JP2015199207A - Image formation device and image formation method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image formation device that improves picture quality by preventing resolution from decreasing.SOLUTION: An image formation device includes a photoreceptor drum 502 where an image corresponding to image data representing density of each pixel is formed; and an exposure head 106 which has a plurality of organic EL elements, and scans the photoreceptor drum 502 with light emitted from the respective EL elements to form an image. The image formation device corrects density of each pixel so that an exposure distribution of exposure value determined by the density in the scanning direction and an exposure distribution in a non-scanning direction are substantially the same, and controls the quantity of light emitted from the organic EL element based upon the corrected density.

Description

  The present invention relates to an image forming apparatus such as a copying machine or a copying machine that forms an image using an electrophotographic process. In particular, the present invention relates to an image forming apparatus including an exposure device that exposes a photoreceptor to form an electrostatic latent image during image formation.

  In an electrophotographic image forming apparatus, an electrostatic latent image is formed on a photosensitive drum by exposing a rotating photosensitive drum with a long exposure head (exposure device) having a plurality of light emitting elements. There is. As the light emitting element, a solid light emitting element such as an LED (Light Emitting Diode) element or an organic EL (Electro-Luminescence) element can be used. The exposure head includes a plurality of light emitting elements (hereinafter referred to as “light emitting element array”) arranged in the direction of the rotation axis of the photosensitive drum, and a rod lens array that forms an image of light of each light emitting element on the photosensitive drum. Prepare. The exposure head irradiates light to the rotating photosensitive drum. Therefore, a light spot is formed on the photosensitive drum with the circumferential direction of the photosensitive drum as the scanning direction.

  The length of the light emitting element array is determined according to the length of the image forming area on the photosensitive drum in the rotation axis direction, and the interval between the light emitting elements is determined according to the resolution of the image forming apparatus. For example, when the resolution of the image forming apparatus is 1200 [dpi (dot per inch)], the pixel interval of the image to be formed is 21.1 [μm] (the digits after the decimal point are omitted). The interval is also 21.1 [μm]. The light emitting element array needs to be longer than the print width in the rotation axis direction of the photosensitive drum, and the number of light emitting elements is determined by the print width and resolution. For example, when the printing width in the rotation axis direction of the photosensitive drum is 297 [mm] and the resolution of the image forming apparatus is 1200 [dpi], the light emitting element array has about 14000 light emitting elements equal to the number of pixels. Arranged.

  An image forming apparatus using such an exposure head uses fewer parts than a laser scanning image forming apparatus that deflects and scans a laser beam with a polygon motor, thereby reducing the size and cost of the apparatus. Is easy. Further, by positioning the focal distance with the surface of the photosensitive drum as the irradiated surface with high accuracy, it is possible to reduce the diameter of the light spot formed by exposure of the photosensitive drum. By reducing the size of the light spot, the resolution of the image forming apparatus can be increased and the sharpness of the image can be increased.

  However, since the light spot is formed by the rotation of the photosensitive drum with the circumferential direction of the photosensitive drum as the scanning direction, the electrostatic latent image may be stretched in the scanning direction to cause image blur. Image blur in the scanning direction causes image detriment such as character proportion deterioration and line width mismatch between vertical and horizontal lines. Even when the response time (rise time, fall time) of the light emitting element is long, the image blur in the scanning direction becomes large. In general, a TFT (Thin Film Transistor) using an organic semiconductor that drives an organic EL element has a low response speed. Therefore, particularly in an exposure head using an organic EL element as a light emitting element, there is a problem that the response speed of the light emitting element is slow and the resolution in the scanning direction is likely to decrease.

  Patent Document 1 discloses a configuration in which a current application circuit is provided to assist the rise of light emission at the light emission start timing of the light emitting element in order to improve the response speed of the light emitting element. In Patent Document 1, the rise time can be improved, but it has no effect on the fall time delay. In addition, it is necessary to provide a current application circuit for each light emitting element, and when the number of light emitting elements is large, the circuit scale is greatly increased, resulting in an increase in cost.

JP 2012-151336 A

  SUMMARY OF THE INVENTION It is a main object of the present invention to provide an image forming apparatus that improves image deterioration by preventing a decrease in resolution.

  An image forming apparatus of the present invention that solves the above-described problem has a photosensitive member on which an image corresponding to image data representing the density for each pixel is formed, and a plurality of light emitting elements, and the light emitted from each light emitting element An exposure device that scans a photoconductor to form the image, and the exposure distribution in the scanning direction and the exposure distribution in the non-scanning direction of the exposure amount determined by the density are substantially the same for each pixel. And control means for controlling the amount of light emitted from the light emitting element based on the corrected density.

According to the present invention,
By making the exposure distributions in the scanning direction and the non-scanning direction substantially the same, image blurring in the scanning direction is prevented. Thereby, it is possible to prevent the degradation of the resolution and improve the image deterioration.

