JP3254275B2 - Image recording device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image recording apparatus for reproducing a halftone image.

[0002]

2. Description of the Related Art An image recording apparatus such as a laser printer which combines an electrophotographic technique and a laser scanning technique can use plain paper and can obtain high-speed and high-quality images. It is widely used in copiers. A general laser printer is provided with a laser scanning optical system using a rotating polygon mirror, and scans and exposes a drum-shaped photoconductor by deflecting a laser beam modulated according to an image signal by the rotating polygon mirror. An electrostatic latent image is formed on a photoreceptor, and the electrostatic latent image is developed with toner, and then transferred and fixed to recording paper to obtain an output image.

FIG. 2 shows an example of a laser optical system used in a laser printer. A light source 11 composed of a semiconductor laser is driven by a modulation circuit in accordance with an image signal, and a laser beam from the semiconductor laser 11 is deflected by a rotary polygon mirror 13 via a lens 12. The laser beam from the rotating polygon mirror 13 is imaged as a minute light spot on a drum-shaped photoconductor 15 by an imaging lens 14 composed of an fθ lens. This light spot is rotated by the polygon mirror 13 and the photosensitive member 1.
The photosensitive member 15 is scanned and exposed by the rotation of the rotation 5 to form an electrostatic latent image of an image. In addition, a light receiving element (synchronous detection element) 16 is arranged outside the image area on the scanning line on the scanning start side to detect a laser beam from the imaging lens 14.
The image writing start position in the main scanning direction is controlled based on the light receiving signal from the controller 6.

In an image recording apparatus having such a laser scanning optical system, in order to digitally reproduce a halftone image, an image signal is compared with a threshold matrix, and the shading of the image is determined by the dot area, that is, the threshold matrix. In general, a pseudo halftone reproduction method, which is an area gradation method for converting into the number of recording pixels in the image, is generally used.

In this pseudo halftone reproduction method, for example, the shading of one pixel or a plurality of pixels in a certain area is compared with a threshold matrix having a threshold value which gradually increases from the center to the outside, and the threshold value of the threshold matrix is determined. Higher density portions are output as black. Therefore, the density of the image is reproduced as the size of the area (rate) (halftone dots are formed). Next, in order to simplify the description, a case where an image signal of one pixel is reproduced by a plurality of dots will be described as an example.

FIG. 3 shows an example of a 4 × 4 threshold matrix. This threshold matrix and a rotary polygon mirror having six reflecting surfaces are used to reproduce a 12 × 12 matrix by a pseudo halftone reproduction method.
FIG. 4 schematically shows an image having the gradation density. 4, the horizontal axis is the direction in which the laser beam is scanned by the rotary polygon mirror, that is, the main scanning direction, and the vertical axis is the direction in which the photoconductor rotates and moves, that is, the sub-scanning direction (the direction perpendicular to the main scanning direction). ), And numerals 1 to 6 written at the left end indicate the numbers of the reflecting surfaces of the rotary polygon mirror for scanning the laser beam.

In this case, if wobble (reflection surface tilt error of the rotating polygon mirror, image shift in the sub-scanning direction due to air fluctuation, etc.) occurs due to the reflection surface tilt error of the rotating polygon mirror,
The length of the cross-shaped pattern in the sub-scanning direction varies. Normally, the tilt of the reflecting surface of the rotating polygonal mirror is a sinusoidal variation with a cycle of one rotation of the rotating polygonal mirror, so that the reproduced image has density unevenness (banding) with a cycle of 12 scanning lines in the sub-scanning direction. The image quality is significantly deteriorated. Here, the 12 scanning lines in the cycle of the density unevenness correspond to the least common multiple of the size 4 of the threshold matrix in the sub-scanning direction and the number 6 of the reflecting surfaces of the rotary polygon mirror.

In order to reduce such density unevenness, in a laser scanning optical system, a variable angle prism is disposed between a collimating lens and a rotating polygon mirror, and the angle is changed by an electrostrictive element (piezo element). Japanese Patent Application Laid-Open No. 63-313112 discloses a scanning optical device which deflects a laser beam emitted from a light source and converted into parallel light by a collimating lens to correct a change in a scanning pitch on a recording surface .
It is described in the gazette .

On the other hand, in the laser printer, there is a multi-beam scanning system in which a plurality of laser beams are simultaneously scanned by using a laser diode array or the like in response to a recent demand for high speed. A4 per minute on a laser printer
In order to realize an optical system that outputs 100 images of the same size, the speed V _{0 of the} photoconductor is about 500 mm / sec, and one-beam scanning in which only one laser beam is scanned by a rotating polygon mirror. In the method, the rotation speed R (rpm) of the rotating polygon mirror
Is given by the following equation.

R (rpm) = V ₀ × DPI × 60 / (25.4 × N) (1) where V ₀ is the speed (mm / sec) of the photoconductor, and DPI is 1 inch. The recording density per unit is generally 300 to 400, and N is the number of reflecting surfaces of the rotary polygon mirror, and is generally 5 to 10
It is.

By substituting V ₀ = 500 and DPI = 300 into equation (1), R becomes 59055 (rpm). At such a high rotational speed, a rotating polygon mirror cannot use a conventional ball bearing as a bearing for supporting a rotating shaft, and requires special bearings such as a fluid bearing and a magnetic bearing, resulting in an increase in cost. In addition, the modulation frequency of the laser (semiconductor laser) as the light source increases, and it is necessary to increase the speed of data transfer from the laser control circuit and the host machine to the laser, resulting in an increase in cost. However, when the number of laser beams is set to M by scanning a plurality of laser beams collectively by the multi-beam scanning method, the number of rotations of the rotary polygon mirror and the modulation frequency of the laser can be reduced to 1 / M. , Can greatly reduce costs.

Japanese Patent Application Laid-Open No. Sho 59-112763 discloses an example in which a semiconductor laser array comprising a plurality of semiconductor lasers is used as a light source, and the point where the cross-sectional shapes of the beams emitted from the individual semiconductor lasers are adjacent to each other is recorded on a recording medium. There is described a laser information recording apparatus having an optical system for forming an image, a drive circuit for independently driving each semiconductor laser, and scanning laser beams from a plurality of semiconductor lasers collectively.

[0013]

In the above scanning optical apparatus, a variable angle prism is disposed between a collimating lens and a rotary polygon mirror, and the angle is changed by an electrostrictive element. Since the collimating lens deflects the collimated laser beam and corrects the change in the scanning pitch on the recording surface, it is necessary to control the angle of the variable angle prism with respect to the laser beam very accurately and at high speed. However, there is a disadvantage that the control system becomes complicated.

