JP5476727B2 - Image evaluation method, image evaluation apparatus, and image forming apparatus - Google Patents

Description

  The present invention relates to a technique for measuring an electrostatic latent image in an electrophotographic image forming apparatus such as a copying machine, a printer, a facsimile machine, or a combination of these, and more particularly to the reproducibility of an electrostatic latent image in consideration of a spatial frequency. The present invention relates to an image evaluation method and an image evaluation apparatus for evaluation. The present invention also relates to an image forming apparatus capable of improving the image quality of electrophotography by the image evaluation method and the image evaluation apparatus.

  Input of original data relating to an image in an image forming apparatus or device (especially a digital copying machine as a copying machine or a laser printer as a printer) that applies electrophotographic technology such as a copying machine, a printer, a facsimile machine, or a combination of these. One of the processes of image formation up to final paper output is formation of an electrostatic latent image. An electrostatic latent image is obtained by forming characters, images, and the like made up of small dots on a charged photoreceptor using, for example, a scanning optical system. This electrostatic latent image cannot be visually observed unless the toner is attached, and it is difficult to accurately measure the electrostatic latent image using a general-purpose measuring device.

  A technique for measuring or visualizing this electrostatic latent image is disclosed in Japanese Patent Application Laid-Open No. 2003-295696. In this case, in the case where charged particles (for example, electrons) are used for detection of an electrostatic latent image on a charged photoconductor, the size of the electrostatic latent image can be measured with a high accuracy of μm level by this technique.

In addition, the developing process attaches the toner to the electrostatic latent image on the photoconductor. For example, in Patent Document 2 (Japanese Patent Laid-Open No. 2002-304025), a set of toners after development on the photoconductor Using the average equivalent circle diameter and standard deviation of the body,
CV1 value = standard deviation / average equivalent circle diameter is defined. And
0 <CV1 <179.01 × (average circle equivalent diameter of toner aggregate) ^−1.9031
Has been disclosed.

The above prior art is similar to the present invention described later in that the dot diameter variation is defined by a value obtained by dividing the standard deviation by the average equivalent circle diameter.
However, the conventional technology is a developing device, but the present invention described later is an evaluation method and an evaluation device. In addition, an electrostatic latent image and a toner aggregate (image) after development are different. There is a difference.

Hereinafter, problems to be solved by the present invention will be described.
Characters, images, and the like composed of toner finally fixed on paper through an electrophotographic process are called toner images. Here, a toner image is used in contrast to the above-described electrostatic latent image. There are several items such as shape and density in the evaluation of the toner image, and repetitive evaluation is the evaluation of reproducibility. The reproducibility is whether or not the same toner image is formed when the input values of characters and images are the same and the toner image is repeatedly formed. Here, the input value refers to electronic information when characters and images are formed using, for example, a personal computer (PC). Strictly speaking, there is almost no possibility that the same toner image is formed. This is because the electrophotographic process is not always the same. For example, the toner is fine particles of several μm, has a particle size distribution (different sizes), and has a different shape. Therefore, microscopically, the toner used every time is not the same. Furthermore, there are variations in the formation of the electrostatic latent image, which causes various factors such as variations in the amount of toner attached to the electrostatic latent image. Therefore, for example, the same toner image is expressed with a standard deviation. In contrast, the input values are always the same.

  The average and standard deviation of the measured values obtained by repeatedly forming and measuring the toner image are obtained, and the reproducibility is judged by how much the measured value deviates from that. . Ultimately, it is sufficient if the difference is not visible to the human eye, but in order to manage quality, reproducibility must be grasped numerically. Here, reproducibility is used synonymously with variation.

With respect to the toner image, since the electrostatic latent image does not involve the toner, reproducibility is relatively easy to grasp except for the versatility of the measuring device to be used. In general, the reproducibility of a toner image is inferior to the reproducibility of an electrostatic latent image because of the complexity of toner shape and size and the phenomenon of toner adhesion.
However, the toner image can be easily evaluated using an optical microscope. For example, if image capture and image processing are programmed in advance, automatic measurement is possible under computer control. In addition, repeated measurement is easy.

On the other hand, the electrostatic latent image cannot be observed unless the apparatus described in Patent Document 1 is used. Furthermore, regarding the electrostatic latent image, there is no example in which reproducibility is repeatedly evaluated at present.
This reproducibility also depends on the spatial frequencies that make up characters and images. For electrophotography, it is preferable that high reproducibility can be ensured regardless of the spatial frequency of the image. In order to improve the image quality, it is necessary to know the reproducibility in consideration of the spatial frequency of an image in each process of electrophotography, and to find the cause if deterioration occurs. At this time, this evaluation in the electrostatic latent image is useful.
At present, there is no evaluation method for the reproducibility of the electrostatic latent image including the spatial frequency, and the present invention described later has been reached.

The toner image is formed on the entire surface of the paper. At this time, even if a toner image having the same input value is formed on the entire surface of the paper, the toner image is not the same depending on the location. In addition to the above reproducibility, this also includes the uniformity of the photoreceptor. Evaluation of the electrostatic latent image is also effective in viewing the uniformity of the photoreceptor.
However, there is currently no method for evaluating the reproducibility of the electrostatic latent image for each location of the photoconductor in consideration of the spatial frequency.

  The size of the electrostatic latent image is, for example, an area. However, the spatial frequency evaluated here has a unit of reciprocal length. For this reason, the size of the electrostatic latent image is also easily compared with the spatial frequency when expressed in units of length.

Regarding the method of modulating the spatial frequency of the electrostatic latent image, it is preferable that it can be finely modulated.
In addition, regarding the method for modulating the spatial frequency of the electrostatic latent image, it is preferable that fine modulation is possible even in a low spatial frequency (large dot) region.

The present invention embodies an evaluation apparatus that performs the above evaluation.
In the present invention, the evaluation result regarding the reproducibility of the electrostatic latent image including the spatial frequency is reflected in the image formation to improve the image quality, and an image forming apparatus having this adjusting means is provided. It is.

