving current to the corresponding light emitting element in accordance with a potential difference between a potential at a junction point with the first power source line and the designation potential held by the first holding circuit; second driving circuits provided for each of the light emitting elements, each of the second driving circuits including a second holding circuit that receives and holds the designation potential output from the designation circuit, and connected with the second power source line to supply driving current to the corresponding light emitting element in accordance with a potential difference between a potential at a junction point with the second power source line and the designation potential held by the second holding circuit; and a switching control unit that controls the first and second driving circuits provided for the same light emitting element so as to alternately switch respective states of the first and second driving circuits between a state where one of the first and second driving circuits receives the designation potential while the other driving circuit supplies the driving current, and a state where the other driving circuit receives the designation potential while the one driving circuit supplies the driving current, for each of the horizontal scanning periods.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, an optical writing device and an image forming apparatus according to an embodiment of the present invention will be described with reference to the drawings. However, the scope of the invention is not limited to the illustrated examples.

[1] Configuration of Image Forming Apparatus

Initially, a configuration of the image forming apparatus according to this embodiment is described.

[1-1] Configuration of Image Forming Apparatus

The configuration of the image forming apparatus according to this embodiment is now described.

FIG. 1is a view illustrating a main configuration of the image forming apparatus according to this embodiment. As illustrated inFIG. 1, an image forming apparatus1is a so-called tandem color multi-function peripheral (MFP), and includes a document reading unit100, an image forming unit110, and a feeding unit130. The document reading unit100optically reads a document placed on a document tray101and creates image data of the document, while sending the document by using an automatic document feeder (ADF)102. The image data thus obtained is stored in a control unit112(described later).

The image forming unit110includes imaging units111Y through111K, the control unit112, an intermediate transfer belt113, a pair of secondary transfer rollers114, a fixing device115, a pair of discharge rollers116, a discharge tray117, a cleaning blade118, and a pair of timing rollers119. Toner cartridges120Y through120K are attached to the image forming unit110to supply toner in colors of Y (yellow), M (magenta), C (cyan), and K (black), respectively.

The imaging units111Y through111K receive supply of toner from the corresponding toner cartridges120Y through120K, and form toner images in respective colors of Y, M, C, and K under the control of the control unit112. For example, the imaging unit111Y includes a photosensitive drum121, a charging device122, an optical writing device123, a developing device124, and a cleaning device125. The charging device122uniformly charges an outer circumferential surface of the photosensitive drum121under the control of the control unit112.

The control unit112generates a digital luminance signal to allow light emission from the optical writing device123based on printing image data contained in a received job. The control unit112generates this digital luminance signal by using an ASIC (Application Specific Integrated Circuit, hereinafter referred to as “luminance signal output unit”) contained in the control unit112. The optical writing device123includes light emitting elements arranged in lines in the horizontal scanning direction, as will be described later. The optical writing device123executes optical writing to the outer circumferential surface of the photosensitive drum121by utilizing light emitted from the respective light emitting elements in response to the digital luminance signal generated from the control unit112, and forms an electrostatic latent image.

The developing device124supplies toner to the outer circumferential surface of the photosensitive drum121to develop the electrostatic latent image (visualize the image). Primary transfer voltage is applied to the primary transfer roller126so that a toner image carried on the outer circumferential surface of the photosensitive drum121can be electrostatically transferred to the intermediate transfer belt113by electrostatic attachment (primary transfer). After the primary transfer, the cleaning device125scrapes residual toner remaining on the outer circumferential surface of the photosensitive drum121off the surface by using a cleaning blade, and illuminates the outer circumferential surface of the photosensitive drum121by using a discharging lamp to remove charges from the surface.

The imaging units111M through111K form toner images in colors of M, C, and K, respectively, in manners similar to the foregoing method. These toner images are sequentially transferred to the intermediate transfer belt113by the primary transfer such that the respective toner images are overlapped with each other and forma color toner image on the intermediate transfer belt113. The intermediate transfer belt113is an endless rotating body which rotates in a direction indicated by an arrow A. The intermediate transfer belt113sends the toner image after the primary transfer toward the pair of secondary transfer rollers114.

The feeding unit130includes feeding cassettes131, each of which stores recording sheets S in corresponding sheet size. The feeding unit130supplies the recording sheets S sheet by sheet to the image forming unit110. The recording sheet S supplied from the feeding unit130is conveyed in parallel with the running of the toner image on the intermediate transfer belt113, and passes through the pair of timing rollers119to reach the pair of secondary transfer rollers114. The pair of timing rollers119sends the recording sheet S such that the recording sheet S and the toner image can reach the pair of secondary transfer rollers114at the same time.

