JP2007072335A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus capable of achieving image formation with high resolution and high density by forming latent image distribution which will not deteriorate, while using a beam having a small diameter of 50μm or smaller. <P>SOLUTION: If the quantum efficiency of an image carrier is η, exposure energy is E[J/m<SP>2</SP>], and the area of the beam spot is S[m<SP>2</SP>], if the diameter of a beam spot to perform exposure for forming an electrostatic latent image is 50μm or smaller, Expression (1): 7e-8*η*E/S≤1.7 holds. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an image forming apparatus, and more particularly to an image forming apparatus using an image carrier.

  As an image forming apparatus such as a printer, a facsimile, a copying apparatus, and a multifunction machine of these, an image forming apparatus that forms an image using an electrophotographic process is known. In an image forming apparatus using such an electrophotographic system, high image quality is required, and a high-resolution image is required. Here, the resolution is an index indicating how finely an image can be expressed, and is expressed by the number of drawing dots per unit length (inch) dpi (dots / inch). It may be expressed by a resolution of “scanning direction”) × image carrier traveling direction (hereinafter referred to as “sub-scanning direction”). is there.

  As means for realizing such a high-resolution image, the pulse width and power of the beam for writing the electrostatic latent image on the electrostatic latent image carrier are controlled, or the beam diameter (spot diameter) is reduced. A method of squeezing is used. As a means for obtaining a high density in a high resolution image, a method of setting the pulse width and power of the beam to the maximum value and maximizing the development potential which is the difference between the development bias Vb and the bright potential VL is taken. ing.

However, when the resolution becomes 1200 dpi or higher, it has become a big problem to reduce the spot diameter of the beam. For example, as described in Patent Document 1, the resolution is reduced without reducing the beam diameter. There is an example in which the exposure energy is defined according to the characteristics of the photoreceptor so that the maximum image density can be secured while maintaining the characteristics.
JP 2002-14525 A

In addition, PM (Power Modulate) modulation, PWM (Power Wide Modulate) modulation, etc. for varying the exposure energy are performed in order to improve gradation. However, it is difficult to form a small-diameter dot only by PM and PWM modulation, and it is desired to reduce the diameter of the exposure beam. It has come to be obtained.
JP 2000-187172 A

In addition, there are the following patent documents relating to the present invention.
JP 2001-281934 A JP 2004-001260 A JP 2004-109702 A JP 2004-202682 A JP 2004-287361 A JP 2004-287370 A JP 2004-287371 A JP 09-319161 A Japanese Patent No. 3266054 JP 2000-019756 A JP 2000-108409 A JP 2001-096794 A JP 2004-106365 A JP 2004-170473 A JP 2001-180040 A

As described above, although it is possible to obtain an excellent light spot with a small diameter, as a means for obtaining a high density in a high resolution image having a resolution of 1200 dpi or more, the minimum pixel size capable of resolving the beam spot diameter When the diameter is reduced to 50 μm or less, it has been confirmed that the phenomenon that the dot image cannot be completely stopped despite the small diameter of the beam spot is confirmed. An example is shown in FIG. The example shown in FIG. 12 is an isolated 1 dot image when the beam spot diameter is 16 × 16 μm (main scanning direction spot diameter × sub-scanning direction spot diameter: 1 / e ^{2 of} intensity). In the figure, the circle is the beam spot diameter, and it can be seen that a large dot image is formed with respect to the beam spot diameter.

  The inventor has analyzed the cause of this, and the details will be described later. However, when the beam spot diameter is reduced, the latent image may be extremely diffused due to an increase in writing density accompanying an increase in resolution of the image forming method. found. In such a diffused latent image, the dot image spreads, and it is difficult to form a high-resolution image even if the beam spot diameter is reduced to about the minimum pixel that can be resolved.

  Further, it has been found that the laser beam for exposure is shortened as the beam diameter is reduced, and that the above-described problem becomes more remarkable by increasing the energy of the laser.

  The present invention has been made on the basis of the above-mentioned problems and knowledge, and can form a high-resolution and high-density image by forming a latent image distribution that does not deteriorate while using a small-diameter beam of 50 μm or less. An object is to provide an apparatus.

In order to solve the above problems, an image forming apparatus according to the present invention has a quantum efficiency of η and an exposure energy of an image carrier when a beam spot diameter for performing exposure for forming an electrostatic latent image is 50 μm or less. When E [J / m ² ] and the beam spot area are S [m ² ], the following equation (1) is satisfied.

