Analysis of the reasons for the dot expansion of t

The so-called black and white in black and white image printing refers to the contrast between the ink left on the printed object by the printing ink and the color of the printed object itself, or it can be called monochrome printing. We generally call the color image with R, G, B or C, m, y hues in the image color as color image, and the color image without the above hues as achromatic image, that is, there is no hue, saturation, only brightness in achromatic image. The reproduction of black-and-white image is based on this principle. A full understanding of the reproduction law of black-and-white image will help us further understand the reproduction law of color image

in the process of transferring the ink of the graphic part on the printing plate to the printed object, due to various reasons, For example, the printing plate is in contact with the printed object and the printing ink is transferred "In the past, the company lacked a long-term strategy. The brush pressure, the deformation caused by the rubber blanket in contact with the printing plate and the printed lightweight packaging, the ability of the printed object to absorb and penetrate the ink, and the optical phenomenon of the ink performance, etc., caused the ink transferred to the printed object to deform relative to the printing plate or the graphic part of the original film used to make the printing plate. This phenomenon is manifested in the visual tone In printing, it is point enlargement, and its law is that no matter the size of the point, it is shown as the equidistant enlargement of the edge. Reflected in the image tone change is that the ink density of the highlight area composed of small dots and the dark area composed of large dots does not increase much, while the point increase value at the middle tone, especially at 50%, is the largest

in general, the average point spread value in offset printing is 8%-12% for coated paper printing; About 20% offset paper; More than 30% of paper. However, for the same printing coated paper, the dot gain value of hard lining and air cushion blanket is 8% - 12%; Neutral lining is more than 15%; The soft lining is more than 20%. In addition, improper control of printing pressure, ink volume and ink balance will also change the point enlargement value during printing

dot enlargement is mainly manifested in three aspects in printing control: first, highlight small dots. In printing, generally, 1% - 2% of small dots cannot be printed, while 2% - 3% of small dots will jump to 5% - 7% due to dot enlargement; The second is to secretly highlight big ideas. Due to the expansion of points, more than 85% of points may become 100% on the ground; Third, because the point expansion value at 50% is the largest, and until the conditioning is qualified, it is the transition stage from circular point to square point, so the step will have a very obvious mutation

the above conditions will shift the entire copy gradient curve to the dark tone during the printing process, and there is no point at the highlight dot; Dark areas appear dot and paste, and the rich levels contained in dark tones cannot be correctly expressed. This situation shows that the whole image copy is lost in the level of the highlight area or the dark area, and the copy curve is offset

the factor that causes the image reproduction curve to shift towards the medium dark tone is that human vision does not feel the same about the highlight and dark tone of the image, and the hierarchical tone of equal density reproduction does not appear to be evenly distributed in human eyes, so the medium dark tone must be highlighted in the normal reproduction process of the image. Although this will cause the loss of highlight level, it can fully display the dark level. Although the phenomenon of dot enlargement and the level shift of medium and dark tones are mainly manifested in the printing stage, overcoming these problems can not be solved at the same time by the printing process. The fundamental way to solve these problems is in the prepress production stage, especially in the image scanning stage, it is necessary to correctly set the gamma values of the white field, black field and replication curve of image reproduction according to the actual printing situation, expand and shrink the points displayed in printing in advance, and solve the problems in printing before printing. This is the basic principle of digitalized and standardized printing control