JPH1152272A - Optical scanning optical system and image forming device using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain an optical scanning optical system and an image forming device using the optical system capable of excellently forming an image with a spot diameter and light quantity suitable for each resolution in accordance with the switching of the resolution. SOLUTION: In this device, the state of laser luminous flux emitted from a light source means 1 is converted to another state by a conversion optical element 2, the converted laser luminous flux is guided to a deflecting means 5 after the diameter of the luminous flux is shaped by a diaphragm member 3 having an aperture part, and the laser luminous flux deflected by the deflecting means 5 is guided to a surface to be scanned 7 through an image- formation means 6 so as to optically scan the surface 7. A light shielding member 8 having a light shielding part shielding a part of the laser luminous flux on an optical axis is provided in an optical path between the light source means 1 and the deflecting means 5. By utilizing the light shielding member 8, the effective ratio of a light shielding diameter in a main scanning direction and a subscanning direction is varied at least in one direction.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a scanning optical apparatus and an image forming apparatus using the same, and more particularly, to a spot diameter using a light-shielding member having a light-shielding portion having a predetermined shape for shielding a part of a light beam on an optical axis. It is suitable for devices such as a laser beam printer (LBP) and a digital copying machine, which can form an image with a light amount and a spot diameter suitable for each of the resolutions (or scanning line densities) according to the switching of resolution (or scanning line density). It is something.

[0002]

2. Description of the Related Art Conventionally, in an optical scanning optical system used for an image forming apparatus such as a laser beam printer or a digital copying machine, a laser beam (light beam) which is light-modulated and emitted from a light source means in accordance with an image signal is used, for example. The light is periodically deflected by an optical deflector composed of a polygon mirror, and is converged into a spot on a photosensitive recording medium (photosensitive drum) surface by an imaging means (imaging optical system) having fθ characteristics. Image recording is performed by optical scanning.

FIG. 9 is a schematic view of a main part of an optical scanning optical system used in this type of image forming apparatus.

In FIG. 1, a laser light beam (light beam) emitted from a light source means 91 is converted into a substantially parallel light beam by a collimator lens 92, and the size of the light beam cross section is shaped into a beam by a diaphragm member 93. Is incident on a cylindrical lens 94 having a predetermined refractive power. Of the substantially parallel light beams incident on the cylindrical lens 94, they are emitted as they are in the state of substantially parallel light beams in the main scanning section. The deflecting surface (reflection surface) of the optical deflector 95 formed of a polygon mirror and converged in the sub-scanning section.
An image is formed substantially as a line image near 95a. Then, the laser beam deflected by the deflection surface 95a of the optical deflector 95 is expressed by f
The light is guided onto a photosensitive drum surface 97 as a surface to be scanned through an imaging optical system (fθ lens) 96 having the θ characteristic, and the light deflector 95 is rotated in the direction of arrow A, whereby the photosensitive drum surface is rotated. Optical scanning is performed in the direction of arrow B (main scanning direction) on 97. As a result, the photosensitive drum surface 97 as a recording medium
The image is recorded above.

[0005]

In recent years, high resolution and high image quality have been demanded for laser beam printers, digital copiers and the like, and particularly, high resolution and rich gradation in halftone (intermediate gradation). In order to respond to reproduction, it is required that the spot diameter on the surface of the photosensitive drum, which is the surface to be scanned, be reduced.

Further, in an image forming apparatus capable of switching and outputting a resolution (or scanning line density) in accordance with an image to be output, it is desirable to set an ideal spot diameter and an ideal light amount according to each resolution. .

Here, the spot diameter d on the photosensitive drum surface is represented by the following equation.

D = αFλ (1) In the equation (1), F: F number of the image forming beam (optical system) λ: wavelength of the laser beam α: constant Therefore, the spot diameter d is made smaller. For this purpose, it is necessary to reduce the F-number of the imaging beam as understood from the equation (1),
Alternatively, it is necessary to shorten the wavelength λ of the laser beam.

However, when the F-number of the image forming beam is reduced, the beam width of the laser beam emitted from the collimator lens 92 is increased, and it is necessary to increase the effective diameter of the collimator lens 92. Such a collimator lens has a problem in that the number of lenses used for the collimator lens increases, and the adjustment accuracy becomes strict, so that the manufacturing cost increases.

Further, since the light beam width of the light beam incident on the deflecting surface (reflection surface) of the light deflector 95 is also increased, the effective area of the deflecting surface must be increased. Such an optical deflector not only increases the manufacturing cost but also increases the weight of the optical deflector itself, so that the load of a motor for rotating the optical deflector becomes very large, and as a result, Various problems such as an increase in the cost and power consumption of the motor and an increase in the amount of heat generated from the motor occur.

