JP2003173052A - Image forming apparatus and its image stabilizing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus and image stabilizing method which enable accurate detection of the amount of toner depositing on an image carrier. <P>SOLUTION: An electrode plate 11 composed of the image carrier and an electrode plate 12 which is parallel thereto are arranged and a test pattern TN developed with toner is formed on the electrode plate 11 as the image carrier. The electrostatic capacity between the electrode plates 11 and 12 is detected and the toner mass (toner deposition amount) of the test pattern TN is calculated from the detected electrostatic capacity. On the basis of the calculation result, one or a plurality of control parameters of an imaging means, e.g. an electrostatic charging voltage, an electrostatic charging grid voltage, an exposure lamp voltage, the driving device of a laser device for exposure, the driving voltage of an LED device for exposure, etc., are set to form an image of good quality. Toner between the electrode plates can be removed by applying a magnetic field at the same time as an electric field and a sensor is not stained with particulates of toner, etc., so that an accurate detection result is obtained. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electrophotographic image forming apparatus such as a copying machine and a printer, and an image stabilizing method thereof.

[0002]

2. Description of the Related Art In an image forming apparatus adopting an electrophotographic system such as a copying machine and a printer, one of the factors affecting the image quality is the amount of toner adhering to a photosensitive member. This is because the toner adhesion amount is closely related to the image density. Therefore, in order to form a high quality image, it is required to appropriately control the amount of toner adhered to the photoconductor.

There are roughly two methods for controlling the toner adhesion amount used in the conventional electrophotographic image forming apparatus. The first method is an AIDC (Auto Image D) that directly controls the amount of toner attached to the photoconductor.
This is called "energy control", in which a latent image of a test pattern is formed on a photoconductor and developed with toner, and the toner adhesion amount of the visualized test pattern is optically detected using an image density sensor. .

Then, the relationship between the image density and the charge amount of the photoconductor, that is, the charge voltage of the charger and the charge grid voltage, which have been created in advance by experiments or the like, the image density and the exposure amount, that is, the exposure lamp, the exposure laser device, the exposure From the table showing the relationship with the drive voltage of the LED device for use, determine the charge amount of the photoconductor corresponding to the detected image density, that is, the charge charger voltage or the charge grid voltage, or the exposure amount, Exposure lamp, exposure laser device, exposure LED
The drive voltage of the device is determined (see, for example, Japanese Unexamined Patent Publication No. 5-14729).

The second method is to detect the toner component amount in the developer (the developer is composed of toner and magnetic carrier) in the developing device, that is, the toner concentration by magnetic or other method, and to determine the reference concentration. If the toner density is lower than the reference density, it is judged that the toner is insufficient and the toner is replenished.
The toner density is constantly maintained within a predetermined range to control the amount of toner adhering to the photosensitive member.
uto Toner Density Contro
l).

Further, the remaining amount of toner in the developing device is detected,
If the amount is less than a predetermined amount, there is a method in which toner is replenished to maintain the toner concentration within a predetermined range and the amount of toner attached to the photoconductor is controlled. In order to detect the remaining amount of toner, it is known that a parallel electrode plate is arranged in the toner container and the remaining amount of toner is detected based on the detected value of the electrostatic capacity (Japanese Patent Laid-Open No. 2001-2001). -117344, JP 2
001-92335).

[0007]

In the toner adhesion amount control by the AIDC method described above, the toner adhesion amount of the test pattern is detected by the image density sensor (AIDC sensor) to control the toner adhesion amount. When the toner smoke (fine particles) adheres to the surface, the reflected light of the detection light projected by the toner smoke (fine particles) is dispersed, and the accurate toner adhesion amount cannot be detected.

Even in the toner adhesion amount control by the ATDC method, if the toner smoke (fine particles) adheres to the detection surface of the ATDC sensor, the magnetic field is diffused and the accurate toner concentration cannot be detected.

Further, since the detection circuit for processing the output signals from these sensors exhibits a non-linear characteristic due to a saturated portion occurring in the output signal with respect to the input signal due to the characteristic of the detection circuit, the detection circuit has a wide range of input signals. On the other hand, there is an inconvenience that accurate detection cannot be performed or detection cannot be performed.

