JP3914645B2 - Image forming apparatus and method, and copying machine - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an image forming apparatus and method for forming a color image by electrophotography and a copying machine.
[0002]
[Prior art]
In electrophotographic technology, in order to obtain a high-definition image, the diameter of a beam for optically writing an electrostatic latent image on a photoreceptor is reduced. This reduction in the diameter of the optical writing beam is particularly effective for improving the gradation of the highlight portion.
[0003]
In general, it is known that a high-definition image can be obtained by reducing the diameter of the optical writing beam. In particular, since a black image often outputs characters, there are many occasions where a high-definition image is required. However, in an image forming apparatus having binary optical writing means, if the optical writing density and the exposure energy per dot are kept constant, and only the optical writing beam diameter is reduced, When the area of one dot becomes 1 pixel or less determined by the optical writing density, the desired solid density (or toner adhesion amount on the photosensitive member) is maintained without decreasing the post-exposure potential between dots in a solid image. ) Cannot be obtained.
[0004]
In this regard, in the technique disclosed in Japanese Patent Laid-Open No. 5-150607, the black image is provided with two exposure devices, ie, an optical writing unit for characters and an optical writing unit for pictures. A high-definition image uses an optical writing unit for characters, and a high-density image uses an optical writing unit for pictures.
[0005]
[Problems to be solved by the invention]
However, the technique disclosed in Japanese Patent Laid-Open No. 5-150607 has the disadvantage that the manufacturing cost increases and the apparatus becomes large because two optical writing units are provided.
[0006]
An object of the present invention is to form an image that can develop a high-definition character on a black image, improve the gradation of a highlight portion, reduce the manufacturing cost of the device, and reduce the size of the device. It is an object to provide an apparatus and method and a copying machine.
[0007]
[Means for Solving the Problems]
  According to the first aspect of the present invention, there are provided a plurality of exposure devices for optically writing an electrostatic latent image corresponding to four colors of yellow, magenta, cyan and black on the photosensitive member, and each of the exposure devices formed by the exposure devices. An electrophotographic image forming apparatus for forming a color image by superimposing the four color toners, and a plurality of developing devices that respectively develop the electrostatic latent image with the color toners corresponding to the four colors. An exposure apparatus that supports monochrome black uses 1 / e of the peak value of the light amount distribution obtained by the stationary beam.²The beam spot area defined by the ellipse area is narrower than the beam spot area of the exposure apparatus corresponding to another color, and the optical writing density in the main scanning direction corresponds to the other color. Higher than the optical writing density in the main scanning directionIn addition, each exposure apparatus corresponding to three colors other than four colors or single color black emits an electrostatic latent image for forming a black image by mixing the four colors or the three colors on the photosensitive member. Make up for the lack of density of monochrome black images as needed by writingAn image forming apparatus characterized by the above.
[0008]
  Therefore, the exposure apparatus corresponding to black of the single color has a higher optical writing density in the main scanning direction than the optical writing density in the main scanning direction of the exposure apparatus corresponding to other colors. High-definition characters can be developed, and the gradation of the highlight portion can be improved. In addition, since it is not necessary to equip a plurality of exposure apparatuses for black images, the manufacturing cost of the apparatus can be reduced and the apparatus can be miniaturized.
  In addition, each exposure apparatus corresponding to three colors other than four colors or single color black optically writes an electrostatic latent image for forming a black image by mixing the four colors or the three colors onto the photosensitive member. As a result, the density shortage of the monochrome black image is compensated as necessary. Therefore, when the density is insufficient only with the monochrome black image, the density shortage can be compensated by the mixed color black image.
[0011]
  Claim2The invention described in claim 11In the invention described in item 1, the exposure apparatus that supports monochrome black performs optical writing with an LED array head.
[0012]
Therefore, by using the LED array head in the exposure apparatus for forming a black image, the beam spot area can be easily reduced as compared with the case where a laser diode is used.
[0013]
  Claim3The invention described in claim 12In the invention described in (1), each exposure apparatus corresponding to a color other than black of a single color performs optical writing with a laser diode.
[0014]
Therefore, an image forming apparatus having an excellent cost balance is provided by using an LED array head in an exposure apparatus for forming a black image and using a laser diode having a lower cost than the LED array head in an exposure apparatus for forming other colors. be able to.
