Incremental printing of symbolic information – Light or beam marking apparatus or processes – Scan of light
Reexamination Certificate
2002-04-04
2004-12-21
Pham, Hai (Department: 2861)
Incremental printing of symbolic information
Light or beam marking apparatus or processes
Scan of light
C347S235000, C347S250000, C347S256000
Reexamination Certificate
active
06833855
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a multi-beam scanning optical system and an image forming apparatus using the same. More particularly, the present invention is suitably implemented as an image forming apparatus, such as a laser beam printer and a digital copying machine, in which high-quality printing can be realized at high speed with a relatively simple construction.
2. Description of the Related Art
Hitherto, a scanning optical system used in an image forming apparatus, e.g., a laser beam printer and a digital copying machine, has such a construction that a light beam emitted from a light source is introduced to a deflecting unit through an entrance optical system, and the light beam deflected by the deflecting unit is focused by a scanning optical system into the form of a spot on a photoconductor drum surface, i.e., a surface to be scanned, thereby optically scanning the photoconductor drum surface with the light beam.
With recent improvements in performance and function of an image forming apparatus, a demand for operation at higher speed has increased more and more. One solution of meeting such a demand is to employ a plurality of light sources. For example, Japanese Patent Laid-Open No. 9-54263 proposes a multi-beam scanning optical system in which a light source is constituted as a multi-beam laser chip radiating, from one chip, a plurality of laser beams focused into spots lying on a line.
In that multi-beam scanning optical system, an optical means for beam synchronous detection (BD optical system) is usually disposed just upstream of a stage of writing an image signal for precise control of an image-write start position.
FIG. 18
is a sectional view (main scan section view) of principal part of a conventional multi-beam scanning optical system in the main scan direction. Referring to
FIG. 18
, numeral
51
denotes a light source unit comprising two light emitting portions (light sources) constituted by, e.g., semiconductor lasers. The two light emitting portions are separately arranged from each other in both the main scan direction and the sub-scan direction. Numeral
52
denotes an aperture diaphragm for shaping each of light beams emitted from the two light emitting portions into an optimum beam shape. Numeral
53
denotes a collimator lens for converting the light beams having passed the aperture diaphragm
52
into substantially parallel light beams. Numeral
54
denotes a cylindrical lens that has predetermined refracting power only in the sub-scan direction. The above-mentioned elements, such as the aperture diaphragm
52
, the collimator lens
53
and the cylindrical lens
54
, constitute respective components of an entrance optical system
62
.
Numeral
55
denotes a deflecting unit (optical deflector) that is constituted by a rotating polygon mirror, for example, and is rotated at a constant speed in a direction of arrow PA, shown in
FIG. 18
, by a driving unit (not shown) such as a motor. Numeral
56
denotes a scanning optical system
56
that has the f&thgr; characteristic and comprises two first and second f&thgr; lenses. The scanning optical system
56
has the function of compensating an image plane tilt by holding, in a sub-scan section, a conjugate relationship between the vicinity of a deflecting surface
55
a
of the optical deflector
55
and the vicinity of a photoconductor drum surface
57
as a surface to be scanned.
Numeral
58
denotes a return mirror (referred to as a “BD mirror” hereinafter) for reflecting, toward the side of a synchronous detection sensor
61
(described later), a plurality of light beams (referred to as “BD light beams” hereinafter) used for detecting sync signals to adjust the timing of scan start positions on the photoconductor drum surface
57
. Numeral
59
denotes a slit (referred to as a “BD slit” hereinafter) that is provided in a position optically equivalent to the photoconductor drum surface
57
. Numeral
60
denotes a BD lens for making the BD mirror
58
and the synchronous detection sensor
61
located in a conjugate relationship and for compensating a plane tilt of the BD mirror
58
. Numeral
61
denotes a photosensor (referred to as a “BD sensor” hereinafter) serving as the synchronous detection sensor
61
. The above-mentioned elements, such as the return mirror
58
, the BD slit
59
, the BD lens
60
and the BD sensor
61
, constitute respective components of an optical means for beam synchronous detection (BD optical system).
With the arrangement of
FIG. 18
, beam synchronous detection is performed for each of the BD light beams emitted from the two light emitting portions, and the timing of a scan start position in recording of an image on the photoconductor drum surface
57
is adjusted for each light beam emitted from the light emitting portion by using an output of the BD sensor
61
.
In a multi-beam scanning optical system including a plurality of light emitting portions (light sources), however, if the relative positional relationship between respective light beams emitted from the plurality of light sources in the main scan direction is changed on a surface to be scanned with the progress of scan for various reasons, degradation of a printed image occurs. Also, in spite of that the relative positional relationship between the light emitting portions in the main scan direction is not changed during scan, degradation of a printed image also occurs if write start positions are shifted from each other between the light emitting portions.
Such a phenomenon is conceivably caused by a shift of focus position of the BD light beam on the plane of a BD slit (namely, the BD light beam is not properly focused because it is out of focus on the plane of the BD slit) and a shift of focus position of the scanning light beam on the surface to be scanned.
A shift of focus position of the BD light beam on the BD slit plane will be described below with reference to
FIGS. 19
to
26
. Note that, for the sake of clearer representation, the BD sensor that should be illustrated on the upper side in the figure is omitted from all of
FIGS. 19
to
26
. Also, marginal rays are omitted in
FIGS. 20A
,
23
A,
25
A and
26
A.
FIG. 19A
shows a state at the moment when two light beams (herein, a light beam A emitted from one light emitting portion and a light beam B emitted from the other light emitting portion) are focused just at one end (right end as viewed on the figure) of the BD slit in the main scan direction. The light beam A scanned from the left to the right in
FIG. 19A
does not enter the BD sensor until it reaches just the right end of the BD slit. Upon the light beam A reaching the right end of the BD slit, the BD sensor outputs a signal indicating the incidence of the light beam A. Similarly to the light beam A, the light beam B scanned from the left to the right does not enter the BD sensor until it reaches just the right end of the BD slit, and the BD sensor outputs a signal indicating the incidence of the light beam B upon reaching the right end of the BD slit. The timing of write start positions of the light beams A, B is adjusted by detecting the timed relationship between the two output signals from the BD sensor.
However, if the focus positions of the light beams A, B having passed the BD optical system are relatively shifted &dgr;M, as shown in
FIG. 20A
, in the main scan section to the upstream side looking from the slit, i.e., to the side nearer to the deflecting unit, the following phenomenon occurs and the write start positions of the light beams A, B are shifted from each other. More specifically, at the timing at which the light beam A should be focused at the right end of the BD slit and should start entering the BD sensor unless the light beam A is not defocused, the light beam A already enters the surface of the BD sensor because of defocusing (namely, the defocused light beam A at the proper detection timing in this case is indicated by a broken line on the left side in FIG.
20
A). The light beam A actually starts entering the
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