Regarding shortening the wavelength λ of the laser beam, visible lasers are still generally expensive,
Although used for some high-end machines, it is not suitable for low-cost printers.

When switching the resolution (or scanning line density), it is originally desirable to set the optimum spot diameter and light amount according to the resolution. However, in a conventional image forming apparatus, for example, 400 dpi and 600 dpi are set. For example, when switching the spot size, the spot diameter is not changed according to the resolution.

The light amount is, for example, 600 dpi.
Resolution of 2 such as switching between
In the case where the recording density is doubled with the same spot diameter and the same light amount in the case of changing the recording density by about twice, the density of the output image naturally becomes high. Therefore, it is desirable to reduce the light amount to about half.

However, it is difficult to adjust the light quantity only by the output of a laser light source, for example, a light source. Generally, a laser light source used for a scanning optical system used in an image forming apparatus such as a laser beam printer is inexpensive and has an output of 5%.
It is often a semiconductor laser of about mW. It is desirable to use a semiconductor laser of this class in the range of about 1 mW to about 4.5 mW in consideration of life, stability of output, thermal characteristics, and the like. For example, if the power is about 1 mW to 4.5 mW, the light quantity can be changed by about 4.5 times. Therefore, when the resolution is doubled, the light quantity seems to be able to be simply halved by the laser output. When the light passes through the optical system, the adjustment range of the light amount is significantly reduced due to the variation in the transmittance or the reflectance of each optical component and the variation in the light amount due to the variation in the radiation angle of the semiconductor laser itself, which has the greatest influence. Therefore, there is a problem that it is difficult to even reduce the light amount on the surface to be scanned to half.

According to the present invention, a light-shielding member having a light-shielding portion having a predetermined shape (for example, a circular shape or an elliptical shape) for shielding a part of a light beam on an optical axis is provided in an optical path between the light source means and the deflecting means. By making the effective light-shielding diameter ratio in the main scanning direction and the sub-scanning direction variable in at least one direction using the light-shielding member, the spot diameter of the laser beam on the surface to be scanned can be reduced, It is an object of the present invention to provide an optical scanning optical system capable of favorably forming an image with a light amount and a spot diameter suitable for each of the resolutions (or scanning line densities) according to switching of the resolution (or scanning line density), and an image forming apparatus using the optical scanning optical system.

[0016]

An optical scanning optical system according to the present invention comprises the following steps: (1) The state of a laser beam emitted from a light source is converted into another state by a conversion optical element, and the converted laser beam is A diaphragm member having an opening to shape the light beam diameter and guide the light beam to the deflecting means, and guide the laser light beam deflected by the deflecting means onto the surface to be scanned through the image forming means; In an optical scanning optical system that optically scans a surface,
A light-shielding member having a light-shielding portion that shields a part of the laser beam on the optical axis is provided in an optical path between the light source means and the deflecting means, and the main scanning direction and the sub-scanning direction are provided by using the light-shielding member. Is characterized in that the effective light shielding diameter ratio is variable in at least one direction.

In particular, (1-1) the effective light-shielding diameter ratio is changed when switching the scanning line density or resolution of the optical scanning optical system, and (1-2) the scanning line density is increased. Or, when the resolution is switched to a high resolution, the effective light-shielding diameter ratio is set to be lower than the low-density or low-resolution, or (1-3) the change in the effective light-shielding diameter ratio is the light-shielding diameter ratio. By changing the inclination angle of the member with respect to the optical axis, (1-4) the effective light shielding diameter ratio in the main scanning direction and the sub-scanning direction by the light shielding member is set to P
m (eff) and Ps (eff), the diameters of the aperture of the aperture member in the main scanning direction and the sub-scanning direction are Dm and Ds, respectively, and the light shielding when the light shielding part of the light shielding member is projected on the aperture member. The diameters of the portion in the main scanning direction and the sub-scanning direction are dm (eff) and ds, respectively.
(eff), the effective light shielding diameter ratio Pm (ef)
f) and Ps (eff) are

[0018]

(Equation 2) Or (1-5) the light-shielding member is created by pattern evaporation on glass, and is arranged to be inclined with respect to the optical axis of the light-shielding member, and (1-6) the conversion optical element Is a state of a light beam emitted from the light source means a substantially parallel light beam,
Alternatively, it is converted into a convergent light beam or a divergent light beam.

The image forming apparatus of the present invention comprises: (2) The above (1), (1-1), (1-2), (1-3), (1-4), (1-5), (1) An image is formed using the optical scanning optical system according to any one of 1-6).