[0015]
  Claim4The invention described in claim 13In the invention according to any one of the above, each exposure apparatus corresponding to a color other than a single black color is sub-scanned by the exposure apparatus whose optical writing density in the sub-scanning direction corresponds to another color. It is characterized by being higher than the optical writing density in the direction.
[0016]
Therefore, high-definition characters can be developed for the black image, and the gradation of the highlight portion can be improved.
[0017]
  Claim5The invention described in claim 14In the invention described in any one of the above, an exposure apparatus that supports monochrome black is:SaidThe beam spot area and one dot area on the photosensitive member are made smaller than one pixel area determined by the optical writing density.
[0018]
Therefore, high-definition characters can be developed for the black image, and the gradation of the highlight portion can be improved.
[0021]
  Claim6In the invention described in the above, an image of a document is read for each of a plurality of color separation lights, and each image signal for each read color separation light is subjected to color conversion processing to correspond to four colors of yellow, magenta, cyan, and black, respectively. An image reading apparatus for obtaining a plurality of color image data and an image forming apparatus for forming a color image by an electrophotographic method based on the plurality of color image data, wherein the image forming apparatus includes each color corresponding to the four colors. A plurality of exposure devices for optically writing an electrostatic latent image based on image data onto a photoconductor, and a plurality of development devices for developing each electrostatic latent image formed by each exposure device with color toners corresponding to the four colors. A developing device that forms the color image by superimposing the color toners, and the exposure device corresponding to the monochrome black uses a stationary beam. 1 / e of the peak value of the resulting light intensity distribution²The beam spot area defined by the ellipse area is narrower than the beam spot area of the exposure apparatus corresponding to another color, and the optical writing density in the main scanning direction corresponds to the other color. Higher than the optical writing density in the main scanning directionIn addition, each exposure apparatus corresponding to three colors other than four colors or single color black emits an electrostatic latent image for forming a black image by mixing the four colors or the three colors on the photosensitive member. Make up for the lack of density of monochrome black images as needed by writingThis is a copying machine characterized by that.
[0022]
  Therefore, the exposure apparatus corresponding to black of the single color has a higher optical writing density in the main scanning direction than the optical writing density in the main scanning direction of the exposure apparatus corresponding to other colors. High-definition characters can be developed, and the gradation of the highlight portion can be improved. In addition, since it is not necessary to equip a plurality of exposure apparatuses for black images, the manufacturing cost of the apparatus can be reduced and the apparatus can be miniaturized.
In addition, each exposure apparatus corresponding to three colors other than four colors or single color black optically writes an electrostatic latent image for forming a black image by mixing the four colors or the three colors onto the photosensitive member. As a result, the density shortage of the monochrome black image is compensated as necessary. Therefore, when the density is insufficient only with the monochrome black image, the density shortage can be compensated by the mixed color black image.
[0025]
  Claim7The invention described in claim 16In the invention described in item 1, the exposure apparatus that supports monochrome black performs optical writing with an LED array head.
[0026]
Therefore, by using the LED array head in the exposure apparatus for forming a black image, the beam spot area can be easily reduced as compared with the case where a laser diode is used.
[0027]
  Claim8The invention described in claim 17In the invention described in (1), each exposure apparatus corresponding to a color other than black of a single color performs optical writing with a laser diode.
[0028]
Accordingly, a copier having an excellent cost balance can be provided by using an LED array head for an exposure apparatus for forming a black image and using a laser diode having a lower cost than the LED array head for an exposure apparatus for forming other colors. Can do.
[0029]
  Claim9The invention described in claim 16~8In the invention according to any one of the above, each exposure apparatus corresponding to a color other than a single black color is sub-scanned by the exposure apparatus whose optical writing density in the sub-scanning direction corresponds to another color. It is characterized by being higher than the optical writing density in the direction.
[0030]
Therefore, high-definition characters can be developed for the black image, and the gradation of the highlight portion can be improved.
[0031]
  Claim 10The invention described in claim 16~9In the invention according to any one of the above, the exposure apparatus supporting monochrome black makes the beam spot area and one dot area on the photosensitive member smaller than one pixel area determined by optical writing density. Features.
[0032]
Therefore, high-definition characters can be developed for the black image, and the gradation of the highlight portion can be improved.
