JP2012108198A - Focus adjusting device, image forming apparatus and focus adjusting method - Google Patents

Abstract

The focal length is accurately adjusted from the density of a test pattern image.
A focus adjusting device that adjusts the focus by adjusting the density of a test pattern image, a distance adjusting unit that adjusts a distance between an exposure unit and a photosensitive member, and density detection of the test pattern image. By increasing or decreasing the energy density of the density detection means and exposure means, the energy density changing means for changing the surface potential of the photoconductor relative to the energy density and the energy density changing means saturate the energy density and the surface potential of the photoconductor. In a state where the potential is increased to the potential to be changed, the distance adjustment unit changes the distance between the exposure unit and the photosensitive member, and the density detection unit adjusts the distance to a focal position where the density of the test pattern image is detected to be the lightest. .
[Selection] Figure 5

Description

  The present invention relates to a focus adjustment apparatus that adjusts the focus of a test pattern image by adjusting the density of the test pattern image, an image forming apparatus equipped with the focus adjustment apparatus, and a focus adjustment method.

  LED (Light Emitting Diode) array print heads used in exposure sections of digital copiers and facsimiles have a very shallow depth of focus, so that the distance between the photoreceptor surface and the LED head is within the focal length tolerance. It is necessary to manage the installation accuracy very strictly. However, the distance between the LED head and the surface of the photoconductor is changed because the thickness of the photoconductor changes with time due to wear or the like even if the accuracy of the parts is increased during production and the distance between the surface of the photoconductor and the LED head is set strictly. May change and deviate from the tolerance of the focal length (so-called out-of-focus). Therefore, it is generally known to create a test pattern image on the photoconductor or transfer belt while changing the distance between the surface of the photoconductor and the LED head and adjust the focal length based on the detection result of the test pattern image. ing.

  However, in the conventional methods for determining the quality of the focal length based on the detection result of the test pattern image formed while changing the focal length, depending on the exposure condition (exposure energy, test pattern shape) and the relationship of the charging bias, Since the tendency of the latent image is changed, there is a problem in that it is impossible to accurately determine whether the focal length is good from the surface potential, the density of the toner image, the line width, and the like.

  In order to solve this problem, Patent Document 1 aims at automatically correcting the focal length of an LED printer head in an electrophotographic apparatus using an LED printer head as an exposure means. At the time of adjusting the focal length, a predetermined test pattern is used. A focus adjustment control unit for performing image printing while continuously changing the distance between the photoconductor and the LED printer head by the automatic positioning device for adjusting the focal length, and on the photoconductor, intermediate transfer body, or print medium And a density sensor for detecting the print density of the test pattern image for adjusting the focal length, which is formed by printing while continuously changing the focal length. In the focus adjustment control unit, the density by the density sensor is provided. LED printer head automatic positioning device with the focal length of the position with the lowest density as the optimum focal length Configured to adjust the more focal length is disclosed.

  That is, Patent Document 1 employs a focus adjustment method in which the print density of a test pattern image for adjusting the focal length, which is formed while changing the focal length, is detected by a density sensor, and the focal length is adjusted based on the detected density. ing. In this method, the point at which the density of the test pattern image formed while changing the focal length is the thinnest is regarded as the optimum focal length. However, since this focus adjustment method does not take into consideration the conditions such as the exposure condition and the development bias which are other factors that determine the quality of the focal length, the defocusing always occurs when the density of the test pattern image decreases. In other words, it is impossible to accurately determine whether the focal length is good or not without considering the exposure conditions and development bias, which are other factors that determine the quality of the focal length described above. The problem is still not solved.

  Therefore, the present invention has been made in view of the above problems, and a focus adjustment device that performs focus adjustment of a test pattern image that can stably determine the quality of a focal length, and an image forming apparatus equipped with the focus adjustment device And a focus adjustment method.

