JP4706430B2 - Tandem laser scanning device - Google Patents

Description

  The present invention relates to a tandem-type laser scanning device, and more particularly to a laser scanning device mounted on an image forming apparatus such as a copying machine or a printer that forms a color image in a tandem manner by electrophotography.

  Conventionally, a plurality of photoconductors for forming the three primary colors and the black image are juxtaposed, and the respective color images formed on the photoconductor are sequentially primary-transferred onto the intermediate transfer body and synthesized. Various tandem-type image forming apparatuses are provided in which the composite image is secondarily transferred onto a transfer material.

  As a laser scanning device corresponding to this type of tandem type, Patent Document 1 discloses that a light from a plurality of light sources is deflected by one surface of a deflector (polygon mirror), and then an optical path is separated by a folding mirror. And a technique for forming a color image at high speed by forming an image by guiding it to each photoreceptor via a mirror.

  Further, Patent Document 2 discloses that when light from a plurality of light sources has different incident angles to the deflector in the sub-scanning direction, the imaging magnification in the sub-scanning direction is changed to suppress bow on the photoconductor. Is described. Here, it is disclosed that an aperture for setting light from a light source to a desired spot diameter is provided in front of the deflector.

  By the way, in the tandem-type laser scanning device described in Patent Document 1, if the imaging magnification in the sub-scanning direction between the deflector and the photosensitive member is made uniform for each optical path, there is a great restriction on the arrangement of optical elements. . Therefore, it is conceivable to provide a degree of freedom in the arrangement of the optical elements by making a difference in the imaging magnification in the sub-scanning direction. In this case, as described in Patent Document 2, with respect to the position of the image of the aperture formed near the photoconductor through the optical system after the aperture using the aperture, the distance from the photoconductor is a sub-distance. A difference occurs depending on the imaging magnification in the scanning direction.

As a result, a change in the beam diameter at the time of defocusing causes a difference in accordance with the magnification in the sub-scanning direction. There was a problem that the color tone changed due to a difference in tonality.
JP 2004-70190 A JP 2000-180749 A

  Therefore, an object of the present invention is to provide a degree of freedom in arrangement of optical elements by making a difference in the imaging magnification in the sub-scanning direction, and to eliminate the difference in change in beam diameter on each photoconductor during defocusing. Another object of the present invention is to provide a tandem type laser scanning device capable of obtaining a color image of good quality.

In order to achieve the above object , the present invention provides at least two light sources, one deflector for deflecting light from the light source, and light from the light source in the sub-scanning direction only in the vicinity of the deflector. A first optical system that collects light, an optical path disposed between the light source and the deflector, at least two apertures for restricting light from each of the light sources, and deflected by the deflector In a tandem type laser scanning device comprising: a second optical system that guides each light to each of at least two photoconductors;
In the second optical system, the imaging magnification in the sub-scanning direction between the deflector and the photoconductor is 0.9 or less corresponding to each of the photoconductors, and is different from each other.
An optical element having power in the sub-scanning direction is disposed in the optical path between the aperture and the deflector,
No optical element having power is disposed in the optical path between the optical element and the aperture,
The distance along the principal ray from the aperture to the optical element is longer as the optical path having a smaller absolute value of the imaging magnification in the sub-scanning direction between the deflector and the photoconductor,
It is characterized by.

In the tandem-type laser scanning device according to the present invention, the degree of freedom in arranging the optical elements is ensured by making a difference in the imaging magnification in the sub-scanning direction, and the position of the aperture is changed from the aperture to the optical element. Since the distance along the principal ray is set to be longer for the optical path having a smaller absolute value of the imaging magnification in the sub-scanning direction between the deflector and the photoconductor, the imaging magnification in the sub-scanning direction is different although it is set longer. The position of the image of the aperture formed near the photoconductor through the optical system after the aperture is substantially the same distance from the photoconductor. As a result, even when defocus occurs, the behavior of the beam diameter on the photoconductor becomes almost equal, and the change in gradation and color of the color image is eliminated.

  Embodiments of a tandem laser scanning apparatus according to the present invention will be described below with reference to the accompanying drawings.

(Refer 1st Example and FIGS. 1-10)
FIG. 1 shows a schematic configuration of a tandem type laser scanning apparatus according to a first embodiment of the present invention. This laser scanning device generally includes four semiconductor lasers 1a to 1d, apertures 3a to 3d, a cylinder lens 5, a polygon mirror 6, and four photoconductors 12a in which light deflected by the polygon mirror 6 is juxtaposed. And a scanning optical system 20 that guides the light to 12d to form an image.

