JP6486241B2 - Image forming apparatus - Google Patents

Description

  BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus such as a copying machine, a printer, and a facsimile machine using an electrophotographic system, and more specifically, an electrophotographic system that expresses gradation by pseudo halftone processing using a plurality of photoconductors. The present invention relates to an image forming apparatus.

  As an electrophotographic photosensitive member (photosensitive member) used in an electrophotographic image forming apparatus, an organic material is used as a photoconductive substance (a charge generating substance or a charge transporting substance) because of low cost and high productivity. Organic photoreceptors in which a photosensitive layer (organic photosensitive layer) is provided on a support are widely used. As an organic photoreceptor, a multilayer photosensitive layer formed by laminating a charge generation layer containing a charge generation material and a charge transport layer containing a charge transport material from the advantages of high sensitivity and a variety of material designs. Photoconductors having them are the mainstream.

  Electrical and / or mechanical external forces of charging, exposure, development, transfer, and cleaning are directly applied to the surface of the photoreceptor. Therefore, the surface of the photoconductor tends to be hardened in order to improve the durability of the photoconductor. However, if the strength of the surface layer of the photoconductor is increased, the amount of abrasion of the surface layer of the photoconductor is reduced, so that the releasability is impaired, and the cleaning performance and the durability performance are liable to be problematic. Specific examples of the deterioration in cleaning performance include chattering and turning of the cleaning blade, and further cleaning failure due to blade edge chipping or chipping. Here, chattering of the cleaning blade is a phenomenon in which the cleaning blade vibrates due to an increase in frictional resistance between the cleaning blade and the surface of the photosensitive member. Further, the turning of the cleaning blade is a phenomenon in which the cleaning blade is reversed in the moving direction of the photosensitive member in a high humidity environment. On the other hand, specific examples of the decrease in durability performance include an increase in the amount of wear on the surface layer due to an increase in frictional resistance and the occurrence of scratches due to local pressure concentration.

  Therefore, there is a method of improving the releasability and reducing the frictional force by roughening the surface of the photoconductor to reduce the contact area when the cleaning blade comes into contact. Examples of the method for roughening the surface of the photoreceptor include the following. A method for controlling the drying conditions when forming the surface layer, a method for incorporating particles into the surface layer, a method for polishing using a metal wire brush, a method using a specific cleaning means and toner, a film-like abrasive A method of polishing using a method, a method of performing blasting, and the like.

  On the other hand, a method for controlling the surface shape of the photoconductor in more detail has been proposed. For example, in order to suppress problems such as cleaning performance and rubbing memory, a photoconductor having a predetermined dimple shape has been proposed (Patent Document 1). Further, a method has been proposed in which the surface of the photoreceptor is compression-molded using a well-shaped uneven stamper to form an independent uneven shape on the surface of the photoreceptor with good controllability (Patent Document 2). Furthermore, the thing (patent document 3) which prescribed | regulated the shape and space | interval of a recessed part is proposed. In addition, a device in which the surface shape of the intermediate transfer member is devised (Patent Document 4) has been proposed in order to ensure the slipperiness on the intermediate transfer member carrying the toner image as in the case of the photosensitive member.

  However, when the surface shape of the photosensitive member is controlled as described above, the toner image may be scattered, or the toner image may be distorted or misaligned depending on the surface shape. Then, as described in Patent Document 3 and Patent Document 4, the surface shape of the photoreceptor is regularly formed so as to have a specific periodicity (regularity) (having a peak in a specific frequency band at a spatial frequency). If this occurs, moire may occur due to interference with pseudo halftone processing (halftoning) during image formation.

  To deal with this problem, Patent Document 4 adjusts the surface shape of the intermediate transfer member so that moire is difficult to visually perceive for black, which is the lightest color (lowest lightness color) among cyan, magenta, yellow, and black. Is disclosed.

International Publication No. 2005/093518 JP 2001-0666814 A JP 2010-008898 A Japanese Patent Laid-Open No. 11-198451

  However, in an image forming apparatus that expresses gradation by pseudo-halftone processing, the screen angle of each color does not cause color unevenness in accordance with the degree of color shift (also called “plate shift” or “color shift”). Generally have an angular difference. That is, since yellow, magenta, and cyan images are formed at a screen angle different from that of a black image, color moiré occurs in the configuration of Patent Document 4. Japanese Patent Application Laid-Open No. 2004-228561 describes a moiré regulation for the minimum lightness color and a stipulation for reducing the moire for the maximum lightness color. However, a convex shape is formed on a common intermediate transfer body onto which the toner image of each color is transferred. Therefore, moire occurs even though the interval is 0.5 mm or less.

  Here, since moire is interference between specific frequencies in the spatial frequency, it is only necessary to reduce the periodicity of one, and for example, an error diffusion method may be used. Using this principle, for example, as described above, a method of expressing an error diffusion shape in which a plurality of concave portions are distributed on the surface of the photoreceptor by an error diffusion method to avoid moiré with pseudo halftone processing is conceivable. . However, if dot disturbance occurs due to the surface shape of the photoconductor, the error diffusion pattern is easily noticeable and the graininess (roughness and flickering) may deteriorate. This deterioration in graininess becomes noticeable in black with low brightness that is easily recognized visually.

  In the case of an image forming apparatus having a plurality of photoconductors, it is necessary to consider that an excessive increase in the types of photoconductors is disadvantageous in manufacturing and management.

  Accordingly, an object of the present invention is to suppress moire without excessively increasing the types of photoconductors in a configuration in which gradation is expressed by pseudo halftone processing using a plurality of photoconductors having a plurality of concave portions. An object of the present invention is to provide an image forming apparatus capable of suppressing deterioration of graininess.

  The above object is achieved by the image forming apparatus according to the present invention. In summary, the first aspect of the present invention includes a plurality of photoconductors, and a processing unit that generates gradation of an image formed with toner on each of the plurality of photoconductors by a pseudo halftone process using a screen; Toner image forming means for forming an image with a plurality of different colors of toner on each of the plurality of photoconductors based on the processing result of the processing unit, and on each surface of the plurality of photoconductors, A pattern in which a plurality of independent recesses are formed, and the plurality of recesses have a specific periodicity on the surface of the photoreceptor on which an image is formed with toner of the lowest brightness color among the toners of the plurality of colors The plurality of recesses have a specific periodicity on the surface of the photoreceptor on which images are formed with at least two toners excluding the minimum lightness color among the plurality of color toners. Not substantial It is an image forming apparatus, comprising that are formed in the same pattern.

  According to a second aspect of the present invention, there is provided a plurality of photoconductors, a processing unit that generates a gradation of an image formed with toner on each of the plurality of photoconductors by a pseudo halftone process using a screen, Toner image forming means for forming an image with a plurality of different colors of toner on each of the plurality of photoconductors based on the processing results, and each of the plurality of photoconductors has a plurality of independent images. A plurality of concave portions are formed in a pattern having a specific periodicity on the surface of the photoconductor on which an image is formed with toner of the lowest brightness color among the plurality of color toners. The plurality of concave portions have substantially the same periodicity on the surface of the photoconductor on which images are formed with at least two color toners excluding the minimum lightness color among the plurality of color toners. The putter In are formed, respectively, the pattern is an image forming apparatus, wherein different from the pattern of the plurality of recesses of the photoreceptor surface of the image with a toner of the minimum lightness color is formed.

  According to a third aspect of the present invention, there is provided a plurality of photoconductors, a processing unit that generates a gradation of an image formed with toner on each of the plurality of photoconductors by pseudo halftone processing using a screen, Toner image forming means for forming an image with a plurality of different colors of toner on each of the plurality of photoconductors based on the processing results, and each of the plurality of photoconductors has a plurality of independent images. The plurality of recesses have a specific periodicity on the surface of the photoreceptor on which images are formed with at least two color toners including the lowest lightness color among the plurality of color toners. The image is formed in substantially the same pattern, and the image is formed with at least two toners other than at least two colors including the lowest lightness color among the plurality of color toners. On the surface of the body, an image forming apparatus characterized by being formed respectively in substantially the same pattern in which the plurality of recesses has no specific periodicity.

  According to a fourth aspect of the present invention, there is provided a plurality of photoconductors, a processing unit that generates a gradation of an image formed with toner on each of the plurality of photoconductors by a pseudo halftone process using a screen, Toner image forming means for forming an image with a plurality of different colors of toner on each of the plurality of photoconductors based on the processing results, and each of the plurality of photoconductors has a plurality of independent images. The plurality of recesses have a specific periodicity on the surface of the photoreceptor on which images are formed with at least two color toners including the lowest lightness color among the plurality of color toners. The image is formed with substantially the same pattern, and the image is formed with at least two color toners other than the at least two color toners including the lowest lightness color among the plurality of color toners. On the surface of the body, the plurality of recesses are respectively formed in a pattern having a specific periodicity, and the pattern is formed on each of the photoreceptors on which an image is formed with at least two colors of toner including the minimum lightness color. An image forming apparatus, wherein the pattern is different from the pattern of the plurality of recesses on the surface.

  According to the present invention, in a configuration in which gradation is expressed by pseudo halftone processing using a plurality of photosensitive members having a plurality of concave portions, granularity is suppressed while suppressing moire without excessively increasing the number of types of photosensitive members. Sexual deterioration can be suppressed.

