JP5659565B2 - Optical scanning device, optical scanning method, and image forming apparatus - Google Patents

Description

  The present invention relates to an optical scanning device, an optical scanning method, and the optical scanning device and an image forming apparatus using the optical scanning method.

In recent years, in an electrophotographic image forming apparatus such as a digital copying machine, a printer, a facsimile, or a composite machine of these, an optical scanning device used for image writing is used in order to meet the demand for high-speed writing and high-density writing. The scanning optical system is becoming multi-beam. However, there is a side effect that tends to generate an abnormal image called “banding” with multi-beam. The main factors causing banding are listed below.
(1) Beam pitch non-uniformity due to manufacturing error of each channel of the multi-beam light source.
(2) Light amount non-uniformity due to light amount deviation of each channel of the multi-beam light source.
(3) Beam pitch non-uniformity due to rotational misalignment of the multi-beam light source.
(4) Beam pitch deviation between multiple beams due to deviation in optical magnification of the scanning optical system.
(5) Factors due to latent image density fluctuations due to multibeam reciprocity failure.
(6) Mechanical factors due to vibrations, photoconductor rotation fluctuations, and surface tilt of the polygon mirror.
(7) Other factors such as uneven development.

The internal factors (1) to (5) (pitch and density variation between multiple beams) and the external factors (6) and (7) (noise factors that are not related to multiple beams) are not ideal. Banding does not occur when writing is performed at an ideal speed with a typical beam pitch interval.
By the way, whether banding is conspicuous depends greatly on the perception sensitivity of the human eye, and as shown in FIG. 3, when the image is observed from a position about 300 mm away, the period around 1 mm / cycle particularly peaks. As described above, the sensitivity is high in the range of 0.3 to 5 mm / cycle (that is, 0.2 to 3.3 cycle / mm in spatial frequency). If density irregularities of this period occur in the output image, even if there are slight density irregularities, the human eye is determined as “periodic density irregularities”, and is easily noticeable as so-called banding.

When the number of beams is small and the image data density is not so high (for example, about 600 dpi (dot / inch)), the banding is usually not sufficiently conspicuous for human eyes because the cycle is sufficiently shorter than 0.3 mm / cycle.
For example, when writing 600 dpi image data with 150 lines (150 lpi (line / inch)) with 4-beam writing, the banding cycle is 0.16 mm / cycle, which is very sensitive to human eye sensitivity. Very low.
However, when the number of beams increases (especially when it exceeds 20), it is easy to enter a frequency region with high human eye sensitivity.

Therefore, the following proposals have conventionally been made as methods for reducing banding.
In Patent Document 1 (Japanese Patent Laid-Open No. 2000-37904), since the number of multi-beam channels is a common multiple of the number of unit pixels of the dither matrix, the laser at the end of the array is written at a predetermined position of the dither pattern. Since all the scanning lines have the same strength, banding is not noticeable. However, this method is effective for a specific image pattern with a specific number of lines. However, since the number of lines and the image pattern are actually set differently depending on the user, it is not practical.

In Patent Document 2 (Japanese Patent Laid-Open No. 2002-113903), Patent Document 3 (Japanese Patent Laid-Open No. 2007-23740), and Patent Document 4 (Japanese Patent Laid-Open No. 2007-196460), a difference in density between scanning lines due to reciprocity failure. For reduction, it has been proposed to change the output laser light amount according to the light emission pattern of a plurality of beams.
However, this method can reduce the reduction in density difference between scanning lines due to reciprocity failure, but cannot reduce the effects of beam pitch nonuniformity and light quantity nonuniformity.

In Patent Document 5 (Japanese Patent Laid-Open No. 2008-170640), in order to reduce banding due to fluctuations in the relative position of the laser exposure device and the photosensitive drum due to vibrations and fluctuations in the rotational speed of the photosensitive drum, It has a detection means and a means for setting the amount of emitted light.
However, in this prior art, although a method for correcting mechanical factors by laser light amount adjustment is described, the influence on beam pitch deviation cannot be reduced.

Patent Document 6 (Japanese Patent Laid-Open No. 10-221903) proposes a method of reducing banding accompanying a change with time by changing the screen angle of an image.
However, no proposal has been made regarding banding reduction by multi-beam beam pitch.

  Japanese Patent Application Laid-Open No. 2009-29115 proposes a method for correcting the exposure amount in order to reduce banding due to surface tilt of the polygon scanner.

  As described above, various proposals for reducing banding have been made, but it was necessary to take measures against each one of the many factors causing banding, and no drastic measures were taken. .

  The present invention has been made in view of the above circumstances, and in an optical scanning apparatus or optical scanning method using a multiple beam scanning optical system, there are fluctuation factors such as uneven beam pitch, uneven light amount between light sources, and reciprocity failure. Even in such a case, it is an object (problem) to make it possible to form a high-quality image with no unevenness in density and suppressing occurrence of abnormal images such as “banding”, and further, the optical scanning device or It is an object (problem) to provide an image forming apparatus using an optical scanning method.

In order to achieve the above object, the present invention employs the following solutions.
[1]: a plurality of light sources, a scanning unit that scans a plurality of light beams emitted from the plurality of light sources (hereinafter referred to as multi-beams) in the main scanning direction, and a multi-beam scanned by the scanning unit An optical scanning device comprising a multi-beam scanning optical system having an optical element for focusing on a carrier, and driving the plurality of light sources according to image data to form an image on the image carrier. The number of light sources n is composed of a prime number of n ≧ 41, and when the number of lines (line / inch) of the image data is L, the banding cycle [Band] represented by the following equation is satisfied. (Claim 1).
Band (mm / cycle) = n × 25.4 / L ≧ 5.0

[2]: A plurality of light sources, a scanning unit that scans a plurality of light beams emitted from the plurality of light sources (hereinafter referred to as multi-beams) in the main scanning direction, and a multi-beam scanned by the scanning unit An optical scanning device comprising a multi-beam scanning optical system having an optical element for focusing on a carrier, and driving the plurality of light sources according to image data to form an image on the image carrier. The number of light sources n is a prime number of n ≧ 29, and half the data in the main scanning direction of the image data is shifted by one pixel in the pixel density unit in the sub-scanning direction, and image processing is performed.
When the apparent line number (line / inch) of the image data is L ′, a banding period [Band] expressed by the following equation is satisfied (claim 2).
Band (mm / cycle) = n × 25.4 / L ′ ≧ 10

[3]: In the optical scanning device according to [2], the pixel density unit is 2400 dpi or more .

[4]: [1] to the optical scanning apparatus according to any one of [3], wherein the multi-beam scanning optical system, between a plurality beam by the first scan, interlaced fill in two scans Scanning is performed (claim 4).
[5]: The optical scanning apparatus according to any one of [1] to [4], at least m of the light sources of the n (n ≧ 41 or n ≧ 29) (n ≧ m ≧ 2) One pixel is formed by the light source of (5).
[6]: [1] to the optical scanning apparatus according to any one of [5], wherein the plurality of light sources monolithic surface-emitting lasers arranged two-dimensionally on the same chip, characterized by being used (Claim 6).
[7]: The optical scanning device according to [6], wherein the light sources of the surface emitting laser are two-dimensionally arranged in a point-symmetric manner.
[8]: The optical scanning device according to [6], wherein the surface emitting laser, the number of light sources are use to the image data writing and prime, in addition an auxiliary light source, the 8 multiples of the light source as the total number (Claim 8).
[9]: In the optical scanning device according to [7] or [8], when the arrangement of the light sources of the surface emitting laser is a column V and a row H,
| H−V | / (H + V) <0.2
And the smaller of the column V or the row H is used in the sub-scanning corresponding direction (claim 9).

