Electrophotography – Control of electrophotography process – Of plural processes
Reexamination Certificate
2000-12-12
2001-11-20
Pendegrass, Joan (Department: 2852)
Electrophotography
Control of electrophotography process
Of plural processes
C399S048000, C399S049000
Reexamination Certificate
active
06321043
ABSTRACT:
This invention relates generally to an electrostatographic printing apparatus and, more particularly, concerns the control of developed mass per unit area (DMA) in real time using internal process parameters as actuators.
The basic reprographic process used in an electrostatographic printing apparatus generally involves an initial step of charging a photoreceptor member to a substantially uniform potential. The charged surface of the photoreceptor member is thereafter exposed to a light image of an original document to selectively dissipate the charge thereon in selected areas. This procedure records an electrostatic latent image on the photoreceptor member that corresponds to the original document being reproduced. The latent image is then developed by use of toner particles adhering triboelectrically to carrier granules. The toner particles are attracted away from the carrier granules to the latent image, forming a toner image on the photoreceptor member which is subsequently transferred to a copy sheet. In the case of Charged Area Development (CAD) the development bias voltage and toner polarity are chosen such that toner is attracted to undischarged areas of the photoreceptor; in the case of Discharged Area Development (DAD), the developer bias voltage and toner polarity are chosen such that toner is attracted to discharged areas of the photoreceptor. The copy sheet is then advanced to a fusing station for permanently affixing the toner image to the copy sheet.
In electrostatographic apparatus using a drum-type or an endless belt-type photoreceptor member, the photosensitive surface can contain more than one image at one time as it moves through various processing stations. The portions of the photosensitive surface containing the projected images, so-called “image areas”, are usually separated by a segment of the photosensitive surface called the inter-document space. After charging the photosensitive surface to a suitable charge level, the inter-document space segment of the photosensitive surface is generally discharged by a suitable lamp in the case of CAD development to avoid attracting toner particles. Various areas on the photosensitive surface, therefore, will be charged to different voltage levels. For example, there will be the high voltage level of the initial charge on the photosensitive surface, a selectively discharged image area of the photosensitive surface, and a fully discharged portion of the photosensitive surface between the image areas if CAD development is employed.
The approach utilized for multicolor electrostatographic printing is similar to the process described above. However, rather than forming a single latent image on the photoreceptor 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 photoreceptor surface. Each single color electrostatic latent image is developed with toner of a color complementary thereto and the process is repeated for differently colored images with the respective toner of complementary color. Thereafter, each single color toner image can be transferred to the copy sheet in superimposed registration with the prior toner image, creating a multi-layered toner image on the copy sheet. Finally, this multi-layered toner image is permanently affixed to the copy sheet to form a finished color copy.
In copying or printing systems, such as a xerographic copier, laser printer, or ink-jet printer, a common technique for monitoring the quality of prints is to create a “test patch” of a predetermined desired density. The actual density of the printing material (toner or ink) in the test patch can then be optically measured by a suitable sensor to determine the effectiveness of the printing process in subsequently placing this printing material on the print sheet.
In the case of xerographic devices, such as a laser printer, the surface that is typically of most interest is the charge-retentive surface or photoreceptor, on which the electrostatic latent image is formed. The optical sensor for determining the density of toner on the test patch, which is often referred to as a “densitometer”, is typically disposed along the path of the photoreceptor, directly downstream of the development unit. A routine within the operating system of the printer periodically creates test patches of a desired density at predetermined locations on the photoreceptor by deliberately causing the exposure system thereof to discharge the surface to a predetermined extent.
The test patch is then moved past the developer unit and the toner particles within the developer unit are caused to adhere to the test patch electrostatically. The developed test patch is moved past a densitometer disposed along the path of the photoreceptor, and the light absorption of the test patch is tested. The sensor readings are then used to measure and control the tone response curve (TRC) and make suitable adjustments to the system such as changing developer bias, photoreceptor charging potential, or exposure level to maintain consistent quality.
High quality digital copiers and laser printers are expected to deliver tight control of halftone and solid area image densities. Typical specifications call for very small print delta E variation at every point on the tone response curve over the course of a day. This task is especially challenging for control of black and white images, because reflection-type optical sensors will saturate at a level well below the desired solid area developed mass (DMA) level; as a result, the conventional approach to determine image quality of a solid black area is based upon an inference from halftone measurements.
Conventional approaches to the foregoing problems typically require image quality control systems that employ a high computational complexity or very large numbers of sampled test patches. Accordingly, there is a need for an image quality control system operable in an electrostatographic printing apparatus that is better able to actuate subsystem parameters using the known relations of the physical xerography process to control DMA.
In high quality digital electrophotography, it is important to maintain a stable relationship between halftone input coverage (Cin), effected by the latent image recording device, and the output of the marking process. A measure of this relationship may be expressed as image density or delta E from paper. In desirable practice, the resultant tone response curve should be held within a narrow range over the course of the print job (i.e., within-run) and over the course of a day (i.e., within-day).
Heretofore, for black areas, adequate control of DMA has been less than successful. Even before the image density corresponding to 100% Cin is reached, the reflectance-type optical sensors will saturate. Even small amounts of measurement uncertainty in the system make it very difficult to infer DMA from halftone patch reflectance measurements taken below the saturation level. For example, there may be successful control of TRC output at low and mid-level values of Cin, at which the test patches can be measured with a good signal-to-noise ratio, but the overall DMA variation at 100% Cin can remain much larger than is desirable. Such variation of black solid area quality is a significant source of image degradation even when as many as three black halftone test patches are used. Furthermore, such variation typically increases as the halftone screen frequency is increased.
The root cause of this variation (in the relationship between the halftone test patch reflectance and DMA) is due to the sensitivity of halftone dot shape and size to noise factors such as raster output scanner (ROS) spot size and subtle changes in the shape of the photo-induced discharge curve (PIDC). These changes perturb the relationship between the reflectance of a test patch and the corresponding determination of DMA. These factors can be comparable in magnitude to the sensitivity of the levels of midtone and shadow
Dudley Mark Z.
Pendegrass Joan
Xerox Corporation
LandOfFree
Control of halftone and solid area image quality by way of a... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Control of halftone and solid area image quality by way of a..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Control of halftone and solid area image quality by way of a... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2592995