JP2000071510A - Image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent generation of a laser beam intensity irregularity on a surface to be scanned according to an image height. SOLUTION: The apparatus comprises scanning position detecting means 20-23, 27-30 for detecting the scanning position of a light spot, a memory means 31 provided previously corresponding to the scanning position of the light spot detected by the scanning position detecting means for storing light amount correction data of a light source means, and a control means 2 for controlling the light amount of the light source means corresponding to the scanning position of the light spot based on the light amount correction data stored in the memory means 31.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. 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 machine.

[0002]

2. Description of the Related Art Conventionally, an image forming apparatus such as a laser printer, a digital copying machine, a facsimile machine, or the like, uses a laser beam from a recording light source comprising a laser diode modulated by image data by a scanning means comprising a polygon mirror. A beam scanning type image forming apparatus equipped with a device that scans the photoconductor in the main scanning direction and moves the photoconductor in the subscanning direction to write an image on the photoconductor one line at a time is used.

In this beam scanning type image forming apparatus, a laser beam is deflected at a constant angular velocity by a polygon mirror.
To make the scanning speed on the surface to be scanned constant, fθ
Lenses and fθ mirrors are used. JP-A-6-25
Japanese Patent Application Publication No. 5172 discloses an image forming apparatus that detects a beam intensity on a photoreceptor and controls an exposure amount by a laser beam based on the outstanding result. Tokuhei 6-
No. 14663 discloses an optical scanning apparatus which scans a surface to be scanned without deflecting modulated light from a semiconductor laser by a rotary deflector and making the scanning speed uniform by an fθ lens. A first output intensity control circuit for obtaining a digital reference signal for setting the emission intensity of the semiconductor laser at the time to the first reference value; A second output intensity control circuit for obtaining a digital correction width control signal for setting a reference value for the image scanning, and an image scanning method in which the frequency continuously changes according to a change in the scanning speed on the surface to be scanned. An image scanning clock frequency control circuit for generating a clock,
The image scanning clock frequency control circuit is driven each time the frequency division ratio with respect to the reference clock from the oscillator is switched to generate a digital correction signal.
A down counter, a first D / A converter for D / A converting the correction signal, and D / A conversion of the correction width control signal for gain adjustment of the first D / A converter. , A second D / A converter, and D / A conversion of the reference signal.
, A correction signal analog-converted by the first D / A converter, and a reference value signal analog-converted by the third D / A converter, and An arithmetic circuit that arithmetically modulates the signal with the correction signal to generate an analog signal that changes stepwise in proportion to the change in the scanning speed, and modulates the analog signal obtained by the arithmetic circuit. An optical scanning device characterized in that a semiconductor laser is driven while being modulated by a signal.

Japanese Patent No. 2549012 discloses a modulator for modulating a laser beam emitted from a laser light source based on an image signal and a reference clock, and a first scanning for scanning the modulated laser beam in a main scanning direction. Means for imaging the laser beam to form a beam spot on the surface of the record carrier; second scanning means for relatively moving the record carrier in a direction orthogonal to the main scanning direction; Frequency control means for controlling the frequency of the reference clock applied to the modulation means in response to a change in the main scanning speed of the beam spot caused by the characteristics of the lens; and starting the main scanning of the laser beam for each main scanning. Synchronizing means for synchronizing the position and the generation start time of the reference clock, wherein the frequency control means is a line memo of n words of one word (n is an integer of 2 or more). And a D / A converter and a voltage-controlled oscillator. The line memory stores one address and one word for each predetermined angle of incidence on the fθ lens. Is stored as n-bit data, and the n-bit data read from the line memory is converted to D / D by an addressing signal proportional to the angle of incidence of the fθ lens after the start of each main scan. An image recording apparatus is described in which an A converter converts the analog clock into an analog signal and a voltage-controlled oscillator generates a reference clock having a frequency proportional to the analog signal.

Japanese Patent No. 2571592 discloses an optical scanning device that scans a surface to be scanned by a rotating deflector with modulated light from a semiconductor laser and does not use an fθ lens, and that responds to changes in the scanning speed of the surface to be scanned. An image scanning clock generator for generating an image scanning clock whose frequency changes continuously,
A digital value setting circuit for generating a digital value for changing the output intensity of the semiconductor laser in accordance with a change in the scanning speed of the surface to be scanned; and converting the output value of the digital value setting circuit into an analog value and A digital / analog converter for changing the current of the semiconductor laser in accordance with a value of the digital / analog converter, wherein the gain of the output of the digital / analog converter is adjusted by the output value of the digital / analog converter. An optical scanning device that adjusts the change to be optimum, further comprising a gain adjustment unit that adjusts the gain of the digital / analog converter according to the output of a reference voltage generator. Have been.

