JPH09311589A - Image forming device and method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent an erroneous restoration of the electrification of a photoreceptor caused by surface potential detecting error depending on the sensitivity distribution of a surface electrometer. SOLUTION: In this image forming device as an electrophotographic system, a latent image is formed, developed and transferred, and discharge is performed successively while a photoreceptor 21 is turned. In such a case, uneven electrification distribution data in the width direction of the photoreceptor obtained from a surface potential sensor 1 which can move in the main scanning direction of the photoreceptor 21 is separated to the distribution data of low frequency component where potential change is gentle and the distribution data of high frequency component being the part of sudden potential change based on spatial frequency characteristic obtained from the sensitivity distribution of the sensor 1. Next, the separated low frequency component data and the high frequency component data are respectively inputted in comparators 9 and 11 and compared with 1st and 2nd constants previously set so as to judge based on the compared result whether the uneven electrification is restored or not (that is, the propriety of restoration).

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus having a photoconductor and an image forming method.

More specifically, the present invention relates to an image forming apparatus having an electrophotographic electrostatic latent image forming process and an image forming method.

[0003]

2. Description of the Related Art As is known in the art, an electrophotographic apparatus such as a copying machine or an LBP (laser beam printer) generally has a drum-shaped photosensitive member having a photosensitive layer formed on the surface thereof and a charged outer periphery thereof. Equipment ・ Exposure equipment ・ Development equipment ・
A transfer device, a separation device, a cleaning device, and a pre-exposure device are sequentially arranged, and a fixing device for fixing the toner image transferred to the sheet material is further provided.

In such an electrophotographic apparatus, after the entire photosensitive layer on the surface of the photoconductor is charged to a predetermined potential by the charging device, the photoconductor is rotated and the image corresponding to the original is irradiated by the exposure device. As a result, an electrostatic latent image is formed. The electrostatic latent image formed on the surface of the photoconductor is visualized by applying a developer (toner) by a developing device, and the image is transferred to a sheet material by a transfer device.

The sheet material on which the image visualized by the transfer device is transferred is separated from the photoconductor by the separating device and fixed by the fixing device. The photoconductor is cleaned by the cleaning device to remove the residual developer on the photoconductor.

In order to stabilize the image transferred to the sheet material, there is a method of measuring the potential charged on the photoconductor with an electrometer and optimizing the charge amount and the light amount at the time of exposure based on the result. In well-known and recent digital image forming methods, an electrometer capable of moving in the width direction of the photoconductor is provided, and measurement is performed in the width direction of the photoconductor to correct partial image defects. It has become possible to obtain a proper image in the entire image.

[0007]

However, the above-described conventional example has the following drawbacks with reference to FIGS. 6 and 7.

In the conventional scanning electrometer shown in FIG. 6, the metal wire wire 5 of the corona generating source used in the charger 51 is used.
When 2 is partially soiled, the charging potential of the drum-shaped photoconductor 53 becomes partially uneven as shown in FIG.

When this potential unevenness is measured by the electrometer 55 which is movable in the widthwise direction of the photoconductor by the stepping motor 54, since the detection window of the electrometer has a finite width, the observation data at a certain measurement time is obtained. , The change within the detection width is integrated, and the potential distribution in the photoconductor width direction after detection becomes as shown in FIG. 7B.

That is, the periodic change in the region where the spatial frequency characteristic (MTF) determined by the sensitivity distribution of the electrometer is 100% is faithfully detected (portion A shown in FIG. 7B), but due to the discharge abnormality. Since the MTF is lowered in the portion having a short cycle change or the portion which changes abruptly (the portions B and C shown in FIG. 7B), the surface potential cannot be detected faithfully.

When the above uneven distribution is attempted to be corrected by correcting the laser light amount at the time of exposure based on the uneven potential distribution data detected in such a state, the following inconvenience occurs.

That is, in the area A shown in FIG. 7A, since the actual potential unevenness is accurately detected, it can be accurately repaired, but in the areas B and C shown in FIG. 7B. The error component ((a)-(b)) with respect to the original potential is shown in FIG.
The result is as shown in FIG. In FIG. 7 (c), there is a case where the portion becomes abruptly changed, and even if the potential unevenness is repaired, a wavy density unevenness occurs at the edge portion, resulting in an unsightly result. is there.