  In order to solve the above problems, a focus adjustment apparatus according to the present invention is a focus adjustment apparatus that adjusts the focus of a test pattern image by adjusting the density of the test pattern image printed on the original image of the test pattern. A distance adjusting means for adjusting the distance between the exposure means and the photosensitive member; a density detecting means for detecting the density of the test pattern image; and increasing or decreasing the energy density of the exposing means to thereby increase or decrease the energy density of the photosensitive member. Energy density changing means for changing the surface potential of the photosensitive member, and the exposure by the distance adjusting means in a state where the energy density is increased by the energy density changing means until reaching a potential that saturates the surface potential of the photoreceptor. The density of the test pattern image is changed by the density detecting means by changing the distance between the means and the photosensitive member. And having a concentration adjusting means for adjusting the distance to the focal position is detected with the thinnest, the.

  In the focus adjustment apparatus according to the present invention, the density detection unit includes a density sensor that detects the density of the test pattern image on the photosensitive member, the transfer body, or the print medium, and the density sensor is in a scanning direction. And an average value of the plurality of density sensors provided is detected as the density of the test pattern image.

  Further, in the focus adjustment device according to the present invention, the resolution of the original image of the test pattern and the exposure unit is the same, and the test pattern image includes a print area having a pixel size per unit of the resolution, and the print It is a checkered pattern composed of a plurality of non-printing areas sharing the sides surrounding the four sides of the area.

  In the focus adjusting apparatus according to the present invention, the exposure means has a beam diameter larger than the pixel size, and the beam diameter takes a minimum value when adjusted to the focus position, and the print area and the non-print The distance from the region is more than the minimum beam diameter.

  An image forming apparatus according to the present invention includes the above-described focus adjustment device.

  The focus adjustment method according to the present invention is a focus adjustment method for adjusting the focus of the test pattern image by adjusting the density of the test pattern image printed on the original image of the test pattern. A step of adjusting a distance from the body, a step of detecting the density of the test pattern image, a step of changing the surface potential of the photoconductor relative to the energy density by increasing or decreasing the energy density of the exposure means, By adjusting the distance between the exposure means and the photosensitive member in a state where the energy density is increased to a potential that saturates the surface potential of the photosensitive member by changing the surface potential of the photosensitive member. The focal position at which the density of the test pattern image is detected to be the thinnest by changing the distance and performing the density detection Characterized in that it and a step of adjusting the distance.

  According to the present invention, the focal length can be accurately adjusted from the density of the test pattern image in consideration of other factors (exposure conditions, development bias, etc.) that determine the quality of the focal length.

1 is a configuration diagram illustrating an overall configuration of an image forming apparatus equipped with a focus adjustment apparatus according to an embodiment of the present invention. In the focus adjustment apparatus according to the embodiment of the present invention, it is a diagram showing the relationship between the exposure energy density and the surface potential of the photoreceptor. In the focus adjustment apparatus according to the embodiment of the present invention, (a) the relationship between the position on the photoconductor (position (coordinates) on the surface of the photoconductor) and the exposure intensity when the exposure energy density is small; b) A diagram showing the relationship between the position on the photoconductor (position (coordinates) on the surface of the photoconductor) and the surface potential (latent image potential) of the photoconductor, and (c) in the focal position and in the defocus position. It is a figure explaining the image of the test pattern image in a certain case. In the focus adjustment apparatus according to the embodiment of the present invention, (a) the relationship between the position on the photoreceptor (position (coordinates) on the photoreceptor surface) and the exposure intensity when the exposure energy density is sufficiently large; (B) A diagram showing the relationship between the position on the photoconductor (position (coordinates) on the surface of the photoconductor) and the surface potential (latent image potential) of the photoconductor, and (c) the position at the focal position and the defocus position. It is a figure explaining the image of the test pattern image in the case of. It is a flowchart explaining operation | movement of the focus adjustment method which concerns on embodiment of this invention. FIG. 5 is a diagram illustrating an example of an original image of a test pattern in the focus adjustment apparatus according to the embodiment of the present invention. In the focus adjustment apparatus according to the embodiment of the present invention, for the test pattern image when the toner is actually applied, the exposure beam diameter and the interval between the print area and the non-print area are described.