[0006]

In the above-described beam scanning type image forming apparatus, the scanning speed of the laser beam passing through the fθ lens or the fθ mirror on the surface to be scanned becomes substantially constant.
The intensity of the laser beam on the surface to be scanned 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 beam is emitted from the laser diode to when it reaches the surface to be scanned, varies depending on the incident angle of the laser beam. This is because the thickness of the fθ lens differs depending on the image height.

The intensity of the beam intensity due to the image height is called a shading characteristic. The shading characteristic is usually 10% or more, which affects the density of a formed image. Further, the shading characteristics are determined by the characteristics and arrangement of the optical elements, and variations between image forming apparatuses and variations due to environment such as temperature and humidity are slight.
In the image forming apparatus described in JP-A-6-255172, the beam intensity on the photoreceptor is detected, and the amount of exposure by the laser beam is controlled based on the detection result. However, there is a problem that a control means is required, and control becomes complicated, and the cost is increased.

According to the first aspect of the present invention, the shading characteristics of the optical system can be corrected, and the unevenness in image density due to the difference in beam intensity at each image height can be reduced to improve the image quality. It is an object of the present invention to provide an image forming apparatus capable of performing the following. According to a second aspect of the present invention, there is provided an image forming apparatus capable of achieving the same object as the first aspect of the present invention with a simple configuration and capable of easily rewriting light amount correction data even in a system having a different optical system. The purpose is to provide.

A third object of the present invention is to provide an image forming apparatus capable of achieving the same object as the first object of the present invention with a simple and low-cost configuration. A fourth object of the present invention is to provide an image forming apparatus capable of achieving the same object as the first object of the present invention with a simple and low-cost configuration. The invention according to claim 5 solves the problem that in the invention according to claim 4, there is a risk that the beam intensity is suddenly switched due to the switching of the division of the scanning position and a step occurs in the image density,
It is an object of the present invention to provide an image forming apparatus capable of improving image quality by smoothing a step of image density.

The invention according to claim 6 can correct shading characteristics by the optical system, reduce unevenness in image density due to a difference in beam intensity for each image height, and improve image quality. It is an object of the present invention to provide an image forming apparatus capable of performing the following. According to the seventh aspect of the present invention, it is possible to set the optimum beam intensity for the beam detection means for synchronization detection, to improve the synchronization detection accuracy, and to obtain a good image by reducing the main scanning dot shift. It is an object of the present invention to provide an image forming apparatus capable of performing the above.

According to the present invention, the optimum beam intensity can be set for the beam detecting means for detecting the synchronization, the accuracy of the synchronization detection can be improved, and the deviation of the main scanning dot can be reduced to obtain a good result. It is an object to provide an image forming apparatus capable of obtaining an image. According to a ninth aspect of the present invention, there is provided an image forming apparatus capable of correcting a light amount of a beam firstly incident on a scanning position detecting unit to an appropriate light amount and reliably detecting a scanning position of a light spot. The purpose is to:

[0012]

In order to achieve the above object, the invention according to claim 1 comprises a light source means for generating a light beam, a deflecting means for deflecting and scanning the light beam from the light source means, and A beam scanning type image forming apparatus having a scanning image forming optical means for converging the light beam as a light spot on the surface to be scanned and scanning the surface to be scanned with the light spot. Scanning position detecting means, storage means for storing light amount correction data of the light source means, which is given in advance corresponding to the scanning position of the light spot detected by the scanning position detecting means, and is stored in the storage means. Control means for controlling the light amount of the light source means in accordance with the scanning position of the light spot based on the light amount correction data.

According to a second aspect of the present invention, there is provided the image forming apparatus according to the first aspect, further comprising a writing unit for freely writing light amount correction data in the storage unit.

According to a third aspect of the present invention, in the image forming apparatus according to the first aspect, the scanning position detecting means detects a scanning position obtained by dividing the entire scanning position of the light spot into a predetermined number. A scanning position detecting unit having a resolution, wherein the storage unit is a storage unit that stores light amount correction data for each portion obtained by dividing the entire scanning position of the light spot into a predetermined number.

According to a fourth aspect of the present invention, in the image forming apparatus of the first aspect, reading means for reading light amount correction data from the storage means for each of the divided scanning positions, and reading by the reading means. Digital-to-analog conversion means for converting the light quantity correction data into an analog signal, wherein the control means controls the light quantity of the light source means with an analog signal from the digital / analog conversion means.

According to a fifth aspect of the present invention, there is provided the image forming apparatus according to the fourth aspect, further comprising a low-pass filter for cutting and smoothing a high-frequency component of an analog signal from the digital-to-analog converting means, and the control means. Is for controlling the light quantity of the light source means by the analog signal smoothed by the low-pass filter means.

According to a sixth aspect of the present invention, in the image forming apparatus according to the first aspect, the storage means stores the light quantity correction data such that the beam intensity of the light spot at each scanning position becomes substantially constant. It becomes.