Further, in order to detect the uneven electric potential faithfully, it is possible to accurately detect the electric potential distribution by using an electrometer having a very narrow detection sensitivity distribution. Has a drawback that the cost becomes very large, and the number of detected data is large, so that the cost of the memory for storing the data also increases.

Accordingly, an object of the present invention is to provide
An object of the present invention is to provide an image forming apparatus and an image forming method capable of eliminating erroneous charging repair of a photoconductor due to a surface potential detection error depending on the sensitivity distribution of a surface electrometer.

Another object of the present invention is to provide an image forming apparatus and an image forming method capable of efficiently repairing uneven charging of a photosensitive member without using a highly accurate and highly accurate surface electrometer. Especially.

[0016]

To achieve the above object, the present invention provides an image forming apparatus having a photoconductor, in which the surface potential of the photoconductor is scanned by scanning the charged surface of the photoconductor. A low-frequency extraction unit that extracts a low-frequency component whose surface potential changes slowly based on a spatial frequency characteristic specified by the detection sensitivity distribution of the potential detection unit; It is provided with a discriminating means for discriminating whether or not the compensation for the uneven charging of the photoconductor is effective based on the extraction output of the area extracting means. Here, the discrimination means compares the output value of the low frequency extraction means with a predetermined reference value,
Whether or not the compensation is possible is determined. Further, the light amount correction for forming a latent image is performed based on the output of the extraction means.

Another aspect of the present invention is, in an image forming apparatus having a photoconductor, a potential detection unit for detecting the surface potential of the photoconductor by scanning the surface of the charged photoconductor, and a detection by the potential detection unit. Based on the spatial frequency characteristic specified by the sensitivity distribution, the high frequency extraction means for extracting the high frequency component in which the change of the surface potential is abrupt, and the charging of the photoreceptor based on the extraction output of the high frequency extraction means. It is provided with a determination means for determining whether or not compensation for unevenness is effective. Here, the determination means determines whether or not the compensation is possible by comparing the output value of the high frequency extraction means with a predetermined reference value.

Still another aspect of the present invention is an image forming apparatus including means for uniformly charging a photosensitive member prior to forming a latent image on the photosensitive member, and scanning the surface of the uniformly charged photosensitive member. A potential detecting means for detecting the surface potential of the photoconductor, and a low frequency component in which the change of the surface potential is gradual, based on the spatial frequency characteristic specified by the detection sensitivity distribution of the potential detecting means, The comparison means includes extraction means for extracting a high-frequency component in which the surface potential changes sharply, and comparison means for comparing the low-frequency component and the high-frequency component with first and second reference values, respectively. It is determined whether or not to compensate for the uneven charging of the photoconductor, based on the comparison result of 1. Here, the high frequency component is a component obtained by removing the low frequency component from the surface potential component detected by the potential detecting means. Further, it is preferable to compare the high frequency component with the second reference value after comparing the low frequency component with the first reference value. In addition, the low frequency component and the first
According to the result of comparison with the reference value of, it is possible to adopt a configuration including means for stopping the comparison process of the high frequency component and the second reference value. In addition, it is possible to adopt a configuration including means for compensating for uneven charging of the photoconductor based on the low frequency component.

Further, in the image forming method according to the present invention, the photosensitive member is uniformly charged prior to forming the latent image on the photosensitive member,
The surface potential of the photoconductor is detected by scanning the surface of the photoconductor uniformly charged using a potential sensor, and based on the spatial frequency characteristic specified by the detection sensitivity distribution of the potential sensor, A low-frequency component whose surface potential changes slowly and a high-frequency component whose surface potential changes sharply are extracted, and the low-frequency component and the high-frequency component are respectively compared with first and second reference values. By doing so, it is determined whether or not compensation for uneven charging of the photoconductor is performed.

[0020]

DETAILED DESCRIPTION OF THE INVENTION By implementing the present invention, the following electrophotographic image forming apparatus can be realized.

That is, the photosensitive member is rotated to form a latent image.
In an electrophotographic image forming apparatus that sequentially performs development, transfer, and charge removal, a space that is obtained from the sensitivity distribution of the potential sensor for the photoconductor widthwise electrostatic charge distribution data obtained from the surface potential sensor that can move in the main scanning direction of the photoreceptor. Based on the frequency characteristics, it is separated into low-frequency component distribution data with a gradual potential change and high-frequency component distribution data with a sudden potential change, and the separated low-frequency component data and high-frequency component data are set in advance. The first constant and the second constant are compared with each other, and based on the comparison result, it is determined whether to repair the charge unevenness (that is, whether the repair is possible or not).

