JP2006346863A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus which can obtain a good image without any error of a main scanning magnification and can obtain a good image without a color shift in forming a color image. <P>SOLUTION: When printing is started, a quantity of LD light is set to be one for the time of 600 dpi printing (S201). An interval between two synchronous detections is measured in the state (S202). Adjustment of the main scanning magnification is terminated (S203) by performing correction of a pixel clock. Thereafter, the quantity of LD light is reset to one corresponding to each writing pixel density (S204, S205 and S206). Image data is outputted to carry out the printing operation (S207). <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

The present invention relates to an image forming apparatus such as a copying machine, a printer, a FAX, and a printing machine (all including color) that forms an image by optical writing according to an input image signal, and in particular, an image magnification in the image forming apparatus. Regarding control.

  As this type of technology, for example, the invention disclosed in Patent Literatures 1-3 is known. Among these, in Patent Document 1, for the purpose of providing a multiple beam writing device capable of improving the accuracy of the light intensity of the synchronization detection signal and preventing the deterioration of the image quality, a plurality of light emission sources (multiple laser light sources), A rotating polarizer (rotating scanning means) that simultaneously scans and scans a plurality of writing lights from each light source, an optical lens that constitutes a scanning optical system that scans the plurality of writing lights, and a writing start position on the surface of the photoreceptor. In a multi-beam writing apparatus comprising an optical sensor for generating a synchronizing signal to be defined and a synchronous detection signal generating means having a synchronous detection mirror, a plurality of beams are generated when the optical sensor is irradiated with a light beam to generate a synchronous signal. An invention is disclosed in which is lit.

  Japanese Patent Application Laid-Open No. 2004-228688 corrects variations in the exposure start positions of a plurality of beams on the photosensitive member due to variations in the installation accuracy of the synchronization detection sensor even when the recording density is changed. For the purpose of correcting the variation in the exposure start position on the photosensitive member, when the beam interval in the sub-scanning direction on the photosensitive member is changed according to the recording density, after the synchronization detection sensor detects each of the beams, Change the time to start the beam modulation for each beam, or start the beam modulation after the synchronous detection sensor detects each beam according to the light amount detected by the light amount detection sensor. An invention is disclosed in which the time until this is changed for each beam.

Further, Patent Document 3 discloses a plurality of light emitting elements and light emitting elements for the purpose of preventing a shift in writing start position in the main scanning direction or fluctuation of the output image in the main scanning direction in multi-beam exposure. A driver that activates light emission, a scanner that scans a plurality of beams on the photosensitive member with a positional difference in the main scanning direction and the sub-scanning direction, a sensor that detects light beams outside the exposure main scanning width, and a main scanning signal according to the detection signal In a multi-beam printer including a generator that generates each synchronization signal that defines a writing start position, each sensor generates a lighting signal for generating each light beam for detection, and each lighting signal is supplied to each driver. An invention including a gate and a microcomputer that provides light amount instruction data for making each driver the same sync signal width between each lighting signal is disclosed. It has been.
Japanese Patent Laid-Open No. 2003-0.25626 JP 2001-018444 A JP-A-11-287964

  As conventional multi-beam image forming apparatuses, for example, those disclosed in Patent Documents 1-3 described above are known. However, in these color image forming apparatuses, the amount of laser light is reduced by changing the writing conditions. If it changes, the light quantity of the laser at the synchronization detection position (front end, rear end) also changes. As a result, the falling positions of the leading edge synchronization detection signal and the trailing edge synchronization detection signal change. Therefore, when the main scanning magnification correction is performed by measuring the distance between the two synchronization points, the main scanning is performed. There will be a difference in magnification. This difference causes color misregistration and appears as degradation of image quality. Further, when the main scanning position of the rear end synchronization detection sensor is adjacent to the image data period, the density at the left end of the image may change depending on the writing condition.

The present invention has been made in view of such a state of the prior art, and an object of the present invention is to obtain a good image free of main scanning magnification error, and good in color misregistration in forming a color image. It is to be able to obtain a simple image.

