Incremental printing of symbolic information – Light or beam marking apparatus or processes – Scan of light
Reexamination Certificate
1999-09-02
2001-03-13
Le, N. (Department: 2861)
Incremental printing of symbolic information
Light or beam marking apparatus or processes
Scan of light
C347S256000, C347S243000, C347S259000, C359S197100, C359S206100, C359S216100, C359S662000
Reexamination Certificate
active
06201561
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical scanner for scanning laser beams, a scanning lens, which is an optical component for scanning laser beams, and an image forming apparatus for forming an image by scanning laser beams, and more particularly to an optical scanner having an one-piece comprised scanning lens having the function of an F&thgr; lens used in a laser scanning optical system, a scanning lens and an image forming apparatus.
2. Description of the Related Art
A laser beam scanner is used for a laser printer or a digital copying machine. This laser beam scanner generally comprises: as shown in, for example,
FIG. 3
, a laser beam source
11
; a collimator lens
13
for converting divergent light from the laser beam source
11
into substantially parallel light beams; a slit
15
; a cylinder lens
17
for focusing only parallel light beams in the slow scan direction; a light deflector
19
having a reflective surface near a focal position focused by the cylinder lens
17
for rotating at a constant angular velocity; an F&thgr; lens
21
for moving a light beam deflected at a constant angular velocity on a surface to be scanned at a constant velocity and focusing the light beam in the fast scan direction; and an inclination-correcting cylinder mirror
23
for causing the reflective surface of the light deflector
19
to be conjugate with the surface to be scanned in the slow scan direction and focusing the light beam on the surface to be scanned (see Japanese Published Unexamined Patent Application No. 6-18803).
In the above-described laser beam scanner, the following three functions are requested for the optical system (scanning optical system) on or after the light deflector. The first function is to convert a light beam deflected by the light deflector at a constant angular velocity so as for the light beam to scan on the surface to be scanned at a constant velocity. The second function is to cause the reflective surface of the light deflector to be conjugate with the surface to be scanned in order to prevent the scanning line position on the surface to be scanned from fluctuating in the slow scan direction by means of an error when a plurality of reflective surfaces, which the light deflector has, incline in the slow scan direction. The third function is to cause both substantially parallel light beams in the fast scan direction and divergent light beams in the slow scan direction to form an image on the surface to be scanned.
Such functions required for the scanning optical system have generally been implemented by a plurality of optical elements (two pieces of F&thgr; lenses and a cylinder mirror in the above-described example). In order to reduce the component count in the laser beam scanner and to make the apparatus size compact, however, there have been proposed a large number of techniques for providing one piece of lens with the above three functions (Japanese Published Unexamined Patent Application Nos. 62-138823, 4-50908, 8-190062 and the like) . Almost all of these techniques are applied to an optical system in which an incident light beam on a light deflector and a reflective light beam thereby are in a fast scan surface including the normal of a deflecting reflective surface.
As the scanning optical system using an F&thgr; lens, there are an underfilled optical system and an overfilled optical system.
In the underfilled optical system, the width of a light beam becoming incident on a polygonal rotating mirror is smaller than the polygonal rotating mirror, and the light beam does not protrude outwardly of the reflective surface while the surface to be scanned is being scanned by the light deflector (see the Japanese Published Unexamined Patent Application No. 6-18803). In the underfilled optical system, as shown in
FIG. 4
, all light beams becoming incident on the light deflector are used for deflection and scanning, and therefore, the light beam energy is effectively used. Since the width of a light beam becoming incident on the F&thgr; lens irrespective of any scanning angle is constant, there is a merit that the diameter and quantity of light of an optical spot to be imaged on the surface to be scanned are uniform. Since, however, the width of the reflective surface in the light deflector must be made sufficiently larger than that of the incident light beam, there is a demerit that the diameter of the polygonal rotating mirror will be dramatically large if a number of the reflective surfaces is increased to speed up or the width of the incident light beam on the light deflector is made larger to reduce the optical spot diameter on the surface to be scanned.
On the other hand, in the overfilled optical system, the width of a light beam becoming incident on a polygonal rotating mirror is larger than the polygonal rotating mirror, and the light beam always protrudes outwardly of the opposite sides of the reflective surface used for scanning while the surface to be scanned is being scanned by the light deflector (see the Japanese Published Unexamined Patent Application No. 8-171070). In the overfilled optical system, as shown in
FIG. 5
, part of a light beam becoming incident on the light deflector is used for deflection and scanning, and therefore, a loss in the light beam energy is significant. Since the width of a light beam becoming incident on the F&thgr; lens changes depending upon the scanning angle, there is a demerit that the diameter of an optical spot imaged on the surface to be scanned and power significantly change. Since, however, the width of the reflective surface in the light deflector becomes an opening width of the light beam becoming incident on the F&thgr; lens and the width of the reflective surface of the polygonal rotating mirror becomes smaller, there is a merit that the diameter of the polygonal rotating mirror required is small even if an attempt is made to increase the number of reflective surfaces or to make the optical spot diameter on the surface to be scanned smaller.
In order to reduce the respective demerits of the above-described underfilled and overfilled optical systems, there has been proposed a method of causing a light beam to become incident on the light deflector from near the central portion of the deflection angle of the light deflector as also shown in
FIG. 6
(Japanese Published Unexamined Patent Application Nos. 63-204221 and 9-230274). In the case of the underfilled optical system, there is a merit that the width of the reflective surface can be made smaller and the diameter of the polygonal rotating mirror becomes smaller because the light beam width projected on the reflective surface becomes small. Also, in the case of the overfilled optical system, there is a merit that a change in the width of the incident light beam on the F&thgr; lens based on deflecting reflection becomes smaller and heterogeneity in the optical spot diameter and the quantity of light on the surface to be scanned is improved. Since it is actually necessary to separate the incident light beam on the light deflector from the deflected and reflected light beam, the light beam is caused to obliquely become incident in the slow scan direction although on the optical axis of the F&thgr; lens in the fast scan direction.
Also, for the optical scanner, there has been proposed a technique using a plurality of light beams in order to correspond to plural colors (see the Japanese Published Unexamined Patent Application No. 10-73778). According to this technique, as shown in
FIG. 7
, a plurality of light beams arranged in the slow scan direction are incident on a single light deflector and a plurality of light beams reflected by the light deflector are guided to different positions on the surface to be scanned by respective different mirrors not to thereby increase the number of the light deflectors. Also, in order to cause the plurality of light beams arranged in the slow scan direction to become incident on the same reflecting point on the light deflector, the respective light beams
Fuji 'Xerox Co., Ltd.
Le N.
Oliff & Berridg,e PLC
Pham Hai C.
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