[0035]
  Claim 11The invention described in (1) uses a plurality of exposure apparatuses each corresponding to four colors of yellow, magenta, cyan, and black, and optically writes an electrostatic latent image corresponding to each of the four colors on a photoconductor, And developing each electrostatic latent image with a color toner corresponding to the four colors, and forming an image of a color image in which the four color toners are superimposed. The exposure by an exposure apparatus that supports monochrome black is 1 / e of the peak value of the light amount distribution obtained by the stationary beam.²The beam spot area defined by the ellipse area is narrower than the beam spot area of the exposure apparatus corresponding to the other color, and the optical writing density in the main scanning direction is based on the exposure apparatus corresponding to the other color. Higher than optical writing density in main scanning direction of exposureIn addition, exposure by each of the exposure apparatuses corresponding to three colors other than four colors or single color black is performed by applying an electrostatic latent image for forming a black image by the four colors or the mixed colors of the three colors on the photoreceptor. To compensate for insufficient density of monochromatic black image if necessaryThis is an image forming method.
[0036]
  Therefore, the exposure apparatus corresponding to black of the single color has a higher optical writing density in the main scanning direction than the optical writing density in the main scanning direction of the exposure apparatus corresponding to other colors. High-definition characters can be developed, and the gradation of the highlight portion can be improved. In addition, since it is not necessary to equip a plurality of exposure apparatuses for black images, the manufacturing cost of the apparatus can be reduced and the apparatus can be miniaturized.
In addition, exposure by each exposure apparatus corresponding to three colors other than four colors or single color black is performed by irradiating an electrostatic latent image for forming a black image by the mixed colors of the four colors or the three colors on the photoreceptor. By writing, the shortage of the density of the monochrome black image is compensated as necessary. Therefore, when the density of the monochrome black image is insufficient, the lack of density can be compensated by the mixed color black image.
[0039]
  Claim 12The invention described in claim 11In the invention described in (1), the exposure by the exposure apparatus corresponding to the monochrome black is performed by optical writing with the LED array head.
[0040]
Therefore, by using the LED array head in the exposure apparatus for forming a black image, the beam spot area can be easily reduced as compared with the case where a laser diode is used.
[0041]
  Claim 13The invention described in claim 12In the invention described in (1), the exposure by each exposure apparatus corresponding to a color other than single black is performed by optical writing with a laser diode.
[0042]
Accordingly, an image forming method having an excellent cost balance is provided by using an LED array head for an exposure apparatus for forming a black image and using a laser diode having a lower cost than the LED array head for an exposure apparatus for forming other colors. be able to.
[0043]
  Claim 14The invention described in claim 11~ 13In the invention according to any one of the above, exposure by each exposure apparatus corresponding to a color other than a single black color,
The optical writing density in the sub-scanning direction is higher than the optical writing density in the sub-scanning direction of exposure by an exposure apparatus corresponding to another color.
[0044]
Therefore, high-definition characters can be developed for the black image, and the gradation of the highlight portion can be improved.
[0045]
  Claim 15The invention described in claim 11~ 14In the invention described in any one of the above, in the exposure by the exposure apparatus corresponding to monochrome black, the beam spot area and one dot area on the photosensitive member are made smaller than one pixel area determined by the optical writing density. It is characterized by that.
[0046]
Therefore, high-definition characters can be developed for the black image, and the gradation of the highlight portion can be improved.
[0049]
DETAILED DESCRIPTION OF THE INVENTION
Embodiment 1 of the Invention
FIG. 1 is a front view showing a schematic configuration of the entire tandem type color image forming apparatus 1 according to Embodiment 1 of the present invention. As shown in FIG. 1, the color image forming apparatus 1 is provided with four image processing units 2Y, 2M, 2C, and 2K for yellow (Y), magenta (M), cyan (C), and black (K). ing.
[0050]
Each of the image processing units 2Y, 2M, 2C, and 2K includes a photosensitive member 3, a charging device 4 that charges the photosensitive member 3, and an exposure device 5 (5a) that optically writes an electrostatic latent image on the photosensitive member 3. And developing devices 6Y, 6M, 6C, and 6K that develop the electrostatic latent images on the photoreceptor 3 with toners of Y, M, C, and K, respectively, to form toner images of Y, M, C, and K; , Y, M, C, and K are provided with a transfer device 7 that transfers the toner images to the transfer paper P, and a cleaning device that is not shown in FIG.