Particularly, as the toner particles have become finer recently, the conventional AIDC sensor or the ATDC sensor controls the toner adhesion amount, in addition to the influence of the fumes (fine particles) of the toner adhering to the sensor. Due to the addition of non-linear characteristics of the circuit, the toner adhesion amount cannot be controlled appropriately, and high-quality image formation may not be possible.

Further, in the structure in which the remaining amount of toner is detected based on the electrostatic capacity of the electrode plate arranged in the developing device, the charged toner adheres to the electrode plate and is gradually accumulated, resulting in an electrostatic capacity detection error. However, there is an inconvenience that the remaining amount of toner cannot be accurately detected.

[0012]

SUMMARY OF THE INVENTION The present invention is to solve the above problems. According to the invention of claim 1, an image latent image is formed on an image bearing member, and the latent image is developed with a developer to form an image. In the image forming apparatus including the image forming means, a pair of electrode means for forming a capacitor using the image carrier as one electrode, and a gap between the electrodes in a state where a developer is present between the pair of electrode means. Capacitance detecting means for detecting the capacitance of the image forming means, calculation means for calculating the developer amount based on the detected capacitance, and image forming means for calculating the developer amount based on the developer amount calculated by the calculating means. An image forming apparatus comprising: a control unit that sets an image forming condition.

The image bearing member includes any one of a photoconductor, an intermediate transfer member, a transfer carrier and a recording medium.

Further, the image forming means is provided with magnetic field generating means for generating a magnetic field in a direction orthogonal to the electric field direction generated between the pair of electrode means, and the image forming means is composed of the electrode means and the magnetic field generating means. A developer collecting unit that collects the developer attached to the electrode unit is configured.

The developer collecting means may include a recycling means for recycling the collected developer to the developing means of the image forming means.

According to a fifth aspect of the present invention, there is provided an image stabilizing method for an image forming apparatus comprising an image forming means for forming an image latent image on an image bearing member and developing the latent image with a developer. hand,
It includes the following processing steps:

That is, a step of forming a test pattern on the image bearing member with a developer, a test pattern is disposed between a pair of electrode means for forming a capacitor with the image bearing member as one electrode, and a space between the electrodes. The amount of developer of the test pattern on the image carrier based on the detected capacitance between the electrodes, the amount of developer of the determined test pattern Step of setting the image forming condition of the image forming means based on the above.

[0018]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below. The image forming apparatus itself has a well-known configuration, and detailed description thereof is omitted. However, a charging charger, a developing device,
An image forming unit is configured by arranging an exposure lamp, an exposure laser device, an exposure device such as an exposure LED device, a transfer device, and a cleaner.

Control parameters of such image forming means, for example, charging charger voltage, charging grid voltage, exposure lamp voltage, exposure laser device drive voltage, exposure LED.
The drive voltage of the apparatus and the like are closely related to the image density (toner adhesion amount) formed on the image carrier. Therefore, the control means controls the control parameters of the image forming means so that a test pattern is formed on the image carrier to detect the toner adhesion amount, and an appropriate image density is obtained based on the detection result.

FIG. 1 is a view for explaining an example of the basic structure of the toner adhesion amount detecting device 10, in which an image carrier 11 and an electrode plate 12 are arranged in parallel therewith, and the toner is placed on the image carrier 11. It is assumed that the test pattern TN developed in 1. is formed.

The shapes of the image carrier 11 and the electrode plate 12 are flat for convenience of explanation. That is, the image carrier 11 is a belt-shaped photoreceptor, and the electrode plate 12 is an electrode plate arranged in parallel on the belt-shaped photoreceptor 11. Further, since the image carrier 11 acts as an electrode plate in the detection of the toner adhesion amount, the image carrier 11 will be referred to as an electrode plate 11 in the following description.

Here, it is assumed that the dimensions of each element constituting the toner adhesion amount detecting device are as follows.

Interval between electrode plate 11 and electrode plate 12: Deq, thickness of toner layer of test pattern TN: Dtn, air gap between test pattern TN and electrode plate 12: Dar, area of electrode plate 12: A electrode plate 11 If the distance Deq between the electrode plate 12 and the electrode plate 12 and the area A of the electrode plate 12 are constants, the electrostatic capacitance Ceq between the electrode plate 11 and the electrode plate 12 is the air layer portion between the toner layer and the electrode plate 12. And the electrostatic capacitance Ctn between the electrode plate 11 and the toner layer, which is represented by the following equation (1).

