JP2002086799A - Imaging apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an imaging apparatus which can form a stabilized latent image regardless of the bean spot diameter or imaging process conditions while dealing with high density writing. SOLUTION: As shown on the drawing, image data and a pixel clock are inputted to a synchronization circuit 11. A driving current generating circuit 12 delivers a preset current synchronized with the image clock to a drive circuit 15 thence to a laser diode 16. The laser diode 16 emits a laser beam depending on a PWM modulated pulse width. A saw-tooth supply current is preset and a scanning optical system is controlled depending on the saw-tooth wave. Since it is controlled depending on a preset current, energy density can be increased even at a part of narrow pulse width and overlap of the side lobes of a laser beam can be reduced. Consequently, even a latent image at a low density part can form a deep latent image.

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 using an electrophotographic process, for example, a high resolution image forming apparatus using a laser optical system.

[0002]

2. Description of the Related Art In recent years, image forming apparatuses such as printers, color printers, copiers, and color copiers, especially printing apparatuses that require high-precision images, have been widely used. In these electrophotographic image forming apparatuses, in addition to the demand for improving the printing quality of character images and solid images, the demand for improving image quality and color has also been increasing. For this reason, importance has also been placed on the quality of halftone (halftone) and gradation reproduction. As a method for reproducing the halftone gradation, a method of modulating a laser beam for exposing a uniformly charged photoconductor is employed, a method of performing pulse width modulation (PWM) for each pixel irradiated with the laser beam, and a method of irradiation. A method of modulating the amount of light (PM), a method combining these methods (PWM + PM), and the like have been proposed. Further, image processing such as dither processing, line processing, and error diffusion has been proposed as the gradation processing, and a method of performing exposure by combining these image processing and a laser beam modulation method has been adopted in many cases. Further, in order to improve the image quality, the image quality is improved by increasing the writing density for exposing a uniformly charged photoconductor.

On the other hand, in the electrophotographic process, since various characteristics such as sensitivity characteristics and development characteristics of the photoreceptor fluctuate depending on environmental conditions and durability conditions, colors and densities of a printed image are not reproduced to desired colors and densities. That is, an image may not be stably formed. For this reason, in recent years, the surface potential of a photoconductor after exposure is detected to obtain sensitivity characteristics, or an inspection is performed by developing an electrostatic latent image formed on a predetermined site such as a photoconductor under predetermined conditions under predetermined conditions. Process control for stabilizing the image is performed by detecting the density of the P sensor pattern, which is a pattern, to obtain development characteristics, and feeding back the characteristic change, which is the detection result, to the image forming conditions of the image to be recorded. I have.

Japanese Patent Application Laid-Open No. 2-231161 discloses that a semiconductor laser beam emits an information recording laser beam in an intensity-modulated and pulse-width-modulated state so that an image having a halftone can be recorded. A control method of an information recording laser beam to be changed, wherein a means for changing a laser beam intensity within each one pixel period in accordance with a predetermined change pattern;
A method of controlling an information recording laser beam comprising means for determining the irradiation time of a laser beam in each of the successive one pixel periods based on density information for each pixel is described. Japanese Unexamined Patent Publication No. 3-24167 discloses an optical writing method for modulating a laser beam by image data, in which the laser beam is modulated so that the image data signal and the surface potential distribution of the laser-exposed photoreceptor become substantially equal. In such an optical writing method, there is described an optical writing method in which the edge portion of the modulation is emphasized so that the exposure becomes close to a binary value as the surface potential.

Japanese Patent Application Laid-Open No. 4-77050 discloses an exposure type semiconductor laser driving device in which the light emission intensity is reduced in a low density portion and the light emission intensity is increased in a high density portion. JP-A-3-248174, JP-A-3-24
No. 9672, Japanese Unexamined Patent Publication No. 3-279974,
A laser scanning type image forming apparatus and an image forming apparatus in which a light emission pulse width is proportionally made to correspond to light emission intensity are described. Japanese Patent Application Laid-Open No. H11-254747 discloses that in an image forming apparatus in which a laser output is on / off binary, a detection value of a predetermined detection pattern is changed by changing an on / off duty ratio in one pixel. An image forming apparatus having a method of providing feedback to the user is described.

