JPH09230679A - Image forming device and image forming method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To make vivid letter reproduction and excellent image reproduction possible although it is of a practical light beam sport diameter by specifying the ratio of the light energy width of a responding zone to the light energy width of a blind zone and the ratio of the light energy width of the blind zone to the energy set value of light beam. SOLUTION: A photosensitive body drum 1 is exposed by a right beam 25 from a light beam scan 20 after being uniformly electrified by an electrostatic latent image forming electrifier 2 and an electrostatic latent image is formed on the photosensitive body drum 1. Light electric potential damping characteristic of the photosensitive body 1 is deleted so that the ratio of the light energy width E2 of a responsing zone to the light energy width E1 of a blind zone, E2/E1, may be 0.3 or more but less than 1.3 and the ratio of the light energy width E1 of the blind zone to the energy wetting value E0 of a light beam, E1/E0, may be 0.2 or more but less than 0.6, electrification potential by the electrifier 2 is set, and the light energy by the light bean scan 20 is adjusted.

Description

Detailed Description of the Invention

[0001]

The present invention relates to uniformly charging a photoconductor on which an electrostatic latent image is formed, and irradiating the charged photoconductor with a light beam modulated according to image information. The present invention relates to a so-called digital electrophotographic image forming apparatus for forming an electrostatic latent image on a photoconductor by developing the electrostatic latent image to form a visible image.

[0002]

2. Description of the Related Art In recent years, so-called digital electrophotography in which an electrostatic latent image is formed on a photosensitive member by irradiating the photosensitive member with a light beam carrying image information representing characters and pictures as described above. Image forming apparatuses of the type have been widely adopted in printers and copying machines. In such a digital image forming apparatus, the light beam is turned on and off at a high speed not only when forming a binary image of black and white but also when forming an image having a halftone, so-called a halftone dot structure or a common A method of expressing a halftone by forming an electrostatic latent image of a line structure has been conventionally known. This method has a relatively simple algorithm and can be realized at low cost. It has been widely used in printers and copiers.

[0003]

By forming such an electrostatic latent image having a so-called halftone dot structure or line structure,
In the method of expressing halftone, latent image formation can be performed with a constant light beam spot diameter from a low density part (highlight part) to a high density part in an image and with a constant number of dots or lines per unit length. Done. For this reason, the exposure profile in the highlight portion does not change in a binary (digital) manner such that the exposure energy is on and off, and has an intermediate (analog) exposure energy distribution in which the contrast is lowered. Since it has an exposure profile and the exposure amount itself is small, the reproducibility of dots and lines is poor, resulting in a grainy image with low granularity, and the stability of gradation expression in a hot and humid environment is also poor. is there. Also,
Even when reproducing small characters, especially small Kanji characters, there is a problem that line reproducibility is poor and crushing and blurring occur.

To address this problem, if the number of dots or lines per unit length is reduced to form a latent image in order to improve the contrast of the exposure profile in the highlight part, dots and lines in the highlight part are formed. However, there are drawbacks that the image structure such as dots is easily recognized, and the resolution of characters is lowered to lower the image quality.

A method of making the light beam spot diameter sufficiently small is also conceivable, but in that case, an imaging optical system for condensing the light beam to form the light beam spot on the photosensitive member is very precise and expensive. It becomes a big thing and becomes large-sized, and it is not suitable for practical use. On the other hand, in order to solve this problem, Japanese Patent Laid-Open No. 1-1694
Japanese Laid-Open Patent Publication No. 54-54 discloses that a photoconductor having a photopotential decay characteristic in which the charge potential is not attenuated until the exposure energy intensity reaches a certain level and the charge potential is rapidly attenuated when the exposure energy intensity exceeds a certain threshold value. , A means for improving the contrast of an exposure profile is disclosed. Further, JP-A-2-282277 describes a reproduction of a halftone image using a photosensitive member having similar characteristics.

According to this method, since the charging potential is not attenuated by the weak exposure energy intensity, the weak exposure energy intensity portion which is the hem portion of the exposure profile is cut off, and clear characters can be reproduced. The reproducibility of lines is also good, and the rough image with low granularity is improved, and the image with excellent granularity can be obtained.
However, at this weak exposure energy intensity, since the charging potential changes abruptly when it goes beyond the dead area where the charging potential is not attenuated, the gradation expression including the highlight part becomes sharp, and there is unevenness in the light beam irradiation energy. In that case, there arises a problem that density unevenness is likely to become apparent. In order to suppress the steepness of gradation expression and the appearance of density unevenness, it is essential to make the light beam spot diameter sufficiently small, and as described above, the imaging optical system is extremely precise and expensive. In addition, it becomes large and unsuitable for practical use.

In view of the above-mentioned conventional problems, the present invention provides an image having a practical light beam spot diameter and excellent image reproduction with clear character reproduction and clear dots or lines and no graininess. An object of the present invention is to provide an image forming apparatus and an image forming method capable of realizing reproduction, and capable of stably expressing a highlight portion with high uniformity and stability.

[0008]

SUMMARY OF THE INVENTION An image forming apparatus of the present invention which achieves the above object is a photosensitive member which moves in a predetermined sub-scanning direction and on which an electrostatic latent image is formed, and a charging unit which charges the photosensitive member. By irradiating the charged photoconductor with a light beam modulated according to image information simultaneously or sequentially with respect to a plurality of pixels arranged in a predetermined main scanning direction intersecting with the sub-scanning direction, the photoconductor An exposure unit for forming an electrostatic latent image thereon, and a developer carrying member for carrying a developer on the surface and conveying the developer to a developing position facing the photoconductor, In an image forming apparatus provided with a developing means for developing a visible image on the photoconductor by developing the formed electrostatic latent image, the photoconductor, the charging means and the exposing means are the above-mentioned charging means. Charge potential of photoconductor VH, saturation surface potential of the light attenuation of the photoreceptor VL, the energy set value of the light beam E
0, the surface potential of the photoreceptor is VH or less VH- (VH-V
L) / 5, the light energy width of the insensitive region that remains at a potential of 5 or more is E1, and the surface potential of the photoreceptor is VH- (VH-V
L) / 5 or less VL + (VH−VL) / 5, where E2 is the light energy width of the response region that remains at a potential of 0.3 <E2 / E1 <1.3 and 0.2 <E1 / E0 < It is characterized in that it is adjusted so as to satisfy the relationship of 0.6.

According to the image forming method of the present invention which achieves the above-mentioned object, a charging step of charging a photoconductor on which an electrostatic latent image is formed, which moves in a predetermined sub-scanning direction, a photoconductor on the charged photoconductor is charged. By simultaneously or sequentially irradiating a plurality of pixels lined up in a predetermined main scanning direction intersecting with the sub-scanning direction with a light beam modulated according to image information, an electrostatic latent image is formed on the photoconductor. The exposing step and the developing step of forming a visible image on the photoreceptor by developing the electrostatic latent image formed on the photoreceptor, wherein the charging step and the exposing step include the charging. In the step, the charging potential of the photoconductor is VH, the saturation surface potential of light decay of the photoconductor is VL, the energy setting value of the light beam is E0, and the surface potential of the photoconductor is VH or less VH- (VH-
VL) / 5, the light energy width of the insensitive region which remains at a potential of 5 or more is E1, and the surface potential of the photoconductor is VH- (VH-V
L) / 5 or less VL + (VH−VL) / 5, where E2 is the light energy width of the response region that remains at a potential of 0.3 <E2 / E1 <1.3 and 0.2 <E1 / E0 < It is characterized in that it is adjusted so as to satisfy the relationship of 0.6.

