For how many years now have we heard the request from brand owners for a more colorful image in a printed product? We live in a world of color, and I would argue that just about everyone prefers a more colorful image when possible, as long as it doesn’t look fake or artificial. Think about colors in the real world. With spring comes so many wonderful flowers, plants, and foliage.
The natural saturation, or richness, of color is well beyond what we are capable of reproducing with any print processes, especially conventional ones (offset, flexo, gravure, and screen). We then take pictures with an RGB-based sensor in a digital camera and view those images on an RGB display, both of which are not capable of fully capturing the richness of color in the “real world.” We’re now one step removed from the true colorfulness of the original subject, but it’s still typically very good. Then, to reproduce those images in print, we go to our old friend, the CMYK-based printing process, the process with arguably the smallest gamut (or range of reproducible color) of all color reproduction processes.
With cost always being an issue, ink suppliers are pressured to keep their prices low, so they are forced to use the least expensive pigments they can find, which are usually the least colorful ones. But even if cost was not an issue, the range of color possible with CMYK is smaller than RGB, and even smaller than that of the natural world. We’re now two steps removed from the original color. But CMYK, to its credit, does a fantastic job of reproducing color very effectively and efficiently. To think that just three colors, or crayons (the chromatic primaries CMY), can make the full range of color that we enjoy is amazing!
The trouble comes when we start to compare things to each other, whatever they are. If all we ever did was look at color output in a one-off fashion and not compare it to something else, our lives would be so much easier. But that’s not the case. We value color consistency, so we need to manage color and our expectations across a wide range of output devices, substrates, and, ultimately, printed products.
Digital printing brings a number of interesting prospects. We know how effective and valuable the technology can be with shorter runs, quicker turnarounds, and print customization. But digital printing also typically brings with it a larger range of reproducible colors (or gamut), which gets us a step closer back to the original color that we desire. However, it’s different than the output of conventional print processes, so it challenges that consistency we supposedly value so much. And when printed products are produced across an array of print technologies, the desire is usually to have the color match across all of them, especially when it comes to brand colors.
Before we talk about how best to achieve that consistency across multiple platforms, I would challenge and encourage you to think about taking advantage of larger color gamut devices where and whenever possible, as long as it doesn’t create problems (think about situations when printed products are next to each other versus when they’re not). As long as you can maintain the hue of the desired colors, more saturation is usually not a bad thing. More on that later.
Let’s take a look at the gamuts of both what’s possible with some digital printing processes versus a conventional printing process, and see how they compare. For illustrative purposes, I’ll be using a newer profile developed by Idealliance, called “PrintWide,” that was designed to encompass the gamut of virtually all inks or colorants in all known color printing systems. While there isn’t one printer or system that will reproduce all of the colors in this gamut, it represents what is possible across a range of digital printing systems, especially inkjet.
Figure 1: 2-D and 3-D plots of “PrintWide” and CRPC6 profiles/gamuts.
As shown in Figure 1, PrintWide is compared to CRPC6 (Characterized Reference Printing Condition #6), which represents conventional offset printing on premium coated paper (or what has traditionally been referred to as GRACoL).
These plots show the outer boundary and area/volume of the two profiles. As you can clearly see, the range of color from the PrintWide condition is significantly larger than that of the conventional offset color space. Now, what does that look like with an actual image?
Figure 2 attempts to show the difference between the two profiles. This difference is simulated here because the print version of this article has been printed via web offset with most likely a Grade 3 paper, and for the digital edition, it’s whatever color you’re looking at, be it a computer display, phone, or some other emissive device. The differences are very apparent, mainly with saturation of color.
Figure 2: Shown above is an RGB image converted to both PrintWide (top) and CRPC6 (bottom) color gamuts.
However, some of the colors do appear to be pretty consistent between the two images. Keep in mind that colors within (or inside) the gamut of the smaller-gamut system will usually reproduce consistently between the two, assuming the profiles are similar from a hue point of view. It’s highly saturated colors outside or near the boundary of the higher-gamut condition that result in differences.
But differences in saturation are not as objectionable. The oranges, reds, and blues show this especially well. They are colors that live outside the boundary of the CRPC6 gamut, but within the boundary of the PrintWide gamut. Hence, the difference in color between the two. But, again, colors that are within the smaller gamut system can be reproduced very accurately by both systems.
Process colors and spot colors present different challenges when looking to achieve color consistency across multiple platforms. All print output devices will utilize process colors to reproduce pictorial images and many other graphics, but not all devices will utilize spot colors. Where it gets interesting is when the color of the “crayons,” or base colorants used by the devices, are different.
