Pantone Colour Matching on extra-colour inkjets

01/11/2016

By Simon Eccles


Pantone has published guides to show colour matching using extended gamut inksets

Simon Eccles looks at what Pantone matching means, how it works and why it matters to printers and their clients.

Digital printer manufacturers often state that their devices are able to match a large percentage of Pantone colours. A few years ago it tended to be "about 85%".

Now Epson has claimed that the ten-colour UltraChrome HDX inkset used in its latest SureColor SC-P aqueous printer series can match a remarkable 99% of Pantone values. Here we look at what Pantone matching means, how it works and why it matters to printers and their clients.

Accurate reproduction of brand colours is becoming ever more important for large format inkjet print. No longer is it enough to simply print on larger pieces of media; now the customers expect accurate colours that precisely match other elements in their cross-media marketing campaigns.

Colour management is an important part of making sure that printers' output is accurate and consistent.
The issue of making sure that brand colours match the client's specifications is related to colour management, but slightly separate. If your inks can't hit a brand colour accurately enough to satisfy the customer, then no amount of colour management will fix it.

CMYK limitations

All colour printers use at least cyan, yellow and magenta inks, and nearly all of them have black too. This set of four is abbreviated as CMYK. It is affordably adequate for colour photographs and can be used to produce a respectably high proportion of Pantone colours, but not all of them.

There are many colours that the human eye can perceive that CMYK inks can never achieve. These include intense and often attractive shades of blue, red, green, purple, oranges and rich browns.

The total range of colours that a printing process can achieve is called the gamut. To achieve colours that are not possible in CMYK alone, they need to print with additional coloured inks. In the digital world these are typically orange, green and violet, plus sometimes red and blue.

As a general rule, you'll only need one of these extras plus one or more of the CMYK inks in order to match a particular "special" brand colour.

Pantone as a standard


Pantone Matching System solid colours, as previewed in the colour picking menu in QuarkXPress

For more than 50 years, the Pantone Matching System and its variants have been the de facto standard for describing colour in the design and printing sectors, as well as others such as plastics and paints.

Pantone isn't the only colour model available: in Japan, the Toyo Inks and DIC systems are commonly used. The Trumatch system was designed for digital design, but trails behind Pantone. As everyone has heard of Pantone we'll use that as our main example.

The core Pantone Matching System Plus (PMS+) set comprises more than 1,800 discrete "solid" colours, usually presented as numbered references in very carefully produced Pantone Guides.

These are swatch books showing each colour printed with a "solid" (ie individually pre-mixed) ink, on coated and uncoated papers, plus tear-off colour "chips" or as palettes that can be called up on-screen in computer design programs.

These are swatch books showing each colour printed with a "solid" (ie individually pre-mixed) ink, on coated and uncoated papers, plus tear-off colour "chips" or as palettes that can be called up on-screen in computer design programs.

The complication is many of those PMS+ colours cannot be precisely matched with standard CMYK transparent process inks, but no inkjets allow you to mix your own special ink colours. Well known "impossible" Pantone colours that are popular with designers include Orange 021, Reflex Blue, and Rubine Red.

Impossible colours


This is the Pantone PMS+ solid colour menu in Adobe InDesign

Brand identity designers naturally choose distinctive colours for their logos or other "house" uses. They'll supply a Pantone value with the artwork and expect the printer to match it. Unfortunately many are difficult or impossible to achieve with CMYK.

It's not just designers being awkward or ignorant of print processes, it's often deliberate, to discourage counterfeiting by simple scanning or photocopying of branded documents or packaging.

With screen printing or other conventional (non-digital) print processes, this is no great problem: you just buy or mix a special ink that's the exact colour needed, and print it as a "special" or "spot" solid colour.

Conventional printing presses (letterpress, lithography, flexography, gravure or screen) can print inks of any colour. If you've got colour photography or normal line art, that can be printed by CMYK. Any special brand-colour logos or text is printed using an additional ink as a fifth colour. This costs more, but hey, the client pays.

For conventional (ie non-digital) printing, you can mix your own inks using the "recipes" given in the Pantone Formula Guide. Starting with 13 base coloured inks plus black, you weigh out precise amounts and blend them before putting them in the ink duct. A total of 1,867 Pantone solid colours can be mixed from the base inks. Ink makers often offer popular colours ready-mixed.

