Visual Noise in Printed FM Dot Images

Since the first FM-screening product was launched, more than a dozen years have passed. Its development does not replace modulating and screening as the people concerned predict. The actual application, especially in the domestic traditional printing industry, is developing slowly. . There are many reasons for this, both in terms of process and equipment conditions, as well as reasons for people’s understanding due to insufficient research. Naturally, there are problems with FM screening technology, such as FM screening, which does not produce amplitude modulation. The moiré in the screening mode is more obvious than the visual noise (graininess). Where does the visual noise come from, how does it detect and assess the size of the noise, and how to reduce the noise so that the image of the FM network point can really approach the photograph? This is a theoretical and technical problem that needs urgent research and solution.

FM network distribution characteristics and advantages of FM screening

The traditional structure of AM nets is characterized by the following: the ink dots (exposure points) in the grid are distributed according to certain rules; the centers of the dots are constant and evenly distributed; the tone levels are represented by the dot size (area of ​​dot area). The dots (exposure points) of the FM network are random randomly distributed in the grid; the spacing of the dots randomly changes; the size of the dots remains the same, and the frequency (number) of the dots appearing in the unit area represents the tone level.

The reason why FM screening technology has received wide attention and is applied within a certain range is because of its many advantages. For example, it is possible to better reproduce the level of the bright and dark tone of the manuscript. The level reproduction is uniform and there is no moiré pattern. It is not limited by the angle of outlets and expands the scope of reproduction of gradation. However, the FM screening technology also brings some problems to the printing process and the quality of the printed products. For example, due to the small dots, the dots are easily lost during the printing; the requirements for plate material, printing conditions and printing conditions are more stringent; The value is high and the law of expansion is special. Furthermore, the visual noise generated by FM screening has become one of the key issues that limit the promotion and application of FM screening technology.

Visual noise of FM network image

"Noise" or noise, which was originally used to characterize the sound characteristics of a soundless radio signal. Similarly, it can also be used in the transmission of television signals to describe the drawing characteristics when a channel has no signal. In this case, the image displayed on the TV screen is similar to the particles of the emulsion, so the word noise can be used to describe the graininess of the image, and the noise level becomes a measure of granularity. Due to the smoothness of the paper and the like, the unevenness of the micro-density (fine reflection coefficient) of the printed image is caused, so that the printed image has a certain degree of "roughness", that is, there is graininess. The unevenness of graininess or density (brightness and darkness) is the visual noise of printed images.

The visual noise (graininess) of the printed image is not unique to the image of the FM network point, and the traditional amplitude modulated dot image will also be generated. For a long time, for the amplitude-modulated dot image, the center of attention is to prevent moiré and to reduce the noise. The reason is that the visual noise of the AM dot image is smaller. However, the noise of FM network image is relatively easy to occur, and it is obvious that it cannot be ignored. The visual noise of the FM network image is easily generated in high-profile and mid-tone areas. When the coverage of the dot area is 20%, the human eye has a peak in the sensitivity to visual noise, especially in the same level of flat screen area. Take two kinds of images with different noise levels as an example. The picture with less noise should be more detailed, and the picture with loud noise should be more rough. It is worth mentioning that the visual noise is not a moiré and is not formed by the optical interference between the separation images. The monochromatic print image can also generate noise.

Causes of noise analysis

In general, the visual noise of the FM dot print image is greater than the AM dot image. And the same as the FM screening, the image content, visual noise is also different. Although the image printing process in the image may be small, the noise of the printed image may also be increased. However, the main factor affecting the noise level is obviously the screening method.

The generation and size of the visual noise of the FM network image can be characterized by the average value of the dot density (the number of equipment pixels or the number of inked dots per unit area) in the image, the mean value of the dot diameter change, and the change rate of the dot pitch. When the dot diameter and pitch change rate are small, no visual noise will be generated regardless of the dot density. When the dot density is large enough, no noise will be generated even if the dot diameter and pitch change rate are large. At this time, the human eye has no way of distinguishing between tiny and detailed structures. When the value of the dot density is smaller than the critical state, and the average value of the dot diameter and pitch change rate is greater than its critical state, visual noise is generated. In 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 blue noise. With the further increase in the dot diameter and pitch change rate, white noise has emerged, which is also the most common case. Since human visual characteristics belong to low-pass filters, they are insensitive to blue noise and are not easily noticeable, while white noise can produce a dazzling feeling.

Originally, the size of the FM network is the same, how can there be a change in dot diameter? The main reason is the random distribution of the FM network. During the screening process, the random distribution of dot positions can easily cause the adhesion of dots, resulting in changes in dot size and shape. The circumference of the dot also changes, and the optical magnification of the dots also changes, resulting in FM dots. Some bonded to each other, some separated, some close to each other, some distant, so that the dot diameter and spacing changes, resulting in the noise of the FM network image.

Possible ways to reduce noise

The structural characteristics of the FM network are the irregular distribution of the outlets. From the above analysis, it can be seen that the random distribution of the outlets will bring about visual noise. Therefore, to reduce the noise, efforts must be made to reduce the bonding of outlets and reduce the deviation of outlet distribution distances. After years of research, China has achieved remarkable results in reducing the visual noise of FM network images.

Source: Print Magazine

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