JPH08132675A - Method and apparatus for forming image - Google Patents

Abstract

PURPOSE: To record delicate gradation at a latent image forming stage by so differentiating exposure positions by a light source that the adding states of the energy from the light source become different even if the total exposure time of the light source is the same in the area of a predetermined size. CONSTITUTION: One exposure pattern is designated by a Z address counter 92 from a ROM 91 for storing a plurality of exposure patterns in which the positions of black elements made of (m×n) matrix are differentiated from each other, the values of elements for forming the designated exposure patterns are sequentially designated by X- and Y-address counters 93, 94, and read as 1-bit data DE. A driver 95 controls ON, OFF a semiconductor laser 121 based on the read data. The part on a photosensitive drum corresponding to the ON of the laser 121 is exposed, its potential is lowered, and toner is adhered. As a result, when a developed color is black, the toner adhered part on a record sheet is black. On the contrary, the part of the sheet corresponding to the OFF of the laser 121 becomes white.

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 and apparatus for forming a halftone image by an electrophotographic process.

[0002]

2. Description of the Related Art In an electrophotographic image forming apparatus such as a page printer, a digital copying machine or a facsimile machine, a density pattern method or a dither method is used as a gradation reproduction method for outputting an image having a halftone on a sheet. Are used.

In the density pattern method, one pixel of an original image is made to correspond to a threshold value of each dot (element) of an m × n matrix, and is expressed by a plurality of area patterns different from each other.

The dither method uses m × for one pixel of the original image.
A dot concentration type (Fattening type) in which the threshold value of each dot of the n matrix is made to correspond one-to-one, and the center is used as the nucleus to gradually increase the weight.
And a dot dispersion type (Bayer type) in which dots are dispersed as much as possible.

When a dot-dispersion type matrix is used in the formation of a hard copy image by an electrophotographic process, a high γ is produced due to the influence of the intensity distribution of exposure by laser light or the like.
The recording characteristic is (gamma). Therefore, a dot concentration type matrix is usually used.

However, in the case of using a dot-concentrated matrix, if the matrix size is increased to increase the number of gradations, the resolution is lowered, and if the matrix size is decreased to improve the resolution, the number of gradations is increased. Will decrease.

As a recording method for achieving both gradation and resolution, a method in which a dot concentration type threshold matrix is modified as shown in FIG. 14 and pulse width modulation are combined (IH method). Has been proposed (Electronic Society of Japan, Vol. 25, No. 1, 1986, p. 34). This I
The gradation number r that can be expressed by the H method is calculated by the dot division number R by pulse width modulation and the matrix size m × m.
Including all white, r = m × m × R × 0.5 + 1,
Higher quality output can be obtained compared to the dither method before this.

[0008]

However, even when the IH method is used, high gamma of output characteristics due to the influence of the light intensity distribution of laser light is unavoidable, as in the case of the dot concentrated type. . Therefore, to obtain the linearized output,
Only the necessary steps must be extracted so that the density changes are evenly spaced. Therefore, in that case, the actual number of gradations is lower than the number r of gradations calculated by the above equation, and the image quality is deteriorated accordingly.

As described above, it is difficult to say that the conventional technique has sufficiently achieved both the gradation and the resolution. This is because it is premised that the number of light emitting dots in the matrix area, that is, the total exposure time (total exposure time) must be increased in order to increase the output density. For example, in the case of the above-mentioned IH method, in order to increase the number of gradations r, it is necessary to increase the matrix size m or the number of dot divisions R, but an increase in the matrix size m decreases the resolution. As a result, the increase in the number of divided dots R leads to a situation in which it is difficult to deal with high speed.

As a method for solving this problem, it is conceivable that the laser light is intensity-modulated with three values or more (multivalue). By performing intensity modulation of three levels or more, gradation can be improved without lowering resolution, but it is difficult to ensure stability of the multilevel laser, which leads to cost increase. I will end up.

Therefore, the applicant of the present invention, as the density pattern group or the dither pattern group for reproducing the area gradation, has a plurality of exposure times (the number of exposure spots) but different exposure positions (positions of the exposure spots). An exposure pattern group including an exposure pattern is provided, and a predetermined exposure pattern is selected according to the gradation level of the pixels of the original image to form a latent image. A gradation recording method for expressing gradation has been proposed (Japanese Patent Application No. 5-259).
691). According to this method, the number of gradations can be increased without lowering the resolution as compared with the case where only the exposure time is changed stepwise according to the gradation level.

On the other hand, as a developing method for visualizing the electrostatic latent image formed by exposure, a two-component magnetic brush developing method, a predetermined gap is provided between the developing roller and the photoconductor (latent image carrier). There are proposed a non-contact developing method to be provided, a contact developing method of bringing a toner layer supporting member into contact with a photoconductor, and the like. Among the contact developing methods, the method of contacting a part of the cylindrical thin film externally mounted on the developing roller in the circumferential direction with the photoreceptor (Japanese Patent Laid-Open No. 63-226676) is a method of contacting the developing roller itself. It is possible to perform development without unevenness.