Further, means for repairing uneven charging is provided based on the separated low frequency component distribution data.

Here, the preset threshold value data (first and second constants) can be arbitrarily set, and the threshold value data is changed according to the durable image number or the image quality required for copying, so that the image quality required can be changed. Enables diagnostic imaging of images.

Embodiments of the present invention will be described in detail below with reference to the drawings.

First Embodiment FIG. 1 is a block diagram showing the configuration of a digital copying machine showing a first embodiment of the present invention. In the figure, 1 is a non-contact potential sensor for measuring the surface potential of the photoconductor, 2 is a detection circuit for detecting the output from the potential sensor and amplifying it to an appropriate voltage level, and 3 is a detection circuit 2. A / D that converts the output into digital data with 8-bit resolution, for example
The converter 4 is a memory for temporarily storing the detection data, and 5
Is an LPF (low-pass filter) circuit for processing the data of the detected potential distribution and obtaining the data in which the high frequency component of the change of the distribution is removed, and 6 is the data from the memory 4 and the LP
A subtracter for subtracting the operation result data from the F circuit 5, 7 is a memory for storing the operation result data by the subtractor 6, 8 is a memory for storing the output from the LPF circuit 5, and 9 is data in the memory 8. And a comparator 7 for comparing the data from the comparison data memory 10 with the memory 7
A comparison data memory in which data for comparing the data stored in the memory 8 and the data stored in the memory 8 is stored, 11 is the data in the memory 7 and the comparison data memory 10
A comparator for comparing the data from the above, a control circuit 12 for controlling the entire system, a control panel 13 and a correction coefficient 14 for correcting the exposure amount based on the data in the memory 8. A light quantity correction LUT (look-up table), 15 is an image buffer memory for temporarily storing image data output from a document scanner (not shown), and 16 is gamma correction by converting the image data in the image buffer memory 15 into density data. Image processing circuit to perform, 1
7 is a light quantity correction LU for the output data from the image processing circuit 16.
A multiplier for multiplying the correction coefficient from T14, 18 is a laser drive circuit for controlling the amount of laser light for forming a latent image on the photoconductor, and 19 is a width direction of the laser light source and the photoconductor (mainly An exposure device including a polygon mirror for scanning laser light in the scanning direction) and other optical elements, 20 is a stepping motor for moving the potential sensor 1 in the main scanning direction of the photoconductor, and 21 is an amorphous photosensitive layer. Drum-shaped photoreceptor made of silicon, 2
2 is a charging device, 23 is a developing device, 24 is a transfer device, 25
Is a separating device, 26 is a cleaning device, and 27 is a pre-exposure device.

Hereinafter, the detection of the charging potential unevenness in the main scanning direction of the photoconductor 21, which is the main constituent element of this embodiment, and the data processing method thereof (hereinafter referred to as the diagnostic restoration sequence) will be described.

The main cause of uneven charging in the main scanning direction is uneven corona current due to foreign matter attached to the metal wire lines or grid lines, which are the corona generating sources used in the charging device 22.

Although such foreign matter is regularly cleaned with a wire cleaning tool (not shown), it cannot be sufficiently removed and causes image deterioration such as black spots, black streaks, and white streaks. Therefore, when the power is turned on or a fixed number (eg 2000)
The diagnostic repair sequence is executed every time. Then, the above condition is diagnosed and image restoration is performed.

In the diagnostic restoration sequence, first, the photoconductor 21 is rotated at the normal copying operation speed, and the pre-exposure device 27 removes the charge. Next, the charging device 22 supplies a current to the wire line in the charging device so that the photoconductor 21 has a predetermined potential, for example, + 500V. At this time, if the wire wire has a uniform surface state, the surface potential of the photoconductor 21 is uniformly + 500V over the entire surface. However, when foreign matter adheres to a part of the wire and uniform corona discharge is not performed, uneven charging occurs parallel to the sub-scanning direction.