  Next, embodiments for carrying out the present invention will be described in detail with reference to the drawings. In addition, in each figure, the same code | symbol is attached | subjected to the part which is the same or it corresponds, The duplication description is simplified thru | or abbreviate | omitted suitably.

  FIG. 1 is a configuration diagram showing the overall configuration of an image forming apparatus equipped with a focus adjustment apparatus according to an embodiment of the present invention. The image forming apparatus is a known apparatus such as a copying machine, a facsimile machine, or a printer, and may be any type of image forming apparatus that can use a focus adjusting apparatus to which the present invention is applied. The image forming apparatus in the present embodiment can form a color image.

  First, the outline of the present invention will be described. The present invention detects the density of each test pattern image formed while gradually changing the distance between the writing head and the photoconductor, and determines the relationship between the writing head and the photoconductor from the relationship between the distance and the density of the test pattern image. The focus adjustment apparatus for determining the distance between the following features has the following characteristics.

  In other words, when forming a test pattern image for adjusting the focal length, the exposure beam diameter is set to a sufficiently large amount of light so as to attenuate the photoreceptor surface potential to the residual potential (saturation region).

  Hereinafter, features of the present invention will be described in detail with reference to the drawings. The image forming apparatus 100 is a general tandem color image forming apparatus, and each process cartridge 102a to 102d (for example, cyan, magenta, and yellow) for monochrome black and color (for example, cyan, magenta, yellow) is particularly necessary. If there is not, the process cartridge 102 is detachably attached to the image forming apparatus main body 100, and the exposure apparatuses 103a to 103d (unless there is no special need) in the image forming apparatus main body 100. , Exposure device 103), transfer rollers 101a to 101d (hereinafter referred to as transfer roller 101 unless otherwise required), paper feed tray 104, fixing device 106, and the like.

  The process cartridge 102 is set at a predetermined position of the image forming apparatus main body 100, and the photoconductors 108a to 108d (hereinafter referred to as the photoconductor 108 unless otherwise required) are cylindrical photoconductor drums, and have a linear velocity. Rotating at 50 mm / s. Roller-shaped chargers 110a to 110d as charging means (hereinafter referred to as charger 110 if not particularly required) are pressed against the surface of the photoreceptor 108, and are driven to rotate by the rotation of the photoreceptor 1008. In addition, the photoconductor 108 is uniformly charged to a substantially uniform surface potential by applying a direct current or a bias in which an alternating current is superimposed on the direct current from a high voltage power source (not shown).

  Subsequently, the photoconductor 108 is exposed to image information by an exposure device 103 which is a latent image forming unit, and an electrostatic latent image is formed. The developing units 111a to 111d (hereinafter referred to as the developing unit 111 unless otherwise required) are one-component contact developing units, and the electrostatic latent image of the photoconductor 1008 is generated by a predetermined developing bias supplied from a high voltage power source (not shown). The image is visualized as a toner image. The developing unit 111 initially stores one-component toner.

  Four process cartridges 102 are arranged in parallel. When a full-color image is formed, a visible image is formed in the order of black, cyan, magenta, and yellow, and a visible image of each color is disposed so as to face the photoconductor 108. By applying a predetermined transfer bias supplied from an unillustrated high-voltage power supply for each color (not shown) to the roller 101, a transfer electric field is formed, and the toner image on the photoconductor 108 is sequentially transferred to a sheet as a transfer material, thereby being full color. An image is formed, and thereafter, the image is fixed on paper by heat and pressure by the fixing device 106 and then discharged.

