JPH05297678A - Color image forming method - Google Patents

Abstract

PURPOSE:To reproduce a color image with optimum image density without losing color balance by, when a color with decreased image density exists despite the toner concentration of the almost prescribed value, controlling expo sure conditions for the color. CONSTITUTION:When a color with decreased image density (an amount of attached toner) exists although the concentration of toner has almost the prescribed value, a second CPU 60 makes the amount of attached toner optimum by controlling an emitted light quantity control circuit 54 for the color, and by modulating the intensity of a light source for dot exposure and modulating the pulse width, thereby increasing a dot area when an electrostatic latent image to be developed by a two-component developer is formed by the dot exposure.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention employs a developing method of visualizing an electrostatic latent image using a two-component developer composed of a toner and a carrier of a copying machine or the like to form a color image of a plurality of colors. In particular, the present invention relates to a color image forming method for controlling exposure conditions so as to obtain an appropriate image density.

[0002]

2. Description of the Related Art In a developing device for developing an electrostatic latent image on an image bearing member by using a two-component developer composed of a toner and a carrier, the toner and the carrier in the two-component developer are mixed by weight. The ratio (hereinafter referred to as toner concentration T _C ) greatly affects the developability. For example, if the toner density of the two-component developer is lower than the proper value, the density of the developed image will be low. On the contrary, when the toner density becomes too high, the density of the developed image becomes too high and the so-called fog increases. Therefore, the recorded image on the transfer material transferred from the image carrier becomes unsuitable.

Therefore, in order to always obtain an image having a preferable density, the toner density of the two-component developer is set to an appropriate level,
At the same time, it is necessary to keep the appropriate level constant during development. Therefore, conventionally, there has been proposed a two-component developer concentration control system in which the toner concentration of the two-component developer is kept constant by controlling the toner supply amount. This density control method detects change events such as magnetic permeability change and volume change of two-component developer, change of image density after development, and color change of two-component developer due to toner and carrier having different colors. By doing so, the toner concentration is detected, and the toner replenishment amount is controlled based on the detected toner concentration to maintain the proper toner concentration.

However, in the method of detecting a change event as described above, stable operation for a long period of time has been difficult due to reasons such as erroneous detection and difficulty in compensating for deterioration of the surface of the photosensitive member over time.

As a solution to this problem, Japanese Patent Laid-Open No. 57-1366 has been proposed.
No. 69 “Two-component developer concentration control device” is introduced. This "two-component developer concentration control device" is a detection level that should detect the volume of the two-component developer in the developing device so that the developing effect in the recording device can be stably and appropriately maintained for a long period of time. With a plurality of, separately, by optically detecting the developed image of the reference image formed on the photosensitive surface to detect the developing performance, or the density of the recorded image,
In response to this, the plurality of detection levels are switched and set by the central processing unit so that the toner density of the two-component developer is controlled in order to avoid carrier replenishment as much as possible and to keep the developing performance as constant as possible. It is configured to. For the time being, the above-mentioned object was achieved.

[0006]

However, the contents of the original document to be image-recorded often have variations in blackening rate due to differences in image density and image density, and blackening of line drawings, character images, low-density originals, etc. When a developing operation is executed to continuously form an image of original contents having a low rate, a predetermined image density corresponding to the toner density cannot be obtained even though the toner density is constant, and the toner density is low. -The phenomenon that the correspondence between the density and the image density is broken has occurred. It was also found that this phenomenon is likely to occur in the case of a color electrostatic recording device in which a color image can be reproduced by using a two-component developer of various colors.

Then, when the cause of occurrence of such a phenomenon was considered, the following findings were found.

That is, as a result of analyzing the phenomenon common to the above two examples, the developing sleeve and the stirring means in the developing device are continuously driven to rotate within the developing time during the image forming process in the electrostatic recording apparatus. However, it has been found that this stirring time, in other words, the residence time of the two-component developer in the toner hopper, is commonly related. That is, in the former example, since the original is of low density, the toner consumption amount per unit time is smaller than usual, and the toner retention time becomes longer accordingly. In the latter example, the toner consumption amount of the two-component developer for each color is different, and the residence time of the toner is also different for each color.

Considering that the phenomenon in the above example occurs even when the toner concentration is constant, it is presumed that the phenomenon does not result from a so-called spent toner, which is a state in which toner components are attached around the carrier and the toner charge amount is reduced. However, since it is related to the toner retention time, that is, if the toner retention time becomes long, the chances of friction between the toner and the carrier increase, and it is expected that the toner charge amount will continue to increase. It is considered that the toner charge amount outside the predetermined range affects the image density. This phenomenon will be described below.

FIG. 13A is a diagram showing the transition of toner consumption of each color corresponding to the number of copies in a color image forming apparatus for recording a color image with a two-component developer of a plurality of colors. 13B is a diagram showing the toner charge amount of each color corresponding to the number of copies in the color image forming apparatus, and FIG. 13C is an optical image corresponding to the number of copies in the color image forming apparatus. It is the figure which showed density (CD).

Here, the optical image density (CD) means a constant exposure part formed on the photosensitive drum by irradiation from the scanning optical system based on an image signal corresponding to the reflected light from the standard density plate. The image density corresponds to the reflected light from the standard toner image developed under the electrostatic process condition.

FIG. 13A shows an example of copying a certain color original, and the solid line BK shows the transition of the toner consumption according to the number of copies of the black toner. Consumes only 80 mg per A4 sheet on average, 300
It consumes about 240g per 0 sheets. The solid line Y shows the transition of the toner consumption according to the number of copies of the yellow toner, specifically, the average consumption is 50 mg per sheet,
Consume about 150g per 3000 sheets. The solid line C shows the transition of toner consumption according to the number of copies of cyan toner, specifically, the average consumption is 50 mg per sheet, and about 150 g per 3000 sheets. The solid line M shows the transition of the toner consumption amount according to the number of copies of the magenta toner, specifically, the average consumption is 10 mg per sheet and about 30 g per 3000 sheets. These show that the amount of consumption differs depending on the color of the developer.

The above is an ideal case in which the development characteristics of each color do not change even after copying 3000 sheets.

However, in a normal image recording apparatus,
Due to the demand for simplification of control, the developing sleeve and the stirring means in the developing device are continuously driven to rotate within the developing time during the image forming process. Therefore, as described below, the toner charge amount of each color in the developing device varies due to the toner staying for a long time. An example thereof will be described below.

In FIG. 13B, the solid line BK shows the transition of the toner charge amount Q / m according to the number of copies of the black toner. From this point, the toner charge amount is substantially constant regardless of the number of copies. It is presumed that this is because the toner concentration is controlled so that the toner is sequentially replenished in accordance with the consumption, so that the toner that stays for a long time is small.

