JPH10177144A - Image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To make it possible to form a high quality image by operating the mean deviation value based on the detection result of the deviation value from a deviation value detection means provided on the start end side and the terminal end side in a main scan direction and correcting a scan position in the scan direction based on the operated deviation value. SOLUTION: Scan start points of laser beams L1, L2 deflected by a polygon mirror 3 are detected by an index sensor 6 arranged on a tip side of the scan area, and the scan end points are detected by the index sensor 7 arranged on the end side of the scan area. Thus, the deviation values of both end sides of an image recording area are detected, and averaging operation is performed based on this detection result, and by detecting the optimum deviation values of the scan positions of plural laser beams, the deviation value of the image recording area is detected precisely. By correcting the scan positions of plural laser beams based on this detection result, an image forming device capable of forming a high picture quality image is provided.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus, and more particularly, to an image forming apparatus which simultaneously scans a recording medium in parallel with a main scanning direction by a plurality of light beams to record a plurality of lines at the same time. The present invention relates to a technique for detecting an optical axis shift of the plurality of light beams.

[0002]

2. Description of the Related Art In an image forming apparatus for recording image information by deflecting a laser beam (light beam) modulated on the basis of an image signal by a rotating polygon mirror or the like and scanning it on a recording medium. To increase the speed of
It is known that a configuration in which a plurality of lines are simultaneously recorded using a plurality of laser beams may be used.
188713).

In the case where a plurality of laser beams are simultaneously scanned, the scanning position of each of the plurality of laser beams may be shifted in the main scanning direction or the sub-scanning direction, which may affect accurate image formation. As a countermeasure, a device for detecting and adjusting the scanning position deviation between these beams is known (JP-A-6-270463, JP-A-7-723).
No. 99).

[0004]

Conventionally, the amount of deviation of the scanning positions of a plurality of laser beams has been detected by a beam detection index sensor disposed on the front end side in front of the image recording area in the main scanning direction. ing. However, since the shift amount of the scanning position of the plurality of laser beams is different at the rear end portion of the rear portion of the image recording region from the front end portion of the image recording region, a plurality of scan positions are detected based on the detection data of the shift amount of the front end portion. When the scanning position of the laser beam is adjusted, there is a problem that the rear end portion is shifted with respect to the detection data of the shift amount of the front end portion.

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned problems, and an image forming apparatus configured to perform image recording using a plurality of light beams detects an optimum amount of displacement between scanning positions of a plurality of laser beams. By doing so, it is possible to achieve high precision in detecting the amount of displacement of the image recording area, and to correct the scanning positions of a plurality of laser beams based on the detection result, thereby realizing high-quality image formation. It is an object to provide a forming device.

[0006]

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide an image forming apparatus for simultaneously scanning a recording medium with a plurality of light beams in parallel in a main scanning direction to record a plurality of lines at the same time. Two shift amount detectors provided at the start and end sides of the beam in the main scanning direction for detecting shift amounts of a plurality of light beams in the main scanning direction and / or the sub-scanning direction; Calculating means for calculating an average shift amount based on a detection result of a shift amount from the scanning beam; and a scanning timing and / or a sub-scanning direction of the plurality of light beams in the main scanning direction based on the shift amounts calculated by the calculating means. And a correcting means for correcting the scanning position of the image forming apparatus.

[0007]

Embodiments of the present invention will be described below.

FIG. 1 is a diagram showing an image exposure system of a laser printer as one embodiment of an image forming apparatus according to the present invention. The laser printer of the present embodiment has two internal modulations according to image data. Laser beam (light beam) L1,
L2 is scanned in parallel with the main scanning direction, and two lines are simultaneously recorded on a recording medium.

In FIG. 1, the light source unit 1 is
Two semiconductor lasers 1a and 1b are arranged in one line, and two divergent lights emitted from the light source unit 1 are converted into two parallel laser beams L1 and L2 by the condenser lens 2.

The two laser beams L1 and L2 are applied to a polygon mirror 3, and the two laser beams L1 and L2 deflected by the polygon mirror 3 are charged by a photosensitive member whose surface is uniformly charged via an fθ lens 4. Scanning is performed on a drum (recording medium) 5.

The photosensitive drum 5 is provided with a laser beam L1,
The laser beams L1 and L2 and the photosensitive drum 5 are relatively rotated in the sub-scanning direction (a direction orthogonal to the main scanning direction) to be rotated in synchronization with the main scanning of L2, thereby recording a two-dimensional image. Is performed.