As long as the conventional area gradation reproducing method is used, the gradation recorded at the time of forming the latent image can be reproduced on the sheet by using any of the developing methods described above. However, in the case of performing the gradation recording (exposure) in which the exposure position is different in the exposure area corresponding to one pixel as described above, when the two-component magnetic brush developing method or the non-contact developing method is used, There is a problem in that an excessive amount of toner adheres to the boundary between the exposed portion and the non-exposed portion and the boundary portion becomes thicker (thicker), so that the intended gradation cannot be obtained. That is, it was found that the effect of the gradation recording method proposed by the present applicant was impaired at the developing stage.

The present invention has been made in view of this problem, and reproduces the delicate gradation recorded at the stage of latent image formation by exposure on a sheet, and a high-resolution hard copy image excellent in gradation. The purpose is to enable the formation of.

[0015]

In order to solve the above-mentioned problems, the method according to the invention of claim 1 scans an original image line-sequentially to form a latent image on a photoconductor, and then the latent image is formed. An image forming method for developing an image on a sheet by developing
At the time of forming the latent image, the total irradiation amount of energy applied to the exposure area of a certain size corresponding to each pixel is changed according to the gradation level of each pixel of the original image, and the energy is changed. Of the original image are recorded by making the positions of irradiating the energy different from each other in the plurality of exposure areas having the same total irradiation amount of the cylindrical shape having a toner layer on the surface. Is a method of developing the latent image by bringing a part of the thin film member in the circumferential direction into contact with the photoconductor.

According to a second aspect of the present invention, an image forming method in which an original image is line-sequentially scanned to form a latent image on a photoconductor and then the latent image is developed to form an image on a sheet. In forming the latent image, the total exposure time for an exposure area of a certain size corresponding to each pixel is changed according to the gradation level of each pixel of the original image, and the exposure is performed. Of the original image is recorded by making the exposure positions different from each other with respect to the plurality of exposure regions having the same total time, and a cylindrical thin film member having a toner layer on the surface thereof is formed. In this method, the latent image is developed by bringing a part of the circumferential direction into contact with the photoconductor.

According to a third aspect of the present invention, an image forming method in which an original image is line-sequentially scanned to form a latent image on a photoconductor and then the latent image is developed to form an image on a sheet. In forming the latent image, an exposure pattern including a plurality of exposure patterns having the same total exposure time but different exposure positions in order to determine the exposure position in a matrix area having a certain size. Using a group, an exposure pattern corresponding to a gradation level of each pixel of the original image is selected to record the gradation of the original image, and a circumferential direction of a cylindrical thin film member having a toner layer on the surface thereof. Is a method of developing the latent image by contacting a part of the latent image with the photoconductor.

An apparatus according to a fourth aspect of the present invention comprises an exposure control means for scanning an original image line-sequentially to form a latent image on a photoconductor, and a developing means for visualizing the latent image. An electrophotographic image forming apparatus for forming an image on a sheet, wherein the exposure control means includes an exposure pattern group including a plurality of exposure patterns having the same total exposure time but different exposure positions. The stored pattern ROM,
Exposure pattern designating means for designating an exposure pattern according to the gradation level of each pixel of the input image data,
Data of the designated exposure pattern is stored in the pattern ROM.
From the pattern ROM, and driver means for controlling the intensity of the light source based on the data read from the pattern ROM. The developing means is rotatably provided. A developing roller, a cylindrical thin film member having a longer peripheral length than that of the developing roller, which is externally mounted on the developing roller, rotates together with the developing roller, and part of the circumferential direction is separated from the developing roller. As described above, the thin film member is pressed against the developing roller, and a guide member for bringing the outer surface of the thin film member away from the developing roller into contact with the photoconductor, and a toner layer are formed on the outer surface of the thin film member. And a toner leveling member for doing so.

[0019]

When, for example, a rotating photosensitive drum is exposed in the main scanning direction by a laser beam or a light source such as an LED array, the exposed portion with a certain energy or more is visualized by the toner. At that time, even if the total exposure time by the light source is the same in a region of a certain size, if the exposure position by the light source is different, the addition state of energy from the light source is different, so that it is visualized. The recording areas are different from each other, and recording is performed with different gradations.

The latent image on which gradation is recorded is visualized by a contact developing method in which a circumferential part of a cylindrical thin film member having a toner layer on its surface is brought into contact with a photoconductor.

[0021]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a color page printer 1 according to the present invention.
FIG. 3 is a cross-sectional front view showing the mechanical configuration of FIG.

The color page printer 1 forms an image on a sheet by an electrophotographic process. A photosensitive drum 3, which is an electrostatic latent image carrier, is arranged in the center of the housing 2. The photoconductor drum 3 is a cylindrical body having an organic photoconductor layer on the outer periphery and an outer diameter of 100 mm, and rotates in the direction of arrow a around a shaft (not shown) at a constant system speed.

Around the photosensitive drum 3, a corona discharge type charging charger 4 for uniformly charging the surface of the photosensitive drum 3, a developing device 9 for visualizing an electrostatic latent image as a toner image, and an intermediate portion. A transfer device 14, a cleaning device 26, and a charge eliminating device 28 are arranged. Further, a scanning optical system 6 for forming a latent image is arranged above the charging charger 4. A light beam (laser beam) 7 emitted from the scanning optical system 6 enters an exposure / exposure region 8 of the photosensitive drum 3.

