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Color Tolerances, Process Control, and Managing Expectations
September 1, 2008 By: Jay Sperry Paperboard PackagingIntegrating a color management system from pressroom to customer requires several viewpoints to be considered, each with its own opportunities, challenges, and payoffs. At the heart of an ideal color management system is a well-connected, implicit workflow.
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To address the creation and definition of color standards, several entities need consideration. The primary stakeholders are the customer, designer, prepress, ink maker, and printer/converter.
In almost every case, the farther upstream a universal color definition is placed, the better the predictability and transition across stakeholders. Ideally, corporate or brand colors can be transitioned across media and vendors, while maintaining color consistency. This requires special attention to end use factors such as substrate and printing systems, supporting the need for a defined workflow and communicated expectations.
Let's start using an easy to understand, tangible method for defining the "root" color. This system would use formulated press-ready ink, proofed using a repeatable drawdown method on substrate identical, or closely replicable, to the end use packaging material. The assigned marketing group or appropriate decision makers can use multiple ink formulations, substrate choices, and optimum combinations of both for color evaluation. A pre-existing, printed sample of ink-on-substrate also would fit into this model of determining the root color.
A second method, more commonly used, is the use of a mass-produced, categorized color system such as the Pantone Matching System, or PMS Book. This color system fits well with many front-end work flows and customer evaluations. The main drawback to the PMS system for printers is the implementation of colors across the diverse substrates and printing scenarios that we encounter.
Once the color is decided upon, it is crucial to translate that subjective decision into a permanent, numerical definition for the manufacturing process. Objective color definition methods are almost always measured as spectral values, 30-35 reflective color measurements, but usually are computed to LABCH for communication. It is important to remember that LABCH values are dependant upon the illuminant and observer specifications, commonly D50 and 2º. A frequent color communication breakdown occurs between the ink manufacturer or blender and the production floor, where the ink room uses D65 and 10º for example.
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Specifications
Color measurement specifications such as the above mentioned scenario must be addressed and communicated between parties to avoid confusion and color matching mistakes. Objective color definitions are used in two primary stages of the package printing cycle: the customer expectation or contract proof, and in the pressroom for quality control. In order to evaluate the degree of accuracy of the spot color reproduction, a color tolerancing system needs to be established and understood.
Mathematical formulas have been created to calculate the distance, or Delta (Δ), between the reference color and the sample in an ordered color space defined as CIE LAB. There are several formulas to use depending on the application, ranging from "straight" Delta E (ΔEab), to Delta E CMC (ΔECMC), to the most current model Delta E 2000 (ΔE2000). Complex mathematical formulas are intended to make a ΔE unit of 1 an equal visual perception difference throughout the range of hues, chroma, and lightness (LCH). These formulas will continue to evolve as more research is conducted.
Just like the spectrophotometer settings, the formula or method used for color tolerancing must be communicated to all stakeholders. When a color expectation is set either with a color chip, PMS swatch, or contract proof, the color must be measured and certified against the "root" color reference. Typically a ΔE2000 of 2 is used in this step of the workflow.
The color reproduction cycle now moves into the pressroom, and color tolerancing is used to certify print sample color. In the pressroom, color control often becomes difficult due to the quality and consistency of input variables. For instance, if the substrate has a ΔE of 1.5, and the incoming ink is held to a ΔE of 1.5, and the anilox roller has a ΔE of 2 across the width of the roll, normal pressroom variation may be as high as ΔE of 5.
Normally on a press, these incoming variables along with ink maintenance, prepress, and impression establish the color tolerancing expectation. The best way to understand variability is to monitor process control targets throughout a long production run. You then plot the Delta E over time, and begin to develop a benchmark for color consistency. Try to itemize the variables that are causing color inconsistency, and develop systems to measure and analyze those variables independently, such as incoming substrate, ink and plates.
Determining and establishing the correct target color values for a color management system is a crucial, often difficult, piece of the puzzle. We cannot make assumptions that all parties in the supply chain of a printed package understand what "root" color to aim for. We cannot correctly benchmark the efficiency of a color management system until all parties are aware of the customer's intent, and are properly equipped and trained to manage color consistently.
I recommend as you evolve your color measurement and management workflow, begin by using numerically defined target colors for any color, process or spot, being used in production. Measure this color in routine intervals during production, and get comfortable recording and communicating Delta E and color tolerances. At this step, you also can begin developing troubleshooting techniques based on the numbers. Fortunately, there are numerous tools recently released to allow automatic input of color data into sophisticated software to streamline the measurement, analysis, storage and communication of process quality control.
Once a workflow is built and tested for solid color monitoring, it is time to refine the workflow to include tonal targets. The easiest place to start is with the mid-tone dot reproduction. As screens, vignettes, and images are used in packaging, it is vital to also set expectations and control around color reproduction of the tonal range. When adding dots, required for halftones and screen tints, you add several key indicators for print consistency.
From a workflow standpoint, now there must be clear communication between the pressroom and prepress, resulting in a dot management system. It is good practice to standardize a process control system that uses both the solid swatch and one or more tint swatches, ideally with flexography at the minimum dot and a mid-tone (30 percent) swatch.
In order to use these tint swatches for the color management program, an objective reference color definition must be established. This is the most difficult part of the workflow. An example is a 50 percent screen of Clemson orange.
If I design a graphic area with a 50 percent tone, I need to understand what that color will look like on press. I need to understand how specified tones with a defined printing system reproduce. I also need to understand the intended color of that 50 percent tone, this being the most under-researched part of this workflow.
Assuming my visual decisions of that 50 percent tone are for it to print to a 70 percent (default Photoshop and Illustrator settings), I now need to assign the correct printing plate tonal value, to print to a 70 percent. I then can publish a color definition for a printed 70 percent tone of Clemson orange. So how can I find what a 70 percent tone of Clemson orange looks like? I can either use a small proofer that uses flexographic plates, or I use a press test or printed sample that has a range of defined tones.
It is important to remember that a color management system evolves over time, one step at a time. The best results are found when a team of stakeholders has regular discussions on current issues, challenges and future implementations.
Unfortunately, color control does not happen in a week, month, or even a year. It takes a long-term commitment, with measureable progress and success being made at routine intervals. In the end, color management allows presses to have less downtime in makeready and production, better agreement between prepress and the production floor, smoother communication and proactive solutions with the end customer, and a more trained, quality oriented, efficient work force.
