FM screening, also known as random screening (usually called "FM screening"), that is, all halftone micro dots have very small and the same size, each dot is as small as a single element of the recording device, and is a graphic recorder or The minimum number of dots that the printer can produce, but the average number (or frequency) of dots in the surface area of ​​the image varies with the value of the copied tone. In addition, their spatial distribution is carefully assigned by an algorithm, and the points are distributed according to the statistical estimates of hue and the details of the neighboring parts of the image, without significant accumulation or accumulation of unwanted micro-points. In fact, the dot position is based on "calculated randomness." In conventional AM screening (commonly referred to as "AM screening") method, the distances between the dots are the same and the dot sizes are different; the FM screening changes the distance between the centers of the dots and the dot size according to the brightness and blackness of the input pixels.
The concept of FM screening was proposed many years ago, but until 1993, lithographers were able to get their commercialized products. Since the first manufacturer announced its FM network products in the spring of 1993, many new manufacturers have also introduced themselves. The corresponding product. Although the merits of FM screening are commendable, the actual use of this technology has been slow. Many printing companies are very interested in FM screening, but few printers and plate makers make this process a fixed part of production. .
As a newer technology that appears later than the AM screening technology, FM screening technology has many advantages: 1 The very small dot image generated by FM screening looks smoother, enhances the boundary definition, and can display more. Details; 2 image file size is 25% of AM screening file. (Same image quality); 3 no screen moiré, clearer color definition; 4 low-grade scanning and lower output resolution; 5 faster printing without adjustment; 6 easier to maintain ink balance; 7 thicker ink coverage can get a higher dynamic range; 8 drying time is short; 9 in a thinner layer ink distribution more uniform; 10 sets of quasi-problem does not affect the color balance; (11) screening color version and the original color version of the overlay is not There will be moire, can achieve high-fidelity color (HiFi color) and spot color printing plate to improve the print color gamut; (12) eliminate the midtones of the tone jump.
Because of these advantages, FM screening technology is considered a better alternative to traditional AM screening. However, looking at the current situation in China, we will find that the application of frequency modulation and screening technology is still thundery and rainy. To make full use of the potential advantages of FM network technology, it still takes a long and difficult exploration, due to the frequency-modulated screening algorithm itself. The reason, as well as other factors, make it difficult to popularize FM screening technology:
1. Network expansion. Dot expansion occurs around the perimeter of the outlets, but a large outlet is divided into smaller outlets. The perimeter and area ratio increase. The larger the perimeter, the more the outlet expansion value increases. For any given outlet density, the perimeter in the FM network is It is larger than the perimeter in traditional screens. The extra dot gain is an inherent characteristic of small dots. The perimeter of dots in the middle tone is larger than the one-quarter tone, and thus the dots are also expanded. In the 200l screen, the perimeter of 40% of the outlets increased by 1μm, and the outlet size increased by 1%. The dot enlargement range is in the range of 18%-25% in the halftone image according to the SWOP (web offset press standard). The use of FM screen images will have particularly large dot gains, ranging from 25% to 35% on coated paper and up to 50% on non-coated paper. The challenge is that FM screening is difficult to overcome, although some printing machinery manufacturers provide a software-based conversion curve so that their equipment can compensate for the expected dot gain, it is to change the rendering curve of the imagesetter itself, and This is only one of the links that leads to the expansion of outlets. The basis of the compensation curve is only "expected", and it cannot always be compensated according to the specific features of each manuscript and the specific printing press, resulting in errors at the same time.
2. Has a grainy feel. The appearance of “granules†or sandpaper is the result of random distribution of FM mesh points, especially in the quarter-tone region of the screen. The amplitude modulation network is regular, so the noise generated by it is easily filtered out, and the FM network is Random, random noise is non-filterable. The ideas of the quarter-tone region are easy to notice and give a grainy appearance, but not to the extent that they merge with each other. This is more likely to occur in image regions with a dot size greater than 20 μm and a very smooth image. Combining AM and FM in an image may be the best way to reduce graininess in the future. The smooth tone region of the FM network image is preferably an amplitude modulation network to maintain a smooth appearance of the screen. The tone transition area preferably uses a frequency modulation network to reproduce as much details as possible, particularly in the highlight portion and the tone of about 25%. Since there is no special standard limit for manuscripts to be adapted to these two screening technologies, this method of integration is not professional, and according to some professionals, the output errors caused by this combination are more difficult to troubleshoot.
