JPH11264952A - Optical scanning device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain an optical scanning device which forms a spot shape meeting print conditions on a scanned surface and also changes the spot shape even when the same optical deflector is used. SOLUTION: The optical scanning device which optically scans a surface 5 by guiding the luminous flux emitted by a light source means 1a to the optical deflector 3 after shaping it into nearly parallel luminous flux in horizontal scan section by a 1st optical system 1b and guiding the luminous flux reflected and deflected by the optical deflector 3 to the surface 5 by a 2nd optical system 4 is characterized by that the luminous flux width of the luminous flux generated by the 1st optical system 1b is set wider than the width of the deflection surface of the optical deflector 3 and the reflection factor of the deflecting surface of the optical deflector 3 is set uneven on the deflection surface.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an optical scanning device,
In particular, after the light beam modulated and emitted from the light source means is reflected and deflected by an optical deflector comprising a rotating polygon mirror, optical scanning is performed on the surface to be scanned through scanning optical means having fθ characteristics to record image information. The present invention relates to an optical scanning device suitable for an apparatus such as a laser beam printer or a digital copying machine having an electrophotographic process.

[0002]

2. Description of the Related Art Conventionally, a rotating polygon mirror (hereinafter also referred to as a "polygon mirror") has been used as an optical deflector to reflect and deflect a light beam (light beam) emitted from a light source means such as a semiconductor laser. 2. Description of the Related Art An optical scanning device that optically scans a photosensitive recording medium surface to record image information is widely used in a printer unit such as a laser beam printer or a digital copying machine.

In the case where the light beam is reflected and deflected by the polygon mirror to optically scan the surface to be scanned (recording medium surface), it can be roughly classified into the following two scanning systems.

A scanning method in which scanning is performed using a light beam smaller than the width in the sub-scanning direction of each deflection surface (hereinafter also referred to as "facet") of a polygon mirror.

A scanning method in which a light beam wider than the width of the facet in the main scanning direction is made incident on a polygon mirror to perform scanning.

In the scanning method, the diameter of the light beam reflected and deflected by the polygon mirror is substantially the same as the diameter of the light beam incident on the polygon mirror, and this diameter is equal to the diameter of an aperture provided between the light source and the polygon mirror. ).

In the scanning method, the diameter of the light beam reflected and deflected by the polygon mirror is limited by the width of the facet of the polygon mirror in the main scanning direction, and the light beam emitted outside the facet is scanned by facets before and after the facet. Is reflected and deflected to an invalid area, and then shielded from light.

FIG. 12 is an explanatory diagram showing a state of a light beam reflected and deflected by a polygon mirror in the above-described scanning method.

In FIG. 1, reference numeral 20 denotes an incident light beam incident on a polygon mirror 30 to be described later from a light source unit (not shown), and w denotes the width (diameter) of the incident light beam 20 in the main scanning direction.
It is. Reference numeral 30 denotes a polygon mirror, and a portion corresponding to each side of the regular polygon is a deflection surface (mirror surface), and this deflection surface is referred to as a facet as described above. Reference numeral 31 denotes a facet currently being used to guide a light beam onto the surface of a photosensitive drum (not shown), reference numeral 32 denotes a front side thereof, that is, a facet which has already been scanned, and reference numeral 33 denotes a rear side thereof, that is, a next scanning facet. Facet. Reference numeral 21 denotes a reflected light beam reflected and deflected by the facet 31, and the polygon mirror 30 rotates in the direction of the arrow R around the rotation center C to be deflected and scanned in the direction of the arrow S. a is the width (diameter) of the reflected light beam 21 in the main scanning direction.

As is apparent from FIG. 1, the diameter a of the reflected light beam 21 is limited by the width of the facet, and does not depend on the width w of the light beam 20 incident from a light source unit (not shown).

When the width a of the reflected light flux 21 reflected and deflected by the polygon mirror 30 is set to be the same as the width w of the incident light flux 20, the above-mentioned scanning methods have larger facets than the above-mentioned scanning methods. Therefore, the polygon mirror having the same diameter can have many facets. That is, this means increasing the number of optical scans in one rotation of the polygon mirror, and it can be said that this is a scanning method more suitable for, for example, a high-speed printer or a high-scanning line density printer that requires high-speed scanning. .

