Color Management Basics, Part 1
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Color Management Basics, Part 1

We all know the challenges of poor or no color management. Inaccurate color, heavy ink consumption, inconsistent color and printing artifacts are just a few of the challenges you may face.

By Ray Weiss, Digital Imaging Specialist, SGIA

This series of four articles will delve into the basics of color management, including some basic color theory.

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  • Color, it's everywhere we look. Think of that exotic car (it's red, of course), to the vibrant green of a lush rainforest, all the way to the deep blue of the ocean. Your assignment is to print these memory colors onto hundreds of different surfaces while using a myriad of different processes, oh, and have them all look the same. Piece of cake! Right?

    This installment will define common terms, cover the color management pyramid and suggest tools that you should have to take the guesswork out of the process. Reading this will not make you an expert, but it will get you started down the road to better color, and it will help you improve your processes so that you move towards better, more consistent and repeatable color.

    To start, color management is the process of translating how a color is depicted (proof, digital file) to how it is reproduced by various output devices. The unique color characteristics of the output device are mapped into a profile that is then used to translate the input color from the proof to the color gamut of the output device. Color management is a crucial process when interpreting colors that we can "see" to colors that we can print.

    Before moving forward, one thing to remember is that color theory is just that, theory, meaning you will run into many different opinions on what should be done to get the best color. The goal of this series is for you to come away with a process that you follow, and an understanding of why you are doing it. If you do that, you will be ahead of many shops that rely on troubleshooting as their color management practice.

    The previous color examples of the red car, the green rainforest and the blue ocean all bring to mind the RGB (Red/Green/Blue) color model. RGB is an additive color model in which each element of RGB is added until all three are at peak levels and white is produced. Think of your computer monitor, which when powered down is a black surface. When the power is turned on and an image is displayed, the phosphors of RGB begin to glow in differing amounts of intensity. When all three are fully on, white is what you see. RGB is a device-dependent color model. An RGB monitor can display 16.8 million colors because each element (RGB) has a value from one to 255. Once you do the math, 255 x 255 x 255 means you have the potential for 16.8 million colors. To define the same color across these devices will require some type of color management.

    How We See Color
    In the human eye, vision is based on signals that the brain receives from the rods and cones at the back of the retina. The human eye contains about 120 million rod cells and 6 million cone cells. The rod cells are extremely sensitive and are almost entirely responsible for your peripheral and night vision, and they play almost no role in seeing colors. Cone cells, on the other hand, function best in brighter light and are responsible for color vision. The cone cells in our retina are sensitive to light in a small region of the electromagnetic spectrum that is known as visible light. Visible light corresponds to a wavelength range of 400-700 nanometers (nm), which is the color range of violet through red. The cone cells act as receivers that have been tuned to this slice of the spectrum, and they are responsible for sending signals to the brain. The brain then translates them into the color that we see. The average human eye can see around 10 million colors.

    In contrast, CMY is a subtractive color model. In this model, you start with a white surface and add each of the colors until all three are at 100 percent, which gives you black. If you're thinking, "Wait a minute, you get a muddy mess at best and not black", you are right. Due to impurities in inks, you need to add black or K in the typical CMYK model to get a true black. A CMYK printing device can reproduce around 35,000 colors.

    It's important to note that in the additive or RGB model, the intersection of the colors creates the subtractive colors. Therefore at the intersection of Red and Green you get Yellow, Red and Blue makes Magenta, and Blue and Green makes Cyan. Conversely, in the CMY or subtractive model the intersection of color creates the additive colors. This means that the intersection of Cyan and Magenta makes Blue, Cyan and Yellow makes Green, and Magenta and Yellow makes Red.

    We know that colors that we "see" are really signals from the eye to the brain, which then constructs those colors and images. Fundamentally, color needs three criteria, a light source (think of the sun, a light bulb or the flame from a candle), an object (think of a lemon or that red car), and an observer (think of your eye or a spectrophotometer). Light is either reflected or absorbed by the object, and then is "seen" by your eye or a spectrophotometer (think of an i1 from X-rite* or a SpectroPad from Barbieri*).

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    For those of you taking note of some of these numbers, you would have realized that our eyes and different RGB devices can take in, i.e. see millions of colors, and yet our CMYK printing processes are limited to reproducing around 35,000 colors. So, how do you deal with that huge discrepancy in color? Color management!

    Defining Terms
    To prevent confusion and misunderstandings, it's critical to define the terms that will be appearing in this series.

    Color Gamut: The range of colors that a particular device can display or print.

    Rendering Intent: The process used to take colors that are outside of the color gamut of the device and find a way to translate the difference to a range that the device can reproduce. There are four main rendering intents, and understanding what they are doing in the conversion process is crucial. Let's start with the saturation intent (probably used the least in our industry). This intent tries to produce vivid colors in an image at the expense of color accuracy. The saturation intent is suitable for business graphics like charts, where brightly saturated colors are more important than the exact relationship between colors.

