JP2016126267A - Image formation device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image formation device with which it is possible to obtain the excellent gradation of densities without causing "gradation skipping" in cases where a dither matrix for background exposure is used separately from a dither matrix for images in order to suppress various harmful effects on images.SOLUTION: Provided is an image formation device for exposing a photoreceptor on the basis of data based on a dither matrix as an exposure pattern, the image formation device being provided with, as dither matrixes, a first dither matrix as a first exposure pattern for obtaining a plurality of density gradations in a first area in which a developer image is formed and a second dither matrix as a second exposure pattern for exposing in a second area in which no developer images are formed to the extent that there is no sticking of the developer, with a condition that A≥B being satisfied, where A represents the amount of a potential drop in the photoreceptor corresponding to the lowest density gradation when exposed via the first dither matrix, and B represents the amount of a potential drop in the photoreceptor when exposed via the second dither matrix.SELECTED DRAWING: Figure 12

Description

  The present invention relates to an electrophotographic image forming apparatus.

  In an electrophotographic color image forming apparatus, a light beam such as a laser beam is used to expose a formed image on each charged photosensitive drum to form an electrostatic latent image. A developer image (toner image) is formed on the surface of the photosensitive drum. The toner image formed on the surface of each photoconductive drum is transferred by sequentially superimposing the toner images of each color on the intermediate transfer member, and then transferring the multicolor toner image on the intermediate transfer member onto a recording medium at a time. Is formed.

  When printing a multicolor image with such a color image forming apparatus, when the toner image transferred onto the intermediate transfer member at the upstream station enters the nip of the photosensitive drum at the downstream station, Cause a phenomenon. That is, there is a difference in the resistance value between the photosensitive drum and the intermediate transfer belt between the portion where the toner (developer) of the upstream station is formed and the portion where the toner (developer) is not formed. As a result, the transfer current flowing from the transfer material and the intermediate transfer belt to the photosensitive drum is different between the toner forming portion and the toner non-forming portion, and this difference in the amount of transfer current is caused by the surface potential on the photosensitive drum after transfer. Causes unevenness.

  If the photosensitive drum is charged again with potential unevenness on the surface of the photosensitive drum, the surface potential unevenness cannot be sufficiently filled, and the amount of toner adhering to the surface of the photosensitive drum will be different in the next development process. And density unevenness occurs in the formed image.

  As a method for solving this problem, the charging bias is set high in the charging process, and the surface potential of the photosensitive drum after charging is increased, thereby leveling the surface potential unevenness of the photosensitive drum generated during transfer. There is a method for reducing the density unevenness of the formed image.

  However, in order to stably develop toner on the surface of the photosensitive drum by the development process by increasing the charging bias set value in the charging process and increasing the surface potential of the photosensitive drum, the following is necessary. Become. That is, while increasing the light emission intensity of the light beam to the photosensitive drum in the exposure process, in the non-printing area (non-toner image forming area), the light beam is emitted for a short time so as not to cause excessive toner adhesion, It is necessary to cancel the increase in the surface potential of the photosensitive drum. The light emission control for emitting the light beam to the non-toner image forming region for a short time is called background exposure.

  In background exposure, laser light emission for background exposure is performed by causing a light emitting element such as a laser to emit light in synchronization with an image clock for a non-toner image forming region. Specifically, as shown in FIG. 17, laser light emission of one pixel or less synchronized with the image clock is performed for each pixel. For the light emission control of the background exposure, for example, as shown in FIG. 16, a dither matrix composed of a plurality of pixels continuous in the main scanning direction and the sub-scanning direction is used.

  As described above, the background exposure light emission pattern is embedded in the image data of the non-toner image area in the image area, and the background exposure light emission pattern is assigned to the white image data portion in the dither matrix. Then, background exposure is performed by outputting the image data from the image processing unit to the light emitting element driving circuit.

