JP5256887B2 - Exposure apparatus and image forming apparatus - Google Patents

Description

  The present invention relates to an electrophotographic exposure apparatus and an image forming apparatus including the exposure apparatus.

An electrophotographic image forming apparatus (printer apparatus) has an exposure apparatus for image formation (printing). In recent years, various attempts have been made to use a light emitting element such as an organic EL element as a light source of the exposure apparatus. In an image display device using such a light emitting element for each pixel, a plurality of transistors for selecting a pixel and supplying a driving current for the light emitting element to the light emitting element, a holding capacitor for holding the driving current, etc. Is provided for each pixel. For example, Patent Document 1 discloses a display device having such a configuration. (Patent Document 1)
An outline of a specific operation in this type of display device will be described. Gradation pixel data is written by a current corresponding to display gradation data flowing through a holding capacitor connected between the gate and source of the driving transistor for a period selected by the selection transistor. After the selection transistor is turned off, a signal charge corresponding to the gradation display luminance of the pixel is written in the holding capacitor. By holding this during the display period, a current corresponding to the signal charge is generated. The light emitting element is lit by being supplied from the driving transistor to the light emitting element. A configuration equivalent to the configuration in such a display device can be applied to each pixel of the image forming apparatus.
JP 2003-043995 A

  However, in a light emitting element such as an organic EL element, it is known that when a current is supplied to emit light, the light emission luminance when a constant current is applied gradually decreases due to a change in characteristics due to a change with time of the light emitting element. . Further, in the image forming apparatus, since the light emission luminance and the light emission time are different for each pixel, the degree of change with time is also different for each light emitting element of each pixel. Therefore, in an image forming apparatus provided with this type of display circuit, unevenness in light emission luminance occurs for each pixel with time, and it becomes difficult to form an image with a uniform density.

  The present invention has been made in view of the above-described circumstances, and an object of the present invention is to provide an exposure apparatus capable of suppressing the occurrence of unevenness in image formation by suppressing the influence of changes over time for each pixel. And providing an image forming apparatus including the exposure apparatus.

According to the first aspect of the present invention, there is provided an exposure apparatus for performing exposure according to image data on a photosensitive drum, wherein a plurality of pixels having light emitting elements are arranged in a line shape, and the photosensitive drum is adapted to the image data. A light emitting element array that performs exposure and supplies exposure energy to the photosensitive drum, and a current value and a driving time of a driving current applied to the light emitting elements of each of the plurality of pixels of the light emitting element array. A time-measurement storage unit that stores the integrated value of the product, a characteristic change rate storage unit that stores the current value of the drive current of the light-emitting element and the characteristic of the luminance change rate with respect to the drive time, and exposure of one line on the photosensitive drum 1 line time, the duty consisting ratio of emission time of the light emitting element with respect to 1 line time is divided into a plurality of different division periods with one another, the light emitting element array is the photoreceptor de to 1 line exposure energy required to expose one line in a system is divided and supplied in the plurality of divided periods, the first divided period after the start of the one line time is set as a specific divided period, The duty in the specific division period is set to a value smaller than the duty in other division periods excluding the specific division period in the plurality of division periods, and in the plurality of pixels in the specific division period , Based on the integrated value and the characteristic of the luminance change rate, a current value of a drive current applied to the light emitting elements of pixels other than the specific pixel that is predicted to have the highest luminance change rate is the integrated value. It said set to a value based on the luminance change rate characteristics, and a control unit to bring the rate of change of brightness of the light emitting elements of the other pixels to the luminance change rate of the particular pixel and It is characterized in.

  According to a second aspect of the present invention, in the first aspect of the invention, the plurality of pixels in the light emitting element array are divided into a plurality of pixel groups including a predetermined number of the pixels. The pixel is driven in a time division manner.

  According to a third aspect of the present invention, in the first or second aspect of the present invention, the control unit calculates a current value of a drive current applied to the light emitting element of each pixel in the specific divided period as the integrated value. And an operation for setting a value based on the luminance change rate characteristic over a plurality of line periods in which a plurality of lines are exposed on the photosensitive drum, thereby bringing the luminance change rate of the light emitting element of each pixel close to each other. It is characterized by.

According to a fourth aspect of the present invention, in the invention according to any one of the first to third aspects, the control unit uses the first line time as the divided periods and the first half of the first half and the second half of the second half. The light emitting element of each pixel is divided into two periods, the duty in the first period is made smaller than the duty in the second period, and the first period is set as the specific divided period. The current value of the drive current applied to is set to a value based on the integrated value and the luminance change rate characteristic.

The invention according to claim 5 is the invention according to claim 4, wherein the control unit sets the duty in the second period to three times or more of the duty in the first period .

Invention according to claim 6, set in the invention according to claim 4 or 5, wherein the control unit, the duty in the second period, the same value for the plurality of pixels of said light emitting element array In the second period, the current value of the drive current applied to the light emitting elements of the plurality of pixels is set to the same value.

According to a seventh aspect of the present invention, there is provided an image forming apparatus for performing printing according to image data, wherein a photosensitive drum and a plurality of pixels each having a light emitting element are arranged in a line, and the image data is arranged on the photosensitive drum. A light emitting element array for performing exposure according to the above and supplying exposure energy to the photosensitive drum, and a current value and a driving time of a driving current applied to the light emitting elements of each of the plurality of pixels of the light emitting element array A time storage unit that stores an integrated value of a product of the light source, a characteristic change rate storage unit that stores characteristics of a luminance change rate with respect to a current value and a drive time of the driving current of the light emitting element, and one line for the photosensitive drum. 1 line time for exposing the said duty consisting ratio of emission time of the light emitting element with respect to 1 line time is divided into a plurality of different division periods, the light emitting element array is the sense of One line exposure energy required to expose one line on the body drum is set to be divided and supplied in the plurality of divided periods, and the first divided period after the start of the one line time is set as a specific divided period. The duty in the specific division period is set to a value smaller than the duty in the other division periods other than the specific division period in the plurality of division periods, and in the specific division period, A current value of a drive current applied to the light emitting element of other pixels other than the specific pixel predicted to have the largest luminance change rate based on the integrated value and the luminance change rate characteristic. set to a value based on said value luminance change rate characteristics, and a control unit to bring the rate of change of brightness of the light emitting elements of the other pixels to the luminance change rate of the particular pixel, Characterized by comprising.

