JP6812216B2 - Image forming device - Google Patents

Description

The present invention relates to, for example, an image forming apparatus such as a laser beam printer, a digital copier, and a digital fax machine, and more particularly to an image forming apparatus that forms an image by a light beam.

The electrophotographic image forming apparatus has an optical scanning apparatus for exposing the photoconductor. The optical scanning device reflects a light beam with a rotating multifaceted mirror to scan the surface of the photoconductor, thereby forming an electrostatic latent image on the photoconductor. The light beam reflected by the rotating polymorphic mirror usually scans the photoconductor through a scanning lens having an fθ characteristic. The fθ characteristic is an optical characteristic that forms an image of a light beam on the surface of a photoconductor so that a spot of the light beam moves at a constant velocity on the surface of the photoconductor when the rotating multifaceted mirror is rotating at a constant angular velocity. Means. By using a scanning lens having an fθ characteristic, an appropriate exposure can be performed using an image clock having a fixed period. However, a scanning lens having an fθ characteristic is relatively large and costly. Therefore, for the purpose of miniaturization and cost reduction of the image forming apparatus, it is considered that the scanning lens itself is not used or the scanning lens having no fθ characteristic is used.

Patent Document 1 has a configuration in which electrical correction is performed so that the reproducibility of an image formed on the surface of the photoconductor is constant even when the spot of the light beam does not move on the surface of the photoconductor at a constant velocity. It is disclosed. Patent Document 1 also discloses that the light intensity is controlled to be substantially equal in the scanning line.

Japanese Unexamined Patent Publication No. 2-131212

The image forming apparatus is required to maintain the quality of the formed image to be equal to or higher than a predetermined quality. Here, increasing the reproducibility of the isolated pixels is one of the conditions for maintaining the quality of the image. This also applies, for example, when a scanning lens that does not have the fθ characteristic described in Patent Document 1 is used.

The present invention provides an image forming apparatus that enhances the reproducibility of isolated pixels.

According to one aspect of the present invention, the image forming apparatus is an exposure means for forming an electrostatic latent image on the photoconductor by dividing one pixel into a plurality of pixel pieces and exposing the pixel pieces as a unit. A determination means for determining an isolated pixel surrounded by blank pixels of the electrostatic latent image based on image data, a selection means for selecting a correction target pixel from blank pixels around the isolated pixel, and the correction target pixel. The selection is provided with a correction means for correcting the image data so as to expose the pixel piece of the above , and a storage means for holding information indicating the relationship between the position of the pixel in the main scanning direction and the correction amount of the pixel size. The means determines whether or not the isolated pixel is the size correction target based on the position of the isolated pixel in the main scanning direction and the information held by the storage means, and the isolated pixel is the size correction target. If this is the case, the correction target pixel is selected .

According to the present invention, the reproducibility of isolated pixels can be improved.

The block diagram of the image forming apparatus by one Embodiment. The block diagram of the optical scanning apparatus according to one Embodiment. The figure which shows the relationship between the image height and a partial magnification by one Embodiment. The figure which shows the LSF profile by one Embodiment. The figure which shows the exposure control composition by one Embodiment. The figure which shows the time chart of the synchronization signal and the image signal by one Embodiment, and the dot image of the on-axis image height and the most off-axis image height. The block diagram of the image modulation part by one Embodiment. Explanatory drawing of the screen by one Embodiment. A time chart relating to the operation of the image modulation unit according to one embodiment. Explanatory drawing of halftone processing by one Embodiment. Explanatory drawing of insertion / removal of pixel piece by one Embodiment. The figure which shows the relationship between the electric current and the brightness of the light emitting part by one Embodiment. A time chart illustrating partial magnification correction and brightness correction according to an embodiment. The figure which shows the exposure energy distribution in the on-axis image height and the most off-axis image height by one Embodiment. The block diagram of the isolated pixel control part by one Embodiment. The figure which shows the image including the isolated pixel by one Embodiment. The figure which shows the relationship between the image height and the pixel size by one Embodiment. The image figure of the correction of the isolated pixel by one Embodiment. The figure which shows the determination method of the isolated pixel by one Embodiment, and the pixel to be corrected. The block diagram of the isolated pixel control part by one Embodiment. The figure which shows the selection order of the correction target pixel by one Embodiment. The figure which shows the image including the isolated pixel by one Embodiment. The image figure of the correction of the isolated pixel by one Embodiment. Explanatory drawing of the isolated pixel by one Embodiment. The explanatory view of the relationship between the area and the resolution for determining the isolated pixel by one Embodiment.

Hereinafter, exemplary embodiments of the present invention will be described with reference to the drawings. The following embodiments are examples, and the present invention is not limited to the contents of the embodiments. Further, in each of the following figures, components that are not necessary for the description of the embodiment will be omitted from the drawings.

<First Embodiment>
FIG. 1 is a schematic configuration diagram of an image forming apparatus 9 according to the present embodiment. The laser drive unit 300 of the optical scanning device 400 emits an optical beam 208 based on the image data output from the image signal generation unit 100 and the control data output from the control unit 1. The light beam 208 scans the photoconductor 4 charged by a charged portion (not shown) and forms a latent image on the surface of the photoconductor 4. A developing unit (not shown) develops this latent image with toner to form a toner image. This toner image is fed from the paper feed unit 8 and transferred to a recording medium such as paper which is conveyed by the roller 5 to a position where it comes into contact with the photoconductor 4. The toner image transferred to the recording medium is heat-fixed to the recording medium at the fixing unit 6. The recording medium on which the toner image is fixed is discharged to the outside of the image forming apparatus 9 by the paper ejection roller 7.

2A and 2B are configuration views of the optical scanning apparatus 400 according to the present embodiment, FIG. 2A shows a cross-sectional view in the main scanning direction, and FIG. 2B shows a cross-sectional view in the sub-scanning direction. .. The light beam (luminous flux) 208 emitted from the light source 401 is shaped into an elliptical shape by the aperture diaphragm 402 and is incident on the coupling lens 403. The light beam 208 that has passed through the coupling lens 403 is converted into substantially parallel light and incident on the anamorphic lens 404. The substantially parallel light includes weakly convergent light and weakly divergent light. The anamorphic lens 404 has a positive refractive power in the main scanning cross section and converts the incident light flux into convergent light in the main scanning cross section. Further, the anamorphic lens 404 concentrates the luminous flux in the vicinity of the reflecting surface 405a of the deflector 405 in the sub-scanning cross section, and forms a long line image in the main scanning direction.

Then, the luminous flux that has passed through the anamorphic lens 404 is reflected by the reflecting surface 405a of the deflector (polygon mirror) 405. The light beam 208 reflected by the reflecting surface 405a passes through the imaging lens 406 and is imaged on the surface of the photoconductor 4 to form a predetermined spot-like image (hereinafter referred to as a spot). By rotating the deflector 405 in the direction of arrow A at a constant angular velocity by a driving unit (not shown), the spot moves in the main scanning direction on the scanned surface 407 of the photoconductor 4, and is electrostatically charged on the scanned surface 407. Form a latent image. The main scanning direction is a direction parallel to the surface of the photoconductor 4 and orthogonal to the moving direction of the surface of the photoconductor 4. The sub-scanning direction is a direction orthogonal to the main scanning direction and the optical axis of the luminous flux.

The beam detect (hereinafter referred to as BD) sensor 409 and the BD lens 408 are synchronous optical systems that determine the timing of writing an electrostatic latent image on the scanned surface 407. The light beam 208 that has passed through the BD lens 408 is incident on the BD sensor 409 including the photodiode and detected. The writing timing is controlled based on the timing at which the light beam 208 is detected by the BD sensor 409. The light source 401 of the present embodiment has one light emitting unit, but the light source 401 may be provided with a plurality of light emitting units that can independently control light emission.