[0012]

However, in the scanning method, since the shape of the light beam is determined by the facet of the polygon mirror 30, there is a problem that the degree of freedom is small. That is, the shape of the reflected light beam reflected and deflected by the polygon mirror 30 is always a shape in which both sides are sharply cut by the facet width of the polygon mirror 30. As a result, a beam cross section close to a rectangle is obtained. When the image is formed on the surface of the photosensitive drum, there is a problem that the side lobe is enlarged or the spot shape (spot diameter) cannot be changed according to the characteristics of the printer.

According to the present invention, the light beam width of the light beam formed by the first optical system is formed so as to be wider than the width of the deflecting surface of the optical deflector in the main scanning section, and the reflectance of the deflecting surface is set to be larger. By setting different distributions in the main scanning direction in the deflecting surface, a spot shape (spot diameter) suitable for printing conditions can be formed on the surface to be scanned, and the main scanning direction of the deflecting surface can be adjusted. Are different distributions in the sub-scanning direction within the deflecting surface, and the position of the light beam incident on the deflecting surface and the position of the deflecting surface are relatively variable in the sub-scanning direction. It is another object of the present invention to provide an optical scanning device capable of changing a spot shape even when the same optical deflector is used.

[0014]

An optical scanning device according to the present invention comprises:
(1) The light beam emitted from the light source means is converted into a substantially parallel light beam in the main scanning section by the first optical system and is guided to the optical deflector, and the light beam reflected and deflected by the optical deflector is transmitted to the second optical system. In an optical scanning device for guiding light onto a surface to be scanned by a system and optically scanning over the surface to be scanned, the light beam width of the light beam formed by the first optical system has a light beam width within the main scanning cross section. And the reflectance of the deflecting surface of the optical deflector is set to be non-uniform within the deflecting surface.

In particular, (1-1) the reflectivity of the deflecting surface in the main scanning direction is set so as to be lower at the peripheral portion than at the central portion. Is set to be 60% or less of the reflectance of the central portion, and (1-3) the reflectance of the deflecting surface in the main scanning direction is That the intensity is set to have a local minimum at approximately the center of the reflected and deflected light beam, (1-4)
The reflectivity of the deflecting surface in the main scanning direction is set to have a different distribution in the sub-scanning direction in the deflecting surface, and the position of the light beam incident on the deflecting surface and the position of the deflecting surface are in the sub-scanning direction. It is configured to be relatively variable, (1-
5) In the peripheral portion of the deflecting surface, the region of the reflecting portion is changed in the sub-scanning direction, or (1-6) a light absorbing portion is provided in the peripheral portion of the deflecting surface, (1-7) The ridge of the deflecting surface is chamfered obliquely in the sub-scanning direction.

[0016]

(Embodiment 1) FIG. 1 is a schematic view of a main part of an optical system according to Embodiment 1 of the present invention, and FIG. 2 is one of a plurality of deflecting surfaces of the optical deflector shown in FIG. It is an external view of one deflection | deviation surface.

In FIG. 1, reference numeral 1a denotes light source means, which is made of, for example, a semiconductor laser. Reference numeral 1b denotes a collimator lens as a first optical system having a positive refractive power, for example, converting a light beam (light beam) emitted from the light source 1a into a substantially parallel light beam. An aperture (aperture) 1c limits the outer diameter of the light beam. Each element such as the semiconductor laser 1a, the collimator lens 1b, and the aperture 1c constitutes one element of the light source unit 1. In the present embodiment, the light beam width of the light beam emitted from the light source unit 1 is formed to be wider than the width of a deflecting surface 3a of an optical deflector 3 described later in the main scanning section.
Thus, even if the deflecting surface 3a of the optical deflector 3 moves by a predetermined amount due to rotation, the deflecting surface 3a is set so as not to deviate from the light beam.

Reference numeral 2 denotes a cylindrical lens (cylinder lens) which has a predetermined refracting power in the sub-scanning section, and applies a light beam passing through the aperture 1c to the deflection surface 3a of the optical deflector 3 in the sub-scanning section. The image is formed substantially as a line image (line image elongated in the main scanning direction).