1 is a schematic cross-sectional view of an image forming apparatus. It is a schematic block diagram of a controller (image processing part). It is a flowchart figure of the image processing by a controller. It is a schematic diagram for demonstrating an example of the method of forming the recessed part of a photoreceptor. It is a schematic diagram for demonstrating the other example of the method of forming the recessed part of a photoreceptor. It is a schematic diagram which shows the example of the shape of the recessed part of a photoreceptor. It is a schematic diagram which shows an example of the dither matrix of dot growth. It is a schematic diagram which shows an example of the dither matrix of line growth. It is a schematic diagram for demonstrating the number of lines and angle of the surface shape of a screen and a photoreceptor. It is explanatory drawing which shows the relationship between the angle between lines, and a moire. It is explanatory drawing which shows the relationship between the cycle between lines, and a moire. It is explanatory drawing of an error diffusion method. It is a schematic diagram which shows an example of an error diffusion pattern. It is a schematic diagram of an image for explaining a determination method of moire. It is a graph for demonstrating the spatial frequency analysis in the determination method of a moire. It is a graph for demonstrating the spatial frequency analysis in the determination method of a moire. It is a schematic diagram of an image for explaining a determination method of moire. It is a graph for demonstrating the spatial frequency analysis in the determination method of a moire. It is a graph for demonstrating the spatial frequency analysis in the judgment method of granularity. It is a graph for demonstrating the spatial frequency analysis in the judgment method of granularity. It is a schematic diagram of the image for demonstrating the judgment method of graininess. It is a schematic diagram for demonstrating a rosette pattern. It is a schematic diagram for demonstrating a rosette pattern. It is a schematic diagram which shows an example of the relationship between a screen pattern and the surface shape of a photoreceptor. It is a schematic diagram which shows an example of the relationship between a screen pattern and the surface shape of a photoreceptor.

  Hereinafter, an image forming apparatus according to an embodiment of the present invention will be described in more detail with reference to the drawings.

1. 1 is a schematic cross-sectional view of an image forming apparatus 100 according to an embodiment of the present invention. The image forming apparatus 100 of the present embodiment is a tandem laser beam printer that employs an intermediate transfer method and can form a full-color image using an electrophotographic method. The image forming apparatus 100 according to the present embodiment converts a full-color recorded image into a recording material (transfer material, paper, etc.) according to an image signal from a device such as a personal computer (PC) connected to the apparatus main body 110. Recording medium, sheet).

  The image forming apparatus 100 includes first, second, and third image forming units SY, SM, SC, which form chromatic color images of yellow (Y), magenta (M), and cyan (C). A fourth image forming unit SBk that forms a black (Bk) color image that is an achromatic image is provided. If there is no particular need to distinguish between elements having the same or corresponding functions and configurations in each of the image forming units 1Y, 1M, 1C, and 1Bk, Y at the end of a code representing an element for any color, A general description will be given with M, C, and Bk omitted.

  The image forming unit S includes a photosensitive drum 1 that is a rotatable drum-type electrophotographic photosensitive member (photosensitive member). The photosensitive drum 1 is rotationally driven in the direction of arrow R1 (clockwise) in the drawing. As will be described in detail later, a plurality of independent recesses are formed on the surface of the photosensitive drum 1. In the image forming unit S, the following devices are arranged around the photosensitive drum 1 in order along the rotation direction. First, a charger 2 as a charging unit is arranged. Next, an exposure device (laser scanner) 3 as an exposure unit is arranged. Next, a developing device 4 as a developing unit is arranged. Next, a primary transfer roller 5 as a primary transfer unit is disposed. Next, a cleaning device 6 as a cleaning means is arranged. Next, a pre-exposure device 7 is disposed as pre-exposure means.

  Further, the image forming apparatus 100 includes an intermediate transfer belt 8 that is configured by an endless belt as an intermediate transfer member so as to face each photosensitive drum 1 of each image forming unit S. The intermediate transfer belt 8 is stretched around a plurality of support rollers, and is driven to rotate in the direction of arrow R2 in the figure. On the inner peripheral surface side of the intermediate transfer belt 8, the primary transfer rollers 5 described above are arranged to face the photosensitive drums 1 of the image forming units S. The primary transfer roller 5 is pressed (biased) toward the photosensitive drum 1 through the intermediate transfer belt 8 to form a primary transfer portion N1 where the photosensitive drum 1 and the intermediate transfer belt 8 are in contact with each other. A secondary transfer device 9 is disposed on the outer peripheral surface side of the intermediate transfer belt 8. The secondary transfer device 9 has a secondary transfer belt 91 formed of an endless belt as a recording material conveying member. The secondary transfer belt 91 is stretched around a plurality of support rollers and is driven to rotate in the direction of arrow R3 in the figure. The secondary transfer belt 91 is in contact with the intermediate transfer belt 8 at a position facing the secondary transfer counter roller 81 which is one of the plurality of support rollers of the intermediate transfer belt 8 to form a secondary transfer portion N2. To do.

  Further, the image forming apparatus 100 includes a fixing device 10 including a fixing roller 10 a and a pressure belt 10 b on the downstream side in the transport direction of the recording material P transported by the secondary transfer belt 91.

  At the time of image formation, the photosensitive drum 1 is rotationally driven around a rotating shaft (not shown) at a predetermined process speed (circumferential speed) in the direction of arrow R1 in the drawing. The surface of the photosensitive drum 1 is charged almost uniformly by a charger 2 to a predetermined potential having a predetermined polarity. The surface potential of the photosensitive drum 1 at this time is defined as a charging potential (non-exposed portion potential) VD. The surface of the charged photosensitive drum 1 is scanned and exposed by the exposure device 3. The exposure device 3 irradiates the photosensitive drum 1 with a predetermined amount of laser light from a laser chip based on an image signal sent from a controller 14 described later. Then, at the portion irradiated with the laser beam on the surface of the photosensitive drum 1, the electric charge held by the charging process is removed. As a result, an electrostatic latent image (electrostatic image) is formed on the photosensitive drum 1. The surface potential of the photosensitive drum 1 in the portion irradiated with the laser light at this time is set as an exposure portion potential VL. The electrostatic image formed on the photosensitive drum 1 is developed (visualized) with toner as a developer by the developing device 4. Inside the developing device 4, a developing sleeve as a developer carrier is provided. When a predetermined developing bias (developing voltage) is applied to the developing sleeve, the toner carried on the developing sleeve corresponds to the electrostatic image on the photosensitive drum 1 at the opposite portion between the photosensitive drum 1 and the developing sleeve. Then fly to and adhere to the photosensitive drum 1. As a result, a toner image is formed on the photosensitive drum 1. When the DC component of the developing bias is Vdc and the potential difference between this Vd and the exposed portion potential VL is the developing contrast Vcont, the toner to be developed increases as Vcont increases.

  The toner image formed on the photosensitive drum 1 is electrostatically transferred onto the intermediate transfer belt 8 by the action of the primary transfer roller 5 in the primary transfer portion N1. At this time, the primary transfer roller 5 is applied with a primary transfer bias (primary transfer voltage) having a polarity opposite to the toner charging polarity (normal charging polarity) during development. For example, when forming a full-color image, toner images of each color of yellow, magenta, cyan, and black are transferred onto the intermediate transfer belt 8 so as to be sequentially superimposed at each primary transfer portion N1. Toner remaining on the photosensitive drum 1 after the primary transfer step (primary transfer residual toner) is removed from the photosensitive drum 1 by the cleaning device 6 and collected. Further, the surface of the photosensitive drum 1 after the removal of the primary transfer residual toner is irradiated with light by the pre-exposure device 7 to remove residual charges, and is charged again by the charger 2.

  Here, in the present embodiment, the cleaning device 6 includes a cleaning blade 61 as a cleaning member and a cleaning container 62 arranged so as to contact the surface of the photosensitive drum 1. The cleaning blade 61 is a plate-like member that is formed of an elastic material such as urethane rubber and extends along the longitudinal direction (rotational axis direction) of the photosensitive drum 1. The cleaning blade 61 is in contact with the surface of the photosensitive drum 1 such that the tip in the counter direction, that is, the free end is directed upstream in the rotation direction (surface movement direction) of the photosensitive drum 1. Then, the cleaning blade 61 rubs the surface of the rotating photosensitive drum 1 and physically scrapes the adhering matter such as the primary transfer residual toner on the surface of the photosensitive drum 1 from the surface of the photosensitive drum 1 for cleaning. The container 62 is accommodated.

  The toner image formed on the intermediate transfer belt 8 is electrostatically transferred onto the recording material P carried and conveyed on the secondary transfer belt 91 in the secondary transfer portion N2. At this time, a secondary transfer bias (secondary transfer) having a polarity opposite to the normal charging polarity of the toner is applied to the secondary transfer belt 91 via the secondary transfer roller 91 which is one of the plurality of support rollers. Transfer voltage) is applied. The toner (secondary transfer residual toner) remaining on the intermediate transfer belt 8 after the secondary transfer process is removed from the intermediate transfer belt 8 and collected by a belt cleaning device (not shown).

  The recording material P to which the toner image has been transferred is conveyed to the fixing device 10 and is heated and pressurized by being nipped and conveyed by the fixing roller 10a and the pressure belt 10b, and the toner image is fixed thereon. . Thereafter, the recording material P is discharged (output) to the outside of the apparatus main body 110.

  In this embodiment, the image forming apparatus 100 can perform image formation in a full color mode capable of forming a full color image and a monochrome mode for forming a monochrome image. In the full color mode, the intermediate transfer belt 8 is brought into contact with all the photosensitive drums 1. In the monochrome mode, the intermediate transfer belt 8 is separated from the photosensitive drums 1Y, 1M, and 1C for yellow, magenta, and cyan (hereinafter also referred to as “color”) by a separation mechanism (not shown). This suppresses the color photosensitive drums 1Y, 1M, and 1C from being worn during the formation of the black and white image. In this embodiment, in order to make the durability of the black photosensitive drum 1Bk and the color photosensitive drums 1Y, 1M, and 1C substantially the same, the outer diameter (diameter) of the black photosensitive drum 1Bk is: It is larger than the outer diameter of the color photosensitive drums 1C, 1M, and 1Y. More specifically, the outer diameter of the black photosensitive drum 1Bk is 80 mm, and the outer diameters of the color photosensitive drums 1Y, 1M, and 1C are 50 mm (however, in Example 4 described later, black photosensitive drum 1Bk). The drum 1Bk is also changed to one having an outer diameter of 50 mm.)

