Patent Application
20110274455
1. Field of the Technology
The present disclosure relates to systems and methods for reducing print defects in electrostatically formed images.
2. Description of the Prior Art
The electrophotographic printing process is well known. Typically, electrostatic imaging and printing processes include several distinct stages. These stages typically include some or all of (1) charging, (2) exposing, (3) developing, (4) transferring, (5) fusing, and (6) cleaning. An electrophotographic printing system typically includes a printer or a marking engine. The printer or marking engine may include a photoconductive belt or drum as a photoconductive surface.
In the charging stage, a uniform electrical charge is uniformly deposited on the surface of the photoconductive belt or drum to electrostatically sensitize the photoconductive surface. The electrophotographic exposing stage includes rotating or moving the charged photoconductive surface to an exposure station, where the charged portion of the photoconductive surface is exposed to light from, for example, a scanning laser beam. By modulating the light beam, an electrostatic latent image of variable electrostatic potential is recorded on the photoconductive surface. The light beam is modulated with an input serial stream so that individual picture elements or pixels of the image represented by the data stream are exposed on the photoreceptor to form the latent image.
The electrophotographic developing stage uses a developer material, such as a toner powder, which is attracted to the latent image in varying densities depending varying electrostatic potential of the latent image. In the transferring and fusing stages, the toner powder image is transferred to a copy sheet, and finally the toner powder image is heated and/or pressed to permanently fuse the powder image to the copy sheet in the image configuration. In the electrophotographic cleaning stage, the photoconductive surface of toner is cleaned and the charge images are discharged so that the process can be reliably repeated.
It is well known in the prior art of process control to schedule solid area, uniform halftones or background in a test patch. Some printers contain many test patches. During the print run, each test patch may be scheduled to have single halftone that would represent a single byte value on the tone reproduction curve. This is a complicated way to increase the data bandwidth required for the process control loops. It also consumes customer toner for printing many test patches. For example, U.S. Pat. No. 5,060,013 discloses a control system using test patches at different locations within the image frame on the photoreceptor. A plurality of sensors are arranged to sample the test areas in defined columns of the frame and measurements coordinated with the location of the test area.
It is also known in the prior art, for example, U.S. Pat. No. 4,341,461 to provide two test targets, each having two test patches, selectively exposed to provide test data in the photoreceptor image area for control of the toner dispensing and bias control loops. In this system, the test patches are imaged in interdocument zones on the photoreceptor. In addition, U.S. Pat. No. 5,450,165 discloses the use of incoming data or customer image data as a test patch. In particular, incoming data is polled for preselected density conditions to be used for test patches to monitor print quality.
Defects in the scanning and printing process can arise from one or more of the stages described above. Defects may occur in the development stage, where parts of the processed image will have regions of diminished toner density. Such regions of diminished intensity tend to occur in electrostatic printing at an interface of two objects of an image having different gray levels, and therefore different electrostatic potential and toner densities. The object that would result in a higher toner density can “steal” toner from the region that would result in a lower toner density, creating a toner “starvation” or “white space” defect.
It is also known, as disclosed in U.S. Pat. No. 7,295,349 that, as the electrostatic latent image passes serially through the development process, some images may contain features that have a darker solid region on top of a lighter solid region. The lead edge of the dark solid is the edge that passes first through the development stage. The trail edge of a dark solid is the edge that passes last through the development state. In this process, the white space can occur either on the lead edge of the interface, the trail edge of the interface, or both, and that the magnitude of the white space can be different. U.S. Pat. No. 7,295,349 discloses methods that compensate for the white space defect by modifying the input image pixel intensity and/or the input image bitmap. The input image intensity values in lighter regions that precede or occur near a light-to-dark transition to a dark object are raised above the input image intensity values. Thus, when printed, the printed image intensity values in such regions are higher than the corresponding image intensity values.
The present disclosure is a simple method to compensate for degradation of developed images at the borders of halftone and solid areas, known as the starvation effect. The starvation effect is the scavenging by the developer brush of toner from a previously developed portion of a half tone area. The method is applicable in existing development control systems already using solid area and halftone patches, by using a starvation test patch and a small aperture specular sensor. However, it should be understood that the disclosure is not limited to existing control schemes and would apply to other development control systems where toner depletion at the boundary of dissimilar image portions was an issue.