[0020]

FIG. 1 shows an optical scanning optical system according to a first embodiment of the present invention, for example, a laser beam printer (LBP).
FIG. 3 is a cross-sectional view (main scanning cross-sectional view) of a main part in the main scanning direction when used in an image forming apparatus such as that shown in FIG.

In FIG. 1, reference numeral 1 denotes light source means, for example, a semiconductor laser. Reference numeral 2 denotes a conversion optical element, which is a laser beam (light beam) emitted from the light source unit 1.
Is converted into a substantially parallel light beam, a convergent light beam, or a divergent light beam. Reference numeral 3 denotes an aperture member (aperture stop) having an opening 3a having a shape to be described later, and shaping the size of the cross section of the laser beam.

Reference numeral 8 denotes a light-shielding member, which has a light-shielding portion 8a having a shape to be described later. The diameter is reduced. In the present embodiment, the effective light blocking diameter ratio for blocking the light beams in the main scanning direction and the sub-scanning direction by the light blocking member is variable in at least one direction.
In accordance with the switching, image formation is performed with a light amount and a spot diameter suitable for each. The light-shielding member 8 is formed by evaporating a pattern on a sheet glass, and is arranged to be inclined with respect to the optical axis L of the light-shielding member.

Reference numeral 4 denotes a cylindrical lens having a predetermined refractive power only in the sub-scan section. Reference numeral 5 denotes an optical deflector, which is, for example, a polygon mirror (rotating polygon mirror), and is rotated at a constant speed in the direction of arrow A by driving means (not shown) such as a motor. 6 is f as imaging means
An imaging optical system (fθ lens system) having -θ characteristics;
The laser beam deflected by the optical deflector 5 forms an image on a photosensitive drum (recording medium) surface 7 as a surface to be scanned.

In this embodiment, the laser beam emitted from the light source means 1 is converted by the conversion optical element 2 into a substantially parallel beam (or a convergent beam or a divergent beam).
Main scanning direction (deflection direction of optical deflector) by aperture member 3
And the sub-scanning direction (direction orthogonal to the deflection direction of the optical deflector)
At this time, the size of the light beam cross section is beam-shaped. After that, a part of the laser beam near the optical axis is shielded by the light shielding member 8 and enters the cylindrical lens 4. Of the substantially parallel light beams incident on the cylindrical lens 4, they are emitted as they are in the state of substantially parallel light beams in the main scanning section. In the sub-scan section, the light converges and forms a substantially linear image on the deflection surface (reflection surface) 5a of the optical deflector 5. Thereafter, the laser beam deflected by the deflecting surface 5a is condensed on the photosensitive drum surface 7 via the image forming optical system 6, and the optical deflector 5 is rotated in the direction of arrow A, whereby the photosensitive drum surface 7
The top is scanned at a constant speed in the direction of arrow B (main scanning direction). Thus, an image is recorded on the photosensitive drum surface 7 as a recording medium.

FIG. 2 is an explanatory diagram showing a specific shape (aperture shape) of the opening 3a of the diaphragm member 3 shown in FIG.
The shape of the opening 3a is arbitrarily determined according to the spot diameter on the photosensitive drum surface 7, and is usually circular, elliptical, or elliptical. In FIG.
m is the diameter of the opening 3a in the main scanning direction, and Ds is the diameter of the opening 3a in the sub-scanning direction.

FIG. 3 shows the light shielding portion 8 of the light shielding member 8 shown in FIG.
a. The shape of the light shielding portion 8a is, for example, circular or elliptical with the optical axis as the center. In the figure, dm is the diameter of the light shielding portion 8a in the main scanning direction, and ds is the diameter of the light shielding portion 8a in the sub scanning direction.

FIG. 4 shows that the laser beam emitted from the light source means 1 is converted into a substantially parallel beam by the conversion optical element 2, and the beam is shaped by the stop member 3 and the light shielding member 8 inclined at a predetermined angle with respect to the optical axis L. It is the principal part schematic diagram which showed the appearance. In the figure, the same elements as those shown in FIG. 1 are denoted by the same reference numerals.

In the figure, dm (eff) is the diameter of the light-shielding portion 8a of the light-shielding member 8 in the main scanning direction when the light-shielding portion 8a is projected onto the diaphragm member 3, and dm is the diameter of the light-shielding portion 8a at the position of the light-shielding member 8. This is the diameter in the main scanning direction.

FIG. 5 is an explanatory diagram showing an effective light shielding shape by the light shielding member 8 at the position of the diaphragm member 3. In this figure, the same elements as those shown in FIGS. 2 and 4 are denoted by the same reference numerals.

