JP2009258577A - Optical scanning device and image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To determine and improve a ghost effect resulting from conjugation of a rotary polygon mirror and a scanned surface with respect to a ghost image by double reflection which is likely to appear in an optical scanning device with higher-definition. <P>SOLUTION: The optical scanning device includes one or two or more light sources 1 emitting luminous fluxes; a first optical system 2 which couples the luminous fluxes from the light sources; a second optical system 3 which collects the luminous flux from the first optical system into a substantially linear shape long in a main scanning direction; a rotary polygon mirror 4 which has a deflective reflection surface in the vicinity of the substantially linear convergence part and deflects the luminous flux by the deflecting reflection surface; a scanned surface 7; and a third optical system 5 which has one or two or more lenses and converges the luminous flux deflected by the rotary polygon mirror onto the scanned surface as an optical spot. When a predetermined condition is satisfied by single reflection each from different two faces 5a and 5b among optical surfaces of the lenses 5 within the third optical system which act as reflective surfaces in general optical spot convergence, reflection reducing coating is applied to one or both of the different two surfaces. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an optical scanning device, and an image forming apparatus such as a digital copying machine, a laser printer, an optical plotter, a facsimile machine, or a multifunction machine having these mounted thereon.

  The light beam from the light source device is deflected by the optical deflector, and the deflected light beam is condensed toward the surface to be scanned by the scanning optical system disposed after the light deflector. An optical scanning apparatus that forms an optical spot and optically scans a surface to be scanned with the optical spot is used in an image forming apparatus such as a digital copying machine, a laser printer, an optical plotter, a facsimile machine, or a complex machine thereof. The technology is also widely known.

Here, what constitutes the actual state of the surface to be scanned is, for example, a photosensitive image carrier (for example, a photoreceptor) having photoconductivity.
As an image forming apparatus, an image carrier made of, for example, a drum-shaped photosensitive member is arranged facing a transfer medium (transfer paper, intermediate transfer member, etc.), and emitted from a light source device corresponding to the image carrier. The light beam is deflected by one optical deflector, the image carrier is exposed by a scanning optical system corresponding to the image carrier, and latent images are formed. These latent images are visualized with a developer, and then a visible image is formed. An image can be obtained by transferring and fixing to a transfer medium.
In addition, as a full-color image forming apparatus, for example, four image carriers made of a drum-shaped photoconductor are arranged in the transport direction of a transfer medium (transfer paper, intermediate transfer member, etc.), and these image carriers are arranged. A light beam emitted from a plurality of light source devices corresponding to the body is deflected by one optical deflector, and each image carrier is simultaneously exposed by a plurality of scanning optical systems corresponding to each image carrier to form a latent image. After visualizing these latent images with different color developers such as yellow, magenta, cyan, black, etc., these visible images are transferred onto the same transfer medium one after another, and fixed. An image can be obtained.

An object of the present invention is to improve a “ghost image” that is one factor that degrades image quality in an image forming apparatus that forms a monochrome image or a color image as described above.
Here, the “ghost image” is an image carrier made up of a photoconductor that is the surface to be scanned when the light beam is reflected by an optical surface of a scanning lens, a metal part for holding the scanning lens, or the like. In addition to the latent image for the purpose of image formation, it refers to a light spot that causes a latent image unrelated to the image to be formed.
Usually, the light beam that forms a “ghost image” is mostly a light beam reflected by a metal part or the like for holding a scanning lens or the like. Therefore, in most cases, an extremely simple method is adopted in which reflection is suppressed by taking measures such as coloring the above-described metal parts with a color that is difficult to reflect, so that a “ghost image” cannot be formed.

However, since the reflected light beam from the optical surface of the scanning lens is generated on the optical surface that should originally be transmitted, it is not possible to adopt a simple method that has been taken with metal parts or the like.
For example, as a problem in Patent Document 1, there is a problem of a ghost image caused by a single reflection on an optical surface to be originally transmitted, such as a soundproof glass and a scanning lens.
Therefore, Patent Documents 1 to 4 propose means for solving the problem of the ghost image caused by the single reflection on the optical surface.

JP 2006-235213 A JP 2006-20371 A JP 2004-272230 A JP 2007-017915 A

As described above, various means for solving the problem of the ghost image due to the single reflection on the optical surface have been proposed. Conventionally, the ghost image due to the double reflection on the optical surface to be originally transmitted is Due to the low reflectivity of the optical surface, the energy was low and this was not a problem.
For example, the reflectance from the optical surface of a scanning lens made of a medium having a refractive index of 1.5 is about 4% (it slightly varies depending on the light incident angle on the surface and the polarization state of light). The energy of the ghost image reflected once from the optical surface has an energy of about 4% as compared with a normal light spot. Compared to this, the energy of the ghost image of the double reflection is about 0.16%, and the energy is much weaker than that of the single reflection.
However, a ghost image caused by double reflection, which did not cause this problem, may become a problem due to the recent high definition of the image forming apparatus.

