Patent Application
20110313707
The present invention is directed to systems and methods for correcting measurements obtained using an in-line spectrophotometer from color test patches in the presence of a banding defect in a digital document reproduction device.
In a typical multi-function document reproduction system, a photoconductive drum or photoreceptor belt rotates at an angular velocity and, as the photoconductive drum rotates, the drum is electrostatically charged. A latent image is exposed line-by-line onto the photoconductive drum using a scanning laser and, for instance, a rotating polygon mirror. The latent image is developed by electrostatically adhering toner particles to the photoconductive drum. The developed image is transferred from the photoconductive drum to the output media such as paper. The toner image on the paper is fused to the paper to make the image on the paper permanent. The surface of the photoconductive drum is cleaned to remove any residual toner on the surface of the photoconductive drum. Typically, the printing device drives the photoconductive drum using a motor drive system or a motor drive train. The motor drive system has a substantial amount of external loading because it typically drives the auxiliary rollers and transports the paper through a series of gear trains. With the additional external loading, as well as periodic disturbances due to imperfections in the series of gear trains, the motor drive system imparts a varying velocity on the photoconductive drum. The varying photoconductive drum velocity causes scanline spacing variation in the printed image. The scanline spacing variation is a significant contributor of artifacts in the marking process. For example, halftone banding caused by scanline spacing variation is one of the most visible and undesirable artifacts, appearing as light and dark streaks across a printed page perpendicular to the process direction. Such one dimensional image density variation in the process direction are often periodic and can result from errors in the mechanical motion of rotating components within a marking engine. These components may be gears, pinions, and rollers in the charging and development subsystems, photoreceptors and their drive trains, or the ROS polygon.
Many Xerox production color systems (iGen3 and iGen4, DC8002, DC7002) now include an In-Line-Spectrophotometer (ILS). Systems such as the DC8002/7002 are susceptible to a banding defect. Current ILS implementations do not include mechanisms to robustly deal with banding. The control loop for color management uses the ILS for measurement of the current system state. If a patch falls within an area with banding on a print, the system will read and calibrate according to the readings for that patch. Both dot linearization and characterization (ICC profile generation) use the ILS patch reads to generate new dots and profiles in-situ. The accuracy of the profiles and channel-by-channel correction are limited by the accuracy of the patch measurements. The accuracy of these measurements is dependent on several factors including: ILS repeatability, ILS accuracy, and xerographic variation. ILS repeatability has been shown to be very low and ILS accuracy has been addressed previously via the Spectral Component Analysis (SCA) matrix.
Accordingly, what is needed in this art are increasingly sophisticated systems and methods for creating a dynamic document comprising constant color patch targets whose distance between patch repeats is optimal for reducing banding noise in ILS measurements.
The following U.S. patents, U.S. patent applications, and Publications are incorporated herein in their entirety by reference.
“Method And Apparatus For Controlling Non-Uniform Banding And Residual Toner Density Using Feedback Control”, U.S. Pat. No. 7,054,568 to Mizes et al.
Systems And Methods For Correcting Banding Defects Using Feedback And/Or Feed Forward Control', U.S. Pat. No. 7,058,325 to Hamby et al.
“Method And System To Compensate For Banding Defects”, U.S. Publication No. 20090046325, to Paul et al.
What is disclosed is a novel system and method for correcting for In-Line-Spectrophotometer (ILS) measurements of constant value patches in the presence of a banding defect.
In one example embodiment, the present method for correcting measurements using an in-line spectrophotometer color correction system in the presence of a banding defect involves the following. A constant value color patch is first printed with a document reproduction system having an in-line spectrophotometer color correction system. An ILS data stream is received in response to the in-line spectrophotometer interacting with the constant value patch and the ILS data stream is analyzed to determine a frequency of at least one structured noise component in the document reproduction system that is due to process banding. In one embodiment, analyzing the ILS data stream comprises performing a Fast Fourier Transform of each L*a*b* component in the ILS data stream and identifying a peak frequency of each L* a* and b* channel of the document reproduction system associated with the structured noise components. Peak frequencies are then compared to determine a common frequency across all of the channels. A banding wavelength, λBand, is determined from the common frequency. Thereafter, a distance between repeats of a color patch target is adjusted as a function of λBand such that measurements of the color patch target repeats by the ILS system are synchronized to the banding wavelength. If a single peak frequency has been identified in multiple channels, then that single peak frequency is determined to be a frequency of the structured noise component. An embodiment for adjusting the distance between is provided and described herein in further detail.