As shown in FIG. 2, the imaging lens 406 has two optical surfaces (lens surfaces), an incident surface 406a and an exit surface 406b. The imaging lens 406 is configured such that the luminous flux deflected by the reflecting surface 405a scans the scanned surface 407 with desired scanning characteristics in the main scanning cross section. Further, the imaging lens 406 has a configuration in which the spot of the laser beam 208 on the surface to be scanned 407 has a desired shape. The imaging lens 406 can be a plastic molded lens formed by injection molding. Alternatively, the imaging lens 406 can be a glass mold lens. Since the molded lens can easily form an aspherical shape and is suitable for mass production, the productivity and optical performance can be improved by adopting the molded lens as the imaging lens 406.

The imaging lens 406 does not have the so-called fθ characteristic. That is, when the deflector 405 is rotating at a constant angular velocity, the spot does not move at a constant velocity on the surface to be scanned 407. By using the imaging lens 406 having no fθ characteristic, the imaging lens 406 can be arranged close to the deflector 405 (at a position where the distance D1 is small). Further, the imaging lens 406 having no fθ characteristic can have a smaller length in the main scanning direction (width LW) and the optical axis direction (thickness LT) than the imaging lens having the fθ characteristic. Therefore, the size of the optical scanning device 400 can be reduced. Further, in the case of a lens having fθ characteristics, there may be a sharp change in the shape of the entrance surface and the exit surface of the lens when viewed in the main scanning cross section, and if there is such a shape restriction, a good result is obtained. Image performance may not be obtained. On the other hand, since the imaging lens 406 does not have the fθ characteristic, there is little sudden change in the shape of the incident surface and the exit surface of the lens when viewed in the main scanning cross section, so that good imaging performance is achieved. Obtainable.

The scanning characteristics of the imaging lens 406 according to this embodiment are represented by the following equation (1).
Y = (K / B) × tan (Bθ) (1)
In the equation (1), θ is the scanning angle (scanning angle of view) by the deflector 405, Y is the condensing position (image height) of the spot on the surface to be scanned 407, and K is the axial image. It is an imaging coefficient at high, and B is a coefficient (scanning characteristic coefficient) that determines the scanning characteristic of the imaging lens 406. In the present embodiment, the on-axis image height is the image height (Y = 0) on the optical axis, and corresponds to the scanning angle θ = 0. Further, the off-axis image height corresponds to an image height other than the on-axis image height, that is, a scanning angle θ ≠ 0. Further, the most off-axis image height corresponds to the image height (Y = + Ymax, −Ymax) when the scanning angle θ becomes the maximum (maximum scanning angle of view). The scanning width W, which is the width in the main scanning direction of a predetermined region (scanning region) on which a latent image can be formed on the surface to be scanned 407, is represented by W = | + Ymax | + | −Ymax |. The center of the predetermined region is the on-axis image height, and the end is the most off-axis image height.

Here, the imaging coefficient K is a coefficient corresponding to f at scanning characteristic (fθ characteristic) Y = fθ when parallel light is incident on the imaging lens 406. That is, the imaging coefficient K is a coefficient for making the focusing position Y and the scanning angle θ proportional to each other when a light flux other than parallel light is incident on the imaging lens 406, as in the fθ characteristic. Supplementing the scanning characteristic coefficient, the equation (1) when B = 0 is Y = Kθ, which corresponds to the scanning characteristic Y = fθ of the imaging lens used in the conventional optical scanning apparatus. Further, since the equation (1) when B = 1 is Y = Ktanθ, it corresponds to the projection characteristic Y = ftanθ of a lens used in an imaging device (camera) or the like. That is, in the equation (1), by setting the scanning characteristic coefficient B in the range of 0 ≦ B ≦ 1, it is possible to obtain the scanning characteristic between the projection characteristic Y = ftanθ and the fθ characteristic Y = fθ.

Here, when the equation (1) is differentiated by the scanning angle θ, the scanning speed of the luminous flux on the scanned surface 407 with respect to the scanning angle θ can be obtained as shown in the following equation (2).
dY / dθ = K / (cos ² (Bθ)) (2)

Further, by dividing the equation (2) by the velocity dY / dθ = K at the axial image height and subtracting 1, the following equation (3) is obtained.
(1 / (cos ² (Bθ))) -1 = tan ² (Bθ) (3)
Equation (3) expresses the amount of deviation (partial magnification) of the scanning speed of each off-axis image height with respect to the scanning speed of the on-axis image height. In the optical scanning apparatus 400 according to the present embodiment, the scanning speed of the luminous flux is different between the on-axis image height and the off-axis image height except when B = 0.

FIG. 3 shows the relationship between the image height and the partial magnification when the scanning position on the scanned surface 407 is fitted with the characteristic of Y = Kθ. In the present embodiment, by giving the scanning characteristics shown in the equation (1) to the imaging lens 406, as shown in FIG. 3, the scanning speed gradually increases from the on-axis image height to the off-axis image height. The partial magnification is large because it is faster. The partial magnification of 30% means that the irradiation length of the surface to be scanned 407 in the main scanning direction becomes 1.3 times when light is irradiated for a unit time. Therefore, if the pixel width in the main scanning direction is determined at a fixed time interval determined by the cycle of the image clock, the pixel density will differ between the on-axis image height and the off-axis image height.

Further, as the image height Y moves away from the on-axis image height and approaches the most off-axis image height (the larger the absolute value of the image height Y), the scanning speed gradually increases. As a result, the unit length is scanned when the image height is near the most off-axis image height, rather than the time required to scan the unit length when the image height on the scanned surface 407 is near the on-axis image height. The time required is shorter. This is because, when the emission brightness of the light source 401 is constant, the unit length when the image height is near the most off-axis image height is larger than the total exposure amount around the unit length when the image height is near the on-axis image height. It means that the total exposure amount around is smaller.

That is, when the optical configuration is as described above, the partial magnification with respect to the main scanning direction and the variation in the total exposure amount per unit length may not be appropriate for maintaining good image quality. Therefore, in the present embodiment, in order to obtain good image quality, the above-mentioned partial magnification correction and brightness correction for correcting the total exposure amount per unit length are performed.

FIG. 5 is a block diagram showing an exposure control configuration in the image forming apparatus 9. The image signal generation unit 100 receives image data from a host computer (not shown) and generates a VDO signal 110 corresponding to the image data. The image signal generation unit 100 also has a function of correcting the pixel width. The control unit 1 controls the image forming apparatus 9 and the brightness of the light source 401. The laser driving unit 300 causes the light source 401 to emit light by supplying a current to the light emitting unit 11 of the light source 401 based on the VDO signal 110.

When the image signal generation unit 100 is ready to output the image data, the image signal generation unit 100 instructs the control unit 1 to start image formation through the serial communication 113. When the control unit 1 is ready for image formation, the control unit 1 transmits the TOP signal 112, which is a sub-scanning synchronization signal, and the BD signal 111, which is a main scanning synchronization signal, to the image signal generation unit 100. When the image signal generation unit 100 receives the synchronization signal, it outputs the VDO signal 110, which is an image signal, to the laser drive unit 300 at a predetermined timing. The main constituent blocks of the image signal generation unit 100, the control unit 1, and the laser drive unit 300 will be described later.

FIG. 6A is a timing chart of various synchronization signals and image signals when an image forming operation corresponding to one page of a recording medium is performed. Time elapses from left to right in the figure. The "HIGH" of the TOP signal 112 indicates that the tip of the recording medium has reached a predetermined position. When the image signal generation unit 100 receives the "HIGH" of the TOP signal 112, it transmits the VDO signal 110 in synchronization with the BD signal 111. The light source 401 emits light based on the VDO signal 110 to form a latent image on the photoconductor 4. In FIG. 6A, for the sake of simplification of the figure, the VDO signal 110 is shown to be continuously output across the plurality of BD signals 111. However, in reality, the VDO signal 110 is output during a predetermined period between the output of the BD signal 111 and the output of the next BD signal 111.

The partial magnification correction method by the image signal generation unit 100 will be described. Prior to the explanation, the factor of the partial magnification and the correction principle will be described with reference to FIG. 6 (B). FIG. 6B is a diagram showing the timing of the BD signal 111 and the VDO signal 110, and the dot image of the latent image on the scanned surface 407. Time elapses from left to right in the figure. When the image signal generation unit 100 receives the rising edge of the BD signal 111, it transmits the VDO signal 110 after a predetermined timing so that a latent image can be formed at a position separated from the left end of the photoconductor 4 by a desired distance. Then, the light source 401 emits light based on the VDO signal 110, and a latent image corresponding to the VDO signal 110 is formed on the surface to be scanned 407.

