JP3402875B2 - Optical scanning device - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical scanning device used in an image forming apparatus such as a laser printer or a laser facsimile.

[0002]

2. Description of the Related Art Generally, an optical scanning device of this type deflects a light beam from a laser light source by a polygon mirror,
An image is formed as a light spot on the surface to be scanned by the image forming lens system.

A semiconductor laser or the like is often used as a laser light source, and divergent light emitted from the laser light source is converted into a substantially parallel light beam by a collimator lens, and the outer shape of the light beam is limited by the aperture. The light beam whose outer shape is limited is deflected by a polygon mirror that rotates at a constant angular velocity and enters the imaging lens system. The imaging lens system has an fθ characteristic that a light beam deflected at a constant angular velocity scans a surface to be scanned arranged at a predetermined interval at a constant velocity, and forms a minute light spot over the entire scanning area. Thus, it is necessary that the field curvature be well corrected.

Further, since the polygon mirror has processing errors on the mirror surface and vibrations of the rotation axis, many image forming lens systems correct the scanning position deviation in the direction perpendicular to the main scanning direction, that is, in the sub-scanning direction. A tilt correction function is provided for this purpose. Therefore, the imaging lens system is an anamorphic lens system having different imaging characteristics in the main scanning direction and the sub-scanning direction, and at least one surface of these imaging lens systems is a toric surface having sufficient accuracy. And is required to be a cylindrical surface.

Conventionally, an image forming lens system is processed by a glass material so as to have a toric surface and a cylindrical surface, and an antireflection film is applied to this kind of glass lens by vapor deposition or the like. On the other hand, since processing of a glass lens is difficult and expensive, a synthetic resin lens that is low in cost and capable of correcting aberrations in a free shape has been widely used in recent years.

[0006]

However, it is necessary to apply an antireflection film to a synthetic resin lens as in the case of a glass lens. However, when applying an antireflection film to a synthetic resin lens, a gas is removed from the synthetic resin lens. However, there is a problem in that the synthetic resin lens is thermally deformed and the film is easily peeled off. Therefore, the synthetic resin lens is currently used without suppressing the reflection loss.

Further, since the reflection loss increases / decreases in accordance with the incident angle of the light beam, there arises a problem that the light amount changes depending on the scanning position. Therefore, in the image forming apparatus using this type of optical scanning device, the density of the image changes depending on the scanning position, which makes it difficult to reproduce a high-definition and faithful image.

An object of the present invention is to solve the above-mentioned problems and to provide an optical scanning device which can make the light amount distribution on the surface to be scanned uniform by using an inexpensive synthetic resin lens having no antireflection film. To provide.

[0009]

An optical scanning device according to the present invention for achieving the above object comprises a deflecting means for deflecting a light beam from a light source means, and a light beam from the deflecting means on a surface to be scanned. In the optical scanning device including an image forming lens system for forming an image on the optical axis, the image forming lens system includes at least one synthetic resin lens, and at least one of the synthetic resin lenses has a transmittance at the optical axis. It is characterized in that it is set to a predetermined value or less.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described in detail with reference to the illustrated embodiments. FIG. 1 is a main part configuration diagram of the first embodiment, in which a cylindrical lens 2 and a polygon mirror 3 are sequentially arranged in a traveling direction of a light beam from a light source unit 1 including a semiconductor laser 1a, a collimator lens 1b, and an aperture 1c. The imaging lens 4 and the surface to be scanned 5 are arranged in the traveling direction of the light beam which is arranged in the direction of the arrow and is deflected by the polygon mirror 3. The surface to be scanned 5 is the surface of the photoconductor when the optical scanning device of the present embodiment is installed in an image forming apparatus using an electrostatic copying mechanism or the like.

