Incremental printing of symbolic information – Light or beam marking apparatus or processes – Scan of light
Reexamination Certificate
2000-03-09
2004-11-23
Pham, Hai (Department: 2861)
Incremental printing of symbolic information
Light or beam marking apparatus or processes
Scan of light
C347S250000
Reexamination Certificate
active
06822666
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a multibeam scanning optical apparatus and also to a color image-forming apparatus. A multibeam scanning optical apparatus is an apparatus adapted to optically scan the surface of an object by means of a plurality of light beams that are emitted from one or more than one light sources, deflected by an optical deflector and transmitted by way of a scanning optical system having an f&thgr; characteristic. A color image-forming apparatus is an apparatus adapted to record image information obtained by scanning the surface of an image carrier by means of deflected light. Such apparatus find applications in the field of laser beam printers (LBPs) and digital copying machines having a feature of carrying out a color electrophotography process.
2. Related Background Art
In conventional scanning optical apparatus to be used for image-forming apparatus such as laser beam printers and digital copying machines, the light beam emitted from a light source and optically modulated according to the image signal applied to it is periodically deflected by a light deflector typically comprising a rotary polygon mirror and then focussed on the surface of a photosensitive recording medium (photosensitive drum) to produce a spot of light there by means of a focussing optical system having an f&thgr; characteristic, which optical system is then used to scan the surface of the-recording medium and record the image on the recording medium.
FIG. 1
of the accompanying drawings schematically illustrates a principal portion of a known scanning optical apparatus.
Referring to
FIG. 1
, a divergent light beam emitted from light source
81
is substantially collimated by a collimator lens
82
and restricted for quantity by a diaphragm
83
before it enters a cylindrical lens
84
that is made to have a predetermined refractive power only in the sub-scanning direction. The substantially collimated light beam entering the cylindrical lens
84
is then made to exit the lens as a beam substantially collimated in the main-scanning plane, while it is converged in the sub-scanning plane to produce a linear image on the deflection plane (reflection plane)
85
a
1
of a light deflector
85
comprising a rotary polygon mirror.
Then, the light beam deflected and reflected by the deflection plane
85
a
1
of the light deflector
85
is led to the surface
88
of a photosensitive drum to be scanned by way of a scanning optical element having an f&thgr; characteristic (f&thgr; lens)
86
so that the surface
88
of the photosensitive drum is optically scanned in the direction indicated by arrow
88
a
(main-scanning direction) to record the scanned image as the light deflector
85
is driven to rotate in the sense of arrow
85
a.
In order to accurately control the starting point of the operation of drawing the image for the scanning optical apparatus, the light beam deflected by the light deflector
85
is partly taken out and entered to beam detector (BD) sensor
92
by way of the scanning optical element
86
, a beam detector (BD) mirror
95
and a slit
91
immediately prior to the start of writing the image signal. Then, the output signal of the beam detector (BD) sensor
92
is used to regulate the timing and the spot at which the operation of drawing the image on the surface of the photosensitive drum is started.
In recent years, as a result of technological development in the field of image forming apparatus involving the use of an electrophotography process particularly in terms of high speed and high resolution, there is an ever-increasing demand for multibeam scanning optical apparatus comprising a multibeam laser device having a plurality of light emitting sections and scanning optical apparatus of the type employing a plurality of scanning optical apparatus as so many units in order to realize high speed color image formation as shown in
FIG. 2
that illustrates a tandem type color image forming apparatus where a plurality of scanning optical apparatus are operated simultaneously for different colors in order to record image information on respective photosensitive drums as well as hybrid type color image forming apparatus realized by combining apparatus of the above identified types. In
FIG. 2
, reference numerals
111
,
112
,
113
and
114
denote respective scanning optical apparatus and reference numerals
121
,
122
,
123
and
124
denote respective photosensitive drums operating as so many image carriers, whereas reference numerals
131
,
132
,
133
and
134
denote respective developing units and reference numeral
141
denotes a conveyor belt.
Since the manufacturing cost of such scanning optical apparatus is vital, the scanning optical element (f&thgr; lens) is typically prepared by plastic molding without the process of compensating, if any, the chromatic aberration of magnification.
However, in the case of a multibeam scanning optical apparatus adapted to form a final image by means of light beams of a multibeam laser having a plurality of light emitting sections, the plurality of light beams can show discrepancies in terms of magnification to consequently degrade the quality of the produced image due to various factors including those listed below:
(1) variances of the initial wavelengths of a plurality of light beams emitted from the multibeam laser;
(2) variances of the wavelength of the plurality of light beams caused by mode hopping of the multibeam laser that is attributable to environmental changes; and
(3) fluctuations in the refractive index of the plastic lens also attributable to environmental changes.
In
FIG. 1
, there are also shown the image region of the known scanning optical apparatus and the displacements of the focussing positions of the light beams as detected at the start of drawing the image when the wavelength of the light source of laser B (as observed by the laser beam emitted from the light source) is modified relative to the wavelength of the light source of the laser A. Note that, while only the focussing points of laser A and laser B are indicated by A and B respectively in
FIG. 1
, there are actually more focussing points that are not illustrated in FIG.
1
.
In an actual image, the displacements of focussing positions on the surface being scanned due to variations of magnification (wavelength) do not give rise to any jittering along the left edge of the image but they do along the right edge of the image as shown in
FIG. 3
to consequently degrade the recorded image. This is because the timing and the spot at which the operation of drawing the image on the surface of the photosensitive drum is started are regulated (synchronized) at the side of starting the scanning operation as pointed out above. More specifically, referring to
FIG. 1
, the detection light beam to be detected by the BD sensor
92
of the known scanning optical apparatus strikes aslant the f&thgr; lens
86
shared with the light beams for scanning the surface to be scanned and, if the wavelength of the laser beam emitted from the laser A differs from that of the light beam emitted from the laser B, they produce a relative deviation of the focussing positions equal to that of the starting points of scanning the surface to be scanned. Thus, the starting points of scanning of the light beams can be made to agree with each other by detecting a part of each of the light beams and so controlling the scanning timing of the beam as to correct the relative deviation of the focussing positions due to the difference of the wavelengths on the basis of the signals obtained by detecting the laser beams. However, it will be appreciated that, as a result, a remarkable jittering phenomenon can appear at the terminating points of scanning.
A similar problem arises in the scanning optical apparatus of tandem type color image forming apparatus. More specifically, when magnification discrepancies arises among a number of scanning optical apparatus, a relative deviation of registration occurs over the range from the center toward
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