Reference numeral 3 denotes an optical deflector. The polygonal prism has a wall surface formed of a deflecting surface (mirror surface), for example, a polygon mirror (rotating polygon mirror), and is driven by driving means (not shown) such as a polygon motor in the figure. It is rotating at a constant speed in the direction of arrow A. In the present embodiment, the reflectance of the facet (deflecting surface) 3a of the polygon mirror 3 is set so as to be non-uniform within the facet 3a. That is, the reflectance of the facet 3a with respect to the light source wavelength in the main scanning direction is set to be lower at the peripheral portion 3a2 than at the central portion 3a1, and the reflectance of the peripheral portion 3a2 in the main scanning direction of the facet 3a is set at the central portion 3.
The reflectance is set to be 60% or less with respect to the reflectance of a1.

Reference numeral 4 denotes a second optical system, which comprises a single anamorphic lens (scanning lens) using a plastic aspheric surface and receives a light beam based on image information reflected and deflected by the facet 3a of the polygon mirror 3. An image is formed on the photosensitive drum surface 5 as a scanning surface. The scanning lens 4 has a so-called fθ characteristic in which a light beam deflected at a constant angular velocity in the main scanning plane and incident thereon is focused on a spot moving on the photosensitive drum surface 5 at a constant velocity. In the above, the light beam condensed in this plane by the cylindrical lens 2 or the deflection surface 3a illuminated by this light beam
A so-called tilt correction function that brings the image forming relationship between the image forming apparatus and the photosensitive drum surface 5.

In the present embodiment, the light beam emitted from the semiconductor laser 1a is converted into a substantially parallel light beam by the collimator lens 1b, the outer diameter of the light beam is restricted by the aperture 1c, and the light beam enters the cylindrical lens 2. Of the substantially parallel light beam incident on the cylindrical lens 2, the light beam is emitted as it is in the main scanning section. Further, in the sub-scan section, the light converges and forms an almost linear image (a linear image elongated in the main scanning direction) on the facet 3a of the polygon mirror 3. And facet 3a of polygon mirror 3
The light beam reflected and deflected by the light beam is focused on the photosensitive drum surface 5 via a scanning lens (fθ lens) 4 to form a spot image. By rotating the polygon mirror 3 in the direction of arrow A, the light beam is reflected on the photosensitive drum surface 5. Is optically scanned in the direction of arrow B (main scanning direction). Thus, an image is recorded on the photosensitive drum surface 5 as a recording medium.

In this embodiment, as shown in FIG. 2, the reflectivity of the facet 3a of the polygon mirror 3 with respect to the light source wavelength in the main scanning direction is reduced from the central portion (bright portion) 3a1 to the peripheral portion (dark portion) 3a2. You have set. That is,
In the present embodiment, the reflectance of the facet 3a of the polygon mirror 3 is not constant over the entire surface, and the reflectance is high at the central portion 3a1 in the main scanning direction of the facet 3a, and the peripheral portion 3a
2 is set so as to gradually decrease. This change in reflectance can be realized by depositing different substances, controlling the film thickness, applying a light-shielding paint, or the like.

FIG. 3 shows a facet 3a of the polygon mirror 3.
FIG. 4 is an explanatory diagram showing the intensity distribution of the reflected light beam reflected and deflected by, where the vertical axis represents the reflectance, and the horizontal axis represents the position in the main scanning direction inside the light beam.

In FIG. 3, a dotted line A represents the intensity distribution of the reflected light beam when a conventional polygon mirror having a constant reflectance in the facet is used, and solid lines B and C represent the reflected light beam intensity when the present embodiment is applied. It is an intensity distribution.

FIGS. 4A, 4B and 4C show the main scanning direction when the reflected light flux having the intensity distribution shown in FIG. 3 is formed on the photosensitive drum surface by the same scanning lens. FIG. 4 is an explanatory diagram showing a cross section of the spot intensity distribution of FIG.