Digital matching


Epson's SureColor SC-P7000 features gamut-extending inks including violet

Things got more complicated when digital printers came along, especially inkjets. Their fluid ink is formulated and made to precise tolerances. It's not practical to mix your own, and unless you're talking very high volumes, you won't find an ink manufacturer willing to make special colours for you.

The good news is that inkjet CMY inks (forget black, or K, for now) are generally purer in colour than the ISO standard colours for conventional inks. A litho cyan is much too green, magenta is too blue and yellow is too green.

Because inkjet chemists haven't been hampered by decades of compromise standards, they've usually been able to achieve CMYs that are much closer to the theoretical ideal. You still won't get all the Pantone values out of purer CMY, but you'll get more than from litho inks.

Until recently inkjets that offer extended colour gamuts have usually been aqueous ink models used for proofing or for vibrant photographic and fine art work.

They may have six, eight, ten or 12 ink channels in multiple print heads. Typically the gamut-increasing colours are orange, green and violet. A few manufacturers (including HP on some DesignJets) offer red, green and blue as well as orange.

Solvent inkjets for signage sometimes have extra colours too. For instance, the Mimaki JV300 can run the SS21 eco-solvent nine-ink set that includes gamut-extending orange as well as tone-smoothing light cyan, light magenta, light black and white.

Epson's UltraChrome HDX ink set (that it claims reproduces 99% of PMS colours) is called Light Black, Photo Black, Matte Black, Cyan, Light Cyan, Yellow, Vivid Magenta, Vivid Light Magenta, Violet, Orange, Green, Magenta, Violet, Orange and Green. The extended gamut is achieved by the orange green and violet inks as well as by purer cyan and yellow, plus vivid magenta (which is more red than usual).

These extended gamut colours are transparent "process" inks, not solid "spot" colours. The halftone process of varying dot sizes allows different colour combinations can't be reached by CMY alone.

Unfortunately there is no ISO or any other standard for extended gamut on inkjets. Every manufacturer and ink maker does its own thing. It would be impractical for Pantone to give out separation values for every inkjet or other digital printer on the market, so it doesn't.

Profiles

So, in a world of digital artwork and digital printing with lots of colours, how do you match those Pantone values?

With any luck, your Rip or printer supplier will have worked it out for you, so you don't have to. Most if not all Rip manufacturers work with Pantone to create "look-up tables" or profiles for every Pantone value.

If the driver sees "Pantone 281C" as the name of a spot colour in an artwork file, it will look this up to find the ink tints needed to reproduce the colour as closely as possible with your particular printer and media combination. This depends on your printer being calibrated and profiled for the media of course, but that applies to any issue of colour accuracy.

Extended Gamut guides

For extended gamut printers, this has recently become somewhat easier, as this year Pantone published its Extended Gamut system for the first time. This lets you match 1,729 of the PMS solid colours by using standard process CMYK plus orange, green and/or violet process inks.

The Pantone Plus Extended Gamut Guide is one of the familiar fan-out swatchbooks showing the 1,729 colours, including tints. If you buy this guide it includes an access code for Pantone Color Manager software which lets you import XG palettes into most popular design programs.

The Pantone Extended Gamut process only produces an exact match with either its own specification of orange, green and violet inks (which are licensed to ink makers), or though Rips that have Pantone licenses that include XG support. So far the ink specifications are only available for litho and not inkjet.

However, this shouldn't be a problem, according to Pantone, which says: "The purpose of the XG guide book is to give design teams a good indication of what an extended gamut split of the Pantone coated guide will look like, rather than to become de facto standards themselves."

"We expect that the process will be that the user continues to target the Pantone coated guide master colour, only using their process split, just as they would from four-colour."

Replacement colours

Pantone's intention isn't so much that Rips will understand the new XG colour numbers, more that users will specify the Pantone Plus number, having visually confirmed that it can be matched, using the XG guide for comparison.

The Rip uses a method called replacement colour. When it sees a particular colour name it uses a pre-set combination of colours from its profile. If it's a Pantone number, it will have all those already in the profile.

Some Rips also take particular RGB or CMYK values, or a range of values and replace those with pre-set combinations of printer ink colours. This would in principle allow precise matching of a brand colour within a photograph, but this might take some experimentation and proofing to get it right.

Conclusion

Pantone matching matters more and more, but fortunately, it's getting easier. In an ideal world there would be a standard for extended gamut that everyone could work to, but so far there isn't The Pantone XG guides are a step in the right direction, but you'll still need to exercise eyeballs, spectrophotometers and skills to achieve the best results.

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