You Can’t Control What You Can’t Measure

The role of color process control plays in enhancing reproduction quality and productivity in package printing cannot be overstated. Without appropriate process control, costly rework, waste, and unnecessary delays in production can result, potentially undermining the salability of the product. However, as the old adage says: you can't control what you can't measure. In the pressroom, a variety of measures can be applied to calibrate the output of computer-to-plate systems and minimize variation on press. These include statistical sampling, visual inspection, density spot checks, or full sheet scans, depending on the customer's expectations and the nature of the job at hand.

Materials created in the prepress department, however, require the use of robust, reliable measurement devices designed to detect and measure color changes at various stages in the workflow before problems wind up on press. Generally, four types of instrumentation measure these parameters:

1) Densitometers: for pure CMYK analysis/ink density measurement and determining the strength of a color based on the ink film thickness.

2) Colorimeters: break down color into numeric values using the CIE XYZ color space or one of its derivatives, such as CIE L*a*b or CIE L*u*v.

3) Spectrophotometers: measure data within the color spectrum (spectral data), are capable of gathering the most complete color data, and provide the most accurate and useful color information. Brand and special spot colors require analysis with colorimeters or spectrophotometers to ensure the best possible match.

4) Plate analyzers: measure for dot area, screen ruling, dot size, and screen angle.

Properly applied, these solutions substantially reduce, and in many cases virtually eliminate, the need to absorb the cost of rejected print jobs and missed customer deadlines caused by subjective color shift, dot gain issues, or plates with faulty compensation curves.

How they work

Instrumentation converts the visual appearance of color into numerical values that can be accurately communicated anywhere in the world for any printing process, independent of substrate. Although handheld devices work differently from integrated color profiling tools, both are needed—color profiling to provide a "fingerprint" of the printing process, and handheld instruments to standardize the process across the workflow and for other purposes. Neither is an appropriate substitute for the other.

"The experience of color is subjective, no matter how trained the eye becomes," explains Chris Wallace, global accounts representative for X-Rite, Inc. (www.xrite.com). "Visual assessment is needed for the evaluation of color, but color communication is accomplished digitally." X-Rite is the developer of the Color Exchange Format (CfX), a file format designed to accurately and unambiguously communicate all commercially relevant aspects of color across devices, applications, and geographies.

In the packaging world, establishing the customer's or brand owner's expectations is the first step to matching color. "Some use a tight tolerance and some are very loose about it," Wallace explains, "but instrumentation can't tell you how to feel about a color. That can only be done by asking the customer to assess the printed matter inside a light box to determine which samples are pleasing and which are not." Once the samples have been winnowed down, instruments are employed to document the L*a*b value of the sample against the L*a*b of the reference to determine the deviation of the color from the standard (delta E).

Don't forget maintenance

But, how do we know that the selected measurement device can achieve the desired level of accuracy? Regular calibration is key, Wallace says. X-Rite offers the NetProfiler system that enables users to automatically test, measure, and profile their instruments over the Internet. Using sophisticated software and certified physical standards, the system reportedly takes just minutes to produce performance statistics on every instrument within a network. As an alternative, he adds, customers can have their instruments re-certified annually through on-site services provided by companies such as X-Rite. Best practices for the optimum use of measuring devices call for measurement conditions to be set to standards regarding luminescence, measurement angle, and substrate and other parameters.

Wallace considers measurement techniques in printing and packaging "pretty well standardized" by now, and said he expects the future will bring the development of more integrated, open-loop scanning systems. pP