Here, a case where the light source 401 is made to emit light for the same period at the on-axis image height and the most off-axis image height based on the VDO signal 110 to form a dot-shaped latent image will be described. The size of this dot corresponds to one dot of 600 dpi (width of 42.3 um in the main scanning direction). As described above, the optical scanning device 400 has an optical configuration in which the scanning speed at the end portion (out-of-axis image height) is faster than that at the central portion (on-axis image height) on the scanned surface 407. As shown as the toner image A, the latent image dot1 having the most off-axis image height is enlarged in the main scanning direction as compared with the latent image dot2 having the axial image height. Therefore, in the present embodiment, as the partial magnification correction, the period and time width of the VDO signal 110 are corrected according to the position in the main scanning direction. That is, by partial magnification correction, the emission time interval of the most off-axis image height is shortened as compared with the emission time interval of the on-axis image height, and as shown as toner image B, the latent image dot3 of the out-of-axis image height and the on-axis Make the latent image dot4 of the image height the same size. Such correction makes it possible to form dot-shaped latent images corresponding to each pixel at substantially equal intervals in the main scanning direction.

FIG. 7 is a block diagram showing an example of the image modulation unit 101 of the image signal generation unit 100. The density correction processing unit 121 stores a density correction table for density correction, and performs density correction of image data input based on the density correction table. The halftone processing unit 122 performs screen (dither) processing of image data and conversion processing for expressing the density in the image forming apparatus 9. The isolated pixel control unit 140 performs correction processing on the isolated pixel. Although the details will be described later, the isolated pixel correction processing by the isolated pixel control unit 140 is performed on the image data 141 before the halftone processing by the halftone processing unit 122, but the image data after the halftone processing is performed. The configuration may be performed for 143. When performed before the halftone processing, the density correction processing unit 121 outputs the image data 141 after the density correction to the isolated pixel control unit 140. The isolated pixel control unit 140 outputs the processed image data 142 to the halftone processing unit 122 via the density correction processing unit 121 or directly. On the other hand, when the image data is performed after the halftone processing, the density correction processing unit 121 outputs the image data 141 after the density correction to the halftone processing unit 122. Then, the halftone processing unit 122 outputs the image data 143 after the halftone processing to the isolated pixel control unit 140. The isolated pixel control unit 140 outputs the processed image data 143 to the PS conversion unit 123 via the halftone processing unit 122 or directly. In the following description, the PS conversion unit 123 shall always receive data from the halftone processing unit 122, and this signal shall be referred to as a signal 129.

FIG. 8A is an example of the screen used by the halftone processing unit 122, and the density is expressed by a 200-line matrix 153 having 3 pixels in each of the main scanning direction and the sub scanning direction. The white portion in the figure is the portion where the light source 401 is not emitted (off), and the black portion is the portion where the light source 401 is emitted (on). The matrix 153 is provided for each gradation, and the gradation increases (the density becomes darker) in the order indicated by the arrows. In the present embodiment, one pixel 157 is a unit that divides image data in order to form one dot of 600 dpi on the surface to be scanned 407. As shown in FIG. 8B, in the state before the pixel width is corrected, one pixel is divided into 16 pixel pieces, and the light emission of the light source 401 is switched on / off in units of the pixel pieces. That is, one pixel can express 16 steps of gradation. Further, the order in which a plurality of pixel pieces of one pixel are turned on can be freely controlled according to the density. 8 (C) to 8 (F) are diagrams for explaining the order in which the pixel pieces are turned on. FIG. 8C shows a type in which a pixel piece grows from the center to both ends, FIG. 8D shows a type in which the pixel piece grows from left to right, and FIG. 8E shows a type in which the pixel piece grows from right to left. FIG. 8F shows a type that grows from both ends to the center. Note that FIG. 8 (A) shows an example of screen growth using FIGS. 8 (C) to 8 (E).

Returning to FIG. 7, the PS conversion unit 123 converts the input parallel 16-bit signal 129 into a serial signal 130. The FIFO 124 receives the serial signal 130, stores it in a line buffer (not shown), and after a predetermined time, outputs the serial signal as a VDO signal 110 to the laser drive unit 300 in the subsequent stage. The write and read control of the FIFA 124 is performed by the frequency control unit 128 controlling the write enable signal WE131 and the read enable signal RE132. The frequency control unit 128 performs this control based on the partial magnification information received from the CPU 102 via the CPU bus 103. The PLL unit 127 supplies the clock (VCLKx16) 126, which is obtained by multiplying the frequency of the clock (VCLK) 125 corresponding to one pixel by 16 times, to the PS conversion unit 123 and the FIFA 124.

Next, the operation after the halftone processing of the block diagram of FIG. 7 will be described with reference to the time chart related to the operation of the image modulation unit 101 shown in FIG. As described above, the PS conversion unit 123 takes in the multi-valued 16-bit signal 129 from the halftone processing unit 122 in synchronization with the clock 125, and sends the serial signal 130 to the FIFA 124 in synchronization with the clock 126. The FIFO 124 captures the signal 130 only when the WE signal 131 is valid “HIGH”. When the image is shortened in the main scanning direction to correct the partial magnification, the frequency control unit 128 partially disables the WE signal to "LOW" so that the FIFA 124 does not capture the serial signal 130. Control. That is, the pixel pieces are extracted. FIG. 9 shows an example in which one pixel is usually composed of 16 pixel pieces, and one pixel piece is extracted from the 1st pixel in the figure and is composed of 15 pixel pieces.

Further, the FIFO 124 reads the accumulated data in synchronization with the clock 126 (VCLKx16) only when the RE signal 132 is valid “HIGH”, and outputs the VDO signal 110. When the image is lengthened in the main scanning direction to correct the partial magnification, the frequency control unit 128 partially disables the RE signal 132 to "LOW", so that the FIFA 124 does not update the read data and the clock 126. The data one clock before is continuously output. That is, a pixel piece having the same data as the data of the adjacent pixel piece on the upstream side with respect to the main scanning direction processed immediately before is inserted. FIG. 9 shows an example in which one pixel is usually composed of 16 pixel pieces, and two pixel pieces are inserted into the 2nd pixel in the figure to be composed of 18 pixel pieces. The FIFO 124 has been described as a circuit having a configuration in which the output is not in the Hi-Z state but the previous output is continued when the RE signal is set to invalid "LOW".

10 and 11 are views for explaining from the parallel 16-bit signal 129, which is the input image of the halftone processing unit 122, to the VDO signal 110, which is the output of the FIFA 124, using an image. FIG. 10A is an example of a multi-value parallel 8-bit image signal input to the halftone processing unit 122. Each pixel has 8-bit density information. The pixel 150 has density information of F0h, the pixel 151 has density information of 80h, the pixel 152 has density information of 60h, and the white background portion has density information of 00h. FIG. 10B is a screen, which grows from the center along line 200, as described in FIG. FIG. 10C is an image image of the image signal of the parallel 16-bit signal 129 after the halftone processing, and each pixel 157 is composed of 16 pixel pieces as described above.