  The present invention has been made in view of the above circumstances, and a rotating polygon mirror and a surface to be scanned are used for a ghost image by double reflection (particularly conspicuous for those including a biconvex lens) that is likely to appear in an optical scanning device in which high definition is advanced. Provides a means to "determine" and "improve" the effects of ghosts caused by the conjugation of the ghosts, and by taking this measure, foreseeing ghost problems without trial production, In order to provide an optical scanning device that can solve the problem with the above measures, an image forming device that can perform high-definition image formation by mounting the optical scanning device is provided. For the purpose.

In order to achieve the above object, the present invention employs the following solutions.
The first means of the present invention includes one or a plurality of light sources that emit a light beam, a first optical system that couples the light beam from the light source, and a light beam from the first optical system that is long in the main scanning direction. A second optical system that condenses linearly, a rotary polygon mirror that has a deflecting / reflecting surface in the vicinity of the substantially linear condensing portion, deflects a light beam by the deflecting / reflecting surface, a surface to be scanned, and 1 Alternatively, in the optical scanning device having a third optical system that has a plurality of lenses and condenses the light beam deflected by the rotary polygon mirror as a light spot on the surface to be scanned, the optical surface of the lens in the third optical system In the normal light spot condensing, among the two different surfaces, when the following condition (Equation 1) is satisfied by one-time reflection from two different surfaces among the surfaces acting as refractive surfaces, 1 or 2 with anti-reflection coating .

  Here, λ is the wavelength, Dpi is the pixel density in the main scanning direction, ω 0 is the beam spot diameter at the beam waist position of the light beam composed of the reflected light, and ΔX is the distance from the beam waist position.

  According to a second means of the present invention, in the optical scanning device of the first means, the beam spot diameter ω 0 at the beam waist position of the light beam made of the reflected light and the beam spot diameter at a distance ΔX from the beam waist position. For ωg, the following conditional expression is satisfied.

  The third means of the present invention includes one or a plurality of light sources that emit a light beam, a first optical system that couples the light beam from the light source, and a light beam from the first optical system that is long in the main scanning direction. A second optical system that condenses linearly, a rotary polygon mirror that has a deflecting / reflecting surface in the vicinity of the substantially linear condensing portion, deflects a light beam by the deflecting / reflecting surface, a surface to be scanned, and 1 Alternatively, in the optical scanning device having a third optical system that has a plurality of lenses and condenses the light beam deflected by the rotary polygon mirror as a light spot on the surface to be scanned, the optical surface of the lens in the third optical system In the normal light spot condensing, when the following conditions (formula 1 ′) are satisfied by one-time reflection from two different surfaces among the surfaces acting as refracting surfaces, the two different surfaces are used. Of these, one or two surfaces are provided with anti-reflection coating To.

Here, λ is the wavelength, Dpi is the pixel density in the main scanning direction, ω 0 is the beam spot diameter at the beam waist position of the light beam composed of the reflected light, and ΔX is the distance from the beam waist position.
N1 and n2 indicate the refractive indexes of the medium before and after each of the two different surfaces, and R1 and R2 indicate the reflectances at the respective surfaces.

  According to a fourth means of the present invention, in the optical scanning device of the third means, the beam spot diameter ω 0 at the beam waist position of the light beam composed of the reflected light and the beam spot diameter at a distance ΔX from the beam waist position. For ωg, the following conditional expression is satisfied.

According to a fifth means of the present invention, in the optical scanning device according to any one of the first to fourth means, the third optical system has at least one lens whose main scanning cross-sectional shape is a biconvex surface. And
According to a sixth means of the present invention, in the optical scanning device of any one of the first to fifth means, the combined refractive power due to the reflection of the specific two surfaces in the sub-scanning section of the third optical system. Satisfies the following conditions.
(Synthetic refractive power by reflection of two specific surfaces) ≧ 0

According to a seventh means of the present invention, in the optical scanning device according to any one of the first to sixth means, a microstructure having a anti-reflection function is provided two-dimensionally instead of the anti-reflection coating. Features.
According to an eighth means of the present invention, in the optical scanning device according to any one of the first to seventh means, the lens in the third optical system has a rough surface outside the effective diameter. To do.
According to a ninth means of the present invention, in the optical scanning device according to any one of the first to seventh means, the lens in the third optical system is a coating having an anti-reflection effect on the surface outside the effective diameter. It is characterized by giving.

According to a tenth means of the present invention, in the optical scanning device according to any one of the first to ninth means, the light source can be rotationally adjusted around the light emitting direction.
According to an eleventh means of the present invention, in the optical scanning device according to any one of the first to tenth means, the third optical system includes two or more lenses and a plurality of plane mirrors. Features.
The twelfth means of the present invention is the optical scanning device according to any one of the first to eleventh means, wherein at least one lens in the third optical system has a surface whose main scanning cross-sectional shape is non-arc. It is characterized by having.
Furthermore, the thirteenth means of the present invention is the optical scanning device of any one of the first to twelfth means, wherein at least one lens in the third optical system has a free-form main scanning sectional shape. It has a certain surface.