In the figure, ds (eff) is the diameter of the light shielding portion 8a in the sub-scanning direction when the light shielding portion 8a of the light shielding member 8 is projected on the diaphragm member 3.

Here, the shape of the light-shielding portion 8a of the light-shielding member 8 is set to Pm (eff) and Ps (eff) with respect to the aperture 3a of the aperture member 3 in terms of the effective light-shielding diameter ratio in the main scanning direction and the sub-scanning direction, respectively. ), The light-shielding diameter ratios Pm (eff) and Ps (eff) are

[0032]

(Equation 3) Is defined.

It is generally known that the spot diameter on the photosensitive drum surface 7 can be reduced by blocking the laser beam passing through the central portion of the diaphragm member 3 as described above.

For example, FIG. 6 shows a result of a simulation performed by the present inventor as an example. Here, for the sake of simplicity, the simulation is performed on the assumption that dm (eff) = ds (eff), that is, the effective light blocking diameter ratio in the main scanning direction and the sub-scanning direction is equal. Here, Pm (eff) = P
The diffraction limit spot diameter when s (eff) = 0 is set to 60 μm. As shown in the figure, it can be understood that the spot diameter can be reduced as the effective light blocking diameter ratio is increased.

In this embodiment, when the scanning line density is changed to high density or the resolution is changed to high resolution, the effective light blocking diameter ratio is increased compared to low density or low resolution. That is, at high resolution (or high density), the amount of light on the surface of the photosensitive drum 7 is low (or low density).
And the spot diameter becomes smaller.

In this embodiment, the effective light blocking diameter ratio is changed when the scanning line density or the resolution of the optical scanning optical system is switched. For example, as described above, the scanning line density is increased or the resolution is increased. When switching to high resolution, the effective light-shielding diameter ratio is increased compared to low density or low resolution. The effective change of the light-shielding diameter ratio is performed by changing the inclination angle of the light-shielding member with respect to the optical axis by changing means (not shown).

Here, for example, 600 dpi and 1200 dpi
FIG. 7 shows a case where the resolution is switched as i.
This will be described with reference to FIG. 7 and FIG.
The same elements as those shown in FIG. 5 are denoted by the same reference numerals.

FIGS. 8A and 8B are a main scanning sectional view showing the arrangement of the light shielding member 8 at 600 dpi and an explanatory diagram showing an effective light shielding shape at the position of the stop member 3. FIGS. In FIG. 2A, the light shielding member 8 is arranged to be inclined with respect to the main scanning direction (or the optical axis L direction).
And its inclination angle α with respect to the main scanning direction (or the optical axis L
By appropriately setting the inclination angle with respect to the effective light-shielding diameter ratio, a small spot diameter and light amount corresponding to a 600 dpi image are obtained.

On the other hand, FIGS. 7A and 7B show 1200
FIG. 7 is a main scanning cross-sectional view illustrating the arrangement of the light shielding member 8 when the resolution is switched to dpi, and an explanatory diagram illustrating an effective light shielding shape at the position of the diaphragm member 3. 8A, the inclination angle α of the light shielding member 8 with respect to the main scanning direction is shown in FIG.
The effective light-shielding diameter ratio is set large by making it smaller than that in FIG. By increasing the area that blocks the laser beam, the amount of light on the photosensitive drum surface can be reduced to more than half and the spot diameter in the main scanning direction can be reduced, resulting in higher resolution. A suitable spot shape can be obtained.

As described above, in the present embodiment, the light shielding member 8 is provided in the optical path between the semiconductor laser 1 and the optical deflector 5 as described above, and the effective light in the main scanning direction is utilized by using the light shielding member 8. By making the light-shielding diameter ratio variable, the spot diameter of the laser beam on the scanned surface 7 can be reduced, and the light amount and spot diameter suitable for each can be changed according to the switching of the resolution (or scanning line density). Image formation can be performed.

In this embodiment, the light shielding member 8 is arranged to be inclined with respect to the main scanning direction. However, the present invention is not limited to this. For example, the light shielding member 8 may be inclined in the sub scanning direction, or may be relatively inclined in both directions. It may be arranged. Thereby, the effective light-shielding diameter ratio in the sub-scanning direction can also be made variable.

In this embodiment, there is no particular limitation on the changing means for changing the inclination angle of the light shielding member when switching the resolution (or the scanning line density). If the light reflected on the surface of the light-shielding member returns to the semiconductor laser again, the operation of the semiconductor laser becomes unstable. Therefore, the light-shielding member is always arranged inclined with respect to the optical axis L. Is good. Thereby, the operation stability of the semiconductor laser 1 can be improved.