In order to achieve the above object, the first means includes leading edge synchronization detecting means for generating a synchronization detection signal serving as a reference for the main scanning line, and trailing edge synchronization detecting means for detecting the position of the trailing edge of one line. Then, each of the light beams emitted from each of the plurality of light sources and modulated by the image signal is irradiated onto the photoconductor via the scanner optical system to form an image on the photoconductor, and the leading edge synchronization detection signal and Measure between the edge synchronization detection signal, adjust the main scanning magnification based on the measurement result, and measure the distance between the leading edge synchronization detection signal and the trailing edge synchronization detection signal in the image forming apparatus that changes the laser light quantity according to the writing condition When performing the main scanning magnification correction, the main scanning magnification correction is performed with a constant laser light quantity.

A second means is characterized in that, in the first means, the writing condition includes at least one of a writing print density and a writing printing speed, and the laser light quantity is constant irrespective of the writing condition.

The third means includes a leading edge synchronization detecting means for generating a synchronization detection signal serving as a reference for the main scanning line, and a trailing edge synchronization detecting means for detecting the position of the trailing edge of one line, and each of the plurality of light sources. Each of the light beams emitted from the light and modulated by the image signal is irradiated onto the photoconductor via the scanner optical system to form an image on the photoconductor, and the distance between the front end synchronization detection signal and the rear end synchronization detection signal is measured. And
In the image forming apparatus that adjusts the main scanning magnification based on the measurement result and changes the laser light amount according to the writing condition, the laser light amount is kept constant during the synchronization detection period, and the main scanning position is adjusted according to the shading characteristics of the optical system. The laser light quantity is changed stepwise.

According to a fourth means, in the third means, the write condition includes at least one of a write print density and a write print speed, and the laser light quantity is constant during the synchronization detection period even under different write conditions. It is characterized by.

The fifth means is characterized in that, in the third or fourth means, the synchronization detection period is a leading edge synchronization detection period and a trailing edge synchronization detection period.

The sixth means is that in any one of the first to fifth means, when the main scanning position of the trailing edge synchronization detecting means is adjacent to the image data period, the trailing edge synchronization is performed only when the main scanning magnification is adjusted. It is characterized in that the laser light amount in the detection period is made constant regardless of the writing conditions.

According to the present invention, when performing the main scanning magnification correction for measuring the distance between the leading edge synchronization detection signal and the trailing edge synchronization detection signal, the laser light quantity is made constant regardless of the writing condition. The shift in timing at the time of signal detection is eliminated, thereby making it possible to obtain a good image free from a scanning magnification error, and obtaining a good image free from color shift in forming a color image.

  Embodiments of the present invention will be described below with reference to the drawings.

In FIG. 1, the color image forming apparatus according to the present embodiment has different colors (yellow: Y
, Magenta: M, cyan: C, black: K) image forming unit 100Y, 1
00M, 100C, and 100K are arranged in this order from the upstream side to the downstream side in the transport direction along the transport belt 2 that transports the transfer paper 1. The conveyor belt 2 is stretched between a driving-side conveyance roller 3 and a driven-side conveyance roller 4, and is driven to rotate in the direction of the arrow by the rotation of the driving-side conveyance roller 3. A paper feed tray 5 in which the transfer paper 1 is stored is provided below the conveyance belt 2. The transfer sheet at the uppermost position among the stored transfer sheets 1 is fed during image formation and is attracted onto the transport belt 2 by electrostatic attraction. The adsorbed transfer paper 1 is conveyed to the first image forming unit (yellow) 100Y, where yellow image formation is performed.

The first image forming unit (yellow) includes a photosensitive drum 6Y, a charger 7Y, an exposure unit 8, a developing unit 9Y, and a photoconductor cleaner 10Y arranged around the photoconductor drum 6Y.

The surface of the photosensitive drum 6Y is uniformly charged by the charger 7Y, and then exposed by the exposure device 8 with the laser beam 11Y corresponding to the yellow image, thereby forming an electrostatic latent image. The electrostatic latent image formed on the photosensitive drum 6Y is developed by the developing unit 9Y, and a toner image is formed on the photosensitive drum 6Y. This toner image is transferred by the transfer device 12Y at a position (transfer position) where the photosensitive drum 6Y contacts the transfer paper 1 on the transport belt 2, and forms a single color (yellow) image on the transfer paper 1. After the transfer, the photoreceptor drum 6Y is cleaned with unnecessary toner remaining on the drum surface by the photoreceptor cleaner 10Y to prepare for the next image formation.