1 is an overall configuration diagram of an image forming apparatus. (A), (b) is a configuration explanatory view of the exposure head. Structure explanatory drawing of an organic EL element array. (A), (b) is a structure explanatory drawing of an organic EL element. The connection relation figure of an exposure head and a controller. The arrangement of luminous points. The timing chart of a reference voltage and a chip select signal. (A)-(c) is explanatory drawing of the light spot of 1 pixel. Explanatory drawing of a filter process. Functional block diagram of the controller. The block diagram of an exposure modulation part. Explanatory drawing of a filter constant. The hardware block diagram of a two-dimensional filter. A free chart showing image formation processing. FIG. 4 is an exemplary diagram of a filter constant table representing a correspondence relationship between a light amount and a filter constant.

  Hereinafter, embodiments will be described in detail with reference to the accompanying drawings.

[First Embodiment]
The image forming apparatus of this embodiment includes an exposure device in which a plurality of organic EL elements are arranged on a substrate as light emitting elements, and the surface of the photosensitive drum is exposed by the exposure device. The image forming apparatus adjusts the amount of blur of the image in the scanning direction by the exposure device and in the non-scanning direction perpendicular to the scanning direction by performing filter processing on the image data representing the image. The image data includes information necessary for image formation such as color, density, exposure amount, and the like for each pixel of the image to be formed. In addition to the organic EL element, an LED element may be used as the light emitting element of the exposure device.

<Configuration of image forming apparatus>
FIG. 1 is an overall configuration diagram of an image forming apparatus according to the present exemplary embodiment. The image forming apparatus includes a scanner unit 500, an image forming unit 503, a fixing unit 504, and a paper feeding / conveying unit 505. The operation of these components of the image forming apparatus is controlled by a printer control unit (not shown). Under the control of the printer control unit, the scanner, image forming, fixing, and paper feeding / conveying processes described below are smoothly performed.

  The scanner unit 500 irradiates light on a document placed on a document table and optically reads a document image. The scanner unit 500 converts the read document image into an electrical signal and generates image data.

  The image forming unit 503 performs image forming processing according to the image data generated by the scanner unit 500. The image forming unit 503 includes an exposure head 106 that is an exposure device, a photosensitive drum 502 that is a drum-shaped photosensitive member, and a transfer belt 511. The exposure head 106 emits light according to image data to expose the photosensitive drum 502. Four exposure heads 106 are provided (exposure heads 106a, 106b, 106c, 106d). Corresponding to the exposure head 106, four photosensitive drums 502 are also provided. The photosensitive drum 502 is driven to rotate, and the surface is charged by a charger. The photosensitive drum 502 is exposed by the exposure head 106 after the surface is charged, whereby an electrostatic latent image corresponding to the image data is formed. Since the photosensitive drum 502 is exposed while rotating, the circumferential direction of the photosensitive drum 502 becomes the scanning direction. The electrostatic latent image is developed with toner. As a result, a toner image is formed on the photosensitive drum 502.

  On the four photosensitive drums 502, toner images are formed with toners of different colors. In this embodiment, four color toner images of cyan (C), magenta (M), yellow (Y), and black (K) are formed. The toner images formed on the respective photosensitive drums 502 are sequentially transferred onto the transfer belt 511 so as to overlap each other. As a result, a full-color toner image without color misregistration is formed on the transfer belt 511. The residual toner remaining on each photosensitive drum 502 after the transfer is collected.

  The paper feeding / conveying unit 505 includes a paper feeding tray 107 on which paper for image printing is set, a manual feed tray 509, and an external paper feeding device 508. The paper feeding / conveying unit 505 matches the timing of image forming processing by the image forming unit 503. The sheet is conveyed to the image forming unit 503. The sheet feeding / conveying unit 505 conveys the sheet to the image forming unit 503 in accordance with, for example, the end timing of the transfer of the toner image to the transfer belt 511. The toner image formed on the transfer belt 511 is transferred to the sheet conveyed to the image forming unit 503. The sheet on which the toner image is transferred is conveyed to the fixing unit 504.

  The fixing unit 504 is configured by a combination of a roller and a belt, and includes a heat source such as a halogen heater. The fixing unit 504 melts and fixes the toner image transferred to the paper by heat and pressure. The sheet on which the toner image is fixed by the fixing unit 504 is discharged to the outside of the image forming apparatus by the discharge roller 510.

<Configuration of exposure head>
FIG. 2 is a diagram illustrating the configuration of the exposure head 106. FIG. 2A shows the arrangement of the exposure head 106 with respect to the photosensitive drum 502. FIG. 2B shows a light condensing state of the light emitted from the exposure head 106 on the photosensitive drum 502. The exposure head 106 and the photosensitive drum 502 are each attached to the image forming apparatus by an attachment member (not shown).