According to a seventh aspect of the present invention, in the image forming apparatus of the first aspect, the scanning position detecting means is disposed at a position before an effective writing area in the main scanning direction, and the light beam from the deflecting means is provided. Beam receiving means for generating a beam detection signal upon receiving the beam, wherein the control means determines the intensity of the beam of the light spot at the scanning position near the beam detecting means by the intensity of the beam of the light spot at the image height in the effective writing area. The correction is made independently of the intensity.

According to an eighth aspect of the present invention, in the image forming apparatus according to the first aspect, the control means controls the light amount of the light source means for each scanning position only in an effective writing area for forming an image. In the invalid writing area, the light quantity of the light source is controlled to be a predetermined light quantity independently of the light quantity control in the effective writing area.

According to a ninth aspect of the present invention, in the image forming apparatus according to the first aspect, the control means does not depend on the scanning position until the scanning position of the light spot is first detected. Is to control the light amount of the light source means so as to have a constant light amount sufficient to detect the scanning position of the light spot.

[0021]

2 to 4 show a component arrangement and an optical path of a writing optical system according to an embodiment of the present invention. Semiconductor laser (hereinafter referred to as LD) unit 1 as a light source unit
Has a light source control unit 2 composed of an LD control plate, an LD 3 as a light source, a collimating lens 4, and an aperture 5. The LD control plate 2 constitutes an LD modulation unit as a light source modulation unit, and uses a light amount designated by a light amount correction signal from a print control unit 6 as an image formation control unit as a reference light amount from an image data input unit (not shown). LD3 is output from the modulation data output from the print control unit 6 based on the image data.
To emit light (laser light) modulated by the modulation data from the LD 3.

The laser light from the LD 3 is collimated into a parallel light beam by the collimating lens 4, shaped into a predetermined size by the aperture 5, and emitted from the LD unit 1. The laser beam emitted from the LD unit 1 passes through a cylinder lens 7 and passes through a polygon mirror shield glass 9 for soundproofing a rotary deflector 8 composed of a polygon mirror as scanning means.

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 a laser beam incident from the polygon mirror shield glass 9. This polygon mirror 8
Is transmitted again through the polygon mirror shield glass 9 and through the two-element fθ lens 11,
The light passes through a valet toroidal lens 12 for correcting the surface tilt of the polygon mirror 8, is reflected by a turning mirror 13, and is further condensed on a photosensitive member 15 as a recording medium through a dustproof glass.

The photoreceptor 15 is, for example, a photoreceptor drum. The photoreceptor 15 is rotated by a rotation driving unit (not shown) and uniformly charged by a charger (not shown). , The image is written and an electrostatic latent image is formed. The electrostatic latent image on the photosensitive drum 15 is developed by a developing device (not shown) into a toner image, transferred to a recording material such as transfer paper by a transfer unit (not shown), and fixed to the recording material by a fixing device.

The laser beam that has passed through the valet toroidal lens 12 is reflected by the turning mirror 1 before the effective writing area.
The light is condensed on the photodetector 16 as beam detecting means without passing through the light detector 3. When the photodetector 16 receives and detects the laser beam from the valet toroidal lens 12, it generates a beam detection signal (also referred to as a synchronization detection signal) XDETP. This beam detection signal XDETP is input to the print control unit 6.

In the case of this embodiment, the reflectance and transmittance of the optical elements such as the polygon mirror 8, the polygon mirror shield glass 9, the fθ lens 11, the valet toroidal lens 12, the folding mirror 13, and the dustproof glass 14 through which the laser beam passes. Since the light utilization rate varies depending on the incident angle of light to the optical element, the intensity of the laser beam varies depending on the image height.

FIG. 5 shows the laser beam intensity (the light spot S of the light spot S) based on the image height on the surface to be scanned (the surface of the photosensitive drum 15) when the LD 3 is illuminated with the same amount of light at the entire image height in this embodiment. Laser beam intensity). In the case of this embodiment, the photodetector 16 has the image height 0 at the center of the photosensitive drum 15 in the main scanning direction, and the photodetector 16 is provided at the position of the image height -160, from the image height -150 to the image height +150. The range is the effective writing area.

As shown in FIG. 7, the photodetector 16 receives a laser beam from the valet toroidal lens 12 and converts the output signal of the PIN photodiode 17 into a PIN photodiode 17 as a light receiving element for photoelectrically converting the laser beam. A digital signal conversion circuit 18 converts the signal into an ON / OFF digital signal. When the PIN photodiode 17 receives the laser beam from the valet toroidal lens 12, it outputs a low-level synchronization detection signal XDETP. As described above, the IC in which the light receiving element 17 and the electric circuit 18 are contained in one package is used, but the light receiving element 17 and the electric circuit 18 may be separated. The optical detector 16 may use an optical fiber for guiding a laser beam from the valet toroidal lens 12.