  Here, it is preferable to provide a means for adjusting the expression (1) according to the change in the film thickness of the image carrier that occurs with time, and in this case, exposure energy, beam spot diameter, and charging It is preferable to adjust at least one of the potentials.

  Further, it is preferable to provide means for adjusting so that the formula (1) is satisfied with respect to the fluctuation of the charging potential caused by the environmental fluctuation. In this case, at least one of exposure energy, beam spot diameter, and charging potential is provided. Is preferably adjusted.

  According to the image forming apparatus of the present invention, since the expression (1) is satisfied when the beam spot diameter for performing exposure for forming an electrostatic latent image is 50 μm or less, carrier diffusion is suppressed. Thus, a latent image distribution that does not deteriorate is obtained, and high-resolution and high-density image formation can be performed using a small-diameter beam of 50 μm or less.

Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.
First, in order to understand the present invention, when the beam spot diameter is reduced to 50 μm or less, which is about the smallest pixel that can be resolved, as shown in FIG. The cause of the phenomenon that the dot image cannot be fully drawn will be described.

In order to analyze this phenomenon, we compared the potential profiles of latent images. However, since it is difficult to measure the latent image potential profile, the latent image potential profiles in each condition were compared by simulation.
First, the simulation method will be described. The movement of carriers (charges) inside the photoconductor is affected by the Coulomb repulsion between carriers, recombination of carriers, and the mobility of carriers inside the photoconductor. Therefore, calculation and analysis were performed by `` latent image formation simulation '' that considered all these effects.

The physical model used for the analysis consists of (1) calculation of exposure amount by Gaussian laser beam, and (2) calculation of charge carrier generation and its transport process.
First, in the calculation of the exposure amount in (1), the exposure profile I (x, y) when exposed by a laser diode or LED is approximated by the following equation (2) when the LD light profile is approximated by a Gaussian distribution. It can be calculated by integrating in the X direction by the moving distance (Vx × pointing time) in the lighting time. The beam spot diameter is 1 / e ² diameter of the static beam exposure intensity.

  Here, the exposure energy and the exposure profile in the case of using the LD will be described, but the description can be made even in the case of using a solid-state imaging device such as LEDA. In this case, since scanning is not performed in the main scanning direction, the scanning speed Vx = 0.

  Next, the charge carrier generation (2) and its transport process are governed by the following expressions (3), (4), and Poisson equation (5), which are continuous positive and negative carriers.

  Here, n, μ, E, Γ, r, ε, e are carrier number density, mobility, electric field strength, carrier generation amount per unit time, carrier recombination coefficient per unit time, The dielectric constant and the elementary charge amount are shown. Subscripts p and n indicate positive and negative carriers, respectively.

  Charge carriers were assumed to be generated uniformly in the layer, taking into account the thin CGL layer. For this reason, the carrier generation amount Γ is connected to the incident light intensity F, the quantum efficiency η, and the thickness d of the CGL layer by the relationship of the following equation (6).

  Here, β and hν are the light absorption efficiency in the CGL layer and the energy per photon of the laser beam, respectively. The quantum efficiency η depends on the electric field, and is expressed by η = αEn.

  Further, the carrier recombination term of the second item on the right side of the above equations (3) and (4) is for explaining that the amount of generated carriers decreases experimentally when positive and negative carriers coexist in the same vicinity. Was introduced.

  Of the above physical quantities, the light absorption coefficient β and the recombination coefficient R are calculated by experimental fitting from the surface potential at the time of black solid exposure.

  Before the exposure, it is assumed that the photoreceptor is uniformly charged, and the charge amount on the surface of the photoreceptor is calculated. Thereafter, by calculating the above formula, the charge density distribution on the photoreceptor after exposure is calculated.

  First, the exposure intensity distribution was examined. FIG. 1 shows a beam profile at each beam spot diameter obtained from the equation (2). The solid line is the beam profile (cross-sectional view) when the beam spot diameter is 50 μm, and the broken line is the beam spot diameter of 30 μm.