  Further, the untransferred toner remaining on the photoconductor 108 after the transfer is collected and discarded by the cleaning units 127a to 127d (hereinafter, referred to as the cleaning unit 127 unless otherwise required). There is also a cleanerless system in which the developing unit 111 collects the transfer residual toner without providing the cleaning unit 127 on the photosensitive member 108, and various known cleaning units can be employed in the present invention.

  A sheet as a transfer material is held on the paper feed tray 104 and is conveyed to the registration roller pair 107 by the paper feed roller 105, and is synchronized with the timing at which the front end of the toner image on the photoconductor 108 reaches the transfer portion. Paper is fed and enters the transfer section.

  The transfer belt 120 is stretched by a drive roller 122, a transfer roller 101, and a tension roller 121, and is rotationally driven via the drive roller 122 by a drive motor (not shown). Further, the belt tension is applied by springs on both sides of the tension roller 121. The tension roller 121 is a restricting member that restricts meandering of the transfer belt 120 by inserting flanges (not shown) at both ends.

  Each roller that stretches the transfer belt 120 is supported from both sides of the transfer belt 120 by a transfer belt unit side plate (not shown). Further, the photosensitive member 108 and the transfer belt 120 can be separated at an arbitrary timing by a contact / separation mechanism (not shown).

  A transfer belt cleaning blade 123 performs cleaning by scraping off the toner on the transfer belt 120. The transfer residual toner thus scraped off is stored in the transfer toner waste toner storage section through a toner conveyance path (not shown).

  Reference numerals 126a to 126d denote toner density detecting means for detecting the toner amount of the toner patch formed on the photoreceptor 108 (hereinafter, referred to as toner density detecting means 126 unless otherwise required). The toner density detecting means 126 is an optical sensor composed of a regular reflection type sensor and a diffuse reflection type sensor. Although FIG. 1 shows the case where the optical sensor is formed on a photoconductor, it can also be provided on a transfer body or a print medium (paper).

  The optical sensor is a test detected by a plurality of optical sensors even when the focal length varies in the main scanning direction due to the undulation of the exposure apparatus 103 or the film thickness unevenness of the photoconductor 108. By calculating the average value of the density of the pattern image, these variations are absorbed.

  Although a tandem color image forming apparatus has been described as an example here, the present invention can also be applied to a 4-cycle color image forming apparatus or a monochrome image forming apparatus. Of course, a developing method using two-component toner may be adopted.

  The exposure apparatus 103 is a device (hereinafter also referred to as an LED head) in which a LED element having a desired number of pixels (image width × pixel density), a drive circuit thereof, and a lens that collects light emitted from the LED element are assembled in a casing. In the exposure apparatus, each LED element emits light according to an image signal to form a latent image on the photosensitive member 108. The lens has a large numerical aperture and a short focal length in order to efficiently secure the amount of emitted light. For this reason, the LED head is installed on the photosensitive member 108 at a distance of several mm from the photosensitive member.

  In addition, an array of a plurality of lenses is incorporated in the housing as a lens array. A shape (hole, protrusion, plane, etc.) for mounting the LED head is formed in the housing. A harness for supplying an image signal in accordance with the power source and image data is coupled to the LED head. The distance between the LED head and the photoreceptor 108 can be changed within a predetermined range by a focus adjustment mechanism (not shown).

  Next, regarding the relationship between the exposure energy density, which is an important parameter, and the surface potential of the photoreceptor, in determining various conditions such as exposure conditions and development bias, which are factors that determine the quality of the focal length, which is the point of the present invention explain. FIG. 2 is a diagram showing the relationship between the exposure energy density and the surface potential of the photoreceptor in the focus adjustment apparatus according to the embodiment of the present invention.

  According to FIG. 2, the surface potential of the photoconductor attenuates as the value of the exposure energy density, which is the exposure energy per unit area of the exposure apparatus, is increased. When the exposure energy density becomes a sufficiently large value with a certain value as a boundary, even if the exposure energy density value is further increased, the surface potential of the photosensitive member is not further attenuated and takes a constant value. The surface potential of the photosensitive member at this time is defined as a residual potential Vr.

