Gray balance is the basis for any color separation method to achieve correct color reproduction. If gray balance cannot be achieved during color separation, the gray in the original will no longer be gray after copying, and other colors will inevitably produce color cast. When using digital color separation technology (that is, when converting from an RGB image to a CMYK image), the main factors that determine the gray balance curve are the ink and paper combination, the type of black plate generation, the degree of black plate generation, and the dot expansion curve. When the above four factors are determined, the gray balance relationship is determined. 1. Grey balance and its influencing factors The concept of gray balance can be described as: three color versions of yellow, magenta and cyan generate neutral gray on the printed matter according to the proportion of different dot area ratios. According to the subtractive color rendering theory, the superposition of the maximum saturation of the three primary inks of C, M, and Y should be black. In the same way, the equal superposition of three primary inks with different saturations should also produce grays with different brightness. However, due to the fact that the inks actually used have deficiencies in ink manufacture that are difficult to overcome in terms of hue, saturation, and lightness, the same amount of three-primary color inks cannot achieve neutral gray. In order to make the three primary colors of inks display accurate gray with different lightness, it is necessary to change the ratio of the dot area of ​​the three primary colors of ink according to the characteristics of the ink to achieve the gray balance that is essential for color reproduction (2) Factors affecting gray balance There are many factors that affect gray balance. It can be said that almost every link in the color reproduction process will cause changes in gray balance. The main factors are: 1. Ink characteristics The inks produced by different manufacturers have different gray balance relationships. Before the plate-making, the basic characteristics of the ink (such as hue, saturation, and brightness), and the physical and chemical characteristics of the ink (such as the drying speed and gloss of the ink) need to be measured to obtain The correct gray balance relationship should take these factors into consideration when making plates. In addition, the printability of the ink will also affect the gray balance. For example, changes in the viscosity, rheological characteristics, and ink layer thickness of the ink will affect the color reproduction of the original, resulting in the destruction of the gray balance. 2. Paper characteristics Different types of paper have a great difference in the color rendering ability of the same ink. The gray balance parameters of the three-color inks on the coated paper and offset paper commonly used in offset printing are different. The second main index of paper that affects the gray balance is its whiteness, and the rest also have smoothness, absorption characteristics, gloss, opacity and pH, etc., because the changes of the above factors will affect the correct realization of gray balance. 3. Printing conditions The variety of printing plate materials, the composition and characteristics of the photosensitive liquid, the thickness of the sand, the characteristics of the light source, and the conditions of exposure and development all have an effect on the final gray balance. 4. Printing conditions The impact of printing conditions on gray balance is mainly reflected in two aspects: (1) The printing machine can be included in the printing machine and the factors that affect the gray balance are the type and thickness of the blanket, the type of lining, the printing pressure, the printing speed, the type and accuracy of the printing machine, etc. (2) The influencing factors of fountain solution include fountain solution formulation, water consumption, surface tension and pH value of fountain solution, water temperature, emulsification degree of fountain solution and ink, etc. Among the many factors mentioned above, although the printing and printing conditions have an effect on the gray balance, they cannot be considered when making plates. If all the factors that affect the gray balance are taken into account, it will not work at all because there are too many variable factors. Therefore, the main factors that need to be considered for plate making are the characteristics of ink and paper, which can be attributed to the combination of ink and paper. This is why Photoshop first needs to choose the ink and paper combination when setting the color separation parameters, and then choose other conditions to determine the gray balance. 2. Ink and paper combination Image processing software represented by Photoshop usually uses SWOP (Coated) as the default setting for color separation when performing digital color separation. It refers to the use of inks that conform to the Specifications for Web Offset Publication on coated paper print. In most cases, this method can obtain good color separation results. Although the actual printing process conditions may be different from this, for DTP operators who are not familiar with color separation theory and printing process, this It is really a good choice. Although Photoshop refers to the ink and paper combination as Ink Colors, it actually refers to the type of ink, and also includes the paper to be used when printing. The paper used in the printing industry can be roughly divided into three categories, namely Coated (coated paper, also known as coated paper), Uncoated (non-coated paper, also known as offset paper) and Newsprint (newsprint). The surface smoothness, glossiness, ink absorption, water infiltration and ink transfer characteristics of the three types of paper are different, and the tone characteristics, the amount of ink inking and the amount of ink transfer that can be reproduced during printing are also different. The available ink and paper combinations include SWOP (Coated), AD-LITHO (Newsprint) for newspaper printing, Dainippon Ink, European ink and paper combinations (coated paper, newsprint and offset paper respectively), SWOP In addition to coated paper, there are four types of ink: newsprint and offset paper, Japanese ink (Toyo Inks) and paper combination (respectively coated paper, matt coated paper, newsprint and offset paper). Each ink and paper combination has different default midtone dot expansion values. These values ​​are the empirical statistics after long-term use. If the ink and paper combination used by the user is not found in the list, you can obtain the relevant parameters according to the actual ink and paper combination used by proofing, and establish a custom ink and paper combination. In most cases, the ink characteristics used in printing will not differ much between different printers (printing machines) of the same type. For example, the same kind of Tektronix Phaser Ⅱ printer uses the same ink, but the dot expansion value may be significantly different when it is used on another printer. Therefore, for different printers of the same type, it is mainly necessary to modify the dot expansion value obtained according to the actual use, rather than changing the ink type. For images to be used on digital presses, you may encounter the same problem when separating colors.
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(1) The concept of gray balance
Different ink and paper combinations have different gray balance relationships, which will produce different color separation results.