Many features and advantages of the above-described method will become readily apparent from the following detailed description and accompanying drawings.
The foregoing and other features and advantages of the subject matter disclosed herein will be made apparent from the following detailed description taken in conjunction with the accompanying drawings, in which:
What is disclosed is novel system and method for detecting and correcting for In-Line-Spectrophotometer (ILS) measurements of constant value patches in the presence of banding in multi-function document reproduction systems.
It should be understood that one of ordinary skill in this art should be readily familiar with printer quality monitoring and troubleshooting techniques, particularly those which relate to detecting and quantifying xerographic noise, and analyses of scanned test patterns to determine frequency spectra of structured noise components. Those of ordinary skill would be familiar with the text: “Digital Color Imaging Handbook”, 1st Ed., CRC Press (2003), ISBN-13: 97808-4930-9007, and “Control of Color Imaging Systems: Analysis and Design”, CRC Press (2009), ISBN-13: 97808-4933-7468, both of which are incorporated herein in their entirety by reference.
A “reflectance sensing device”, as used herein, refers to a spectrophotometric device having a plurality of illuminators for illuminating a sample of interest and photoreceptor sensors for measuring reflect light from the sample. Each illuminator is a light source having a respective spectrum range. Example illuminators are Infrared (IR) LED, visible LED, and incandescent lamp. A reflectance sensing device may have illuminators of different colors or a single illuminator (white) with different color filters. The illuminators are switched on/off in a predetermined sequence such that spectral measurements can be obtained in each illuminator's wavelength range.
A “spectrophotometer” is one reflectance sensing device which measures a reflectance over many wavelengths and provides distinct electrical signals corresponding to the different levels of reflected light received from the respective different illumination wavelength ranges using multiple channels. A model-based spectrophotometer is a reflectance sensing device that is able to deduce spectral reflectance information for areas of the spectrum that have not been measured directly by utilizing a mathematical model or fitting parameters. This is in contrast to a “first-principles” device which reports spectral reflectance information measured directly at various wavelengths of interest.
An “ILS color correction system” is a system which employs an in-line spectrophotometer to obtain ILS data from color images.
An “ILS data stream” refers to spectral reflectance information reported by the ILS color correction system. Such information generally comprises a plurality of reflectance values, each corresponding to a wavelength or channel of the spectrophotometer device employed. For example, a Gretag Spectrophotometer outputs 36 reflectance values (1 per channel) evenly spaced at 10 nm intervals over a spectrum of 380 nm to 730 nm. An X-Rite Spectrophotometer outputs 31 reflectance values evenly spaced at 10 nm intervals over a spectrum of 400 nm to 700 nm. Spectral measurements of color test targets may further be converted using known extrapolation algorithms.
A “structured noise component” is a component in a document reproduction system which induces noise into an image which has a periodic element in the process direction.
A “banding defect” is defined as a one dimensional image density variation in the process direction and comprises either horizontal or vertical bands which have a wavelength period that varies from a minimum to a maximum frequency over time. The frequency of the banding can be measured using Fourier analysis. Noise due to process banding can arise from errors in the mechanical motion of rotating components such as, for instance, gears, pinions, rollers in various subsystems, photoreceptors and their drive trains, and the like. The periodic element is usually dominated by particular frequencies which help identify the noise source.
Xerographic noise which comprises two primary components, i.e., a random or “white” component, and a structured noise component. The remaining component of xerographic variation has also been addressed by using several measurements of the same target and averaging them in an effort to decrease the noise inherent in printing (e.g. within run and run-to-run variation). Averaging several patches reduces the unstructured noise component but may not reduce the patch noise introduced by banding. The relationship between the distance between patch repeats and the banding wavelength will determine the efficacy of averaging the structured noise out of the patch. The present system and method analyzes the ILS data stream and attempts to identify structured noise components due to banding. An FFT is performed on each L*a*b* component in the ILS data stream for a single color test page. The peak frequencies from the FFT of the L* a* and b* channels are compared. Common frequencies in all 3 channels indicate a banding component. Once the banding frequencies and the banding wavelength are known, the color patch target are adjusted to create a dynamic test patch document whose distance between patch repeats is synchronized to the banding wavelength. By running a series of synchronized test patches and averaging the results the structured noise, a reduction of banding effects on color calibration is achieved and improved customer satisfaction effectuated. The teachings hereof can readily be made available on a machine as a service tool when enables engineers and/or technicians to know when there is a banding problem (via remote diagnostics). If the banding problem is severe enough, the system can also account for this banding depending on the severity and frequency of the bands.