  According to a fourteenth aspect of the present invention, there is provided an image forming apparatus including the optical scanning device according to any one of the first to thirteenth means.

According to the present invention, for ghosts that are likely to appear in optical scanning devices that are becoming higher in definition (especially those that include biconvex lenses), the effects of ghosts caused by the conjugate of the rotating polygon mirror and the surface to be scanned are reduced. By showing the means to “determine” and “improve”, it is possible to foresee the ghost problem without making a prototype, and to solve the problem with the minimum necessary measures, thus reducing the cost of the product. Can contribute.
Further, the present invention increases the degree of freedom of layout of a high-definition scanning optical system, and in particular, it is possible to employ a biconvex lens that has a low performance deterioration rate with respect to decentration, so that high-quality and compact optical scanning is possible. It becomes possible to provide the configuration of the apparatus.
By mounting this optical scanning device, it is possible to realize an image forming apparatus capable of forming a high-definition image.

Hereinafter, the configuration, operation, and effect of the present invention will be described in detail with reference to the drawings.
An optical scanning device mounted on an image forming apparatus deflects a light beam by an optical deflector such as a rotating polygon mirror (also called a polygon mirror), and the light beam moves on an image carrier (for example, a drum-shaped photosensitive member). Further, an image is formed by further rotating the photoconductor. The moving direction of the beam by this optical deflector is called the main scanning direction, and the moving direction by the rotation of the photosensitive member is called the sub-scanning direction.

Here, the configuration of the scanning optical system in which the ghost image caused by the double reflection described in the above-mentioned problem column is a problem will be described below using an example.
FIG. 1 shows a state in which light from the light source 1 is deflected by the rotating polygon mirror 4 and converges on the photoconductor 7 that is the surface to be scanned by the scanning lens 5.
In the figure, for the sake of simplicity, only the principal ray of each light beam is shown by a solid line, but actually, a light beam having a thickness is incident on the rotary polygon mirror 4 as partially shown by the one-dot chain line in FIG. And converge on the photoreceptor 7.

Now, as shown in FIG. 2, the principal ray shown in FIG. 1 is reflected by the second surface 5b of the scanning lens 5, then reflected by the first surface 5a, and converges on the photosensitive member 7 (this). In the present invention, the relationship between the position where the light beam is reflected on the reflecting surface of the rotary polygon mirror 4 and the light spot condensing position is called conjugate by reflection of the optical surface), and a ghost image is formed. . In particular, when a biconvex lens as shown in FIGS. 1 and 2 is used as the scanning lens 5, the combined power due to reflection is always positive, so this conjugate relationship is easily established.
As described above, the energy of each beam is 0.16% if the reflectance of each surface is 4%. However, since this ghost image forms a ghost image at the same position as the number of spots in one light spot row originally formed on the photosensitive member (in the main scanning direction), the above 0.16% is superimposed. It is thought that it is a ghost image of the generated energy.

  However, since the main scanning cross-sectional shape of a recent scanning lens is actually an aspherical surface or a free-form surface, the reflection position of the rotary polygon mirror 4 and the scanned surface 7 are conjugate from the viewpoint of ray aberration. 2, it is unlikely that a light beam deviating from the center of the scanned surface (light beam outside the optical axis) converges to the same position as the light beam near the optical axis, and only the light beam near the optical axis as shown in FIG. 3. Is expected to converge. In this case, the first surface 5a of the scanning lens 5 in FIG.

  Even though only the light flux near the optical axis converges, a considerable number of spots are formed at substantially the same position in a high-definition image forming apparatus such as 1200 dpi (dots per inch) and 2400 dpi in recent years. The relationship between the intensity of the ghost light and the range in which the intensity is strong is the conditional expressions shown in the first to fourth means of the above-described solving means.

  Usually, when a Gaussian-distributed laser beam is narrowed by a lens, the allowable beam depth on the optical axis can be determined as follows. First, the beam spot diameter ω around the position where the beam is focused is expressed by the following Equation 3.

Here, λ represents a beam wavelength, ω 0 represents a beam waist, and X represents a distance from the beam waist position to a position having a predetermined beam spot diameter (ω). Further, the definition of the beam diameter represents a diameter having an intensity of 1 / e ² of the beam profile. Here, if the beam diameter allowable for the beam waist ω0 is ωg, the depth ΔX that allows the beam diameter can be set to ω = ωg and X = ΔX in Equation 3, and in Equation 4 below, Can be represented.

  In Equation 4, (ωg / ω0) is the rate of change in beam diameter, and at the same time, the square of the reciprocal is the rate of change in beam center intensity.

