Color Control in Digital Print

With the rise of digital print processes, color reproduction has become more of a scientific process and less of the traditional craftsmanship of press operators. The physical color kitchen is largely replaced by a virtual on-screen color lab. Matching a color is no longer the exclusive domain of the trained eye, but is in most cases handled by complex mathematical conversions in black box software. Yet, human perception of a color is still where the discussion starts. And this discussion has been documented in the countless articles written by scientists, professionals, designers, printers and color gurus since the introduction of digitalized color reproduction systems.

“It has to be ‘more’ red”.

“I am looking for a deeper blue”.

“This is not our corporate color”.

“The skin tone is too yellow”.

“Why am I seeing little black dots?”.

“I am certain it looked better on screen”.

In our industry there has never not been a question about the final output. This also holds true for the typical answers to such questions:

“Did you include a profile?”.

“Ah, your color separation is for another process”.

“This file has out of gamut colors”.

“We simply have to desaturate this area here”.

“You forgot to convert the spotcolor to process”.

“Well, it is within tolerance”.

Mathematically and scientifically a color may be processed exactly as defined in a reference standard and may fall well within tolerances. It may even be the best output achievable by a certain printing system. Still, once the first print is made a discussion is likely to begin. This is also the reason that prior to starting a production run, we make samples and proofs. We have come to accept that a color in the artwork is not the same as the color on a screen and is more often than not a different color being printed in the end.

By now, most, if not all print professionals are using color workflows and tools to streamline their reproduction processes. An ICC profile is common in the jargon of the industry, as are calibration, spectrophotometer, linearization, gamut, white point and control strip. So, why is it that even today, with all the know-how and technology available to us there still is this mystical challenge of getting a color right?

Transformation

The emergence of digital printing technologies such as inkjet or toner-based printers and the trend towards mass customization are causing a shift in supply chains and production processes. An example is the replacement of traditional screen printing of labels or sleeves by short-run digital direct to shape systems that print directly onto bottles and cups.

Traditionally, the supplier of the printed material is a different company than the buyer that would stick the final print onto a container. Today, a micro-brewery or a bottling company may wish to integrate the printing process into the last stage of their production line, as to limit stock inventory and enable more freedom towards rapid design changes. With the latest generation direct to shape printing systems it has become a reality to benefit from ultra-fast last moment fulfillment.

Never before has there been such a wealth of printing options; ranging from transfer to contactless, with different types of ink to match substrates, running smaller machines that print one-offs or industrial set-ups that incorporate all stages of printing, fixing and post-treatment. And these are increasingly being used by companies that before had little to do with print production.

It is very important to note that this means that skills and knowledge regarding the entire digital print process would have to be transferred to companies or production environments that previously had very little or no exposure to the print production workflow. Color management is just one of the many aspects that are part of this expertise transfer, but a very crucial one nonetheless.

Short-run textile printing is another example where several conventional market segments see the rise of digital print alternatives. Where, until recently, long-run, high-volume production was the norm, work shifts to short-run, low-volume processes. And while there is an awareness of the opportunities, there is an equal amount of challenges in matching the old with the new.

Controlling Complexity

People expect to get the same color output as before and all qualities need to resemble the original. Yet, the inkjet inks are different, both in how they are applied and what color gamut they can achieve. Many other factors come into play as well: the pre-treatment of the media is different, the color yield is different, print-through or penetration of ink into fabrics is different, migration and adhesion of ink is also a factor to consider. In short, there is no real comparison between the traditional and digital processes, other than that an ink is being applied to a surface.

It is a common misconception that digital printing is an easy “push-of-the-button” process. Fact is that the achieved quality of any digital print is dependent upon a whole range of parameters and variables, which need to be understood and tightly controlled in order to obtain the desired output (think of ink drop size, amount of color channels, type of ink, media tension, production speed, temperature, humidity, to name but a few). And even if all these factors have been considered, there still remains the fact that a print production system comprises hardware, software, inks and media of many different suppliers. All too often, manufacturers deliver top notch products and technologies, but lack the in-depth knowledge of and a holistic view on the entire printing process.