[0006]

However, in an image forming apparatus using a conventional electrophotographic process, the exposure time of a laser beam is changed according to the density of an image by a PWM modulation method in order to reproduce gradation. And the energy density of the laser beam has a distribution that is approximated by a Gaussian distribution, so that not only the exposure time (the width of the dot in the main scanning direction) but also the photoreceptor is irradiated according to the input data of the image density The maximum value of the energy density of the laser beam also changes. Further, in the PM modulation method, the maximum value of the energy density of the laser beam irradiated on the photoconductor is changed by changing the exposure light amount. PW
The same can be said for a system combining M modulation and PM modulation. When the energy density of the laser beam is reduced, the surface potential on the photoreceptor does not reach a threshold required for photosensitivity, and the reproducibility in a low density portion is reduced, thereby causing image deterioration. This problem can be improved by reducing the beam spot diameter of the laser beam for scanning and exposing the photoreceptor surface, but in order to reduce the laser spot diameter, the laser oscillation wavelength must be shortened and the incident light laser It is necessary to increase the diameter,
Since the size and cost of the apparatus are increased and the depth of focus is reduced, it is difficult to realize the apparatus due to the limitation of the mechanical accuracy on the image carrier.

FIG. 7 shows a static exposure distribution and a scanning exposure distribution of a laser beam. The laser beam that scans the photosensitive member has a Gaussian beam spot shape as shown in FIG. 7A when the photosensitive member is not scanned, that is, when the photosensitive member is stationary. However, since the laser beam having such a beam spot shape moves in the direction of the arrow (main scanning direction) in FIG. 7B when actually scanning, the same phenomenon as the enlargement of the spot diameter occurs. Would. Therefore, it is desirable that the spot diameter of the laser beam when stationary is an elliptical beam spot shape long in the sub-scanning direction. FIG. 8 is a diagram showing the relationship between the pulse width and the energy intensity when the laser beam of FIG. 7 is PWM-modulated in the main scanning direction. When the laser beam is PWM-modulated in the main scanning direction, the energy intensity changes as the pulse width increases, as shown in FIG. The change in the energy intensity accompanying the increase in the pulse width continues until the side lobes of the Gaussian distribution disappear, and this time contributes to the size of the beam spot diameter in the main scanning direction.

[0008] Here, when a problem of PWM modulation occurs, when the pulse width is narrow, the energy intensity of the laser beam becomes small, and the latent image on the photosensitive member becomes a shallow latent image.
As shown in FIG. 9, the threshold (threshold level) determined by the developing bias and the developing ability cannot be exceeded, and the variation in the charging potential on the surface of the photoreceptor, the variation in the particle diameter and the charging amount of the toner, Reproduction of the toner dots is remarkably reduced, which leads to a gradation jump in a low density portion. Further, when the pulse width is wide, the high density portion is collapsed due to the overlap with the side lobe of the beam of the adjacent pixel, and the relationship between the pulse width and the image density has a γ characteristic as shown in FIG. On the other hand, the integrated value of the laser light amount to be exposed on the photoreceptor is doubled in the same manner, and the lighting time is reduced by 1 /.
If it is set to 2, the same phenomenon occurs as when the beam spot diameter is reduced in a pseudo manner, and a deep latent image can be formed.
As compared with the case where the beam spot diameter is narrowed, the effect is small, and when the modulation frequency is increased, the reproducible pulse width is about 3 to 4 (nsec), and there is a limit.

In the control method of the laser beam for information recording described in Japanese Patent Application Laid-Open No. 2-231161, the exposure based on a predetermined change pattern is of a cosine wave type, and is reproduced in the tone reproduction on the high density portion side. The reproducibility of the gradation with respect to the input data may not be a straight line, and a linear gradation may not be obtained depending on image forming process conditions. In the optical writing method described in Japanese Patent Laid-Open No. 3-24167, it is not possible to achieve a high writing density unless the beam spot diameter is reduced. In the image forming apparatus described in JP-A-3-248174, the laser-scanning image forming apparatus described in JP-A-3-249672, and the image forming apparatus described in JP-A-3-279974, the laser beam in Due to the decrease in energy density, stable development characteristics may not be obtained.

