The formation of the initial outlets depends on the screen (also known as the screen version, network version). In use, the screen is mounted on the front surface or the upper surface of the photosensitive sheet to divide the incident light upon photographing, and dots having different sizes corresponding to the shades of the original document are formed on the photosensitive sheet. However, the new electronic color separation machine or color desktop publishing system has now been screened without screens.
With a magnifying glass to observe the direct screen color separation of the negatives, copies of the positive and dried plates can be seen, the layout of the image is composed of large and small dots.
1. The continuous improvement of the screen 1 The advent of glass screens.
The glass screen was invented by the Americans in 1886. This type of screen has been used for a long time in photolithography. Before the 1970s, large and medium-sized printing factories in China basically screened with glass screens.
In the production of glass screens, the use of high-quality optical glass, the surface coated with acid-resistant paint film, in the special engraving line drawing machine drawn into a long parallel line, the density of the line is determined according to the screen number of lines . After the engraving, the back of the glass is coated with an acid-resistant protective film and etched with hydrofluoric acid. The eroded recessed part was painted with black oil, and then the acid-resistant film was washed off. This resulted in a transparent and opaque glass lined version. Two such lines are vertically glued together, and a metal frame is inlaid on the edge to make a glass screen.
The number of screen lines refers to the number of transparent lines per inch of width, for example, a commonly used 150 line screen is divided into 300 equal parts within 1 inch, and 150 lines for transparent and opaque lines. Glass screens generally have 60, 80, 100, 133, 150, and 175 lines.
In addition to the disadvantages of bulky, expensive and expensive glazing, glass screens must have a certain distance (net distance) between the screen and the photosensitive film, which can easily cause loss of image hierarchy and affect the sharpness of the screen. Therefore, they are gradually replaced by contact screens.
Touch screen. In the 1960s, China began to introduce contact screens. The contact screen is generally made of a glass screen as a master, and a rigid camera film made of a hard, sexy light film. The contact screen is in the form of a film. It is carefully observed with a magnifying glass and covered with dots. Each circle dot has a very dark core with diminishing edge density. Looking at the arrangement of dots as a whole, there is a transparent hole in the middle of every four black dots, or a black dot in the middle of every four transparent holes. The contact screen has the advantages of low cost, light weight, close contact with the photographic film when in use, reduced level loss in remake, improved clarity of the screen, etc., and is therefore quickly popularized in China.
2. The selection of mesh thickness for the selection of mesh thickness must take into consideration various factors, among which the main factors are the viewing conditions of printed materials, the quality of paper, the suitability of printing, and the oldness of printing equipment.
1 The viewing conditions of the printed matter. Mesh thickness, viewing distance (line of sight) and visual effects are closely related to each other. In general, if the visual range is near, the meshes should be finer; if the visual range is far, the meshes should be thicker. For example, stamps are generally carefully regarded on the palm of your hand. The line of sight is very close. The mesh should be finer. You can use 175 lines or 200 lines. If you print posters, posters, or large-format oil paintings, because they have a long-distance view, they usually view them within a few meters or ten meters away. The use of a thin network cable is not as good as a thick screen. For example, when comparing a large-scale oil painting with gouache (both with the same dimensions and painting content), far-reaching visual effects are much better than those of gouache. This is because the strokes of the oil painting are rough (equivalent to a thick screen). When viewed from a distance, the screen can open the contrast, while the gouache is delicate (equivalent to a thin network cable). It looks very beautiful in the near future, and is plain in the distance.
2 printing paper quality. Paper has a direct impact on the quality of the print. In the case of rough paper, it is better to use a thin cable instead of a thick cable. For example, ordinary 60g (refers to the weight of paper per square meter) of domestic offset paper printing text and black and white images, because of the soft texture of this paper, printing is easy to lint, powder off, easy paste version, such as with thin cable, The image is easy to get stuck; if you take care of the printing effect of the image, you have to reduce the amount of ink. At this time, the printed text is easy to grayish, and it is difficult to take both of them into consideration. In this case, using 100 lines is better than 150 lines. Exquisitely printed albums or calendars and other products are generally made of high-quality coated paper, and most of them use 150 lines or 133 lines.
