Facsimile and static presentation processing – Static presentation processing – Attribute control
Reexamination Certificate
1999-12-16
2004-04-13
Lee, Thomas D. (Department: 2724)
Facsimile and static presentation processing
Static presentation processing
Attribute control
C358S406000, C358S450000
Reexamination Certificate
active
06721061
ABSTRACT:
TECHNICAL FIELD
This invention relates to digital imagesetting and, more particularly, to visual sensors for detecting banding.
BACKGROUND INFORMATION
Printing presses use plates to print ink onto paper and other media. One method used for creating plates is to expose photosensitive film with the matter to be printed. When the film is developed, the matter imaged on the film may be imaged onto a photosensitive plate, sometimes referred to as “burning” a plate. After processing, the plate can be used on a press to print the matter onto a medium. Part of the plate, usually the part defining the image to be printed, retains ink, while the other part of the plate does not. When the plate is introduced to ink and then to paper or other medium, the image is printed on the medium.
In a black and white printing job, there is usually one plate that is used to print black ink. In a color printing job, a different plate may be used for each color ink. A color job may use three colors of ink, usually cyan, magenta, and yellow, which in combination can be used to make other colors. A plate is usually produced for each color ink. Often, in addition to cyan, magenta, and yellow, black ink is also used. An additional plate is then required to print the black ink. Occasionally, one or more colors will be printed separately as well, referred to as a “spot color.” That color will also have its own plate.
Electronic prepress systems have used an imagesetter to receive raster data associated with a plate and to image the raster data onto photosensitive film. In this context, a raster may specify an image by pixels in columns and rows, at a predefined resolution. The film is then used to create a plate. The imagesetter exposes the photosensitive film pixel by pixel. One way that imagesetters image the raster data is to scan a laser across and down a piece of film. Electronics control the laser to expose, or refrain from exposing, each pixel in the raster data. The imagesetter images the pixels on the film in a manner that is precise and repeatable. Recently, platesetters also have been used to create plates directly from raster data without the use of intermediate film. Imagesetters, platesetters and like print engines, including proofers, are also referred to generally as output devices and writing engines.
Modern output devices may write or record images on various media used in image reproduction, including but not limited to photo or thermal sensitive paper or polymer films, photo or thermal sensitive coatings, erasable imaging materials or ink receptive media mounted onto an image recording surface, polymer film or aluminum based printing plate materials. Such media are mounted onto a recording surface which may be planar or curved.
Conventional digital imagesetters include a raster image processor (RIP) which receives signals representing an image to be recorded on the applicable media and converts the signals into instructions to a scanner which scans the recording media to produce the desired image. It is the function of the RIP to process the received signals representing the image into a corresponding instruction set that can be understood by the scanner.
Noise in various components of a writing engine, such as vibrations or inaccurate laser scan mechanisms for example, can result in imaging artifacts, sometimes referred to as banding.
Generally, a large component of banding artifacts are line spacing errors, meaning errors in the uniformity of line spacing, or, in other words,local variations in the address size compared to mean address over a long length of copy. High end recording systems are extremely sensitive to such noise effects. Uniformity of line spacing is often required to be less than +/−⅓ to +/−{fraction (1/10)} pixel error at maximum engine addressability. Often a specification for a high end recording system specification requires that imaging artifacts due to banding be essentially invisible for a range of possible output image patterns. This objective is hard to achieve, and it is equally hard to measure since any visible banding exceeds the threshold of failure. It is thus difficult to certify that a writing engine meets its specification, since it is difficult to determine if the writing engine meets the banding criteria.
The traditional method of qualifying a writing engine for banding is to image a full test page with a halftone test pattern that represents the maximum level of difficulty. For an engine with a maximum addressability of 3600 dpi, for example, a page might be imaged at a 96% tint level at 318 lines per inch. To test banding effects resulting from line spacing uniformity errors in the range of {fraction (1/10)} pixel requires a very dark tint, typically greater than 80% to 90%. Also, it typically requires some single pixel structures in the tint pattern. For these reasons a 96% tint is often used. Even at this high tint value, the low visibility of residual {fraction (1/10)} to ⅕ pixel line spacing uniformity errors requires a large sample size, often greater than 6×6 inches to reliably detect the level of resulting banding error. For this reason a full page often is imaged for evaluation. The imaged media is compared with a separate master pass/fail reference page. Observation conditions, such as lighting, viewing angle, and condition of the master can influence the comparison. The visual appearance of the test page is also sensitive to setup/process conditions of the engine and the media. For example, the visual level of banding is generally very sensitive to exposure. A test can be made to change from pass to fail simply by increasing the exposure setting slightly. This is a critical problem when attempting to provide a consistent standard for banding pass/fail criteria. Generally, the exposure must be within 0.3D density units of correct setting, and the writing engine must be focused so that it is within 5% of its design value for every test to obtain useful banding detection results.
In addition to being difficult, the banding qualification process is time consuming and uses a large amount of media—often a full page for each test. It would therefore be useful to have a more efficient and accurate mechanism to qualify the banding effects of a writing engine.
SUMMARY OF THE INVENTION
A visual indicator is described that can be used to qualify the banding of an writing engine. The visual sensor can be imaged on a media to magnify the effect of banding to make the measurement of banding more apparent to a user or machine vision system.
In general, in one aspect, the invention relates to a visual indicator including a reference portion including an image independent of banding and a banding test portion adjacent to the reference portion including an image sensitive to banding. In one embodiment, the banding test portion includes two patterns which when superimposed together magnify the visibility of banding errors. In another embodiment, the banding test portion includes two patterns which when superimposed by first imaging a first pattern and second imaging a second image on top of the first image together magnify the visibility of banding errors. In another embodiment, the banding test portion includes two patterns which when superimposed together magnify the visibility of adjacent line spacing errors.
In another embodiment, the banding test portion includes two overlaid images imaged at different addressabilities. In another embodiment, the banding test portion includes a first image imaged at a first addressability n, and a second image superimposed on the first image imaged at a second addressability m, where m≧n. In another embodiment, the banding test portion includes a first image imaged at a first addressability n, and a second image superimposed on the first image imaged at a second addressability m, where m is approximately 2n. In another embodiment, the banding test portion includes a first imaged horizontal one-on/one-off lines imaged at a first addressability n, and horizontal two-on/three-off
Agfa Corporation
Heffan Ira
Kelley Edward L.
Lee Thomas D.
Sabourin Robert A.
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