[0051]
The transfer belt 11 is stretched between a drive roller 12 and a driven roller 13 that are rotationally driven by a drive source (not shown), and sequentially conveys the transfer paper P to the image processing units 2Y, 2M, 2C, and 2K. The image processing units 2Y, 2M, 2C, and 2K operate synchronously on the transfer paper P conveyed by the transfer belt 11, and Y, M, C, and K toner images are superimposed on the transfer paper P. To form a full-color image. The transfer paper P on which toner images of Y, M, C, and K are formed is fixed with toner by the fixing device 14. Each unit of the color image forming apparatus 1 is driven and controlled by a controller (not shown) having a microcomputer configuration.
[0052]
Next, the exposure apparatuses 5 and 5a will be described in detail.
[0053]
Among the image processing units 2Y, 2M, 2C, and 2K, the exposure device 5 of the image processing unit 2K uses an LED array head for optical writing, and the exposure devices 5a of the image processing units 2Y, 2M, and 2C are light beams. A laser diode is used for writing.
[0054]
FIG. 2 is a graph showing a light amount distribution state obtained by a stationary beam at the image plane position of the photoreceptor 3. That is, an electrostatic latent image is uniformly formed on the photoreceptor 3 after being uniformly charged by the light irradiated from the exposure devices 5 and 5a. The beam spot when forming the electrostatic latent image is a stationary beam. The graph of FIG. 2 shows the light quantity distribution when grasped as. In FIG. 2, d is 1 / e of the peak value of the light amount distribution.²The width in the main scanning direction is shown. The width in the main scanning direction in this case is the arrangement direction of the LED light emitting elements of the LED array head in the case of the exposure apparatus 5. A 1-dot beam spot area described below is obtained by calculation as an elliptical area assumed by the width d in the main scanning direction and the width in the sub-scanning direction (direction orthogonal to the main scanning direction). Further, the area of one pixel determined by the optical writing density described below is, for example, 42.3 × 42.3 μm in the case of an optical writing density of 600 dpi.²And the area of a square. Further, the 1-dot area on the photoconductor 3 described below was obtained by actually measuring the 1-dot area on the photoconductor 3.
[0055]
Here, in the present embodiment, the area of the beam spot for forming the monochrome black image out of the beam spot for exposing the photoreceptor 3 is larger than the area of the beam spot for forming the image other than the monochrome black. Designed to be smaller. In other words, the exposure device 5 in the black image processing unit 2K is compared with the exposure devices 5a in the other color image processing units 2Y, 2M, and 2C. The area is set to be small. More specifically, the area of the light emitting part of each light emitting element constituting the LED array head of the exposure apparatus 5 is formed smaller than the area of the light emitting part of the laser diode of the exposure apparatus 5a. Thereby, in the light amount distribution of the stationary beam illustrated in FIG. 2, the exposure apparatus 5 uses 1 / e of the peak value.²The width d which is the width in the main scanning direction at the value of N is narrowed, and the beam spot area assumed by the main scanning direction width d and the sub scanning direction width is narrowed.
[0056]
In the present embodiment, the exposure device 5 in the black image processing unit 2K is compared with the exposure devices 5a of the other color image processing units 2Y, 2M, and 2C in both the main scanning direction and the sub-scanning direction. Also, the optical writing density is designed to be high. Further, the area of the beam spot formed on the photosensitive member 3 by the exposure device 5 is set smaller than the area of one pixel determined by the optical writing density. For example, if the optical writing density of the exposure apparatus 5 is 1200 dpi, the area of one pixel is 21 × 21 μm.²However, in this embodiment, the area of the beam spot formed on the photoconductor 3 by the exposure device 5 is 100 π μm, which is less than such an area of one pixel.²It is. At that time, the area of one dot on the photosensitive member 3 is also about 350 μm.²It is. The exposure apparatus 5a in the image processing units 2Y, 2M, and 2C for other colors has an optical writing density of 600 dpi and a beam spot area per dot of 2000π μm.²It is. Therefore, as shown in a schematic diagram of black image dots formed on the photoreceptor 3 shown in FIG. 4 (FIG. 3 is a schematic diagram showing an exposure energy distribution when a solid black image is formed, and FIG. 3 shows dots of a black image formed on the photosensitive member 3 by the beam spot having the exposure energy illustrated in FIG. 3), and the electrostatic latent image formed by the exposure device 5 of the image processing unit 2K is developed. The development dots obtained in this way are extremely high definition, and therefore high definition image formation is performed for black images. Moreover, even if the gradation expression by the area gradation method is used, a higher gradation expression of the highlight portion can be obtained.