  The invention according to claim 8 is the invention according to claim 7, wherein the plurality of pixels in the light emitting element array are divided into a plurality of pixel groups including a predetermined number of the pixels. The pixel is driven in a time division manner.

  The invention according to claim 9 is the invention according to claim 7 or 8, wherein the control unit calculates a current value of a drive current applied to the light emitting element of each pixel in the specific divided period as the integrated value. And an operation for setting a value based on the luminance change rate characteristic over a plurality of line periods in which a plurality of lines are exposed on the photosensitive drum, thereby bringing the luminance change rate of the light emitting element of each pixel close to each other. It is characterized by.

According to a tenth aspect of the present invention, in the invention according to any one of the seventh to ninth aspects, the control unit uses the first line time as the divided periods and the first half of the first half and the second half of the second half. The light emitting element of each pixel is divided into two periods, the duty in the first period is made smaller than the duty in the second period, and the first period is set as the specific divided period. The current value of the drive current applied to is set to a value based on the integrated value and the luminance change rate characteristic.

The invention according to claim 11 is the invention according to claim 10, wherein the control unit sets the duty in the second period to three times or more of the duty in the first period .

Invention of claim 12, wherein, setting in the invention according to claim 10 or 11, wherein the control unit, the duty in the second period, the same value for the plurality of pixels of said light emitting element array In the second period, the current value of the drive current applied to the light emitting elements of the plurality of pixels is set to the same value.

  According to the present invention, it is possible to suppress the occurrence of unevenness in image formation by suppressing the influence of a change with time for each pixel.

An exposure apparatus and an image forming apparatus (printing apparatus) according to an embodiment of the present invention will be described below with reference to the drawings.
FIG. 1 is a diagram illustrating a configuration example of an image forming apparatus using the exposure apparatus according to the present embodiment. In the figure, the image forming apparatus includes a photosensitive drum 1, an exposure device 2 integrally provided with a case portion 2A and a rod lens array portion 2B, a charging roller 3, an eraser light source photosensitive member 4, a cleaning member 5, and a developing roller 6a. The image forming (printing) is carried out by an electrophotographic method, including a developing device 6 including a transfer roller 8, a transfer roller 8, a fixing roller 9, and a conveyor belt 11. Note that reference numeral 7 denotes the printing paper 7.

  The rod lens array unit 2B constituting the exposure apparatus 2 is, for example, a lens array in which SELFOC (registered trademark) lenses are arranged in one row or in a plurality of rows. 1 is a lens unit that forms an image on 1;

  The photosensitive drum 1 is a negatively charged OPC (Organic Photo Conductor) photosensitive member (organic photosensitive member), and the charging roller 3 is a negative charger correspondingly. Further, as will be described in detail later, the exposure apparatus 2 includes a light emitting element array in which a plurality of pixels having light emitting elements are arranged in an array.

  In the image forming apparatus shown in FIG. 1, printing is generally performed by the following steps. First, when the charging roller 3 comes into contact with the surface of the rotating photosensitive drum 1, the contacted surface of the photosensitive drum 1 is uniformly charged with a negative potential.

  Subsequently, the exposure device 2 irradiates the photosensitive drum 1 with light, and the exposure energy based on the integrated value of the irradiation time of the intensity of light emitted from the light emitting element of each pixel is applied to the photosensitive drum 1. Then, an electrostatic latent image is formed on the photosensitive drum 1. Thereafter, toner is attached to the electrostatic latent image by the developing device 6. Then, the toner attached to the electrostatic latent image is transferred to the printing paper 7 by the transfer roller 8.

Hereinafter, such a printing process will be described in detail.
First, a negative high voltage supplied from a charging power source (not shown) is applied to the photosensitive drum 1 by the charging roller 3. As a result, the peripheral surface of the photosensitive drum 1 is uniformly negatively charged, and is in an initialized charged state that is initialized in terms of potential.

  Then, optical writing (exposure) according to the printing information is performed by the exposure device 2 on the photosensitive drum 1 whose peripheral surface is in an initialized charged state. As a result, an electrostatic latent image composed of a negative high potential portion due to initialization charging and a negative low potential portion of about −50 [V] due to exposure is formed on the peripheral surface of the photosensitive drum 1.

  Here, the toner charged in the developing unit 6 and charged to a weak negative potential is rotated and conveyed by the developing roller 6a to the facing portion between the developing roller 6a and the photosensitive drum 1. At this time, a developing bias voltage of about −250 [V], for example, is applied to the developing roller 6a from a power source (not shown).

  Accordingly, between the developing roller 6a to which the developing bias voltage of −250 [V] is applied and the minus low potential portion of about −50 [V] of the electrostatic latent image on the photosensitive drum 1 is about 200 [V]. Is formed.

  Due to the potential difference from the developing voltage in these electrostatic latent images, the negatively charged potential portion in the electrostatic latent image that has a positive polarity relative to the developing roller 6a has a negatively charged toner. Is transferred to form a toner image. The toner image is conveyed to a transfer portion where the photosensitive drum 1 and the transfer roller 8 are opposed to each other by the rotation of the photosensitive drum 1.

  Note that the toner adhesion amount in the toner image formed as described above, that is, the density of the developed image, depends on the exposure amount of the photosensitive drum 1 by the light emitting element of the exposure device 2. It is determined by the amount of potential attenuation on the circumferential surface.