Synchronization detection signal XDET from photodetector 16
P is input to the print control unit 6, and the print control unit 6 combines the image data from the image input unit (not shown) and the forced lighting signal for obtaining the synchronization detection signal as modulation data of the LD 3 to the LD control unit 2. Output. FIG. 1 shows the print control unit 6.
Is shown. The print control unit 6 is not limited to the one shown in FIG. 1, but only a part related to the present invention is shown in FIG.

First, the clock generation circuit 19 is composed of a crystal oscillator or a PLL frequency synthesizer, and generates a print pixel clock LDCLK. The print pixel clock LDCLK is supplied from the clock synchronization circuit 20 to the photodetector 1.
6 synchronization detection signal (synchronization detection pulse signal) XDET
The phase is synchronized with the timing of P to become a laser beam modulation clock LDCLK1.

The synchronization detection pulse signal X from the light detector 16
The DETP is synchronized with the laser beam modulation clock LDCLK1 in the clock synchronization circuit 20, and becomes the main scanning start signal LCLR. The laser beam modulation clock LDCLK1 and the main scanning start signal LCLR are output to an image input unit (not shown), and are used as a clock and a synchronization signal for synchronizing image data with image writing in the image input unit. That is, the image input unit starts the transfer of one line of image data to the print control unit 6 by the main scanning start signal LCLR, and synchronizes the transfer of the image data to the print control unit 6 with the laser beam modulation clock LDCLK1. Do it.

Further, the main scanning start signal LCLR from the clock synchronization circuit 20 is also output to a reset terminal of the first counter 21 to reset the first counter 21. The first counter 21 is a so-called main scanning counter, and is a main scanning start signal LCL from the clock synchronization circuit 20.
This is a binary counter that is reset by R and incremented by the laser beam modulation clock LDCLK1 from the clock synchronization circuit 20, and the count value indicates the main scanning position of the laser beam.

The first counter 21 has a number of bits that does not overflow during the scanning of one line.
This number of bits is 8 for printing paper of 297 mm in the main scanning direction.
If an image is printed at 00 dpi, 14 bits are required. The first counter 21 has two comparators 2
2 and 23 are connected, and the first comparator 22 generates a forced drive signal BD of the LD 3 for synchronous detection of the laser beam.

The first comparator 22 has a CPU (C) serving as numerical value setting means for variably setting a numerical value B to the first comparator 22.
(entral Processing Unit) 24
Are connected via a register 25. The first comparator 22 calculates the count values A and C of the first counter 21.
The PU 24 compares the count value A with the numerical value B variably set in the first comparator 22, and when the count value A exceeds the set value B, the output signal BD becomes active.

The output signal B of the first comparator 22
D is ORed with the image data from the image input unit by the OR gate 26 as a beam detection signal, and the LD modulator 2 drives the LD 3 by the output signal of the OR gate 26 to emit light. Therefore, the LD 3 is forcibly driven by the forcible lighting signal from the OR gate 26 to emit light.

When the laser beam from the LD 3 forcibly turned on by the forcible drive signal from the OR gate 26 enters the light receiving portion of the photodetector 16, a beam detection signal (synchronous) output from the photodetector 16 Detection signal) XDETP
Becomes active. This beam detection signal XDETP
Are synchronized with the laser beam modulation clock LDCLK1 by the clock synchronization circuit 20, are output as the main scanning start signal LCLR, and the first counter 21 is reset.

The forcible drive signal BD of the LD 3 is an output signal of the first comparator 22. The first comparator 22 determines the count value A of the first counter 21 and the CPU 2
The output signal becomes active when the count value A is larger than the set value B by comparing the variable B with a numerical value B which is variably set in step 4, so that when the first counter 21 is reset, the forced drive signal BD Is negated, and the LD 3 is turned off.

When the first counter 21 is reset,
Since the first counter 21 restarts counting the laser beam modulation clock LDCLK1 from the clock synchronization circuit 20, the counting operation of the first counter 21 is repeated for each surface of the polygon mirror 8. At this time,
Forcibly driving the LD 3, the laser beam scanned on the next surface of the polygon mirror 8 reaches the light receiving section of the photodetector 16 after the scanning beam from the polygon mirror 8 passes through the effective writing area. Must be before,
Since it is necessary to prevent flare, the CPU 24 sets the timing so that the laser beam scanned on the next surface of the polygon mirror 8 arrives immediately before the light receiving unit of the photodetector 16.
Set the set value B with.

The second comparator 23 generates an effective write area signal LGATE, and a period during which the effective write area signal LGATE is at a high level indicates an effective write area. The second comparator 23 has a numerical value C,
CPU 24 which is a numerical value setting means for variably setting D
Are connected via a register 25. The second comparator 23 calculates the count values A and C of the first counter 21.
The PU 24 compares the numerical values C and D which are variably set in the second comparator 23 with the output signal LGA during a period in which the count value A is equal to or more than the set value C and equal to or less than the set value D.
TE becomes active.