In the ordinary one-beam scanning method, the laser beam is perpendicularly incident on the reflecting surface of the rotary polygon mirror in the sub-scanning section, but in the multi-beam scanning method, the laser beam is slightly incident on the reflecting surface of the rotating polygon mirror. At an angle. That is, in general, as shown in FIG. 5, the curvature of the scanning line becomes larger toward the end of the plurality of laser beams scanned simultaneously (as the distance from the optical axis of the fθ lens increases). Therefore, when image recording is performed using such a laser scanning optical system, the scanning position shift of the scanning line in the sub-scanning direction fluctuates with the number M of laser beams as a cycle. This scanning position shift also causes exposure unevenness on the recording medium (photoconductor), and becomes apparent as density unevenness in a halftone image on an image. Further, in the multi-beam scanning method, the period of the scanning position shift due to the tilting of the reflecting surface of the rotating polygon mirror has an effect of multiplying M times. Therefore, the density fluctuation due to the tilting of the reflecting surface of the rotating polygon mirror is lower than that of the one-beam scanning method. It becomes a frequency thing and becomes visually noticeable.

SUMMARY OF THE INVENTION It is an object of the present invention to provide an image recording apparatus having a simple structure capable of improving the above-mentioned drawbacks, reducing density unevenness and obtaining a high-quality halftone image.

[0016]

According to a first aspect of the present invention, an image signal is compared with a threshold matrix, and a plurality of light beams are modulated based on the comparison result. In an image recording device that collectively scans a plurality of light beams with a rotating polygon mirror and reproduces a halftone image with the plurality of light beams from the rotating polygon mirror,
Size or repetition of the threshold matrix in the sub-scanning direction
The relationship between the period n, the number M of the light beams, and the number N of reflection surfaces of the rotary polygon mirror is set such that n / (N × M) is an integer of 1 or more.

According to a second aspect of the present invention, an image signal is compared with a threshold matrix, and a plurality of light beams are modulated based on the comparison result. The modulated plurality of light beams are collectively scanned by a rotary polygon mirror. In the image recording apparatus for reproducing a halftone image by using a plurality of light beams from the rotary polygon mirror, the size or repetition period n of the threshold matrix in the sub-scanning direction, the number M of the light beams, the rotation The relationship with the number N of reflection surfaces of the polygon mirror is set so that (N × M) / n is an integer of 2 or more.

According to a third aspect of the present invention, in the image forming apparatus of the second aspect, n / M is set to an integer of 1 or more.

According to a fourth aspect of the present invention, the image signal is compared with a threshold matrix, and a light beam is formed based on the comparison result.
Modulates and scans this modulated light beam with a rotating polygon mirror
And the light beam from the rotating polygon mirror produces a halftone image.
In a sub-scanning direction size or repetition period n of the threshold matrix and the number N of reflection surfaces of the rotary polygon mirror, so that N / n becomes 2, and detection means for detecting a rotational position of the rotary polygon mirror, the rotating multi by the detection signal of the detection means
Of the reflective surface of the surface mirror is obtained by a means for forming a halftone dot center of the halftone image in two opposite reflection surfaces tilt error is small.

According to a fifth aspect of the present invention, an image signal is compared with a threshold matrix, and a plurality of light beams are modulated based on the comparison result. The modulated plurality of light beams are collectively scanned by a rotary polygon mirror. In the image recording apparatus for reproducing a halftone image by using a plurality of light beams from the rotary polygon mirror, the size or repetition period n of the threshold matrix in the sub-scanning direction, the number M of the light beams, the rotation set the relation between the reflection surface speed n of the polygon mirror to the (n × M) / n = 2, and a detection means for detecting a rotational position of said rotary polygonal mirror, wherein the rotating polygon by the detection signal of the detection means those having a means for forming a halftone dot center of the halftone image in the two reflecting surfaces tilt error is small among the reflecting surfaces of the mirror <br/>.

According to a sixth aspect of the present invention, in the image recording apparatus of the fifth aspect, n / M is set to an integer of 1 or more.

[0022]

According to the first aspect of the present invention, the relationship among the size or repetition period n of the threshold matrix in the sub-scanning direction, the number M of light beams, and the number N of reflecting surfaces of the rotary polygon mirror is n / (N ×
M) is set to be an integer of 1 or more. Therefore, the pixel area does not fluctuate due to the fluctuation of the scanning line pitch due to the tilting of the rotary polygon mirror, and a halftone image with uniform density without density unevenness in the sub-scanning direction is obtained, and the configuration is simple. . In addition, the size of the threshold matrix in the sub-scanning direction or the repetition period n is necessarily an integral multiple of the period M of the pitch variation of the scanning line due to the bending of the scanning line due to the scanning of the plurality of light beams by the rotating polygon mirror. Density non-uniformity in the sub-scanning direction does not occur due to fluctuations in scanning line pitch due to line bending.

According to the second aspect of the present invention, the relationship among the size or repetition period n of the threshold matrix in the sub-scanning direction, the number M of light beams, and the number N of reflecting surfaces of the rotating polygon mirror is represented by (N ×
M) / n is set to be an integer of 2 or more. For this reason, a low-frequency density fluctuation due to the tilting of the rotating polygon mirror does not occur, and a high-quality halftone image in which the density fluctuation is inconspicuous can be obtained.

According to a third aspect of the present invention, in the image forming apparatus of the second aspect, n / M is set to an integer of 1 or more. Therefore, density unevenness in the sub-scanning direction does not occur due to scan line pitch fluctuation due to scan line bending.

According to the fourth aspect of the present invention, the relationship between the size or repetition period n of the threshold matrix in the sub-scanning direction and the number N of reflection surfaces of the rotary polygon mirror is set so that N / n becomes 2. The rotation position of the rotary polygon mirror is detected by the detection unit. Rotating multi-plane by the detection signal of this detection means
Dot center of the halftone image is formed by the two opposing reflective surface inclination error is small among the reflecting surface of the mirror. Therefore,
Density unevenness caused by scanning line pitch fluctuation due to the tilting of the rotary polygon mirror is favorably reduced, and a high-quality halftone image with less density unevenness in the sub-scanning direction can be obtained. In addition, the size of the threshold matrix in the sub-scanning direction or the repetition period n is necessarily an integral multiple of the period M of the pitch variation of the scanning line due to the bending of the scanning line due to the scanning of the plurality of light beams by the rotating polygon mirror. Density non-uniformity in the sub-scanning direction does not occur due to fluctuations in scanning line pitch due to line bending.

According to the fifth aspect of the present invention, the relationship among the size or repetition period n of the threshold value matrix in the sub-scanning direction, the number M of light beams, and the number N of reflecting surfaces of the rotating polygon mirror is (N ×
M) / n = 2, and the rotation position of the rotary polygon mirror is detected by the detection means. The halftone dot center of the halftone image in the two reflecting surfaces tilt error is small among the reflection surfaces of the rotary polygonal mirror by the detection signal of the detecting means is formed. Accordingly, the density unevenness caused by the fluctuation of the scanning line pitch due to the tilting of the rotary reflecting mirror is favorably reduced, and a high-quality halftone image having no density unevenness in the sub-scanning direction can be obtained.