  In the present embodiment, a non-contact charger is used as the black charger 2Bk. Specifically, a corona charger disposed as a non-contact charger facing the photosensitive drum 1Bk is used. On the other hand, in the present embodiment, contact chargers are used as the color chargers 2Y, 2M, and 2C. Specifically, a charging roller disposed in contact with the photosensitive drums 1Y, 1M, and 1C as a contact charger is used.

  In the present embodiment, the black image forming unit SBk includes a potential sensor 11Bk as a potential detection unit between the exposure position by the exposure device 3Bk and the development position by the development device 4Bk in the rotation direction of the photosensitive drum 1Bk. Is arranged. The surface potential of the black photosensitive drum 1Bk (the potential on the photosensitive drum 1Bk) is feedback-controlled using the potential sensor 11Bk. That is, the controller 14 controls the voltage applied to the charger 2Bk so that the surface potential of the photosensitive drum 1Bk acquired by the potential sensor 11Bk and the target potential substantially coincide with each other. On the other hand, in the present embodiment, the color image forming units SY, SM, and SC do not include a potential sensor. Therefore, the surface potentials of the color photosensitive drums 1Y, 1M, and 1C are controlled using known discharge current control.

  In the present embodiment, a plurality of different color toners are respectively applied to a plurality of photoconductors by the charger 2, the exposure device 3, and the developing device 4 of each image forming unit S based on the processing result of a pseudo halftone processing unit to be described later. The toner image forming means for forming an image is configured.

2. Electrophotographic photoreceptor (photoreceptor)
Next, the electrophotographic photosensitive member (photosensitive member) will be described. In the present embodiment, the photoreceptor includes a support and an organic photosensitive layer (hereinafter also simply referred to as “photosensitive layer”) provided on the support. In general, a cylindrical (drum type) organic photosensitive member in which a photosensitive layer is formed on a cylindrical support is widely used as the photosensitive member, but a belt-like or sheet-like shape is also possible. In the present embodiment, a drum-type organic photoreceptor (photosensitive drum) is used.

  Even if the photosensitive layer is a single-layer type photosensitive layer containing the charge transport material and the charge generation material in the same layer, the charge generation layer containing the charge generation material and the charge transport layer containing the charge transport material Separated layered (functionally separated type) photosensitive layers may be used. From the viewpoint of electrophotographic characteristics, a laminated photosensitive layer is preferred. In addition, the laminated type photosensitive layer is a reverse layer type in which the charge transport layer and the charge generation layer are laminated in this order from the support side, even if it is a normal layer type photosensitive layer in which the charge generation layer and the charge transport layer are laminated in this order from the support side. It may be a photosensitive layer. In the case of employing a laminated photosensitive layer, the charge generation layer may have a laminated structure, and the charge transport layer may have a laminated structure. Furthermore, a protective layer can be provided on the photosensitive layer for the purpose of improving durability.

  First, the support will be described. As a material for the support, any material that exhibits conductivity (conductive support) may be used. For example, a support made of metal (made of alloy) such as iron, copper, gold, silver, aluminum, zinc, titanium, lead, nickel, tin, antimony, indium, chromium, aluminum alloy, and stainless steel can be given. Moreover, the said metal support body and plastic support body which have the layer which formed the film by vacuum deposition of aluminum, an aluminum alloy, and an indium oxide tin oxide alloy can also be used. Also, a support in which conductive particles such as carbon black, tin oxide particles, titanium oxide particles, and silver particles are impregnated into plastic or paper together with an appropriate binder resin, or a plastic support having a conductive binder resin is provided. It can also be used.

  The surface of the support may be subjected to a cutting process, a roughening process, or an alumite process for the purpose of preventing interference fringes due to scattering of laser light.

  Between the support and an intermediate layer or photosensitive layer (charge generation layer, charge transport layer) described later, there is a conductive layer for the purpose of preventing interference fringes due to scattering of laser light and covering scratches on the support. It may be provided.

  The conductive layer may be formed using a conductive layer coating liquid in which carbon black, a conductive pigment or a resistance adjusting pigment is dispersed and / or dissolved in a binder resin. You may add the compound which carries out hardening polymerization by the heating or radiation irradiation to the coating liquid for conductive layers. The surface of a conductive layer in which a conductive pigment or a resistance adjusting pigment is dispersed tends to be roughened.

  The film thickness of the conductive layer is preferably 0.2 μm or more and 40 μm or less, more preferably 1 μm or more and 35 μm or less, and even more preferably 5 μm or more and 30 μm or less.

  Examples of the binder resin used for the conductive layer include the following. Polymers / copolymers of vinyl compounds such as styrene, vinyl acetate, vinyl chloride, acrylic ester, methacrylic ester, vinylidene fluoride, trifluoroethylene, polyvinyl alcohol, polyvinyl acetal, polycarbonate, polyester, polysulfone, polyphenylene oxide, Polyurethane, cellulose resin, phenol resin, melamine resin, silicon resin and epoxy resin.

  Examples of the conductive pigment and the resistance adjusting pigment include particles of metals (alloys) such as aluminum, zinc, copper, chromium, nickel, silver, and stainless steel; those deposited on the surface of plastic particles. Alternatively, zinc oxide, titanium oxide, tin oxide, antimony oxide, indium oxide, bismuth oxide, indium oxide doped with tin, or metal oxide particles of tin oxide doped with antimony or tantalum may be used. These may be used alone or in combination of two or more. When two or more types are used in combination, they may be simply mixed, or may be in the form of a solid solution or fusion.

  An intermediate layer having a barrier function or an adhesive function may be provided between the support or the conductive layer and the photosensitive layer (charge generation layer, charge transport layer). The intermediate layer is formed to improve the adhesion of the photosensitive layer, improve the coating property, improve the charge injection property from the support, and protect the photosensitive layer from electrical breakdown.

  Examples of the material for the intermediate layer include the following. Polyvinyl alcohol, poly-N-vinylimidazole, polyethylene oxide, ethyl cellulose, ethylene-acrylic acid copolymer, casein, polyamide, N-methoxymethylated 6 nylon, copolymer nylon, glue and gelatin. The intermediate layer can be formed by applying a coating solution for intermediate layer obtained by dissolving these materials in a solvent and drying it.

  The film thickness of the intermediate layer is preferably 0.05 μm or more and 7 μm or less, and more preferably 0.1 μm or more and 2 μm or less.

  Next, the photosensitive layer will be described. Examples of the charge generating material used in the photosensitive layer include the following. Pyrlium, thiapyrylium dyes; phthalocyanine pigments having various central metals and various crystal systems (α, β, γ, ε, X type, etc.); anthanthrone pigments; dibenzpyrenequinone pigments; pyranthrone pigments; An azo pigment such as trisazo; an indigo pigment; a quinacridone pigment; an asymmetric quinocyanine pigment; a quinocyanine pigment; These charge generation materials may be used alone or in combination of two or more.

  Examples of the charge transport material used in the photosensitive layer include the following. Examples include pyrene compounds, N-alkylcarbazole compounds, hydrazone compounds, N, N-dialkylaniline compounds, diphenylamine compounds, and triphenylamine compounds. Triphenylmethane compounds, pyrazoline compounds, styryl compounds, stilbene compounds.

  When functionally separating the photosensitive layer into a charge generation layer and a charge transport layer, the charge generation layer can be formed by the following method. That is, first, the charge generating material is dispersed by a method using a homogenizer, an ultrasonic dispersion, a ball mill, a vibration ball mill, a sand mill, an attritor, or a roll mill together with a binder resin and a solvent of 0.3 to 4 times (mass ratio). . The charge generation layer coating solution obtained by dispersion is applied. By drying this, a charge generation layer can be formed. The charge generation layer may be a vapor generation film of a charge generation material.

  The charge transport layer can be formed by applying a charge transport layer coating solution obtained by dissolving a charge transport material and a binder resin in a solvent, and drying it. In addition, among the above charge transport materials, those having film formability alone can be formed as a charge transport layer by itself without using a binder resin.

  Examples of the binder resin used for the charge generation layer and the charge transport layer include the following. Polymers and copolymers of vinyl compounds such as styrene, vinyl acetate, vinyl chloride, acrylic acid ester, methacrylic acid ester, vinylidene fluoride, trifluoroethylene, polyvinyl alcohol, polyvinyl acetal, polycarbonate, polyester, polysulfone, polyphenylene oxide, Polyurethane, cellulose resin, phenol resin, melamine resin, silicon resin and epoxy resin.

  The thickness of the charge generation layer is preferably 5 μm or less, more preferably 0.1 μm or more and 2 μm or less.

  The thickness of the charge transport layer is preferably 5 μm or more and 50 μm or less, and more preferably 10 μm or more and 35 μm or less.

  In order to improve the durability performance, which is one of the characteristics required for the photoreceptor, the material design of the charge transport layer serving as the surface layer is important in the case of the above-described function-separated photoreceptor. Examples include using high-strength binder resins, controlling the ratio between plastic charge transport materials and binder resins, and using polymer charge transport materials. It is effective to form the surface layer with a curable resin in order to express the above.

