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BOW CORRECTION FOR LASER SCANNERS

IP.com Disclosure Number: IPCOM000026764D
Original Publication Date: 1993-Aug-31
Included in the Prior Art Database: 2004-Apr-06
Document File: 4 page(s) / 160K

Publishing Venue

Xerox Disclosure Journal

Abstract

In most designs, the laser of the printer is incident on a polygon from out of a plane containing the polygon. This geometry leads to scan bow for a typical ROS to be symmetric about the center of a scan line. Scan line bow is the displacement perpendicular to the average fast-scan direction. A graph of the displacement from a straight scan line as a function of a normalized half-scan line for a typical ROS is shown in Figure 1. Proper designs build in correction that compensate for scan bow so that the actual scan bow in the image plane is usually less than 0.030 inches. However, scan bow continues to limit the scan line width and forces compromises in the optical design in order to keep scan bow acceptably small. Although, scan bow in the 0.030 inch range is usually not a problem for single color printers, it does add complications to the registration for multiple color printers and for single pass color printers were separate scanners write each color. Scan line bow is inherent to any ROS operated out of a plane containing a scanning element.

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Page 1 of 4

XEROX DISCLOSURE JOURNAL

BOW CORRECTION FOR LASER SCANNERS U.S. C1.346/160 John R. Andrews

Proposed Classification

Int. C1. Gold 15/00

DISPLACEMENT FROM STRAIGHT 37

LINE SCAN

0.0

0 NORMALIZED HALF-SCAN LENGTH \

1

FIG. I

11

IT I

XEROX DISCLOSURE JOURNAL - VO~. 18, NO. 4 July/August 1993 369

3\

FIG.2

[This page contains 1 picture or other non-text object]

Page 2 of 4

BOW CORRECTION FOR LASER SCANNERS (Cont'd)

Most raster output scanner (ROS) designs use a polygon to implement scanning in electrophotographic printers. In most designs, the laser of the printer is incident on a polygon from out of a plane containing the polygon. This geometry leads to scan bow for a typical ROS to be symmetric about the center of a scan line. Scan line bow is the displacement perpendicular to the average fast-scan direction. A graph of the displacement from a straight scan line as a function of a normalized half-scan line for a typical ROS is shown in Figure 1. Proper designs build in correction that compensate for scan bow so that the actual scan bow in the image plane is usually less than 0.030 inches. However, scan bow continues to limit the scan line width and forces compromises in the optical design in order to keep scan bow acceptably small. Although, scan bow in the 0.030 inch range is usually not a problem for single color printers, it does add complications to the registration for multiple color printers and for single pass color printers were separate scanners write each color. Scan line bow is inherent to any ROS operated out of a plane containing a scanning element.

Disclosed is a optical element having a wedge-angle that varies along the scan line to convert a bowed scan into a straight scan line. The optical element or prism is shaped to introduce a compensating optical deflection as a function of scan angle in order to compensate for unwanted bow displacement at points along the exposure path of a beam of light. The prism corrects the displacement perpendicular to the average fast-scan direction that occurs in typical ROS systems. Such an optical component can completely eliminate scan bow, which consequently improves registration in single pass color printing systems. The prism could be added to a ROS system at a very low cost by simply placing a relatively thin film on any optical component currently in the system.

Figure 2 shows scanning arrangement 2 with incident light 3 reflecting off of polygon 11. Prism 4 in arrangement 2 has a large wedge angle that corrects large displacements and a small wedge angle that corrects small displacements. Using prism 4 straight scan line 5 can be generated from an initial scan line 6 that is substantially bowe...