  Background exposure is not limited to the prevention of image density unevenness during multicolor image printing as described above. It is also used in the following cases where an electrostatic latent image (for example, an image edge portion) in which the drum surface potential changes sharply on the photosensitive drum is formed. That is, when this image edge portion is developed by the developing device, the image is formed thinner than the electrostatic latent image originally formed on the photosensitive drum, and the colors and colors of the images formed adjacent to each other with different colors are formed. In this case, a phenomenon occurs in which a white gap that should not be present is generated. Hereinafter, this phenomenon is referred to as a white gap.

  In the case of monochromatic image formation, since there is no adjacent color, there is no problem even if the image is slightly thinned. However, in the case of color image formation, there is a problem. For example, in the case of an image in which a cyan band and a black band are adjacent to each other, an image that should originally be adjacent to a cyan band and a black band is an image in which a cyan image is a black image. Are also formed thin. For this reason, the final image on the recording material has a gap between the cyan color and the black color.

  In this case, as a background exposure, the thinning of the image can be prevented by causing the light emitting element of the laser scanner to emit light for a short time in the non-toner image forming region in the entire printable region so as not to cause excessive toner adhesion. .

  Furthermore, in addition to the case described above, background exposure is also used as a countermeasure against inversion fog that occurs when the potential difference (back contrast) between the developing bias potential and the primary charging bias is large (Patent Document 1).

  Here, when the pixels that perform background exposure are continuous, a drive current with a minute time interval flows through the light source repeatedly at a constant cycle. Then, a current flows through the light source drive circuit and the power line cable supplying current to the light source, and a high frequency noise voltage is generated by the inductance component of the power line. Then, high-frequency noise having a frequency included in the noise voltage resonates with a power line cable or the like, and a part of the electromagnetic energy of the high-frequency noise is radiated as electromagnetic waves into the space using the power line cable or the like as an antenna. The electromagnetic waves radiated in this way are called unwanted radiation noise.

  Thus, in the exposure pattern in which the pixels that perform background exposure are continuous (FIG. 18A), a large amount of unnecessary radiation noise is generated due to the light emission period in the main scanning direction of the laser (FIG. 18B). )).

JP 2003-323012 A

  Here, it is considered that a dither matrix is provided separately from the image dither matrix as an exposure pattern in which pixels that perform background exposure are not continuous, and the frequency components generated by the exposure pattern are dispersed in frequency to suppress the generation of unnecessary radiation noise. It is done.

  However, in a configuration that suppresses the generation of unnecessary radiation associated with background exposure by using a background exposure dither matrix in which frequency components generated by the light emission pattern are dispersed in frequency apart from the image dither matrix, Create a challenge. In a low-density halftone image, for example, image data that is greater than 0% and less than or equal to about 5%, the potential setting on the photosensitive drum changes, and a halftone image that should originally be able to obtain continuous gradation is “ This is a problem that “tone jump” (density jump) occurs.

  By mixing a low-density halftone image and a non-toner image formation area, the background dither matrix and the image dither matrix have a low level, and the drum surface potential cannot be gently connected. . That is, the relationship between the drum surface potential after background exposure (for example, −450 V) and the drum surface potential in the image dither matrix is as shown in FIG. As a result, “gradation skip” occurs, and good gradation is not obtained.

  An object of the present invention is to use a background exposure dither matrix separately from the image dither matrix in order to suppress various adverse effects of the image, and to produce a gradation having a good density without causing “jumping of gradation”. An object of the present invention is to provide an image forming apparatus capable of obtaining the characteristics.

  In order to achieve the above object, an image forming apparatus according to the present invention uses a photoconductor, a charging unit for charging the photoconductor, and a dither matrix as an exposure pattern for the photoconductor charged by the charging unit. An image forming apparatus comprising: an exposure unit that performs exposure based on data; and a developing unit that forms a developer image by attaching a developer to the photoreceptor on which a latent image is formed by the exposure unit. The matrix is not formed with a first dither matrix as a first exposure pattern for obtaining a plurality of density gradations in a first region where a developer image of the photoconductor is formed, and the developer image of the photoconductor. And a second dither matrix as a second exposure pattern for performing exposure by the exposure means to such an extent that the developer does not adhere to the photoconductor in the second region, A potential drop amount of the photosensitive member corresponding to the lowest density gradation when exposed through the dither matrix is A, and B a potential drop amount of the photosensitive member when exposed through the second dither matrix. In this case, the condition of A ≧ B is satisfied.