  The charge transport layer itself can be composed of a curable resin. Further, it is possible to form a curable resin layer as the second charge transport layer or protective layer on the above-described charge transport layer. The properties required for the curable resin layer are both the strength of the film and the charge transport capability, and are generally composed of a charge transport material and a polymerized or crosslinkable monomer or oligomer. In some cases, it is also possible to use conductive fine particles whose resistance is controlled for the purpose of imparting charge transport capability.

  As the charge transport material, known hole transport compounds and electron transport compounds can be used. Examples of the polymerizable or crosslinkable monomer or oligomer include a chain polymerization material having an acryloyloxy group and a styrene group, and a sequential polymerization material having a hydroxyl group, an alkoxysilyl group, and an isocyanate group. From the viewpoint of the obtained electrophotographic characteristics, versatility, material design, and production stability, a combination of a hole transporting compound and a chain polymerization material is preferable. Furthermore, both a hole transporting group and an acryloyloxy group are included in the molecule. A system that cures the compound is particularly preferred.

  As the curing means, known means using heat, light, and radiation can be used. In the case of the charge transport layer, the thickness of the cured layer is preferably 5 μm or more and 50 μm or less, more preferably 10 μm or more and 35 μm or less, as described above. In the case of the second charge transport layer or protective layer, the thickness is preferably 0.1 μm or more and 20 μm or less, and more preferably 1 μm or more and 10 μm or less.

  Various additives can be added to each layer of the photoreceptor. Examples of the additives include deterioration inhibitors such as antioxidants and ultraviolet absorbers, organic resin particles such as fluorine atom-containing resin particles and acrylic resin particles, and inorganic particles such as silica, titanium oxide, and alumina.

3. Next, in this embodiment, the surface shape of a photoconductor that is a drum type organic photoconductor (photosensitive drum) will be described.

  In this embodiment, the photoreceptor has a plurality of independent recesses (concave shape) on the surface, and (i) a circular shape having an opening diameter of 20 μm or more and 80 μm or less and a depth of 0.5 μm or more and 8 μm or less. The area ratio having the concave shape per unit area is 10% or more and 30% or less.

  As will be described in detail later, the present invention is characterized by the period and pattern of the recesses on the surface of the photoconductor, and the shape of the recesses is arbitrary. For example, a known one described in Patent Document 3 is used. May be the same. Therefore, the shape of the recess is not limited to a circle, and may be an ellipse shape, a honeycomb shape, or a prism shape as described in Patent Document 3. 6A to 6G show examples of the concave shape. 6A to 6G, d represents the opening diameter of the recess. Further, the recesses on the surface of the photoreceptor may have different shapes, sizes, or depths. Further, as for the recesses on the surface of the photoreceptor, all the recesses may have the same shape, size, or depth. Further, the recesses of the photoconductor may be a combination of recesses having different shapes, sizes or depths, and recesses having the same shape, size or depth.

  The recess is formed on at least the surface of the photoreceptor. The region where the concave portion on the surface of the photoconductor is formed may be the entire surface of the photoconductor or a part of the surface of the photoconductor. However, in order to exhibit good performance, it is preferable that a recess is formed at least in a region in contact with the cleaning blade on the surface of the photoreceptor. Details of the period and pattern of the recesses on the surface of the photoreceptor will be described later.

  Here, for the expression of the cleaning performance of the surface of the photoreceptor by the cleaning member, it is important to consider the influence of the toner (toner particles and external additive) on the two elements of the photoreceptor and the cleaning member. In general, good cleaning performance means that the toner remaining on the surface of the photoconductor without being transferred to the transfer medium is interposed between the cleaning blade and the surface of the photoconductor, and the frictional resistance generated between the two is reduced. It is considered that the state is expressed by reducing. However, depending on the electrophotographic process, the toner interposed between the cleaning blade and the surface of the photoreceptor may become extremely small. For example, it is considered that the frictional resistance between the cleaning blade and the surface of the photosensitive member is particularly likely to increase during large-scale printing of a pattern having a low printing density or during monochromatic continuous printing in a tandem electrophotographic system. Therefore, the above-described deterioration in cleaning performance and durability performance tend to be problematic. Furthermore, there may be a problem of fusion caused by an increase in frictional resistance. Such a problem generally tends to become more prominent as the mechanical strength of the surface layer of the photoconductor becomes higher and the peripheral surface of the photoconductor becomes harder to wear.

  On the other hand, the contact area between the photosensitive member and the cleaning blade is reduced by providing the concave portion on the surface of the photosensitive member as in the present embodiment. As a result, the frictional resistance between the photoreceptor and the cleaning blade tends to decrease, the cleaning performance is maintained well, and the occurrence of various image defects is suppressed. In particular, it is possible to suppress an increase in the frictional resistance between the cleaning blade and the photosensitive member, which is likely to occur at the time of mass printing of a pattern having a low printing density and at the time of monochromatic continuous printing in a tandem electrophotographic system. Thereby, the cleaning performance is maintained satisfactorily. When the area ratio of the recess is less than 10%, it is difficult to obtain the above effect. Such a surface shape of the photoconductor works most effectively when a photoconductor whose surface is not easily worn is applied. This is because, as described above, a photoreceptor whose surface is difficult to wear is highly durable, but problems such as cleaning performance become remarkable.

  The concave portion on the surface of the photoreceptor can be measured using, for example, a commercially available laser microscope, optical microscope, electron microscope, or atomic force microscope.

  As the laser microscope, for example, the following devices can be used. Ultra-depth shape measurement microscope VK-8550, ultra-depth shape measurement microscope VK-9000, and ultra-depth shape measurement microscope VK-9500 (all manufactured by Keyence Corporation); surface shape measurement system Surface Explorer SX-520DR type machine ( (Manufactured by Ryoka System Co., Ltd.); scanning confocal laser microscope OLS3000 (manufactured by Olympus Corporation); real color confocal microscope Oplitex C130 (manufactured by Lasertec Corporation).

  As the optical microscope, for example, the following devices can be used. Digital microscope VHX-500 and digital microscope VHX-200 (both manufactured by Keyence Corporation); 3D digital microscope VC-7700 (manufactured by OMRON Corporation).

  As the electron microscope, for example, the following devices can be used. 3D Real Surface View Microscope VE-9800, and 3D Real Surface View Microscope VE-8800 (both manufactured by Keyence Corporation); Scanning Electron Microscope Conventional / Variable Pressure SEM (manufactured by SII Nanotechnology Co., Ltd.) Scanning electron microscope SUPERSCAN SS-550 (manufactured by Shimadzu Corporation).

  As the atomic force microscope, for example, the following devices can be used. Nanoscale hybrid microscope VN-8000 (manufactured by Keyence Corporation); scanning probe microscope NanoNavi station (manufactured by SII Nanotechnology Inc.); scanning probe microscope SPM-9600 (manufactured by Shimadzu Corporation) ).

  Using the microscope, the number of recesses, the major axis diameter, and the depth in the measurement visual field can be measured with a predetermined magnification. Furthermore, the average major axis diameter, average depth, and area ratio of the apertures per unit area can be obtained by calculation.

As an example, a measurement example using an analysis program by the Surface Explorer SX-520DR type machine will be described. A photoconductor to be measured is placed on a work table, tilted to adjust the level, and three-dimensional shape data of the peripheral surface of the photoconductor is captured in a wave mode. At that time, the magnification of the objective lens may be set to 50 times, and the field observation of 100 μm × 100 μm (10000 μm ² ) may be performed. By this method, the surface of the photoconductor to be measured is divided into four equal parts in the rotation direction of the photoconductor, and divided into 25 equal parts in a direction perpendicular to the rotation direction of the photoconductor. In addition, a square region having a side of 100 μm is provided for measurement. Next, the contour line data of the surface of the photoreceptor is displayed using a particle analysis program in the data analysis software.

The hole analysis parameters of the recess, such as the shape of the recess, the major axis diameter, the depth, and the opening area, can be optimized depending on the formed recess. For example, when observing and measuring a recess having a major axis diameter of about 10 μm, the major axis diameter upper limit may be 15 μm, the major axis diameter lower limit may be 1 μm, the depth lower limit may be 0.1 μm, and the volume lower limit may be 1 μm ³ or more. Then, the number of recesses that can be identified as recesses on the analysis screen is counted, and this is used as the number of recesses.

Further, with the same visual field and analysis conditions as described above, the total aperture area of the recess is calculated from the total aperture area of each recess obtained using the particle analysis program, and the aperture area of the recess is calculated from the following formula: You may calculate an area ratio (Here, what is simply described as an area ratio shows this opening part area ratio.).
{Total aperture area of recess / (Total aperture area of recess + Total area of non-recess)} × 100]

4). Method for Forming Surface Shape of Photoreceptor Next, a method for forming the surface shape of the photoreceptor will be described. The method for forming the surface shape of the photoreceptor is not particularly limited as long as it is a method capable of forming the above-described recess. As an example of the method for forming the surface shape of the photoconductor, there is a method for forming the surface shape of the photoconductor by laser irradiation having output characteristics having a pulse width of 100 ns (nanoseconds) or less. In addition, there is a surface shape forming method in which a shape transfer is performed by pressing a mold having a predetermined shape against the surface of the photoreceptor.

  A method for forming the surface shape of the photoreceptor by laser irradiation having an output characteristic in which the pulse width is 100 ns (nanoseconds) or less will be described. Specific examples of the laser used in this method include an excimer laser using a gas such as ArF, KrF, XeF or XeCl as a laser medium, or a femtosecond laser using titanium sapphire as a medium. Further, the wavelength of the laser beam in the laser irradiation is preferably 1,000 nm or less.