The transfer sheet 1 having the single color (yellow) transferred by the first image forming unit (yellow) 100Y in this way is conveyed by the conveying belt 2 to the second image forming unit (magenta) 100M. Here again, the toner image (magenta) formed on the photosensitive drum 6M is transferred onto the transfer paper 1 in a similar manner. The transfer paper 1 is further conveyed to the third image forming unit (cyan) 100C and the fourth image forming unit (black) 100K, and similarly formed M, C, and K toner images are sequentially transferred to color. Form an image. The transfer paper 1 on which the color image is formed by passing through the fourth image forming unit 100K is peeled off from the conveying belt 2, fixed by the fixing device 13, and then discharged. Note that the image forming process elements of the second to fourth image forming units 100M, 100C, and 100K are shown by changing the subscripts indicating the colors with respect to the first image forming unit 100Y, and the description of the overlapping units is omitted. The same applies hereinafter.

FIG. 2 is a schematic view of the optical unit (optical writing unit) constituting the exposure device 8 as viewed from above in FIG.

In the figure, the light beams from the LD unit BK16 and the LD unit Y17 pass through the cylinder lenses CYL_BK18 and CYL_Y19, enter the lower surface of the polygon mirror 22 by the reflection mirror BK20 and the reflection mirror Y21, and the polygon mirror 22 rotates. Thus, the light beam is deflected, passes through the fθ lens BKC23 and the fθ lens YM24, and is folded back by the first mirror BK25 and the first mirror Y26.

On the other hand, the light beams from the LD unit C27 and the LD unit M28 are CYL_C2
9 and CYL_M30, enters the upper surface of the polygon mirror 22, and the polygon mirror 22 rotates to deflect the light beam, pass through the fθ lens BKC23 and the fθ lens YM24, pass through the first mirror C31 and the first mirror M32. Wrapped by

Cylinder mirrors CYM_BKC33 and C are located upstream from the writing position in the main scanning direction.
YM_YM34 and sensor BKC35 and sensor YM36 are provided, and the light beams that pass through fθ lens BKC23 and fθ lens YM24 are CYM_BKC33 and CY.
The light is reflected and collected by the M_YM 34 and enters the sensor BKC 35 and the sensor YM 36. These sensors BKC35 and YM36 are synchronization detection sensors for synchronizing in the main scanning direction. Further, similarly to the upstream side, cylinder mirrors CYM_BKC37 and CYM_YM38, as well as sensors BKC39 and sensor YM40 are provided on the downstream side of the image area in the main scanning direction. The light is reflected and collected by the CYM_YM 38 and enters the sensor BKC 39 and the sensor YM 40. Therefore, in this embodiment, the synchronization detection sensors on the upstream side (front end side) in the main scanning direction are the sensors BKC35 and YM36, and the synchronization detection sensors on the downstream side (rear end side) are the sensor BKC39 and the sensor YM40.

Further, in the light beams from the LD unit BK16 and the LD unit C27, the common CYM_BKC33 and the sensor BKC35 on the writing side and the common CYM_ on the end side
BKC37 and sensor BKC39 are used. The same applies to the LD unit Y17 and the LD unit M28. Since light beams of two colors are incident on the same sensor, by making the incident angles of the polygon mirrors 22 of the light beams of the respective colors different,
The timing at which each light beam enters each sensor is changed and output as a pulse train in time series. As can be seen from the figure, BK and C and Y and M are scanned in opposite directions.

FIG. 3 is a block diagram showing the configuration of the pixel clock generator in this embodiment. The pixel clock is a clock used for writing image data. As can be seen from the figure, a PLL unit 51 is provided for each color. In the PLL unit 51, a reference clock (REFCLK) is provided by a 1 / M frequency divider 52.
) And the 1 / N frequency divider 54 for each color pixel clock (* _WCLK) (hereinafter, “*” indicates each color K, M, C, Y). A signal obtained by dividing N by N is input to the phase comparator + LPF + VCO 53, and each color high frequency clock (* _PLLCLK) is generated. Then, based on each color leading edge synchronization detection signal (* _DETP_N) from the synchronization detection sensor, the 1 / K divider 55 divides the high frequency clock by K to generate a pixel clock (* _WCLK), and generates an internal synchronization signal The unit 56 generates an internal synchronization signal (* _PSYNC_N) synchronized with the pixel clock. Each color image forming unit operates with the pixel clock and the internal synchronization signal.

Pixel clock frequency is fwclk = PLLclk / K
= (Frefclk / M × N) / K (1)
It becomes.