After being charged by the charging device 22, the stepping motor 20 is operated, the potential sensor 1 starts moving along the photoconductor 21 in the main scanning direction, and the photoconductor surface potential is measured. The observed surface potential is converted into an appropriate voltage by the detection circuit 2 and then quantized by the A / D converter 3. Here, the sampling interval is, for example, 1 m
m and the quantization step is 8 bits. The sampling area is at least the width of the sheet material used, and for example, when the A4 size sheet is passed in the direction parallel to the rotation axis direction of the photoconductor, the area of 297 mm or more is sampled. The memory sizes of the memories 4, 7 and 8 have a capacity capable of storing data in an area determined by the maximum sheet passing width.

Next, the data processing after sampling will be described.

FIG. 2A shows an example of the surface potential distribution of the photoconductor 21 when it is charged by the charging device 22 with dirty wire lines, where the X axis is the position in the photoconductor main scanning direction. , Y axis represents the surface potential. When such potential unevenness in the main scanning direction is sampled by the potential sensor 1, depending on the MTF characteristic (spatial frequency characteristic) determined by the sensitivity distribution of the potential sensor 1, it becomes impossible to accurately respond to a minute potential change.

FIG. 3 is a diagram showing the MTF characteristic of the potential sensor 1. In the case of this figure, a drop in the potential of 0.5 cycle / cm, that is, a width of about 10 mm or more can be detected almost accurately, but a fine potential change below that cannot be detected faithfully.

FIG. 2B shows the distribution of potentials sampled by such a potential sensor 1. Each point (x address) in the X-axis direction and potential data (D ₀ ) at that point are temporarily stored in the memory 4 and then the LPF circuit 5
Removes high-frequency components (portions that change drastically). Here, the LPF circuit 5 has an IIR configuration and has a Butterworth characteristic, and its cutoff frequency Fc (spatial frequency of 70%) and attenuation characteristic are determined by the MTF characteristic of the potential sensor 1. For example, as shown by the dotted line in FIG. 3, frequency components higher than the spatial frequency of 0.75 cycles / cm are cut.

The result of such an operation is shown in FIG.
Shown in The calculation result (D ₁ ) is stored in the memory 8. Further, the data in the memory 4 and the data in the memory 8 are subtracted by the subtracter 6 between the data having the same address. The output data (D ₂ ) is stored in the memory 7.

On the other hand, the calculation result at each x address is input to the comparator 9 and compared with the data S from the comparison data memory 10. That is, it is determined whether or not the potential drop is too large to correct. For example, in the case of an image exposure system, the dark potential becomes the background, and therefore the judgment criterion is S, which is a value corresponding to the potential at which a black stripe starts to appear on a white background due to a decrease in the dark potential (V _D ).
And

When the number N in which D ₁ is equal to or lower than the level S is equal to or greater than the predetermined value N ₀ , the operation panel 13 displays a warning. Here, the values of N ₀ and S can be obtained from the allowable limit of the output image member and set from the operation panel.

Next, the subtracter 6 extracts the difference between D ₀ and D _{1 at} the same x address. The result (D ₂ ) is stored in the memory 7. D ₂ is a component that cannot be accurately detected by the potential sensor 1, and corresponds to a portion where the potential change (positive and negative) due to abnormal discharge of the wire is severe. Therefore, data exceeding the data k (± k shown in FIG. 2D) set in the comparison data memory 10, that is, | D ₂
The comparator 11 detects the data that satisfies |> k, and determines whether or not the exceeded number M is a predetermined value M ₀ or more.
M ₀ and k are the operation panel 13 depending on the requested image.
Any value can be set from outside.

If M or M ₀ or more, streak-like density unevenness is conspicuous in the solid part of the halftone even after the repairing operation described later, so that fact is displayed on the operation panel 12.

When N <N ₀ and M <M ₀ , the laser light amount is corrected at the time of latent image formation by the following operation.

Potential distribution data of the memory 8 (FIG. 2 (c))
Based on the above, the data for correcting the decrease in the potential with respect to the x address is written in the light amount correction LUT 14. Here, the correction data is obtained from a previously known relationship between the laser light amount and the latent image potential, based on the difference amount from D _M shown in FIG. 2C. That is, coefficient values for performing a correction for reducing the amount of laser light are written as a table in a portion where the charging potential is low. When forming a latent image, the x address and the laser writing data are referred to, and the correction data is extracted from the table and output to the multiplier 17. This data is retained until the next image diagnostic sequence is performed.