FIG. 11 shows an example in which a pixel piece is inserted to stretch an image and an image obtained by extracting an image piece, focusing on an area 158 of 8 pixels in the main scanning direction of FIG. 10C with respect to the serial signal 130. Is shown as an example of shortening. FIG. 11A is an example of increasing the partial magnification by 8%. By inserting a total of 8 pixel pieces into a group of 100 consecutive pixel pieces at equal or substantially equal intervals, the pixel width is changed so as to increase the partial magnification by 8%, and the latent image is mainly scanned. Can be stretched in the direction. FIG. 11B is an example of reducing the partial magnification by 7%. By extracting a total of 7 pixel pieces at equal or substantially equal intervals for a group of 100 consecutive pixel pieces, the pixel width is changed so as to reduce the partial magnification by 7%, and the latent image is mainly scanned. Can be shortened in the direction. In this way, in the partial magnification correction, by changing the pixel width in which the length in the main scanning direction is less than one pixel, the dot-shaped latent images corresponding to each pixel of the image data are substantially evenly spaced with respect to the main scanning direction. To be able to form. It should be noted that the substantially equal intervals in the main scanning direction include those in which the pixels are not completely arranged at equal intervals. That is, as a result of the partial magnification correction, the pixel spacing may vary slightly, and the pixel spacing may be evenly spaced on average within a predetermined image height range. As described above, when the pixel pieces are inserted or extracted at equal or substantially equal intervals, the difference in the number of pixel pieces constituting the pixel is compared with the number of pixel pieces constituting the pixel between two adjacent pixels. It becomes 0 or 1. Therefore, it is possible to suppress variations in the image density in the main scanning direction when compared with the original image data, so that good image quality can be obtained. Further, the position where the pixel piece is inserted or extracted may be the same position for each scanning line (line) with respect to the main scanning direction, or the position may be shifted.

As described above, the larger the absolute value of the image height Y, the faster the scanning speed. Therefore, in the partial magnification correction, the above-mentioned pixel pieces are inserted and / or excerpted so that the image becomes shorter as the absolute value of the image height Y becomes larger (the length of one pixel becomes shorter). In this way, latent images corresponding to each pixel can be formed at substantially equal intervals in the main scanning direction, and the partial magnification can be appropriately corrected.

Here, a method of correcting the partial magnification in the main scanning direction by inserting and removing different pixel pieces for each scanning position by the frequency control 128 shown in FIG. 7 has been described. Further, the frequency control unit 128 may be a method of correcting the partial magnification by controlling the frequency for each scanning position by separately using a PLL or the like without using the pixel piece insertion / extraction control. In the present embodiment, the imaging lens 406 does not have the fθ characteristic. However, the imaging lens 406 or another lens (not shown) may have an insufficient fθ characteristic, and the fθ characteristic may be supplemented by electrically correcting the partial magnification.

Subsequently, the luminance correction will be described. The reason for performing the brightness correction is that the total exposure amount (integrated light amount) to one pixel by the light source 401 is corrected so that the length of one pixel becomes shorter as the absolute value of the image height Y becomes larger by the partial magnification correction. ) Decreases as the absolute value of the image height Y increases. In the luminance correction, the luminance of the light source 401 is corrected so that the total exposure amount (integrated light amount) for one pixel is constant at each image height.

The control unit 1 of FIG. 5 has an IC 3 incorporating a CPU core 2, an 8-bit DA converter 21, and a regulator 22, and together with the laser drive unit 300, constitutes a brightness correction unit. The laser drive unit 300 includes a memory 304, a VI conversion circuit 306 that converts a voltage into a current, and a laser driver IC 9, and supplies a drive current to the light emitting unit 11 of the light source 401. The partial magnification information is stored in the memory 304, and the information on the correction current supplied to the light emitting unit 11 is stored. The partial magnification information is partial magnification information corresponding to a plurality of image heights with respect to the main scanning direction. In addition, instead of the partial magnification information, it may be the characteristic information of the scanning speed on the surface to be scanned.

Next, the operation of the laser drive unit 300 will be described. The IC 3 adjusts and outputs the voltage 23 output from the regulator 22 based on the information of the correction current for the light emitting unit 11 stored in the memory 304. The voltage 23 becomes the reference voltage of the DA converter 21. Next, the IC 3 sets the input data of the DA converter 21 and outputs the luminance correction analog voltage 312 in synchronization with the BD signal 111. The VI conversion circuit 306 in the subsequent stage converts the luminance correction analog voltage 312 into a current 313 and outputs it to the laser driver IC9. In the present embodiment, the IC 3 outputs the luminance correction analog voltage 312, but a DA converter may be mounted on the laser drive circuit 300 to generate the luminance correction analog voltage 312 in the vicinity of the laser driver IC 9.

The laser driver IC 9 controls ON / OFF of the light emission of the light source 401 by switching whether the current IL is passed through the light emitting unit 11 or the dummy resistor 10 according to the VDO signal 110. The current IL supplied to the light emitting unit 11 is obtained by subtracting the current 313 from the current Ia set in the constant current circuit 15. The current Ia flowing through the constant current circuit 15 is automatically adjusted by feedback control so that the brightness detected by the photodetector 12 becomes the desired brightness Papc1. This automatic adjustment is a so-called APC (Automatic Power Control). The value of the variable resistor 13 is adjusted so that it is input to the laser driver IC 9 as a desired voltage when the light emitting unit 11 emits light to a predetermined brightness at the time of factory assembly.

FIG. 12 is a graph showing the characteristics of the current and the brightness of the light emitting unit 11. The current Ia required for the light emitting unit 11 to emit light with a predetermined brightness changes depending on the ambient temperature. Graph 51 in FIG. 11 shows the relationship between the current and the brightness at a standard temperature, and Graph 52 shows the relationship between the current and the brightness in a high temperature environment. It is known that in a laser diode generally used as a light emitting unit 11, the current Ia required to output a predetermined brightness changes depending on the environmental temperature, but the efficiency (slope in the figure) hardly changes. .. That is, in order to emit light at a predetermined brightness Papc1, the current value indicated by point A as the current Ia is required in the standard temperature environment, whereas the current value indicated by point C as the current Ia is required in the high temperature environment. It becomes. As described above, the laser driver IC 9 automatically adjusts the current Ia supplied to the light emitting unit 11 so as to have a predetermined brightness Papc1 by monitoring the brightness with the photodetector 12 even if the environmental temperature changes. Since the efficiency does not change even if the environmental temperature changes, the brightness is twice as high as that of Papc1 by subtracting the predetermined currents ΔI (N) and ΔI (H) from the current Ia for emitting light at the predetermined brightness Papc1. Can be reduced to. In addition, in FIG. 12, it is changed by 0.74 times. Note that ΔI (N) and ΔI (H) are substantially the same values regardless of the environmental temperature. In the present embodiment, the brightness of the light emitting unit 11 is gradually increased from the on-axis image height to the most out-of-axis image. Therefore, the central portion emits light at the brightness indicated by points B and D in FIG. Then, it emits light with the brightness indicated by points A and C.

The brightness correction is performed by subtracting the current Id corresponding to the predetermined currents ΔI (N) and ΔI (H) from the current Ia automatically adjusted to emit light at a desired brightness. As described above, the larger the absolute value of the image height Y, the faster the scanning speed. Then, as the absolute value of the image height Y increases, the total exposure amount (integrated light amount) to one pixel decreases. Therefore, in the luminance correction, the luminance is corrected so that the larger the absolute value of the image height Y is, the larger the luminance is. Specifically, the current 313 is set to decrease as the absolute value of the image height Y increases, so that the current IL increases as the absolute value of the image height Y increases. In this way, the brightness can be appropriately corrected. Although the method of correcting the total exposure amount of one pixel so as to match at each image height has been described above, for example, the exposure amount of the line image or the patch image may be corrected so as to match at each image height. Further, in the present embodiment, the description is given on the premise that the luminance correction is performed, but the configuration may be such that the luminance correction is not performed.