[10]: A plurality of light sources, a scanning unit that scans a plurality of light beams emitted from the plurality of light sources (hereinafter referred to as multi-beams) in the main scanning direction, and a multi-beam scanned by the scanning unit In the optical scanning method of forming an image on the image carrier by driving the plurality of light sources according to image data, using a multi-beam scanning optical system having an optical element for condensing on the carrier. The number of light sources n is composed of a prime number of n ≧ 41, and when the number of lines (line / inch) of the image data is L, the banding cycle [Band] represented by the following equation is satisfied. (Claim 10).
Band (mm / cycle) = n × 25.4 / L ≧ 5.0

[11]: A plurality of light sources, a scanning unit that scans a plurality of light beams emitted from the plurality of light sources (hereinafter referred to as multi-beams) in the main scanning direction, and a multi-beam scanned by the scanning unit In an optical scanning method of forming an image on the image carrier by driving the plurality of light sources according to image data using a multi-beam scanning optical system having an optical element that focuses on the carrier.
An image processing is performed in which the number n of light sources of the multi-beam is a prime number of n ≧ 29, and half of the image data in the main scanning direction is shifted by one pixel in the pixel density unit in the sub-scanning direction. ,
When the apparent line number (line / inch) of the image data is L ′, a banding cycle [Band] represented by the following equation is satisfied (claim 11).
Band (mm / cycle) = n × 25.4 / L ′ ≧ 10

[12]: In the optical scanning method according to [11], the pixel density unit is 2400 dpi or more (claim 12).

[13]: [10] to an optical scanning method according to any one of [12], wherein the multi-beam scanning optical system, between a plurality beam by the first scan, interlaced fill in two scans Scanning is performed (claim 13).
[14]: The optical scanning method according to any one of [10] to [13], n pieces at least m of the (n ≧ 41 or n ≧ 29) of the light source (n ≧ m ≧ 2) One pixel is formed by the light source of (14).
[15]: [10] to an optical scanning method according to any one of [14], wherein the plurality of light sources monolithic surface-emitting lasers arranged two-dimensionally on the same chip, characterized by being used (Claim 15).
[16]: In the optical scanning method according to [15], the light sources of the surface-emitting laser are two-dimensionally arranged in point symmetry.
[17]: The optical scanning method according to [15], wherein the surface emitting laser, the number of light sources are use to the image data writing and prime, in addition an auxiliary light source, the 8 multiples of the light source as the total number (Claim 17).
[18]: In the optical scanning method according to [16] or [17], when the arrangement of the light sources of the surface emitting laser is a column V and a row H,
| H−V | / (H + V) <0.2
And the smaller of the column V or the row H is used in the sub-scanning corresponding direction (claim 18).

[19]: an image bearing member, an image forming apparatus having a writing means for forming a latent image on the image bearing member, as the writing means, according to any one of [1] to [9] An optical scanning device is provided (claim 19).
[20]: an image bearing member, an image forming apparatus having a writing means for forming a latent image on the image bearing member, said writing means, according to any one of [10] to [18] An optical scanning method is used (claim 20).

Since the banding period is dominantly determined by the least common multiple of the number of light sources and the number of image lines, as described in [1] and [10] of the solution, the number of light sources is a prime number, so Banding interference can be brought closer to the low frequency side. Furthermore, in order to shift to a frequency sufficiently lower than 1 mm / line having high visual sensitivity, a total number of beams of at least 29 and a prime number is effective.
For example, when writing a line number of 200 lpi, substituting it into the equations described in [1] and [10] of the solving means, the banding period is
n × 25.4 / L = 41 × 25.4 / 200 = 5.2 (mm / cycle)
Thus, it becomes possible to shift to a period that is not sufficiently conspicuous for human eyes.

As described above, by making the number of light sources a prime number, banding interference with the number of image lines can be brought to the low frequency side. Furthermore, in order to shift to a frequency sufficiently lower than 1 mm / line having high visual sensitivity, a total number of beams of at least 29 and a prime number is effective.
In solutions [1] and [10], the banding cycle is set to 5 (mm / cycle) or more, so that it is sufficiently acceptable for business documents and the like, but is insufficient for realizing an image equivalent to offset printing. . Therefore, solutions [2] and [11] propose a method for realizing a banding cycle of 10 mm / cycle or more.
For example, to write the number of lines 200 lpi, the image processing becomes 50lpi corresponding to the long period of 1/4 the number of lines apparent, is substituted into the formulas described [2] and [11] of the solution, The banding cycle is
n × 25.4 / L = 29 × 25.4 / 50 = 14.7 (mm / cycle)
Thus, it becomes possible to shift to a period that is not sufficiently conspicuous for human eyes.

  Even if the pixel shift is performed in units of a pixel density of 2400 dpi or more, it cannot be determined with the resolution of the human eye. Therefore, as in [3] and [12] of the solving means, the pixels in the dither are shifted, and the period until the pixels are shifted in the reverse direction again is set to the apparent number of lines, thereby increasing the period. It becomes possible.

  As described above, when the number of light sources is a prime number, an image can be formed without fail by two interlaced scans. Therefore, as in the solution [4] and [13], banding caused by disturbance such as surface tilt of the scanning means (polygon mirror, etc.) is inconspicuously diffused by an interlaced scanning method in which an image is formed by two scans. be able to. Note that jumping three or more times increases the memory capacity of a RAM or the like, which increases the problem of configuring the electrical system.

In the solutions [5] and [14], there is no banding even when one pixel is formed by a plurality of light sources, and high-speed and high-density writing can be realized.
Also, as in [6] to [8] or [15] to [17] of the solution, is capable of high-density mounting by a surface emitting laser (VCSEL), it is possible to high-density writing.
Further, as in the solution means [9] and [18], when the arrangement of the light sources of the surface emitting laser is a column V and a row H,
| H−V | / (H + V) <0.2
By using the smaller one of the column V or the row H in the sub-scanning corresponding direction, it is possible to provide an optical scanning device and an optical scanning method that are less affected by optical aberrations and suppress banding. .

  As described above, according to the present invention, even when there are fluctuation factors such as uneven beam pitch, uneven light amount between light sources, and reciprocity failure in a multi-beam scanning optical system, there is no uneven density and “banding”. Since the optical scanning device or the optical scanning method capable of forming a high-quality image in which the occurrence of abnormal images such as the above is suppressed can be provided, the solution means using the optical scanning device or the optical scanning method [19] In the image forming apparatus of [20], high-density and high-quality image formation can be performed.

It is a figure which shows the structural example of the writing optical system of the optical scanning device used for an image forming apparatus, and its drive control means. It is a figure which shows an example of the multi-beam light source using a surface emitting laser. It is a figure which shows an example of the relative sensitivity with respect to the image period of a human eye. It is a figure which shows an example of the banding period in the case of outputting 2 dot lines by 2400 dpi and the number of lines of 200 lpi, (a) is a figure which shows 42 beams, (b) is a figure which shows the example of 41 beams which are prime numbers. It is a figure which shows the result of having calculated | required the banding period generate | occur | produced from 29 beams to 62 beams. It is a figure which shows an example of a banding period in the case of outputting by 2400 dpi, the number of lines of 200 lpi, and 2 dot lines by the interlaced scanning by 41 beams. It is a figure which shows the example of an arrangement | sequence when 41 light sources are arranged in point symmetry. It is a figure which shows the example of an arrangement | sequence when arranging 41 light sources including an auxiliary light source. It is a block diagram which shows an example of a laser drive system. It is a figure which shows an example of a banding period when the number of apparent lines is lengthened by image processing. It is a figure which shows an example of the banding period of a 2nd Example. It is the figure which showed the banding period of FIG. 11 with the perceptual banding value. It is a figure which shows the result of having calculated | required the banding period generate | occur | produced by 23 beams-62 beams. It is a figure which shows an example of a banding period in the case of outputting 2dot lines by interlaced scanning with 29 beams at 2400 dpi, a line number of 200 lpi, and an apparent line number of 50 lpi. (A) is a figure which shows the example of an arrangement when 29 light sources are arranged point-symmetrically, (b) is a figure which shows the example of an arrangement when an auxiliary light source is arranged in 29 light sources. 1 is a diagram illustrating a configuration example of an image forming apparatus according to the present invention.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
First, an example of an optical scanning device will be described.
FIG. 1 is a diagram showing a configuration example of a writing optical system (scanning optical system) of an optical scanning device used in a general image forming apparatus such as a laser printer or a digital copying machine using an electrophotographic process and its drive control means. is there. Referring to FIG. 1, laser light emitted from a semiconductor laser unit 1001 that is a light source unit is deflected and scanned in the main scanning direction by a rotating polygon mirror 1002 that is a scanning unit, and an optical element such as a scanning lens 1003. Then, a light spot is formed on an image carrier (photoconductive photosensitive member) 1004 which is a scanned medium, and the photosensitive member 1004 is exposed to form an image (electrostatic latent image). At this time, the phase synchronization circuit 1009 sets the modulation signal generated by the clock generation circuit 1008 to a phase synchronized with the photodetector 1005 that detects the light of the semiconductor laser deflected and scanned by the polygon mirror 1002. That is, the phase synchronization circuit 1009 generates a phase-synchronized image clock (pixel clock) for each line based on the output signal of the photo detector 1005 and supplies it to the image processing unit 1006 and the laser drive circuit 1007. To do. In this way, the semiconductor laser unit 1001 performs the operation of the semiconductor laser via the laser driving circuit 1007 according to the image data generated by the image processing unit 1006 and the image clock whose phase is set for each line by the phase synchronization circuit 1009. By controlling the light emission time, the electrostatic latent image on the scanned medium (photoconductor) 1004 can be controlled.