  The photoconductors 12a to 12d are for forming the three primary colors and the black image, and the images of the respective colors formed on the photoconductors 12a to 12d are transferred and synthesized on an intermediate transfer belt (not shown). . Such a color image forming method is known as a tandem type, and the description thereof is omitted.

  The laser light emitted from each of the semiconductor lasers 1a to 1d becomes parallel light by the collimator lenses 2a to 2d, and after being regulated by the apertures 3a to 3d, is superposed in the main scanning direction Y by the plane mirror array 4. The light is condensed only in the sub-scanning direction Z by the cylinder lens 5. Thereafter, the laser beam is deflected by the polygon mirror 6 in the main scanning direction Y at an equal angular velocity, refracted by the common scanning first lens 7, and then corresponding to the four branched optical paths to the respective photoconductors 12a to 12d. Images are formed on the photoreceptors 12a to 12d via the first folding mirrors 8a to 8d, the second folding mirrors 9a to 9d, the second scanning lenses 10a to 10d, and the window glasses 11a to 11d.

  FIG. 2 shows an optical path in a cross section in the sub-scanning direction Z of the laser scanning apparatus according to the first embodiment. Until the laser beam is deflected by the polygon mirror 6, the respective optical paths have the same height in the sub-scanning direction Z, and the angles of the principal rays in the sub-scanning direction Z are different.

  Further, the distances from the second lenses 10a to 10d to the photoconductors 12a to 12d are different for each optical path. In the case of a tandem type laser scanning device, since the amount of black toner consumed is larger than that of other color toners, only the black toner hopper may be large. In the first embodiment, the apparatus is made compact by assigning an optical path with “a” for black image formation. Hereinafter, the four optical paths are distinguished and expressed as “a”, “b”, “c”, and “d”, respectively.

  On the other hand, the positions of the second lenses 10a to 10d having power in the sub-scanning direction Z are different for each optical path, and the imaging magnification in the sub-scanning direction Z of the scanning optical system in each optical path is different for each optical path. ing. As a result, the degree of freedom in designing each optical element constituting the scanning optical system 20 is increased.

  FIG. 3 shows a developed state of the optical path “a” in the laser scanning apparatus according to the first embodiment. Similarly, FIG. 4 shows a developed state of the optical path “b”. Similarly, FIG. 5 shows a developed state of the optical path “c”. Similarly, FIG. 6 shows a developed state of the optical path “d”. In each of these drawings, the plane and fixed mirrors are omitted, and are drawn so as to be optically equivalent to the original optical system. In each figure, (A) is a cross-sectional view in the main scanning direction Y, (B) is a cross-sectional view in the sub-scanning direction Z, and (C) is a cross-sectional view in the sub-scanning direction Z of the light source unit optical system.

  Tables 1 to 7 below show construction data of each optical element in the optical path “a”. Tables 8 and 9 show construction data of each optical element in the optical path “b”, and the surfaces 1 to 8 have the same shape as the optical element in the optical path “a”. Tables 10 and 11 show construction data of each optical element in the optical path “c”, and the surfaces 1 to 8 have the same shape as the optical element in the optical path “a”. Tables 12 and 13 show construction data of each optical element in the optical path “d”, and the surfaces 1 to 8 have the same shape as the optical element in the optical path “a”. In addition, the coefficient which is not described in Tables 1 to 13 is 0.

  In these construction data, the surface shape of the axisymmetric aspheric surface is expressed by the following equation (1).

  Further, the surface shape of the free-form surface is expressed by the following equation (2).

  FIG. 7 shows the positions of the aperture conjugate images 13a to 13d in the laser scanning apparatus according to the first embodiment, and is superimposed on the developed optical path in the Y cross section in the main scanning direction after the apertures 3a to 3d. Aperture conjugate images 13a to 13d shown here are images in the sub-scanning direction Z section at a deflection angle corresponding to the center of the image. In the first embodiment, the imaging magnification in the sub-scanning direction Z of the scanning optical system is made different for each optical path. However, the apertures 3a to 3d are set to the optimum positions for each optical path as described below. The positions of the aperture conjugate images 13a to 13d are aligned.

  In FIG. 7, the positions of the window glasses 11a to 11d are aligned at one place. Actually, the positions of the window glasses 11a to 11d are different in each optical path. However, since the window glasses 11a to 11d are flat plates having no power, even if only the position is changed without changing the angle, the optical action is achieved. Does not change.