According to a sixth aspect of the present invention, in the image recording apparatus of the fifth aspect, n / M is set to an integer of 1 or more. For this reason, the density unevenness in the sub-scanning direction caused by the fluctuation of the scanning line pitch due to the bending of the scanning line does not occur.

[0028]

6 to 8 show a part of an example of a laser scanning optical system. In this example, a multi-beam scanning method is used. In a semiconductor laser array 21 as a light source, a plurality of laser beams emitted from a plurality of light emitting points slightly separated in the sub-scanning direction are converted into substantially parallel lights by a collimating lens 22. Then, the light is narrowed in the sub-scanning direction by the cylinder lens 23 to the vicinity of the reflection surface of the rotary polygon mirror 24. The laser beam on the optical axis of the collimating lens 22 and the cylinder lens 23 is incident on the reflecting surface 24a of the rotating polygon mirror 24 in the direction perpendicular to the sub-scanning direction, and the other laser beams are rotated at a slightly inclined angle. 24, and is incident on the reflection surface.

The rotary polygon mirror 24 deflects and scans the laser beam from the cylinder lens 23 by rotation, and the rotary polygon mirror 2
The laser beam from 4 is formed as a minute light spot on a recording medium 26 made of a photoreceptor by an imaging lens 25 made of an fθ lens, and each light spot has a predetermined pitch in the sub-scanning direction according to the recording density. Thus, an electrostatic latent image is formed on the recording medium 26. Recording medium 2
Reference numeral 6 is driven by a motor to rotate in the sub-scanning direction.

The imaging lens 25 is an anamorphic fθ lens having a different focal length between the main scanning direction and the sub-scanning direction. The reflecting surface of the rotary polygon mirror 24 and the recording medium 26 are substantially conjugated in the sub-scanning direction. And serves to reduce the fluctuation of the scanning line pitch due to the scanning position shift due to the angular error (tilting of the reflecting surface) of each reflecting surface of the rotary polygon mirror 24 with respect to the rotation axis. However, since this geometrical conjugate relationship cannot be perfect, the fluctuation of the scanning line pitch cannot be completely eliminated by correcting the imaging lens 25. The cylinder lens 23 has a function of controlling the laser beam diameter on the recording medium 26 in the sub-scanning direction. The light receiving element (synchronous detection element) 27 is provided outside the image area on the scanning line on the scanning start side to detect a laser beam from the imaging lens 25, and an output signal from the light receiving element 27 is output as a synchronization signal. An image writing start position in the main scanning direction is controlled based on the synchronization signal.

As shown in FIG. 9, the rotary polygon mirror 24 comprises a polygon mirror 31 for deflecting and scanning the laser beam and a scanner motor 32 for rotating the polygon mirror. Reflective surface 3
1a and the polygon mirror mounting surface 32c on the rotor of the scanner motor 32
Caused by touch in the thrust direction. Most of the two factors of the inclination of the reflecting surface of the rotary polygon mirror 24 are caused by processing errors (for example, the inclination due to the clearance between the central axis of the inner diameter of the polygon mirror 31 and the processing axis during the processing of the polygon mirror, the scanner motor 32). Due to the inclination in chucking when machining the rotor of a lathe)
The inclination of the reflecting surface of the rotating polygon mirror 24 becomes a sine wave with one rotation of the rotating polygon mirror 24 as a cycle. FIG. 10 schematically shows the inclination of the reflecting surface of a rotary polygon mirror having six reflecting surfaces.

The first embodiment of the present invention is an embodiment of the first aspect of the present invention. In the above laser scanning optical system, the semiconductor laser array 21 is a semiconductor laser array comprising two semiconductor lasers, and The polygon mirror 24 has four reflecting surfaces. Hereinafter, the first embodiment will be described.

The image signal input as shown in FIG.
The value is compared with the threshold value matrix 29 by the
By converting into a value, the shading of the image is converted into the area of the dot, that is, the number of recording pixels in the threshold matrix 29. The LD modulation circuit 30 drives and controls each of the semiconductor lasers of the semiconductor laser array 21 at a timing based on the synchronizing signal by the image signal from the binarization circuit 28, and the laser beam intensity-modulated from these semiconductor lasers by the image signal. Is ejected to form an electrostatic latent image of an image on the recording medium 26. Although not shown, the electrostatic latent image on the recording medium 26 is developed with toner by a developing device, transferred to recording paper by a transfer device, and then fixed by a fixing device to obtain an output image.

FIG. 11 shows the structure of the threshold matrix 29. FIGS. 12 and 13 show examples of the density level (image signal) of the original image to be exposed on the recording medium 26 and the exposure pattern of the recording medium 26. In the first embodiment, the number M of laser beams emitted from the light source 21 is two, the number N of reflecting surfaces of the rotary polygon mirror is four, and the size of the threshold matrix 29 is eight.
(Main scanning direction) × 8 (sub-scanning direction), and (black) within halftone dots on the recording medium 26 according to the density level of the image.
The number of dots, that is, the halftone dot area is set. Since the rotating polygon mirror 24 simultaneously scans two laser beams on each reflecting surface, the period of the scanning position shift in the sub-scanning direction due to the falling of the reflecting surface of the rotating polygon mirror 24 is N × M = 4 × 2 = 8 (scanning). Line). Accordingly, the period of the scanning position shift in the sub-scanning direction due to the tilt of the reflecting surface of the rotary polygon mirror 24 and the size of the threshold matrix 29 in the sub-scanning direction are the same, and the influence of each halftone dot by the scan line pitch shift is the same. .

Specifically, as shown in FIG. 14, the scanning position shift in the sub-scanning direction is +
Becomes the side, the reflective surface of a number 4 of the rotary polygonal mirror 24 - becomes the side. As a result, the cruciform pattern of the image as shown in FIG. 12 (here, the pattern of the 12th gradation) is deformed in the sub-scanning direction as shown in FIG. 13, but each cruciform pattern is deformed the same. Accordingly, there is no change in the dot area (and the dot shape) between the dots at the same density level due to the change in the pitch of the scanning line due to the inclination of the reflecting surface of the rotary polygon mirror 24.
An output image with a uniform density can be obtained.

In the first embodiment, M = 2, N = 4 and the size of the threshold matrix in the sub-scanning direction or the repetition period n is n = 8. However, when M = 2, N = 4, n = 8. = 1
6, 24,... (N / (N × M) = 2, 3,...)
It is clear that the same effect as in the first embodiment can be obtained. That is, also in these cases, n is an integral multiple of the period of the scanning position shift in the sub-scanning direction due to the tilt of the reflecting surface of the rotating polygon mirror 24, and the scanning position shift due to the tilting of the reflecting surface of the rotating polygon mirror 24 causes a difference between the halftone dots. Does not change the dot area. In addition, not only when M = 2 and N = 4, n / (N
× M) = integer (an integer of 1 or more) can achieve the same effect. The threshold matrix in the sub-scanning direction
The repetition period n is the distance between halftone dot centers in the sub-scanning direction.
That is.