  In FIG. 2, in a region where the surface potential of the photoconductor does not decay to the residual potential Vr, that is, a region where the value of the exposure energy density is small, the value of the latent image potential which is the surface potential of the photoconductor is large due to the charging bias before exposure. fluctuate. In the following description, a region where the value of the surface potential (latent image potential) of the photoconductor varies with respect to the value of the exposure energy density is an “unsaturated region”, and has no variation (attenuates to the residual potential Vr. The region is called “saturated region”.

  First, in a non-saturated region, that is, a region where the value of the surface potential (latent image potential) of the photoconductor varies with respect to the value of the exposure energy density, pay attention to one LED element. Think. Here, FIG. 3 shows (a) a position on the photoconductor (position (coordinates) on the surface of the photoconductor) and exposure intensity when the exposure energy density is small in the focus adjusting apparatus according to the embodiment of the present invention. FIG. 5B is a diagram showing the relationship between the position on the photoreceptor (position (coordinates) on the photoreceptor surface) and the surface potential (latent image potential) of the photoreceptor, and FIG. It is a figure explaining the image of the test pattern image in the case where it exists in a case, and when it exists in a focus shift position.

  Referring to FIG. 3A, when the light emission amount of the LED as the light source is constant, when the distance between the LED head and the photoconductor 108 is deviated from the focal length (hereinafter, “defocus”). The waveform of the exposure intensity distribution gently spreads compared with the case where the focal point is in focus (hereinafter referred to as “focal point position”), so that the beam diameter emitted from the LED becomes larger. However, the exposure intensity has a maximum value at the focal position.

  When the exposure energy density per LED element is in a non-saturated region, as shown in FIG. 3B, the surface potential (latent image potential) of the photoreceptor is a waveform of the exposure intensity distribution in FIG. The shape looks like a traced shape. That is, in the case of the focal position, the potential attenuation is larger than in the case of defocusing, but the waveform shape is sharp. When the entire latent image potential is averaged, the accumulated value of the latent image potential is almost the same in the case of the focal position and in the case of the defocus. It is difficult to distinguish between the two.

  Next, referring to FIG. 3C, when the amount of toner adhesion in the test pattern image when the exposure energy density is in the non-saturated region is compared, in the case of the focal position, the toner adheres to a small area in a dark area. In the case of defocusing, the toner adheres relatively lightly over a wide area. In this case, whether the toner is reproduced darkly in the test pattern image depends on parameters that may fluctuate such as exposure conditions and development bias. It is difficult to determine which of these cases.

  Next, in a saturated region, that is, a region where the surface potential (latent image potential) value of the photoconductor does not vary with respect to the value of the exposure energy density, attention is paid to a certain LED element with reference to FIG. Think about it. Here, FIG. 4 shows (a) the position on the photosensitive member (position (coordinate) on the surface of the photosensitive member) and the exposure intensity when the exposure energy density is large in the focus adjusting apparatus according to the embodiment of the present invention. FIG. 5B is a diagram showing the relationship between the position on the photoreceptor (position (coordinates) on the photoreceptor surface) and the surface potential (latent image potential) of the photoreceptor, and FIG. It is a figure explaining the image of the test pattern image in the case where it exists in a case, and when it exists in a focus shift position.

  When the exposure energy density per LED element is in the saturated region, as shown in FIG. 4A, the exposure intensity distribution is sharp at the focal position and defocused as in the case where the exposure energy is in the unsaturated region. Then, it becomes a gentle waveform shape, and the exposure intensity becomes the maximum value at the focal position.

  However, when the exposure energy density per LED element is in the saturation region, the surface potential (latent image potential) of the photoconductor is sufficiently large as shown in FIG. The waveform shape of the latent image potential distribution in the case of position and in the case of defocus is not affected by the exposure intensity, and a difference in the exposure beam diameter appears remarkably between them.