3. The tolerance of the entire production process becomes smaller. Due to the expansion of outlets and smaller outlets, FM outlets cannot tolerate dust in the working environment when outputting, and the latitude is usually smaller. The problems caused by dust and dirt in the AM screening process are not significant. In the case of FM screening, they will Caused a big problem. The entire work area must be kept clean to avoid fine particles during reproduction. When the entire system is using small-sized dots, small dust particles will cause defects that cannot be ignored. The micro-dots in the FM network are equal to or less than the dust in the AM network. Traditionally, over-exposure plates can eliminate them. This is very noticeable in FM screening and cannot be eliminated by over-exposure because this This will cause small outlets to be exposed together. When printing, a consistent and uniform vacuum pressure is required to ensure the consistency between the printing plate and the entire printing plate. A slight change in vacuum pressure will cause “hot spotsâ€, which is more than the amplitude modulation network. The same "hot spot" got more attention. The environment of the printing shop has also become critical during printing on the upper version. Paper fluffs, particles in the air, and flying fibrous materials from the paper may cause the printing plate or the rubber roller to be dirty and make the prints clear. Defects. Because the effect of increasing the ink film thickness on the size of FM dots is not as significant as that of the amplitude modulation network, it is difficult to improve the quality of the images with poor color separation on the printing press. In the printing process, the ink is adjusted to compensate for the scan. Defects are simply not possible, because the dots are too small to print properly, and it is easy to paste. If you want to improve the lack of density caused by the scan by adding ink, the adjustment during printing in the AM screening is very easy to implement. The amplitude of the AM network is large, and the spacing between the points is also large. There is room for adjustment, and the FM network cannot. Having said that, I couldn't help but mention a little about the working environment. I went to a lot of prepress, printing, and post-press processing places. The pre-press work environment was OK, but printing and post-press processing were hard to make Complimentary, there are not a few printing and finishing operations. In a large workshop, nearly ten machines such as printing presses, paper cutters, die-cutting machines, laminators, binders, etc., come together with dust, smoke, The strange smell of chemical gases, static electricity, and noise can't be imagined here. The problems of stoppages and dirty inks, and the resulting uneven inks, etc., can cause printers to suffer a lot, as do other operators. Although this is a digression, at least such “bad†environments print high quality prints with AM screens, not to mention the use of environmentally demanding FM screens. . This is like having a high-speed car without a freeway, and the car's speed is still not rising. The printing industry is facing the challenge of environmental issues while adopting new technologies and new equipment. This is also a serious topic of global concern.
4. Film properties. High-contrast film is critical for suppressing dot gain because there may be partially exposed areas around the tiny FM network. All films have a part of the exposed transition area around the fully exposed dot, which will cause the dot size to become larger. The FM network itself is small, and its slight expansion will cause significant results. Using high-contrast film can reduce the area of ​​partial exposure, thereby reducing the potential for dot gain in this stage of the process. Since the inherent latitude in the FM network replication process is small, the use of the widest exposure range of the film gives the best dot reproduction. However, at present, whether DuPont, Kodak and other films in the use of FM screening is not satisfactory, but fortunately these manufacturers are gradually improving their product performance.
5. Proofing. Current proofing technology is the use of conventional proofing technology of analog materials. It is used to accurately reflect the changes in the structure and size of the modulation sites on the printing press. The behavior of AM outlets is fundamentally different from the behavior of FM outlets. The current proofing technology cannot reflect the frequency of FM outlets on the printing press. behavior. It is difficult to achieve accurate FM network proofing, and users cannot adjust the traditional proofing system to reflect the behavior of FM outlets. Accurate proofing of FM screen images is still an unresolved problem because existing proofing systems cannot reproduce extremely small dot sizes or reflect excessive dot gains, and only developed a matching proofing method. Screen technology can be recognized. Some analog systems can more accurately predict the behavior of the FM network than other systems, but the granular toner used in these analog systems is too large to reproduce the slender printing dots used in the FM screening and cannot be accepted as a contract proof. The future of FM network proofing lies in electronic proofing, because electronic non-network proofing is similar to the way in which screens in FM screens are placed on substrates. This may produce good results, but electronic proofing technology is still not perfect.