[0010]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a diagram showing the photopotential decay characteristics of a photoconductor used in an image forming apparatus of the present invention. FIG. 1 shows the photopotential attenuation characteristics in which the horizontal axis represents the exposure energy for the photoconductor and the vertical axis represents the photoconductor surface potential. In FIG. 1, VH represents the charging potential of the photoconductor, and VL represents the saturated surface potential of the light attenuation of the photoconductor, that is, the surface potential of the photoconductor after the photopotential is sufficiently attenuated by exposure and then the potential decay of the light energy. Ratio is 0.5V
The potential set to m ² / mJ is represented, and the energy setting value E0 of the light beam represents the energy of the light beam that is output when a 100% image signal is exposed.

Now, when the exposure energy is increased from 0, the surface potential of the photosensitive member keeps a substantially constant value at first, but when the exposure energy reaches a certain value, the surface potential of the photosensitive member starts to decrease. A region where the amount of attenuation of the surface potential of the photoconductor is 20% or less of VH-VL is called a dead region, and the light energy width of the dead region is represented by E1. The exposure energy is further increased and the amount of attenuation of the surface potential of the photoconductor is 2 of VH-VL.
From the point of exceeding 0%, the photosensitive member surface potential is VH-VL of 2
The region until it is attenuated below the level of 0% is called the response region, and the light energy width of the response region is represented by E2.

As shown in FIG. 1, in the image forming apparatus of the present invention, the ratio E2 / E1 of the light energy width E2 of the response area to the light energy width E1 of the dead area is 0.3 or more and 1.
3 or less, and the ratio E1 / E0 of the width E1 of the dead area to the energy setting value E0 of the light beam is 0.
The photopotential decay characteristic of the photoconductor is selected so that the value is 2 or more and 0.6 or less, the charging potential by the charging unit is determined, and the light energy by the exposure unit is adjusted. Light energy width E of the response area with respect to light energy width E1 of the dead area
By setting the ratio E2 / E1 of 2 to 0.3 or more and 1.3 or less, the charging potential does not suddenly become zero even when the dead zone is exceeded and the response zone is reached. Instability of gradation expression due to steep gradation expression or uneven light irradiation is suppressed, and the ratio E1 of the light energy width E1 of the dead area to the energy setting value E0 of the light beam.
By setting / E0 to 0.2 or more and 0.6 or less, even if a practical light beam spot is used, the weak exposure energy intensity portion of the exposure profile is cut off by the dead area, so clear character reproduction and good It is possible to reproduce various dots and lines.

FIG. 2 is a graph showing the relationship between E2 / E1 of the photosensitive member used in the image forming apparatus of the present invention and gradation stability and line / character reproducibility. That is, FIG.
The ratio E2 / E1 of the light energy width E2 of the response area to the light energy width E1 of the dead area is taken on the horizontal axis of (a), and the left vertical axis is an index showing gradation stability (color uniformity). In-plane color difference ΔE, and the vertical axis on the right side is an edge width (edge width) as an index showing line / character reproducibility.
h: unit μm). The exposure condition at this time is 2
00 line / 25 μm.

Here, a method of measuring the in-plane color difference ΔE and the edge width will be described. FIG. 3 shows the in-plane color difference ΔE.
FIG. Using the image forming apparatus to be measured, three types of uniform halftone images having a 15 cm square size and an image area ratio Cin of 10%, 20%, and 30% are output on the recording paper. For each time, nine hues (CIE Lab space) as shown in FIG. 3 are measured, and the vector length of the two furthest points on the color space in each image area ratio is represented as an in-plane color difference ΔE.

FIG. 4 is an explanatory view of the edge width,
FIG. 4A is an enlarged view of the line drawing, and FIG. 4B is its AA ′.
It is a graph which shows the image density of a cross section. As shown in FIG. 4B, the image density of the line drawing does not show a sharp change near the edges of the base portion and the colored portion, but changes in a gentle curve. The distance between the intersections of the maximum density level and the minimum density level of the tangent line at the center of the maximum density and the minimum density of the change curve is defined as edge width (μm). The edge width is an index that quantitatively expresses the sharpness of the line drawing edge, that is, the blur of the line drawing.When the edge width is small, it is a sharp line drawing, and when the edge width is large, it is a blurred line drawing. .

In the region where the ratio E2 / E1 is less than 0.3, the light energy width E2 in the response region is extremely smaller than the light energy width E1 in the dead region as shown in FIG. Since it suddenly becomes zero, the gradation expression in the extreme highlight part (Cin <10%) becomes sharp and the color difference increases, as shown by the dashed line in the upper left part of the graph of FIG. 2A, and If there is unevenness in the irradiation energy of the light beam, density unevenness is likely to become apparent.

On the other hand, in the region where the ratio E2 / E1 exceeds 1.3, the light energy width E1 in the dead region becomes smaller than the light energy width E2 in the response region as shown in FIG. As with the case of a photoreceptor that does not have a region, clear characters are not reproduced and good dots and lines are not reproduced. For example, when the ratio E2 / E1 is 0.1, the edge width is 26 μm, while the ratio E2 / E1 is 3.0.
The edge width is 45 μm (not shown) when increased up to. In the present invention, the upper limit of the ratio E2 / E1 can be maintained at the edge width of 30 μm or less 1.3.
However, the ratio E2 / E1 is preferably 1.0 or less from the viewpoint of line / character reproducibility. Also, the ratio E2 / E1
2 exceeds 1.3, as indicated by the broken line in the upper right part of the graph of FIG.
Gradation stability (color uniformity) because the color difference at 0%) increases
From this point as well, the ratio E2 / E1 is preferably 1.3 or less.

On the other hand, when the ratio E1 / E0 of the light energy width E1 in the dead region to the light beam energy set value E0 is less than 0.13 (corresponding to approximately 1 / e ² ), the light beam energy set value E0 is set. Compared with, the light energy width E1 in the dead area is smaller and the weak exposure energy intensity portion is not cut off sufficiently, so clear characters or good dots or lines cannot be reproduced, or Since the energy setting value E0 of the light beam is set excessively as compared with the light energy width E1, the image density is saturated in the middle and high density portions, and good gradation cannot be reproduced.

FIG. 5 shows the energy setting value E0 of the light beam.
Ratio of light energy width E1 in the dead region to E1 / E0
5 is a graph showing the relationship between the line reproducibility and lines. In addition,
Here, the edge width (μm) was selected as an index showing line / character reproducibility. As shown in FIG. 5A, the ratio E1 / E0 of the light energy width E1 of the dead region to the energy setting value E0 of the light beam is 0.13 (approximately 1 / e ²
(Corresponding to 1)), the edge width is reduced to 30 μm or less, the edge of the line or character becomes too sharp, and part of the edge is cut off. Therefore, in the present invention, the lower limit of the ratio E1 / E0 is set to 0.2.

On the other hand, as shown in FIG. 5 (b), when the ratio E1 / E0 of the light energy width E1 in the dead area to the energy setting value E0 of the light beam exceeds 0.6, the light energy width in the dead area is exceeded. E1 is too large and the number of cut-off portions is large, and the edge width is below the range of 50 ± 10 μm, and lines or dots are thinned or blurred, or the light energy width E1 of the dead area.
Since the energy setting value E0 of the light beam is set to be too small as compared with the above, the low density portion cannot be reproduced, and the image density in the high density portion is insufficient, so that good gradation reproduction cannot be performed.