Ceq = 1 / {(1 / Car) + (1 / Ctn)} (1) Capacitance C of the air layer portion between the toner layer and the electrode plate 12
ar is determined by the permittivity ratio εtn of the toner, and the electrode plate 11
The capacitance Ctn between the toner layer and the toner layer is the permittivity ratio εar of air.
And the following relational expression holds.

Ctn = (εtn · A) / Dtn ... (2) Car = (εar · A) / Dar (3) As described above, since the area A of the electrode plate 12 and the distance Deq between the electrode plates 11 and 12 are known values, the electrode plate 1
If the electrostatic capacitance Ctn between 1 and the toner layer is detected, the thickness Dtn of the toner layer can be expressed by the following equation (4) by modifying the equation (2).

Dtn = (εtn · A) / Ctn (4) Since the toner density Rtn is a known value, the toner adhesion amount M
tn (mass) can be expressed by the following equation (5).

Mtn = Rtn · A · Dtn = Rtn · A · (εtn · A) / Ctn (5) That is, the toner adhesion amount Mtn (mass) is the electrostatic capacitance C of the toner.
It can be calculated from the reciprocal of tn.

FIG. 2 shows an example of the detection circuit 13 of the toner adhesion amount detection device 10. The detection circuit is composed of an operational amplifier AM, and an electrode plate 11 and an electrode plate 12 are provided between the input terminal and the ground terminal. A signal indicating the electrostatic capacitance between them is input. Frequency f proportional to the reciprocal of the detected capacitance from the output terminal
The constant voltage pulse is output, and the relationship between the capacitance C and the frequency f is expressed by the following equation. However, α is a constant.

C = α (1 / f) (6) Next, the toner adhesion amount detection processing will be described. First, in the preparation stage, the toner adhesion amount is separately measured in advance for a plurality of test pattern TN samples. Then, with respect to the sample test pattern TN, the electrostatic capacity is detected by the toner adhesion amount detection device described above, the electrostatic capacity C calculated based on the frequency f of the constant voltage pulse output from the detection circuit, and the measured sample test pattern TN
And the amount of toner adhered thereto are recorded as a reference table.

Next, with respect to the test pattern TN created in the course of the image forming process, the electrostatic capacity is detected by the toner adhesion amount detection device, and the calculation is performed based on the frequency f of the constant voltage pulse output from the detection circuit. The value of the electrostatic capacitance C is searched for on the above reference table, and the toner adhesion amount corresponding to the electrostatic capacitance C is obtained.

As a result, the toner adhesion amount of the test pattern TN created during the image forming process can be obtained.
Based on this toner adhesion amount, the image forming conditions of the image forming means, that is, the control parameters of the image forming means, such as the charging charger voltage, the charging grid voltage, the exposure lamp voltage, the exposure laser device drive voltage, and the exposure LED device Any one or a plurality of control parameters such as drive voltage are set.

FIG. 3 is a diagram showing an example of the measurement result of the detection circuit of the toner adhesion amount detection device, in which the horizontal axis represents the toner layer thickness (μm) and the vertical axis represents the electrostatic capacitance (nF). In this measurement, the distance between the electrodes (the distance De between the electrode plate 11 and the electrode plate 12 De
q) was 5 mm, the area A of the electrode plate 12 was 625 mm ² , the dielectric constant εtn of the toner was 2.5, and the electrostatic capacitance change was measured when the thickness of the toner layer was changed.

As is apparent from FIG. 3, the electrostatic capacitance changes linearly with respect to the change in the thickness of the toner layer in a wide range, that is, the change in the toner adhesion amount, and the detection result of the detection circuit is non-linear. It turns out that there is no sex.

In this detection device, the wider the electrode area A,
Although the detection sensitivity is high, the gain of the detection circuit can be adjusted by adding an amplification circuit.

Further, the fluctuation of the dielectric constant εtn of the toner due to the change of the environmental temperature and the environmental humidity is small enough to be ignored, and the influence of the environmental change on the detection result is slight. Similarly, the change in the charge amount of the toner due to the change in the ambient temperature or the ambient humidity also changes the temperature / humidity from 10 ° C / 15% to 30 ° C / 85
% Is about 15 μC / g, and when converted to capacitance, the toner particle size is 4 to 4 in the above temperature / humidity range.
In the case of 10 μm, about 3 to 50 nF (1 nF = 1 × 10
^-15 F), which is a very small amount, and the change in the amount of charge of the toner due to changes in the environment is also small.

