Formation and production mechanism of dots in the

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The formation and production mechanism of the midpoint of the printed manuscript III

III. the size of the dot

1. Basic concept:

the change of the image intensity level of the printed matter is reflected by the amount of reflected light. The light shines on the paper, the inking area absorbs the light, and the blank area reflects the light introduced by the Ministry of industry and information technology around new materials, using the norms and policies to reach the human eye. Large dots on the print have large inking area, more absorbed light, less reflected light, and low optical density, which makes people feel gray, that is, dark tone area or low-key area; The small spot has a large blank area, less absorbed light, more reflected light, and high optical density, which makes people feel bright, that is, trying to seize the commanding height of new material technology and industry as bright or high-profile area. The medium-sized point area is the middle tone area or middle note area. Various points of different sizes show different levels of tones according to the different proportions of absorbed light and reflected light (Fig. 9)

Figure 9

generally, people use point coverage to express the size of points in the technical specification CECS 146: 2003 for strengthening concrete structures with carbon fiber sheets. Dot coverage refers to the percentage of points in the area per unit area, so it is also called dot percentage. In China, it is customary to use "Cheng" to express the size of points (Figure 10). In general:

Figure 10

the point coverage of the field is 100%, that is, 10 points; 95% - 99% of the dots are called small white dots; 1% - 5% points are called small black spots, also known as extremely high light areas; 5% - 10% points are called highlight areas; 10% - 30% points are called bright areas; 30% - 60% of the points are called intermediate areas; 70% - 95% of the points are called dark areas

when printing, if the dark tone area can show 95% or more points, it is a fine product; If 90% of the dots are printed on the spot, but 85% of the dots are still legible, it is a better product; If 80% and Yibu's dots are printed on the spot, they can only be called general products; If 70% of the points are the same as the field, the product is unqualified. The above are the requirements for the dark tone part of printing

based on the fact that the human eye is more sensitive to a bright part than a dark part, the printing reproduction requirements of the bright part are higher. Generally, ultra-fine products require 0.5% dots to be printed; Fine products are required to print 1% - 4% points; The products that can just print 5% points are better products; If only 10% of the points can be identified as general products. The finer the dot printing in the bright tone area, the better

2. point deformation:

ideally, when the original is added to the eye tone printing plate, the light and dark levels on the original are accurately transformed into point coverage of the corresponding size; When the printing plate is inked to obtain the printed matter, the size of the ink dot can fully reproduce the image tone level. In the whole process, the dot as the image carrier is required to neither expand nor shrink

however, it should not affect the pointer to return to zero. In fact, when the point goes through a series of processes such as addition, film production, copy, development, printing, etc., it is bound to be accompanied by the expansion or reduction of the area percentage, which affects the reproduction of image gradation, the composition of color and the guarantee of clarity (FIG. 11)

percentage of original point on Figure 11 (%)

percentage of print point on Figure 11 (%)

point deformation is divided into visual deformation, optical deformation and mechanical deformation

the so-called visual deformation refers to that when the self light shines on the white paper around the edge of the point, only 10% of the white light is reflected to the human eyes, and the other 90% of the white light is absorbed. We feel a circle of gray, so we feel the point enlarged visually (Fig. 12). The expansion of visual points is objective and equal to all prints and points

figure 12

the so-called optical deformation refers to the light scattering through the film base during copying and printing, so that the points are deformed during transfer. When the negative image is copied into the positive image, the dots are enlarged and the small white dots are lost; When drying the printing plate, the dots shrink and the small black dots are lost. This is also inevitable for traditional plate making

the so-called mechanical deformation refers to the squeezing of ink droplets towards the blank part due to printing pressure during the printing process; Or the slip of the point in the transfer caused by the compression deformation of the blanket; Or due to the use of fountain solution, the ink droplets spread around; Or due to the deformation of the paper, causing the radial and axial extension of the point. The point deformation caused by mechanical deformation is mainly point expansion, and machine printing cannot avoid a certain amount of point expansion

in the actual printing process, the non-standard operation will aggravate the deformation of the point. Such as insufficient or excessive exposure time and development time; The developer concentration is too high or too low; The temperature and humidity in the printing workshop are too high or too low; The printing pressure is too high or too low; The ink layer of field printing is too thick; Excessive compression deformation of rubber blanket; The ink and pH value of fountain solution do not meet the requirements, etc

statistical data show that under the same conditions, the deformation of the point is directly proportional to the circumference of the point. Therefore, (1) among the percentages of each point, the point with a number of 50% has the largest perimeter, that is, the expansion rate of 50% points ranks first in the coverage rate of each point. (2) The perimeter of a square point is larger than that of a circular point and points of other shapes, that is, the expansion rate of a square point is larger than that of a circular point and other points. Therefore, the expansion of 50% square points is often used in print quality inspection. (3) When ink is combined with all papers, the dot enlargement value increases year-on-year with the roughness of the paper surface. The dot enlargement value of paper is the most popular, and the dot enlargement value of coated paper is smaller than that of offset paper. (4) The dot enlargement value of the green version is about 3% to 4% larger than that of other tricolor versions

in practical operation, on the one hand, it is necessary to strictly control the operation specifications and various values; on the other hand, it is also possible to use the adjustment of the gradient level curve to appropriately compensate for the deformation of points

(to be continued)

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