At the time of copy output, image data read from a scanner device (not shown) is temporarily stored in the image buffer memory 15 for a predetermined number of lines or all lines (full screen) in the main scanning direction, and the processing speed is adjusted. Done. Next, density data conversion and gamma correction are performed by the image processing circuit 16 in the next stage, and then the multiplier 17
And output data from the light amount correction LUT 14 and a multiplication process are performed.

The image data thus corrected is input to the laser drive circuit 18, modulated according to the image data so as to have a predetermined light amount, and a latent image is formed on the photoconductor 21 by the exposure device 19. .

The latent image formed in this way has fine streak-like unevenness that is inconspicuous, but since the light amount is faithfully corrected for a large period or unevenness of inclination, a high-quality output image is always obtained. .

Embodiment 2 In the first embodiment, in order to store the data of D ₀ , D ₁ and D ₂ , each memory is provided with
Although the individual arithmetic circuit (LPF circuit, subtractor) is used, as shown in FIG. 4, the circuit is configured so that the dedicated digital signal processor 101 (DSP) can perform the filtering and the judgment as to whether the restoration is possible or not. The structure may be simplified.

In FIG. 4, 102 is a threshold value, K, S
And a memory for storing values such as tap coefficients for filtering, and these constants can be arbitrarily set externally via the controller 12. 103 is
Memory used as a work area for computation (R
AM).

The DSP 101 first performs a filtering process based on the data stored in the memory 4. The threshold S is used to determine whether the first repair is possible. That is, the determination based on the low frequency component as described above is performed, and if it is determined to be impossible here, a signal is output to the control circuit 12 and the calculation is stopped.

On the other hand, if it is judged to be possible, the subtraction processing of D ₀ and D ₁ is performed to judge the high frequency component. If it is determined to be acceptable, the light amount correction calculation is performed using the data D ₁ , the correction coefficient is obtained, and the result is output to the light amount correction LUT 14.

Third Embodiment In the above-described second embodiment, when there is uneven sensitivity in the circumferential direction of the photoconductor or uneven charging in the circumferential direction due to mounting accuracy of the photoconductor, the measurement error becomes large. . Therefore, at the time of installation, that is, before the charging device is contaminated, the circumferential charge unevenness is measured, and the result is stored in advance in the circumferential unevenness data memory 104 as shown in FIG. By subtracting, it is possible to obtain more accurate correction data.

Before performing the diagnostic restoration sequence, a black and white striped pattern latent image having different resolution is formed by the pattern generator 105 of FIG. 5, the potential distribution is read by the potential sensor 1, and the DSP 101 performs FFT analysis. As a result, that is, the optimum filter constant may be obtained from the MTF data so that the best restoration can always be performed even if the potential sensor 1 deteriorates with time.

In such an embodiment, the structure of the filter, the sampling interval, and the number of quantization bits are not limited to the above example. Further, the charging device is not limited to a device that performs corona discharge, and may be a roller charging device, so that not only the defect of the charging device but also the latent image unevenness due to the defect of the exposure device, the laser drive circuit and the like can be similarly repaired.

The present invention is not limited to a copying machine, but can be applied to a printing apparatus such as an LBP (laser beam printer).

According to each of the above-described embodiments, the potential unevenness distribution data in the main scanning direction obtained from the photoconductor surface potential sensor movable in the main scanning direction of the photoconductor is changed based on the spatial frequency characteristic of the potential sensor. The low frequency component is separated into the low frequency component and the high frequency component, which is the part where the potential changes suddenly due to abnormal discharge of the charging device, and the separated low frequency component data and high frequency component data are compared with preset reference data. However, based on the comparison result, it is possible to repair the uneven charge.
Since it was judged as impossible and the uneven charging was repaired by using the data of the low frequency component so as not to repair the part where the potential change is large and the error is large at the time of repairing, the charging device is deteriorated. A stable output image with no unevenness can always be obtained. Moreover, this also makes it possible to extend the maintenance interval of the service person.

[0054]

As described above, according to the present invention, in compensating the charge amount of the photoconductor, erroneous repair of the potential of the photoconductor due to the surface potential detection error depending on the sensitivity distribution of the surface electrometer is eliminated. You can

Further, it is possible to efficiently repair the uneven charging of the photosensitive member without using a highly accurate and highly accurate surface electrometer.

Further, even if the charging device or the like is deteriorated, a stable output image without unevenness can always be obtained. Moreover, this also makes it possible to extend the maintenance interval of the service person.