  According to the present invention, when a dither matrix for background exposure is used separately from the image dither matrix in order to suppress various image adverse effects, a gradation with a good density is generated without causing “gradation skip”. Sex is obtained.

1 is a schematic configuration diagram of an image forming apparatus according to an embodiment of the present invention. 1 is a block diagram illustrating an image forming control system of an image forming apparatus according to an embodiment of the present invention. It is a functional block diagram of the image processing flow at the time of printing. FIG. 3 is a diagram illustrating a growth order in a dither matrix of the image forming apparatus according to the first embodiment. FIG. 3 is a diagram illustrating a dither matrix of the image forming apparatus according to the first embodiment. FIG. 3 is a diagram illustrating a position control matrix of the image forming apparatus according to the first embodiment. FIG. 3 is a diagram illustrating a PWM table of the image forming apparatus according to the first embodiment. It is a figure which shows the output result by the PWM table of the image forming apparatus of 1st Embodiment. It is a figure which shows the pixel of the non-toner image formation area which concerns on 1st Embodiment. It is a figure which shows the background exposure pattern which concerns on 1st Embodiment. FIG. 3 is a diagram illustrating a PWM table of the image forming apparatus according to the first embodiment. It is a figure which shows the output result of the image forming apparatus of 1st Embodiment. It is a figure which shows the outline of the dither matrix of the image forming apparatus of 2nd Embodiment. It is a figure which shows the dither matrix of the image forming apparatus of 2nd Embodiment. FIG. 6 is a diagram illustrating a relationship between a drum potential drop amount and a light emitting region of an image forming apparatus according to a second embodiment. It is a figure which shows the outline of the background exposure pattern in a comparative example. It is a figure explaining the light emission control of the laser in a prior art example. It is a figure which shows the background exposure pattern and radiation noise in a prior art example. It is a figure explaining the subject in this invention.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. The following embodiments are merely examples, and are not intended to limit the technical scope of the present invention only to them.

<< First Embodiment >>
(Image forming device)
FIG. 1 is a schematic configuration diagram relating to a color laser printer 50 as an image forming apparatus according to the first embodiment of the present invention. As shown in the figure, the color laser printer 50 includes a plurality of photosensitive drums 5 (5Y, 5M, 5C, and 5K) that are first image carriers, and in turn, an intermediate that is a second image carrier. Continuously multiple transfer is performed on the transfer belt 3 (transfer object). This is a quadruple drum type (inline type) printer that obtains a full-color print image. This image forming apparatus is a 600 dpi, 20 ppm printer that supports output up to A4 size (210 mm × 297 mm) paper.

  The intermediate transfer belt 3 is an endless endless belt, suspended on a driving roller 12, a tension roller 13, an idler roller 17, and a secondary transfer counter roller 18, and rotated at a process speed of 115 mm / sec in the direction of the arrow in the figure. doing. As the material of the intermediate transfer belt 3, polyimide, polyamide, polycarbonate (PC), polyvinylidene fluoride (PVDF), or the like is used. The drive roller 12, the tension roller 13, and the secondary transfer counter roller 18 are support rollers that support the intermediate transfer belt 3.

  Four photosensitive drums 5 (5Y, 5M, 5C, 5K) are arranged in series in the moving direction of the intermediate transfer belt 3 corresponding to each color. The photosensitive drum 5Y having a yellow developing device (developing means) is uniformly charged to a predetermined polarity and potential by a primary charging roller (charging means) 7Y during the rotation process. Then, by receiving the image exposure 4Y by the image exposure means (exposure means) 9Y, an electrostatic latent image corresponding to the first color (yellow) component image of the target color image is formed. Next, the electrostatic latent image is developed with yellow toner (developer) as the first color by the developing roller 8Y.