3 printability. In a nutshell, printability refers to the adaptation of printing, such as paper, ink, blankets, printing plates, and other factors. In offset printing, the graphic ink on the plate is first embossed on the blanket cylinder and then imprinted on the paper by the blanket cylinder. In this process of ink transfer, the most prominent change is that the dots embossed on the paper are generally enlarged compared with the dots on the printing plate, and the most extensive expansion is about 50% (50%) of the dots. Why does 50% of outlets expand at most? After research and analysis by relevant experts, it was found that the number of outlets has a direct relationship with the length of outlets. The upper values ​​in Fig. 1 refer to the ratio of the side length of the halftone dot to the center distance of the two halftone dots. From Figure 1, the ratio of the side lengths of the 50% dots is 2.82 (square dots) and 2.45 (circular dots) respectively, so the 50% square dot enlarges the most, and the value at the time of printing is generally (ten. 15% of the network. In other words, all 50% square dots on the layout have been expanded to 65% of the dots. Other percentage points have been expanded to varying degrees after printing (see Figure 2).
It can also be seen from FIG. 1 and FIG. 2 that the expanded value after printing is basically the same under the same printing conditions regardless of whether it is a thick network line or a thin network line. However, if we use the same black-and-white manuscript to make two layouts of the same area, but the screen lines used are different in thickness (using 150 lines and 175 lines), it seems that the images on the two plates are basically the same, but on the two plates. The total number of outlets is much different. Since the total number of outlets of a thin network line greatly exceeds the total number of outlets of a coarse network line, the total length of the outlets of the thin network line also greatly exceeds the total length of the outlets of the coarse network line. This is why thin-line screen printing is difficult to master and easy to paste.
In summary, the selection of the thickness of the cable must take into account various factors, not the finer the cable. In the late 1950s, many units in China had used photographic zooming and zooming techniques to engage in thin cable products. Some of them carried out 200 lines, some also engaged in 250 lines and 300 lines, and most of them got 500 lines. The result was printed. The product is not exactly the same. Through summing up experience, people have realized that engaging in technical work cannot violate scientific laws and realistic conditions.
3. AM booths and FM outlets In recent years, color desktop systems have introduced new technologies for FM screening. For the sake of narrative convenience, the nets used for the old technology are called AM nets, and the FM nets are called FM nets. Observing with a magnifying glass, it can be seen that the old-fashioned modulation network points are composed of large and small dots. The second is whether large or small outlets, or between small outlets and large outlets. The center distance between them is the same. The full range of FM outlets is the same size of outlets, but the outlets are small; the shades of the screen are represented by the density of the outlets, and the outlets in light tones are sparse, and the outlets in dark tones are dense. Compared with AM outlets, FM outlets have the following advantages:
1 The performance level is delicate and realistic. For example, the high-light level of the screen is expressed by the amplitude-modulated dot network. The extremely high light can only be dotless (absolutely net), the sub highlights are covered with fine dots, the level is like a ladder, and they look very clear; while the FM dots are used. The performance of the small outlets can be more coordinated and natural.
2 overcome the screen out of the moire phenomenon. There are various angles in the amplitude of the AM network. Overlapping dots of several different angles often produce moire and affect the beauty of the picture. The distribution of the FM dots on the layout is neither angle nor regular, so dots of several colors are overprinted. Together will not produce moire, the picture looks flat.
3 overcome the middle tone hard phenomenon. The network of 45% to 60% of the AM sites expanded at the time of printing, so the middle tone of the screen was prone to level and level, which made the middle tone level hard. For example, large portrait images, the red version of the neck at the neck are generally more than 40%, when the printing is not well mastered, will expand these outlets to more than 60%, resulting in "blood neck" ugly picture. The FM network is better to solve the problem of the middle tone.
4 The combination of FM network and high-fidelity technology can make the color reproduction realistic. The old process method has poor color reproduction effects on the original, such as gold, silver, metal plating color, pearl color, and the like. The color desktop system uses FM network dots and high-fidelity technology, which can reproduce these colors vividly and achieve realistic effects.
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