In recent years, the demand for higher printing speed (image forming speed) and higher image quality has increased, and in response to this, the speed of a polygon motor that rotates a deflector (polygon mirror), which is a scanning means, and laser modulation have been increased. Although it has been dealt with by speeding up the pixel clock as a reference clock, both speeds are approaching the limits, and the conventional method cannot be used.
Therefore, the use of a multi-beam using a plurality of light sources is used for speeding up. In the multi-beam optical scanning method, the rotational speed of the polygon motor and the pixel clock frequency can be reduced by increasing the number of light beams that can be scanned simultaneously by deflecting the deflector, enabling high-speed and stable optical scanning and image formation. It becomes.
As a light source constituting the multi-beam, a method of combining a single beam laser chip or a method of using an LD array in which a plurality of light emitting elements are incorporated in one laser chip is used. When a surface emitting laser (VCSEL: surface emitting semiconductor laser) configured on the same chip is used, the number of light sources can be greatly increased.

FIG. 2 shows an example of a multi-beam light source using a surface emitting laser.
In the optical scanning device shown in FIG. 1, a light source unit (semiconductor laser unit) 1001 is a semiconductor laser array (more specifically, in which a plurality of light sources (a plurality of semiconductor lasers) are arranged in a lattice shape as shown in FIG. Is composed of, for example, a surface emitting laser in which a plurality of light sources are arranged in a grid on the same chip), the arrangement direction of the plurality of light sources is relative to the rotation axis of a deflector such as a polygon mirror in FIG. The arrangement and angle of the light source unit 1001 are adjusted so as to have a certain angle θ.

  At this time, in FIG. 2, four light sources in the vertical arrangement direction a are light sources a1, a2, a3, a4 from the left, and two light sources a2, a3, for example, among these four light sources a1, a2, a3, a4. Consider a case where one pixel is formed by light emission and scanning (considering a case where one pixel is formed by one virtual light source array made of four light sources). When the pixel density to be formed is 600 dpi, the distance between the two light sources is equivalent to 2400 dpi, and the light source density is four times the pixel density. Therefore, in this case, for example, by changing the light quantity ratio of a plurality of light sources constituting one pixel, it becomes possible to shift the center of gravity position of the pixels in the sub-scanning direction, thereby realizing highly accurate pixel formation exceeding the light source density. it can.

As described above, by forming the scanning optical system of the optical scanning device into a multi-beam, high-speed and high-accuracy pixel formation can be realized, but with the multi-beam conversion, an abnormal image called “banding” is generated. There are easy side effects.
For example, the internal factors (pitch and density variation between multi-beams) shown in the following (1) to (5) generate frequency banding depending on the number of light sources of multi-beams.
(1) Beam pitch non-uniformity due to manufacturing error of each channel of the multi-beam light source.
(2) Light amount non-uniformity due to light amount deviation of each channel of the multi-beam light source.
(3) Beam pitch non-uniformity due to rotational misalignment of the multi-beam light source.
(4) Beam pitch deviation between multiple beams due to deviation in optical magnification of the scanning optical system.
(5) Factors due to latent image density fluctuations due to multibeam reciprocity failure.

It is considered that the banding period [Band] generated by the factor between the multi-beams is substantially expressed by the following equation.
Band (mm / cycle) = LCM (n, D / L) × 25.4 / D (0)
Here, n: number of light sources of multi-beams, D: pixel density (dot / inch), L: number of lines of image data (line / inch), LCM indicates the least common multiple.

  In multi-beam scanning, generally, as the number of beams increases, human visual sensitivity tends to be noticeable with a period in the region of 0.5 mm / cycle to 3 mm / cycle. As an example, with 32 beams, the pixel density is 1200 dpi, and 150 In the case of a line, from the above equation (0), it can be seen that the banding period is 0.7 mm / cycle and it is easy to generate very noticeable banding.

  In general, recent printers, copiers, or multi-function machines of these types require high-speed and high-density writing of 1200 dpi or more and over 50 ppm, and high-resolution writing of about 150 to 240 lines is also required. However, as described above, when the number of multi-beams increases (especially when the number of beams exceeds 20), interference tends to occur between the number of beams and the image period in the vicinity of 1 mm / cycle. Usually, to reduce the internal factors (pitch and density fluctuation between multiple beams) among the main factors of banding, improve the beam pitch adjustment accuracy during the assembly adjustment of the scanning optical system, and the light intensity variation of each channel However, it is difficult to sufficiently suppress banding in consideration of fluctuations due to temperature fluctuations and deterioration over time.

Therefore, the present invention solves the above-mentioned problem in the optical scanning device, and even in the case where there are fluctuation factors such as uneven beam pitch, uneven light amount between light sources, and reciprocity law failure in a multiple beam scanning optical system, density unevenness. Therefore, it is possible to form a high-quality image in which abnormal images such as “banding” are suppressed.
Hereinafter, specific examples of the present invention will be described.

[Example 1-1]
In the first embodiment of the present invention, a plurality of light sources (for example, a multi-beam light source 1001 as shown in FIGS. 1 and 2) and a plurality of light beams (multi-beams) emitted from the plurality of light sources are subjected to main scanning. Scanning means for scanning in the direction (polygon mirror 1002 shown in FIG. 1) and multi-beams scanned by the scanning means are condensed on an image carrier (photoconductive photosensitive member 1004 shown in FIG. 1). An optical scanning device (or optical scanning method) that uses a multi-beam scanning optical system having an optical element (scanning lens 1003 or the like) and drives the plurality of light sources in accordance with image data to form an image on the image carrier. , The number n of light sources of the multi-beams is composed of a prime number of n ≧ 41, and the line number (line / inch) of the image data is L, the banding period [Band represented by the following formula: Is satisfied It is characterized by a door.
Band (mm / cycle) = n × 25.4 / L ≧ 5.0 (1)

First, by making the number n of light sources of the multi-beam light source 1001 a prime number, it becomes possible to shift the interference between the beam period (the number of beams) and the image period to the long period side where the sensitivity of the human eye is sufficiently reduced.
In other words, when the number of light sources n is a prime number, the least common multiple LCM in the above equation (0) is a simple multiplication of the beam number n and the image period (D / L). Can be simplified.
On the other hand, when the number of beams is not a prime number, there is a natural number that is divisible by the image period, so depending on the image period (or the number of lines), it cannot be shifted to the long period side, and a period with high human eye sensitivity appears. Therefore, the number of lines cannot be selected freely.
Here, since the number of lines of the printer output is usually selected by the user in a range of 150 lpi to 240 lpi, the image forming apparatus provides an image in which banding is not noticeable corresponding to any number of lines. There is a need.
In addition, as described above, when the number of conventional beams is small, banding occurred only at a short cycle (0.5 mm / cycle or less), which is not as big a problem as a multi-beam.

Here, FIG. 4 shows an example of a banding cycle when 2 dot lines are output at 2400 dpi and 200 lpi. First, FIG. 4A shows an example of 42 beams, and FIG. 4B shows an example of 41 beams which are prime numbers.
The line width and density of each dot line fluctuate due to internal disturbance (pitch and density fluctuation between multi-beams). As a result, the same beam combination of 42 beams is repeated every 84 pixels (every two scans). The banding period of 0.9mm / Cycle appears.
In the 41 beam, the same beam combination is repeated every 492 pixels (every 13 scans), and a banding period of 5.2 mm / cycle appears.

