Incremental printing of symbolic information – Light or beam marking apparatus or processes – Scan of light
Reexamination Certificate
2002-01-16
2003-08-19
Gutierrez, Diego (Department: 2859)
Incremental printing of symbolic information
Light or beam marking apparatus or processes
Scan of light
C347S251000
Reexamination Certificate
active
06608643
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of Invention
This invention is generally related to an image forming apparatus which uses multi-beam raster output scanners (ROS) to form images on a medium.
2. Description of Related Art
Prealigned dual and quad laser diodes are very expensive. While prealigned dual laser diodes are desirable in xerographic based electronic printers and copiers, due to cost considerations, individual laser diodes are normally used.
FIGS. 1 and 2
illustrate top and side views, respectively, of a conventional rotating polygon-based optical system
100
and a known rotating polygon
140
. It should be appreciated that the functions described below equally apply to most rotating polygon-based systems, independently of the number of light sources used.
As shown in
FIGS. 1 and 2
, the ROS optical system
100
includes a pair of sagittally offset laser diodes
102
and
103
that emit laser beams
121
and
123
, respectively. The laser beams
121
and
123
emitted by the laser diodes
102
and
103
are collimated by a collimator lens
110
. A sagittal aperture stop
120
is placed in a position where the laser beams
121
and
123
cross the system optical axis
500
, to control the aperture size, which in turn controls the spot size on the photoreceptor image plane
182
. The input cylinder optical elements
130
and
131
focus the laser beams
121
and
123
on the surface of the current polygon facet
144
of the rotating polygon
140
. After reflecting from the current polygon facet
144
, the laser beams
121
and
123
pass through the F&thgr; lens
150
. The F&thgr; lens
150
, in general, has relatively low power in the tangential meridian. The F&thgr; lens
150
focuses the laser beams
121
and
123
in the tangential meridian to control the scan linearity in terms of uniform spot displacement per unit angle of polygon rotation. A sagittal aperture stop
160
is placed in a position where laser beams
121
and
123
again cross the system optical axis
500
.
A motion compensating optical element (MCO)
170
then reimages the focused laser beams
121
and
123
from the current polygon facet
144
onto the photoreceptor image plane
182
at a predetermined position, independently of the polygon angle error or tilt of the current facet
144
. Such compensation is possible because the focused laser beams
121
and
123
are stationary “objects” before the F&thgr; lens
150
and the motion compensating optical (MCO) element
170
. Although, due to a polygon tilt or wobble, the laser beams
121
and
123
are reflected to different positions of the post polygon optics aperture for each different facet of the rotating polygon, the beams
121
and
123
are imaged to the same position on the photoreceptor image plane
182
.
SUMMARY OF THE INVENTION
In rotating polygon, ROS-based xerographic copiers and printers, distortions occur from several sources of beam spacing errors. The sources of beam spacing errors in multi-beam rotating polygon based optical systems illustrated in
FIG. 2
are optical and/or mechanical in nature. Beam spacing errors fall into one of the following categories: residual errors in the nominal design, thermal effects, vibration, and fabrication and wear errors in the various optical and mechanical components in the system.
Nominal differential bow is a source of residual beam spacing error. Even if the components were perfectly fabricated and assembled, beam-to-beam differential bow error will be present because the optical design cannot completely eliminate image distortion, as illustrated in
FIGS. 3 and 4
. Variations in ambient temperature produce changes in the refractive index, position, and thickness of optical components. These changes cause differences in scan line shape and position, as shown in
FIGS. 5 and 6
. Mechanical vibrations result in changes in scan line position, which can lead to beam spacing error.
FIGS. 3-6
illustrate the various types of errors which can be introduced by differential scan line bow.
FIG. 3
shows a barrel type bow distortion. Specifically,
FIG. 3
shows the center of curvatures of a pair of bowed scan lines
185
and
187
located on opposite sides of an ideal scan line
189
in such a fashion that the bowed scan lines create a barrel distortion. This occurs whether the bowed scan lines
185
and
187
have the same or different radius of curvature.
FIG. 4
shows a pin cushion type bow distortion. Specifically,
FIG. 4
shows the center of curvature of the bowed scan lines
185
and
187
are also on the opposite side of the ideal scan line
189
(with the same or different radii). However, the arrangement of the bowed scan lines
185
and
187
relative to each other forms a pin cushion distortion. Again, this occurs whether the bowed scan lines
185
and
187
have the same or different radii of curvature.
FIG. 5
shows the ideal scan line
189
as a dashed line. In
FIG. 5
, first bowed scan line
187
has a first radius of curvature which is different from the radius of curvature of the second bowed scan line
185
.
FIG. 6
shows bowed scan line
185
superimposed over the bowed scan line
187
. As shown in
FIG. 6
, the bowed scan line
185
has a center of curvature which is on the opposite side of the ideal scan line
189
from the center of curvature of the bowed scan line
187
. As can be seen from
FIGS. 3-6
, the bow appears as a displacement of a scan line in the process direction as the line extends in the fast scan direction.
As shown in
FIG. 7
, there are shown a plurality of dashed lines representing ideal raster scan line paths
175
across a photoreceptor. The scan line spots
121
′ and
123
′ and
121
″ and
123
″, are shown with respect to each other and with respect to the ideal scan line path
175
. Ideally, the raster scan line spots
121
′,
123
′,
121
″ and
123
″ travel across the photoreceptor within the corresponding ideal scan line paths
175
. However, due to the factors discussed above, the raster scan line spots
121
′,
121
″,
123
′, and
123
″ often, if not usually, do not travel within the ideal scan line paths
175
.
As can be seen on the left side of
FIG. 7
, the raster scan spots
121
″ and
123
″ are separated from each other by a distance Y and do not lie within ideal scan line paths
175
. On the right side of
FIG. 7
, the raster scan spots
121
′ and
123
′ overlap by a distance X. It should be appreciated that, due to bow and the like, as the raster scan spots
121
′,
121
″,
123
′, and
123
″ move across the photoreceptor, the distortions shown in
FIGS. 3-6
develop.
Fabrication variations in material parameters, component geometry, and assembly, manifested in misalignment, improper beam conditioning and defocusing, result in both uniform and non-uniform variation of the beam spacing across the image plane. Local variations in the photoreceptor and tilt errors among the various facets
141
-
148
of the polygon mirror
140
, for example, produce variation in process direction beam position from scan to scan. Curvature error in the lenses can produce either a widening or narrowing of the distance between scanning beams. All of the optical elements of a multi-beam rotating polygon-based optical system
100
may therefore introduce a degree of beam-to-beam spacing error. The combination of errors creates an error unique to each machine, and is commonly referred to as the constant beam-to-beam spacing error.
It also should be appreciated, however, that the constant beam-to-beam spacing error is constant over a limited time period, such as that of several scans to that of hours, days or even longer. That is, the constant beam-to-beam spacing error slowly changes over time. The component parts of the multi-beam rotating polygon-based optical system
100
and the assembly tolerances of those parts tend to slowly deteriorate over time, thus imparting a variable quality to the otherwise constant beam-to-beam spacing error. Conseq
Costanza Daniel W.
German Kristine A.
Hubble, III Fred F.
Loce Robert P.
Lofthus Robert M.
Gutierrez Diego
Oliff & Berridg,e PLC
Verbitsky Gail
Xerox Corporation
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