  In this way, a method in which toner is developed in a portion where an electrostatic latent image is formed by image exposure is referred to as a “reversal development method”. The yellow image formed on the photosensitive drum 5Y enters the primary transfer nip portion with the intermediate transfer belt 3. In the primary transfer nip portion, a voltage application member (primary transfer roller) 10Y is brought into contact with and contacted with the back side of the intermediate transfer belt 3. The voltage application member 10Y has a primary transfer bias power supply (not shown) so that a bias can be applied independently at each port.

  The intermediate transfer belt 3 first transfers yellow at the first color port, and then sequentially transfers magenta, cyan, and black colors at each port from the photosensitive drums 5M, 5C, and 5K corresponding to the above-described steps. To do. The four color toner images transferred onto the intermediate transfer belt 3 rotate and move in the direction of the arrow (clockwise) in FIG.

  On the other hand, the recording material P stacked and stored in the paper feed cassette 1 is fed by the paper feed roller 2, conveyed to the nip portion of the registration roller pair 6, and temporarily stopped. The recording material P once stopped is supplied to the secondary transfer nip by the registration roller pair 6 in synchronization with the timing when the four color toner images formed on the intermediate transfer belt 3 reach the secondary transfer nip. . The toner image on the intermediate transfer belt 3 is transferred onto the recording material P by applying a voltage (about +1.5 kV) between the secondary transfer roller 11 and the secondary transfer counter roller 18.

  The recording material P onto which the toner image has been transferred is separated from the intermediate transfer belt 3 and sent to the fixing device 14 via the conveyance guide 19 where it is heated and pressurized by the fixing roller 15 and the pressure roller 16. Thus, the toner image is melted and fixed on the surface. Thereby, a four-color full-color image is obtained. Thereafter, the recording material P is discharged from the pair of discharge rollers 20 to the outside of the apparatus, and one printing cycle is completed. On the other hand, the toner remaining on the intermediate transfer belt 3 without being transferred to the recording material P in the secondary transfer portion is removed by the cleaning unit 21 disposed on the downstream side of the secondary transfer portion.

  The image forming apparatus of the present embodiment uses a negative photosensitive drum and toner. The photosensitive drum has a charging potential (Vd) of −500 V, a development potential (Vdc) of −300 V, and exposure when the laser emits all light. The potential (Vl) is set to -100V. When minute light emission for background exposure, which will be described in detail later, is performed, −450 V is obtained as the background potential (Vbg). The laser spot diameter is about 60 μm on the photosensitive drum. In addition, the size of one pixel used for the dither processing described later is about 42 μm × about 42 μm.

(Control system for image formation)
FIG. 2 is a block diagram illustrating a configuration example of an image forming control system according to the first embodiment. A controller 501 provided in the color laser printer 50 controls the entire color laser printer 50. The controller 501 executes printing by controlling the color laser printer 50 according to a print job input via the network 502 in accordance with a request from the server 503 or the client PC 504 via the network 502.

(Image processing flow)
FIG. 3 is a functional block diagram illustrating an image processing flow during printing. The CPU 505 of the controller 501 executes various kinds of image processing including halftone image formation by halftone processing 508 and left / right control 509 by executing a program stored in the ROM 507 using the RAM 506 as a work memory. The memory 510 may be allocated to the RAM 506 or another memory such as a hard disk.

  The halftone processing 508 performs, for example, multilevel dither processing on image data having a bit depth of 8 bits (256 gradations) read from the memory 510, and image data having a bit depth of 5 bits (32 gradations), which will be described in detail later. Convert to The left / right control 509 uses a position control matrix corresponding to the dither matrix used by the halftone processing 508 for the multi-value dither processing, and adds 2-bit position control data representing the dot growth direction, which will be described in detail later. Specifically, it is added to the MSB (Most Significant Bit) side of the image data output by the halftone processing 508. The controller 501 sends 7-bit image data to which the position control data is added to the color laser printer 50.

(Dither matrix)
Next, a dither matrix used by the halftone processing 508 for multi-value (5-bit) dither processing will be described. In general, a color laser printer employs a dither matrix having a different setting for each color. In the following description, a configuration related to an arbitrary color (for example, black color) will be described as a representative.