[Brief description of drawings]

FIG. 1 is a configuration block diagram of a copying machine according to a first embodiment of the present invention.

FIG. 2 is a diagram for explaining processing of potential distribution data in the present embodiment.

FIG. 3 is a diagram showing spatial frequency characteristics of a potential sensor and an LPF circuit.

FIG. 4 is a diagram for explaining a data processing unit according to a second embodiment.

FIG. 5 is a diagram for explaining a data processing unit according to a third embodiment.

FIG. 6 is a diagram for explaining a scanning electrometer.

FIG. 7 is a diagram for explaining a conventional problem.

[Explanation of symbols]

 1 Non-contact type surface potential sensor 2 Detection circuit 3 A / D converter 4, 7, 8 Memory 5 LPF circuit 6 Subtractor 9, 11 Comparator 10 Comparison data memory 12 Control circuit 13 Operation panel 14 Light intensity correction LUT 15 Image buffer Memory 16 Image processing circuit 17 Multiplier 18 Laser drive circuit 19 Exposure device 20 Stepping motor 21 Photoconductor 22 Charging device 23 Developing device 24 Transfer device 25 Separating device 26 Cleaning device 27 Pre-exposure device

Claims (11)

[Claims] 1. An image forming apparatus having a photoconductor, comprising: a potential detection unit that detects the surface potential of the photoconductor by scanning the surface of the charged photoconductor; and a detection sensitivity distribution of the potential detection unit. Based on the specified spatial frequency characteristics, a low-frequency extraction unit that extracts a low-frequency component whose surface potential change is gradual; An image forming apparatus comprising: a determining unit that determines whether or not compensation is effective. 2. The image forming apparatus according to claim 1, wherein the determination unit determines whether the compensation is possible by comparing an output value of the low frequency extraction unit with a predetermined reference value. . 3. The image forming apparatus according to claim 1, wherein the light amount correction for forming a latent image is performed based on the output of the extracting unit. 4. An image forming apparatus having a photoconductor, comprising: a potential detection unit for detecting the surface potential of the photoconductor by scanning the surface of the charged photoconductor; and a detection sensitivity distribution of the potential detection unit. High frequency extraction means for extracting a high frequency component in which the surface potential changes abruptly based on the specified spatial frequency characteristics, and compensation for uneven charging of the photoconductor based on the extraction output of the high frequency extraction means. An image forming apparatus, comprising: a determining unit that determines whether or not is valid. 5. The image forming apparatus according to claim 4, wherein the determination unit determines whether or not the compensation is possible by comparing an output value of the high frequency extraction unit with a predetermined reference value. . 6. An image forming apparatus having means for uniformly charging the photosensitive member prior to forming a latent image on the photosensitive member, wherein the photosensitive member is scanned by scanning the surface of the uniformly charged photosensitive member. A potential detection unit that detects the surface potential of the body, and a low-frequency component whose change in the surface potential is gentle based on the spatial frequency characteristics specified by the detection sensitivity distribution of the potential detection unit, and the change in the surface potential. And a comparing means for comparing the low-frequency component and the high-frequency component with first and second reference values, respectively, based on the comparison result by the comparing means. The image forming apparatus is configured to determine whether or not to compensate for the uneven charging of the photoconductor. 7. The high-frequency component according to claim 6,
An image forming apparatus comprising a component obtained by removing the low frequency component from a component of the surface potential detected by the potential detecting means. 8. The image forming apparatus according to claim 6, wherein after comparing the low frequency component with the first reference value, the high frequency component is compared with the second reference value. 9. The method according to claim 8, further comprising: stopping the comparison process between the high frequency component and the second reference value according to the comparison result between the low frequency component and the first reference value. An image forming apparatus comprising means. 10. The image forming apparatus according to claim 8, further comprising means for compensating for uneven charging of the photosensitive member based on the low frequency component. 11. The surface of the photoconductor by uniformly charging the photoconductor prior to formation of a latent image on the photoconductor and scanning the surface of the photoconductor uniformly charged using a potential sensor. The potential is detected, and based on the spatial frequency characteristics specified by the detection sensitivity distribution of the potential sensor, a low frequency component in which the surface potential changes gently and a high frequency component in which the surface potential changes rapidly are Image formation characterized by extracting and comparing the low-frequency component and the high-frequency component with first and second reference values, respectively, to determine whether or not compensation for uneven charging of the photoconductor is performed. Method.