As described above, exposure corresponding to image data is performed simultaneously on two lines, and an electrostatic latent image is formed on the photosensitive drum 5.
(Recording medium). Then, a toner charged to the opposite polarity is attached to the electrostatic latent image, and development is performed. Thereafter, the recording paper is superimposed on the toner image, and a corona charger reverses the polarity of the corona charging polarity from the back side of the recording paper. The toner image is transferred to the recording paper by applying a polar charge to the recording paper.

The scanning start point of the laser beams L1 and L2 deflected by the polygon mirror 3 is detected by an index sensor 6 disposed at the leading end of the scanning area, and the scanning end point is located at the end of the scanning area. Is detected by the index sensor 7 disposed at

As shown in FIG. 2, the index sensors 6 and 7 are integrally provided with four sensors (light beam detecting means) A to D for individually outputting detection signals. D is arranged side by side in the main scanning direction,
The laser beams L1 and L2 are scanned in the order of A → B → D → C.

The light beam detection area (light receiving area) of each of the sensors A to D is formed in a right triangle. Then, the sensor A forms a right angle included in the detection area of the right triangle 2
The long side of the sides is the edge on the starting end side in the main scanning direction, and the long side is arranged so as to be orthogonal to the main scanning direction (parallel to the sub-scanning direction). Further, the sensor B is configured such that the hypotenuse of the detection area of the right-angled triangle is the edge on the starting end side in the main scanning direction, and that the hypotenuse obliquely intersects the main scanning direction at an angle formed by the long side and the hypotenuse. Be placed. Further, the sensor D is arranged such that the arrangement state of the detection area of the sensor A is upside down when the sub-scanning direction is up and down.
Further, the sensor C is disposed such that its detection area is axially symmetric with respect to the axis along the sub-scanning direction with respect to the sensor A.

The sensors A and C shown in FIG. 2 are arranged such that the longer side of the two sides forming a right angle is perpendicular to the main scanning direction, and the longer side is in the main scanning direction. It may be configured to be arranged in parallel with.

That is, by the arrangement of the sensors A to D,
The edges of the sensors A to D on the start side in the main scanning direction are parallel to each other along the sub-scanning direction, and the sensors B and C are not parallel to each other. The direction of the inclination with respect to the main scanning direction is reversed.

In FIG. 2, the detection start position of the laser beam L1 by the sensor A (the position where the beam detection signal rises) is indicated as a1, and the detection start position of the laser beam L2 is indicated as a2. Sensors B to D
Start detection positions of the laser beams L1 and L2 by b1,
b2, c1, c2, d1, and d2.

In this embodiment, the displacements of the laser beams L1 and L2 in the main scanning direction and the sub-scanning direction are measured using the sensors A to D as shown in the flowchart of FIG.

First, a method for measuring the amount of deviation of the distance between the laser beams L1 and L2 in the sub-scanning direction will be described in detail.

The program shown in the flowchart of FIG. 3 is executed every time the power of the laser printer is turned on. First, only the laser beam L1 is turned on,
Scanning is performed in the same manner as in normal image recording (S1).

The laser beam L1 is transmitted from the sensor A
ＤD, a time (detection time difference) T1 from the rising edge (b1) of the beam detection of the sensor B to the rising edge (c1) of the beam detection of the sensor C (see FIG. 4).
Is measured (S2).

Next, only the laser beam L2 is turned on instead of the laser beam L1, and scanning is performed in the same manner as during normal image recording (S3).

Then, similarly, the laser beam L2
Scans over the sensors A to D, a time T2 from the rise (b2) of the beam detection of the sensor B to the rise (c2) of the beam detection of the sensor C (see FIG. 4).
Is measured (S4).

When the measurement of the times T1 and T2 is completed, the absolute value T3 of the time difference between the times T1 and T2 is calculated.

Further, the deviation between the reference value of the time difference T3 corresponding to the case where the distance between the laser beams L1 and L2 in the sub-scanning direction is in a normal state and the time difference T3 actually obtained in the above processing is calculated as It is determined as a value corresponding to the amount of deviation of the interval (S5).

It is preferable that the reference value can be arbitrarily changed and set via the operation unit of the laser printer.