FIG. 13 is a timing chart for explaining the partial magnification correction and the brightness correction described above. The memory 304 of FIG. 5 stores partial magnification information 317 of the optical scanning device 400. This partial magnification information 317 may be measured and stored in each device after assembling the optical scanning device 400, or when there is little variation between the individual devices, typical characteristics are stored without being measured individually. You may. The CPU core 2 reads the partial magnification information 317 from the memory 304 via the serial communication 307 and sends it to the CPU 102 of the image signal generation unit 100. The CPU 102 generates partial magnification correction information 314 based on the partial magnification information 317 and sends it to the frequency control unit 128 of the image modulation unit 101. In FIG. 13, a case where a partial magnification of 35% occurs at the most off-axis image height when the on-axis image height is used as a reference is described as an example. In this example, the partial magnification correction information 314 sets the 17% point to zero magnification correction, the most off-axis image height to -18%, and the on-axis image height to + 17%. Therefore, as shown in the figure, in the main scanning direction, the pixel pieces are extracted near the end where the absolute value of the image height is large to shorten the image length, and the pixel pieces are inserted near the center where the absolute value of the image height is small. The image length is extended. As described with reference to FIG. 11, in order to correct the out-of-axis image height by -18%, 18 pixel pieces are extracted from 100 pixel pieces, and the on-axis image height is corrected by + 17%. Inserts 17 pixel pieces into 100 pixel pieces. As a result, when viewed with reference to the vicinity of the on-axis image height (center), it is substantially the same as the case where 35 pixel pieces are extracted from 100 pixel pieces near the outermost image height (edge). It becomes a state, and the partial magnification of 35% can be corrected. That is, the period during which the spot of the light beam 208 moves on the scanning surface 407 by the width of one pixel (42.3 um (600 dpi)) is 0.74 times the off-axis image height. By inserting and removing pixel pieces having a width of less than one pixel in this way, the pixel width can be corrected and latent images corresponding to each pixel can be formed at substantially equal intervals in the main scanning direction.

Note that, based on the on-axis image height, the pixel pieces are not inserted or extracted in the vicinity of the on-axis image height, and the extraction ratio of the pixel pieces may be increased as the image height approaches the most off-axis image height. On the contrary, based on the out-of-axis image height, the pixel pieces are not inserted or excerpted near the out-of-axis image height, and the pixel piece insertion ratio is increased as the image height approaches the on-axis image height. Is also good. However, as described above, it is better to insert and remove the pixel pieces so that the pixels having the image height between the on-axis image height and the most off-axis image height have the reference pixel width (width of 16 pixel pieces). Get better. That is, the smaller the absolute value of the difference between the reference pixel width and the pixel width of the pixel from which the pixel piece is inserted, the more faithful the original image data is with respect to the image density in the main scanning direction. can get.

Further, although the pixel width by inserting and removing the pixel piece has been described here, the pixel width may be corrected by changing the frequency in each section as described above. When the frequency is changed, as shown in FIG. 8B, all the pixel pieces having 16 pixels can be used for gradation control.

Further, for the luminance correction, the partial magnification information 313 and the correction current information of the memory 304 are read out before the image formation. Then, the CPU core 2 in the IC 3 generates the luminance correction value 315, and the luminance correction value 315 for one scan is stored in a register (not shown) in the IC 3. Further, the output voltage 23 of the regulator 22 is determined and input to the DA converter 21 as a reference voltage. Then, in synchronization with the BD signal 111, the brightness correction value 315 stored in a register (not shown) is read out, so that the brightness correction analog voltage 312 is transmitted from the output port of the DA converter 21 to the VI conversion circuit 306 in the subsequent stage. Feed and convert to current 313. As shown in FIG. 13, since the brightness correction value 315 changes according to the change in the irradiation position (image height) of the laser beam on the scanned surface, the current value 313 also changes according to the laser beam irradiation position. To. This controls the current IL.

The brightness correction value 315 generated by the CPU core 2 based on the partial magnification information 317 and the correction current information is set so that the current 313 becomes smaller as the absolute value of the image height Y increases. Therefore, as shown in FIG. 13, the current IL increases as the absolute value of the image height Y increases. In other words, the current 313 changes during one scan, and the current IL decreases toward the center of the image (the smaller the absolute value of the image height Y). As a result, as shown in the figure, the amount of laser light output by the light emitting unit 11 is such that the brightness of the out-of-axis image height is emitted at Papc1 and the brightness of the on-axis image height is 0.74 times the brightness of Papc1. It will be corrected.

4 (A) to 4 (C) are views showing an optical waveform and a main scanning LSF (Line Spread Function) profile. The optical waveform and the main scanning LSF profile show the case where the light source 401 emits light at a predetermined brightness and a period at each of the on-axis image height, the intermediate image height, and the most off-axis image height. In the optical configuration of the present embodiment, the scanning speed at the most off-axis image height is 135% of that at the on-axis image height, and the partial magnification of the most off-axis image height with respect to the on-axis image height is 35%. The optical waveform is the emission waveform of the light source 401. The main scanning LSF profile is an integral of the spot profile formed on the surface to be scanned 407 in the sub-scanning direction by emitting light with the above-mentioned optical waveform while moving the spot in the main scanning direction. This indicates the total exposure amount (integrated light amount) on the scanned surface 407 when the light source 401 is made to emit light with the above-mentioned optical waveform.

FIG. 4A shows a state in which the above-mentioned partial magnification correction and brightness correction are not performed. In FIG. 4A, the light source 401 emits light for a period T3 at a brightness P3 regardless of the image height. Here, the period T3 is a period required for the main scan for one pixel (42.3 μm) at the axial image height. In FIG. 4A, it can be seen that the main scanning LSF profile is enlarged and the peak of the integrated light amount is lowered as the image height shifts from the on-axis image height to the off-axis image height. FIG. 4B shows a case where only partial magnification correction is performed. That is, the light is emitted at the brightness P3 regardless of the image height, but the light emitting period is shortened from the on-axis image height to the off-axis image height. In FIG. 4B, the enlargement of the main scanning LSF profile that occurs toward the off-axis image height is suppressed. However, since the light emission time is shortened toward the off-axis image height, it can be seen that the peak of the integrated light amount is further lowered as compared with FIG. 4 (A). FIG. 4C shows a case where partial magnification correction and brightness correction are performed. That is, the light emitting time is shortened and the light emitting brightness of the light emitting unit 11 is increased toward the off-axis image height. In FIG. 4 (C), as compared with FIG. 4 (B), the decrease in the peak of the integrated light amount that occurs toward the off-axis image height is suppressed, and the bloat is also suppressed. Although the LSF profiles of the on-axis image height, the intermediate image height, and the out-of-axis image height in FIG. 4C do not completely match, the total exposure amount of each pixel is substantially the same.

As described above, by performing the partial magnification correction and the brightness correction, it is possible to perform exposure that prevents image deterioration without using a scanning lens having an fθ characteristic. However, as described above, even when the partial magnification correction and the brightness correction are performed, the LSF profile of the on-axis image height and the LSF profile of the most off-axis image height do not completely match. Due to this LSF profile variation, the reproducibility of pixels differs depending on the position in the main scanning direction. This phenomenon will be described below.

FIG. 14 shows the exposure energy distribution of the isolated pixels when the partial magnification correction and the brightness correction are performed. In the present embodiment, the isolated pixel means a pixel to which toner is attached by exposure. Further, the blank pixels of the pixels around the blank pixels mean that the pixels do not adhere to the toner by not exposing the pixels, or the pixels do not adhere the toner by exposing the pixels at an exposure amount to which the toner does not adhere. The exposure energy distribution of the isolated pixels is consistent with the LSF profile. The sum of the exposure energies of the isolated pixels at the on-axis image height and the most off-axis image height (integrated value in the main scanning direction) is the same, but the pixel diameters (spot diameters) are different. For example, as shown in FIG. 14, it is assumed that the width of one pixel in the main scanning direction is "the width when the exposure energy is 0.3". That is, it is assumed that the toner adheres to the portion where the exposure energy applied to the photoconductor 4 is 0.3 or more. In this case, the pixel width W21a of the outermost image height is wider than the pixel width W21b of the on-axis image height. Assuming that the width of one pixel in the main scanning direction is "the width at which the exposure energy is 0.2", the magnitude relationship between the outermost image height and the on-axis image height is reversed.

