JPH05183712A - Image forming method - Google Patents

Abstract

PURPOSE:To prevent the generation of stripe structure in which toner dots appear in connection, to improve gradation, and in addition, to obtain an excellent image having a large dynamic range at the time of forming the image by laser beam scanning subjected to intensity modulation. CONSTITUTION:The setting positions and the focal lengthes of the cylindrical lenses 433, 436 of a scanning optical system 430 are set so that the shape of the spot of a laser beam on a photosensitive body becomes long in a main scanning direction. Then, at the time when image density data corresponding to a picture element from an image density data storage circuit are read out, and after D/A-converting it, an intensity-modulated signal is generated by an intensity control circuit, and a semiconductor laser 431 is emission-controlled by this signal, it is discriminated whether the image is a character area or a half tone area by an image discrimination circuit, and in the case of the character area, the maximum quantity of writing light of the semiconductor laser 431 is made larger than in the case of the half tone area.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming method for recording an image using a laser scanning optical system.

[0002]

2. Description of the Related Art An image forming method is known in which one or a plurality of laser beams modulated by a recording signal are scanned on a photosensitive member to record information based on the recording signal on the photosensitive member. .. As the scanning means of the above method, there is a technical means for performing dot exposure in which a laser beam is reflected by a rotary polygon mirror that rotates at a constant speed by a motor, transmitted through an fθ lens, and scanned on a photosensitive member in a form narrowed down to a minute spot. Are known.

Further, as the image forming means, a photosensitive drum which is rotated at a constant speed by a motor is installed such that the drum axis and the scanning direction of the laser beam are set in parallel, and the rotation of the photosensitive drum is a sub-scanning direction. It is known that a latent image of an image is formed on the peripheral surface of the photosensitive drum that is uniformly charged in advance with the scanning of.

In such an image forming method, the light intensity distribution of the spot of the laser beam on the photosensitive drum has a substantially Gaussian curve as shown in FIG. 6, and the portion having a certain threshold value (s) or more is developed. At this time, a change in the potential at which toner adheres will occur. Therefore, in the latent image formed by the spot of the laser beam, when the light intensity of the spot changes, the diameter of the dot of the toner image to be developed (hereinafter referred to as toner dot) changes. Further, when the light emission time becomes long, it becomes large not only in the main scanning direction but also in the sub scanning direction as shown in FIG.

In order to reproduce a halftone image, a modulation signal that changes according to the density data corresponding to a pixel is obtained, the semiconductor laser is driven by this intensity modulation signal, and a laser beam emitted from a semiconductor laser with a change in intensity is generated. When a latent image is formed by being incident on the photoreceptor, the size of the latent image dot changes depending on the light intensity, and a multi-valued minute circular or elliptical latent image can be obtained. The dot-shaped toner dots obtained by developing this latent image can form a halftone image having gradation similar to the halftone dots obtained by halftone dot printing using a screen in printing.

[0006]

However, in the above-mentioned image forming method, the elliptical shape which is long in the sub-scanning direction has conventionally been used as the spot shape of the laser beam. Therefore, the toner dot formed by the spot of such shape is
As shown in FIG. 10, a vertical stripe structure is likely to appear by connecting in the main scanning direction, and a part such as a photograph such as a halftone human skin becomes very unsightly. Also, in the area of a character image or line drawing consisting of thin lines (hereinafter this area is referred to as the character area), unlike the area of a halftone image such as a photograph (hereinafter this area is referred to as the halftone area), the spot of the laser beam is the same spot Since the number of passages is small, the potential change is small. For this reason, if an image is formed with the same maximum writing amount of light from the semiconductor laser, the size of the toner dots will differ between the character area and the halftone area, and the maximum writing area that maintains the gradation of the halftone area appropriately. With the amount of light, the gradation of the character area is poor and a sufficient density cannot be obtained. Further, if the writing maximum light amount of the semiconductor laser is increased so that the density of the character area is sufficient, there is a problem that the halftone area is quickly saturated and the gradation is deteriorated.

The object of the present invention is to prevent the appearance of an unsightly stripe structure in image formation by laser beam scanning with intensity modulation, to improve the gradation of both the character area and the halftone area, and to provide a wide dynamic range. To obtain a higher quality image by having a range.

