Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
1999-12-20
2001-01-23
Phan, James (Department: 2872)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S217200, C347S255000, C347S249000
Reexamination Certificate
active
06178031
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to scan line non-linearity in a Raster Output Scanning (ROS) system and, more particularly, to a method for calculating frequency shifts to correct the scan line non-linearity.
Printing systems utilizing lasers to reproduce information are well known in the art. The printer typically uses a Raster Output Scanner (ROS) to expose the charged portions of a photosensitive medium, such as a photoreceptor, to record an electrostatic latent image on the photosensitive medium.
A plurality of ROS units can be used in a color xerographic ROS printer. Each ROS forms a scan line for a separate color image on a common photoreceptor belt. Each color image is developed in overlying registration with the other color images from the other ROS units to form a composite color image which is transferred to an output sheet. Registration of each scan line of the plurality of ROS units requires each image to be registered to within a 0.1 mm circle or within a tolerance of ±0.05 mm.
A typical prior art raster output scanning system
10
of
FIG. 1
includes a light source
12
for generating a light beam
14
and scanning means
16
for directing the light beam
14
to a spot
18
at a photosensitive medium
20
. The scanning means
16
also serves to move the spot
18
along a scan line
22
of specified length at the photosensitive medium
20
. For that purpose, the scanning means
16
in the illustrated scanner system
10
includes a rotatable polygon mirror with a plurality of light reflecting facets
24
(eight facets being illustrated) and other known mechanical components that are depicted in
FIG. 1
by the polygon
16
rotating about a rotational axis
26
in the direction of an arrow
28
.
The light source,
12
, such as a laser diode, emits a modulated coherent light beam
14
of a single wavelength. The light beam
14
is modulated in conformance with the image information data stream contained in the video signal sent from image output light source control circuit
30
to the light source
12
.
The modulated light beam
14
is collimated by a collimating lens
32
, then focused by a cross-scan cylindrical lens
34
to form a line on a reflective facet
24
of the rotating polygon mirror
16
.
The polygon mirror
16
is rotated around its axis of rotation by a conventional motor (not shown), known to those of ordinary skill in the art.
The beam
14
reflected from the facet
24
then passes through the f-theta scan lenses
36
and the anamorphic wobble correction lens
38
.
The f-theta scan lens
36
consists of a negative plano-spherical lens
40
, a positive piano-spherical lens
42
, and the cross-scan cylinder lens
44
. This configuration of f-theta scan lenses has sufficient negative distortion to produce a linear scan beam. The light beam will be deflected at a constant angular velocity from the rotating mirror which the f-theta scan lens optically modifies to scan the surface at a constant linear velocity.
The f-theta scan lens
36
will focus the light beam
14
in the scan plane onto the scan line
22
on the photosensitive medium
20
.
After passing through the f-theta scan lens
36
, the light beam
14
then passes through a wobble correction anamorphic lens element
38
. The wobble correction optical element can be a lens or a mirror and is sometimes referred to as the “motion compensating optics”. The purpose of optical element
38
is to correct wobble along the scan line generated by inaccuracies in the polygon mirror/motor assembly.
The wobble correction lens
38
focuses the light beam in the cross-scan plane onto the scan line
22
on the photosensitive medium
20
.
As the polygon
16
rotates, the light beam
14
is reflected by the facets
24
through the f-theta and wobble correction lenses and scans across the surface of the photosensitive medium in a known manner along the scan line
22
from a first end
46
of the scan line
22
(Start of Scan or “SOS”) past a center (the illustrated position of the spot
18
) and on to a second end
48
of the scan line
22
(End of Scan or “EOS”). The light beam exposes an electrostatic latent image on the photosensitive member
20
. As the polygon
16
rotates, the exposing light beam
14
is modulated by circuit
30
to produce individual bursts of light that expose a line of individual pixels, or spots
18
, on the photosensitive member
20
.
Ideally, the ROS should be capable of exposing a line of evenly spaced, identical pixels on the photosensitive medium
20
. However, because of the inherent geometry of the optical system of the ROS, and because manufacturing errors can cause imperfections in the facets of a polygon mirror, obtaining evenly spaced, identical pixels can be problematic.
“Scan non-linearity” refers to variations in spot velocity occurring as the spot moves along the scan line during the scan cycle. Scan linearity is the measure of how equally spaced the spots are written in the scan direction across the entire scanline. Typical scan linearity curves start at zero position error at one end of a scan having a positive lobe of position error across the scanline, cross the center of scan with zero position error and then have a negative lobe of position error across the remainder of the scanline toward the other end of the scan. Scan linearity curves may have image placement errors of zero at several locations across the scanline. Ideally, the curve would be at zero across the entire scanline.
Scan non-linearity is typically caused by system geometry or a velocity variation of the scanning means. The speed at which the focussed exposing light beam travels across the scan line on the photosensitive medium
50
is called the spot velocity.
Without some means to correct for the inherent scan non-linearity caused by the geometry of the ROS system, the spot velocity will vary as the light beam scans across the photosensitive medium. A scanner having a multifaceted rotating polygon, for example, directs the light beam at a constant angular velocity. But the spot is farther from the polygon facets at the ends of the scan line than it is at the center and so the spot velocity will be higher towards the ends of the scan line, and lower towards the center of the scan line.
Some raster output scanners compensate for such non-linearity electronically using a variable frequency pixel clock (sometimes called a scanning clock). The pixel clock produces a pulse train (i.e., a pixel clock signal) that is used to turn the light beam emitted by the light source on and off at each pixel position along the scan line. Varying the clock frequency and thereby the timing of individual pulses in the pulse train serves to control pixel placement along the scan line. If the frequency of the pixel clock signal is constant, the resulting pixels will be positioned further apart at the edges of the photosensitive medium, and closer together towards the center of the photosensitive medium. That will more evenly space the pixels and thereby at least partially compensate for what is sometimes called pixel position distortion (i.e., uneven pixel spacing caused by scanner non-linearity).
The light source control circuitry
30
serves as an electronic control system for controlling the light beam
14
in order to produce the pixels along the scan line
22
. The control system may, for example, be configured using known componentry and design techniques to produce a control signal for activating the light beam at each of a plurality of desired pixel positions along the scan line (e.g., the central portion of each pixel position being evenly spaced at {fraction (1/300)} inch intervals for 300 dpi resolution or being evenly spaced at {fraction (1/600)} inch intervals for 600 dpi resolution, etcetera).
Preferably, the control system is configured so that the control signal defines a pixel interval for each pixel position and so that the pixel interval defined by the control signal varies proportionately according to spot velocity, i.e., a higher frequency at the ends of the scan line th
Rahnavard Mohammad H.
Rauch Russell B.
Phan James
Propp William
Xerox Corporation
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