[0057]
On the other hand, while high-definition image formation and gradation expression of a high highlight portion can be obtained, as is apparent from FIG. 4, a gap is generated between the optical writing positions of the black image. For this reason, it is not suitable for high-density image expression. Therefore, in the present embodiment, when it is desired to obtain a high-density image, exposure by the exposure device 5 in the image processing units 2Y, 2M, and 2C of colors other than black and development are performed, and yellow, magenta, and cyan Black in which an image obtained by superimposing color toner images develops color (or monochromatic black may be superimposed), that is, mixed color black is used. Here, since the exposure apparatus 5 of the image processing units 2Y, 2M, and 2C of colors other than black has a beam spot diameter larger than the beam spot diameter for monochromatic black, the formed dots are mixed color black. Larger diameter than monochrome black.
[0058]
FIG. 5 is a graph showing the relationship between image density and document density in a single color black image and a mixed color black image. As shown in FIG. 5, the γ curve (dotted line (1)) with respect to the image density in the single color black rises rapidly at the position of ID 1.0 in the mixed color black (dotted line (2)). Thereby, in the present embodiment, a beam spot diameter of about 50 × 50 μm is obtained as the beam spot diameter for mixed color black. In this case, the lighting time of the light emitting element of the exposure apparatus 5a is 40 μsec, and the stationary beam power at this time is 2 mW.
[0059]
FIG. 6 is a flowchart for explaining the flow of processing for determining whether to form a mixed color black image or a single color black image as described above in the color image forming apparatus 1. As shown in FIG. 6, when image data is input (step S1), the color image forming apparatus 1 performs arithmetic processing and stores image gradation information of each color in a RAM of a controller (not shown) (step S2). ). Here, the optical writing density of the exposure apparatus 5 corresponding to monochrome black is 1200 dpi, and the optical writing density of the exposure apparatus 5a corresponding to other colors is 600 dpi. Therefore, the 1200 × 4 4 × 4 matrix is 600 dpi. This corresponds to a 2 × 2 matrix. In this case, the gradation expression uses an area gradation method that expresses gradation with a 4 × 4 matrix of 1200 dpi, and a dither method or an error diffusion method is used as a specific method. As the number of gradations, there are two types of black, monochrome black and mixed color black, on the premise that the gradation is expressed by a 1200 × 4 4 × 4 matrix, so 4 × 4 + 2 × 2 + 1 = 21 gradations. Can be expressed. Therefore, for black, a solid black image has a gradation number of 21/21. On the other hand, as illustrated in FIG. 5, when the monochrome black and the mixed color black are compared, the γ curve with respect to the document density changes rapidly at the position of ID 1.0. Therefore, ID 1.0, that is, the number of gradations 17/21 is set as a threshold (step S3), and if the number of gradations is less than that, a black image is formed with monochromatic black (step S4). When the above is reached, a black image is formed with mixed color black (step S5).
[0060]
By performing such control, when high-definition black image formation is required, a monochrome black image as shown on the left side in FIG. 7 is formed, and formation of a high-density black image is required. In this case, a mixed color black image as shown on the right side in FIG. 7 is formed. Further, even if the black gradation image is expressed by area gradation of a 1200 dpi 4 × 4 matrix (corresponding to a 600 dpi 2 × 2 matrix), since the number of gradations is as large as 21, for example, good gradation expression in a highlight portion. Is possible. Thus, according to the present embodiment, it is possible to achieve both high-definition image formation and high-density image formation.
[0061]
As described above, of the image processing units 2Y, 2M, 2C, and 2K, the exposure device 5 of the image processing unit 2K uses the LED array head for optical writing. In comparison, the beam spot area can be easily reduced.
[0062]
Further, while an LED array head is used for the exposure device 5 of the image processing unit 2K, each exposure device 5a of the image processing units 2Y, 2M, and 2C uses a laser diode for optical writing. A forming apparatus can be provided.