[0043]

According to the present invention, as described above, in the optical path between the light source means and the deflecting means, a light shielding portion of a predetermined shape (for example, a circular shape or an elliptical shape) for shielding a part of the light beam on the optical axis is provided. A light-shielding member having an effective light-shielding diameter ratio in at least one direction in the main scanning direction and the sub-scanning direction using the light-shielding member. Specifically, switching of resolution (or scanning line density) is performed. In this case, for example, by increasing the effective light blocking diameter ratio at a high resolution as compared with a low resolution, the light amount can be reduced without changing the output of a semiconductor laser as a light source, which is suitable for a high resolution. An optical scanning optical system and an image forming apparatus using the optical scanning optical system, which can obtain an image with a high density and can also obtain a higher quality image at a high resolution because the spot diameter can be reduced at the same time. Achievement Rukoto can.

[Brief description of the drawings]

FIG. 1 is a main scanning sectional view of a first embodiment of the present invention.

FIG. 2 is an explanatory diagram showing a shape of an aperture stop according to the first embodiment of the present invention.

FIG. 3 is an explanatory view showing a shape of a light shielding portion according to the first embodiment of the present invention.

FIG. 4 is an explanatory diagram showing a state in which beam shaping is performed by a light shielding unit in the first embodiment of the present invention.

FIG. 5 is an explanatory diagram showing an effective light shielding shape at the position of the aperture member according to the first embodiment of the present invention.

FIG. 6 is an explanatory diagram showing a relationship between an effective light blocking diameter ratio and a spot diameter.

FIG. 7 illustrates a resolution (600 dp) according to the first embodiment of the present invention.
Explanatory drawing showing the state of inclination of the light shielding member at the time of switching i) and explanatory drawing showing the effective light shielding shape at the position of the diaphragm member.

FIG. 8 illustrates a resolution (1200 dp) according to the first embodiment of the present invention.
Explanatory drawing showing the state of inclination of the light shielding member at the time of switching i) and explanatory drawing showing the effective light shielding shape at the position of the diaphragm member.

FIG. 9 is a main scanning sectional view of a conventional optical scanning optical system.

[Description of Signs] 1 Light source means 2 Conversion optical element 3 Aperture member 4 Cylindrical lens 5 Deflection means (optical deflector) 6 Imaging means (fθ lens system) 7 Scanned surface (photosensitive drum surface) 8 Light shielding member

Claims (8)

[Claims] 1. A laser beam emitted from a light source means is converted into another state by a conversion optical element, and the converted laser beam is deflected by shaping its beam diameter by an aperture member having an opening. An optical scanning optical system that guides the laser beam deflected by the deflecting unit onto the surface to be scanned through the image forming unit, and optically scans the surface to be scanned. A light-shielding member having a light-shielding portion that shields a part of the laser beam on the optical axis is provided in an optical path between the light source and the deflecting means, and an effective light in the main scanning direction and the sub-scanning direction is provided by using the light-shielding member. An optical scanning optical system, wherein a light shielding diameter ratio is variable in at least one direction. 2. The optical scanning optical system according to claim 1, wherein the effective light-shielding diameter ratio is changed when switching the scanning line density or the resolution of the optical scanning optical system. 3. The effective light-shielding diameter ratio is increased when the scanning line density is switched to a high density or when the resolution is switched to a high resolution. Optical scanning optical system. 4. The optical scanning optical system according to claim 1, wherein the effective change of the light shielding diameter ratio is performed by changing an inclination angle of the light shielding member with respect to an optical axis. 5. The effective light-shielding diameter ratio of the light-shielding member in the main scanning direction and the sub-scanning direction is Pm (eff) and Ps (ef), respectively.
f), the diameters of the opening of the aperture member in the main scanning direction and the sub-scanning direction are Dm and Ds, respectively, and the main scanning direction and the sub-scanning direction of the light shielding unit when the light shielding unit is projected onto the aperture member. Assuming that the diameters in the scanning direction are dm (eff) and ds (eff), respectively, the effective light-shielding diameter ratios Pm (eff) and Ps (eff)
Is: The optical scanning optical system according to claim 1, 2, 3, or 4, wherein 6. The optical scanning optical system according to claim 1, wherein the light shielding member is formed by pattern vapor deposition on glass, and is arranged to be inclined with respect to the optical axis of the light shielding member. 7. The optical scanning optical system according to claim 1, wherein said conversion optical element converts a state of a light beam emitted from said light source means into a substantially parallel light beam, a convergent light beam, or a divergent light beam. 8. An image forming apparatus for forming an image using the optical scanning optical system according to claim 1.