Radiation imagery chemistry: process – composition – or product th – Including control feature responsive to a test or measurement
Reexamination Certificate
1998-10-09
2002-04-23
Chea, Thorl (Department: 1752)
Radiation imagery chemistry: process, composition, or product th
Including control feature responsive to a test or measurement
C430S204000, C430S305000, C430S617000, C430S619000, C358S296000
Reexamination Certificate
active
06376138
ABSTRACT:
DESCRIPTION
The present invention relates to an improved rendering technique for halftone images and corresponding improved materials, used for generating a screened reproduction of a continuous tone image by means of an electronic screening modulation of the original. An example of the technique is given for a thermal recording material.
BACKGROUND OF THE INVENTION
Thermal imaging or thermography is a recording process wherein images are generated by the use of imagewise modulated thermal energy.
Direct thermal thermography is concerned with materials which are substantially not photosensitive, but are sensitive to heat or thermosensitive. Imagewise applied heat is sufficient to bring about a visible change in a thermosensitive imaging material.
Most of the “direct” thermographic recording materials are of the chemical type. On heating to a certain conversion temperature, an irreversible chemical reaction takes place and an image is produced by a change of the local optical density.
U.S. Pat. No. 5,424,182 discloses a thermal imaging material and preparation.
Many reproduction methods are only capable of reproducing a small number of stable image tones. For example, offset printing is only capable of printing two stable tone values i.e. deposit ink or not. In order to reproduce images having continuous tones, a halftoning or screening technique is used. In the graphic arts environment, halftoning techniques convert density values of tints and images into a geometric distribution of binary dots that can be printed. The eye is not able to see the individual halftone dots, and only sees the corresponding “spatially integrated” density value. In a more general context, halftoning techniques can be seen as methods to convert “low spatial, high tonal resolution information” into an equivalent of “high spatial, low tonal resolution information”. The qualifiers “low” and “high” have to be seen on a relative scale in this context. In amplitude-modulation screening the halftone dots, that together give the impression of a particular tone, are arranged on a fixed geometric grid. By varying the size of the halftone dots, the different tones of images can be simulated. Consequently, this technique can also be called “dot-size modulation screening”. This class of halftoning technique is traditionally used in combination with a digital film recorder. A typical digital film recorder employs a scanning laser beam that exposes a photosensitive material at high resolution. The “grid” that defines the resolution at which the laser beam can be switched on or off, usually has an element size in the range of {fraction (1/1800)} of an inch. The photosensitive material can be a photographic film from which a printing plate is later prepared by means of photomechanical techniques. The smallest addressable unit on a recorder is often called a “micro dot”, “recorder element”, or “rel”. Its size is referred to as the recorder “pitch”. As illustrated in
FIG. 3
a,
a dot-size modulated halftone dot is made up of a clustered set of recorder elements.
The most important characteristics of a screening or halftoning technique for faithfully reproducing continuous tone information include:
1) The image rendering characteristics, more specifically the capability of the technique to render spatial detail in the original image content without the introduction of artifacts such as moiré, textures and noise, as well as the capability to render a full range of tones;
2) The photomechanical characteristics of the halftone dots produced by the method, which determine how consistently halftone dots can be recorded, copied or duplicated in the different steps of the photomechanical preparation of the printing plates; and,
3) The behaviour of the halftones on an offset printing press.
The class of amplitude-modulation halftoning, with some of its variants, will now be reviewed in the light of the above characteristics, and their advantages and disadvantages will be discussed.
Amplitude-modulation screening has as its major advantages that it has excellent photomechanical reproduction characteristics, and that, for screens with rulings up to 200 lines/inch, it prints predictably on offset presses. An important disadvantage of amplitude-modulation screening, however, is the fact that unwanted patterns can occur within the halftoned image. Depending on their origin, these patterns are called subject moiré, colour moiré or internal moiré. Subject moiré results from the geometric interaction between periodic components in the original subject matter and the halftone screen itself. Methods addressing subject moiré are disclosed in e.g. U.S. Pat. No. 5,130,821, EP-A-0 369 302 and EP-A-0 488 324. These methods do not, however, completely solve the problem.
Colour moiré results from interferences between the a halftones of the different colour separations of the image. The use of screen angles for the different colour separations shifted by 60 degrees one with respect to the other has been suggested to address this problem. Several disclosures relate to the problem of generating screens with these angles or close approximations thereof. See e.g. U.S. Pat. Nos. 4,419,690, 4,350,996, 4,924,301 and 5,155,599. Other combinations of angles, frequencies or relative phases of the halftone dot patterns for the different colour separations have also been used to overcome the same problem, as described e.g. in U.S. Pat. Nos. 4,443,060, 4,537,470 and EP-A-0-501 126.
Internal moiré refers to patterns resulting from the geometric interaction of the halftone screen with the addressable grid on which they are rendered. Methods to reduce internal moiré are usually based on the introduction of a random element that breaks up or “diffuses” the phase error that periodically builds up as a consequence of the frequency and angle relation between the halftone screen and the addressable grid on which it is rendered. Examples of such techniques are disclosed in U.S. Pat. Nos. 4,499,489, 4,700,235, 4,918,622, 5,150,428 and WO-A-9 004 898.
EP-A-0 734 147, EP-A-0 774 857 and EP-A-0 734 148 provide methods for generating screened reproductions of continuous tone images with improved reproduction characteristics.
These three patent applications disclose methods for improving the image quality by complicating the modulation of the laser. These solutions not only tend to complicate the manufacturing process, they are also time-consuming and costly.
Another screening technique is referred to as frequency modulation halftoning. Whereas in amplitude modulation screening the halftone dots, that together give the impression of a particular tone, are arranged on a fixed geometric grid and are their size is varied to simulate the different tones of images, in frequency modulation screening the distance between the halftone dots is modulated rather than their size. The arrangement of exposed microdots according to frequency modulation screening is shown in
FIG. 3
b.
According to
FIG. 3
b
frequency-modulation halftone dots consist of individual recording elements. Each black square in
FIG. 3
b
represents one frequency-modulation halftone dot, occupying exactly one “rel”. The whole
FIG. 3
b
shows a region or set of dispersed black frequency-modulation halftone dots, covering each one individual recording element. In the prior art methods, the size of a frequency-modulated halftone dot equals the size of a recorder element.
Various frequency-modulation screening techniques have been disclosed and they can be divided into the following subclasses:
(1) Point-to-point thresholding based techniques;
(2) Error Diffusion techniques (and their variations); and,
(3) Special techniques, such as that disclosed in DE-A-2 931 098, and further developed in U.S. Pat. No. 4,485,397. The most representative example of point-to-point thresholding is the halftoning based on the “Bayer” dither matrix. See BAYER, B. E. “An Optimum Method for Two-level Rendition of Continuous-tone Pictures”, New-York: Proc. IEEE International Conference on Communications, Conference Record, 1973, pp
Bosschaerts Jacobus
Delabastita Paul
Horsten Bart
Schellekens Luc
Agfa-Gevaert
Chea Thorl
Hoffman, Warnick & D'Alessandro LLC
Merecki John A.
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