The solid line Y shows the transition of the toner charge amount Q / m according to the number of copies of the yellow toner, and the solid line C shows the toner charge amount Q / m according to the number of copies of the cyan toner.
, The toner charge amount is
It is slightly increasing with the increase in the number of sheets. This is because the toner concentration control makes it possible to successively
Toner that replenishes but stays for a long time
Is presumed to be due to a slight residual amount exceeding the allowable range.

The solid line M shows the transition of the toner charge amount Q / m according to the number of copies of the magenta toner, and the toner charge amount rises extremely as the number of copies increases. It is presumed that this is because toner is replenished successively according to consumption by controlling toner concentration, but a large amount of toner that remains for a long time exceeds the allowable range.
As described above, when each color developer is loaded, the difference in toner charge amount between the color developers is small and does not affect the developability, but as the copying is continued, each color developer is Due to the difference in the consumption amount of the toner, there is a difference in the amount of toner that stays for a long time.
This shows that the toner charge amount differs between the color developers and the developability also varies.

The following phenomenon occurs due to the difference in toner charge amount between the color developers.

In FIG. 13 (c), the solid line BK shows the transition of the optical image density (CD) according to the number of copies of the black toner. From this, the optical image density (CD) is substantially constant regardless of the increase in the number of copies. That is, the developability is substantially constant. The solid line Y shows the transition of the optical image density (CD) according to the number of copies of the yellow toner, and the solid line C
Is the optical image density (C
D) of the optical image density (C
D) is slightly increasing due to an increase in the number of copies, but gradually increasing. This indicates that the yellow and cyan toners are slight, but the developability is lowered. The solid line M is the optical image density (CD) according to the number of copies of the magenta toner.
The optical image density (CD) rises extremely as the number of copies increases. This indicates that the magenta toner extremely reduces the developability.

As described above, in the image forming apparatus in which the stirring in each color developing device is continuously performed over the entire developing time, there is a difference in the consumption amount of each color developing agent, and each color developing existing in the developing device is different. Due to the difference in the stirring time of the agent, the developability is affected. Therefore, there is a problem in that the image density is reduced in the case of a single color reproduced by the image recording apparatus, and in color copying, a desired amount of each color toner is not developed and the color balance of the color image is disturbed.

The present invention has been made in view of the above problems, and an object thereof is to reduce or increase the image density between the color developers regardless of maintaining the toner density substantially constant at a predetermined value. In the case of occurrence of the above, the present invention provides a color image forming method capable of reproducing an image with proper image density and without disturbing color balance by detecting this and controlling the exposure condition to a predetermined value.

[0022]

In order to achieve the above object, the present invention forms an electrostatic latent image on a uniformly charged photoreceptor by image exposure by dot exposure and forms the electrostatic latent image. In a color image forming method in which a color toner image composed of a plurality of colors is laminated by developing with a two-component developer for each color, the two-component developer causes The dot area is enlarged when the electrostatic latent image developed by the two-component developer is formed by dot exposure for the color in which the toner adhesion amount when the electrostatic latent image is developed is lower than a predetermined amount. ..

[0023]

The photosensitive member is uniformly charged, and an electrostatic latent image is formed on the photosensitive member by imagewise exposure by dot exposure. The electrostatic latent image is developed with a two-component developer for each color to form a plurality of images. Color toner images of different colors are stacked.

At this time, a color in which the toner adhesion amount when the electrostatic latent image of the reference density is developed by the two-component developer is reduced below a predetermined amount, although the toner density is substantially constant,
That is, with respect to the color whose developability has deteriorated, the dot area when the electrostatic latent image developed by the two-component developer is formed by dot exposure is enlarged, so that the development when the two-component developing means performs the development is performed. The developability is properly restored by increasing the potential gap.

[0025]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A color image forming method according to an embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 is a block diagram of a color image forming apparatus to which a color image forming method according to an embodiment of the present invention is applied. The color image forming apparatus 100 includes a photosensitive drum 1, which is a latent image carrier, and a photosensitive member. First to control the rotation of the drum 1
Motor driver 10, encoder 11, four developing devices 2
0BK, 20C, 20M, 20Y, high-voltage power supply 40, transfer device 42, charger 41, reading optical system 51, scanning optical system 5
2. Optical system drive circuit 53, irradiation light amount control circuit 54, hopper drive circuit 55, first CPU 50, second CPU 60, optical density measuring means 61, developing bias circuit 62 and four developing devices 20BK, 20C, 20M and 20Y respectively. Toner concentration detecting means 63BK, 63C, 6 provided respectively
3M and 63Y.

When a copy button (not shown) is pressed, the scanning optical system 52 is controlled under the control of the first CPU 50.
A light source emits light based on an image signal corresponding to the image density of the original image from the above to irradiate the photosensitive drum 1 with light, thereby forming an electrostatic latent image on the photosensitive surface of the photosensitive drum 1. Latent image development device 20BK, 20C, 20M, 20Y
The toner images of a plurality of colors are stacked (superposed) on the photosensitive drum 1 by sequentially developing with, and the transfer device 42 is discharge-driven based on the registration signal to transfer the toner image onto the transfer paper. .. After that, the transfer paper is fixed to form a reproducible image that can be stored. That is, black B
K, cyan C, magenta M, and yellow Y are used as basic colors, and other colors are reproduced by appropriately selecting and stacking these basic colors. In this case, the order of superposing the colors other than the basic color is cyan C, magenta M, and yellow Y in order from the lower layer. The toner images of a plurality of colors may be superposed on the transfer drum instead of on the photosensitive drum 1.

The photosensitive drum 1 uses a drum-shaped conductive support made of aluminum and having a diameter of 150 mm, and a thickness of 0.1 μm made of an ethylene vinyl acetate copolymer on the support.
m is an OPC photosensitive member formed by laminating an intermediate layer of m and a photosensitive layer having a film thickness of 35 μm. When the photosensitive layer is irradiated with light, the surface potential drops. Therefore, if the photosensitive drum 1 is previously uniformly charged to a predetermined potential and then the light based on the light and shade of the original image is irradiated, the surface potential of the photosensitive drum 1 becomes non-uniform, and a portion having a lowered potential is formed. be able to.
This is what is called an electrostatic latent image. The photoconductor drum 1 is not limited to this, and may have another structure such as a photoconductor made of amorphous silicon. Here, for convenience of description, the OPC photosensitive member described above will be described. The latent image carrier is not limited to the photosensitive drum 1, but may be a belt-shaped member.