The above relationship can be theoretically obtained by the above equation (0) or (1).
(A) In the case of 42 beams,
Band (mm / cycle) = LCM (42,2400 / 200) × 25.4 / 2400 = 0.9
And
(B) In the case of 41 beams, since n is a prime number, equation (1) can be applied,
Band (mm / cycle) = 41 × 25.4 / 200 = 5.2
It becomes.
As described above, if the number of light sources is a prime number, the light source can be shifted to the long frequency side.

FIG. 5 shows the result of obtaining the banding period generated from 29 beams to 62 beams based on the equations (0) and (1). Furthermore, the area where the banding cycle Band (cycle / mm), which is particularly conspicuous and cheap to the human eye, falls within the following formula is indicated by gray hatching.
5 (cycle / mm) ≤ Band ≤ 5.0 (cycle / mm) (2)

As described above, by using a light source having a prime number of 41 beams or more, it is possible to obtain a condition in which banding is not conspicuous even in the range of 120 to 240 lines.
However, if it is not a prime number even with 41 beams or less, it will fall within the range of the above equation (2) with any number of lines, resulting in a period with high human eye sensitivity. You cannot choose freely.
Further, the beam of 41 beams or less falls within the range of the above expression (2) particularly in the vicinity of 200 lpi with high resolution. Also, since the number of beams decreases, it is not suitable for high speed and high density.

  As described above, according to the present embodiment, banding can be shifted to a spatial frequency region (long period side) that is not noticeable to human eyes, and good image quality without abnormal images can be maintained.

[Example 1-2]
By the way, in the case of banding, external factors such as vibrations, photoconductor rotation fluctuations, polygon mirror surface tilts, and other factors such as uneven development (noise factors that occur regardless of multi-beams) The banding period is different from the above equation (0).

For example, surface tilt and vibration of a polygon mirror of a polygon scanner occur in a cycle that depends on the number of mirror surfaces, the photoconductor occurs in a cycle corresponding to the circumference of the photoconductor, and external vibration is a vibrating element (such as a folding mirror). In the development, banding occurs in a cycle corresponding to the developing member (such as rotation of a screw).
In order to cope with these external factors, it is difficult only by the means of the above-mentioned embodiment 1-1.

Therefore, there is an interlaced scanning method as a method for making density fluctuations due to external factors as inconspicuous as possible. The interlaced scanning method (also referred to as an interlaced method) is a method in which a plurality of beams are written with a plurality of scanning times so that the plurality of beams do not overlap each other for each scanning.
This interlaced scanning method has an advantage that even if density unevenness occurs due to an external factor, an image is formed by two to three scans, and thus density unevenness due to an external factor can be dispersed, so that banding is not noticeable. . Furthermore, since the interval between the light sources can be arranged wider than that of the normal adjacent scanning method, there is an advantage that the occurrence of thermal and electrical crosstalk between the light sources can be suppressed.

  On the other hand, if the beam pitch interval of the interlaced scanning itself is shifted, the interval between the first scan and the second scan becomes non-uniform and appears periodically, so that internal factors (pitch and density fluctuation between multiple beams) ) On the other hand, it is vulnerable to banding. However, this side effect can be avoided at the same time by using the system of Example 1-1.

Therefore, in the present embodiment, it is possible to perform scanning corresponding to both an internal factor and an external factor of banding by performing interlaced scanning in addition to the method of Example 1-1.
Furthermore, if the number of light sources is a prime number as in Example 1-1, a plurality of beams can always be filled with two scans.
With respect to the first scan, the beam No of the second scan is scanned at the position [(n + 1) / 2].
When the number of beams is an even number, an image cannot generally be formed by two scans, and three or more scans are required. However, when the number of beams is a prime number, an image can always be formed by two scans. In the interlace scanning, it is necessary to store image data in the memory for the number of scans. For this reason, skipping three or more scans increases the memory capacity of the RAM and the like, which increases the problem on the electrical system (expansion of circuit scale and cost increase).

FIG. 6 shows an example of a banding cycle in the case where 2400 dpi, 200 lpi, and 2 dot lines are output by interlaced scanning with 41 beams. Similar to the adjacent scanning of the 41 beams in FIG. 4B described above, the beam combination is repeated every 492 pixels (every 13 scans), and a 5.2 mm / Cycle banding cycle appears. At this time, if there is an internal disturbance of 41 beams (pitch and density variation between multiple beams), banding that is noticeable to human eyes is generated.
The above equation (1) can also be applied to interlaced scanning,
Band (mm / cycle) = 41 × 25.4 / 200 = 5.2 (mm / Cycle)
It becomes.
As described above, if the number of light sources is a prime number, the light source can be shifted to the long frequency side.

  According to the present invention, since the interlaced scanning can be completed in two scans, it is possible to realize a scanning optical system that has a small memory capacity and can cope with both internal and external factors.

[Example 1-3]
In laser scanning writing, the pixel density and the writing density do not necessarily match. For example, a method is known in which the pixel density of image data is further divided, one pixel (one dot) is formed by a plurality of beams, and writing is performed at a high writing density.
For example, in the multi-beam light source 1001 using the surface emitting laser shown in FIG. 2, the light source pitch is arranged at an interval of 4800 dpi, and one pixel is 1200 dpi, and is composed of four beams. The first pixel P1 is formed by a column of light sources a1 to a4, and the second pixel P2 is formed of a column b of light sources b1 to b4.
In this embodiment, the number of light sources is a prime number of 41 beams or more, but in FIG.

An image forming apparatus that forms one pixel with a plurality of beams is advantageous in high-speed writing because forming one pixel with a plurality of beams is advantageous in terms of light quantity compared to forming with one beam.
Further, since one pixel can be further divided into higher resolutions, it can be applied to corrections such as smoothness of slant lines (jaggy reduction), improvement of character sharpness, bending by image processing, and inclination correction.
However, while the above-mentioned advantages are provided, the combination of beams forming one pixel changes sequentially, so that internal factors such as uneven beam pitch on the image plane of multiple beams, uneven light quantity between light sources, and reciprocity law failure (multi-beam) The image density for each pixel is also likely to change sequentially due to the influence of the pitch and density fluctuations between them. Since the combination of beams forming one pixel is often repeated periodically in the sub-scanning direction, image density fluctuations also appear periodically, and periodicity such as so-called “banding”, which is easily perceived by the human eye. It is a problem that an abnormal image is easily generated.

  Therefore, in this embodiment, “one pixel is formed by at least m (n ≧ m ≧ 2) light sources out of n (n ≧ 41) light sources”, so that the banding period is reached. Can be shifted to a low frequency region insensitive to human eyes, and even when a single pixel is formed by a multi-beam light source, there is no banding and high-speed and high-density writing can be realized.

[Example 1-4]
In order to perform high-density writing (particularly 1200 dpi or more) with a beam having 41 or more light sources, it is necessary to set the light source pitch in the sub-scanning direction to be narrow.
However, the so-called “edge emitting laser”, which has been widely used in the past, is difficult to realize. An edge-emitting laser is a semiconductor laser that is composed of a horizontal resonator whose reflection mirror is an end face cut out by cleaving a substrate on which a semiconductor is laminated and grown, and the direction in which the light resonates is parallel to the substrate surface. Since the beam formation is arranged in one row, if the light source pitch is narrowed, it becomes easy to be affected by thermal and electrical crosstalk, and if it is attempted to avoid the influence, it becomes longer in one row, so that optical aberration causes Beam spot diameter, beam pitch unevenness, etc. are likely to deteriorate.

  On the other hand, a so-called “surface emitting laser” is a semiconductor laser in which the direction of light resonance is perpendicular to the substrate surface, and the emitted light is also emitted perpendicular to the substrate surface. By using the “surface emitting laser”, the number of beams of 29 or more can be two-dimensionally arranged, so that the apparent light source pitch in the sub-scanning direction can be set narrow even if the interval between the light sources is wide. At this time, since the intervals between the light sources can be set wide, thermal damage and thermoelectric crosstalk can be sufficiently suppressed.

  Therefore, in this embodiment, in addition to the configurations of Embodiments 1-1 to 1-3, “the monolithic surface emitting lasers two-dimensionally arranged on the same chip are used as the n light sources”. Thus, by using a “surface emitting laser” as the light source of the optical scanning device, the light source pitch in the sub-scanning direction of the multi-beam light source can be narrowly arranged, so that even for 41 or more multi-beams, 1200 dpi or more It is possible to realize an optical scanning device suitable for high-density writing.