  Next, as described above, when the toner image is conveyed to the transfer unit, the printing paper 7 is conveyed to the transfer unit by the conveyance belt 11. In the transfer portion, the toner image is transferred onto the printing paper 7 by the transfer roller 8.

  The printing paper 7 onto which the toner image has been transferred in this manner is conveyed further downstream by the conveying belt 11, and after the toner image is thermally fixed by the fixing roller 9, the printing paper 7 is discharged to the outside of the image forming apparatus. The

  Further, after the toner image is transferred onto the printing paper 7, the residual toner is removed from the peripheral surface of the photosensitive drum 1 by the cleaning member 5, and is further uniformly 0 (zero) [V] by the eraser light source photosensitive member 4. To be charged for the charging roller 3.

  In the exposure unit 2, the case portion 2 </ b> A emits light composed of organic EL elements along the axial direction of the photosensitive drum 1, which is the main scanning direction of exposure scanning on the photosensitive drum 1 shown in FIG. 1. A light emitting element array in which a large number of pixels having elements is arranged in a line is provided.

  FIG. 2 shows the configuration of the drive circuit for the exposure head section provided in the case section 2A. In the drawing, for the sake of simplicity, the light emitting element array on the exposure head substrate is assumed to be composed of all 24 pixels. However, as described above, the actual exposure head substrate has a print resolution of, for example, 1200 [dpi]. ], The printing apparatus corresponding to A4 paper (long side 297 [mm]) has a configuration of about 14,000 pixels.

  In the figure, in the case unit 2A of the exposure apparatus 2, a data analysis / timing control unit (control unit) 20 to which image data to be printed is supplied, a data driver 21, a select generation unit 22, an exposure head substrate 23, an anode A generation unit 24, a timing storage unit 25, and a characteristic change table (characteristic change rate storage unit) 26 are provided. A total of six pixel groups a1 to a4, b1 to b4,..., F1 to f4 divided by four adjacent pixels are arranged on the exposure head substrate 23.

  When performing exposure in accordance with image data, the data analysis / timing control unit 20 receives image data from a raster image processor (not shown), a horizontal control signal HSYNC that matches the paper size and transport speed, and a vertical control signal. Then, data is transferred to the data driver 21 in accordance with these control signals.

  The data driver 21 generates a plurality of drive currents Idrva to Idrvf having current values based on the gradation data sent from the data analysis / timing control unit 20 and supplies them to the exposure head substrate 23.

  The select generation unit 22 generates a plurality of selection signals (Select1 to Select4) for selecting the pixels in accordance with the control signal from the data analysis / timing control unit 20 and supplies the selection signals to the exposure head substrate 23. .

On the other hand, the anode generation unit 24 also generates a plurality of holding signals (Anode1 to Anode4) for setting the light emitting elements of each pixel to the light emission state in accordance with the control signal from the data analysis / timing control unit 20, and the exposure head substrate 23. To supply.
The timekeeping storage unit 25 is an integrated value of the product of the current value of the driving current applied to the light emitting elements of each pixel and the driving time (fermentation time) for all the pixels of the light emitting element array formed on the exposure head substrate 23. Remember.

  The characteristic change table 26 stores the current value of the drive current of the light emitting elements of the light emitting element array of the exposure head substrate 23 and the luminance change rate with respect to the drive time in a table format.

  The data analysis / timing control unit 20 is supplied with image data to be printed, and the integrated value for each pixel stored in the timing storage unit 25 and the current value of the driving current of the light emitting element stored in the characteristic change table 26 And a calculation means for calculating a correction driving value corresponding to the luminance change rate of the light emitting element of each pixel predicted based on the luminance change rate with respect to the driving time, and the correction for the gradation data corresponding to the image data The corrected gradation data multiplied by the drive value is sent to the data driver 21.

  Further, the data analysis / timing control unit 20 generates and sends a reset signal to each pixel of the exposure head substrate 23 as will be described later.

FIG. 3 is a view showing a circuit configuration of the exposure head substrate 23 shown in FIG. FIG. 4 is a drive timing chart as a comparison target for explaining the effect of the present invention.
FIG. 3 shows a pixel group a consisting of four pixels a1 to a4 and a part of a pixel group b consisting of four pixels b1 to b4 among the pixel groups a to f on the exposure head substrate 23 shown in FIG. (Pixels b1, b2).

  Each pixel is composed of, for example, a thin film transistor, and includes a selection transistor 33 as a pixel selection switch, an organic EL element 34 as a light emitting element, and a driving transistor 31 for driving the organic EL element 34, which is composed of a thin film transistor. And a holding transistor 30 formed of a thin film transistor and a holding capacitor 32 for holding a signal charge corresponding to the display luminance of the pixel.

  In the exposure head substrate 23 shown in FIG. 3, a plurality of driving signal lines to which each driving voltage is applied, a plurality of selection signal lines to which each selection signal is applied, and a plurality of holding signals to which each holding signal is applied. With lines.

  During the pixel selection period in which the selection signal applied to each selection signal line is set to the high level, the drive currents Idataa to Idataf having current values based on the gradation data are supplied from the data driver 21 to the respective drive signal lines. The charge corresponding to the current that flows to the drive transistor 31 through the transistor 33 and flows through the current path of the drive transistor 31 is accumulated in the holding capacitor 32.

  Then, during the holding period in which the selection signal is set to the low level and the holding signal applied to the holding signal line is set to the high level, a current corresponding to the charge accumulated in the holding capacitor 32 is supplied from the driving transistor 31 to the organic EL element 34. , And the organic EL element 34 is configured to emit light with a luminance corresponding to the current value of the current.