The CPU 24 converts the distance from the photodetector 16 to the start position of the effective write area and the distance from the photodetector 16 to the end position of the effective write area as the number of clocks as numerical values C and D in advance. Since the data is written to the register 25 and given to the second comparator 23, the effective write area signal LGATE indicates that the scanning position of the laser beam is in the effective write area. The valid write area signal LGATE from the second comparator 23 is applied to a second counter 27.

When the count value A of the first counter 21 matches the set value C, the second comparator 23 outputs a pulse signal indicating that the scanning position of the laser beam is at the start position of the effective writing area. The pulse signal is ORed with the output signal of the comparator 30 by the OR gate 29. The output signal of the OR gate 29 is applied to the reset input terminal of the second counter 27 and the count clock input terminal of the third counter 28.

The second counter 27 and the third comparator 30 constitute means for dividing the entire effective writing area into a plurality of parts at equal intervals according to the image height. First, the second counter 27
Is the effective write area signal L from the second comparator 23
GATE is input as an enable signal, and the count operation is not performed during a period when the effective write area signal LGATE is negative, that is, outside the effective write area.

When the valid write area signal LGATE from the second comparator 23 becomes active, the second counter 27 is reset by the output signal of the OR gate 29 and at the same time the laser from the clock synchronization circuit 20 is reset. The counting of the beam modulation clock LDCLK1 is started. The count value of the second counter 27 is output to the third comparator 30, and the third comparator 30 is a numerical value setting means C for variably setting the numerical value F thereto.
The PU 24 is connected via the register 25.

The third comparator 30 compares the count value E of the second counter 27 with a numerical value F variably set in the third comparator 30 by the CPU 24,
When the count value E matches the set value F, a pulse signal WIDTH for dividing the effective write area is generated. CPU2
In 4, the width of dividing the effective write area at equal intervals is converted into the number of clocks LDCLK <b> 1 and written to the register 25 in advance as a numerical value F.

The pulse signal WIDTH from the third comparator 30 is supplied to the second comparator 2 by the OR gate 29.
3 is ORed with a pulse signal indicating that the scanning position of the laser beam from 3 is at the start position of the effective writing area. The output signal of the OR gate 29 is output to the reset input terminal of the second counter 27 and to the 3 is applied to the count clock input terminal of the counter 28. The second counter 27
The pulse is reset by the pulse signal WIDTH that divides the effective writing area into a plurality of parts, which is input from the third comparator 30 via the OR gate 29, and restarts the counting operation.
Therefore, the counting operation of the second counter 27 is repeated until the effective write area signal LGATE from the second comparator 23 becomes negative for each width that divides the effective write area at equal intervals. Will generate a pulse signal WIDTH that divides the effective writing area at equal intervals.

The third counter 28 receives the valid write area signal LGATE from the second comparator 23 into the reset input terminal and outputs the negative write area signal LGATE from the second comparator 23 during the negative period. That is, 0 is output outside the valid writing area. When the valid write area signal LGATE from the second comparator 23 is activated, the laser beam scanning is applied from the second comparator 23 to the count clock input terminal of the third counter 28 via the OR gate 29. A pulse signal indicating that the position is at the start position of the effective writing area is input, the counter 28 is incremented, and the counter 28 outputs 1.

Next, the third comparator 30 outputs the pulse signal WIDTH for dividing the effective writing area for each width for dividing the effective writing area at equal intervals while the laser beam scans the effective writing area. And output. The pulse signal WIDTH from the third comparator 30 is supplied to the OR gate 29.
, And is input to the count clock input terminal of the third counter 28, and while the laser beam scans the effective writing area, the third counter 28 is incremented by the pulse signal WIDTH.

When the scanning position of the laser beam exceeds the effective writing area, the effective writing area signal LGATE from the second comparator 23 becomes negative, and the third counter 2
The count value of 8 returns to 0. As described above, the count value of the third counter 28 indicates the scanning position where the effective writing area is scanned by the laser beam and is divided at equal intervals.
The counting operation of the counter 28 is repeated for each surface of the polygon mirror 8.

The count value of the third counter 28 is inputted as an address signal to a RAM 31 as storage means.
The address of the RAM 31 is specified by the count value of the third counter 28. The RAM 31 stores a numerical value variably set by the CPU 24 as a data table, and the CPU 24 controls reading and writing of data.
The CPU 24 outputs the light amount correction data that makes the beam intensity of the light spot at each scanning position substantially constant as each light amount correction data corresponding to each scanning position obtained by dividing the effective writing area in advance at each address (third position) of the RAM 31. Counter 28
At the address specified by the count value of (1).

When scanning the effective writing area with the laser beam, the RAM 31 reads the light quantity correction data corresponding to each scanning position of the laser beam from the address specified by the count value of the third counter 28. This light quantity correction data is converted from digital to analog by a digital / analog converter (DAC) 32 to become an analog voltage corresponding to the light quantity correction data, and is converted to a low-pass filter (LPF).
The signal 33 is input to the LD modulator 2 as a light amount correction signal LDLVL.

