FM screening technology and its application
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The emergence of Frequency Modulated Halftone Dot (FM network point) and its advantages in resolution, no moiré and image and text processing have received widespread attention from the printing industry. With the improvement of materials and supporting processes, it will become a better than traditional AMDF (AmplitudeModulatedHalftone) Dot, referred to as AM network point), the main screening method. Conventional screening technology uses the method of changing the dot size to reproduce the color and layer change of the continuous tone original. Compared with the meaning in the radio technology, changing the size of the dot corresponds to the amplitude modulation in the radio technology. For this reason, the FM screening technology After the appearance, people used the network generated by the conventional screening technology as the AM network.
The difference between FM network technology and AM network technology is that it does not change the size of the dot. Instead, the computer generates the same size points according to the pixel values of the digital image. The minimum diameter of these points can be equal to the recording accuracy of the device. . A set of finite number of point groups in a halftone unit constitutes a very special form of mesh, but it has no fixed shape, because the positions where these points appear are random, sometimes called "random dots", for example two When the numerically identical pixels form a dot by the FM screening technique, although the number of recorded dots is the same, they appear differently. The sum of the area constituting a frequency-modulated recording point is equal to the size of a conventional halftone dot, and its area ratio is also equal to that of a conventional halftone dot. The amplitude modulation dot is the color tone effect of the image by the size of the dot. The frequency of the dot appearing in the space and the number of the internet spot per unit area are constant. The image tone is expressed by changing the dot area, that is, the spot of the bright spot is small, and the dark spot is The dot size is large; the area of the frequency modulation dot is fixed, and the image tone is expressed by changing the density (density) of the dot in the space.
Compared with AM network points, FM network points have many advantages, mainly in:
1. Eliminate the occurrence of "clay" "Moiré" is the result of periodic structural interactions. Color printing is usually done by accurately overprinting four independent screen images, which are printed in four colors of cyan, magenta, yellow and black, respectively, using amplitude modulation and screening techniques, and regularly ordered dots and dots. The simplification will inevitably lead to the angle of the network cable, which inevitably produces "moire" in the process of overprinting. With FM screening technology, the probability of occurrence of the network is random, the arrangement of the dots is irregular, and there is no longer a regular spatial dot structure. Therefore, after the several color plates are superimposed, there is no such thing as "clay". Interference fringes, which also eliminate the possibility of "marriage" due to interference with certain regular stripes of the image itself (such as the bark of a fence or the teeth of a shark) that interfere with the screen.
2. It can improve the color reproduction ability. Because the frequency distribution of the FM network is irregular, there is no problem of the cable angle, and the problem of “mosquito” is solved. Therefore, colors other than cyan, magenta, yellow and black can be used to expand the color gamut of the color, that is, More than four colors, increase the printing of spot color inks, achieve high fidelity (HiFi) color printing, reduce the color difference between printed matter and original, and improve the color reproduction ability. This is extremely helpful for increasing profits in the print commercial market. In addition, because the frequency modulation dots are irregularly distributed, the dots can be prevented from overlapping each other, and the ink can be directly printed on the substrate as much as possible, so that a larger amount of ink can be used, and the color effect can also be increased.
3. High resolution In the AM network technology, the number of common dot lines indicates the fineness of the dot image. The most basic unit of the subtle level is determined by the number of network lines. According to the formula:
Gray level = (output resolution DPI / number of lines LPI) 2 +1
The number of network lines is far lower than the final resolution of the system. Obviously, the amplitude modulation network must sacrifice its proper resolution in order to obtain sufficient level of expressiveness. This weakness does not exist in the FM network technology. Because the FM network technology uses the density of the network, that is, the number of dots in the unit length (DPL) indicates the fineness of the dot image, and the blackening point of the FM dot is the image. The "element" of the hue is also the most basic unit of subtle level of expression. Therefore, it can be said that the FM network has a higher resolution than the AM screen under the same system resolution.
4. High definition In the amplitude modulation screening technology, the number of dot lines determines the sharpness. The higher the number of network lines, the smaller the output dot area and the better the resolution. However, usually the number of network cables is too large, and the copying technique is difficult. Therefore, the output images generated by the amplitude modulation screening technology must be emphasized at a subtle level, that is, USM. Due to the extremely small dot size used in FM screening, the image reproduction is high-definition, and the copying effect is close to the continuous-adjusted photographic draft, especially the subtle levels of the bright and dark parts are well reproduced. High-quality images can also be obtained by using FM network technology on low-resolution imagesetters, while AM-band technology is more demanding than the resolution of the banks. FM screening technology is a brand-new screening technology, which has many advantages. It can be used for both fine prints and low-precision newspaper printing. It has attracted wide attention from the printing industry and has great development prospects. (Greater China Graphic Arts Network)
At present, FM technology has some technical problems in the process, equipment and materials.
1. Network expansion problem. Dot enlargement refers to the phenomenon of halftone dot spread when printing with ink. Images using FM dots will have a particularly large dot spread, which is 25% to 35% when printed on coated paper and as high as 50% when printed on uncoated paper, seriously affecting image reproduction.
2. Quality control is difficult. Due to the problem of dot enlargement, the printed image will be dark, and the middle tone will be shifted to the darkness. In the printing process, the performance of the middle and dark adjustment parts is difficult to grasp.
3. There are high requirements for materials and machines for post-printing, printing, etc. At present, domestically produced materials and processes are difficult to meet the requirements of FM network for printing suitability. At present, high-end color printing commonly uses 175LPI or 200LPI amplitude modulation outlets, and the dot size range is generally 2% to 98%. In a sense, this is now There are limits on materials (print plates, blankets, inks, paper, etc.) that can be achieved in conventional processes. The FM screen image is composed of 2% or smaller dots corresponding to 200LPI amplitude modulation dots, so it is difficult to realize FM dot reproduction by using existing materials and processes.
While there are some unresolved issues with FM screening technology, there are many benefits to using this technique in many image processing applications. In the following projects you can decide if you can use FM screening technology:
1. Direct plate making or direct printing. FM screening requires special attention to environmental quality to ensure the control of the expansion of outlets. The use of a direct platesetter or direct printing system can reduce the number of steps in the process that affect quality, thus helping to maintain the dot size.
2.High color. FM screening technology is excellent for performing fine tone transitions (especially for highlights and shadows). In order to preserve the tonal details of high-level color images, it is best to use FM dot technology from input to output.
3. Images with complex details. The use of AM dots in printing often does not show very fine detail images. Use FM dots to get a minimum dot size that helps preserve the most complex details.
4. HiFi high fidelity color and spot color printing. HiFi color improves print gamut by using plates of more than four colors. As we have already described, printing with more than four colors and printing with AM dots can increase the risk of overprinting and moiré. FM screening eliminates moiré and is best suited for HiFi color and CMYK based additions to metallic, fluorescent or glazed plates.