FIG. 15 is a block diagram of the isolated pixel control unit 140 in the present embodiment. The isolated pixel detection unit 281 receives determination information 283 from a storage medium (not shown) provided in the CPU 102 or the image modulation unit 101, detects the presence or absence of isolated pixels in the image data 141 or the image data 143, and indicates the detection result. The detection signal 285 is output. The image data 141 is image data after density correction output by the density correction processing unit 121. Further, the image data 143 is the image data after the halftone processing output by the halftone processing unit 122. That is, in the present embodiment, the isolated pixel control unit 140 performs processing on the image data after the density correction processing or the halftone processing. Then, the isolated pixel correction unit 282 selects a correction target pixel from the blank pixels around the isolated pixel indicated by the detection signal 285. Then, the image data 141 or 143 is corrected so as to expose the pixel piece of the selected correction target pixel, thereby adjusting the size of the isolated pixel. Then, the image data 142 or 144 after the isolated pixel correction processing is output. The image data 142 is an output signal when the image data 141 is processed, and the image data 144 is an output signal when the image data 143 is processed. In the present embodiment, when the isolated pixel correction processing is performed on the image data after the density correction processing, the halftone processing is prohibited for the isolated pixel and the pixel to be corrected.

FIG. 16 shows an example of isolated pixels. In FIG. 16, the black-painted pixels are isolated pixels, and the white-painted pixels are blank pixels. FIG. 16A is a pattern in which there are two blank pixels between the isolated pixels in the main scanning direction and three blank pixels between the isolated pixels in the sub-scanning direction. FIG. 16B is a pattern in which one blank pixel is provided between the isolated pixels in each of the main scanning direction and the sub scanning direction. FIG. 17B shows the relationship between the image height position and the pixel size when the toner adheres to the region of “exposure energy = 0.3” or more. FIG. 17C shows the relationship between the image height position and the pixel size when the toner adheres to the region of "exposure energy = 0.2" or more. As shown in FIGS. 17B and 17C, the pixel size varies depending on the image height position. In the present embodiment, as shown in FIG. 17A, correction is performed so that the size is constant regardless of the image height position.

The isolated pixel detection unit 281 detects isolated pixels based on, for example, determination information 283, which is a matrix of 5 pixels each for main scanning and sub-scanning shown in FIG. 19A. Specifically, if the toner adheres to the pixel of interest #M and the toner does not adhere to the pixels 321 around it, the pixel of interest #M is determined to be an isolated pixel. The isolated pixel detection unit 281 notifies the isolated pixel correction unit 282 of the isolated pixel detected in this way by a detection signal 285.

The isolated pixel correction unit 282 is corrected from two pixels 323 adjacent to the central pixel #M in the sub-scanning direction and two pixels 324 adjacent to the central pixel #M in the main scanning direction shown in FIG. 19B. Select a pixel. That is, in this example, the correction target pixel is selected from the isolated pixel and four blank pixels adjacent to each other in the main scanning direction and the sub-scanning direction. However, it is also possible to select the correction target pixel from the eight blank pixels around the isolated pixel. When the pixel 323 is used as the correction target pixel, the two pixels 326 adjacent to the pixel 323 in the sub-scanning direction are not set as the correction target pixels by other isolated pixels. Further, in this example, when toner is adhered to a certain pixel and the periphery of the pixel is a blank pixel, the pixel is regarded as an isolated pixel. However, when the toner does not adhere to the periphery of the 2 × 2 pixels, these 2 × 2 pixels can be combined into an isolated pixel.

Next, the isolated pixel correction unit 282 processes the correction target pixel according to the correction information 284 held by the storage unit (not shown) of the image signal generation unit 100. The correction information 284 is information indicating the relationship between the image height position and the size correction amount. For example, the information indicating the image height position and the pixel size shown in FIGS. 17B or 17C is an example of the correction information 284. In FIG. 17B, the size at the on-axis image height is 1.0, and the size at the most off-axis image height is 0.5. In this case, based on the relationship shown in FIG. 17B, the isolated pixel correction unit 282 attaches toner to the pixel piece of the pixel to be corrected at the out-of-axis image height, thereby reducing the size of the isolated pixel. It can be determined that it is 0, that is, doubled. Note that FIGS. 17B and 17C show information regarding the width in the main scanning direction, but in this example, this is also applied to the width in the sub-scanning direction. However, it is also possible to separately provide correction information 284 indicating the image height position and the size correction amount in the main scanning direction and correction information 284 indicating the image height position and the size correction amount in the sub-scanning direction. .. Further, for example, the correction information 284 can be the image height position, the number of pixel pieces to be added so as to be exposed around the pixel for adjusting the pixel size, and the information indicating the direction thereof. Here, the directions of the pixel pieces to be added are the main scanning direction and the sub-scanning direction with respect to the isolated pixels, but when the correction target pixel is selected from the eight blank pixels around the isolated pixels, the diagonal direction is also Can be specified. Even if it is an isolated pixel, if the size is 1.0 at the position in the main scanning direction, the isolated pixel correction unit 282 does not select a pixel to be corrected for the isolated pixel, and therefore, the isolated pixel It does not resize. Further, in the present embodiment, the information indicating the relationship between the image height position and the pixel size is set as the correction information 284. However, other parameters related to the variation in the size of the isolated pixels, such as the rotational angular velocity of the deflector 405, can be used in place of the image height to provide correction information 284. Note that FIGS. 17B and 17C are examples, and the position where the pixel size becomes the maximum or the minimum depending on the image height may be the intermediate image height.

18 (A) is an image in which the isolated pixel of FIG. 16 (A) is corrected, and FIG. 18 (B) is an image in which the isolated pixel of FIG. 16 (B) is corrected. In the example of FIG. 18A, blank pixels above and below (sub-scanning direction) and left and right (main scanning direction) of the isolated pixels are set as correction target pixels, thereby correcting the size of the isolated pixels. On the other hand, in the example of FIG. 18B, only the blank pixels on the left and right (main scanning direction) of the isolated pixels are set as the correction target pixels, and the size of the isolated pixels is corrected by this. First, the selection of the pixel to be corrected and the pixel piece to which the toner is attached to the pixel to be corrected will be described. Two blank pixels adjacent to the isolated pixel in the main scanning direction can always be selected as correction target pixels. However, the pixel piece to which the toner is attached is selected so as not to be continuous with the pixel piece to which the toner of another pixel is attached in the main scanning direction. For example, as shown in FIGS. 18A and 18B, the size can be adjusted by sequentially adhering toner from a pixel piece continuous with the isolated pixel. However, if it is not continuous with the pixel piece to which other toner is attached, the toner may be attached to the pixel piece which is not continuous with the isolated pixel in the main scanning direction. For example, as shown in FIG. 18B, if the same blank pixel is not continuous with the pixel piece to which the toner of another pixel is attached in the main scanning direction, the same blank pixel may be used as a correction target pixel of a different isolated pixel. it can.

On the other hand, a blank pixel adjacent to the isolated pixel in the sub-scanning direction can be selected as a correction target pixel when the other pixel adjacent to the blank pixel in the sub-scanning direction does not have a pixel piece to which toner adheres. In FIG. 18A, a blank pixel adjacent to the isolated pixel in the sub-scanning direction is adjacent to another blank pixel in the direction opposite to the isolated pixel, and thus can be a correction target pixel. On the other hand, in FIG. 18B, a blank pixel adjacent to the isolated pixel in the sub-scanning direction is not a correction target pixel because it is adjacent to another isolated pixel in the direction opposite to the isolated pixel. As shown in FIG. 18A, the size of the isolated pixel and the pixel to be corrected adjacent to each other in the sub-scanning direction can be adjusted by adding a pixel piece to which toner is attached from the central portion. .. However, the structure may be such that the toner is adhered from the pixel pieces other than the central portion.

The number of pixel pieces to which the toner is attached is determined based on the correction information 284. Further, depending on the size correction amount, blank pixels diagonally above or diagonally below the isolated pixels can be set as correction target pixels. Further, if the number of pixel pieces indicated by the correction information 284 cannot be added within the limits of the selection of the correction target pixel and the limitation of the pixel pieces to be exposed, the pixel pieces are added within these limits. For example, in FIG. 18B, since an adjacent blank pixel in the sub-scanning direction cannot be a correction target pixel, even if the correction information 284 indicates that a pixel piece is added in the sub-scanning direction, the correction target pixel Due to selection restrictions, pixel fragments are not added in the sub-scanning direction. In any case, among the pixel pieces of the pixel to be corrected of a certain isolated pixel, the pixel piece to which the toner is attached is the pixel piece to which the toner of the other pixel is attached (toner is applied to correct the other isolated pixel). It should not be adjacent to (including the pixel pieces to be attached).