Further, when n / (N × M) = integer (an integer of 1 or more), n / M is necessarily an integer. In other words, the size n of the threshold matrix in the sub-scanning direction is an integral multiple of the number M of laser beams, and the halftone dot area between the halftone dots at the same density level is (dots) due to the scanning position shift in the sub-scanning direction due to the bending of the scanning line. The point shape does not change.
Accordingly, it is possible to obtain an extremely high-quality intermediate image free from unevenness in density due to the inclination of the reflecting surface of the rotary polygon mirror 24 and the bending of the scanning line. Further, since a plurality of laser beams are simultaneously deflected and scanned by the rotary polygon mirror 24, the recording speed can be remarkably improved as compared with the ordinary one-beam scanning system.

Next, a second embodiment of the present invention will be described. The second embodiment is an embodiment of the invention described in claim 2, and in the first embodiment, (N × M) / n =
Integer (an integer of 2 or more), that is, the number of laser beams M ×
The number N of reflection surfaces of the rotary polygon mirror was set to the size of the threshold matrix in the sub-scanning direction or an integral multiple of the repetition period n. Accordingly, as described above, in the one-beam scanning method of M = 1, density unevenness of an output image occurs with a scanning line having the least common multiple of n and N as a cycle, but in the multi-beam scanning method of M ≧ 2, the rotating polygon mirror is used. The period of the scanning position shift due to the reflection surface inclination error is the period of N × M scanning lines. For this reason, normally, the density unevenness caused by the reflection surface tilt error of the rotary polygon mirror is a low-frequency density unevenness having a scan line of the least common multiple of N × M and n (> N × M). Will be very noticeable visually. However, in the second embodiment, (N ×
M) / n = integer (an integer of 2 or more), so N ×
The least common multiple of M and n can be N × M, and the period of density unevenness caused by the reflection surface tilt error of the rotating polygon mirror becomes the period of N × M scanning lines, causing low-frequency density unevenness. Never do.

In the second embodiment, specifically, in the first embodiment, a threshold matrix of 3 (main scanning direction) × 3 (sub scanning direction) is used as the threshold matrix 29 as shown in FIG. In addition, n = 3 and N = 6 using a rotating polygon mirror having six reflecting surfaces. Since the number M of laser beams is 2, (N × M) / n = 4, and the condition of (N × M) / n = integer (an integer of 2 or more) is satisfied. 10 due to the falling of the reflecting surface of the rotary polygon mirror 24.
The scanning position shift in the sub-scanning direction as shown in FIG. 7 occurs on the + side on the reflecting surfaces of the rotating polygon mirror 24 with the numbers 2 and 3 and on the-side on the reflecting surfaces 5 and 6 of the rotating polygon mirror 24. Occurs. Then, the cross-shaped pattern of the image (here, the pattern of the fifth gradation) is as shown in FIG. 16 when there is no scanning position shift, but due to the above-mentioned scanning position shift, as shown in FIG. Deform. This modification has a cycle of 12 scanning lines, and the cycle of the density unevenness of the image is also the cycle of 12 scanning lines, so that low-frequency density unevenness does not occur. On the other hand, for example, using a 5 × 5 threshold matrix, n = 5
, The least common multiple of N × M = 12 and n = 5 is 60
Visually conspicuous density unevenness in a low frequency is generated for a scan line and period.

Next, a third embodiment of the present invention will be described. The third embodiment is an embodiment of the third aspect of the present invention. In the second embodiment, (N × M) / n =
Integer (an integer of 2 or more), that is, the number of laser beams M ×
The number N of reflecting surfaces of the rotating polygon mirror is set to an integer multiple of the size of the threshold matrix in the sub-scanning direction or the repetition period n, and n / M is set to an integer of 1 or more. Accordingly, by setting (N × M) / n = integer (an integer of 2 or more), low-frequency density unevenness does not occur due to the falling of the reflecting surface of the rotary polygon mirror 24 as in the second embodiment. .

Further, since n / M is set to an integer of 1 or more, density unevenness in the sub-scanning direction does not occur due to the scanning line bending by the multi-beam scanning method. In other words, a scanning position shift occurs due to the scanning line bending by the multi-beam scanning method, and the period is the period of the M scanning lines. Therefore, by setting the size n of the threshold matrix in the sub-scanning direction to be an integral multiple of M (an integer of 1 or more), each halftone dot of the image is affected by the scanning position deviation due to the scanning line bending an integer number of times. In addition, a uniform halftone image can be obtained without changing the area (and shape) between the halftone dots.

In the third embodiment, specifically, in the second embodiment, a threshold matrix of 4 (main scanning direction) × 4 (sub scanning direction) is used as the threshold matrix 29 as shown in FIG. N = 4, N =
6, M = 2, (N × M) / n = 3, n / M = 2, and (N × M) / n = integer (an integer of 2 or more) and n / M =
The condition of an integer (an integer of 1 or more) is satisfied.

The cross-shaped image pattern (12th gradation) is not deformed as shown in FIG. 19 when there is no scanning position shift, but as shown in FIG. Due to the shift in the scanning position in the sub-scanning direction, the cross-shaped image pattern (12th gradation) is deformed as shown in FIG. This modification has a cycle of 12 scan lines, and the cycle of density unevenness is also the cycle of 12 scan lines, so that low-frequency density unevenness does not occur.

Further, the two laser beams are transmitted to the rotating polygon mirror 2.
Scanning position bending as shown in FIG. 23 occurs due to scanning line bending as shown in FIG. That is, when the distance in the sub-scanning direction between two laser beams is set with reference to the position (the center of the image in the main scanning direction) where the scanning line interval is minimum, the scanning position shift at both ends of the image It is generated alternately in the + and-directions with two scanning lines as one cycle. However, as shown in FIG. 21, the area does not fluctuate between the halftone dots due to the scanning position shift, and the density unevenness due to the scanning bending occurs.
I do not .

Next, a fourth embodiment of the present invention will be described. This fourth embodiment is an embodiment of the invention described in claim 4, and has a laser scanning optical system as shown in FIG.
In the first embodiment, the light source 21 is a semiconductor laser.
And a rotating polygon mirror 24 having six reflecting surfaces is used as N = 6, and the rotating position sensor 33 composed of a reflection type sensor is used as the rotating polygon mirror 24.
The rotation position of the rotary polygon mirror 24 is detected by detecting the mark provided above. The rotation position sensor 33 is not limited to the reflection type sensor, and may be any sensor that detects the rotation position of the rotary polygon mirror 24, that is, the reflection surface 24a used for scanning, and may be attached to the scanner motor 32 side. .

As shown in FIG. 10, the tilt of the reflecting surface of the rotating polygon mirror 24 is close to a sine wave with one rotation of the rotating polygon mirror 24 as a cycle. Therefore, in the rotating polygon mirror 24 having an even number of surfaces, there are two reflection surfaces with small inclination,
The two reflecting surfaces are mutually facing (facing) reflecting surfaces. These two reflecting surfaces correspond to the reflecting surfaces Nos. 1 and 4.