  FIG. 4 is a diagram showing the growth order corresponding to each position in the dither matrix. Each of the nine baskets is numbered to indicate the growth order from 1 to 9. In the drawing, the left-right direction of the dither matrix represents the main scanning direction of the laser, and the up-down direction represents the sub-scanning direction of the laser. FIG. 5 is a diagram showing a dither matrix for multi-value (5-bit) dither processing according to the growth order shown in FIG.

  The “1” (first) data shown in FIG. 4 is described at the center in the left and right direction and the vertical direction of FIG. 5, and the “2” (second) data shown in FIG. "3" (third) data shown in FIG. As described above, the dither matrix shown in FIG. 5 is also composed of 9 tiles like the matrix shown in FIG. 4, and 31 threshold values corresponding to levels 1 to 31 are set in each tile.

  The halftone processing 508 selects a wrinkle to be referred to in the dither matrix shown in FIG. 5 according to the coordinates of the pixel of the input image data, and based on the threshold value of the selected wrinkle, outputs the 5-bit output image data of the pixel decide. That is, the input image data (input pixel value) is compared with the selected threshold value of the eyelid, and a level that is equal to or higher than the corresponding threshold value and lower than the threshold value by one level is determined as output image data. When the same threshold value is described across a plurality of levels, the output image data is determined to be the highest level that is less than the threshold value of one level and the same threshold value.

(Position control matrix)
FIG. 6 is a diagram illustrating an example of a position control matrix used by the left / right control 509 to add position control data. The number of ridges in the position control matrix is the same as that of the dither matrix, and each ridge corresponds to each ridge of the dither matrix. The left / right control 509 selects a tile to be referred to in the position control matrix according to the coordinates of the pixel of the image data after dithering, and the image after dithering the 2-bit position control data stored in the selected basket. It is added to the MSB side of the data and 7-bit image data is output. Note that the position control matrix shown in FIG.

  “R”, “C”, and “L” shown in FIG. 6 indicate dot growth directions, “R” indicates growth from the right end to the left end of the pixel, and “C” indicates growth from the center to both ends of the pixel. “L” means growth from the left end of the pixel toward the right end. The 2-bit position control data is, for example, R = “01”, C = “00”, and L = “10”.

(PWM table)
FIG. 7 is a diagram (PWM table) showing an example of a pulse signal generated by the color laser printer 50 by PWM control based on output image data of each pixel. The color laser printer 50 performs PWM control by dividing the input 7-bit image data into a lower 5-bit image signal (PWM value) and an upper 2-bit growth control signal (C, L, R) for each pixel. And generate a pulse signal. An integer value between 0 (non-light emission) and 255 (total light emission) is assigned to each level 0 to 31 as the PWM value. As a result, the color laser printer 50 emits a laser and controls the growth of the toner dot image for each pixel.

  FIG. 8 is a diagram for explaining an output result when an image is formed by the above-described image forming apparatus. Light emission patterns at gradation values 0 (level 0), 25 (level 3), 99 (level 12), 148 (level 18), 206 (level 25), and 255 (level 31) are shown. In the figure, one square indicates one pixel, and each square corresponds to each square of the dither matrix. The gray portion displayed in each cage is the laser emission region. As shown in the figure, in the image forming apparatus of this embodiment, as the gradation value increases, a good halftone image can be obtained by increasing the PWM value of each pixel in the pixel order of FIG.

  Note that the dither matrix shown in FIG. 5, the position control matrix shown in FIG. 6, and the PWM table shown in FIG. 7 are held in the ROM 507.

(Dither matrix for background exposure)
In this embodiment, a dither matrix M0 (second dither matrix) for background exposure is used separately from the image dither matrix M1 (first dither matrix) as shown in FIG. In other words, a dither matrix for background exposure is provided separately from the above-mentioned dither matrix for images in order to disperse the frequency components generated by the light emission pattern in terms of frequency and suppress the generation of unnecessary radiation associated with background exposure. It is done.

The image dither matrix M1 is formed for input data with a printing rate greater than 0%,
The dither matrix M0 for background exposure is formed for input data having a printing rate of 0%.

