Elaborate on outlets (2)

Fourth, the shape of the dot

The shape of the dots is related to the visual effect of the printed product when expressing different image layers, and also has a certain influence on the printability. The most commonly used dot shapes are square dots, round dots and chain dots. As shown in Figure 16. Sometimes, in order to produce special visual effects, some special dots are also used, such as linear dots, cross dots, concentric dots, brick dots, wave dots, oval dots, sand dot dots, etc.

Figure 16

The square dot is a traditional dot shape, and the dot is square, which has strong ability of reflecting layers and rich picture tone. However, the square corners are easily deformed by optical and chemical effects; the square dots have the same perimeter, and the circumference of the square dots is larger than other various dots; the dot expansion rate is the highest; when the dot coverage rate is 50%, the dots just take the angle, and the dot expansion angle The dots overlap, and the corners come off when the dots are reduced. When the signal bar detects the print quality, 50% square dots are used to observe the dot copying in the midtone position, which is very simple and intuitive, as shown in Figure 16. Based on the characteristics of square dots to express fine levels, lack of rich midtone levels, poor transition, and not soft pictures, it is often used for duplication of images with good texture and relatively flat midtones.

The circular dots are just tangent to the arc at 70%, and the dots below 70% are independent, and among the same percentage dots below 70%, the circumference of the circular dots is the smallest and the dot expansion rate is the smallest. Therefore, circular dots have a unique effect on expressing bright tones and midtone levels. However, more than 70% of the circular dots are in contact with an arc, and the dot expansion rate is high. When the signal bar detects the printing quality, 75% of the square dots are used to observe the duplication of the dots in the dark tone portion, as shown in Figure 16. Therefore, at 70% tone, the image of the circular dot will have vacancies and skip levels. The tone level at the dark tone is very easy to blur, the tone loss is serious, and the level reproducibility is poor. Based on this, circular dots are often used in the image duplication of high- and mid-tone areas of the picture, which require high copy requirements and lack of dark tone levels.

The chain-shaped dots are diamond-shaped, and the two diagonals of the dots are not equal in length, as shown in Figure 16, when the dot percentage is 35%, the long diagonal line is just angled, and the short diagonal line is off; when the dot percentage is At 65%, the short diagonal was just angled and the long diagonal was off. Therefore, in the midtone region of 35% to 65%, there are two small jumps in the density of the picture, unlike square dots and round dots, there is only one large jump at 50% and 70%, respectively. Therefore, the transition of the midtone density of the chain dots is very soft and rich in layers. It is especially suitable for the reproduction of manuscripts of landscapes and people, and is conducive to the reproduction of large areas of soft tones of skin tones, flowers, leaves and other areas on the screen.

As for the brick-shaped dots, as the name suggests, it is suitable for expressing buildings; concentric dots are suitable for expressing vortex effects; wave-shaped dots are suitable for expressing water landscape; linear dots are suitable for expressing trunks and bamboo forests.

In printing and copying, according to the different manuscript screens and artistic effects to be selected, selecting the appropriate dot shape will help to improve the quality of the printed copy and achieve a unique aesthetic.

5. Number of outlets

The number of dots is a unit that represents the thickness of the dots, that is, the number of dots that can be accommodated per unit length, also known as the number of screen lines or the number of screens, the unit is: line / cm (l / cm) or line / inch ( l / in).

Table 1: The relationship between different products and the number of outlet lines

Generally speaking, the higher the number of dot lines, the more dots per unit area, the smaller the dot unit area. For the same dot coverage, the smaller the ink droplets, the clearer and finer the expressed image, and the higher the picture quality. As shown in Figure 17, the number of dots on the left is greater than the number of dots on the right.

Figure 17

Therefore, for different products with different quality requirements, choose different dots and lines.

Of course, the selection of dots and lines is not absolute. It is also necessary to make overall arrangements according to various factors such as the purpose of the printed matter, the type of paper, and the resolution of the device, and to select the appropriate dots and lines reasonably.

1. Determine the number of screens according to the purpose of the printed product

If the printed matter is for reading books, pictures, etc., the observation distance is close, it is appropriate to use high screen number; if the printed product is used for outdoor posters and advertising pictures, the observation distance is long, the low screen number can be used, the effect is good And easy to print. The theory is analyzed based on the visual principle of available outlets. Table 2 shows the number of dot lines applicable for different line-of-sight.

