JP4616735B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to an optical scanning device included in an image forming apparatus such as a digital copying machine or a laser printer, and more particularly to an apparatus having a function of switching a print dot density.

  In an optical scanning device that scans a plurality of laser beams, a technique for switching the print dot density is known. For example, there is a configuration in which an afocal anamorphic zoom lens is mounted (see, for example, Patent Document 1). With this zoom lens, the print dot density is switched by changing the imaging magnification in the sub-scanning direction between the light source and the rotary polygon mirror. In addition, an axially symmetric optical system is removably moved in the optical path, and there is a configuration in which the interval between a plurality of beams and the beam are switched by this insertion / extraction (see, for example, Patent Document 2).

JP-A-57-54914

Japanese Patent Laid-Open No. 10-206762

  In any of the systems disclosed in Patent Documents 1 and 2, the spot diameter changes simultaneously with the switching of the print dot density. That is, when the printing dot density is increased, the spot diameter is decreased.

  In addition, each optical component in the optical scanning device has manufacturing errors, adjustments, and placement errors, and the scanning surface may not always coincide with the focal point. When the focus is shifted from the scanning plane, the spot diameter increases. When there is the same defocus, the smaller the spot diameter at the focal position, the greater the rate of change of the spot diameter at the time of defocus. For this reason, depending on the amount of defocus and the print dot density to be switched, the higher the print dot density, the larger the spot diameter.

  Hereinafter, a change in the spot diameter at the time of defocus will be specifically described. For example, it is assumed that a light source having a wavelength of 488 nm is used and 480 dpi is obtained when two lenses are not inserted into the optical axis, and 600 dpi when inserted.

  If the focal position deviation between the scanning surface and the scanning spot is 0, both the scanning interval and the scanning spot diameter are about 53 μm at 480 dpi and about 42 μm at 600 dpi. However, if the focal position of the scanning surface and the scanning spot is shifted by 5 mm due to an adjustment error or the like, the scanning interval becomes the same value as described above, but the scanning spot diameter is about 79 μm and 600 dpi at 480 dpi according to the following formula. Sometimes about 85 μm.

  For this reason, the spot diameter of 600 dpi, which is originally a small spot diameter, becomes a large spot diameter. When the spot diameter is increased, the line width and the like are increased, and good image quality cannot be obtained.

  On the other hand, from the viewpoint of variation in error, the influence becomes greater as the spot diameter becomes smaller. For this reason, when the spot diameter is reduced, there is a variation in line width that is thick at a certain scanning position and appropriate at a certain scanning position (unstable image quality). At this time, the overall printing impression is deteriorated.

  Originally, when switching to high density, it is preferable to reduce the spot diameter according to the print dot density. However, when the spot diameter is reduced, the error variation is affected as described above.

  For this reason, it is conceivable to make the spot diameter constant even when the printing density is changed so as not to be affected by any influence.

  The present invention has a problem that when the defocusing on the photosensitive member occurs when the high dot density is smaller than the low dot density, the high dot density becomes larger than the low dot density. It is an object of the present invention to solve this problem and to provide an image forming apparatus capable of stable printing.

  In order to solve the above problems, in the present invention, a plurality of light sources capable of switching the modulation frequency and the amount of light, a collimator lens that makes light from the light sources substantially parallel rays, and a rotating polygon mirror that deflects and scans the rays from the light source And an image forming apparatus having an optical scanning device that inserts and removes two lenses, which are a concave lens and a convex lens, on the optical axis between the light source and the rotating polygon mirror according to the modulation frequency of the light source. It is arranged between the light exit surface of the lens and the rotary polygon mirror so that the light quantity of the light source is increased when the modulation frequency of the light source is high.

  With the configuration of the present invention, even when the printing dot density, that is, the scanning interval is switched, the spot diameter and the light amount do not change, and even if there is a defocus, the high dot density printing is stable without a larger spot diameter. An image forming apparatus capable of printing was obtained.

  FIG. 2 shows an example of an image forming apparatus when the present invention is applied. A drum-shaped photoreceptor 18 that is a recording medium for forming a toner image is driven to rotate. The photosensitive member 18 is uniformly charged to a specific polarity by the charging device 10 and then exposed to a laser beam from an optical scanning device 11 described later to form an electrostatic latent image.