  In this background exposure, the laser emits a small amount of light to such an extent that the toner does not adhere to the photosensitive drum (is not visualized). Laser light emission is performed for a light emission time of one pixel or less, and specifically, by setting a light emission width of about 10% in one pixel, the surface potential of the photosensitive drum is changed from −500 V (Vl) to −450 V (Vd). Reduce the background exposure by -50V. As described above, the surface potential of the photosensitive drum before development can be set to an appropriate potential at which the toner is not developed.

  As shown in FIG. 9, the light emission pattern (exposure pattern) of the background exposure has a light emission pattern of the non-toner image formation region (region where the developer image is not formed) composed of a plurality of pixels continuous in the scanning direction. Arranged so as not to be continuous in the scanning direction. Specifically, FIG. 10 shows an example of a dither matrix for background exposure. In adjacent pixels in the main scanning direction, the light emission width and the light emission region are changed by shifting the light emission start or light emission end timing. Further, in the sub scanning direction, the light emission pattern formed in the main scanning direction is shifted pixel by pixel, and the light emission pattern is not continuous in the sub scanning direction.

  Thus, in the present embodiment, the light emission pattern (exposure pattern) for background exposure is an exposure area of one pixel or less for each pixel and is not continuous in the sub-scanning direction. Further, the generation of unnecessary radiation noise is less than the exposure pattern for images.

(Dither matrix potential setting)
As described above, a mixture of low-density halftone images and non-toner image formation areas results in a mixture of low-level dither matrices for background exposure and image dither matrices, and a gentle drum surface potential. I can't connect them together. That is, the relationship between the drum surface potential after background exposure (for example, −450 V) and the drum surface potential in the image dither matrix is as shown in FIG. As a result, “gradation skip” occurs, and good gradation is not obtained.

  Therefore, in this embodiment, the potential drop of the photoconductor when the image dither matrix is formed on the photosensitive drum, and the potential of the photoconductor when the dither matrix for background exposure is formed on the photoconductor drum. It was set to be more than the amount of descent. That is, A is the potential drop of the photosensitive member when the image dither matrix is formed on the photosensitive drum, and B is the potential drop of the photosensitive member when the background exposure dither matrix is formed on the photosensitive drum. When A ≧ B. The amount of potential drop from −500 V to the position a in FIG. 19 corresponds to A, and the amount of potential drop from −500 V to the position a in FIG.

  When A = B, A indicates the level 0 of the image dither matrix, and the potential drop amount of the photoconductor when the level 0 of the image dither matrix is formed on the photosensitive drum is equivalent to the dither matrix for background exposure. This is equivalent to the potential drop of the photosensitive member when it is formed on the photosensitive drum. When A> B, the following is indicated. That is, A indicates a level greater than level 0 of the image dither matrix. When the image dither matrix at that time is formed on the photosensitive drum, the potential drop amount of the photosensitive member indicates the background exposure dither matrix. Greater than potential drop when formed above.

  In these cases, even if the non-toner image forming area and the low-density halftone image are mixed, “halftone skip” does not occur in the halftone image.

  On the other hand, when A <B, it is shown in FIG. The potential drop amount of the photosensitive member when the level 0 of the image dither matrix is formed on the photosensitive drum is smaller than the potential drop amount of the photosensitive member when the dither matrix for background exposure is formed on the photosensitive drum. At this time, when a non-toner image forming region and a low-density halftone image coexist, “gradation skip” occurs in the halftone image from which continuous gradation should be obtained.

  Here, in this embodiment, each pixel is centered at a PWM value of 25 so that the drum surface potential after background exposure (Vbg = −450 V) is obtained as the level 0 drum surface potential of the image dither matrix. , Left or right light emission. In this embodiment, even if the level 0 of the image dither matrix is set to the PWM value 25, it is equivalent to the drum surface potential (Vbg) after the background exposure, so the toner does not cause charge adhesion (not visualized). . In the image forming apparatus of the present embodiment, if the PWM value is up to about 60, the toner is not charged and adhered on the photosensitive drum by laser emission (not visualized).