The collimator lens 1b is an anamorphic lens having a positive power only in the direction perpendicular to the paper surface, that is, in the sub-scanning direction, and the divergent light from the semiconductor laser 1a is made into a substantially parallel light beam. Aperture 1
The symbol c limits the outer shape of the light beam that has passed through the collimator lens 1b, and the cylindrical lens 2 focuses the light beam only in the sub-scanning direction and forms an image on the polygon mirror 3 as a focal line parallel to the paper surface. There is. Polygon mirror 3
Is arranged so as to have a reflection point near the focal line of the cylindrical lens 2 and is rotated in the direction A at a constant angular velocity to deflect the light beam.

The image forming lens 4 has a so-called fθ characteristic for forming an image of a light beam, which is deflected and incident at a constant angular velocity in the main scanning direction, on the surface to be scanned 5 as a light spot moving at a constant velocity. The image forming lens 4 is provided with a so-called tilt correction function for forming an image forming relationship between the mirror surface of the polygon mirror 3 on which the light beam condensed in the sub-scanning direction is incident and the surface to be scanned 5. . In order to satisfy this tilt correction function with one lens, the imaging lens 4 is an anamorphic aspherical lens made of synthetic resin.

Here, since the optical path length passing through the imaging lens 4 differs depending on the angle of view, the amount of light lost due to internal loss also changes depending on the angle of view. This change becomes more remarkable as the internal loss per unit length increases. Become. Therefore, in the imaging lens 4 of this embodiment, by setting the internal loss to an appropriate amount,
The change in reflection loss is corrected. Therefore, when the imaging lens 4 is manufactured, a synthetic resin material having a low transmittance is used, or an appropriate amount of dye or fine particles is mixed with the synthetic resin material to set the internal loss to an optimum value. To be done.

Table 1 below shows the relationship between the light spot position, that is, the reflectance and transmittance of the imaging lens 4 with respect to the scanning position, and the light amount change rate at the scanning position. It shows how much it changes with the amount of light on the optical axis.

[0015]     Table 1 Scanning position and light amount change rate in the first embodiment                   First side Second side Scan Position Incident Angle Reflectance Incident Angle Reflectance Transmittance Change     mm degree% degree %%%    105.8 49.9 11.7 8.1 4.4 84.4 -7.9     70.6 38.7 7.8 13.5 4.8 87.8 -4.2     45.9 27.6 5.8 11.7 4.6 89.9 -2.0      0.1 0.0 4.2 0.0 4.2 91.7 0.0    -46.7 -27.9 5.8 -11.9 4.6 89.8 -2.0    -83.5 -43.2 9.1 -13.1 4.7 86.6 -5.5   -105.4 -49.7 11.6 -9.2 4.5 84.5 -7.9

Here, the transmittance does not include internal loss. Further, the surface to be scanned 5 can print over a length of 210 mm, and the scanning position is 0 mm on the optical axis, the scanning start side is negative, and the scanning end side is positive. Furthermore, the incident angle is not the angle of the light beam with respect to the optical axis, but the angle of the light beam with respect to the surface normal of the position where the light beam enters the imaging lens 4. Since the light beam which is the laser light is linearly polarized, the reflection and refraction at the imaging lens 4 are performed only by the S-polarized component.

The reflection loss Rs at the imaging lens 4 under such conditions is expressed by the following equation. Rs ＝ {− sin (θ ₁ −θ ₂ ) / sin (θ ₁ + θ ₂ )} ²・ ・ ・ (1)

Where θ ₁ is the incident angle with respect to the surface normal, θ
₂ is the refraction angle with respect to the surface normal. Since the relationship between the incident angle θ ₁ and the refraction angle θ ₂ follows Snell's law, the refraction angle θ ₂ is omitted in Table 1 and the refractive index is 1.52.

As can be seen from Table 1, the incident angle of the imaging lens 4 largely changes on the first surface and changes from 0 degree on the optical axis to about 50 degrees at the outermost part. On the other hand, the reflection loss, that is, the reflectance increases from about 4% on the optical axis to about 12% at the peripheral portion. Therefore, when a conventional lens is used as the imaging lens 4, the amount of light when the scanning position is on the optical axis is
The amount of light when the scanning position is in the peripheral portion is reduced by about 8%. Therefore, in the conventional lens, the scanned surface 5
It becomes impossible to make the above light amount uniform, and it becomes impossible to obtain uniform image density in an image forming apparatus equipped with a conventional optical scanning device.