The problems to be solved by the present invention are summarized below.
A first object of the present invention is to provide an image evaluation method relating to reproducibility of an electrostatic latent image in consideration of a spatial frequency.
The second problem is to provide an image evaluation method relating to the reproducibility of an electrostatic latent image, including the uniformity of a photoreceptor.
A third problem is to provide an image evaluation method for reproducibility of an electrostatic latent image that appropriately represents a spatial frequency.
A fourth problem is to provide an image evaluation method for an electrostatic latent image in which the spatial frequency of the electrostatic latent image can be easily modulated.
A fifth problem is to provide an image evaluation method for an electrostatic latent image that is easy to modulate the spatial frequency of the electrostatic latent image in a region having a lower spatial frequency.
The sixth problem is to provide an image evaluation apparatus for evaluating the reproducibility of an electrostatic latent image.
The seventh problem is to provide an image forming apparatus having means for reflecting an evaluation result of electrostatic latent image reproducibility.

In order to solve the above problems, the present invention employs the following solutions.
According to a first aspect of the present invention, there is provided an image evaluation method in which an electrostatic latent image is formed on a charged photosensitive member by light irradiation, and the electrostatic latent image is measured with charged particles to perform image evaluation. Forming a plurality of different electrostatic latent images with different spatial frequencies in order to define reproducibility of the electrostatic latent image with respect to the size of the electrostatic latent image at any point on the body; The measurement step for measuring the electrostatic latent image formed by the forming step and the charge eliminating step for eliminating charges on the photosensitive member are repeated, and the reproducibility of the electrostatic latent image is obtained for each spatial frequency. (Claim 1).

The second solving means of the present invention is characterized in that, in the image evaluation method of the first solving means, the image evaluation is performed at a plurality of points on the photoconductor.
The third solving means of the present invention is the image evaluation method of the first or second solving means, wherein the size of the electrostatic latent image is obtained by dividing the area of the electrostatic latent image by the circumference ratio. It is defined by a square root (claim 3).
Further, the fourth solving means of the present invention is the image evaluation method according to any one of the first to third solving means, wherein the spatial frequency of the electrostatic latent image is changed according to the intensity of the light irradiation. (Claim 4).
Further, according to a fifth solving means of the present invention, in the image evaluation method according to any one of the first to third solving means, the spatial frequency of the electrostatic latent image is obtained by making the light irradiation adjacent to each other a plurality of times. (5).

  A sixth solving means of the present invention is an apparatus for performing image evaluation using the image evaluation method of any one of the first to fifth solving means, and at least charged particles for charging the photoreceptor. Generating means, means for irradiating light on the photoconductor, means for observing the electrostatic latent image formed on the photoconductor, and means for determining reproducibility of the electrostatic latent image (Claim 6).

  According to a seventh aspect of the present invention, there is provided an image forming apparatus, the static image obtained by the image evaluation method of any one of the first to fifth solving means or the image evaluation apparatus of the sixth solving means. It has means for reflecting the reproducibility of the electrostatic latent image on the image formation (claim 7).

According to the image evaluation method of the first solving means, the reproducibility of electrostatic latent images having different spatial frequencies can be evaluated.
According to the image evaluation method of the second solving means, electrostatic latent images can be formed at different positions on the photoconductor. In addition to the formation of electrostatic latent images at different locations on the photoconductor, repeated measurements taking into account the spatial frequency of the electrostatic latent image can be used to reduce the spatial frequency at different positions on the photoconductor. It is possible to evaluate reproducibility in consideration.
According to the image evaluation method of the third solving means, it is possible to provide a method for evaluating the reproducibility of the electrostatic latent image that appropriately represents the spatial frequency by defining the equivalent circle diameter and using this.
According to the image evaluation method of the fourth solving means, it is possible to provide an electrostatic latent image evaluation method in which the spatial frequency of the electrostatic latent image can be easily modulated by changing the intensity of irradiation light.
According to the image evaluation method of the fifth solving means, by forming a plurality of dots adjacent to each other, the spatial frequency modulation of the electrostatic latent image can be easily performed in a region having a lower spatial frequency. A method can be provided.

  According to the image evaluation apparatus of the sixth solving means, it is possible to provide an evaluation apparatus capable of evaluating including the reproducibility of the electrostatic latent image in consideration of the spatial frequency and the variation of the location on the photosensitive member in addition to this. .

  According to the image forming apparatus of the seventh solving means, there is means for reflecting the evaluation result of the electrostatic latent image reproducibility, and the evaluation result regarding the reproducibility of the electrostatic latent image including the spatial frequency is reflected in the image formation. In addition, it is possible to provide an image forming apparatus capable of achieving high image quality.

1 is a schematic configuration diagram of an image forming apparatus showing an embodiment of the present invention. FIG. 2 is a diagram illustrating an example of a photoconductor according to the present invention, and is a diagram schematically illustrating a layer configuration of the photoconductor and a state of charging and light irradiation. It is a schematic perspective view which shows an example of a scanning optical system. It is a figure for demonstrating the reproducibility of an electrostatic latent image. It is a figure for demonstrating the reproducibility of an electrostatic latent image. It is a figure which shows typically the dot of three different spatial frequencies. It is a figure for demonstrating the evaluation method of the reproducibility in a different place of a drum-shaped photoconductor. It is explanatory drawing of the evaluation method at the time of using the small piece of a photoreceptor as a sample. It is explanatory drawing of a circle equivalent diameter in case an electrostatic latent image is elliptical. It is a figure which shows an example of the relationship between the spatial frequency of the circle equivalent diameter of an electrostatic latent image, and electrostatic latent image reproducibility. It is a figure which shows an example of the relationship between the output of LD, and a circle equivalent diameter. It is a figure which shows an example of the relationship between the output of LD, and a spatial frequency. It is explanatory drawing in the case of making a dot with the low spatial frequency of an electrostatic latent image adjacent to a some dot. It is a figure which shows typically the structural example of the image evaluation apparatus which concerns on this invention. It is a figure which shows an example of the relationship between the spatial frequency of three types of photoconductors, and reproducibility. It is a figure which shows an example of the relationship between LD output setting value and the exposure energy density on a photoconductor.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