When the main scanning magnification correction is performed, the two (front and rear) synchronization detection sensors 35, 39, 3
The pixel clock frequency is measured by measuring the passing time of 6,40 by counting with the high frequency clock, and adjusting the values of M and N so that the count value matches a preset reference count value. Make changes.

FIG. 4 is a timing chart showing the relationship between the synchronization detection signal and the LD light quantity of the image forming apparatus according to the present embodiment, and FIG. 5 is a flowchart for conventional printing. In the flowchart of FIG. 5, the light intensity of the LD 16, 17, 27, 28 is set for each of the writing pixel density of 600 dpi or 1200 dpi (steps S101, 102, 103), and the synchronization detection sensors 35, 36 on the front end side and the rear end side are set. The synchronization detection sensors 39 and 40 perform measurement between two synchronization points (step S104). Next, main scanning magnification correction is performed (step S105). In the main scanning magnification correction, the measurement result between the two synchronization points and the reference value are compared, M and N are substituted into the equation (1), PLL is set, image data is output (step S106), and printing is completed. It is supposed to be.

The two color light beams incident on the sensor BKC35 and the sensor YM36 are converted into KC (Y
M) A tip synchronization detection signal is generated and input to a writing control unit (not shown). In the writing control unit, the tip synchronization detection signal common to the two colors is separated for each color as shown in the timing chart of FIG. 4, and the separated signal is used as the tip synchronization signal of that color, and the main scanning of each color is performed. Use as a writing start reference. Similarly, the two-color light beams incident on the sensor BKC39 and the sensor YM40 also generate a KC (YM) rear-end synchronization detection signal at the rear end side of the main scan, and after each color separated by the writing control unit. It becomes an end synchronization detection signal. The magnification of main scanning is measured by counting between two synchronization detection signals of each color with a high frequency clock. If the magnification is large, the count value increases, and if it is small, the count value decreases.

FIG. 6 shows a typical circuit example of the synchronization detection sensor. The synchronization detection sensor is provided on the scanning line of the LD, and a pulse signal (in the period when the LD scans the PD (photodiode) in FIG.
Synchronous detection signal) is output. The waveform of the synchronization detection signal depends on the time during which the LD scans the PD and the light amount of the LD.

For example, consider a case where the image forming apparatus according to the present embodiment prints at two pixel densities of 600 dpi and 1200 dpi. In that case, the scanning speed in the sub-scanning direction is 2 times the linear speed of the photosensitive member.
It is assumed that the rotation speed of the polygon mirror 22 is not changed, and the scanning clock (pixel clock) in the main scanning direction is changed to 1: 2. 1200dp
Since the dot of i is smaller and usually a better image can be obtained by printing with a smaller amount of LD than at 600 dpi, the amount of light is set to be smaller.

In the case of 600 dpi, P depends on the light quantity of the LD that scans the tip synchronization detection sensors 35 and 36.
Assuming that the output of D (Vpd) has a waveform as shown in FIG. 7, the synchronization detection signal Ssync is generated by the comparator com using the reference voltage (Vref) as a threshold. In the case of 1200 dpi, since the light quantity of the LD is set small, when the LD scans the synchronization detection sensors 35 and 36, the falling position of the synchronization detection signal Ssync is delayed by a as shown in FIG. Similarly, on the rear end side, the waveform of the synchronization detection signal Ssync varies depending on the light quantity of the LD that scans the rear end synchronization detection sensors 39 and 40, but the light efficiency of the writing optical system is slightly different from the front end side. 600 dpi and 1200
The difference in the falling position of the rear end synchronization detection signal at dpi is b. At this time 600 dpi
When the main scanning magnification is adjusted at 1200 dpi, a difference of (a−b) occurs in the count value between the leading edge and trailing edge synchronization detection signals. For this reason, the main scanning magnification is changed by the difference, and in the color image forming apparatus, the difference also causes color misregistration.

In order to solve this problem, in the present image forming apparatus, when adjusting the main scanning magnification as shown in FIG. 4, the LD light amount is always the same as in 600 dpi printing, and a difference occurs in the falling position of the synchronization detection signal Ssync. Control not to. FIG. 8 is a flowchart showing a processing procedure according to this embodiment controlled in this way. In this processing procedure, when printing is started, the LD light quantity is first set to 600 dpi printing (step S201). In this state, the two synchronization detection intervals 35, 36, 39, and 40 are measured (step S202), the pixel clock is corrected, and the main scanning magnification adjustment is completed (step S203). Thereafter, the LD light amount corresponding to each writing pixel density is reset (steps S204, 205, and 206), the image data is output, and the printing operation is performed (step S207). By processing in this way, there is no difference in the measurement result between the two-point synchronization detections depending on the writing condition, and printing can be performed at an appropriate main scanning magnification.

