The application of CIE LAB to study trapping efficiency

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Title: The application of CIE LAB to study trapping efficiency
Author: Malikhao, Patchanee
Abstract: It has been known that apparent trapping percentages caused by changing printing sequence (or printing order) affect the color appearance of the overprint areas. However, no other studies in the past were done in a standard color measurement system- that is the CIE system. This thesis was conducted in order to learn how the selected printing sequences yellow- magenta-cyan-black, black-cyan-magenta-yellow, yellow-magenta-cyan-black, and cyan-magenta-yellow-black- of a monotack process ink yield the color gamuts on the CIE LAB color space. Since the two sequences: yellow-magenta-cyan-black and cyan-magenta-yellow-black, were recommended to be adopted as standard sequences for SWOP inks, this reseach attempted to prove that these sequences printed by a web-offset press yielded the similar color gamut on the CIE LAB color space. How the abovementioned selected printing sequences affected the changes in the apparent trapping percentages were also studied. The color differences between the color patches printed by changing printing orders and those of the ideal inks were calculated and compared with one another in order to find out whether the overprint colors visually differed from one another or not. Moreover, how the trapping changed the color appearance of the print from what was expected in terms of hue angles and chroma shifts were studied. Last but not least, the colorimetric trapping percentages were also studied. It can be concluded from the findings that the changes of ink sequence of the monotack ink in the web-offset printing caused the wet-trapping percentages change. Thus caused the hue angle and chroma shifts which resulted in different color gamuts. It was proved that the two sequences yellow-magenta-cyan-black and cyan-magenta-yellow-black did not have the similar color gamut on the CIE LAB color space; they produced visually different green. The relationships between ink trapping percentages and visually different color appearance in this study can be explained as follows. It is found that the trapping of yellow over magenta did not yield different color appearances from the trapping of magenta over yellow. The trapping of cyan over yellow yielded different color appearances from the trapping of yellow over cyan, but the trapping of cyan over magenta did not yield different color appearances from the trapping of magenta over cyan. The densitometric apparent trapping percentages calculated from the Preucil's equation failed to explain the relationships between the red trapping and the color difference of the red printed patches from the red ideal ink. The colorimetric trapping percentages using the Preucil’s concept was proved invalid. Larger sample sizes and other ink sequences are recommended for the next study.
Record URI: http://hdl.handle.net/1850/10145
Date: 1988-02

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