[Explanation of electrophotography and electrophotographic process]
First, an outline of a general electrophotographic process will be described.
The electrophotographic process includes the following (1) charging, (2) exposure, (3) development, (4) transfer, (5) cleaning, and (6) fixing.
(1) Development: The surface of a photoconductor as a photoconductor is charged using a charger in a dark place.
(2) Exposure: The charged photosensitive member is irradiated with light, and the charge of the portion irradiated with the light is removed, thereby forming an electrostatic latent image. Currently, negatives are the mainstream in digital equipment.
(3) Development: Toner that is fine particles charged with a polarity opposite to that of the electrostatic latent image is electrostatically attached to the electrostatic latent image.
(4) The paper as a recording medium is superimposed on the developed toner image, and a charge on the back side of the paper is applied to the paper with a charge opposite to the charged polarity of the toner, and the toner is transferred to the paper by electrostatic force. .
(5) Cleaning: Residual toner remaining on the photoconductor without being transferred is removed using a cleaner such as a blade or a magnetic brush.
(6) Fixing: The toner adhering to the transfer paper is sent to a heat roller fixing machine and fixed by heat and pressure.

(Description of image forming apparatus)
FIG. 1 shows an example of the configuration of an image forming apparatus provided with means for performing the processes (1) to (6).
1, this image forming apparatus includes a photoconductor (OPC) 71, a charging charger 72, exposure (exposure means) 73, toner (developing means) 74, a recording medium (paper such as transfer paper) 75, a transfer charger 76, A fixing roller 77, a cleaning blade 78, and a static elimination light source 79 are provided. Corresponding to the above process, (1) charging of the photoreceptor 71 is performed by the charging charger 72, (2) exposure 73 is performed by exposure means (not shown) (for example, a scanning optical system described later), and (3) development. Is performed by electrostatically attaching the toner 74 of the developing means to the photosensitive member 71. (4) The transfer is performed by transferring the toner on the photosensitive member to the paper 75 by the transfer charger 76. (5) Cleaning is performed by removing the toner remaining on the photosensitive member by the cleaning blade 78, and (6) fixing is performed by fixing the toner on the paper 75 by the fixing roller 77. Further, light remaining from the light source 79 for charge removal is discharged to remove the charge remaining on the photoreceptor 71.

(Description of toner)
There are several methods for the development of the electrophotographic process (3), and toner / developer is classified based on the methods. First, there are a dry type and a wet type, and the dry type is the mainstream in terms of high speed. The dry type includes a one-component developer and a two-component developer. One-component developers include non-magnetic toner and magnetic toner. The two-component developer includes a toner and a carrier. This toner includes non-magnetic toner and magnetic toner. The toner is mostly composed of a polymer and contains a colorant and the like. The size is from several μm to a dozen μm. The carrier is a metallic material such as iron powder and has a size of several tens to several hundreds of μm.

(Explanation of photoconductor)
The photoconductor is a photoconductive material. At present, organic electrophotographic photoreceptors (OPCs) are mainly used from the viewpoint of low cost and easy processability. In general, OPC is made of a layer structure. An intermediate layer is provided on a conductive support (conductive layer), a charge generation layer (CGL) is stacked thereon, and a charge transport layer (CTL: Charge Transfer Layer) is further stacked. . This configuration (normal layer configuration) is a negative charging system. In addition, the structure in which the order of stacking CGL and CTL is reversed is positively charged. There is also a single-layer photoreceptor using a material in which charge generation and charge transport are mixed. The intermediate layer is provided to prevent charge leakage. FIG. 2 schematically shows the layer structure, charging, and light irradiation. For neutralization, a white light source or a light emitting diode (LED) is used.

(Description of FIG. 2)
In FIG. 2, which is a case of a normal layer configuration, the photoreceptor (OPC) is configured by laminating a conductive layer (conductive support) 121, a charge generation layer (CGL) 122, and a charge transport layer (CTL) 123, In the figure, an electron (negative charge) 126, a hole (positive charge) 127, an irradiation light 124, and a movement direction 125 of the hole in the charge transport layer are shown.

  When light is applied to the photosensitive member (OPC) whose surface is charged, the CGL 122 absorbs the light and generates positive and negative charges 126 and 127. As for the positive and negative charges, one charge (for example, positive charge (hole) 127) is injected into the CTL 123 and the other charge (for example, negative charge (electron) 126) is injected into the conductive support 121 by the electric field on the surface. The charge (for example, positive charge) injected into the CTL 123 passes through the CTL 123 and reaches the surface of the photoreceptor, and cancels the charge (for example, negative charge) on the surface. Therefore, in electrophotography, this principle is used, and the irradiated light is scanned two-dimensionally to form character and image patterns on the photoreceptor. According to this pattern, the above-described charge generation and movement occurs, and the surface charge is canceled. This is an electrostatic latent image. Generally, the thickness of the CGL 122 is about sub μm. The CTL 123 is several tens of μm. The electrostatic latent image is a character or an image, but the minimum unit is a dot.

(Description of recording medium)
In the above electrophotographic process (6), the toner is mainly fixed on a fixed-size transfer paper, a cardboard, a postcard or the like, but there are also polymer materials such as an overhead projector (OHP) sheet as a recording medium.

(Explanation of exposure means)
An example of a scanning optical system which is one of the exposure means is shown in FIG. The scanning optical system shown in FIG. 3 includes a light source 111, a collimating lens 112, a cylindrical lens 113, a first mirror 114, an optical deflector (eg, a polygon mirror) 115, a first scanning lens 116, and a second scanning lens. 117, the second mirror 118, and the like are provided, but this is an example, and the configuration and arrangement are not limited thereto. The polygon mirror 115 rotates at a high speed of several tens of thousands rpm (round per minute) around the rotation axis 1151 and scans the light incident on the polygon mirror 115. This scanning direction is called the main scanning direction. This is the longitudinal direction of the photoconductor 119. Further, the photoconductor 119 rotates around the rotation shaft 1191. This direction is called the sub-scanning direction. An electrostatic latent image formed on the photosensitive member by light irradiation is, for example, 1192. Here, one dot of the electrostatic latent image is exaggerated and drawn greatly. Further, the photosensitive member itself may not be included in the scanning optical system. In addition, an opening 120 is provided immediately before the light source 111 for adjusting the beam diameter. The shape of the aperture 120 includes a square, a circle, a rectangle, and the like. In the figure, it is a rectangle.

