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FLEXO Magazine : March 2014
TECHNOLOGIES & TECHNIQUES Printing Tolerances Using Deltas to Control Color & Results by Julian Fernandez The goal of any printer should be the ability to print in a consistent and repeatable way at the lowest possible cost. As in any other industry, a standardized process is needed to achieve this objective. It is simply not possible to start talking about standardization if we don’t start defining the tolerance of the process. The following research will give printers and prepress companies, as well as brand owners and graphic designers, an idea on how to define and set printing tolerances based on real color differences. The research includes actual data collected from 22 different printers and includes 9 different printing conditions. DEFINITIONS & BASIC KNOWLEDGE In order to understand the results from the paper, the read- er should be familiar with the following terminology: Delta E is a metric that was created by the International Commission on Illumination in 1976 to define the difference or distance between two colors. It allows people to quantify a notion that would otherwise be described by adjectives. The original formula, also known as the “classic” formula, is linear in all directions. This means that it won’t consider the colors the formula analyzes. It only cares about the distances between them. Delta E 2000 is the most recent formula to calculate Delta E. The human eye is more sensitive to noticing differences in certain colors like grays and less sensitive to other colors like those high in chroma (bright colors). This more recent formula takes this into consideration and, thus, is more realistic to the way we perceive color differences. Density by definition is the degree of color or darkness of an image. For calibrated inks, the concept of density refers to the amount of ink that is transferred onto the substrate. Printing density is important during the pressrun because it is the most important variable to manage in order to keep the run under control. In order to control density, we use a reflection densitometer. For a CMYK printing process, the density read will depend on the color. Density must be read in a solid patch of that color that is printed on a chart, normally on the sides, bottom or top of the press sheet depending on the process and layout. The press operator must know which are the values for each color and its tolerances. Solid densities values are defined by ISO Standard 12647 for many printing process and substrates. However it is very likely that the user won’t find the exact process or material being printed. In this case, it is possible to use a similar one as a reference. Otherwise the densities must be calculated. Dot Gain is properly known as Tonal Value Increase. It is a phenomenon that causes printed materials to look darker than intended. It is caused by halftone dots growing in the area between the original printing film and the final printed result. Dot gain calculation is an important part of a CMYK color model. Dot gain is also a very important parameter to define and include in the control strip. In order to keep the pressrun un- der control, dot gain must be measured in one or more tonal patches. It is a common practice to measure a 50 percent dot, but it is also common to measure 40 percent and 60 per- (+-‐0 .5) (+-‐0 .5) (+-‐0.5) 1.4 -‐ 1.5 1.35 -‐ 1.45 0 .95 -‐ 1.05 0 1.45 1.4 1 1 1.4 1.35 0.95 2 1.4 1.45 0.95 3 1.4 1.35 1.05 4 1.4 1.45 1.05 5 1.5 1.35 0.95 6 1.5 1.45 0.95 7 1.5 1.35 1.05 8 1.5 1.45 1.05 Image 1 38 FLEXO MARCH 2014 www.flexography.org