  In addition, reset transistors 35 and 36 are provided in each drive signal line, and the reset transistors 35 and 36 are turned on at the initial stage of the selection period to discharge the charge charged in the holding capacitor 32.

A horizontal control signal HSYNC shown in FIG. 4 is a synchronizing signal in the main scanning direction (axial direction of the photosensitive drum 1) of the printing apparatus, and a dot formation processing time (one line time) in which an active to active period is allowed for one line. Become. For example, if the printing resolution is 1200 [dpi] in both the main scanning direction and the conveyance direction, the printing speed is 40 [sheets / minute], and the distance between sheets is 50 [mm], an A4 horizontal size light source unit is used.
((25.4mm / 1200dpi) / (210mm x 40 sheets + 50mm x 40 sheets)) / 60
≈122 [μ seconds].

  The data driver 21 is configured so that exposure energy necessary for exposing the photosensitive drum 1 to form a latent image can be obtained during a period (one line time) in which the horizontal control signal HSYNC is active to active. The pixel is driven to emit light for a predetermined time.

  If the light emission time is set to a sufficiently short period with respect to one line time, each pixel can emit light a plurality of times during one line time, and it is possible to make the arranged pixels emit light in a time-sharing manner, and Desirable from an implementation perspective.

  In this case, as shown in FIG. 2 and FIG. 3, the selection signals Select 1 to 1 connected to the anodes of a plurality of pixels 34 by the output from one data driver 21 and generated by the select generation unit 22 on the cathode side. Select4 is shifted within one line time.

  In FIG. 2, as an example, the data driver 21 drives the four pixels a1 to a4 of the pixel group a with one drive signal line Idrva, and the supply lines to the exposure head substrate 23 are a total of six lines Idrva to Idrvf. However, the present invention is not limited to this.

  Similarly, the selection signal lines Select1 to Select4 from the select generation unit 22 to the exposure head substrate 23 and the holding signal lines Anode1 to Anode4 from the anode generation unit 24 to the exposure head substrate 23 are also for four pixels of each pixel group. Although four lines are output at a time, the present invention is not limited to this.

  Therefore, in the exposure head substrate 23, 24 (= 4 × 6) pixels are formed in a row in the horizontal direction (scanning direction). In the exposure head substrate 23 using an actual organic EL, pixel elements corresponding to the paper size are formed in one or a plurality of columns.

  FIG. 4 shows a control timing when the circuit configuration as described above is controlled by a general driving method. The signal charge corresponding to the drive current is written to the holding capacitor 32 in the circuits of the pixels a1, b1,..., F1 in each pixel group during the period when the selection signal Select1 is at “H” level. The current corresponding to the signal charge written in the holding capacitor 32 is supplied from the drive transistor 31 to the organic EL element 34, so that the organic EL element 34 emits light during the period in which the holding signal Anode1 is at "H" level. To do.

  Similarly, the pixels a2, b2,..., F2 in each pixel group when the selection signal Select2 is “H” level, and the pixels a3, b3 in each pixel group when the selection signal Select3 is “H” level. ,..., F3 are output from the data driver 21 in accordance with the grayscale data of the pixels a4, b4,..., F4 in each pixel group during the period when the selection signal Select4 is at "H" level. Thus, the light emitting elements of the corresponding pixels emit light during the period in which the holding signals Anodes 2 to 4 are at the “H” level thereafter.

Hereinafter, problems in the above-described general driving method will be described.
It is known that the light emission luminance of the organic EL element gradually decreases as the driving time elapses. The plurality of organic EL elements 34 constituting the exposure head substrate 23 are driven to be lit based on the gradation of the image data corresponding to the arranged position, and are driven for each pixel position. Therefore, the accumulated value of the light emission time for each pixel is different.

  Therefore, when the plurality of organic EL elements 34 constituting the exposure head substrate 23 are all driven with the same drive current, the actual light emission luminance varies depending on the degree of change over time for each pixel. Brightness unevenness occurs, and unevenness occurs in image formation (printing). And the malfunction that this becomes conspicuous gradually with progress of time arises.

  The present invention is intended to improve the above problems. Hereinafter, a driving method according to an embodiment of the present invention will be described.

FIG. 5 shows control timing when the circuit configuration shown in FIG. 3 is controlled by the driving method according to the embodiment of the present invention.
Here, the data analysis / timing control unit 20 exposes the photosensitive drum 1 by each light emitting element of the light emitting element array of the exposure head substrate 23 to form a latent image, and exposure energy supplied to the photosensitive drum 1. The light emission period of the light emitting element of each pixel is divided into a plurality of lines within one line time so that the drive current value for each pixel is adjusted during each divided light emission period. Drive control is performed so that desired exposure energy can be obtained and the luminance change rate of each pixel is uniform.

  In other words, FIG. 5 shows a case where one line time is divided into two parts before and after as an example. Here, the first half of the divided period is called a first period (first period), and the second half is called a second period (second period).

  The data analysis / timing control unit 20 then sets the ratio (duty) of the light emission period (first light emission period) within one line time set in the first period and the light emission period (second light emission) set in the second period. (Period) ratio (duty), the integrated value of the product of the current value of the driving current of the light emitting element and the driving time, the current value of the driving current of the light emitting element, and the luminance change rate with respect to the driving time. The value of the drive current applied to the light emitting element of each pixel is appropriately set according to the luminance change rate of the light emitting element and the corrected drive value calculated by the calculation means corresponding to the predicted luminance change rate of the light emitting element. It is set and driven so as to reduce the variation in the luminance change rate of the light emitting element of each pixel. FIG. 5 shows a case where the duty in the first period is 12.5% and the duty in the second period is 37.5%.