As described above, by correcting the pixel size, it is possible to suppress variations in pixel reproducibility depending on the image height position. In the present embodiment, it is assumed that the imaging lens 406 of the optical scanning apparatus 400 does not have the fθ characteristic, and therefore the partial magnification correction processing and the brightness correction processing are performed. However, regardless of the characteristics of the imaging lens 406, an image forming apparatus having an optical scanning device in which the size of isolated pixels formed of pixels of a predetermined size or smaller formed of a single pixel or a plurality of pixels changes according to the image height. The present invention can be applied to the above.

<Second embodiment>
Subsequently, the second embodiment will be described focusing on the differences from the first embodiment. In the first embodiment, when the periphery of one exposed pixel is an unexposed pixel, the one exposed pixel is regarded as an isolated pixel. In the present embodiment, when one or more pixels to be exposed exist in the predetermined area and the pixels around the exposed pixels in the predetermined area are not exposed, that is, blank pixels. The exposed pixels in the predetermined area are defined as isolated pixels. FIG. 24 is an explanatory diagram of isolated pixels of the present embodiment. In the present embodiment, the range of 2 pixels in each of the main scanning direction and the sub scanning direction is set as a predetermined area. In the total of 26 pixel patterns shown in FIG. 24, the black pixels indicate the pixels to be exposed, that is, the pixels to which the toner is attached. Further, the shaded pixel indicates a pixel to which toner is not adhered among the eight pixels around the center pixel. The white pixel indicates a pixel other than the periphery of the center pixel and to which toner is not adhered. The horizontal line pixels are pixels other than those around the center pixel, and indicate pixels to which toner may or may not be adhered. The black pixels in the pixel pattern of FIG. 24 are all within the range of 2 pixels in the main scanning direction and the sub-scanning direction (4 pixels in total), and toner is not adhered to the surrounding pixels. Therefore, in the present embodiment, the isolated pixel detection unit 281 determines that each of the pixels shown in black in FIG. 24 is an isolated pixel.

It should be noted that the predetermined area for determining the isolated pixel can be defined not by the number of pixels but by, for example, the actual length. For example, it is assumed that the isolated pixel is determined by setting a square region having a length of about 84.7 μm in each of the main scanning direction and the sub scanning direction as a predetermined region. 84.7 μm is 600 dpi, which is the length of two pixels. Therefore, in this case, as shown in FIG. 25A, the area of 2 pixels × 2 pixels is the area for determining the isolated pixel. However, in the case of 300 dpi, as shown in FIG. 25 (B), the isolated pixel is determined for each pixel region. Further, in the case of 1200 dpi, as shown in FIG. 25 (C), the isolated pixel is determined for each region including a total of 16 pixels of 4 pixels × 4 pixels. In the following, as shown in FIG. 24, the present embodiment will be described by taking as an example a case where isolated pixels are determined for each region of 2 pixels × 2 pixels.

Also in this embodiment, the isolated pixel detection unit 281 can detect isolated pixels based on the determination information 283, which is a matrix of 5 pixels each for the main scan and the sub scan shown in FIG. 19A, for example. Specifically, the pixel to which the toner is attached is determined from the region including a total of 4 pixels of 2 pixels each in the main scanning direction and the sub-scanning direction, including the pixel of interest #M to which the toner is attached. Then, when the attention pixel #M to which the toner is attached and the pixels around the pixel to which the determined toner is attached are all blank pixels, the attention pixel #M and the pixel to which the determined toner is attached are isolated. It can be determined as a pixel. The method for determining isolated pixels is not limited to the method using the matrix shown in FIG. 19A, and any algorithm can be used.

Also in this embodiment, the correction target pixel is selected from the blank pixels around the isolated pixel. However, not all of the surrounding blank pixels can be used as correction target pixels, and there is a limit to the blank pixels that can be selected as correction target pixels. In FIG. 24, the shaded blank pixels indicate pixels that can be selected as correction target pixels. On the other hand, in FIG. 24, the white and horizontal line pixels indicate pixels that cannot be selected as correction target pixels. Further, the horizontal line pixels indicate pixels that are not corrected by other isolated pixels. As described above, in the present embodiment, among the surrounding blank pixels, the pixels that can be selected as the correction target pixels are determined according to the pattern of the isolated pixels determined in units of the predetermined area. Among the surrounding blank pixels, the pixels that can be selected as the correction target pixels are shown in the correction information 284. Further, the number of pixel pieces to be exposed by the pixel to be corrected is also shown in the correction information 284 as in the first embodiment.

As described above, in the present embodiment, one or more exposed pixels in a predetermined area surrounded by blank pixels are defined as an isolated pixel group including one or more isolated pixels. Then, among the surrounding blank pixels, the correction target pixel is selected from the predetermined blank pixels determined by the pattern of the isolated pixel group. Then, the size of the isolated pixel group is adjusted by exposing the pixel piece of the selected correction target pixel. It should be noted that the correction information 284 indicates from which blank pixel the correction target pixel is selected and which pixel piece of the correction target pixel is exposed. When exposing pixels in a small area surrounded by blank pixels, the spot diameters of the exposed pixels vary even if partial magnification correction and brightness correction are performed, as in the case of the isolated pixels of the first embodiment. Therefore, by correcting the size of these isolated pixels, it is possible to suppress variations in pixel reproducibility depending on the image height position.

<Third Embodiment>
Subsequently, the third embodiment will be described focusing on the differences from the first embodiment. The partial magnification correction process and the brightness correction process in this embodiment are the same as those in the first embodiment. FIG. 20 is a block diagram of the isolated pixel control unit 140 according to the present embodiment. The same reference numerals are used for the same parts as those of the isolated pixel control unit 140 of the first embodiment shown in FIG. 15, and the description thereof will be omitted. In the present embodiment, the isolated pixel correction unit 282 uses the ranking information 330 stored in a storage unit (not shown) of the image signal generation unit 100 for selecting the correction target pixel. FIG. 21 shows an example of the ranking information 330. The ranking information 330 shown in FIG. 21 corresponds to the growth order of the screen in the halftone processing shown in FIG. 8A. That is, it corresponds to the order of adding the pixel pieces to which the toner is attached according to the increase in the density. FIG. 22 is an example of an image including isolated pixels. In FIG. 22, the black-painted pixel is an isolated pixel, the white-painted pixel is a blank pixel, and the number of each pixel is the growth rank of the screen indicated by the rank information 330.

In the present embodiment, the rank information 330 is used in determining the correction target pixel from the blank pixels around the isolated pixel. 23 (A) to 23 (E) show images of correction of isolated pixels with respect to FIGS. 22 (A) to 22 (C), respectively. Note that FIG. 23 (A) is a correction image of FIG. 22 (A), FIGS. 23 (B) and (C) are correction images of FIG. 22 (B), and FIGS. 23 (D) and (E). ) Is a correction image of FIG. 22 (C).

As shown in FIG. 23 (A), in the present embodiment, basically, among the left and right pixels of the isolated pixel, the pixel whose order is younger and the pixel above and below the isolated pixel, the one whose order is younger. Is the pixel to be corrected. For example, in FIG. 23A, the pixel on the left side and the pixel on the upper side of the isolated pixel are the correction target pixels. FIG. 23 (B) also shows a case where the correction target pixel is determined by the same method as in FIG. 22 (A).

On the other hand, in FIG. 23B, the blank pixel between the second isolated pixel and the third isolated pixel from the left in the main scanning direction is the correction target pixel of each of the second isolated pixel and the third isolated pixel. It has become. In this case, for example, as shown in FIG. 23C, it is possible to prevent one blank pixel from becoming two different correction target pixels. For example, with respect to the isolated pixel having the rank 2 in FIG. 23C, the blank pixel having the rank 1 in the main scanning direction is set as the correction target pixel. As a result, for the isolated pixel having the rank of 3, the blank pixel having the rank of 2 is set as the correction target pixel instead of the blank pixel having the rank of 1 in the main scanning direction.

