At present, the most important part of color offset quality control is color control. The quality of color reproduction directly affects the quality of prints.
The print copy process of a color manuscript can usually be expressed as a combination of color decomposition and color reproduction of image information. In practice, the presence of color differences is inevitable due to the unsatisfactory conditions. The color difference mainly comes from three aspects. One is the color shift caused by the original itself due to the shooting process and materials, and the color shift caused by discoloration (color attenuation) of the color medium; the second is the color difference during the color decomposition process, mainly the light source, lens, color filter, and photomultiplier. Errors such as tubes and photosensitive materials; third, color differences in the color reduction process, mainly errors in paper, ink, and printing processes.
In addition, the quality of the prepress plate has a direct impact on the quality of the printed matter. Moreover, the reasons for the color difference during the plate making process are also quite numerous. No analysis is performed here, but only when using the offset printing as a color reproduction method. Several major factors of color quality are described in detail.
1 Network changes
Color prints are used to reproduce the color and tone of originals using overprinting of yellow, blue and green plates with different dot angles. A black version is also added to make up for the lack of dark tone. In offset printing, due to various reasons, it is inevitable that outlets will be expanded.
Changes in dot size can cause changes in the color of prints, and it is worth noting that the depth of printed matter is not always linear with the percentage of dots. At the end where the optical density is small, the effect of the increase in the dot area on the hue density is much smaller than the end where the optical density is large. In other words, to make a difference between two brighter tones, it is necessary to make a large change in coverage area of ​​the dot area; on the contrary, to make two darker tones distinguishable, dot area coverage is only It can be achieved with minor changes. It also shows that in the bright tuning area, small changes in the percentage of dots have less influence on the hue and the eyes are less noticeable; in the darkening tone, slight changes in the percentage of the dots will cause significant changes in the hue. Therefore, the effect of the dot size change on the image copy color is complicated. In actual operation, the control of the dot size of the dark tone portion is particularly important.
2 ink
The structure and properties of the ink itself have a direct effect on the reproduction of the color.
In color printing, the quality of prints is often measured by the ink's on-site density, dot gain, print contrast, and overprint rate. These parameters are closely related to the ink film thickness on the print sheet. The ink layer is too thin, the ink cannot completely cover the printing area of ​​the substrate, the color is necessarily light and the gloss is not enough. Usually, the thickness of the ink film is about 1.0 μm, and the thickness of the ink film increases, and the density does not increase. On the contrary, because the ink layer is too thick, the dot network will increase, the level will be parallel, the tone and color will undergo major changes, resulting in color cast. Therefore, controlling the ink film thickness is an important means of color reproduction. The thickness of the ink film is not only restricted by the nature of ink and paper, but also affected by printing conditions and environmental factors. To sum up, there are mainly the following points: printing pressure, printing speed, ink supply, paper performance, blanket quality, and temperature and humidity. Different inks have different ink film thicknesses due to the difference in their performance, such as different coloring power, in order to achieve a certain required density value.
Therefore, controlling the ink film thickness is the key, and the ink's own defects should also be understood. It is known that the ideal three-primary inks are mixed in equal proportions to produce neutral ash, and the actually used three primary inks are mixed in equal proportions, and they cannot produce neutral ash, and only produce dark colors. The correct reproduction of the gray balance is the basis of color printing, and the ink itself is not ideal, which obviously brings certain difficulties to the correct reproduction of colors.
At the same time, the range of colors (color gamut) that the ink overprint can reproduce is the smallest of all color gamuts. The inks produced by different manufacturers and the different types of inks produced by the same manufacturer are different in the range of colors that they can reproduce. When printing, it is necessary to select inks with a large color gamut as much as possible. The larger the color gamut, the smaller the loss of color information, and the higher the fidelity of color reproduction; conversely, the smaller the color gamut, the greater the loss of color information. Correct reproduction of colors.
3 Paper
Different printing materials have different printing aptitudes. The smoothness, ink absorption, surface strength, whiteness, sizing, and stretchability of the paper are all closely related to the quality of the printed matter. This paper mainly discusses the effect of paper's smoothness and whiteness on color reproduction.
The smoothness of the paper determines the degree of contact between the paper and the plate. Paper smoothness and ink transfer rate are proportional to a certain range. When the smoothness is high, even if the ink layer of the printing plate is very thin, the ink connecting material will quickly form a fixed ink layer on the paper surface, which is favorable for overprinting of the next printing color, but when the ink layer is increased to a certain thickness The transfer rate no longer rises. Coated paper has a higher degree of smoothness than offset paper, and can still obtain prints with even color even when the thickness of the ink layer is slightly changed. On the other hand, offset paper is inferior and the transfer rate is low, which is not conducive to the reduction of color.
The whiteness of paper, the degree of whiteness of paper, is the ability of paper to reflect the visible light at each wavelength. Printing paper, except for a few colored papers, is mostly white, and the whiteness of different papers is not the same. For printing, of course, the higher the whiteness of the paper, the better. Only the paper that is nearly pure white can reflect all the visible light, and the color characteristics of the ink can be fully utilized. The yellow, blue, or reddish color of the paper can cause color shifts in the print, affecting the hue, lightness, and saturation of the print. The whiteness of the paper also affects the contrast in the picture layer. The whiteness of the paper is good, and the layering and sharpness of the picture are correspondingly improved. On the contrary, the color of the printed product is dark and the contrast is poor. (to be continued)
The print copy process of a color manuscript can usually be expressed as a combination of color decomposition and color reproduction of image information. In practice, the presence of color differences is inevitable due to the unsatisfactory conditions. The color difference mainly comes from three aspects. One is the color shift caused by the original itself due to the shooting process and materials, and the color shift caused by discoloration (color attenuation) of the color medium; the second is the color difference during the color decomposition process, mainly the light source, lens, color filter, and photomultiplier. Errors such as tubes and photosensitive materials; third, color differences in the color reduction process, mainly errors in paper, ink, and printing processes.
