JP2009042432A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus configured to correct potential unevenness on the surface of an image carrier by outputting a toner image for measuring potential unevenness so as to be hardly influenced by a developing means, and predicting the potential unevenness on the surface of the image carrier from the density of the toner image. <P>SOLUTION: The image forming apparatus performs following processing: (i) charging the surface of the image carrier 1, (ii) exposing the surface of the image carrier charged to a predetermined latent image potential, (iii) outputting a toner image for at least one turn of the image carrier by developing the latent image potential with predetermined developing bias, and (iv) detecting the density of the toner image, at a plurality of latent image potentials different in potential level, and two-dimensional potential unevenness on the surface of the image carrier is corrected by controlling a correction means 3 based on two-dimensional distribution of the obtained density of a plurality of toner images. The developing bias for outputting the toner image at the plurality of latent image potentials is changed in accordance with the latent image potential. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an image forming apparatus such as an electrophotographic copying machine or an electrophotographic printer. Specifically, the surface of the image carrier is uniformly charged, the latent image is exposed according to the image data input to the charging surface of the image carrier surface, and the potential of the image carrier surface is changed, thereby forming an image. The present invention relates to an image forming apparatus that performs. In particular, the present invention relates to an image forming apparatus capable of suitably performing correction for uniformizing the charging potential on the surface of the image carrier and the exposure potential when image exposure is performed on the surface of the image carrier.

  In an image forming apparatus such as an electrophotographic copying machine or a laser beam printer, an image carrier is uniformly charged by a charger, and a latent image is formed on a charged surface of the image carrier by an exposure device in accordance with input image data. The latent image is developed with toner by a developing device. The developing device has a developing sleeve and causes the toner attached to the developing sleeve to fly and adhere to the image carrier. In recent years, with the advancement of digital technology, such image forming apparatuses are rapidly shifting to color images and improving the quality of output images. In particular, in the world of DTP (DeskTop Publishing / DeskTop Prepress), there is a strong demand for the color stability and in-plane uniformity of output products. Therefore, in the image forming apparatus, various techniques have been proposed in order to realize various calibration techniques and electrophotographic process stabilization including charging, exposure, development, transfer, and fixing. This is because, for example, in the surface of the output image, film thickness unevenness and sensitivity unevenness of the image carrier, longitudinal unevenness of the charging device, longitudinal unevenness of the developing device and unevenness of the developing sleeve, and various other processes in the transfer process and the fixing process. Unevenness is caused by overlapping, and various correction techniques have been proposed. Among these, the unevenness caused by the image carrier is often because the unevenness pattern is relatively stable because it is unique to the photoconductor, and it is difficult to reduce film thickness unevenness and sensitivity unevenness for manufacturing reasons. Correction techniques have been proposed.

  In addition, with recent advances in electrophotographic technology, image forming apparatuses are highly demanded by the market for color stability and in-plane uniformity. Therefore, a technique for correcting at a level of several volts is desired.

  Here, the unevenness caused by the image carrier will be described from the viewpoint of the configuration and the manufacturing method of the image carrier.

  As the image carrier, a function separation type or a single layer type having a two-layer structure of a charge generation layer and a charge transport layer on a conductive support substrate as the lowermost layer is used. In addition, as a material constituting the photoconductor, an organic photoconductor composed of an organic material (hereinafter referred to as OPC / Organic Photo Conductor) or a photoconductor of selenium (Se) or silicon (Si) called an inorganic photoconductor is used. It is also possible.

  As a manufacturing method of the OPC photoreceptor, a solution in which the raw material is dissolved is sequentially applied to a substrate, and a film is formed by removing a substrate coated by spray coating or a substrate immersed in the solution. The dipping method to form etc. can be used. At this time, the film quality such as the film thickness and the raw material density in the film is adjusted by the viscosity of the solution used for forming the film and the drawing speed when dipping. If the film characteristics at that time are not uniform, uneven potential on the surface of the photoreceptor when charged and unevenness of the exposure potential upon exposure occur. In addition, when there is unevenness in hardness, unevenness in charging potential or exposure potential occurs due to unevenness in shaving when repeatedly output.

  As described in Patent Document 1, a method for producing an inorganic photoreceptor, for example, an amorphous silicon (a-Si) photoreceptor, is a vacuum deposition method, a sputtering method, an ion plating method, a thermal CVD method, a photo CVD method. A film formation method such as a plasma CVD method can be used. Among these, a plasma CVD method in which a source gas is decomposed by direct current, high frequency, or microwave glow discharge to form an a-Si deposited layer on a support has been put into practical use. When a photoconductor film is generated using such a film forming method, as in OPC, unevenness in film thickness and film quality occurs, and uneven charging potential and uneven exposure potential occur in the surface of the photoconductor.

  Furthermore, as described in Patent Document 2, when a light transmission insulating overcoat layer is laminated to improve the strength of the film and thereby extend its life, the thickness and quality of the added film are improved. Due to the unevenness, uneven charging potential and uneven exposure potential in the surface of the photoreceptor are increased.

  As described above, it is inevitable that the image carrier has in-plane unevenness, and various correction techniques have been proposed.

  Patent Literature 3 and Patent Literature 4 disclose a technique for making the laser exposure portion potential (exposure potential) in the axial direction of the photosensitive member uniform by correcting the laser lighting time according to the exposure portion potential characteristics. This can be done by correcting the PWM signal using a table corresponding to the exposure potential characteristics.

  Patent Document 5 discloses a technique in which exposure is performed with a constant amount of light after charging, and sensitivity unevenness in the moving direction of the photosensitive member is measured with a potential sensor to correct the exposure amount. In this correction method, the corrected exposure amount is converted into an analog voltage by a digital-analog converter as an 8-bit laser power value for each pixel. By inputting the voltage value obtained by comparing the analog voltage and the reference voltage to the base of the transistor, the same effect can be obtained by determining the laser driving current value corresponding to the laser power value.

  Patent Document 6 describes a technique for dividing a latent image area on a photosensitive member into two-dimensional segments and correcting the copy density for each segment. Patent Document 7 describes a technique in which a latent image region on a photoconductor is divided into two-dimensional segments and the amount of exposure light is corrected for each segment.

  In Patent Document 8, Patent Document 9, Patent Document 10, Patent Document 11, Patent Document 12, Patent Document 13, and Patent Document 14, non-uniformity of sensitivity of a photoreceptor due to a plurality of movable potential sensors and / or density sensors is disclosed. The measurement method is described.

Patent Document 15 discloses a laser control method for correcting sensitivity unevenness on the entire surface of a photoreceptor.
Japanese Patent Publication No. 60-35059 JP-A-60-67951 JP 63-49778 A JP-A-63-49779 JP 2000-267363 A JP 05-188707 A JP 2002-067387 A JP 05-165295 A JP 05-244483 A Japanese Patent Laid-Open No. 06-003911 Japanese Patent Laid-Open No. 06-011931 Japanese Patent Laid-Open No. 06-130767 Japanese Patent Laid-Open No. 06-266194 JP 2004-258482 A JP 2004-223716 A

  As described above, a number of techniques have been proposed for uniformity in the image plane, particularly regarding unevenness of the image carrier. However, as described above, due to the recent high market demand for color stability, the accuracy required for the correction technology is increasing, and a correction technology of several volts at the potential level of the image carrier is desired.

  In the prior art, in addition to the correction means, there is a big problem particularly in the means for measuring potential unevenness information. When general potential measurement is performed, an error of about several volts is generated in consideration of the stability of the distance between the measurement surface and the measurement probe. Especially in the case of measuring means installed in an image forming apparatus where installation space is limited and mechanical intersections are limited, a larger error occurs, and measurement of potential unevenness information at several V level is very Have difficulty.

  As a means for measuring potential unevenness, there has also been proposed a method for converting from the toner density when an image is actually output. However, in this case, it is impossible to measure the potential unevenness of the charged potential Vdark portion where the toner does not adhere to the image carrier. Further, when the toner density of the exposed portion potential Vlight portion where the toner adheres exceeds the amount of toner that can be developed on the developing sleeve, there is a problem that the potential cannot be accurately estimated. The main reasons are that if the amount of toner that can be developed on the developing sleeve is exceeded, the toner will not be loaded even if the potential drops further, and the sensitivity of the change in the toner concentration with respect to the toner amount is low. This is because the potential cannot be estimated properly.

  The gradation region on the high density side, such as Vlight, is susceptible to fluctuation factors on the developing device side, which becomes a problem when performing unevenness correction on the image carrier alone. The fluctuation factors on the developing device side include M / S as the amount of toner on the developing sleeve, Q / M as the charged charge amount of toner, and SD gap (Sleeve-Drum) as the distance between the developing sleeve and the image carrier. There are fluctuations in Gap). A method of simultaneously measuring and correcting the unevenness of the developing device is also conceivable, but if unevenness occurs in the circumferential direction due to the eccentricity or rotational unevenness of the developing sleeve or the image carrier, the unevenness on the image carrier side The relationship of each time changes and cannot be corrected. Here, the above symbols are M: toner weight, S: area where toner is developed, and Q: average charge amount of toner.

  In addition, when actually correcting at several V level, it is necessary to correct based on potential unevenness information at a plurality of potential levels, and potentials at a plurality of potential levels in the vicinity of the charging potential or the exposed portion. Measurement of unevenness is required. In particular, when using a photoconductor that is liable to be uneven in charge potential, for example, an a-Si photoconductor that has a fast potential decay rate due to dark decay, a plurality of potential levels including the vicinity of the charge potential Vdark are used. It is desired to measure potential unevenness information. This is because, as a cause of the potential unevenness, the charging potential unevenness due to dark decay and the sensitivity unevenness upon light irradiation are combined in combination. It is difficult to predict potential unevenness at various potential levels from Vdark to Vlight from potential unevenness at a specific potential. Potential unevenness in the vicinity of the charging potential (Vdark) where dark decay is the main factor, and exposure portion potential (Vlight) unevenness due to the difference in sensitivity due to light irradiation, more preferably an intermediate potential level between Vdark and Vlight ( The potential unevenness at Vhalf) is measured. It is preferable to predict a composite potential unevenness including unevenness due to dark decay and unevenness due to a difference in sensitivity from the obtained result. Here, Vhalf refers to an intermediate potential level of about Vhalf≈ (Vdark + Vlight) / 2.

