Electrophotography – Control of electrophotography process – Forming test image
Reexamination Certificate
2002-11-14
2004-06-01
Grainger, Quana M. (Department: 2852)
Electrophotography
Control of electrophotography process
Forming test image
C399S301000, C399S038000, C399S039000, C399S041000, C399S046000
Reexamination Certificate
active
06744997
ABSTRACT:
FIELD OF THE INVENTION
This invention pertains to inline color laser imaging devices, as well as electophotographic development processes, and in particular to methods and apparatus for reducing calibration time in an inline color imaging device.
BACKGROUND OF THE INVENTION
Color printing by an electrophotographic printer is achieved by scanning a digitized image onto a photoconductor. Typically, the scanning is performed with diodes which pulse a beam of energy onto the photoconductor. The diodes can be for example laser diodes or light emitting diodes (LEDs). The photoconductor typically comprises a drum or a belt coated with a photoconductive material capable of retaining localized electrical charges. Each localized area capable of receiving a charge corresponds to a pixel. Each pixel is charged to a base electrical charge, and then is either exposed or not exposed by the laser, as dictated by the digital data used to pulse the laser. Exposing a pixel corresponds to electrically altering (typically discharging) the localized area from the base electrical charge to a different electrical charge. One charge will attract toner, and the other charge will not. In this manner, toner is selectively transferred to the photoconductor. In most electrophotographic printing processes, the exposed (electrically discharged) pixels attract toner onto the photoconductor. This process is known as discharge area development (DAD). However, in some electrophotographic printing processes the toner is attracted to the un-discharged (i.e., charged) area on the photoconductor. This latter type of electrophotographic printing is known as charge-area-development (CAD). For purposes of discussion, it will be assumed that DAD is used, although the present invention is not limited to DAD.
Once the photoconductor has had the desired toner transferred to it, the toner is then transferred to the finished product medium. This transfer can direct, or it can be indirect using an intermediate transfer device. The finished product medium typically comprises a sheet of paper, but can also comprise a transparency. After the toner is transferred to the finished product medium, it is processed to fix the toner to the medium. This last step is normally accomplished by thermally heating the toner to fuse it to the medium, or applying pressure to the toner on the medium. Any residual toner on the photoconductor and/or the intermediate transfer device is removed by a cleaning station, which can comprise either or both mechanical and electrical means for removing the residual toner.
There are a variety of known methods for selectively attracting toner to a photoconductor. Generally, each toner has a known electrical potential affinity. Selected areas of the photoconductor are exposed from a base potential to the potential for the selected toner, and then the photoconductor is exposed to the toner so that the toner is attracted to the selectively exposed areas. This latter step is known as developing the photoconductor. In some processes, after the photoconductor is developed by a first toner, the photoconductor is then recharged to the base potential and subsequently exposed and developed by a second toner. In other processes, the photoconductor is not recharged to the base potential after being exposed and developed by a selected toner. In yet another process, the photoconductor is exposed and developed by a plurality of toners, then recharged, and then exposed and developed by another toner. In certain processes, individual photoconductors are individually developed with a dedicated color, and then the toner is transferred from the various photoconductors to a transfer medium which then transfers the toner to the finished product medium. The selection of the charge-expose-develop process depends on a number of variables, such as the type of toner used and the ultimate quality of the image desired.
Image data for an electrophotographic printer (which will also be known herein as a “laser printer”), including color laser printers, is digital data which is stored in computer memory. The data is stored in a matrix or “raster” which identifies the location and color of each pixel which comprises the overall image. The raster image data can be obtained by scanning an original analog document and digitizing the image into raster data, or by reading an already digitized image file. The former method is more common to photocopiers, while the latter method is more common to printing computer files using a printer. Accordingly, the technology to which the invention described below is applicable to either photocopiers or printers. Recent technology has removed this distinction, such that a single printing apparatus can be used either as a copier, a printer for computer files, or a facsimile machine. In any event, the image to be printed onto tangible media is stored as a digital image file. The digital image data is then used to pulse the beam of a laser in the manner described above so that the image can be reproduced by the electrophotographic printing apparatus. Accordingly, the expression “printer” should not be considered as limiting to a device for printing a file from a computer, but should also include a photocopier capable of printing a digitized image, regardless of the source of the image.
The raster image data file is essentially organized into a two dimensional matrix. The image is digitized into a number of lines. Each line comprises a number of discrete dots or pixels across the line. Each pixel is assigned a binary value relating information pertaining to its color and potentially other attributes, such as density. The combination of lines and pixels makes up the resultant image. The digital image is stored in computer readable memory as a raster image. That is, the image is cataloged by line, and each line is cataloged by each pixel in the line. A computer processor reads the raster image data line by line, and actuates the laser to selectively expose a pixel based on the presence or absence of coloration, and the degree of coloration for the pixel.
The method of transferring the digital raster data to the photoconductor via a laser, lasers or LEDs is known as the image scanning process or the scanning process. The scanning process is performed by a scanning portion or scanning section of the electrophotographic printer. The process of attracting toner to the photoconductor is known as the developing process. The developing process is accomplished by the developer section of the printer. Image quality is dependent on both of these processes. Image quality is thus dependent on both the scanning section of the printer, which transfers the raster data image to the photoconductor, as well as the developer section of the printer, which manages the transfer of the toner to the photoconductor.
A typical in-line color laser printer utilizes a plurality of (typically 4) laser scanners to generate a latent electrostatic image for each color plane to be printed. This allows for four colors to be imaged on a transfer medium and then transferred to the finished product medium. The four color planes typically printed, and which are generally considered as necessary to generate a relatively complete palate of colors, are yellow, magenta, cyan and black. That is, the color printer is typically provided with toners in each of these four colors. These colors will be known herein as the “primary colors”. Some printers have the capability of printing one base color on top of another on the same pixel, so as to generate a fuller palate of finished colors. One method to accomplish this is to provide four photoconductors, one for each primary color, used in conjunction with an intermediate transfer belt. This configuration is described more fully below with respect to the prior art apparatus shown in FIG.
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In the scanning process, a laser is scanned from one edge of the photoconductor to the opposing edge and is selectively actuated or not actuated on a pixel-by-pixel basis to scan a line of the image onto the photoconductor. Th
Burkes Theresa A.
Clifton George B.
Grainger Quana M.
Hewlett--Packard Development Company, L.P.
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