  In the PWM table, as shown in FIG. 11, an integer value between 25 (light emission equivalent to background exposure) and 255 (all light emission) is assigned to each level 0 to 31.

  FIG. 12 is a diagram for explaining an output result when image formation is performed by the image forming apparatus of the present embodiment. In FIG. 12, light emission patterns at background exposure, gradation values 25 (level 0), 47 (level 3), 114 (level 12), 159 (level 18), 210 (level 25), and 255 (level 31) are shown. Show. As shown in the figure, in the image forming apparatus of this embodiment, as the gradation value increases, a good halftone image can be obtained by increasing the PWM value of each pixel in the pixel order of FIG. In particular, on the low level side, “gradation skip” (density jump) does not occur, and a gentle gradation can be secured.

(Effect of this embodiment)
As described above, the image forming apparatus according to the present embodiment has a configuration that solves various image problems (such as white gaps) due to background exposure with reduced unwanted radiation noise.

  Furthermore, a dither matrix for background exposure and a dither matrix for images are provided. Then, the potential drop A of the photosensitive member corresponding to the lowest density gradation when exposed through the image dither matrix, and the potential drop of the photosensitive member exposed through the dither matrix for background exposure. For B, the condition of A ≧ B is satisfied. That is, the photosensitive drum surface potential in the light emission pattern formed on the photosensitive drum by the image dither matrix is set to be equal to or higher than the drum surface potential (Vbg) after background exposure. As a result, it is possible to obtain gradation with good density without causing “gradation skip” (density jump).

<< Second Embodiment >>
The configuration of the image forming apparatus according to the second embodiment of the present invention will be described below. The same members as those in the first embodiment are denoted by the same reference numerals, and description thereof is omitted. The image forming system, the image processing apparatus, the image processing flow, and the like are the same as those in the first embodiment, and thus the description thereof is omitted. In the image forming apparatus of this embodiment, the level 0 drum surface potential of the image dither matrix is more accurately matched with the drum surface potential (Vbg) after background exposure, thereby further improving the good gradation. It can be obtained stably.

(Net light emission area of dither matrix)
According to the study by the present inventor, it has been found that the drum surface potential can be adjusted more accurately by combining the net light emitting region in consideration of the light emission loss of the light emitting region. The light emission loss occurs at the rise and fall of the laser light emission, and a region obtained by subtracting the portion is defined as a net light emission region.

  Hereinafter, the net light emitting region will be described in detail. FIG. 13 shows a dither matrix in which light emission regions having the same light emission width A are formed in the adjacent four pixels 1 to 4. The pixels 1 and 4 are disposed in the center of the pixel, the pixel 2 is disposed on the left side of the pixel (upstream in the scanning direction), and the pixel 3 is disposed on the right side of the pixel (downstream in the scanning direction).

  For the pixels 1 to 4, the laser movement is shown below the pixels 1 to 4. The laser is turned on / off in accordance with the light emitting area of the pixel. The operating waveform of the laser is as shown in FIG. Accordingly, the light emitting area where the laser is lost in the pixel 4 is a shaded area, and the net light emitting area is the light emission width B portion. In addition, when the light emitting regions are continuous at the pixel boundary as in the pixels 2 and 3, it can be considered that there is no laser loss at the continuous portion, and the net light emitting region of the pixels 2 and 3 has the light emission width A + B. .

(Dither matrix pattern)
The light emission pattern of the dither matrix in this embodiment is shown in FIG. FIG. 14A is a level 0 dither matrix of the image dither pattern, and FIG. 14B is a background exposure dither matrix. The loss part of the laser is indicated by diagonal lines, and the net light emission area is indicated by gray.

  Since the dither matrix of level 0 of the dither pattern for images has a light emitting region only in the center pixel, the laser loss is once for each rise / fall. Since the dither matrix for background exposure does not count the light emitting region that is continuous at the pixel boundary portion of the broken-line circle frame portion, the laser rise is 5 times and the fall is 4 times. The laser loss was estimated to be about 3%.

