JP6257398B2 - Image forming apparatus - Google Patents

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.

  An image forming apparatus using an electrophotographic process includes an exposure device that irradiates a laser beam to form an electrostatic latent image on a photoreceptor. The exposure machine deflects and scans the laser beam emitted from the semiconductor laser to form an image on the surface of the photoreceptor. The exposure machine includes a light source unit that emits laser light by a semiconductor laser, a deflection scanning unit that deflects and scans laser light from the light source unit by a polygon mirror, and guides laser light from the light source unit to the deflection scanning unit and deflects it by the deflection scanning unit. An optical unit that forms an image of the scanned laser beam on the photosensitive member is provided.

  In such an image forming apparatus, it is inevitable that distortion occurs due to manufacturing errors and assembly errors of components and supports of the exposure machine. Due to the distortion, the light intensity distribution (hereinafter referred to as “spot shape”) that forms an image on the surface of the photoreceptor may be deformed. The deformation of the spot shape occurs differently depending on the position in the scanning direction (longitudinal direction) on the photoreceptor. The deformation of the spot shape appears as a change in the density or line width of the output image, and is visually recognized as image quality unevenness. As another exposure machine, there is one that exposes the surface of a photoreceptor using a light source such as a plurality of LEDs (Laser Electric Diodes) arranged on a line. Even in such an exposure machine, similar image quality unevenness may be visually recognized due to variations in the spot shape of each light source.

  Patent Documents 1 and 2 disclose techniques for correcting such image quality unevenness. In Patent Document 1, light amount correction data calculated in advance is stored for each position in the longitudinal direction on the photoconductor so that density unevenness is corrected and uniformed, and light amount correction is performed based on the light amount correction data. Do. In Patent Document 2, as a method of correcting density unevenness caused by unevenness of the potential attenuation characteristic of the photoconductor, a characteristic table is stored over the entire surface of the photoconductor, and light quantity correction is performed according to the characteristic table.

JP 2007-62100 A JP 2002-67387 A

  The techniques of Patent Documents 1 and 2 only adjust the total amount of light emitted to the target pixel, and cannot match the light amount distribution. The “target pixel” is a pixel that is currently a processing target (exposure target). Since the light quantity distribution cannot be matched, the reproduction of isolated points and fine lines, and the gamma characteristics change. Therefore, the amount of light emitted to the surrounding pixels (a plurality of surrounding pixels including the target pixel) is controlled by PWM (Pulse Width Modulation) so that the integrated light amount distribution obtained by integrating the light amount distributions of the peripheral pixels matches the desired spot shape. There has been proposed a correction method.

  However, when the light emission amount to the peripheral pixels is corrected by PWM control, the number of correction candidates (hereinafter referred to as “correction degree of freedom”) is finite. In this case, the correction degree of freedom is limited to the PWM division number and the control range (range of surrounding pixels). From the finite number of correction degrees of freedom, the condition that the accumulated light amount distribution of the peripheral pixels is closest to the desired spot shape is selected and used.

  However, even when the light emission amount of the peripheral pixels is corrected by PWM control, the integrated light amount distribution of the peripheral pixels does not always match the spot shape, and a shape error may occur. Further, the center of gravity position of the accumulated light amount distribution of the peripheral pixels is preferably the center position of the target pixel, but the center of gravity error occurs due to a shift. Due to the occurrence of correction errors (shape error and gravity center error), a sufficient correction effect by PWM control cannot be obtained.

  As the degree of freedom of correction determined according to the number of PWM divisions and the control range is lower, the correction error becomes larger. Although the correction error can be reduced by increasing the number of PWM divisions, in this case, the apparatus becomes complicated and the cost increases.

  In order to solve the above problems, it is a main object of the present invention to provide an image forming apparatus that corrects image unevenness caused by deformation of a spot shape by appropriately adjusting a light emission amount.

  The image forming apparatus of the present invention that solves the above problems includes a photoconductor on which an image is formed by light irradiation, an exposure device that irradiates the photoconductor with light whose light emission amount is determined by pixel data, and the photoconductor. Based on the input pixel data for the upper pixel of interest and an error from a predetermined light amount distribution caused by the processing for the immediately preceding pixel, peripheral pixel data of peripheral pixels that are peripheral pixels including the target pixel is acquired. And a first calculation means for calculating an error caused by the peripheral pixel data, and a second calculation means for generating the pixel data of each pixel according to the peripheral pixel data acquired by the first calculation means. And.

