Since large format digital printing offers solutions to print on almost any type of material, there are some challenges when it comes to managing color appearance. But understanding the limitations for some substrate- and ink combinations should help you reach a satisfactory end result.
Interior decorations is one of the most rapidly expanding business areas for digital printing, since a designer or vendor can produce bespoke products in very short runs, without the penalties that come with traditional printing, such as high initial start-up costs. Digital printing has overcome most of the early problems regarding quality, and today can actually offer superior quality, for instance when reproducing photographic images, compared to conventional screen printing. But with all printing methods, every type of substrate, be it textiles, wood, metal, glass, plastic or paper, needs the right type of ink. With digital printing, the ink determines which type of digital printer to use. And to achieve a very wide range of colors, standard CMYK process colors might not suffice.
Photographic images and vector graphics
There are two main categories of graphics, but they can both be color managed. Logos, patterns and line art illustrations are typically created and saved as vector graphics, and as such can be enlarged or scaled in size freely, without loss of quality. Vector graphics are usually monochromatic and often colored using spot colors, like for example one of the Pantone colors. This is the first challenge in color management for digital wide format production: how to best convert those spot colors to a matching mix of the CMYK process colors.
While many types of interior decorations, like wallpapers, traditionally use patterns made up of a number of elements, each using a certain spot color when printing in conventional screen, this may not be possible in digital printing. As a designer you need to be aware that the CMYK color gamut only matches about 65-70% of the spot colors, depending on substrate and type of ink. If you are used to working with spot colors and need to convert existing designs to process colors, you might have to accept that some of those spot colors can’t be matched a 100% accurately just with CMYK.
Checking for an accurate match
The way you can check if the CMYK version of the colors will match the spot color, is to measure the colors with a spectrophotometer. The unit for color difference is called ∆E (Delta E) and ideally a reproduced color shouldn’t deviate more than let’s say 2.5 ∆E, or we will notice a slight difference in hue. Most people will probably say that when the color difference exceeds ∆5 it’s not the correct color anymore. But this all depends on if the person has better than average color vision.
If you work with and assess colors on a professional level, you should take the F-M100 test, to check if your color discrimination capacity really is flawless.
If you are about to produce interior decorations that must match a certain spot color, or series of spot colors, you need to check with the print service provider if their digital printers can use an extended ink setup. This typically adds orange, green and violet to the base CMYK process colors. Now the digital printer suddenly can reproduce close to 99% of the special spot colors, again depending on what substrate is being used.
RGB to CMYK conversions
When reproducing photographic elements, the challenge in color management is not directly related to spot colors as such, but rather how accurately photos are converted from RGB to the CMYK process colors. The primary colors in the additive gamut are Red, Green and Blue, and those colors are achieved in print using a mix of Cyan, Magenta and Yellow (the black ink is called K as in Key color, and doesn’t affect the hue, only the dynamic range of the shadow areas). The CMYK color system is subtractive, so Red, Green and Blue colors are the secondary colors achieved when two or more of the primary colors are mixed. These secondary colors are less saturated than the primary colors, and red, orange, green, blue and violet, etc. are difficult, sometimes impossible, to match in CMYK. If part of your wild format artwork is photos where green, orange and violet hues are essential for the design, you should use a print service provider with digital presses capable of extending the ink set beyond CMYK to include at least Orange Green and Violet. This is called the extended color gamut, and is fast being embraced by more and more manufacturers and added to some of their digital printer models.
One of the advantages of digital printing versus conventional printing is that sample prints can be made in the same device as will be used for final production. As long as your print service provider uses relevant prepress software and raster image processors to handle the required color management, you should be able to specify which parts of your designs need to match named spot colors. Make sure the proofs (sample prints) are made in the same device as will be used for actual production, and check the proofs carefully. Among the workflow solutions on the market, and hugely popular for large format production, is the EFI Fiery RIP, which has excellent functionality for handling spot colors.
Test your color vision!
When we said that most people will say that a color deviation of more than ∆E5 is unacceptable, it actually depends on how good your color vision is. If you do a lot of critical color evaluations, you should check it because only about 10% of both men and women have perfect color vision, or superior color discrimination capacity as it is called.
The most common color vision test is the FM100 test, the Farnsworth-Munsell hue test. Originally the test consisted of a hundred different color samples which the tested person had to arrange in color sequence. Later it was found that stable results could be achieved with only 85 tiles, but the name was kept.
The F-M 100 test goes way beyond just testing if you are color blind. While more men than women are what is called color blind (poor color discrimination capacity), most of us, both women and men, have a color discrimination capacity spread from quite poor, to average, to fairly good. Some of us have problems with greens that are close to each other, others have more problems with reds and orange hues, and others with shades of blue. If you work with colors on a professional level, it’s well worth taking the F-M 100 test.
When we are happy with our print samples, the colors match fine. But when we hang or place our lovely interior decoration where it’s supposed to go, the colors seem somehow to be different! You have experienced metamerism. This phenomenon happens when the pigments used in dyes or inks react differently to the different lights used to view them. Metamerism is fairly well known in the graphics arts industry, and was one of the main reasons why it was decided to use a standardized light in viewing booths when evaluating proofs (samples) and final prints. The ISO 3664 standard specifies viewing conditions for photos and prints, using a standardized daylight called D50 at 5000 K. The unit K stands for Kelvin, and describes how ‘warm’ or ‘cold’ the light appears for the viewer.
There are other such standards, for example D65 at 6500 K, a ‘colder’ light than D50, and used in RGB standards like Adobe RGB and sRGB. An important part of the D50 specification is the spectral distribution and brightness of the light source. When prints are measured or viewed in for example an office or shop, the colors may look very different, depending on the spectral distribution of the light. So if you are concerned about how your interior decorations look where they will be seen or sold, that you’ll have to take the trouble to evaluate them there, on site. It’s not only the spectral distribution of the light in the lamps that matters, it’s also important to have the correct brightness. Too low light intensity and we don’t see much of any color.
There are solutions around the corner to handle situations of different light conditions, to adjust or adapt in the software used for color management. But until the ICC standard is updated, we need to prepare as best we can and evaluate the prints in the final lighting environment. The adjustments needed are more or less manual tweaks and fine tuning. This is possible to do, but quite time consuming. ICC version 5, also called iccMAX, may simplify those types of adjustments, but this is still in the future albeit a near future.