  Equation 2 is a ghost image reflected twice on the 4% reflecting surface (energy is 0.0016 times that of the original beam), and main scanning is performed at a position (light intensity ω0 / ωg) away from the beam waist position by ΔX. The structure in which the ghost is particularly problematic is that the energy when the ghost image is formed at the same position by the resolution in the direction is 4% or more of the original beam. Here, ω 0 represents the beam waist of the ghost light that is reflected twice by the reflecting surface and reaches the surface to be scanned, and ω g represents the beam spot diameter at a position separated by ΔX from the position of the beam waist, and the beam of the ghost light At the waist position, the central intensity of the light spot of the ghost light is the strongest. The energy of the ghost image, which is usually a problem, cannot be generally described because it varies depending on the energy of the light beam forming the ghost image and the material of the photoreceptor, but at least 4% in view of the problem of the ghost image caused by the conventional single reflection. Care should be taken when there is more energy.

Now, the above equation 1 is derived from the above equation 2.
0.0016 × (ω0 / ωg) ² × Dpi ≧ 0.04
↓
0.0016 × Dpi / 0.04 ≧ (ωg / ω0) ²
↓
0.04 × Dpi ≧ (ωg / ω0) ² (Formula 5)
Is derived by substituting Equation 5 into Equation 4.

  Now, this ghost image is a problem, but if the design value is at hand, it is considered to investigate from there, or to investigate the light spot position by experiment. However, if the configuration of the image forming apparatus is difficult to check by experiment, the shape may be checked by measuring the shape near the optical axis as shown in FIG. At this time, it is sufficient to examine the vicinity of the optical axis within a range of about ± 5 mm from the optical axis.

  After the problem surface is identified by such a method, the energy of the ghost image becomes less than 4% by applying the anti-reflection coating to only one or two of the two problem surfaces. This method does not require any anti-reflection coating on all lenses, especially when the number of lenses increases, and contributes to cost reduction. Further, since it is not necessary to avoid the condition of the above formula 1 in the layout of the optical scanning device, it is possible to contribute to an improvement in design freedom.

Further, as a technique other than coating, there is no problem even if a fine structure as described in Patent Document 2 is provided on the optical surface.
By using the present invention, there is no restriction on the layout of optical components as in Patent Document 3.
Moreover, it is not necessary to use a special member like patent document 4 by using this invention.

As another method, the light intensity of the ghost image can be weakened by spreading the light spot of the ghost image on the photosensitive member without converging in the sub-scanning direction. In order to implement this, it is desirable that the combined power of at least the two specific surfaces is 0 or more.
Here, FIG. 4 shows a sub-scanning sectional shape of the scanning optical system of FIG. Since the scanning lens 5 has a bi-convex cross-sectional shape, the scanning lens 5 has positive power in the sub-scanning cross section. In FIG. 4, the optical path for forming a normal beam spot is indicated by a solid line.
In the sub-scan section shown in FIG. 4, the position of reflection by the deflecting reflection surface of the rotary polygon mirror and the photoconductor 7 are normally set conjugate. Therefore, when passing through the scanning lens 5 having a positive power, the light flux is Converge on the photoreceptor. Therefore, when the light is reflected by two surfaces having a total power of 0 or more, it converges once before the photosensitive member and then diverges.
When the first surface 6a passes through a reflection surface having a negative power, such as the second scanning lens 6 of the scanning optical system shown in FIG. 9A, the reflection from the rotary polygon mirror 4 by reflection as compared with the normal optical path. Although the optical path length to the convergence point has increased, the distance from the rotary polygon mirror 4 to the convergence point also increases due to the negative power, so there is a possibility of convergence.

  In addition, there is a method of roughening the outside of the effective diameter. For example, as shown in FIG. 9B, if the area of the thick line of the scanning lens (first scanning lens) 5 is roughened or painted black, even if ghost light converges on the photosensitive member, much energy is consumed. -Can be weakened.

Further, by rotating the light source 1, the polarization direction of the laser beam can be changed to change the reflectance on the optical surface in the third optical system. Since this makes it possible to weaken the ghost light, it is desirable that the light source 1 be configured to be rotatable around, for example, an arrow (light emission direction) emitted from the light source in FIG.
Further, by using a laser diode array (LDA) for the light source 1, it is possible to realize high speed and high density. At that time, in order to obtain a desired sub-scanning beam pitch on the photosensitive member surface, it is necessary to incline the LDA. At this time, the polarization direction becomes nearly parallel to the main scanning direction, and the same effect as described above can be obtained. it can. Of course, a surface emitting LD (VCSEL) may be used as the light source.

  By using a method other than the coating described above, there is a possibility that the problem of ghost light can be solved without applying the anti-reflection coating (of course, it is more effective if several methods are used in combination).

[Example 1]
Hereinafter, more specific embodiments of the optical scanning device of the present invention will be described.
FIG. 5 is a schematic block diagram of an optical scanning device showing a first embodiment of the present invention.
This optical scanning device includes a laser diode (hereinafter referred to as LD) that is a light source 1, a coupling lens 2 that makes a divergent light beam from LD1 a parallel light beam, and light that spreads in a direction perpendicular to the paper surface of FIG. And a rotating polygon mirror 4 as an optical deflector, a scanning lens 5 as an fθ lens, and a photoconductor 7 as a surface to be scanned. If the optical scanning device is mounted on an image forming apparatus, an image can be obtained by developing an optical latent image on a photosensitive member with a developing device and fixing the image on a transfer medium such as transfer paper (image). A configuration example of the forming apparatus will be described later).
As indicated by the alternate long and short dash line in FIG. 5, a light beam having a thickness is actually reflected by the rotary polygon mirror 4 and converged on the photoconductor 7 by the scanning lens 5. By the rotation of the rotary polygon mirror 4, this convergent light beam scans on the photosensitive member in the main scanning direction.

