JP2002361922A - Imaging apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an imaging apparatus in which high image quality excellent in reproducibility of a microdot and gradation can be realized without complicating an exposing unit even when an a-Si photosensitive body is used. SOLUTION: A semiconductor laser is employed in an exposing unit, and the rising part of optical output from the semiconductor laser is provided with an overshoot of overexposure intensity P1 for holding an electrostatic latent image strongly by overexposing an amorphous silicon photosensitive body. Following to the overshoot, an exposure intensity P2 for forming an electrostatic latent image corresponding to the gradation on the amorphous silicon photosensitive body depending on the pulse width corresponding to the gradation of dots forming the image is provided. A microdot can be reproduced by the overshoot of overexposure intensity P1 and gradation can be reproduced by the exposure intensity P2 .

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus (electrophotographic apparatus) such as a copying machine, a printer or a facsimile using amorphous silicon (hereinafter referred to as "a-Si") as a photosensitive member. Even when using an a-Si photoreceptor, which has conventionally been difficult to achieve a high resolution of 1200 dpi or more because of a low ability to hold an electrostatic latent image, excellent reproducibility of fine dots and excellent gradation are obtained. The present invention relates to an image forming apparatus capable of realizing high image quality.

[0002]

2. Description of the Related Art Conventionally, in an image forming apparatus using an electrophotographic method, such as a copying machine, a printer, and a facsimile, the surface of a photoreceptor is uniformly charged in the dark, and then exposed by an exposure apparatus using a laser beam or the like. A desired electrostatic latent image is formed on the charged photoreceptor surface by exposing the body, and toner is supplied to the electrostatic latent image to visualize the electrostatic latent image to form an image. Then, the image visualized by the toner is transferred to a recording medium and fixed to perform printing, and
The untransferred toner is removed by a cleaning means or the like.

As a photoreceptor used in such an image forming apparatus, an OPC (organic photoreceptor) is conventionally known. However, the OPC photosensitive member has a problem in durability because the photosensitive member surface is soft and the photosensitive layer is easily worn by the cleaning means, the toner, and the recording medium. Therefore, the photoconductor surface is harder than the OPC photoconductor,
Amorphous silicon (a-S) is considered from the viewpoints of wear resistance, durability, function retention (maintenance), and environmental considerations.
i) Photoreceptors have been widely used.

[0004] Recently, there has been a strong demand for such an image forming apparatus to have higher speed, higher image quality and higher resolution.
However, the a-Si photoreceptor has a volume resistivity of 1 × 10 ^{9 to 11 as} compared with the OPC photoreceptor (organic photoreceptor).
(Ωcm), and the ability to hold the electrostatic latent image written on the photoreceptor surface by the exposure device is low.
It has been considered difficult to obtain high-resolution image quality of dpi or more, that is, a dot-to-dot pitch of 21.1 μm or less. In particular, when the total film thickness of the a-Si photoreceptor is 20 μm or more, the dark decay characteristic becomes faster than that of the OPC photoreceptor, so that the above-described phenomenon of image blur or flow of a latent image becomes remarkable, and the resolution decreases. There was a problem that was significant.

Therefore, in order to improve the reproducibility of the minute dots, the electrostatic latent image must be maintained from the time when the electrostatic latent image is formed by the exposure device until the electrostatic latent image reaches the developing area.
That is, it is necessary to prevent the flow of the electrostatic latent image.
As described above, the flow of the electrostatic latent image depends on the film thickness of the photoconductor and the dark decay characteristics, but is also greatly affected by the light emission rising characteristics of the laser. That is, by overexposing the photosensitive member with laser light, the electrostatic latent image is strongly held. However, when the photoreceptor is overexposed, the gradation of the halftone image is lost and the image quality tends to be poor in contrast.
Therefore, the emission intensity of the laser for obtaining an image quality with excellent reproducibility of minute dots and the emission intensity of the laser for obtaining an image quality with excellent gradation are required to be different. An exposure apparatus having emission intensity is required.

