Optical scanning device

Details Optical scanning device Optical scanning device

1999-12-06

2001-10-16

Schuberg, Darren (Department: 2872)

Optical: systems and elements

Deflection using a moving element

Using a periodically moving element

C359S206100, C359S217200, C347S241000, C347S242000

Reexamination Certificate

active

06304360

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical scanning device for use in an image formation device such as a laser printer or a digital copying machine that forms an image by scanning a laser beam according to an image information and exposing the image on a photoreceptor. Specifically, it relates to an optical scanning device suitable for use in a multicolored image formation device that superposes plural images formed by plural laser beams to form one image.
2. Description of the Related Art
In the conventional image formation device using the electrographic system, it is a usual exercise to scan a light beam according to an image information by an optical scanning device and form a latent image on an electrified photoreceptor, and to transfer a developed image acquired by developing this latent image to a printing paper and form the image.
Recently, accompanied with the spread of colored documents, there has been developed a full-colored image formation device that forms the developed images of each of black (K), yellow (Y), magenta (M), cyan (C) by the electrographic system, and sequentially transfers these developed images to form a full colored image.
Specially for applications that require a high speed in image formation, a so-called tandem system full-colored image formation device is being developed, which contains plural independent image formation devices, continuously transfers the developed images formed by these devices to a single transfer medium, and forms a full-colored image in one cycle.
A conventional full-colored image formation device will now be explained with reference to FIG.
5
A and FIG.
5
B.
As shown in
FIG. 5A
,
FIG. 5B
, an image formation device
118
K, image formation device
118
Y, image formation device
118
M, and image formation device
118
C are provided in the order of black (B), yellow (Y), magenta (M), cyan (C), from the upstream in the paper feeding direction of a paper conveyance belt
114
. To each of the image formation devices, a sub-unit constituting the electrographic process is configured around a photosensitive drum
122
as the image carrier.
In this device, first an electrification device
124
electrifies the photosensitive drum
122
, and then an optical scanning device
120
scans to expose a laser beam according to the image information onto the photosensitive drum
122
to form a latent image.
Next, after a development device
126
develops the latent image, the developed image is transferred to a printing paper
112
that is conveyed at a constant speed.
This process is carried out in the order of K, Y, M, C, and a fixing device not illustrated fixes the transferred image to the paper
112
, which thereafter is ejected.
The optical scanning device
120
possesses a configuration such that a laser beam source (not illustrated) emits a light beam according to image information, a rotational polygon mirror
128
deflects the light beam with a constant angular velocity, and two pieces of f&thgr; lenses
130
form an image on the photosensitive drum
122
as a beam spot scanning at a constant speed.
And, in the configuration shown in
FIG. 5A
,
FIG. 5B
, the optical scanning devices
120
each have the polygon mirrors
128
individually.
And, the Japanese Published Unexamined Patent Application No. Sho 62-189420 mentions that, since the aforementioned conventional colored image formation device contains plural optical scanning devices, there are problems of high cost, size expansion of the device, disturbances of the images, and the like. And, it also mentions that the above problems can be solved by employing one motor for driving to rotate plural polygon mirrors, and by integrating the optical scanning devices into one package.
There are, however, the following problems in the optical scanning device disclosed in the Japanese Published Unexamined Patent Application No. Sho 62-189420:
(1) As shown in
FIG. 1
,
FIG. 2
,
FIG. 5
in the specification of the published application, the provision of plural polygon mirrors in the device increase the cost. Further, since the polygon mirrors each have differences in the flatness of each reflecting surface and the angle of inclination of each reflecting surface in the sub-scanning direction, there occur differences in the degree of blooming for each color, the fluctuations of the vertical lines, and the condensation and rarefaction of the scanning lines (the so-called, banding); and in consequence the superposed colored image can be unacceptable.
(2) As shown in the embodiment in FIG.
1
through
FIG. 5
in the specification of the published application, the f&thgr; lenses are laid out for the number of the beams, which increases the cost (this is because the beam needs to be transmitted through the optical axis in the general f&thgr; lenses, in order to secure the optical performance).
(3) In the embodiment that employs four polygon mirrors (see
FIG. 1
,
FIG. 2
in the specification of the published application), a giant load is imposed on the motor as a rotation drive unit, and it could be the unrealistic in view of the cost and the power consumption.
(4) As shown in
FIG. 6
, in the embodiment that employs both sides of polygon mirrors
140
at the same time and reduces the number of the polygon mirrors
140
to two, though it is realistic in practical use, since the optical components including f&thgr; lenses
142
, reflection mirrors
144
, and the like are arrayed on both sides of the polygon mirrors
140
, the entire optical scanning device becomes enormously large in the lateral direction.
Although slight miniaturization can be realized by using multiple pieces of the reflection mirrors
144
, not only the cost is increased for the increased number of the reflection mirrors
144
, but also the adjustment mechanism for preventing the optical path deflection or the high-accuracy mechanical dimension is inevitably required, which involves a significant cost increase.
SUMMARY OF THE INVENTION
In view of the aforementioned circumstances, the present invention provides an optical scanning device for use in a multicolored image formation device that superposes plural images formed by plural light beams to produce one image, with a low cost and small size by minimizing the number of the components, and with high performance.
According to an aspect of the present invention, the optical scanning device, used in a multicolored image formation device that superposes a plurality of images formed on photoreceptors by plural light beams to obtain one multicolored image, has a single deflector that contains a single polygon mirror and a single rotation drive unit for rotating the polygon mirror, and reflects and deflects the light beams incident on the polygon mirror in a main scanning direction, plural light sources that emit the plural light beams, respectively, on the polygon mirror from both sides of the polygon mirror in a direction transverse to a rotation axis thereof, and two pairs of f&thgr; lenses that are disposed on the both sides of the polygon mirror in the direction transverse to the rotation axis thereof, and at least transmit the light beams reflected and deflected by the polygon mirror to form spot images on the photoreceptors. In the device, the plural light beams are made incident on and emitted from each of both sides of the polygon mirror in the direction transverse to the rotation axis thereof.
The optical scanning device is able to reflect and deflect the light beams falling on the polygon mirror in the main scanning direction, by the rotation drive unit rotating the polygon mirror with a constant angular velocity.
The light beam reflected and deflected on the polygon mirror transmits through the f&thgr; lens to thereby converge into a spot with a specific size on the photoreceptor, which is scanned with a virtually constant speed.
Here, the number of the deflectors that deflect the light beams with a constant angular velocity in the direction corresponding to the main scanning direction is reduced to one, t

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