The motor driver 10 is a circuit that mainly controls the drive of a main motor (not shown) that rotates the photosensitive drum 1. Based on a control signal from the first CPU 50, the motor driver 10 turns on the rotation speed and rotation of the main motor.・ Off control.

The encoder 11 generates a pulse signal having a predetermined width corresponding to the rotation phase of the photosensitive drum 1, and the first CP
Output to U50. As a result, the first CPU 50 detects the rotation phase of the photosensitive drum 1.

The reading optical system 51 includes a proof lamp (not shown) integrally formed with the first scanning mirror, and a second scanning mirror (V mirror) which moves at a speed ratio of 1/2 of that of the first scanning mirror.
The original is scanned while the optical path length in front of the lens is always kept constant. As a result, the reading optical system 51 forms an image of the reflected light from the document on the platen glass on the light receiving portion of the solid-state image sensor, and outputs the output signal from the solid-state image sensor to the optical system drive circuit 53 as an image information signal. To do.

When the scanning optical system 52 is an analog type,
A known scanning system is used. For example, although not shown, an illumination lamp integrally formed with the first scanning mirror, a second scanning mirror (V mirror) that moves at a speed ratio of 1/2 of the first scanning mirror, and the like, and an optical path in front of the lens The original is scanned while the length is always kept constant. A standard density plate corresponding to, for example, optical reflection density CD = 1.0 is provided at one lower end of the platen glass, and this is irradiated by an irradiation lamp,
After the standard latent image is formed on the photosensitive drum 1, it can be developed with toner. Further, the irradiation intensity can be adjusted by controlling a so-called CVR, which is a power source for lighting the illumination lamp.

As a so-called digital scanning optical system, a method using a similar reading scanning system or a method of storing image data forming a standard density portion in a memory and forming a standard latent image based on this data There is. In the case of the digital system, the scanning optical system 52 as an image exposing means for forming an electrostatic latent image is a laser scanning system including a polygon mirror or the like for rotationally scanning a laser from a semiconductor laser 52b modulated by an image signal. Alternatively, a fixed scanning system using an LED array or liquid crystal may be used.

Further, in the digital scanning optical system, there are an intensity modulation method and a pulse width modulation method as a light modulation method of a laser emitted from the scanning optical system. The intensity modulation method is a method of adjusting the irradiation intensity by adjusting a current conducted to, for example, a semiconductor laser, and the pulse width modulation method is a method of continuously and periodically applying electrical image data such as a triangular wave. It is a method of adjusting the irradiation light amount (irradiation area per pixel) by slicing with a variable reference wave and binarizing.

The optical system drive circuit 53 is a circuit including a circuit for controlling a mechanical system such as a polygon mirror, and an image processing circuit for processing an image signal from the reading optical system 51. For example, the optical reflection densities 0 to 1. Store standard image density data that changes stepwise or continuously up to 0 on ROM,
The scanning optical system 52 is controlled to form a standard latent image based on this data. In this embodiment, the standard image density data corresponds to the optical reflection density of 1.0, but is not limited to this.

The irradiation light amount control circuit 54 is a circuit for controlling a current conducted to a semiconductor laser which constitutes a part of the optical system drive circuit 53 in the intensity modulation method, and in the pulse width modulation method, electric image data. Is a circuit for slicing with a reference wave, such as a triangular wave, which is continuously and periodically variable, and binarized.

Developing devices 20BK, 20C, 20M, 20Y
Are loaded with two-component developers of black, cyan, magenta, and yellow, respectively, and the mechanical configurations are exactly the same except that the loaded two-component developers have different color components. That is, the developing devices 20BK, 20C, 20M, 2
As shown in FIG. 2, 0Y is a sleeve 2 containing a magnet roller 20b having an N pole and an S pole in a developing tank 20a formed of a lower casing and an upper casing.
0c, a regulating member 20d made of a rigid member provided in the upper casing so as to be in pressure contact with the sleeve 20c, and screw-shaped first and second stirring rollers 20e, 20f.
And a scraper 20g for scraping the two-component developer from the sleeve 20c. The two-component developer is composed of toner and carrier, and the toner is attached to the carrier and conveyed to the position of the sleeve 20c.
Is conveyed to the developing area, which is a position facing the photoconductor drum 1. Further, in order to replenish the developing tank 20a with new toner, the replenishment port 20h communicates with a toner hopper (not shown), and the hopper drive circuit 55 is responsive to the toner replenishment signal.
It is configured to include a toner conveying screw (not shown) driven by. In addition, instead of the regulation member 20d, a thin layer forming means including a magnetic rod or a magnetic plate may be provided.

The first agitating roller 20e has a shape that conveys the two-component developer toward the front side of the recording paper, and the second agitating roller 20f has a shape that conveys the two-component developer toward the inner side of the recording paper. ing. In addition, the first and second stirring rollers 2
By providing a partition wall between 0e and 20f, the two-component developer can be smoothly circulated and locally retained. As a result, the two-component developer conveyed to the developing area can be replaced, and the developing conditions are stabilized.

The scraper 20g is rotatably supported by rollers and is provided so as to come into pressure contact with / separate from the sleeve 20c. At the time of pressure contact, the two-component developer which has passed through the developing area and consumed toner is scraped from the sleeve 20c. ..

The sleeve 20c is provided with a developing bias circuit 62 for applying a voltage having an AC bias component and a DC bias component via a protective resistor in order to prevent image fogging.

The developing bias circuit 62 includes a sleeve 20c.
The toner in the two-component developer conveyed to the developing area by the electrostatic latent image formed on the surface of the photosensitive drum 1 is moved by the electrostatic force generated by the electrostatic latent image formed on the surface of the photosensitive drum 1. Includes an AC power supply for applying an AC bias for vibrating between the sleeve 20c and the photoconductor 1 and a high voltage DC power supply for applying a DC bias. As described above, the developing bias circuit 62 generates an oscillating electric field between the sleeve 20c and the photosensitive drum 1, and the toner particles vibrate between the sleeve 20c and the photosensitive drum 1. Even if the photosensitive drum 1 does not come into contact with the photosensitive drum 1, it is possible to form a toner image by toner particles on the photosensitive drum 1 efficiently and clearly. The developing bias circuit 62 can change the developing bias according to a control signal from the second CPU 60.

The high voltage power supply circuit 40 is a circuit for applying a predetermined high voltage to the transfer device 42 and the charging device 41, and is a second CP.
Under the control of U60, the initial charging potential of the photoconductor drum 1 can be changed by changing the voltage value applied to the charger 41.