On the other hand, in a normal writing optical system, if the LD emits light with a constant light amount in the main scanning direction, a difference occurs in the LD light amount on the surface of the photoreceptor due to the shading characteristics of the optical system. The amount of LD light on the photoreceptor surface is high at the center in the main scanning direction and low at the end in the main scanning direction. Therefore, in order to make the LD light amount uniform on the surface of the photoreceptor, the image quality can be improved by correcting the light amount output from the LD as shown in FIG. Note that correction of the amount of light output from the LD is also referred to as shading correction.

FIG. 12 shows an example of the configuration of the shading correction control unit. One line is divided into equal divisions (15 divisions in the example—see FIG. 9), and an LD reference voltage corresponding to the light amount set in each area 1 to 15 is output. The LD driver can emit light with a light amount corresponding to the LD reference voltage. Further, the position at which shading correction is started based on the synchronization detection signal is determined by the value of the main scanning counter 121 (FIG. 9C). Up to that point, light is emitted with the same amount of light as in the tip synchronization detection period, and when the shading start position set by the shading start position control unit 122 is reached, the area width (d) set by the division width counter 123 is counted. . When the area 1 ends, the division number counter 124 repeats the process up to the area 15. The shading correction data output unit 125 outputs data corresponding to the LD light amount setting value for each area preset at the timings of the areas 0 to 15. The data is converted into an analog signal (voltage) in the DAC 126 and input to the LD driver as the LD light amount reference voltage via the low-pass filter 127. In the example, a DAC 126 is used, but a similar function may be achieved using PWM.

As shown in FIG. 9, in the normal case where the LD light amount is lower than 600 dpi at 1200 dpi, the LD light amount is dropped at a constant rate for the entire line, so that the LD rate is also maintained at the same rate even during the synchronization detection period.
The amount of light decreases. Therefore, when the main scanning magnification adjustment is performed as described with reference to FIG. 7, a difference in magnification occurs between 600 dpi and 1200 dpi. In order to solve this problem, as shown in FIG. 10, even at 1200 dpi, the LD light quantity (area 0) at the tip synchronization detection position.
The difference in main scanning magnification adjustment is eliminated by setting the LD light amount (area 14) at the rear end synchronization detection position to the same value as at 600 dpi.

FIG. 11 is a timing chart showing the relationship between the synchronization detection signal of the image forming apparatus and the LD light quantity when the position of the rear end synchronization detection is adjacent to the image data area. As shown in the figure, when the position of the rear end synchronization detection is adjacent to the image data area, in order to bring the LD light amount to the level at 600 dpi in the area 13, it is changed at the time of the area 12 as shown by the dotted line. If not, it will not follow. However, since part of the area 12 is an image data area, the LD light quantity at the left end of the main scanning is high in this waveform, and the density is high compared to other parts. In order to eliminate this, when adjusting the main scanning magnification, the same LD light amount is set to 600 dpi and 1200 dpi in the leading edge and trailing edge synchronization detection period as shown by the dotted line in the figure. Since the detection signal is not used, control is performed as shown by a solid line. That is, the control timing is changed at the time of image printing when performing the main scanning magnification adjustment.

As described above, according to the present embodiment, even when the laser light amount changes due to the change of the writing condition, the laser light amount in the synchronization detection period (front end and rear end) is made the same when adjusting the main scanning magnification. The distance between two synchronous detections can be measured accurately, and a good image free from main scanning magnification error can be obtained. Thereby, in the color image forming apparatus, it is possible to obtain a good image without color misregistration.

In addition, since the laser light quantity at the main scanning position is provided in a stepwise manner, the laser light quantity can be made constant only during the synchronization detection period (front end and rear end) regardless of the writing conditions. Thereby, the timing for adjusting the main scanning magnification can be freely selected. For example, it is possible to measure the interval between two-point synchronization detection during image printing and reflect it in the main scanning magnification between papers, thereby reducing the time required for magnification adjustment.