As a technique for obtaining such high-resolution image quality, for example, Japanese Unexamined Patent Publication No. Hei 9-114206 discloses a method of generating a waveform having a sharp peak within the writing time of each pixel by an output control wave generating means. In addition, an apparatus that controls the emission light of laser light with this waveform to concentrate exposure energy, prevents interference between pixels without changing the beam diameter of laser light, and improves the reproducibility of fine lines is shown. ing.

Japanese Patent Application Laid-Open No. 2000-127498 discloses that when a laser beam is subjected to pulse width modulation to reproduce a gradation, the laser beam is prevented from being reproduced accurately in a highlight portion and a dark portion due to a Gaussian distribution. For,
The surface of the photoreceptor is charged to a potential VH1 higher than the potential VH at which the toner does not adhere, and the laser is exposed at a background exposure intensity of intensity 1 at which the surface potential of the photoreceptor is VH. Is reduced to a potential lower than the surface potential VL suitable for attaching toner, and an exposure intensity of excessive intensity 2 can be generated.
And the exposure of intensity 2 are given in combination, thereby controlling the surface potential of the image area to VL and controlling the contrast to be large, and transmitted in a software or hardware manner. By correcting the image information, the contrast after exposure is increased without modulating the laser beam intensity in multiple steps, without reducing the beam diameter, and preventing ripples from occurring. Is improved so that reproducibility and gradation of minute dots can be obtained.

[0008]

However, Japanese Patent Application Laid-Open Nos. 9-114206 and 2000-127 disclose these problems.
The technique described in Japanese Patent Publication No. 498 is not supposed to use the a-Si photosensitive member described above.
0 dpi or more, that is, the pitch between dots is 21.1 μm
It is not for obtaining the following high-resolution image quality.
Further, these techniques are disclosed in, for example, Japanese Patent Application Laid-Open No. 9-114206.
The technique disclosed in the above publication requires output control wave generation means for generating a waveform having a sharp peak within the writing time of each pixel, and requires complicated control. Also, Japanese Patent Application Laid-Open
The technology described in Japanese Patent Application Laid-Open No. 0-127498 is based on the background exposure intensity of intensity 1 where the photoconductor surface potential is VH by a laser beam, and the intensity of the background exposure is higher than intensity 1. Suitable surface potential VL
A mechanism for providing an excessive intensity 2 for lowering the potential to a lower potential, and an exposure intensity of 2 or an exposure OFF
A mechanism to generate a pulse is required, which is soft,
It has to be complicated in terms of hardware.

In view of the above circumstances, the present invention can realize high image quality excellent in reproducibility of fine dots and gradation without using an a-Si photosensitive member without complicating the exposure apparatus. It is an object to provide an image forming apparatus.

[0010]

According to the present invention, there is provided a photosensitive member using amorphous silicon having a photosensitive layer as a thin film.
An exposure device configured to form an electrostatic latent image with a semiconductor laser that provides a light output corresponding to each dot forming the image on the photoconductor, and an image forming apparatus that forms an image by electrophotography. The light output of the semiconductor laser constituting the exposure apparatus is over-exposed at an over-exposure intensity P1 for over-exposing the amorphous silicon photoreceptor having the photosensitive layer as a thin film at the rising portion of the light output to strongly hold an electrostatic latent image. A shoot and an exposure intensity P2 for forming electrostatic dots corresponding to the gradation on the amorphous silicon photoreceptor with a pulse width corresponding to the gradation of the dots constituting the image; It is characterized in that it is possible to reproduce minute dots by overshoot and to reproduce gradation by exposure intensity P2.

By doing so, the overshoot of the overexposure intensity P1 for overexposing the amorphous silicon photoreceptor to strongly retain the electrostatic latent image is generated by simply giving a sharp rising pulse to the semiconductor laser. Therefore, as described in JP-A-9-114206 and JP-A-2000-127498, output control wave generating means for generating a waveform having a sharp peak within the writing time of each pixel and generating a plurality of intensities are provided. It is not necessary to use a complicated mechanism in terms of software and hardware such as a mechanism for turning off the exposure and a mechanism for turning off the exposure.
Also, an image forming apparatus that can realize high image quality with excellent gradation can be provided.

The semiconductor laser used in the exposure apparatus of the image forming apparatus according to the present invention has a pulse width of an overshoot of the exposure intensity P1. Strength P1
The overshoot pulse width is set to 50 ns or less, and the ratio of the exposure intensities P1 and P2 is set to 1.2 to 5.
It is characterized by being set to 0.