  According to the present invention, in order to obtain peripheral pixel data in consideration of an error from a predetermined light amount distribution caused by processing on the immediately preceding pixel, the light emission amount is appropriately adjusted, and is generated by deformation of the spot shape. Image unevenness can be corrected.

1 is a configuration example diagram of an image forming apparatus. Explanatory drawing of the deformation | transformation state of a spot shape. (A), (b) is a figure showing integrated light quantity distribution. The figure showing the shift | offset | difference of the gravity center position of a dot. The figure showing the relationship between a pixel space | interval and a spot diameter. Functional block diagram of the controller. FIG. 4 is an exemplary diagram of a spot diameter table. FIG. 4 is an exemplary diagram of a peripheral pixel data table. Explanatory drawing of the surrounding pixel data obtained by 1st calculation. Explanatory drawing of a 2nd calculation. The flowchart of the 1st calculation. (A), (b) is explanatory drawing of a spot diameter. Explanatory drawing of a spot diameter error. (A)-(c) is explanatory drawing of deterioration of the image quality by a spot diameter error. Functional block diagram of the controller. FIG. 4 is an exemplary diagram of a peripheral pixel data table. The flowchart of the 1st calculation.

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

[First Embodiment]
<Configuration of image forming apparatus>
FIG. 1 is an exemplary configuration diagram of an image forming apparatus according to the present embodiment. The image forming apparatus 100 scans a photosensitive member with a laser beam to perform image forming processing. The image forming apparatus 100 performs PWM control on the light emission amount of the laser light according to the pixel data of each pixel input from the controller 400. For this purpose, the image forming apparatus 100 includes a PWM signal generation unit 260 and a laser drive circuit 270. The pixel data is data for determining the amount of laser light emitted for each pixel. The controller 400 inputs the pixel data of all the pixels to the image forming apparatus 100 as image data of one image.

  The image forming apparatus 100 includes image forming units 250a, 250b, 250c, and 250d in order to form images of each color of cyan (C), magenta (M), yellow (Y), and black (K). The image forming unit 250a forms a cyan toner image. The image forming unit 250b forms a magenta toner image. The image forming unit 250c forms a yellow toner image. The image forming unit 250d forms a black toner image. The image forming units 250a, 250b, 250c, and 250d are provided along the intermediate transfer belt 210. Intermediate transfer belt 210 is driven to rotate in the direction of arrow R1. The intermediate transfer belt 210 is rotationally driven to sequentially transfer (primary transfer) the toner images formed by the image forming units 250a, 250b, 250c, and 250d.

  The toner image transferred to the intermediate transfer belt 210 is transferred (secondary transfer) to a recording medium such as a sheet conveyed in the direction of the arrow R2 by the secondary transfer unit 220. As a result, the toner image is transferred to the recording medium. Residual toner remaining on the intermediate transfer belt 210 after the secondary transfer is removed by the intermediate transfer belt cleaner 240.

  The recording medium on which the toner image is transferred is conveyed to the fixing unit 230. The fixing unit 230 performs, for example, thermocompression bonding of the toner image transferred to the recording medium. Thereafter, the recording medium is discharged outside the image forming apparatus 100. In this way, the image forming process on the recording medium is performed.

  The configuration of the image forming units 250a, 250b, 250c, and 250d will be described. As described above, the image forming unit 250a forms a cyan toner image, the image forming unit 250b forms a magenta toner image, the image forming unit 250c forms a yellow toner image, and the image forming unit 250d forms a black toner image. The toner images are formed in parallel in the order of the image forming units 250a, 250b, 250c, and 250d, with the timing being shifted by a certain time. The color of the image formed by the image forming units 250a, 250b, 250c, and 250d is not limited to this, and for example, light ink or clear ink may be used. Further, the arrangement of the image forming units 250a, 250b, 250c, and 250d is not limited to that shown in FIG. Hereinafter, the configuration of the image forming unit 250a will be described. Since the image forming units 250b, 250c, and 250d have the same configuration as the image forming unit 250a, description thereof is omitted.

  The image forming unit 250a includes a photosensitive drum 251a that is a drum-shaped photosensitive member. Around the photosensitive drum 251a, a charger 252a, an exposure unit 253a, a developing unit 254a, and a cleaner 256a are provided.