Therefore, in the present invention, the ratio E1 / E0 is set to 0.
6 or less, but from the viewpoint of line / character reproducibility, the ratio E
1 / E0 is preferably 0.5 or less. Further, the photoreceptor in the image forming apparatus of the present invention, on the substrate,
A charge generation layer that generates charges, a photopotential decay control layer that stores the injected charges when the injected charge amount is relatively small, and transports the injected charge when the injected charge amount is relatively large, It is also preferable to have a photosensitive layer composed of a charge transport layer for transporting.

Also, in the image forming method of the present invention,
By doing so, the photopotential attenuation control layer accumulates the photocarriers generated from the charge generation layer and starts releasing the photocarrier when the accumulated amount reaches saturation. The light energy width of the dead region is determined by the storage capacity of the photopotential decay control layer. On the other hand, the potential is attenuated when the photocarriers that have passed through the photopotential decay control layer reach the surface of the photoconductor. That is, the light energy width of the response region, which represents the attenuation of the potential, is determined by the photocarrier generation capability of the charge generation layer. In this way, the light energy width in the dead area and the light energy width in the response area can be controlled almost independently,
0.3 <E2 / E1 <1.3 and 0.2 <E1 /
It becomes possible to adjust so as to satisfy the relationship of E0 <0.6.

Further, in the photoreceptor of the image forming apparatus of the present invention, the thickness of the photopotential attenuation control layer is x, the thickness of the photosensitive layer, that is, the charge generation layer, the photopotential decay control layer,
It is also a preferable embodiment that the thickness of each layer is adjusted so that 0.01 ≦ x / L ≦ 0.5, where L is the total thickness of the charge transport layer.

In the region where x / L is less than 0.01,
Since the photopotential attenuation control layer is too thin, the light energy width E1 in the dead area is narrowed, and it is not possible to reproduce clear characters and good dots and lines like the photoconductor having no dead area. . On the other hand, in the region where x / L exceeds 0.5, since the photopotential attenuation control layer is too thick, it takes time for the photocarriers to move inside the photopotential attenuation control layer and the potential is not attenuated. The optical energy width E2 of is increased and the condition of E2 / E1 <1.3 cannot be satisfied. In addition, the time required for the potential to decay after irradiation with light exceeds 300 msec, which is a general practical limit, and is not suitable for practical use. By setting x / L to be 0.01 or more and 0.5 or less, it is possible to realize a photoconductor having a dead zone and suitable for practical use.

In the image forming apparatus of the present invention, the photopotential decay control layer comprises a charge transporting domain dispersed in an electrically inactive matrix, and the photopotential decay control layer contains a charge transporting property. Domain volume ratio is 20%
It is also preferable that it is adjusted to 50% or less. When the volume ratio of the charge transporting domain exceeds 50%, it becomes difficult to accumulate photocarriers, and the light energy width E1 of the dead region decreases, so that E2 / E1 <1.3 cannot be satisfied, and thus the dead region is provided. Like no photoconductor,
It is not possible to reproduce clear characters and good dots and lines. 50% volume ratio of charge transport domain
By the following, it becomes possible to sufficiently accumulate the photocarriers and form the light energy width E1 in the dead region. On the other hand, when the volume ratio of the charge transporting domain is less than 20%, the passage of photocarriers generated in the charge generation layer starts to be blocked, and the potential is not attenuated,
The light energy width E2 of the response region increases, and E2 / E1 <
The condition of 1.3 cannot be satisfied. A large amount of light energy is irradiated to attenuate the potential, and the light energy width E1 of the actual dead area is relatively small even if it does not change, which means that the photosensitive body does not have a dead area. Similarly, it becomes impossible to reproduce clear characters and good dots and lines.

Further, in the image forming apparatus of the present invention, the exposure means is configured such that the beam diameter of the light beam in the main scanning direction is D _bh , the pixel pitch in the main scanning direction is D _Sh , and the beam in the sub scanning direction is _Assuming that the diameter is D _bv and the pixel pitch in the sub-scanning direction is D _sv , the above relationship is satisfied so that D _bh / D _Sh ≤1 / 2 0.7 _{≤D bv} / D _{sv ≤1.3.} It is also a preferable embodiment that the photoconductor is irradiated with a light beam. Here, the pixel pitch in the main scanning direction refers to a repeating pitch in pixel units of the potential distribution in the main scanning direction when a halftone electrostatic latent image is formed, and the pixel pitch in the sub scanning direction is It refers to the repeating pitch in pixel units of the potential distribution in the sub-scanning direction when a halftone electrostatic latent image is formed. Further, the beam diameter means a diameter of a circle formed by connecting the points where the light energy intensity when the light beam is applied to a plane is 1 / e ² = 13.5% of the maximum value.

As described above, by setting the exposure means so as to satisfy the relationship of D _bh / D _Sh ≤1 / 2, the gradation expression including the highlight part can be expressed more smoothly and stably. To be done. The lower limit of D _bh / D _Sh is not particularly limited from the viewpoint of favorably and stably expressing the gradation expression including the highlight portion. On the other hand, in the sub-scanning direction, if the beam diameter is made too small, toner adheres only to the ratio of the beam diameter in the sub-scanning direction to the pixel pitch in the sub-scanning direction, the maximum density that can be expressed decreases, and the dynamic gradation is reduced. Since the range is narrowed, it is preferable that D _bv / D _sv is 0.7 or more. The reason why the upper limit of D _bv / D _sv is set to 1.3 is that if the beam diameter in the sub-scanning direction is expanded to exceed 1.3, crosstalk between adjacent pixels in the sub-scanning direction becomes a problem. Is.

Further, the developing means in the image forming apparatus of the present invention brings the developer, in which the magnetic particles and the toner are mixed, into contact with or close to the photoconductor at the developing position, so that the toner is exposed to the light. It is also one of the preferred embodiments that the toner is attached on the body in a pattern corresponding to the electrostatic latent image formed on the photoreceptor. Here, "contact" and "proximity" in the above-mentioned "... bringing the developer into contact with or close to the photoconductor ..." means the distance between the photoconductor and the developer carrying member is S, When the thickness of the developer layer at the developing position is T, 1 ≦ T /
The state of satisfying S is called “contact”, and 0.8 ≦ T / S <
The state that satisfies 1 is called “proximity”. By adopting the method of developing in such a contact or proximity state, faithful reproduction with high followability to the electrostatic latent image is performed, and clearer character reproduction and a sense of roughness due to clear dots and lines. It is possible to obtain a high-granularity image with no image.
On the other hand, when a non-contact development method is adopted in which the developer is placed away from the photoconductor and the toner is allowed to fly to develop, the toner is scattered and clear lines, dots, and lines are reproduced. Have difficulty.

Further, the developing means in the image forming apparatus of the present invention is capable of developing with any color toner among a plurality of color toners, and 1
Each electrostatic latent image is developed with any one color toner, and this image forming apparatus sequentially superimposes a visible image with each color toner formed on the photoconductor onto a predetermined transfer target. It is also one of the preferable embodiments to be provided with a transfer means for performing such transfer.

Here, as the above-mentioned photoconductor, a charge generation layer for generating charges and a charge injected when the injected charge amount is relatively small are stored on the substrate and when the injected charge amount is relatively large. It is preferable to use a photoreceptor having a photosensitive layer composed of a photopotential decay control layer that transports the injected charges and a charge transport layer that transports the charges. FIG. 6 is a schematic configuration diagram of a color copying machine incorporating an embodiment of the image forming apparatus of the present invention.

A document is set downward on the document reading unit 10, and an image described on the document is photoelectrically read by the document reading unit 10 to form an image signal. The light beam scanning unit 20 is turned on based on the image signal,
An off-modulated light beam is generated and the light beam 25
Is repeatedly scanned on the photoconductor drum 1 rotating in the direction of the arrow in the main scanning direction perpendicular to the rotation direction (sub-scanning direction) of the photoconductor drum 1, that is, in the direction perpendicular to this paper surface.