The optical density measuring means 61 uses a tungsten lamp as a light source, and the light from the tungsten lamp reflected by the reference toner image on the photosensitive drum 1 is received by a light receiving portion such as a photodiode. And B (blue), G (green), R on the entire surface of the light receiving part.
A (red) three-color filter is provided so that the color can be switched according to the color of the image density signal. The optical density measuring means 61 is not limited to this, and as a light source, three colors L of B (blue), G (green) and R (red) L
It is also possible to use an ED and omit the above three-color color filter on the entire surface of the light receiving portion.

Developing devices 20BK, 20C, 20M, 20Y
Toner concentration detecting means 63BK and 63 provided respectively in the
C, 63M, and 63Y detect the change in the magnetic permeability of the two-component developer, so that the corresponding developing devices 20BK, 20C, and
Although the toner concentration in the two-component developer loaded in 20M or 20Y is detected, the toner concentration may be detected by detecting the volume level of the two-component developer. Here, for convenience of explanation, the toner density detecting means for detecting the toner density by the change of the magnetic permeability will be described.

The first CPU 50, which controls the image forming process in sequence, has a built-in image forming program for executing the image forming process, and the image forming process is performed by a start signal issued in response to pressing of the copy button. The program is started to execute the image forming process.

The second CPU 60 has a first look-up table T1 in which the data of the image density CD and the toner density T _{C at} an appropriate toner charge amount are associated in a one-to-one relationship.
A second lookup table T2 in which the data of the image density CD and the development potential gap at an appropriate toner charge amount are associated in a one-to-one relationship, and an image density control program including a toner density determination program and an exposure condition control program, It is stored in the built-in ROM 60a. Here, the development potential gap is, in the regular development method,
The potential difference between the initial charge potential V _H and the developing bias V _B V _H -
V _B , which is the developing bias V _B in the reversal developing method.
And a potential difference V _B -V _L of the exposed portion potential V _L.

The toner density determination program is composed of black density BK, cyan color C, magenta color M and yellow color Y detected by toner density detection means 63BK, 63C, 63M, 63Y.
It is determined whether or not the toner density in the two-component developer is less than or equal to a predetermined value. When the toner density is less than or equal to the predetermined value, the toner density NG signal is output and the toner density is not less than or equal to the predetermined value. In this case, the toner concentration OK signal is sent to the first CPU 50.
Is a program to send to. The first CPU 50
Is a toner concentration OK signal received from the second CPU 60,
Alternatively, it has a toner density judgment result register R1 for setting the toner density NG signal for each color. When the signal in this toner density judgment result register R1 is the toner density NG signal, the two components corresponding to that color are provided. To keep the toner concentration of the developer constant, the toner is supplied from the corresponding toner hopper to the corresponding developing tank 20a.

The exposure condition control program program is a toner program.
Despite keeping the density substantially constant at a predetermined value, the detection signal from the optical density detecting means 61 causes a decrease in image density for a certain color (only a single color or a multiple color).
When the increase is indicated, the program detects this and controls the exposure condition (development potential gap) by referring to the second lookup table T2 to maintain the lowered image density at a predetermined value. Here, the decrease / increase of the image density is detected by detecting the reflected image density of the toner image on the photosensitive drum 1 corresponding to the standard density plate, and the image density C as a comparison reference read from the first lookup table T1.
It is detected by comparing with D = about 1.0 [toner concentration T _C = 7.5 (wt%)]. At this time, the reference image density is set in the reference image density register R2.

The photosensitive drum 1 for forming a color image will be described below.
The upper toner layer will be described with reference to FIGS.

FIG. 3 is a sectional view of the toner image on the photosensitive drum 1 in each color reproduction when the toner charge amount of each color developer is appropriate. 3 (a), FIG. 3 (b), and FIG.
(C) shows a cross-sectional view of a toner image when cyan, magenta, and yellow are reproduced at a predetermined optical reflection image density, and the toner adhesion amounts are substantially the same.
This toner adhesion amount is an appropriate value. FIG. 3D shows a sectional view of a toner image reproducing blue at a predetermined optical reflection image density, and FIG. 3E shows a sectional view of a toner image reproducing green at a predetermined optical reflection image density. 3F is a cross-sectional view of a toner image in which red is reproduced at a predetermined optical reflection image density. These toner images have substantially the same toner adhesion amount in each color toner layer, and the color balance is good. , Blue, green, and red are reproduced, and the image density is in a sufficient state. FIG. 3G shows a sectional view of a toner image in which black is reproduced by superimposing three colors of cyan, magenta, and yellow at the optical reflection image density CD = 1.0. The toner adhesion amount of each color is proper, and the color balance is Is also excellent.

FIG. 4 is a schematic diagram showing a state in which only the toner charge amount of magenta exceeds an appropriate value. 4 (a), 4 (b), 4 (c), 4 (d), 4 (e), 4
In (f), the cyan layer and the yellow layer have proper toner adhesion amounts. Therefore, as shown in FIGS. 4 (a) and 4 (c), the image density is sufficient for cyan and yellow single color reproduction. In addition, the green can be reproduced as shown in FIG. 4 (e). On the other hand, FIG. 4 (b), FIG. 4 (d),
In FIG. 4F, the magenta layer is smaller than the proper toner adhesion amount. Therefore, a sufficient density cannot be obtained in the magenta single color reproduction shown in FIG. Figure 4
(D) is a cross-sectional view of a toner image in which blue is reproduced at a predetermined optical reflection image density. Although the toner adhesion amount of the cyan toner layer is sufficient, the toner adhesion amount of the magenta toner layer is more than an appropriate value. It's getting less. This is a blue that is close to cyan with poor color reproduction. Figure 4 (f)
Shows a cross-sectional view of a toner image in which red is reproduced at a predetermined optical reflection image density. The toner adhesion amount on the yellow toner layer is sufficient, but the toner adhesion amount on the magenta toner layer is less than an appropriate value. .. In this state, red cannot be reproduced sufficiently, resulting in red with a large amount of yellow, that is, orange.