Various of the above-mentioned and further features and advantages will be apparent to those skilled in the art from the specific apparatus and its operation or methods described in the example below, and in the claims. Thus, there will be a better understanding from this description of specific embodiments, including the drawing figures, wherein:
In such electrophotographic printing, the step of conveying toner to the latent image on the photoreceptor is known as “development.” The object of effective development of a latent image on the photoreceptor is to convey toner particles to the latent image in a controlled manner so that the toner particles effectively adhere electrostatically to the charged areas on the latent image.
When the developer material is placed in a magnetic field, the carrier beads with toner particles thereon form what is known as a magnetic brush, wherein the carrier beads form relatively long chains which resemble the fibers of a brush. This magnetic brush is typically created by means of a “developer roll.” The developer roll is usually in the form of a cylindrical sleeve rotating around a fixed assembly of permanent magnets. The carrier beads form chains extending from the surface of the developer roll, and the toner particles are electrostatically attracted to the chains of carrier beads.
When the magnetic brush is introduced into a development zone adjacent to the electrostatic latent image on the photoreceptor, the electrostatic charge on the photoreceptor will cause the toner particles to be pulled off the carrier beads and onto the photoreceptor.
Typically, solid area development control is established by creating toner control patches of single desired density. Control patches are created using the primary scanning laser beam or an alternate light source, such as a patch generator, to discharge the photoreceptor to the proper development field. The actual developed mass per unit area (DMA) of the toner on the control patches is then optically measured to determine the effectiveness of the printing process in placing the toner on the print sheet. Typically, a specular reflectance sensor is used for determining the density of the toner on a control patch. Both solid area and halftoned control patches of varying densities, including a black solid area control patch, can be used to assure color quality control. Solid patches are represented on a Solid Area Developability Curve and halftoned patches are represented on a Tone Reproduction Curve (TRC).
With reference to
As pixels of the latent halftone image area 12 advance past the brush 16, as shown in the bottom view of
However, with reference to
This is illustrated in the bottom view of
With reference to
In particular, the control procedure is activated as shown at 22 and the first step is to develop starvation test patches as illustrated at block 24. The charge on the photoreceptor for the test patches and the location of the patches on the photoreceptor is a selected design parameter for a particular control. At block 26, a small aperture sensor reads the developed image at a solid area/halftone boundary and, as illustrated at block 28, the sensor signal representing the developed boundary is compared to a standard test patch.
The degree of difference between test development and actual development is determined. If actual development is not within specification, there will be a modification of the toner concentration target as shown in block 30. The modification of the toner concentration target generates a change in suitable developer components to increase or decrease the amount or level of toner in the developer, as illustrated at block 32. In particular, if there is unacceptable scavenging of toner from a portion of the halftone area, the toner concentration level in the developer will be lowered.
If the starvation level is determined at block 28 to be within specification, then the next step in the control is to develop multilevel Tone Reproduction Curve patches as shown in block 34. First the solid area density of the developed image is sensed as illustrated at block 36. This measurement is then checked or compared with the test or standard patches to determine whether or not the solid area development of the system meets specification, as shown in block 38. If not in specification, actuators for providing solid area toner coverage are adjusted to provide appropriate solid coverage as shown in block 40.
If the solid area coverage of the measured developed image is within specification, the next step is then to check the overall developed halftone coverage, the tone reproduction curves (TRC), as illustrated at 42. This sensed degree of development is compared to the multilevel TRC patches to determine if the halftone development is within specification. If not, there is a modification of the tone reproduction curve Look Up Table (LUT) as shown in block 46. If within specification, the final step is to print the developed image shown at 48.
It should be apparent, therefore, that while specific embodiments of the present disclosure have been illustrated and described, it will be understood by those having ordinary skill in the art to which this invention pertains, that changes can be made to those embodiments without departing from the spirit and scope of the disclosure. Further, the claims, as originally presented and as they may be amended, encompass variations, alternatives, modifications, improvements, equivalents, and substantial equivalents of the embodiments and teachings disclosed herein, including those that are presently unforeseen or unappreciated, and that, for example, may arise from applicants/patentees and others. Unless specifically recited in a claim, steps or components of claims should not be implied or imported from the specification or any other claims as to any particular order, number, position, size, shape, angle, color, or material.