  The exposure head 106 includes an organic EL element array 601 that is a light emitting element array composed of a plurality of organic EL elements, a substrate 602 on which the organic EL element array 601 is mounted, and a rod lens array 603. The organic EL element array 601 includes organic EL elements arranged in a line along the rotation axis direction of the photosensitive drum 502. The organic EL element array 601, the substrate 602, and the rod lens array 603 are attached to the housing 604 and integrally configured. The exposure head 106 performs focus adjustment and light amount adjustment of each spot (exposure position) as a single unit. At the time of exposure, the photosensitive drum 502 rotates. Therefore, the light emitted from each of the plurality of organic EL elements scans the photosensitive drum 502 in the circumferential direction of the photosensitive drum 502, which is a direction perpendicular to the organic EL elements.

  The rod lens array 603 is an optical system having an erecting equal magnification optical characteristic that irradiates the irradiated surface (the surface of the photosensitive drum 502) with the same magnification relationship with the light of the organic EL element array 601. The exposure head 106 is arranged such that the distance between the photosensitive drum 502 and the rod lens array 603 and the distance between the rod lens array 603 and the organic EL element array 601 are a predetermined focal length. As a result, a light spot corresponding to the light emitting surface shape and arrangement position of the organic EL element array 601 is formed on the photosensitive drum 502. As the light emitting surface size of the organic EL element increases, the light spot on the photosensitive drum 502 also increases. Moreover, the resolution of the exposure head 106 is increased by reducing the interval between the organic EL elements. For example, in the case of the exposure head 106 having a resolution of 1200 [dpi], the organic EL elements are arranged on the substrate 602 at an interval of 21.16 [μm].

  When the exposure head 106 is assembled, focus adjustment for adjusting the focal length and light amount adjustment for adjusting the light amount of each organic EL element are performed. In the focus adjustment, the attachment position of the rod lens array 603 is adjusted so that the distance between the rod lens array 603 and the organic EL element array 601 becomes a desired value. In the light amount adjustment, each organic EL element is caused to emit light sequentially, and the drive current of each organic EL element is adjusted so that the light condensed through the rod lens array 603 has a predetermined light amount.

  The organic EL element is a known material and structure, and there are no restrictions on the material and structure as long as they have similar light emission characteristics. FIG. 3 is an explanatory diagram of the configuration of the organic EL element array 601. The organic EL element 300 includes a TFT driving unit 302, a light emitting unit 306, and electrode patterns 303 and 309. Each organic EL element 300 acquires a control signal for controlling light emission from the controller. The TFT drive unit 302 of each organic EL element 300 controls the emission of light from the light emitting unit 306 according to the acquired control signal. The organic EL element array 601 is an active matrix driving system including such an organic EL element 300 for each light emitting point.

  FIG. 4 is an explanatory diagram of the configuration of the organic EL element 300. The organic EL element 300 of the present embodiment has a bottom emission structure, but is not limited thereto, and may have a top emission structure.

  FIG. 4A is a cross-sectional view of the organic EL element 300 on the substrate 602. The substrate 602 of this embodiment is made of a transparent glass material having a high light transmittance. The light generated in the organic EL element 300 is emitted from the substrate 602 side. On the substrate 602, a TFT driving unit 302 and an electrode pattern 303 are formed. Since the organic EL element 300 has a bottom emission structure and light is emitted from the electrode pattern 303 side, the electrode pattern 303 is composed of a metal oxide film having a high light transmittance such as ITO (Indium Tin Oxide).

  The electrode pattern 309 is formed at a position facing the electrode pattern 303 with the light emitting unit 306 of the organic EL element interposed therebetween, and is connected to the ground layer of the substrate 602. The electrode pattern 309 is positioned opposite to the light emission direction. Therefore, the electrode pattern 309 does not need to use a material with high light transmittance such as ITO, and is configured by a metal oxide film having conductivity such as Al.

  The light emitting unit 306 of the organic EL element includes a hole injection layer 304, a hole transport layer 305, a light emission layer 313, an electron transport layer 307, and an electron injection layer 308, and a voltage applied between the electrode patterns 303 and 309. Emits light in response to.

  The hole injection layer 304 is formed on the electrode pattern 303 using MoO3 (molybdenum trioxide), CuPc (copper furocyanine), or the like as a material, and holes are transferred to the hole transport layer 305 according to the voltage applied from the electrode pattern 303. Inject. The hole transport layer 305 is formed on the hole injection layer 304 using α-NPD (naphthylphenyldiamine) or the like as a material, and supplies holes to the light emitting layer 313.

  The electron injection layer 308 is formed below the electrode pattern 309 using Li 2 O (lithium oxide) or the like as a material, and injects electrons into the electron transport layer 307 according to the voltage applied from the electrode pattern 309. The electron transport layer 307 is formed below the electron injection layer 308 using Alq 3 (Tris aluminum) or the like as a material, and supplies electrons to the light emitting layer 313.

  The light emitting layer 313 emits light with an intensity corresponding to the amount of electrons supplied from the electron transport layer 307 and the amount of holes supplied from the hole injection layer 304. The light emitting layer 313 is a layer formed of an organic light emitting material, and a known organic material that emits light in a wavelength region corresponding to the photosensitive characteristics of the photosensitive drum 502 is used.

