Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
2002-06-06
2004-12-14
Chang, Audrey (Department: 2872)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S205100, C347S244000, C347S259000
Reexamination Certificate
active
06831764
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a light scanning device and an image forming apparatus using the same and, more particularly, to a light scanning device which is suitable for a laser beam printer or digital copying apparatus which has, for example, an electrophotographic process of reflecting/deflecting (deflecting/scanning) a light beam, optically modulated and emitted from a light source means, by using a deflecting element formed from a rotating polyhedral mirror or the like, and recording image information by optically scanning a surface to be scanned through an imaging optical system having f-&thgr; characteristics.
2. Description of Related Art
In a conventional scanning optical device for a laser beam printer (LBP) or the like, the light beam which is optically modulated in accordance with an image signal and emitted from a light source means is cyclically deflected by a light deflector formed from, e.g., a rotating polyhedral mirror (polygon mirror), and the light beam is focused into a spot on the surface of a photosensitive recording medium (photosensitive drum) and optically scanned on the surface by an imaging optical system having f-&thgr; characteristics.
FIG. 11
is a schematic view showing the main part of a conventional light scanning device.
Referring to
FIG. 11
, the divergent light beam emitted from a light source means
71
is converted into a substantially parallel light beam by a collimator lens
72
, and the light beam is limited by a stop
73
to be incident on a cylindrical lens
74
having predetermined refracting power only in the sub scanning direction. Of the substantially parallel light beam incident on the cylindrical lens
74
, the light in a main scanning section emerges without any change. The light in a sub-scanning section is focused and substantially formed into an almost line image on a deflecting surface (reflecting surface)
75
a
of a light deflector
75
.
The light beam reflected/deflected by the deflecting surface
75
a
of the light deflector
75
is guided onto the photosensitive drum surface
78
serving as a surface to be scanned through an imaging optical system (f-&thgr; lens system)
76
having f-&thgr; characteristics. By rotating the light deflector
75
in the direction indicated by an arrow A, a photosensitive drum surface
78
is optically scanned to record image information.
In such a light scanning device, in order to record high-resolution image information, for example, the following requirements need to be satisfied. The curvature of the image surface is properly corrected throughout the surface to be scanned. Distortion characteristics (f-&thgr; characteristics) with uniform velocity characteristics are set between a scanning angle &thgr; and an image height in the Y direction. A spot diameter on the image surface at each image height is uniform. Conventionally, various types of light scanning devices that satisfy such optical characteristics or correction optical systems (f-&thgr; lenses) have been proposed.
As laser beam printers and digital copying apparatuses have decreased in size and cost, similar requirements are imposed on light scanning devices.
As an arrangement that satisfies such requirements, a light scanning device having an f-&thgr; lens formed from a single lens is disclosed in, for example, Japanese Patent Application Laid-Open Nos. 4-50908 and 9-33850.
According to Japanese Patent Application Laid-Open No. 4-50908, a high-order aspherical surface is used in the main scanning direction of an f-&thgr; lens to relatively properly correct aberration characteristics. However, since the magnification in the sub scanning direction between a light deflector and a surface to be scanned is not uniform, the spot diameter in the sub scanning direction tends to change with a change in image height.
According to Japanese Patent Application Laid-Open No. 9-33850, the curvatures of at least two of the lens surfaces of an f-&thgr; lens in the light scanning device in the sub scanning direction continuously change at the effective portion of an imaging lens along the main scanning direction independently of the curvature in the main scanning direction. This makes it possible to control the position of the principal plane in the sub scanning direction by bending the two surfaces and make the sub scanning magnification at each image height uniform, thereby making the spot diameter uniform.
In the above proposal, in order to make the sub scanning magnification uniform, the position of the principal plane is so controlled as to make the main scanning uniform by bending at least two surfaces. Although this allows completely independent setting of a main scanning shape and sub scanning shape, requirements such as suppression of an increase in lens thickness tend to make the lens shape in the main scanning direction have a relatively large aspherical surface amount.
A lens having a large aspherical surface amount in the main scanning direction like the one described above is subjected to considerable deteriorations in optical performance due to the arrangement errors of the respective lens surfaces and lens. Of the deteriorations in optical performance, scanning line bending in the sub scanning direction, in particular, cannot be corrected by adjusting mirrors and the like arranged in the device body, unlike scanning line height deviation, scanning line inclination, and the like. This therefore poses a serial problems. In order to suppress scanning line bending to a low level, the respective lens surfaces and the lens need to be arranged with high precision in accordance with design values or an adjusting mechanism needs to be provided for the lens to adjust the arrangement according to the design values.
FIG. 13
is a graph in which a scanning line bending amount is plotted at each image height, with an optical axis regarded as an origin, in the scanning lens disclosed as the first embodiment (
FIG. 12
) in Japanese Patent Application Laid-Open No. 9-33850 when each surface and the lens block are decentered by 50 &mgr;m in the Z direction (a direction perpendicular to the optical axis and the main scanning direction). In
FIG. 12
, the reference characters S
1
to S
8
respectively identify a light source, a collimator lens incident surface (plane), a collimator lens exit surface, a cylinder lens incident surface (plane), a cylinder lens exit surface, a deflection surface, a scanning lens incident surface and a scanning lens exit surface. In
FIGS. 13 and 14
, Block, R
1
, R
2
respectively refer to a lens, an incident surface and an exit surface.
FIG. 14
shows the scanning line bending amounts in a case wherein each surface and the lens block are tilted by 3′ around a straight line (rotation axis) which passes through the respective surface vertexes (the surface vertex on the incident side of the lens in the case of the lens block) and is parallel to the main scanning direction.
In a scanning lens of a type which has a large aspherical surface amount in the main scanning direction and also has power in the sub scanning direction on the two surfaces, a very large scanning line bending amount is produced by decentering in the Z direction. In addition, the scanning line bend sensitivity level with respect to a tilt is high. Obviously, therefore, even if the performance of the lens is compensated in terms of design, the lens does not reach a practical level when it actually processed and mounted in a housing.
In a color image forming apparatus in which light scanning devices are respectively arranged in correspondence with four photosensitive devices (photosensitive drums), and latent images are formed on them by laser beams to form Y (yellow), M (magenta), C (cyan), and Bk (black) original images on the corresponding photosensitive device surfaces, since the four color images, i.e., the Y, M, C, and Bk images, formed on the respective photosensitive device surfaces are superimposed on a transfer medium such as a paper sheet, if scanning lines of the light scan
Ishihara Keiichiro
Kato Manabu
Sato Hiroshi
Shimomura Hidekazu
Allen Denise S.
Canon Kabushiki Kaisha
Chang Audrey
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