The pair of secondary transfer rollers114is constituted of a pair of rollers to which secondary transfer voltage is applied. The pair of secondary transfer rollers114is pressed against each other to form a secondary transfer nip portion. The toner image on the intermediate transfer belt113is electrostatically transferred to the recording sheet S (secondary transfer) at this transfer nip portion. The recording sheet S to which the toner image has been transferred is sent to the fixing device115. Residual toner remaining on the intermediate transfer belt113after the secondary transfer is further conveyed in the direction of the arrow A, and scraped by the cleaning blade118to be discarded.

The fixing device115fixes the toner image to the recording sheet S by heating and fusing the toner image. The recording sheet S to which the toner image has been fixed by fusing is discharged to the discharge tray117by the pair of discharge rollers116.

The control unit112controls the foregoing processes and other operation of the image forming apparatus1, including operation of a not-shown operation panel. The control unit112transmits and receives image data to and from other devices such as a personal computer (PC), and receives printing jobs. The control unit112includes a facsimile modem to transmit and receive image data to and from other facsimile machines via facsimile lines.

Instead of the configuration discussed herein, a transfer charger or a transfer belt may be employed in transferring toner images in place of the transfer rollers. In addition, in removing residual toner from the intermediate transfer belt113, a cleaning brush or a cleaning roller may be employed in place of the cleaning blade118.

[2] Configuration of Optical Writing Device123

A configuration of the optical writing device123is hereinafter described.

FIG. 2is a cross-sectional view illustrating optical writing operation performed by the optical writing device123. As illustrated inFIG. 2, the optical writing device123includes an OLED panel200and a rod lens array (SLA: Selfoc Lens Array)202, both components of which are housed in a holder203. OLEDs201corresponding to a number of light emission dots are mounted on the OLED panel200and arranged in lines in the horizontal scanning direction. Each of the OLEDs201emits optical beams L, while the rod lens array202converges the optical beams L on the outer circumferential surface of the photosensitive drum121.

FIG. 3is a schematic plan view of the OLED panel200, accompanied with a cross-sectional view taken along a line A-A′, and a cross-sectional view taken along a line C-C′. A schematic plan view part inFIG. 3shows the OLED panel200from which a sealing plate (described later) is removed.

As illustrated inFIG. 3, the OLED panel200includes a TFT substrate300, a sealing plate301, a source IC302, and others. A number of OLEDs are disposed on the TFT substrate300and arranged in lines in the horizontal scanning direction. The TFT substrate300has a substrate surface on which the OLEDs are arranged. This surface is provided as a sealing area to which the sealing plate301is attached with spacer frame bodies303interposed between the sealing area and the sealing plate301.

This structure produces a sealed condition of the sealing area, into which dry nitrogen or the like is charged to avoid contact between the sealing area and the outside air. A moisture absorbent may be further sealed into the sealing area for absorbing moisture. The sealing plate301may be made of sealing glass, for example, or material other than glass.

The source IC302is mounted on the TFT substrate300in an area out of the sealing area. A luminance signal output unit310of the control unit112inputs a digital luminance signal to the source IC302via a flexible wire311. The source IC302converts the digital luminance signal into an analog luminance signal (hereinafter abbreviated as “luminance signal”), and inputs the converted signal to driving circuits provided for each of the OLEDs. The driving circuits generate driving current for the OLEDs in accordance with the luminance signal. According to this embodiment, the luminance signal is a voltage signal.

According to this embodiment, 15,000 OLEDs are arranged in lines on the TFT substrate300, and divided into 150 light emission blocks each of which contains 100 OLEDs.

FIG. 4is a block diagram illustrating a main circuit configuration on the TFT substrate300. As illustrated inFIG. 4, a dot circuit array400is formed on the TFT substrate300. The dot circuit array400includes, for each light emission block, a shift register401, and dot circuits402provided for each of OLEDs405, and receives input of control signals from an SEL circuit, a φSH circuit, and a DAC circuit contained in the source IC302.

The shift register401provided for each of the light emission blocks sequentially designates a dot circuit to which a luminance signal is to be written. The shift register401also includes a logic circuit for controlling operation of the corresponding dot circuits402.FIG. 5illustrates a main configuration of the shift register401. The shift register401, having received input of an SEL signal and a φSH signal from the SEL circuit and the φSH circuit of the source IC, produces a /SEL signal corresponding to an inverse signal of the SEL signal at a NOT element502.

