Optical scanner

Details Optical scanner Optical scanner

2001-05-15

2003-01-07

Spyrou, Cassandra (Department: 2872)

Optical: systems and elements

Deflection using a moving element

Using a periodically moving element

C359S205100, C359S216100, C347S232000

Reexamination Certificate

active

06504639

ABSTRACT:

TECHNICAL FIELD
The present invention relates to an optical scanner used in a laser beam printer, a laser facsimile, a digital copier, or the like.
BACKGROUND ART
Many of optical scanners used in laser beam printers and the like each include a semiconductor laser as a light source, a first image formation optical system for linearly focusing a light beam from the light source on an optical deflector to compensate for the tilt of a deflection surface of the optical deflector, a polygon mirror as the optical deflector, and a second image formation optical system for forming uniform spots on a surface to be scanned at a constant speed.
A second image formation optical system in a conventional optical scanner includes a plurality of large glass lenses called an “f&thgr; lens” and hence, had problems of difficulty in size reduction and of high cost. In order to achieve reductions in size and cost, an optical scanner as described in JP 11-30710 A has been proposed that includes one curved mirror used as a second image formation optical system.
Schematically, it is described that a light beam from the curved mirror is guided directly to an image surface in the above-mentioned optical scanner. In the optical scanner, however, the reflection angle of a light beam reflected by the curved mirror is small and accordingly, it has been necessary to dispose a reflecting mirror between the curved mirror and a photoconductive drum to actually guide a light beam to the photoconductive drum. In addition, since a cross-sectional shape in a sub scanning direction of the curved mirror described in JP 11-30710 A is not a circular arc shape but a quadratic polynomial shape, processing and measurement of the curved mirror are difficult.
JP 11-153764 A discloses an optical scanner including a second image formation optical system composed of one curved mirror alone without requiring a reflecting mirror. However, suitable conditions for guiding a light beam directly to a surface to be scanned have not been clear. In addition, it is necessary to allow a light beam to be incident obliquely on the curved mirror at a large angle therebetween to guide the light beam directly to the surface to be scanned, but this causes a considerable beam aberration at the mirror plane. However, there has been no sufficient countermeasure to such a considerable beam aberration.
DISCLOSURE OF INVENTION
In view of the above-mentioned problems, the present invention is intended to provide an optical scanner having excellent optical performance, guiding a light beam from one curved mirror directly to a photoconductive drum without requiring a reflecting mirror, and allowing the curved mirror to have a shape permitting relatively easy processing and measurement.
An optical scanner according to a basic configuration of the present invention includes: a light source unit for emitting a light beam; an optical deflector for deflecting the light beam from the light source unit so as to cause scanning; a first image formation optical system for forming a line image on a deflection surface of the optical deflector; and a second image formation optical system composed of one curved mirror. The first image formation optical system is disposed between the light source unit and the optical deflector. The second image formation optical system is disposed between the optical deflector and a surface to be scanned. A light beam from the first image formation optical system is incident obliquely on a plane that is parallel to a main scanning direction and contains a line normal to the deflection surface of the optical deflector. A light beam from the optical deflector is incident obliquely on a plane (hereinafter referred to as a “YZ plane”) that is parallel to the main scanning direction and contains a line normal to a vertex of the curved mirror. In addition, a conditional formula of 10<&thgr;M<35 is satisfied, wherein &thgr;M indicates an angle in degrees between the YZ plane and a central axis of a light beam traveling toward the curved mirror from the optical deflector.
According to this configuration, the second image formation optical system is composed of one mirror and reflects a light beam at a large reflection angle satisfying a relationship of 10<&thgr;M. Hence, the degree of freedom in arrangement increases, no reflecting mirror is required, and a light beam can be guided directly to the surface to be scanned. The upper limit of the above-mentioned conditional formula defines a range in which beam aberration can be compensated.
In the above-mentioned configuration, a light beam is incident obliquely on the curved mirror at a large angle satisfying the relationship of 10<&thgr;M. This causes a large beam aberration at a mirror plane. In order to compensate for this aberration, in the optical scanner of the present invention, a light beam from the first image formation optical system is allowed to be incident obliquely on a plane that is parallel to the main scanning direction and contains the line normal to the deflection surface of the optical deflector, and the tilt angle of the light beam is limited in a range in which the beam aberration can be compensated.
In order to obtain an excellent spot in the above-mentioned configuration, in a cross section in a sub scanning direction, an angle made by a light beam reflected by the curved mirror with respect to an incident light beam from the deflection surface is set to be negative when a direction of an angle made by a reflected light beam reflected by the deflection surface with respect to an incident light beam from the first image formation optical system is regarded as positive. With this configuration, the reflected light beam and the incident light beam are positioned in the positive and negative directions, respectively. Hence, the beam aberration caused by the oblique incidence is compensated and thus an excellent spot can be obtained.
In order to obtain a more excellent spot, in the above-mentioned configuration, the following conditional formula (1) is allowed to be satisfied:
1.6
<&thgr;M/&thgr;P+
0.98
L/
(
L+D
)<2.2  (1),
wherein &thgr;P indicates an angle in degrees between the line normal to the deflection surface and the light beam from the first image formation optical system, L a distance from the deflection surface to the vertex of the curved mirror, and D a distance from the curved mirror to the surface to be scanned.
When the positional relationship among the first image formation optical system, the optical deflector, and the second image formation optical system satisfies the above-mentioned conditional formula, the beam aberration caused by the oblique incidence of a light beam can be compensated suitably. The aberration is caused in an oblique direction when the conditional formula is not satisfied.
In order to achieve a higher resolution, it is desirable that the following conditional formulae (2) and (3) are satisfied:
1.86
<&thgr;M/&thgr;P+
0.98
L/
(
L+D
)<1.94  (2);
and
0.48
<L/
(
L+D
)<0.75  (3).
When the conditional formula (2) is satisfied, the beam aberration caused by the oblique incidence of a light beam can be compensated further suitably. When the conditional formula (3) is satisfied, the beam aberration can be compensated even at a reflection angle satisfying the relationship of 10<&thgr;M. When the lower limit is not satisfied, the beam aberration occurs. On the contrary, when the upper limit is not satisfied, a beam diameter in the sub scanning direction considerably varies between the scanning center and periphery. In such cases, it is difficult to obtain a high resolution.
In the optical scanner with the above-mentioned basic configuration, the curved mirror has a cross section with a circular arc shape in the sub scanning direction. With this configuration, the curved mirror is allowed to have a shape permitting relatively easy processing and measurement.
It can be considered to allow the curved mirror to have a shape with different radii of main and sub c

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