FIG. 3A shows a spot formed by a reflected light beam having an intensity distribution indicated by a dotted line A in FIG. 3, and in this case, a central maximum, that is, another pole is provided on both sides of a so-called main lobe. The value, the so-called side lobe, has appeared. The height of this side lobe reaches about 5% of the maximum intensity of the spot when the intensity distribution in the light beam in the main scanning direction is almost constant. Depending on the application of the optical scanning system, the height of the side lobe is the non-printing portion adjacent to the printing portion. , Or in the middle part of the adjacent print dots, exposure does not fall sufficiently, the contrast decreases,
As a result, the image quality is significantly reduced.

On the other hand, in the present embodiment, for example, in the solid line B in FIG. 3, the reflectivity of the facet in the main scanning direction is reduced at the peripheral portion with respect to the central portion, and the peripheral intensity of the reflected light beam in the main scanning direction is reduced at the central intensity. The intensity distribution of the spots is shown in FIGS. 3B and 3C, respectively, since the peripheral intensity is reduced to about 0 as shown by the solid line C in FIG. It looks like this. At this time, the height of the side lobe is 3% or less of the maximum intensity, and the effect of the side lobe is practically eliminated.

That is, the reflected light flux reflected and deflected by the facet gradually decreases in the peripheral portion in the main scanning direction.
Since it approaches a Gaussian distribution, it can suppress side lobes that occur when the reflected light flux forms an image.

As described above, in the present embodiment, the reflectance of the facet 3a of the polygon mirror 3 is not constant over the entire surface as described above, and the reflectance is high at the central portion 3a1 in the main scanning direction of the facet 3a, and the peripheral portion 3a2 , The influence of side lobes can be practically eliminated, and a good spot shape can be formed on the surface to be scanned.

(Embodiment 2) FIG. 5 shows Embodiment 2 of the present invention.
FIG. 4 is an explanatory diagram showing facets of a polygon mirror in FIG.

The present embodiment is different from the first embodiment in that the reflectivity of the facet of the polygon mirror with respect to the light source wavelength in the main scanning direction has a minimum intensity at the approximate center of the light beam reflected and deflected by the facet. It is set to. Other configurations and optical functions are substantially the same as those of the first embodiment, and thus, similar effects are obtained.

That is, in this embodiment, the reflectivity of the facet 53a of the polygon mirror is not constant over the entire surface.
The reflectance of the facet 53a with respect to the light source wavelength in the main scanning direction is changed from the peripheral portion (bright portion) 53a2 to the central portion (dark portion) 53a.
It is set to be lower at a1.

FIG. 6 is an explanatory diagram showing the intensity distribution of the reflected light beam reflected and deflected by the facet. The vertical axis shows the reflectance, and the horizontal axis shows the position inside the light beam in the main scanning direction.

FIG. 7 is an explanatory diagram showing a cross section of the spot intensity distribution in the main scanning direction when the reflected light having the intensity distribution shown in FIG. 6 is imaged on the photosensitive drum surface by the scanning lens.

According to the present embodiment, as shown in FIG. 7, the height of the side lobe of the spot is larger than that of the conventional example (FIG. 4A), but the diameter of the main lobe is smaller. Such spots (super-resolution spots) can produce an effect in improving the resolving power in an image forming method in which printing is performed only when, for example, a predetermined amount or more of exposure has occurred.

(Embodiment 3) FIG. 8 shows Embodiment 3 of the present invention.
FIG. 4 is an explanatory diagram showing facets of a polygon mirror in FIG.

The present embodiment differs from the first embodiment in that the reflectance (reflectance distribution) of the facets of the polygon mirror with respect to the light source wavelength in the main scanning direction is different in the sub-scanning direction within the facets. And the incident position of the light beam with respect to the facet and the position of the facet (polygon mirror) are relatively variable in the sub-scanning direction. Other configurations and optical functions are substantially the same as those of the first embodiment, and thus, similar effects are obtained.

That is, in the present embodiment, as shown in the figure, the reflectance of the facet 83a of the polygon mirror is not constant over the entire surface, and the reflectance of the facet 83a with respect to the light source wavelength in the main scanning direction is changed to the peripheral portion (dark portion) 83a2. The central portion (bright portion) 83a1 is set to be higher, and the area of the reflective portion (reduced portion) 83b in the peripheral portion 83a2 is reduced from the upper portion to the lower portion (in the sub-scanning direction) of the facet 83a. On the contrary, the area of the reflection portion 83b in the peripheral portion 83a2 is increased from the lower portion to the upper portion, and the incident position of the light beam on the facet 83a and the position of the facet 83a are sub-scanned. It is configured to be relatively variable in the direction.