FIG. 6 shows, as a reference example, a light source corresponding to an A4 horizontal size with a print resolution of 1200 [dpi] in the main scanning direction and the transport direction, a printing speed of 40 [sheets / minute], and a sheet spacing of 50 [mm]. The unit is exposed for the purpose of forming an image on the surface of the photosensitive drum 1 with a photosensitivity of the photosensitive drum 1 of 0.3 [μJ / cm ² ] and an efficiency of the organic EL element 34 of 2.2 [cd / A]. When the light transmission degree of the lens array provided in the unit is set to 6 [%], the gradation data is the highest gradation when the characteristics of the light emitting elements of each pixel of the light emitting element array of the exposure head substrate 23 are uniform. In order to obtain the required exposure energy, the driving current required for causing the light emitting element of each pixel to emit light at the maximum gradation according to the conventional general driving method is equal to the lighting time within one line time. It is the table | surface shown for every ratio (duty).

  Since the light emission luminance of the organic EL element 34 is proportional to the current value, the current value of the drive current when the gradation data is a gradation other than the highest gradation is the drive at the highest gradation described above. The current is multiplied by a ratio corresponding to the gradation value.

  The exposure energy supplied to the photosensitive drum 1 by the exposure device 2 corresponds to the product of the light emission luminance and the light emission time of the light emitting elements of the light emitting element array. As shown in FIG. 6, the duty value is small, and the shorter the time during which the organic EL element 34 emits light in one line time, the larger the driving current is required.

  In addition, as described above, when the drive current supplied to each pixel is the same, there is no problem when the characteristics of the light emitting elements of each pixel are uniform, but the light emission of each pixel depends on the degree of change over time for each pixel. When the characteristics of the elements are not uniform, the light emission luminance differs from pixel to pixel, resulting in luminance unevenness.

  On the other hand, in this embodiment, as shown in FIG. 5, one line time is divided into a first period and a second period.

  FIG. 7 shows that in this embodiment, when the characteristics of the light emitting elements of the respective pixels of the light emitting element array are uniform, the exposure energy required when the gradation data is the highest gradation is defined as the first period of one line period. It is a table | surface which shows the relationship of the drive current for every duty of a 1st period and a 2nd period to obtain in a 2nd period.

  Here, the configuration of each part related to exposure of the exposure head substrate 23 and the photosensitive drum 1 is the same as that shown in FIG. Also in this case, the current value of the driving current when the gradation data is a gradation other than the highest gradation is obtained by multiplying the driving current at the highest gradation by a ratio corresponding to the gradation value. It becomes.

  That is, in this embodiment, as shown in FIG. 7, a 25 [%] image is formed in the first half period shown in FIG. 7A, and 75 [%] in the second half period shown in FIG. %] Drive to form an image.

  As in the case shown in FIG. 6, in order to obtain a constant exposure energy, a smaller drive value requires a larger drive current. In the present embodiment, as shown in FIG. 5, the duty in the first period and the duty in the second period are set to different values, the duty in the first period is 12.5%, and the duty in the second period is 37.5%. .

  In this case, as can be seen from FIGS. 7A and 7B, when the characteristics of the light emitting elements of each pixel of the light emitting element array are uniform, the drive current in the first period and the drive current in the second period The value will be the same.

Next, a change characteristic of the light emitting element due to a change with time will be described.
FIG. 8 shows the change rate with respect to the drive time (light emission time) when the current value of the drive current is changed to a predetermined rated value and twice the value as the time-varying characteristics of the organic EL element 34. (Relative value when the initial state is 100).

  In the figure, I is a drive current value of a predetermined value, and when lighting is driven with a duty A [%] during one line time, II is A / 2 [%], which is half of the duty in the case of A Thus, when the drive current is set to be twice the predetermined value of I and the light emission brightness immediately after the start of driving is the same as in the case of I, each brightness is reduced to a half value (50 [% ] Is a time-dependent change characteristic until it becomes.

  This embodiment is based on the fact that the luminance change rate of the light emitting element within a certain time can be changed by changing the current value of the drive current as in the characteristic shown in FIG. In the first period of the first half of one line period, the current value of the drive current supplied to the pixel having the largest luminance change rate is, for example, the gradation of the gradation data corresponding to when the light emitting element has the initial characteristics. Set the rated value according to the value, and change the current value of the drive current supplied to each other pixel according to the luminance change rate of the light emitting element of each pixel from the rated value according to the gradation value of the gradation data By changing (increasing) the values in accordance with the correction drive values, the luminance change rates of the light emitting elements of these pixels are made to be approximately equal. In the second half of the second period, driving is performed by setting the current value of the driving current supplied to each pixel to the same value with a rated value corresponding to the gradation value of the gradation data, for example.

  That is, in the first period, the current value of the drive current supplied to a pixel having a relatively low luminance change rate of the light emitting element among all the pixels is set to the current value of the drive current supplied to a pixel having a high luminance change rate of the light emitting element. Thus, the change in the characteristics of the pixels having a relatively low luminance change rate of the light-emitting element is accelerated, so that the luminance change rates of the respective pixels are substantially aligned at the time when the first light emission period of the first period is completed. Is.

  In this case, the occurrence of unevenness in image formation can be suppressed. However, since the light emitting element of each pixel undergoes a light emission operation by flowing a drive current, the characteristics change somewhat, so the drive current is set to the rated value. In this case, the light emission luminance of the light emitting element of each pixel is lowered overall, and when used in an image forming apparatus, the print density gradually becomes lighter overall, but the change is gradual. Therefore, it is not conspicuous.

An example of a specific operation in the present embodiment will be described in more detail with reference to the drawings.
FIG. 9 is a diagram for explaining the concept of the driving method in the present embodiment, and FIG. 10 is a table of the luminance change rate with respect to the current value of the driving current of the light emitting element stored in the luminance change table 26 and the driving time. An example is shown.