[0013] That is, when the pulse width W ₂ of the overshoot portion P1 is equal to or greater than 50 ns, with the gradation failure occurs in the dots for light emission intensity is high, the value of P1 P
If the value is more than 5 times the value of 2, the light emission intensity of the overshoot portion P1 is too strong, and a gradation defect occurs. Therefore, as described in claim 2, the pulse width of the overshoot of the exposure intensity P1 is 50 ns or less, and
By setting the ratio of P2 and P2 to 1.2 to 5.0, it is possible to provide an image forming apparatus capable of realizing high image quality with excellent reproducibility of fine dots and gradation. When the pulse width of the overshoot is 50 ns or less, the pulse width does not include 0 ns and naturally indicates a value larger than 0 ns.

The oscillation wavelength of the semiconductor laser is, as set forth in claim 3, characterized in that the oscillation wavelength λ of the semiconductor laser ranges from 600 nm to 720 nm.

That is, the a-Si photosensitive member has a light absorption peak near about 680 nm of the spectral sensitivity characteristic,
m or less, or 720 nm or more, the sensitivity characteristics are reduced and a semiconductor laser having a higher emission intensity must be used, and interference fringes are more likely to occur, and once exposed portions are recharged. Sometimes, a so-called optical memory, which is lower than the original set potential, becomes prominent. Therefore, as described in claim 3 above,
By using the wavelength within the range of 600 to 720 nm,
Good results can be obtained in terms of spectral sensitivity characteristics, interference fringes, optical memory, and the like, and an image forming apparatus capable of realizing high image quality with excellent reproducibility of fine dots and excellent gradation can be provided.

The amorphous silicon photoconductor and the surface potential used in the present invention have a three-layer structure of a carrier blocking layer, a photosensitive layer, and a surface protection layer. Having a photosensitive layer thickness of 10 μm to 20 μm and a surface potential of +2
The range is from 00V to + 500V.

That is, when the film thickness of the photoreceptor is 20 μm or more, the moving speed of the heat carrier is increased and the dark decay characteristic is reduced. As a result, the flow of the latent image toward the surface of the photoreceptor tends to occur. In addition to causing a decrease in resolution, the thicker the photoreceptor, the longer the film formation time and the higher the cost, and the longer the film formation time, the higher the probability of adhesion of foreign matters and the like, and the lower the yield. On the other hand, when the thickness of the photoconductor is less than 10 μm, the charging ability of the photoconductor is low, and it is difficult to obtain a predetermined surface potential. Dependent), the dielectric breakdown occurs due to repeated charging, resulting in a black spot image, and the irregular reflection of the laser light on the surface of the conductive substrate causes interference fringes in the half pattern. .

Therefore, when the photosensitive layer has a thickness in the range of 10 to 20 μm as described in claim 4, the photosensitive layer has excellent charging ability, withstand voltage, dark decay characteristics, and manufacturing cost, and has reproducibility of fine dots. High image quality with excellent gradation can be realized. If the surface potential of the photoreceptor is less than +200 V, the developing electric field becomes insufficient, and it becomes difficult to secure image density. On the other hand, if the voltage exceeds +500 V, there are problems such as insufficient charging ability depending on the thickness of the photoreceptor, easy occurrence of black spots due to dielectric breakdown, and increase in the amount of ozone generated. When the thickness is reduced, the charging ability of the photoconductor tends to decrease correspondingly. Therefore, the value of the surface potential is increased by +200
By setting the voltage in the range of V to +500 V, it is possible to obtain excellent image quality with excellent charging ability, prevention of dielectric breakdown, increase of ozone generation, etc., and excellent reproducibility of fine dots and gradation. Can be provided.

[0019]

Embodiments of the present invention will be illustratively described in detail below with reference to the drawings. However, the dimensions, materials, shapes, relative arrangements, and the like of the components described in this embodiment are not intended to limit the scope of the present invention to them, unless otherwise specified. This is just an example.