In the semiconductor laser driving device described in Japanese Patent Application Laid-Open No. 4-77050, stable development characteristics may not be obtained due to a decrease in the energy density of a laser beam in a low density portion. In the image forming method and the apparatus described in Japanese Patent Application Laid-Open No. 6-87234, the apparatus becomes complicated due to having the modulation operation circuit in the low density section, and the count of the operation section must be changed depending on the image forming process conditions. And the processing becomes complicated. JP 8
In the image forming apparatus described in Japanese Patent Application Publication No. -185021, the light sensitivity is low at a low light amount, and the light sensitivity increases when the light amount exceeds a predetermined light amount. I will. JP-A-11-25474
In the image forming apparatus described in Japanese Patent Application Laid-Open No. 7-297, it can be applied only to a binary image forming apparatus, and it is not possible to realize a high writing density.

An object of the present invention is to provide an image forming apparatus which is not affected by the beam spot diameter and the image forming process conditions, can cope with high-density writing, and can form a stable latent image. It is.

[0012]

According to the first aspect of the present invention, a scanning optical system for scanning and exposing a photosensitive member uniformly charged based on a received image signal with a laser beam, and each of the received image signals is provided. Pulse width modulation means for pulse width modulation of the exposure time of the scanning optical system, which is scanned and exposed on the photoconductor in synchronization with pixels, in accordance with the density of an image, and in synchronization with each pixel of the received image signal An intensity modulator for intensity-modulating the intensity of a laser beam scanned and exposed by the scanning optical system according to a predetermined pattern, wherein when the pulse width modulation by the pulse width modulator is narrow, the intensity modulation means performs the scanning. When the intensity of the optical system is increased and the pulse width modulation by the pulse width modulation unit is wide, the intensity modulation unit achieves the object by reducing the intensity of the scanning optical system.

According to a second aspect of the present invention, in the first aspect of the present invention, the predetermined pattern that the intensity modulating means intensity-modulates in synchronization with each pixel of the received image signal is:
The above object is achieved by being based on a predetermined reference wave used for the pulse width modulation means.

According to a third aspect of the present invention, in the first or second aspect of the present invention, the potential detecting means for detecting a surface potential of the photosensitive member, which has been scanned and exposed by the scanning optical system, after the exposure, Pulse width selection means for selecting a set value of the pulse width modulated by the pulse width modulation means based on the potential detected by the electrical detection means,
The above object is achieved by further providing

According to a fourth aspect of the present invention, in the first or second aspect of the present invention, the exposed electrostatic latent image of the photosensitive member, which is scanned and exposed by the scanning optical system, is visualized as a document image. Visualizing means, density detecting means for detecting the density of the original image visualized by the visualizing means, and a pulse modulated by the pulse width modulating means based on a potential detected by the density detecting means. The above object is attained by further comprising a pulse width selecting means for selecting a set value of the width.

[0016]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments according to the present invention will be described below in detail with reference to FIGS. FIG. 1 is a block diagram showing a control circuit of a scanning optical system of the image forming apparatus according to the present embodiment. The control circuit includes a synchronization circuit 11, a drive current generation circuit 12, a data table 13, a pulse width generation circuit 14, a drive circuit 15, and a laser diode 16. As shown in FIG. 1, the image data and the pixel clock are input to the synchronization circuit 11. The drive current generation circuit 12 outputs a preset current synchronized with the image clock to the drive circuit 15. When the drive circuit 15 supplies the current to the laser diode 16, the laser diode 16
A laser beam is emitted according to the WM-modulated pulse width. Here, as the preset current, a sawtooth-like supply current as shown in FIG. 2 is set, and the scanning optical system is controlled in accordance with the sawtooth wave. Since the control is performed in accordance with the preset current, the energy density can be increased even in the portion where the pulse width is narrow as shown in FIG. 3, and the overlap of the side lobes of the laser beam can be reduced. A deep latent image can be formed. Further, in the high-density portion, the increase in energy due to the increase in pulse width can be reduced, and the surface potential curve on the photoreceptor can be reduced as shown in FIG.
It can be made the curve of the inclination like. as a result,
The surface potential of the photoreceptor can be formed on a straight line.