  FIG. 9 shows driving of the first pixel a1 of the pixel group a of the exposure head substrate 23 and the first pixel b1 of the pixel group b, and the luminance change rate of the pixel a1 is smaller than the luminance change rate of the pixel b1. Show. FIG. 9A shows the exposure energy corresponding to the product of the light emission luminance and the light emission period of each pixel, and FIG. 9B shows the selection signal (Select signal) and the holding signal (Anode signal) for each pixel. Reference numeral 9 (C) denotes drive current (Idrva, Idrvb) supplied to each pixel.

  As shown in FIG. 9C, the data analysis / timing control unit 20 makes the current value of the drive current Ia1 applied to the pixel a1 in the first period larger than the current value of the drive current Ib1 applied to the pixel b1. The current value of the drive current Ib1 has, for example, a rated value that emits light with a luminance corresponding to a gradation signal corresponding to pixel data when the light emitting element of the pixel has initial characteristics.

  Then, in the first light emission period of the first period, since the drive current Ia1 having a current value larger than the drive current Ib1 applied to the pixel b1 is caused to flow through the pixel a1, light is emitted as shown in FIG. At the beginning of the light emission period, the light emission luminance of the light emitting element of the pixel a1 is higher than the light emission luminance of the light emitting element of the pixel b1.

  However, as shown in FIG. 8, the larger the current value of the drive current, the faster the change in light emission luminance with time (decrease in light emission luminance). Therefore, although the pixel b1 is also supplied with the driving current Ib1 and emits light, a certain temporal change occurs during the first light emission period, but the current value of the driving current Ia1 supplied to the pixel a1 is stored in the time measuring storage unit 25. Based on the integrated integrated value and the luminance change rate stored in the characteristic change table 26, the luminance change rate of the pixel b1 at the same timing as the luminance change rate of the pixel a1 when the first light emission period ends. Set to a value that is equivalent to or close to.

  In the subsequent second period, as shown in FIG. 9C, the current values of the drive currents Ia1 and Ib1 supplied to the pixels a1 and b1 are the same, for example, set to rated values. At this time, since the pixels a1 and b1 are driven with the same current for the same period, the luminance change rates of the pixels a1 and b1 in the second light emission period of the second period are equal.

  As a result, the exposure energy supplied to the photosensitive drum 1 by the pixels a1 and b1 in the one line period corresponds to the pixel a1 and the pixel b1 in the first light emission period of the first period, as shown in FIG. However, there is almost no difference between the pixel a1 and the pixel b1 in the second light emission period of the second period.

  Since the exposure energy supplied in the second light emission period is set larger than the exposure energy supplied in the first light emission period, the exposure supplied by the pixel a1 and the pixel b1 in this one line period. The energy can be set to substantially the same value, and unevenness in image formation can be suppressed.

  FIG. 9 shows the concept of the driving method of the present invention. Since the time of the first light emission period is actually very short, the light emission as shown in FIG. The decrease in light emission luminance due to changes in the element over time does not necessarily occur only in one first light emission period, but by repeating the same control operation over a plurality of line periods, FIG. ) Is realized.

  Next, a method for setting the current value of the drive current Ia1 supplied to the pixel a1 based on the integrated value stored in the timekeeping storage unit 25 and the luminance change rate stored in the characteristic change table 26 will be described.

  Here, the timekeeping storage unit 25 records the integrated value of the product of the current value of the drive current and the drive time of the light emitting elements of all the pixels of the light emitting element array formed on the exposure head substrate 23. Further, in the characteristic change table 26, as shown in FIG. 10, the current value of the drive current of the light emitting element is used as a parameter, and the relationship between the drive time and the luminance change rate for the current values of a plurality of drive currents is stored in a table format. Has been.

  Prior to the operation of transferring data to the data driver 21, the data analysis / timing control unit 20 first refers to the integrated values of all the pixels stored in the time storage unit 25 and performs the light emission operation in the next one line period. A pixel (referred to as the worst pixel) that is predicted to have the largest integrated value and a maximum luminance change rate among the pixels, and the integrated value of the worst pixel of the other pixels. The difference is extracted.

  Next, referring to the characteristic change table 26 based on the integration value of the worst pixel, the luminance change rate of the light emitting element of the worst pixel is estimated, and the difference from the integration value of the worst pixel of the pixel other than the worst pixel is calculated. Based on the characteristic change table 26, the luminance change rate of the light emitting elements of the pixels other than the worst pixel is equal to the luminance change rate of the light emitting elements of the worst pixel in the state after the first light emission period of the first period has elapsed. Alternatively, a correction drive value is calculated as a correction value of the drive current value for each pixel, which is necessary to obtain an approximate value.

  The data analysis / timing control unit 20 sets the value of the drive current for each pixel according to the value of the corrected drive value for each pixel calculated in this way, and supplies the corresponding gradation data to the data driver 21. .

  The data driver 21 supplies the pixels a1, b1,..., F1 in each pixel group from the data analysis / timing control unit 20 when the selection signal Select1 is at the “H” level during the first period. Corresponding to the gradation data, the drive currents Idrva = Ia1, Idrvb = Ib1,..., Idrvf = If1 for supplying 25% of the exposure energy (divided exposure energy) necessary for printing one line are exposed. The signal is supplied to each drive signal line of the head substrate 23, and the corresponding signal charge is written to the holding capacitor 32 of each pixel.

  Then, by supplying a current corresponding to the signal charge written in the holding capacitor 32 from the drive transistor 31 to the organic EL element 34, first, the holding signal Anode1 is at the “H” level in the first period (first light emission). During the period, the organic EL element 34 is caused to emit light.

  Similarly, the gradation data supplied from the data analysis / timing control unit 20 to the pixels a2, b2,..., F2 in each pixel group when the selection signal Select2 is at the “H” level in the first period. , Idrvva = Ia2, Idrvb = Ib2,..., Idrvf = If2 for forming 25% of the image to be printed is supplied to each driving signal line of the exposure head substrate 23, and the corresponding signal is supplied. Charge is written into the holding capacitor 32 of each pixel.