The paradox is that in today’s industry we seem to have more technology available to us to control color reproduction, including all the tools for making profiles, preparing artwork and driving print heads, yet at the same time the complexity of integrated systems leads to challenges we can’t seem to fully address. We have come to rely on solutions that function quite well on paper, but are no match for the increased demands of the many new application fields.

Technical Challenges

One distinct feature of printed labels and sleeves is that all color application and therefore also color measurement is performed on a flat substrate, usually paper or foil stock. Spectrophotometers to measure targets for profiling are standardized instruments and most RIPs have in-built profiling technologies for quickly achieving highly accurate color profiles. One could say that this technology is quite mature with very little advances or new developments necessary.

If one wants to profile a new set of inks for a given substrate, the process simply involves printing out a range of target files, measuring the individual color patches (usually an automated process as well) and generating the profiles. The spectrophotometer is placed directly onto the flat surface and a measurement of a particular color or a whole range of colors can be performed with ease. Most spectrophotometers are designed to shine a light source directly onto the substrate at an angle and measure the reflected light perpendicular to the substrate. To generate a profile of good quality, hundreds of individual color combinations must be measured, with bigger targets easily reaching 1,000 and more color patches. Since each patch needs to be of a size suitable to the aperture of the measuring device (e.g. 5×5 mm), all patches together quickly reach the size of an A3 page.

Whilst this is no challenge as long as one prints onto a flat paper substrate, profiling, for example, directly onto a beer bottle with its very limited “real estate” for the application of color patches, becomes a real problem. Furthermore, no readily available spectrophotometer on the market is specifically designed to be used with non-flat articles, as the geometry of aperture, substrate and optics and the process of emitting and capturing the light from round surfaces simply has not been intended for this purpose. On glass or PET, for example, printed colors (even with an underlying thick coat of white ink) are not totally opaque, causing a lot of light emitted by the spectrophotometer to pass straight through the substrate and not reflect back into the optic, leading to erroneous readings and a wrong color measurement.

As with the example of the profiling of bottles, in textile there is a similarity to the complexity level.

Measuring color on textiles is far more difficult than with flat surfaces like paper. Fabrics are structured surfaces, with a multitude of construction types (i.e. open or closed, woven or knitted) and various types of fibers (cotton, polyester, nylon, etc.). Light scattering and reflection come into play when using spectrophotometers to calibrate and generate profiles. One of the difficulties is for instance a semi-transparent fabric that cannot be accurately measured when the surface it is placed on shines through or influences the light that is reflected to the measurement device. Another example is the color density difference of the same color and the same volume of ink on a fabric that has a slightly thinner or thicker thread in a slightly different weave. A droplet of ink could penetrate the media deeper, resulting in a less vibrant color on the surface.

Different types of fibers need different types of ink, and, for instance, the definition of CMYK of an acid ink is not the same as the CMYK of a sublimation ink. A typical traditional textile design is a repeat pattern filled with color book reference colors: these are based on ink recipes, rather than RGB or CMYK values. As a result, the considerable time-consuming process in the digital textile print production workflow is proofing designs for color accuracy to warrant color matching.

Answering the question

When it is a matter of printing an RGB photo to a flat paper surface, results are seemingly achieved without effort. But when the print process and the specific application characteristics are highly complex, it can’t be regarded in the same way. It is not merely a matter of a quick measurement that will be an instant success. It takes the right technology to work. And it takes a specialist who is able to combine the expertise of the color scientist and the view of the trained eye and applies that to the practical production workflow while considering the application implications.

Even though color control is working quite well in some areas, in others it is still far from perfect. Technical requirements aside, a successful implementation largely depends on the transfer of know-how and expertise to those that are newly introduced to digital print production workflows. As long as new application possibilities and technology developments are changing the way we produce printed materials, a standardized norm is unlikely to become available in every segment. Logically, the need for industry support and education will continue to exist.

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The observations in this article come from the industry expert initiative LMNS. This article has been published in print (SIP Magazine) and online at www.lmns.nl/articles 

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