The developing device 9 is composed of a developing device holder 10 and four developing devices 11Y, 11M, 11C and 11K of a one-component contact developing type which are detachably mounted on the developing device holder 10. These developing devices 11Y, 11M, 11
Yellow toner, magenta toner, cyan toner, and black toner are stored in C and 11K, respectively.
The developing devices 11Y, 11M, 11C, and 11K are mounted so as to be able to move back and forth toward the photoconductor drum 3, and the developing sleeve 53, which is a developer bearing member, is mounted in the developing area 13.
Y, 13M, 13C, and 13K are provided so that they can be switched between a developing state in which they contact the outer peripheral surface of the photosensitive drum 3 and a non-developing state in which they are separated from the photosensitive drum 3. At the time of printing, the developing devices 11Y, 11M, 11C and 11K are selectively brought into a developing state.

The intermediate transfer device 14 is an endless transfer belt 1 onto which the toner image formed on the photosensitive drum 3 is transferred.
5, a transfer roller 17 that presses the outer peripheral surface of the transfer belt 15 against the outer peripheral surface of the photosensitive drum 3 in the primary transfer area 16.
A support roller 18 that supports the transfer belt 15 together with the transfer roller 17, a tension roller 19 that supports the transfer belt 15 and applies a predetermined tension, and a secondary transfer area 2
The second transfer roller 21 is in contact with the outer peripheral surface of the transfer belt 15 at 0, and the scraper 22 for cleaning the outer peripheral surface of the transfer belt 15. The transfer belt 15 is made of a semi-conductive (electrical resistance value: 10 ⁷ Ω · cm) resin material or metal material, and has a length of 628 mm (2 times the peripheral length of the photoconductor).
Double), a seamless belt having a width of 340 mm and a thickness of 150 μm.
It is moved in the direction of arrow b by the rotational force transmitted to. A detection hole 23 is formed in the green portion of the transfer belt 15. The detection hole 23 is detected by a reference position detection sensor 24 including a light emitting element and a light receiving element, and the drive timing of various devices is controlled based on the output signal of the reference position detection sensor 24. The transfer rollers 17 and 21 are the photosensitive drum 3, the transfer belt 15, and the transfer belt 15 respectively.
In order to secure a sufficient nip width between the sheet and the sheet, it is preferable to use a flexible material such as expanded polyethylene. The primary transfer roller 17 is made of a conductive material (10 ⁶ Ω · cm) and the secondary transfer roller 21 is made of a semiconductive material (˜10 ⁷ Ω · c).
It is preferably composed of the material of m).

On the other hand, a paper feeding device 30 is arranged below the housing 2. The paper feeding device 30 includes a cassette paper feeding unit 31 and a manual paper feeding unit 32. The paper (not shown) accommodated in the cassette paper feed unit 31 is supplied to the timing roller pair 37 through the passage 34 by the rotation of the paper feed roller 33, and the paper inserted into the manual paper feed unit 32 is fed by the paper feed roller 35. By the rotation, it is supplied to the timing roller pair 37 via the passage 36. The timing roller pair 37 transfers the sheet to the secondary transfer position 20 at a predetermined timing.
Send to. At the secondary transfer position 20, the toner image on the transfer belt 15 is transferred onto the paper.

The sheet on which the toner image is transferred is conveyed to the fixing device 39 by the sheet conveying device 38 below the photosensitive drum 3. The fixing device 39 includes a heat roller 40 having a built-in heater, and a pressing roller 41 that comes into pressure contact with the heat roller 40, and heats the toner image to fix it on the paper. The sheet after fixing is discharged to the sheet discharge tray 43 on the upper surface of the housing 2 through the passage 42.

Next, the developing units 11Y, 11M, 11C, 11
The configuration of K will be described in detail. Since the four developing devices have the same basic configuration, the developing device 11 is representatively shown here.
Y will be described.

FIG. 2 is a sectional front view of the developing device 11Y.
The developing device 11Y has a developing roller 53 in an opening 52 on the front surface side (photosensitive drum 3 side) of a flat housing 51. The developing roller 53 includes a developing sleeve 5 made of a thin film in a cylindrical shape having a peripheral length slightly longer than its peripheral length.
4 is exterior. The material of the developing sleeve 54 is resin such as polycarbonate or nylon, or metal such as nickel.

