Optical scanner

Details Optical scanner Optical scanner

2000-05-11

2003-05-20

Phan, James (Department: 2872)

Optical: systems and elements

Deflection using a moving element

Using a periodically moving element

C359S212100, C347S242000

Reexamination Certificate

active

06567201

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to an optical scanner to be used for an image-forming apparatus such as laser beam printer or laser facsimile machine.
2. Related Background Art
Optical scanners to be used for image-forming apparatus are adapted to reflect and deflect a light beam such as a laser beam by means of a rotary polygon mirror rotating at high speed so as to make a scanning light beam. The obtained scanning light beam is focussed on a photosensitive member arranged on a rotary drum to form an electrostatic latent image there. Then, the electrostatic latent image is turned into a visible toner image by means of a developing machine, which toner image is then transferred onto a recording medium such as a sheet of recording paper. Thereafter, the toner on the recording medium is heated and fixed to complete the printing process.
FIG. 1
of the accompanying drawings schematically illustrates a known typical optical scanner E
o
. Referring to
FIG. 1
, the laser beam (light beam) emitted from a semiconductor laser
101
is collimated in a lens barrel
100
and then converged to a linear light beam by means of a cylindrical lens
130
. Then, the light beam is deflected by a rotary polygon mirror
102
to scan in a predetermined direction (main-scanning direction) that is perpendicular to the axis of rotation of the polygon mirror and subsequently focussed on a photosensitive member
215
arranged on a rotary drum by means of imaging lenses
103
a
and
103
b
. As the light beam striking the photosensitive member
215
is made to scan in the main-scanning direction by the rotation of the rotary polygon mirror
102
and also in the sub-scanning direction by the rotation of the rotary drum, it forms an electrostatic latent image on the photosensitive member.
As the above-described scanning operation for writing image information on the photosensitive member
215
of the rotary drum is repeated, there can arise a problem that the starting point of the writing cycle may be displaced form cycle to cycle due to possible division errors of the reflecting planes of the rotary polygon mirror
102
. To avoid this problem, the scanning light beam coming form the rotary polygon mirror
102
is reflected by a BD mirror and led into a BD sensor when the bet gets to the end of the plane being scanned. Then, the controller of the BD sensor transforms the introduced light beam into a scan start signal. The semiconductor laser
101
is so arranged that it starts another write modulation cycle upon receiving the scan start signal.
The imaging lenses
103
a
and
103
b
are a spherical lens and a toric lens and have a so-called f&thgr; function of transforming the scanning light beam made to move at a constant angular velocity by the rotary polygon mirror
102
into a scanning light beam moving at a constant velocity in the main-scanning direction on the rotary drum. The rotary polygon mirror
102
, the motor for driving the polygon mirror
102
, the imaging lenses
103
a
and
103
b
are contained in an optical cabinet
110
whose top opening is hermetically sealed by means of a lid member
120
having a radiator panel
121
. In
FIG. 1
, the radiator panel
121
and the lid member
120
are indicated by broken lines to show their positions.
As a result of the technological development in recent years, image-forming apparatus are made to operate at high speed to produce high density images. However, this technological trend requires the rotary polygon mirror to operate with a large number of revolutions per unit time.
On the other hand, as the rotary polygon mirror is driven to rotate at high speed, there arises problems including those of vibrations, noises and heat generated particularly by the bearing and the motor of the rotary polygon mirror. The noises generated by the rotary polygon mirror and the motor can end up with abnormal vibratory sounds. Some known optical scanners are provided in the inside thereof with a noise absorber to minimize the noise problem. The heat generated by the motor can by turn significantly raise the temperature of the optical cabinet to consequently degrade the performance of the motor and that of the optical components housed in the optical cabinet. Although the heat problem may be partly dissolved by arranging radiator fins and/or a heat exchanger in the inside, such an arrangement can greatly increase the dimensions of the apparatus and raise the assembling cost.
An effective way of alleviating the noise problem and the vibration problem of the rotary polygon mirror is to reduce the number of revolutions per unit time of the rotary polygon mirror. Meanwhile, so-called multi-beam deflection scanners adapted to use a plurality of light sources and hence so many light beams simultaneously are known. Such scanners are designed to produce high density images at high speed if the rotary polygon mirror is driven with a relatively small number of revolutions per unit time. If, for instance, two light beams are used, the photosensitive member of the rotary drum is exposed to two light beams simultaneously. Therefore, the time required for the photosensitive member to be fully exposed to light can be reduced to a half of the time required for the exposure process of the photosensitive member using a single light beam. Similarly, if four light beams are used, the number of revolutions per unit time of the rotary polygon mirror can be reduced to a quarter of that of the rotary polygon mirror using a single light beam.
Thus, the use of a plurality of light sources is effective to avoid the problems arising form the-high speed operation of the rotary polygon mirror and including those of vibrations, noise and heat. However, with the known technology, a plurality of light sources are arranged independently in the optical cabinet so that each of them requires a cumbersome operation of aligning the optical axis and securing it to the cabinet by means of screws. Therefore, the use of a plurality of light sources inevitably entails an increased number of parts to be assembled. Additionally, each of the light sources requires a considerable space for accommodating the screws securing it to the cabinet to make the latter dimensionally remarkable.
Then, the light beams emitted from the plurality of light sources are deflected by the rotary polygon mirror to scan the photosensitive member in the main-scanning direction with regular intervals separating them. As a result, the photosensitive member is exposed to light.
In order for any adjacently located ones of the plurality of light beams to be separated from each other accurately by a predetermined distance, all the light sources have to be accurately positioned in and secured to the optical cabinet. Then, positioning the light source accurately is a painstaking operation and requires a large optical cabinet.
More specifically, in order to realize a pixel density of 600 dpi, for instance, the intervals separating the scanning lines on the photosensitive member have to be regulated so as to be equal to 42.3 &mgr;m. Then, the operation of aligning the optical axis of each of the light sources is time consuming. Thus, there is a demand for an optical scanner that provides an improved efficiency for assembling.
SUMMARY OF THE INVENTION
In view of the above identified problems of the prior art, it is therefore an object of the present invention to provide an optical scanner designed to comprise a plurality of light sources for the purpose of high speed and high density printing that allows a simplified operation for securing the light sources in position and requires only a limited space for the light sources to reduce the size of the optical cabinet and the cost of assembling it.
Another object of the present invention is to provide an optical scanner comprising a plurality of light sources that are unitized with respective collimator lenses to allow a simplified operation for securing the light source units to the optical cabinet and the use of only a limited space to accommodate the li

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