  As the light source 111, a laser diode (LD) is used. In recent years, light emitting diodes (LEDs) and vertical surface emitting lasers (VCSELs) have also been used. Note that optical scanning can be divided using several LDs. If VCSEL is used, the optical scanning can be divided into several tens, which is effective for increasing the resolution of an image.

Also, the size of the dots on the photoreceptor is determined by the desired resolution. The higher the resolution, the smaller the dot size. This may require high specifications not only for the light source 111 but also for the optical system. The factor that determines the size of the dot is the size of the aperture 120. Although depending on the performance of the optical system, assuming that the aberration is sufficiently corrected, if the size of the aperture 120 is reduced, the dot becomes larger. Conversely, if the size of the aperture 120 is increased, the dot becomes smaller.
For example, an LD is used as the light source 111 and the wavelength λ is λ = 655 nm.

  The charged potential of the photoconductor 119 is about −600 V to −1000 V, and the photoconductor 119 may be one mounted on a commercially available laser printer, digital copier, or the like. Although FIG. 3 shows a cylindrical photoconductor, it may be a plate, sheet, or endless belt.

(Explanation of the first solution)
First, the definition of reproducibility of an electrostatic latent image will be described.
FIG. 4A shows a part 1 of the charged photoconductor, the writing light 2, and the electrostatic latent image 3 of the dots formed. This corresponds to the drawing of only a part of the periphery of the photosensitive member 119 of the scanning optical system shown in FIG.

First, an electrostatic latent image 3 of dots is formed by irradiating an arbitrary point on a charged photoconductor to form an electrostatic latent image 3 of dots. This process is repeated as many times as desired. If it is repeated n times (n is a positive integer), an electrostatic latent image of a series of dots is obtained as shown in FIG. The horizontal arrangement represents repetition. As shown in FIG. 4B, the electrostatic latent image of the dot has a different shape and size each time it is repeated (may be referred to as dot variation). However, in FIG. 4B, the drawing is very exaggerated.
The difference in the shape and size of the dots is due to all of the scanning optical system, the photoconductor, and the measurement system.
With respect to the size of the electrostatic latent image of dots, an average value and a standard deviation are obtained for the number of repetitions n. Here, the value obtained by dividing the standard deviation by the average value is defined as the reproducibility of the electrostatic latent image.
That is,
Reproducibility of electrostatic latent image = standard deviation / average value (Equation 1)
It is.

  When the size of the electrostatic latent image 3 of dots shown in FIG. 4A changes every measurement, and the change is large, the value of (Equation 1) becomes large. Conversely, if the size of the electrostatic latent image does not change even after repeated measurements, this value becomes small. If the reproducibility is good, this value is small, and if the reproducibility is bad, this value is large.

The spatial frequency is generally defined by a periodic pattern such as a line and pair or a cycle. In order to define with dots, dots are formed one by one as shown in FIG. 5, and “present” and “absent” of dots are defined as one cycle (in FIG. 5, the horizontal arrangement of dots is a space). (This is not a repetition of FIG. 4). When the size of one dot in the scanning direction is 600 dpi (dpi: dot per inch), this one cycle is 300 dpi. 300 dpi is 0.0847 mm. The spatial frequency is the reciprocal of 11.81 cycles · mm ⁻¹ . This indicates that there are this number of periodic patterns per mm. In FIG. 5, the dots are arranged in the horizontal direction, but the same applies to the vertical direction.

  FIG. 6 schematically shows dots having three different spatial frequencies. The dots become large, medium, and small according to the low, medium, and high spatial frequencies. In FIG. 6, the arrangement of dots in the horizontal direction represents repetition. In the first solution of the present invention described above, electrostatic latent images are repeatedly formed and measured by changing the spatial frequency of dots as shown in FIG. That is, after a dot having a certain spatial frequency is measured n times, another dot having a different spatial frequency is measured n times, and this is sequentially performed. Alternatively, the number of measurements may be fixed first (n = i, i is a positive integer), and measurement may be performed while changing the spatial frequency, and after this is completed, the number of measurements may be increased (n = i + 1).

  As shown in FIG. 4B, the dot size varies, but the spatial frequency change is adjusted to exceed this variation (details will be described in the description of the fourth and fifth solving means described later). Describe).

  In the first solving means, the “size of the electrostatic latent image” is, for example, an area. However, the amount is not limited to the area, and may be an amount having the unit of length described in the third solving means.

  According to the image evaluation method of the first solving means of the present invention, the reproducibility of the electrostatic latent image is defined with respect to the size of the electrostatic latent image at an arbitrary point of the photoconductor, and the spatial frequency is sequentially changed. The reproducibility of the electrostatic latent images having different spatial frequencies can be evaluated by forming the electrostatic latent image, performing measurement, and obtaining the reproducibility of the electrostatic latent image for each spatial frequency.

(Explanation of second solution)
The formation of an electrostatic latent image differs from place to place on the photoconductor, and therefore, evaluation of reproducibility at different places on the photoconductor is important for knowing the variation depending on the place. FIG. 7 shows an example in which a drum-shaped photoconductor 41 is used. Light irradiation 42 is performed on the charged drum-shaped photoconductor 41 at different locations on the photoconductor. The light irradiation is performed using a scanning optical system having a configuration as shown in FIG. FIG. 7 depicts only the vicinity of the photoreceptor 119 of FIG. In FIG. 7, the photoconductor is depicted as if it was exposed simultaneously, but there is actually a time difference. The drum-shaped photoconductor 41 has a diameter of about 60 mm and a length of about 300 mm, for example.