FIG. 6 is a diagram illustrating only the principal ray of the light beam in the scanning optical system of the optical scanning device shown in FIG. 5 in order to show the convergence of the ghost light in an easily understandable manner.
As shown in FIG. 6, on the second surface 5b of the scanning lens 5, the light beam is normally refracted and travels toward the photoreceptor 7, but is partially reflected (the amount of reflected light follows Fresnel's law). Head to the first surface 5a. The light reflected by the first surface 5a of the scanning lens 5 has a conjugate relationship between the “reflection position of the rotating polygon mirror 4” and the “photosensitive member (scanned surface) 7” due to the reflection of the first surface 5a and the second surface 5b. If it is, it goes to the center of the surface to be scanned (the center of the photosensitive member in the main scanning direction in the figure).

  If the refractive index of the scanning lens 5 is 1.5, the maximum reflectance of each surface is 4%. Therefore, the illuminance on the photosensitive member of the ghost image of each beam is 0.16% compared to a normal light spot. Degree. However, since a ghost image is formed at the same position as the spot that scans the photoconductor 7 in a row, the illuminance on the photoconductor is superimposed and the ghost image appears remarkably.

If the resolution in the main scanning direction of this optical scanning device is 1200 dpi, a maximum of 1200 light spots are formed on the photoconductor 7 per inch (2.54 cm), so that a light spot is formed on the photoconductor in one scan. If the range to do is 30 cm,
1200 × 30 / 2.54≈14173 ghost images are formed at the same position. When this is 2400 dpi, 28346 ghost images that are twice that are formed at the same position. (Because there is actually a ray aberration, it cannot be shot at the same position, but it is a guideline for considering the relationship between the resolution and the illuminance of the ghost image.)

In the first embodiment, for example, by applying the anti-reflection coating to the second surface 5b of the scanning lens 5, the reflectance of the second surface is reduced to, for example, 0.1%, so that the light energy of one ghost image is normally reduced. It is possible to reduce the light spot to 0.004% (0.001 × 0.04), which is very effective.
Of course, the anti-reflection coating is provided on both the first surface 5a and the second surface 5b of the scanning lens 5 so that the light energy of one ghost image is 0.0001% (0.001) with respect to a normal light spot. If it is reduced to (× 0.001), there is almost no possibility of generating a remarkable ghost image.
Further, if a range in which the intensity of the ghost image is strong is obtained from Equation 1 described in the first means, and it is not necessary to apply a coating or a fine structure to the optical surface, it is more effective not to implement it.

In this embodiment, the refractive index is 1.5. However, the refractive index of the plastic lens actually used in the scanning lens is about 1.58. At this time, the reflectance of the optical surface is about 5.1%. At this time, as in the above description, when formulas 1 and 2 are assembled,
Equation 2 takes the form:

よって式１は以下の形になる。

Therefore, Formula 1 has the following form.

  In this way, in consideration of the change in reflectance due to the refractive index of an optical member such as a scanning lens, Expressions 1 and 2 are expanded to Expressions 1 'and 2' below.

However, n1 and n2 indicate the refractive indexes of the medium on the two specific surfaces before and after each surface, and R1 and R2 indicate the reflectance on each surface.
For example, in the case of the scanning lens 5 of Example 1, n1 is air (n1 = 1) or a medium having a refractive index of 1.5 and n2 is the same for each surface of the scanning lens.
Further, if it is difficult to measure the refractive index, there is no problem even if R is directly measured.

[Example 2]
Next, Example 2 has the configuration of Example 1, and includes two scanning lenses as the third optical system. Depending on the layout, there is of course no problem in miniaturizing the entire optical scanning device by inserting a folding mirror (not shown) between the first scanning lens and the second scanning lens and between the second scanning lens and the photosensitive member. .
A specific configuration of the second embodiment is shown in FIG. The scanning lens 5 on the rotary polygon mirror side is a first scanning lens, and the scanning lens 6 on the photoreceptor side is a second scanning lens. The first surface 6a of the second scanning lens 6 is a free-form surface. As in the first embodiment, a thick light beam indicated by a one-dot chain line is actually reflected on the rotary polygon mirror 4 and converged on the photosensitive member. In FIG. 8 showing a ghost image, the path of the ghost light is easy to see. As shown, only the chief ray is shown.