  Therefore, referring to FIG. 4C, when the amount of toner adhesion in the test pattern image when the exposure energy density is in the saturation region is compared, in the case of the focal position, that is, when the exposure beam diameter is minimized. In the test pattern image, toner adhesion is minimized, and in the case of defocusing, that is, as the exposure beam diameter increases, the toner adhesion in the test pattern image increases. It is possible to judge.

  Next, a focus adjustment method using the focus adjustment device will be specifically described. FIG. 5 is a flowchart for explaining the operation of the focus adjustment method according to the embodiment of the present invention.

  First, in step (hereinafter referred to as “S”) 50, when the focus position adjustment is started, the energy density of the exposure unit is increased in S51. In S52, it is determined whether or not the surface potential (latent image potential) of the photoreceptor has reached the saturation region.

  In S52, if it is determined that the surface potential (latent image potential) of the photoreceptor has reached the saturation region (S52: Y), the process proceeds to S53, and it has not reached the saturation region (it is still a non-saturation region). If it is determined (S52: N), the process proceeds to S51, and the exposure energy density of the exposure means is increased again.

  In S53, the distance between the exposure means and the photoconductor is adjusted, and in S54, the toner density of the test pattern image is detected by the optical sensor. Then, in S55, it is determined whether or not the toner density of the test pattern image has become the lightest.

  If it is determined in S55 that the toner density of the test pattern image has become the lightest (S55: Y), the process proceeds to S56, and the focus position adjustment is completed. If it is determined in S55 that the toner density of the test pattern image has not reached the minimum density (S55: N), the process returns to S53, the distance between the exposure means and the photosensitive member is adjusted again, and in S54 the optical density is adjusted. The sensor detects the toner density of the test pattern image.

  Next, an example of the original image of the test pattern used in the focus adjustment apparatus of the present invention will be described. 6A to 6C are diagrams illustrating examples of original images of test patterns in the focus adjustment apparatus according to the embodiment of the present invention. In using the focus adjustment apparatus of the present invention, the resolution of the original image of the test pattern and the lens array on which the LED head is mounted must be the same. Then, as shown in FIG. 6, the original image of the test pattern is composed of a 1 pixel size printing area of this resolution and a plurality of non-printing areas sharing sides surrounding the 1 pixel size printing area. It is a checkered pattern.

  Next, a specific test pattern image when the toner is actually applied to the original test pattern image used in the focus adjustment apparatus of the present invention will be described. FIG. 7 is a diagram for explaining the exposure beam diameter and the interval between the print area and the non-print area for the test pattern image when the toner is actually applied in the focus adjustment apparatus according to the embodiment of the present invention. . When actually creating a test pattern image, the exposure beam diameter may be set to be larger than the length of one side defined by the resolution of the lens array on which the LED head is mounted. Therefore, when dots for one pixel are formed with a strong exposure energy density as in the embodiment of the present invention, the dot diameter becomes larger than the original one pixel. Therefore, for example, in a pattern in which a print area and a non-print area are repeated every other pixel as in the original image of the test pattern shown in the left diagram of FIG. 7B, the test pattern image is crushed even at the focus position. End up.

  7A and 7B, the left figure is the original image of the test pattern, the right figure is the test pattern image when the toner is actually applied, and the exposure beam diameter is the length of one side of one pixel. The case where it is almost twice this is shown.

  The left figure in FIG. 7A is an original image of the test pattern in which the interval between the print areas is equal to or larger than the exposure beam diameter, and FIG. 7B is one pixel (less than the exposure beam diameter) in the print area. It is an original image of a test pattern. Since the test pattern image is formed in a size corresponding to the exposure beam diameter, toner is applied to the end of the print area about half the difference between the exposure beam diameter and the length of one side of one pixel.

