The Construction of Optimum Gray Balance Combinations for Sheetfed Offset Lithography

• Main Author: 吳玉如
• Other Authors: 謝顒丞
• Format: Others
• Language: zh-TW
• Published: 2004
• Online Access: http://ndltd.ncl.edu.tw/handle/zszw3z

碩士 === 國立臺灣藝術大學 === 廣播電視學系碩士班應用媒體藝術組 === 92 === Gray balance is the first important step undertaken by color separators, in determining the color content and contrast that the final printed piece will have. It is a significant factor in determining what the overall color gamut will be. Some of the important factors that influence gray balance and color for the offset lithographic printing process include paper, dampening solution, printing plate, and printing blanket. Proper gray balance ensures that a tone of appropriate. Cyan (C), Magenta (M) and Yellow (Y) tint values is visually perceived as neutral gray. Theoretically, equal amount of CMY should produce neutral gray; unfortunately, pigments of printing inks that comprise the process colors are not as pure as the theory suggests. They possess some hue error. Therefore, in order to achieve the most neutral grays, optimum combinations of CMY must be empirically determined. This study was an experimental research in nature. The purposes of the study was determined the optimum CMY combinations that produce the most neutral gray for highlights, midtones, and shadows. More than fifty company members of the Printing Industry of Taiwan (PIT) were invited to participate in the study. Twenty-three companies, representing the north, central and the south of Taiwan submitted a total of 65 sets of printed samples for this study; 34 sets were printed on coated paper and 31 sets on uncoated paper. Participants were provided with an original test form created specifically for this experiment on a CD-ROM in digital format or a set of conventional litho film. Each participant was asked to submit at least 100 printed sheets on both coated and uncoated paper respectively. Then, each stock was systematically randomly selected for a sample of 35 sheets each. At least, a total of 2275 printed sheets were sampled for this study (1190 for coated and 1085 for uncoated stocks). A GATF Gray Balance Chart was used as a process standardization target for determining the three-color dot requirements for cyan, magenta, and yellow films to reproduce a neutral scale at four different tone values (7%, 30%, 60%, 80% tints). X-Rite 530 spectrodensitometer was used for measuring the Gray Balance Chart to investigate the colorimetric values (L*a*b* color) of the squares selected as most neutral, or to measure various squares to find the most neutral squares (where the a* and b* values are closest to zero, or minimum chroma values). Furthermore, X-Rite 530 spectrophotometer was used to measure the various print attributes for the purpose of exploring the relationship between the minimum chroma values and the values of the print attributes. The results of the study can be used as the standard for offset printers to set the gray balance reference for their in-house color reproduction and management.