2. Determine the number of screen lines according to the paper type

When we choose the number of screens, we should consider the paper used. The recommended data is as follows:

The surface of the coated paper is smooth, and use 175 lines / inch (that is, 70 lines / cm) and more screen lines.

For non-coated paper, use 125-170 lines / inch (that is, 50 lines / cm or more) for the number of screen lines.

The newsprint uses screen lines of 100 lines / inch (ie 40 lines / cm) and below.

3. Determine the number of screen lines according to the scanner resolution

When the scanner scans the continuous adjustment original, the optical information on the original becomes digital information usable by the computer, that is, pixels. When printing, these pixels are arranged and combined into the desired mesh tone image.

The higher the resolution of the scanner, the smaller the pixels, and the more pixels expressing the image, the more realistic the reproduction of the original. The size of the pixels is consistent with the number of screen lines of the screen print. The higher the number of screen lines, the finer the screen dots. Therefore, the resolution of the scanner is proportional to the number of screen lines of the image. According to the analysis of the scanning characteristics of the scanner and the dot angles of the yellow, magenta, cyan, and black plates during four-color printing:

Number of screen lines = scanner resolution * 2/3 That is, when continuously adjusting the image screen, the number of screen lines input should be lower than the resolution of the scanner.

4. The output resolution of the laser imagesetter determines the number of screen lines

Research shows that the gray level of the image = (output resolution / screening line number) 2 + 1.

In order to make the gray level of the image change delicately, that is, the screen tone is rich in layer, the output resolution should be high, but the number of screen lines of the image should be small.

For example, a laser phototypesetter with an output resolution of 2400 dots / inch (ie 2400dpi), when the number of screen lines is 150 lines / inch (ie 150lpi), the generated mesh can be reproduced (2400/150) 2 + 1 = 257 At this level, a mesh unit contains exactly 16x16 laser printing dots; but if the number of screen lines is 200 lines / inch (ie 200lpi), the generated mesh can only be reproduced (2400/200) 2 + 1 = 145 levels, at this time, a mesh unit just contains 12 * 12 laser printing points.

Therefore, the resolution of the laser imagesetter is fixed, and it is necessary to determine the appropriate number of screen lines according to the requirements of the image gray level, and the higher the density of the non-screen lines, the better.

5. Determine the number of screens added according to other factors

The number of screen lines is also related to the overprint accuracy of the printing machine. Only through a high-quality printing machine, a high-quality hard blanket, a high-quality fountain solution control, and a high-quality machine adjustment can the mesh of a high dot count be realized. Adjust prints. The same semi-formed dots have high dot lines and small black dots. If the machine is not good, the pressure control is uneven, the compression deformation of the blanket is large, and the dot slip is serious, these small black dots are easily lost, resulting in serious layer defects. .

Based on the above analysis, when determining the number of screen lines for the mesh tone image, all factors should be considered as a whole, summarized in the following table:

Six, the outlet angle

The dot angle refers to the angle between the dot arrangement line and the horizontal line.

The human eye's vision is the most beautiful and comfortable for the 45-degree dot angle, and the image performance is the most stable and not stiff; the most uncomfortable for the 0-degree or 90-degree dot angle, even if the dot number is high, the eyes can still Feel the line of dots and linear patterns, which affects the expressive power of the picture; for the dot angle of 15 degrees or 75 degrees, the human eye feels average.

Therefore, for monochrome mesh tone prints, we of course choose 45-degree dots.

However, for multi-color mesh-adjusted printed products, the dot angle of each color version must be staggered to prevent the occurrence of moire.

Based on this, the angle of the color network cable commonly used in the world is as follows:

(1) For two-color printing, use a 45-degree screen angle for the dark version or the primary color version and 15 degrees for the light-color version or the secondary color version.

(2) For three-color printing, the dark version or main color version uses a 45-degree screen angle, and the other two-color version uses 15 degrees and 75 degrees.

(3) For four-color printing, the main color version uses a 45-degree screen angle, the yellow version uses a 90-degree screen angle, and the other two colors use 15-degree and 75-degree screen angles, respectively.