In this embodiment, the preset supply current has been described as a sawtooth wave current waveform, but the present invention is not limited to this, and a triangular wave or the like may be set. When a pulse width is generated in a pulse width generation circuit in a PWM modulator, a sawtooth wave or a triangular wave may be used. If a sawtooth wave or a triangular wave that generates this pulse width is used for the drive current generation circuit 12 of the laser diode 16, the circuit can be shared, and the resolution can be more efficiently improved.

Further, there is a case where the surface potential of the photosensitive member is not linearly reduced with respect to the energy applied to the photosensitive member due to the light attenuation characteristics of the photosensitive member. Also,
Its characteristics also deteriorate due to temperature, humidity, changes over time, and the like. Therefore, in the present embodiment, a control method as shown in FIGS. 5 and 6 is performed. FIG. 5 is a diagram showing feedback control of the pattern generation circuit. As shown in FIG. 5, the exposure unit 22 forms an electrostatic latent image on the surface of the photoconductor 23 based on a pattern set in the pattern generation circuit 21 in advance. Then, the surface potential detecting means 24
Measures the potential of the photoconductor 23,
The laser diode is controlled by ensuring the linearity of the surface potential and feeding back the result to the exposure means 22. The output corrected by the feedback control is the pulse width of the pulse width modulation. Therefore, the number of data for modulating the laser beam is desirably a value larger than the number of data input to the light modulation unit.
By processing the 3-bit data (8 gradations) input to the light modulation section with 8 bits (256 gradations) in the light modulation section, the accuracy of the correction value can be increased.

FIG. 6 is a diagram showing a modified example of the feedback control of the pattern generation circuit. As shown in FIG. 6, the exposure unit 32 develops the toner on the photoconductor 33 based on the pattern preset in the pattern generation circuit 31, and the image density detection unit 34 detects the density of the image developed on the photoconductor 33. By doing so, feedback control may be performed. When toner is attached to the latent image formed on the photoreceptor 33 by a developing device, a carrier and toner are used.
Compared to the component development, the one-component development using only the toner has a density curve with γ even at the same surface potential.
Therefore, by using the exposure control method of the present embodiment, it is possible to perform exposure with a characteristic in which the potential curve of the intermediate density portion is inclined, and the reproducibility of the intermediate density portion can be improved.

As described above, even when a relatively large beam spot diameter is used as compared with the writing density, it is possible to provide a high-resolution image forming apparatus by reducing the overlap of the side lobes having a Gaussian distribution. Can be. Also, P
By sharing a part of the WM generation circuit and a part of the PM circuit, the circuit can be simplified. To provide a high-resolution image forming apparatus by monitoring a surface potential of an exposed photoconductor and selectively selecting a pulse width for PWM modulation based on image data, thereby exposing a γ curve of the photoconductor linearly. Can be. Further, the density of the exposed photoreceptor after being visualized is detected, and the PWM
By selectively selecting the pulse width to be modulated, a high-resolution image forming apparatus can be provided by exposing the γ curve of the photoconductor linearly. According to the present embodiment, it is possible to provide an image having a halftone at a high resolution even when a developing method with a steep γ such as one-component development is used.

[0021]

According to the first aspect of the present invention, when the pulse width modulation by the pulse width modulation means is narrow, the intensity modulation means increases the output value of the scanning optical system, and when the pulse width modulation by the pulse width modulation means is wide. Since the intensity modulating means reduces the output value of the scanning optical system, the overlap of the side lobes of the laser beam can be reduced, and the latent image in the low density portion can also form a deep latent image.

According to the second aspect of the present invention, the predetermined pattern which is intensity-modulated in synchronization with each pixel of the image signal received by the intensity modulation means is based on a predetermined reference wave preset in the pulse width modulation means. Therefore, a high-resolution image forming apparatus can be provided.

According to a third aspect of the present invention, there is provided a potential detecting means for detecting a surface potential of the photosensitive member scanned and exposed by the scanning optical system after exposure, and a pulse width modulation based on the potential detected by the electrical detecting means. And a pulse width selection means for selecting a set value of the pulse width modulated by the means, so that the gamma curve of the photoreceptor is linearly exposed,
A high-resolution image forming apparatus can be provided.