On the side walls supporting both ends of the developing roller 53 and the phenomenon sleeve 54, a guide member made of an elastic material is in contact with the outer peripheral surface of the rear portion of the phenomenon sleeve 54 on the side opposite to the photosensitive drum 3 with the developing roller 53 interposed therebetween. (Not shown) is provided, and the phenomenon sleeve 54 is urged by the guide member.
The outer peripheral surface of the rear part is in close contact with the roller 53, and the phenomenon sleeve 54
The slack portion 55 is formed on the front portion of the sheet, that is, on the side of the photoconductor. Further, on the outer peripheral surface of the upper portion of the phenomenon sleeve 54, a plate-shaped blade 56 having one end fixed to the ceiling of the housing 51.
However, they are in contact with each other via a restriction portion 57 having a thickness of about 1 mm provided on the free end side. The restriction portion 57 may be formed of a soft elastic material different from the material of the blade 56.
Bend the tip end of the
You may make it contact with 4. When the restricting portion 57 and the blade 56 are made of different materials, it is preferable that the blade 56 is made of stainless steel, ribbon copper, phosphor bronze or the like, and the restricting portion 57 is made of an abradable material such as silicon or nylon.

Behind the phenomenon sleeve 54, a toner supply roller 59 is arranged. This toner supply roller 5
An outer peripheral portion 9 is made of, for example, foamed urethane. In the flat toner storage portion 60 located behind the toner supply roller 59, the toner transport mechanism 6 for moving the toner stored therein toward the toner supply roller 59.
1 is provided.

The toner transport mechanism 61 includes support members 62, 62 arranged in the vicinity of both side walls of the housing 51 and extending in the front-rear direction (left-right direction in FIG. 2) and a lateral direction (front-back direction of the paper surface in FIG. 2). ), And has a ladder-shaped conveying member 64 including seven feeding members 63 whose both ends are fixed to the supporting members 62 and 62.

The two feeding members 63 on the front side are fixed to the bottom surface of the housing 51 with an angle of inclination which is closer to the vertical than the other five feeding members 63, so that the conveying force of the toner to the front side is further increased. It's big. Long holes 65 extending in the vertical direction are formed in the front portions of the support members 62, 62, and a support shaft 66 that traverses the toner accommodating portion 60 is formed in the long holes 65.
Is inserted. Further, in the support shaft 66, a portion 67 penetrating the elongated hole 65 is bent in a U-shape. As a result, the U-shaped portion 67 turns as the support shaft 66 rotates,
The transport member 64 moves in the front-back direction and the vertical direction.

When the developing device 11Y having the above-mentioned configuration is arranged at the developing position, the developing roller 53, the toner supply roller 59, and the gear (not shown) fixed to the end portions of the support shaft 66 are driven by the drive motor. Are connected, and the arrows c, respectively
Rotate in the d and e directions. Further, the transport member 64 repeats the toner feeding operation by the rotation of the support shaft 66. That is, when the U-shaped portion 67 of the support shaft 66 is above in the movement locus, the U-shaped portion 67 abuts the upper end of the elongated hole 65,
The transport member 64 moves rearward with its front portion lifted. At this time, the rear end side of the support member 62 moves rearward while sliding on the bottom surface 68 of the toner accommodating portion 60, and the feeding member 63 is kept in a non-contact state with the bottom surface 68. Next, when the front portion of the conveying member 64 comes into contact with the bottom surface 68 of the toner accommodating portion 60 due to the movement of the U-shaped portion 65 accompanying the rotation of the support shaft 66, the movement of the U-shaped portion 67 causes the feeding member 63 to move the toner. It moves forward while sliding on the bottom surface 68 of the housing portion 60.
When the U-shaped portion 67 again comes into contact with the upper end portion of the elongated hole 65, the front portion of the transport member 64 moves backward while being lifted.

By the movement of the conveying member 64,
The toner, which is the one-component developer housed inside the developing device 11Y, moves toward the toner supply roller 59 as the transport member 64 moves forward. On the other hand, when the transport member 64 moves rearward, only the rear end portion of the transport member 64 is in contact with the bottom portion of the toner container 60, and the feed member 63 is kept away from the bottom surface 68. The toner located at the bottom is not conveyed backward. An elastic body such as rubber, sponge, or film is provided below the supporting member 62 and the feeding member 63, and when the conveying member 64 contacts the bottom surface 68. It is preferable to soften the impact and sound.

The toner conveyed near the toner supply roller 59 is supplied to the developing sleeve 54 by the rotation of the toner supply roller 59, and is supported on the outer peripheral surface of the developing sleeve 54. The developing sleeve 54 is in close contact with the developing roller 53 by means of a guide member that is pressed against both ends of the developing sleeve 54.
It rotates in the direction of arrow c with the rotation of 3. Then, the toner layer covering the outer periphery of the developing sleeve 54 is regulated by the regulating portion 57 of the blade 56 and adjusted to have a constant thickness. Also,
The toner is charged to have a predetermined polarity and potential by coming into contact with the blade regulating portion 57. Since the blade 56 is arranged so that a frictional force is applied from the fixed portion to the free end by the contact between the developing sleeve 54 and the regulating portion 57, a stable contact state with the developing sleeve 54 is obtained. can get.

The toner held on the outer surface of the developing sleeve 54 is carried to the developing area 13Y by the rotation of the developing sleeve 54 driven by the developing roller 53. Then, the toner adheres to the latent image formed on the surface of the photosensitive drum 3 by electrostatic attraction to form a toner image.