  The photosensitive drum 251a has an organic photoconductor layer with a negative polarity on the outer peripheral surface, and rotates in the direction of arrow R3. The charger 252a receives a negative voltage and irradiates the surface of the photosensitive drum 251a with charged particles, thereby charging the surface of the photosensitive drum 251a to a uniform negative potential.

  The photosensitive drum 251a whose surface is charged is irradiated with laser light from an exposure unit 253a. The exposure machine 253a emits laser light according to the image data input from the controller 400. The controller 400 inputs image data to the PWM signal generation unit 260. The PWM signal generation unit 260 generates a PWM signal for instructing the lighting pulse width of the laser beam for each pixel according to the image data, and inputs the PWM signal to the laser driving circuit 270. The number of PWM divisions can be arbitrarily determined. The laser driving circuit 270 drives the exposure unit 253a according to the PWM signal to irradiate the laser beam. By irradiating the laser beam, an electrostatic latent image corresponding to the image data is formed on the surface of the photosensitive drum 251a.

  The developing device 254a supplies a negatively charged developer (toner in the present embodiment) to the photosensitive drum 251a using a developing roller that rotates at a substantially constant speed. The developing device 254a supplies cyan toner to the photosensitive drum 251a. As a result, toner adheres to the electrostatic latent image formed on the surface of the photosensitive drum 251a, and the electrostatic latent image is reversely developed to form a toner image. In the present embodiment, the developing roller is formed by coating a conductive rubber whose electric resistance is adjusted by dispersing carbon on an aluminum cylinder.

  On the photosensitive drum 251a on which the toner image is formed, the toner image is transferred to the intermediate transfer belt 210 by the primary transfer unit 255a. The primary transfer unit 255 a primarily applies a negatively charged toner image to the intermediate transfer belt 210 by applying a positive charge. The residual toner image remaining on the photosensitive drum 251a after the primary transfer is removed by the cleaner 256a. After the primary transfer, the secondary transfer and fixing steps are performed as described above. Hereinafter, the photoconductive drums 251a, 251b, 251c, and 251d are referred to as photoconductive drums 251 when they are not distinguished.

<Change in spot shape>
The spot shape changes depending on the imaging position of the laser light irradiated onto the photosensitive drum 251. FIG. 2 is a diagram illustrating a change state of the spot shape depending on the position of the photosensitive drum 251 in the longitudinal direction. The photosensitive drum 251 is scanned with laser light in the longitudinal direction to form an electrostatic latent image. Therefore, the spot shape changes depending on the position on the photosensitive drum 251 in the longitudinal direction. In FIG. 2, the spot shape at the end in the longitudinal direction of the photosensitive drum 251 changes in the circumferential direction of the photosensitive drum 251 as compared with the spot shape near the center. In the present embodiment, a case where such a change in spot shape is corrected will be described.

  When the spot shape changes, if all the pixels are irradiated with the same light amount, the light amount distribution differs for each pixel due to the difference in spot diameter. In the present embodiment, dot formation is performed with a set of a plurality of pixels, and the light amount distribution is adjusted by changing the weight of pixel data of each pixel. Pixel data of each pixel is determined by the first calculation and the second calculation performed by the controller 400. The spot shape is corrected by the first calculation and the second calculation. In the first calculation, pixel data (peripheral pixel data) to be attached to surrounding pixels is calculated for all the pixels in order to correct changes in the spot shape. In the second calculation, the pixel data of each pixel is determined by adding the peripheral pixel data calculated for all the pixels in the first calculation.

  FIG. 3 shows peripheral pixel data calculated by the first calculation and an integrated light amount distribution when an image is formed based on the peripheral pixel data. The peripheral pixels are composed of pixels adjacent to the target pixel in the circumferential direction of the photosensitive drum 251. 3A shows a case where the spot diameter is small, and FIG. 3B shows a case where the spot diameter is large.

  As shown in FIG. 3A, when the spot diameter is small, the pixel data of the target pixel is distributed to a large number of peripheral pixels (peripheral pixels) around the target pixel to increase the integrated light amount distribution in the peripheral pixels. Can be spread. In FIG. 3A, the peripheral pixel data is set to “1”, “1”, “1”, that is, the pixel data is distributed to the peripheral pixels with a weight of 1: 1: 1, thereby increasing the spot diameter. spread.