The LPF 33 cuts and smoothes the high frequency components of the analog signal from the DAC 32. As a result, it is possible to prevent the intensity of the laser beam from being switched due to the switching of the division of the effective writing area, thereby preventing the occurrence of a step in the image density and smoothing the step in the image density. LD modulator 2
The LD 3 is modulated and driven by the modulation data from the OR gate 26.

The LD modulation drive of the LD modulation section 2 is called pulse width modulation for controlling the pulse width of one dot in accordance with the modulation data, power modulation for controlling the light amount of one dot in accordance with the modulation data, or A combination of pulse width modulation and power modulation disclosed in Japanese Patent Application Laid-Open No. 2-243363, filed by the present applicant, may be used. The LD modulating unit 2 controls the drive current of the LD 3 by controlling the output current of the monitoring photodiode 34 built in the LD unit 1 and monitoring the light amount of the LD 3.
3 is controlled. Further, the LD modulation section 2 includes an LPF 3
The reference light amount of the LD 3 is controlled by the light amount correction signal LDLVL from 3 in accordance with the scanning position of the laser beam.

This embodiment is an apparatus having shading characteristics as shown in FIG. 5, and writes light amount correction data into the RAM 31 such that the characteristics are opposite to the shading characteristics as shown in FIG. By correcting the light amount of the LD 3 with the correction data, the beam intensity of the light spot on the surface to be scanned becomes constant. FIG. 8 is a time chart of the synchronization detection pulse signal XDETP and the light quantity correction signal LDLVL when the effective writing area is divided into 15 equal parts when the effective writing area is divided at equal intervals. Light intensity correction signal L
The DLVL is light amount correction data # 1 to # 15 having characteristics opposite to shading characteristics in a plurality of regions obtained by dividing the effective writing region into fifteen equal parts, and a constant light amount correction in regions other than the effective writing region. This is data # 0.

The photodetector 16 is composed of a PIN photodiode 17 and a digital signal conversion circuit 18. In order to generate a beam detection pulse XDETP with accurate timing, the amount of received light must be appropriate. Since the timing of the beam detection pulse XDETP may vary depending on the amount of received light, the optimal amount of received light of the photodetector 16 does not always match the amount of laser beam applied to the photoconductor 15 for image formation.

Therefore, the CPU 24 sets the data to be written at the address 0 of the RAM 31 independently of the data corresponding to the beam power for irradiating the photosensitive drum 15 and sets the RA
The data to be written to address 0 of M31 is
Is set to an appropriate value so that the light amount of the laser beam incident on the light detector 16 becomes a constant light amount sufficient for the photodetector 16 to detect the laser beam. Therefore, the light quantity correction data # 0 in the area other than the effective writing area is data that makes the light quantity of the laser beam incident on the photodetector 16 appropriate, and the light quantity of the laser beam incident on the photodetector 16 is appropriate. become.

When the power is turned on or when the LD 3 is once turned off and turned on again, it is not known where the scanning position of the laser beam is before the first beam detection signal is obtained. Therefore, when the CPU 24 does not know where the scanning position of the laser beam is located until the first beam detection signal is obtained, such as when the power is turned on or when the LD 3 is once turned off and turned on again, the CPU 24 obtains the beam detection signal first. Until the above, the count start value of the first counter 21 is set to a value larger than the set value D regardless of the scanning position of the laser beam. Therefore, since the output signal of the OR gate 26 is not activated, the light amount correction signal LDLVL is at a level based on the light amount correction data at the address 0 of the RAM 31, and the light amount of the laser beam first incident on the photodetector 16 is equal to the light detection amount. Table 16
Has a constant light amount sufficient to detect the laser beam.

As described above, in this embodiment, the LD unit 1 as a light source unit for generating a light beam composed of a laser beam, the polygon mirror 8 as a deflection unit for deflecting and scanning the light beam from the light source unit, A light beam from the means is condensed as a light spot on the surface of the photoreceptor 15 as a surface to be scanned, and a scanning lens comprising an fθ lens 11, a valet toroidal lens 12, and a folding mirror 13 for scanning the surface to be scanned with the light spot. A light detector 1 as a scanning position detecting means for detecting a scanning position of the light spot in a beam scanning type image forming apparatus having an image optical means.
6, clock synchronization circuit 20, counters 21, 27, 2
8, comparators 23 and 30, and OR gate 29;
RAM 3 as storage means for storing light amount correction data of the light source means provided in advance corresponding to the scanning position of the light spot detected by the scanning position detection means.
1 and an LD modulator 2 as control means for controlling the light quantity of the light source means in accordance with the scanning position of the light spot based on the light quantity correction data stored in the storage means. The amount of light generated by the means can be corrected based on a given data table (light amount correction signal in the RAM 31) corresponding to the image height to be scanned. For this reason, shading characteristics by the optical system can be corrected, and unevenness in image density due to a difference in beam intensity for each image height can be reduced, and image quality can be improved.

