Since the advent of the first FM screening product, more than a dozen spring and autumn have passed. Its development does not replace the amplitude modulation screening as dominated by the relevant people. The actual application, especially in the domestic traditional printing field, has developed slowly . There are many reasons for this, including the problems of process and equipment conditions, as well as the reasons for people's understanding due to insufficient research, and naturally the problems of FM screening technology, such as FM screening will not produce amplitude modulation The moiré in the screening method, but the visual noise (graininess) is more obvious. Where does the visual noise come from, how to detect and evaluate the noise size, and how to reduce the noise so that the FM dot image can really be close to the photo. This is a theoretical and technical problem that needs to be researched and solved.
Distribution characteristics of FM network points and advantages of FM screening
The structural characteristics of traditional amplitude modulation dots are: the inked dots (exposure dots) in the grid are distributed according to a certain rule; the center distance of the dots is constant and uniformly distributed; the tone level is expressed by the dot size (dot area ratio). The dots (exposure points) of FM dots are randomly distributed randomly in the grid; the pitch of dots changes randomly; the size of dots is unchanged, and the tone level is expressed by the frequency (number) of dots in a unit area. The reason why the FM screening technology is widely concerned by everyone and is applied in a certain range is because of its many advantages. For example, it can better reproduce the layers of the original light and dark tones, and the layer reproduction is uniform without generating moire. It is not limited by the angle of the dot, and expands the range of tone reproduction. However, the FM screening technology also brings some problems to the printing process and print quality. For example, due to the small dots, the dots are easy to be lost during printing; the requirements on the plate material, printing conditions and printing conditions are more demanding; the dots are expanded The value is high and the expansion law is special. Furthermore, the visual noise generated by FM screening has also become one of the key issues limiting the promotion and application of FM screening.
Visual noise of FM dot image
The term "noise" or noise was originally used to characterize the sound characteristics when there were no sound signals during radio broadcasting. Similarly, it can also be used to describe the drawing characteristics of a channel when there is no signal in the transmission of TV signals. In this case, the image displayed on the TV screen is similar to the particles of the photosensitive emulsion, so the term noise can be used to describe the granularity of the image, and the noise level has become a measure of granularity. Due to the smoothness of the paper and other reasons, the micro-density (micro-reflection coefficient) of the printed image is not uniform, which gives the printed image a certain degree of "roughness", that is, there is graininess. The unevenness of graininess or density (light and dark) is the visual noise of the printed image.
The visual noise (graininess) of the printed image is not unique to the FM dot image, and the traditional AM dot image will also be generated. For a long time, for the dot image, the center of people's attention is to prevent moire, and the noise reduction is less, because the visual noise of the amplitude dot image is smaller. The noise of the FM dot image is more likely to occur, and it is more obvious and cannot be ignored. The visual noise of the FM dot image is easy to be generated in the high-tone and middle-tone parts. When the coverage of the dot area is 20%, the sensitivity of the human eye to visual noise peaks, especially in the flat mesh area at the same level. Comparing two FM dot images with different noise levels, the picture with less noise is more delicate, and the picture with more noise is rougher. It is worth mentioning that visual noise is not moire, and is not caused by optical interference between color-separated images. Monochromatic printed images can also generate noise.
Causes of noise
Generally speaking, the visual noise of FM dot print images is greater than that of AM dot images. And the same as the FM screening, the same image content, the visual noise is also different. Although a small image-making process may increase the noise of the printed image, the main factor affecting the noise size is obviously the screening method. The generation and size of visual noise in the FM dot image can be characterized by the dot density (the number of device pixels or inked dots per unit area) in the image, the average change in dot diameter and the average change rate in dot spacing.
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When the change rate of dot diameter and pitch is small, no visual noise will be generated regardless of the dot density; when the dot density is large to a certain degree, no noise will be generated despite the large change rate of dot diameter and pitch, because this At this time, the human eye can no longer distinguish extremely small structures. When the value of the dot density is smaller than the critical state, and the average value of the dot diameter and the pitch change rate is greater than the critical state, visual noise is generated. Within this range, when the change rate of dot diameter and pitch is small, the dot density is small as pink noise, and the dot density is large as blue noise. With the further increase of the dot diameter and the pitch change rate, white noise appears, which is also the most common. Since the visual characteristics of the human eye are low-pass filters, they are not sensitive to blue noise and are not easily noticeable, while white noise produces a dazzling feeling.
Originally, the size of FM dots was the same, how could there be a change in dot diameter? The main reason is the random distribution of FM dots. During the screening process, the random distribution of dot positions can easily cause the bonding of dots, which can cause the size and shape of the dots to change, as well as the perimeter of the dots, and the optical expansion of the dots. Some are bonded to each other, some are separated, some are closer, and some are farther apart, causing the dot diameter and spacing to change, thereby causing noise in the FM dot image.
Possible ways to reduce noise
The structural characteristics of FM dots are the irregular distribution of dots. From the above analysis, it can be seen that the random distribution of dots will bring visual noise. Therefore, to reduce the noise, we must work hard to reduce the bonding of dots and the deviation of the distribution distance of dots. After years of research, China has achieved significant results in reducing the visual noise of FM dot images.
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