In the figure, A and B are light beams incident on the facet 83a. Since the light beam A is incident on the upper part of the facet 83a and is reflected and deflected as shown in the figure, the intensity distribution of the reflected light beam has a greater decrease in the amount of light at the end compared to the light beam B. Since the light beam B is incident on the lower portion of the facet 83a and is reflected and deflected, the intensity distribution of the reflected light beam has a smaller decrease in the amount of light at the end compared to the light beam A. For example, when the light beams A and B form an image on the photosensitive drum surface via the same scanning lens, the spot diameter of the light beam A becomes larger than the spot diameter of the light beam B as shown in FIG.

In the conventional facet having a uniform reflectance over the entire surface, or in the case where the size of the spot diameter is almost determined by the facet as in the first and second embodiments, the size of the spot diameter is large. However, the incident position of the light beam on the facet 83a of the polygon mirror and the position of the facet 83a are relatively variable in the sub-scanning direction using the polygon mirror according to the present embodiment. With this configuration, the spot diameter (spot shape) can be changed.

For example, when the polygon mirror according to the present embodiment is applied to a printer capable of switching the resolution, as described above, the incident position of the light beam on the facet 83a of the polygon mirror and the position of the facet 83a are set in the sub-scanning direction. By making it relatively variable, for example, when obtaining a high-resolution print, the light beam is made to enter the lower part of the facet 83a, and conversely, when obtaining a low-resolution print, the light beam is applied to the upper part of the facet. If the light is incident, the spot diameter can be changed according to the resolution.

(Embodiment 4) FIG. 10 is an explanatory view showing facets of a polygon mirror according to Embodiment 4 of the present invention.

The difference between the third embodiment and the third embodiment is that in the third embodiment, the spot shape (spot diameter) using the same polygon mirror is changed, and the continuously changing reflectance distribution is scanned in the main scanning direction. In this embodiment, a reflectance distribution that changes discontinuously is used. Other configurations and optical functions are substantially the same as those of the third embodiment, and thus, similar effects are obtained.

That is, in the figure, reference numeral 10b denotes a light absorbing portion provided in the peripheral portion (black portion) 10a2,
0a from the upper part to the lower part (in the sub-scanning direction)
0a2 is formed to be small. The light absorbing portion 10b is formed on the facet 10a with a light-shielding paint or the like. A and B are light beams incident on the facet 10a. Light flux A is facet 10 as shown in FIG.
The light beam B enters the upper part of the facet 10a and is reflected and deflected, so that the width of the reflected light beam becomes Wa. The light beam B enters the lower part of the facet 10a and is reflected and deflected, and the width of the reflected light beam becomes Wb.

When the reflected light flux reflected and deflected by the facet 10a is imaged on the photosensitive drum surface using the same scanning lens, the size of the image spot (spot diameter) is inversely proportional to the width of the reflected light flux. Therefore, the spot diameter due to the light beam A becomes larger than the spot diameter due to the light beam B. Therefore, when the polygon mirror according to the present embodiment is applied to, for example, a printer capable of switching the resolution, the incident position of the light beam on the facet 10a of the polygon mirror and the position of the facet 10a are relatively variable in the sub-scanning direction. With this configuration, the spot diameter can be easily changed as in the third embodiment.

FIG. 11 is a schematic view of a main part of a polygon mirror according to a fifth embodiment of the present invention.

In this embodiment, the difference from the above-described fourth embodiment is that, in the above-described fourth embodiment, the width of the reflected light beam is changed by the light absorbing portion formed on the facet. The configuration is such that the reflected light beam does not enter the scanning lens. Other configurations and optical functions are substantially the same as those of the fourth embodiment.
Thereby, a similar effect is obtained.

That is, in the present embodiment, as shown in FIG.
3b is chamfered obliquely in the sub-scanning direction.
By making the shape of 13a a trapezoid, an effect similar to that of the fourth embodiment is obtained.