[0008]

SUMMARY OF THE INVENTION The above object is to provide an image forming method in which an image is recorded by scanning with a laser beam whose intensity is modulated by density data corresponding to pixels. This is achieved by an image forming method characterized by being an ellipse having a major axis in the main scanning direction.

Further, the maximum writing amount of the laser beam is made larger than the halftone area in the character area based on the discrimination between the character area and the halftone area, or the character area and the halftone area. According to a preferred embodiment of the present invention, the duty ratio of the control pulse for changing the light amount for the time of the control pulse based on the determination is set to be larger in the character area than in the halftone area.

[0010]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The configuration of an image forming apparatus of an embodiment to which the present invention is applied will be described. FIG. 1 is a perspective view showing a schematic configuration of the image forming apparatus of this embodiment.

The image forming apparatus 400 has an intensity based on a modulation signal obtained from an analog image density signal obtained by D / A converting digital image density data from a computer or a scanner after uniformly charging a photoconductor. A dot-shaped electrostatic latent image is formed by the modulated laser beam spot, and this is reverse-developed with toner to form a toner image consisting of toner dots. A color toner image is formed, this color toner image is transferred, separated and fixed to obtain a color image.

The image forming apparatus 400 includes a drum-shaped photoconductor (hereinafter, simply referred to as a photoconductor) 401 that rotates in the direction of the arrow.
And a scorotron charger 402 that applies a uniform charge onto the photosensitive member 401, a scanning optical system 430, developing devices 441 to 444 loaded with yellow, magenta, cyan, and black toner, and a transfer device including a scorotron charger. 462, separator 463, fixing roller 464, cleaning device 470, and static eliminator 474.

The scanning optical system 430 collimates the laser light emitted from the semiconductor laser 431 into parallel laser light into a laser beam. This laser beam is reflected and deflected by a rotary polygon mirror 434 that rotates at a constant speed, and a minute amount is formed on a peripheral surface of a photosensitive member 401, which is a uniformly drum-shaped photosensitive member, by an fθ lens 435 and cylindrical lenses 433 and 436. Image exposure is performed by scanning with a laser beam focused into an elliptical spot. Here, the fθ lens 435 is a correction lens for performing constant-speed optical scanning, and the cylindrical lenses 433, 4
Reference numeral 36 is an element that corrects the variation of the spot position due to the surface tilt of the rotary polygon mirror 434, and determines the spot shape of the laser beam on the photoconductor by its installation position and focal length, and in the present invention, it is long in the main scanning direction. It is set to be an ellipse. 437 is a scanning mirror for reflecting the laser beam, 438
Is an index mirror and 439 is an index sensor. The index signal from the index sensor 439 detects the start of scanning by the laser beam and the surface position of the rotary polygon mirror 434 that rotates at a predetermined speed to detect the period in the main scanning direction. As a result, the spot of the laser beam scans the photoconductor 401 in parallel with the drum axis.

The spot shape of the laser beam on the photoconductor 401 is an ellipse that is long in the main scanning direction as shown in FIG. 1 / e ² i.e. the length of the minor axis diameter of equal intensity line of 13.5% strength) a, when the length of the major axis b, and the length of one side of the pixel is d is, 0.3 d ≦ a ≦ 1.0d Further, the flatness a / b is preferably 0.3 ≦ a / b ≦ 0.9, and particularly preferably 0.5 ≦ a / b ≦ 0.8.

In the present invention, since the spot shape of the laser beam on the photosensitive member 401 is elongated in the main scanning direction as shown in FIG. 2, the toner dots should be connected in the sub scanning and main scanning directions in the low density portion. There is no. When the image density is high, the images may be connected in the main scanning direction as shown in FIG.

The photosensitive member 401 used in this embodiment is a photosensitive member having a high γ characteristic, and a specific configuration example thereof is shown in FIG.

As shown in FIG. 8, the photosensitive member 401 comprises a conductive support 401A, an intermediate layer 401B and a photosensitive layer 401C. Photosensitive layer 40
The thickness of 1C is about 5 to 100 μm, preferably 10 to 50
μm. The photosensitive member 401 uses a drum-shaped conductive support 401A made of aluminum having a diameter of 150 mm, and the conductive support 40
0.1A thickness of ethylene-vinyl acetate copolymer on 1A
A μm intermediate layer 401B is formed, and a film thickness of 35 μm is formed on the intermediate layer 401B.
It is configured by providing a photosensitive layer 401C of μm.