    Next up is the perceptual intent. This intent aims to preserve the visual relationship between colors so that they are perceived as natural to the human eye, even though the color values themselves may change. This intent takes out-of-gamut colors and moves them to the closest area on the gamut, and may then move in-gamut colors so that the relationship is preserved with those out-of-gamut colors. This intent is suitable for photographic images with out-of-gamut colors where accuracy of saturated colors is not important.

    Then you have the colorimetric intent (this one comes in two flavors, absolute and relative). Reproducing an image colorimetrically means having colors mapped as accurately as possible. Out-of-gamut colors are "clipped" as they are brought to the edge of the gamut of the printer. Absolute colorimetric is used for proofing. Out-of-gamut colors are shifted to the gamut edge, leaving colors inside of the gamut unchanged. This intent will simulate the paper color, if necessary, by adding a "tint" of that color. The aim is to maintain color accuracy, though it comes at the expense of preserving relationship between colors. Relative colorimetric compares the extreme highlight of the source-color space to that of the destination-color space and shifts all colors accordingly. Out-of-gamut colors are shifted to the closest reproducible color at the edge of the gamut. Relative colorimetric preserves more of the original colors in an image than perceptual, and relative colorimetric is the standard rendering intent for printing in North America and Europe.

    Keep in mind that many of the popular RIP software will use a rendering intent that they have determined to be the best for their software, not necessarily the best for what you are trying to accomplish. Find where these settings are located within the RIP software that you use, and experiment with the different intents to see how it affects your results. Also be sure to develop a test print that has a mix of vector and raster images so that you have something to use for an evaluation tool as you work through your color management settings.

    ICC Profile: An ICC Profile is a snapshot in time that includes the printer and inks, the media or substrate used, and the RIP software. The current ICC profile is a set of data that characterizes a color input or output device, or a color space. The profile defines a mapping between the device source or target color space and a Profile Connection Space (PCS). The PCS is either CIELAB (L*a*b*) or CIEXYZ. Think of a map that serves as a guide for color.

    The ICC (International Color Consortium) recently unveiled iccMAX, which is a new color management system that allows for flexibility in the PCS. The current ICC v2 and v4 profiles have fixed D50 colorimetry (defines the illuminant condition) and they use CIE 1931 as the Standard Colorimetric Observer. For more information on this new reference condition, visit www.color.org.

    L*a*b*: L*a*b* is a device-independent color space and includes all perceivable colors. The easiest way to describe it is to think of the L as the lightness axis (L* of 0 is the darkest black, and L* of 100 is the brightest white). Next is a*, which represents the axis of green and red with a* of 0 being a neutral gray (negative values along the a* axis are green, and positive values represent red). The other axis is b*, which is the yellow and blue axis. Remember b* of 0 is neutral gray, and negative b* values are blue and positive b* values are yellow.

    Metamerism: This term is used to describe a situation where two color samples, when viewed with one light source, do not match when viewed with another light source. The reason for this is pretty obvious when you think back to the three criteria of color, a light source, an object and an observer. If the light source changes, it would stand to reason that the color would almost certainly change, which is exactly what occurs with metamerism. You can combat metamerism in a couple of different ways. One is to have a standard light source for viewing proofs, the only problem is that this does not address the metamerism that will occur when your customer takes their print from your calibrated light booth with D50 lights back to their store using D65 fluorescent lights (or even worse, incandescent lights). By the way, D50 and D65 are just two measurement standards for lighting and the "temperature" of the light that is emitted. D50 is defined as daylight or Kelvin 5000, and D65 daylight at noon or Kelvin 6500.

    The other thing you can do to help with metamerism is to understand GCR (Gray Component Replacement). GCR is one of the last steps in creating a profile that tells the RIP where to start black replacement for CMY in neutral colors. When you use CMY to create the neutral colors, the effects of metamerism are more pronounced, and substituting black ink reduces the effect. The downside is that you have to be wary of bringing black in too early, particularly with printers that use large droplet sizes, because you can create a peppering effect.

    What's Next
    In the next installment of this series, I'll cover more of the tools, spectrophotometers, primarily, as well as the color management pyramid. These articles will help you put a process in place to get you on the path to predictable, consistent and repeatable color.

    Ray Weiss, Digital Imaging Specialist for SGIA, recently joined the Association in 2014. He provides solutions and technical information on digital printing and digital imaging issues as well as digital equipment, materials, and vendor referrals. Ray started his career in offset and digital imaging after eight years in the USAF in the security field. His career began in the graphics industry with a typesetting and prepress business in Washington, DC, which grew into an offset print operation in Maryland. He then moved into sales, training, support, and service in the wide format industry for over 10 years. Ray has extensive experience with many of the major RIP software and printer manufacturers, and worked closely with the Smithsonian Institution to implement a color managed workflow in their Exhibits department.

    This article appeared in the SGIA Journal, March / April 2016 Issue and is reprinted with permission. Copyright 2016 Specialty Graphic Imaging Association (www.sgia.org). All Rights Reserved.

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