[0049]

According to the present invention, as described above, the light beam width of the light beam formed by the first optical system is formed to be wider than the width of the deflecting surface of the optical deflector in the main scanning section. By setting the reflectivity of the deflecting surface so as to have a different distribution in the main scanning direction in the deflecting surface, a spot shape (spot diameter) suitable for printing conditions can be formed on the surface to be scanned. The reflectivity of the deflecting surface in the main scanning direction is differently distributed in the sub-scanning direction within the deflecting surface, and the position of the light beam incident on the deflecting surface and the position of the deflecting surface are relatively variable in the sub-scanning direction. With such a configuration, it is possible to achieve an optical scanning device that can change the spot shape even when the same optical deflector is used.

[Brief description of the drawings]

FIG. 1 is a schematic view of a main part of a first embodiment of the present invention.

FIG. 2 is an external view of a facet of a polygon mirror according to the first embodiment of the present invention.

FIG. 3 is a distribution diagram illustrating an intensity distribution of a reflected light beam according to the first embodiment of the present invention.

FIG. 4 is an explanatory diagram showing a spot shape according to the first embodiment of the present invention.

FIG. 5 is an external view of a facet of a polygon mirror according to a second embodiment of the present invention.

FIG. 6 is a distribution diagram illustrating an intensity distribution of a reflected light beam according to the second embodiment of the present invention.

FIG. 7 is an explanatory diagram showing a spot shape according to a second embodiment of the present invention.

FIG. 8 is an external view of a facet of a polygon mirror according to a third embodiment of the present invention.

FIG. 9 is a distribution diagram showing an intensity distribution of a reflected light beam according to the third embodiment of the present invention.

FIG. 10 is an external view of a facet of a polygon mirror according to a fourth embodiment of the present invention.

FIG. 11 is an external view of a polygon mirror according to a fifth embodiment of the present invention.

FIG. 12 is an explanatory diagram showing a state of a light beam reflected and deflected by a polygon mirror in a conventional optical scanning device.

[Explanation of symbols]

Reference Signs List 1 light source unit 1a light source means (semiconductor laser) 1b first optical system (collimator lens) 1c aperture 2 cylindrical lens 3 optical deflector (polygon mirror) 3a deflection surface (facet) 4 second optical system (scanning lens) 5 Scanned surface (photosensitive drum surface) 53a, 83a, 10a, 113a Deflection surface (facet) 113 Optical deflector (polygon mirror)

Claims (8)

[Claims] 1. A light beam emitted from a light source means is converted into a substantially parallel light beam in a main scanning section by a first optical system and guided to an optical deflector, and the light beam reflected and deflected by the light deflector is converted to a second light beam.
In the optical scanning device for guiding light onto the surface to be scanned by the optical system and optically scanning the surface to be scanned, the light beam width of the light beam formed by the first optical system has the light beam width within the main scanning cross section. An optical scanning device formed so as to be wider than a width of a deflecting surface of a deflector, wherein a reflectance of the deflecting surface of the optical deflector is set to be non-uniform in the deflecting surface. 2. The optical scanning device according to claim 1, wherein the reflectance of the deflecting surface in the main scanning direction is set to be lower at a peripheral portion than at a central portion. 3. The optical scanning device according to claim 2, wherein the reflectance of the peripheral portion of the deflection surface in the main scanning direction is set to be 60% or less of the reflectance of the central portion. apparatus. 4. The reflectance of the deflecting surface in the main scanning direction is set so as to have a minimum intensity at substantially the center of a light beam reflected and deflected by the deflecting surface. Optical scanning device. 5. The reflectance of the deflecting surface in the main scanning direction is set to have a different distribution in the sub-scanning direction in the deflecting surface.
4. The optical scanning device according to claim 2, wherein an incident position of the light beam on the deflecting surface and a position of the deflecting surface are relatively variable in the sub-scanning direction. 6. The optical scanning device according to claim 5, wherein the area of the reflection portion changes in the sub-scanning direction at the periphery of the deflection surface. 7. The optical scanning device according to claim 5, wherein a light absorbing portion is provided at a peripheral portion of the deflecting surface, and a region of the light absorbing portion changes in the sub-scanning direction. . 8. The optical scanning device according to claim 5, wherein a ridge of the deflection surface is chamfered obliquely in a sub-scanning direction.