When the photosensitive member 41 is small, or when a small piece is cut out from the drum-shaped photosensitive member 41, the position of light irradiation may be fixed and the photosensitive member may be moved. FIG. 8 shows an example of this, and a small piece 51 of the photosensitive member is placed on the sample stage 52. The small pieces 51 of the photoconductor are charged after being placed on the sample stage 52. The sample stage 52 can be moved in the x and y directions by a stepping motor or a piezoelectric element (not shown). Assume that the incident position of the irradiation light 53 from the scanning optical system is constant. When the sample stage 52 is slightly moved in the x direction (FIGS. 8A to 8B), the formation position of the electrostatic latent image on the photosensitive member is opposite to the direction in which the sample stage 52 is moved in the x direction. Move to. Further, when the sample stage 52 is slightly moved in the x direction (from (b) to (c) in FIG. 8), the formation position of the electrostatic latent image further moves. The same applies to the y direction.
Further, if the sample stage 52 is made sufficiently large or a holding jig is provided, a drum-shaped photosensitive member as shown in FIG. 7 can be mounted.

  As described above, according to the image evaluation method of the second solving means, the image evaluation is performed at a plurality of points on the photoconductor, thereby forming and evaluating the electrostatic latent image at different positions on the photoconductor. Can do. In addition to the formation of electrostatic latent images at different locations on the photoconductor, repeated measurements taking into account the spatial frequency of the electrostatic latent image can be used to reduce the spatial frequency at different positions on the photoconductor. It is possible to evaluate reproducibility in consideration.

(Explanation of third solution)
When an electrostatic latent image is formed on a photoconductor with a scanning optical system, it often has a dot shape (ellipse) close to an ellipse. This is a case where an aperture 120 having a rectangular shape is provided immediately after the LD light source 111 of the scanning optical system shown in FIG. The dot shape at this time is as shown in FIG. Assuming that the area when the ellipse is S, if the shape close to the ellipse is a perfect circle without changing the area and the radius is r, πr ² = S (π is the circumference ratio) ). Therefore, r = (S / π) ^1/2 . Here, 2r is referred to as an equivalent circle diameter.

In the example of the first solving means described above, the size of the electrostatic latent image is, for example, an area. However, in the third solution, r having a length dimension or a circle equivalent diameter 2r is used.
Here, the spatial frequency is the reciprocal of the length, and the size of the electrostatic latent image is preferably represented by the length. Further, the beam diameter of the scanning optical system is often expressed as, for example, 45 μm × 50 μm, and it is more appropriate to express the size of the electrostatic latent image as a length.

  According to the image evaluation method of the third solving means, the size of the electrostatic latent image is defined by a length dimension defined by a square root obtained by dividing the area S of the electrostatic latent image by the circumference ratio π. Alternatively, by using the equivalent circle diameter 2r, it is possible to provide a method for evaluating the reproducibility of the electrostatic latent image that appropriately represents the spatial frequency.

(Specific examples of the first to third solving means)
An example of a graph of the spatial frequency and reproducibility of the equivalent circle diameter of the electrostatic latent image is shown in FIG. Assume that the measurement result for a photoconductor is (1) in the figure. Now, based on this, the inferior photoreceptor has the characteristics (2), and the superior photoreceptor has the characteristics (3). An ideally good photoconductor has no change in reproducibility with respect to the spatial frequency and has a small absolute value of the reproducibility.

  According to the image evaluation method of the present invention, the characteristics of the photoconductor can be known by obtaining the reproducibility with respect to the spatial frequency. Further, the superiority or inferiority of the photoconductor can be known by using this evaluation method between a plurality of photoconductors of the same type as in product inspection or between a plurality of photoconductors of different types as in product comparison.

  Also, FIG. 10 does not have to be the result for different photoconductors, and can be viewed as measurements at different locations on the same photoconductor. If the photoreceptor is made uniform, for example, the straight lines (1) to (3) in the figure all overlap. The inclination of the straight line and the deviation of the intercept represent the nonuniformity of the photoreceptor. According to this evaluation method, since a spatial frequency and reproducibility can be obtained at a plurality of different locations with respect to a certain photoconductor, the nonuniformity of the photoconductor can be known.

  In FIG. 10, the spatial frequency is limited, and the characteristics of the photoconductor are approximated by a straight line. However, when the spatial frequency increases, the characteristic may become a curve. It is not necessarily limited to a straight line.

(Explanation of the fourth solution)
In the fourth solution of the present invention, the light emission intensity of the LD light source of the scanning optical system is changed in order to change the spatial frequency of the electrostatic latent image of the dots.
Specifically, the size of the electrostatic latent image of the dots was changed by changing the output of the LD light source 111 of the scanning optical system shown in FIG. FIG. 11 shows an example of the relationship between the output of the LD and the equivalent circle diameter. The horizontal axis in FIG. 11 is the LD output set value, and the vertical axis is the equivalent circle diameter. It can be seen that there is a linear relationship between the output of the LD and the equivalent circle diameter. Therefore, the circle equivalent diameter can be finely adjusted by adjusting the LD output.
Further, FIG. 12 is a graph in which the reciprocal of the equivalent circle diameter is taken and the spatial frequency is plotted on the vertical axis.

  According to the image evaluation method of the fourth solving means, the electrostatic latent image evaluation method in which the spatial frequency of the electrostatic latent image can be easily modulated by changing the spatial frequency of the electrostatic latent image according to the intensity of light irradiation. Can be provided.

(Explanation of fifth solution)
In the fifth solution of the present invention, in order to change the spatial frequency of the electrostatic latent image of dots, a plurality of dots are made adjacent to each other. As an example, as shown in FIG. 13, 1 dot is formed adjacent to 2 × 2 = 4 dots, and this is defined as 1 dot. In this way, dots having a low spatial frequency can be formed.
This is because the dot size cannot be increased in the method of adjusting the intensity of irradiation light described in the fourth solution, for example, because the LD output has an upper limit. For example, at the right end of the graph of FIG. 11, the dot can be enlarged only to a circle equivalent diameter of about 160 μm. On the other hand, in the fifth solving means, it is possible to enlarge the dots to about 320 μm, which is about twice in one direction. The area is four times. However, since the dots are brought into contact with each other, the actual size is less than this, but it can be surely made larger than 160 μm.