In this embodiment, the storage means 3
1 is provided with the CPU 24 as a writing means for freely writing the light quantity correction data, so that the light quantity correction data corresponding to the image height is a table in the memory 31, and the light quantity correction data can be freely written in this table. it can. Therefore, the above effect can be achieved with a simple configuration, and the light amount correction data can be easily rewritten even in a system having a different optical system.

In this embodiment, the scanning position detecting means is a scanning position detecting means having a discrete detection resolution for detecting a scanning position obtained by dividing all the scanning positions of the light spot into a predetermined number. Since the storage means is a storage means for storing the light quantity correction data for each part obtained by dividing the entire scanning position of the light spot into a predetermined number, the data table of the storage means is divided into an appropriate number according to the image height and the division is performed. Light amount correction data designating light amount correction can be given to each of the selected portions, and the above-described effect can be achieved with a simple and low-cost configuration.

Further, in this embodiment, the CPU 24 as reading means for reading the light quantity correction data from the storage means for each of the divided scanning positions (each image height), and the light quantity correction data read by the reading means. DA as a digital / analog conversion means for converting data into an analog signal
C32, and the control means controls the light amount of the light source means by an analog signal from the digital / analog conversion means, so that the above-mentioned effect can be achieved with a simple and low-cost configuration.

In this embodiment, the digital
A low-pass filter means 33 for cutting and smoothing high-frequency components of the analog signal from the analog conversion means,
Since the control unit controls the light amount of the light source unit based on the analog signal smoothed by the low-pass filter unit, there is no step in the image density due to the sudden change of the beam intensity at the switching of the scanning position division. In addition, it is possible to improve the image quality by smoothing the step of the image density.

Further, in this embodiment, the storage means is constituted by storage means for storing light amount correction data such that the beam intensity of the light spot at each scanning position (each image height) becomes substantially constant. The shading characteristics can be corrected, the unevenness in image density due to the difference in beam intensity at each image height can be reduced, and the image quality can be improved.

In this embodiment, the scanning position detecting means is arranged at a position before the effective writing area in the main scanning direction, and generates a beam detection signal when receiving a light beam from the deflecting means. The control means includes a light detector 16 as a detection means, and the control means controls the beam intensity of the light spot at the scanning position near the beam detection means independently of the beam intensity of the light spot at the image height in the effective writing area. Since the correction is performed, the beam intensity within the effective scanning period is set to the optimum intensity for improving the print image quality, and the beam intensity for synchronization detection is different from the optimal beam intensity during the effective scanning period. An optimum beam intensity can be set in the vicinity of the beam detecting means, so that synchronization detection accuracy can be improved, and a main scanning dot shift can be reduced to obtain a good printed image.

In this embodiment, the control means controls the light amount of the light source means for each scanning position (for each image height) only in an effective writing area for forming an image, and controls an ineffective writing area. In this example, the light intensity of the light source means is controlled to be a constant light amount independently of the light intensity control in the effective writing area. Therefore, the beam intensity within the effective scanning period is optimized for improving the print image quality. Intensity can be set, and the optimum beam intensity different from the optimum beam intensity during the effective scanning period can be set in the vicinity of the beam detection means for the beam detection means for synchronization detection, thereby improving the synchronization detection accuracy. In this embodiment, it is possible to reduce the main scanning dot deviation and obtain a good print image. In this embodiment, the control means keeps the scanning position intact until the scanning position of the light spot is first detected. Without Since the light quantity of the light source means is controlled so that the scanning position detecting means has a constant light quantity enough to detect the scanning position of the light spot, the light quantity of the beam first entering the scanning position detecting means is appropriately adjusted. Correction can be made, and the scanning position of the light spot can be reliably detected.

[0065]

As described above, according to the first aspect of the present invention, with the above configuration, the shading characteristics of the optical system can be corrected, and the unevenness in image density due to the difference in beam intensity at each image height. Can be reduced, and the image quality can be improved. According to the second aspect of the present invention, with the above configuration, the effect of the first aspect of the present invention can be obtained with a simple configuration, and the light amount correction data can be easily rewritten even in a system having a different optical system.

According to the third aspect of the present invention, with the above configuration, the effects of the first aspect of the invention can be obtained with a simple and low-cost configuration. According to the invention of claim 4,
With the above configuration, the effect of the invention according to claim 1 can be obtained with a simple and low-cost configuration. According to the fifth aspect of the present invention, with the above-described configuration, the image intensity does not change suddenly due to the sudden change of the beam intensity due to the switching of the division of the scanning position. The image quality can be improved by smoothing the steps.

