From a technical point of view, Moire fringe is the visual result of interference between two lines or two objects at a constant angle and frequency. When the human eye can't distinguish between these two lines or two objects, it can only see the interference pattern. This optical phenomenon is called moire fringe. Moire fringe phenomenon is caused by signal sampling frequency close to the resolution of photoreceptor. The usual solution is to use a low-pass filter to block signals higher than the resolution of the photoreceptor, and its side effect is to reduce the imaging resolution. Therefore, designers should compromise between resolution and moire fringe when designing low-pass filters. Because D70 uses a weak low-pass filter in front of CCD, it improves the imaging resolution and increases the probability of moire fringe, which is also common in other DSLR.
Recommended solution: avoid shooting regular patterns close to the camera resolution in the scene, or remove them later with software.
Screen printing and moire fringe
Source: Guangdong Printing, No.2, 2003
Author: Bai Guangdong Henghui Industrial Company
I. Understanding Moire Fringe
Moire fringe is an optical phenomenon first discovered by French researcher Mr. Moire in the18th century. From a technical point of view, Moire fringe is the visual result of interference between two lines or two objects at a constant angle and frequency. When the human eye can't distinguish between these two lines or two objects, it can only see the interference pattern. This optical phenomenon is called moire fringe.
Moire fringe is a potential problem in halftone screen printing. The so-called halftone printing is a method of decomposing a continuous-tone manuscript into dots with different sizes by photography or other methods to show the hierarchy. Dark dots are represented by printing larger dots, and bright dots are represented by printing smaller dots. Moire fringes will appear between dots of the same color, especially between dots of multi-color printing or four-color printing.
Moire fringes formed between dots are the same problem in screen printing at all levels. Dots and screens can also form another form of moire fringes, and the distribution of moire fringes on the screen can produce unrecognizable patterns which are obviously different from the original.
Moire fringes can be produced from three aspects:
1. Interference between two-color or multi-color points;
2. Interference between dots of different colors and network cables;
3. As an additional factor, the interference caused by the characteristics of the printed object itself. The purpose of using moire fringe protection system is to predict moire fringe according to the number of meshes, screening lines, printing colors and screening angles you choose.
Second, avoid moire fringes.
It is possible to predict and analyze moire fringe by mathematical calculation, and the calculation result is only theoretical moire fringe. Moire fringes that actually affect screen printing are visible moire fringes that are harmful to printing results. Moire fringe protection system provides a simple visual control tool for screen printing machine. Using this tool, moire fringes can be avoided at any step of the copying process.
Serilor？ Log contains two sets of tools for predicting moire fringes:
1. Screen mesh simulator is a set of film for simulating screen mesh. The number of analog meshes depends on the test films you order-advanced test films and basic test films. Two sets of specimens include four items: code names are Mg-S-B: 100t/cm,10t/cm, 120T /cm, 130T /cm, (255t/ cm. For coarse screen printing or special screen printing of fabrics, advanced samples are MG-S-C: 77t/cm, 8 1T /cm, 85T /cm and 90T/cm；; ; Mg-s-d: 48t/cm，54T /cm，6 1T /cm，68T /cm。
Specimens for special use and high voltage shielding need to be specially ordered.
2. Moire fringe recognizer/indexer. By overlapping the indexer with the film, the mesh number and color separation angle of the film can be detected.
Two sets of tools are used alternately, which can comprehensively analyze mesh number/screening line number/dot angle and moire function. You need to install a glass workbench with stable light in the darkroom, which can provide a high-intensity and stable light source. Moire fringes are difficult to find in weak light.
Two sets of protective tools for moire fringes can help you choose the screen correctly, and also help you choose the color separation parameters (screen number and angle) and printing parameters-both the whole process control from the manuscript to the printed matter requires important parameter control.
3. The following examples can help you understand the use of Moore protective tools.
① Select the correct screen for the color separator provided by customers. First, put the screen simulation piece on the glass workbench, and then put the color separator in the selected screen area in turn. The color separator must be completely aligned with the corresponding position of the simulated paper. When multi-color or four-color printing is used, each color separation should be checked on the grid simulator.
In some special cases, moire fringe will appear very obviously in one of the four color separations. Experience has proved that these problems can be solved by replacing this color separator with a different screen. The number of grids used can be selected by the Ganan standard specimen, which is the closest, but will not produce moire fringes. Note: This is your last solution to this problem. In some cases, it will not work, and it will change the amount of ink deposition and cause additional troubles: drying speed, screen clarity and color deviation.
② The mesh number cannot be changed, but the color separator can be changed. Due to special technical factors, you may not be able to change the mesh number (printing parameters and ink deposition amount have been set). At this time, you should modify the color separation parameters to avoid moire fringes.
Use a mesh simulation sheet that matches the selected mesh number, and cover the Moire fringe indexer on the mesh simulation sheet according to the screening angle of the color separator, so as to determine the number of screening lines used by your color separator. When you turn the indexer, different superposition will produce moire fringes. If moire fringe only appears in the highlight area (below 10% dot percentage), the printed image will hardly be affected. However, when moire fringe appears at all gray levels or at 50% dot, the screening angle must be changed. When visible moire fringes appear, it is necessary to switch to the number of screening lines or color separation, and moire fringes will not appear until all color separation plates coincide with the screen.
Please check the recommended value of the scanner before selecting the screening angle. These screening angles are different for different printing processes (offset printing and flexography) and are also related to the country. For example, the screening angle of offset printing in North America is almost the same as the European standard DIN 16547 (1), but they belong to different color plates. As shown in the figure (omitted):
European flexographic screening angle
North American offset printing screening angle
European standard DIN 16547 (1)
European standard DIN 16547 (2)
The figure also shows that it is very important to keep the three deepest primary colors 30 degrees away from each other. But the specific screening angle is not static. Many printers have improved the screening angle with color splitters and used them for screen printing. The biggest help that can be obtained by using moire fringe protection system is to judge the matching relationship between mesh number, screening line number and screening angle by visual inspection.
(3) The mesh number and color separation cannot be changed. In some cases, the screen printing machine cannot change the mesh number or change the color separation. At this time, there are two ways to check the moire fringe. One is to change the screen tension (adjust the mesh number), and the other is to change the screen tension angle. Moire protection system will guide you to change the tension angle of screen to prevent Moire fringe. Place the stepper on the grid simulation board and rotate the stepper until moire fringes are generated. The tilt angle on the indexer is the net stretching angle that can avoid moire fringes.
4. In other aspects, there are several reasons for moire fringes, which are related to the printing process, although the moire fringe protection system is not designed to detect such moire fringes. It can still help you identify these moire fringes.
① screen tension. If the screen tension is too low or unstable in the printing process, moire fringe will appear, and you will see moire fringe will appear in the fixed position of the pattern, resulting in dot enlargement and dot reduction. To engage in color printing, the tension of the screen must reach 20 Newton/cm.
② Selection of scraper. Improper selection of scraper angle or hardness can also cause moire. Excessive scraper pressure or poor sharpness of scraper edge contour will cause printing distortion. In order to avoid these problems, it is recommended to use the following three-layer hardness composite scraper for printing.