FIG. 5 shows a case where a signal for modulating the light source for image exposure is produced by the pulse width modulation method in the image recording apparatus of this embodiment when only the toner charge amount of magenta exceeds the appropriate value. By changing the reference pulse width to change the image exposure amount per pixel, the latent image potential (exposure portion potential) formed on the photoconductor drum 1 is adjusted to adjust the toner adhesion amount during development. FIG. 6 is a schematic diagram for correcting the color balance by controlling the color reproducibility and optimizing the color reproducibility of each color. In this case, since there is no problem of disturbing the color balance in the reproduction of green, the description thereof will be omitted. Figure 5
(A) is a schematic diagram in which the magenta toner layer is corrected at a predetermined optical reflection image density, and shows that the area of the toner layer is enlarged. FIG. 5B is a cross-sectional view of a toner image that reproduces blue at a predetermined optical reflection image density. Although the cyan toner layer has a sufficient toner adhesion amount, the magenta toner layer has a proper toner adhesion amount. It is smaller, but the area of the toner layer is larger,
The color balance is well corrected. Therefore, blue is reproduced properly. FIG. 5C is a cross-sectional view of a toner image in which red is reproduced at a predetermined optical reflection image density. The toner adhesion amount of the yellow toner layer is sufficient, but the toner adhesion amount of the magenta toner layer is appropriate. Although it is less than the value, the toner layer area is wide and the color balance is corrected well. Therefore, red is reproduced properly.

FIG. 6 shows the image forming apparatus of this embodiment when only the toner charge amount of magenta exceeds an appropriate value.
When the light source for image exposure is modulated using the intensity modulation method, 1
By varying the standard image exposure amount per pixel, the latent image potential (exposure portion potential) formed on the photoconductor drum 1 is adjusted to control the toner adhesion amount during development and reproduce each color. It is a schematic diagram which correct | amends a color balance by optimizing a sex. In this case, since the problem of disturbing the color balance does not occur in green reproduction, the description thereof will be omitted. FIG. 6A is a schematic diagram in which the magenta toner layer at a predetermined optical reflection image density is corrected, and shows that the toner adhesion amount of the toner layer is sufficient. Figure 6 (b)
Shows a cross-sectional view of a toner image in which blue is reproduced at a predetermined optical reflection image density. The cyan toner layer and the magenta toner layer have a sufficient toner adhesion amount, and the color balance is well corrected. Therefore, blue is reproduced properly. FIG. 6C is a cross-sectional view of a toner image in which red is reproduced at a predetermined optical reflection image density. The toner adhesion amount of the yellow toner layer and the magenta toner layer is sufficient, and the color balance is corrected well. .. Therefore,
Properly reproduce red.

FIG. 7 is a schematic diagram for correcting the color balance by the intensity modulation method in the image recording apparatus of the present embodiment in the same manner as in FIG. 6 when the toner charge amount of magenta exceeds the appropriate value. Is. FIG. 7A shows the optical reflection image density C before correction.
FIG. 9 is a schematic diagram in which the toner layer is corrected when black is reproduced at D = 1.0, and only the toner adhesion amount of the magenta toner layer is reduced, the color balance is lost, and it is reproduced as brown. FIG. 7B shows the optical reflection image density CD =
FIG. 7 is a schematic diagram in which the color balance at the time of black reproduction is corrected by the intensity modulation method at 1.0 as in the case of FIG. 6, and FIG. 7C shows the pulse width modulation method at the optical reflection image density CD = 1.0. It is a schematic diagram in which the color balance when reproducing black is corrected. All of these are reproduced as black.

FIG. 8 is a schematic diagram showing a state in which the toner charge amounts of cyan and magenta have exceeded appropriate values in the image recording apparatus of this embodiment. In FIG. 8C, the yellow toner image has a proper toner adhesion amount. However, FIG.
In (b), the toner layers of cyan and magenta are less than the proper toner adhesion amount. Therefore, it is not possible to obtain a sufficient density in monochrome reproduction. Figure 8 (d)
Shows a cross-sectional view of a toner image in which blue is reproduced at a predetermined optical reflection image density, and the cyan toner layer and the magenta toner layer are less than the proper toner adhesion amount, but the color balance is not lost. Therefore, it is reproduced as light blue. FIG. 8E is a cross-sectional view of a toner image in which green is reproduced at a predetermined optical reflection image density. The yellow layer has a sufficient toner adhesion amount, but the cyan layer has less than an appropriate value. Therefore, the color balance is lost and it is reproduced as yellowish green. Figure 8
(F) is a cross-sectional view of a toner image in which red is reproduced at a predetermined optical reflection image density, and the yellow layer has a sufficient toner adhesion amount, but the magenta layer has less than an appropriate value. Therefore, the color balance is lost and the color is reproduced as orange. FIG. 8G is a cross-sectional view of a toner image in which black is reproduced at the optical reflection image density CD = 1.0. The yellow layer has a sufficient toner adhesion amount, but the magenta layer and the cyan layer have less than an appropriate value. Is becoming Therefore, the color balance is lost and it is reproduced as a light brown color. The first C
The PU 50 is a first counter C that counts the number of copies.
T1, a second counter CT2, and a third counter CT3 are built-in, and by a detection signal from a paper discharge sensor or the like,
The first counter CT1 counts the number of continuous copies during continuous copying, and the second counter CT2 indicates the number of copies indicating the timing for executing the exposure condition control program, that is, the total number of copies after the previous exposure condition control is performed. To count. Specifically, the exposure condition control program is executed every time 1000 copies are made. The third counter CT3 executes the exposure condition control program and then determines whether or not the image density returns to a predetermined level by the control, that is, the exposure condition (image exposure to obtain a desired developing potential gap. The exposure value for the level control (hereinafter, simply referred to as the exposure value) The exposure value return judgment copy number indicating the timing of the return judgment is counted. Specifically, after executing the exposure condition control program, it is determined whether or not the image density has recovered to a predetermined level at the time of copying 50 sheets, and the exposure value is returned to the initial value based on the determination. There is. Note that the first counter CT1, the second counter CT2, and the third counter CT3 are down counters, and the number of continuous copies, the total number of copies, and the number of copies for determining the exposure value return judgment are set, respectively, and count down them. Count value is "0"
When it becomes, it is assumed that the set number of copies have been made.

Next, the image recording operation of the image recording apparatus will be described with reference to the flowcharts of FIGS. 9 and 10. 9 is a main flow executed by the first CPU 50, and FIG. 10 is a control flow executed by the second CPU 60 (that is, an exposure value for image exposure level control to obtain a desired developing potential gap). is there.

The power is turned on by pressing a main switch (not shown). As a result, the first CPU 5
0 and the second CPU 60 are activated (step F-1 in FIG. 9).

The first CPU 50 drives the motor driver 10 to start the main motor, and connects the first and second stirring rollers 20e, 20f of the developing devices 20BK, 20C, 20M, 20Y to this drive system. And agitate the two-component developer of each color. Further, the heater of the fixing device (not shown) is heated to a predetermined temperature. Also, the first CPU 50
Is connected to the drive system of the main motor to rotate and drive the photosensitive drum 1, and the charger 41 and a static eliminator (not shown) charge the photosensitive drum 1 and repeat static elimination. Pre-processing such as adjusting Further, in the pre-processing, the continuous copy number A set by the user by a predetermined button operation is set to the first counter C.
Set to T1 (step F-2).