As shown in FIG. 8, a ghost image is formed at the center of the surface to be scanned by the first reflection on the first surface 6a of the second scanning lens 6 and the second reflection on the first surface 5a of the first scanning lens 5. The
As described above, since the first surface 6a of the second scanning lens 6 is a free-form surface, even if the reflecting surface of the rotary polygon mirror 4 and the photoconductor 7 are conjugated in the vicinity of the optical axis, the first scanning lens in FIG. The light beam that passes through the periphery of the light beam 5 does not travel toward the photoreceptor 7 even if it is reflected by the first surface 5 a of the first scanning lens 5, but travels in a direction that does not occur.
In the second embodiment, as in the first embodiment, the resolution in the main scanning direction and the illuminance (superimposed) of the ghost image are substantially proportional.

In Example 2, since there are two scanning lenses, there are four surfaces that can be coated. However, by applying the anti-reflection coating to the first surface 5a of the first scanning lens 5, the light energy of one ghost image can be reduced, so that a significant ghost may occur with a minimum number of coating surfaces. Can be dropped. Of course, by coating both the first surface 5a of the first scanning lens 5 and the first surface 6a of the second scanning lens 6, the possibility of further ghosting can be reduced.
As described in the eleventh means described above, when there are two or more lenses, the combination of two possible faces as the specific two faces described in the second means is one lens (one pattern). Therefore, the present invention is particularly effective when the number of lenses is two or more.

As described above, for the ghost (especially conspicuous for the one including a biconvex lens) that is likely to appear in an optical scanning device with higher definition, the reflecting surface of the rotary polygon mirror 4 and the photosensitive member (scanned surface) 7 are conjugated. A means to "determine" and "improve" the effects of ghosts.
This makes it possible to foresee the ghost problem without making a prototype, and to solve the problem with the minimum necessary measures, thereby contributing to the cost reduction of the product.
In addition, the present invention increases the degree of freedom of layout of a high-definition scanning optical system, and in particular, a biconvex lens having a low performance deterioration rate with respect to decentration (as can be seen from FIG. It is easy to produce a ghost, and it is difficult to adopt a high-definition optical scanning device unless the determination / improvement method as in the present invention is used. I can expect. That is, since the biconvex lens distributes positive power to each surface, the declination angle of the light beam on one surface is small, and the influence of decentering of the first surface and the second surface is small. For this reason, the scanning optical system can be reduced in size by adopting a biconvex lens having a weak decentration effect even with the same power.

[Example 3]
Next, a configuration example of the image forming apparatus according to the present invention will be described.
FIG. 10 is a schematic configuration diagram of an image forming apparatus showing an embodiment of the present invention, and schematically shows an example of a laser printer. This laser printer has a photoconductive photosensitive member formed in a drum shape as the image carrier 7. Around the photoreceptor 7, a charging roller 10 as a charging unit, a developing device 30, a transfer roller 40 as a transfer unit, and a cleaning device 50 are arranged. In this embodiment, the contact-type charging roller 10 with less ozone generation is used as the charging means, but a corona charger using corona discharge can also be used as the charging means. Further, as the latent image forming unit, the optical scanning device 20 having the configuration described in the first and second embodiments is provided, and the photosensitive member 7 that is a scanned surface is disposed between the charging roller 10 and the developing device 30. On the other hand, exposure is performed by optical scanning of the laser beam LB. In FIG. 10, reference numeral 60 denotes a paper feed cassette, reference numeral 61 denotes a paper feed roller, reference numeral 62 denotes a registration roller, reference numeral 63 denotes a fixing device, reference numeral 64 denotes a paper discharge roller, and reference numeral 65 denotes a paper discharge tray.

  When forming an image, the photosensitive member 7 is rotated at a constant speed in the clockwise direction, the surface thereof is uniformly charged by the charging roller 10, and the electrostatic latent image is exposed by light writing by the laser beam LB of the optical scanning device 20. Is formed. The formed electrostatic latent image is a so-called “negative latent image”, and the image portion is exposed. This electrostatic latent image is reversed and developed by the developing device 30, and a toner image is formed on the photoreceptor 7. The paper feed cassette 60 containing transfer paper is detachable from the main body of the image forming apparatus, and the uppermost one of the stored transfer paper is fed by the paper feed roller 61 when mounted as shown in the figure. The transferred transfer paper is fed to the registration roller 62 at the leading end. The registration roller 62 feeds the transfer paper to the transfer unit at the same timing as the toner image on the photoconductor 7 moves to the transfer position. The transferred transfer paper is superimposed on the toner image at the transfer portion, and the toner image is electrostatically transferred by the action of the transfer roller 40. The transfer paper on which the toner image has been transferred is fixed on the toner image by the fixing device 63, passes through the conveyance path, and is discharged onto the paper discharge tray 65 by the paper discharge roller 64. Further, after the toner image is transferred, the surface of the photoreceptor 7 is cleaned by the cleaning device 50 to remove residual toner, paper dust, and the like.

  In the image forming apparatus configured as described above, the latent image is formed on the photosensitive member 7 by using the optical scanning device described in the first and second embodiments as the latent image forming unit, and developed by the developing device 30 to be visualized. Therefore, it is possible to form a high-quality image that is not affected (or reduced) by the ghost image. Therefore, it is possible to realize an image forming apparatus that has excellent resolution and high definition and high reliability.