As the conductive support 401A, a drum of aluminum, steel, copper or the like having a diameter of about 150 mm is used. It may be a metal belt such as a nickel belt made by the Chu method. Further, the intermediate layer 401B withstands a high charge of ± 500 to ± 2000 V as a photoconductor, for example, in the case of positive charge, it blocks injection from the electroconductive support 1C to obtain excellent light attenuation characteristics due to an avalanche phenomenon. As described above, it is preferable that the intermediate layer 401B has hole mobility, so that, for example, Japanese Patent Application No.
It is preferable to add 10% by weight or less of the positively chargeable charge transport material described in the specification of No. 5. As the intermediate layer 401B,
Usually, for example, the following resins used for the photosensitive layer for electrophotography can be used.

(1) Vinyl-based polymers such as polyvinyl alcohol (poval), polyvinyl methyl ether and polyvinyl ethyl ether, (2) polyvinyl amine, poly-N
-Vinylimidazole, polyvinylpyridine (quaternary salt), polyvinylpyrrolidone, vinylpyrrolidone-vinyl acetate copolymer and other nitrogen-containing vinyl polymers, (3) polyethylene oxide, polyethylene glycol, polypropylene glycol and other polyether polymers, (4)
Acrylic acid-based polymers such as polyacrylic acid and its salts, polyacrylamide and poly-β-hydroxyethyl acrylate, (5) Polymethacrylic acid and its salts, methacrylic acid-based polymers such as polymethacrylamide and polyhydroxypropylmethacrylate Polymer, (6) ether cellulose polymer such as methyl cellulose, ethyl cellulose, carboxymethyl cellulose, hydroxyethyl cellulose, hydroxypropyl methyl cellulose,
(7) Polyethyleneimine-based polymers such as polyethyleneimine, (8) Polyalanine, polyserine, poly-L-glutamic acid, poly- (hydroxyethyl) -L-glutamine, poly-δ-carboxymethyl-L-cysteine, polyproline , Poly-amino acids such as lysine-tyrosine copolymer, glutamic acid-lysine-alanine copolymer, silk fibroin and casein, (9) starch acetate,
Starch and its derivatives such as hydroxyethyl ethyl starch, starch acetate, hydroxyethyl starch, amine starch, and phosphate starch, (10) Mixing water and alcohol such as polyamide soluble nylon and methoxymethyl nylon (8 type nylon) Polymer soluble in solvent.

The photosensitive layer 401C is basically prepared by mixing a phthalocyanine fine particle having a diameter of 0.1 to 1 μm, which is made of a photoconductive pigment, and an antioxidant and a binder resin without using a charge transport material together, using a solvent for the binder resin. It is formed by dispersing the solution to prepare a coating solution, coating the coating solution on the intermediate layer, drying and optionally heat treatment.

When the photoconductive material and the charge transport substance are used in combination, the photoconductive pigment and 1 of the photoconductive pigment are used.
/ 5 or less, preferably 1/1000 to 1/10 (weight ratio) of a small amount of a charge transport substance, and dispersed in a photoconductive material, an antioxidant and a binder resin to form a photosensitive layer. By using such a high-γ photoconductor, a sharp latent image can be formed despite the spread of the beam diameter, and recording with high resolution can be effectively performed.

In this embodiment, the color toner image is transferred to the photosensitive member 401.
Since they are superposed on each other, a photosensitive member having infrared spectral sensitivity and an infrared semiconductor laser are used so that the laser beam from the scanning optical system is not blocked by the color toner image.

Next, the light attenuation characteristics of the high γ photoconductor used in this embodiment will be described.

FIG. 7 is a graph showing the characteristics of the high γ photoconductor. In the figure, V ₁ is a charging potential (V), V ₀ is an initial potential (V) before exposure, and L ₁ is a laser beam irradiation light amount (μJ / cm) required for the initial potential V ₀ to be attenuated to 4/5. ² ) and L ₂ represent the irradiation light amount (μJ / cm ² ) of the laser beam required for the initial potential V ₀ to be attenuated to ⅕.