FIG. 4 is a view for explaining the structure of the toner collecting device 15 for collecting the residual toner adhering to the electrode plate of the toner adhesion amount detecting device.

As described above, the voltage is applied between the electrode plates 11 and 12 arranged in parallel with each other, and the toner adhesion amount adhered onto the electrode plate 11 is detected.
If the residual toner adhering to the above is not removed, the next detection of the toner adhering amount will be hindered.

Therefore, in order to remove the residual toner, the following means are adopted. That is, a voltage is applied between the electrode plates 11 and 12 to generate an electric field E in the direction of arrow e, and
When the magnetic field B is generated in the direction of arrow b (direction from the front to the back with respect to the paper surface) orthogonal to the electric field E, the electric field E
An electromagnetic force (Lorentz force) is generated in a direction orthogonal to the direction of the magnetic field B and the magnetic field B.

The generated electromagnetic force acts on the residual toner attached to the electrode plate 11, and the residual toner moves in the direction of the electromagnetic force and separates from the electrode plate 11. As shown in FIG. 4, if the moving direction of the residual toner coincides with the direction of gravity, the residual toner can be stored in the recovered toner container 16 by the action of gravity, and a special recovery mechanism is not required. At this time,
It is advisable to provide a smoke-emission prevention shield 17 between the ends of the electrode plates 11 and 12 and the opening of the collected toner container to prevent the toner from scattering.

Further, the residual toner separated from the electrode plate 11 may be accommodated in a recovery toner container by a suction device, or the recovery toner may be returned to the developing device to recycle the toner.

A known electromagnet having a coil wound around an iron core can be used to generate a magnetic field. The magnetic field strength is set to a magnetic field strength that exceeds a limit value at which the toner attached to the electrode plate 11 starts to move by controlling the magnitude of the current supplied to the coil.

By periodically changing at least one of the electric field and the magnetic field, it is possible to give vibration to the residual toner adhering to the electrode plate, so that the residual toner is removed from the electrode plate by sieving. Can be promoted.

FIG. 5 is a view for explaining an example of a structure for generating a magnetic field between the two electrode plates 11 and 12, and the electrode plate 11
It is assumed that a voltage is applied between points 12 and 12 and an electric field E is generated in the direction of arrow e. The magnetic pole of the known electromagnet 18 is arranged toward the gap between the electrode plates 11 and 12, and a magnetic field is generated in the arrow b direction orthogonal to the arrow e direction which is the direction of the electric field E. The number of electromagnets 18 arranged is arbitrary. Here, a sine wave AC voltage is applied to the electromagnet to generate an AC magnetic field and the magnetic field is periodically changed.

In addition, as a means for periodically changing either the electric field or the magnetic field, a pulsed voltage may be applied between the electrode plates or a pulsed voltage may be supplied to the electromagnet.

Further, with the above-described structure for removing the residual toner, it may not be possible to sufficiently remove the toner having a low chargeability. As a countermeasure against this, the residual toner adhered to the electrode plate may be used as a pretreatment for the toner removal processing. It is preferable that the residual toner is removed by previously detecting the electrostatic capacitance, and subtracting and correcting the electrostatic capacitance of the residual toner from the detected value of the toner adhesion amount based on the test pattern.

In the toner adhesion amount detecting device 10 shown in FIG. 1, the image carrier 11 has been described as a belt-shaped photoconductor.
It may be a cylindrical photoreceptor. 6A and 6B are views for explaining an example of the configuration and arrangement of the image carrier and the electrode plate in the case where the image carrier is a cylindrical photosensitive member. FIG. 6A is a perspective view thereof, and FIG. Is a sectional view thereof.

As is apparent from FIGS. 6A and 6B, the electrode plates 22 having an arcuate cross section are arranged at a predetermined distance D so as to be parallel to the outer surface of the cylindrical photosensitive member 21. Has been done. The arc shape of the electrode plate 22 does not have to be concentric with the cylindrical photoconductor 21, and may be any configuration as long as the electrostatic capacitance does not change depending on the rotation angle of the cylindrical photoconductor 21. Reference numeral TN indicates a toner test pattern formed on the cylindrical photosensitive member 21.