6. The dot size is too small. Photographic screen processing methods provided by imagesetter manufacturers generally allow only one or two fixed dot sizes for a given output resolution. These dots are used for non-coated paper and newsprint or for screen printing. When it seems too small, it is difficult to control the expansion of outlets. Small dots are also one of the causes of noise interference in low-contrast images, but the larger dots increase the graininess of the image, and the smallest micro-dots are too small to be reproduced properly by many printers. Even if you get small dots on the plate, it is difficult to accurately transfer the ink to the paper, because the fine dot size is only available on the new, expensive printing press by an experienced operator. This kind of replication is feasible.
7. Printers and customers' opinions. For printers, using FM screening technology means purchasing RIP with the ability to compute and randomly place dots, requiring an additional investment, followed by some other harsh working conditions, environment, and staffing. The increase in quality requirements increases investment, printers also consider whether these extra investments can bring them greater profits when investing, and they will not be easy when there are no fixed customers who require FM screening. Investment. Even if the printers make the money, those investment costs usually exceed the extra cost of the customer's advantage of random grid output. Because most of the high-quality customers require high-quality output of 200 LPI or more through AM screening technology, they will not be required to pay extra funds for methods that do not bring them much benefit. Even if the customer is willing to pay extra funds, it is not the final structure of the image output with FM screening that is better than the result of the AM screening. For example, when printing a large-area flat field color, the result of the FM screen printing is The impression of stains is not present when printing with the AM screen output.
The above is an analysis of the potential shortcomings of FM screening technology and why it cannot be popularized. There are actually many other factors, such as the country’s economic status and the overall national quality. Although there are so many defects and factors in the use of FM screening technology, Affecting its popularity, but as a new technology, it has the superiority unmatched by traditional screening methods. However, there are many external conditions that affect its use. There are still many areas for improvement, such as Many people are still studying better screening algorithms. These algorithms are based on the advantages of combining AM and FM. Some algorithms have been applied locally, such as those used in screen printing. Wait.
We have reason to believe that the currently difficult to popularize FM screening technology will advance with the development of science and technology, especially India.
The concept of FM screening was proposed many years ago, but until 1993, lithographers were able to get their commercialized products. Since the first manufacturer announced its FM network products in the spring of 1993, many new manufacturers have also introduced themselves. The corresponding product. Although the merits of FM screening are commendable, the actual use of this technology has been slow. Many printing companies are very interested in FM screening, but few printers and plate makers make this process a fixed part of production. .
As a newer technology that appears later than the AM screening technology, FM screening technology has many advantages: 1 The very small dot image generated by FM screening looks smoother, enhances the boundary definition, and can display more. Details; 2 image file size is 25% of AM screening file. (Same image quality); 3 no screen moiré, clearer color definition; 4 low-grade scanning and lower output resolution; 5 faster printing without adjustment; 6 easier to maintain ink balance; 7 thicker ink coverage can get a higher dynamic range; 8 drying time is short; 9 in a thinner layer ink distribution more uniform; 10 sets of quasi-problem does not affect the color balance; (11) screening color version and the original color version of the overlay is not There will be moire, can achieve high-fidelity color (HiFi color) and spot color printing plate to improve the print color gamut; (12) eliminate the midtones of the tone jump.