  Further, with regard to the timing for performing the above correction, more frequent correction timing in real time is required because of the high demand for the correction level.

  Therefore, in the image forming apparatus, there is a demand for a technique that allows the user to easily and more accurately measure the potential unevenness on the surface of the image carrier while the image forming apparatus is operating in the market. Specifically, in order to correct the potential unevenness on the surface of the image carrier, when measuring the potential unevenness, when using a method for estimating the potential by outputting an image, highly sensitive measurement and potential prediction are performed. It is hoped that it will be possible.

  An object of the present invention is to output a toner image for measuring potential unevenness under conditions that are hardly affected by the developing means, predict potential unevenness on the surface of the image carrier from the density of the toner image, and detect potential unevenness on the surface of the image carrier. An object of the present invention is to provide an image forming apparatus that corrects the above.

The configuration for achieving the object of the present invention is as follows.
An image forming apparatus for forming an image on a recording material, comprising a cylindrical image carrier having photoconductivity, charging means for charging the surface of the image carrier, and exposing the surface of the image carrier after charging. An image exposing means for forming a latent image, a developing means for developing the latent image by attaching toner to the latent image on the surface of the image carrier, and correcting two-dimensional potential unevenness on the surface of the image carrier. Correction means, and
(I) a treatment for charging the surface of the image carrier;
(Ii) a process of exposing the surface of the image carrier after charging so as to have a predetermined latent image potential;
(Iii) processing for developing the latent image potential with a predetermined developing bias to output a toner image over at least one turn of the image carrier;
(Iv) processing for detecting the density of the toner image;
The processes from (i) to (iv) are performed with a plurality of latent image potentials having different potential levels, and the correction means is controlled based on the two-dimensional distribution of the density of the obtained toner images, In the image forming apparatus for correcting the two-dimensional potential unevenness on the surface of the image carrier,
The developing bias when outputting a toner image with a plurality of latent image potentials is changed according to the latent image potentials.

The configuration for achieving the object of the present invention is as follows.
An image forming apparatus for forming an image on a recording material, comprising a cylindrical image carrier having photoconductivity, charging means for charging the surface of the image carrier, and exposing the surface of the image carrier after charging. Image developing means for forming a latent image, and developing means for developing the latent image by attaching toner to the latent image on the surface of the image carrier,
(I) a treatment for charging the surface of the image carrier;
(Ii) a process of exposing the surface of the image carrier after charging so as to have a predetermined latent image potential;
(Iii) processing for developing the latent image potential with a predetermined developing bias to output a toner image over at least one turn of the image carrier;
(Iv) processing for detecting the density of the toner image;
The processes from (i) to (iv) are performed at a plurality of latent image potentials having different potential levels, and the image exposure means is controlled based on the two-dimensional distribution of the density of the obtained toner images. In the image forming apparatus for correcting the two-dimensional potential unevenness on the surface of the image carrier,
The developing bias when outputting a toner image with a plurality of latent image potentials is changed according to the latent image potentials.

The configuration for achieving the object of the present invention is as follows.
An image forming apparatus for forming an image on a recording material, comprising a cylindrical image carrier having photoconductivity, charging means for charging the surface of the image carrier, and exposing the surface of the image carrier after charging. An image exposing means for forming a latent image, a developing means for developing the latent image by attaching toner to the latent image on the surface of the image carrier, and correcting two-dimensional potential unevenness on the surface of the image carrier. Correction exposure means,
(I) a treatment for charging the surface of the image carrier;
(Ii) a process of exposing the surface of the image carrier after charging so as to have a predetermined latent image potential;
A process of developing (iii) with a predetermined developing bias and outputting a toner image over at least one turn of the image carrier;
(Iv) processing for detecting the density of the toner image;
The processes from (i) to (iv) are performed at a plurality of latent image potentials having different potential levels, and the correction exposure means is controlled based on the two-dimensional distribution of the density of the obtained toner images. In the image forming apparatus for correcting the two-dimensional potential unevenness on the surface of the image carrier,
The developing bias when outputting a toner image with a plurality of latent image potentials is changed according to the latent image potentials.

  According to the present invention, a toner image for measuring potential unevenness is output under conditions that are not easily influenced by the developing means, and potential unevenness on the surface of the image carrier is predicted from the density of the toner image, and potential unevenness on the surface of the image carrier is detected. An image forming apparatus capable of correcting the above can be provided.

  In the following description, regarding the members constituting the image forming apparatus, the longitudinal direction is a direction orthogonal to the recording material conveyance direction on the surface of the recording material. The longitudinal direction is also the axial direction of the image carrier. The circumferential direction is a direction along the outer peripheral surface (surface) of the image carrier. The longitudinal width is the length in the longitudinal direction.

  In this embodiment, potential unevenness of a photosensitive drum as a photosensitive member is measured by toner density, correction data is created based on the measured unevenness information, and the exposure amount to the image carrier is controlled. Thus, potential unevenness is corrected. As a method for correcting potential unevenness, the first means is a method of controlling the amount of laser light used for image exposure. As a method for correcting the potential unevenness, there is a method in which the second means includes an exposure means for correcting the potential unevenness separately from the image exposure, and controls the laser light quantity of the exposure means for the correction.

  Hereinafter, preferred embodiments of the present invention will be described with reference to the drawings. However, the constituent elements described in this embodiment are merely examples, and are not intended to limit the scope of the present invention only to them.

[Example]
(1) Configuration of Entire Image Forming Apparatus FIG. 1 is a structural model diagram showing an image forming apparatus according to the present invention. This image forming apparatus is an electrophotographic copying machine. A is a copying machine main body, and B is a color image reading device (color reader) mounted on the copying machine main body A.

  The color image reading device B is for performing color separation photoelectric reading of a color image document to be copied. That is, a reading unit 22 incorporating a document illumination lamp, a short focus lens array, and a CCD sensor scans and moves the color document G placed on the document table 21 while illuminating the image surface of the color document G. Thereby, the original surface reflected light of the illumination scanning light is imaged by the short focus lens array and enters the CCD sensor. The CCD sensor includes a light receiving unit, a transfer unit, and an output unit. The CCD light-receiving unit performs color separation photoelectric reading of the color image and converts the optical signal into a charge signal. The transfer unit sequentially transfers the signal to the output unit in synchronization with the clock pulse. The output unit converts the charge signal into a voltage signal. Amplification, low impedance output. The obtained analog signal is subjected to known image processing, converted into a digital image signal (image data), and sent to a control circuit section (control means) 11 on the copying machine main body A side.

  The copying machine main body A has a drum-shaped electrophotographic photosensitive member (hereinafter referred to as a photosensitive drum) 1 which is a cylindrical image bearing member having photoconductivity. Around the outer peripheral surface (surface) of the photosensitive drum 1, there are a charging unit 2, an image exposure unit 3, a color developing device 4, a transfer unit 5, a fixing unit 6 and a cleaning unit 7. . Reference numeral 9 denotes a feeding cassette on which the recording material P is loaded and stored. A conveying unit 10 includes conveying rollers 10a and 10b, a transfer drum 10c, a conveying belt 10d, and the like. The control circuit unit 11 drives and controls the photosensitive drum 1, the charging unit 2, the image exposure unit 3, the color developing device 4, the transfer unit 5, the fixing unit 7, the transport unit 10, and the like.

  When an image output signal is sent to the control circuit unit 11 from an operation unit (not shown) provided in the copying machine main body A or an external device (not shown) such as a personal computer, the photosensitive drum 1 rotates in a predetermined direction in the arrow direction. Rotates at speed (process speed). The outer peripheral surface (surface) of the photosensitive drum 1 is uniformly charged by the charging means 2 to a potential of minus 300V to 700V, preferably 500V to 600V. At this time, the photosensitive drum 1 is a functionally separated type or a single layer type having a two-layer structure such as a charge generation layer and a charge transport layer with a conductive drum-shaped support base as a lowermost layer. Can be used.

  As the charging means 2 for charging the surface of the photosensitive drum 1 which is the surface of the image carrier, a corona charging method using a corona charger comprising a wire and an electric field control grid is used. In addition, there is a roller charging method in which a charging roller brought into contact with the photosensitive drum 1 is charged by applying a direct current or a superimposed bias of direct current and alternating current. An injection charging system in which magnetic particles are carried on a magnetic roller, rotated by contact with an image carrier, and charged by directly injecting charges onto the surface of the photosensitive member by applying a bias thereto. and so on. An example of potential setting for obtaining a charging potential of 500 V in each charging method will be given. In the case of corona charging, a setting of an applied current of 1000 μA to the corona wire and a grid potential of 500V can be used. In the case of roller charging, settings of DC bias 500 V and AC bias 1 kHz / 400 Vpp can be used. In the case of injection charging, settings of DC bias 500 V and AC bias 1 kHz / 400 Vpp can be used. When using a photoconductor with a high dark decay speed such as an a-Si photoconductor, the potential of the corona charging grid bias, roller charging DC bias, and injection charging DC bias is attenuated from the charging position to the developing position. It is necessary to set up by raising the amount.

  As the image exposure means 3 that exposes the surface of the image carrier after charging (the surface of the photosensitive drum 1) to form a latent image, a scanning optical system including a rotatable deflection mirror is used. The image exposure means 3 causes the semiconductor laser to blink on the surface of the photosensitive drum 1 in accordance with the image data. Then, the same charge pattern as the laser irradiation pattern is generated, and the generated charge cancels out the charged charge charged in the charging process, thereby forming a desired electrostatic latent image pattern on the surface of the photosensitive drum 1. The scanning optical system includes a scanner type using the above-described semiconductor laser, an LED that performs image exposure via a self-focus lens as a condensing device, and other elements such as an EL element and a plasma light emitting element. Optical systems can also be used.