FIG. 1 is a view schematically showing the structure of an image forming apparatus according to the present invention, and FIG. 2 is a view showing an example of the structure of an a-Si photosensitive drum similarly used in the image forming apparatus according to the present invention. FIG. 3 is a diagram for explaining an output waveform of the laser beam in the image forming apparatus according to the present invention, FIG. 4 illustrates the reproducibility of the relationship between the pulse width W ₂ and the minute dots of overshoot portion P1 of the laser beam graph for, 5 graphs for explaining the pulse width W ₂ and the gradation of the relationship overshoot portion P1, 6 levels P1 and P2 of the laser light ratio (P1 / P2) and the small dots FIG. 7 is a graph for explaining the relationship of reproducibility, and FIG. 7 shows laser light levels P1 and P2.
8 is a graph for explaining the relationship between the ratio (P1 / P2) and the number of gradations. FIG. 8 is a block diagram of a control circuit of a semiconductor laser constituting an exposure apparatus in the image forming apparatus of the present invention.

In FIG. 1, 1 is an a-Si photosensitive drum, 2 is a charger such as a scorotron, 3 is an exposure device including a semiconductor laser (not shown), 4 is a developing device, 5 is a transfer roll, 6 is cleaning means, 7 is an erasing means, 8 is an optical signal, and in FIG.
Is a carrier blocking layer, 22 is a photosensitive layer, and 23 is a surface protective layer. In FIG. 8, reference numeral 50 denotes an image signal supplied to the image forming apparatus; 51, a gray-scale signal generator for generating a gray-scale signal of each dot from the image signal; 52, a pulse width required by the gray-scale signal pattern generator for generating a pulse, 53 of the semiconductor laser driver, the overshoot control circuit for controlling the pulse width W ₂ and the emission intensity of the overshoot portion P1 of the semiconductor laser 54, 55 is a semiconductor laser.

As shown in FIG. 2, the a-Si photoreceptor 1 is composed of a plurality of layers including a carrier blocking layer 21, a photosensitive layer 22, and a surface protective layer 23 formed on a conductive substrate 20, as shown in FIG. Use the configured one. In the following description, the film thickness of the a-Si photoconductor 1 is referred to as the film thickness from the surface of the conductive substrate 20 to the photoconductor surface, that is, the carrier blocking layer 21, the photosensitive layer 22, and the surface protection layer. 23.

In the present invention, the thickness of the photosensitive layer 22 on the conductive substrate 20 in the a-Si photosensitive member 1 is preferably set to 20 μm or less. That is, when the thickness of the photoreceptor is 20 μm or more, the moving speed of the heat carrier is increased, and the dark decay characteristic is reduced. As a result, the flow of a latent image toward the surface of the photoreceptor is likely to occur, and the resolution is reduced. Cause you to It is known that not only the a-Si photoreceptor, but also the OPC photoreceptor, the thinner the thickness of the photoreceptor, the higher the resolution is. In terms of cost, the thicker the photoreceptor, the longer the deposition time. The cost of the photosensitive member is smaller and the quality is more stable.

On the other hand, if the thickness of the photosensitive layer 22 is less than 10 μm, the charging ability of the photosensitive member is low, and it is difficult to obtain a predetermined surface potential. Further, since the pressure resistance performance of the photoconductor is proportional to the total film thickness (in particular, it depends on the photoconductive layer 22), dielectric breakdown due to repeated charging occurs and a black spot image is formed. Is irregularly reflected, causing a problem that interference fringes occur in the half pattern. Therefore, the thickness of the photosensitive layer is preferably in the range of 10 to 20 μm from the viewpoints of charging ability, withstand voltage, dark decay characteristics, manufacturing cost and quality.

As a more preferred embodiment of the photoreceptor,
The thickness of the surface protective layer 23 constituting this photoreceptor is 0.0
It is preferable that the thickness be in the range of 5 to 5 μm. That is, when the thickness of the surface protective layer 23 is less than 0.05 μm, the characteristics such as the charging ability, abrasion resistance, and environmental resistance of the photosensitive layer 22 tend to decrease. If the thickness of the layer 23 exceeds 5 μm, the film-forming time becomes long, which is disadvantageous in terms of cost. Therefore, the surface protective layer 2
3 is more preferably set to a value within the range of 0.1 to 3 μm from the balance of charging ability, abrasion resistance, environmental resistance, film formation time and the like.