The preferred range of L ₂ / L ₁ is _{1.0 <L 2 / L 1 ≦} 1.5.

In this embodiment, V ₁ = 1000 (V), V ₀ = 950
(V), L ₂ / L ₁ = 1.2. Also, the photoconductor potential in the exposed area is 1
It is 0V.

The photosensitivity at the position corresponding to the mid-exposure period when the light attenuation curve attenuates the initial potential (V ₀ ) to 1/2 is E1 /
2, and the photosensitivity at the position corresponding to the initial exposure stage when the initial potential (V ₀ ) is attenuated to 9/10 is E9 / 10, (E1 / 2) / (E9 / 10) ≧ 2 is preferable. A photoconductive semiconductor that gives a relationship of (E1 / 2) / (E9 / 10) ≧ 5 is selected. Here, the photosensitivity is defined by the absolute value of the potential decrease amount with respect to the minute exposure amount.

As shown in FIG. 7, in the light attenuation curve of the photoconductor 401, the absolute value of the differential coefficient of the potential characteristic, which is the photosensitivity, is small when the amount of light is small, and sharply increases as the amount of light increases. Specifically, as shown in FIG. 7, the light attenuation curve shows a nearly flat light attenuation characteristic in the early stage of exposure due to poor sensitivity characteristics for a short period of time. The sensitivity becomes an ultra-high γ characteristic that drops almost linearly. Specifically, the photoconductor 401 is considered to obtain a high gamma characteristic by utilizing the avalanche phenomenon under the high charge of +500 to + 2000V. That is, the carriers generated on the surface of the photoconductive pigment in the early stage of exposure are effectively trapped in the interface layer between the pigment and the coating resin, and the light attenuation is surely suppressed. It is understood that an avalanche phenomenon occurs.

In the present invention, it is preferable that the photoconductor has a high γ characteristic, but a photoconductor having a normal amount of light and a potential decrease may be used.

FIG. 4 is a block diagram showing an embodiment of the image processing circuit used in the image forming apparatus to which the present invention is applied.

The image processing circuit 1000 of this embodiment is a circuit which constitutes the drive circuit of the scanning optical system 430 of FIG. 1, and comprises an image data processing circuit 100, a modulation signal generating circuit 200, and a raster scanning circuit 300.

The image data processing circuit 100 is a circuit for interpolating and outputting the edge portion of the font data, and comprises an input circuit 110 and a font data generating circuit 12 which are composed of a computer.
0, font data storage circuit 130, interpolation data generation circuit 14
Consists of 0, the character code signal from the input circuit 110,
The size code signal, the position code signal and the color code signal are sent to the font data generating circuit 120. The fond data generation circuit 120 selects an address signal from four types of input signals and sends it to the font data storage circuit 130. The font data storage circuit 130 sends the font data corresponding to one character corresponding to the address signal to the font data generation circuit 120. Font data generation circuit 120
Sends the font data to the interpolation data generation circuit 140. The interpolation data generation circuit 140 interpolates jaggedness or jumps of the image density data generated at the edge portion of the font data using the intermediate density and sends the interpolated data to the image density data storage circuit 210 composed of a frame memory. As for the generated color, the corresponding color is converted into Y, M, C, and BK density data according to the color code. In this way, the font is bitmap-developed in each frame memory in a state where each color has the same shape but different density ratios.

The modulation signal generation circuit 200 includes an image density data storage circuit 210, a read circuit 220, an image discrimination circuit 240, and a D / A.
Circuit 250, strength control circuit 261, reference clock generation circuit 280
Composed of.

The reference clock generation circuit 280 is a reference clock pulse generation circuit and generates a clock pulse having the frequency of the pixel clock. Reference clock generation circuit 280
The clock output from is referred to as a reference clock DCK ₀ for convenience, and is output to the read circuit 220 and the D / A circuit 250.

The image density data storage circuit 210 is a normal page memory (hereinafter simply referred to as the page memory 210), a RAM (random access memory) for storing in page units, and at least one page (for one screen). It has a capacity for storing image density data corresponding to corresponding multi-valued pixels. Further, if it is an apparatus adopted for a color printer, it will have a page memory for storing image density signals corresponding to a plurality of color components, for example, yellow, magenta, cyan and black color components.