  The color developing device 4 has a rotary 4 b supported by a rotation shaft 4 a parallel to the rotation center of the photosensitive drum 1. The rotary 4b includes four developing devices 4Y, 4M, 4M, 4M, 4M, 4M, 4M, 4M, 4M, 4M, 4M, 4M, 4M, and 4M as developing means for developing the latent image by attaching toner to the latent image on the surface of the image carrier. 4C and 4B (hereinafter referred to as 4 (Y-B)) are detachably mounted. The developing devices 4 (Y to B) are sequentially moved from a predetermined standby position to a developing position facing the surface of the photosensitive drum 1 by the rotation of the rotary 4b. The developing device 4 (Y to B) forms a developing nip portion with the surface of the photosensitive drum 1 at the developing position. The electrostatic latent image pattern formed on the surface of the photosensitive drum 1 is sent to the developing nip portion according to the rotation of the photosensitive drum 1, and the toner of each color is supplied from the developing device 4 (Y to B) at the developing nip portion. Developed. In the developing device 4 (Y to B), the developing nip portion is a developing sleeve 4Ys · 4Ms · 4Cs · 4Bs (hereinafter referred to as 4 (Ys to Bs) provided at a distance of several tens to several hundreds μm from the surface of the photosensitive drum 1. )) And an area sandwiched between the outer peripheral surface (surface) and the surface of the photosensitive drum 1. A magnetic carrier electrostatically carrying non-magnetic toner is conveyed to the developing nip portion according to the rotation of the developing sleeve 4 (Ys to Bs) containing a magnet, and used for the developing process. A high frequency electric field (developing bias) of several KHz / hundreds to several KV is applied to the developing sleeve 4 (Ys to Bs) from a developing bias power source (not shown). As a result, the toner (toner particles) carried on the magnetic carrier flies and reciprocates between the developing sleeve 4 (Ys to Bs) and the surface of the photosensitive drum 1, thereby obtaining a toner image corresponding to the electrostatic latent image pattern. . As an example of the developing bias, a good output image can be obtained by using a rectangular bias of 9 kHz / 800 Vpp, setting the potential difference between Vdark and Vdc to 120 V, and the potential difference between Vdc and Vlight to 250 V.

  As a developing method at this time, a magnetic one-component non-contact developing method in which magnetic toner is conveyed by a magnetic force and is developed in a non-contact manner on the surface of the photosensitive drum 1 at the developing nip portion, or the photosensitive drum 1 at the developing nip portion. A magnetic contact development method in which development processing is performed by contacting the surface can also be applied. Further, non-magnetic toner is regulated by a developing blade (not shown) to be frictionally charged, transported to the surface of the developing sleeve 4 (Ys to Bs) and transported to the developing nip portion, and the toner is developed in a non-contact manner. A one-component non-contact development method can also be applied. Alternatively, other development methods such as a non-magnetic one-component contact development method in which development processing is performed by contacting the surface of the photosensitive drum 1 at the development nip portion can also be used.

On the other hand, the recording paper (recording material) P conveyed one by one from the feeding cassette 9 by the conveying rollers 10a and 10b is formed on the outer peripheral surface (front surface) of the transfer drum 10c rotated at substantially the same speed as the photosensitive drum 1. Retained. The recording paper P is introduced into a transfer nip portion between the surface of the photosensitive drum 1 and the surface of the transfer drum 10c by the rotation of the transfer drum 10c, and is nipped and conveyed by the photosensitive drum 1 and the transfer drum 10c at the transfer nip portion to be fixed. Sent to means 6. In the process in which the recording paper P is nipped and conveyed at the transfer nip portion, the toner image on the surface of the photosensitive drum 1 is transferred to the recording paper P by the transfer means 5 provided inside the transfer drum 10c. As the transfer method at this time, a transfer method using an electric force or a mechanical force can be used. As a method for performing transfer using electric force, there is a corona transfer method in which transfer is performed by applying a DC bias having a polarity opposite to the charged polarity of the toner using a corona wire. In addition to this, a roller transfer method in which a roller having a member having an electric resistance value of 10 ⁵ to 10 ^{12 on} the surface layer is brought into contact and a bias having a polarity opposite to that of the toner is applied.

  The recording paper P that has exited the transfer nip is heated and fixed on the surface of the recording paper P by receiving heat and pressure from the fixing means 6. The fixing system uses a roller fixing system in which a pair of cylindrical fixing rolls are in contact with each other and a recording paper is passed between them, a belt fixing system in which a suspended belt is rotated and fixed, or a non-contact lamp. Various systems such as a flash fixing system for fixing can be used.

  After the toner image is transferred, the surface of the photosensitive drum 1 is removed by a blade-like cleaning member 7a provided in a cleaning device 7 as a cleaning unit, so that residual toner remaining on the surface of the photosensitive drum 1 is removed. To be served.

(2) Outline of correction of potential unevenness on the surface of the photosensitive drum In the image forming apparatus of this embodiment, in the process of attaching the toner to the surface of the photosensitive drum, the potential unevenness is accurately determined from the toner density in consideration of the following two points. It is easy to measure. That is, the toner amount has a very high sensitivity to the potential on the surface of the photosensitive drum, and the image output can be easily performed by the user with a single button. To do.

  First, a method for predicting potential unevenness from toner density will be described with reference to FIG.

  2A shows the potential of the surface of the photosensitive drum 1 and the output toner density when the latent image pattern is developed using a latent image pattern having a spatial frequency of 200 lines (200 line / inch). It shows the relationship. (B) shows the relationship between the potential of the surface of the photosensitive drum 1 and the output toner density when the latent image pattern is developed using a latent image pattern having a spatial frequency of 150 lines (150 line / inch). Is. (C) shows the relationship between the potential of the surface of the photosensitive drum 1 and the output toner density when the latent image pattern is developed using a latent image pattern having a spatial frequency of 300 lines (300 lines / inch). Is. (D) shows the relationship between the potential of the surface of the photosensitive drum 1 and the output toner density when the latent image pattern is developed using the latent image pattern of the analog latent image.

  Here, the latent image pattern of an analog latent image (analog latent image pattern) is a flat latent image pattern like an analog latent image by performing image drawing with a laser spot diameter sufficiently larger than the image writing pitch. Refers to those formed. Specifically, image writing was performed with a laser beam having a spot diameter of 60 μmφ which is much larger than the pitch length of 10.400 μm of 2400 dpi with respect to the image writing pitch of 2400 dpi to obtain an analog latent image pattern. Here, the laser spot diameter refers to a spot diameter having a light quantity of 1 / e2 from the peak light quantity of the light quantity distribution of the laser beam. The relationship between the surface potential of the photosensitive drum (hereinafter referred to as the photosensitive member potential) and the toner density in the case of an analog latent image pattern obtained in this manner indicates the basic characteristics of development. . The Vdc potential, which is the DC component of the development bias, is substantially centered, the toner density, which is the amount of toner, has changed to an S-shape, and the slope of the development characteristics, the so-called development gamma, is the most when the photoreceptor potential is near Vdc. It turns out that it is steep. The state at that time will be described with reference to FIG. 2 correspond to (a), (b), (c), and (d) of FIG. 2, respectively, and the left image is a latent image pattern and the right image is a developed toner image. As shown in (d), in the case of an analog latent image pattern, it is clear that toner is not placed on a certain potential. On the other hand, in the digital patterns (a), (b), and (c), all are expressed in gradation in terms of area, and as the spatial frequency becomes lower in accordance with the spatial frequency, the highlight to shadow portion is changed. It can be seen that the toner amount changes linearly.

  From the above, it can be understood that when the spatial frequency of an image (latent image pattern) is set to a high frequency, when the photoreceptor potential is predicted from the toner density, highly sensitive prediction is possible.

  Also, when actually outputting an image, if the spatial frequency of the image is too high, it is not possible to express an appropriate gradation, so it may be necessary to output the image with a lower spatial frequency of the image. Understandable.

  In this embodiment, the developing characteristic has a steeper change with respect to the change in the photoreceptor potential. By using the developing characteristic in the vicinity of Vdc surrounded by the broken-line square in FIG. It realizes prediction of body potential.

  As a specific means for that purpose, a photosensitive member potential with high sensitivity can be predicted by performing development processing with Vdc approaching the photosensitive member potential to be measured.

  Further, as shown in FIG. 2D, as a basic characteristic of development, the linearity of the development characteristic is high in the region of ± 50 V of development Vdc. Accordingly, it can be understood that when Vdc is brought close to the photoreceptor potential to be measured, it is more preferable to set the potential within a range of ± 50 V with respect to the photoreceptor potential.

  It is preferable to measure the latent image potential at a plurality of locations. At this time, it is preferable to perform the measurement at a charging potential Vdark, an exposure portion potential Vlight, and an intermediate exposure portion potential Vhalf for forming a halftone image. It is. Here, Vhalf is defined as a potential Vhalf ≈ (Vdark + Vlight) / 2 near the middle between Vdark and Vlight.

  It is preferable to measure the potential with a plurality of latent image potentials, as described above, “When using an a-Si photoconductor having a fast potential decay rate due to dark decay, including the vicinity of the charge potential Vdark, It is desirable to measure potential unevenness information at a plurality of potential levels, which is a combination of two factors: unevenness of charging potential due to dark decay and sensitivity unevenness when irradiated with light. It is difficult to predict potential unevenness at various potential levels from Vdark to Vlight from potential unevenness at a specific potential, potential unevenness in the vicinity of the charged potential (Vdark) where dark decay is the main factor, Uneven exposure portion potential (Vlight) unevenness, which is mainly caused by a difference in sensitivity due to light irradiation, more preferably an intermediate potential level (Vhalf) between Vdark and Vlight. Measuring the potential unevenness. Then, consisting unevenness due to a difference of unevenness and sensitivity due to the dark decay than the results obtained, it is preferable. "In order to make predictions of complex potential non-uniformity. Therefore, it is intended to accurately predict potential unevenness generated by a combination of dark attenuation and sensitivity unevenness at the time of exposure based on a plurality of latent image potential measurement results.