  It can be seen that as the beam spot diameter becomes smaller, the beam profile becomes sharper and the peak value at the center increases. When exposure with the same energy is performed in such beams having different beam spot diameters, the exposure intensity at the central portion of the small-diameter beam becomes stronger. When the image carrier is exposed to light, the energy of the light excites carriers in the charge generating material (here, the layer) inside the image carrier. Depending on the electric field strength inside the image carrier, when the energy is strong and the electric field strength is large, more carriers are generated. The generated + carriers move to the surface of the image carrier through the charge transfer layer, and are neutralized with the negative charges charged on the image carrier to form a latent image. When the carrier moves to the surface of the image carrier, the carrier is diffused by an electric field generated inside the image carrier to spread the latent image, that is, the latent image is deteriorated.

  When a beam spot diameter of 50 μm or more is used, the spread of the latent image does not have a great influence. However, if the beam diameter is further reduced, the exposure profile becomes sharper and the exposure area is reduced. However, since the amount of generated carriers is increased, the latent image is likely to be deteriorated.

  Next, a latent image profile at a beam spot diameter of 16 × 16 μm is shown in FIG. A solid line indicates a beam profile cross-sectional view, and a dotted line indicates a latent image distribution (volume charge density distribution of a photoreceptor surface layer of 0.3 μm) in the beam profile.

Next, the latent image diameter when the film thickness of the image carrier and the beam spot diameter are changed is examined. Here, the latent image diameter was the e- ² diameter of the latent image charge density distribution obtained by simulation.
The relationship between the beam spot diameter and the latent image diameter at each film thickness is shown in FIG. The 45-degree straight line in the figure represents that the beam spot diameter and the latent image diameter coincide. As the beam spot diameter is reduced, it can be confirmed that the latent image diameter tends to increase away from the 45 degree straight line. As the beam spot diameter decreases, the amount of carriers generated inside the image carrier increases, the coulomb repulsive forces of the generated carriers repel each other, and the latent image deteriorates because the charges move sideways. It can be confirmed.

  As described above, when the beam spot diameter is reduced, the image is likely to deteriorate. However, as shown in FIGS. 4 and 5, the latent image is extremely diffused due to the increase in the writing density accompanying the increase in the resolution of the image forming method. I was able to confirm. In such a latent image, it is difficult to form a high resolution image with a dot image spreading. That is, under such conditions that the latent image is very diffused, the obtained dot image is significantly deteriorated and it is difficult to achieve high image quality.

  Therefore, in order to obtain a high-quality image, it is necessary to set appropriate conditions while observing the effects of both sharpness and diffusion of the latent image by reducing the beam spot diameter.

  Thus, the spread of the latent image was examined by using the beam spot diameter, exposure energy, image carrier, and charged potential thereof. Under each condition, the reproducibility of the isolated 1-dot image is evaluated, and “◯” indicates that the dot is reproduced in a desired size, and “×” indicates that the image is deteriorated (not reproduced or too wide). ". The results are shown in Table 1.

  Table 2 shows the results of performing the above-described exposure simulation and calculating the latent image diameter. Further, when the relationship between each condition, the ratio of the latent image diameter and the beam spot diameter was examined, it was found that there is a correlation as shown in FIG.

From the results shown in Table 2, it can be seen that when the latent image diameter / beam spot diameter is larger than 3, the reproducibility of the one-dot image is lowered. Further, as shown in FIG. 6, when the reproducibility of a one-dot image is good, quantum efficiency × exposure energy / beam spot area (= A) and latent image diameter / beam spot diameter (= B) are
7E-8 * A + 1.3 ≦ 3
That is,
7e-8 * η * E / S ≦ 1.7
It was confirmed that a good one-dot image could be formed when

Accordingly, in an image forming apparatus having an image carrier, a charging unit for charging the image carrier, and an exposure unit for forming an electrostatic latent image on the image carrier based on image information, an electrostatic latent image is formed. When the beam spot diameter for exposure for forming is 50 μm or less, the quantum efficiency of the image carrier is η, the exposure energy is E [J / m ² ], and the beam spot area is S [m ² ]. When equation (1) is satisfied, carrier diffusion can be suppressed and a latent image distribution that does not deteriorate can be formed.

Next, an example of an image forming apparatus to which the present invention is applied will be described with reference to FIG.
The image forming apparatus includes an image carrier 1 that is a photoconductive photoconductor formed in a cylindrical shape. Around the image carrier 1, a charging roller 2 as a charging unit, a developing device 3, A transfer roller 4 and a cleaning device 5 are arranged. An optical scanning device 6 is provided, and the image carrier 1 is exposed downstream of the charging roller 2.