  The light emitting unit 306, the electrode patterns 303 and 309, and the TFT driving unit 302 are sealed with resist films 311 and 312 to prevent moisture from entering inside and to have an electrical insulating property. . As a sealing method, besides a method of forming a resist film, a known sealing technique in which a sealing structure is made of a glass material and a moisture adsorbent is sealed in the sealed space may be used.

  Such a light emitting unit 306 is manufactured by a vacuum deposition method when a low molecular material is used as the material of the light emitting layer 313. In the vacuum evaporation method, the material of the low-molecular organic EL element 300 vaporized by the evaporation source penetrates from the opening of the metal mask that is in close contact with the substrate 602 to form a film. By arbitrarily designing the mask pattern of the metal mask used in each manufacturing process, the light emitting unit 306, the TFT driving unit 302, the electrode patterns 303, 309, and the like can be manufactured to have arbitrary design values.

  Note that in the case where a polymer material is used as the material of the light-emitting layer 313 of the light-emitting portion 306, the light-emitting layer 313 may be formed by an inkjet printing method. In the inkjet printing method, a light-emitting element can be manufactured in an arbitrary shape by applying a polymer organic EL material at a predetermined position, similarly to the vacuum deposition method.

  FIG. 4B is a circuit configuration diagram of the TFT drive unit 302. The TFT drive unit 302 includes two TFTs 401 and 403 and a capacitor 402.

  A reference voltage Vr is input to the source electrode of the TFT 401, and a chip select signal Cs is input to the gate electrode. The reference voltage Vr and the chip select signal Cs are control signals for controlling light emission input from the controller. When the chip select signal Cs is turned on, the gate of the TFT 401 is turned on, the reference voltage Vr is transmitted to the drain electrode of the TFT 401, and the capacitor 402 is charged so as to be substantially the same voltage as the reference voltage Vr. The TFT 403 has a source electrode connected to the power source and a gate electrode connected to the capacitor 402, and transmits a stable voltage corresponding to the gate voltage to the drain electrode side. The drain electrode of the TFT 403 is connected to the light emitting unit 306 of the organic EL element 300. By the TFT drive unit 302, the organic EL element 300 emits light with a drive voltage corresponding to the reference voltage Vr held in the capacitor 402 when the chip select signal Cs is turned on.

  In the exposure head 106 having such a configuration, the light quantity can be controlled for each pixel by inputting the chip select signal Cs and the reference voltage Vr to the TFT driving unit 302 for each organic EL element 300.

FIG. 5 is a connection diagram of the exposure head 106 and the controller 610 that inputs a control signal to the exposure head 106. In the present embodiment, an organic EL element array 601 in which 16 organic EL elements 300 are arranged in the rotation axis direction of the photosensitive drum 502 will be described. FIG. 6 is a layout diagram of the light emitting points E1 to E16. One light emitting point is constituted by the light emitting unit 306 of one organic EL element 300. The organic EL element array 601 employs a time-division exposure method in which each of the light emitting points E1 to E16 sequentially emits light, and acquires a signal indicating the light emission intensity sequentially for every four light emitting points from the controller 610. The controller 610 includes a CPU (Central Processing Unit), a RAM (Random Access Memory), and a ROM (Read Only).
Memory). The controller 610 provides various data to the exposure head 106 when performing a filtering process described later.

  A set of light emitting points E1 to E16 is defined as a line element 620. The light emitting point E1 is located at the left end of the line element 620, and the light emitting point E16 is located at the right end of the line element 620. On the substrate 602, a wiring pattern 608 for transmitting a control signal instructing light emission to the organic EL element array 601 is provided. The control signal is input from the controller 610 to the connector 606 via the bundled wire 611 and transmitted to the wiring pattern 608. The wiring pattern 608 includes a reference voltage line and a chip select line. The light emission points E1 to E16 are controlled in light emission timing by a combination of signals input from the reference voltage line and the chip select line. The reference voltage line is a signal line that transmits an analog signal (reference voltage Vr) corresponding to image data corresponding to each light emitting point. The reference voltage line transmits image data to a plurality of light emitting points with one signal line. The chip select line transmits a chip select signal Cs for selecting a light emitting point. The light emitting point selected by the chip select signal Cs permits the input of an analog signal (reference voltage Vr) from the reference voltage line.

  In the present embodiment, four reference voltage lines (reference voltage lines Vr1-1 to Vr1-4) are wired for every four light emitting points. Further, chip select lines C1-1 to C1-4 are wired at a rate of one for four light emitting points. The drive circuit that drives the four light emitting points is enabled by the chip select signal Cs. The light emission of the four light emitting points is independently controlled by the reference voltage lines Vr1-1 to Vr1-4.