The shift register401further produces a φA signal at an OR element501based on the SEL signal and the φSH signal, and produces a φB signal at an OR element503based on the /SEL signal and the φSH signal. The SEL signal and the /SEL signal are used in selecting a power source line through which driving current is supplied to the corresponding OLED405, as will be discussed later. The φA signal and the φB signal are used in determining whether or not the luminance signal is to be written.

Each of the dot circuits402includes the OLED405and a dot driving circuit403. The dot driving circuit403is constituted of driving circuits404A and404B of dual systems A and B. The driving circuits404A and404B receive power supply via power source lines VcA and VcB of the dual systems A and B, respectively, as lines extending from a constant voltage source Vc. The power source lines VcA and VcB are branched from each other in the vicinity of the DAC circuit (source IC302) outside the dot circuit array400, and wired after the branch to the driving circuits404A and404B, respectively, with no junction between the power source lines VcA and VcB.

Driving current of an amount corresponding to the luminance signal received from the DAC circuit is supplied to the OLED405via the driving circuit404A or404B designated based on the SEL signal and the /SEL signal.

According to this embodiment, the TFT substrate300is formed in the following procedures. Initially, the dot circuit array400not including the OLEDs405is formed on a glass substrate. Then, the OLEDs405are formed. Subsequently, the source IC302is mounted to complete the TFT substrate300.

[3] Configuration of Driving Circuits404

A configuration of the driving circuits404is hereinafter described. The driving circuits404A and404B of the A and B systems have a common configuration. Accordingly, in the following description, signs “A” and “B” are not given to the reference numerals of the driving circuits404A and404B.

FIG. 6is a circuit diagram illustrating a main configuration of the driving circuits404. As illustrated inFIG. 6, each of the driving circuits404includes selector switches601and604, a capacitor602, and a TFT603. According to this embodiment, each of the selector switches601and604is a TFT.

The capacitor602and the selector switch601function as an S/H circuit unit, and hold a potential difference between the luminance signal output from the DAC circuit, and the constant voltage source Vc. The respective selector switches601disposed on wires extending from a corresponding signal line to the capacitors602are switched in accordance with the control signals φA and φB received from the shift register401, so that the luminance signal is input only to the selected capacitor602and held therein.

According to this embodiment, the signal line extending from the DAC circuit to the driving circuits404is provided as a common line for the A and B systems. However, the signal line from the DAC circuit may be separately provided for each of the A and B systems.

The TFT603supplies, to the OLED405, driving current corresponding to the luminance signal held in the capacitor602, in response to the luminance signal applied between a source and a drain of the TFT603.

The selector switch604is disposed between a drain terminal of the TFT603and the OLED405. The selector switch604functions as a driving current control unit which supplies driving current to the OLED405only from the driving circuit404of the system A or B2selected by the shift register401.

When the selector switch604is disposed on the circuitry in the range from the power source line Vc to the TFT603, gate voltage Vg of the TFT603is variable in accordance with variations of conduction-state characteristics of the selector switch604. In this case, the accuracy of the driving current amount to be supplied may decrease.

Moreover, in manufacturing the OLED panel200, OLEDs need to be formed on the upper part of TFTs previously formed on the glass substrate. Accordingly, when the selector switch604is disposed on the cathode side of the OLED405, the OLED405comes to the position of the circuitry in the range from the TFT603to the selector switch604.

In this case, the selector switch604needs to be further formed on the upper part of the OLED405, for example, after the OLED405is formed on the upper part of the TFT603. Accordingly, a connection step is additionally required for connecting the OLED405and the selector switch604, in which case design and manufacture become difficult.

On the other hand, when the selector switch604is disposed on the circuitry in the range from the TFT603to the OLED405as in this embodiment, the following advantages are offered:(a) reduction of variations of the driving current amount resulting from variations of the conduction-state characteristics of the selector switch604; and(b) easy formation of the circuitry.

[4] Operation of Optical Writing Device123

Operation of the optical writing device123is hereinafter described. Every light emission block operates in a similar manner, wherefore operation of one of the light emission blocks is only discussed herein.

FIG. 7is a timing chart showing an example of exposing operation executed by the one light emission block. The light emission block exposes the outer circumferential surface of the photosensitive drum121line by line.FIG. 7shows exposing operation executed from the mth line to the (m+2)th line.