Here, since the slack portion 55 of the developing sleeve 54 which is in contact with the photosensitive drum 3 is not in contact with the developing roller 53, the developing sleeve 54 is soft and has an appropriate nip width and is even. 3 to form a toner image having a uniform thickness capable of faithfully reproducing the gradation recorded during latent image formation. Further, even if a difference in peripheral speed is provided between the photosensitive drum 3 and the developing sleeve 54, the toner image is not destroyed.

The gradation recording method for gradation reproduction in the color page printer 1 will be described in detail below.
FIG. 3 is a perspective view showing the outline of the scanning optical system 6.

The semiconductor laser 121 is controlled based on image information read from an image memory of a control unit (not shown) and outputs a binary-modulated light beam. The light beam is collimated by the collimating lens 122 and is deflected by the rotating polygon mirror 123. The polarized light beam forms an image on the photosensitive drum 3 by an image forming lens 124 called an fθ lens, and performs beam scanning.
At the time of this beam scanning, the light at the beginning of each scanning line of the light beam is reflected by the mirror 125, and the detector 1
Lead to 26. The detection signal from the detector 126 is used as a synchronizing signal for scanning in the H direction (horizontal direction).

FIG. 4 shows the modulation circuit 90 of the semiconductor laser 121.
FIG. 5, and FIG. 5 is a diagram showing the timing of address designation. In FIG. 4, the modulation circuit 90 has a pattern R
It is composed of an OM 91, a Z address counter 92, an X address counter 93, a Y address counter 94, and a driver 95.

The pattern ROM 91 stores a large number of exposure patterns PS composed of an m × n matrix.
One exposure pattern P by the Z address counter 92
S is designated, and the value of each element (cell) that constitutes the designated exposure pattern PS is sequentially designated by the X address counter 93 and the Y address counter 94.
It is read as bit data DE. The exposure pattern PS will be described in detail later.

The Z address counter 92 performs Z adjustment for each image data DG, that is, for each pixel based on the image data DG read from an image memory (not shown) or the like.
The address AZ is generated and output. The image data DG is
The digital signal is, for example, about 6 bits (64 gradations) or 8 bits (256 gradations) indicating the gradation level (quantized density) of each pixel of the original image.

The X address counter 93 counts the pixel clock signal SC, and the count value is used as the X address A.
Output as X. The pixel clock signal SC is a clock signal having a frequency k times (k is the number of divisions) the image clock signal. That is, k pixel clock signals SC are input while one image data DG is input. The X address counter 93 repeats counting within a predetermined value range according to the number of elements EM arranged in the lateral direction (row direction, horizontal direction, or main scanning direction) of the exposure pattern PS described later. For example, the exposure pattern PS
If the size is 4 (vertical) × 16 (horizontal), counting is repeated in the range of 0 to 15, as shown in FIG.

The Y address counter 94 counts the horizontal synchronizing signal SH issued every scanning of one line, and outputs the count value as the Y address AY. The Y address counter 94 repeats counting within a predetermined value range according to the number of elements EM in the vertical direction (column direction, vertical direction, or sub-scanning direction) of the exposure pattern PS described later. For example, the size of the exposure pattern PS is 4 (vertical) × 16
If it is (horizontal), as shown in FIG. 5, counting is repeated in the range of 0 to 3.

The driver 95 uses the data DE read from the pattern ROM 91 to drive the semiconductor laser 12
1 is controlled on / off. Next, the exposure pattern stored in the pattern ROM 91 will be described.

The exposure pattern has m dots D vertically and horizontally.
Each dot DT of the square matrix made of T is divided into k in the horizontal direction, and each of the divided small regions is a matrix of size m × (m × k) that is an element EM. Each element EM takes a value of "0" or "1". When it is "0", the semiconductor laser 121 is off, and when it is "1", the semiconductor laser 121 is on. Therefore, the portion of the photoconductor drum 3 corresponding to the turning on of the semiconductor laser 121 is exposed, its potential is lowered, and the toner adheres to the portion. As a result, for example, when the developing color is black, the toner-attached portion on the recording paper becomes black. On the contrary, the part of the recording paper corresponding to the turning off of the semiconductor laser 121 becomes white.
Therefore, the element EM with a value of “0” is replaced with the “white element
W) ”and an element EM having a value of“ 1 ”is a“ black element (EMB) ”
There is a thing.

The point to be noted here is that
This means that the area of the image recorded in black on the recording paper by the two black elements EMB depends on the position of the black elements EMB. More specifically, the black element EMB
Differs depending on whether the element adjacent to is the white element EMW or the black element EMB, and also differs depending on whether the adjacent position is left or right or up and down. The reason will be described in detail later.

FIG. 6 is a diagram showing an example of the exposure pattern PSA,
7 is a diagram showing a state of irradiation energy corresponding to the exposure pattern PSA shown in FIG. 6, FIG. 8 is a diagram showing a recording state on a recording sheet corresponding to the exposure pattern PSA shown in FIG. 6, and FIG.
FIG. 6 is a diagram showing a distribution state of irradiation energy due to lateral movement of a light beam. Note that FIGS. 7 and 8 are obtained by simulation, and FIG. 8 shows a recording state when binarized with 5.2 erg as a threshold value.