By increasing the area of the electrode plate 22 having an arcuate cross section, the roundness of the outer surface of the cylindrical photosensitive member 21 and the curved surface shape of the electrode surface of the electrode plate 22 having an arcuate cross section vary. Also, it is possible to reduce the influence on the detection accuracy in detecting the electrostatic capacitance.

7 to 9 are diagrams showing examples of test patterns formed on the image carrier in the toner adhesion amount detecting device. 7A is a front view of the binary pattern, FIG. 7B is an enlarged side view showing the positional relationship between the pattern and the electrode plate formed on the image carrier, and FIG. 7C is the image carrier. FIG. 3 is a diagram for explaining the electrostatic capacitance between the body and the electrode plate. In this figure, there are 4 toner patterns (n = 4) and 45 toner-free patterns (m = 45).

FIG. 8A is a front view of the binary halftone dot pattern, FIG. 8B is an enlarged side view showing the positional relationship between the pattern and the electrode plate formed on the image carrier, and FIG. c) is a diagram for explaining the electrostatic capacity between the image carrier and the electrode plate. In this figure, there are 16 toner patterns (n = 16) and 33 patterns without toner (m = 33). Has been done.
In this case, the electrostatic capacity increases as much as the toner pattern increases compared to the binary pattern.

FIG. 9A is a front view of the multi-valued pattern, FIG. 9B is an enlarged side view showing the positional relationship between the pattern and the electrode plate formed on the image bearing member, and FIG. 9C. [FIG. 3] is a diagram for explaining electrostatic capacitance between an image carrier and an electrode plate. The toner adhesion amount differs depending on the density gradation that differs for each pattern, the toner thickness differs for each pattern as shown in FIG. 9B, and the electrostatic capacitance also changes according to the toner adhesion amount. It is shown.

Next, detection of data relating to the water content and material of the recording medium will be described. If the electrostatic capacity of the recording medium is detected by a configuration similar to that of the toner adhesion amount detection device 10 described above, data regarding the water content and material of the recording medium, which is one of the control factors of the image forming process in the image forming apparatus, can be obtained. Obtainable.

FIG. 10 is a view for explaining an example of the structure of a detection device for detecting the water content or material of the recording medium. It shows the transfer belt 31 forming the first electrode plate and the transfer belt 31 arranged at a position distant from the transfer belt 31. The recording medium P is passed between the two electrode plates 32, and a capacitance detector 34 is connected between the first electrode plate (transfer belt) 31 and the second electrode plate 32. In FIG. 10, the toner image TN is formed on the transfer belt 31 that constitutes the first electrode plate.

To detect the water content of the recording medium, first,
Pass the recording medium P under the temperature and humidity of the standard environment,
The capacitance is detected by the capacitance detector 34, the dielectric ratio of the recording medium under the temperature and humidity of the standard environment is obtained from the detected capacitance, and the dielectric ratio and the water content of the recording medium are recorded. I'll do it.

For a recording medium used in a work environment other than the standard environment, the dielectric constant is obtained by the above-mentioned detection device, and this is compared with the dielectric ratio of the recording medium previously recorded in the standard environment to determine the inclusion of the recording medium. The amount of water can be calculated.

By treating the water content of the obtained recording medium as one of the control factors of the image forming process, the stability of the formed image can be enhanced.

[0057]

As described above, according to the present invention, a pair of electrode means for forming a capacitor with the image carrier as one electrode is used for detecting the toner adhesion amount of the test pattern formed on the image carrier. Is what you use. A test pattern is arranged between the pair of electrode means to detect the electrostatic capacitance between the electrodes, and the developer amount of the test pattern on the image carrier is calculated and determined based on the detected electrostatic capacitance, The image forming condition of the image forming means is set based on the determined developer amount.

Since the developer amount of the test pattern is calculated and determined based on the electrostatic capacity of the electrode means, as in the conventional AIDC sensor or ATDC sensor, the developer smoke (fine particles).
There is no stain on the sensor due to, for example, and the detection result does not become inaccurate.

The developer adhered to the electrode means is easily discharged from the electrode means by generating a magnetic field in a direction orthogonal to the electric field direction generated between the electrode means by the magnetic field generation means. Therefore, there is no possibility that the developer adhering to the electrode means is accumulated and the detection result becomes inaccurate. Further, by collecting and recycling the discharged developer, there is no influence on the environment.