The photosensitive drum 1 is uniformly charged by the electrostatic latent image forming charger 2, and then repeatedly scanned in the main scanning direction by the light beam 25 from the light beam scanning unit 20. The light beam is on / off-modulated by the light beam pulse width modulation circuit 30 according to an image signal. The light beam 25 exposes the photosensitive drum 1 to an electrostatic latent image on the photosensitive drum 1. Is formed. The electrostatic latent image formed on the photosensitive drum 1 moves to a developing position facing the rotary developing device 3. The rotary developing device 3 includes yellow, cyan,
It is composed of four developing devices each having magenta and black toner. Each developing device employs a reversal developing method using two-component magnetic brush development. In the present embodiment, the developing device of the reversal development system using the two-component magnetic brush development has been described as an example, but the developing device of the present invention is the reversal development system using the two-component magnetic brush development. However, the present invention is not limited to the above, but can be applied to a developing device using a one-component developer or a developing device of a normal development type. The average particle size of the toner used in this embodiment is 7 μm. The toner particle size may be about 10 μm which is used in a normal electrophotographic process, but from the viewpoint of image quality, it is preferably 7 μm or less. The rotary developing device 3 rotates each time the electrostatic latent image corresponding to each color is developed, and the electrostatic latent image is developed with the toner corresponding to the color. At this time, a bias voltage is applied to the developing roll 3a used for development, and toner adhesion to the background portion of the electrostatic latent image is suppressed.

The toner image obtained by the development reaches the transfer position facing the transfer drum 4 by the rotation of the photosensitive drum 1. Recording paper (not shown) conveyed from the paper tray 11 via a predetermined paper conveyance path 12 is attracted to the transfer drum 4 on the outer periphery of the transfer drum 4 by the operation of the paper suction charger 4a. The toner image formed on the photosensitive drum 1 is transferred to the transfer position facing the photosensitive drum 1 with the rotation of the transfer drum 4 in the direction of the arrow. Is electrostatically transferred onto the recording paper adsorbed on the outer periphery of the transfer drum 4. After the transfer, the photoconductor drum 1 has a cleaner 5
The residual toner is removed by means of the pre-exposure device 6, and the pre-exposure device 6 emits light to eliminate the charge, and the electrostatic latent image forming charger 2 is again provided.
Thus, charging for the next electrostatic latent image formation is performed. On the other hand, while yellow, cyan, magenta, and black toner images are sequentially formed on the photosensitive drum 1, one recording sheet is attracted to the transfer drum 4 and rotates with the rotation of the transfer drum 4. The recording paper attracted to the transfer drum 4 is also conveyed to the transfer position in synchronization with the arrival of the toner image of each color at the transfer position, and the toner images of each color are sequentially superimposed on the one recording paper. Is transferred to

When the transfer of the four color toner images of yellow, cyan, magenta and black onto the recording paper attracted to the transfer drum 4 is completed, the recording paper attracted to the transfer drum 4 is separated by the peeling charger 4c. As a result, the electrostatic attraction with the transfer drum 4 is removed, and the peeling claw 4e separates the transfer drum 4 from the transfer drum 4 to fix it by the fixing device 9, and then the paper is carried out to the outside of the apparatus. On the other hand, the transfer drum 4 from which the recording paper has been peeled is discharged by the discharging charger 4d, and when the image is formed again, the next recording paper is adsorbed in the same manner as described above.

FIG. 7 is a block diagram of the light beam scanning unit.
From the semiconductor laser 21, the light beam pulse width modulation circuit 3
A laser beam controlled to be turned on and off by 0 is emitted, the laser beam is collimated by a collimator lens 22, and the polygon mirror 23 rotates in the arrow direction.
Is repeatedly reflected and deflected by the
A light beam 25 carrying image information is repeatedly scanned on the photosensitive drum 1 in the main scanning direction (vertical direction in FIG. 7) via the fθ lens 24 for adjusting the spot diameter. The start time of each scan by the light beam 25 is sensed by the scan start signal generating optical sensor 26, and the SOS (Start Of Sca) indicating the start of each scan.
n) It is taken in as a signal.

FIG. 8 is a circuit block diagram of the light beam pulse width modulation circuit. As shown in FIG. 8, the light beam pulse width modulation circuit 30 includes a triangular wave oscillator 31 and a comparator 3.
2, a waveform selection circuit 33, and a D / A converter 34 are provided. The triangular wave oscillator 31 generates two types of triangular waves having different repetition frequencies as shown in FIG. In the waveform selection circuit 33, based on the discrimination signal for discriminating between the halftone image and the character image (binary image), a triangular wave having a low repetition frequency is generated for the halftone image and a triangular wave having a high repetition frequency is generated for the character image. The selected triangular wave is transmitted to the comparator 32. On the other hand, the image signal (digital signal) obtained by reading the image on the original by the original reading unit 10 is D / A.
It is input to the converter 34, converted into an analog signal, and input to the comparator 32. The comparator 32 compares the level of the analog image signal with the level of the triangular wave, and converts the level into a binary signal having a pulse width corresponding to the level of the analog image signal. This binary signal is input to the semiconductor laser 21 shown in FIG. 7, and the semiconductor laser 21 emits a laser beam that repeatedly turns on and off according to the binary signal.
In this method, one cycle of the triangular wave corresponds to the pixel pitch in the main scanning direction.

FIG. 9 is a block diagram of a developing device for one of the four developing devices constituting the rotary developing device 3 shown in FIG. This developing device has a cylindrical developing roll 91 that carries the developer by supporting the developer on its surface and rotating the same in a housing 92 that contains the developer in which magnetic particles and toner are mixed, and a developing roll. A developer regulating member 95 that regulates the amount of the developer attached to the surface of 91, and screw augers 93 and 94 that stir and convey the developer by rotating to supply the developer to the developing roll 91 are provided. There is.

The developing roll 91 has a built-in magnet roll which is fixed so as not to rotate, and is provided with a sleeve 97 which is rotatably supported around the magnet roll. The sleeve 97 is arranged so as to face the photoconductor 1 at a distance of 0.5 mm, and development is performed at portions where the surfaces of the sleeve 97 are close to each other. Further, the magnet roll disposed inside has a plurality of magnetic poles 96a, 96b, ..., 96e, and forms a magnetic brush of magnetic particles on the surface of the sleeve 97 by a magnetic field formed between adjacent magnetic poles. To do. Sleeve 97
Conveys the developer by its rotation.

A developing bias voltage in which alternating current is superposed on direct current is applied to the developing roll 91 from a direct current superposed alternating current voltage power source 98, and an electric field formed at a developing position close to the photosensitive drum 1 causes The charged toner adheres to the electrostatic latent image formed on the photosensitive drum 1. This applied voltage has a direct current component of the same polarity as the photosensitive member charging potential, and a development potential contrast (difference between the direct current component of the developing bias voltage and the overall exposure potential of the photosensitive member) with respect to the entire exposed portion potential of the photosensitive drum 1. Is set to about 300V.

The height of the developer layer at the position where the photosensitive drum 1 and the developing roll 91 are closest to each other is 2.0 mm when the developing roll 91 is sufficiently separated from the photosensitive drum 1. The gap between the developer regulating member 95 and the developing roll 91 is set so that The AC component of the developing bias voltage has an amplitude value of 1.2 KV in peak-to-peak value and a frequency of 6 kHz, and its waveform is a rectangular wave.