  FIG. 8 shows the change of the beam diameter in the sub-scanning direction Z accompanying the defocusing of the optical system in the laser scanning apparatus of the first embodiment. The beam diameter is a numerical value calculated at a deflection angle corresponding to the center of the image. Since the positions of the aperture conjugate images 13a to 13d are close to the photoreceptors 12a to 12d, the beam diameter changes in the sub-scanning direction Z at the time of defocusing are different between the minus side and the plus side. In this case, since the aperture conjugate images 13a to 13d are generated on the minus side, the beam diameter is reduced when defocused to the minus side.

  The reason why the aperture conjugate images 13a to 13d are generated in the vicinity of the photosensitive members 12a to 12d is that the imaging magnification in the sub-scanning direction Z of the scanning optical system is low, and this imaging magnification is low. This is to narrow the light flux width in the direction Z. In the first embodiment, since four laser beams are scanned at different angles in the sub-scanning direction Z, the first folding mirrors 8a to 8a are required unless the light flux width of the portion until the optical path is separated is reduced. The optical path separation at 8d becomes impossible. Therefore, the imaging magnification in the sub-scanning direction Z of the scanning optical system is preferably 0.9 or less.

  The optical path can also be separated by increasing the angle difference between the optical paths. In this case, however, the jitter due to the surface tilt of the polygon mirror 6 and the pitch unevenness due to the error in the distance between the axial surfaces of the polygon mirror 6 are large. As a result, the image quality deteriorates.

  In the main scanning direction Y, the position of the aperture conjugate image is far away from the photoconductors 12a to 12d, so that asymmetry due to the defocus direction does not occur.

  FIG. 9 shows changes in the beam diameter in the sub-scanning direction Z accompanying defocusing of the optical system in the laser scanning apparatus according to the first embodiment in curves a to d (same as in FIG. 8), and curve b as a comparative example. ', C', d 'are shown. Curves b ′, c ′, and d ′ as comparative examples are the same as the optical path “a” in the cross section in the main scanning direction Y with respect to the positions of the apertures 3b to 3d in the optical paths “b”, “c”, and “d”. It is changed as follows.

  Table 14 below shows the optical path length from the aperture to the cylinder lens, the optical path length from the aperture conjugate image to the photosensitive member in the sub-scanning direction Z, and the sub-scanning optical system in the first example and the comparative example. The imaging magnification in the scanning direction Z is shown. The movement of the aperture is along the principal ray direction of each optical path. Accordingly, the height of the aperture in the sub-scanning direction Z is different in each optical path.

Table 14 also shows the value of t _k / t, which is 1.00 in the first embodiment. Here, the number of photoconductors is n, and the distance along the principal ray from the aperture conjugate image to the photoconductor by the optical system installed between the aperture and the photoconductor at the deflection angle corresponding to the center of the image is t _k. (1 ≦ k ≦ n), and t of the t _k corresponding to the optical path with the lowest imaging magnification in the sub-scanning direction Z. In this case, it is desirable that the value of t _k / t is 1, that is, the optical path length from the aperture conjugate image to the photosensitive member in the sub-scanning direction Z is equal, but the value of t _k / t is 0.9. To 1.1, it is practical.

  By the way, in the comparative example, the change of the beam diameter is different between the optical paths. Each optical path is assigned to a different color in the tandem laser scanning device, and the behavior of the beam diameter change at the time of defocus is different. For example, the beam diameter changes differently at a temperature change. This means that the gradation changes differently for each color, and the color of the color image changes.

  FIG. 10 shows the positions of the aperture conjugate images 13b ', 13c', 13d 'in the comparative example. Since the apertures of the optical paths “b”, “c”, and “d” are moved to the same position as the optical path “a”, the positions of the conjugate images 13b ′, 13c ′, and 13d ′ are shifted from the positions of the conjugate images 13a. . It can be understood from FIG. 9 that the asymmetry of the beam diameter change increases as the position of the conjugate image approaches the photoconductor.

(Refer 2nd Example and FIGS. 11-13)
FIG. 11 shows a schematic configuration of a tandem type laser scanning apparatus according to the second embodiment of the present invention. In this laser scanning device, the configuration of the optical system after the polygon mirror 6 is the same as that in the first embodiment. Therefore, in the second embodiment, the same members as those in the first embodiment are denoted by the same reference numerals, and redundant description is omitted.

  In the second embodiment, each optical path is superposed in the main scanning direction Y by the free-form curved mirror array 14 instead of the plane mirror array 4 and the cylinder lens 5 in the first embodiment, and the polygon mirror 6 in the sub-scanning direction Z. Concentrating at the same height above.

