Refinements to color management in the digital age
have made many tools available to the narrow web converter to achieve perfection in proofs and on press.
By Liam O’Hara
At this point, “color management” is something that most print professionals are aware of, but they might not be using it or be certain of how to implement it. Of course, printers have been “managing” color for decades, one way or another as they print images, but the current term refers to using press, monitor, scanner, camera and proofer profiles derived from colorimetric data to control color throughout the graphics workflow. This article presents a brief overview of the benefits of a color management system and the requirements to implement color management in a printing facility, as well as the answers to some commonly asked questions.
The main advantages of a color managed system are being able to accurately predict color (to make proofs you can match on press) and achieve that color quickly with minimal waste and setup time. However, the whole thing hinges on having as little variation in the process as possible. Consistency is the key to the whole ballgame. Before a color management system can even be considered, systematic process control must be in place. If your presses and operators are unable to print consistently, the whole effort will be a frustrating exercise in chasing your tail.
For the narrow web printer, process control begins with your suppliers, ensuring that they are delivering a consistent and uniform product. Papers and films should be inspected to ensure that they have the same print characteristics: whiteness, clarity, smoothness, hold-out, surface treatment, etc., based on your printing needs. Inks should have the same density, hue, color strength, and print performance criteria. Your suppliers should be able to document quality checks on their products. If you receive your plates from a trade shop, they should also be able to demonstrate that the plates are made the same way each time, with consistent relief and dot structure. If you make your plates in house, measure the plates you produce. Know what the dot size for your control targets should be on the plate and ensure that it measures the same every time.
At the press, operators should follow standard setup procedures. Blades should be inspected, pH and viscosity must be adjusted to meet specifications, anilox rolls must be cleaned thoroughly after each run to ensure that the cells stay open. Of course, not every label printer follows the same practices, but it is important that all operators within the plant use consistent procedures in order to reduce variation. The FTA’s FIRST document (see sidebar on opposite page) provides recommended procedures that you can adopt if they suit your process, or you may have already determined what works best for you. But if the first shift operator sets up one way and the next person on press sets up differently and runs to his own preferences, consistency is gone and you’re doomed before you’ve begun.
Once you’ve ensured that you can print in a consistent, repeatable manner, you’re in a position to implement a color management system. A color management system is composed of a few essential components: device profiles and what’s called the CMM, or color management module. The CMM is the calculator that uses the profiles to make conversions from the RGB pixels of a digital image to the CMYK dot percentages of a printing press or proofing device. But first, let’s consider profiles.
Generally speaking, profiles are simply big look-up tables that relate how a press, scanner, camera or monitor creates color (RGB pixels or CMYK dot percentages) to the actual color that is produced. For instance, a press profile is made by printing a target composed of specific CMYK combinations and then measuring the color of each swatch on the target. The color can be specified through CIELAB, as shown in Figure 1. A spectrophotometer is most commonly used to measure the color. The targets may contain well over a thousand swatches, which sounds tedious to measure, but many instruments are available to measure these in just a few minutes. Since there is always some variation in the press, measure several sheets throughout the run and average them together before using the data to make a profile.
Figure 1. CIELAB is a three-dimensional color model that allows one to measure and specify color using three axes: L* (light/dark vertical axis) a* (the red/green axis) and b* (the blue/yellow axis).
Figure 2. Each orange block has the same CMY build but with different profiles assigned. In this way, Illustrator is simulating the appearance of this CMY build on different presses/proofing devices.
The other great thing is that once an image is set up, the profiles can be used to ensure that the color stays the same even as you switch from the RGB of the image file to the CMYK of your press. (Actually, that’s an overstatement: The color conversion is limited by the range of colors you can print. Some colors of the RGB file may be outside your range, or color gamut, in which case the profile will attempt to deliver the closest match.) If you have a profile for the scanner (or camera) that captured the image, then the computer “knows” what color the pixels represent. If you have a profile for the press, the press profile can tell you what CMYK combination will hit that color on press. Your computer, be it a Mac or a PC, has its own CMM in place (it may have more than one to choose from). The CMM is the calculator that puts the two profiles together to make the switch from RGB to CMYK. In this way (the “convert to profile” command in PhotoShop, or perhaps in the RIP for the imagesetter/platesetter), the CMM can provide you with color separations for plate making that should come quickly up to color (provided all the press variables – substrate, ink, anilox, and setup – have stayed the same).