Because of these advantages, FM screening technology is considered a better alternative to traditional AM screening. However, looking at the current situation in China, we will find that the application of frequency modulation and screening technology is still thundery and rainy. To make full use of the potential advantages of FM network technology, it still takes a long and difficult exploration, due to the frequency-modulated screening algorithm itself. The reason, as well as other factors, make it difficult to popularize FM screening technology:
1. Network expansion. Dot expansion occurs around the perimeter of the outlets, but a large outlet is divided into smaller outlets. The perimeter and area ratio increase. The larger the perimeter, the more the outlet expansion value increases. For any given outlet density, the perimeter in the FM network is It is larger than the perimeter in traditional screens. The extra dot gain is an inherent characteristic of small dots. The perimeter of dots in the middle tone is larger than the one-quarter tone, and thus the dots are also expanded. In the 200l screen, the perimeter of 40% of the outlets increased by 1μm, and the outlet size increased by 1%. The dot enlargement range is in the range of 18%-25% in the halftone image according to the SWOP (web offset press standard). The use of FM screen images will have particularly large dot gains, ranging from 25% to 35% on coated paper and up to 50% on non-coated paper. The challenge is that FM screening is difficult to overcome, although some printing machinery manufacturers provide a software-based conversion curve so that their equipment can compensate for the expected dot gain, it is to change the rendering curve of the imagesetter itself, and This is only one of the links that leads to the expansion of outlets. The basis of the compensation curve is only "expected", and it cannot always be compensated according to the specific features of each manuscript and the specific printing press, resulting in errors at the same time.
2. Has a grainy feel. The appearance of “granules†or sandpaper is the result of random distribution of FM mesh points, especially in the quarter-tone region of the screen. The amplitude modulation network is regular, so the noise generated by it is easily filtered out, and the FM network is Random, random noise is non-filterable. The ideas of the quarter-tone region are easy to notice and give a grainy appearance, but not to the extent that they merge with each other. This is more likely to occur in image regions with a dot size greater than 20 μm and a very smooth image. Combining AM and FM in an image may be the best way to reduce graininess in the future. The smooth tone region of the FM network image is preferably an amplitude modulation network to maintain a smooth appearance of the screen. The tone transition area preferably uses a frequency modulation network to reproduce as much details as possible, particularly in the highlight portion and the tone of about 25%. Since there is no special standard limit for manuscripts to be adapted to these two screening technologies, this method of integration is not professional, and according to some professionals, the output errors caused by this combination are more difficult to troubleshoot.
3. The tolerance of the entire production process becomes smaller. Due to the expansion of outlets and smaller outlets, FM outlets cannot tolerate dust in the working environment when outputting, and the latitude is usually smaller. The problems caused by dust and dirt in the AM screening process are not significant. In the case of FM screening, they will Caused a big problem. The entire work area must be kept clean to avoid fine particles during reproduction. When the entire system is using small-sized dots, small dust particles will cause defects that cannot be ignored. The micro-dots in the FM network are equal to or less than the dust in the AM network. Traditionally, over-exposure plates can eliminate them. This is very noticeable in FM screening and cannot be eliminated by over-exposure because this This will cause small outlets to be exposed together. When printing, a consistent and uniform vacuum pressure is required to ensure the consistency between the printing plate and the entire printing plate. A slight change in vacuum pressure will cause “hot spotsâ€, which is more than the amplitude modulation network. The same "hot spot" got more attention. The environment of the printing shop has also become critical during printing on the upper version. Paper fluffs, particles in the air, and flying fibrous materials from the paper may cause the printing plate or the rubber roller to be dirty and make the prints clear. Defects. Because the effect of increasing the ink film thickness on the size of FM dots is not as significant as that of the amplitude modulation network, it is difficult to improve the quality of the images with poor color separation on the printing press. In the printing process, the ink is adjusted to compensate for the scan. Defects are simply not possible, because the dots are too small to print properly, and it is easy to paste. If you want to improve the lack of density caused by the scan by adding ink, the adjustment during printing in the AM screening is very easy to implement. The amplitude of the AM network is large, and the spacing between the points is also large. There is room for adjustment, and the FM network cannot. Having said that, I couldn't help but mention a little about the working environment. I went to a lot of prepress, printing, and post-press processing places. The pre-press work environment was OK, but printing and post-press processing were hard to make Complimentary, there are not a few printing and finishing operations. In a large workshop, nearly ten machines such as printing presses, paper cutters, die-cutting machines, laminators, binders, etc., come together with dust, smoke, The strange smell of chemical gases, static electricity, and noise can't be imagined here. The problems of stoppages and dirty inks, and the resulting uneven inks, etc., can cause printers to suffer a lot, as do other operators. Although this is a digression, at least such “bad†environments print high quality prints with AM screens, not to mention the use of environmentally demanding FM screens. . This is like having a high-speed car without a freeway, and the car's speed is still not rising. The printing industry is facing the challenge of environmental issues while adopting new technologies and new equipment. This is also a serious topic of global concern.