On the other hand, Table 2 below shows the rate of change in the amount of light, which is the internal loss of the imaging lens 4 with respect to the scanning position, for each transmittance on the optical axis.

[0021]     Table 2 Internal loss of the imaging lens in the first example Scan position Optical path length Optical path length ratio Transmittance on optical axis%                                     90 85 80 75 70     mm mm Light intensity change rate%    105.8 4.134 0.376 1.27 4.63 8.31 12.33 16.78     70.6 9.333 0.848 0.31 1.11 1.96 2.86 3.84     45.9 10.498 0.954 0.09 0.33 0.59 0.85 1.14      0.1 11.000 1.000 0.00 0.00 0.00 0.00 0.00    -46.7 10.520 0.956 0.09 0.32 0.56 0.82 1.09    -83.5 8.423 0.766 0.47 1.71 3.04 4.46 5.99   -105.4 5.031 0.457 1.10 4.02 7.18 10.64 14.43

Here, the transmittance on the optical axis includes internal loss,
The rate of change in light quantity does not include surface reflection. Further, the transmittance is related to the internal loss, the surface reflection component is irrelevant, but it is difficult to measure only the transmittance,
In addition to the internal loss, the total surface reflection loss of the first surface and the second surface is 8%, and the transmittance is represented. Therefore, a transmittance of 80% on the optical axis means a total of 12% of internal loss and 8% of surface reflection loss.

If an internal loss of Fo% occurs in the reference optical path length of the imaging lens 4, the internal loss Fk at the optical path length ratio K is given by the following equation. Fk = {1- (1-Fo / 100) ^k } × 100 (2)

As shown in Table 2, as the absolute value of the scanning position increases, the rate of change of the light quantity increases positively and the light quantity increases. On the other hand, the conventional transmittance on the optical axis is 90%.
With the above lenses, the rate of change in light quantity is only a little over 1%. Therefore, in this embodiment, the transmittance on the optical axis is 85% or less, preferably 75 to 85%.

The light beam emitted from the semiconductor laser 1a is collimated by the collimator lens 1b, the outer shape is limited by the aperture 1c, and the cylindrical lens 2 forms a linear image on the polygon mirror 3. The light beam deflected by the polygon mirror 3 is imaged as a light spot on the scanned surface 5 by the imaging lens 4. At this time, when the transmittance on the optical axis of the imaging lens 4 is 85%, the rate of change in the amount of light in the peripheral portion of the scanning position is corrected from minus 8% in Table 1 to a little over 4% in Table 2. As a result, the rate of change is suppressed to minus 4%.

As described above, in the first embodiment, when the on-axis transmittance of the imaging lens 4 is 85 to 75%, the light amount change rate on the scanned surface 5 is as shown in Table 1. Maximum value −
7.9% is the maximum value for each transmittance shown in Table 2 4.63%
Or corrected by 8.31% or 12.33% and finally 4
It is possible to suppress the change in the light amount of about%.

Therefore, in the first embodiment, a synthetic resin lens having no antireflection film can be used as the imaging lens 4. Further, even if the incident angle of the light beam incident on the imaging lens 4 changes, the light amount distribution at the scanning position can be made uniform, and high-definition and faithful image reproduction becomes possible.