  The excimer laser is laser light emitted in the following steps. First, high energy such as discharge, electron beam or X-ray is applied to a mixed gas of a rare gas such as Ar, Kr or Xe and a halogen gas such as F or Cl to excite the above elements. And combine them. Thereafter, excimer laser light is emitted when dissociating by falling to the ground state. Examples of the gas used in the excimer laser include ArF, KrF, XeCl, and XeF, and any of them may be used. In particular, KrF or ArF is preferable.

More specifically, the method of forming the recesses on the surface of the photoreceptor by the laser irradiation can be performed as follows. As shown in FIG. 4A, a mask in which a laser light blocking part a and a laser light transmitting part b are appropriately arranged is used. Only laser light that has passed through the mask is condensed by the lens and irradiated onto the workpiece, so that a recess having a desired shape and arrangement can be formed. Since a large number of recesses within a certain area can be processed simultaneously instantly regardless of the shape and area of the recesses, the process can be performed in a short time. Laser irradiation using a mask processes several mm ² to several cm ² per irradiation. In laser processing, as shown in FIG. 4B, first, the photosensitive member is rotated by a workpiece rotating motor d. By rotating the laser irradiation position in the axial direction of the photoconductor by the workpiece moving device e while rotating, for example, it is possible to efficiently form the recesses over the entire surface of the photoconductor. The depth of the recess can be adjusted within the above-described desired range depending on the irradiation time and the number of times of irradiation with the laser beam. According to this method, it is possible to realize rough surface processing with high controllability of the size, shape, and arrangement of the recesses, and with high accuracy and high flexibility.

  Further, in the method for forming the surface shape of the photoconductor by laser irradiation, the same mask pattern can be used to form recesses in a plurality of portions on the surface of the photoconductor or the entire surface of the photoconductor by the above-described formation method. By this method, a highly uniform recess can be formed on the entire surface of the photoreceptor. As a result, the mechanical load on the cleaning blade is uniform. The period and pattern of the concave portions of the photoreceptor, which will be described in detail later, can be changed by setting the mask pattern.

Next, a method for forming a surface shape in which the mold having the predetermined shape is brought into pressure contact with the surface of the photoreceptor to transfer the shape will be described. Fig.5 (a) is a figure which shows the example of the schematic of the press-contact shape transcription | transfer apparatus by a mold. After the predetermined mold B is attached to the pressure device A that can repeatedly press and release, the mold B is brought into contact with the photoconductor C at a predetermined pressure to transfer the shape. A mold B longer than the entire circumference of the photoconductor C is attached to the pressure device A, and the photoconductor C is rotated and moved while applying a predetermined pressure to the photoconductor C. It is possible to form a predetermined dimple shape over the entire circumference. As another example, a sheet-shaped mold may be sandwiched between a roll-shaped pressurizing device and a photoreceptor, and surface processing may be performed while feeding the mold sheet. Note that the mold or the photoreceptor may be heated for the purpose of efficiently transferring the shape. In examples described later, the surface shape of the photoconductor is formed by this method. More specifically, the temperature is controlled to 110 ° C., and the mold is pressed against the photoconductor with a pressure of 50 kg / cm ² , The shape was transferred in the circumferential direction.

  The material, size, and shape of the mold itself can be selected as appropriate. The material is metal or resin film with fine surface processing, silicon wafer patterned surface with photoresist, resin film with fine particles dispersed, resin film with a predetermined fine surface shape. Can be mentioned. An example of the mold shape is shown in FIG. As shown in the lower diagram of FIG. 5B, the mold shape is a convex shape, and a concave portion is formed in the photosensitive member by contacting the photosensitive member. The period and pattern of the concave portions of the photosensitive member, which will be described in detail later, can be changed by the above patterning setting. In addition, an elastic body can be installed between the mold and the pressure device for the purpose of imparting pressure uniformity to the photoconductor.

  Desired recesses can be formed by performing the aforementioned laser processing or press-contact shape transfer processing using a mold on the photoreceptor having the surface layer as described above.

5. Image Processing Unit Next, the controller (image processing unit, image forming controller) 14 included in the image forming apparatus 100 will be described with reference to the block diagram of FIG. 2 and the flowchart of FIG.

  A PDL (Page Description Language) image signal such as PS or PCL from an image input device such as a PC is received by the input I / F 101. The input I / F 101 has a LAN line interface such as 100BASE-T and a USB interface, and inputs PDL data to the image forming controller 14 (s201).

  The RIP (Raster Image Processor) 102 converts the PDL information input via the input I / F 101 into 1200 dpi bitmap information (s202). The image information converted into bitmap information by the RIP 102 is sent to a CMS (Color Management System) 103.

  The CMS 103 receives an input color signal such as C'M'Y'K 'from RGB via L * a * b * or C'M'Y'K' from CMYK via L * a * b *. A color designated by a printer driver (not shown) or the like (also referred to as a color space) is converted (s203). Since RGB and CMYK are device-dependent color spaces, they are once converted into L * a * b *, which is a device-independent color space, and then converted into C′M′Y′K ′ suitable for the image forming apparatus 100. Converted.

  The halftoning unit (pseudo halftone processing unit) 104 performs resolution conversion from 1200 dpi to 2400 dpi using a zero-order interpolation (nearest neighbor) method (s204). Further, the halftoning unit 104 performs 2400 dpi binary halftoning (pseudo halftone processing) on the image data (s205), and sends the image data to the image forming unit via the output I / F unit 107. Then, the image forming unit forms an image (s206, s207).

  The CPU 105 plays a role of controlling the image processing, and issues an instruction to temporarily store information in the memory 106 as necessary.

6). Half toning (pseudo halftone processing)
Next, halftoning (pseudo halftone processing) will be described.

  In this embodiment, halftoning is performed using a known processing method called a dither matrix method. The dither matrix method is roughly the following processing method. A dither matrix (screen) in which threshold values as shown in FIG. 7 are described is applied to the input image signal. Then, the input image signal is processed so as to be printed if it is darker than the threshold and not printed if it is thinner than the threshold. In the dither matrix method, control of the number and angle of screens in halftoning, the growth method (dots and lines), etc., is used in many image forming apparatuses because the coefficient of the dither matrix may be changed.

  FIG. 7 is a typical dither matrix, and shows a dot growth square dither (square dot screen, dot screen) of 2400 dpi, binary, 170 lpi (line / inch), and an angle of 45 °, which is also used in the examples described later. ing. The portion shown in gray in FIG. 7 represents a pixel on which a dot is formed when a uniform image signal of 25 levels out of 255 levels is input. By changing the threshold matrix as shown in FIG. 8, development from dot growth to line growth (line screen) is also easy.

  In the embodiment described later, among the halftoning conditions, the number of screen lines, the angle, and the growth method are changed. As shown in FIGS. 8 and 9, the number of lines and the angle of the screen can be changed by optimizing the size of the dither matrix, and the growth method can be changed by optimizing the order of the thresholds.

  Note that, as shown in FIGS. 9A and 9B, the number of lines and the angle of the screen are defined by lines passing (connected) through print pixels (halftone dots) by the dither matrix or the like. FIG. 9A schematically shows a dot screen, and FIG. 9B schematically shows a line screen. Further, as shown in FIGS. 9C and 9D, the number of lines and the angle of the surface shape of the photosensitive drum 1 are defined by a line passing (connected) through the recess. FIG. 9C shows a dot pattern in which the concave portions are arranged at substantially equal intervals in the vertical and horizontal directions, and FIG. The line pattern currently arranged is shown typically.

7). Moire Next, moire caused by the surface shape of the photosensitive drum 1 will be described.

  As described above, moire may occur due to the halftoning and the periodicity of the recesses formed on the surface of the photosensitive drum 1. That is, when a concave portion is formed on the surface of the photosensitive drum 1, a laser beam irradiation position shift at the time of exposure, toner disturbance at the time of development, contact with the intermediate transfer belt 8 at the time of primary transfer, a non-contact portion, etc. For these reasons, the toner image may be disturbed or the transfer efficiency may be locally different. When the concave portions are regularly formed on the surface of the photosensitive drum 1 so as to have a specific periodicity (regularity) (having a peak in a specific frequency band in the spatial frequency), the above-described disturbance or transfer of the toner image. A phenomenon such as a local difference in efficiency is likely to occur according to the periodicity of the recesses on the surface of the photosensitive drum 1. Therefore, moire may occur due to interference between this phenomenon and halftoning.

  Moire occurs when two or more patterns having a specific periodicity are combined. Moire is likely to occur when an angle between specific periodic patterns is close or when there is a slight difference in the number of lines.

  FIGS. 10A to 10D show the occurrence of moiré when the angle between two line patterns is different. As shown in FIGS. 10A to 10D, as the angle of the two patterns becomes closer, the cycle of the intersection of the lines of the two patterns becomes a low frequency (low cycle), so that it is easily recognized as moire. 10A to 10D intersect at the above angles of 90 °, 45 °, 15 °, and 5 °, respectively, and the two patterns have the same number of lines. 10A to 10D, when each of the two patterns is a line pattern, 90 ° has the highest moiré cycle (high cycle).

  Further, when two line patterns having a line number difference of 20% as shown in FIG. 11A are overlapped at the same angle, moire as shown in FIG. 11B is generated. However, even if the line number difference is 20%, moire does not occur as shown in FIG. 11C when the angle between the two line patterns is set to 45 °, for example.

  As described above, it is possible to avoid moiré by providing an angle difference between patterns having specific periodicity. From this point of view, when the concave portion is formed on the surface of the photosensitive drum 1 so as to have a specific periodicity, it is desirable to do the following in order to suppress moire. That is, when a line screen is used in halftoning, it is desirable to optimize the surface shape of the photosensitive drum 1 so that the angle difference from the screen angle is 30 ° or more and 150 ° or less. This angular difference is more desirably 90 °. Further, when a dot screen is used in halftoning, the surface shape of the photosensitive drum 1 is optimized so that the angle difference from the screen angle is 30 ° or more and 60 ° or less because there are two directions of the cycle. It is desirable to do. This angular difference is more preferably 45 °.

  In order to make the moire inconspicuous, the number of lines of the surface shape of the photosensitive drum 1 should be an integer multiple of the number of lines of the screen used in halftoning, particularly in the case of a dot screen, an integer multiple or double the root. Is desirable.

  On the other hand, as another method of suppressing moire, there is a method of performing processing for eliminating periodicity on one of two patterns that are the source of moire. FIG. 12A shows an example of a configuration that realizes the above processing using an error diffusion circuit and a delay attenuation filter (FIG. 12B is a typical Floyd & Steinberg filter). This method is used for high-frequency images with many fine lines such as pseudo-halftone processing when copying a periodic document such as a printed matter or a design diagram represented by CAD, and suppresses moire. be able to.

  When image information is input to the circuit shown in FIG. 12A, the result of the delay attenuation filter is reflected to the target pixel, and binarization is performed by the gradation unit. The binarized pixel of interest grasps an error (difference) in density with respect to the input image signal, and distributes an error corresponding to a predetermined coefficient to the right, lower left, lower, and lower right in FIG. Using the fact that the degree of error distribution becomes non-periodic, it becomes a substitute for noise. Therefore, an aperiodic dot pattern (error diffusion pattern) can be generated.

  As described above, in order to suppress moire due to interference with halftoning, it is conceivable that the concave portion on the surface of the photosensitive drum 1 is expressed by the error diffusion pattern using the above principle.

8). Outline of moire suppression in this embodiment As described above, as a method of suppressing moire, as described above, a method in which the angle difference between patterns having a specific periodicity is equal to or greater than a certain value, or one pattern is represented by an error diffusion pattern. There is a method of making a pattern having no specific periodicity (non-periodic, random). Table 1 below shows the moire suppression performance and granularity when the concave portion on the surface of the photosensitive drum 1 is a pattern having a specific periodicity and when it is a non-periodic pattern (error diffusion pattern). There is a relationship as shown. In Table 1, ◎ is very good, ○ is good, Δ is practical but slightly inferior, and × is inferior. A method for determining moire and graininess will be described later.

  As described above, when the concave portion on the surface of the photosensitive drum 1 is used as an error diffusion pattern, if the disorder of dots occurs due to the surface shape of the photosensitive drum 1, the error diffusion pattern is easily noticeable and the graininess is deteriorated. is there. That is, since the error diffusion pattern does not have a specific frequency band in the spatial frequency as shown in FIG. 13, the moire suppression performance is good (◎), but the graininess is slightly inferior (△), the image is flickering, flickering, etc. May give an impression.

  Accordingly, in the present embodiment, for black which is about 15 in brightness and has the lowest (darkest) color (lowest brightness) among the four colors, the disturbance of the toner image due to the error diffusion pattern tends to become noticeable due to graininess. Therefore, the error diffusion pattern should not be adopted for the surface shape of the black photosensitive drum 1Bk.

  The lightness of the color material used in the image forming apparatus 100 of the present embodiment is about 15 for black, 25 for cyan, 23 for magenta, and about 88 for yellow. Therefore, in this embodiment, priority is given to suppressing the deterioration of the granularity of black as mentioned above. However, when the brightness of the color material other than black is lower than the brightness of black, the surface shape of the photosensitive drum 1 can be determined with priority given to the suppression of the deterioration of the graininess of the minimum brightness color that is the color. The acceptance / rejection of the surface shape pattern of the photosensitive drum 1 is summarized in Table 2 below.

  Further, regarding the setting of the surface shape of the photosensitive drum 1, it is necessary to consider that it is disadvantageous from the viewpoint of cost and time if there is a color designation at the time of manufacturing the photosensitive drum 1, at the time of shipment from the factory, lot management, and replacement. . Therefore, it is desirable to share the photosensitive drum 1 with at least two colors to suppress an excessive increase in the types of photosensitive drums 1.

9. Next, a method for determining moire and graininess will be described.

9-1. Moire Determination Method In the present embodiment, moire is a moire that is an interference between a pattern of a recess intentionally formed on the photosensitive drum 1 and a pseudo halftone process (also referred to as “SCR” here). It is important to suppress this. An example of a method for determining moire as an index for this will be described.

  (1) An image of a recess (here, angle 5 °, line) formed on the photosensitive drum 1 and an SCR image (here, area ratio 30%, 150 lpi, angle 45 °, line) are defined pixels. (Here, 1200 dpi, 512 × 512 pixels) information is prepared. The image processing software binarizes the concave image and the SCR image, and deletes the concave image from the SCR image. An image from which the concave image is deleted from the SCR image is subjected to a two-dimensional FFT (Fast Fourier Transform) process, a one-dimensional process is performed, and converted into spatial frequency information. FIG. 14A schematically shows the SCR image, and FIG. 14B schematically shows an image obtained by deleting the concave image from the SCR image. Further, the solid line in FIG. 15 shows the result of the two-dimensional FFT processing (power spectrum with the number of lines (lpi) on the horizontal axis) of the image in which the concave image is deleted from the SCR image. In addition, the dashed-dotted line in FIG. 15 shows the result of the FFT process of the SCR image for reference. The synthesized image may be obtained by directly observing the developed photosensitive drum 1 or may be synthesized with the screen and analyzed on the digital data that is the basis of the mold formed on the photosensitive drum 1.

(2) Next, the VTF characteristic (visual spatial frequency characteristic), which is the eye resolving power sensitivity characteristic, is multiplied by the result of the two-dimensional FFT processing of the image in which the concave image is deleted from the SCR image. FIG. 16A shows the VTF characteristics, and FIG. 16B shows the result of multiplying the VTF characteristics. Moire refers to a sense of discomfort felt when a plurality of periodic patterns interfere with each other and the interference pattern is generated on the low frequency side where the eye sensitivity is good. In general, even if an interference pattern occurs on the high frequency side where there is no eye resolving power, it is not called moire. In order to quantitatively evaluate this phenomenon, it is necessary to consider the sensitivity characteristics of the eyes. In addition, the VTF characteristic used the VTF function which the following Dooley advocated, and the observation conditions were 300 mm.
VTF = 5.05 × exp (−0.138 × u) × (1-exp (−0.1 × u))
u = f × R × π / 180 (cycles / degree)
f: Spatial frequency (cycles / mm)
R: Observation distance (mm) = 300 mm

  When the same processing as in the case where the angle of the surface shape of the photosensitive drum 1 is 5 ° is performed for the cases where the angle is 15 °, 45 °, and 90 °, the results shown in FIGS. 17 and 18 are obtained. It is done. FIGS. 17A to 17C schematically show images obtained by deleting the concave images at the angles of 15 °, 45 °, and 90 ° from the SCR images, respectively. FIG. 18 shows a comparison of processing results similar to those in FIG. 16B when the surface shape angle of the photosensitive drum 1 is 5 °, 15 °, 45 °, and 90 °.

  The peak at 150 lpi in FIG. 18 is a screen component. A portion higher than the screen component (a portion indicated by an arrow in FIG. 18) on the lower line number side than this peak can be determined as moire. This determination method is merely an example, and for example, the half-value portion of the peak of the screen component (the * portion in FIG. 18) may be determined as moire. From the results shown in FIG. 17, the moire is remarkable when the surface shape angle of the photosensitive drum 1 is 5 ° and 15 °, and the moire is sufficiently when the surface shape angle of the photosensitive drum 1 is 45 ° and 90 °. It turns out that it is suppressed. As described above, the pattern having a specific periodicity of the concave portion of the photosensitive drum 1 is based on the pattern, the screen corresponding to the photosensitive drum 1 to which the pattern is applied, and the spatial frequency analysis based on the visual spatial frequency characteristics. Can be set. In this case, typically, the pattern of the concave portion of the photosensitive drum 1 can be set so that there is no peak due to moire in a lower frequency region than the peak due to the screen.

  Moire can be determined by the above frequency analysis. This determination method was used to determine whether or not to adopt the configuration of an example described later. However, in the present invention, the moire determination method is arbitrary, and other known moire determination methods may be used.

9-2. Next, an example of a graininess judgment method will be described. The granularity includes various methods such as RMS granularity, dot area standard deviation, integration of frequency characteristics after the FFT analysis, and IFFT (Inverse Fast Fourier Transform) once again to return to the real image space to obtain standard deviation. Is known. Here, a method of determining the graininess by integrating each frequency power spectrum after the FFT analysis result will be described.

  (1) FFT analysis is performed on the SCR image (here, 150 lpi, angle 90 °, line). FIG. 19A shows the result (power spectrum 1) (dashed line).

  (2) In the same manner as described in the above moire determination method, the concave image (here, 300 dpi, error diffusion pattern) is deleted from the SCR image, and FFT analysis is performed. FIG. 19A shows the result (power spectrum 2) (solid line).

  (3) The power spectrum 1 is subtracted from the power spectrum 2 which is the FFT analysis result. FIG. 19B shows the result.

  (4) The result of subtracting the power spectrum 1 from the power spectrum 2 is multiplied by the VTF characteristic. FIG. 20A shows the result.

  (5) The power spectrum of each frequency band is integrated to obtain a granularity index.

  By the above calculation, it is possible to quantitatively evaluate the granularity in consideration of the eye sensitivity characteristics. This determination method was used to determine whether or not to adopt the configuration of the example described later.

