Test equipment for color printing

Image analysis – Image transformation or preprocessing – Changing the image coordinates

Reexamination Certificate

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Details

C430S300000

Reexamination Certificate

active

06332047

ABSTRACT:

BACKGROUND OF THE INVENTION
Modern colour printing is a high quality business, and more printing companies are introducing testing to ensure that what they print is what the customer requires. This has lead to the increase in the demand for accurate test equipment.
Changes in the types and uses of technology within the printing industry have meant that the available techniques for testing have outstripped existing test equipment. This has lead to the requirement for a new type of test instrument.
Until approximately 5 years ago, and still with some of the smaller printing companies, the following methods were used.
An original artwork is digitised by a device called a scanner to obtain a transparency or picture and, via optical and electronic techniques, an image is produced in a computer readable digital form. The digital image is then manipulated by a computer system and, once completer converted into a form that can be projected onto a piece of film. The film contains the four basic printing colours (Cyan, Magenta, Yellow and Black) and is imaged and developed. The next stage is to make printing plates (one for each colour) from the film which plates are then used to produce the printed picture.
The film is produced by a device called an imagesetter, which uses a laser to mark the film ready for processing; this creates “dots” that make up a printed image. Control of the film production process is achieved by measuring the size and spacing of the dots created on the film and feeding this information back to the imagesetter. Thus, if an area that the imagesetter believes to be 50% is measured at 46%, the measured data is entered into the imagesetter. It will then adjust the exposure level of the laser to compensate. Current test instruments for this technique are called densitometers and they measure one criteria, the density of the printed area, and calculate the percentage dot area. These instruments work by shining a light at the film (or a test proof on paper) and measuring the light transmitted by the film (or reflected by the test proof). The amount of light transmitted/reflected signifies the density of the area being tested.
The goal for some years has been to eliminate the film stage and image directly onto the printing plates, thereby eliminating the associated costs, time and variations introduced through the number of processes involved. Methods to do this have been available in recent years, but the ability to measure accurately the imaged plate has been a problem, with the result that printers are loath to make use of these methods.
The method of correction of errors in dot size/spacing on a printing plate are the same as for the film, with an imagesetter producing the plate in the same way as film. However, as there is no satisfactory method of measuring the dot size/spacing on a printing plate, it is not possible to enter the actual data to allow compensation. Several manufacturers of densitometers have tried to use existing approaches to measure plates. However, with the grained aluminium plates used almost universally today, conventional densitometers are not suitable because of the immense amount of light scatter that causes inaccurate (and inconsistent) readings. This lack of accurate and reliable measuring equipment is the major reason that direct to plate technology has not been taken up widely by the printing industry.
BRIEF SUMMARY OF THE INVENTION
An object of the present invention is to provide an alternative method and apparatus for reliable imaging directly onto the printing plates.
According to this invention, a method of measuring an imaged printing plate comprises providing an even light over the area of the imaged plate to be measured, enlarging said area via magnifying means, obtaining an image of the enlarged area via a c.c.d. camera, capturing electronic signals fed from the camera via an electronic framegrabber to provide an electronic image representing the enlarged area, and feeding the captured image to a microprocessor for analysis to determine the ratio of etched area to surface area.
It will be appreciated that this invention includes apparatus required for effecting the above method, including a source for providing even light, means for magnifying the area of the plate to be measured, a c.c.d. camera for producing an enlarged image of said area, a framegrabber for capturing the enlarged image in electronic form, and a microprocessor for analysing and determining the ratio of etched to surface area.
As mentioned briefly above, a modern printing plate comprises an aluminium alloy base onto which an image (in the form of a pattern of dots) it etched. In order to effect testing, in addition to the actual image, a test strip of target dot areas are added to each plate. These targets are printed at “known” dot percentages and are used to compare actual values against expected. If correct the printer can proceed; if outside of a desired tolerance, the actual values can be entered on the imagesetter to allow a new compensated plate to be produced. As discussed above, the problem with measuring the plate using conventional densitometers is that the surface to be measured does not reflect light in a consistent manner. The effects of grain and surface treatment cause light to stray in an unpredictable manner. In addition, as the chemicals used in the development process age, the colour of the base plate changes, causing errors in the densitometer readings.
In accordance with this invention, this problem is overcome though the use of a c.c.d. camera, operating through a magnifier, linked into a PC and framegrabber to capture images of the plate. These captured images are then analysed to determine how much of the area viewed is etched and how much is surface. Even lighting is important and is preferably provided by a diffused, or infra-red light source.


REFERENCES:
patent: 3971316 (1976-07-01), Schank et al.
patent: 4357403 (1982-11-01), Shimada et al.
patent: 4878114 (1989-10-01), Huynh et al.
patent: 5413672 (1995-05-01), Hirotsuji et al.
patent: 5659479 (1997-08-01), Duley et al.
patent: 5966553 (1999-10-01), Nishitani et al.

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