[Example 1-5]
When forming multiple beams on the image plane (photosensitive member) through the scanning optical system, each multi-channel beam is easily affected by optical aberration because it passes through an optical path away from the optical axis of each optical system. . In particular, deviations in the beam spot diameter are likely to occur between the beams, the beam spacing is non-uniform, and beam pitch unevenness is likely to occur. Such beam spot diameter deviation and beam pitch unevenness tend to cause banding.

Therefore bright in the present embodiment is characterized in "each light source of the surface emitting laser according to embodiment 1-4, are arranged two-dimensionally are in point symmetry to." As a result, multi-channels can be integrated two-dimensionally, and each beam is transmitted through the vicinity of the optical axis of the optical system, so that it is not easily affected by optical aberrations and suppresses the occurrence of beam spot diameter deviation and beam pitch unevenness. It is possible to suppress the cause of banding.

  As an example in which prime light sources are arranged point-symmetrically, 41 arrangement examples are shown in FIG. In FIG. 7, the prime light sources are arranged point-symmetrically and are also affected by optical aberrations, so that the beam pitch interval can be kept uniform. In addition, when mounting packages, the wiring can be arranged radially in a point-symmetric manner (the wiring can be shortened), so that the impedance can be kept almost uniform and the optical waveform is less likely to become dull. is there.

[Example 1-6]
In the multi-beam laser light emission drive control, digital signals of image data are transmitted in parallel to each beam, and D / A conversion is performed at the final stage to drive each semiconductor laser element. Therefore, it is efficient to form the digital transfer data with 2 ^ n, and it is usually desirable to perform signal processing of a multiple of 8.
Here, in this embodiment, the surface emitting laser in Embodiment 1-5 has a light source of 41 or more prime numbers used for writing image data, and further includes an auxiliary light source to have a light source that is a multiple of 8 in total. And

Here, since the beam actually written to the image data is a prime number, it is not a multiple of 8. Therefore, it is characterized in that efficient data transfer is performed by generating a virtual bit signal corresponding to the auxiliary light source.
FIG. 8 shows an example of an arrangement of light sources, and FIG. 9 shows an example of a block diagram of a laser drive system of this example. In FIG. 8, seven auxiliary light sources are provided for 41 beams used for actual writing, for a total of 48 beams. Since 48 beams are multiples of 8, it is convenient for digital signal transmission. In the example shown in FIG. 9, 1200 dpi image data is written to 4800 dpi, density conversion is performed, image distortion is corrected by a dot position correction function, and diagonal line smoothing (jaggy correction) is provided.
Here, the auxiliary light source may be used for a purpose other than image data, for example, a sync signal detection beam, or may be a dummy light source that does not actually emit light.

[Example 1-7]
As described above, when multiple beams are imaged on the image plane (photosensitive member) through the scanning optical system, optical aberration becomes a problem.
When the arrangement becomes long in the main scanning direction, it is necessary to widen the writing width of the scanning optical system (make the scanning lens have a wide angle of view), and optical design that satisfies optical aberrations becomes difficult. In the case of a two-dimensional arrangement, in order to adjust the beam pitch in the sub-scanning direction, the entire light source is generally rotated and adjusted around the optical axis. However, the rotation adjustment accuracy must be extremely high. It is difficult to compensate for temperature fluctuations and changes with time.
Further, when the arrangement becomes long in the sub-scanning direction, each beam passes through a lens position away from the optical axis, which causes the aforementioned beam spot diameter deviation and beam pitch unevenness. Particularly at the peripheral image height, main scanning jitter tends to occur simultaneously and trapezoidal distortion occurs.
As described above, there are problems in both directions, but generally the problem in the sub-scanning direction is larger.

Therefore, in this example, when the number of arrangements of the surface emitting lasers in Example 1-5 and Example 1-6 is column V and row H,
| H−V | / (H + V) <0.2 (3)
And the smaller of the column V or the row H is used in the sub-scanning corresponding direction.

The arrangement of the surface emitting lasers of this example is the same as that shown in FIG. 7 or FIG. 8, and both are arrangements satisfying the above-mentioned expression (3).
In FIG. 7, the array includes V: 7 columns and H: 7 rows including the sky light source.
| H−V | / (H + V) = | 7−7 | / (7 + 7) = 0
In and, H and V in FIG. 7 may be used either for the same number in the sub-scanning.
Moreover, in FIG. 8, it is an array of V: 6 columns and H: 8 rows including the auxiliary light source,
| H−V | / (H + V) = | 8−6 | / (8 + 6) = 0.14
And (2) are satisfied. As a result, it is possible to realize an optical scanning device and an optical scanning method using a scanning optical system that is hardly affected by optical aberrations and suppresses banding.

[Example 2-1]
In the second embodiment of the present invention, a plurality of light sources (for example, the multi-beam light source 1001 shown in FIGS. 1 and 2) and a plurality of light beams (multi-beams) emitted from the plurality of light sources are arranged in the main scanning direction. Scanning means for scanning (polygon mirror 1002 shown in FIG. 1) and an optical element for focusing the multi-beams scanned by the scanning means on the image carrier (photoconductive photosensitive member 1004 shown in FIG. 1) In an optical scanning device (or optical scanning method) that uses a multi-beam scanning optical system having (scanning lens 1003 or the like) and drives the plurality of light sources according to image data to form an image on the image carrier. The number of light sources n of the multi-beam is configured by a prime number of n ≧ 29, and has an image processing for making the apparent number of lines of the image data longer, and the number of apparent lines of the image data (line / inch) is L ' In this case, the banding cycle [Band] represented by the following formula is satisfied.
Band (mm / cycle) = n × 25.4 / L ′ ≧ 10 (4)

When the number n of light sources of the multi-beam light source 1001 is a prime number, the least common multiple LCM in the above-described equation (0) is a simple multiplication of the number n of beams and the image period (D / L). It can be simplified as an expression.
On the other hand, when the number of beams is not a prime number, there is a natural number that is divisible by the image period, so depending on the image period (or the number of lines), it cannot be shifted to the long period side, and a period with high human eye sensitivity appears. Therefore, the number of lines cannot be selected freely.

On the other hand, when the number of beams is not a prime number, there is a natural number that is divisible by the image period, so depending on the image period (or the number of lines), it cannot be shifted to the long period side, and a period with high human eye sensitivity appears. Therefore, the number of lines cannot be selected freely.
Since the number of lines of the printer output is normally selected by the user in the range of 150 lpi to 240 lpi, the image forming apparatus needs to provide an image in which banding is not conspicuous for any number of lines.
In addition, as described above, when the number of conventional beams is small, banding occurred only at a short cycle (0.5 mm / cycle or less), which is not as big a problem as a multi-beam.

In the first embodiment of the present invention, the banding cycle is set to 5 mm / cycle or more, which is acceptable for business documents that are often viewed at a distance of about 300 mm, but is 300 mm or more apart from images such as offset printing or posters. When looking, 5mm / cycle is not enough. Particularly for fields and poster manuscripts that require high quality image quality, it is necessary to increase the banding cycle to at least 10 mm / cycle.
Therefore, in this embodiment, in order to further increase the banding cycle, a method of increasing the apparent number of lines in image processing is proposed.

First, specific means for increasing the apparent number of lines in image processing will be described.
As shown in FIG. 10A, in contrast to the case where 2 dot lines are output at 2400 dpi and the number of lines of 200 lpi, in this embodiment, the pixel shift is 2400 dpi units (ie, 10.6 μm). Shift half and again shift image shift in the opposite direction.
In this way, the number of basic lines 200 lpi can be converted to 50 lpi by image processing such as image shift, and the period can be increased. Here, if the image shift is 2400 dpi or more (that is, 10.6 μm or less), it cannot be determined by the human eye that the pixel is shifted.
Further, although the apparent number of lines is 50 lpi, the image data itself maintains 200 lpi, so that the resolution of the image is not deteriorated.

An example of the banding period of the second embodiment is shown in FIG. In FIG. 11A, by setting the number of light sources to be a prime number of 29, the same combination of 29 beams is repeated at a cycle of 349 pixels (every 13 scans). It becomes possible to shift the interference of the period to 3.7 mm to the long period side.
This period is substituted into the above equation (4),
29 × 25.4 / 200 = 3.7
Can be obtained as
However, this cycle is at a level that can be sufficiently discerned by the human eye.