That is, when the positions b1 and c1 in the sub-scanning direction where the laser beam L1 is detected by the sensors B and C are assumed as reference positions, for example, the scanning position of the laser beam L2 is lower in FIG. Suppose that it has shifted to the side. In this case, the positions b2 and c2 in the sub-scanning direction where the laser beam L2 is detected by the sensors B and C are in the main scanning direction as the distance between the detection start end edges of the sensors B and C goes downward in FIG. By being configured to spread on both sides, the position b2 is shifted to the scanning start end, and conversely, the position c2
2 is shifted to the end of the scan, so that the time T2
Becomes longer, and the time T3 becomes longer than the reference.

Therefore, if the deviation between the time difference T3 and the reference value is obtained, the amount of deviation of the interval between the laser beams L1 and L2 can be calculated based on the information on the scanning speed and the angle of the oblique sides of the sensors B and C. Can be done.

It should be noted that the laser beams L1,
In the case of detecting the shift amount of the interval in the sub-scanning direction of L2, the time difference caused by the shift varies depending on the angle at which the oblique sides of the sensors B and C obliquely intersect the main scanning direction, and the angle shown in FIG. B ° is set as acute as possible. In other words, it is desirable that the interval between the oblique sides of the detection areas of the sensors B and C changes abruptly along the sub-scanning direction.
Further, the angle B ° may be determined based on the adjustment accuracy of the scanning position, the resolution of time measurement, the limitation on the size of the sensor, and the like.

The measurement results of the times T1 and T2,
Information such as the finally calculated shift amount may be displayed on a display unit provided in the laser printer.
By doing so, it is possible to easily adjust the scanning position of the laser.

Incidentally, the measurement of the times T1 and T2 is as follows.
In this embodiment, the process is performed as shown in FIG.

FIG. 6 shows a case where the time difference (time between a1 and d1) at which the laser beam L1 is detected by the sensors A and D is measured.
The same applies to other combinations.

In FIG. 6, the reference clock clk is 1 /
16 types of delay clocks dl0, sequentially delayed by 16 periods
(Reference clock) to dl15 are generated using a digital delay line. In FIG. 6, only the clocks clk, dl1, dl2, dl8, dl12, and dl15 are shown, and other delay clocks are not shown. For example, if the clock synchronized with the rise a1 of the detection signal of the sensor A (the clock that rises first immediately after the rise of the detection signal) is the clock dl8,
The rising at the time of the synchronization is set as the first count, and then the rising of the clock dl8 is sequentially counted.

During this counting, if the detection signal of the sensor D rises and the clock synchronized with the rise (d1) of this detection signal is the clock dl12, the number of the rising edges of the clock dl8 up to that point (sensor A) Clock dl8 synchronized with the detection signal (a1)
The phase difference between the clock dl8 and the clock dl12 (4/16 cycle, which can be expressed as a delay clock number = dl4) is obtained by multiplying the value obtained by subtracting 1 from the clock cycle (including the rising edge of the clock). The added value becomes the output time difference (the interval between a1 and d1) of the detection signals of the sensors A and D. In the above-described displacement detection in the sub-scanning direction, the times T1 and T2 are obtained as the clock count number and the delay clock number as described above, while the reference time corresponding to the specified value of the interval is also used as the clock count number. In the calculation of the time difference T3, the count number and the delay clock number may be calculated respectively.

Next, a specific example of a circuit which measures time as described above and detects a deviation based on the measurement result will be described with reference to FIG.

In FIG. 7, the outputs of the sensors B and C are indicated by the phase director (1) 11 and the phase director (1).
(2) Output to 12 and respectively.

On the other hand, a reference clock clk is input to the digital delay line 13, and the clocks dl0 to dl15 are output from the digital delay line 13.

The phase director (1) 1
At 1, (2) 12, the delay clocks dl0 to dl15 synchronized with the rising edges of the detection signals of the sensors B and C are respectively detected (see FIG. 6), and the detection results are output to the phase difference calculation unit 14.

The phase difference calculator 14 calculates the phase difference (1/16 cycle unit) between the clock synchronized with the detection timing (b1 or b2) of the sensor B and the clock synchronized with the detection timing (c1 or c2) of the sensor C. ), That is, sensor B,
A fraction within the clock cycle of the detection interval of C is obtained, and the result is latched by the latch circuit 18 according to a one-shot pulse generated by the one-shot circuit 31 from the detection signal of the sensor C.

The phase director (1) 11
Is also output to the clock selector 15, and the clock selector 15 selectively outputs a delay clock synchronized with the detection signal of the sensor B to the counter 16.