Process Colors: In looking at process colors, recall the differences seen in the 2-D and 3-D plots, along with the visual difference between the two images. The primary difference between the PrintWide profile/gamut and the CRPC6 profile/gamut is in the base CMYK colorants and overprints. PrintWide is based on inkjet inks, while CRPC6 is based on sheetfed offset inks. Both are CMYK, but the differences are drastic. One is dye-based (inkjet) in most cases, and the other is pigment-based (offset).
The purity and chromaticity you can achieve with inkjet inks are phenomenal and extend far beyond what conventional CMYK inks can achieve. The result, then, is a significant difference in color when reproducing the same graphic (as seen in the previous images) via the two ink systems and processes, if allowed to print natively. And therein lie the challenges and difficulties in achieving a match between the two.
Let’s take a look at the six main chromatic colors in process color printing: cyan, magenta, and yellow (process color primaries), and red, green, and blue (overprint primaries). The table below shows the colorimetric values of these six solids as seen through the same two profiles used in the earlier example.
Zeroing in on Lightness (L*), Chroma (C*) and Hue Angle (h°) to evaluate the colors, the differences are very apparent. Of particular note are the differences (Deltas) in Chroma in all colors, Hue Angle for the red and blue overprints, and Lightness in the blue overprint. In all cases, the primary colors in PrintWide are much more chromatic than CRPC6. For the red overprint, in addition to the change in Chroma, the hue of the PrintWide red is 12 degrees more yellow (less blue) than CRPC6, which is very significant. And for the blue overprint, it is a much darker, deeper, and redder shade (over 16 degrees!) of blue than in CRPC6. These differences are then summarized with Delta E 2000, which shows overall color difference. In all cases, the Delta E differences are significant. These differences can also be seen graphically in the next image.
Figure 3: This plot shows the differences between PrintWide and CRPC6. In addition to Chroma differences, note the Hue Angle differences as denoted with the white lines.
In Figure 3, note the differences in the angles of the white lines that go through the red and blue overprints for each condition. These represent the Hue angle differences between the two profiles. Also note the differences in the length of the white lines between the two. They represent the differences in Chroma. What's interesting, and good, is how well the Hue angles of the other four sets of primaries/overprints line up with each other.
The bottom-line impact here is that while any CMYK build that is close to being a solid will be more chromatic with the PrintWide color space, the blues and reds will have a color shift between these two print conditions. And that could be objectionable. As mentioned before, when Hue angles can be maintained between print conditions, with the difference being mainly in Chroma (and hopefully minimal differences in Lightness) the visual differences, while probably somewhat noticeable, will not be as objectionable as differences in Hue angle.
Spot Colors: Spot color inks are most utilized by the conventional printing processes to help produce colors that are not achievable with process color inks. They provide a fairly simple solution and are much more color stable on press (as opposed to a process-color build of the same color, if achievable). They do require a dedicated printing unit like any other ink, but the benefits are great.
In digital printing, while there are exceptions where spot color inks can be formulated (some electrophotographic applications), most systems, especially inkjet, utilize a fixed set of inks that include CMYK and oftentimes additional inks (light cyan, light magenta, perhaps an orange, green, violet, etc.) in an expanded gamut approach to maximize the device’s color gamut. And even though a file may only contain CMYK, the devices will utilize all their inks to match the colors as best as possible, within their gamut capability.
The same approach is used with spot colors. Colorimetric (L*a*b*) values of the target color(s), as contained in the graphic file, are processed through the RIP, and the devices do their best to match the colors accurately. They should all hit the hue of the target color well; it’s just a matter of whether they can hit the Lightness and Chroma of the color as well.
Figure 4: The Pantone Coated color library plotted as points against CRPC6 and PrintWide. Note how many more colors are outside the gamut of CRPC6 as compared to PrintWide.
Returning to the two profiles we’ve been evaluating, Figure 4 shows a plot of the Pantone Coated Library in relation to the PrintWide and CRPC6 profiles.
For spot colors that are within a particular gamut, they can be matched well. But for colors outside the gamut, there will be compromises in color accuracy to varying levels, depending on how far out-of-gamut the color is. But again, if the differences can be limited mainly to Chroma, the result can be reasonably good. And just because a color is outside a particular gamut, it doesn’t mean it won’t be acceptable.