The developer used here is a mixture of toner having an average particle size of 7 μm and magnetic particles (ferrite carrier) having an average particle size of 50 μm, and the toner concentration is 7%. FIG. 10 is a diagram schematically showing a conventional basic idea regarding tone reproduction and a basic idea according to the present invention.

In a conventional so-called analog copying machine, as shown in FIG. 10A, a photosensitive member which responds linearly to exposure energy is ideal in terms of gradation reproduction of a halftone image. . In addition, the above-mentioned JP-A-1-169
In the electrophotographic apparatus for digital exposure described in Japanese Laid-Open Patent Publication No. 454, 1982, Japanese Patent Laid-Open No. 2-282277, and the like, as shown in FIG. , Is ideal because of the clear reproduction of character images.

On the other hand, the present embodiment is shown in FIG.
As shown in, a photosensitive member having both a dead region where the photosensitive member surface potential hardly responds to the exposure energy and a linearly changing response region is used, and the exposure energy at 100% exposure is set to the photosensitive member surface potential. By setting it in the saturated portion of (3), a character image and a halftone image can be expressed well under practical beam exposure conditions.

Hereinafter, FIG. 10 (a), FIG. 10 (b), and FIG.
The reproduction of dots and lines according to each basic idea of 0 (c) will be described, and the basic idea of FIG. 10 (c) will be compared with the basic idea (comparative example) of FIGS. 10 (a) and 10 (b). It is shown that (the present embodiment) is excellent. FIG. 11 is a diagram showing an exposure energy profile of a light beam that exposes a photoconductor.

11 (a), 11 (b) and 11
(C) is the light beam spot diameter d_b (Μm) is constant
Then, the distance d between adjacent pixels_p (Μm) and light beams
Pot diameter d _b When the ratio of D is D, the value of D is 1
It is the result when / 1, 1/2, 1/3, which figure
Also, the distance d between pixels_p Lighting pulse width for (μm)
(%) (To be exact, the optical beam pulse width modulation circuit 30 (see FIG.
Laser beam is emitted from the semiconductor laser 21.
The time it takes to control the ON state of the photosensitive drum 1
The value converted to scanning distance on the surface) is 50%, 20%, 10
The exposure energy profile at% is shown
You.

As is apparent from FIG. 11A, the exposure energy profile is not a binary (digital) profile but a dull (analog) profile in which a weak exposure energy intensity portion exists. Further, as the pulse width (%) is reduced, the contrast of the exposure energy profile decreases. FIG. 12 is a diagram showing the characteristics of a conventional photoconductor used in a so-called analog copying machine or the like, which is a photoconductor having a characteristic according to the concept of FIG.

When a halftone image is reproduced by using a photoconductor having an approximately linear photopotential decay characteristic as shown in FIG. 12, the surface potential profile of the photoconductor is shown in FIG. It became almost similar to the exposure energy profile shown and became blunt (analog-like)
It becomes the surface potential profile. 11 (b) and FIG.
As can be seen from (c), as the value of D is reduced to 1/2 or 1/3, the rounding is improved, but it is still analog, and the developed toner easily scatters and has a rough feeling. The image becomes a halftone image with low granularity. Further, when trying to reproduce a character image, the character image is configured by repeating on / off of lines so that the lighting pulse width (%) is 50% so that the character image is prominent in kanji characters.
Since it is similar to the exposure energy profile at%, the developed toner is likely to be scattered and the character quality lacks sharpness, as in the halftone image reproduction described above.

On the other hand, FIG. 13 shows the above-mentioned JP-A-1-169.
FIG. 11 is a diagram showing the characteristics of a photoconductor that realizes the basic idea (see FIG. 10B) proposed in Japanese Patent Laid-Open No. 454 or Japanese Patent Laid-Open No. 2-282277. In addition, FIG. 14 shows FIG.
Also, the exposure energy profile when D = 1/1 (FIG. 14A) and the photoconductor having the photopotential attenuation characteristics shown in FIG. 13 have the exposure energy profile shown in FIG. 14A. FIG. 15 is a diagram showing a result (FIG. 14B) of simulating a potential profile of an electrostatic latent image formed on the photoconductor when exposed by a light beam.

When a photoconductor having a photopotential decay characteristic that responds rapidly with a predetermined threshold energy as shown in FIG. 13 is used, the region exceeding the threshold energy shown in FIG. As shown in (b), the contrast of the exposure energy profile is maintained, the surface potential profile of the photoconductor is digital, and the developed toner is less likely to scatter, and clear character reproduction and clear dots or grain due to lines are observed. It is possible to reproduce halftone images with excellent degree.

However, the image is not reproduced at all in the region where the threshold energy is not exceeded. That is, the value of D is 1/1
In the case of, even a highlight area of 20% is not reproduced. D
As the value of is reduced to 1/2 or 1/3, gradation expression in the highlight region becomes possible, but it is still necessary to reduce the beam diameter. Particularly, in order to represent a 10% image and secure the dynamic range of gradation, the value of D is set to 1
The beam diameter must be reduced until it becomes about / 4.
It is possible to reproduce the highlight by setting the output of the light beam high, but in that case, the dynamic range for gradation reproduction is narrowed, which results in saturation of the density in the middle and high density areas.

As is conventionally known, in order to form a high quality image, the expression of the highlight part is important,
Due to the recent development of DTP (hard copy of displayed screen (Desk Top Publishing)), it is necessary to accurately express the highlight part in particular, but it is acceptable as an image quality level of a character image.
Assuming the realization of an image forming apparatus having a pixel density of dpi (dot per inch), when a gradation with a pulse width of 10% is expressed by narrowing the beam diameter,
The beam diameter in the main scanning direction is required to be about 20 μm, which is practically difficult to realize.

FIG. 15 is a diagram showing characteristics of the photoconductor that can be adopted in the embodiment of the present invention. When the photoconductor having the photopotential decay characteristics shown in FIG. 15 is used, if the value of D is about ½, dots and lines having a pulse width of 10% can be stably expressed. The surface potential profile of
It is more digital and, as a result, less scattered toner is developed.

In the image forming apparatus of the present invention, by using the photoconductor having such a photopotential decay characteristic,
It is possible to obtain smooth and stable gradation expression, and in the image forming apparatus that forms a color image, the effect of stabilizing the hue of the highlight portion is obtained. 16 (a),
16 (b) and 16 (c) respectively show a photoconductor showing the approximately linear photopotential decay characteristic shown in FIG. 12, and FIG.
A photoconductor having a binary photopotential decay characteristic shown in FIG. 6 and a photoconductor having the photopotential decay characteristic shown in FIG. 15 were scanned with a light beam modulated by an image signal corresponding to the same highlight portion. It is a simulation of the potential profile of the electrostatic latent image formed on each photosensitive member.

16 (a), 16 (b) and 16
As can be seen by comparing (c), this embodiment (FIG.
By (c), a clear electrostatic latent image is formed without missing the highlight portion. This allows
It achieves both smooth gradation expression of highlights, clear character reproduction, and image reproduction with excellent granularity due to clear dots and lines.

[0055]

EXAMPLES Examples of the present invention and comparative examples will be described below. FIG. 17 is a sectional view showing the structure of the photoconductor. Hereinafter, the photoconductors used in Examples and Comparative Examples of the present invention will be described with reference to FIG. 17, and then Examples and Comparative Examples of the present invention will be described.