By the way, as described above, the second CPU 60
Is a toner density detecting means 63BK, 63C, 63 corresponding to each color at all times based on the toner density determination program.
It is determined whether or not the toner concentration detection signal from M, 63Y has become equal to or less than the set value of 7.5 wt%, and 7.5 wt%
% Or less, toner concentration NG signal, 7.5 wt
If it is not less than%, the toner concentration OK signal is set to the first
It is transmitted to the toner density determination result register R1 of the CPU 50.

Therefore, the first CPU 50 reads the signal set in the toner concentration determination result register R1 (step F-3) and determines whether or not the color of the toner concentration NG signal exists (step F-4). .. As a result, if there is a color of the toner density NG signal, toner is replenished for that color (step F-5) and the process proceeds to step (F-6). On the other hand, if all the toner density OK signals for each color, the process immediately proceeds to step (F-6) to determine whether or not the countdown value in the second counter CT2 is "0", thereby making a total copy. It is determined whether or not the count value of the number of sheets has reached the total number of copies, 1000, which is related to the setting. As a result, if the total number of copies has not reached 1000, the process proceeds to step (F-9). If the total number of copies reaches 1000, the first CPU 50
Sets the total number of copies set to "1000" in the second counter CT2 (step F-7), generates an image density control interrupt, and outputs an image density control command signal to the second CPU 60 ( Step F-8).

Therefore, the second CPU 60 is connected to the first CPU 5
In response to the image density control command signal from 0, the optical system drive circuit 53 is driven, and the scanning optical system 52 causes the exposure unit (standard electrostatic latent image) corresponding to the optical reflection density CD = 1.0 corresponding to each color. Are sequentially formed on the photosensitive drum 1 (step F-81 in FIG. 10). Then, each developing device 20BK, 20
By driving C, 20M, and 20Y, the standard electrostatic latent image of each color is visualized as a toner image to form a standard toner image of each color (step F-82). Next, the second CPU 60
Are toner density detecting means 63BK, 63C, 63M, 6
The reference image density CD corresponding to the toner density detection signal of each color from 3Y is read from the first lookup table T1 and set in the reference image density register R2 (step F-83).

Then, the optical image density measuring means 61 is activated to take in the actual optical image densities of the respective colors, and it is compared with the reference image densities of the corresponding colors held in the reference image density register R2. That is, it is determined whether or not there is a color whose developability is deteriorated (step F-84). As a result, when there is a color whose developability has deteriorated, it is judged whether or not the exposure value for controlling the image exposure level of that color is the initial value (step F-85). That is, if the exposure value for controlling the image exposure level of that color has not been changed, the change amount of the exposure value for controlling the image exposure level corresponding to the actual optical image density is used as the second lookup table T.
It is read from step 2 (step F-86). Then, the exposure value for image exposure level control is changed based on the read amount of change in the exposure value for image exposure level control (step F-8).
7) Return. The exposure value for controlling the image exposure level is changed by adjusting the irradiation light amount by the irradiation light amount control circuit 54 to control the exposure part potential _VL, which will be described in detail later.

On the other hand, when the exposure value for controlling the image exposure level is not the initial value, the exposure value for controlling the image exposure level is changed, as will be apparent from the following description, but the change still results in proper developability. It means that it has not recovered, and in this case returns immediately. However, this return is for all colors whose developability has deteriorated.
85) and after performing the process of step (F-86).

If it is determined in step (F-84) that the developability has not deteriorated for all colors, whether the exposure value for image exposure level control for all colors is the initial value or not. Is determined (step F-88). as a result,
If there is a color whose exposure value for image exposure level control is not the initial value, it means that the developability was restored appropriately as a result of changing the exposure value for image exposure level control. The exposure value for controlling the image exposure level of is returned to the initial value (step F-89), and the process returns. On the other hand, if the exposure value for image exposure level control for all colors is the initial value, the developability of all colors is appropriate from the beginning, and there is no need to change the exposure value for image exposure level control. It returns as it is.

Therefore, the first CPU 50 executes the image forming process based on the image forming program (FIG. 9).
Step F-9). Then, the countdown value of the continuous copy number of the first counter CT1, the second counter CT2
The countdown value of the total number of copies is subtracted by 1 (step F-10), and it is determined whether the exposure value for image exposure level control is the initial value for all colors (step F-11). As a result, if the exposure value for controlling the image exposure level of all colors is the initial value, the setting is made by determining whether the countdown value of the continuous copy number of the first counter CT1 is “0”. It is determined whether or not the continuous copy number A has been reached (step F-1).
2), the countdown value of the first counter CT1 is "0"
When the number of continuous copies A is reached, the process ends. On the other hand, if the number of continuous copies A has not been reached, the process returns to step (F-3) to make continuous copies for that number.

In step (F-11), when it is determined that there is a color whose exposure value for image exposure level control is not the initial value, that is, the second CPU 60 changes the exposure value for image exposure level control. If there is a color that has not been restored to the initial value after that, the third counter CT3
The number of copies B for judging the return of the exposure value is subtracted by 1 (step F-13). Next, the countdown value (A) of the continuous copy number A of the first counter CT1 is set to the third counter C.
T3 exposure value return judgment It is judged whether or not it is equal to or more than the countdown value (B) of the number of copies B (step F-1).
4). As a result, if the countdown value (A) of the continuous copy number A is equal to or larger than the countdown value (B) of the exposure value return determination copy number B, the countdown value (B) of the exposure value return determination copy number B of the third counter CT3. ) Is "0", it is determined whether or not the exposure value return determination copy number B related to the setting has been reached (step F-15). As a result, if the exposure number return determination copy number B has not been reached, it is clear that the set number of continuous copy number A has not been completed. Therefore, in order to continue continuous copy, step (F-9 ) Return to.

On the other hand, when the countdown value (B) of the third counter CT3 is "0", indicating that the number of copies B for the exposure value return judgment relating to the setting has been reached, the third counter CT3 is exposed. The value recovery judgment copy number B is set (step F-16). Then, the process returns to step (F-8) in order to return the changed exposure value for image exposure level control to the initial value.