  When performing image formation, the photoconductive image carrier 1 rotates clockwise in the figure, the surface thereof is uniformly charged by the charging roller 2, and writing is performed by scanning the laser 6a from the optical scanning device 6. Upon exposure, an electrostatic latent image is formed. The electrostatic latent image formed on the image carrier 1 is a “negative latent image”, and the image portion is exposed. The electrostatic latent image is reversely developed by the developing device 3 with toner having the same polarity as the charged polarity of the image carrier 1, and a toner image is formed on the image carrier 1.

  On the other hand, the uppermost sheet of the transfer member 7 is fed by a paper feed roller 12 from a cassette 8 containing a transfer member 7 such as paper or OPC mounted on the image forming apparatus main body. The transferred transfer member 7 is fed at its leading end to the registration roller pair 13 and sent to the transfer unit at the same timing as the toner image on the image carrier 1 moves to the transfer position.

  The transferred transfer member 7 is superposed on the toner image at the transfer portion by the transfer roller 4, and the toner image is electrostatically transferred by the action of the transfer roller 4. The transfer member 7 to which the toner image has been transferred is sent to the fixing device 14, where the toner image is fixed, and is discharged onto the discharge tray 17 by the discharge roller pair 16 through the conveyance path 15. On the other hand, the surface of the image carrier 1 after the toner image is transferred is cleaned by the cleaning device 5 to remove residual toner, paper dust, and the like.

  The toner image can be transferred not only by a roller but also by a belt, a charger, a brush, or the like, and can also be performed via an intermediate transfer medium such as an intermediate transfer belt. Although an example of a monochrome image forming apparatus is shown here, a color image forming apparatus having a plurality of developing units on an image carrier, or a color tandem type image forming having a plurality of image carriers and an image forming unit. The present invention can also be applied to an apparatus.

  The image forming apparatus having such a configuration is improved so that the photosensitive member type, the charging potential, the beam spot diameter, and the beam power can be varied, and an isolated one-dot image under the condition that the above expression (1) is satisfied. The dot was reproduced as “◯”, and the image was deteriorated (not reproduced or spread too much) as “X”. The results are shown in Table 3.

As shown in Table 3, it was confirmed that a good image can be formed under the condition that the expression (1) is satisfied.

  On the other hand, when the image forming apparatus as described above is used over time, the image carrier 1 is worn, the sensitivity of the photoreceptor is lowered, and the image is deteriorated. When the film thickness decreases, the electric field strength inside the photosensitive member increases, and accordingly, the photosensitive member sensitivity increases, so that the formula (1) is not satisfied, and the image deteriorates.

  Therefore, image deterioration can be prevented by detecting the film thickness of the image carrier 1, adjusting other conditions in accordance with the film thickness variation, and establishing the relational expression (1).

Therefore, another embodiment of the present invention for performing such adjustment will be described with reference to FIGS. FIG. 8 is a block diagram illustrating a control unit of the image forming apparatus according to the embodiment, and FIG. 9 is a flowchart of processing for adjusting exposure energy.
Here, the control unit 20 includes a controller 21 configured by a microcomputer that controls the entire image forming apparatus, and an engine control unit 22 for controlling each unit (engine) involved in image formation. Yes.

  The controller 21 includes an image memory 23, a font memory 24, and the like. A host 30 is connected to the controller 21, and the controller 21 receives image information from the host 30 and performs processing for the printer function.

  The image forming apparatus also includes a scanner 25 that reads an original image. The scanner 25 operates in response to a drive signal from the engine control unit 22, and image information read by the scanner 25 is sent to the controller 21. As a result, the controller 21 performs processing for the digital copying machine function.

  That is, the controller 21 that has received the image signal from the host 30 or the scanner 25 develops it in the image memory 23 and transmits control / write data to the engine control unit 22 in accordance with the drive signal from the operation panel 26. At this time, if the image signal is text data from the host 30, an appropriate font is called from the font memory 24 as necessary, and image data (character data) according to the called font is developed in the image memory 23. . On the other hand, the engine control 22 that has received control / write data transfer from the controller 21 sends drive signals to drive motors, clutches, and solenoids 27 that serve as drive sources for various movable parts such as a paper feeder and an image carrier. To control the driving of them, and a driving signal is applied to the high-voltage power supply circuit 28 for the charging device and the developing device to control the driving thereof.