Reference is now briefly being made to
Reference is now made to
The color tandem architecture 200 includes a multi-color image forming device with a plurality of print stations arranged in series, each of which transfers a different color toner image of a multicolor image to an intermediate transfer member 250. A first photoreceptor drum 210a includes a charging device 220a, an exposing device 230a, a developer device 240a and a cleaning device 270a. A single color toner image formed on first photoreceptor drum 210a is transferred to intermediate transfer belt 250 by first transfer corotron 254a. Transfer belt 250 is wrapped around rollers 251, 253 which tension the transfer belt and are also driven to move belt 250 in the direction of arrow 255. Second, third and fourth photoreceptors 210b, 210c, 210d also include charging, exposing, developing, and cleaning devices (not shown) to form and transfer second, third and fourth single-color toner images to belt 250 (on top of each other) using transfer corotrons 254b, 254c, 254d. Typically, these would include separate stages for each of cyan, magenta, yellow and black colorants. Although four stages are shown, fewer or greater stages can be present. The multicolor image that is formed on belt 250 is then transferred to receiving media 212, such as paper, by corotron 258. Media 212 moves in the direction of arrow 214 through fusing station 272.
After the transfer of the multi-color image to the receiving material 212, a residue of the multicolor image, represented as residual toner patch 276, may remain on the intermediate belt 250. Upon completion of transfer of the image to media 212, the intermediate belt passes in contact with backing plate 285 which aids in retaining a shape of the belt, and then passes through cleaning station 260 to remove the residual toner. The intermediate belt 450 then advances around to re-engage photoreceptors 410a-d as known in the art.
The color tandem architecture of
Controller 290 provides data and control to the several photoreceptors 210a-d and associated components. Controller 290 further accepts signals from other sources and produces page sync signals used to assemble multiple images into coherent time domain samples. A page sync recorder (not shown) stores various time stamps that indicate a relative time between the start of printed of toner images and/or pages of which images are captured by either an internal printer sensor or offline scanner. Analysis processor 292 is a special purpose computer system capable of performing various aspects of the present method as described with respect to the flow diagram of
The present method analyzes the ILS data stream received from various of the above-described sensors and identifies structured noise components due to banding. Once the banding frequencies and the banding wavelength are known, a distance between color patch targets on a printed sheet are adjusted to create a dynamic test patch document whose distance between patch repeats is synchronized to the banding wavelength.
Reference is now being made to the flow diagram of
At step 302, a constant value color patch is printed with a document reproduction system having an ILS color correction system which is intended to be analyzed for a banding defect. One such system is shown and discussed with respect to the architecture of
At step 304, an ILS data stream is received in response to an in-line spectrophotometer sensing device interacting with the constant value patch.
At step 306, the ILS data stream is analyzed to determine a frequency of at least one structured noise component that is due to process banding. In one embodiment, analyzing the ILS data stream to determine a frequency of the at least one structured noise component due to process banding involves performing a Fast Fourier Transform of each L*a*b* component in the ILS data stream and then identifying a peak frequency of each L* a* and b* channel of the document reproduction system associated with the structured noise component. The peak frequencies are then compared to determine a common frequency across all of the channels. The wavelength λBand is determined from the common frequency. If only a single peak frequency has been identified in multiple channels then that single peak frequency is determined to be a frequency of the structured noise component. In one embodiment,
At step 308, a distance between repeats of a color patch target is adjusted as a function of a wavelength λBand of the determined frequency of the structured noise component such that measurements by the in-line spectrophotometer of the color patch target repeats are synchronized to the banding wavelength. Thereafter, further processing stops.
In one embodiment, a distance between color patch target repeats is given by:
where N is an integer and depending on a value of N, repeats of a given color patch target may lie on a same page or on different pages, and where K is the number of repeats. This assures that each color is sampled at equal intervals of a cycle.
