Incremental printing of symbolic information – Light or beam marking apparatus or processes – Scan of light
Reexamination Certificate
2003-02-12
2004-11-30
Pham, Hai (Department: 2861)
Incremental printing of symbolic information
Light or beam marking apparatus or processes
Scan of light
C347S244000
Reexamination Certificate
active
06825870
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates in general to a scanning optical apparatus and an image forming apparatus using the same. In particular, the present invention is suitable for an image forming apparatus, such as a laser beam printer, a digital copying machine or a multi-function printer having an electrophotographic process, which is adapted to deflect light beam emitted from a light source by a deflecting element to scan optically a surface to be scanned through a scanning optical element having the f&thgr; characteristics thereby to record image information.
2. Related Background Art
Heretofore, in a scanning optical apparatus such as a laser beam printer, a light beam which has been optically modulated in accordance with an image signal to be emitted from the light source are periodically deflected by an optical deflector including a rotating polygon mirror for example to be converged on a surface of a photosensitive recording medium (photosensitive drum) in a spot-like shape by a scanning optical element having the f&thgr; characteristics and then the surface of the recording medium is optically scanned with the deflected luminous fluxes to carry out the image recording.
FIG. 16
is a perspective view schematically showing construction of a main portion of a conventional scanning optical apparatus. In the figure, diverged light beam emitted from a light source
91
are made substantially the parallel light beam or converged light beam by a collimator lens
92
, and then the luminous fluxes (quantity of light) are shaped by an aperture stop
93
to be inputted into a cylindrical lens
94
which has a refracting power only in a sub scanning direction. The luminous flux entered into the cylindrical lens
94
is emitted within a main scanning cross section in its entirety, while it is converged within the sub scanning cross section to be imaged substantially in the form of a line image in the vicinity of a polygon mirror surface
95
a
of an optical deflector
95
constituted by a rotating polygon mirror.
Then, the light beam which has been reflected and deflected by the polygon mirror surface
95
a
of the optical deflector
95
are introduced onto a photosensitive drum surface as a surface
98
to be scanned through an f&thgr; lens system (scanning optical element)
96
having the f&thgr; characteristics. Then, the photosensitive drum surface
98
is scanned in a direction (main scanning direction) indicated by an arrow B with the deflected light beam by rotating the optical deflector
95
in a direction indicated by an arrow A to thereby record the image information.
In such a scanning optical apparatus, for the optical correction of the surface tilt of an optical deflector within the sub scanning cross section, it is necessary to establish an optical conjugate relationship (imaging relationship) between the vicinity of the deflecting surface of the optical deflector and the surface to be scanned.
FIG. 17
is a cross sectional view of a main portion in the sub scanning direction from the deflecting surface to the surface to be scanned. In the case where the deflecting surface
95
a
and the surface
98
to be scanned have perfectly the conjugate relationship in such a manner, under the condition in which the spherical aberration of the scanning optical element
96
is less and also a quantity of surface tilt of the deflecting surface is less, as indicated by dotted lines, the light beam deflected by the deflecting surface
95
a
having the surface tilt is necessarily returned back onto an optical axis and hence has no displacement in the sub scanning direction on the surface
98
to be scanned.
However, in the optical deflector such as a polygon mirror in which the deflecting surface is apart from the rotational center as shown in
FIG. 18
, the deflecting surface is moved back and forth depending on the deflected field angle for deflection. Thus, if the light beam within the sub scanning cross section is wanted to be imaged on the surface to be scanned in the whole field angle, then it is difficult to perfectly establish the conjugate relationship between the deflecting surface
95
a
and the surface
98
to be scanned in the whole field angle. By the way, in the figure, the same constituent elements as those shown in
FIG. 16
are designated with the same reference numerals. Then, reference numeral
71
designates a line image obtained through the cylindrical lens
94
, reference numeral
81
designates a polygon mirror surface for deflecting the light beam on the optical axis, reference numeral
82
designates a polygon mirror surface for deflecting the outermost off-axis light beam on the side opposite to the light source side, and reference numeral
83
designates a polygon mirror surface for deflecting the outermost off-axis light beam on the light source side.
Thus, since the conjugate relationship is perfectly established in the image height at which the line image
71
obtained through the cylindrical lens
94
coincides with the deflecting surface, the irradiation position shift due to the surface tilt is not caused. However, the variation in the irradiation position due to the surface tilt is large in the vicinity of the optical axis and in the vicinity of the optical path most outside the optical axis in each of which the distance from the line image
71
to the deflecting surface is long. This variation in the irradiation position due to the surface tilt causes nonuniformity in pitch in the sub scanning direction with the cycle of the number of polygon mirror surfaces and hence becomes a problem in terms of recording image information with high accuracy.
On the other hand, in recent years, a multi-beam scanning optical apparatus for scanning simultaneously a surface with a plurality of light beams emitted from a multi-beam light source has been proposed in order to cope with the high speed and the high definition. However, since in this apparatus, a certain quantity of pitch error originally remains among a plurality of light beams, there is encountered a problem in that the irradiation position shift due to the surface tile largely influences an image.
Furthermore, there has been proposed a color image forming apparatus in which four photosensitive bodies (photosensitive drums) are used for the high speed operation of a color copying machine, and scanning optical apparatuses are arranged therein, respectively, to form latent images using laser beams thereby to form images of a manuscript having Y(yellow), M(Magenta), C(cyanogen), and Bk(black), respectively, on the surfaces of the corresponding photosensitive bodies. However, in this apparatus, there is encountered a problem in that the number of lines of used mesh dot patterns and the screen angles are variegated, and thus in a part of the patterns, the irradiation position shift due to the above-mentioned surface tilt appears in the form of a Moiré pattern in an image to degrade remarkably the image quality.
SUMMARY OF THE INVENTION
In the light of the foregoing, the present invention has been made in order to solve the above-mentioned problems associated with the prior art, and it is, therefore, an object of the present invention to provide a scanning optical apparatus and an image forming apparatus using the same adapted to image recording of high quality which are capable of improving the function of correcting the surface tilt while meeting the curvature of field within a sub scanning cross section by utilizing an easy and simple method, and of suppressing the irradiation position shift due to the surface tilt of a deflecting surface of a deflector to the less degree.
According to one aspect of the invention, a scanning optical apparatus includes: a first optical element for converting a state of light beam emitted from light source means into another state; a second optical element for converting the light beam from the first optical element into a line image elongated in a main scanning direction; a deflecting element for deflecting and scanning the l
Canon Kabushiki Kaisha
Fitzpatrick ,Cella, Harper & Scinto
Pham Hai
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