  At this time, when the image output is performed with the latent image potential of Vdark or Vlight set near the latent image potential for measuring the development Vdc, the setting of the development bias may become extremely high or low. As a result, discharge leaks in the apparatus and carrier adhesion to the photosensitive drum may occur. In such a case, a means for outputting an image with a plurality of latent image potentials instead of measuring a significant latent image potential such as Vdark or Vlight is preferable. Examples of the plurality of latent image potentials include a latent image potential between Vdark and Vhalf, a latent image potential near Vhalf, and a latent image potential between Vhalt and Vlight.

  At this time, it is also an important factor to measure the toner density in an area where the relationship of the toner density to the toner amount is linear. Specifically, the potential unevenness in the toner density area of 0.3 to 1.0. Is preferably measured. This is because, as shown in FIG. 4, the inclination of the reflection density with respect to the toner loading amount changes gradually as the density increases, and the higher the reflection density inclination with respect to the toner loading amount, the more accurate the measurement. Because it becomes possible. For the measurement of the toner density, a measurement method based on “ISO 5 / 3-Status A Density” is used. In this example, the above-described SpectroLino manufactured by GretagMachbeth was used, and the concentration was measured with the Status-A setting. For measurement of the in-plane two-dimensional distribution, the concentration was measured using SpectroChart. “SpectroChart” is a system name including “Lino Stage” which performs measurement while moving SpectroLino and SpectroLino as concentration measuring machines.

  Further, as in this embodiment, the potential unevenness in the vicinity of Vlight, which is originally output with a solid density, is developed with the toner amount in the halftone area as the toner amount. It becomes possible to measure potential unevenness. The fluctuation factors on the developing device side include M / S, which is the amount of toner on the developing sleeve, Q / M, which is the charged amount of toner, and SD gap (Sleeve-Drum), which is the distance between the developing sleeve and the photosensitive drum. There are fluctuations in Gap).

  FIG. 5 shows the development gamma characteristic when the M / S in the developing device changes. As can be seen from the figure, compared to the gradation in the highlight to halftone area, the gradation in the halftone to high density area is greatly affected. Here, the highlight to halftone area is an area of 0.0 to 0.8 with the toner density measured by the above method, and the halftone to high density area is 0 to the toner density measured by the above method. .8 to 1.6 region. This tendency is the same when Q / M changes. This shows the following. That is, when measuring the unevenness of the potential of the photosensitive drum using a developing device that tends to cause fluctuation factors in the plane of the output image when the image is repeatedly output in the longitudinal direction or circumferential direction of the photosensitive drum. The potential unevenness measurement image is output at the gradation in the highlight to halftone region. This makes it possible to measure the potential unevenness with higher accuracy.

  In this embodiment, paying attention to the above characteristics, when measuring potential unevenness information of a plurality of potential levels, development is performed by changing the development bias to an area where the gradient of development gamma increases with respect to the potential level to be measured. Thus, highly accurate potential unevenness measurement is easily performed.

(3) Configuration for Measuring Potential Unevenness on Surface of Photosensitive Drum A configuration for measuring potential unevenness on the surface of photosensitive drum 1 will be described in detail.

  The control circuit unit 11 measures the potential unevenness on the surface of the photosensitive drum 1 in accordance with a signal when the user who is an operator selects the potential unevenness measurement mode from the operation unit or at a predetermined periodic timing. The image output operation program is executed.

That is, the control circuit unit 11 performs the following processes (i) to (iv) with a plurality of latent image potentials having different potential levels, and based on the two-dimensional distribution of the density of the obtained toner images. Two-dimensional potential unevenness correction on the surface of the photosensitive drum 1 (image carrier surface) is performed. The potential unevenness is corrected by the image exposure means 3 as a correction means. Then, the developing bias when outputting the toner image at a plurality of latent image potentials is changed according to the latent image potential. The development bias is changed by controlling the development bias power source in accordance with the latent image potential.
(I) A process for charging the surface of the photosensitive drum 1 (image carrier surface).
(Ii) A process of exposing the charged surface of the photosensitive drum 1 (image carrier surface) to a predetermined latent image potential.
(Iii) Processing for developing the latent image potential with a predetermined developing bias and outputting a toner image over at least one revolution of the photosensitive drum (image carrier).
(Iv) Processing for detecting the density of the toner image.

  First, in order to explain the measurement of potential unevenness on the surface of the photosensitive drum 1, the charging potential is Vdark, the exposure portion potential is Vlight, the DC component of the developing bias is Vdc, and the potential for measuring the potential unevenness (latent image potential) is This will be described as Vht. Here, the notation of Vht is used for a symbol of an intermediate potential (a halftone region that is neither solid white (Vdark) nor solid black (Vlight)) used for potential measurement.

  The definition of words used in this embodiment will be described with reference to FIG. FIG. 6 plots input data (image data) on the horizontal axis and the surface potential of the photoreceptor on the vertical axis.

  The surface potential of the photosensitive drum 1 when the image data is 00h / 8 bits and no light irradiation is performed (hereinafter referred to as the photosensitive member surface potential), that is, the photosensitive member surface potential when only charging is performed is the charging potential / Vdark. It is defined as (Vd). The photosensitive member surface potential when the image data is FFh / 8 bits and the maximum amount of light irradiation is performed is defined as exposure portion potential / Vlight (Vl). An intermediate potential region between the charging potential and the exposed portion potential is defined as halftone potential / Vhalflne (Vht). In this embodiment, an intermediate potential approximately halfway between the charging potential / Vdark (Vd) and the exposure portion potential / Vlight (Vl), which is one measure when measuring the surface potential of the photosensitive member, is Vhalf [Vhalf≈ (Vdark≈Vlight) / 2].

  The relationship between a plurality of target potential levels (latent image potential) and the developing bias at that time when measuring potential unevenness will be described with reference to FIG.

  First, as the first (1st step) potential level, an electrostatic latent image is formed at the potential level of the halftone region used in the normal process setting, and the halftone image is developed using the developing bias set in the normal process setting. Get.

  As the second (2nd step) potential level, an electrostatic latent image is formed at a potential level on the Vdark side of the normal process setting, and the DC component of the developing bias at that time is output when the first potential level is output. An image is output with the potential difference set to about 1 between Vdc and Vht.

  Subsequently, as the third (3rd step) potential level, an electrostatic latent image was formed at a potential level on the Vlight side of the normal process setting, and the DC component of the developing bias at that time was output as the second potential level. Similarly to the case, the image output is performed with the potential difference set to about 1. As the AC component of the developing bias at this time, it is preferable to use the conditions used in normal image formation because the toner particles fly back and forth and adhere to the surface of the photosensitive drum 1. Here, the DC component of the developing bias is set to a condition different from usual, such as Vdark and Vlight. Therefore, as described above, when various adverse effects such as in-machine discharge and carrier particle adhesion occur, the potential from the peak to the peak of the AC component can be lowered.

  That is, the charged potential on the surface of the image carrier is Vdark, the exposed portion potential on the surface of the image carrier after charging is Vlight, and the intermediate potential between the charged potential Vdark and the exposed portion potential Vlight is Vhalf = (Vdark + Vlight) / 2. To do. At this time, the plurality of latent image potentials having different potential levels are: the potential level V01 on the exposure portion potential Vlight side (Vlight <V01 <Vhalf) and the potential level V02 on the charging potential Vdark side (Vhalf <V02 <Vdark). It is to be prepared.

  By outputting an image under such conditions, not only can an image be output under the condition of the highest sensitivity to potential fluctuations as shown in FIG. 2, but also the fluctuation factors of the developing device 4 (Y to B). It is possible to measure the unevenness of the surface potential of the photoreceptor which is difficult to receive.

  That is, in view of the basic characteristics of development, which is the development characteristic for the analog latent image shown in FIG. 2D, the conditions having the highest sensitivity to changes in potential, that is, the development Vdc and the latent image potential of the photosensitive drum 1 are used. Are substantially the same or development processing is performed within ± 50V. Thereby, it can be understood that an image can be output under a condition with high sensitivity to a change in potential on the surface of the photosensitive drum 1 (potential unevenness).

  The laser light emission signal in the image exposure means 3 used for latent image formation at this time is a pulse signal PWM-controlled at the highest resolution driving pitch of the image forming apparatus. For example, in the case of a 1200 dpi 8-bit device, a PWM signal in which a pulse width of about 70 h to 90 h is set with a 1200 dpi driving pitch is output to the entire image surface. In the case of an image forming apparatus having a plurality of resolutions as drive signals, forming a latent image at a high spatial frequency as shown in FIG. 2 is more sensitive to changes in toner density with respect to potential changes. Therefore, it is preferable to form a latent image pattern by PWM driving at the highest resolution that the image forming apparatus can have. As shown in FIG. 2, it is more preferable to form the latent image pattern with an analog pattern because the sensitivity of the change in toner density with respect to the change in potential is higher. However, changing the driving current of the semiconductor laser and using it at a setting far from the rated current value is not preferable from the viewpoint of the stability of laser emission. In this case, it is preferable to devise measures such as reducing light with a filter while continuously emitting light at the rated current value. In addition, forming a latent image with light other than the light source used for actual image drawing is not preferable when there is uneven exposure amount of the optical system. It is generally preferable to form a latent image pattern with a light source actually used for image drawing.

  As a spatial frequency at the time of actual image output, if a pattern as shown in FIG. 2D where the change in the toner adhesion amount is large with respect to the surface potential of the photosensitive member is used, the level as shown in FIG. The tone becomes steep and good image formation cannot be performed. The output image for measuring the potential unevenness can be measured with higher sensitivity if it is performed with a high frequency pattern. However, it is desirable to set the spatial frequency to an appropriate setting during actual image output, and in general, to form an image with a lower frequency pattern than during potential unevenness measurement.

