Streamlining Color Consistency Part 1: Getting a Handle on the Variables

Printing high impact graphics is a goal of any and all print service providers (PSPs). The wide array of choices in wide-format digital printing devices present many opportunities, but also present some challenges. Color not only needs to be high impact, but it must also be consistent: consistent within a single print, throughout a print run, from one run to the next, from one device to another, and even from one plant or PSP to another. Color must also be accurate to an agreed upon color target, be it for the process color imagery, or brand/spot colors. Understanding the variables and knowing how to address them while streamlining color accuracy and consistency are critical for PSPs looking to grow their market share and opportunities.

In this first part of a two-part series, we’re going to review the variables encountered with wide-format printing that impact color. Then, in the second part, we’re going to provide an overview of strategies and tools that will help you control color across your devices and your operation.

Variation, Deviation, Consistency, Accuracy, and Precision

Before getting into the weeds of variables that impact color, it is important to first understand the types of color variation, because doing so will help direct you on where to address the situation you might be facing. You must first understand the target, or reference, you are shooting for. You must then manage the printing process so you hit that target. And then you must maintain that color throughout the run, and run-after-run — this is where the concepts of accuracy and precision come in. Both are born out of measurement systems where sample measurements are taken, then compared to a reference value. But the concepts can apply to situations that are assessed subjectively as well, like color.

To understand the difference between accuracy and precision, think of a dart board. When the darts are thrown, there are two aspects to evaluating where they landed. The first is how tight the grouping of darts is — precision; and the second is how closely they land in relation to the bullseye — accuracy.

To understand the difference between accuracy and precision, think of a dart board. The bullseye is the target. When the darts are thrown, there are two aspects to evaluating where they landed. The first is how tight of a grouping of darts, which is precision (or think of it as consistency). The second is how closely they land in relation to the bullseye, which is accuracy. Ideally, you want both: a tight (precise) cluster of darts very close (accurate) to the bullseye. But you can have neither or a combination of the two. And depending on the results, you will need to adjust your process accordingly.

Maybe there’s a systemic flaw in the mechanics of how you throw the darts; you just can’t throw a dart the same way twice. This, again, speaks to precision. Or maybe you have issues with how you aim the darts, which speaks to accuracy. Another example would be the scope on a rifle. You may be the best shot from a precision, or grouping, point of view, but if your scope is not calibrated to the rifle, you could be well off the target, or inaccurate.

Bringing these concepts into the wide-format printing world, think of a situation where print after print is showing a fair degree of color variation. That’s an issue of precision, or consistency/repeatability. There may be issues with the heads, or nozzles, or any number of things that render the device not repeatable. But if the consistency from print-to-print appears to be good, yet all of the prints don’t match the agreed upon target, then that’s an issue of accuracy. There could be a mismatch between the profile used for the production run and the target condition. Even if the proper ink set is targeted in a profile, maybe the white point of the substrate is different between the profile and substrate. Or there could be calibration or other issues. But it’s always the fault of prepress — remember that.

Understanding the Big Variables

Unlike our friends in the conventional print process world (offset, flexography, gravure, screen), where there are so many process-specific variables to tackle, many of the variables — and the control of those variables — in the digital printing world, especially inkjet, are engineered into the hardware and software. That’s not to say there aren’t still variables and user-based actions that won’t affect the print output, but they are far fewer. For the purposes of this article, we’re going to focus mainly on what could affect accuracy, or achieving the desired result, in comparison to the target, or reference.

A 2D plot (in the L*a*b* color space of a bunch of profiles. These represent a variety of devices, ink systems, and substrates, and the difference between them is significant.

There are two primary components used when trying to describe, or characterize, a device’s color: gamut and profile. Color gamut describes the range of color a particular device can achieve on a specific substrate with a particular ink system/set of colorants. Color profiles are actual files (in a standardized format) that mathematically represent the color output of a condition, where a “condition” is a combination of substrate, ink system, and printing technology. Profiles utilize the L*a*b* color space as a connection between CMYK input and color output. They are used to convert between color spaces (like from an RGB digital photo to CMYK) and to color manage devices so their color output is accurate to a target condition. I can’t emphasize this latter part enough. The profile really serves as the proverbial “stake in the ground,” and again, is supposed to accurately represent the color output of a particular device (condition). It also feeds process control efforts, serving as the target/reference condition when evaluating actual print output against the target condition. More on this in Part 2 of this series.

To illustrate how color gamuts and profiles can be used to represent various print conditions, take a look at the plot in Figure 1. Here we have a 2D plot (in the L*a*b* color space; just the a*b* slice) of a bunch of profiles. These represent a variety of devices, ink systems, and substrates.

The difference between all these profiles is significant and would have a huge impact on color. For example, if the same graphics file were output on the devices these profiles represent, there would be an inordinate amount of color difference between them, which would certainly be objectionable to a customer.

There are differences in chroma (or saturation), hue (color shifts), and lightness. Managing color across various devices (with their own, native, and unique output) is critical to achieving color consistency between them, which we will also address in Part 2.

The Importance of the Right RIP

RIPs, or Raster Image Processors, are effectively the hardware (or software) that processes a graphics file, applies color management, screens it (if necessary), and processes it to a bitmap for output. They can drive platesetters, digital presses, and/or proofing systems (and probably more). The reason RIPs are called out here is because of the large impact they have on color output.

RIPs are like language interpreters, where they take input from a graphics file, then process and interpret what’s in the file (be it Postscript, PDF, etc.) and render it for output. But unfortunately, not all RIPs perform the same way. Given the same input file, you may get different results out the back-end of a RIP. In the earlier days of Postscript, it was not uncommon to have differences in text-wrapping, line breaks, overprints, gradient rendering … all sorts of things. So when you had one RIP for, say, a proofing system, then another RIP for platemaking, you could have differences between the two that, if not caught, could turn into a nightmare on press and the potential for wrongly printed jobs with rejects and unhappy customers. Not good. Times are much better now, but we’re still not entirely out of the woods. Many of the glaring differences in output have been eliminated, but there still remain differences in how various RIPs render color.

And many RIPs are married to specific devices, meaning you don’t necessarily get to choose the RIP. So, if your shop has multiple wide-format devices from different manufacturers (and even sometimes from the same manufacturer), you are most likely using different RIPs, and they may lead to differences in color rendering, which can be tricky when trying to load-balance a job and run it on multiple devices. The best solution is to use the same RIP for multiple output devices, if possible.

In Part 2 of this series, we will review the various tools, solutions, and practices to help you streamline your workflow so you can optimize and achieve accurate and precise color across your devices and your operation.