As illustrated inFIG. 7, the φSH signal repeats both an H state and an L state 100 times to sequentially select the 100 OLEDs405constituting the one light emission block during one horizontal scanning period (Hsync).

The SEL signal holds either an H state and an L state during one horizontal scanning period to control on and off of the selector switch604B of the B system. In the H state of the SEL signal, the selector switch604B is turned on, and driving current is supplied from the driving circuit404B of the B system to the OLED405. On the other hand, in the L state of the SEL signal, the selector switch604B is turned off, in which condition no driving current is supplied from the B system.

The /SEL signal is an inverse signal of the SEL signal, and controls on and off of the selector switch604A of the A system. In an H state of the /SEL signal, the selector switch604A is turned on, and driving current is supplied from the driving circuit404A to the OLED405. On the other hand, in an L state of the /SEL signal, the selector switch604A is turned off, in which condition no driving current is supplied from the A system.

A φA(n) signal controls the selector switch601A included in the driving circuit404A in the A system of the nth (n=1 through 100) dot driving circuit403. In an H state of the φA(n) signal, the selector switch601A is turned on, and a luminance signal is written to the capacitor602A. In an L state of the φA(n) signal, the selector switch601A is turned off, in which condition luminance signal writing to the capacitor602A is inhibited.

Similarly, a φB(n) signal controls the selector switch601B included in the driving circuit404B in the B system of the nth (n=1 through 100) dot driving circuit403. In an H state of the φB(n) signal, the selector switch601B is turned on, and a luminance signal is written to the capacitor602B. In an L state of the φB(n) signal, the selector switch601B is turned off, in which condition luminance signal writing to the capacitor602B is inhibited.

During the horizontal scanning period for exposing the mth line, the H-state SEL signal is input, for example. In this case, the A system is designated for the writing period, while the B system is designated for the driving period. Every time the φSH signal comes to the H state, a luminance signal is sequentially written to the capacitor602A of the nth A system and held therein.

As illustrated inFIG. 8A, the driving circuit404A of the A system in this case does not supply driving current to the OLED405. Accordingly, no current flows in the power source line VcA of the A system, wherefore a potential VcA(n) at a junction point between the nth driving circuit404A and the power source line VcA does not drop. As a result, the potential at the junction point VcA(n) becomes substantially equivalent to a constant voltage Vc, wherefore a potential difference between the constant voltage Vc and a luminance signal Vdac (m) is accurately written to the capacitor602A.

On the other hand, the driving circuit404B of the B system supplies, to the OLED405, driving current corresponding to a potential difference written to the capacitor602B during the horizontal scanning period for the (m−1)th line in a manner similar to the foregoing method. As a result, current flows in the power source line VcB, wherefore a junction point potential VcB(n) drops.

However, under the condition that the selector switch601B has been turned off, the potential difference between the terminals of the capacitor602B is maintained without variation, and applied to the TFT604B as gate voltage VgB. In this case, the gate voltage VgB of the TFT603B is not affected by the voltage drop of the junction point potential VcB(n), wherefore non-uniformity of luminance resulting from a voltage drop does not occur.

Subsequently, in the horizontal scanning period for exposing the (m+1)th line, the SEL signal comes to the L state. As a result, the writing period of the A system is switched to the driving period, while the driving period of the B system is switched to the writing period. In this case, the driving circuit404A of the A system supplies driving current to the OLED405while not affected by the voltage drop of the junction point potential VcA(n) as illustrated inFIG. 8B. On the other hand, the junction point voltage VcB (n) does not drop in the driving circuit404B of the B system, wherefore the potential difference between the constant voltage Vc and a luminance signal Vdac(m−1) is accurately written to the capacitor602B.

During the horizontal scanning periods for exposing the (m+2)th line and further lines, processes similar to the foregoing processes are alternately repeated. After completion of these processes, exposure of an entire printing image ends.

[4] Modified Examples

While the embodiment of the present invention has been described, it is intended, as a matter of course, that the present invention should not be limited to the embodiment described herein. For example, the following modifications may be made.

(1) According to this embodiment, the selector switch604is disposed on the circuitry in the range from TFT603to the OLED405. However, needless to say, the present invention is not limited to this example. Instead, the following configuration may be employed.

FIG. 9is a circuit diagram illustrating a main configuration of the driving circuits404according to a modified example. As illustrated inFIG. 9, each of the selector switches604is disposed on the circuitry in the range from the capacitor602to the gate electrode of the TFT603. According to this configuration, the driving circuits404execute similar operation based on control signals similar to the corresponding signals of the foregoing embodiment.