  Here, an image carrier current value detection unit 40 is connected to the engine control unit 22, and a detection signal is input to the engine control unit 22. The image carrier current detection unit 40 takes a current from a rotation shaft (not shown) of the image carrier, digitally converts it, and sends it to the engine control unit 22. That is, the current value flowing into the image carrier 1 is detected by the image carrier current detection unit 40, and this current value is proportional to the capacitance of the photosensitive layer which is an insulator during the charging process. Is inversely proportional to the film thickness of the photosensitive layer. Therefore, the value of the current flowing into the image carrier 1 detected by the image carrier current detector 40 is inversely proportional to the film thickness of the photosensitive layer. Therefore, the engine control unit 22 obtains the film thickness of the photosensitive layer based on the detection signal from the image carrier current detection unit 40 by arithmetic processing.

  Further, as an image carrier film thickness detecting means, a table (not shown) of the engine control unit is provided in a memory (not shown) for defining the relationship between the film thickness variation amount of the image carrier and the usage time. It is also possible to perform control such that the film thickness is predicted and recognized as the actual film thickness.

  Therefore, the engine control unit 22 detects the film thickness of the image carrier based on the detection signal in the image carrier current detection unit 40 as shown in FIG. When the amount of film thickness variation is large and Equation (1) is not satisfied, that is, the writing condition (exposure energy) by the exposure apparatus according to the amount of film thickness variation is expressed by Equation (1). Adjust so that it holds.

  As described above, in addition to recognizing the film thickness of the image carrier by the current flowing through the image carrier, it is also possible to recognize the film thickness of the image carrier from the current value flowing through the charging means. In this case, the portion of the image carrier current value detection unit 40 in FIG. 8 becomes the charging current value detection unit.

  That is, the value of the current flowing through the charging unit 2 is proportional to the sum of the electrostatic capacity Co of the image carrier and the electrostatic capacity Cc of the charging unit 2. Since each electrostatic capacity is inversely proportional to the film thickness of each layer, the film thickness of the image carrier can be calculated from the value of the current flowing into the charging means 2 detected by the charging means current detection section. Based on this detection signal, the film thickness of the image carrier can be obtained by arithmetic processing.

  Therefore, by adjusting the exposure energy for the image carrier so that the expression (1) is satisfied according to the wear of the film thickness of the image carrier thus obtained, a high-quality image stably over time. Can be formed.

  It is also possible to adjust the charging potential or the beam spot diameter instead of the exposure energy. When adjusting the charging potential, the voltage or potential applied to the charging means instead of the writing variable means in FIG. May be changed according to the film thickness of the photosensitive member. Further, when the beam spot diameter is adjusted according to the film thickness, for example, a beam diameter changing plate is used.

  Thus, still another embodiment of the present invention in which the beam spot diameter is changed using the beam diameter changing plate will be described with reference to FIGS. FIG. 10 is an explanatory plan view showing the configuration of the optical system in the exposure apparatus in the embodiment, and FIG. 11 is an explanatory plan view showing the configuration of the light source device.

  This optical system includes a light source device 51, a polygon mirror 52, a scanning lens 53, a cylindrical lens pair 57, and a lens barrel 58. Among these, the light source device 51 further includes first and second laser diodes 511 and 512. The light source device 51 emits laser beams 501 and 502 output from the first and second laser diodes 511 and 512 at regular intervals in the sub-scanning direction (direction perpendicular to the paper surface) while the optical axis thereof coincides with the main scanning direction. And exit toward the cylindrical lens pair 57 so as to be substantially parallel. The light source device 51 changes the beam diameter of the laser beams 501 and 502 according to the resolution.

  The laser beams 501 and 502 are focused on the deflection surface (mirror surface) of the polygon mirror 52 that rotates about the rotation shaft 52a while being condensed in the sub-scanning direction via the cylindrical lens 57 held by the lens barrel 58. To do. The reason why the cylindrical lens 57 collects light in the sub-scanning direction is to correct the tilting of the deflection surface. The polygon mirror 52 is rotationally driven around a rotation shaft 52a by a polygon motor (not shown), and deflects the laser beam in the main scanning direction. The scanning lens 53 includes a toroidal lens 531 and an f-θ lens 532, and irradiates the laser beam reflected from the polygon mirror 52 onto the image carrier in a focused state.