According to the sixth aspect of the invention, with the above configuration, the shading characteristics of the optical system can be corrected, and the unevenness in image density due to the difference in beam intensity at each image height can be reduced. Image quality can be improved. According to the seventh aspect of the present invention, with the above configuration, it is possible to set the optimum beam intensity for the beam detection means for synchronization detection, to improve the synchronization detection accuracy, and to reduce the main scanning dot deviation. And a good image can be obtained.

According to the eighth aspect of the present invention, with the above configuration, the optimum beam intensity can be set for the beam detecting means for detecting the synchronization, the accuracy of the synchronization detection can be improved, and the main scanning dot shift can be achieved. And a good image can be obtained. According to the ninth aspect of the present invention, with the above configuration, it is possible to correct the light amount of the beam first incident on the scanning position detecting means to an appropriate light amount, and it is possible to reliably detect the scanning position of the light spot. .

[Brief description of the drawings]

FIG. 1 is a block diagram illustrating a configuration of a print control unit according to an embodiment of the present invention.

FIG. 2 is a diagram showing a component arrangement and an optical path of the writing optical system of the embodiment.

FIG. 3 is a diagram showing a writing optical system of the embodiment.

FIG. 4 is a perspective view showing a writing optical system of the embodiment.

FIG. 5 is a diagram illustrating a laser beam intensity according to an image height on a surface to be scanned when the LD is illuminated with the same light amount at the entire image height in the embodiment.

FIG. 6 is a characteristic diagram showing a light amount correction characteristic of the embodiment.

FIG. 7 is a block diagram showing a photodetector of the embodiment.

FIG. 8 is a time chart showing a synchronization detection pulse signal XDETP and a light quantity correction signal LDLVL when the effective writing area is divided into 15 equal parts in the embodiment.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 LD unit 2 LD modulation section 8 Polygon mirror 15 Photoconductor 11 fθ lens 12 Barreloidal lens 13 Folding mirror 16 Photodetector 20 Clock synchronization circuit 21, 27, 28 Counter 23, 30 Comparator 29 OR gate 29 31 RAM 24 CPU 32 DAC 33 Low-pass filter means

Claims (9)

[Claims] A light source for generating a light beam; a deflecting device for deflecting and scanning the light beam from the light source device; a light beam from the deflecting device being condensed as a light spot on a surface to be scanned; In a beam scanning type image forming apparatus having scanning image forming optical means for scanning a surface to be scanned, a scanning position detecting means for detecting a scanning position of the light spot, and a light spot of the light spot detected by the scanning position detecting means. A storage unit for storing light amount correction data of the light source unit, which is given in advance corresponding to the scanning position, and based on the light amount correction data stored in the storage unit, An image forming apparatus comprising: a control unit that controls a light amount of a light source unit. 2. An image forming apparatus according to claim 1, further comprising a writing unit for freely writing light amount correction data in said storage unit. 3. An image forming apparatus according to claim 1, wherein said scanning position detecting means has a discrete detection resolution for detecting a scanning position obtained by dividing all scanning positions of said light spot into a predetermined number. The image forming apparatus is characterized in that the storage means is storage means for storing light amount correction data for each part obtained by dividing the entire scanning position of the light spot into a predetermined number. 4. An image forming apparatus according to claim 1, wherein said light amount correction data read out from said storage means is read out for each of said divided scanning positions, and said light amount correction data read out by said read out means is analogized. An image forming apparatus, comprising: a digital / analog converting unit for converting a signal into a signal; wherein the control unit controls a light amount of the light source unit based on an analog signal from the digital / analog converting unit. 5. An image forming apparatus according to claim 4, further comprising low-pass filter means for cutting and smoothing a high-frequency component of an analog signal from said digital-to-analog conversion means, wherein said control means includes a low-pass filter means. An image forming apparatus, wherein the light amount of the light source means is controlled by a smoothed analog signal. 6. An image forming apparatus according to claim 1, wherein said storage means comprises storage means for storing light amount correction data such that a beam intensity of a light spot at each scanning position becomes substantially constant. Image forming device. 7. An image forming apparatus according to claim 1, wherein said scanning position detecting means is arranged at a position before an effective writing area in a main scanning direction, and detects a light beam from said deflecting means. A beam detecting means for generating a signal, wherein the control means corrects the beam intensity of the light spot at the scanning position near the beam detecting means independently of the beam intensity of the light spot at the image height in the effective writing area. An image forming apparatus. 8. An image forming apparatus according to claim 1, wherein said control means performs light amount control of said light source means for each scanning position only in an effective writing area for forming an image, and an invalid writing area. In the image forming apparatus, the light amount of the light source unit is controlled to be a predetermined light amount independently of the light amount control in the effective writing area. 9. The image forming apparatus according to claim 1, wherein the control means controls the scanning position of the light spot by the scanning position detecting means until the scanning position of the light spot is detected, regardless of the scanning position. An image forming apparatus, wherein the light amount of the light source means is controlled so as to have a constant light amount sufficient to detect a position.