Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
2000-01-24
2001-01-09
Phan, James (Department: 2872)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S216100, C359S217200, C347S233000, C347S243000
Reexamination Certificate
active
06172788
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a light scanning device and more particularly to a light scanning device wherein a plurality of light beams are irradiated, the plurality of irradiated light beams are deflected by a deflecting means and divided scanning is carried out on each scanning line by the deflected plural light beams, and a light scanning device for use in digital image forming apparatuses for electrophotography such as laser printers, laser copiers, and the like.
Further, this invention also relates to an optical device which is used in such image recording apparatus as laser printers, digital copiers and the like and in which light beam is swept according to image information so as to scan and expose a photoconductor, and a scanning method of the optical device, and more specifically to an optical device in which a single scanning line on the photoconductor is scanned in two divisions with two light beams at the same time and a scanning method of the optical device.
2. Description of the Related Art
In an ordinary optical scanning apparatus of the prior art, the face width of a polygon mirror is larger than the beam width in the main scanning direction of a light beam striking the polygon mirror. This face width is designed so as to cover the whole portion of the incident light beam regardless of any scanning angle (the so-called underfilled optical system).
In this underfilled optical system, as shown in
FIG. 28
, when an incident light beam having a beam width D0 in the main scanning direction is deflected by the polygon mirror, the beam width in the main scanning direction of a deflected light beam (deflection beam) is equal to the beam width of the incident light regardless of the scanning position. That is, assuming that the beam widths in the main scanning direction of the deflection beams leading to Start Of Scan (SOS), Center Of Scan (COS) and End Of Scan (EOS) are Ds, Dc, De respectively, D0=Ds=Dc=De is established. Consequently, the light volume and beam diameter (the effective diameter of a light spot converged on a photoconductor) at each scanning position are equalized thereby minimizing deterioration of image quality.
In recent years, the demand for higher recording speeds and resolutions has increased in such image recording apparatuses as laser beam printers, digital copiers, and the like using a light scanning device. To meet this demand for higher recording speeds and resolutions, a method may first be considered in which the time for a light beam to scan a single scanning line on the photoconductor is reduced by increasing the rotation speed of the polygon mirror.
However, there is a problem to be solved to achieve this idea. That is, usually the polygon mirror is rotated directly by a driving motor and currently, the upper limit of the rotation speed of the driving motor is 15,000 rpm (when ball bearings are used). However, this high speed motor is actually difficult to utilize because of the large increase in production costs. Even if pneumatic bearings are used, the limit is 40,000 rpm. Thus, there is an upper limit to the speeds and resolutions able to be obtained by increasing the rotation speed of the polygon mirror.
Increasing the number of deflecting faces of the polygon mirror can also be considered. However, if the number of deflecting faces increases, the diameter of the polygon mirror increases so that it is difficult to drive it with an ordinary driving motor. If, for example, under the underfilled optical system, it is intended to scan an A3 size sheet and ensure a beam diameter of about 60 &mgr;m on the photoconductor, if the number of the faces of the polygon mirror is more than 10, the diameter of the polygon mirror exceeds 100 mm. To solve this problem, Japanese Patent Application Laid-Open (JP-A) No. 50-93719 has disclosed an overfilled optical system as a technology for avoiding an enlargement of the polygon mirror diameter (see FIG.
25
).
As shown in
FIG. 25
, the light scanning device disclosed in the aforementioned patent comprises a light beam generating means
81
, a modulation means
82
A, a flat/convex cylindrical lens
86
A having a curvature in a scanning direction, a focusing lens
88
A, a polygon mirror
90
A, an incline correcting cylindrical lens
92
, and a photoconductor drum
94
. According to the aforementioned patent, it is desirable that the number of the deflecting faces of the polygon mirror
90
A is 20-30 and the scanning angle (±&agr;) is ±12-18°.
According to the overfilled optical system, by expanding the beam width in the main scanning direction of light beam striking the polygon mirror beyond the face width of the polygon mirror as shown in
FIG. 29
, the diameter of the polygon mirror can be reduced, thereby making it possible to avoid an enlargement of the polygon mirror diameter even if the number of the deflecting faces thereof is increased.
However, if the number of the deflecting faces of the polygon mirror
90
A is increased so as to increase the speed, the scanning angle at which the light beam is scanned by one deflecting face is inevitably decreased. Thus, the scanning width at a fixed distance from the polygon mirror
90
A is decreased as the scanning angle is decreased. That is, to secure the same scanning width as in the prior art, the distance from the polygon mirror
90
A to the photoconductor drum
94
needs to be increased, and the size of the light scanning device needs to be enlarged. For example, as is shown in
FIG. 26
, if it is intended to obtain a scanning width of 297 mm which is equivalent to A3 size paper, with the scanning angle (±&agr;) being ±2~18°, the focal length f of the optical system exceeds 500 mm.
Although in the prior art, the central value of the beam diameter on the photoconductor drum
94
is assumed to be approximately 150 &mgr;m, currently, as resolutions are intensified, this value has commonly come to approximately 60 &mgr;m. Even in the overfilled optical system, if the number of the deflecting faces of the polygon mirror is 20-30 as shown in
FIG. 27
, the internal circle diameter of the polygon mirror exceeds 60 mm, and therefore this polygon mirror is difficult to rotate with a cheap motor. In
FIG. 27
, it is assumed that the scanning width is 297 mm and the beam diameter is 5 &mgr;m.
As described above, even if the overfilled optical system is employed, there is a limit in the increase in speed, the increase in resolution, and the reduction in size able to be obtained.
Therefore, as an art in which a high speed and high resolution are realized while a small size is also attained, Japanese Patent Application Laid-Open (JP-A) No. 63-47718 has disclosed an optical device (light scanning device) in which divided scanning is performed on the surface of the photoconductor in the main scanning direction. This patent does not mention anything about overfilled optical systems.
In this optical device (light scanning device), the first half of an image area on the photoconductor is scanned with one laser beam and the second half thereof is scanned with an other laser beam. Because divided scanning is performed on the same scanning line with two laser beams, the number of the deflecting faces of the polygon mirror can be increased thereby increasing the print speed.
That is, in this optical device (light scanning device), as shown in
FIG. 30
, two light beams are projected from two laser beam sources onto the same point on the polygon mirror
9
so that the laser beams are perpendicular to the same deflecting face thereof and have different incident angles. Consequently, the divided regions
10
,
11
on the photoconductor are scanned at the same time with two deflection beams from the polygon mirror
9
. In this optical device, the difference in incident angle between two laser beam sources to a deflecting face is assumed to be &thgr;/2 while the entire scanning angle to a plane to be scanned of the photoconductor
3
is &thgr;. Divided scanning is performed on t
Sekikawa Yoshihito
Suzuki Takayoshi
Toda Tsuneo
Finnegan Henderson Farabow Garrett & Dunner L.L.P.
Fuji 'Xerox Co., Ltd.
Phan James
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