FIG. 25 shows an example of a threshold value matrix 29 for reproducing a halftone image. This threshold matrix 29 is a threshold matrix of 3 (main scanning direction) × 3 (sub scanning direction), and has a dot concentration type configuration. Size n of the sub-scanning direction of the threshold matrix 29 is set to half of the reflection surface speed N of the rotary polygonal mirror 24 (N / n = 2) , the laser by the reflecting surface of the numbers 1, 4 of the rotating polygon mirror 24 The image corresponding to the second row of the threshold matrix 29 is formed by the beam scanning, that is, the center of the halftone dot is formed. Therefore, since the center of the halftone dot is formed by laser beam scanning of the reflection surface having a small scanning position shift due to the tilting of the rotary polygon mirror 24, it is possible to reduce density unevenness due to a tilt error of the rotary polygon mirror 24 in an image. it can.

[0048] Figure 26-28 3 shows the (main scanning direction) × 3 1 gray level of the halftone patterns in the case of using the threshold matrix (the sub-scanning direction) schematically. FIG. 26 shows a rotating polygon mirror 24.
FIG. 27 shows a rotating polygon mirror 2
FIG. 28 shows a case where the reflection surface of the rotary polygon mirror 24 has a tilt of 4 and the central portion of the halftone dot is formed by the reflection surface of the rotary polygon mirror 24 having a small tilt (the case of the fourth embodiment) . This is a case where the central part of a halftone dot is formed by a reflection surface with a large fall. As is apparent from FIGS. 26 to 28, in the case of FIG. 28, the interval between the centers of the halftone dots fluctuates, so that the density unevenness appears remarkably in the cycle of six scanning lines. As shown in FIG. 27, in the fourth embodiment, since the fluctuation of the interval between the halftone dot centers is small, the density unevenness in the period of six scanning lines is not so noticeable.

FIGS. 44 to 46 similarly show 3 (main scanning direction) ×
3 5 th gradation of halftone patterns in the case of using a threshold matrix (sub scanning direction) shown schematically. Fig. 44 shows a rotating multi-sided surface
FIG. 45 shows a case where the reflection surface of the mirror 24 does not fall.
The reflecting surface of the surface mirror 24 is tilted, and
When the center of the halftone dot is formed with a reflective surface
FIG. 46 shows the rotary polygon mirror 24.
When the central part of the halftone dot is
You. As is clear from FIGS.
Since the interval between the centers of the halftone dots fluctuates, the darkness is
The degree unevenness appears remarkably. As shown in FIG.
In the example, since the fluctuation of the interval between the halftone dot centers is small,
Periodic density unevenness is not so noticeable. FIG.
(A) to (a) in the main scanning direction / sub-scanning
When an image is formed repeatedly in the directions shown in FIGS.
46 (b), the density unevenness is easily noticeable in FIG.
5 (b) and FIG. 46 (b).
I understand. This is because the rotating polygon mirror 24 has a small inclination.
This is the effect of forming the center of the halftone dot on the launch surface.
Note that except in the fourth embodiment, obtained by simulation the concentration distribution obtained by actual exposure conditions, which was frequency analysis, density unevenness of the period of 6 scan lines in the sub-scanning direction is the black (white) solid All gradations (1 to 1
8) reduced to less than half.

If the number N of reflection surfaces of the rotating polygon mirror 24 is an even number and the size of the threshold matrix in the sub-scanning direction or the repetition period n is half of the number N of reflection surfaces of the rotating polygon mirror 24, the center of the halftone dot is determined. A similar effect can be obtained by forming the reflecting surface of the rotary polygon mirror 24 with laser beam scanning on a reflecting surface having a small scanning position shift due to an error of the reflecting surface tilting.

Next, in the fourth embodiment, actually a small reflecting surfaces of inclination error of the rotary polygonal mirror 24 to the center of the halftone dot
The means for forming will be described. As shown in FIGS. 47 and 48, the input image signal is compared with the threshold matrix 29 by the binarization circuit 34 to be binarized, so that the density of the image is equal to the area of the dot, that is, in the threshold matrix 29. It is converted to the number of recording pixels. LD modulation circuit 30
Is the semiconductor laser 2 based on the image signal from the binarization circuit 34.
1 is controlled, and a laser beam intensity-modulated by an image signal is emitted from the semiconductor laser 21 so that the recording medium 26
An electrostatic latent image of the image is formed thereon. Although not shown, the electrostatic latent image on the recording medium 26 is developed with toner by a developing device, transferred to recording paper by a transfer device, and then fixed by a fixing device to obtain an output image.

At this time, the surface selection circuit 35 changes the reflection surface 24a used for laser beam deflection scanning of the rotary polygon mirror 24 by the rotation position detection signal from the rotation position sensor 33 and the synchronization signal from the synchronization detection element 27, and the rotation surface. A reflection surface having a small tilt error of the mirror 24 and a central portion of a halftone dot are selected so as to correspond to each other so that density unevenness due to displacement of a scanning line is reduced, and a surface selection signal is generated. The binarizing circuit 34 binarizes the image signal. Accordingly, the reflection surface of the rotary polygon mirror 24 having a small inclination error and the center of the halftone dot correspond to each other, so that the density unevenness due to the displacement of the scanning line is reduced. In this case, the inclination of the reflection surface of the polygon mirror 33 may be measured in advance, and a mark (a mark detected by the rotation position sensor 33) may be added to the polygon mirror 33 based on the measurement data. The rotation position sensor 3
Instead of 3, a detecting means for detecting the inclination of the reflecting surface of the rotary polygon mirror 24 is installed integrally with or separately from the synchronous detecting element 27, and a signal from this detecting means and the synchronous detecting element 2 are provided.
The surface selection signal may be generated by selecting the reflection surface 24a used for laser beam deflection scanning of the rotary polygon mirror 24 by the synchronization signal from 7 so as to reduce the density unevenness due to the displacement of the scanning line. .

Next, a fifth embodiment of the present invention will be described. The fifth embodiment is an embodiment of the invention described in claim 5, and is different from the fourth embodiment in that the first embodiment
Similarly, a semiconductor laser array is used as the light source 21.
Rutotomoni, the product of the number of reflection surfaces N number of beams M and the rotating multifaceted mirror 24 is set to an even number, i.e., either one or both of N and M is set to an even number (N is also set to an odd number Is possible) and half of M × N is the threshold matrix 2
9 is set equal to the size in the sub-scanning direction or the repetition period n.