  Tables 15 and 16 shown below show construction data of each optical element in the optical path “a”, and the optical elements after the surfaces 1 to 3 and the polygon mirror 6 have the same shape as in the first embodiment. Tables 17 and 18 show construction data of each optical element in the optical path “b”, and the optical elements after the surface 1 to surface 3 and the polygon mirror 6 have the same shape as in the first embodiment. Tables 19 and 20 show construction data of each optical element in the optical path “c”, and the optical elements after the surfaces 1 to 3 and the polygon mirror 6 have the same shape as in the first embodiment. Tables 21 and 22 show construction data of each optical element in the optical path “d”, and the optical elements after the surfaces 1 to 3 and the polygon mirror 6 have the same shape as the first embodiment.

  FIG. 12 shows the positions of the aperture conjugate images 13a to 13d in the laser scanning apparatus according to the second embodiment, although the positions are different from the aperture conjugate images 13a to 13d in the first embodiment shown in FIG. In the second embodiment, there is no difference between the optical paths.

Table 23 below shows the optical path length from the aperture to the free-form curved mirror array, the optical path length from the aperture conjugate image to the photoconductor in the sub-scanning direction Z, and the sub-scanning direction of the scanning optical system in the second embodiment. The imaging magnification at Z (which is 0.9 or less in each optical path and is different) is shown. Table 23 also shows the value of t _k / t. In the second embodiment, the value of t _k / t is desirable 1.00.

  FIG. 13 shows changes in the beam diameter in the sub-scanning direction Z accompanying the defocusing of the optical system in the laser scanning apparatus according to the second embodiment. Since the positions of the aperture conjugate images 13a to 13d are aligned between the optical paths, there is no difference in the change in the beam diameter between the optical paths.

(Refer 3rd Example and FIGS. 14-17)
FIG. 14 shows a schematic configuration of a tandem type laser scanning apparatus according to a third embodiment of the present invention. This laser scanning device is configured such that the polygon mirror 6 is disposed substantially at the center, and two laser beams are incident on the polygon mirror 6 from the left and right. The optical system is configured in a mirror image state with respect to a plane passing through the rotation center axis of the polygon mirror 6 and parallel to the center axis of the photoconductors 12a to 12d. The other configuration is the same as that of the first embodiment. In the third embodiment, the same members as those of the first embodiment are denoted by the same reference numerals, and redundant description is omitted.

  Tables 24 to 28 shown below show construction data of each optical element in the optical path “a”, and the surfaces 1 to 3 have the same shape as the first embodiment. Tables 29 to 31 show construction data of each optical element in the optical path “b”. Surfaces 1 to 3 have the same shape as the first example, and surfaces 5 to 8 are optical paths in the third example. It has the same shape as the optical element “a”. The optical paths “c” and “d” are mirror images of the optical paths “a” and “b”, and data description is omitted.

  FIG. 15 shows an optical path in a cross section in the sub-scanning direction Z of the laser scanning apparatus according to the third embodiment. In FIG. 15, only the optical paths “a” and “b” are shown, and the optical paths “c” and “d” having a mirror image relation are omitted. In the third embodiment, a difference is provided in the optical path length from the second lenses 10a to 10d to the photoconductors 12a to 12d for convenience of arrangement of the folding mirrors 8a to 8d and 9b to 9c. This is effective for reducing the thickness of the apparatus. In the third embodiment, the optical path lengths from the window glasses 11a to 11d to the photoconductors 12a to 12d are set to be the same between the optical paths.

  FIG. 16 shows the positions of the aperture conjugate images 13a to 13d in the laser scanning apparatus according to the third embodiment. In the third embodiment, in order to make the imaging magnification in the sub-scanning direction Z uniform in the entire system from the semiconductor lasers 1a to 1d to the photoconductors 12a to 12d, the focal lengths of the cylinder lenses 5a to 5d are made different for each optical path. ing.

  That is, cylinder lenses 5a to 5d having power in the sub-scanning direction are disposed in the optical path between the apertures 3a to 3d and the polygon mirror 6, and between the cylinder lenses 5a to 5d and the apertures 3a to 3d. No optical element having power is disposed in the optical path, and the distance along the principal ray from the apertures 3a to 3d to the cylinder lenses 5a to 5d is the distance between the polygon mirror 6 and the photoconductors 12a to 12d. The optical path with a smaller absolute value of the imaging magnification in the sub-scanning direction is set longer. Thus, the positions of the apertures 3a to 3d are appropriately set, and the positions of the aperture conjugate images 13a to 13d are aligned.