In addition, this configuration decreases loads such as parasitic capacitance and element resistance which may be produced on the drain electrode side of the TFT603, and thus achieves higher light emission responsiveness. Accordingly, image quality such as contrast and MTF (Modulation Transfer Function) improves.

(2) According to this embodiment, the selector switch604is controlled via the shift register401. However, needless to say, the present invention is not limited to this example. The selector switch604may be controlled directly by the source IC302or other components positioned outside the dot circuit array400, for example.

(3) According to this embodiment, the control unit112generates the digital luminance signal to allow light emission from the optical writing device123based on the printing image data contained in the received job. However, needless to say, the present invention is not limited to this example. Instead, the following configuration may be employed.

The TFT603constituting each of the driving circuits404has characteristic variations, wherefore driving current may vary even when the same gate voltage is applied. However, when the gate current is adjusted for each of the TFTs603based on examinations of characteristic variations of the TFTs603carried out beforehand, desired driving current is allowed to be supplied to the respective OLEDs405.

For this purpose, the control unit112stores variation data obtained by the examinations, and allows the luminance signal output unit310to adjust the digital luminance signal based on the variation data. More specifically, at the same gate voltage, the control unit112outputs a digital luminance signal indicating higher luminance to the TFT603which receives smaller driving current, and outputs a digital luminance signal indicating lower luminance to the TFT603which receives larger driving current.

This method realizes high image quality regardless of the characteristic variations of the TFTs603.

(4) According to this embodiment, the example of the tandem color multifunction peripheral has been discussed. However, needless to say, the present invention is not limited to this example. The present invention is applicable to a color apparatus of a type other than the tandem type, or a monochrome apparatus. Moreover, similar advantages are offered when the present invention is applied to a printer, a copy machine equipped with a scanner, or a facsimile machine having a communication function.

An optical writing device and an image forming apparatus according to the present invention are useful devices having a function of an optical writing device utilizing organic LEDs and capable of preventing non-uniformity of light intensity.

According to an embodiment of the present invention, the driving circuit which receives the designation potential does not supply driving current. In this case, a voltage drop does not occur in the power source line connected with the driving circuit. Accordingly, non-uniformity of light emission from the light emitting elements is avoidable, wherefore image quality increases. Moreover, the driving circuit supplying driving current does not receive a new designation potential within the corresponding horizontal scanning period. In this case, light emission duty becomes the maximum, wherefore the life of the light emitting elements increases.

According to an embodiment of the present invention, the first and second power source lines are connected to the constant voltage source at a common power supply point. In this case, a reference potential at which no driving current flows is equalized between the first and second power source lines, when the corresponding driving circuits are connected only to the one side of each of the first and second power source lines with respect to the power supply point. Accordingly, the potential difference between the designation potential and the reference potential is stabilized, wherefore reduction of non-uniformity of luminance and noise is achievable. In addition, cost reduction based on reduction of the number of power sources is also achievable.

According to an embodiment of the present invention, the switching control unit preferably includes selector switches provided for each circuitry in a range from the designation circuit to the holding circuit, and a range from the driving circuit to the light emitting element. According to this configuration, variations of the driving current amount decreases in comparison with a structure which disposed the selector switch for each circuitry in a range from the power source line to the driving circuit. Moreover, manufacture is easier than in a structure which disposes the light emitting element for each circuitry in a range from the driving circuit to the selector switch.

According to an embodiment of the present invention, the switching control unit preferably includes selector switches provided for each circuitry in a range from the designation circuit to the holding circuit, and a range from the holding circuit to the driving circuit. According to this configuration, light emission responsiveness of the light emitting elements improves as a result of decrease in parasitic capacitance. Accordingly, the time required for forming an electrostatic latent image becomes shorter. In addition, manufacture of each circuitry becomes easier.

According to an embodiment of the present invention, when a correcting unit is provided as a unit that corrects the designation potential in accordance with characteristic variations of each of the driving circuits, image quality further improves.

According to an embodiment of the present invention, the light emitting elements are preferably OLEDs. It is further preferable that each of the driving circuits and the selector switches is a thin film transistor.

According to an embodiment of the present invention, an image forming apparatus according to an aspect of the present invention includes the optical writing device according to an aspect of the present invention. According to this configuration, the foregoing advantages are offered.