[0063]
[Embodiment 2 of the Invention]
FIG. 8 is a longitudinal sectional view showing an outline of a digital color copying machine 50 according to the second embodiment of the present invention. The color image forming apparatus 1 is mounted on the digital color copying machine 50. In FIG. 8, members having the same reference numerals as those in FIG. 1 are members similar to those of the color image forming apparatus 1. The detailed explanation is omitted.
[0064]
As shown in FIG. 8, the digital color copying machine 50 is roughly divided into a scanner unit 51 and a printer unit 52.
[0065]
The scanner unit 51 implements the image reading apparatus of the present invention, and reads an image of a document for each of red, green, and blue color separation lights. The image signal read from the original is subjected to color conversion processing by an image processing unit (not shown) and converted into four color image signals of Y, M, C, and K.
[0066]
In the printer unit 52, the paper feed roller 53 feeds the transfer paper P from one of the paper feed cassettes 54, and the registration roller 55 sends the transfer paper P to the transfer belt 11 at the timing when the transfer paper P is aligned with the leading edge of the image. Further, the transfer paper P on which the toner is fixed by the fixing device 14 is discharged by a paper discharge roller 56.
[0067]
According to the digital color copying machine 50 configured as described above, since the color image forming apparatus 1 described above is provided, the same operations and effects as the color image forming apparatus 1 can be achieved.
[0068]
【The invention's effect】
  According to the first aspect of the present invention, the exposure apparatus corresponding to black of a single color has an optical writing density in the main scanning direction of an exposure apparatus whose optical writing density in the main scanning direction corresponds to another color. Since it is higher, it is possible to develop high-definition characters with respect to the black image and to improve the gradation of the highlight portion. In addition, since it is not necessary to equip a plurality of exposure apparatuses for black images, the manufacturing cost of the apparatus can be reduced and the apparatus can be miniaturized.
In addition, each exposure apparatus corresponding to three colors other than four colors or single color black optically writes an electrostatic latent image for forming a black image by mixing the four colors or the three colors onto the photosensitive member. As a result, the density shortage of the monochrome black image is compensated as necessary. Therefore, when the density is insufficient only with the monochrome black image, the density shortage can be compensated by the mixed color black image.
[0070]
  Claim2The invention described in claim 11In the invention described in 1), by using the LED array head in the exposure apparatus for forming a black image, the beam spot area can be easily reduced as compared with the case where a laser diode is used.
[0071]
  Claim3The invention described in claim 12In the invention described in the above, image formation with an excellent cost balance is achieved by using an LED array head for an exposure apparatus for forming a black image and using a laser diode having a lower cost than the LED array head for an exposure apparatus for forming other colors. An apparatus can be provided.
[0072]
  Claim4The invention described in claim 13In the invention described in any one of the above, it is possible to develop high-definition characters with respect to a black image, and it is possible to improve the gradation of the highlight portion.
[0073]
  Claim5The invention described in claim 14In the invention described in any one of the above, it is possible to develop high-definition characters with respect to a black image, and it is possible to improve the gradation of the highlight portion.
[0075]
  Claim6In the invention described in the above, the exposure apparatus that supports monochrome black has a higher optical writing density in the main scanning direction than the optical writing density in the main scanning direction of the exposure apparatus that supports other colors. Thus, it becomes possible to develop high-definition characters with respect to the black image, and the gradation of the highlight portion can be improved. In addition, since it is not necessary to equip a plurality of exposure apparatuses for black images, the manufacturing cost of the apparatus can be reduced and the apparatus can be miniaturized.
In addition, each exposure apparatus corresponding to three colors other than four colors or single color black optically writes an electrostatic latent image for forming a black image by mixing the four colors or the three colors onto the photosensitive member. As a result, the density shortage of the monochrome black image is compensated as necessary. Therefore, when the density is insufficient only with the monochrome black image, the density shortage can be compensated by the mixed color black image.
[0077]
  Claim7The invention described in claim 16In the invention described in 1), by using the LED array head in the exposure apparatus for forming a black image, the beam spot area can be easily reduced as compared with the case where a laser diode is used.
[0078]
  Claim8The invention described in claim 17In the invention described in the above, a copier having an excellent cost balance by using an LED array head for an exposure apparatus for forming a black image and using a laser diode having a lower cost than the LED array head for an exposure apparatus for forming other colors. Can be provided.