Specifically, the rotating polygon mirror 24 having five reflecting surfaces is used, and the threshold matrix 29 is 5 (main scanning direction) × 5 (sub scanning direction) as shown in FIG.
Is used. That is, N =
5, n = 5, M = 2, and M × N / 2 = n = 5 . The scanning position shift in the sub-scanning direction as shown in FIG. 30 occurs due to the inclination of the reflecting surface of the rotary polygon mirror 24. The scanning position shift (tilt of the reflecting surface) in the sub-scanning direction is minimized on the reflecting surface of the rotating polygon mirror 24 with the number 1 and becomes the second smallest on the reflecting surfaces of the rotating polygon mirror 24 with the numbers 3 and 4. The maximum is at the reflection surfaces of Nos. 2 and 5 of 24. In this case, the surface selection circuit 35 changes the reflection surface 24 a used for laser beam deflection scanning of the rotary polygon mirror 24 by the rotation position detection signal from the rotation position sensor 33 and the synchronization signal from the synchronization detection element 27. A surface selection signal is generated by scanning the laser beam with the reflection surface of No. 1 and the reflection surface of No. 3 or 4 so that a dot center is formed, and a binary value is generated in accordance with the surface selection signal. The conversion circuit 34 binarizes the image signal. Accordingly, the reflection surface of the rotary polygon mirror 24 having a small inclination error and the center of the halftone dot correspond to each other, so that the density unevenness due to the displacement of the scanning line is reduced.

FIGS. 31 to 33 schematically show a halftone dot pattern (dotted line) of the fifth gradation when a threshold matrix of 5 (main scanning direction) × 5 (sub scanning direction) is used. FIG. 31 shows a halftone dot pattern when the reflecting surface of the rotary polygon mirror 24 does not fall,
FIG. 32 shows a halftone dot pattern when the center of the halftone dot is formed by the reflection surface of the rotary polygon mirror 24 having a small tilt (in the case of the fifth embodiment), and FIG. 33 shows a large tilt of the rotary polygon mirror 24. This is a halftone dot pattern when the center of the halftone dot is formed on the reflection surface. As is apparent from FIGS. 31 to 33, in the case of FIG. 33, the interval between the centers of the halftone dots varies, so that the density unevenness appears remarkably in the cycle of 10 scanning lines. As shown in FIG. 32, in the fifth embodiment, the fluctuation of the interval between the halftone dot centers is small, so that the density unevenness in the period of 10 scanning lines is not so noticeable.

In the sixth embodiment of the present invention, the rotary polygon mirror 24 having six reflecting surfaces is used in the fifth embodiment, and the size or repetition of the threshold matrix 29 in the sub-scanning direction is used. Assuming that the cycle n is 6, FIG.
A threshold matrix of 6 (main scanning direction) × 6 (sub scanning direction) as shown in FIG. 5 is used. In the sixth embodiment, a scanning position shift in the sub-scanning direction as shown in FIG. 34 occurs due to the reflection surface of the rotary polygon mirror 24 falling. The displacement of the scanning position in the sub-scanning direction (the inclination of the reflecting surface) is minimized at the reflecting surfaces of the rotating polygon mirror 24 with the numbers 1 and 4. In this case, the surface selection circuit 35 uses the rotation position detection signal from the rotation position sensor 33 and the synchronization signal from the synchronization detection element 27 to
The reflection surface 24a used for the laser beam deflection scanning of
A surface selection signal is generated by scanning the laser beam with the reflecting surfaces of the rotary polygon mirror 24 with the numbers 1 and 4 so that a dot center is formed, and a binarizing circuit 34 is generated in accordance with the surface selection signal. Performs binarization of the image signal. Therefore, the rotating polygon mirror 2
The non-uniformity of the density due to the displacement of the scanning line is reduced by the correspondence between the reflection surface having a small inclination error and the center of the halftone dot.

FIGS. 36 to 38 schematically show halftone dot patterns (dotted lines) of the twelfth gradation when a threshold matrix of 6 (main scanning direction) × 6 (sub scanning direction) is used. FIG. 36 shows a halftone dot pattern when the reflecting surface of the rotating polygon mirror 24 does not fall, and FIG.
FIG. 38 shows a halftone dot pattern when the center of a halftone dot is formed by the reflection surface (in the case of the sixth embodiment).
This is a halftone dot pattern when the center of the halftone dot is formed by a reflective surface having a large inclination. As is apparent from FIGS. 36 to 38, in the case of FIG.
Density unevenness remarkably appears in the scanning line cycle. As shown in FIG. 37, in the sixth embodiment, since the variation in the interval between the centers of the halftone dots is small, the density unevenness in the cycle of 12 scanning lines is not so noticeable.

According to a fifth aspect of the present invention, (N × M) / n =
2, the number N of reflecting surfaces of the rotary polygon mirror 24 is set to 6
The number n of beams is not limited to two. For example, N: M: n is 4: 2: 4, 4: 3: 6, 4:
4: 8, 5: 2: 5, 5: 4: 10, 5: 8: 20,
6: 2: 6, 6: 3: 9, 6: 4: 12, and the like. Then, the surface selection circuit 35 changes the reflecting surface 24 a used for laser beam deflection scanning of the rotating polygon mirror 24 by the rotation position detection signal from the rotation position sensor 33 and the synchronization signal from the synchronization detection element 27 to tilt the rotation polygon mirror 24. A reflection surface having a small error and a central portion of a halftone dot are selected so as to correspond to each other so as to reduce density unevenness due to displacement of a scanning line, and a surface selection signal is generated, and binarized in accordance with the surface selection signal. The circuit 34 binarizes the image signal. Therefore, the reflection surface of the rotary polygon mirror 24 having a small inclination error and the center of the halftone dot correspond to each other, so that the density unevenness due to the displacement of the scanning line is reduced.

Next, a seventh embodiment of the present invention will be described. The seventh embodiment is an embodiment of the invention according to claim 6, wherein (N × M) / n = 2 and n / M
M is set to an integer greater than or equal to one. Specifically, this seventh
In the embodiment, in the fourth embodiment, the first
A semiconductor laser array is used as the light source 21 as in the embodiment.
At the same time, a threshold matrix of 6 (main scanning direction) × 6 (sub-scanning direction) is used as the threshold matrix 29, and N = 6, n = 6, M = 2 and M × N / 2 = n =
6. In the seventh embodiment, similarly to the sixth embodiment, as shown in FIGS. 36 to 38, it is possible to satisfactorily reduce the density unevenness due to the tilt of the reflecting surface of the rotary polygon mirror 24.

Furthermore, in the seventh embodiment, since n / M is set to the integer 3, the density unevenness does not occur due to the scanning position deviation due to the scanning line bending. That is, by simultaneously scanning two laser beams simultaneously on the reflecting surface of the rotary polygon mirror 24, a scanning line bend occurs as schematically shown in FIG. 40, and a scanning position shift occurs as shown in FIG. However, as shown in FIG. 39, since each halftone dot undergoes the same fluctuation due to a scanning position shift due to a scanning line bend,
Density unevenness due to scanning line bending does not occur.