  According to the image evaluation method of the fifth solving means, the spatial frequency of the electrostatic latent image is changed by performing light irradiation adjacent to each other multiple times and forming a plurality of dots adjacent to each other. It is possible to provide a method for evaluating an electrostatic latent image in which spatial frequency modulation is easy in a region having a lower spatial frequency.

(Explanation of sixth solving means)
FIG. 14 schematically shows a configuration example of the image evaluation apparatus described in the sixth solving means of the present invention. The basic configuration of the image evaluation apparatus 151 includes an electron gun 152, a condenser lens 153, a scanning lens 154, an objective lens 155, a light irradiation optical system 156, a sample 157, a sample stage 164, an electron detector 163, a vacuum pump 159, and an interface. 158, an electronic calculator 160 (computer 160a and display 160b).

  The means for generating charged particles for charging the photosensitive member (sample 157) described in the sixth solving means is specifically an electron gun 152 and lenses 153, 154, 155, and irradiates the photosensitive member with light. The means is specifically the light irradiation optical system 156, and the means for observing the electrostatic latent image of the dot formed at an arbitrary position on the photoreceptor is specifically the electron gun 152 and the lenses 153 and 154. 155 and the electron detector 163.

  The electron gun 152 may be used in, for example, a scanning electron microscope (SEM). The electron gun 152 includes a hot filament, a field emission, etc. Any of them may be used.

  Considering charging the photoconductor with electrons using the electron gun 152, it is necessary to place the electron gun, photoconductor and others in a vacuum. This is not unique to the apparatus and is similar to the SEM. For this reason, the whole apparatus is evacuated using a vacuum pump 159. In FIG. 14, only one vacuum pump 159 is drawn for convenience, but a plurality of rotary pumps, turbo molecular pumps, ion sputtering pumps, and the like are used in combination depending on the degree of vacuum to be achieved.

The charged particle is generally an electron having a negative polarity. However, ions may be used like an ion gun used in FIB (Focused Ion Beam). This is an ion species such as Ga ⁺ .

  The condenser lens 153, the scanning lens 154, and the objective lens 155 are all magnetic field lenses or electrostatic lenses. The charged particle beam is scanned two-dimensionally on the sample 157 by the scanning lens 154. Here, the sample 157 is a photoconductor. Reference numeral 161 denotes an electrostatic latent image formed on the photoconductor. This finally allows a person to observe on the display 160b through the computer 160a of the electronic computer 160.

Electrons are preferably used for charging the photoreceptor. When electrons are used, the charged potential on the surface of the photoconductor mainly depends on the electron acceleration voltage and the irradiation time. The higher the electron acceleration voltage, the higher the charged potential on the surface of the photoreceptor. In the present invention, it is necessary to make the conditions equivalent to those for electrophotography. The conditions are that the acceleration voltage (Vacc) is about 1.0 to several kV, and the time is about several tens of seconds to several tens of minutes. When the charging potential is measured using, for example, a general-purpose surface potential meter, a value of about −600 to −1000 V is obtained. The charging potential is equivalent to that of an actual copying machine or printer.
Further, instead of changing Vacc, the charging potential may be adjusted by applying a bias voltage to the photosensitive member.

  The light irradiation optical system 156 is a means for forming an electrostatic latent image having an arbitrary pattern by irradiating the photosensitive member with light. Specifically, the scanning optical system is as shown in FIG. In FIG. 14, the optical system is inside the apparatus, but may be outside (in the atmosphere). In this case, in order to maintain the vacuum, light from the scanning optical system may be guided to the photoconductor (sample) 157 by providing a transparent window such as glass having high transmittance in the apparatus. Moreover, it is preferable to provide an antireflection film on the glass surface of the window.

  In FIG. 14, the sample photoconductor 157 is depicted as a flat plate (or sheet), but it may be a cylinder or a drum (photoconductor 119 in FIG. 3). The sample stage 164 can move in three dimensions x, y, and z. The sample stage 164 is moved to an arbitrary position where an electrostatic latent image of dots is formed in the x and y directions. In the case of SEM, z is a working distance (working distance).

  The principle of forming an electrostatic latent image on the photoreceptor 157 is as shown in FIG. The means for observing the electrostatic latent image preferably uses electrons. This uses the electron gun 152 used for charging. However, the current value needs to be weakened. For example, assuming that the current value at the time of charging is several nA, the current value at the time of observation is several pA, which is about one thousandth. This is to prevent the electrostatic latent image from being charged again and erased by electron irradiation. Moreover, the electrostatic latent image disappears faster as the electron beam having a higher current value is irradiated.

  The electron detector 163 is a secondary electron detector. The observation electrons reach the photoconductor 157 and emit secondary electrons from the photoconductor 157. The principle that an electrostatic latent image can be measured is that secondary electrons are not emitted so much from the place where the electrostatic latent image is formed, and secondary electrons are often emitted from places where the electrostatic latent image is not formed. It depends. Secondary electrons are detected by the secondary electron detector 163. The secondary electron detector 163 includes a phosphor that emits light when irradiated with electrons, a light guide, a photomultiplier that multiplies the emitted light, and the like. This is a scanning type detector, and follows the scanning of the electron beam on the photosensitive member.

  Each of the above means is controlled by the computer 160a of the electronic computer 160 via the interface 158. Detailed control of each means is performed by software stored in a memory in the computer.

  The image evaluation apparatus configured as described above may have apparatus restrictions. For example, the scanning range of the electron beam cannot be widened, or the size of the sample (photosensitive member) is limited. With respect to the scanning range of the electron beam, it is possible to scan a certain range (several mm × several mm) if a resolution of several nm is sacrificed as compared with a general-purpose SEM. However, many photoreceptors are several centimeters in diameter and several tens of centimeters in the longitudinal direction. It is difficult to scan the entire surface of the photoreceptor at once. Partial scanning is repeated by moving the photosensitive member. If a photoconductor of this size is used as a sample, the sample stage 164 becomes large and the vacuum chamber needs to have a large capacity. Therefore, it is necessary to devise measures such as cutting out and measuring the photoconductor. When the photoconductor is cut out and measured, the scanning can be performed in a very narrow range.

