Incremental printing of symbolic information – Light or beam marking apparatus or processes – Scan of light
Reexamination Certificate
1999-11-24
2001-09-18
Pham, Hai C. (Department: 2861)
Incremental printing of symbolic information
Light or beam marking apparatus or processes
Scan of light
C347S233000, C347S116000
Reexamination Certificate
active
06292208
ABSTRACT:
BACKGROUND OF THE INVENTION
A. Field of the Invention
The present invention relates generally to scanning systems. More particularly, this invention relates to the use of imagers to create of xerographic images.
B. Description of the Related Art
Electrophotographic printers wherein a laser scan line is projected onto a photoconductive surface are well known. In the case of laser printers, facsimile machines, and the like, it is common to employ a raster output scanner (ROS) as a source of signals to be imaged on a precharged photoreceptor (a photosensitive plate, belt, or drum) for purposes of xerographic printing. The ROS provides a laser beam which switches on and off according to digital image data associated with the desired image to be printed as the beam moves or scans, across the photoreceptor. Commonly, the surface of the photoreceptor is selectively, imagewise discharged by the laser in locations to be printed white, to form the desired image on the photoreceptor. The modulation of the beam to create the desired latent image on the photoreceptor is facilitated by digital electronic data controlling a modulator associated with the laser source. A common technique for effecting this scanning of the beam across the photoreceptor is to employ a rotating polygon surface (the surface of the polygon typically being a mirror or other reflective surface); the laser beam from the ROS is reflected by the facets of the polygon, creating a scanning motion of the beam, which forms a scan line across the photoreceptor. A large number of scan lines on a photoreceptor together form a raster of the desired latent image. Once a latent image is formed on the photoreceptor, the latent image is subsequently developed with a toner, and the developed image is transferred to a copy sheet and fixed, as in the well-known process of xerography.
FIG. 1
shows the basic configuration of a scanning system
100
used, for example, in an electrophotographic printer or facsimile machine. A laser source
10
produces a collimated laser beam, also referred to as a “writing beam”,
12
which is reflected from the facets of a rotating polygon
14
. Each facet of the polygon
14
in turn deflects the writing beam
12
to create an illuminated beam spot
16
on the pre-charged surface of photoreceptor
18
. The system may further include additional optical elements such as focusing lenses. The energy of the beam spot
16
on a particular location on the surface of photoreceptor
18
, corresponding to a picture element (pixel) in the desired image, discharges the surface for pixels of the desired image which are to be printed white. In locations having pixels which are to be printed black, the writing beam
12
is at the moment of scanning interrupted, such as by a modulator
11
controlled by imagewise digital data, so the location on the surface of photoreceptor
18
will not be discharged. It is to be understood that gray levels are typically imaged in like manner by utilizing exposure levels intermediate between the “on” and “off” levels. Thus, digital data input into laser source
10
is rendered line by line as an electrostatic latent image on the photoreceptor
18
.
When the beam spot
16
is caused, by the rotation of polygon
14
, to move across photoreceptor
18
, a scan line
20
of selectively discharged areas results on photoreceptor
18
. In
FIG. 1
, the photoreceptor
18
is shown as a rotating drum, but those skilled in the art will recognize that this general principle, and indeed the entire invention described herein, is applicable to situations wherein the photoreceptor is a flat plate, a moving belt, or any other configuration. The surface of photoreceptor
18
, whether it is a belt or drum, moves in a process direction (as indicated by the arrow drawn on the side of the drum
18
); the motion of spot
16
through each scan line
20
is transverse to the process direction (as indicated by the arrow drawn on the surface of the drum
18
and below scan line
20
). The periodic scanning of beam spot
16
across the moving photoreceptor
18
creates an array of scan lines
20
, called a raster
22
, on the photoreceptor
18
, forming the desired image to be printed. One skilled in the art will appreciate that such a configuration will typically further include any number of lenses and mirrors to accommodate a specific design.
In a rotating-polygon scanning system, there is a practical limit to the rate at which digital information may be processed to create an electrostatic latent image on a photoreceptor. One practical constraint on the speed of a system is the maximum polygon rotation speed. It can be appreciated that high quality images require precision placement of the raster scan lines as well as exact timing to define the location of each picture element or pixel along each scan. In a conventional polygon scanner, this precision is achieved by holding very close mechanical tolerances on the polygon geometry and the rotational bearings supporting the polygon body and drive motor. Experience has shown that beyond about 20,000 RPM, precision ball bearings with the required closeness of fit have limited life and are impractical in many scanner applications. As a result, exotic alternatives such as air bearings are sometimes used, but these represent a substantial increase in engineering complexity and maintenance, and hence cost. Another constraint is the size of the polygon itself; it is clear that the forces associated with high speed rotation increase with the diameter of the object being rotated. In particular, both the stored energy and the gyroscopic forces that must be restrained by the bearings increase with the square of the diameter. It is therefore prudent to limit the polygon size to maximize bearing life as well as reduce the potential for damage should a bearing fail at high speed.
In addition to practical constraints, the speed of a printer must be considered in conjunction with other competing desirable characteristics of a printer, particularly resolution. In purely optical terms, there is a trade-off between speed and resolution in a scanning system. The higher the resolution, that is, the more pixels that are designed to form a latent image of a given size, the lower the numerical aperture of the optical system required in order to define the pixels accurately. This trade off can be summarized by a derived equation for an under filled system relating the angular velocity &ohgr; of a polygon having a mean diameter D to the desired pixel size (that is, the inverse of resolution) &Dgr;x:
D&ohgr;
2
=[L&lgr;P
2
/&Dgr;x
3
][(60/&pgr;)
2
k/
2
][E/&khgr;]
The other variables in this equation are as follows: L is the length of the intended scan path, which in this context is the width of the photoreceptor across which the scan line is formed. P is the process speed, in inches per second, of the photoreceptor motion in the machine. k is a constant which depends on the intensity profile of the beam (for example, under a certain convention, the usable pixel size is dependent on a focused concentration where 86% of the total power of the beam is focused within a circular area of a given size). E is an efficiency, factor relating to the proportion of the “circumference” of the polygon which is practically usable for scanning purposes, i.e., because the numerical aperture for a given resolution &Dgr;x requires a specific beam width at the polygon, the beam will not be reflected usefully for a certain portion of the time when the beam is focused near the ends of the facets of the polygon. The larger the ratio of facet length to beam width, the larger the proportion of the polygon rotation which is usable for scanning purposes. &khgr; is the ratio of reflected scan angle to rotational scan angle, which depends on whether the facets of the polygon are parallel or oriented at 45 degrees to the axis of the polygon. If the facets are parallel, as in the illustrated case, then &khgr; is equal to
2. There are some designs in which the facets of the polygo
Fork David Kirtland
Lofhus Robert M.
Sun Dcai
Finnegan Henderson Farabow Garrett & Dunner
Pham Hai C.
Xerox Corporation
LandOfFree
Sensing system to allow side-by-side writing of photonic... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Sensing system to allow side-by-side writing of photonic..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Sensing system to allow side-by-side writing of photonic... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2506587