Therefore, in the present invention, as shown in FIG. 11B, the apparent number of lines is set to 50 lpi by pixel shift, and the same beam combination of 29 beams is repeated at a period of 1393 pixels (every 49 scans). The banding period is increased to 14.7 mm and exceeds 10 mm / Cycle, which makes it less noticeable to human eyes.
Substituting this period into equation (4)
29 × 25.4 / 50 = 14.7
Can be obtained as In this method, in addition to the basic period of 14.7 mm, there are many interference conditions such as 8.9 mm, but since the banding period is not equal, the banding density is relatively small.

Whether banding is actually conspicuous is often determined by an index called perceptual banding value. As shown in FIG. 3, the perceptual banding is an evaluation value that depends on the sensitivity of the human eye and the density parameter of the banding. FIG. 12 is a diagram showing FIG. 11 in terms of perceptual banding values.
In FIG. 12A, the basic spectrum of 3.7 mm / Cycle rises and exceeds the standard value that banding is allowed (this standard value is defined by the manufacturer).
In FIG. 12B, a basic spectrum of 14.7 mm rises, but the spectrum size can be suppressed to a standard value or less because of a long period with a relatively low sensitivity of the human eye. In addition to the basic spectrum of 14.7 mm, a large number of banding spectra have risen, but since the banding concentration is not high as described above, the banding spectrum is kept below the standard value.

FIG. 13 shows the result of obtaining the banding period generated from 23 beams to 62 beams based on the above equations (0) and (4). Furthermore, the area where the banding cycle Band (cycle / mm), which is particularly conspicuous and cheap to the human eye, falls within the following formula is indicated by gray hatching.
5 (cycle / mm) ≦ Band ≦ 5.0 (cycle / mm) (5)

By using a light source having a prime number of 29 beams or more and reducing the apparent number of lines by image shift, it is possible to obtain a condition in which banding is not conspicuous even in an actual image line number range of 120 to 240 lines.
However, if it is not a prime number even with 29 beams or more, it will fall within the range of the above formula (5) with any number of lines, and it will become a period with high sensitivity of human eyes, so the user can freely set the number of image lines Can not choose.

  As described above, according to the present embodiment, the banding can be shifted to the spatial frequency region (long period side) that is inconspicuous to human eyes, especially when high image quality equivalent to offset printing is required or for posters. Can maintain good image quality without abnormal images.

[Example 2-2]
By the way, in the case of banding, external factors such as vibrations, photoconductor rotation fluctuations, polygon mirror surface tilts, and other factors such as uneven development (noise factors that occur regardless of multi-beams) The banding period is different from the above equation (0).

  For example, surface tilt and vibration of a polygon mirror of a polygon scanner occur in a cycle that depends on the number of mirror surfaces, the photoconductor occurs in a cycle corresponding to the circumference of the photoconductor, and external vibration is a vibrating element (such as a folding mirror). In the development, banding occurs in a cycle corresponding to the developing member (such as rotation of a screw). In order to cope with these external factors, it is difficult only with the means of Example 2-1.

Therefore, there is an interlaced scanning method as a method for making density fluctuations due to external factors as inconspicuous as possible. The interlaced scanning method (also referred to as an interlaced method) is a method of writing a plurality of scanning times between a plurality of beams so that the plurality of beams do not overlap each other for each scanning.
This interlaced scanning method has an advantage that even if density unevenness occurs due to an external factor, an image is formed by two to three scans, and thus density unevenness due to an external factor can be dispersed, so that banding is not noticeable. . Furthermore, since the interval between the light sources can be arranged wider than that of the normal adjacent scanning method, there is an advantage that the occurrence of thermal and electrical crosstalk between the light sources can be suppressed.

  On the other hand, if the beam pitch interval of the interlaced scanning itself is shifted, the interval between the first scan and the second scan becomes non-uniform and appears periodically, so that internal factors (pitch and density fluctuation between multiple beams) ) On the other hand, it is vulnerable to banding. However, this side effect can be avoided simultaneously by using the method of Example 2-1.

Therefore, in the present embodiment, it is possible to perform scanning corresponding to both an internal factor and an external factor of banding by performing interlaced scanning in combination with the method of Example 2-1.
Furthermore, as in Example 2-1, if the number of light sources is a prime number, a plurality of beams can always be filled with two scans.
With respect to the first scan, the beam No of the second scan is scanned at the position [(n + 1) / 2].
When the number of beams is an even number, an image cannot generally be formed by two scans, and three or more scans are required. However, when the number of beams is a prime number, an image can always be formed by two scans. In the interlace scanning, it is necessary to store image data in the memory for the number of scans. For this reason, skipping three or more scans increases the memory capacity of the RAM and the like, which increases the problem on the electrical system (expansion of circuit scale and cost increase).

FIG. 14 shows a case where 2400 dpi, 200 lpi lines, an apparent line number of 50 lpi, and 2 dot lines are output by 29 beams with interlaced scanning. Similarly to the adjacent scanning of 29 beams in FIG. 11 (b), the same combination of 29 beams is repeated at a cycle of 1393 pixels (every 49 scans), and the banding cycle is 14.7 mm / Cycle. A period appears. At this time, if there is an internal disturbance of 29 beams (pitch and density fluctuation between multiple beams), banding that is noticeable to human eyes is generated.
The above equation (4) can also be applied to interlaced scanning,
Band (mm / cycle) = 29 × 25.4 / 50 = 14.7 (mm / Cycle)
It becomes.

As described above, if the number of light sources is a prime number and the apparent number of lines is reduced by an image processing means such as an image shift, it is possible to further shift to the long frequency side.
According to the present embodiment, since the interlaced scanning can be completed in two scans, it is possible to realize a scanning optical system having a small memory capacity and corresponding to both internal factors and external factors.

[Example 2-3]
In laser scanning writing, the pixel density and the writing density do not necessarily match. For example, a method is known in which the pixel density of image data is further divided, one pixel (one dot) is formed by a plurality of beams, and writing is performed at a high writing density.
For example, in the multi-beam light source 1001 using the surface emitting laser shown in FIG. 2, the light source pitch is arranged at an interval of 4800 dpi, and one pixel is 1200 dpi, and is composed of four beams. The first pixel P1 is formed by a column of light sources a1 to a4, and the second pixel P2 is formed of a column b of light sources b1 to b4.
In the present embodiment, the number of light sources is a prime number of 29 beams or more, but in FIG.

An image forming apparatus that forms one pixel with a plurality of beams is advantageous in high-speed writing because forming one pixel with a plurality of beams is advantageous in terms of light quantity compared to forming with one beam.
Further, since one pixel can be further divided into higher resolutions, it can be applied to corrections such as smoothness of slant lines (jaggy reduction), improvement of character sharpness, bending by image processing, and inclination correction.
However, while the above-mentioned advantages are provided, the combination of beams forming one pixel changes sequentially, so that internal factors such as uneven beam pitch on the image plane of multiple beams, uneven light quantity between light sources, and reciprocity law failure (multi-beam) The image density for each pixel is also likely to change sequentially due to the influence of the pitch and density fluctuations between them. Since the combination of beams forming one pixel is often repeated periodically in the sub-scanning direction, image density fluctuations also appear periodically, and periodicity such as so-called “banding”, which is easily perceived by the human eye. It is a problem that an abnormal image is easily generated.

  Therefore, in this embodiment, “one pixel is formed by at least m (n ≧ m ≧ 2) light sources among n (n ≧ 29) light sources”. Can be shifted to a low frequency region insensitive to human eyes, and even when a single pixel is formed by a multi-beam light source, there is no banding and high-speed and high-density writing can be realized.

[Example 2-4]
In order to perform high-density writing (particularly 1200 dpi or more) with 29 or more beams, it is necessary to set the light source pitch in the sub-scanning direction to be narrow.
However, the so-called “edge emitting laser”, which has been widely used in the past, is difficult to realize. An edge-emitting laser is a semiconductor laser that is composed of a horizontal resonator whose reflection mirror is an end face cut out by cleaving a substrate on which a semiconductor layer is grown. The direction of light resonance is a semiconductor laser parallel to the substrate surface. Since the beam formation is arranged in a line, if the light source pitch is narrowed, it becomes easy to be affected by thermal and electrical crosstalk. Spot diameter, beam pitch unevenness, etc. are likely to deteriorate.