[0079]
  Claim9The invention described in claim 16~8In the invention described in any one of the above, it is possible to develop high-definition characters with respect to a black image, and it is possible to improve the gradation of the highlight portion.
[0080]
  Claim 10The invention described in claim 16~9In the invention described in any one of the above, it is possible to develop high-definition characters with respect to a black image, and it is possible to improve the gradation of the highlight portion.
[0082]
  Claim 11In the invention described in the above, the exposure apparatus that supports monochrome black has a higher optical writing density in the main scanning direction than the optical writing density in the main scanning direction of the exposure apparatus that supports other colors. Thus, it becomes possible to develop high-definition characters with respect to the black image, and the gradation of the highlight portion can be improved. In addition, since it is not necessary to equip a plurality of exposure apparatuses for black images, the manufacturing cost of the apparatus can be reduced and the apparatus can be miniaturized.
In addition, exposure by each exposure apparatus corresponding to three colors other than four colors or single color black is performed by irradiating an electrostatic latent image for forming a black image by the mixed colors of the four colors or the three colors on the photoreceptor. By writing, the shortage of the density of the monochrome black image is compensated as necessary. Therefore, when the density of the monochrome black image is insufficient, the lack of density can be compensated by the mixed color black image.
[0084]
  Claim 12The invention described in claim 11In the invention described in 1), by using the LED array head in the exposure apparatus for forming a black image, the beam spot area can be easily reduced as compared with the case where a laser diode is used.
[0085]
  Claim 13The invention described in claim 12In the invention described in the above, image formation with an excellent cost balance is achieved by using an LED array head for an exposure apparatus for forming a black image and using a laser diode having a lower cost than the LED array head for an exposure apparatus for forming other colors. A method can be provided.
[0086]
  Claim 14The invention described in claim 11~ 13In the invention described in any one of the above, it is possible to develop high-definition characters with respect to a black image, and it is possible to improve the gradation of the highlight portion.
[0087]
  Claim 15The invention described in claim 11~ 14In the invention described in any one of the above, it is possible to develop high-definition characters with respect to a black image, and it is possible to improve the gradation of the highlight portion.
[Brief description of the drawings]
FIG. 1 is a front view showing an outline of a color image forming apparatus according to Embodiment 1 of the present invention;
FIG. 2 is a graph showing a light amount distribution state obtained by a stationary beam at the position of a photoreceptor image surface.
FIG. 3 is a schematic diagram showing an exposure energy distribution when a solid black image is formed.
4 is a schematic diagram showing dots formed on a photoconductor by a beam spot having exposure energy illustrated in FIG. 3; FIG.
FIG. 5 is a graph showing a relationship between image density and document density in a single color black image and a mixed color black image.
FIG. 6 is a flowchart of processing for determining whether to form a single color black image or a mixed color black image in the color image forming apparatus.
FIG. 7 is a schematic diagram showing a contrast between a monochrome black image and a mixed color black image.
FIG. 8 is a longitudinal sectional view showing an outline of a digital color copying machine according to a second embodiment of the present invention;
[Explanation of symbols]
1 Image forming device
5 Exposure equipment
5a Exposure equipment
6Y Developer
6M Developer
6C Development device
6K developing device
50 copier
51 Image reader
P transfer paper

Claims (15)

A plurality of exposure apparatuses for optically writing an electrostatic latent image corresponding to four colors of yellow, magenta, cyan and black on each photoconductor;
A plurality of developing devices for developing each electrostatic latent image formed by each exposure device with color toners corresponding to the four colors,
In an electrophotographic image forming apparatus for forming a color image obtained by superimposing the four color toners,
The exposure apparatus corresponding to the monochrome black is an exposure apparatus whose beam spot area defined by an elliptical area of 1 / e ² of the peak value of the light amount distribution obtained by the stationary beam corresponds to another color. The optical writing density in the main scanning direction is narrower than the beam spot area and higher than the optical writing density in the main scanning direction of the exposure apparatus corresponding to the other color, and