The material constituting the photosensitive layer 22 is as follows:
Especially limited if it is amorphous silicon (a-Si)
A-Si, a
-Examples of inorganic materials such as SiC, a-SiO, and a-SiON
Can be shown. Among these materials, a-SiC is a special feature.
High resistance, better charging ability, abrasion resistance, ring resistance
The photosensitive layer material in the present embodiment
It is suitable as a material. Further, among a-SiC, Si and C
(Carbon), that is, a-Si
_(1-X)C_X(X value is less than 0.3-1)
And a-Si_{( 1-X)}C_X(The value of X is 0.5 to 0.5.
95 or less). This is because such a-S
i_(1-X)C_X(The value of X is 0.5 to 0.95 or less)
0¹²-10 ¹³With a particularly high resistance of Ωcm
The flow of the latent image in the direction of the photoreceptor surface is small,
This is because they have excellent maintenance ability and moisture resistance.

The surface potential (charging potential) of the photoreceptor is preferably set to a value in the range of +200 to + 500V. If the surface potential of the photoreceptor is less than +200 V, the developing electric field becomes insufficient and it becomes difficult to secure image density. On the other hand, if the voltage exceeds +500 V, there are problems such as insufficient charging ability depending on the thickness of the photoreceptor, easy occurrence of black spots due to dielectric breakdown, and increase in the amount of ozone generated. In particular, when the film thickness is reduced, the charging ability of the photoreceptor tends to decrease correspondingly. Therefore, from the viewpoint of the balance between the developability and the charging ability of the photoreceptor, the value of the surface potential is +200 V to +500.
The value is preferably within the range of V, more preferably +
That is, it is set in the range of 200V to + 300V.

Next, the light source of the exposure apparatus 3 according to the present invention can obtain a high light output with a small driving voltage, and can easily adjust the pitch (N) and the wavelength. A semiconductor laser is used. The wavelength of the semiconductor laser is preferably between 600 and 720 nm. This is because there is a light absorption peak near about 680 nm in the spectral sensitivity characteristic of the a-Si photoreceptor and 600 n
If it is less than m, or more than 720 nm, the sensitivity characteristic will be reduced and a semiconductor laser with higher emission intensity must be used. In addition, interference fringes are easily generated, and when an exposed portion is recharged, a so-called optical memory in which the potential becomes lower than the original set potential becomes remarkable. Therefore, from the viewpoint of spectral sensitivity characteristics, interference fringes, optical memory, and the like, the wavelength of the irradiation light to be used is preferably set to a value within the range of 600 to 720 nm.

The intensity of the light emitted from the semiconductor laser of the exposure device 3 is measured until the electrostatic latent image formed by exposing the a-Si photosensitive drum 1 reaches the developing area without flowing. It must be strong enough to be retained. As described above, the flow of the electrostatic latent image depends on the film thickness of the photoconductor and the dark decay characteristics, but is greatly affected by the emission rise characteristics of the semiconductor laser. The electrostatic latent image is strongly retained.
However, when the photoreceptor is overexposed, the gradation of the halftone image is lost and the image quality tends to be poor in contrast. Therefore, the emission intensity of the laser for obtaining an image quality with excellent reproducibility of fine dots and the emission intensity of the laser for obtaining an image quality with excellent gradation are required to be different. In this method, this problem was solved by controlling to give two kinds of light emission intensities by utilizing overshoot in the optical waveform output of one semiconductor laser. That is, in an exposure apparatus in a general image forming apparatus, a CR circuit is usually used so as to prevent such an overshoot of a semiconductor laser, but in the present invention, this overshoot is positively used, As described above, two kinds of light emission intensities are provided in a pseudo manner.

FIG. 3 shows an optical waveform output of the semiconductor laser obtained by such control. Pulse width W ₁ of the entire of the optical waveform output of Figure 3, a-S
Control is performed so as to correspond to the dot diameter of the image formed on the i photosensitive drum 1. That is, the light emission time t (pulse width W ₁ , unit: ns) of one dot corresponds to a resolution of 1200 dp.
i, the peripheral speed of the a-Si photosensitive drum 1 is D (mm /
s), the width of the recording medium is S (mm), the length is T (mm),
When one inch is set to 25.4 mm and recording is performed by passing the recording medium in the length direction, the following expression (1) is used.