  Next, FIG. 15 shows the relationship between the drum potential drop after the background exposure and the light emitting area. FIG. 15A shows the relationship between the drum potential drop after background exposure and the net light emission area, and FIG. 15B shows the relationship between the drum potential drop after background exposure and the conventional light emission area. Yes. It can be seen that the drum potential drop after the background exposure has a correlation with the net emission region. In the present embodiment, since the drum potential drop after background exposure is 50 V, the net light emitting area is about 9.5%.

  As described above, in the present embodiment, in the first (for image) and second (for background exposure) exposure patterns, the light emitting area in which the rising and falling portions of the exposure unit are deleted is defined as the net light emitting area. . In this case, the net light emission area of the first (image) dither matrix is greater than or equal to the net light emission area of the second (background exposure) dither matrix.

(Effect of this embodiment)
As described above, the image forming apparatus according to the present embodiment has a configuration that solves various image problems (such as white gaps) due to background exposure with reduced unwanted radiation noise.

  Furthermore, a dither matrix for background exposure and a dither matrix for images are provided. Then, the potential drop A of the photosensitive member corresponding to the lowest density gradation when exposed through the image dither matrix, and the potential drop of the photosensitive member exposed through the dither matrix for background exposure. For B, the condition of A ≧ B is satisfied. That is, the photosensitive drum surface potential in the light emission pattern formed on the photosensitive drum by the image dither matrix is set to be equal to or higher than the drum surface potential (Vbg) after background exposure. As a result, it is possible to obtain gradation with good density without causing “gradation skip” (density jump).

  Further, the net light emitting area excluding the laser loss part is matched in the light emitting area of level 0 of the dither matrix for background exposure and the dither matrix for image. As a result, the drum surface potential of level 0 of the dither matrix for images can be more accurately matched to the drum surface potential (Vbg) after background exposure, and good gradation can be obtained more stably. Can do.

(Modification)
As mentioned above, although preferable embodiment of this invention was described, this invention is not limited to these embodiment, A various deformation | transformation and change are possible within the range of the summary. Incidentally, the functions, materials, shapes and relative arrangements of the components described in the present embodiment are not intended to limit the scope of the present invention only to those unless otherwise specified.

(Modification 1)
For example, the background exposure herein is not for a particular application.

5 .... Photosensitive drum, 7 .... Primary charging roller, 8 .... Developing roller, 9 .... Exposure means, M1..Dither matrix for image, M0..Dither matrix for background exposure

Claims (6)

A photoreceptor,
Charging means for charging the photoreceptor;
Exposure means for exposing the photosensitive member charged by the charging means based on data by a dither matrix as an exposure pattern;
Developing means for forming a developer image by attaching a developer to the photoreceptor on which a latent image is formed by the exposure means;
An image forming apparatus having
The dither matrix is
A first dither matrix as a first exposure pattern for obtaining a plurality of density gradations in a first region where a developer image of the photoreceptor is formed;
A second dither matrix as a second exposure pattern for performing exposure by the exposure means to such an extent that the developer does not adhere to the photoconductor in a second region where the developer image of the photoconductor is not formed;
With
A potential drop amount of the photoconductor corresponding to the lowest density gradation when exposure is performed through the first dither matrix is A,
When the potential drop amount of the photoconductor when exposing through the second dither matrix is B,
An image forming apparatus satisfying a condition of A ≧ B.   The image forming apparatus according to claim 1, wherein the first dither matrix is formed with respect to input data having a printing rate greater than 0%.   The image forming apparatus according to claim 1, wherein the second dither matrix is formed for input data having a printing rate of 0%.   4. The image forming apparatus according to claim 1, wherein the second exposure pattern generates less unnecessary radiation noise than the first exposure pattern. 5.   5. The image formation according to claim 1, wherein the second exposure pattern is an exposure area of one pixel or less for each pixel and is not continuous in the sub-scanning direction. 6. apparatus.   In the first and second exposure patterns, when the light emitting area from which the rising and falling portions of the exposure means are deleted is used as the net light emitting area, the net light emitting area of the first dither matrix is the second light emitting area. The image forming apparatus according to claim 1, wherein the image forming apparatus is at least a net light emitting region of the dither matrix.