  In contrast, a so-called “surface emitting laser” is a semiconductor laser in which the direction in which light resonates is perpendicular to the substrate surface, and the emitted light is also emitted perpendicular to the substrate surface. By using the “surface emitting laser”, even 29 or more beams can be two-dimensionally arranged, so that the apparent light source pitch in the sub-scanning direction can be set narrow even if the intervals between the light sources are wide. At this time, since the intervals between the light sources can be set wide, thermal damage and thermoelectric crosstalk can be sufficiently suppressed.

  Therefore, in this embodiment, in addition to the configurations of Embodiments 2-1 to 2-3, “the monolithic surface emitting lasers two-dimensionally arranged on the same chip are used as the n light sources”. Thus, by using a “surface emitting laser” as the light source of the optical scanning device, the light source pitch in the sub-scanning direction of the multi-beam light source can be narrowly arranged, so that it is 1200 dpi or more for 29 or more multi-beams. It is possible to realize an optical scanning device suitable for high-density writing.

[Example 2-5]
When forming multiple beams on the image plane (photosensitive member) through the scanning optical system, each multi-channel beam is easily affected by optical aberration because it passes through an optical path away from the optical axis of each optical system. . In particular, deviations in the beam spot diameter are likely to occur between the beams, the beam spacing is non-uniform, and beam pitch unevenness is likely to occur. Such beam spot diameter deviation and beam pitch unevenness tend to cause banding.

Therefore, in this embodiment, are the features to "each light source of the surface emitting laser according to embodiment 2-4 are two-dimensionally arranged in point symmetry to." As a result, multi-channels can be integrated two-dimensionally, and each beam is transmitted through the vicinity of the optical axis of the optical system, so that it is not easily affected by optical aberrations and suppresses the occurrence of beam spot diameter deviation and beam pitch unevenness. It is possible to suppress the cause of banding.

  As an embodiment in which prime light sources are arranged point-symmetrically, 29 arrangement examples are shown in FIG. In FIG. 15A, the prime light sources are arranged point-symmetrically and are also affected by optical aberrations, so that the beam pitch interval can be kept uniform. In addition, when mounting packages, the wiring can be arranged radially in a point-symmetric manner (the wiring can be shortened), so that the impedance can be kept almost uniform and the optical waveform is less likely to become dull. is there.

[Example 2-6]
In the multi-beam laser light emission drive control, digital signals of image data are transmitted in parallel to each beam, and D / A conversion is performed at the final stage to drive each semiconductor laser element. Therefore, it is efficient to form the digital transfer data with 2 ^ n, and it is usually desirable to perform signal processing of a multiple of 8.
Here, in this embodiment, the surface emitting laser in Embodiment 2-5 has a light source of a prime number of 29 or more used for writing image data, further including an auxiliary light source, and has a light source that is a multiple of 8 in total. And

Here, since the beam actually written to the image data is a prime number, it is not a multiple of 8. Therefore, it is characterized in that efficient data transfer is performed by generating a virtual bit signal corresponding to the auxiliary light source.
FIG. 15B shows the arrangement of the light sources of this embodiment, and FIG. 9 shows an example of a block diagram of the laser drive system of this embodiment.
In FIG. 15B, three auxiliary light sources are provided for 29 beams used for actual writing, and a total of 32 beams are provided. Since 32 beams are multiples of 8, it is convenient for digital signal transmission. In the example shown in FIG. 9, 1200 dpi image data is written to 4800 dpi, density conversion is performed, image distortion is corrected by a dot position correction function, and diagonal line smoothing (jaggy correction) is provided.
Here, the auxiliary light source may be used for a purpose other than image data, for example, a sync signal detection beam, or may be a dummy light source that does not actually emit light.

[Example 2-7]
As described above, when multiple beams are imaged on the image plane (photosensitive member) through the scanning optical system, optical aberration becomes a problem.
When the arrangement becomes long in the main scanning direction, it is necessary to widen the writing width of the scanning optical system (make the scanning lens have a wide angle of view), and optical design that satisfies optical aberrations becomes difficult. In the case of a two-dimensional arrangement, in order to adjust the beam pitch in the sub-scanning direction, the entire light source is generally rotated and adjusted around the optical axis. However, the rotation adjustment accuracy must be extremely high. It is difficult to compensate for temperature fluctuations and changes with time.
Further, when the arrangement becomes long in the sub-scanning direction, each beam passes through a lens position away from the optical axis, which causes the aforementioned beam spot diameter deviation and beam pitch unevenness. Particularly at the peripheral image height, main scanning jitter tends to occur simultaneously and trapezoidal distortion occurs.
As described above, there are problems in both directions, but generally the problem in the sub-scanning direction is larger.

Therefore, in this example, when the number of arrangements of the surface emitting lasers in Example 2-5 and Example 2-6 is column V and row H,
| H−V | / (H + V) <0.2 (6)
And the smaller of the column V or the row H is used in the sub-scanning corresponding direction.

The arrangement of the surface emitting lasers of this example is the same as that shown in FIG. 15A or 15B, and both satisfy the above expression (6).
In FIG. 15, the array includes V: 5 columns and H: 7 rows including the sky light source.
| H−V | / (H + V) = | 7−5 | / (7 + 5) = 0.16
And (6) are satisfied. As a result, it is possible to realize an optical scanning device and an optical scanning method using a scanning optical system that is hardly affected by optical aberrations and suppresses banding.

Next, an embodiment of the image forming apparatus according to the present invention will be described.
FIG. 16 is a schematic configuration diagram illustrating a mechanical configuration of the image forming apparatus according to the present exemplary embodiment. The image forming apparatus 100 according to the present exemplary embodiment includes an optical scanning device 102 including a plurality of light source devices, a polygon mirror 102a, and optical elements such as an optical element, and an image forming unit 112 including a photosensitive drum, a charging device, a developing device, and the like. The transfer unit 122 includes an intermediate transfer belt and the like. The optical scanning device 102 includes a plurality of VCSELs as a plurality of light source devices. In the embodiment shown in FIG. 16, a plurality of light beams (multi-beams) emitted from a plurality of light source devices (not shown in FIG. 16) are once condensed by a first cylindrical lens (not shown) and scanned. The light is deflected to the reflection mirror 102b by the polygon mirror 102a.

In the image forming apparatus 100, the post-object type optical scanning device 102 that does not use an fθ lens as a scanning lens is configured. However, a configuration in which an fθ lens is used as a scanning lens may be used.
In this optical scanning device 102, a polygon mirror 102a common to the four scanning optical systems is used, and the light beam L is cyan (C), magenta (M), yellow (Y), black in the illustrated embodiment. The number corresponding to each color of (K) is generated, reflected by the reflecting mirror 102b, and condensed again by the second cylindrical lens 102c, and then the photosensitive drums 104a, 106a, 108a, 110a are exposed. Each scanning optical system has the same configuration as that shown in FIG.

  Since the irradiation of the light beam L is performed using a plurality of optical elements as described above, the timing is synchronized in the main scanning direction and the sub-scanning direction. Hereinafter, the main scanning direction is defined as the light beam scanning direction, and the sub-scanning direction is defined as a direction orthogonal to the main scanning direction.

  Each of the photosensitive drums 104a, 106a, 108a, and 110a includes a photoconductive layer including at least a charge generation layer and a charge transport layer on a conductive drum such as aluminum. The photoconductive layer is disposed corresponding to each of the photosensitive drums 104a, 106a, 108a, and 110a, and is charged with a surface charge by the chargers 104b, 106b, 108b, and 110b including a corotron, a scorotron, or a charging roller. Is granted.

  The electrostatic charges imparted on the photosensitive drums 104a, 106a, 108a, 110a by the respective chargers 104b, 106b, 108b, 110b are imagewise exposed by the light beam L to form an electrostatic latent image. The electrostatic latent images formed on the photoconductive drums 104a, 106a, 108a, and 110a are developed by the developing devices 104c, 106c, 108c, and 110c including a developing sleeve, a developer supplying roller, a regulating blade, and the like. Is formed.