[0031]

(Equation 1)

The light emission time t (pulse width W ₁ ) of _one dot is shorter by the number of divisions when the number of dots in the sub-scanning direction is divided and displayed in a multi-valued image. P1 in FIG. 3 is an overshoot at the rise of the light pulse, and its level is an overexposure level at which the electrostatic latent image formed on the a-Si photosensitive drum 1 is strongly held as described above. , P2 are levels at which normal tone reproduction is possible. The pulse width _{W 2} of the overshoot portion (P1) is controlled to 50ns or less.

[0033] That is, when the pulse width W ₂ of the overshoot portion P1 is equal to or greater than 50 ns, poor gradation dot because emission intensity is high occurs. Therefore preferably, it is better to set the pulse width W ₂ to a value in the range below 20 ns. The relationship between the overshoot portion P1 and the level P2 at which normal tone reproduction is possible is set such that the value of P1 is not more than five times the value of P2. This allows the value of P1 to be P2
If the value is 5 times or more, the light emission intensity of the overshoot portion P1 is too strong, and a gradation defect occurs.
Preferably, it is appropriate to set the value in the range of 1.2 to 2.0 times.

[0034] In the graph shown this from FIG. 4 to that shown experimentally in Figure 7, Figure 4 shows the reproducibility of the relationship between the pulse width of the overshoot portion P1 of the laser light W ₂ and the minute dots graph, Figure 5, graph, Figure 6 shows the pulse width W ₂ and the gradation of the relationship overshoot portion P1 is reproduced ratio level P1 of the laser light and P2 and (P1 / P2) of small dots FIG. 7 is a graph showing the relationship between the ratio (P1 / P2) of the laser light levels P1 and P2 and the number of gradations. In FIGS. 4 and 5,
The intensity levels P1 and P2 of the laser light are such that P1 is 1.
It is set to 5 times, 6, 7, the pulse width _{W 2} of the overshoot P1 can is set to 10 ns.

First, as can be seen from FIG. 4, the reproducibility level of the minute dot is determined by the pulse width W _{2 of the} overshoot P1.
There is substantially constant at 5 exceeds 10 ns, also the number of gradations, as can be seen from FIG. 5, the pulse width W ₂ of the overshoot P1 is below 200 gradations around beyond the 20 ns. Further, as can be seen from FIG. 6, when the ratio (P1 / P2) exceeds 2.0, the reproducibility level of the minute dots based on the ratio (P1 / P2) of the laser light levels P1 and P2 is substantially constant at 5. As can be seen from FIG. 7, the number of gradations is less than 200 gradations when the ratio (P1 / P2) exceeds 4.0.

[0036] Therefore, as described above, it is preferable that the pulse width W ₂ of the overshoot portion P1 is set with the following values 20 ns, The ratio of the overshoot level P1 and the gradation reproduction levels P2 is, P1 Is suitably set to 5 times or less, preferably 1.2 to 2.0 times the value of P2. In the above description, the pulse width of the overshoot is set to 50 ns or less, or 20 ns or less.
Although the value has been described as being equal to or less than ns, this value naturally does not include the pulse width of 0 ns and indicates a value larger than 0 ns.

As a method for causing the overshoot, a method generally used such as applying a steep rising pulse to the semiconductor laser and utilizing the impedance of the semiconductor laser itself may be used. This control will be described with reference to the block diagram of the control circuit of the semiconductor laser shown in FIG. 8. First, a gradation signal generator 51 generates a gradation signal of each dot from an image signal 50 given to the image forming apparatus. The pattern generator 52 generates a pulse pattern having a pulse width corresponding to the gradation according to the gradation signal using a normal PWM (pulse width modulation) technique or the like. On the other hand, the overshoot control circuit 54 controls the semiconductor laser 5 based on the oscillation start current of the semiconductor laser 55 to be used, the amount of overshoot, and the impedance.
The semiconductor laser driver 53 controls the semiconductor laser driver 53 so that the pulse width and the light amount of the overshoot described above are obtained, and shapes the pulse sent from the pattern generator 52 and sends it to the semiconductor laser 55. Therefore, the semiconductor laser 5
5, the light output as described above with reference to FIG. 3 is obtained.