Example 4
FIG. 11 is a schematic configuration diagram of an image forming apparatus according to another embodiment of the present invention. This image forming apparatus is an example of a tandem type full-color laser printer.
In FIG. 11, a transport belt 41 that transports transfer paper (not shown) fed from a paper feed cassette 60 disposed in the horizontal direction is provided on the lower side of the apparatus. The conveyor belt 41 is stretched around a plurality of rollers 42 and 43. Above the conveyor belt 41, a yellow (Y) photoreceptor 7Y and a magenta (M) photoreceptor 7M are image carriers. The cyan (C) photoconductor 7C and the black (K) photoconductor 7K are arranged at equal intervals in order from the upstream side in the transport direction. In the following, yellow, magenta, cyan, and black are distinguished by Y, M, C, and K in the code.

The photoconductors 7Y, 7M, 7C, and 7K that are image carriers are all formed to have the same diameter, and process members that perform image formation according to the electrophotographic process are sequentially disposed around the photoreceptors. Taking the yellow photoreceptor 3Y as an example, a charging charger 10Y as a charging means, an optical scanning device 20Y, a developing device 30Y, a transfer charger 40Y as a transfer means, a cleaning device 50Y, and the like are sequentially arranged. The same applies to the other photoconductors 7M, 7C, and 7K.
That is, this image forming apparatus uses the photoconductors 7Y, 7M, 7C, and 7K as scanning surfaces set for the respective colors, and a pair of optical scanning devices 20Y, 20M, 20C, and 20K is provided for each of them. 1 correspondence relationship.

  As these optical scanning devices, those having the configurations described in the first and second embodiments can be used independently, or the optical deflector (rotating polygon mirror) 4 is shared and each scanning optical device is used. The scanning lens 5 in the system can be shared for optical scanning of the photoconductors 7M and 7Y, and another scanning lens 5 can be shared for optical scanning of the photoconductors 7K and 7C.

  Around the conveyance belt 41, a registration roller 62 and a belt charging charger 44 are provided on the upstream side of the photoreceptor 7Y, and a belt separation charger 45 and a charge removal charger are provided on the downstream side of the photoreceptor 7K. 46, a belt cleaning device 47 and the like are provided. A fixing device 63 is provided downstream of the belt separation charger 45 in the transport direction, and is connected to a paper discharge tray 65 by a paper discharge roller 64.

  In the image forming apparatus having such a configuration, for example, in the full color mode, each of the optical scanning devices 20Y is based on the image signals of Y, M, C, and K for each of the photoreceptors 7Y, 7M, 7C, and 7K. , 20M, 20C, and 20K, an electrostatic latent image is formed. These electrostatic latent images are developed with toners of the corresponding developing devices 30Y, 30M, 30C, and 30K to form toner images, which are sequentially transferred onto transfer paper that is electrostatically attracted onto the transport belt 41 and transported. Are superimposed. The four color toner images transferred onto the transfer paper in a superimposed manner are fixed as a full color image by the fixing device 63, and the fixed transfer paper is discharged onto a discharge tray 65 by a discharge roller 64.

  By using the optical scanning apparatus having the configuration described in the first and second embodiments as the image forming apparatus having the above-described configuration and operation, high-quality image formation that is not affected by (or reduced by) a ghost image is performed. Thus, an image forming apparatus capable of forming a high-definition full-color image can be realized.

It is a figure which shows the structural example of the scanning optical system of the optical scanning device in which the ghost image by twice reflection becomes a problem. It is a figure which shows the example in which a ghost image is formed with the scanning optical system shown in FIG. FIG. 2 is a diagram showing an example in which the first surface of the scanning lens is an aspherical surface in the scanning optical system shown in FIG. 1. It is a figure which shows the subscanning cross-sectional shape of the scanning optical system of FIG. 1 is a schematic configuration diagram of an optical scanning device according to a first embodiment of the present invention. FIG. 6 is a diagram illustrating only a principal ray of a light beam for easy understanding of the ghost light convergence in the scanning optical system of the optical scanning device shown in FIG. 5. It is a schematic block diagram of the optical scanning device which shows the 2nd Example of this invention. FIG. 8 is a diagram illustrating only a principal ray of a light beam in the scanning optical system of the optical scanning device illustrated in FIG. 7 in order to easily show the degree of convergence of ghost light. (A) is a figure which shows the subscanning cross-sectional shape of the scanning optical system of FIG. 8, (B) is a figure which shows the area | region outside the effective diameter of a scanning lens. 1 is a schematic configuration diagram of an image forming apparatus showing an embodiment of the present invention. It is a schematic block diagram of the image forming apparatus which shows another Example of this invention.