The counter 16 measures the interval between the rising edges b1 (b2) and c1 (c2) of the outputs of the sensors B and C by counting the clock output from the clock selector 15. The counting section of the counter 16 is
Flip flop 17 to which outputs of sensors B and C are input
Is controlled by the

The count value of the counter 16 is latched by the latch circuit 18 with a one-shot pulse generated from the detection signal of the sensor C.

Thus, for example, the laser beam L1
A time T1 which is a detection interval of the sensors B and C when only the laser beam L2 is turned on is measured and stored in the latch circuit 18. Subsequently, a time T2 when only the laser beam L2 is turned on is measured and latched. It is stored in the circuit 18.

In the circuit configuration shown in FIG. 7, the functions as the first time difference measuring means and the second time difference measuring means are as follows.
The phase directors (1) 11, (2) 12, digital delay line 13, phase difference calculator 14, clock selector 15, counter 16, flip-flop 17, latch circuit
18, realized by the one-shot circuit 31.

When the times T1 and T2 are obtained as the clock count number and the clock phase difference, the time difference calculating section 19 as the time deviation calculating means separately calculates the difference between the times T1 and T2 using the count number and the clock phase difference. The operation is performed, and the result is temporarily stored in the latch circuit 20.

Then, a shift calculating section 21 as a sub-scanning direction shift detecting means compares the reference value given via the operation section with the data stored in the latch circuit 20, and outputs the laser beams L1 and L2. Is calculated in the sub-scanning direction (the amount of change in the interval), and the calculation result is output to the display unit.

In the above description, only the sensors B and C of the sensors A to D are used and the laser beams L1 and L2 are used.
The processing for detecting the shift in the sub-scanning direction has been described. After this processing, the scanning positional relationship (shift in the main scanning direction) of the laser beams L1 and L2 in the main scanning direction is detected, and based on the detection result. Each laser beam L
1, it is preferable to control the writing position by L2,
For this purpose, sensors A and D are provided.

The processing for detecting the shift in the main scanning direction is shown in the flowchart of FIG. 3 after the detection of the shift in the sub-scanning direction.

First, only the laser beam L1 is turned on (S6), and the time difference between the rise (a1) when the sensor A detects the laser beam L1 and the rise (d1) when the sensor D detects the laser beam L1. T5 (see FIG. 8) is measured (S7).

Here, since the edges of the light beam detection areas of the sensors A and D on the start side in the main scanning direction are parallel to the sub scanning direction (perpendicular to the main scanning direction), the time difference T5 is different from the sub scanning direction. , And is determined only by the interval between the edges of the sensors A and D on the starting end side in the main scanning direction and the scanning speed.

Next, the scanning of the laser beams L1 and L2 is started simultaneously, and only the laser beam L1 is incident on the sensor A and only the laser beam L2 is incident on the sensor D. Scanning is performed while performing mask control (S8), and a time difference T6 between a rising edge (a1) at which the laser beam L1 is detected by the sensor A and a rising edge (d2) at which the laser beam L2 is detected by the sensor D is detected.
(See FIG. 8) is measured (S9).

The mask control is performed for each laser beam L1,
The control may be performed by turning on and off the L2, or the laser beams L1 and L2 may be selectively incident on the sensors A and D by using a polarizing element or the like.

Here, when the laser beams L1 and L2 are scanned without being shifted in the main scanning direction, the time differences T5 and T6 should be the same time, for example, a delay in the scanning of the laser beam L1. When the laser beam L2 is scanned in this way, the delay is obtained as T6-T5 (= T7) (S10: see FIG. 8).

The circuit for detecting the actual amount of deviation in the main scanning direction based on the measurement of T5 and T6 and the measurement result is based on the method described in the measurement of the amount of deviation in the sub-scanning direction (FIGS. 6 and 7). See).

The flowchart shown in FIG. 20 shows the contents of a shift correction program showing execution of shift correction in the main scanning direction and the sub-scanning direction in accordance with the shift amount detection results of the index sensors 6 and 7.

First, the shift amount of the index sensor 6 in the main scanning direction and the sub-scanning direction is detected according to the shift detection program of FIG. 3 (S100). Subsequently, the shift amount of the index sensor 7 in the main scanning direction and the sub-scanning direction is detected (S101).

Here, the average value of the shift amounts detected from the index sensors 6 and 7 is calculated, and the averaged data is used as the shift amounts in the main scanning direction and the sub-scanning direction (S
102).

Based on the deviation calculated by the averaging operation, the deviation of the laser beams L1 and L2 is corrected (S103).