The CMM does more than convert RGB to CMYK. It can also help you hold a color match as you make a proof. Since the press profile can tell the CMM what color the CMYK dots on the plate will produce, the profile for the proofing device can tell the CMM what dot percentages need to go to the proofer in order to make that same color and simulate the press performance.
Color management offers another tool when creating color separations. Using gray component replacement, or GCR, allows the prepress operator some choices about how to create the separations. Most profiling software allows the user to determine how much black is used to create the image. One can limit the black to dark shadows (a “skeleton black”) or allow black to create all the tonality that creates the shape and detail of the image – or just some of it. The result is that you can print the same image using different strategies – for instance, if you can’t break the halftone black and the solid coverage black onto different plates, you may choose to limit black to the shadows so that you can run a heavier coverage for the solids without making the image “go dark.” On the other hand, if you can allow black to carry the image detail, then you can have a much more “color stable” image. To illustrate this, picture a label depicting bread. Magenta and yellow create the main bread color, a golden color. Cyan is used on top of the magenta and yellow to tone the image and create shadows and texture. Now if the cyan density rises for some reason, the bread gets blue spots – moldy bread. If black were used to create the shadows instead of cyan, the image might get darker, but never get moldy (Figure 3). There are also benefits of using less ink (black instead of building shadows with CMY) and more forgiving registration (most of the detail is in one plate). Profiles make it much easier to “engineer” these choices to help make the job run easier.
This quick overview may make the process seem simple (print a target, measure the patches), but there are a couple of catches. For instance, most profiles assume that you will print screens all the way down to 0 percent, which is a real mistake for flexo – you need to ensure that a minimum dot is assigned to the image to ensure that no harsh highlight breaks occur in the print.
Also, never forget that for the profile to be precise, it is important that all variables stay the same. Obviously, if you change the anilox roll volume, the press will print differently. If it is printing differently from the press run that was used to create the profile, the profile will fail to give the right CMYK combinations for the new ink volume, and the color won’t match your expectations or the proof. If you change the ink set, then the color won’t match. Even simply switching the printing sequence can throw the profile off. Consistency is paramount to success. This means that if you use different substrates, and different anilox rolls, and different inks, you must have profiles for each condition. So that can be a big investment in test runs and measurement. It is without a doubt an investment, but if color fidelity is important, the investment is necessary for success.
Speaking of investment, what do you need to implement your own system? First, the QC system and standard operating procedures must be in place to minimize variation. A spectrophotometer is needed to gather the color data for the profile. Spectrophotometers run anywhere from $3,000 and up, depending on the degree of functionality you require. Some spectros can measure print and the monitor, others only print. Some measure point-by-point, which can require an hour or more to measure the target, others can scan the target in a matter of a few minutes. Profiling software packages are available from a number of companies, and can cost from $3,000 to $7,000.
One thing of interest to packaging printers is that a few profiling systems allow you to make custom profiles employing spot colors as well as process colors. Of these, some require that you run profiling targets specifically for those inks; others, like the ProfileMaker 5 Packaging Suite from X-Rite, allow you to substitute colors into previous profiles. In other words, you can take an existing CMYK profile and tell it to use Pantone 186 Red instead of magenta. Remarkably, it can recalculate a new profile with considerable accuracy. However, the use of spot colors leads to another catch for a color managed system: creating proofs for spot colors.
Figure 3 shows an RGB image reproduced with a UCR Skeleton Black (top) and a 100 percent GCR full black (bottom). The UCR image has most of the shadows created with cyan, whereas the GCR image has the shadows built primarily out of black with no cyan.
Last, if you are looking at implementing color management, work with your vendors to help you set up your system. This overview simply presents some general trends and practices; you’ll want an experienced hand to guide you. If you use a trade shop for your plates, they should be well versed in color management prepress. Your ink vendors should be knowledgeable of color management practices as well, and there are numerous consultants that can assist.
Of course, training is also available from several colleges and universities. Clemson offers hands-on color management training specifically for narrow web flexography on a regular basis, as well as a wide variety of classes on flexography and brand color management. Visit http://graphics .clemson.edu/itc for information on current programs.
If you want to read more about implementing color management, Understanding Color Management by Abhay Sharma and Real World Color Management by Bruce Fraser, Chris Murphy and Fred Bunting are excellent books on the subject.