Explanation of symbols

1: Light source 2: Coupling lens (first optical system)
3: Cylinder lens (second optical system)
4: Polygon mirror (optical deflector)
5: Scanning lens (first scanning lens)
5a: first surface 5b: second surface 6: second scanning lens 6a: first surface 6b: second surface 7: photoconductor (scanned surface)
7Y, 7M, 7C, 7K: photoconductor (image carrier)
10: Charging roller (charging means)
10Y, 10M, 10C, 10K: Charger (charging means)
20: Optical scanning device 20Y, 20M, 20C, 20K: Optical scanning device 30: Developing device 30Y, 30M, 30C, 30K: Developing device 40: Transfer roller (transfer means)
40Y, 40M, 40C, 40K: transfer charger (transfer means)
41: Conveying belt 42, 43: Roller 44: Belt charging charger 45: Belt separation charger 46: Static elimination charger 47: Belt cleaning device 50Y, 50M, 50C, 50K: Cleaning device 60: Paper feed cassette 61: Paper feed roller 62: Registration roller 63: Fixing device 64: Paper discharge roller 65: Paper discharge tray

Claims (14)

One or more light sources that emit luminous flux;
A first optical system for coupling a light beam from the light source;
A second optical system for condensing the light beam from the first optical system in a substantially linear shape long in the main scanning direction;
A rotating polygon mirror that has a deflecting reflecting surface in the vicinity of the substantially linear condensing unit and deflects the light beam by the deflecting reflecting surface;
A scanned surface;
In an optical scanning apparatus having a third optical system that has one or a plurality of lenses and condenses a deflected light beam by the rotating polygon mirror as a light spot on the scanned surface,
On the optical surface of the lens in the third optical system, the following conditions (formula 1) are satisfied by one-time reflection from two different surfaces among the surfaces that act as refracting surfaces in normal light spot condensing. When you are satisfied
An optical scanning device characterized in that one or two of the two different surfaces are provided with an anti-reflection coating.

Here, λ is the wavelength, Dpi is the pixel density in the main scanning direction, ω 0 is the beam spot diameter at the beam waist position of the light beam composed of the reflected light, and ΔX is the distance from the beam waist position. The optical scanning device according to claim 1,
A beam spot diameter ω0 at the beam waist position of the luminous flux composed of the reflected light, and
About the beam spot diameter ωg at a distance ΔX from the beam waist position,
An optical scanning device characterized in that the following conditional expression is satisfied:

One or more light sources that emit luminous flux;
A first optical system for coupling a light beam from the light source;
A second optical system for condensing the light beam from the first optical system in a substantially linear shape long in the main scanning direction;
A rotating polygon mirror that has a deflecting reflecting surface in the vicinity of the substantially linear condensing unit and deflects the light beam by the deflecting reflecting surface;
A scanned surface;
In an optical scanning apparatus having a third optical system that has one or a plurality of lenses and condenses a deflected light beam by the rotating polygon mirror as a light spot on the scanned surface,
On the optical surface of the lens in the third optical system, the following conditions (formula 1 ′) are obtained by one-time reflection from two different surfaces among the surfaces that act as refracting surfaces in normal light spot focusing. )
An optical scanning device characterized in that one or two of the two different surfaces are provided with an anti-reflection coating.

Here, λ is the wavelength, Dpi is the pixel density in the main scanning direction, ω 0 is the beam spot diameter at the beam waist position of the light beam composed of the reflected light, and ΔX is the distance from the beam waist position.
N1 and n2 indicate the refractive indexes of the medium before and after each of the two different surfaces, and R1 and R2 indicate the reflectances at the respective surfaces. The optical scanning device according to claim 3.
A beam spot diameter ω0 at the beam waist position of the luminous flux composed of the reflected light, and
About the beam spot diameter ωg at a distance ΔX from the beam waist position,
An optical scanning device characterized in that the following conditional expression is satisfied:

The optical scanning device according to any one of claims 1 to 4,
The third optical system has at least one lens having a biconvex surface in a main scanning cross section. In the optical scanning device according to any one of claims 1 to 5,
In the sub-scan section of the third optical system, the combined refractive power due to the reflection of the two specific surfaces satisfies the following condition.
(Synthetic refractive power by reflection of two specific surfaces) ≧ 0 The optical scanning device according to any one of claims 1 to 6,
An optical scanning device characterized in that, instead of the anti-reflection coating, a fine structure having an anti-reflection function is provided two-dimensionally. The optical scanning device according to any one of claims 1 to 7,
The lens in the third optical system has a rough surface outside the effective diameter. The optical scanning device according to any one of claims 1 to 7,
An optical scanning device characterized in that the lens in the third optical system is coated on the surface outside the effective diameter with an anti-reflection effect. In the optical scanning device according to any one of claims 1 to 9,
An optical scanning device characterized in that the light source can be rotated and adjusted around a light emitting direction. The optical scanning device according to any one of claims 1 to 10,
The third optical system comprises two or more lenses and a plurality of plane mirrors. The optical scanning device according to any one of claims 1 to 11,
The at least one lens in the third optical system has a surface whose main scanning cross section has a non-arc shape. The optical scanning device according to any one of claims 1 to 12,
At least one lens in the third optical system has a surface whose main scanning cross-sectional shape is a free-form surface.   An image forming apparatus comprising the optical scanning device according to claim 1.

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