Furthermore, since the laser light amount at the rear end synchronization detection position is controlled to be constant only when adjusting the main scanning magnification, even when the main scan position of the rear end synchronization detection sensor is adjacent to the image data period, Accurate main scanning magnification adjustment can be performed without changing the density.

1 is a diagram illustrating an outline of an overall configuration of a color image forming apparatus according to an embodiment of the present invention. It is the schematic which looked at the optical unit (optical writing unit) in FIG. 1 from upper direction. It is a block diagram which shows the structure of the pixel clock generation part in this embodiment. 6 is a timing chart showing a relationship between a synchronization detection signal and an LD light amount of the image forming apparatus according to the present embodiment. It is a flowchart which shows the process sequence of the conventional main scanning magnification correction. It is a circuit diagram which shows the circuit structure of the synchronous detection sensor in this embodiment. It is a figure which shows the output waveform of the detection output of a synchronous detection sensor in this embodiment, and a synchronous detection signal. It is a flowchart which shows the process sequence of the main scanning magnification correction | amendment in this embodiment. It is a timing chart which shows the relationship between the synchronous detection signal, the image data area | region, and the LD light quantity in 600 dpi and 1200 dpi implemented conventionally. 6 is a timing chart showing the relationship between the synchronization detection signal, the image data area, and the LD light quantity at 600 dpi and 1200 dpi in the present embodiment. The timing chart showing the relationship between the synchronization detection signal, the image data area, and the LD light quantity at 600 dpi and 1200 dpi in this embodiment, shows the case where the output timing of the image data and the trailing edge synchronization detection signal is close. It is a block diagram which shows the structure of the shading correction control part in this embodiment.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Transfer paper 2 Conveyor belt 6Y, 6M, 6C, 6K Photosensitive drum 7Y, 7M, 7C, 7K Charger 8 Exposure device (optical unit)
9Y, 9M, 9C, 9K Developer 10Y, 10M, 10C, 10K Photoconductor cleaner 51 PLL section 52 1 / M frequency divider 53 Phase comparator 54 1 / N frequency divider 55 1 / k frequency divider 56 Internal synchronization Signal generation unit 57 PWM clock generation unit 58 PWM data control unit 100Y, 100M, 100C, 100K Image formation unit 121 Main scanning counter 122 Shading start position control unit 123 Division width counter 124 Division number counter 125 Shading correction data output unit 126 DAC
127 Low-pass filter

Claims (6)

Tip synchronization detection means for generating a synchronization detection signal which is a reference of the main scanning line;
Rear end synchronization detection means for detecting the position of the rear end of one line;
And each of the light beams emitted from each of the plurality of light sources and modulated by the image signal is irradiated onto the photoconductor via the scanner optical system to form an image on the photoconductor, and the tip synchronization detection signal In the image forming apparatus that measures the main scanning magnification based on the measurement result and changes the laser light amount according to the writing condition,
An image forming apparatus, wherein when performing main scanning magnification correction by measuring a distance between the leading edge synchronization detection signal and the trailing edge synchronization detection signal, the main scanning magnification correction is performed with a constant laser light amount. 2. The writing condition includes at least one of a writing print density and a writing printing speed, and the laser light quantity is constant regardless of the writing condition.
The image forming apparatus described. Tip synchronization detection means for generating a synchronization detection signal which is a reference of the main scanning line;
Rear end synchronization detection means for detecting the position of the rear end of one line;
And each of the light beams emitted from each of the plurality of light sources and modulated by the image signal is irradiated onto the photoconductor via the scanner optical system to form an image on the photoconductor, and the tip synchronization detection signal In the image forming apparatus that measures the main scanning magnification based on the measurement result and changes the laser light amount according to the writing condition,
An image forming apparatus, wherein a laser light amount is constant during a synchronization detection period, and a laser light amount at a main scanning position is changed stepwise in accordance with a shading characteristic of the optical system. 4. The image forming apparatus according to claim 3, wherein the write condition includes at least one of a write print density and a write print speed, and the laser light quantity is constant during the synchronization detection period even under different write conditions. . The image forming apparatus according to claim 3, wherein the synchronization detection period is a leading edge synchronization detection period and a trailing edge synchronization detection period. When the main scanning position of the trailing edge synchronization detection means is adjacent to the image data period, the laser light quantity in the trailing edge synchronization detection period is made constant regardless of the writing condition only when the main scanning magnification is adjusted. The image forming apparatus according to any one of claims 1 to 5.

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