As an example, the graph of
where Lavg is the average L* of that patch, Lac is the structured noise due to banding at a given wavelength, and v(k) is the uncorrelated noise.
If we average all of the L*(k) together, we get:
In other words, all of the sinusoidal components will cancel out and only the average L* with the uncorrelated noise remains. Since the distance between repeats is a function of the banding wavelength the calibration target needs to be created dynamically in a system if the banding is measured dynamically. The patch colors remain the same but the patch order is permuted to meet the distance between similar colors according to Eq. (1). The same document cannot be submitted K times, as the desired distance of Eq. (1) may not be met.
Once the distance between color patch repeats has been synchronized to the wavelength of the identified structured noise component due to process banding, subsequent measurements by the device's color control system have improved ILS repeatability and ILS accuracy.
Reference is now being made to
In
The computations necessary to establish and/or to determine adjustment of individual image formation parameters such as, for example, selection from among the individual available thresholds and/or dilation parameters, may be implemented within a circuit in the image forming device itself. Alternatively, such computations may be performed on a programmable general purpose computer, special purpose computer, program microprocessor or microcontroller, or other like digital signal processing devices. These other like digital signal processor may include, but are not limited to, peripheral integrated circuit elements, ASIC, or other integrated circuits, hard-wired electronic or logic circuit, or the like, or may even be manipulated through manual adjustment of one or more operating parameters and/or user-adjustable input parameters that may be associated with one or more of the operating parameters of the system and methods disclosed. It should be appreciated that, given the required inputs, to include, but not be limited to, appropriate information regarding thresholds and/or inputs regarding device settings, and the like, and may include inputting software algorithms or any combination of software, hardware and/or firmware control parameters to implement the individual devices and/or modules hereof in varying combinations.
It will be appreciated that the above-disclosed and other features and functions, or alternatives thereof, may be desirably combined into many other different systems or applications. Various presently unforeseen or unanticipated alternatives, modifications, variations, or improvements therein may become apparent and/or subsequently made by those skilled in the art which are also intended to be encompassed by the following claims. Accordingly, the embodiments set forth above are considered to be illustrative and not limiting. Various changes to the above-described embodiments may be made without departing from the spirit and scope of the invention. The teachings hereof can be implemented in hardware or software using any known or later developed systems, structures, devices, and/or software by those skilled in the applicable art without undue experimentation from the functional description provided herein with a general knowledge of the relevant arts.
Moreover, the methods hereof can be implemented as a routine embedded on a personal computer or as a resource residing on a server or workstation, such as a routine embedded in a plug-in, a photocopier, a driver, a scanner, a photographic system, a xerographic device, or the like. The methods provided herein can also be implemented by physical incorporation into an image processing or color management system. Furthermore, the teachings hereof may be partially or fully implemented in software using object or object-oriented software development environments that provide portable source code that can be used on a variety of computer, workstation, server, network, or other hardware platforms. One or more of the capabilities hereof can be emulated in a virtual environment as provided by an operating system, specialized programs or leverage off-the-shelf computer graphics software such as that in Windows, Java, or from a server or hardware accelerator or other image processing devices.
One or more aspects of the methods described herein are intended to be incorporated in an article of manufacture, including one or more computer program products, having computer usable or machine readable media. The article of manufacture may be included on at least one storage device readable by a machine architecture or other xerographic or image processing system embodying executable program instructions capable of performing the methodology described herein. The article of manufacture may be included as part of a xerographic system, an operating system, a plug-in, or may be shipped, sold, leased, or otherwise provided separately either alone or as part of an add-on, update, upgrade, or product suite.
It will be appreciated that various of the above-disclosed and other features and functions, or alternatives thereof, may be combined into other systems or applications. Various presently unforeseen or unanticipated alternatives, modifications, variations, or improvements therein may become apparent and/or subsequently made by those skilled in the art which are also intended to be encompassed by the following claims. Accordingly, the embodiments set forth above are considered to be illustrative and not limiting.
Various changes to the above-described embodiments may be made without departing from the spirit and scope of the invention. The teachings of any printed publications including patents and patent applications, are each separately hereby incorporated by reference in their entirety.