  When the spot diameter is large as shown in FIG. 3B, the accumulated light quantity distribution in the peripheral pixels is reduced by distributing the pixel data of the pixel of interest to the three peripheral pixels (peripheral pixels) around the pixel of interest. Can be spread. In FIG. 3B, the spot diameter is reduced by setting the peripheral pixel data to “1”, “2”, and “0”, that is, by distributing the pixel data to the peripheral pixels with a weight of 1: 2: 0. To do.

  By selecting peripheral pixel data having different weights with respect to the spot diameter in this way, even if the spot diameter is different at each position, the integration in the peripheral pixels (“1 × 3” peripheral pixels in FIG. 3). The light quantity distribution can be made uniform. That is, the light quantity distribution for each pixel is represented by the thin line in FIG. 3, but the accumulated light quantity distribution becomes the thick line in FIG. 3 by integrating the peripheral pixels, and even if the spot diameters are different. Light quantity distribution.

  However, when different PWM control is performed for pixels adjacent in the circumferential direction, a deviation (error) occurs in the barycentric position of the light amount distribution forming the dots. FIG. 4 is a diagram illustrating such a shift. The center of gravity position of the accumulated light amount distribution forming the dots ideally overlaps the center of the target pixel, but in reality, there may be a deviation in the circumferential direction. Due to the deviation from the center of gravity position of the ideal integrated light amount distribution, the distance from other adjacent dots changes, the gamma characteristic changes, and image quality deterioration such as image density unevenness occurs.

  In this embodiment, in order to suppress image quality deterioration due to an error in the center of gravity position of the light amount distribution, an error in the center of gravity position that occurs when performing spot shape correction processing is calculated, and the error calculated in the processing for subsequent pixels Offset. For this reason, in the present embodiment, the peripheral pixel data to be selected is determined according to the pixel data of the target pixel, the spot diameter at the position of the target pixel, and the error of the centroid position due to the processing for the immediately preceding pixel.

  In this embodiment, processing is performed using “1 × 3” peripheral pixel data, but the size of the peripheral pixel data is not limited to this. For example, as shown in FIG. 5, when the spot diameter is larger than the pixel interval, a plurality of spots overlap. In this case, by increasing the number of pixels included in the peripheral pixels and performing calculations including the adjacent pixels and further adjacent pixels, the light amount distribution is corrected for larger spot diameter fluctuations.

  FIG. 6 is a functional block diagram of the controller 400. The controller 400 includes an image input unit 410, a color separation processing unit 420, a gamma correction processing unit 430, a halftone processing unit 440, and a spot diameter correction processing unit 450. These functional blocks are realized by, for example, a CPU (Central Processing Unit) executing a predetermined computer program.

  The image input unit 410 acquires multi-value (for example, 8 bits for each color) RGB data from the outside. The color separation processing unit 420 converts the RGB data acquired by the image input unit 410 into CMYK data. The gamma correction processing unit 430 performs gamma correction processing on the CMYK data with reference to a density correction table stored in advance. The CMYK data is converted into C1M1Y1K1 data by the gamma correction process. The halftone processing unit 440 performs intermediate modulation processing on the C1M1Y1K1 data and converts it into, for example, 2-bit C2M2Y2K2 data. Note that the number of bits of each data can be arbitrarily determined.

  The spot diameter correction processing unit 450 includes a first calculation unit 451, a second calculation unit 452, a peripheral pixel data storage unit 453, and a spot diameter storage unit 454. The spot diameter correction processing unit 450 receives C2M2Y2K2 data as input pixel data, and performs spot shape correction processing by the first calculation and the second calculation described above.

  The spot diameter storage unit 454 stores a spot diameter table representing the relationship between the position of the photosensitive drum 251 in the longitudinal direction and the spot diameter (integrated light amount distribution) at each position. FIG. 7 is an exemplary view of a spot diameter table. The position in the longitudinal direction set in the spot diameter table is represented by a relative position in the longitudinal direction by normalizing the length in the longitudinal direction to “2” with reference to the longitudinal center of the photosensitive drum 251. The spot diameter is set in advance, for example, by measuring it during assembly adjustment.