  A developing device 12 is disposed downstream of the exposure position. The toner image on the photosensitive member 18 developed by the developing device 12 is charged on the back surface of the printing paper 13 conveyed by the conveying device 14 with a polarity opposite to that of the toner by the transfer device 15. The upper toner image is transferred onto the printing paper 13.

  After the transfer, the printing paper 13 is peeled off from the photoreceptor 18 by a peeling means (not shown), and the toner that has not been transferred is removed by the cleaning device 16.

  On the other hand, the printing paper 13 peeled off from the photoreceptor 18 is conveyed to the fixing device 17. The fixing device 17 includes a heat roll that is heated and a pressure roller that is in pressure contact with the heat roll. Therefore, the toner image held on the printing paper 13 is melted under pressure and fixed by the action of pressure. The printing paper 13 that has been subjected to the fixing process is discharged outside the image forming apparatus, assuming that the image formation has been completed.

  FIG. 3 shows the configuration of the optical scanning device 11 of FIG. A light beam 21 emitted from a plurality of laser light sources 20 is collimated by a collimator lens 22, passes through a cylindrical lens 23 having a predetermined curvature only in the scanning direction, is deflected and scanned by a rotary polygon mirror 25 through a spherical lens 24, The light is reflected by the folding mirror 29 via the Fθ lens 26 and formed on the photosensitive member 18 (not shown) to form an electrostatic latent image. A part of the light beam deflected and scanned is incident on the sensor 28 by the mirror 27, and modulation for writing light beams emitted from the plurality of laser light sources 20 is started by an output signal from the mirror 27.

  Next, an optical system between the plural laser light sources 20 and the rotary polygon mirror 25 is shown in FIGS. In the figure, the case of 4-beam scanning is shown.

  FIG. 4 is a sectional view in the scanning direction. One of the light beams emitted from the plurality of laser light sources 20 is illustrated. The light beam that has passed through the collimator lens 22 has its beam width widened by the cylindrical lens 23 and the spherical lens 24, and reaches the rotating polygonal mirror 25 as a parallel light beam. If the focal length of the cylindrical lens 23 is a and the focal length of the spherical lens 24 is b, the distance between the principal points of the cylindrical lens 23 and the spherical lens 24 may be a + b. At this time, the beam width is expanded b / a times.

  FIG. 5 is a sectional view in the sub-scanning direction. One of the light beams emitted from the multiple laser light sources 20 is illustrated. The light beam that has passed through the collimator lens 22 passes through the cylindrical lens 23 and is condensed on the rotary polygon mirror 25 by the spherical lens 24. When the focal length of the spherical lens 24 is b, the distance between the spherical lens 24 and the rotary polygon mirror 25 is b.

  FIG. 6 is a cross-sectional view in the scanning direction. The principal rays (center lines) 37 of four rays out of the rays emitted from the plurality of laser light sources 20 are illustrated. After passing through the collimator lens 22, the four principal rays intersect once with each other and become parallel with the cylindrical lens 23, and intersect with each other on the rotating polygonal mirror 25 with the spherical lens 24. If the focal length of the collimator lens 22 is c and the focal length of the cylindrical lens 23 is a, the distance between the collimator lens 22 and the cylindrical lens 23 may be c + a.

  FIG. 7 shows a switching lens group. In this example, the lens group includes two cylindrical lenses having the same cross-sectional shape in the scanning direction, and is mounted on a base 30 described later. The insertion / removal direction of this lens is set to the generatrix direction of the cylindrical lens. The switching lens set has a two-lens configuration. The focal length of the first lens 32 is d, the focal length of the second lens 33 is e, the principal point distance of the two lenses is L, and the two switching lens sets. Is 480 dpi when removed from the optical axis, and 600 dpi when inserted into the optical axis, d, e, and L satisfying equations (1) and (2) may be used.

-E / d = 600/480 (1)
L = e + d ……………………… (2)
Note that the value of L is preferably adjusted and fixed so as to absorb manufacturing errors in the focal lengths of the first lens 32 and the second lens 33. If d = −80 mm, e = 100 mm and L = 20 mm are obtained from the equations (1) and (2). If this lens is inserted, the print dot density can be switched from 480 dpi to 600 dpi. If the scanning interval at 600 dpi is 42.3 μm and the spot diameter in the sub-scanning direction is 42.3 μm, the scanning interval at 480 dpi is 52.9 μm and the spot diameter is 52.9 μm.