  One evaluation of the electrostatic latent image is charging, exposure, measurement, and static elimination. This series of operations is repeated a desired number of times. The latent image data obtained by the measurement is stored in a memory in the computer. The processing in the computer is, for example, obtaining an area and an equivalent circle diameter for each number of measurements, and obtaining an average value, a standard deviation, and a ratio (reproducibility) thereof. Such processing and calculation can be easily performed by using general-purpose software.

This evaluation of the electrostatic latent image relates to the size and spatial frequency of a certain electrostatic latent image. When this series of evaluations is completed, the spatial frequency is then changed. This is performed by the method described in the fourth solution means or the fifth solution means. After changing the spatial frequency, the same repeated measurement is performed.
Further, the position of forming the electrostatic latent image on the photosensitive member is changed by the method described in the second solving means.

  Thus, according to the image evaluation apparatus of the sixth solving means, at least means for generating charged particles for charging the photoconductor (electron gun 152, condenser lens 153, scanning lens 154, objective lens 155) and , Means for irradiating light on the photoconductor (light irradiation optical system 156), means for observing the electrostatic latent image formed on the photoconductor (electron detector 163, electronic computer 160), and electrostatic latent image Evaluation means capable of evaluating including the reproducibility of the electrostatic latent image in consideration of the spatial frequency and the variation of the location on the photosensitive member. An apparatus can be provided.

(Explanation of seventh solving means)
FIG. 1 shows an example of an electrophotographic image forming apparatus. However, in the image forming apparatus according to the present invention, the image evaluation method or the sixth solution of any one of the first to fifth solving means described above. Means for reflecting the reproducibility of the electrostatic latent image obtained by the image evaluation apparatus.
Specifically, at the time of inspection of the photoreceptor, many points are measured on the photoreceptor to obtain reproducibility. An electronic table of the coordinates and reproducibility of each point is prepared and stored in the memory of the image forming apparatus configured as shown in FIG. 1, and the table is referred to as necessary and reflected in image formation. .

  Thus, according to the image forming apparatus of the seventh solving means, there is means for reflecting the evaluation result of the electrostatic latent image reproducibility, and the evaluation result regarding the reproducibility of the electrostatic latent image including the spatial frequency is obtained. Since it is reflected in image formation, an image forming apparatus capable of achieving high image quality can be provided.

  Next, more specific examples of the present invention will be described.

[Example 1] (Examples of first, third, and sixth solving means)
(1) The reproducibility was evaluated using a photoconductor used in a commercially available copying machine as a sample (photoconductor A).
The photoconductor A was a drum-shaped photoconductor, which was evaluated by cutting it out to about 2 cm × 2 cm. The repetition is 10 times. At this time, the average value of the area of the electrostatic latent image of the dots was 5600 μm ² , and the standard deviation was 100 μm ² . Therefore, the reproducibility is 0.018.
Further, the average value in the case of the equivalent circle diameter was 84.6 μm, and the standard deviation was 0.8 μm. Therefore, the reproducibility is 0.0095. The spatial frequency at this time is 5.9 cycle · mm ⁻¹ .

Next, the dot size was changed and reproducibility was evaluated. The repetition is also 10 times. At this time, the average value of the area of the electrostatic latent image of the dots was 16000 μm ² , and the standard deviation was 200 μm ² . Accordingly, the reproducibility is 0.013. The average value when the equivalent circle diameter was 142.8 μm, and the standard deviation was 0.7 μm. Therefore, the reproducibility is 0.0049. The spatial frequency at this time is 3.5 cycle · mm ⁻¹ .
Therefore, it can be seen that the larger the electrostatic latent image of the dots, in other words, the lower the spatial frequency, the better the reproducibility.

(2) The same reproducibility evaluation was performed using another different type of photoconductor (photoconductor B) as a sample (photoconductor B).
The photoreceptor B is also a drum-shaped photoreceptor, and was cut out and used for evaluation. The repetition is 10 times. Average value of areas of the electrostatic latent image dot is 6600μm ^2, the standard deviation was 200 [mu] m ^2. Therefore, the reproducibility is 0.03. Further, the average value in the case of the equivalent circle diameter was 92 μm, and the standard deviation was 1 μm. Accordingly, the reproducibility is 0.01. The spatial frequency is 5.5 cycle · mm ⁻¹ .

Next, the dot size was changed and reproducibility was evaluated. The repetition is also 10 times. At this time, the average value of the area of the electrostatic latent image of dots was 16800 μm ² , and the standard deviation was 200 μm ² . Therefore, the reproducibility is 0.011. In addition, the average value in the case of the equivalent circle diameter was 146.4, and the standard deviation was 0.9 μm. Therefore, the reproducibility is 0.0061. The spatial frequency is 3.4 cycle · mm ⁻¹ .

As described above, it can be seen that the larger the electrostatic latent image of the dots, in other words, the lower the spatial frequency, the better the reproducibility.
Further, the above (1) and (2) are performed at substantially the same spatial frequency. Therefore, comparison is possible, and it can be seen that the photoreceptor A is superior to the photoreceptor B.
In both (1) and (2), the larger dot is 2 × 2 = 4 dots.

[Example 2] (Examples of first, second, third, fifth, and sixth solving means)
In addition to the above photoconductors A and B, two types of photoconductors (photoconductor C and photoconductor D) were evaluated for reproducibility at 9 different locations on the photoconductor. In the same manner as in Example 1, the average value and standard deviation were obtained for the area of the electrostatic latent image of dots and the equivalent circle diameter. However, it is related to the data of all 27 points which are repeated 3 times and multiplied by 9 different places. The reproducibility was obtained from the average value and the standard deviation. The equivalent circle diameter is shown in Table 1 below.