The exposure patterns PSA1 to PSA4 shown in FIG.
Each dot DT of the 3 × 3 dot square matrix is divided into four in the horizontal direction to form a 3 × 12 matrix, and “0” is shown in white and “1” is shown in black for each element EM.

The semiconductor laser 121 is turned on corresponding to one black element EMB, and the semiconductor laser 121 irradiates the photosensitive drum 3 with a light beam. Therefore, the single black element EMB irradiates the photosensitive drum 3 with the light beam for a unit time, that is, the exposure. The total number of black elements EMB is the total exposure time.

Here, each dot DT of the square matrix corresponds to one pixel of the image data DG, and by dividing each dot DT into four, the semiconductor laser 1
21 is pulse width modulated. Light beam
For example, with respect to one dot DT, the half-value width in a stationary state is about 60 μm in both the main scanning direction and the sub-scanning direction.
It has a circular shape of m. The energy of the light beam is high in the central part, and the energy is gradually lower toward the periphery. Further, the light beam scans the photosensitive drum 3 for one line in the main scanning direction and then scans the next line in the same manner. Therefore, for each element EM of the exposure pattern PSA, the element EM in the same row is used. Are scanned continuously, but for elements EM whose columns are different from each other, the scanning is performed spatially spaced apart from each other, that is, discontinuously.

As a result, when the black elements EMB are laterally adjacent to each other, the irradiation energies of the light beams are added to each other to increase, and as a result, the maximum irradiation energy increases as shown in FIG. On the other hand, when the black elements EMB are adjacent to each other in the vertical direction, the contribution of the irradiation energy is small, and therefore the maximum irradiation energy does not become large. That is,
Even if the number of black elements EMB is the same, in other words, the total exposure time is the same, the maximum irradiation energy is large when the black elements EMB are continuously arranged in the lateral direction, and the black elements EMB are The maximum irradiation energy is smaller than that in the case of lining up in the vertical direction. Therefore, even if the total exposure time is the same, an electrostatic latent image is formed on the photosensitive drum 3 with different irradiation energy.

Then, when the electrostatic latent images formed with such different irradiation energies are developed, the irradiation energies exceeding a certain threshold are applied depending on the sensitivity of the photosensitive drum 3 and the magnitude of the developing bias. The toner adheres only to the received portion to be visualized, and a black image is formed.

As a result, even if the number of black elements EMB is the same, the area of the image formed on the recording paper PP differs depending on the arrangement position. That is, the black element EM
A plurality of exposure patterns PS having the same total number of Bs
By making the positions of the black elements EMB different from each other with respect to A, recording with different gradations is performed.

In FIG. 6A, the exposure pattern PSA
In No. 1, the total number of elements EM is "4", and one dot DT in the center is recorded in black. In this case, as shown in FIG. 7 (A), the irradiation energy spreads in a substantially concentric circle. When the threshold value is 5.2 erg, as shown in FIG. 8 (A), A circular black image FG1 is formed in the center.

In FIGS. 6B to 6D, in the exposure patterns PSA2 to PSA4, the total number of the elements EM is "5".
That is, 5/4 dots are recorded. However, these differ in the position (that is, pattern) of the element EM. That is, in the exposure pattern PSA2, five black elements EMB are arranged continuously in the lateral direction,
In the exposure pattern PSA3, one black element EMB is arranged in a row above the other four black elements EMB, and in the exposure pattern PSA4, two black elements EMB are arranged in a row above the other three black elements EMB. Has been done.

Therefore, as shown in FIGS. 7B to 7D, the irradiation energy of the exposure pattern PSA2 in which the five black elements EMB are continuous in the lateral direction becomes the largest, and the exposure pattern PSA4 with a small number of continuous patterns is obtained. Irradiation energy is the smallest. As a result, as shown in FIGS. 8B to 8D, the image FG2 formed by the exposure pattern PSA2 is the largest and the image FG4 formed by the exposure pattern PSA4 is the smallest. If this is shown numerically, the exposure patterns PSA1 ...
The dot area ratios [%] of the images FG1 to 4 in No. 4 are “2.6”, “8.0”, “5.0”, and “1.3”, respectively.
6 to 8, as is clear from the comparison between (A) and (D), when the number of black elements EMB of the exposure pattern PS is different from each other, the number of black elements EMB is small depending on the arrangement position. In some cases, the area of the image recorded on the recording paper PP becomes larger.

As described above, even if the matrix size and the dot division number R are the same, the gradation can be changed by changing the position of the element EM. That is,
The number of gradations can be increased without lowering the resolution. Therefore, the total number of elements EM and each element EM
Linearized gradation characteristics can be obtained by combination with the position of. Moreover, since the intensity modulation of two or more levels is not performed, it can be implemented at a low cost without causing a problem of laser stability.

Although a large number of exposure patterns PS are stored in the pattern ROM 91, when the image data DG has, for example, 64 gradations, 6 including all white and all black.
Four exposure patterns PS are stored.