The operation of the image forming apparatus of the present invention having the above-described structure is as follows.
Is charged by the scorotron charger 2, and then, as described with reference to FIG. 8, the laser output light from the exposure device 3 is output by the image signal 50, and as shown in FIG. W ₂ is 20ns
Hereinafter, the exposure level is set to be an overexposure level that strongly holds the electrostatic latent image formed on the a-Si photosensitive drum 1, and is followed by a level P2 at which normal tone reproduction is possible. The optical signal 8 controls a-
The Si photoreceptor 1 is exposed. Then, a-Si
The electrostatic latent image formed on the photoreceptor 1 is visualized with toner of the developing device 4, the toner image is transferred to a recording medium by a transfer roller 5, fixed by a fixing device (not shown), and discharged. .
Then, the toner remaining on the a-Si photoreceptor 1 without being transferred onto the a-Si photoreceptor 1 is removed by a cleaning unit 6, and the residual charge on the a-Si photoreceptor 1 is further removed by an erasing unit 7 to form the next image. Prepare.

In this manner, the image is formed by the image forming apparatus according to the present invention. Hereinafter, the reproducibility and the gradation of the fine dots performed based on the specifications described above will be described. The results of the experiment will be described.

In the following experiment, as an example, the carrier blocking layer was 3 μm, the photosensitive layer was 10 μm, and the surface protective layer was 1 μm.
An a-Si photosensitive drum having a diameter of 30 mm (manufactured by Kyocera Corporation) having a diameter of 4 μm was prepared, and a printer manufactured by Kyocera Corporation, FS-
Attached to a 3750 modified machine (A4 compatible, output speed 18 sheets / min), the pulse width of the overshoot of the semiconductor laser in the exposure apparatus was 10 ns, and the value of P1 was 0.45.
The values of mW and P2 were set to 0.30 mW, that is, P1 was set to 1.5 times P2, and the drum peripheral speed was set to 120 mm / s. In this case, the emission time of one dot is 18 ns according to the above equation (1).

Then, 255 kinds of dither patterns were prepared for confirming the gradation, and the number of dither patterns was visually confirmed. As a result, even when the a-Si photosensitive member was used, 1200 dither patterns were used.
The reproducibility of one dot of dpi was good, and it was confirmed that an image excellent in gradation was obtained.

As described above, the overshoot of the overexposure intensity P1 for strongly holding the electrostatic latent image by overexposing the amorphous silicon photoreceptor is provided, and the exposure of the pulse width corresponding to the gradation of the dots constituting the image is provided. By providing the intensity P2, the reproduction of minute dots and the reproduction of gradation become possible, and the overshoot can be generated simply by giving a steep rising pulse to the semiconductor laser. Publication No.
As in the case of Japanese Patent Application Laid-Open No. 00-127498, software such as an output control wave generating means for generating a waveform having a sharp peak within the writing time of each pixel, a mechanism for generating a plurality of intensities, and a mechanism for turning off exposure, It is not necessary to use a complicated mechanism in terms of hardware, and it is possible to provide an image forming apparatus capable of realizing high image quality with excellent reproducibility of fine dots and gradation.

[0043]

As described above, according to the first aspect of the present invention, the overshoot of the overexposure intensity P1 for overexposing the amorphous silicon photoreceptor to strongly retain the electrostatic latent image is merely a sharp rise. Can be generated simply by giving the pulse of the semiconductor laser to the semiconductor laser.
-114206 and Japanese Patent Application Laid-Open No. 2000-127498, output control wave generation means for generating a waveform having a sharp peak within the writing time of each pixel, a mechanism for generating a plurality of intensities, and Therefore, it is not necessary to use a complicated mechanism such as a software-based or hardware-based mechanism, and it is possible to provide an image forming apparatus capable of realizing high image quality with excellent reproducibility of fine dots and excellent gradation.

