Electrophotography – Control of electrophotography process – Of plural processes
Reexamination Certificate
2003-01-15
2004-02-24
Ngo, Hoang (Department: 2852)
Electrophotography
Control of electrophotography process
Of plural processes
C399S053000
Reexamination Certificate
active
06697582
ABSTRACT:
BACKGROUND
The present invention relates to electrophotographic printing. It finds particular application in conjunction with a method and system for controlling a printing device's tone reproduction curve (TRC). The invention helps minimize contouring and maximize a number of shades or colors available for an output image. The invention will be described in reference to a xerographic print engine. However, the invention is also. amenable to other electrophotographic processes, such as for example, ionographic print engines and like applications.
Electrophotographic copiers, printers and digital imaging systems typically record an electrostatic latent image on an imaging member. The latent image corresponds to the informational areas contained within a document being reproduced. In xerographic systems, a uniform charge is placed on a photoconductive member and portions of the photoconductive member are discharged by a scanning laser or other light source to create the latent image. In ionographic print engines the latent image is written to an insulating member by a beam of charge carriers, such as, for example, electrons. However it is created, the latent image is then developed by bringing a developer, including colorants, such as, for example, toner particles into contact with the latent image. The toner particles carry a charge and are attracted away from a toner supply and toward the latent image by an electrostatic field related to the latent image, thereby forming a toner image on the imaging member. The toner image is subsequently transferred to a physical media, such as a copy sheet. The copy sheet, having the toner image thereon, is then advanced to a fusing station for permanently affixing the toner image to the copy sheet.
The approach utilized for multi-color electrophotographic printing is substantially identical to the process described above. However, rather than forming a single latent image on the photoconductive surface in order to reproduce an original document, as in the case of black and white printing, multiple latent images corresponding to color separations are sequentially recorded on the photoconductive surface. Each single color electrostatic latent image is developed with toner of a color complimentary thereto and the process is repeated for differently colored images with the respective toner of complimentary color. Thereafter, each color toner image can be transferred to the copy sheet in superimposed registration with the other toner images, creating, for example, a multi-layered toner image on the copy sheet. This multi-layer toner image is permanently affixed to the copy sheet in substantially conventional manner to form a finished copy.
An image to be rendered (an input image) is received in the form of, or is transformed into the form of, a set of contone values. For example, each contone can have a value ranging from 0 to 255 (in eight bit systems) or from 0 to 4095 (in higher resolution twelve bit systems). The contone values are indicative of how much colorant should be applied to the output medium in order to render a small portion of the image. For example, zero may indicate that no colorant should be applied to a small portion of the medium and a contone value of 255 may indicate that the entire area associated with a halftone cell should be covered with toner. Often, an ideal relationship between contone values and the amount of colorant applied to the medium is a linear one. That is, typically an ideal or target tone reproduction curve (TRC), which relates input contone values to, for example, colorant density applied to the print medium, relates each possible contone value to a unique and incrementally proportional amount of colorant.
Some electrophotographic systems include a hierarchical control scheme in an attempt to provide an actual tone reproduction curve (TRC) that is as close as possible to the ideal or target tone reproduction curve (TRC). For example, some electrophotographic systems include what are referred to as level 1 control loops for maintaining electrophotographic actuators at associated set points, level 2 control loops for selecting set points for the level 1 control loops, and level 3 controls for compensating for residual differences or errors between the actual TRC and the target TRC in spite of the efforts of the level 2 control loops.
Xerographic actuators include, for example, cleaning field strength or voltage, development field strength or voltage, imager or laser power, and AC wire voltage associated with some developers. For example, in some xerographic environments level 1 control loops include electrostatic voltmeters (ESV) for measuring charge voltage generated by charge applied to a photoconductive member. For instance, the ESV measure the charge applied in the area of test patches in inter document or inter page zones (IPZ) of the photoconductor. If measured voltages, such as, for example, a discharged area voltage, or a cleaning voltage of an area surrounding a discharged area deviate from set point values, level 1 control loops adjust xerographic actuators to return the measured voltages to set point potentials. For example, the level 1 control loops vary a charge or bias voltage applied to elements of a developer to adjust a resulting development field and/or cleaning filed. Additionally, the level 1 control loops may adjust a laser power to return a related discharge field back toward a discharge field set point.
Level 2 control loops include, for example, infrared densitometers (IRD). In xerographic environments, and perhaps in other electrophotographic environments, infrared densitometers are also known as Enhanced Toner Area Coverage Sensors (ETACS). The infrared densitometers or ETACS are used to measure, for example, the density of toner or colorant applied to or developed on the photoconductive member. For instance, a set of test patches is written in an interdocument or interpage zone on the photoconductor. The test patches are developed and the amount or density of colorant or toner present in the test patches is measured. If the amount of colorant or toner in a test patch is incorrect or varies from a target test patch density, the level 2 control loops generate or select one or more new set points for the xerographic actuators of the level 1 control loops.
For instance, if a high-density test patch, such as a test patch corresponding to a target density of 100 percent (e.g., contone value 255), includes too little colorant or toner (is less dense than the target density), then the level 2 control loop may increase a set point related to the generation of a development field.
If the measured or actual density of a low-density test patch, or a test patch associated with a low-target density, such as, for example, 10 percent (e.g., a contone value of 25 or 26), includes more colorant or toner than is indicated by the associated target density, the level 2 controls may select or determine a new set point for a level 1 control loop associated with controlling a cleaning field voltage. For instance, increasing the cleaning field may reduce a toner density measured in a next low-density test patch.
If an infrared densitometer measures a deviation from a midrange target density in an associated test patch, the level 2 controls may select or determine a new set point for a level 1 controller responsible for regulating laser power.
The level 2 control loops strive to maintain the actual densities of test patches at desired or target levels. The assumption is that by adjusting the level 1 actuator set points to maintain the densities of a few test patches at target levels, an entire actual TRC will be maintained at or near an ideal or target TRC.
However, due to environmental and system changes, such as, for example, temperature, humidity, system age, wear, thermal expansion and contraction, toner quality and toner sources, the actual TRC of a system can become nonlinear. Therefore, anchoring an actual TRC to an ideal or target TRC at a few points, such as the high, low and midrange targe
Fay Sharpe Fagan Minnich & McKee LLP
Ngo Hoang
Xerox Corporation
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