The readout circuit 220 reads out image density data corresponding to pixels in a unit of one scanning line continuous from the image density data storage circuit (page memory) 210 in synchronization with the reference clock DCK ₀ by using the index signal as a trigger, It is sent to the discrimination circuit 240 and the D / A circuit 250.

The image discriminating circuit 240 is a circuit for discriminating whether the image is a character region or a halftone region. When the image is discriminated as a character region, the intensity control circuit 26
A signal for increasing the maximum writing light quantity of the semiconductor laser 431 is sent to 1.

The image discriminating circuit 240 successively differentiates the image density data corresponding to, for example, consecutive pixels, and when the differential value is α or more or −α or less when the specific value is α, the image is a character area. If the differential value is α or more, it means that the edge is on the left side of the scanning line, and if it is −α or less, it means that the edge is on the right side in the scanning line direction. be able to.

The D / A circuit 250 latches the image density data only while the processing of the image discrimination circuit 240 is being executed,
This is a circuit for D / A converting image density data.

The intensity control circuit 261 is a control circuit that changes the intensity of the laser beam according to the density data corresponding to the pixel input via the D / A circuit 250, and the control signal from the image discrimination circuit 240. The maximum writing light amount is switched by. One line of the intensity modulated signal thus obtained is sent to the raster scanning circuit 300 as one unit.

The raster scanning circuit 300 includes, in addition to the semiconductor laser 431, a lighting control circuit, an index detection circuit, a polygon mirror driver, etc., which are not shown. The lighting control circuit oscillates the semiconductor laser 431 with the modulation signal from the intensity control circuit 261. The index detection circuit detects the surface position of the rotary polygon mirror 434 that rotates at a predetermined speed based on the index signal from the index sensor 439, and performs optical scanning by a raster scanning method at a cycle in the main scanning direction.

The polygon mirror driver rotates the DC motor at a predetermined speed and rotates the rotary polygon mirror 434 at a constant angular speed.

Next, the image forming process of the image forming apparatus 400 will be described.

First, the photoconductor 401 is neutralized by the static eliminator 474 and then uniformly charged by the scorotron charger 402. An electrostatic latent image corresponding to yellow is formed on the photoconductor 401 by the yellow data (8b) from the image density data storage circuit 210.
It is formed by irradiation with a laser beam whose intensity is modulated by it's digital density data). The electrostatic latent image corresponding to yellow is developed by the first developing device 441, and a dot-shaped first toner image (yellow toner image) is formed on the photoconductor 401. This first toner image is not transferred to the recording paper, but is retracted to the cleaning device 470.
Underneath, and again on the photoconductor 401 the scorotron charger 4
It is charged by 02.

Then, the intensity of the laser beam is modulated by magenta data (8-bit digital density data), and the modulated laser beam is irradiated on the photoconductor 401 to form an electrostatic latent image. This electrostatic latent image is transferred to the second developing device 44.
The second toner image is developed to form a second toner image (magenta toner image). Similarly to the above, the third developing device 443
Are sequentially developed to form a third toner image (cyan toner image), and a three-color toner image sequentially stacked on the photoconductor 401 is formed. Finally, a fourth toner image (black toner image) is formed, and a four-color toner image sequentially formed on the photoconductor 401 is formed.

These four-color toner images are transferred by the operation of the transfer device 462 onto the recording paper supplied from the paper feeding device.

The recording paper carrying the transferred toner image is separated by a separator.
The sheet is separated from the photoconductor 401 by 463, is conveyed by a guide and a conveyor belt (not shown), is carried into a fixing roller 464, is heated and fixed, and is discharged to a sheet discharge tray. On the other hand, the photoconductor 401 that has completed the transfer is cleaned by the cleaning device 470 that has been released, and is prepared for the next image formation.

FIG. 5 is a block diagram showing another image processing circuit 1000 of this embodiment.