  When the logic of the chip select signal Cs of the chip select line C1-1 is high, the light emitting points E1 to E4 are selected. The light emission point E1 is controlled to emit light by controlling the drive current according to the reference voltage Vr of the reference voltage lines Vr1-4 that are analog signals. Similarly, the light emission of the light emitting points E2, E3, E4 is controlled by the reference voltage lines Vr1-3, Vr1-2, Vr1-1. At this time, the logic of the chip select signals Cs1 to C1-3 other than the chip select line C1-1 is low, and the light emitting points E5 to E16 are set to the reference voltage lines Vr1-1 to Vr1-. The reference voltage Vr from 4 is ignored. As described above, among the plurality of chip select lines C1-1 to C1-4, only the chip select signal of one chip select line is in the high state, and only the light emitting point selected by the chip select signal Cs has the reference voltage Vr. Receive.

  FIG. 7 is a timing chart of the reference voltage Vr and the chip select signal Cs. In this embodiment, since there are four reference voltage lines, the reference voltage Vr is simultaneously input to the four light emitting points.

  In the present embodiment, the light emitting points E1 to E16 sequentially emit light four by four from the left end portion to the right end portion of the substrate 602. The logic of the chip select signal Cs and the reference voltage Vr is switched at a constant cycle. Assuming that the light emission timings of the light emission points E1 to E4 at the left end of the substrate 602 are T1, and the light emission timings of the light emission points E5 to E8 to be emitted next are T2, the light emission points are sequentially switched from the left end of the substrate 602 to the right end in order of T1 and T2. . Here, the group of light emitting points that are simultaneously switched is a group selected by the common chip select signal described above. A cycle in which light emission is sequentially controlled from the left end portion to the right end portion of the substrate 602 is one line cycle, and light emission control is sequentially performed again from the left end portion to the right end portion of the substrate 602 in the next line. The time for one line cycle is set so that the time interval for moving the position of the surface of the photosensitive drum 502 by 21.1 [μm] matches the one line cycle time interval.

<Correction by filter processing>
In the present embodiment, in order to adjust the blurring amount of the image in the scanning direction and the non-scanning direction by the exposure head 106, a filter process is performed in the non-scanning direction. The filtering process is performed when a light spot is formed on the photosensitive drum 502. FIG. 8 is an explanatory diagram of a light spot of one pixel. The filter constant is determined from the distribution of the light amount when forming the light spot of one pixel.

  FIG. 8A is an explanatory diagram of a light spot in the scanning direction. As described above, in the erecting equal-magnification optical system using the rod lens array 603, the light emitted from the light emitting surface of the organic EL element 300 is directly irradiated onto the surface of the photosensitive drum 502 that is the irradiated surface. For this reason, the shape of the light-irradiated portion (light spot) on the surface of the photosensitive drum 502 is substantially the same as the shape of the light emitting surface if the photosensitive drum 502 is stopped. Since the light emitting surface of the organic EL element 300 of this embodiment is square, the light spot when the photosensitive drum 502 is stopped is also square.

  When forming a light spot of one pixel, the organic EL element 300 emits light for the time of irradiation for one pixel in the scanning direction. Since the photosensitive drum 502 rotates at a constant speed, a light spot corresponding to the scanning distance of one pixel is formed on the surface of the photosensitive drum 502 by light emission of the organic EL element 300 with the circumferential direction of the photosensitive drum 502 as a scanning direction. It is formed. The spot width of the light spot in the non-scanning direction that is the rotational axis direction of the photosensitive drum 502 is substantially equal to the width of the light emitting surface of the organic EL element 300.

  That is, the spot width in the scanning direction of the light spot is determined by the rotational speed of the photosensitive drum 502 and the light emission time of the organic EL element 300, and the width of the light spot in the non-scanning direction depends on the width of the light emitting surface of the organic EL element 300. Determined. When an image is formed at a resolution of 1200 [dpi], the size of the light emitting surface of the organic EL element 300 is determined so that the spot width is 21.16 [μm]. The pixel pitch interval determined by the resolution is 21.16 [μm].

  FIG. 8B is a diagram illustrating temporal characteristics of the light emission amount when the organic EL element 300 emits light for one pixel. In the organic EL element 300, there is a delay in light emission and extinction due to the response characteristics of the TFT drive unit 302 and the light emitting unit 306. The rise time Tr when the organic EL element 300 shifts from the light-off state to the light-emitting state and the fall time Tf when it shifts from the light-emitting state to the light-off state vary depending on the drive current and the light emission amount of the organic EL element 300. In the present embodiment, the filter constant for the filter process is determined based on the relationship between the rise time Tr and the fall time Tf with respect to the light emission amount of the organic EL element 300 obtained experimentally.

  FIG. 8C is a diagram showing the exposure distribution in the scanning direction of the light that exposes the photosensitive drum 502 when the light spot is formed. The exposure head 106 forms a light spot so that the light spots of pixels adjacent in the scanning direction overlap. The shape of the light spot is uniquely determined by the shape of the light emitting surface of the organic EL element 300, the scanning distance of the organic EL element 300, the rising time Tr of the organic EL element 300, and the falling time Tf, as described with reference to FIG. It is decided. In the present embodiment, the light emitting surface is a square having a side of 21.16 [μm], and the scanning distance is 21.16 [μm]. Therefore, the light spot has a length in the scanning direction of 42.32 [μm] and a width of two pixels of 1200 [dpi]. As a result, the 0.5-pixel spectral spot overlaps the pixels adjacent in the scanning direction.