FIG. 23 (D) is a correction image corresponding to the pixel pattern of FIG. 22 (C). As described above, when the vertical blank pixel is used as the correction target pixel, the vertical blank pixel is not set as the correction target pixel because the pixel to which the other toner is attached and the area to which the toner is attached are continuous. Others are the same as in FIG. 23 (A). On the other hand, FIG. 23 (E) shows a case where one blank pixel is determined not to be a correction target pixel of two different isolated pixels.

As described above, in the present embodiment, the correction target pixel is selected based on the rank indicated by the rank information 330. By making the ranking information 330 correspond to the growth order of the screen in the halftone processing, the size of the isolated pixel can be corrected by the pixel piece at an appropriate position in consideration of the halftone processing, and thus good image quality can be obtained. As described in the second embodiment, the present embodiment can also be applied to the case where one or a plurality of exposed pixels in a predetermined area surrounded by blank pixels are isolated pixels. Specifically, the pixels to be corrected are set in order from the pixel having the youngest screen growth order in the halftone processing.

[Other Embodiments]
The present invention supplies a program that realizes one or more functions of the above-described embodiment to a system or device via a network or storage medium, and one or more processors in the computer of the system or device reads and executes the program. It can also be realized by the processing to be performed. It can also be realized by a circuit (for example, ASIC) that realizes one or more functions.

4: Photoreceptor, 400: Optical scanning device, 281: Isolated pixel detection unit, 282: Isolated pixel correction unit

Claims (18)

Photoreceptor and
An exposure means for forming an electrostatic latent image on the photoconductor by dividing one pixel into a plurality of pixel pieces and exposing the pixel pieces as a unit.
A determination means for determining isolated pixels surrounded by blank pixels of the electrostatic latent image based on image data, and
A selection means for selecting a pixel to be corrected from blank pixels around the isolated pixel, and
A correction means for correcting the image data so as to expose a pixel piece of the correction target pixel,
A storage means for holding information indicating the relationship between the position of the pixel in the main scanning direction and the correction amount of the pixel size, and
Equipped with a,
The selection means determines whether or not the isolated pixel is a size correction target based on the position of the isolated pixel in the main scanning direction and the information held by the storage means, and the isolated pixel is of the size. An image forming apparatus characterized in that the pixel to be corrected is selected as the correction target . The image forming apparatus according to claim 1, wherein the correction means determines a pixel piece to be exposed of the correction target pixel based on the information held by the storage means. In the correction means, the pixel piece to be exposed by the correction target pixel is not adjacent to the pixel piece to be exposed by a pixel different from the isolated pixel adjacent to the correction target pixel. The image forming apparatus according to claim 1 or 2 , wherein the image forming apparatus is determined. When the correction target pixel is a pixel adjacent to the isolated pixel in the main scanning direction, the correction means selects the pixel pieces to be exposed by the correction target pixel in order from the pixel pieces adjacent to the isolated pixel. The image forming apparatus according to claim 2 or 3 . The fourth aspect of the present invention is characterized in that the correction means determines the number of pixel pieces of the correction target pixel to be exposed in order from the pixel pieces adjacent to the isolated pixels based on the information stored in the storage means. The image forming apparatus described. The correction means is characterized in that, when the correction target pixel is a pixel adjacent to the isolated pixel in the sub-scanning direction, the pixel piece to be exposed by the correction target pixel is selected from the central pixel piece. Item 6. The image forming apparatus according to any one of Items 1 to 5 . Photoreceptor and
An exposure means for forming an electrostatic latent image on the photoconductor by dividing one pixel into a plurality of pixel pieces and exposing the pixel pieces as a unit.
A determination means for determining isolated pixels surrounded by blank pixels of the electrostatic latent image based on image data, and
A selection means for selecting a pixel to be corrected from blank pixels around the isolated pixel, and
A correction means for correcting the image data so as to expose a pixel piece of the correction target pixel,
With
The number of pixels pieces exposure of the correction target pixel, images forming device you being determined in accordance with the main scanning direction of the position of said isolated pixels. The determination means determines the isolated pixels of the electrostatic latent image based on the image data after the density correction processing.
The image forming apparatus according to any one of claims 1 to 7 , wherein the correction means corrects the image data after the density correction process. Further, a processing means for performing halftone processing on the image data corrected by the correction means is provided.
The image forming apparatus according to claim 8 , wherein the processing means does not target the isolated pixel and the correction target pixel for the halftone processing. Photoreceptor and
An exposure means for forming an electrostatic latent image on the photoconductor by dividing one pixel into a plurality of pixel pieces and exposing the pixel pieces as a unit.
A determination means for determining isolated pixels surrounded by blank pixels of the electrostatic latent image based on image data, and
A selection means for selecting a pixel to be corrected from blank pixels around the isolated pixel, and
A correction means for correcting the image data so as to expose a pixel piece of the correction target pixel,
A processing means that performs halftone processing on the image data corrected by the correction means, and
With
The determination means determines the isolated pixels of the electrostatic latent image based on the image data after the density correction processing.
The correction means corrects the image data after the density correction process, and corrects the image data.
The processing means does not target the isolated pixel and the correction target pixel for the halftone processing.
The exposure means forms the electrostatic latent image on the photoconductor by scanning the photoconductor with light in the main scanning direction based on the image data after the halftone processing by the processing means.
The scanning speed of the light is not constant
The exposing unit adjusts the brightness of the light in accordance with the scanning position of the light in the main scanning direction, and images forming device you and performs insertion of pixel pieces. The determination means determines the isolated pixels of the electrostatic latent image based on the image data after the density correction processing and the halftone processing.
The image forming apparatus according to any one of claims 1 to 7 , wherein the correction means corrects image data after the density correction process and the halftone process. Photoreceptor and
An exposure means for forming an electrostatic latent image on the photoconductor by dividing one pixel into a plurality of pixel pieces and exposing the pixel pieces as a unit.
A determination means for determining isolated pixels surrounded by blank pixels of the electrostatic latent image based on image data, and
A selection means for selecting a pixel to be corrected from blank pixels around the isolated pixel, and
A correction means for correcting the image data so as to expose a pixel piece of the correction target pixel,
With
The determination means determines the isolated pixels of the electrostatic latent image based on the image data after the density correction processing and the halftone processing.
The correction means corrects the image data after the density correction process and the halftone process.
The exposure means forms the electrostatic latent image on the photoconductor by scanning the photoconductor with light in the main scanning direction based on the image data corrected by the correction means.
The scanning speed of the light is not constant
The exposing unit adjusts the brightness of the light in accordance with the scanning position of the light in the main scanning direction, and images forming device you and performs insertion of pixel pieces. The determination means according to any one of claims 1 to 12 , wherein when the periphery of one exposed pixel is a blank pixel, the one exposed pixel is determined to be the isolated pixel. The image forming apparatus described. When the determination means has a plurality of exposed pixels in a predetermined region and the periphery of the plurality of exposed pixels is a blank pixel, the determination means determines the plurality of exposed pixels as the isolated pixel. The image forming apparatus according to any one of claims 1 to 12 , characterized in that. The image forming apparatus according to any one of claims 1 to 14 , wherein the selection means selects a correction target pixel from blank pixels adjacent to the isolated pixel in the main scanning direction and the sub-scanning direction. The image forming apparatus according to any one of claims 1 to 15 , wherein the selection means selects the correction target pixel from the blank pixels around the isolated pixel according to a predetermined order. The selection means is characterized in that, among the blank pixels adjacent to the isolated pixel in the sub-scanning direction, the blank pixel adjacent to the other pixel having the pixel piece to be exposed in the sub-scanning direction is not selected as the correction target pixel. The image forming apparatus according to any one of claims 1 to 16 . The isolated pixel is a pixel to which toner is attached by exposure by the exposure means.
The blank pixel is a pixel to which the toner is not adhered because the exposure means is not exposed, or a pixel to which the toner is not adhered by being exposed at an exposure amount that the exposure means does not adhere to the toner. The image forming apparatus according to any one of claims 1 to 17 .