  FIG. 1 shows an embodiment of the present invention. The position where the switching lens set is inserted / removed is preferably after the collimator lens 22 as shown in FIG. This is because this position is parallel light in both the scanning direction and the sub-scanning direction as shown in FIG. 4 and FIG. The feature of the present invention is that the slit 31 is mounted between the switching lens set and the rotary polygon mirror 25, and the light quantity of the light source is increased when the modulation frequency of the light source 20 is high. As a result, even if the printing dot density, that is, the scanning interval is switched, the scanning spot diameter and the amount of scanning light do not change. .

  Specific examples are shown below. 600 dpi when the switching lens set is inserted and 480 dpi when the switching lens set is removed. If the light beam width in the sub-scanning direction when the switching lens group is removed is 4 mm, if the sub-scanning direction width of the slit 31 is 4 mm, even if there is a 5 mm focal position shift as shown in the problem, The spot diameter at 480 dpi is 79 μm, and the same spot diameter as 79 μm can be obtained even at 600 dpi. This is because the relationship between the spot diameter and the beam width has the following relationship.

d = 4 × λ × f / (π × D) (3)
d is the spot diameter, λ is the wavelength, f is the focal length of the lens, π is the circumference, and D is the beam width.

  From the above equation (3), if the beam width at 480 dpi is 4 mm and the beam width is 4 mm at 600 dpi, the spot diameter is the same.

  There is no problem because the optical system magnification is changed by the switching lens and the scanning interval is appropriately changed. When the modulation frequency is high, that is, when the printing dot density is high, the amount of light blocked by the slit 31 is larger than when the modulation frequency is low. Therefore, it is necessary to increase the light amount of the light source main body by the same amount as the blocked amount.

  For example, in the above case, if the amount of light passing through the slit 31 at 480 dpi is 100%, light spreads by two lenses at 600 dpi, so 20% of light is blocked by the slit. Therefore, at 600 dpi, the amount of light passing through the slit 31 is about 80% of that at 480 dpi. Therefore, what is necessary is just to control so that the light quantity of a light source main body may be enlarged 20%. Actually, since it is also affected by the transmittance of the two lenses, it is only necessary to adjust the light amount of the light source so that the light amount coming out from the slit 31 does not change even when the switching lens set is inserted and removed.

  In this embodiment (FIG. 1), the slit 31 is mounted between the cylindrical lens 23 and the first and second lenses. However, the slit 31 may be provided between the cylindrical lens 23 and the rotary polygon mirror 25. There is a similar effect.

  In the above embodiment, 600 dpi is set when the switching lens set is inserted, and 480 dpi is set when the switching lens group is not inserted. However, the same effect as the above embodiment can be obtained even if the reverse is set. Further, the present invention is not limited to the combination of 600,480 dpi.

It is a figure which shows the laser light source apparatus of this invention. 1 is a diagram illustrating a configuration of an image forming apparatus. It is a figure which shows the structure of an optical scanning device. It is sectional drawing of the scanning direction of the lens system of an optical scanning device. It is sectional drawing of the subscanning direction of the lens system of an optical scanning device. It is sectional drawing of the scanning direction of the lens system of an optical scanning device. It is a figure which shows a switching lens group.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 ... Charging device, 11 ... Optical scanning device, 12 ... Developing device, 13 ... Printing paper, 14 ... Conveying device, 15 ... Transfer device, 16 ... Cleaning device, 17 ... Fixing device, 18 ... Photoconductor, 20 ... Multiple lasers Light source, 21 ... light beam, 22 ... collimator lens, 23 ... cylindrical lens, 24 ... spherical lens, 25 ... rotating polygon mirror, 26 ... Fθ lens, 27 ... mirror, 28 ... sensor, 29 ... folding mirror, 30 ... base, 31 ... slit, 32 ... first lens, 33 ... second lens, 35 ... light ray, 36 ... light ray, 37 ... principal ray.

Claims (1)

A plurality of light sources capable of switching the modulation frequency and the amount of light, a collimator lens that makes light from the light sources substantially parallel rays, a rotating polygon mirror that deflects and scans the light rays from the light source, and light between the light source and the rotating polygon mirror In an image forming apparatus having an optical scanning device that inserts and removes two lenses consisting of a concave lens and a convex lens on the axis according to the modulation frequency of the light source,
An image forming apparatus, wherein a slit is disposed between a light exit surface of the two lenses and a rotary polygon mirror, and the light amount of the light source is increased when the modulation frequency of the light source is high.

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