  A region β in FIG. 8C represents a region of a pixel (target pixel) to which the exposure head 106 irradiates (exposes) light, and the region α and the region γ are adjacent to the target pixel in the scanning direction. This represents a region of pixels (peripheral pixels). Assume that the integrated values of the exposure amounts of the regions α, β, and γ are Pα, Pβ, and Pγ. Moreover, let the ratio which normalized each integrated value P (alpha), P (beta), P (gamma) with the total value of integrated value P (alpha), P (beta), P (gamma) be filter constant C (alpha), C (beta), C (gamma) used for a filter process. The filter constants Cα and Cγ represent the ratio of the amount of light diffused to pixels adjacent in the scanning direction of the target pixel with respect to the total amount of light emitted from the organic EL element.

  The filtering process diffuses light to pixels adjacent in the non-scanning direction as well as light diffusion to pixels adjacent in the scanning direction of the target pixel. That is, the filtering process diffuses light so that the pixels adjacent to each other in the non-scanning direction have an exposure distribution similar to the exposure distribution in the regions α and γ in FIG. FIG. 9 is a diagram for explaining the filtering process using nine (3 × 3) pixels. Each pixel has a density a to k determined by image data. The densities a to k determine the exposure amount on the photosensitive drum 502 by the exposure head 106. For example, the higher the density, the greater the exposure amount.

  In the filter processing, the densities a to k are corrected according to the filter constants Cα, Cβ, and Cγ. In FIG. 9, the density of each pixel in the first row is first multiplied by filter constants Cα, Cβ, and Cγ. For example, “a * Cα”, “a * Cβ”, and “a * Cγ” are respectively calculated by multiplying the density a of the upper left pixel by a filter constant. “A * Cβ” is added to the density a of the upper left pixel, and “a * Cα” and “a * Cγ” are added to the densities of other pixels adjacent to the upper left pixel in the non-scanning direction. This calculation is performed for all pixels. Thereafter, the filter processing is completed by correcting the density of each pixel according to the calculation result.

The density of each pixel after the filter processing is calculated by the equation (1). Dtar 'is the density of the pixel of interest after filtering, Dtar is the density of the pixel of interest before filtering, Df is the density of the pixel adjacent to the upstream of the pixel of interest in the non-scanning direction, and Dr is non-scanning of the pixel of interest This is the density before filtering of pixels adjacent downstream in the direction.
Dtar '=
Df * Cγ + Dtar * Cβ + Dr * Cα Formula (1)

When the density E after the filter processing of the middle left pixel, which is set to the density e by the image data in FIG. 9, is expressed by Expression (1), Expression (2) is obtained.
E
= A * Cγ + e * Cβ + i * Cα Equation (2)

  Thus, by performing the filter processing so that the exposure distribution in the scanning direction and the exposure distribution in the non-scanning direction are substantially the same, the amount of blur of the light spot in the scanning direction and the non-scanning direction can be made substantially equal. . That the exposure distribution is substantially the same means that the difference in the shape of the exposure distribution (exposure amount, change amount, change rate) is within a predetermined range.

  FIG. 10 is a functional block diagram showing functions implemented in the controller 610 for performing such filter processing. Each function provided in the controller 610 is realized by execution of a predetermined computer program or hardware. The controller 610 acquires image data from the scanner unit 500 and performs light emission control of the exposure head 106 according to the acquired image data. The controller 10 includes an image processing unit 631, a memory control unit 632, a memory 633, an exposure modulation unit 634, and a pattern conversion unit 635.

  The image processing unit 631 divides the image data acquired from the scanner unit 500 for each color. The image processing unit 631 executes conversion processing into pixel data corresponding to an image based on image data and screen processing corresponding to each color. The memory control unit 632 writes the image data processed by the image processing unit 631 into the memory 505 and reads out the written image data from the memory 5050 according to the execution timing of the image forming process. The memory control unit 632 sends the read image data to the exposure modulation unit 634.

  The exposure modulation unit 634 filters the image data input from the memory control unit 632 and sends the image data to the pattern conversion unit 635. The pattern conversion unit 635 converts the image data processed by the exposure modulation unit 634 into binary data. The pattern conversion unit 635 rearranges the transfer order of the image data according to the order of the dots of the organic EL elements of the exposure head 106, and inputs the control signals according to the image data to the exposure head 106 in the order of rearrangement.

  The filtering process by the exposure modulation unit 634 will be described. FIG. 11 is a block diagram of the exposure modulation unit 634. The exposure modulation unit 634 includes a two-dimensional filter 2002 and a filter constant generation unit 2004. The two-dimensional filter 2002 uses filter constants assigned to each of a pixel (target pixel) k (0, 0) forming an image and a peripheral pixel k (m, n) positioned around the target pixel. Then, the image data of the target pixel is corrected.