According to the fourth aspect of the present invention, there is provided a visualizing means for visualizing the exposed electrostatic latent image of the photosensitive member scanned and exposed by the scanning optical system as a document image, and a visualizing means for visualizing the image by the visualizing means. Density detecting means for detecting the density of the original image, and pulse width selecting means for selecting a set value of a pulse width modulated by the pulse width modulating means based on the potential detected by the density detecting means. Therefore, it is possible to provide a high-resolution image forming apparatus by exposing the γ curve of the photoconductor linearly.

[Brief description of the drawings]

FIG. 1 is a block diagram illustrating a control circuit of a scanning optical system of an image forming apparatus according to an embodiment.

FIG. 2 is a diagram showing an example of a supply current according to the present embodiment.

FIG. 3 is a diagram showing a relationship between a pulse width and an energy density according to the present embodiment.

FIG. 4 is a diagram showing a relationship between a pulse width and an image density according to the present embodiment.

FIG. 5 is a diagram illustrating feedback control of a pattern generation circuit.

FIG. 6 is a diagram illustrating a modified example of the feedback control of the pattern generation circuit.

FIG. 7 is a diagram showing a static exposure distribution and a scanning exposure distribution of a laser beam.

8 is a diagram illustrating a relationship between a pulse width and an energy intensity when the laser beam of FIG. 7 is PWM-modulated in the main scanning direction.

FIG. 9 is a diagram showing a relationship between a pulse width and an energy density.

FIG. 10 is a diagram showing a relationship between a pulse width and an image density.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 11 Synchronization circuit 12 Drive current generation circuit 13 Data table 14 Pulse width generation circuit 15 Drive circuit 16 LD (laser diode) 21, 31 Pattern generation circuit 22, 32 Exposure means 23, 33 Photoconductor 24 Surface potential detection means 34 Image density detection means

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) G03G 15/04 G03G 15/04 120 5C072 H04N 1/113 H04N 1/04 104Z 5C074 1/29 F term (reference ) 2C262 AA05 AA17 AB07 BB01 BB11 BB31 BB49 GA03 GA21 GA34 GA47 2C362 AA54 AA63 AA66 CA02 CA11 2H027 DA02 DA09 EA02 EC03 2H045 DA28 2H076 AB21 AB22 AB75 DA06 DA07 DA11 DA19 5C072 AA03 H15A02 AA03 BA15A02 A05 X CC26 DD07 DD08 DD11 EE11 GG12

Claims (4)

[Claims] 1. A scanning optical system for scanning and exposing a photosensitive member uniformly charged based on a received image signal with a laser beam, and scanning exposure on the photosensitive member in synchronization with each pixel of the received image signal. Pulse width modulation means for pulse width modulating the exposure time of the scanning optical system according to the density of an image, and the intensity of a laser beam scanned and exposed by the scanning optical system in synchronization with each pixel of the received image signal. And intensity modulation means for intensity modulation according to a predetermined pattern, when the pulse width modulation by the pulse width modulation means is narrow,
The intensity modulation means increases the intensity of the scanning optical system,
When the pulse width modulation by the pulse width modulation means is wide,
The image forming apparatus according to claim 1, wherein the intensity modulation unit reduces the intensity of the scanning optical system. 2. A predetermined pattern in which the intensity modulation means intensity-modulates in synchronization with each pixel of the received image signal,
2. The image forming apparatus according to claim 1, wherein said image forming apparatus is based on a predetermined reference wave used for said pulse width modulation means. 3. A potential detecting means for detecting a surface potential after exposure of the photoreceptor scanned and exposed by the scanning optical system; and a pulse width modulating means based on the potential detected by the electric detecting means. 3. The image forming apparatus according to claim 1, further comprising: a pulse width selection unit that selects a set value of the pulse width to be set. 4. An image developing means for visualizing an exposed electrostatic latent image of the photoconductor scanned and exposed by the scanning optical system as an original image, and an original image visualized by the visualizing means. Concentration detecting means for detecting the density of the pulse width, and pulse width selecting means for selecting a set value of a pulse width modulated by the pulse width modulating means based on the potential detected by the concentration detecting means. The image forming apparatus according to claim 1, wherein: