JP2004272126A - Image forming device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a beam scanning type image forming device in which a shading correction function is simply realized with a low cost composition. <P>SOLUTION: The beam scanning type image forming device having a light source means 1 which generates a laser beam, a deflection means 8 which deflects and scans the laser beam emitted from the light source means 1, and scanning and focusing optical means 11, 12 and 13 which focus the laser beam emitted from the deflection means 8 on a plane to be scanned 15 and scan the plane with the laser beam. A correction means of the shading characteristic which is a laser intensity dislocation in a laser beam scanning width on the scanning and focusing optical means 11, 12 and 13 is composed of a pulse width modulation means 17 which generates a pulse corresponding to the quantity of the laser beam and a low-pass filter 18 which smoothes the output from the pulse width modulation means. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a beam scanning type image forming apparatus such as a laser printer, a digital copying machine, and a facsimile apparatus, and particularly to a shading correction technique.
[0002]
[Prior art]
2. Description of the Related Art As an image forming apparatus such as a laser printer, a digital copying machine, and a facsimile machine, a laser beam is scanned in a main scanning direction on a surface to be scanned such as a photoconductor by a scanning means such as a polygon mirror, and the surface to be scanned is scanned in a sub-scanning direction. And a beam scanning type image forming apparatus equipped with a function of writing an image one line at a time on a photoreceptor.
In this beam scanning type image forming apparatus, a laser beam is deflected at a constant angular speed by a polygon mirror, and an fθ lens or an fθ mirror is used to keep the scanning speed on the surface to be scanned constant. The scanning speed of the laser beam passing through the fθ lens or the fθ mirror on the surface to be scanned becomes constant, but the intensity of the laser beam on the photoreceptor surface varies depending on the image height. This is because the light utilization efficiency, such as the reflectance and transmittance, of the optical elements such as glass, lenses, and mirrors that passes from when the laser light is emitted from the laser diode to the surface of the photoreceptor varies depending on the incident angle of the laser light. This is because the thickness of the lens differs depending on the image height. Such intensity of the light beam intensity due to the image height is called a shading characteristic, and since it affects the density of a formed image, the beam scanning type image forming apparatus is required to correct the shading characteristic.
As a method relating to the shading correction method, for example, as an earlier application of the same applicant as the present application, an effective writing area is divided, light amount correction data corresponding to each divided area is written in advance in an address of a RAM, and laser light At the time of scanning the effective writing area, the light amount correction data corresponding to each divided area is read from the designated address, and the analog voltage converted by the digital / analog converter (DAC) is input to the LD modulation unit as a light amount correction signal, Techniques for adjusting the amount of laser light have been proposed.
[0003]
[Problems to be solved by the invention]
As described above, conventionally, in order to correct shading, an effective writing area is divided, light amount correction data corresponding to each divided area is converted by a digital / analog converter (DAC), and the converted analog voltage is converted. A technique has been proposed in which a laser light amount is input as a light amount correction signal to an LD modulator to adjust the laser light amount.
However, in the prior art, the conversion of the light quantity correction data by the DAC is performed inside the LD write control IC, and it takes much time to create and verify the LD write control IC, and furthermore, the chip area increases and the cost increases. Was a factor.
SUMMARY OF THE INVENTION It is an object of the present invention to provide a beam scanning type image forming apparatus that can easily realize a shading correction function with a low-cost configuration.
[0004]
[Means for Solving the Problems]
In order to achieve the above object, the invention according to claim 1 includes a light source means for generating a laser beam, a deflecting means for deflecting and scanning the laser light from the light source means, and a laser beam from the deflecting means to be scanned. A beam scanning type image forming apparatus having a scanning image forming optical means for converging and scanning a surface, wherein the shading characteristic correcting means which is a laser intensity displacement within a laser beam scanning width in the scanning image forming optical means is a laser. The most main feature of the image forming apparatus is a pulse width modulation means for generating a pulse corresponding to the light amount, and a low-pass filter for smoothing the output from the pulse width modulation means.
According to a second aspect of the present invention, in the image forming apparatus of the first aspect, the main feature is an image forming apparatus in which the time constant of the low-pass filter is variable.
[0005]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings.
FIG. 1 is a front view showing a writing optical system of an image forming apparatus according to an embodiment of the present invention, and FIG. 2 is a top view showing the same together with a control system. The configuration will be described together with the operation.
Laser light as a light source is shaped and emitted from an LD 2 (light source means) of a laser diode (LD) unit 1. The laser beam passes through a cylinder lens 7 and passes through a polygon mirror shield glass 9 for soundproofing a polygon mirror 8 (deflecting unit) as a scanning unit. The polygon mirror 8 is driven to rotate by a polygon motor 10 as a rotation drive unit, rotates at a predetermined rotation speed, and deflects and scans the laser light incident from the polygon mirror shield glass 9.
The laser light from the polygon mirror 8 passes through the polygon mirror shield glass 9 again, passes through the fθ lens 11, passes through the valet toroidal lens 12 for correcting the surface tilt of the polygon mirror 8, and passes through the folding mirror 13. The light is reflected, passes through the dustproof glass 14, and is condensed on the photosensitive member 15 (scanning surface) as a recording medium. The laser light from the polygon mirror 8 is incident on the beam detector 16 as a beam detection signal.
The photoconductor 15 is, for example, a photoconductor drum. The photoconductor 15 is driven to rotate by a rotation driving unit, is uniformly charged by a charger, and is repeatedly scanned in a main scanning direction by a laser beam to write an image. To form an electrostatic latent image. The electrostatic latent image on the photosensitive drum 15 is developed into a toner image by a developing device, and the toner image is transferred to a recording material such as transfer paper by a transfer unit and fixed to the recording material by a fixing device.
[0006]
In the case of this embodiment, the laser light is deflected at a constant angular velocity by the polygon mirror 8 and passes through the polygon mirror 8, the polygon mirror shield glass 9, fθ in order to keep the scanning speed on the surface to be scanned constant. Optical elements such as a lens 11, a valet toroidal lens 12, a folding mirror 13, and a dustproof glass 14 are used. In the claims, the scanning image forming optical means means the fθ lens 11, the valet toroidal lens 12, and the folding mirror 13.
Since the light use efficiency such as the reflectance and the transmittance of these optical elements differs depending on the incident angle of light, the intensity of the laser light varies depending on the image height. Such intensity of the light beam intensity due to the image height is called shading characteristics, and affects the density of the formed image.
[0007]
FIG. 3 shows the laser beam intensity according to the image height on the surface to be scanned (the surface of the photosensitive drum 15) when the LD 2 is illuminated with the same amount of light at the entire image height. With the image height 0 as the center of the photosensitive drum 15 in the main scanning direction, there are left and right effective writing areas. The beam intensity on the surface to be scanned becomes constant by correcting the amount of laser light so that the shading characteristic in FIG. 3 is opposite to that shown in FIG.
In this embodiment, the light amount correction data corresponding to each scanning position of the laser beam is read at the time of scanning the effective writing area of the laser beam. The pulse width modulation (PWM) unit 17 (pulse width modulation means) of the LD write control IC 6 shown in FIG. 2 performs pulse width modulation corresponding to the light amount correction data. This PWM output is smoothed by a low-pass filter (LPF) 18 external to the LD write control IC and becomes a light amount correction signal of an analog voltage to correct the laser light amount.
At this time, if the time constant of the LPF 18 is too small, the PWM output becomes difficult to be smoothed, and the ripple increases, and if the time constant of the LPF 18 is too large, the light amount correction signal will Despite the following, the time constant of the LPF 18 is derived and set to an optimum value, so that a light quantity correction signal that changes smoothly as shown in FIG. 4 can be obtained.