Each exposure apparatus corresponding to three colors other than four colors or single color black optically writes an electrostatic latent image for forming a black image by the four colors or the mixed colors of the three colors on the photosensitive member. An image forming apparatus that compensates for a lack of density of a monochrome black image as necessary.   The image forming apparatus according to claim 1, wherein the exposure apparatus supporting monochrome black performs optical writing with an LED array head.   The image forming apparatus according to claim 2, wherein each exposure apparatus corresponding to a color other than a single black performs optical writing with a laser diode.   Each exposure apparatus corresponding to a color other than single black has a higher optical writing density in the sub-scanning direction than an optical writing density in the sub-scanning direction of an exposure apparatus corresponding to another color. The image forming apparatus according to claim 1. To have an exposure apparatus corresponding to a single color black claim 1-4, characterized by less than one pixel area determined one dot area on the beam spot area and photosensitive member in the optical writing density The image forming apparatus according to 1. A document image is read for each of a plurality of color separation lights, and each image signal for each read color separation light is subjected to color conversion processing, and a plurality of color image data corresponding to four colors of yellow, magenta, cyan, and black are obtained. An image reading device to obtain;
An image forming apparatus that forms a color image by electrophotography based on the plurality of color image data,
The image forming apparatus includes:
A plurality of exposure apparatuses for optically writing an electrostatic latent image based on each color image data corresponding to the four colors onto each photoconductor;
A plurality of developing devices for developing each electrostatic latent image formed by each exposure device with color toners corresponding to the four colors,
The color images are formed by superimposing the color toners,
The exposure apparatus corresponding to the monochrome black is an exposure apparatus whose beam spot area defined by an elliptical area of 1 / e ² of the peak value of the light amount distribution obtained by the stationary beam corresponds to another color. The optical writing density in the main scanning direction is narrower than the beam spot area and higher than the optical writing density in the main scanning direction of the exposure apparatus corresponding to the other color, and
Each exposure apparatus corresponding to three colors other than four colors or single color black optically writes an electrostatic latent image for forming a black image by the four colors or the mixed colors of the three colors on the photosensitive member. A copying machine characterized in that it compensates for insufficient density of a monochrome black image as necessary.   7. The copying machine according to claim 6, wherein the exposure apparatus that supports monochrome black performs optical writing with an LED array head.   8. The copying machine according to claim 7, wherein each exposure apparatus corresponding to a color other than monochrome black performs optical writing with a laser diode.   Each exposure apparatus corresponding to a color other than single black has a higher optical writing density in the sub-scanning direction than an optical writing density in the sub-scanning direction of an exposure apparatus corresponding to another color. A copier according to any one of claims 6 to 8.   10. The exposure apparatus supporting monochrome black makes the beam spot area and one dot area on the photosensitive member smaller than one pixel area determined by optical writing density. The copying machine described in 1. A step of optically writing an electrostatic latent image corresponding to each of the four colors on a photoconductor using a plurality of exposure apparatuses corresponding to four colors of yellow, magenta, cyan and black,
Developing each electrostatic latent image with a color toner corresponding to the four colors,
In an electrophotographic image forming method for forming a color image obtained by superimposing the four color toners,
In the exposure by the exposure apparatus corresponding to the monochrome black, the beam spot area defined by the elliptical area of 1 / e ² of the peak value of the light amount distribution obtained by the stationary beam corresponds to the other color. It is narrower than the beam spot area of the apparatus, and its optical writing density in the main scanning direction is higher than the optical writing density in the main scanning direction of exposure by the exposure apparatus corresponding to the other color, and is four colors or single color Exposure by each exposure apparatus corresponding to three colors other than black is necessary by optically writing an electrostatic latent image for forming a black image by the above four colors or a mixed color of the three colors on the photoreceptor. An image forming method characterized in that a shortage of density of a monochrome black image is compensated according to the above.   The image forming method according to claim 11, wherein the exposure by the exposure apparatus corresponding to the monochrome black is performed by optical writing with an LED array head.   The image forming method according to claim 12, wherein the exposure by each exposure apparatus corresponding to a color other than a single black color is performed by optical writing with a laser diode.   Exposure by each exposure apparatus corresponding to a color other than black of the single color is based on the optical writing density in the sub-scanning direction of the exposure by the exposure apparatus corresponding to the other color in the sub-scanning direction. The image forming method according to claim 11, wherein the image forming method is high.   15. The exposure by an exposure apparatus corresponding to monochrome black makes the beam spot area and one dot area on the photosensitive member smaller than one pixel area determined by optical writing density. The image forming method according to any one of the above.

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