  Further, when the development bias is changed in this way, it is necessary to consider the influence of development on the non-image area on the surface of the photosensitive drum 1. As shown in FIG. 8, the relationship between the image forming area 41 and the non-image forming area 42 on the surface of the photosensitive drum 1 and the longitudinal width 43 of the developing sleeve 4 (Ys to Bs) is as follows. In general, the relationship of the longitudinal width 43 is satisfied. Here, the image forming area is a range to be printed when an image is actually output. Light irradiation is sequentially performed on the image forming area 41 as indicated by reference numeral 44, and the photoreceptor potential is lowered.

  Further, the relationship of the longitudinal widths of other members in contact with the surface of the photosensitive drum 1 will be described with reference to FIG. FIG. 9 is a diagram schematically showing the relationship between the longitudinal widths of the photosensitive drum 1 and members (peripheral members) necessary for the electrophotographic process. When the longitudinal width 50, the charging width 51, the image drawing width 52, the developing width 53, the cleaning width 54, and the paper width 55 of the photosensitive drum 1 are set, it is necessary to set the developing width 53 <the cleaning width 54. This is because if the cleaning member 7a is not arranged wider than the width to which the toner can adhere, the toner remains on the surface of the photosensitive drum 1 and the peripheral members are contaminated with the toner. Here, the charging width 51 indicates the longitudinal width of the charging roller as the charging unit 2. The cleaning width 54 refers to the longitudinal width of the cleaning member 7a. The paper width 55 refers to the longitudinal width of the recording material P. Further, it is necessary to set the developing width 53 <the charging width 51. This prevents the peripheral member from being contaminated with toner when the toner adheres to an uncharged area on the surface of the photosensitive drum 1 (an area outside the development width 53 (non-charged area)). Because. Here, the developing width 53 indicates the longitudinal width of the developing sleeve 4 (Ys to Bs). Further, it is necessary to set the image drawing width 52 <the developing width 53. Similarly, if the toner adheres to the portion where the potential is lowered by exposure, the peripheral member is contaminated with the toner. Here, the image drawing width 52 refers to the longitudinal width of the image forming area 41 (FIG. 8). For example, in the case of the 2nd step shown in FIG. 7, the toner adheres to the region 2 of the charging potential Vdark that is not normally developed by changing the developing bias. In order to remove the toner by the cleaning member 7a, it is necessary to set the developing width 53 <the cleaning width 54. The relationship 54 between the developing width 53 and the cleaning width is such that when the intermediate transfer belt 10e (FIG. 25) is used or when the transfer drum 10c (FIG. 1) is used, the intermediate transfer belt 10e and the transfer drum 10c are cleaned. Necessary as a mechanism. As another countermeasure, the toner adhering to the outer side 2 from the end of the image forming area 41 on the surface of the photoconductive drum 1 is applied to the end of the surface of the photoconductive drum 1 corresponding to the adhering toner area after development to before transfer. It is also possible to provide a cleaning member only.

  Thus, by setting the longitudinal width of each member, the toner image on the surface of the photosensitive drum 1 can be output (transferred) onto the recording material.

  Further, the potential unevenness may be measured by detecting the density of the toner image on the surface of the photosensitive drum 1 without transferring the toner image on the surface of the photosensitive drum 1 to the recording material P. In this case, by canceling the operation of the transfer means during measurement, it is possible to measure the potential unevenness without adversely affecting the toner attached to the outer side 2 from the end of the image forming area 41 on the surface of the photosensitive drum 1.

  A similar problem occurs in the 3rd step for measuring the Vlight side. In this case, as shown in FIG. 10A, an electric field 71 in the direction opposite to the developing direction is generated in the region outside 73 from the end of the image forming region 41. When the electric field 71 exceeds a certain value, the magnetic carrier exceeds the binding force of the magnetic field generated from the developing sleeve 4 (Ys to Bs) when the two-component development system that transports the magnetic carrier by magnetic force is used. Then, the magnetic carrier is attracted toward the photosensitive drum by an electric force, causing a phenomenon called carrier adhesion on the surface of the photosensitive drum 1. Also in this case, the carrier adhering to the surface of the photosensitive drum 1 can be removed by setting the cleaning member 7a so that the developing width 53 <the cleaning width 54. However, if carrier particles that are harder and larger than the toner particles are caught on a member that is in contact with the surface of the photosensitive drum 1, such as the cleaning member 7a, there is a problem that the surface of the photosensitive drum 1 is damaged, which is not preferable. For this reason, as shown in FIG. 10B, it is more preferable to reduce the potential 72 by exposing a region 74 from the end of the image forming region 41 to the developing width 53, thereby preventing carrier adhesion. Therefore, it is preferable that the scanning width of the image exposure means 3 has a scanning width equal to or greater than the longitudinal width of the developing means (developing device) 4 (Y to B) and can be exposed.

  Further, regarding the output image at this time, it is necessary to associate the position of the photosensitive drum 1 in the rotation direction with the toner image. Therefore, the reference position of the photosensitive drum 1 is identified using a Beam Detect sensor (hereinafter referred to as BD sensor) 127 (FIG. 17) installed on the side surface of the photosensitive drum 1. In this embodiment, before performing image output for measuring potential unevenness, the photosensitive drum 1 is returned to the reference position, and the leading edge of the recording paper P for image writing is adjusted so as to overlap the reference position. As another method, a method of identifying the reference position by changing the exposure pulse width or exposure intensity at the timing of the reference position during image output, for example, turning on the FFh or increasing the exposure power may be used. it can.

  The potential unevenness is calculated by detecting the density of the three-level toner image of the 1st step, 2nd step, and 3rd step thus obtained, and measuring the two-dimensional distribution of the density of the toner image. For color measurement, it is necessary to measure a plurality of points at a uniform pitch in the two-dimensional surface of the surface of the photosensitive drum 1. Therefore, an automatic colorimeter (not shown) such as “GretagMachbeth / SpectroChart (SpectroLino)” or “X-Rite / Eye-One iO” may be used as the toner density measuring means. In this embodiment, SpectroChart is used as the toner concentration measuring means. The A3 size measurement toner image is measured at 26.4 mm pitch in the circumferential direction and 30 mm pitch in the longitudinal direction, so that 10 pieces of colorimetric data in total of 10 points each in the circumferential direction and the longitudinal direction. Acquired. In the present embodiment, since the photosensitive drum 1 having a diameter of 84 mm is used, the colorimetric data is obtained at 10 points in the circumferential direction by measuring at a 26.4 mm pitch.

  As another color measurement method, it is also possible to perform sampling manually using a handy colorimeter. It is also possible to use a drum-type image reading scanner that reads an image by rotating and moving an image sample using one fixed reading head. It is also possible to use a flatbed type reading scanner in which an image sample is placed on a platen and scanned by a reading head arranged in a line. It is also preferable from the viewpoint of productivity to use a reading scanner attached to the image forming apparatus. In many cases, these various colorimetric methods are trade-offs between high accuracy and complexity, and it is possible to select a toner concentration measurement method in view of the balance between measurement accuracy and simplicity. .

  Further, as shown in FIG. 4, the transmission density is relatively linear even in a high density area where the inclination with respect to the toner application amount is relatively high. It is also possible to convert to unevenness.

  The obtained toner image density (hereinafter referred to as toner density) unevenness information is converted into potential unevenness information using a one-dimensional LUT (look-up table). At this time, the unevenness information of the developing device 4 (Y to B) is also combined with the unevenness of the potential unevenness measurement image. In order to suppress the influence of the unevenness of the developing device 4 (Y to B), the photosensitive drum 1 is attenuated to 0 V, and the unevenness information of the developing device 4 (Y to B) is measured under a condition that is not affected by the charging characteristics and the photosensitive characteristics. To do. And it is also possible to use as a correction | amendment at the time of converting the nonuniformity of the obtained image for potential nonuniformity measurement into potential nonuniformity information. This is because the potential distribution on the surface of the photosensitive drum 1 subjected to the charging process or the exposure process is affected by various uneven factors such as the film thickness, sensitivity, and potential attenuation behavior of the photosensitive drum 1. This is because it is difficult to extract only the unevenness of the developing device 4 (Y to B). The object is to make the photosensitive drum 1 less susceptible to various unevenness factors by setting the photosensitive drum 1 to 0V. Here, as means for setting the potential of the photosensitive drum 1 to 0 V, natural potential attenuation due to being left, neutralization by bringing a neutralizing member into contact with the surface of the photosensitive drum 1, and the like can be considered. In the case of using a contact-type charging method, it is also preferable that the charging member is set to 0 V and the potential of the photosensitive drum 1 is discharged to 0 V. Although it is relatively easy to extract unevenness information in the longitudinal direction of the developing device 4 (Y to B) by such a method, unevenness information is extracted for unevenness in the rotation direction of the developing sleeve 4 (Ys to Bs). Difficult to do. Therefore, the phase of the rotation direction of the photosensitive drum 1 and the phase of the rotation direction of the developing sleeve 4 (Ys to Bs) are shifted, toner images are output to a plurality of recording papers P, and the obtained output images are averaged. By adopting the method, a method that does not consider the unevenness in the circumferential direction is adopted. Further, a home position sensor for detecting the position in the rotation direction of the developing sleeve 4 (Ys to Bs) may be provided to correct development unevenness in the rotation direction of the photosensitive drum 1.

  Further, as shown in FIG. 1, an image forming apparatus that forms a full-color image of cyan, magenta, yellow, and black using a single photosensitive drum 1 is used to output an image for measuring potential unevenness for each color. A technique for reducing the influence of unevenness of the developing devices 4 (Y to B) of the respective colors by averaging the potential unevenness obtained from the obtained image for measuring potential unevenness can also be used.