  FIGS. 21A and 21B show a photosensitive drum 1 in which a concave portion of an error diffusion pattern having an opening diameter of 84 μm (300 dpi) and an opening diameter of 21 μm (1200 dpi) is formed, and a line screen of 150 lpi and an angle of 90 °, respectively. The output image image when used is shown. In addition, the frequency characteristics similar to FIG. 20A in each case of FIG. 21A and FIG. 21B are respectively indicated by the solid line and the alternate long and short dash line in FIG. It can be seen that when the size of the concave portion of the error diffusion pattern is large, graininess is likely to occur in a low frequency band with high eye sensitivity.

10. Examples Hereinafter, the present invention will be described in more detail with reference to specific examples according to the present embodiment.

Example 1
Table 3 below shows the setting of the screen and the setting of the surface shape of the photosensitive drum 1 for each color in the halftoning of the present embodiment.

  In this embodiment, the surface shape angle of the photosensitive drum 1Bk is set to −45 ° (135 °) so as to be orthogonal to the 45 ° line screen for black, which is the minimum brightness color among cyan, magenta, yellow, and black. ). Since the line screen and the surface shape of the photosensitive drum 1Bk are orthogonal components having the same number of lines (169 lpi), they have the same relationship as in FIG. 10A and do not generate frequency components other than the screen frequency of 169 lpi. Therefore, moire can be suppressed satisfactorily. Further, since the concave portions are regularly formed on the black photosensitive drum 1Bk so as to have a specific periodicity, a black image with good graininess can be obtained.

  On the other hand, in this embodiment, in the color image forming portions SY, SM, and SC, concave portions of the error diffusion pattern shown in FIG. 13 are formed on the photosensitive drums 1Y, 1C, and 1K. Therefore, moire can be satisfactorily suppressed for yellow, magenta, and cyan.

  In this embodiment, the photosensitive drums 1Y, 1M, and 1C for each color of cyan, magenta, and yellow can be shared. Such common use is advantageous in reducing the cost and time of manufacturing the photosensitive drum 1 because only one mold member is required and a color-specific package is required. In addition, when the photosensitive drum 1 is shipped from the factory, it is not necessary to specify each color in terms of inventory management, which is advantageous in reducing cost and time. For this reason, in this embodiment, priority is given to the suppression of moire and the suppression of cost and time above the granularity for cyan, magenta, and yellow, which have relatively high brightness and the deterioration of the granularity is not noticeable.

  As described above, in this embodiment, a plurality of concave portions are formed in a pattern having a specific periodicity on the surface of the photoreceptor on which an image is formed with toner of the lowest brightness color among the plurality of colors of toner. Further, in this embodiment, a plurality of recesses have substantially no specific periodicity on the surface of the photoconductor on which an image is formed with at least two toners excluding the lowest lightness color among the toners of a plurality of colors. Are formed in the same pattern. In particular, in the present embodiment, the at least two color toners are toners of all colors except the minimum lightness color among the plurality of color toners. Here, the pattern of the concave portions of the photosensitive member being substantially the same means that the number of lines and the angle of the surface shape of the photosensitive member are substantially the same (may be different within an allowable error range). As long as they can be used in common as the color photoconductors, it is not limited to being completely the same.

  As described above, according to this embodiment, it is possible to suppress moire while suppressing deterioration of black graininess. In addition, by sharing the color photosensitive drums 1Y, 1M, and 1C, it is possible to reduce costs and time for manufacturing and management. As a result, it is possible to achieve both cost reduction and high image quality for manufacturing and management.

(Example 2)
Table 4 below shows the setting of the screen and the setting of the surface shape of the photosensitive drum 1 for each color in the halftoning of the present embodiment.

  In the present embodiment, the following points are mainly changed from the first embodiment. A dot screen of 45 ° is used for black, and concave portions are regularly formed so as to have a specific periodicity in the photosensitive drum 1M for magenta having the lowest brightness among cyan, magenta, and yellow.

  In this embodiment, there are three types of photosensitive drums 1, and the graininess can be improved for magenta, which has the lowest brightness among cyan, magenta, and yellow, and the deterioration of graininess is noticeable next to black. Thereby, for example, the skin color reproduction of a person is improved.

  In this embodiment, for black and magenta, the angle difference between the screen and the surface shape of the photosensitive drum 1 is set to 45 ° for black in order to suppress moire between regular dot patterns.

  When the dot patterns as shown in FIGS. 22A and 22B are overlapped, a kind of moire called a rosette pattern as shown in FIG. 22C may also occur. . Compared to the suppression of moire in the case of line patterns, the above-mentioned rosetta pattern has a minimum moire pattern in each direction regularly, so there is no sense of incongruity. However, as in the case of the line patterns, when the angle becomes close as in the case of the dot patterns as shown in FIGS. 23A and 23B, the uncomfortable feeling as shown in FIG. Some Rosetta patterns may occur. Therefore, even in the case of a dot growth screen, it is desirable to make an angle difference with respect to the surface shape of the photosensitive drum 1, and the angle difference is preferably 30 to 60 ° as described above. In the case of a dot growth screen (square dot dither in this embodiment), there is a frequency band on the opposite side of 90 °, and therefore, the angle difference that can be taken is smaller than in the case of a line growth screen. Therefore, in this embodiment, for black and magenta, the angle difference between the screen angle and the surface shape angle of the photosensitive drum 1 is set to 45 °.

  As described above, in this embodiment, a plurality of concave portions are formed in a pattern having a specific periodicity on the surface of the photoreceptor on which an image is formed with toner of the lowest brightness color among the plurality of colors of toner. Further, in this embodiment, a plurality of concave portions are formed in a pattern having a specific periodicity on the surface of the photoreceptor on which an image is formed with at least one color toner excluding the lowest lightness color among the plurality of color toners. ing. This pattern is different from the pattern of the plurality of recesses on the surface of the photoreceptor on which an image is formed with toner of the lowest brightness color. In particular, in the present embodiment, the at least one color toner includes a toner having the lowest lightness among the colors other than the lowest lightness color among the plural color toners.

  As described above, according to the present exemplary embodiment, there are three types of photosensitive drums 1, but moire can be suppressed while suppressing deterioration in granularity of black, which is the lowest lightness color, and magenta, which has the next lightness. . In addition, by sharing at least the yellow and cyan photosensitive drums 1Y and 1C, it is possible to appropriately reduce the cost and time for manufacturing and management.

(Example 3)
Table 5 below shows the setting of the screen and the setting of the surface shape of the photosensitive drum 1 for each color in the halftoning of the present embodiment.

  In this embodiment, black is a 90 ° line screen. For this reason, setting the angle of the surface shape of the photosensitive drum 1 for black to 0 ° is advantageous for suppressing moire. However, considering the ease of chattering of the cleaning blade 61, it is not preferable to form the recesses on the photosensitive drum 1Bk in a line shape along the main scanning direction. This is because the cleaning blade 61 may vibrate in the concave portion, which may cause cleaning failure. Therefore, in this embodiment, for black, the angle of the surface shape of the photosensitive drum 1Bk is 45 °, and the angle difference between the screen and the surface shape of the photosensitive drum 1Bk is 45 °.

  In the present embodiment, concave portions are regularly formed on the photosensitive drums 1M and 1C so as to have a specific periodicity for magenta having the lowest brightness among cyan, magenta, and yellow and cyan having the next lowest brightness. Yes. As a result, the deterioration of graininess can be further suppressed, and the cyan and magenta photosensitive drums 1C and 1M can be shared.

  In this embodiment, cyan and magenta are 45 ° and 135 ° line screens, respectively. That is, as described in the second embodiment, in the case of a square dot screen, there are also dots at 90 ° positions of a certain dot. Therefore, regarding the moire suppression, attention must be paid to the 90 ° period. On the other hand, this is not present in the case of line screens. As shown in FIG. 24, in the case of a line pattern, moire can be avoided up to three regular patterns. Therefore, in this embodiment, the cyan screen angle is 45 °, the magenta screen angle is 135 °, and the surface shape angles of the cyan and magenta photosensitive drums 1C and 1M are 90 °.

  As described above, in this embodiment, a plurality of concave portions are formed in a pattern having a specific periodicity on the surface of the photoreceptor on which an image is formed with toner of the lowest brightness color among the plurality of colors of toner. Further, in this embodiment, a plurality of concave portions substantially have a specific periodicity on the surface of the photoreceptor on which images are formed with at least two color toners excluding the lowest lightness color among the plurality of color toners. Each is formed in the same pattern. This pattern is different from the pattern of the plurality of recesses on the surface of the photoreceptor on which an image is formed with toner of the lowest brightness color. In particular, in this embodiment, the at least two color toners include a color having the lowest brightness among the colors except the lowest brightness color, and a color having the next lowest brightness. In this embodiment, a plurality of recesses have no specific periodicity on the surface of the photoreceptor on which images are formed with toners of all colors except the minimum lightness color and the toner of at least two colors. It is formed with a pattern.

  As described above, in this embodiment, there are three types of photosensitive drums 1 and the screen angles of cyan and magenta are limited. However, the graininess of the cyan, which has the third lowest lightness among yellow, magenta, cyan and black, is deteriorated. Moire can be suppressed while suppressing the above. Further, by sharing at least the cyan and magenta photosensitive drums 1C and 1M, the cost and time for manufacturing and management can be appropriately reduced.

Example 4
Table 6 below shows the setting of the screen and the setting of the surface shape of the photosensitive drum 1 for each color in the halftoning of the present embodiment.