In the seventh embodiment, the semiconductor laser array 21 that generates three laser beams is used, and the LD modulation circuit 30 uses the image signal from the binarization circuit 34 to control each of the semiconductor laser arrays 21. The drive of the semiconductor laser is controlled, and a threshold matrix of 9 (main scanning direction) × 9 (sub-scanning direction) is used as the threshold matrix 29, so that M × N / 2 = n = 9 and n / M = 3. You can also. According to a sixth aspect of the present invention, (N × M) / n =
2. Set n / M = integer (an integer of 1 or more).
The number N of reflecting surfaces of the rotating multiple mirror 24 is not limited to six.
Further, the number of beams n is not limited to a few.

In the above embodiment, the screen angle is 0. However, the present invention can be applied to the case where the screen angle exists. For example, FIG. 42 shows that the screen angle is 4
An example of a threshold matrix capable of simulating a halftone image of 32 gradations at 5 degrees is shown, and an image ( 16th gradation) as shown in FIG. 43 is obtained.

[0063]

As described above, according to the first aspect of the present invention, an image signal is compared with a threshold value matrix, and the comparison result is obtained.
An image recording apparatus that modulates a plurality of light beams based on a plurality of light beams, scans the modulated plurality of light beams collectively by a rotating polygon mirror, and reproduces a halftone image by the plurality of light beams from the rotating polygon mirror. , The relationship between the size or repetition period n of the threshold value matrix in the sub-scanning direction, the number M of light beams, and the number N of reflection surfaces of the rotating polygon mirror is represented by n.
Since / (N × M) is set to be an integer of 1 or more, the pixel area does not fluctuate due to the fluctuation of the scanning line pitch due to the tilting of the rotary polygon mirror, and the density is not uniform in the sub-scanning direction. A halftone image with a high density can be obtained and the configuration is simple. In addition, the size of the threshold matrix in the sub-scanning direction or the repetition period n is necessarily an integral multiple of the period M of the pitch variation of the scanning line due to the bending of the scanning line due to the scanning of the plurality of light beams by the rotating polygon mirror. Density non-uniformity in the sub-scanning direction does not occur due to fluctuations in scanning line pitch due to line bending.

According to the second aspect of the present invention, the image signal is compared with the threshold matrix, a plurality of light beams are modulated based on the comparison result , and the modulated plurality of light beams are collectively collected by the rotating polygon mirror. In the image recording apparatus for scanning and reproducing a halftone image by using a plurality of light beams from the rotary polygon mirror, the size or repetition period n of the threshold matrix in the sub-scanning direction, the number M of the light beams, Since the relationship between the number N of reflection surfaces and the number N of reflection surfaces of the rotary polygonal mirror is set so that (N × M) / n is an integer of 2 or more, low-frequency density fluctuation due to surface tilt of the rotary polygonal mirror does not occur. A high-quality halftone image with less noticeable fluctuation is obtained.

According to the third aspect of the present invention, in the image forming apparatus according to the second aspect, since n / M is set to an integer of 1 or more, a change in the pitch of the scanning line due to the bending of the scanning line causes a change in the sub-scanning direction. No density unevenness occurs.

According to the fourth aspect of the present invention, the image signal is compared with the threshold value matrix, and the optical
And modulates the modulated light beam with a rotating polygon mirror.
Scans the light beam from this rotating polygon mirror,
In the image recording apparatus for reproducing the image of the above, the size of the threshold matrix in the sub-scanning direction or the repetition period n;
A detecting means for setting the relationship between the number N of reflection surfaces of the rotating polygon mirror and N / n to be 2, and detecting a rotation position of the rotating polygon mirror;
Since a halftone unit to form a halftone dot center portion of the image <br/> with two opposite reflective surfaces tilt error is small among the reflection surfaces of the rotary polygon mirror, scanning by tilt of the rotating polygon mirror Density unevenness caused by line pitch fluctuation is successfully reduced,
A high-quality halftone image with less density unevenness in the sub-scanning direction can be obtained.

According to the fifth aspect of the present invention, the image signal is compared with the threshold matrix, a plurality of light beams are modulated based on the comparison result , and the modulated plurality of light beams are collectively collected by the rotating polygon mirror. In the image recording apparatus for scanning and reproducing a halftone image by using a plurality of light beams from the rotary polygon mirror, the size or repetition period n of the threshold matrix in the sub-scanning direction, the number M of the light beams, The relationship with the number N of reflecting surfaces of the rotary polygon mirror is expressed as (N × M) / n =
Set 2, and a detection means for detecting a rotational position of said rotary polygonal mirror, the rotation by the detection signal of the detection means
Two reflecting surfaces of the reflecting surfaces of the polygon mirror with a small surface tilt error
In so and means for forming a halftone dot center of the halftone image, density unevenness caused by pitch fluctuation of the scanning lines due to tilt of the rotating reflector is satisfactorily reduced, without sub-scanning direction density unevenness A high quality halftone image can be obtained.

According to the sixth aspect of the present invention, in the image recording apparatus according to the fifth aspect, since n / M is set to an integer of 1 or more, the sub-scanning direction caused by the fluctuation of the scanning line pitch due to the bending of the scanning line. Does not occur.

[Brief description of the drawings]

FIG. 1 is a block diagram illustrating a circuit unit according to a first embodiment of the present invention.

FIG. 2 is a perspective view showing an example of a laser scanning optical system.

FIG. 3 is a diagram illustrating an example of a threshold matrix.

FIG. 4 is a diagram schematically illustrating an image of a twelfth gradation expressed by a pseudo intermediate reproduction method.

FIG. 5 is a diagram schematically illustrating an example of scanning line bending by a multi-beam scanning method.

FIG. 6 is a front view showing an example of a laser scanning optical system.

FIG. 7 is a plan view showing the laser scanning optical system.

FIG. 8 is a rear view showing a part of the laser scanning optical system.

FIG. 9 is a perspective view showing the rotary polygon mirror of the first embodiment.

FIG. 10 is a diagram schematically illustrating an example of a reflection surface tilt of a rotary polygon mirror.

FIG. 11 is a diagram showing another example of a threshold matrix.

FIG. 12 is a diagram showing an example of an original exposure pattern.

FIG. 13 is a diagram illustrating an example of an exposure pattern when the reflecting surface of the rotary polygon mirror is tilted.

FIG. 14 is a diagram schematically showing another example of the reflection surface of the rotary polygon mirror falling down.

FIG. 15 is a diagram showing a threshold matrix according to the second embodiment of the present invention.

FIG. 16 is a diagram showing an example of an exposure pattern when the reflection surface of the rotary polygon mirror does not fall.

FIG. 17 is a diagram showing an example of an exposure pattern when the reflecting surface of the rotary polygon mirror is tilted.

FIG. 18 is a diagram showing a threshold matrix according to the third embodiment of the present invention.

FIG. 19 is a diagram illustrating an example of an exposure pattern when the reflection surface of the rotary polygon mirror is not tilted.

FIG. 20 is a diagram illustrating an example of an exposure pattern when the reflecting surface of the rotary polygon mirror is tilted.