From the photoconductor A to the photoconductor K and the photoconductor M, which will be described below, the potential attenuation is less likely to occur with respect to a weak light input, and the potential attenuation occurs in response to a certain light input. The photoconductor has a photopotential decay characteristic and exhibits a so-called S-shaped photopotential decay characteristic in which the photopotential decay curve with respect to the input exposure energy has an inflection point.
Further, the photoconductor L exhibits a so-called J-shaped potential attenuation that is generally used and is approximately proportional to the amount of incident light.
It is a function-separated type photoreceptor including a charge generation layer and a charge transport layer using an organic semiconductor.

The photopotential decay control layer referred to in the present invention is a layer in which charge transporting domains are dispersed in an electrically inactive matrix, and has two phases, an electrically inactive phase and a charge transporting phase. It is characterized by a heterogeneous system consisting of Specific examples of the photopotential decay control layer include a material having a charge-transporting ability dispersed in an insulating resin (electrically inactive matrix) in a microcrystalline (charge-transporting domain) state. , But is not limited to this.

The charge-transporting layer referred to in the present invention comprises a charge-transporting matrix, and the charge-transporting phase 1
It is characterized by a homogeneous system consisting of phases. Specific examples of the charge transport layer include, but are not limited to, those obtained by solid-solving a material having a charge transport ability in a binder resin in a molecular state, and a charge transporting polymer.

(Photoreceptor A) This photoreceptor A is shown in FIG.
As shown in (a), it comprises a conductive support 111, a charge generation layer 112, a photopotential attenuation control layer 113, and a charge transport layer 114. As the conductive support 111, an aluminum cylindrical pipe was used. The charge generation layer 112 is
It consists of dichlorotin phthalocyanine pigment, which is a photoconductive pigment, and polyvinyl butyral resin. Each of them is mixed and dispersed in a solvent at a ratio of pigment: 2, resin: 1, and coated on an aluminum pipe to obtain 0.2 μm. The charge generation layer 112 was formed.

The photopotential attenuation control layer 113 is composed of hexagonal selenium fine particles and vinyl chloride-vinyl acetate copolymer resin, and is mixed / dispersed in a solvent and coated on the charge generation layer 112 to have a thickness of 2 μm. Photopotential decay control layer 113
Was formed. The volume ratio of the hexagonal selenium fine particles in this photopotential attenuation control layer was about 30%. The average particle size of the hexagonal selenium fine particles was 0.05 μm.

Next, a coating solution prepared by dissolving a compound having a molecular weight of 80,000, which is a repeating unit represented by the following structural formula (1), which is a polymer charge transport material, in a solvent is applied to the photopotential attenuation control layer 1 described above.
A charge transport layer 114 having a film thickness of 20 μm was formed by applying it on 13 by a dip coating method and drying. This photoconductor exhibits the photopotential decay characteristics shown in FIG. Also,
The ratio x / L of the film thickness x of the photopotential attenuation control layer to the total film thickness L of this photoreceptor was 0.090.

[0062]

Embedded image

(Photoreceptor B) Photoreceptor A except that the addition amount of the hexagonal selenium fine particles was changed to change the volume ratio of the hexagonal selenium fine particles in the photopotential attenuation control layer from 30% to 35%.
A photoconductor was prepared in the same manner as in. (Photoreceptor C) The volume ratio of the hexagonal selenium fine particles in the photopotential attenuation control layer was changed to 3 by changing the addition amount of the hexagonal selenium fine particles.
A photoconductor was prepared in the same manner as the photoconductor A except that the content was changed from 0% to 40%.

(Photoreceptor D) The amount of the hexagonal selenium fine particles added was changed to change the volume ratio of the hexagonal selenium fine particles in the photopotential attenuation control layer from 30% to 20%. A photoreceptor was prepared in the same manner as the photoreceptor A except that the thickness was changed to 1.2 μm. The ratio x / L of the film thickness x of the photopotential attenuation control layer to the total film thickness L of this photoconductor was 0.056.

(Photoreceptor E) By changing the amount of hexagonal selenium fine particles added, the volume ratio of the hexagonal selenium fine particles in the photopotential attenuation control layer was changed from 30% to 45%, and the film of the photopotential attenuation control layer was changed. Change the thickness to 5 μm and change the thickness of the charge transport layer to 15 μm.
A photoconductor was prepared in the same manner as photoconductor A except that m was changed. The ratio of the film thickness x of the photopotential attenuation control layer to the total film thickness L of this photoconductor: x / L was 0.248.

(Photoreceptor F) Photoreceptor E except that the addition amount of the hexagonal selenium fine particles was changed to change the volume ratio of the hexagonal selenium fine particles in the photopotential attenuation control layer from 45% to 55%.
A photoconductor was prepared in the same manner as in. (Photoreceptor G) By changing the addition amount of hexagonal selenium fine particles, the volume ratio of the hexagonal selenium fine particles in the photopotential attenuation control layer was adjusted to 2
A photoconductor was prepared in the same manner as the photoconductor D except that the content was changed from 0% to 15%.

(Photoreceptor G) Photoreceptor D except that the amount of hexagonal selenium fine particles added was changed to change the volume ratio of hexagonal selenium fine particles in the photopotential attenuation control layer from 20% to 15%.
A photoconductor was prepared in the same manner as in. (Photoreceptor H) A photoreceptor was prepared in the same manner as the photoreceptor A except that the film thickness of the photopotential attenuation control layer was changed to 0.5 μm. The ratio of the film thickness x of the photopotential attenuation control layer to the total film thickness L of this photoconductor: x / L was 0.024.

(Photoreceptor I) The film thickness of the photopotential attenuation control layer is set to 1
A photoreceptor was prepared in the same manner as the photoreceptor A except that the thickness was changed to 0 μm and the thickness of the charge transport layer was changed to 10 μm. Ratio of the film thickness x of the photopotential attenuation control layer to the total film thickness L of this photoconductor:
x / L was 0.049. (Photoreceptor J) A photoreceptor was prepared in the same manner as the photoreceptor A except that the film thickness of the photopotential attenuation control layer was changed to 0.2 μm. The ratio of the film thickness x of the photopotential attenuation control layer to the total film thickness L of this photoreceptor: x / L was 0.009.

(Photoreceptor K) The film thickness of the photopotential attenuation control layer is set to 1
A photoreceptor was prepared in the same manner as the photoreceptor C except that the thickness was changed to 5 μm and the thickness of the charge transport layer was changed to 5 μm. Ratio of film thickness x of photopotential attenuation control layer to total film thickness L of this photoconductor: x
/ L was 0.74. (Photoreceptor L) The photoreceptor L is, as shown in FIG.
Conductive support 111, intermediate layer 115, charge generation layer 11
2. The charge transport layer 114. Conductive support 1
As 11, a cylindrical pipe made of aluminum was used. For the intermediate layer 115, a 0.2 μm thick methoxymethylol nylon resin was used. Charge generation layer 11
No. 2 was the same as the photoconductor A except that the hydroxygallium phthalocyanine pigment was used instead of the dichlorotin phthalocyanine pigment. The charge transport layer 114 is the same as the photoconductor A. This photoconductor exhibits the photopotential decay characteristics shown in FIG.

(Photoreceptor M) The photoreceptor M is disclosed in
No. 169454, the conductive support 11 is made based on the technique disclosed in FIG. 17 (b).
1, an intermediate layer 115, and a photoconductor layer 116. As the conductive support 111, an aluminum cylindrical pipe was used. The thickness of the intermediate layer 115 is 0.2 μm.
The methoxy methylolated nylon resin of was used. The photoconductor layer 116 is mainly composed of phthalocyanine fine particles which are a photoconductive pigment having a particle diameter of 0.1 to 1.0 μm, and is mixed and dispersed by using a heat-effective resin made of polyester and a solvent, and is then dispersed on the intermediate layer 115. It is formed by applying, coating and drying. The layer thickness of the photoconductor layer 116 was 25 μm. This photoconductor exhibits the photopotential decay characteristics shown in FIG.