  Table 1 shows the reproducibility at different locations on the photoconductor, and it can be seen that the reproducibility is better when the spatial frequency is smaller. The reproducibility when evaluated at the same place on the photoreceptor is shown in Table 2 below. In the case of reproducibility at the same place in Table 2, the number of repetitions is 10 times, which is the same evaluation as in Example 1. Compared with the values in Table 1, it can be seen that the reproducibility is good. Therefore, it can be seen that reproducibility is poor at different locations on the photoreceptor. The larger dot is 2 × 2 = 4 dots as one dot.

Moreover, what plotted the result of Table 2 on the graph is shown in FIG. When the photoconductors B, C, and D are compared at a spatial frequency of about 4 to 6 (cycle · mm ⁻¹ ) where the measured value is present, C, D, and B are obtained from the better. Further, when evaluated from the inclination of the approximate straight line, the photoconductor D having a small straight line inclination is excellent. If the absolute value of the reproducibility of the photoconductor D is small, the photoconductor is better.

[Example 3] (Examples of the fourth and sixth solving means)
11 and 12 show changes in the equivalent circle diameter or spatial frequency of the electrostatic latent image of the dots when the light emission intensity (setting value) of the LD is changed from 380 to 830 mV. The photoconductor used is that used in commercially available copying machines and printers (photoconductor A). First, it can be seen from the graph of FIG. 11 that the equivalent circle diameter can be linearly changed by the emission intensity of the LD. When approximated by a straight line, the slope of the straight line is 0.0777, and the intercept is 96.389. From this, it can be seen that, for example, when a dot with an equivalent circle diameter of 150 μm is required, the emission intensity of the LD may be about 690 mV. There is a linear relationship shown in FIG. 16 between the light emission intensity (set value) of the LD and the exposure energy density (mJ / m ² ) on the photoreceptor.

  When the reproducibility was obtained at three different locations on the same photoconductor using the photoconductor B, the results shown in Table 3 below were obtained. The repetition at each point is 10 times. The set value of the LD output was 385 mV and was constant regardless of the location. It can be seen that the reproducibility varies depending on the location, especially at location 2. Further, the coordinates on the photoconductor for each of the locations 1, 2, and 3 can be obtained.

[Embodiment 4] (Embodiment of Seventh Solution)
As an example of means for reflecting the evaluation result on the image formation of the image forming apparatus, many points are measured on the photosensitive member at the time of the inspection of the photosensitive member to obtain reproducibility. Then, an electronic table of the coordinates and reproducibility of each point is prepared and stored in the memory of an electrophotographic image forming apparatus such as a copying machine or printer having the configuration shown in FIG. The table is referred to and reflected in image formation. For example, the LD output is always adjusted at a position with poor reproducibility.
In this way, it is possible to provide an image forming apparatus capable of obtaining a certain reproducibility and improving the image quality.

  The following Table 4 summarizes some characteristics of the photoconductors A to D used in the above examples.

1: Part of a charged photoconductor 2: Writing light 3: Electrostatic latent image 41, 71: Photoconductor (OPC)
42: Light irradiation 51: Small piece of photoconductor 52: Sample stage 53: Irradiation light 72: Charger charger 73: Exposure (exposure means)
74: Toner (developing means)
75: Paper (recording medium)
76: Transfer charger 77: Fixing roller 78: Cleaning blade 79: Light source for static elimination 111: Light source (LD)
112: Collimating lens 113: Cylindrical lens 114: First mirror 115: Polygon mirror (optical deflector)
116: first scanning lens 117: second scanning lens 118: second mirror 119: photoconductor 121: conductive layer 122: charge generation layer (CGL)
123: Charge transport layer (CTL)
124: Irradiation light 126: Electron (negative charge)
127: Hall (positive charge)
151: Image evaluation device 152: Electron gun 153: Condenser lens 154: Scan lens 155: Objective lens 156: Light irradiation optical system 157: Sample (photosensitive member)
158: Interface 159: Vacuum pump 160: Electronic calculator 160a: Computer 160b: Display 163: Electronic detector 164: Sample stage

JP 2003-295696 A Japanese Patent Laid-Open No. 2002-304025

Claims (7)

In an image evaluation method for forming an electrostatic latent image by irradiating light on a charged photoreceptor and measuring the electrostatic latent image with charged particles to perform image evaluation,
Formation of an electrostatic latent image at various points on the photoconductor, defining the reproducibility of the electrostatic latent image with respect to the size of the electrostatic latent image, and sequentially forming a plurality of different electrostatic latent images with different spatial frequencies. Repeating a step, a measuring step for measuring an electrostatic latent image formed by the forming step, and a static eliminating step for neutralizing charges on the photoconductor,
An image evaluation method, wherein the reproducibility of the electrostatic latent image is obtained for each spatial frequency. The image evaluation method according to claim 1,
An image evaluation method, wherein the image evaluation is performed at a plurality of points on the photoconductor. The image evaluation method according to claim 1 or 2,
The size of the electrostatic latent image is defined by a square root obtained by dividing the area of the electrostatic latent image by the circumference ratio. The image evaluation method according to any one of claims 1 to 3,
An image evaluation method, wherein the spatial frequency of the electrostatic latent image is changed according to the intensity of the light irradiation. The image evaluation method according to any one of claims 1 to 3,
An image evaluation method, wherein the spatial frequency of the electrostatic latent image is changed by performing the light irradiation adjacently a plurality of times. An apparatus for performing image evaluation using the image evaluation method according to claim 1, comprising at least:
Means for generating charged particles for charging the photoreceptor;
Means for irradiating light on the photoreceptor;
Means for observing an electrostatic latent image formed on the photoreceptor;
Means for determining reproducibility of the electrostatic latent image;
An image evaluation apparatus comprising:   It has a means to reflect the reproducibility of the electrostatic latent image obtained by the image evaluation method as described in any one of Claims 1 thru | or 5, or the image evaluation apparatus of Claim 6 in image formation. An image forming apparatus.

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