Here, when a mechanism capable of adjusting the scanning position in the sub-scanning direction is provided as disclosed in, for example, JP-A-63-50809, the deviation amount calculated by the deviation correction program is calculated. By adjusting the scanning position of the laser beams L1 and L2 in the sub-scanning direction based on the information, the interval between the laser beams L1 and L2 in the sub-scanning direction is corrected to a specified value (S105).

On the other hand, the shift correction in the main scanning direction is performed by delaying the writing time of the laser beam L2 with respect to the writing by the laser beam L1 according to the shift correction program. By delaying by an average value of the delay time T7), image recording can be performed without being shifted in the main scanning direction by the two laser beams L1 and L2 that are shifted in the main scanning direction. The writing position may be controlled such that the generation of the horizontal synchronization signal corresponding to the laser beam L2 is delayed by the delay time T7 'with respect to the generation of the horizontal synchronization signal corresponding to the laser beam L1 (S104). When T7 'is a negative value, the generation of the horizontal synchronization signal corresponding to L2 may be advanced by T7'.

When the shift amount in the sub-scanning direction obtained according to the shift correction program is obtained as the count number of the delay clock and the clock phase difference, the horizontal synchronizing signal is adjusted based on the count number of the clock. , The deviation obtained as the clock phase difference is the laser beam L from the delay clocks dl10 to dl15.
The adjustment may be made by selecting a dot clock corresponding to 1, L2.

By the way, the shapes and combinations of the detection areas of the sensors A to D used for detecting the displacement in the sub-scanning direction and the main scanning direction are not limited to those shown in FIG. For example, as shown in FIGS. 9 to 12, the configuration of the sensors A to D may be used.

That is, in order to detect the displacement in the sub-scanning direction, it is sufficient that there is a combination of sensors in which the edges of the light beam detection area on the starting end side in the main scanning direction are not parallel to each other. In order to detect the deviation in the direction, it is only necessary that there is a combination of sensors in which both edges of the light beam detection area on the start side in the main scanning direction are orthogonal and parallel to the main scanning direction. If the edge on the starting end side in the main scanning direction is defined, the shape of the detection area may be triangular or square.

Further, in the above description, four sensors A to D are used, which are a sensor pair used for detecting a shift in the sub-scanning direction and a sensor pair used for detecting a shift in the main scanning direction.
As shown in FIG. 3, three sensors A to C can perform similar functions.

That is, in the configuration shown in FIG. 13, the two sensors A and B formed in the rectangular light beam detection area are arranged such that the edges of the detection areas on the starting end in the main scanning direction are orthogonal to the main scanning direction. On the other hand, the sensor C in which the light beam detection area is formed in a triangular shape is arranged so that the oblique side on the main scanning start end side obliquely intersects the main scanning direction.

Then, with the sensor configuration shown in FIG.
When detecting a shift in the sub-scanning direction, for example, a combination of the sensor B and the sensor C is used (see FIG. 14).

Here, the edge of the detection region of the sensor B on the starting end side in the main scanning direction is orthogonal to the main scanning direction, but the edge of the detection region of the sensor C on the starting end side of the main scanning direction is oblique in the main scanning direction. Since they intersect, if a shift occurs in the sub-scanning direction,
This appears as a change in the time difference detected by each of the sensors B and C, whereby a shift in the sub-scanning direction can be detected (see FIG. 14).

However, when the edge of the detection area of the sensor B on the starting end side in the main scanning direction is orthogonal to the main scanning direction, the position shown in FIG.
Alternatively, as compared with the case where the combination of the sensors B and C shown in FIG. 4 (or FIGS. 9 to 12) is used, the shift in the sub-scanning direction does not greatly affect the time.
Alternatively, as in the sensors B and C shown in FIG. 4 (or FIGS. 9 to 12), the edges of the detection area on the starting end side in the main scanning direction obliquely cross each other in the main scanning direction, and the inclination direction is It is preferable to use a combination of different sensors. Further, in the case of detecting a shift in the main scanning direction by the sensors A to C as shown in FIG. 13, as shown in FIG.
Detection can be performed in the same manner as described above using a combination of the sensors A and B.

That is, three sensors A as shown in FIG.
C, both the combination in which the edges of the light beam detection area on the starting end side in the main scanning direction are non-parallel to each other and the combination in which the edges are parallel along the sub-scanning direction are included. In other words, in other words, if there is a sensor pair that is parallel in the sub-scanning direction and one sensor that obliquely intersects with the main scanning direction, it is possible to use both the shifts in the sub-scanning direction and the main scanning direction. Therefore, the combination of the three sensors A to C is not limited to the configuration in FIG. 13, and various modes as shown in FIGS. 16 to 19 are easily assumed.