  FIG. 12 is an explanatory diagram of filter constants. FIG. 12 is a (15 × 15) filter constant matrix representing the filter constants of the pixel of interest k (0, 0) and the peripheral pixels k (m, n). The filter constant generation unit 2004 generates a filter constant used for filter processing by the two-dimensional filter 2002 using the filter constant matrix. Note that the filter constant generation unit 2004 generates a filter constant matrix according to the rise time Tr and the fall time Tf of the organic EL element 300.

  FIG. 13 is a hardware configuration diagram of the two-dimensional filter 2002. The two-dimensional filter 2002 includes 14 first-in first-out memory (FIFO) memories 5001 to 5014, a shift register unit 5015, a multiplier unit 5016, and an adder unit 5017. The two-dimensional filter 2002 is synchronously controlled by a clock that synchronously controls the controller 610.

  The 14 FIFO memories 5001 to 5014 are connected in series and are line memory buffers capable of storing image data having the number of pixels corresponding to one line period. The FIFO memories 5001 to 5014 sequentially output the image data input from the scanner unit 500 to the shift register unit 5015 in synchronization with the clock.

  The shift register unit 5015 includes registers arranged in (15 × 15). Fifteen registers D0_0 to D14_0 are assigned as the first-stage shift register group. The second to fifteenth stage shift register groups are similarly configured. The register D0_0 of the first-stage shift register group is connected to the FIFO memory 5001 and sequentially receives image data (pixel data) corresponding to one pixel from the FIFO memory 5001. Similarly, the registers D1_0 to D13_0 of the first-stage shift register group also sequentially receive pixel data from the FIFO memories 5002 to 5013 connected thereto. The register D14_0 of the first-stage shift register group directly receives pixel data from the memory control unit 632. The image data is input from the memory control unit 632 to the register D14_0 and the FIFO memory 5014.

  Pixel data of one pixel is input to each register of the shift register unit 5015. Pixel data of the target pixel is input to the register D7_7. Pixel data of peripheral pixels is input to other registers.

  The multiplier unit 5016 includes (15 × 15) multipliers M0_0 to M14_14. Each of the multipliers M0_0 to M14_14 corresponds to one register of the shift register unit 5015, and pixel data of one pixel is input from the corresponding register. The corresponding filter constants from the filter constant generation unit 2004 are also input to the multipliers M0_0 to M14_14. Each multiplier M0_0 to M14_14 multiplies pixel data and a filter constant. Each multiplier M0_0 to M14_M14 sends the multiplication result to the adder unit 5017.

  The adder unit 5017 includes adders A0 to A15. The adders A0 to A14 add the multiplication results output from the multipliers M0_x to M14_x. The adder A15 adds the addition results of the adders A0 to A14. The addition result of the adder A15 is output from the two-dimensional filter 2002 as image data after filtering of the target pixel.

<Filter constant>
The filter constants Cα, Cβ, and Cγ in this embodiment are switched according to the amount of light from the exposure head 106. FIG. 14 is a free chart showing image forming processing including switching processing of filter constants Cα, Cβ, and Cγ, which is executed by the controller 610. Note that the filter constants Cα, Cβ, and Cγ are stored in advance in a storage device such as a RAM provided in the controller 610.

  When the image forming process is started, the controller 610 sets the light amount adjusted to keep the image density constant in the exposure head 106 (S100). The amount of light is set by a known method. For example, a density detection sensor is provided in the image forming apparatus to detect the image density, and the light quantity is set so that the detection result has a predetermined density.

  The controller 610 acquires filter constants Cα, Cβ, and Cγ from the storage device according to the set light amount (S101). FIG. 15 is an exemplary diagram of a filter constant table showing a correspondence relationship between the light amount stored in the storage device and the filter constants Cα, Cβ, and Cγ. The filter constant corresponding to each light quantity is obtained experimentally as described above. In FIG. 15, the filter constant is set in increments of 10 [%] of the light amount. However, if the light amount adjustment resolution of the image forming apparatus is finer than 10 [%], it may be set more finely.

  The controller 610 performs the filter process according to the expression (1) for all the pixels, and holds the processing result in the RAM or the like (S102). After the filtering process, the controller 610 determines that printing is started by transporting the sheet fed by the sheet feeding / conveying unit 505 to a predetermined position (S103: Y). When printing is started, the controller 610 controls the amount of light emitted from the exposure head 106 based on the image data and the filter processing result stored in the RAM, and forms an image (S104). The controller 610 repeats the processing from step S103 until all jobs are completed (S105).

  In the present embodiment as described above, the amount of blur of the light spot in the scanning direction and in the non-scanning direction can be made substantially equal by the filtering process in the non-scanning direction for each pixel. As a result, it is possible to suppress adverse effects such as a loss of character proportion due to a difference in the amount of blur between the scanning direction and the non-scanning direction, and a mismatch between vertical and horizontal line widths. Further, since the adjustment is performed by the filter processing, it is possible to adjust the amount of blur with a simple and inexpensive configuration rather than providing an optical adjustment mechanism.