10 and 11 are diagrams showing examples of the exposure pattern PSB stored in the pattern ROM 91. Exposure patterns PSB0-23 shown in FIGS.
Is a square matrix of 4 × 4 dots, and each dot DT is divided into four in the horizontal direction to form a 4 × 16 matrix, and “0” is shown in white and “1” is shown in black for each element EM. is there.

These exposure patterns PSB0-23 are
The exposure patterns of 24 gradation levels (level 0 to level 23) on the low density side in the exposure pattern of 64 gradations are shown, and the Z addresses AZ are 0 to 23, respectively. 64 exposure patterns PS including these exposure patterns PSB
B is stored in the pattern ROM 91. These exposure patterns PSB are selected so that linear output characteristics can be obtained for the scanning optical system 6 having the following optical characteristics.

Resolution: Main scanning 2400 DPI Sub scanning 600 DPI Image clock: 8.9 MHz Laser emission intensity: 0.23 mW Still beam diameter (half width): 60 μm for both main and sub scanning Z address counter 92, 64 gradation image data The Z address AZ corresponding to the value of DG is generated, and one of the exposure patterns PSB is designated by this.
The X address counter 93 and the Y address counter 94 specify the X address AX and the Y address AY, and the element EM of each exposure pattern PSB is sequentially data D.
It is read as E, and the semiconductor laser 12 is read according to the value.
1 is on / off controlled. The Z address AZ, the X address AX, and the Y address AY are combined as needed, and the pattern ROM 91 is combined with the combined address.
Addressing is performed.

As a result, using the same scanning optical system 6, IH
A linearized output can be obtained without changing the resolution and the number of gradations, as compared with the case where gradation reproduction is performed by a conventional method such as a method. When the number of gradations is 64, the pattern ROM
It is necessary to store 64 exposure patterns PS in 91, but the element EM (data DE) of each exposure pattern PS is 1
Since it is a bit, the data amount does not increase as compared with the conventional one, and it does not become a cost problem. Rather, since the number of gradations can be determined by the number of exposure patterns PS, the number of gradations can be set in small steps, and the amount of data can be reduced as compared with the conventional method, which is advantageous in terms of cost. .

In the above embodiment, each element EM is set to 2
Although the value is set, it can be set to two or more. By setting the number to two or more, it is possible to further increase the number of gradations and improve the image quality. Also, the number of divisions k is reduced,
By compensating for that by multi-valued, it is possible to lower the frequency of the pixel clock signal SC while maintaining the same number of gray scales, so that there is a margin in the operation of each part, which results in higher operation reliability and cost. It is possible to reduce the amount.

FIG. 12 shows a ternarized exposure pattern PSC.
FIG. 13 is a diagram showing an example of the above, and FIG. 13 is a diagram showing a state of irradiation energy corresponding to the exposure pattern PSC2 shown in FIG. 12B and a recording state on the recording paper PP.

Exposure patterns PSC1 to PSC3 shown in FIG.
Is a 3 × 12 matrix in which each dot DT of a 3 × 3 dot square matrix is divided into four in the horizontal direction similarly to the exposure pattern PSA shown in FIG. 6, and “0” is assigned to each element EM thereof. Is white, "1" is gray, and "2" is black. The element EM having a value of "1" may be referred to as "gray element (EMG)".

The semiconductor laser 121 is turned on in correspondence with one gray element EMG or black element EMB, but the black element EM
When it is B, it is turned on so as to have the rated strength, and when it is the ash element EMG, it is turned on so as to have the strength half the rated value.

According to such an exposure pattern PSC,
Even if the total number of the black elements EMB and the gray elements EMG is the same, the area of the image FG recorded on the recording paper PP depends on the ratio of the black elements EMB and the gray elements EMG and their arrangement positions. It will be different. Therefore, the number of gradations can be increased more than in the case of the above-mentioned exposure pattern PSA.

In the exposure pattern PSC1, the area of the image FG recorded at the same threshold is the exposure pattern PS.
Between A1 and the exposure pattern PSA2, the exposure pattern PSC2 is smaller than the exposure pattern PSA3, and the maximum irradiation energy is lower than the threshold value in the exposure pattern PSC3, the image FG is not recorded. The dot area ratio [%] of the image FG in these exposure patterns PSC1 to 3
Are "5.1", "3.8", and "0.0", respectively.

In the above embodiment, the semiconductor laser 1
Although the scanning optical system 11 using 21 has been described as an example,
Even in an optical system using an LED array, liquid crystal, PLZT, etc., since there is a light intensity distribution on the photoconductor drum 3, the dot area ratio changes depending on the exposure position as in the case of using the semiconductor laser 121. It is possible to increase the number of gradations and improve the linearity of gradation characteristics. In the above embodiment, one pixel of the image data DG is associated with one dot DT of the exposure pattern PS on a one-to-one basis, but one pixel of the image data DG may be associated with one exposure pattern PS. Further, one pixel of the image data DG may correspond to a plurality of dots DT of the exposure pattern PS, or one pixel of the image data DG may correspond to one or a plurality of elements EM.

In the above-mentioned embodiment, the case of the exposure pattern PS in which the screen angle is 0 and becomes thicker from one point has been described, but it is obvious that other types of exposure patterns can be applied. For example, in the case of full-color printing, it is conceivable to use an exposure pattern with a different screen angle for each color, as in normal printing.