Further, since the detection circuit for detecting the electrostatic capacity of the electrode means exhibits a linear characteristic with respect to the developer concentration in a wide range, the control of the image density based on the detection result can be performed accurately and easily. Can be done.

[Brief description of drawings]

FIG. 1 is a diagram illustrating an example of a configuration of a toner adhesion amount detection device according to an embodiment of the present invention.

FIG. 2 is a diagram illustrating an example of a detection circuit of the toner adhesion amount detection device.

FIG. 3 is a diagram showing an example of a measurement result of a detection circuit of the toner adhesion amount detection device.

FIG. 4 is a diagram illustrating a configuration of a recovery device that recovers residual toner that has adhered to an electrode plate of the toner adhesion amount detection device.

FIG. 5 is a diagram illustrating an example of a configuration for generating a magnetic field between two electrode plates.

FIG. 6 is a diagram illustrating an example of the configuration and arrangement of an image carrier and an electrode plate when the image carrier is a cylindrical photosensitive member.

FIG. 7 is a diagram illustrating a positional relationship between a test pattern on an image carrier and an electrode plate, and a capacitance (in the case of a binary pattern).

FIG. 8 is a diagram illustrating a positional relationship between a test pattern on an image carrier and an electrode plate and electrostatic capacitance (in the case of a binary halftone dot pattern).

FIG. 9 is a diagram illustrating a positional relationship between a test pattern on an image carrier and an electrode plate, and a capacitance (in the case of a multi-valued pattern).

FIG. 10 is a diagram illustrating an example of the configuration of a detection device that detects the water content or material of a recording medium.

[Explanation of symbols]

10 Toner adhesion amount detection device 11 Image bearing member (belt-shaped photosensitive member) (electrode plate) 12 electrode plate 13 Capacitance detection circuit 15 Toner collection device 16 Collected toner container 17 Shield for smoke prevention 18 Electromagnet 21 Cylindrical photoconductor 22 Electrode plate with arcuate section 31 transfer belt (first electrode plate) 32 Second electrode plate 34 Capacitance detector

   ─────────────────────────────────────────────────── ─── Continued front page    F-term (reference) 2H027 DA10 DC00 DC02 DD02 DE00                       DE04 DE10 EA01 EA02 EC03                       EC06                 2H077 AA37 AC16 DA04 DA15 DA47                       DA59 DB12 DB13                 2H134 GA20 HF00 JA03 JA11 JB01

Claims (5)

[Claims] 1. An image forming apparatus comprising image forming means for forming an image latent image on an image carrier and developing the latent image with a developer to form an image, wherein the image carrier is used as one electrode of a capacitor. A pair of electrode means for forming an electrostatic capacitance, a capacitance detecting means for detecting a capacitance between the electrodes in a state where a developer is present between the pair of electrode means, and a capacitance based on the detected capacitance. An image forming apparatus comprising: a calculation unit that calculates a developer amount; and a control unit that sets an image forming condition of the image forming unit based on the developer amount calculated by the calculation unit. 2. The image forming apparatus according to claim 1, wherein the image bearing member includes any one of a photosensitive member, an intermediate transfer member, a transfer conveying member, and a recording medium. 3. The image forming means is provided with magnetic field generating means for generating a magnetic field in a direction orthogonal to the electric field direction generated between the pair of electrode means, and the image forming means is composed of the electrode means and the magnetic field generating means. The image forming apparatus according to claim 1, further comprising a developer collecting unit configured to collect the developer attached to the electrode unit. 4. The image forming apparatus according to claim 3, wherein the developer collecting unit includes a recycling unit that recycles the collected developer to the developing unit of the image forming unit. 5. An image stabilizing method for an image forming apparatus, comprising an image forming means for forming an image by forming an image latent image on an image carrier and developing the latent image with a developer. Including. Forming a test pattern on the image carrier with a developer, and disposing a test pattern between a pair of electrode means forming a capacitor with the image carrier as one electrode to reduce the capacitance between the electrodes. A step of detecting, a step of calculating and determining a developer amount of the test pattern on the image carrier based on the detected electrostatic capacitance between the electrodes, and an image formation based on the determined developer amount of the test pattern. Step of setting the image forming condition of the means.