  In this embodiment, the outer diameter of the black photosensitive drum 1Bk is also set to 50 mm, which is the same as the outer diameter of the color photosensitive drums 1Y, 1M, and 1C. Sharing photosensitive drums with the same outer diameter as much as possible is advantageous from the viewpoint of manufacturing and management costs and time, but there are problems in suppressing moire and deterioration of graininess. In this embodiment, two types of photosensitive drums 1 are set according to the settings shown in Table 6. Since the settings for cyan and magenta in the present embodiment are the same as those in the third embodiment, description thereof is omitted.

  The setting for black and yellow in this embodiment will be described. In this embodiment, black is a 90 ° line screen, and yellow is a 165 ° line screen. The surface shape of the photosensitive drum 1 for black and yellow was a line pattern having an angle of 135 °. Thereby, although the screen angle is limited for black and yellow, moire can be suppressed as shown in FIG. 25 while suppressing deterioration of graininess. Further, by sharing the photosensitive drums 1 for black and yellow, cyan and magenta, two types of photosensitive drums 1 can be used, and manufacturing and management costs and time can be reduced. it can.

  As described above, in this embodiment, a plurality of concave portions have a specific periodicity on the surface of the photoreceptor on which images are formed with at least two color toners including the lowest lightness color among the plurality of color toners. Are formed in the same pattern. Further, in this embodiment, a plurality of concave portions are specified on the surface of the photoconductor on which images are formed with at least two color toners other than at least two color toners including the lowest lightness color among the multiple color toners. Each of the patterns having the periodicity is formed. The pattern is different from the pattern of the plurality of recesses on the surface of the photoreceptor on which images are formed with at least two colors of toner including the lowest brightness color.

  In this embodiment, in the configuration in which the outer diameter of the photosensitive drum 1 is 50 mm for all four colors, the screen conditions are defined to suppress the moire and the deterioration of the graininess. In the case of dot growth on the screen, for example, a moiré pattern may be suppressed by adopting a random pattern for the surface shape of the photosensitive drum 1 for colors with relatively high brightness such as cyan and yellow. At this time, the surface shape of the photosensitive drum is adjusted so that the moire can be suppressed and the deterioration of the graininess can be suppressed in the same manner as in this embodiment for black, which is the lowest brightness color, and magenta, which has the next lowest brightness. It is desirable to optimize. As described above, when a plurality of recesses are formed in substantially the same pattern having a specific periodicity on the surface of the photoreceptor on which images are formed with toners of at least two colors including the lowest lightness color, respectively. In addition, the following may be used. That is, among the toners of a plurality of colors, the plurality of recesses do not have a specific periodicity on the surface of the photoreceptor on which an image is formed with at least two colors of toner other than at least two colors including the lowest brightness color. Each pattern is formed in substantially the same pattern. At this time, the toners of at least two colors other than at least two colors including the lowest lightness color may be toners of all colors except for at least two colors including the lowest lightness color among the plural color toners.

  As described above, according to the present exemplary embodiment, moire and graininess deterioration are suppressed for black, which is the lowest brightness color, and at least moire is suppressed for other colors, and at least the photosensitive drum 1 having the same outer diameter is used. Two colors can be shared. As a result, it is possible to achieve both cost reduction and high image quality for manufacturing and management.

  Even when the screen setting and the surface shape setting of the photosensitive drum 1 of the first to third embodiments are adopted, the photosensitive drums 1 for yellow, magenta, cyan, and black have the same outer diameter. You may do it. However, as described above, for example, the photosensitive drum 1Bk for black has a different outer diameter (other index may be used) from the photosensitive drum 1 for other colors for a reason different from moire or graininess. is there. In this case, since the surface shape of the photosensitive drum 1 is different from that of the photosensitive drum 1 for other colors, there is no disadvantage in terms of cost and time in manufacturing and management. It is particularly suitable to do.

DESCRIPTION OF SYMBOLS 1 Photosensitive drum 2 Charging device 3 Exposure apparatus 4 Developing apparatus 5 Primary transfer roller 6 Cleaning apparatus 8 Intermediate transfer belt

Claims (16)

A plurality of photoreceptors;
A processing unit that generates a gradation of an image formed of toner on each of the plurality of photoconductors by a pseudo halftone process using a screen;
Toner image forming means for forming an image with a plurality of different colors of toner on each of the plurality of photoconductors based on the processing result of the processing unit;
Have
A plurality of independent recesses are formed on the respective surfaces of the plurality of photoconductors,
The plurality of recesses are formed in a pattern having a specific periodicity on the surface of the photoconductor on which an image is formed with toner of the lowest brightness color among the plurality of color toners,
The plurality of concave portions have substantially no specific periodicity on the surface of the photoconductor on which images are formed with at least two color toners excluding the minimum lightness color among the plurality of color toners. An image forming apparatus, each of which is formed in a pattern.   2. The image forming apparatus according to claim 1, wherein the at least two color toners are toners of all colors except the minimum brightness color among the plurality of color toners.   The plurality of recesses are formed in a pattern having a specific periodicity on the surface of the photoreceptor on which an image is formed with at least one color toner excluding the minimum lightness color among the plurality of color toners, The image forming apparatus according to claim 1, wherein the pattern is different from a pattern of the plurality of concave portions on the surface of the photoconductor on which an image is formed with the toner of the minimum brightness color.   4. The image forming apparatus according to claim 3, wherein the at least one color toner includes a toner of a color having the lowest brightness among the colors of the plurality of colors excluding the minimum brightness color. 5. A plurality of photoreceptors;
A processing unit that generates a gradation of an image formed of toner on each of the plurality of photoconductors by a pseudo halftone process using a screen;
Toner image forming means for forming an image with a plurality of different colors of toner on each of the plurality of photoconductors based on the processing result of the processing unit;
Have
A plurality of independent recesses are formed on the respective surfaces of the plurality of photoconductors,
The plurality of recesses are formed in a pattern having a specific periodicity on the surface of the photoconductor on which an image is formed with toner of the lowest brightness color among the plurality of color toners,
On the surface of the photoconductor on which images are formed with at least two toners excluding the minimum lightness color among the toners of the plurality of colors, the plurality of concave portions have substantially the same pattern having a specific periodicity. And the pattern is different from the pattern of the plurality of recesses on the surface of the photoconductor on which the image is formed with the toner of the minimum brightness color.   The image forming apparatus according to claim 5, wherein the at least two colors of toner include a color having the lowest lightness among colors excluding the minimum lightness color, and a color having the next lowest lightness.   The plurality of recesses are formed in a pattern having no specific periodicity on the surface of the photoconductor on which images are formed with toners of all colors except the minimum lightness color and the toner of at least two colors. The image forming apparatus according to claim 5, wherein the image forming apparatus is an image forming apparatus. A plurality of photoreceptors;
A processing unit that generates a gradation of an image formed of toner on each of the plurality of photoconductors by a pseudo halftone process using a screen;
Toner image forming means for forming an image with a plurality of different colors of toner on each of the plurality of photoconductors based on the processing result of the processing unit;
Have
A plurality of independent recesses are formed on the respective surfaces of the plurality of photoconductors,
On the surface of the photoconductor on which images are formed with at least two color toners including the lowest lightness color among the plurality of color toners, the plurality of concave portions have substantially the same pattern having a specific periodicity. Each formed,
Among the plurality of color toners, the plurality of recesses have a specific periodicity on the surface of the photoreceptor on which an image is formed with at least two color toners other than at least two colors including the lowest lightness color. An image forming apparatus, wherein the image forming apparatuses are formed in substantially the same pattern.   The toner of at least two colors other than at least two colors including the minimum lightness color is a toner of all colors excluding at least two colors including the minimum lightness color among the plurality of color toners. The image forming apparatus according to 8. A plurality of photoreceptors;
A processing unit that generates a gradation of an image formed of toner on each of the plurality of photoconductors by a pseudo halftone process using a screen;
Toner image forming means for forming an image with a plurality of different colors of toner on each of the plurality of photoconductors based on the processing result of the processing unit;
Have
A plurality of independent recesses are formed on the respective surfaces of the plurality of photoconductors,
On the surface of the photoconductor on which images are formed with at least two color toners including the lowest lightness color among the plurality of color toners, the plurality of concave portions have substantially the same pattern having a specific periodicity. Formed,
Among the plurality of color toners, the plurality of recesses have a specific periodicity on the surface of the photoconductor on which an image is formed with at least two color toners other than the at least two color toners including the lowest lightness color. Each of which has a pattern different from the pattern of the plurality of recesses on the surface of the photoconductor on which an image is formed with toner of at least two colors including the minimum lightness color. Image forming apparatus.   In the spatial frequency analysis based on the screen corresponding to the photoconductor to which the pattern is applied and the visual spatial frequency characteristics, the pattern having the specific periodicity is in a region having a frequency lower than the peak corresponding to the screen. The image forming apparatus according to claim 1, wherein the image forming apparatus has no peak corresponding to moire.   When the screen corresponding to the photoconductor to which the pattern having the specific periodicity is applied is a line screen, the angle difference between the pattern having the specific periodicity and the screen is 30 ° to 150 °. The image forming apparatus according to claim 1, wherein the image forming apparatus is provided.   When the screen corresponding to the photoconductor to which the pattern having the specific periodicity is applied is a dot screen, the angle difference between the pattern having the specific periodicity and the screen is 30 ° to 60 °. The image forming apparatus according to claim 1, wherein the image forming apparatus is provided.   The image forming apparatus according to claim 1, wherein the pattern having no specific periodicity is an error diffusion pattern.   The image forming apparatus according to claim 1, wherein the minimum brightness color is black.   16. The plurality of photoconductors are cylindrical, and the photoconductor corresponding to the lowest lightness color among the plurality of photoconductors has an outer diameter larger than that of the other photoconductors. The image forming apparatus according to claim 1.