FM network points: irregular distribution performance image shade level (on)
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In the process of image reproduction, the traditional screening method is used to express the shade level of the image. The printing technicians have achieved a very high level of achievement after more than a century of unremitting efforts. However, this screening method, due to the color overprinting, will appear many many regular rose flower pattern markings, and sometimes even moiré may appear. At the same time, due to the decomposition of the dots, as the number of screens decreases, the sharpness of the image is drastically reduced, and the characteristics of the original are lost. In recent years, with the research of high-fidelity copying technology, ultra-fine dot printing of more than 120L/cm has appeared, and it is possible to accurately obtain image effects of fine-level reproduction and continuous adjustment. However, no matter how fine, the characteristics of the dots are that there are always different positive or negative points in the image part, and there are always fatal weaknesses. The structure of the rose flower pattern, that is, the pattern phenomenon caused by the screen angle It is independent of the number of network lines. At the same time, if the number of network lines exceeds a certain limit, not only will it bring more difficulties to copying, but also the tone value, especially the loss of highlights is more serious, and the resolution also has certain limitations, which is the biggest drawback of the traditional screening method.
Characteristics of FM outlets
In the 1970s, foreign countries began to study a frequency modulation screening method. Unlike traditional screening, printed dots are calculated based on the statistics of the hue and level of adjacent parts in the image. The inside is irregularly distributed randomly, and the size is the same (mostly 7-40μm), which is used to express the shade level of the image. Because the technical conditions at that time could not meet a large number of computing needs, it was postponed until the 1980s, with the development of electronic computers, the computing power was greatly improved, and this screening method was realized. However, the current FM screening is only a first-order function random screening, the point size is equal, the dot distance change and the photo image are deposited by halogen particles (0.5-5μm) of different sizes and distances to form a tone or color. Still different. Therefore, people continue to study, if the second order function (Second order function) random screening, the distance and area of the point can be randomly changed, then the second-order function screening and the original photo in the mechanism of the tone Very similar, the replica will be more realistic, completely close to the effect of the photo image. Of course, the computational complexity of second-order functions for random screening is larger and more complex, but it is believed to be achievable in the near future.
In fact, at this stage, people adopting first-order function FM screening has shown incomparable superiority over traditional screening methods, in order to use computer algorithms to control the arrangement of dots, which can actually eliminate the unsightly pattern periodicity. Structure, more suitable for image reproduction above the three primary colors. Due to the fine-tuning of the FM screening points, such as the use of 15μm diameter points, you can undoubtedly copy 1%-99% of the tone value, so the replica resolution is high, the image tone is soft, the layer is rich, the image is realistic, There is no Moire effect, this is the reason why the FM screening method can get the positive charm of the printing experts.
Minimum dot size
In the FM screening technology, the only key parameter is the size of the dot, which is much less than the art printing parameters of the traditional screening. It does not have the problem of dot angle, number of mesh lines, and dot shape. The size of the FM dots is only related to the recording accuracy of the exposure device. In theory, a 1200dpi photo-recording spot is a square point with a side length of 1/1200in, but the spot that is actually exposed to the photographic film by laser is a circle that is slightly smaller than the square. 21 μm, which is equivalent to a 1.2% dot of 60 L/cm, or a square point of 1.5%.
From the current output accuracy of the imagesetter, it is completely guaranteed to produce the FM dot size that meets the printing requirements. The problem depends on the size of the dots that can be printed in the printing. Therefore, the FM screening is based on computer hardware, but rather printing. technology. Therefore, the minimum size that can be printed in printing is a critical factor, but its values vary depending on the level of printing equipment, materials, processes and technology. According to reports, it is assumed that in the case of a certain resolution, a spot is taken as an ink dot, and foreign countries can perform high-quality printing on smooth paper, the smallest printing dot can reach 8 μm, and can also be printed on rough paper. 40μm pips. However, in China, it is quite difficult to test the dots at less than 20μm. It is obvious that the gap is relatively large.
Choice of printed materials
Due to the finer frequency and different structure of the FM network, there are high requirements for printing materials such as ink, paper, and blanket.
a. Ink: The FM dots are almost completely isolated and randomly distributed in the image area. Even in the dark part, you can use the calculation method to make as many duplicate dots as possible (at most, only some of the dots may be close to each other). When the hue of the traditional screen is more than 50%, the dots will start to connect or connect. different. At a small ink dot of 20 μm, the ink must exhibit good viscoelasticity on the printing plate, blanket and transfer to paper. When the external force is intermittently or completely stopped, the inked point can immediately restore the original structure and remain isolated. Features do not fall, so this ink touch denaturation, yield value and viscosity are larger.
Since the dots are fine, the ink layer may not be very thick, and the density may be affected. For this, the ink is required to have a coloring power, a high color saturation, a fine particle size, and a strong water resistance. From the overall effect, it is necessary to develop inks suitable for FM dot printing.
b, paper. Paper and ink are two inseparable aspects of printing. In order to ensure that the FM network is not lost, in addition to the general requirement of good whiteness and high surface tensile strength, it is especially important for the smoothness and thickness uniformity of the paper. According to foreign reports, the high-grade coated paper can print dots of 8μm size. The paper with a rough surface is only about 30 μm, so choose a smooth paper with high smoothness.