  That is, the non-printing area surrounded by the printing area is narrowed by applying toner by an amount corresponding to the difference between the exposure beam diameter and the length of one side of one pixel. In order to leave at least one pixel of non-printing area as a test pattern image, the width of the non-printing area needs to be secured from the beginning by the exposure beam diameter.

  In the example of FIG. 7, the non-printing area is narrowed by just one pixel by toner application. Since the non-printing area of the original image of the test pattern in FIG. 7A on the left has a width of two pixels, even if the non-printing area is narrowed by one pixel due to the toner application, the remaining one pixel is non-printing. Remains as a print area, and monochrome lines can be resolved. In the case of FIG. 7B, since the non-printing area of the original image of the test pattern is originally only one pixel wide as shown in the left figure, the non-printing area is crushed by the application of toner, and the printing area and the non-printing area. Is difficult to distinguish.

  The present invention has been described above by the preferred embodiments of the present invention. While the invention has been described with reference to specific embodiments thereof, various modifications and changes can be made to these embodiments without departing from the broader spirit and scope of the invention as defined in the claims. is there.

DESCRIPTION OF SYMBOLS 100 Image forming apparatus 101 Transfer roller 103 Exposure device 104 Paper feed tray 105 Paper feed roller 106 Fixing device 107 Registration roller pair 108 Photoreceptor 110 Charger 111 Developing means 120 Transfer belt 121 Tension roller 122 Drive roller 123 Cleaning blade 126 Toner density detection Means 127 Cleaning means

JP 2004-306454 A

Claims (6)

A focus adjusting device that adjusts the focus of the test pattern image by adjusting the density of the test pattern image printed on the original image of the test pattern,
Distance adjusting means for adjusting the distance between the exposure means and the photosensitive member;
Density detecting means for detecting the density of the test pattern image;
Energy density changing means for changing the surface potential of the photoreceptor relative to the energy density by increasing or decreasing the energy density of the exposure means;
In the state where the energy density is increased by the energy density changing means until reaching a potential that saturates the surface potential of the photoreceptor, the distance adjusting means changes the distance between the exposure means and the photoreceptor, A focus adjustment apparatus comprising: density adjustment means for adjusting the distance to a focus position at which the density of the test pattern image is detected as being the lightest by the density detection means.   The density detection means includes a density sensor that detects the density of the test pattern image on the photoconductor, transfer body, or print medium, and a plurality of density sensors are provided in the scanning direction. The focus adjustment apparatus according to claim 1, wherein an average value of the density sensor is detected as a density of the test pattern image.   The original image of the test pattern and the exposure unit have the same resolution, and the original image of the test pattern shares a print area that is a pixel size per unit of the resolution and a side that surrounds the print area. The focus adjustment device according to claim 1, wherein the focus adjustment device is a checkered pattern composed of a plurality of non-printing areas.   The exposure means has a beam diameter larger than the pixel size, and when adjusted to the focal position, the beam diameter takes a minimum value, and the interval between the print area and the non-print area is the minimum value. The focus adjustment apparatus according to claim 1, wherein the focus adjustment apparatus is spaced apart by a beam diameter or more.   An image forming apparatus comprising the focus adjusting apparatus according to claim 1. A focus adjustment method for adjusting the focus of the test pattern image by adjusting the density of the test pattern image printed on the original image of the test pattern,
Adjusting the distance between the exposure means and the photoreceptor;
Performing density detection of the test pattern image;
Changing the surface potential of the photoreceptor relative to the energy density by increasing or decreasing the energy density of the exposure means;
By adjusting the distance between the exposure means and the photosensitive member in a state where the energy density is increased to a potential that saturates the surface potential of the photosensitive member by changing the surface potential of the photosensitive member. And a step of adjusting the distance to a focal position where the density of the test pattern image is detected to be the thinnest by changing the distance and performing the density detection.

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