Incremental printing of symbolic information – Light or beam marking apparatus or processes – Scan of light
Reexamination Certificate
2002-06-03
2004-10-12
Stephen, Juanita (Department: 2853)
Incremental printing of symbolic information
Light or beam marking apparatus or processes
Scan of light
C359S206100
Reexamination Certificate
active
06803942
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. 17
is a schematic view showing the main part of a conventional light scanning device.
Referring to
FIG. 17
, the divergent light beam emitted from a light source means
1
is converted into a substantially parallel light beam by a collimator lens
2
, and the light beam is limited by a stop
3
to be incident on a cylindrical lens
4
having predetermined refracting power only in the sub scanning direction. Of the substantially parallel light beam incident on the cylindrical lens
4
, the light in a main scanning cross-section emerges without any change. The light in a sub-scanning cross-section is focused and substantially formed into an almost line image on a deflecting surface (reflecting surface)
5
a
of a light deflector
5
.
The light beam reflected/deflected by the deflecting surface
5
a
of the light deflector
5
is guided onto the photosensitive drum surface
8
serving as a surface to be scanned through an imaging optical system (f-&thgr; lens system)
6
having f-&thgr; characteristics. By rotating the light deflector
5
in the direction indicated by an arrow A, the photosensitive drum surface
8
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 Y. 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, a scanning line bend 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 serious problem. In order to suppress a scanning line bend 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.
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 scanning devices corresponding to the respective photosensitive devices bend, errors are produced in the shapes of the scanning lines among four colors. This causes color misregistration on the image on the transfer medium, and hence results in a considerable deterioration in image quality.
FIG. 18
shows a scanning line moving amount on the surface to be scanned when a scanning optical element and each optical element surface are decentered (by 50 &mgr;m) in a direction (sub scanning direction) perpendicular to the main scanning direction in an embodiment disclosed in Japanese Patent Application Laid-Open No. 9-33850. As is obvious from
FIG. 18
, when an optical surface is decentered, a large scanning line bend occurs, and an improvement in arrangement precision or decentering adjustment is required to obtain a high-quality image.
Referring to
FIG. 18
, the R
1
curve represents a scanning line bend amount when only the incident surface of an f-&thgr; lens
6
is decentered in the sub scanning direction; the R
2
curve, a scanning line bend amount when only the exit surface of the f-&thgr; lens
6
is decentered; and the block curve, a scanning line bend amount when both the incident and exit surfaces of the f-&thgr; lens
6
are decentered.
SUMMARY OF THE INVENTION
The present invention has been made to solve the above problems, and has as its object to provide a light scanning device which has an f-&thgr; lens formed from a single lens, and can suppress a scanning line bend caused by the arrangement error of the lens to a low level by optimally shaping the f-&thgr; lens, and an image forming apparatus using the device.
An object of the present invention is to provide a light scanning device having an imaging optical system for guid
Ishihara Keiichiro
Kato Manabu
Sato Hiroshi
Shimomura Hidekazu
Canon Kabushiki Kaisha
Fitzpatrick ,Cella, Harper & Scinto
Mouttet Blaise
Stephen Juanita
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