Incremental printing of symbolic information – Electric marking apparatus or processes – Electrostatic
Reexamination Certificate
1998-03-23
2001-01-23
Braun, Fred L. (Department: 2852)
Incremental printing of symbolic information
Electric marking apparatus or processes
Electrostatic
C399S051000
Reexamination Certificate
active
06177948
ABSTRACT:
BACKGROUND
1. Field of the Invention
This invention relates generally to electrophotographic printers and related processes. More particularly, the invention provides for an apparatus and method that controls the toner pile height of each picture element (PEL) of a printed character independently to reduce smearing on the printing medium.
2. Description of Related Art
In electrophotographic printers, photoreceptor material is electrostatically charged, is exposed by a light source to form an image, the image is developed using toner, and finally, the image on the photoreceptor is transferred onto a print medium such as paper. The stored data to be printed is image pattern data that is held in digitized form in memory within the printer. After the photoreceptor has been electrostatically charged, a voltage source, such as that which controls the light source, discharges areas of the photoreceptor to alter the electrostatic fields in a pattern that mirrors the image pattern data stored in memory. When the powder or toner carries a charge of the same polarity as the charge pattern on the photoreceptor, the toner is electrostatically attracted to areas of the photoreceptor which are discharged to produce an image, such as characters, lines or large patches of print that produce large images. As a result, the heavily discharged areas receive large, usually black, toner deposits and produce dark areas, and the lighter discharged areas will receive lesser amounts of toner deposits to produce gray images. The above describes the commonly employed, discharged-area-development (DAD) process, by which the highly charged areas of the photoreceptor represent white background and the discharge areas represent areas in which toner is to be deposited. The charged area development process (CAD) may also be used. In the CAD process, the toner carries a charge opposite in polarity to the charge pattern on the photoreceptor and the toner is deposited on the charged areas of the photoreceptor. In the CAD process, the light source discharges the white background areas. Since a typical printed page has about ten times more white space than print, the CAD process is less commonly employed.
Print Quality Enhancement (PQE) is a well known form of image enhancement that is used to improve print quality. Image pattern data is organized such that each bit in memory represents a specific picture element (PEL) of the image to be printed on the printed material. PQE relies on an imaging system wherein the electrostatic field of each PEL to be imaged on the photoreceptor may be altered independently. Prior to PQE technology, electrophotographic printers could only turn “on” or turn “off” a voltage controlling the light source for each PEL, and there was no modulation, and therefore, no independent control of toner deposition from PEL to PEL within each PEL. With PQE, each dot or PEL represents either white background or an area to be printed black. A PQE algorithm generates an exposure value for each PEL based on neighboring PELs. The print quality improvement results by precisely exposing each PEL according to PEL data surrounding it.
Electrostatic field gradients control the amount of toner deposited, and these gradients are largest at the edges of print, such as the entire width of thin character or line strokes, or the edge PELs of large printed patches, for example, photographs. This phenomenon is commonly referred to as “edge effect.” It is a well known problem that significantly more toner, that is, a greater pile height of toner, is deposited at the periphery or edge of print, either at the edge PELs of a large printed patch, or across an entire character or line made up of thin strokes.
When image receiving material such as paper is rolled out of a printer on a continuous roll, then it can be either cut and stacked or folded and stacked. Because of the relative motion and pressure that occurs in the stacking operation, the high toner pile height of characters fractures and smears the paper.
U.S. Pat. No. 4,967,211 calls for balancing toner deposits made on printed images. If the image pattern data is recognized as solid area data, the exposure at the edge PELs is modulated to prevent excess toner deposition. Also, if the image pattern data is recognized as character or line data, the exposure of the entire character stroke or line stroke is modulated to prevent excessive toner deposition. The problem with reducing toner deposition on lines and characters, however, is the reduction in visible size of the printed character or line because PELs of the entire width are modulated. Another problem is the reduction in optical density. Optical density is typically reduced to below a value of 1.0. Optical density is extremely important for obtaining high character legibility and print quality. Changes in optical density that are greater than about 1.2 are not visually noticeable to the eye.
A printing apparatus and a method of printing are needed to reduce smearing of printed characters without noticeably reducing the size of the character or the optical density of the character, and while leaving large print patches unaffected.
SUMMARY OF THE INVENTION
This invention provides an electrophotographic printing apparatus and a method of printing that reduces smear of printed characters and lines on printed material while yielding an optical density that is greater than 1.2. The electrophotographic printer herein reduces toner pile height of interior areas of printed characters according to logic that compares at least one mask matrix with predetermined image pattern data to be printed.
A program is provided in the printer to perform logical operations that distinguish character and line strokes from patch print, for example photographs, made up of gray level image data. The program determines whether the target PEL to be printed belongs to a character or line, and more specifically, whether the target PEL data corresponds to an area interior of a character or line, that is, a non-edge PEL. The program comprises at least one mask matrix, and performs logical operations on a PEL-by-PEL basis that compares the mask matrix to the predetermined image pattern data, stored in memory, on a PEL-by-PEL basis. The electrophotographic printer apparatus also comprises a controller that may modulate the voltage of the photoreceptor to affect the amount of toner to be applied for each PEL of data. The toner pile height is reduced if the program makes a determination that the target PEL data corresponds to an area interior of a character.
In the preferred embodiment of the present invention, the printer apparatus controls the voltage of the light source to vary the electrostatic fields and thus the exposure of the photoconductor. The program compares the stored image pattern data to the mask matrix, and if there is a total match for every PEL of the mask matrix, the controller modulates the voltage of the light source, such as a laser light source or a light emitting diode array (LED) source, for example. When the drum covered by the photoreceptor rotates from the imaging station to the developing station, less toner is applied than would otherwise be applied and the toner pile height of the printed character is thereby reduced.
In another embodiment of the invention, the photoreceptor exposure, and therefore toner deposition may be controlled by initially controlling the drum charging voltage in conjunction with modulation of the light source voltage.
In yet another embodiment of the invention, the pile height of toner may also be controlled by varying the bias voltage at the developing station in conjunction with modulation of the light source.
REFERENCES:
patent: 4403257 (1983-09-01), Hsieh
patent: 4460909 (1984-07-01), Bassett et al.
patent: 4882252 (1989-11-01), Kawamura et al.
patent: 4967211 (1990-10-01), Colby et al.
patent: 5357583 (1994-10-01), Sato et al.
D. Estabrooks, M. J. Stanich, “Print Quality Enhancement in Electrophotographic Printers”.
High-End Printing—Boulder; MJS-SCPTECH5-1; Aug. 6, 1997.
Jul. 1970 p.
Estabrooks Dennis C.
Zable Jack L.
Braun Fred L.
Hogg William N.
International Business Machines - Corporation
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