  In the present embodiment, the example in which the filtering process is performed on the region for three pixels including the pixel adjacent to the target pixel in the non-scanning direction has been described. When the exposure distribution diffuses to a pixel that is two pixels or more away from the target pixel, the filter processing range may be expanded by setting a filter constant so that the data is diffused to a region that is two pixels or more away.

  In this embodiment, the filter operation is performed so as to lower the frequency characteristics in the non-scanning direction by diffusing data in the non-scanning direction. On the other hand, enhancement processing may be performed in the scanning direction, and filtering processing may be performed so as to enhance frequency characteristics. In this case, a filter having an edge enhancement effect in the scanning direction may be used. When performing the enhancement process, the amount of blur in the scanning direction can be reduced by reducing the amount of blur in the scanning direction.

  106: exposure head, 502 ... photosensitive drum, 300 ... organic EL element, 302 ... TFT drive unit, 303, 309 ... electrode pattern, 304 ... hole injection layer, 305 ... hole transport layer, 306 ... light emitting unit, 307 ... Electron transport layer, 308 ... Electron injection layer, 601 ... Organic EL element array, 602 ... Substrate, 603 ... Rod lens array, 604 ... Housing

Claims (10)

A photoconductor on which an image corresponding to image data representing the density of each pixel is formed;
An exposure device that has a plurality of light emitting elements and scans the photosensitive member with light emitted from each light emitting element to form the image;
The density of each pixel is corrected so that the exposure distribution in the scanning direction and the exposure distribution in the non-scanning direction of the exposure amount determined by the density are substantially the same, and based on the corrected density, the light emitting element Control means for controlling the amount of light emitted from,
Image forming apparatus. The control unit corrects the density according to a filter constant representing a ratio of a light amount diffused in a scanning direction of a target pixel with respect to a total light amount emitted from the light emitting element.
The image forming apparatus according to claim 1. The filter constant is determined based on a rise time and a fall time when the light emitting element emits light,
The image forming apparatus according to claim 2. The control means corrects the density of the pixel of interest according to a value calculated from the density of a pixel adjacent in the scanning direction of the pixel of interest and the filter constant.
The image forming apparatus according to claim 2. The control means includes a first filter constant representing a ratio of a light amount irradiated to the target pixel with respect to a total light amount emitted from the light emitting element, and a light amount diffused to a pixel adjacent upstream in the scanning direction of the target pixel. The pixel is corrected in accordance with a second filter constant representing a ratio of the second filter constant and a third filter constant representing a ratio of the amount of light diffused to pixels adjacent downstream in the scanning direction of the target pixel.
The image forming apparatus according to claim 2. The control means calculates the density of the target pixel, the product of the density and the first filter constant, the product of the density of a pixel adjacent downstream in the scanning direction of the target pixel and the second filter constant, And correcting by the sum of the product of the density of the pixel adjacent in the scanning direction upstream of the target pixel and the third filter constant,
The image forming apparatus according to claim 5. The apparatus further comprises storage means for storing the first filter constant, the second filter constant, and the third filter constant according to the amount of light emitted from the light emitting element.
The control unit obtains, from the storage unit, a first filter constant, a second filter constant, and a third filter constant corresponding to a light amount of a light emitting element that irradiates the target pixel with light. ,
The image forming apparatus according to claim 5 or 6. The photoconductor is in the form of a drum, and when rotated, the exposure device irradiates light to form the image,
The exposure device scans the photosensitive member with a circumferential direction of the photosensitive member as a scanning direction,
The image forming apparatus according to claim 1. An exposure device that has a photoconductor on which an image corresponding to image data representing density for each pixel is formed, and a plurality of light emitting elements, and scans the photoconductor with light emitted from each light emitting element to form the image. And storage means for storing a filter constant representing a ratio of the amount of light diffused in the scanning direction of the target pixel to be exposed with respect to the total amount of light emitted from the light emitting element, corresponding to the amount of light emitted from the light emitting element. A method executed by an image forming apparatus comprising:
Adjusting the amount of light emitted from each light emitting element to make the image density constant;
Obtaining a filter constant corresponding to the adjusted light amount from the storage means;
Correcting the density of each pixel according to the acquired filter constant so that the exposure distribution in the scanning direction and the exposure distribution in the non-scanning direction of the exposure amount determined by the density are substantially the same;
Controlling the amount of light emitted from the light emitting element of the exposure device based on the corrected density, and performing image formation,
Image forming method. A first filter constant representing a ratio of a light amount irradiated to the target pixel with respect to a total light amount emitted from the light emitting element; a light amount diffused to a pixel adjacent upstream in the scanning direction of the target pixel; And a third filter constant representing a ratio of the amount of light diffused to pixels adjacent downstream in the scanning direction of the target pixel,
The image forming apparatus determines the density of the pixel of interest by multiplying the density by the first filter constant, and by multiplying the density of a pixel adjacent downstream in the scanning direction of the pixel of interest by the second filter constant. , And the sum of the products of the density of the pixel adjacent upstream in the scanning direction of the target pixel and the third filter constant,
The image forming method according to claim 9.