In addition, the quality of the prepress plate has a direct impact on the quality of the printed matter. Moreover, the reasons for the color difference during the plate making process are also quite numerous. No analysis is performed here, but only when using the offset printing as a color reproduction method. Several major factors of color quality are described in detail.
1 Network changes
Color prints are used to reproduce the color and tone of originals using overprinting of yellow, blue and green plates with different dot angles. A black version is also added to make up for the lack of dark tone. In offset printing, due to various reasons, it is inevitable that outlets will be expanded.
Changes in dot size can cause changes in the color of prints, and it is worth noting that the depth of printed matter is not always linear with the percentage of dots. At the end where the optical density is small, the effect of the increase in the dot area on the hue density is much smaller than the end where the optical density is large. In other words, to make a difference between two brighter tones, it is necessary to make a large change in coverage area of ​​the dot area; on the contrary, to make two darker tones distinguishable, dot area coverage is only It can be achieved with minor changes. It also shows that in the bright tuning area, small changes in the percentage of dots have less influence on the hue and the eyes are less noticeable; in the darkening tone, slight changes in the percentage of the dots will cause significant changes in the hue. Therefore, the effect of the dot size change on the image copy color is complicated. In actual operation, the control of the dot size of the dark tone portion is particularly important.
2 ink
The structure and properties of the ink itself have a direct effect on the reproduction of the color.
In color printing, the quality of prints is often measured by the ink's on-site density, dot gain, print contrast, and overprint rate. These parameters are closely related to the ink film thickness on the print sheet. The ink layer is too thin, the ink cannot completely cover the printing area of ​​the substrate, the color is necessarily light and the gloss is not enough. Usually, the thickness of the ink film is about 1.0 μm, and the thickness of the ink film increases, and the density does not increase. On the contrary, because the ink layer is too thick, the dot network will increase, the level will be parallel, the tone and color will undergo major changes, resulting in color cast. Therefore, controlling the ink film thickness is an important means of color reproduction. The thickness of the ink film is not only restricted by the nature of ink and paper, but also affected by printing conditions and environmental factors. To sum up, there are mainly the following points: printing pressure, printing speed, ink supply, paper performance, blanket quality, and temperature and humidity. Different inks have different ink film thicknesses due to the difference in their performance, such as different coloring power, in order to achieve a certain required density value.
Therefore, controlling the ink film thickness is the key, and the ink's own defects should also be understood. It is known that the ideal three-primary inks are mixed in equal proportions to produce neutral ash, and the actually used three primary inks are mixed in equal proportions, and they cannot produce neutral ash, and only produce dark colors. The correct reproduction of the gray balance is the basis of color printing, and the ink itself is not ideal, which obviously brings certain difficulties to the correct reproduction of colors.
At the same time, the range of colors (color gamut) that the ink overprint can reproduce is the smallest of all color gamuts. The inks produced by different manufacturers and the different types of inks produced by the same manufacturer are different in the range of colors that they can reproduce. When printing, it is necessary to select inks with a large color gamut as much as possible. The larger the color gamut, the smaller the loss of color information, and the higher the fidelity of color reproduction; conversely, the smaller the color gamut, the greater the loss of color information. Correct reproduction of colors.
3 Paper
Different printing materials have different printing aptitudes. The smoothness, ink absorption, surface strength, whiteness, sizing, and stretchability of the paper are all closely related to the quality of the printed matter. This paper mainly discusses the effect of paper's smoothness and whiteness on color reproduction.
The smoothness of the paper determines the degree of contact between the paper and the plate. Paper smoothness and ink transfer rate are proportional to a certain range. When the smoothness is high, even if the ink layer of the printing plate is very thin, the ink connecting material will quickly form a fixed ink layer on the paper surface, which is favorable for overprinting of the next printing color, but when the ink layer is increased to a certain thickness The transfer rate no longer rises. Coated paper has a higher degree of smoothness than offset paper, and can still obtain prints with even color even when the thickness of the ink layer is slightly changed. On the other hand, offset paper is inferior and the transfer rate is low, which is not conducive to the reduction of color.
The whiteness of paper, the degree of whiteness of paper, is the ability of paper to reflect the visible light at each wavelength. Printing paper, except for a few colored papers, is mostly white, and the whiteness of different papers is not the same. For printing, of course, the higher the whiteness of the paper, the better. Only the paper that is nearly pure white can reflect all the visible light, and the color characteristics of the ink can be fully utilized. The yellow, blue, or reddish color of the paper can cause color shifts in the print, affecting the hue, lightness, and saturation of the print. The whiteness of the paper also affects the contrast in the picture layer. The whiteness of the paper is good, and the layering and sharpness of the picture are correspondingly improved. On the contrary, the color of the printed product is dark and the contrast is poor. (to be continued)
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