When it is determined in step (F-14) that the countdown value (A) of the continuous copy number A is smaller than the countdown value (B) of the exposure value return determination copy number B, the first counter CT1. By determining whether or not the countdown value (A) of the continuous copy number A is 0, it is determined whether or not the continuous copy number A according to the setting has been reached (step F-17). As a result, if the continuous copy number A has not been reached, the process returns to step (F-9) to perform continuous copy for the continuous copy number A. On the other hand, when the countdown value (A) of the first counter CT1 is “0” and the continuous copy of the set continuous copy number A is completed, the process ends. In this case,
The exposure value for controlling the image exposure level does not return to the initial value and ends while being changed. However, when the next continuous copy is performed, the step (F-11) is passed through the step (F-13). ) The subsequent processing is executed, and at that time, the exposure value for image exposure level control is returned to the initial value.

Returning the exposure value for image exposure level control to the initial value in step (F-88) of FIG. 10 has the following meaning. That is, as described above, the first CPU
The second CPU 60 executes the image forming process after the exposure value for image exposure level control is changed by the second CPU 60.
If copying is continued for a while while the exposure value for controlling the image exposure level is kept low, the developability may become too high. In this state, the toner having a high toner charge amount in the two-component developer is consumed and new toner supply is started. Therefore, after the exposure value for image exposure level control was changed, it was confirmed that the developability was properly restored by executing the image density control program again when the predetermined number (for example, 50) of copy numbers was completed. After that, the exposure value for image exposure level control related to the change is returned to the initial value.

Next, the exposure value (exposure condition) for controlling the image exposure level of FIG. 10 by the second CPU 60 in the case of reversal development.
The control operation will be specifically described. Here, the photosensitive drum 1 is an OPC photosensitive member, and the developer is a two-component developer including an insulating magnetic coating carrier having a particle size of 40 μm and a toner having a particle size of 8.5 μm. Is an example of controlling to maintain 7.5 (wt%) ± 0.5 (wt%)). Further, the developing bias has only the DC component, and the initial charging potential V _H = −700 (V),
And the developing bias V _B = −500 (V),
This is an example of controlling the exposure part potential _VL by adjusting the output value (irradiation light amount) of the semiconductor laser by the intensity modulation method by the irradiation light amount control circuit 54. In this embodiment, the optical reflection density of black toner is not measured. This is Figure (d)
This is because the optical reflection density is less likely to decrease for black as shown in FIG. However, also for black, the exposure condition may be controlled by measuring the optical reflection density similarly to cyan, magenta, and yellow.

In the comparison calculation in step (F-84), if the reference data held in the register for each color and the measurement data from the optical reflection density measuring means 61 are substantially the same, the developing devices 20C, 20M and 20Y are loaded. Cyan, magenta, and yellow developer charge amounts Q / m = -24, -23, and -24 (μC / g) are appropriate values, and the image density is CD = 1.0. The exposure value V _B -V _L is −450 (V). In this state, the toner charge amount of each color is at or near an appropriate value. This electrostatic process condition is common to each color when a new developer is loaded. If the reference data and the measurement data substantially match as described above, there is no need to adjust the exposure value V _B -V _L for controlling the image exposure level. Therefore, as described above, as electrostatic conditions corresponding to the developing density CD = 1.0, the electrostatic initial charging potential V _H = −700 (V), the exposed portion potential _VL = −50 (V), the developing bias. V _B = -500
(V), the exposure value for the image exposure level control V _B -V _L = -4
50 (V), output value of semiconductor laser LD = 1.0 (m
The initial value of W) remains unchanged.

In this case, a toner image can be efficiently and clearly formed on the photosensitive drum 1 by the toner particles. Therefore, each color toner has an image density of about 0 to about 1.2.
Since the contrast is sufficiently wide and the adhesive force between the photosensitive drum 1 and the toner is appropriate, it is possible to perform a clear image reproduction without image deviation during the subsequent image forming process. As shown in FIG. 3G, it is possible to reproduce the color with proper image density and without disturbing the color balance.

However, in the comparison calculation in step (F-84), for example, if the magenta measurement data from the optical image measuring means 61 is much lower than the reference data held in the register, the toner charge amount is -25 (an appropriate value). μC
/ G), which is estimated to be a value larger than, for example, about 30.
It is 5 μC / g or more. In such a case, the developability is deteriorated, and unless the developability is enhanced by controlling the exposure value for controlling the image exposure level and increasing the development potential gap, the results shown in FIGS. As shown in (f), sufficient image density and contrast cannot be obtained.

Therefore, for the color whose developability has deteriorated, the second CPU 60 controls the irradiation light amount control means 64 to lower the exposure portion potential V _L, and thereby the exposure value V _B -V for image exposure level control. Increase _L to increase developing power. Specifically, in the above example, the output value LD of the conductor laser is black,
Cyan and yellow are still 1.0 (mW) as described above, but magenta is adjusted to 1.5 (mW) to lower the exposed portion potential _VL . As a result, FIG.
As shown in (1), since the magenta toner adhesion amount becomes an appropriate value and the developing property becomes high, the developing power is corrected.
Therefore, it is possible to obtain sufficient image density and contrast according to the original image, and as shown in FIGS. 6B and 6C, it is possible to reproduce colors without disturbing the color balance. As shown in FIG. 8, when the developability of cyan and magenta decreases, the output values of the semiconductor laser for cyan and magenta corresponding to the image density CD = 1.0 are 1.5 (mW), respectively. ), The same effect was obtained by setting 1.8 (mW). Also, cyan and magenta,
When the developability of all the colors of yellow is lowered, the output values of the semiconductor lasers for cyan, magenta and yellow corresponding to the image density CD = 1.0 are 1.3 respectively.
The same effect was obtained by setting (mW), 1.5 (mW) and 1.8 (mW).

Next, an example of performing exposure value control for image exposure level control by utilizing the pulse width modulation method in the case of reversal development will be described. Before that, the pulse width modulation method will be described.

When the pulse width modulation method is performed, the semiconductor laser 52a of the optical scanning system 52 is driven by the irradiation light amount control circuit 54 as shown in FIG. For example, in the case of a digital copying machine, a PWM signal for image reproduction is created as follows. That is, when the optical image data reflecting the document information is incident on the CCD 54d, the CCD 54d
Photoelectrically converts the optical image data. The electrical image data is read out by the analog processing circuit 54e and subjected to shading correction and the like, and then the A / D converter 54.
A / D conversion is performed at f. Then, the digital processing circuit 54g performs gamma correction, edge enhancement, etc., and inputs the result to the comparator 54h.