According to the second aspect of the present invention, the pulse width of the overshoot of the exposure intensity P1 is set to 50.
ns or less, and the ratio between the exposure intensities P1 and P2 is 1.2
From 5.0 to 5.0, it is possible to prevent the occurrence of a gradation defect in the dots due to an excessively high light emission intensity, and to realize an image forming apparatus capable of realizing high image quality with excellent reproducibility of minute dots and gradation. Can be provided.

According to the third aspect of the present invention, the oscillation wavelength of the semiconductor laser is 600 to 720 nm.
By this, the sensitivity characteristics of the a-Si photoreceptor are degraded due to wavelengths outside this range, so that a semiconductor laser having a higher emission intensity must be used, interference fringes, optical memory, and the like can be prevented. An image forming apparatus capable of realizing high image quality with excellent reproducibility of fine dots and excellent gradation can be provided.

According to the present invention, the thickness of the photosensitive layer is in the range of 10 to 20 μm, and the surface potential of the amorphous photosensitive member is from +200 V to +200 V.
By setting the range to 500 V, the charging ability, the withstand voltage, the dark decay characteristic, and the manufacturing cost are excellent, the insulation breakdown is prevented, the ozone generation amount is prevented from increasing, and the reproducibility of the minute dots and the gradation property are improved. An image forming apparatus capable of realizing excellent high image quality can be provided.

[Brief description of the drawings]

FIG. 1 is a diagram schematically illustrating the configuration of an image forming apparatus according to the present invention.

FIG. 2 shows a-Si used in the image forming apparatus according to the present invention.
FIG. 3 is a diagram illustrating an example of a structure of a photoconductor drum.

FIG. 3 is a diagram illustrating an output waveform of a laser beam in the image forming apparatus according to the present invention.

4 is a graph for explaining the reproducibility of the relationship between the pulse width W ₂ and the minute dots of overshoot portion P1 of the laser light.

5 is a graph illustrating a pulse width W ₂ and the gradation of the relationship overshoot portion P1.

FIG. 6 shows the ratio of the laser beam levels P1 and P2 (P1 / P2).
6 is a graph for explaining the relationship between P2) and the reproducibility of minute dots.

FIG. 7 shows the ratio of the laser beam levels P1 and P2 (P1 / P2).
9 is a graph for explaining the relationship between P2) and the number of gradations.

FIG. 8 is a block diagram of a control circuit of a semiconductor laser constituting an exposure device in the image forming apparatus of the present invention.

[Explanation of symbols]

P1 Overshoot intensity of overshoot P2 Exposure intensity for forming an electrostatic latent image according to gradation W ₁ Pulse width of one dot constituting _one image W ₂ Pulse width of overshoot

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) G03G 15/04 G03G 15/04 120 15/043

Claims (4)

[Claims] 1. An electrostatic latent image is formed by a photosensitive member using amorphous silicon having a photosensitive layer as a thin film, and a semiconductor laser for providing a light output corresponding to each dot constituting an image to the photosensitive member. An image forming apparatus for forming an image by an electrophotographic method, comprising: an exposure device configured to form an image by an electrophotographic method. An overshoot of the overexposure intensity P1 for overexposing the body to hold the electrostatic latent image strongly and a pulse width corresponding to the gradation of the dots constituting the image correspond to the gradation on the amorphous silicon photoconductor. And an exposure intensity P2 for forming electrostatic dots. The overshoot of the overexposure intensity P1 enables the reproduction of minute dots and the exposure intensity P2 enables reproduction of gradation. Image forming apparatus, characterized in that the. 2. A pulse width of an overshoot of an exposure intensity P1 in a semiconductor laser constituting the exposure apparatus,
2. The method according to claim 1, wherein the exposure intensity is set to be less than 50 ns, and the ratio between the exposure intensities P1 and P2 is set to 1.2 to 5.0.
2. The image forming apparatus according to 1. 3. The image forming apparatus according to claim 1, wherein the semiconductor laser has an oscillation wavelength λ in a range from 600 nm to 720 nm. 4. The amorphous silicon photoreceptor has a three-layer structure of a carrier blocking layer, a photosensitive layer, and a surface protective layer. The photosensitive layer has a thickness of 10 μm to 20 μm, and has a surface potential in a range of +200 V to +500 V. The image forming apparatus according to claim 1, wherein