  The developer carried on the photosensitive drums 104a, 106a, 108a, and 110a is transferred onto the intermediate transfer belt 114 that moves in the direction of arrow A by the conveying rollers 114a, 114b, and 114c. The intermediate transfer belt 114 is conveyed to the secondary transfer unit while carrying C, M, Y, and K developers. The secondary transfer unit includes a secondary transfer belt 118 and conveying rollers 118a and 118b. The secondary transfer belt 118 is conveyed in the direction of arrow B by the conveyance rollers 118a and 118b. An image receiving material 124 such as high-quality paper or a plastic sheet is supplied to the secondary transfer portion from an image receiving material storage portion 128 such as a paper feed cassette by a conveying roller 126.

  The secondary transfer unit applies a secondary transfer bias to transfer the multicolor developer image carried on the intermediate transfer belt 114 onto the image receiving material 124 held by suction on the secondary transfer belt 118. The image receiving material 124 is supplied to the fixing device 120 along with the conveyance of the secondary transfer belt 118. The fixing device 120 includes a fixing member 130 such as a fixing roller including silicone rubber, fluorine rubber, and the like. The fixing device 120 pressurizes and heats the image receiving material 124 and the multi-color developer image to form an image as a printed matter (print) 132. Output to the outside of the forming apparatus 100. After the multicolor developer image is transferred, the transfer belt 114 is supplied to the next image forming process after the transfer residual developer is removed by the cleaning unit 116 including a cleaning blade.

  In the image forming apparatus of the present embodiment having the above-described configuration, an optical scanning device (optical scanning method) using the laser light source, the scanning optical system, and the laser driving system having the characteristics of the above-described first or second embodiment. By using it, it is possible to provide a high-quality image in which the occurrence of abnormal images such as banding is suppressed.

100: Image forming apparatus 102: Optical scanning apparatus 102a: Polygon mirror (scanning means)
102b: reflection mirror 102c: second cylindrical lens 104a, 106a, 108a, 110a: photosensitive drum (image carrier)
104b, 106b, 108b, 110b: Charger 104c, 106c, 108c, 110c: Developer 112: Image forming unit 114: Intermediate transfer belt 114a, 114b, 114c: Conveying roller 118: Secondary transfer belt 120: Fixing device 122: Transfer unit 1001: Multi-beam light source (surface emitting semiconductor laser unit, etc.)
1002: Polygon mirror (scanning means)
1003: Scanning lens (optical element)
1004: Photoconductor (image carrier)
1005: Photodetector 1006: Image processing unit 1007: Laser drive circuit 1008: Clock generation circuit 1009: Phase synchronization circuit

JP 2000-37904 A JP 2002-113903 A JP 2007-23740 A JP 2007-196460 A JP 2008-170640 A Japanese Patent Laid-Open No. 10-221903 JP 2009-29115 A

Claims (20)

A plurality of light sources, scanning means for scanning a plurality of light beams (hereinafter referred to as multi-beams) emitted from the plurality of light sources in the main scanning direction, and the multi-beams scanned by the scanning means on the image carrier In an optical scanning device comprising a multi-beam scanning optical system having an optical element for condensing, and driving the plurality of light sources according to image data to form an image on the image carrier,
When the number of light sources n of the multi-beam is configured as a prime number of n ≧ 41 and the number of lines (line / inch) of the image data is L, a banding period [Band] represented by the following formula is obtained. An optical scanning device characterized by satisfying.
Band (mm / cycle) = n × 25.4 / L ≧ 5.0 A plurality of light sources, scanning means for scanning a plurality of light beams (hereinafter referred to as multi-beams) emitted from the plurality of light sources in the main scanning direction, and the multi-beams scanned by the scanning means on the image carrier In an optical scanning device comprising a multi-beam scanning optical system having an optical element for condensing, and driving the plurality of light sources according to image data to form an image on the image carrier,
An image processing is performed in which the number n of light sources of the multi-beam is a prime number of n ≧ 29, and half of the image data in the main scanning direction is shifted by one pixel in the pixel density unit in the sub-scanning direction. ,
An optical scanning device characterized by satisfying a banding cycle [Band] represented by the following equation when L ′ is an apparent number of lines (line / inch) of the image data.
Band (mm / cycle) = n × 25.4 / L ′ ≧ 10 The optical scanning device according to claim 2.
The optical scanning device according to claim 1, wherein the pixel density unit is 2400 dpi or more. In the optical scanning device according to any one of claims 1 to 3,
The multi-beam scanning optical system performs interlaced scanning in which a plurality of beams obtained by the first scanning are filled by the second scanning. In the optical scanning device according to any one of claims 1 to 4,
An optical scanning device, wherein one pixel is formed by at least m (n ≧ m ≧ 2) of n light sources. In the optical scanning device according to any one of claims 1 to 5,
A monolithic surface-emitting laser in which the plurality of light sources are two-dimensionally arranged on the same chip is used. The optical scanning device according to claim 6.
Each light source of the surface emitting laser is two-dimensionally arranged in a point-symmetric manner. The optical scanning device according to claim 6.
The surface-emitting laser has a light source used for writing image data as a prime number, and further includes an auxiliary light source, and has a light source that is a multiple of 8 in total. The optical scanning device according to claim 7 or 8,
When the arrangement of the light sources of the surface emitting laser is a column V and a row H,
| H−V | / (H + V) <0.2
An optical scanning device characterized by using the column V or the row H in the sub-scanning corresponding direction. A plurality of light sources, scanning means for scanning a plurality of light beams (hereinafter referred to as multi-beams) emitted from the plurality of light sources in the main scanning direction, and the multi-beams scanned by the scanning means on the image carrier In an optical scanning method of forming an image on the image carrier by driving the plurality of light sources according to image data using a multi-beam scanning optical system having a condensing optical element,
When the number of light sources n of the multi-beam is configured as a prime number of n ≧ 41 and the number of lines (line / inch) of the image data is L, a banding period [Band] represented by the following formula is obtained. An optical scanning method characterized by satisfying.
Band (mm / cycle) = n × 25.4 / L ≧ 5.0 A plurality of light sources, scanning means for scanning a plurality of light beams (hereinafter referred to as multi-beams) emitted from the plurality of light sources in the main scanning direction, and the multi-beams scanned by the scanning means on the image carrier In an optical scanning method of forming an image on the image carrier by driving the plurality of light sources according to image data using a multi-beam scanning optical system having a condensing optical element,
An image processing is performed in which the number n of light sources of the multi-beam is a prime number of n ≧ 29, and half of the image data in the main scanning direction is shifted by one pixel in the pixel density unit in the sub-scanning direction. ,
An optical scanning method characterized by satisfying a banding cycle [Band] represented by the following equation when the apparent number of lines (inch / inch) of the image data is L ′.
Band (mm / cycle) = n × 25.4 / L ′ ≧ 10 The optical scanning method according to claim 11.
The optical scanning method, wherein the pixel density unit is 2400 dpi or more. The optical scanning method according to any one of claims 10 to 12,
The optical scanning method, wherein the multi-beam scanning optical system performs interlaced scanning in which a plurality of beams obtained by the first scanning are filled by the second scanning. The optical scanning method according to any one of claims 10 to 13,
An optical scanning method, wherein one pixel is formed by at least m (n ≧ m ≧ 2) light sources out of n light sources. The optical scanning method according to any one of claims 10 to 14,
An optical scanning method using a monolithic surface emitting laser in which the plurality of light sources are two-dimensionally arranged on the same chip. The optical scanning method according to claim 15, wherein
An optical scanning method characterized in that the light sources of the surface emitting laser are two-dimensionally arranged in point symmetry. The optical scanning method according to claim 15, wherein
The surface-emitting laser has a light source used for writing image data as a prime number, and further includes an auxiliary light source, so that the total number of light sources is a multiple of eight. The optical scanning method according to claim 16 or 17,
When the arrangement of the light sources of the surface emitting laser is a column V and a row H,
| H−V | / (H + V) <0.2
Satisfying the above, and the smaller of the column V or the row H is used in the sub-scanning corresponding direction. In an image forming apparatus comprising an image carrier and writing means for forming a latent image on the image carrier,
An image forming apparatus comprising the optical scanning device according to claim 1 as the writing unit. In an image forming apparatus comprising an image carrier and writing means for forming a latent image on the image carrier,
An image forming apparatus using the optical scanning method according to claim 10 as the writing unit.

Images