  Next, the configuration and procedure for changing the beam diameter in the light source device 51 will be described. FIG. 11 is an enlarged view of the light source device 51 in the optical system shown in FIG. In the light source device 51, first and second laser diodes 511 and 512, collimator lenses 513 and 514, beam diameter changing plates 515 and 516, a beam splitter 519, and the like are arranged on a base 510.

  The first and second laser diodes 511 and 512 output laser beams from different directions. For the first and second laser beams 501 and 502 emitted from the first and second laser diodes 511 and 512, the conditions of the intermediate optical system are set so that the diameter of the beam spot on the surface of the photosensitive drum is r. The In addition, the diameter of the first and second laser beams 501 and 502 at the intensity is D. Further, the emission positions of the first and second laser beams 501 and 502 are fixed so that the center distance in the sub-scanning direction between the two beam spots incident on the photosensitive drum surface by these laser beams is r.

  The beam splitter 519 reflects the laser beam 501 in a direction orthogonal to the incident direction, transmits the laser beam 502, and makes the optical axes of the laser beams 501 and 502 substantially parallel to each other. 57 is incident. The beam diameter changing plates 515 and 516 shield the peripheral portions of the laser beams 501 and 502 and change the diameters of the beams incident on the cylindrical lens pair 57. The beam diameter changing plates 515 and 516 are held on the rails 517 and 518 so as to be slidable in the directions of the arrows, respectively. It is moved by an actuator mechanism (not shown). The movement of the beam diameter changing plates 515 and 516 is executed by a signal from an optical system control unit (not shown).

  Further, when the charged potential of the image carrier varies with environmental changes, the sensitivity of the photosensitive member may vary and the image may deteriorate. In such a case, a high-quality image can be obtained by adjusting the exposure energy and the beam spot diameter so that the relational expression (1) is satisfied.

It is explanatory drawing of the beam profile in each beam spot diameter used for description of the analysis of the cause in which a big image is formed with respect to a beam spot diameter. It is explanatory drawing which shows an example of the latent image profile in the beam spot diameter of 16 * 16 micrometers similarly. It is explanatory drawing which shows an example of the relationship between the beam spot diameter and latent image diameter in each film thickness similarly. It is explanatory drawing which shows an example of the latent image profile which similarly generate | occur | produces image degradation. It is explanatory drawing which similarly shows an example of the relationship between a beam spot diameter and a latent image diameter. It is explanatory drawing which shows the relationship between the ratio of quantum efficiency x exposure energy / beam spot area, latent image diameter, and beam spot diameter in the present invention. 1 is a schematic configuration diagram illustrating an example of an image forming apparatus to which the present invention is applied. It is a block diagram of the control part of the image forming apparatus which concerns on other embodiment of this invention. It is a flowchart which similarly shows exposure energy adjustment processing. It is a plane explanatory view showing a configuration of an optical system of an image forming apparatus according to still another embodiment of the present invention. It is a plane explanatory view similarly used for description of a light source device. It is explanatory drawing which shows the example of an isolated 1 dot image at the time of beam spot diameter 16x16micrometer use.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Image carrier 2 ... Charging device 3 ... Optical scanning device 40 ... Photoconductor current value detection part 51 ... Light source device

Claims (5)

In an image forming apparatus comprising: an image carrier; a charging unit that charges the image carrier; and an exposure unit for forming an electrostatic latent image on the image carrier based on image information.
When the beam spot diameter for performing exposure for forming the electrostatic latent image is 50 μm or less, the quantum efficiency of the image carrier is η, the exposure energy is E [J / m ² ], and the beam spot area is S [m ² ], An image forming apparatus characterized in that the following expression (1) holds:
  The image forming apparatus according to claim 1, further comprising means for adjusting the expression (1) so that the expression (1) is satisfied according to a change in the film thickness of the image carrier that occurs with a change over time. Forming equipment.   3. The image forming apparatus according to claim 2, wherein at least one of exposure energy, beam spot diameter, and charging potential is adjusted.   2. The image forming apparatus according to claim 1, further comprising means for adjusting the expression (1) so as to satisfy a change in charging potential caused by an environmental change. 5. The image forming apparatus according to claim 4, wherein at least one of exposure energy, beam spot diameter, and charging potential is adjusted.