  Then, by supplying a current corresponding to the signal charge written in the holding capacitor 32 from the driving transistor 31 to the organic EL element 34, first, the holding signal Anode2 is at the “H” level in the first period (first light emission). During the period, the organic EL element 34 is caused to emit light.

  Similarly, in the subsequent pixels a3, b3,..., F3 and the pixels a4, b4,..., F4, the gradation data supplied when the selection signals Select3 and Select4 are at the “H” level in the first period. Is written in the holding capacitor 32 of each pixel, and the organic EL element 34 is caused to emit light during a period (first light emission period) in which the subsequent holding signals Anode3 and Anode4 are at “H” level.

  Next, when the selection signal Select1 is at the “H” level during the second period, the data driver 21 applies the data analysis / timing control unit 20 to the pixels a1, b1,..., F1 in each pixel group. Corresponding to the supplied gradation data, drive currents Idrva = Ia′1, Idrvb = Ib′1,... For supplying 75% (division exposure energy) of exposure energy required for printing one line. Idrvf = If′1 is supplied to each drive signal line of the exposure head substrate 23, and the corresponding signal charge is written into the holding capacitor 32 of each pixel.

  Then, the current corresponding to the signal charge written in the holding capacitor 32 and corrected according to the deterioration is supplied from the driving transistor 31 to the organic EL element 34, so that the holding signal Anode1 is “H” in the second period. The organic EL element 34 is caused to emit light during the level period (second light emission period).

  Similarly, when the selection signal Select2 is at “H” level in the second period, the gradation data supplied from the data analysis / timing control unit 20 to the pixels a2, b2,..., F2 in each pixel group. Corresponding to the drive current Idrva = Ia′2, Idrvb = Ib′2,..., Idrvf = If′2 for supplying 75% of the exposure energy (divided exposure energy) required for printing one line. The signal is supplied to each drive signal line of the exposure head substrate 23 and the corresponding signal charge is written into the holding capacitor 32 of each pixel.

  Then, a current corresponding to the signal charge written in the holding capacitor 32 is supplied from the driving transistor 31 to the organic EL element 34, whereby the holding signal Anode2 is at the “H” level in the second period (second light emitting period). ), The organic EL element 34 is caused to emit light.

  Similarly, in the subsequent pixels a3, b3,..., F3 and the pixels a4, b4,..., F4, the gradation data supplied when the selection signals Select3 and Select4 are at the “H” level in the second period. Is written in the holding capacitor 32 of each pixel, and the organic EL element 34 is caused to emit light during a period (second light emitting period) in which the subsequent holding signals Anode3 and Anode4 are at “H” level.

  As described above, according to the embodiment, it is possible to continuously maintain image formation (printing) at a uniform density as a whole by suppressing the influence of temporal changes that differ from pixel to pixel.

  Further, in the embodiment, the control for reducing the difference in the luminance change rate of the light emitting elements of each pixel and aligning the luminance change rates of the light emitting elements of each pixel is performed only in the first period for forming, for example, 25% of the image. I was supposed to.

  That is, control is performed to adjust the current value of the drive current so that the rate of change in luminance is equalized during a period when the rate of image formation is lower, that is, during a period when the amount of signal charge originally written in accordance with image data is small. Therefore, it is possible to increase the ratio of forming an image with the luminance change rate of each pixel being uniform, and it is possible to further suppress the occurrence of a density difference between the pixels.

  In the embodiment, the duty of the first period is 12.5%, the duty of the second period is 37.5%, 25% of the image is formed in the first period, and 75% of the image is formed in the second period. However, the present invention is not limited to this, and other duty values and other values of the image formation rate may be used. However, in order to achieve the effect of the present invention, it is necessary to make the image formation rate in the second period larger than the image formation rate in the first period.

  In addition, the present invention is not limited to the above-described embodiments, and various modifications can be made without departing from the scope of the invention at the implementation stage. Further, the functions executed in the above-described embodiments may be combined as appropriate as possible. The above-described embodiment includes various stages, and various inventions can be extracted by an appropriate combination according to a plurality of disclosed constituent elements. For example, even if some constituent requirements are deleted from all the constituent requirements shown in the embodiment, if the effect is obtained, a configuration from which the constituent requirements are deleted can be extracted as an invention.

1 is a diagram illustrating a configuration of an image forming apparatus according to an embodiment of the present invention. 2 is a diagram showing a configuration of a drive circuit of an exposure head unit according to the same embodiment. FIG. 2 is a view showing a circuit configuration of an exposure head substrate according to the embodiment. FIG. The timing chart which shows the control timing in the case of controlling with the general drive method in the circuit of FIG. The timing chart which shows the control timing in the case of controlling with the drive method which concerns on the same embodiment with the circuit of FIG. The figure which shows the relationship of the drive current for every duty in the case of controlling with the drive method of FIG. The figure which shows the relationship of the drive current for every duty in the first period and the second period in the case of controlling with the drive method of FIG. 5 according to the embodiment. The figure which shows the change rate with respect to light emission time when the electric current value of the drive current of the light-emitting luminance of an organic EL element is changed. The figure for demonstrating the concept of the drive method in the embodiment. The figure which shows an example of the table of the luminance change rate with respect to the electric current value of the drive current of the light emitting element memorize | stored in a characteristic change table, and drive time.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Photosensitive drum, 2 ... Exposure apparatus, 2A ... Case part, 2B ... Rod lens array part, 3 ... Charging roller, 4 ... Eraser light source photoconductor, 5 ... Cleaning member, 6 ... Developing device, 7 ... Printing paper, DESCRIPTION OF SYMBOLS 8 ... Transfer roller, 9 ... Fixing roller, 11 ... Conveyor belt, 20 ... Data analysis / timing control unit, 21 ... Data driver, 22 ... Select generation unit, 23 ... Exposure head substrate, 24 ... Anode generation unit, 25 ... Timekeeping Storage unit 26... Characteristic change table 30... Holding transistor 31... Driving transistor 32 .. holding capacitor 33... Selection transistor 34.