[0073]

According to the present invention, fine gradations can be recorded at the stage of latent image formation by exposure without deteriorating the resolution and increasing the cost, and the recorded fine gradations can be recorded. Can be faithfully reproduced on paper, and a high-resolution hard copy image with excellent gradation reproducibility can be obtained.

[Brief description of drawings]

FIG. 1 is a sectional front view showing a mechanical configuration of a color page printer according to the present invention.

FIG. 2 is a sectional front view of a developing device.

FIG. 3 is a perspective view showing an outline of a scanning optical system.

FIG. 4 is a block diagram of a modulation circuit of a semiconductor laser.

FIG. 5 is a diagram showing timing of address designation.

FIG. 6 is a diagram showing an example of an exposure pattern.

7 is a diagram showing a state of irradiation energy corresponding to the exposure pattern shown in FIG.

FIG. 8 is a diagram showing a recording state on a recording paper corresponding to the exposure pattern shown in FIG.

FIG. 9 is a diagram showing a distribution state of irradiation energy due to lateral movement of a light beam.

FIG. 10 is a diagram showing an example of an exposure pattern stored in a pattern ROM.

FIG. 11 is a diagram showing an example of an exposure pattern stored in a pattern ROM.

FIG. 12 is a diagram showing an example of a three-valued exposure pattern.

13 is a diagram showing a state of irradiation energy corresponding to the exposure pattern shown in FIG. 12B and a recording state on a recording sheet.

FIG. 14 is a diagram for explaining a conventional gradation recording method by the IH method.

[Explanation of symbols]

 1 Color Page Printer (Image Forming Device) 3 Photosensitive Drum (Photosensitive Member) 11Y, 11M, 11C, 11K Developing Device (Developing Means) 51 Housing (Guide Member) 53 Developing Roller 54 Developing Sleeve (Cylindrical Thin Film Member) 57 Regulation unit (toner leveling member) 90 Modulation circuit (exposure control means) 91 Pattern ROM 92 Z address counter (exposure pattern designating means) 93 X address counter (address designating means) 94 Y address counter (address designating means) 95 Driver ( Driver means) PS, PSA, PSB, PSC exposure pattern

Claims (4)

[Claims] 1. An image forming method for forming a latent image on a photosensitive member by scanning an original image line-sequentially, and then developing the latent image to form an image on a sheet. At the time of formation, the total irradiation amount of energy applied to an exposure area of a certain size corresponding to each pixel is changed according to the gradation level of each pixel of the original image, and the total irradiation amount of the energy is changed. The gradation of the original image is recorded by making the positions of irradiating the energy different from each other on the plurality of exposure areas having the same number, and the cylindrical thin film member having the toner layer on the surface is recorded. An image forming method, wherein the latent image is developed by bringing a part of the circumferential direction into contact with the photoconductor. 2. An image forming method for forming a latent image on a photoconductor by scanning an original image line-sequentially, and then developing the latent image to form an image on a sheet. At the time of formation, according to the gradation level of each pixel of the original image, while changing the total exposure time for the exposure area of a certain size corresponding to each pixel,
A cylindrical thin film having a toner layer formed on the surface thereof by recording the gradation of the original image by making the exposure positions different from each other with respect to the plurality of exposure regions having the same total exposure time. An image forming method, wherein the latent image is developed by bringing a part of a member in a circumferential direction into contact with the photoconductor. 3. An image forming method for forming a latent image on a photoconductor by scanning an original image line-sequentially and then developing the latent image to form an image on a sheet. At the time of formation, in order to determine the exposure position in the matrix area of a constant size, an exposure pattern group including a plurality of exposure patterns having the same total exposure time but different exposure positions from each other is used. The gradation of the original image is recorded by selecting an exposure pattern according to the gradation level of each pixel, and a part of the cylindrical thin film member provided with a toner layer on the surface in the circumferential direction is used as the photosensitive member. An image forming method, wherein the latent image is developed by bringing them into contact with each other. 4. An image forming apparatus for forming an image on a sheet, comprising: an exposure control unit for scanning an original image line-sequentially to form a latent image on a photoconductor; and a developing unit for visualizing the latent image. And a pattern ROM that stores an exposure pattern group including a plurality of exposure patterns that have the same total exposure time but different exposure positions from each other. Exposure pattern designating means for designating an exposure pattern according to the gradation level of each pixel of the image data, and the data of the designated exposure pattern in the pattern ROM.
From the pattern ROM, and driver means for controlling the intensity of the light source based on the data read from the pattern ROM. The developing means is rotatably driven. A developing roller, and a cylindrical thin film member having a longer peripheral length than the developing roller and externally mounted on the developing roller; and a part of the circumferential direction of the developing roller that rotates together with the developing roller. The thin film member is pressed against the developing roller so as to be separated from the developing roller, and a guide member for bringing the outer surface of the thin film member away from the developing roller into contact with the photoconductor, and a toner layer on the outer surface of the thin film member. An image forming apparatus comprising: a toner leveling member for forming.