In Table 1, charging potential: Vh, developing bias potential: Vb, of Examples and Comparative Examples using the respective photoconductors,
100% exposed portion potential: VL ', photoconductor type, ratio of light energy width E2 of response area to light energy width E1 of insensitive area: E2 / E1, photopotential attenuation control layer relative to total film thickness L of photoreceptor Ratio of film thickness x: x / L, photopotential decay control layer thickness and charge transport layer thickness, and photopotential decay control layer domain%
Is shown.

[0072]

[Table 1]

Table 2 shows the evaluation results of the gradation characteristics of Examples 1 to 7 and Comparative Examples 1 to 6. The light beam output was adjusted so that a 10% pulse width input signal could be represented. In addition, a beam diameter in the main scanning direction was 42 μm, and a 300-line vertical line screen was used. As is clear from Table 1 and Table 2,
If a photosensitive member such as the photosensitive member M having a ratio E2 / E1 of less than 0.3 is used, which responds very sharply, under the above conditions,
It is difficult to obtain good gradation reproduction characteristics.

[0074]

[Table 2]

Table 3 also shows the granularity of halftone images corresponding to a pulse width input signal of 20% in Examples 1 to 7 and Comparative Examples 1 to 6, and
The evaluation results of the reproducibility of 6-point characters in Gothic font are shown.

[0076]

[Table 3]

As is clear from Tables 1 and 3, the ratio E2 /
In the case of a photoconductor in which E1 is larger than 1.3,
As shown in FIG. 6A, the profile of the surface potential of the photoconductor is likely to be analog, so that the toner is likely to be scattered, the granularity of the halftone image is deteriorated, and the characters are crushed significantly. The ratio E2 / E1 is 1.
It is more preferably 0 or less.

When the evaluation results of Tables 2 and 3 are viewed from the constitutional conditions of the photoconductor, in order to make the ratio E2 / E1 1.3 or less, the thickness x of the photopotential attenuation control layer and the photoconductive layer are set. Of the thickness L: x / L is preferably in the range of 0.01 to 0.5, and the volume ratio of the charge transporting domain in the photopotential attenuation control layer is 20% or more and 50% or less. Is preferred.

FIG. 18 is a diagram showing the evaluation results of Tables 2 and 3. If 0.3 <E2 / E1 <1.3,
Both the granularity / character reproducibility and gradation characteristics are maintained in a substantially good state. In Table 4, the charging potential: Vh, the developing bias potential: Vb, of the examples and comparative examples using the respective photoconductors,
100% exposed area potential: VL ', and the ratios E2 / E1 and E1 / E0 at that time are shown.

[0080]

[Table 4]

Table 5 shows Examples 8 to 1 shown in Table 4.
Under each condition of 0 and Comparative Example 7 to Comparative Example 10, the charging potential Vh is ± with respect to the values shown in Table 4.
The results of evaluation of changes in gradation characteristics when the voltage is changed by 20 V are shown. This evaluation simulates a case where the charging characteristics of the photoconductor change due to environmental changes, changes over time, and the like.

[0082]

[Table 5]

As is clear from Tables 4 and 5, the ratio E1 /
When E0 is larger than 0.6, that is, when the surface potential of the photoconductor is not sufficiently saturated with respect to the exposure energy at the time of 100% exposure, the photoconductor and the bias are set. The gradation characteristics, especially the shadow part (high-density part) are likely to fluctuate with respect to changes in the potential. Also, the ratio E1
When / E0 is less than 0.2, that is, when the exposure energy setting value at 100% exposure is excessively large with respect to the photopotential decay characteristic of the photoconductor, the gradation with respect to the change in the charging potential of the photoconductor is obtained. The characteristics, especially the highlight part (low density part), are likely to change. Further, in the case of a photoconductor having a ratio E2 / E1 of less than 0.3, even if the ratio E1 / E0 is appropriate, density fluctuations in the intermediate density portion occur with changes in the charging potential of the photoconductor.
That is, if the ratio E1 / E0 is set between 0.2 and 0.6, gradation variation is small and stable from the highlight portion to the shadow portion (high density portion).

Table 6 shows the conditions of Example 1 and Examples.
Beam diameter D in the main scanning direction from condition 1_bhAnd in the main scanning direction
Pixel pitch D_shRatio D_bh/ D_sh1/3 (Example 1
1) of the gradation characteristics when changed to 1 (Comparative Example 11)
The evaluation results are shown. Ratio D_bh/ D _shIs at least 1/2 or less
Then linear from the highlight area to the shadow area
It can be seen that the gradation can be expressed.

[0085]

[Table 6]

Table 7 shows the conditions of the first embodiment and the beam diameter D _bv in the sub-scanning direction, the pixel pitch D _{sv in the} sub-scanning direction and the ratio D _bv / D _sv of 0.7 (from the conditions of the first embodiment). Example 1
2), 1.3 (Example 13), 0.5 (Comparative Example 12),
The evaluation result of the gradation characteristic when changed to 1.5 (Comparative Example 13) is shown. It can be seen that if the ratio D _bv / D _sv is in the range of 0.7 to 1.3, the dynamic range is secured from the highlight portion to the shadow portion, and gradation expression can be performed well.

[0087]

[Table 7]

Table 8 shows gradation images of gray patch images obtained by superimposing halftone images of yellow, magenta, and cyan under the experimental conditions of Example 2, Comparative Example 5, and Comparative Example 6. It is a result of evaluating the gray balance in the entire gradation image and the color unevenness of each gray color patch image when created.

[0089]

[Table 8]

As described above, according to the embodiment of the present invention, the gray balance of a color image is maintained well and the occurrence of color unevenness is suppressed. (Example 15) Gradation characteristics and granularity were evaluated in the same manner as in Example 2 except that the light beam scanning unit 20 shown in Fig. 6 was replaced with an LED array exposure device that performs exposure with arrayed LEDs. Regarding gradation characteristics, it was possible to reproduce gradation linearly from the highlight area to the shadow area. In addition, good results were also obtained regarding granularity. (Comparative Example 16) Gradation characteristics and granularity were evaluated in the same manner as in Comparative Example 1 except that the light beam scanning unit 20 shown in FIG. 6 was replaced with an LED array exposure apparatus that performs exposure with arrayed LEDs. Regarding gradation characteristics, gradation can be linearly reproduced from the highlight area to the shadow area, but regarding the granularity, toner scattering is remarkable and the image is rough.

[0091]

As described above, according to the image forming apparatus and the image forming method of the present invention, a character image can be formed with a sufficient image quality while having a practical light beam spot diameter, for example, a light spot diameter of 30 μm or more. A halftone image having a wide dynamic range, for example, gradation expression of 50 gradations or more, can be achieved while maintaining the number of dots or the number of lines of 300 dpi or more for reproduction, and a halftone image excellent in granularity. It can be reproduced.

[Brief description of drawings]

FIG. 1 is a diagram showing a photopotential decay characteristic of a photoconductor used in an image forming apparatus of the present invention.

FIG. 2 is an E of a photoconductor used in the image forming apparatus of the present invention.
2 is a graph showing the relationship between 2 / E1 and gradation stability and line / character reproducibility.

FIG. 3 is an explanatory diagram of an in-plane color difference ΔE.

FIG. 4 is an explanatory diagram of edge widths.

FIG. 5 is a graph showing the relationship between the line / character reproducibility and the ratio E1 / E0 of the light energy width E1 in the dead region to the energy setting value E0 of the light beam.