Claims (12)

An exposure apparatus that performs exposure according to image data on a photosensitive drum,
A plurality of pixels having light emitting elements are arranged in a line, and a light emitting element array that performs exposure according to the image data to the photosensitive drum and supplies exposure energy to the photosensitive drum;
A time storage unit that stores an integrated value of a product of a current value and a driving time of a driving current applied to the light emitting elements of each of the plurality of pixels of the light emitting element array;
A characteristic change rate storage unit that stores a characteristic of a luminance change rate with respect to a current value and a drive time of the drive current of the light emitting element;
One line time for exposing one line on the photosensitive drum is divided into a plurality of divided periods having different duty ratios, which are ratios of the light emitting time of the light emitting elements to the one line time, and the light emitting element array is arranged on the photoconductor. One line exposure energy required to expose one line on the drum is set to be divided and supplied in the plurality of divided periods, and the first divided period after the start of the one line time is set as a specific divided period, The duty in the specific division period is set to a value smaller than the duty in other division periods excluding the specific division period in the plurality of division periods, and in the plurality of pixels in the specific division period , other image other than the specific pixel and the brightness change rate based on the characteristics of the luminance change rate and the cumulative values are predicted largest Of the current value of the drive current applied to the light emitting element, set to a value based on the integrated value and the luminance change rate characteristics, the brightness change rate of the light emitting element of the other pixel of said particular pixel A control unit that approaches the rate of change in luminance ;
An exposure apparatus comprising:   2. The plurality of pixels in the light emitting element array are divided into a plurality of pixel groups including a predetermined number of the pixels, and the pixels of the pixel groups are driven in a time division manner. Exposure equipment.   The control unit performs an operation of setting a current value of a drive current applied to the light emitting element of each pixel in the specific division period to a value based on the integrated value and the luminance change rate characteristic. The exposure apparatus according to claim 1, wherein the exposure is performed over a plurality of line periods in which a plurality of lines are exposed to bring the luminance change rate of the light emitting element of each pixel close to each other. The control unit divides the one line time into two divided periods, a first half of the first period and a second half of the second period, and makes the duty in the first period smaller than the duty in the second period. Then, the first period is the specific divided period, and the current value of the drive current applied to the light emitting element of each pixel in the first period is a value based on the integrated value and the luminance change rate characteristic. 4. The exposure apparatus according to claim 1, wherein the exposure apparatus is set as follows. The exposure apparatus according to claim 4, wherein the control unit sets the duty in the second period to be three times or more of the duty in the first period . Wherein the control unit, the said duty in the second period, and set to the same value for the plurality of pixels of said light emitting element array, a driving current applied to the light emitting element of said plurality of pixels in the second period The exposure apparatus according to claim 4, wherein the current values of are set to the same value. An image forming apparatus that performs printing according to image data,
A photosensitive drum;
A plurality of pixels having light emitting elements are arranged in a line, and a light emitting element array that performs exposure according to the image data to the photosensitive drum and supplies exposure energy to the photosensitive drum;
A time storage unit that stores an integrated value of a product of a current value and a driving time of a driving current applied to the light emitting elements of each of the plurality of pixels of the light emitting element array;
A characteristic change rate storage unit that stores a characteristic of a luminance change rate with respect to a current value and a drive time of the drive current of the light emitting element;
One line time for exposing one line on the photosensitive drum is divided into a plurality of divided periods having different duty ratios, which are ratios of the light emitting time of the light emitting elements to the one line time, and the light emitting element array is arranged on the photoconductor. One line exposure energy required to expose one line on the drum is set to be divided and supplied in the plurality of divided periods, and the first divided period after the start of the one line time is set as a specific divided period, The duty in the specific division period is set to a value smaller than the duty in other division periods excluding the specific division period in the plurality of division periods, and in the plurality of pixels in the specific division period , other image other than the specific pixel and the brightness change rate based on the characteristics of the luminance change rate and the integrated value is predicted to largest Of the current value of the drive current applied to the light emitting element, set to a value based on the integrated value and the luminance change rate characteristics, the brightness change rate of the light emitting element of the other pixel of said particular pixel A control unit that approaches the rate of change in luminance ;
An image forming apparatus comprising:   8. The plurality of pixels in the light emitting element array are divided into a plurality of pixel groups composed of a predetermined number of the pixels, and the pixels of the pixel groups are driven in a time division manner. Image forming apparatus.   The control unit performs an operation of setting a current value of a drive current applied to the light emitting element of each pixel in the specific division period to a value based on the integrated value and the luminance change rate characteristic. The image forming apparatus according to claim 7, wherein the rate of change in luminance of the light emitting element of each pixel is made close to each other over a plurality of line periods during which a plurality of lines are exposed. The control unit divides the one line time into two divided periods, a first half of the first period and a second half of the second period, and makes the duty in the first period smaller than the duty in the second period. Then, the first period is the specific division period, and the current value of the drive current applied to the light emitting element of each pixel in the first period is a value based on the integrated value and the luminance change rate characteristic. 10. The image forming apparatus according to claim 7, wherein the image forming apparatus is set as follows. The image forming apparatus according to claim 10, wherein the control unit sets the duty in the second period to three times or more of the duty in the first period . Wherein the control unit, the said duty in the second period, and set to the same value for the plurality of pixels of said light emitting element array, a driving current applied to the light emitting element of said plurality of pixels in the second period The image forming apparatus according to claim 10, wherein the current values are set to the same value.

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