  Further, the estimation of the potential unevenness from the toner density at this time can be realized by using a LUT prepared in advance. In addition, as shown in FIGS. 11 and 12, the potential sensors 221 and 231 arranged in the image forming apparatus measure the potential at the representative point in the longitudinal direction of the surface of the photosensitive drum 1. It is also preferable to correct the measured value by comparing with the potential level of the surface of the photosensitive drum 1 estimated from the toner density. In FIG. 11, reference numeral 221 denotes a potential sensor fixedly disposed at nine points in the longitudinal direction of the photosensitive drum 1. In FIG. 12, reference numeral 231 indicates that the potential sensor can be moved in the longitudinal direction of the photosensitive drum 1.

  Further, by measuring the potential value of the surface of the photosensitive drum 1 and the potential level obtained from the toner density in an interpolating manner, it is possible to measure potential unevenness with higher accuracy. That is, a potential sensor value having a relatively accurate absolute value of the potential on the surface of the photosensitive drum 1 and a potential information obtained from a relatively accurate toner density for a relative level difference in the surface of the photosensitive drum 1. By combining the above, it becomes possible to measure potential unevenness with higher accuracy.

  The potential unevenness table thus obtained is input to the laser drive control unit (see FIG. 17) of the image forming apparatus. In this embodiment, potential unevenness information is input to three types of potential targets for a total of 100 points of 10 points in the longitudinal direction × 10 points in the circumferential direction of the photosensitive drum 1. In this way, potential unevenness data is stored by dividing the surface of the photosensitive drum 1 into a two-dimensional matrix and directly storing unevenness information of each divided portion for each target potential using a two-dimensional table. .

  As another storage method, the longitudinal direction and the circumferential direction of the photosensitive drum 1 are approximated by multiplying the image conveyance direction (the circumferential direction of the photosensitive drum 1) and the longitudinal direction of the photosensitive drum 1 by one-dimensional unevenness, respectively. It is also possible to use a method of storing each using a one-dimensional table.

  In general, in a cylindrical image carrier, unevenness due to the respective directions tends to occur in the major axis direction and the circumferential direction of the cylinder, and the major axis direction in the entire area of the image carrier surface is likely to occur. In some cases, potential unevenness information can be predicted by multiplying the potential unevenness characteristic in the circumferential direction. Therefore, depending on the potential unevenness characteristic of the cylindrical image carrier, the amount of memory for storing the potential unevenness information can be reduced by approximating by multiplying the longitudinal and circumferential one-dimensional potential unevenness tables. However, effective correction is possible.

  A correction table is obtained by calculation from the obtained potential unevenness information. As shown in FIG. 13, potential unevenness data is acquired at three levels of potentials of 1st step, 2nd step, and 3rd step, linear linear approximation is performed as shown in FIG. 14 for each position, and estimated potentials for each input data are obtained. A straight line 101 is obtained. The correction amount of the laser power is calculated from the difference 103 between the estimated potential corresponding to the input data and the target potential line 102 and the inclination 104 of the estimated potential line. The correction amount of the laser power at this time can also be obtained by calculation at the image drawing timing for each pixel. Further, it is also possible to realize the result calculated in advance by storing the result in the LUT for each region and each gradation as shown in FIG.

  At this time, correction can be performed by increasing or decreasing the laser power only in the direction of decreasing the potential. Therefore, it is necessary to perform charging control in advance such that the minimum value of the charging potential becomes the target potential.

  In this embodiment, the potential is corrected by changing the laser power. However, a method of correcting the potential by changing the pulse width can be used. Alternatively, various correction methods such as a method of correcting by combining pulse width and laser power, and a method of increasing / decreasing multi-value data of an image according to the correction amount at the stage of image processing can be used.

  The method for changing the laser power used in this example will be described in detail.

  In this embodiment, as the image exposure means 3 for performing image exposure, a scanner type optical system is used in the scanning optical device. As shown in FIG. 16, the scanning optical device 3 includes a semiconductor laser 111, a polygon mirror 114 that rotates at high speed, and a collimator lens 112 that collimates the light beam L emitted from the semiconductor laser 111. Further, the scanning optical device 3 has a cylinder lens 113 that condenses the parallel light beam L on the surface of the polygon mirror 114 and the light beam L deflected by the polygon mirror 114 on the surface of the photosensitive drum 1 at a constant speed. Fθ lens group 115 for irradiating with. The scanning optical device 3 includes a PD sensor 119 that detects part of the laser light inside the semiconductor laser 111. Then, the APC control of the semiconductor laser 111 is performed using the detection signal of the PD sensor. As a result, stable image recording can be performed while suppressing fluctuations in the laser power due to disturbances such as a temperature rise due to laser emission.

  The semiconductor laser 111 blinks in response to a light emission signal of a laser driver, which will be described later, upon receiving a time-series digital image signal output from the reading unit 22 of the image reading unit A or a personal computer. The light beam L emitted from the semiconductor laser 111 is reflected and deflected on the polygon mirror 114 rotating at a constant speed. The deflected light beam L is reflected by the folding mirror 116 by the fθ lens group 115, is imaged in a spot shape on the surface 117 of the photosensitive drum 1, and is scanned at a constant speed in a predetermined direction 118. The writing position in the scanning direction at this time is controlled so that the image signal is always written from the same position by the detection signal of the PD sensor 119 provided at the end of the optical scanning area.

  As a semiconductor laser driving method in this embodiment, a method called PWM (Pulse Width Modulation) control, which controls the amount of emitted light by changing the emission pulse width, can be used. In addition, a method of controlling the amount of emitted light by changing laser power called PM (Power Modulation) control can be used. Alternatively, various control methods such as a method of controlling the amount of emitted light by using the above-described PWM control and PM control in combination can be used.

  FIG. 17 shows an example of a circuit of the laser drive control unit. By using a laser chip 120 composed of a laser 121 and a PD sensor 122 and applying two current sources of a bias current source 123 and a pulse current source 124 to the laser chip 120, the emission characteristics of the laser 121 are improved. Yes. In addition, in order to stabilize the light emission of the laser 121, the bias current source 123 is fed back using the output signal from the PD sensor 122 as described above to automatically control the bias current amount. At the time of image drawing, the data writing position on the sub-scanning side of the photosensitive drum 1 is controlled by the sequence controller 126 and the writing position on the main scanning side is detected by the BD sensor 127. Then, by controlling the above data based on the detection signal (BD signal) of the BD sensor 127, the laser 121 is blinked at a desired timing, thereby writing an image.

  In the above description, the image exposure means 3 performs correction means for correcting two-dimensional potential unevenness. As another method, it is also possible to provide correction exposure means for correcting potential unevenness separately from exposure for image formation. The scanning optical system can be used in the same way as an exposure unit that divides a region such as an LED and controls an exposure amount, and an image exposure unit. As the correction position of the correction exposure means, it can be installed after charging to before exposure, after exposure to before development, etc., and a predetermined correction amount is applied to the surface of the photosensitive drum 1 with reference to the home position position of the photosensitive drum 1. The potential unevenness is corrected by exposure.

  FIG. 18 shows an example of a flowchart of potential unevenness correction on the surface of the photosensitive drum 1 used in the image forming apparatus of this embodiment.

  When correcting the charging potential by light exposure, it is impossible to correct the direction in which the charging potential is increased as described above. For this reason, regarding the setting of the absolute value of the charging potential, it is necessary that the minimum value of the charging potential is higher than the target potential. Regarding this point, as shown in S1 to S3, when measuring the charged potential unevenness (S1), the lowest potential is compared with the target potential (S2). If the measured minimum potential is lower than the target potential (NO in S2), the charging condition is reset according to the difference (S3), and the potential unevenness data of the charging potential is measured again. The above (S1) to (S3) are repeated, and when the minimum value of the charging potential becomes higher than the target potential (YES in S2), the operation shifts to an output operation for potential unevenness measurement image (S4), and the output image is displayed. Input to an automatic colorimeter provided outside the image forming apparatus. The automatic colorimeter measures the toner image on the surface of the photosensitive drum 1 (S5), and converts potential unevenness from the obtained toner density (colorimetric data) by the LUT (S6). Then, the potential unevenness information is input to the image forming apparatus (S7), and the potential unevenness on the surface of the photosensitive drum 1 is corrected based on the potential unevenness information (S8). As a result, an output image without potential unevenness can be obtained when an image is actually formed on the recording paper P.

In the above S1 to S3, the charging process is performed with the potential setting so as to satisfy the predetermined target potential so that the minimum potential ≧ the target potential, and this is read by the potential sensor. If the measured minimum potential of the plurality of potentials is below the target potential, the charging process is performed again under different conditions, and this is repeated until the minimum potential ≧ target potential.
At this time, the potential sensor 221 is arranged in the longitudinal direction of the photosensitive drum 1 as shown in FIG. 11, or the potential sensor 231 is arranged so as to be movable in the longitudinal direction of the photosensitive drum 1 as shown in FIG. In the case of being present, the lowest potential among these potentials is set to the lowest potential. When the potential sensor is fixed to about one in the longitudinal direction of the photosensitive drum 1, the minimum potential can be inferred from the relationship between the potential unevenness distribution measured in advance and the potential at the longitudinal position. It is. When the potential sensor is fixed to about one in the longitudinal direction of the photosensitive drum 1 and only the potential in the circumferential direction of the photosensitive drum 1 at a specific location can be measured, potential unevenness of the photosensitive drum 1 is preliminarily determined in the image forming apparatus. Measure outside. It is also preferable to perform the correction process in consideration of the potential distribution of the photosensitive drum 1 and the potential measurement result of the representative point in the longitudinal direction of the photosensitive drum 1.

  A specific example of the image forming apparatus according to the present embodiment will be described below.

[Example 1]
An 84 mmφ a-Si photosensitive member was used as the photosensitive drum. The a-Si photosensitive member has a linear EV curve (Energy-Voltage curve) characteristic as shown in 102 of FIG. 14, in which the attenuation characteristic of the photosensitive member surface potential with respect to the integrated exposure intensity, which is input data.
The potential unevenness on the surface of the photosensitive drum showing such characteristics is measured at three potential levels as shown in FIG. 13, and the surface of the photosensitive drum is divided into two-dimensional areas, and the potential characteristics of each area are determined. The potential unevenness was corrected by changing the laser power. In this case, as can be seen from the chart shown in FIG. 18, before the output of the potential unevenness measurement image, the minimum value (minimum potential) of the charging potential on the surface of the photosensitive drum is the target charging potential (target The charging conditions are optimized so as to be larger than the potential.