Given these differences across the various platforms of print technologies, what can one be expected from a color consistency point of view? Throw in the impact of the substrate on color (it’s considered the fifth color in process color printing) and metamerism (how different color pairs match under one light source, but not another), and we’ve got quite the challenge on our hands.
But there are some things you can do to help minimize the differences and achieve a very good level of common appearance across print technologies. There are two main ways to address the situation, depending on the print technology: G7 and color management.
G7 Calibration: G7, or “near neutral” calibration, as it’s referred to generically since there are other similar approaches, is the leading and most widely adopted press calibration methodology. It has been mainly applied in the conventional printing processes — where plate curves are part of the equation — but has applications in any and all process color (CMYK) printing systems.
The main premise is that it helps you achieve a “shared common appearance,” meaning a good visual match between different printing presses, print processes, and substrates, while recognizing the innate differences between them. In fact, all the CRPCs (1 – 7) represent the G7 condition. The more similar the two conditions are to each other (for example, the same printing technology, same substrate, and same ink system, just different presses), the better result you’ll get. The more different (for example, sheetfed offset on uncoated paper versus wide-format UV inkjet on vinyl), the worse the result. But, for this example, the solution will be addressed in the next section.
At its core, G7 works by calibrating multiple devices to a common specified condition via two key attributes:
However, as has been illustrated, the native differences between print processes/technologies and their ink systems create challenges that the G7 methodology alone cannot solve. It’s not a shortcoming of the approach, just a reality of the situation. One of the things that G7 specifies is the color of the primaries, referencing ISO 12647-2. There, the colorimetric values (L*a*b*) of CMYKRGB are all specified for different paper grades. In effect, it levels the playing field to some extent. If every system were to use the same “crayons,” then as a start you have a consistent foundation to build upon. But, when the inks are so different, no plate curves will solve that, and this is where you need color management.
Color Management: When it comes to addressing the bigger challenges in matching color between devices, technologies, and substrates, color management is the solution. However, color management is a broad topic and how it gets used depends on the situation.
Converting an RGB image to CMYK is a form of color management. Converting from one CMYK working space to another is another form of color management. And driving a proofing system to match a target print condition is yet another form of color management. These all share the utilization of ICC profiles, which are the core component of color management. They basically create that link between color spaces, be it RGB to CMYK or CMYK to CMYK. Utilizing the L*a*b* color space, they in effect create a fancy lookup table that ties values (RGB or CMYK) to L*a*b* values so color can be translated from one color space, or condition, to another.
Figure 5: This diagram shows how the larger PrintWide gamut would need to be mapped down to the smaller CRPC6 gamut for color consistency between devices that would utilize these profiles.
The key application of color management, as it applies to the topic of achieving color consistency across multiple platforms is two-fold. One, to constrain, limit, or “map” the larger gamut down to the smaller one. And two, to color-adjust, or match color between the two conditions. For example, when you are looking to achieve a color match between a large gamut device, like an inkjet printer (of whatever size and technology), and a conventional printing process like offset, the gamut, or range of color, of the inkjet device must be clipped, or limited, in terms of the color it can be allowed to produce.
It’s what happens every day with inkjet proofing systems. The color capability of inkjet printers is far beyond what an offset press can produce with just CMYK, so if the goal is to have the inkjet proof match, or simulate, the offset press output, then it can’t be allowed to produce color beyond what the offset press can produce. Additionally, whatever color differences exist inside the gamuts must be addressed. This is done via the ICC profile that represents the targeted output printing condition, in this case an offset press with whatever ink system, substrate, and screening that will be used for the final print output.
Where there can be trouble is when the color expectation and targets are set with a higher-gamut device, then a lower-gamut device is asked to match them. Using the example above, if an inkjet printer or press is allowed to print in its full high-gamut glory, and then offset is expected to match it, forget it. More color cannot be pulled out of a device than it is capable of. Yes, more color can be gained, to an extent, by running higher densities and even utilizing expanded gamut, but these bring other challenges and complexities and still won’t get an offset condition to the gamut level of inkjet.
So, when you want to achieve color consistency across multiple platforms, you must understand each devices color capabilities, their inherent color gamuts, and the impact of substrate. Then, use the right tool(s) to calibrate and/or color manage the device(s) to a shared common appearance condition.
And, if you want to fully embrace the capabilities of these high-gamut devices as part of an overall print campaign, go for it, but understand there will be some differences. Just keep the hues and gray balance the same between the devices as best as possible, have realistic expectations on the output and level of consistency between them, and enjoy the color!
Bill Pope is the Vice President, Technical Services, at PRINTING United Alliance.
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