Table 32 below shows the optical path length from the aperture to the cylinder lens, the optical path length from the aperture conjugate image to the photoconductor in the sub-scanning direction Z, and the sub-scanning direction Z of the scanning optical system in the third embodiment. The imaging magnification is shown. Table 32 also shows the value of t _k / t. In the third embodiment, the value of t _k / t is desirable 1.00.

  FIG. 17 shows changes in the beam diameter in the sub-scanning direction Z accompanying the defocusing of the optical system in the laser scanning apparatus according to the third embodiment. Since the positions of the aperture conjugate images 13a to 13d are aligned between the optical paths, there is no difference in the change in the beam diameter between the optical paths.

(Other examples)
The tandem laser scanning device according to the present invention is not limited to the above-described embodiments, and various modifications can be made within the scope of the gist thereof.

  In particular, the configuration of the light source unit and the configuration of the optical system after the polygon mirror are arbitrary.

1 is a perspective view showing a schematic configuration of a first embodiment of a laser scanning device according to the present invention. It is an optical path figure in the subscanning direction cross section of 1st Example. The developed state of the optical path “a” of the first embodiment is shown, (A) is a cross section in the main scanning direction, (B) is a cross section in the sub scanning direction, and (C) is a cross section in the sub scanning direction of the light source unit optical system. . FIG. 2B shows a developed state of the optical path “b” in the first embodiment, where (A) is a cross section in the main scanning direction, (B) is a cross section in the sub scanning direction, and (C) is a cross section in the sub scanning direction of the light source unit optical system. . FIG. 3A shows a developed state of the optical path “c” of the first embodiment, where FIG. 5A is a cross section in the main scanning direction, FIG. 5B is a cross section in the sub scanning direction, and FIG. . The developed state of the optical path “d” of the first embodiment is shown, (A) is a cross section in the main scanning direction, (B) is a cross section in the sub scanning direction, and (C) is a cross section in the sub scanning direction of the light source unit optical system. . It is explanatory drawing which shows the position of the aperture conjugate image in 1st Example. It is a graph which shows the change of the beam diameter of the subscanning direction accompanying the defocus in 1st Example. It is a graph which shows the change of the beam diameter of the subscanning direction accompanying the defocus in a 1st Example and a comparative example. It is explanatory drawing which shows the position of the aperture conjugate image in a comparative example. It is a perspective view which shows schematic structure of 2nd Example of the laser scanning apparatus based on this invention. It is explanatory drawing which shows the position of the aperture conjugate image in 2nd Example. It is a graph which shows the change of the beam diameter of the subscanning direction accompanying the defocus in 2nd Example. It is a perspective view which shows schematic structure of 3rd Example of the laser scanning apparatus based on this invention. It is an optical path figure in the subscanning direction cross section of 3rd Example. It is explanatory drawing which shows the position of the aperture conjugate image in 3rd Example. It is a graph which shows the change of the beam diameter of the subscanning direction accompanying the defocus in 3rd Example.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1a-1d ... Semiconductor laser 2a-2d ... Collimator lens 3a-3d ... Aperture 4 ... Planar mirror array 5, 5a-5b ... Cylinder lens 6 ... Polygon mirror 7 ... Scanning 1st lens 8a-8d, 9a-9d ... Folding mirror DESCRIPTION OF SYMBOLS 10 ... Scanning 2nd lens 12a-12d ... Photoconductor 13a-13d ... Aperture conjugate image 14 ... Free-form surface mirror array

Claims (3)

At least two light sources;
One deflector for deflecting light from the light source;
A first optical system for condensing light from the light source only in the sub-scanning direction in the vicinity of the deflector;
At least two apertures disposed in an optical path between the light source and the deflector for restricting light from each of the light sources;
A tandem type laser scanning device comprising: a second optical system that guides each light deflected by the deflector to each of at least two photoconductors;
In the second optical system, the imaging magnification in the sub-scanning direction between the deflector and the photoconductor is 0.9 or less corresponding to each of the photoconductors, and is different from each other.
An optical element having power in the sub-scanning direction is disposed in the optical path between the aperture and the deflector,
No optical element having power is disposed in the optical path between the optical element and the aperture,
The distance along the principal ray from the aperture to the optical element is longer as the optical path having a smaller absolute value of the imaging magnification in the sub-scanning direction between the deflector and the photoconductor,
A tandem type laser scanning device characterized by the above. The tandem type laser scanning device according to claim 1, wherein the first optical system is installed in common with respect to the at least two light sources. The tandem-type laser scanning device according to claim 1, wherein the first optical system is individually installed with respect to the at least two light sources.

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