  After the charging conditions are optimized, a toner image for measuring potential unevenness is output. FIG. 7 shows the output setting of the 3-step potential unevenness measurement image used at this time.

First, as the first step (1st), a normal setting of a charging potential (Vdark) −500 Vd and a developing bias DC component of −350 Vdc was used. Then, uniform exposure was performed so that the surface potential of the potential unevenness measurement potential (Vht) −300 Vht was obtained, and an image was output. [1st Step: -500Vd / -300Vht / -350Vdc]
As a developing method at this time, a two-component developing system was used, and a high frequency AC bias of 8 kHz / 1 kVpp was superimposed on a DC bias as a developing bias. Here, 1 kVpp indicates a potential difference of Peak to Peak with a rectangular bias. Further, as an exposure method for forming a latent image potential of Vht, the scanning optical device 3 shown in FIG. Thus, uniform exposure was performed. When outputting an image for measuring potential unevenness, the reference position of the photosensitive drum is moved to the start position in advance. Then, from the position near 50 mm of the front edge of the A3 sheet, image formation is started using the reference position of the photosensitive drum as a starting point for image drawing, thereby obtaining a halftone image for one rotation of the photosensitive drum. Here, the halftone image for one rotation of the photosensitive drum is output in order to measure two-dimensional potential unevenness in the entire circumferential direction of the photosensitive drum. That's why.

Subsequently, similarly, as the second step (2nd), a potential setting of a charging potential (Vdark) −500 Vd and a DC component of the developing bias −450 Vdc was used. Then, uniform exposure was performed by PWM driving with a pulse width of 20 h, whereby the potential unevenness measurement potential (Vht) −400 Vht was set, and image output was performed in the same manner as in the first step. [2nd Step: -500 Vd / -400 Vht / -450 Vdc]
Subsequently, as a third step (3rd), a potential setting of a charging potential (Vdark) −500 Vd and a DC component of the developing bias −200 Vdc was used. Then, uniform exposure was performed by PWM driving with a pulse width of E0h, thereby setting the potential unevenness measurement potential (Vht) to 150 Vht, and image output was performed as in the first step. [3rd Step: -500 Vd / -150 Vht / -200 Vdc]
The obtained three-level halftone images were subjected to color measurement at 10 points in the longitudinal direction of the photosensitive drum, 10 points in the circumferential direction, and 100 points in total using SpectroChart of GretagMachbeth. The obtained colorimetric result is converted into a potential value using a one-dimensional LUT and input to the image forming apparatus.

  In the image forming apparatus, an estimated potential straight line 102 is obtained in each region as shown in FIG. 14 based on the inputted potential unevenness. Then, a laser power value to be corrected is obtained from the difference 103 between the estimated potential level corresponding to the input data and the target potential line 102 and the inclination 104 of the estimated potential line. In this embodiment, as shown in FIG. 19, four potential unevenness measurement points 142, 143, 144, and 145 that are in close proximity from each pixel position 141 are referred to, and the correction laser power at these four points is determined by the above method. calculate. Then, the correction values of the laser power at each pixel point are derived by averaging the correction values of the laser power by multiplying the ratios of the distances 146, 147, 148, and 149 from each of the four points.

  According to the obtained laser power correction value, the laser power is corrected by the laser drive control unit shown in FIG.

  As a result, the in-plane unevenness converted from the density unevenness to 9 V is obtained by performing the correction processing according to the present embodiment on the in-plane unevenness 24 V on the surface of the photosensitive drum before correction, which is converted from the density unevenness. Reduced. As a result, the in-plane color difference of the cyan monochrome output image can be kept within ΔE = 1.57.

[Example 2]
The image forming apparatus according to the present embodiment has the same configuration as that of the image forming apparatus according to the first embodiment except that the Vdark and Vlight potentials are measured when the potential unevenness measurement image is output. The image forming apparatus according to this embodiment measures the Vdark and Vlight potentials, thereby improving the accuracy of the estimated potential straight line 161 shown in FIG. 20 and correcting the in-plane potential unevenness with higher accuracy. .

  FIG. 21 shows potential settings in the image forming apparatus of this embodiment.

First, as the first step (1st), a normal setting of a charging potential (Vdark) −500 Vd and a developing bias DC component of −350 Vdc was used. Then, uniform exposure is performed so that the surface potential of the potential unevenness measurement potential (Vht) −300 Vht is obtained, and an image is output. The developing method at this time is the same as in Example 1. [1st Step: -500Vd / -300Vht / -350Vdc]
Subsequently, as a second step (2nd), a charging potential (Vdark) −500 Vd, and a potential setting of a developing bias DC component −450 Vdc were used. Then, the potential unevenness measurement potential (Vht) was kept at −500 Vht without performing exposure, and image output was performed. [2nd Step: -500 Vd / -500 Vht / -450 Vdc]
Subsequently, as the third step (3rd), a potential setting of a charging potential (Vdark) −500 Vd and a DC component of the developing bias −150 Vdc was used. The potential unevenness measurement potential (Vht) was subjected to uniform exposure so that the entire surface was fully lit to obtain a surface potential of -100 Vht, which is the Vlight potential, and image output was performed. [3rd Step: -500Vd / -100Vht / -150Vdc]
As a result, the in-plane potential unevenness converted from the density unevenness was reduced to 8V by performing the correction process according to the present embodiment on the in-plane unevenness 24V before correction converted from the density unevenness. As a result, the in-plane color difference of the cyan single color output image can be kept within ΔE = 1.40.

[Example 3]
The image forming apparatus shown in the present embodiment is implemented except for the point that the image for developing unevenness correction is simultaneously output (S4a) simultaneously with the output of the potential unevenness measuring image (S4), as shown in the flowchart of FIG. The configuration is the same as that of the image forming apparatus of Example 1. The image forming apparatus shown in the present embodiment can perform correction processing with higher accuracy by adding development unevenness correction when performing colorimetry to toner density / potential conversion LUT processing outside the image forming apparatus. It is.

  Specifically, the image forming apparatus of the present embodiment outputs a potential unevenness measurement image and at the same time performs development processing at a 0 V potential that is not easily affected by the potential unevenness of the photosensitive drum, whereby the density unevenness of the developing device is obtained. Measure. This is because the electric characteristics of the photoconductive drum attenuated to 0 V over time include the charging ability when a charged charge is applied, and the dark attenuation characteristic in which the electric potential is attenuated while moving to the development position after being charged. This is because it is difficult to be affected by light sensitivity characteristics when irradiated with light. For static characteristics such as film thickness and dielectric constant, the effect at 0V remains, but when forming an image of highlight to halftone area, especially when forming an electrostatic latent image with an analog pattern, The impact is limited. Therefore, it is difficult to be influenced by the characteristic unevenness of the photosensitive drum, and the unevenness information of the developing device can be measured.

  As the potential setting at that time, as shown in FIG. 23, the photosensitive drum is allowed to stand for 1 minute so that the potential is attenuated to 0 V, the DC component Vdc of the developing bias is set to −50 V, and the development unevenness correcting image is displayed. Output. The obtained image is transferred from the color measurement to the toner density / potential conversion LUT process outside the image forming apparatus, and when the toner density is converted into the potential value, the unevenness of the developing device is corrected, thereby achieving higher accuracy. Potential unevenness can be calculated.

  As a result, the in-plane potential unevenness converted from the density unevenness is reduced to 6 V by performing the correction process according to the present embodiment on the in-plane potential unevenness 24 V converted from the density unevenness. . As a result, the in-plane color difference of the cyan single color output image can be kept within ΔE = 1.05.

[Example 4]
In the image forming apparatus shown in this example, an 84 mmφ OPC photosensitive member was used as the photosensitive drum. As indicated by reference numeral 192 in FIG. 24, the OPC photoconductor exhibits an EV curve (Energy-Voltage curve) characteristic in which the attenuation characteristic of the photoconductor surface potential with respect to the integrated exposure intensity as input data changes exponentially. Is. As described above, when a photosensitive drum having a non-linear potential decay characteristic with respect to the exposure amount is used, an error increases if the estimated potential is obtained by linear approximation. Therefore, in this embodiment, as shown in FIG. 24, potential unevenness is measured at five potential levels and approximated by a multi-order function to obtain a non-linear estimated potential characteristic and correct the potential unevenness. It is.

  As a result, even for a photoconductor that exhibits non-linear potential decay characteristics such as OPC, the in-plane unevenness 24V before correction converted from the density unevenness is converted from the density unevenness by performing the correction processing according to this embodiment. The in-plane potential unevenness was reduced to 9V. As a result, the in-plane color difference of the cyan monochrome output image can be kept within ΔE = 1.57.

[Others]
The image forming apparatus of the present embodiment is not limited to the image forming apparatus shown in FIG. 1, and may be a tandem type image forming apparatus as shown in FIG.

  FIG. 25 is a structural model diagram of an example of a tandem type copying machine. Members that are functionally the same as those of the copier in FIG. 1 are given the same reference numerals, and description thereof is omitted.

  In the copying machine shown in FIG. 25, first to fourth image forming stations SY for forming toner images of each color of yellow Y, magenta M, cyan C, and black K as image forming means on the copying machine main body A side. · SM · SC · SK (hereinafter referred to as S (Y to K)). Each station S (Y to K) has a photosensitive drum 1 as an image carrier. Around each photosensitive drum 1, there are a charging unit (charging unit) 2, an image exposure unit (scanning optical device) 3, a developing unit 4 as a developing unit, a transfer unit (transfer roller) 5, and a cleaning unit. A (cleaning device) 7 is provided. An intermediate transfer belt 10e constituting a part of the conveying means 10 is disposed so as to face the surface of the photosensitive drum 1 of each station S (Y to K). The transfer belt 10e is stretched around two axes of a driving roller 10f and a driven roller 10g. A transfer nip portion is formed between the photosensitive drum 1 and the conveying belt 7 by disposing the transfer roller 5 so as to face each of the photosensitive drums 1 with the transfer belt 10 e interposed therebetween.