FIG. 6 is a schematic configuration diagram showing a color copying machine to which the present invention is applied.

FIG. 7 is a schematic configuration diagram showing a light beam scanning unit.

FIG. 8 is a circuit block diagram showing an example of a pulse width modulation circuit.

FIG. 9 is a schematic view showing a structure of a developing device.

FIG. 10 is a schematic diagram illustrating a difference between the basic idea of the conventional image forming apparatus and the basic idea of the present invention.

FIG. 11 is a diagram showing an exposure energy profile.

FIG. 12 is a graph showing photoresponse characteristics and bias settings of a conventional photosensitive member.

FIG. 13 is a diagram illustrating a problem in a conventional image forming apparatus.

FIG. 14 is a diagram showing an exposure energy profile (a) and a potential profile of an electrostatic latent image.

FIG. 15 is a diagram showing the photopotential attenuating property and bias setting of the photoconductor that can be adopted in the present embodiment.

FIG. 16 is a diagram showing a potential profile of an electrostatic latent image.

FIG. 17 is a sectional view showing the structure of a photosensitive drum.

FIG. 18 is a diagram showing evaluation results of granularity and gradation characteristics.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Photoconductor drum 2 Electrostatic latent image forming charger 3 Rotating developing device 3a Developing roll 4 Transfer drum 4a Recording material adsorbing charger 4b Transfer charger 4c Peeling charger 4d Discharging charger 4e Peeling claw 5 Cleaner 6 Pre-exposure device 8 Potential sensor 9 Fixing device 10 Document reading unit 11 Paper tray 12 Paper transport path 20 Light beam scanning unit 21 Semiconductor laser 22 Collimator lens 23 Polygon mirror 24 fθ lens 25 Light beam 26 Scanning start signal generation sensor 30 Pulse width Modulator 31 Triangle wave oscillator 32 Comparator 33 Waveform selection circuit 34 D / A converter 91 Developing roll 92 Housing 93, 94 Screw auger 95 Developer regulating member 96a, 96b, ..., 96e Magnetic pole 97 Sleeve 98 DC superimposed AC voltage Power supply 111 Conductive support 112 Electric charge Namaso 113 light potential attenuation control layer 114 the charge transport layer 115 intermediate layer 116 photosensitive layer

Front Page Continuation (72) Inventor Nobuyuki Kato 430 Sakai, Nakai-cho, Ashigarakami-gun, Kanagawa Green Tech Nakakai Fuji Xerox Co., Ltd.

Claims (10)

[Claims] 1. A photoconductor that moves in a predetermined sub-scanning direction and on which an electrostatic latent image is formed, a charging unit that charges the photoconductor, and a predetermined photoconductor on the charged photoconductor that intersects the sub-scanning direction. Exposure means for forming an electrostatic latent image on the photoconductor by irradiating a plurality of pixels lined up in the main scanning direction simultaneously or sequentially with a light beam modulated according to image information,
And a developer carrying member carrying a developer on the surface and carrying the developer to a developing position facing the photoconductor, by developing the electrostatic latent image formed on the photoconductor. In an image forming apparatus including a developing unit for forming a visible image on the photoconductor, the photoconductor, the charging unit, and the exposing unit set the charging potential of the photoconductor by the charging unit to VH, The saturated surface potential of light attenuation is VL, the energy setting value of the light beam is E0, and the light energy width of the dead region where the surface potential of the photoconductor is VH or lower and VH− (VH−VL) / 5 or higher is E1. , The surface potential of the photoconductor is V
H- (VH-VL) / 5 or less VL + (VH-VL) / 5
The optical energy width of the response region that remains at the above potential is E2
The image forming apparatus is adjusted so that the relations of 0.3 <E2 / E1 <1.3 and 0.2 <E1 / E0 <0.6 are satisfied. 2. The photosensitive member, the charging unit, and the exposing unit have a relationship of 0.3 <E2 / E1 <1.0 instead of the relationship of 0.3 <E2 / E1 <1.3. The image forming apparatus according to claim 1, wherein the image forming apparatus is adjusted so as to be satisfied. 3. A charge generating layer for generating charges on a substrate and a charge generating layer for storing a charge injected when the injected charge amount is relatively small, and an injected charge when the injected charge amount is relatively large. 2. The image forming apparatus according to claim 1, further comprising a photosensitive layer including a photopotential decay control layer that transports the generated charges and a charge transport layer that transports the charges. 4. The photosensitive member satisfies 0.01 ≦ x / L ≦ 0.5, where x is the thickness of the photopotential attenuation control layer and L is the thickness of the photosensitive layer. The image forming apparatus according to claim 3, wherein the thickness of each layer is adjusted. 5. The photopotential decay control layer is formed by dispersing charge transport domains in an electrically inactive matrix, and the volume fraction of the charge transport domains in the photopotential decay control layer is 20%. The image forming apparatus according to claim 3, wherein the image forming apparatus is adjusted to 50% or less. 6. The exposing means comprises:
The beam diameter in the main scanning direction is D_bh, The pixel pixel in the main scanning direction
D_Sh, The beam diameter in the sub-scanning direction is D _bv, Said vice
The pixel pitch in the scanning direction is D_svAnd then D_bh/ D_Sh≤ 1/2 0.7 ≤ D_bv/ D_svThe light beam is directed onto the photoconductor so that the relationship of ≦ 1.3 is satisfied.
The irradiation according to claim 1, wherein
Image forming device. 7. The developing means brings the toner, in which magnetic particles and toner are mixed, into contact with or in proximity to the photoconductor at the developing position, so that the toner is removed.
The image forming apparatus according to claim 1, wherein the image forming apparatus is attached on the photoconductor in a pattern corresponding to an electrostatic latent image formed on the photoconductor. 8. The developing means is capable of developing any color toner of a plurality of color toners, and one color toner for each electrostatic latent image. The image forming apparatus is provided with a transfer means for transferring the visible images of the toners of the respective colors formed on the photosensitive member so as to be sequentially superposed on a predetermined transfer target. The image forming apparatus according to claim 1, wherein: 9. A charging step of charging a photosensitive member on which an electrostatic latent image is formed, which moves in a predetermined sub-scanning direction, and a predetermined main scanning direction intersecting the sub-scanning direction on the charged photosensitive member. By irradiating a plurality of lined pixels simultaneously or sequentially with a light beam modulated according to image information,
An exposure step of forming an electrostatic latent image on the photoconductor, and a developing step of forming a visible image on the photoconductor by developing the electrostatic latent image formed on the photoconductor, In the charging step and the exposing step, the charging potential of the photoconductor in the charging step is VH, the saturated surface potential of light attenuation of the photoconductor is VL, the energy setting value of the light beam is E0, and the surface of the photoconductor is Potential is VH or less VH- (V
H-VL) / 5, the light energy width of the dead region that remains at a potential of E1 or more is E1, and the surface potential of the photoconductor is VH- (VH
When the optical energy width of the response region that stays at a potential of −VL) / 5 or less and VL + (VH−VL) / 5 or more is E2, 0.3 <E2 / E1 <1.3 and 0.2 <E1 / E0 An image forming method, wherein the image forming method is adjusted so as to satisfy the relationship of <0.6. 10. The photoconductor as a photoconductor, comprising a charge generation layer for generating charges on a substrate, and storing the injected charges when the injected charge amount is relatively small and injected when the injected charge amount is relatively large. 10. The image forming method according to claim 9, wherein a photoconductor having a photosensitive layer composed of a photopotential decay control layer for transporting the generated charges and a charge transport layer for transporting the charges is used.