  The peripheral pixel data storage unit 453 stores a peripheral pixel data table representing peripheral pixel data corresponding to the pixel data (input pixel data) of the target pixel and the spot diameter. FIG. 8 is an exemplary diagram of a peripheral pixel data table. Pixel data of the target pixel is distributed to pixel data of peripheral pixels. Weighting is performed for each peripheral pixel, and pixel data of the target pixel is distributed according to the weighting. For example, pixel data “3” of the target pixel is distributed into pixel data “1”, “2”, and “0” of peripheral pixels. In the peripheral pixel data table, the barycentric position (corrected barycentric position) based on the integrated light quantity distribution when an image is formed based on each peripheral pixel data is also recorded. The corrected barycentric position is expressed as a relative position in the circumferential direction in which the distance between adjacent pixels is normalized to “1” with the target pixel as a reference. The center-of-gravity shift due to the weighting of surrounding pixels is corrected by the corrected center-of-gravity position. The corrected barycentric position is an example of a characteristic value.

  The first calculation unit 451 performs the first calculation described above for each pixel of the C2M2Y2K2 data using the spot diameter table and the peripheral pixel data table. The first calculation unit 451 performs peripheral pixel data according to the pixel data (input pixel data) of the target pixel and the spot diameter corresponding to the position of the target pixel read from the spot diameter storage unit 454 by the first calculation. Are read from the peripheral pixel data table. FIG. 9 is an explanatory diagram of peripheral pixel data obtained by the first calculation. FIG. 9 shows an example in which the peripheral pixel data is read with the pixels “a” to “i” as the target pixel. For example, “1 × 3” peripheral pixel data “a1” to “a3” are read for the pixel “a”.

The second calculation unit 452 performs a second calculation based on the calculation result (peripheral pixel data) by the first calculation unit 451 and generates pixel data of each pixel. In the first calculation described above, “1 × 3” peripheral pixel data is acquired for each pixel. For this reason, the number of pixels after the first calculation increases with respect to the number of pixels. In the second calculation, in order to equalize the number of pixels input / output to / from the spot diameter correction processing unit 450, an addition process with selected peripheral pixel data is performed on neighboring pixels. FIG. 10 is an explanatory diagram of the second calculation. In the second calculation, the calculation result of the peripheral pixel and the calculation result of the target pixel are added. The formula is shown below.
(Second calculation result) = (Upper pixel) 3+ (Target pixel) 2+ (Lower pixel) 1

For example, as a result of the second calculation for the pixel of interest e in FIG. 9, the pixel data of the pixel E is obtained by Calculated.
E = b3 + e2 + h1

  Thus, the number of pixels input / output to / from the spot diameter correction processing unit 450 can be made equal by the second calculation. However, when the number of pixels input to the spot diameter correction processing unit 450 is one third of the number of beams, the pixel data of each pixel is obtained based on the first calculation result without performing the second calculation. It may be generated.

  FIG. 11 is a flowchart of the first calculation executed by the first calculation unit 451 of the controller 400.

The first calculation unit 451 selects a pixel to be corrected (S901). In the present embodiment, correction processing is performed on all pixels. Here, one pixel to be processed (target pixel) is selected from all the pixels. The first calculation unit 451 that has selected the target pixel calculates the target center-of-gravity position of the dot formed including the target pixel (S902). As described with reference to FIG. 4, the PWM control based on the peripheral pixel data causes an error in the center of gravity position of the dots. Therefore, it is necessary to shift the position of the center of gravity of the dots in a direction that cancels the deviation. A target center-of-gravity position as a target at that time is calculated and set by the following equation using a center-of-gravity error described later.
(Target centroid position) =-(centroid error of previous pixel)

  However, the target center-of-gravity position is “0” for the pixel that is first processed in each scanning line. The target center-of-gravity position is expressed as a relative position in the circumferential direction in which the distance between adjacent pixels is normalized to “1” with the target pixel as a reference.

  After determining the target center-of-gravity position, the first calculation unit 451 acquires the spot diameter of the pixel of interest (S903). The first calculation unit 451 refers to the spot diameter table (see FIG. 7) stored in the spot diameter storage unit 454 and acquires a spot diameter corresponding to the position of the photosensitive drum 251 in the longitudinal direction of the target pixel. . When the spot diameter corresponding to the position of the target pixel is stored in the spot diameter table, the first calculation unit 451 acquires the spot diameter. When the spot diameter corresponding to the position of the target pixel is not stored in the spot diameter table, the first calculation unit 451 acquires the spot diameter corresponding to the position closest to the position of the target pixel, for example.