FIG. 21 is a diagram illustrating an example of an exposure pattern when a scanning line is bent.

FIG. 22 is a diagram schematically illustrating an example of scanning line bending by a two-beam scanning method.

FIG. 23 is a diagram schematically illustrating an example of a scanning position shift due to a scanning line bending.

FIG. 24 is a diagram schematically showing an example of scanning line bending by a multi-beam scanning method.

FIG. 25 is a diagram showing a threshold matrix according to the fourth embodiment of the present invention.

FIG. 26 is a diagram illustrating an example of an exposure pattern when the reflecting surface of the rotary polygon mirror is not tilted.

FIG. 27 is a diagram showing an example of an exposure pattern in the case where the reflecting surface of the rotary polygon mirror is tilted and the center of the halftone dot is formed by the reflecting surface with a small tilt of the rotary polygon mirror.

FIG. 28 is a diagram showing an example of an exposure pattern when a central portion of a halftone dot is formed on a reflective surface of a rotating polygon mirror with a large inclination.

FIG. 29 is a diagram showing a threshold matrix according to the fifth embodiment of the present invention.

FIG. 30 is a diagram showing the reflection surface tilt of the rotary polygon mirror in the fifth embodiment.

FIG. 31 is a diagram illustrating an example of an exposure pattern when the reflection surface of the rotary polygon mirror is not tilted.

FIG. 32 is a diagram showing an example of an exposure pattern in the case where the reflecting surface of the rotary polygon mirror is tilted and the center of the halftone dot is formed by the reflecting surface with a small tilt of the rotary polygon mirror.

FIG. 33 is a diagram showing an example of an exposure pattern in the case where the center of a halftone dot is formed on a reflective surface of a rotating polygonal mirror with a large inclination.

FIG. 34 is a diagram illustrating an example of a reflection surface tilt of a rotating polygon mirror.

FIG. 35 is a diagram illustrating an example of a threshold matrix.

FIG. 36 is a diagram showing an example of an exposure pattern when the reflection surface of the rotary polygon mirror is not tilted.

FIG. 37 is a diagram showing an example of an exposure pattern in a case where the reflecting surface of the rotary polygon mirror is tilted and the center of the halftone dot is formed by the reflecting surface with a small tilt of the rotary polygon mirror.

FIG. 38 is a diagram showing an example of an exposure pattern when a central portion of a halftone dot is formed on a reflective surface of a rotating polygon mirror where the reflection surface has a large inclination.

FIG. 39 is a diagram illustrating an example of an exposure pattern when a scanning line is bent.

FIG. 40 is a diagram schematically showing an example of scanning line bending by a two-beam scanning method.

FIG. 41 is a diagram illustrating an example of a scan position shift due to a scan line curve.

FIG. 42 is a diagram illustrating an example of a threshold matrix of a screen angle of 45 degrees.

FIG. 43 is a diagram illustrating an example of an exposure pattern.

FIG. 44 is a diagram illustrating an example of an exposure pattern when the reflection surface of the rotary polygon mirror is not tilted.

FIG. 45 is a diagram showing an example of an exposure pattern in the case where the reflecting surface of the rotating polygon mirror is tilted and the center of the halftone dot is formed by the reflecting surface of the rotating polygon mirror having a small tilt.

FIG. 46 is a diagram showing an example of an exposure pattern in a case where a central portion of a halftone dot is formed on a reflection surface of a rotating polygon mirror where the reflection surface has a large inclination.

FIG. 47 is a block diagram showing a circuit section of the fourth embodiment.

FIG. 48 is a timing chart of the fourth embodiment.

[Explanation of symbols]

 Reference Signs List 21 semiconductor laser array 24 rotating polygon mirror 28, 34 binarization circuit 29 threshold matrix 30 LD modulation circuit 33 rotation position sensor 35 surface selection circuit

──────────────────────────────────────────────────の Continuing on the front page (51) Int.Cl. ⁷ identification symbol FI H04N 1/405 H04N 1/40 C (58) Investigated field (Int.Cl. ⁷ , DB name) H04N 1/04-1 / 207 B41J 2/435-2/48 G02B 26/10-26/10 109 G03B 27/72-27/80 G03G 13/04-13/056 G03G 15/04-15/056 H04N 1/23-1/31 H04N 1/40-1/409

Claims (6)

(57) [Claims] An image signal is compared with a threshold matrix, a plurality of light beams are modulated based on a result of the comparison , and the plurality of modulated light beams are collectively scanned by a rotating polygon mirror. In the image recording apparatus for reproducing a halftone image by using a plurality of light beams from a plurality of light beams, a size or a repetition period n of the threshold matrix in the sub-scanning direction;
An image recording apparatus, wherein the relationship between the number M of light beams and the number N of reflecting surfaces of the rotary polygon mirror is set such that n / (N × M) is an integer of 1 or more. 2. An image signal is compared with a threshold matrix, a plurality of light beams are modulated based on the comparison result , and the modulated plurality of light beams are collectively scanned by a rotating polygon mirror. In the image recording apparatus for reproducing a halftone image by using a plurality of light beams from a plurality of light beams, a size or a repetition period n of the threshold matrix in the sub-scanning direction;
An image recording apparatus, wherein the relationship between the number M of light beams and the number N of reflecting surfaces of the rotating polygon mirror is set so that (N × M) / n is an integer of 2 or more. 3. The image forming apparatus according to claim 2, wherein n
An image recording apparatus, wherein / M is set to an integer of 1 or more. 4. An image signal is compared with a threshold matrix, and a light beam is modulated based on a result of the comparison.
The beam is scanned by a rotating polygon mirror, and from this rotating polygon mirror
In the image recording apparatus for reproducing a halftone image by the light beam of the above, the relationship between the size or repetition period n of the threshold matrix in the sub-scanning direction and the number N of reflection surfaces of the rotary polygon mirror is set to N / n = 2. set to be, and a detection means for detecting a rotational position of said rotary polygonal mirror, with two opposing reflecting surfaces tilt error is small among the reflection surfaces of the rotary polygonal mirror by the detection signal of the detection means Means for forming a halftone dot center portion of a halftone image. 5. An image signal is compared with a threshold matrix, and a plurality of light beams are modulated based on a result of the comparison. The plurality of modulated light beams are collectively scanned by a rotating polygon mirror. In the image recording apparatus for reproducing a halftone image by using a plurality of light beams from a plurality of light beams, a size or a repetition period n of the threshold matrix in the sub-scanning direction;
Detection means for setting the relationship between the number M of the light beams and the number N of reflection surfaces of the rotary polygon mirror to (N × M) / n = 2, and detecting the rotational position of the rotary polygon mirror; the halftone dot centers of a halftone image with two reflecting surfaces tilt error is small among the reflection surfaces of the rotary polygonal mirror by the detection signal of the detection means
An image recording apparatus, comprising: means for forming . 6. The image recording apparatus according to claim 5, wherein n
An image recording apparatus, wherein / M is set to an integer of 1 or more.