The same parts as those in the block diagram of FIG. 4 are represented by the same reference numerals, and detailed description thereof will be omitted. In the circuit of FIG. 4, the writing maximum light amount of the semiconductor laser 431 is changed between the character area and the halftone area to improve the density of the character area and the gradation of the halftone area. The above-mentioned object is achieved by changing the length of the control pulse for lighting. That is, it is a circuit for improving the image quality by sending a control pulse having a duty ratio suitable for the region to the lighting control circuit 262 according to the determination of the image determining circuit 240. The pulse A generation circuit 291 is a circuit for generating a control pulse (pulse A) having a duty ratio suitable for the halftone region, and the pulse B generation circuit 292 is a control pulse (pulse having a larger duty ratio than the pulse A suitable for the character region). This is a circuit for generating B). As for the control pulses of the pulses A and B, a control circuit having a duty ratio suitable for the area is selected by a selection circuit 255 operated by a selection signal from the image discrimination circuit 240 and sent to the lighting control circuit 262. It is possible to increase the image density by changing the light emission time for each time and increasing the light emission time in the character area as shown by the dotted line in FIG. In the character area, the pulse width is set to 2 to 4 times that of the intermediate area, and by doing so, a reliable latent image format of isolated points can be performed and character reproduction can be improved.

In the above embodiment, the image discriminating circuit 240 discriminates whether the image is a character region or a halftone region and automatically switches the maximum writing light quantity of the semiconductor laser 431 or the duty ratio of the control pulse. It is needless to say that it can be performed independently or simultaneously with the automatic switching. In addition, characters or halftone processing may be uniformly performed on the entire image according to the type of the image.

Although the above-described image processing circuit 1000 is described as a laser printer, it is not limited to this, and the color scanner 1 is used instead of the image data processing circuit 100.
51, A / D conversion circuit 152, density conversion circuit 153, masking U
If the image data processing circuit 150 including the CR circuit 154 and the like is replaced with a circuit for inputting image density data corresponding to pixels from the scanner and performing image processing, it can be applied to other image forming apparatuses such as copying machines. You can

[0052]

As described above, in the image forming method of generating the intensity modulation signal from the image density data corresponding to the pixel and recording the image by the modulation signal,
The spot shape of the laser beam is an ellipse that is long in the main scanning direction, and the writing maximum light amount of the semiconductor laser is larger in the character area of the image than in the halftone area, so the unsightly stripe structure that appears in the low-density area is prevented. It was possible to provide an excellent image forming method having a wide dynamic range in which the density of the character region of the image to be formed is large and the gradation of the halftone region is improved.

[Brief description of drawings]

FIG. 1 is a perspective view showing a schematic configuration of an embodiment of an image forming apparatus to which the present invention is applied.

FIG. 2 is a plan view showing a spot shape of a laser beam of the present invention on a photoconductor.

FIG. 3 is a plan view showing a case where the laser beam spots of FIG. 2 are connected.

FIG. 4 is a block diagram showing an example of an image processing circuit of the image forming apparatus of FIG.

5 is a block diagram showing another example of an image processing circuit of the image forming apparatus of FIG.

FIG. 6 is a diagram showing a relationship between a light intensity distribution of a beam spot and a dot diameter.

FIG. 7 is a graph showing characteristics of the high γ photoconductor used in this example.

FIG. 8 is a cross-sectional view showing a specific configuration example of a high-γ photoconductor used in this example.

FIG. 9 is a diagram showing a change in toner dot size due to a conventional laser beam.

FIG. 10 is a diagram showing a connection state of toner dots by a conventional laser beam.

[Explanation of symbols]

 100 Image data processing circuit 200 Modulation signal generation circuit 210 Image density data storage circuit (page memory) 220 Readout circuit 240 Image discrimination circuit 250 D / A circuit 261 Strength control circuit 262 Lighting control circuit 280 Reference clock generation circuit 300 Raster scanning circuit 400 Image forming device 430 Scanning optical system 431 Semiconductor laser

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification code Office reference number FI technical display location G03G 15/04 116 9122-2H

Claims (3)

[Claims] 1. An image forming method in which an image is recorded by scanning with a laser beam whose intensity is modulated by density data corresponding to pixels, wherein a spot shape of the laser beam on a photoconductor has a major axis in a main scanning direction. An image forming method characterized by being an ellipse. 2. The image forming method according to claim 1, wherein the maximum writing amount of the laser beam is set to be larger in the character area than in the halftone area based on the discrimination between the character area and the halftone area. 3. The image forming method according to claim 1, wherein the duty ratio of the control pulse for causing the laser to emit light for a predetermined time based on the discrimination between the character area and the halftone area is set to be larger than that in the halftone area in the character area.