When it is not necessary to detect a shift in the main scanning direction, it is sufficient to provide only a sensor pair whose light beam detection area has a non-parallel end edge on the starting end side in the main scanning direction. Become.

Further, three laser beams L1, L2, L
Even in a configuration where three lines are simultaneously recorded using three,
For example, the shift in the sub-scanning direction is detected by using a pair of sensors for the two laser beams L1 and L2 in the same manner as described above, and further, the shift is detected for the two laser beams L1 and L3. Since the deviation of each of the laser beams L2 and L3 in the sub-scanning direction can be detected with reference to the above, the configuration is not limited to the configuration using the two laser beams L1 and L2.

[0073]

As described above, according to the image forming apparatus of the present invention, the scanning position deviation amount between a plurality of laser beams is detected not only on the front end side in front of the image recording area but also on the rear end side. Similarly, by arranging the index sensor so that the shift amount can be detected, the shift amount at both ends of the image recording area can be detected. Based on the detection result, an averaging operation is performed, and an optimum plurality of laser beams are obtained. By detecting the shift amount of the scanning position, high accuracy of the shift amount detection of the image recording area is achieved, and by correcting the scanning positions of a plurality of laser beams based on the detection result, high-quality image formation. Can be provided.

[Brief description of the drawings]

FIG. 1 is a perspective view showing an image exposure system according to one embodiment of the present invention.

FIG. 2 is a diagram showing details of an index sensor.

FIG. 3 is a flowchart illustrating detection of an optical axis shift in main and sub scanning directions.

FIG. 4 is a view for explaining displacement detection in the sub-scanning direction.

FIG. 5 is a diagram for explaining the characteristics of displacement detection in the sub-scanning direction.

FIG. 6 is a time chart for explaining time measurement using a clock.

FIG. 7 is a block diagram showing a circuit configuration for detecting an optical axis shift in the sub-scanning direction.

FIG. 8 is a view for explaining characteristics of displacement detection in the main scanning direction.

FIG. 9 is a diagram showing another embodiment of the sensor configuration.

FIG. 10 is a diagram showing another embodiment of the sensor configuration.

FIG. 11 is a diagram showing another embodiment of the sensor configuration.

FIG. 12 is a diagram showing another embodiment of the sensor configuration.

FIG. 13 is a diagram illustrating a sensor configuration including three sensors.

FIG. 14 is a diagram showing displacement detection in the sub-scanning direction by three sensors.

FIG. 15 is a diagram illustrating detection of displacement in the main scanning direction by three sensors.

FIG. 16 is a diagram showing another embodiment of the sensor configuration.

FIG. 17 is a diagram showing another embodiment of the sensor configuration.

FIG. 18 is a diagram showing another embodiment of the sensor configuration.

FIG. 19 is a diagram showing another embodiment of the sensor configuration.

FIG. 20 is a flowchart illustrating shift correction in the main and sub-scanning directions.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 light source unit 1 a, 1 b semiconductor laser 2 condenser lens 3 polygon mirror 4 fθ lens 5 photosensitive drum 6, 7 index sensor 11, 12 phase director 13 digital delay line 14 phase difference calculator 15 clock selector 16 counter 18, 20 Latch circuit 19 Time difference calculator 31 One-shot circuit

Claims (2)

[Claims] 1. An image forming apparatus for simultaneously recording a plurality of lines by simultaneously scanning a recording medium in parallel with a main scanning direction with a plurality of light beams, comprising: A shift amount detecting means for detecting shift amounts of the plurality of light beams in the main scanning direction and / or the sub-scanning direction; and an average shift based on the shift amount detection results from the two shift amount detecting means. Calculating means for calculating the amount; and correcting means for correcting the scanning timing in the main scanning direction and / or the scanning position in the sub-scanning direction of the plurality of light beams based on the shift amount calculated by the calculating means. An image forming apparatus comprising: 2. The two shift amount detecting means includes at least three light beam detecting means arranged side by side in the main scanning direction, and the start side of the light beam detecting area of each of these light beam detecting means in the main scanning direction. 2. The image forming apparatus according to claim 1, wherein the edges have both a combination that is non-parallel to each other and a combination that is parallel to each other.