On the other hand, the reference wave generating circuit 54i generates a triangular wave having a predetermined frequency, and the comparator 5 uses this triangular wave as a reference wave.
Input in 4h. Therefore, the comparator 54h creates a pulse width modulation signal PWM based on the input electric image data and the reference wave, and inputs it to the laser drive circuit 54c. Then, the laser drive circuit 54c controls the light amount of the semiconductor laser 54b, that is, the size (latent image area) of each dot exposure portion corresponding to the image density CD = 1.0 based on the pulse width modulation signal PWM.

When the comparator 54h creates the pulse width modulation signal PWM based on the inputted electric image data and the reference wave, the pulse width obtained by binarizing the electric image data by the reference wave is given by: Although the pulse width modulation signal PWM corresponding to the image density CD = 1.0 is used, the image density CD =
By adding a DC component (bias) to the reference wave in order to change the binarization of the image data corresponding to 1.0 by the reference wave, for example, the pulse width modulation signal PWM corresponding to the image density CD = 1.2 Can be changed.

That is, FIGS. 12A and 12B show how a plurality of levels of electrical image data S are converted into a PWM signal by the reference wave W which is a triangular wave. The numerical values shown in FIG. 12B are the electrical image data S
Shows the value when is expressed by 8 bits. When the level of the electrical image data S decreases, the pulse width modulation signal PW
Since M becomes longer and the latent image area becomes wider, the image density becomes higher. On the contrary, when the level of the electrical image data S rises, the pulse width modulation signal PWM becomes shorter and the latent image area becomes narrower, so that the image density becomes lighter.

In this embodiment, the reference value of the pulse width modulation signal PWM corresponding to the image density CD = 1.0 is 192/256 gradation for black and 128/256 gradation for cyan, magenta and yellow. I am trying. And
When only the magenta developability deteriorates, as shown in FIG. 5, only the reference value of the pulse width modulation signal PWM corresponding to the image density CD = 1.0 for magenta is set to 192 /
It has been changed to 256 gradations. Further, as shown in FIG. 8, when the developability of cyan and magenta deteriorates, the reference values of the pulse width modulation signal PWM corresponding to the image densities CD = 1.0 for cyan and magenta are 192 respectively.
/ 256 gradation and 224/256 gradation are changed. Further, when the developability of all of cyan, magenta, and yellow decreases, the pulse width modulation signal PWM corresponding to the image density CD = 1.0 for cyan, magenta, and yellow.
The reference values of 160/256 gradation and 192/2
It has been changed to 56 gradations and 224/256 gradations.

The color image forming apparatus of this embodiment is
Since the process of superposing the toner on the photosensitive drum 1 over a plurality of layers is adopted, the developing potential of the upper layer is lowered due to the toner potential caused by the lower toner layer previously formed on the photosensitive drum 1. In some cases. In such cases,
It has been found to be advantageous to control the exposure value so that the developing power of the upper yellow toner, which will be developed later, is slightly stronger than that of the lower cyan toner.

As described above, in the present embodiment, when the toner density is kept substantially constant at the predetermined value, when the image density is lowered, this is detected and the exposure condition is controlled to the predetermined value. As a result, the strength of the developing power can be adjusted. Therefore, sufficient image density and contrast corresponding to the color original image can be obtained, and the reproducibility of color balance is improved.

[0083]

As described above, according to the color image forming method of the present invention, when the toner image density is decreased or increased even though the toner density is maintained substantially constant at a predetermined value. Further, by detecting this and controlling the exposure condition to a predetermined value, a color image can be reproduced with an appropriate density.

[Brief description of drawings]

FIG. 1 is a block diagram of a color image forming apparatus to which a color image forming method according to an embodiment of the present invention is applied.

FIG. 2 is a sectional view of a developing device of the color image forming apparatus shown in FIG.

FIG. 3 is a schematic diagram showing a cross section of a toner image when reproducing each color when the toner charge amount of each color developer is appropriate.

FIG. 4 is a schematic diagram illustrating a cross section of a toner image when reproducing each color when only the magenta toner charge amount exceeds an appropriate value.

FIG. 5 is a schematic diagram showing a state in which color balance is corrected by pulse width modulation when only the magenta toner charge amount exceeds an appropriate value.

FIG. 6 is a schematic diagram showing a state in which the color balance is corrected by intensity modulation when only the magenta toner charge amount exceeds an appropriate value.

FIG. 7 is a schematic diagram showing a state in which black is reproduced when only the magenta toner charge amount exceeds an appropriate value.

FIG. 8 is a schematic diagram showing a cross section of a toner image when reproducing each color when the toner charge amounts of cyan and magenta exceed appropriate values.

FIG. 9 is a flowchart showing an image recording operation in the image forming method of the present invention.

FIG. 10 is a flowchart showing an image density control operation in the image forming method of the present invention.

FIG. 11 is a block diagram showing a configuration of an irradiation light amount control circuit when a pulse width modulation method is used.

FIG. 12 is a diagram for explaining the principle of the pulse width modulation method.

FIG. 13A is a diagram showing the relationship between the number of copies and toner consumption for each color used for color image formation, and FIG. 13B is the number of copies and toner for each color used for color image formation. It is a diagram showing the relationship with the charge amount,
FIG. 7C is a diagram showing the relationship between the number of copies and the image density for each color used for color image formation.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 Photoconductor drum 20BK, 20C, 20M, 20Y Developing device 50 1st CPU 60 2nd CPU 61BK, 61C, 61M, 61Y Toner density detecting means 62 Developing bias circuit 63 Optical density measuring means 52 Scanning optical system 54 Irradiation light quantity control circuit

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification code Internal reference number FI Technical indication location H04N 1/29 G 9186-5C (72) Inventor Kunihisa Yoshino 2970 Ishikawa-cho, Hachioji-shi, Tokyo Konica Corporation Inside the company

Claims (4)

[Claims] 1. An electrostatic latent image is formed on a uniformly charged photosensitive member by image exposure by dot exposure, and the electrostatic latent image is developed with a two-component developer for each color to form a plurality of colors. In the color image forming method of stacking color toner images, the toner adhesion amount when the electrostatic latent image of the reference density is developed by the two-component developer is smaller than the predetermined amount, even though the toner density is substantially constant. A color image forming method, comprising: expanding a dot area when an electrostatic latent image developed with a two-component developer is formed by dot exposure with respect to a reduced color. 2. The color image forming method according to claim 1, wherein the dot area is enlarged by increasing the enlargement ratio for the upper layer color. 3. The color image forming method according to claim 1, wherein the enlargement of the dot area is performed by intensity modulation of a light source that performs the dot exposure. 4. The color image forming method according to claim 1, wherein the dot area is enlarged by pulse width modulation of a light source that performs the dot exposure.