FM Screening Technology and Its Application

The emergence of Frequency Modulated Halftone Dots (FM stands) and its advantages in the resolution, non-profiling, and graphic and textual integration have received widespread attention from the printing industry. With the improvement of materials and supporting technologies, it will become a major screening method that is superior to traditional amplitude modulated dots (AM). The conventional screening technology uses the method of changing the dot size to reproduce the color and gradation changes of the continuous tone manuscript. Compared with the meaning in the radio technology, the size of the changing dot corresponds to the amplitude modulation in the radio technology. For this reason, the FM screening technology is used. After the advent of people, the sites created by conventional screening technologies are called AM sites.

The difference between the FM network technology and the AM network technology lies in that it does not change the size of the network, but the computer generates the same size of the pixels in the digital image. The minimum diameter of these points can be equal to the recording accuracy of the equipment. . A set of finite point groups in a halftoning unit constitutes a very special form of dots, but it does not have a fixed shape because the positions of these dots appear to be random and are sometimes referred to as "random dots", for example two Pixels that are exactly equal in value use FM screening to form dots. Although the number of recorded dots is the same, they appear in different positions. The sum of the areas that make up an FM recording point is equal to the size of a regular dot, and its area ratio is also equal to that of a regular dot. AM sites are the effect of the tonality of the image by the size of the site. The frequencies of the sites in the space and the number of Internet sites per unit area are constant, and the tonality of the image is expressed by changing the area of ​​the site. That is, the site of the bright tone is small and the tone is dark. The network is large; the area of ​​the FM network is fixed, and the tone of the image is expressed by changing the distribution density (density) of the network in the space.

There are many advantages of FM network sites compared with AM sites, which are mainly manifested in:

1. Eliminate the occurrence of "rhizome" The "rhizome" is the result of periodic structural interactions. Color printing is usually accomplished by accurately overprinting four separate screen images. These images are printed using four inks of cyan, magenta, yellow, and black respectively. The screens are screened using the amplitude modulation and screening technique. The singulation will inevitably produce screen angles, and inevitably produce "rhizome" in the process of overprinting. With the frequency modulation screening technology, the probability of network appearance is random, the arrangement of network points is irregular, and there is no regular spatial network structure. Therefore, after a few color plates are superimposed, there will be no “rhized pattern”. Interference fringes, it can also eliminate the possibility of "moiré" caused by some regular stripes of the image itself (such as the wood bars of the fence or shark's teeth) interfere with the screen.

2. Improves color reproduction capability Because the distribution of FM network points is irregular, there is no network line angle problem, and the problem of "rhizome" is solved. Therefore, colors other than cyan, magenta, yellow and black can be used to expand the color gamut range of colors. That is, more than four colors are used to increase spot color ink printing and achieve high fidelity (HiFi) color printing, reducing the color gap between prints and originals, and improving color reproduction capabilities. This is extremely helpful for the printing business market to increase profits. In addition, due to the irregular distribution of FM network dots, the overlap of dots can be avoided, and the ink can be directly printed on the substrate as much as possible. Therefore, a larger amount of ink can be used and the effect of increasing the color effect can also be achieved.

3. Resolution High In the AM network technology, the commonly used dot lines represent the fineness of the dot image, and the most basic unit of the subtle level is determined by the number of screen lines, according to the formula:

Gray scale = (Output resolution DPI/LPI) 2+1

The number of network lines is much lower than the final resolution of the system. Obviously, AM sites must sacrifice their resolution to obtain sufficient levels of expressive force. However, this weakness does not exist in the FM network technology, because the FM network technology uses the denseness of the network points, that is, the number of network points per unit length (DPL) to represent the fineness of the network image, and the blackening point that constitutes the FM network point is to form an image. The “elements” of the hue are at the same time the most basic elements of the subtle level of expression. Therefore, it can be said that, under the condition of the same system resolution, the FM network points have higher resolution than the AM network points.

4. High-definition In the AM screening technology, the dot line number determines the sharpness. The higher the cable number, the smaller the dot area of ​​the output and the better the sharpness. However, usually the number of network lines is too large and the copying technology is more difficult. Therefore, the output images generated by the amplitude modulation screening technology have to be emphasized at the subtle level, namely USM. Because the screen area used by FM screening is particularly small and the image reproduction is of high definition, the copying effect is close to the continuous adjustment of the photographic film, especially the subtle levels of highlights and shadows are well reproduced. High-quality images can also be obtained by adopting the frequency modulation dot spotting technique on the low-resolution imagesetter, while the amplitude modulation dot-matrix technology requires a higher resolution of the contrasting rowing machine. FM screening technology is a brand new screening technology with many advantages. It can be used not only in fine prints but also in low-precision newspaper printing. It is widely concerned by the printing industry and has great development prospects.

At present, FM screening technology has some technical problems in the aspects of technology, equipment, and materials.

1. The problem of network expansion. Dot enlargement refers to the phenomenon of halftone dot diffusion when printing with ink. The image using the FM dot network will have a particularly large dot gain, which is 25% to 35% when printing on coated paper, and up to 50% when printing on non-coated paper, which seriously affects the reproduction of the image.

2. Quality control is difficult. Due to the existence of dot expansion problems, the printed image will be dark, and the middle tone will be shifted to the dark tone. In the printing process, the performance of the mid tone and dark tone is not easy to grasp.

3. There are high requirements for the material and machine of the posterior sequence such as printing and printing. At present, domestically produced materials and processes are difficult to meet the requirements of FM radio outlets for printing suitability. At present, high-end color printing is commonly used for 175LPI or 200LPI AM outlets. The outlet range is generally between 2% and 98%. In a sense, this is now There are limit values ​​that materials (print plates, blankets, ink, paper, etc.) can reach in traditional processes. The image of the FM network is formed by aggregating 2% or less of the 200LPI AMs. Therefore, it is difficult to use the existing materials and processes to reproduce the FM.

Although FM screening technology still has some unresolved issues, there are many benefits to using this technology in many image processing applications. In the following items you can decide whether you can use FM screening technology:

1. Direct plate or direct printing. FM screening requires extra attention to the quality of the environment in order to ensure the control of network expansion. The use of a direct plate maker or a direct printing system can reduce the steps that affect the quality of the process and thus help keep the dot size constant.

2. High color. FM screening technology is outstanding in the performance of fine tone transitions (especially highlights and shadows). In order to preserve the tone details of high-level color images, it is best to use FM dot technology from the input to the output.

3. Images with complex details. The use of amplitude-modulated dots in printing often does not perform well with very fine detail images. Using FM dots allows for extremely small dot sizes to help keep the most complex details.

4.HiFi high-fidelity color and spot color printing. HiFi Color improves print gamut by using plates of four or more colors. As we have mentioned, printing with more than four colors and printing with AM dots will increase the risk of overprinting and moiré. FM screening eliminates moire, and is most suitable for HiFi color and CMYK based on adding metallic, fluorescent or gloss printing plates

Source: PACK.CN