  The first calculation unit 451 that has acquired the spot diameter acquires peripheral pixel data (S904). The first calculation unit 451 selects the peripheral pixels stored in the peripheral pixel data storage unit 453 according to the pixel data (input pixel data) of the target pixel, the target center coordinates calculated in S902, and the spot diameter acquired in S903. Obtain peripheral pixel data from the data table. The first calculation unit 451 has the corrected centroid position closest to the target centroid position calculated in S902 among the pixel data of the target pixel and the peripheral pixel data corresponding to the spot diameter acquired in S903 from the peripheral pixel data table. Select. The first calculation unit 451 acquires the selected peripheral pixel data and the corresponding corrected centroid position. When there are a plurality of corresponding peripheral pixel data, the first calculation unit 451 selects an arbitrary one according to a predetermined rule.

The first calculation unit 451 that has acquired the peripheral pixel data and the corrected barycentric position calculates a barycentric error (S905). The first calculation unit 451 calculates a centroid error from the following equation based on the target centroid position calculated in S902 and the corrected centroid position acquired in S904.
(Centroid error) = (corrected centroid position)-(target centroid position)

  The first calculation unit 451 performs the above-described processing of S901 to S905 for all pixels. When the processing is completed for all the pixels, the first calculation is finished (S906: Y).

  As described above, in the present embodiment, the pixel data of the target pixel and the peripheral pixels are calculated according to the spot diameter, and an image having a uniform density is formed on dots formed by a plurality of pixels. In addition, the center-of-gravity error generated by the calculation for each pixel is calculated, and the error is canceled by the processing for the subsequent pixels. Thereby, even if it is a simple structure, generation | occurrence | production of a gravity center error can be suppressed. In addition, an image with little variation in density can be formed.

  The above has been described in the case where the spot shape at the end of the photosensitive drum 251 is deformed in the circumferential direction as compared with the center spot shape. In addition, even when the spot shape is deformed in the longitudinal direction of the photosensitive drum 251, the correction can be performed by the same process as described above. Further, when the spot shape is deformed in the circumferential direction and the longitudinal direction of the photosensitive drum 251, correction can be performed by the same processing.

[Second Embodiment]
In the first embodiment, paying attention to the fact that an error in the center of gravity occurs during the spot shape correction process, this process is canceled by the process for the subsequent pixels. In the second embodiment, focusing on the fact that a spot diameter error occurs during the spot shape correction process, the subsequent pixel process cancels this.

  In the second embodiment, similarly to the first embodiment, image formation is performed by scanning laser light on the photosensitive drum 251 using the image forming apparatus 100 of FIG. In the semiconductor laser, the light emission amount is PWM-controlled. Further, the spot shape formed at the end in the longitudinal direction of the photosensitive drum 251 is deformed in the circumferential direction as compared with the spot shape formed at the center in the longitudinal direction of the photosensitive drum 251 as shown in FIG. .

FIG. 12 is an explanatory diagram of a spot diameter according to the second embodiment. As shown by a broken line in FIG. 12A, the spot diameter has a constant width centered on the center of gravity of the light amount distribution in the entire region in the longitudinal direction of the photosensitive drum 251 and in the circumferential direction of the photosensitive drum 251. The integral light quantity obtained by integrating the light quantity by (integration width) is represented by a width that becomes a predetermined reference value. FIG. 2B is a diagram illustrating the relationship between the integrated light amount and the integration width. The reference value of the integrated light quantity is expressed by the following equation using an error function erf, for example.
(Reference value) = (total integrated light quantity) × erf (2 ^ (1/2))

  In the spot shape correction process, the spot diameter is fixed regardless of the position of the photosensitive drum 251 in the longitudinal direction. That is, by performing PWM control on the light emission amount to the peripheral pixels, correction is performed so that the spot diameter (integrated spot diameter) based on the integrated light amount distribution of the peripheral pixel data matches a desired value. When the peripheral pixels are corrected by PWM control, there are only two degrees of freedom for correction: the PWM division number and the control range. In this way, the finite number of correction degrees of freedom is selected and used so that the integrated spot diameter is closest to the desired value. However, the integrated spot diameter does not always match the desired spot diameter.

