Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
2000-11-28
2004-12-21
Cherry, Euncha P. (Department: 2872)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S205100
Reexamination Certificate
active
06833939
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of Invention
The present invention relates to a light scanning method and a light scanning device, and more particularly to a light scanning method in a light scanning device which deflects plural light beams emitted from a light source by reflection surfaces of a deflector, and simultaneously main-scans by the plural light beams deflected by the deflector a surface to be scanned, and to a light scanning device.
2. Description of the Related Art
Image recording apparatuses for recording images by light beams such as laser printers and electronic photocopiers are widely used. In such image recording apparatus, a photosensitive material is scanned by a light beam by means of a light scanning device.
In the light scanning device, generally, the light beam emitted from the semiconductor laser is modulated according to image data, and, via a collimator lens or the like, is incident on the reflection surfaces of a rotary polygonal mirror rotating at a predetermined speed.
By rotation of this rotary polygonal mirror, the light beam is deflected while its incident angle is being changed continuously, and the light beams scans the photosensitive material. The light beam reflected by the reflection surfaces of the rotary polygonal mirror is guided to the photosensitive material via an f&thgr; lens, a cylinder mirror (or cylinder lens), and the like, and scans the photosensitive material at a constant speed and is focused on the photosensitive material. As a result, an image is exposed and recorded on the photosensitive material.
Recently, to meet the need for higher speeds of image forming apparatus, improvements in the scanning speed of the light scanning device have been desired. As a technology for improving the speed of a light scanning device, the simultaneous scanning method has been known in which plural scanning lines are scanned simultaneously by one scanning by using plural light beams.
In designing the optical system of the light scanning device of this simultaneous scanning system (hereinafter called plural-beam scanning optical system), the bow difference and pitch deviation between the plural light beams must be considered.
FIG. 9
shows bow difference occurring in the case of scanning with two light beams, and
FIG. 10
shows pitch deviation when scanning with two light beams. In the diagrams, the broken line indicates an ideal scanning line position, and the solid line represents an actual scanning line position.
The bow difference means changes in the interval between two light beams in the subscanning direction depending on the scanning position in the main scanning direction, due to a difference in the curvature of the scanning lines formed by the light beams. The pitch deviation means widening or narrowing of the interval between scanning lines when two light beams are scanned simultaneously in a state in which the interval in the subscanning direction between the light beams deviates from a prescribed value. When scanning plural scanning lines simultaneously in one scanning by using plural light beams, if such bow difference or pitch deviation occurs, the image becomes uneven in the subscanning direction, and the image quality is lowered. Such bow difference and pitch deviation must be suppressed in order to obtain image output of high image quality.
Generally, bow difference and pitch deviation are more likely to occur the further the plural beams are apart from the optical axis of the optical system. Conventionally, the plural beams were usually only two or at most four beams which were arranged in a row. When the light emitting points outputting the respective light beams were arranged in a row at the light source, even if the interval between the actual light emission points was wide (see distance “a” in FIG.
11
), by inclining the entire light source obliquely in the subscanning direction, the interval between light emission points in the subscanning direction could be apparently narrowed (see distance “b” in FIG.
11
), and deviation of the plural beams from the light source could be kept small. It was therefore relatively easy to decrease the bow difference and pitch deviation.
However, when the number of beams was further increased to further improve the speed, if the bow difference and pitch deviation were decreased by inclining the light source in the subscanning direction, on the contrary, the problem arose that the plural beams were widely apart in the mainscanning direction. Specifically, when the plural beams were apart in the main scanning direction, since the timing of each beam scanning a same point in the main scanning direction was different, the required buffer memory capacity for aligning the pixel positions in the main scanning direction increased, and a problem arose that the width of the reflection surfaces of the rotary polygonal mirror in the main scanning direction had to be increased substantially.
According to the technology disclosed in Japanese Patent Application Laid-Open (JP-A) No.5-294005, by disposing beams in a two-dimensional arrangement by using a VCSEL (Vertical Cavity Surface Emitting Laser Diode) as the light source, even if the number of beams is increased, the beams are prevented from greatly moving away from the optical axis of the optical system. However, in the case of the light source having light emission points disposed in a two-dimensional arrangement, unlike the conventional one-row arrangement light source, the interval between light emission points in the subscanning direction cannot be apparently narrowed by inclining the entire light source, and hence it is desired to solve the problems of bow difference and pitch deviation optically. So far, several methods have been proposed for reducing the bow difference or pitch deviation optically.
Technology for Reducing Bow Difference
1) Using a f sin &thgr; lens: JP-A No. 1-163717
2) Reducing the subscanning direction lateral magnification rate of an optical system: JP-A No. 2-54211, JP-A No. 9-281421
3) Canceling mutual bow differences occurring at plural positions: JP-A No. 2-129614, JP-A No. 8-118725
4) Specifying intersecting positions of plural beams: JP-A No. 6-18802, JP-A No. 7-209596, JP-A No. 9-274152
5) Limiting tolerance of bow difference: JP-A No. 6-202019
6) Balancing subscanning direction image plane curvature and bow difference: JP-A No. 7-199109
7) Using an aspherical lens: JP-A No. 1-180510, JP-A No. 8-297256, JP-A No. 9-33850, JP-A No. 9-146030, JP-A No.10-333069, JP-A No. 11-84285
8) Aligning plural beams in the bow direction: JP-A No. 10-68898, JP-A No. 10-293260
Technology for Reducing Pitch Deviation
1) Making plural beams incident, in parallel, onto the surface to be scanned: JP-A No. 7-209596, JP-A No. 9-274151
Thus, various technologies for reducing either one of bow difference and pitch deviation have been proposed, but in an actual light scanning device of a simultaneous scanning system, both bow difference and pitch deviation must be suppressed. Of the aforementioned technologies, only the technology disclosed in JP-A No. 7-209596 refers to both bow difference and pitch deviation.
The technology disclosed in JP-A No. 7-209596 is as shown in FIG.
12
. Light beams
104
,
106
emitted from laser diodes
100
,
102
are incident on a rotary polygonal mirror A through a collimator lens
108
and a cylindrical lens
110
. After the light beams
104
,
106
incident on the rotary polygonal mirror A are reflected by the reflection surfaces of the rotary polygonal mirror A, they pass through an anamorphic motion compensation optical system (MCO)
112
,
114
having an f&thgr; lens, a cylindrical lens, and a cylindrical mirror, and leave the light scanning device in a state parallel to the system axis (optical system optical axis)
116
. That is, occurrence of bow difference and pitch deviation is suppressed by making the main exit beam of the light scanning device have a telecentric profile.
The cause of occurrence of bow difference in the plural-beam scanning optical system is, as mentioned above, that the li
Cherry Euncha P.
Morgan & Lewis & Bockius, LLP
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