4. Film properties. High-contrast film is critical for suppressing dot gain because there may be partially exposed areas around the tiny FM network. All films have a part of the exposed transition area around the fully exposed dot, which will cause the dot size to become larger. The FM network itself is small, and its slight expansion will cause significant results. Using high-contrast film can reduce the area of ​​partial exposure, thereby reducing the potential for dot gain in this stage of the process. Since the inherent latitude in the FM network replication process is small, the use of the widest exposure range of the film gives the best dot reproduction. However, at present, whether DuPont, Kodak and other films in the use of FM screening is not satisfactory, but fortunately these manufacturers are gradually improving their product performance.
5. Proofing. Current proofing technology is the use of conventional proofing technology of analog materials. It is used to accurately reflect the changes in the structure and size of the modulation sites on the printing press. The behavior of AM outlets is fundamentally different from the behavior of FM outlets. The current proofing technology cannot reflect the frequency of FM outlets on the printing press. behavior. It is difficult to achieve accurate FM network proofing, and users cannot adjust the traditional proofing system to reflect the behavior of FM outlets. Accurate proofing of FM screen images is still an unresolved problem because existing proofing systems cannot reproduce extremely small dot sizes or reflect excessive dot gains, and only developed a matching proofing method. Screen technology can be recognized. Some analog systems can more accurately predict the behavior of the FM network than other systems, but the granular toner used in these analog systems is too large to reproduce the slender printing dots used in the FM screening and cannot be accepted as a contract proof. The future of FM network proofing lies in electronic proofing, because electronic non-network proofing is similar to the way in which screens in FM screens are placed on substrates. This may produce good results, but electronic proofing technology is still not perfect.
6. The dot size is too small. Photographic screen processing methods provided by imagesetter manufacturers generally allow only one or two fixed dot sizes for a given output resolution. These dots are used for non-coated paper and newsprint or for screen printing. When it seems too small, it is difficult to control the expansion of outlets. Small dots are also one of the causes of noise interference in low-contrast images, but the larger dots increase the graininess of the image, and the smallest micro-dots are too small to be reproduced properly by many printers. Even if you get small dots on the plate, it is difficult to accurately transfer the ink to the paper, because the fine dot size is only available on the new, expensive printing press by an experienced operator. This kind of replication is feasible.
7. Printers and customers' opinions. For printers, using FM screening technology means purchasing RIP with the ability to compute and randomly place dots, requiring an additional investment, followed by some other harsh working conditions, environment, and staffing. The increase in quality requirements increases investment, printers also consider whether these extra investments can bring them greater profits when investing, and they will not be easy when there are no fixed customers who require FM screening. Investment. Even if the printers make the money, those investment costs usually exceed the extra cost of the customer's advantage of random grid output. Because most of the high-quality customers require high-quality output of 200 LPI or more through AM screening technology, they will not be required to pay extra funds for methods that do not bring them much benefit. Even if the customer is willing to pay extra funds, it is not the final structure of the image output with FM screening that is better than the result of the AM screening. For example, when printing a large-area flat field color, the result of the FM screen printing is The impression of stains is not present when printing with the AM screen output.
The above is an analysis of the potential shortcomings of FM screening technology and why it cannot be popularized. There are actually many other factors, such as the country’s economic status and the overall national quality. Although there are so many defects and factors in the use of FM screening technology, Affecting its popularity, but as a new technology, it has the superiority unmatched by traditional screening methods. However, there are many external conditions that affect its use. There are still many areas for improvement, such as Many people are still studying better screening algorithms. These algorithms are based on the advantages of combining AM and FM. Some algorithms have been applied locally, such as those used in screen printing. Wait.
We have reason to believe that the currently difficult to popularize FM screening technology will advance with the development of science and technology, especially India.
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