FIG. 2 is a block diagram of the essential parts of the second embodiment.
Instead of the imaging lens 4 of the first embodiment, a first lens 6
And the second lens 7 is arranged. The first lens 6 is a concave synthetic resin lens, and the transmittance on the optical axis is 85% or less as in the first embodiment. The second lens 7 is a convex glass lens or a synthetic resin lens. The polarization direction of the semiconductor laser 1a is set so that the reflection and refraction by the first lens 6 and the second lens 7 are performed by the P-polarized component. Here, the reflection loss Rp due to the P-polarized component is expressed by the following equation. Rp = {tan (θ ₁ −θ ₂ ) / tan (θ ₁ + θ ₂ )} ²・ ・ ・ (3)

In general, the reflection loss in the S-polarized component increases as the incident angle increases, but the reflection loss in the P-polarized component Rp
Decreases until it reaches a certain angle of incidence, then once increases to 0 and then increases again. This incident angle is called Brewster's angle, and the Brewster's angle of a medium having a refractive index of 1.5 placed in the air is about 56 degrees. However, the incident angle rarely exceeds the Brewster's angle in the imaging lens system, and in the synthetic resin lens system using the P-polarized component,
Contrary to the case of using the S-polarized component shown in the embodiment, the reflection loss Rp decreases and the amount of light on the scanned surface 5 increases as the angle of view increases. However, in the second embodiment, this increase in the amount of light is solved by providing the concave first lens 6 with a correction function.

That is, since the optical path length of the first lens 6 which is a concave synthetic resin lens is longer in the peripheral portion than on the optical axis, the internal loss of the light beam passing through the peripheral portion of the first lens 6 is reduced. Grows larger. For this reason, in the second embodiment using the P-polarized component, an increase in the amount of light in the peripheral portion due to an increase in the incident angle is used, and a concave synthetic resin lens is used for the first lens 6, and the peripheral portion is used. The surface to be scanned 5 due to an increase in transmission loss of
It is possible to suppress the change in the amount of light above.

FIG. 3 is a block diagram of the essential parts of the third embodiment.
The light beam that has passed through the imaging lens 4 of the first embodiment is focused by the folding mirror 8 and then imaged on the scanned surface 5. Further, the amount of light on the surface to be scanned 5 is suppressed based on the reflection characteristic of the folding mirror 8. In this case, whether the amount of light increases or decreases according to the angle of view is not generally determined depending on the film configuration of the folding mirror 8. However, when the light amount decreases according to the angle of view, a convex synthetic resin lens with low transmittance is used for the imaging lens 4, and when the light amount increases according to the angle of view, the concave synthetic resin lens is used. A lens may be used.

When a synthetic resin material is mixed with a dye or fine particles in order to increase the internal loss of the synthetic resin lens, flare due to diffusion may occur in the light spot. Therefore, it is desirable to use a dye that does not cause diffusion in the synthetic resin material.

[0033]

As described above, in the optical scanning device according to the present invention, at least one of the image forming lens systems is provided with the synthetic resin lens, and at least one of the synthetic resin lenses has an optical axis. Since the transmittance is set to the predetermined value or less, a predetermined amount of internal loss can be given to the synthetic resin lens. Therefore, the light amount distribution on the surface to be scanned can be made uniform without applying an antireflection film to the synthetic resin lens, and a high-definition and high-quality image can be provided. Also,
Since a synthetic resin lens can be used and the work of depositing the antireflection film can be omitted, the cost can be reduced.

[Brief description of drawings]

FIG. 1 is a configuration diagram of a main part of a first embodiment.

FIG. 2 is a main part configuration diagram of a second embodiment.

FIG. 3 is a configuration diagram of a main part of a third embodiment.

[Explanation of symbols]

1 Light source unit 3 polygon mirror 4 Imaging lens 5 Scanned surface 6 first lens 7 Second lens 8 folding mirror

Claims (2)

(57) [Claims] 1. An optical scanning device comprising: a deflecting means for deflecting a light beam from a light source means; and an image forming lens system for forming an image of the light beam from the deflecting means on a surface to be scanned. An optical scanning device characterized in that the lens system includes at least one synthetic resin lens, and the transmittance of at least one of the synthetic resin lenses is set to a predetermined value or less. 2. The optical scanning device according to claim 1, wherein the transmittance along the optical axis is 85% or less.