  FIG. 13 is an explanatory diagram of a spot diameter error that occurs when the peripheral spot is corrected by PWM control and the integrated spot diameter does not match the desired spot diameter. FIG. 13 shows a case where the integrated spot diameter is smaller than the desired spot diameter. The spot diameter error leads to image quality deterioration such as a change in image appearance density. With reference to FIG. 14, image quality deterioration due to spot diameter error will be described. FIG. 14A shows an image when there is no spot diameter error. FIG. 14B shows an image when the spot diameter is smaller than a desired value. In this case, the appearance density of the image is lower than the image without the spot diameter error. FIG. 14C shows an image when the spot diameter is larger than a desired value. In this case, the visible density of the image is higher than the image without the spot diameter error.

  In the second embodiment, when spot shape correction processing is performed, a spot diameter error that occurs is calculated, and the spot diameter error is canceled during correction processing for subsequent pixels. FIG. 15 is a functional block diagram of the controller 401 according to the second embodiment. The controller 401 and the controller 400 of the first embodiment in FIG. 6 are different in the configuration of the spot diameter correction processing unit. The spot diameter correction processing unit 455 of the second embodiment is configured by removing the spot diameter storage unit 454 from the spot diameter correction processing unit 450 of the first embodiment. The first calculation unit 456 performs the first calculation by a process different from that of the first calculation unit 451 of the first embodiment, and the peripheral pixel data table stored in the peripheral pixel data storage unit 457 is also the first. Different from the embodiment. Similar to the first embodiment, these functional blocks are also realized by, for example, the CPU executing a predetermined computer program.

  FIG. 16 is an exemplary diagram of a peripheral pixel data table stored in the peripheral pixel data storage unit 457. The peripheral pixel data table records peripheral pixel data corresponding to the position in the longitudinal direction on the photosensitive drum 251. The position in the longitudinal direction is represented by a relative position in the longitudinal direction with the length in the longitudinal direction normalized to “2” with reference to the center in the longitudinal direction of the photosensitive drum 251. In the peripheral pixel data table, the integrated spot diameter by the integrated light amount distribution when an image is formed based on the peripheral pixel data is also recorded. The integrated spot diameter is acquired and recorded in advance, for example, by measuring during assembly adjustment. In this example, the peripheral pixel data and the integrated spot diameter at the position in the longitudinal direction “−1” are recorded as the peripheral pixel data table. The integrated spot diameter is an example of a characteristic value. The peripheral pixel data storage unit 457 stores a similar peripheral pixel data table for each position in the longitudinal direction.

  FIG. 17 is a flowchart of the first calculation of the second embodiment executed by the controller 401.

  In the second embodiment, when spot shape correction processing is performed, the spot diameter is made constant regardless of the position of the photosensitive drum 251. The target spot diameter (desired spot diameter in FIG. 13) is referred to as “global target spot diameter” in the present embodiment. The first calculation unit 456 first sets the global target spot diameter (S1601). When the pixel data is multivalued, the first calculation unit 456 sets a global target spot diameter for all values.

  The first calculation unit 456 that has set the global target spot diameter selects a pixel to be corrected (S1602). In the present embodiment, correction processing is performed on all pixels. Here, one pixel to be processed (target pixel) is selected from all the pixels. The first calculation unit 456 sequentially selects pixels on the same scanning line in order from one direction.

The first calculation unit 456 that has selected the target pixel sets a local target spot diameter (S1603). The first calculation unit 456 deforms the spot diameter of the integrated light amount distribution formed corresponding to the target pixel by an amount that cancels the spot diameter error that occurred immediately before. The target value of the spot diameter at this time is the “local target spot diameter”. The local target spot diameter is obtained by the following equation, for example, using a spot diameter error of the immediately preceding pixel described later.
(Local target spot diameter) = (Global target spot diameter)-(Spot diameter error)

  However, the spot diameter error is “0” for the pixel to be processed first in each scanning line. When the pixel data is multivalued, the global target spot diameter corresponds to the pixel data of the target pixel.

  The first calculation unit 456 refers to the peripheral pixel data table stored in the peripheral pixel data storage unit 457 and acquires an integrated spot diameter corresponding to the peripheral pixel data corresponding to the position of the target pixel and the local target spot diameter. (S1604). The position of the target pixel is represented with the center in the longitudinal direction of the photosensitive drum 251 as in the first embodiment. The first calculation unit 451 selects the peripheral pixel data corresponding to the position of the pixel of interest in the longitudinal direction of the photosensitive drum 251 and having the integrated spot diameter closest to the local target spot diameter. Then, the selected peripheral pixel data and the corresponding integrated spot diameter are acquired. When there are a plurality of corresponding peripheral pixel data, one is selected according to a predetermined rule.