The reason can be analyzed from the following aspects:

(1) In terms of color rendering performance: among the four colors of yellow, magenta, cyan, and black, yellow has the weakest visual sense, followed by magenta and cyan, and black has the strongest sense. Therefore, when screening traditional photographs in China, the black version is 45 degrees; the magenta version is 15 degrees; the cyan version is 75 degrees; the yellow version is 90 degrees.

(2) From the characteristics of moire: yellow, magenta, and cyan will produce obvious orange, red, or green moire, and the moire produced by yellow and black is quite inconspicuous. Therefore, if the main color of the pattern is red, you can The settings are: Magenta version-45 degrees; Black version-15 degrees; Cyan version-75 degrees; Yellow version-90 degrees; Cyan as the main color, then: Cyan version-45 degrees; Black version-75 degrees; Magenta Version-15 degrees; yellow version-90 degrees.

(3) From the point of view of the original dots of the screen-adjusted manuscript: if the original used is a copy of the printed matter, that is, the screen-tuned manuscript, there are dots on the original. When copying, the color separation and screening will cause overlap , Producing severe moire. At this time, scanning the original should be processed by eliminating the net, that is, changing the scanning aperture to increase it as much as possible to reduce the resolution, or adjust the virtual focal length until the original point of the Internet is not clear, or use the PHOTOSHOP software Eliminate the net command again and again. Of course, these operating methods are effective for eliminating the dots on the original, but it has a certain effect on the color separation and screening quality of the original, reducing the clarity of the product. If the net is not thorough, moire will still occur during printing. If the definition of the product is very high and the moire is not allowed, the dot angle of each color version used for copying should be staggered from the dot angle of the corresponding color version on the original. Such as:

The manuscript is: Magenta version-45 degrees; Black version-15 degrees; Cyan version-75 degrees; Yellow version-90 degrees; Can be changed to: Yellow version-45 degrees; Magenta version-15 degrees; Black version One 75 degrees; the blue one is 90 degrees or other dot angle arrangement.

7. Prospects of Screening Technology

Through the analysis of the four elements of the dot, the mechanism of continuous adjustment of the original copy into the mesh-adjusted print is clarified. This screening technology is commonly used in the current printing industry and is a traditional screening method, also known as AM screening (AM , Amplitude Modulation), the dot pitch is equal, the size is not equal, the screen angle setting is strict, the density of the screen is reproduced through the dot size, and the tone reproduction is stable. However, the printed products are prone to moire, the screen will jump color, be disconnected, and the high requirements of printing overprint accuracy affect its development prospects.

In the more than 100 years of development of modern printing technology, the screening technology has taken a big step unconsciously. The appearance and application of FM screening technology has brought a breeze to the printing industry.

FM (Frequence Modulation) refers to the same dot size and unequal dot pitch within a unit area. The density of dots reflects the tone level of the image. As shown in Figure 18, the picture is an irregular arrangement of very small pixels (about 10-20 microns in diameter), dense dots, high picture density, sparse dots, low picture density, thus reproducing the shades of the original . The smaller the idea, the more high-quality tone, color and clarity can be reproduced. The imaging principle is similar to the precipitation of fine silver salt particles on the film to form an image. The idea is delicate, the color change is soft, and the density is high, as long as The dots are small enough, and their prints and manuscripts can be quite realistic. Of course, the performance of the required computer is also higher. In particular, random and irregular dots completely eliminate the occurrence of moire, and increase the tolerance of overprint accuracy and the tolerance of color number of overprint colors, which can achieve high-fidelity special printing effects that are difficult to achieve with conventional four-color overprint. .

Figure 18

However, because the dots of the FM screening are very small, only 10--20 microns, which is equivalent to 1% -2% of the dots in the general 175-line AM screening products. It is very easy to lose in copying, printing, printing, and The outlets of the FM network are easy to expand, generally between 20% -40%. Therefore, the printing of FM network products has very high requirements on printing operations, processes, equipment, materials, technology, environmental temperature and humidity.

Of course, the development of printing technology will increasingly adapt to the printing requirements of FM network products. Such as:

(1) The waterless offset printing technology greatly reduces the dot expansion rate, and can reproduce finer dots and higher ink density.

(2) CTP direct plate-making technology and digital printing technology eliminate the process of copying and printing, and the dot transfer is more direct and the deformation is smaller.

(3) The application of high-quality equipment and materials can adapt to higher requirements of printing.

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