[0008]
As described above, in this embodiment, the LD unit 1 as a light source unit that generates a light beam composed of laser light, the polygon mirror 8 as a deflecting unit that deflects and scans the light beam from the light source unit, A scanning image forming optical unit including an fθ lens 11, a valet toroidal lens 12, and a return mirror 13 for condensing a light beam as a light spot on the surface of a photoreceptor 15 as a surface to be scanned and scanning the surface to be scanned with the light spot. In the beam scanning type image forming apparatus having: a pulse for performing pulse width modulation corresponding to the scanning position of the laser light, based on light amount correction data of the light source means given in advance corresponding to the scanning position of the laser light. Equipped with the width modulation means 17 and the LPF means 18 for smoothing the PWM output, the shading characteristics of the optical system can be simplified. It can be corrected, the image quality can be improved to be able to beam intensity for each image height to reduce the non-uniformity of the image density due to different.
In this embodiment, since only the pulse width modulation (PWM) signal needs to be output from the LD write control IC 6, the creation and verification of the LD write control IC 6 are facilitated, and the LD write control IC 6 Cost can be reduced.
Further, in this embodiment, by adjusting the value of the LPF 18, the PWM output from the pulse width modulation unit 17 is smoothed, and the laser light amount unevenness at the division boundary of the effective writing area is reduced. Shading correction can be realized. Further, the pulse width modulation unit 17 and the LPF 18 can be used not only for shading correction but also for overall LD light amount adjustment.
According to the present embodiment, the LD write control IC 6 performs only pulse width modulation (PWM), and the low pass filter (LPF) 18 outside the LD write control IC 6 controls the pulse width modulation unit inside the LD write control IC. Since the configuration is such that the PWM output from 17 is smoothed, it is possible to provide a beam scanning type image forming apparatus which can easily realize the shading correction function with a low cost configuration.
[0009]
【The invention's effect】
According to the first aspect of the present invention, light source means for generating laser light, deflecting means for deflecting and scanning the laser light from this light source means, and converging and scanning the laser light from this deflecting means on the surface to be scanned. A beam scanning type image forming apparatus having a scanning image forming optical unit, wherein the shading characteristic correcting unit which is a laser intensity displacement within a laser beam scanning width in the scanning image forming optical unit outputs a pulse corresponding to a laser light amount. The target object can be achieved by being constituted by the generated pulse width modulation means and a low-pass filter for smoothing the output from the pulse width modulation means.
According to the second aspect of the present invention, by adjusting the time constant of the low-pass filter, the PWM output from the pulse width modulation means can be smoothed, and the unevenness in the amount of laser light generated when the pulse width is switched can be eliminated. it can.
[Brief description of the drawings]
FIG. 1 is a front view showing a writing optical system of an image forming apparatus according to an embodiment of the present invention.
FIG. 2 is a top view similarly shown together with a control system.
FIG. 3 is a diagram illustrating a laser beam intensity according to an image height on a surface to be scanned (a surface of a photosensitive drum) when an LD is illuminated with the same amount of light at an entire image height.
FIG. 4 is a diagram showing a light quantity correction signal.
[Explanation of symbols]
1 LD unit (light source means)
8 Polygon mirror (deflection means)
11 fθ lens (scanning optical system)
12. Barrettoroidal lens (scanning optical system)
13. Folding mirror (scanning optical system)
15 Photoconductor (scanned surface)
17 pulse width modulation unit (pulse width modulation means)
18 Low-pass filter

Claims (2)

Beam scanning having light source means for generating laser light, deflecting means for deflecting and scanning the laser light from the light source means, and scanning image forming optical means for condensing and scanning the laser light from the deflecting means on the surface to be scanned Type image forming apparatus, wherein a means for correcting shading characteristics, which is a laser intensity displacement within a laser beam scanning width in a scanning image forming optical means, comprises a pulse width modulation means for generating a pulse corresponding to a laser light amount; An image forming apparatus comprising: a low-pass filter for smoothing an output from a width modulation unit. 2. The image forming apparatus according to claim 1, wherein a time constant of the low-pass filter is variable.

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