The first calculation unit 456 that has acquired the peripheral pixel data calculates an error of the spot diameter (S1605). The first calculation unit calculates an error of the spot diameter according to, for example, the following expression in accordance with the local target spot diameter and the integrated spot diameter.
(Spot diameter error) = (Integrated spot diameter)-(Local target spot diameter)

  The first calculation unit 456 performs the processing of S1602 to S1605 as described above for all pixels. When the processing is completed for all the pixels, the first calculation is finished (S1606: Y).

  By performing the first calculation of the second embodiment as described above, the pixel data of the pixel of interest and the peripheral pixels are calculated according to the spot diameter, and dots composed of a plurality of pixels can be formed with uniform image quality. it can. Further, a spot diameter error caused by the calculation for each pixel is calculated, and the error is canceled in the processing for the subsequent pixels. Thereby, generation | occurrence | production of a spot diameter error can be suppressed with a simple structure. Therefore, it is possible to form a high-quality image with little change in the visible density.

  In the peripheral pixel data table, the integrated spot diameter is acquired and held in advance by measurement or the like during assembly adjustment. However, the present invention is not limited to this. For example, the spot shape at each position may be stored, and after estimating the integrated light amount distribution from the spot shape and peripheral pixel data, the spot diameter of the predicted integrated light amount distribution may be calculated and used.

  In the first and second embodiments, the light amount of each pixel is set by PWM control. However, the present invention is not limited to this, as long as the light emission amount of each pixel can be adjusted in multiple stages. For example, it may be configured to control the laser drive current. Further, in the processing for each pixel, the center-of-gravity error and spot diameter error of the integrated light quantity distribution of the peripheral pixels are calculated and canceled, but the peak light quantity of the integrated light quantity distribution may be constant. That is, the process may be performed so that the integrated light quantity distribution or the characteristic value calculated from the integrated light quantity distribution is constant.

  250a, 250b, 250c, 250d ... image forming unit, 251,251a, 251b, 251c, 251d ... photosensitive drum, 210 ... intermediate transfer belt, 220 ... secondary transfer unit, 230 ... fixing unit

Claims (9)

A photoreceptor on which an image is formed by light irradiation;
An exposure unit that irradiates the photosensitive member with light whose light emission amount is determined by pixel data;
Based on input pixel data for the target pixel on the photoconductor and an error from a predetermined light amount distribution caused by processing to the previous pixel, peripheral pixel data of peripheral pixels that are a plurality of peripheral pixels including the target pixel And a first calculation means for calculating an error caused by the surrounding pixel data,
A second calculation unit that generates the pixel data of each pixel in accordance with the peripheral pixel data acquired by the first calculation unit.
Image forming apparatus. A recording unit that records characteristic values obtained from a light amount distribution based on the peripheral pixel data for a plurality of positions on the photoconductor;
The first calculation means calculates the error based on a characteristic value acquired from the recording means according to the position of the target pixel.
The image forming apparatus according to claim 1. The first calculation means sets a target value of the characteristic value, and acquires the peripheral pixel data according to the target value, the input pixel data, and the position of the target pixel on the photoconductor. It is characterized by
The image forming apparatus according to claim 2. The first calculation means calculates the error from a characteristic value obtained from a light amount distribution based on the acquired peripheral pixel data and the target value,
The image forming apparatus according to claim 3. The first calculation means sets the target value according to an error from a predetermined light amount distribution caused by processing on the immediately preceding pixel,
The image forming apparatus according to claim 3 or 4. The characteristic value is a centroid position of a light amount distribution based on the peripheral pixel data,
The image forming apparatus according to claim 2. The characteristic value is a spot diameter of a light amount distribution based on the peripheral pixel data,
The image forming apparatus according to claim 2. The characteristic value is a peak light quantity of a light quantity distribution based on the peripheral pixel data,
The image forming apparatus according to claim 2. The second calculation means calculates pixel data of the target pixel based on peripheral pixel data obtained from the target pixel and peripheral pixel data obtained from a pixel adjacent to the target pixel.
The image forming apparatus according to claim 1.

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