  The photosensitive drums 1 of the stations S (Y to K) are driven and controlled by a control circuit unit (control means) (not shown) to rotate at a predetermined peripheral speed (process speed) in the direction of the arrow. In synchronization with the photosensitive drum 1, the conveying belt 7 is also rotated by the driving roller 8 a by the rotational driving system in the direction of the arrow at a peripheral speed corresponding to the rotational peripheral speed of each photosensitive drum 1.

  First, in the yellow Y station SY of the first color, the outer peripheral surface (front surface) of the photosensitive drum 1 is uniformly charged to a predetermined potential by the charging roller 2. Next, the surface of the photosensitive drum 1 is scanned and exposed with a laser beam corresponding to the image data of the color original G obtained from the reading unit 22. As a result, an electrostatic latent image corresponding to the image data is formed on the surface of the photosensitive drum 1. The latent image is developed with yellow toner by the developing device 4, and a yellow toner image is formed on the surface of the photosensitive drum 1. A similar image forming process is performed in each of the magenta M, cyan C, and black K stations S (M to K). A toner image of each color is formed on the surface of the photosensitive drum 1 of each station S (Y to K).

  On the other hand, the recording paper P stacked and stored in the feeding cassette 9 is conveyed onto the transfer belt 10e by the conveying rollers 10a and 10b. The recording paper P is conveyed from the transfer nip upstream in the rotation direction to the transfer nip downstream in the rotation direction by the rotation of the transfer belt 10e. During the conveyance process, the toner images on the photosensitive drums 1 are transferred onto the recording paper P in order by the transfer roller 5 on the recording material P at the transfer nip portion.

  At each station S (Y to K), the transfer residual toner remaining on the surface of the photosensitive drum 1 after the transfer of the toner image onto the recording paper P is a cleaning blade (not shown) provided in the cleaning means (cleaning device) 7. Is removed. Thus, the photosensitive drum 1 is repeatedly used for the next image formation.

  The recording paper P separated from the transfer belt 10 e and exiting the transfer nip portion on the downstream side in the rotation direction is introduced into the fixing unit 6. The recording paper P receives heat and pressure from the fixing unit 6 so that the toner image is heated and fixed on the surface of the recording paper P.

  If the above-mentioned “(3) Configuration for measuring potential unevenness on the surface of the photosensitive drum” is applied to the tandem type image forming apparatus, the same effect as the image forming apparatus of the first embodiment can be obtained.

1 is a configuration model diagram of an example of an image forming apparatus. FIG. 4 is a diagram for explaining a relationship of toner density with respect to a photoreceptor surface potential. FIG. 4 is a diagram for explaining a relationship between a latent image pattern and a developed toner image pattern with respect to a photoreceptor surface potential. FIG. 6 is a diagram for explaining a relationship between toner density and toner applied amount. FIG. 6 is a diagram for explaining development gamma characteristics when M / S in the developing device changes. The figure explaining the relationship between charging potential, exposure part potential, and halftone potential. The figure explaining the electric potential setting at the time of outputting the electric potential nonuniformity measurement image. FIG. 3 is a diagram for explaining a relationship between an image forming area on the surface of a photosensitive drum and a developing area. The figure which showed typically the relationship between the photosensitive drum and the longitudinal width of the member required for an electrophotographic process. The figure explaining the potential setting at the time of outputting the image for potential nonuniformity measurement. FIG. 6 illustrates an example of a potential sensor. FIG. 9 illustrates another example of a potential sensor. The figure explaining the electric potential setting at the time of outputting the electric potential nonuniformity measurement image. The figure explaining the calculation method of a laser power correction value. The figure explaining electric potential nonuniformity correction | amendment LUT. FIG. 6 illustrates a scanning optical device. The figure explaining the circuit of a laser control part. 6 is a flowchart of an example of potential unevenness correction on the surface of the photosensitive drum 1; The figure explaining the calculation method of a laser power correction value. The figure explaining the calculation method of a laser power correction value. The figure explaining the potential setting at the time of outputting the image for potential nonuniformity measurement. 6 is a flowchart of an example of potential unevenness correction on the surface of the photosensitive drum 1; The figure explaining the potential setting at the time of outputting the image for potential nonuniformity measurement. The figure explaining the calculation method of a laser power correction value. FIG. 6 is a structural model diagram of another example of an image forming apparatus.

Explanation of symbols

1: Photosensitive drum, 2: Charging unit, 3: Image exposure unit (correction unit), 4Y, 4M, 4C, 4K: Development unit

Claims (15)

An image forming apparatus for forming an image on a recording material, comprising a cylindrical image carrier having photoconductivity, charging means for charging the surface of the image carrier, and exposing the surface of the image carrier after charging. An image exposing means for forming a latent image, a developing means for developing the latent image by attaching toner to the latent image on the surface of the image carrier, and correcting two-dimensional potential unevenness on the surface of the image carrier. Correction means, and
(I) a treatment for charging the surface of the image carrier;
(Ii) a process of exposing the surface of the image carrier after charging so as to have a predetermined latent image potential;
(Iii) processing for developing the latent image potential with a predetermined developing bias to output a toner image over at least one turn of the image carrier;
(Iv) processing for detecting the density of the toner image;
The processes from (i) to (iv) are performed with a plurality of latent image potentials having different potential levels, and the correction means is controlled based on the two-dimensional distribution of the density of the obtained toner images, In the image forming apparatus for correcting the two-dimensional potential unevenness on the surface of the image carrier,
The image forming apparatus, wherein the developing bias when outputting a toner image with a plurality of latent image potentials is changed according to the latent image potentials. An image forming apparatus for forming an image on a recording material, comprising a cylindrical image carrier having photoconductivity, charging means for charging the surface of the image carrier, and exposing the surface of the image carrier after charging. Image developing means for forming a latent image, and developing means for developing the latent image by attaching toner to the latent image on the surface of the image carrier,
(I) a treatment for charging the surface of the image carrier;
(Ii) a process of exposing the surface of the image carrier after charging so as to have a predetermined latent image potential;
(Iii) processing for developing the latent image potential with a predetermined developing bias to output a toner image over at least one turn of the image carrier;
(Iv) processing for detecting the density of the toner image;
The processes from (i) to (iv) are performed at a plurality of latent image potentials having different potential levels, and the image exposure means is controlled based on the two-dimensional distribution of the density of the obtained toner images. In the image forming apparatus for correcting the two-dimensional potential unevenness on the surface of the image carrier,
The image forming apparatus, wherein the developing bias when outputting a toner image with a plurality of latent image potentials is changed according to the latent image potentials. An image forming apparatus for forming an image on a recording material, comprising a cylindrical image carrier having photoconductivity, charging means for charging the surface of the image carrier, and exposing the surface of the image carrier after charging. An image exposing means for forming a latent image, a developing means for developing the latent image by attaching toner to the latent image on the surface of the image carrier, and correcting two-dimensional potential unevenness on the surface of the image carrier. Correction exposure means,
(I) a treatment for charging the surface of the image carrier;
(Ii) a process of exposing the surface of the image carrier after charging so as to have a predetermined latent image potential;
A process of developing (iii) with a predetermined developing bias and outputting a toner image over at least one turn of the image carrier;
(Iv) processing for detecting the density of the toner image;
The processes from (i) to (iv) are performed at a plurality of latent image potentials having different potential levels, and the correction exposure means is controlled based on the two-dimensional distribution of the density of the obtained toner images. In the image forming apparatus for correcting the two-dimensional potential unevenness on the surface of the image carrier,
The image forming apparatus, wherein the developing bias when outputting a toner image with a plurality of latent image potentials is changed according to the latent image potentials.   The image forming apparatus according to claim 1, wherein the developing bias is changed in a direction to approach the latent image potential.   The charged potential on the surface of the image carrier is Vdark, the exposed portion potential on the surface of the image carrier after charging is Vlight, and the intermediate potential between the charged potential Vdark and the exposed portion potential Vlight is Vhalf = (Vdark + Vlight) / 2. Then, the plurality of latent image potentials having different potential levels are the potential level V01 on the exposure portion potential Vlight side (Vlight <V01 <Vhalf) and the potential level V02 on the charging potential Vdark side (Vhalf <V02 <Vdark). The image forming apparatus according to claim 1, further comprising:   4. The toner image according to claim 1, wherein when a toner image is output at a plurality of latent image potentials, the density of the obtained toner image is in the range of 0.3 to 1.0. 5. The image forming apparatus described in 1.   The image forming apparatus according to claim 1, wherein the developing bias is within ± 50 V of the latent image potential.   The image forming apparatus according to claim 1, wherein the developing bias changes only a DC component.   The image forming apparatus according to claim 5, wherein at least one of the latent image potentials is within the charging potential Vdark ± 50V.   The image forming apparatus according to claim 5, wherein at least one of the latent image potentials is within the exposure portion potential Vlight ± 50V.   4. The toner image is output with the surface potential of the image carrier set to 0 V, and unevenness of the developing unit is corrected using the density of the obtained toner image. The image forming apparatus according to claim 1.   When exposing the charged image carrier surface to have a predetermined latent image potential, a latent image pattern having a spatial frequency higher than the spatial frequency of the image used when forming an image on the recording material is used. The image forming apparatus according to claim 1, wherein the image forming apparatus is an image forming apparatus.   The image forming apparatus according to claim 1, wherein the toner image is output onto a recording material, and the density of the toner image is detected.   The image forming apparatus according to claim 1, wherein a density of the toner image is detected on a surface of the image carrier.   The image forming apparatus according to any one of claims 1 to 3, wherein amorphous silicon is used as a material of the image carrier.