Managing colour for success

by all | 9 February 2014 11:13 am

By David Tung
People outside the printed image business do not always appreciate why colour management is so important. To them, red is red and blue is blue. Getting colour right, however, can still be a major challenge in today’s industry, whether using offset lithographic, inkjet proofing or wide-format production printers.

More than ever, time is money. A shop will fall behind in the market if it has to reprint graphics because colours do not turn out as the customer expected or if staff is wasting a lot of time and materials to get a brand colour correct in the first place.

While there are many well-established rules and methodologies for web offset and sheetfed lithographic printing, there has been very little development of this type for colour management for wide- to grand-format production inkjet printers. In fact, no industry standards exist yet.

There is thus a need to introduce some ‘best practices’ to achieve consistent, predictable colours for wide-format graphics, regardless of which specific printer model is being used to create those graphics.

There are three main components to consider when attempting to develop consensus in the wide-format printing industry for colour management standards: expectations, output media and raster image processor (RIP) performance.

Expectations
When it comes to colours of printed graphics, expectations are the most difficult to agree upon. Everyone sees colours a bit differently, depending on their age, eye colour and angle of view, let alone reflections from the shirt they happen to be wearing that day or what they are standing next to. Consensus will require a lot of customer education and understanding.

There needs to be an agreed-upon measure of colour difference acceptability based on an ‘unbiased’ observer. This typically calls for an instrumental reading, using a spectrophotometer to determine the Delta E colour difference, as defined by the International Commission on Illumination (CIE). A Delta E metric of around three is usually considered acceptable for a colour match, but the lower the number and thus the closer the match, the better.

Further, unlike an offset lithographic press, most wide-format digital inkjet printers do not offer the ability to use special colours at will (in effect, to ‘substitute’ a specific colour in the inkwell on a per-job basis). So, it is advisable to define brands’ ‘spot colours’ early in the process.

It is important, for example, to check if the brand colour is (a) an actual spot colour or (b) a cyan, magenta, yellow and key/black (CMYK) ‘build’ of a Pantone spot colour. As a colour matching authority, Pantone issues two volumes of its formula colours, one for actual spot colours and one for CMYK reproductions. So, if a spot colour is really a CMYK ‘build,’ it is important to refer to the second volume to get the best match; or, if possible, change it to an actual Pantone spot colour.

One of the most overlooked aspects of colour management is how the final printed graphics will be viewed. The sign industry, after all, creates some prints that are glimpsed from across a busy street at 80 km/h (50 mph) and others that are stared at by customers as they wait in line in a store. These are certainly two very different viewing conditions, each with different quality requirements for wide-format graphics, but both require stable, predictable, repeatable output.

In some cases, print service providers (PSPs) install fluorescent overhead lighting in both their production areas and viewing booths, so illumination is controlled and standardized wherever their employees check on their graphics’ colours, which helps match them to brands and deliver finished jobs in a timely and cost-sensitive manner. The best way to achieve these objectives, however, is to develop profiles for the output media.

Output media
One of the blessings and curses of wide-format printing is the almost unlimited number of options for output media, including myriad flexible and rigid materials. Pretty much anything can be placed under the printhead carriage and decorated with graphics, so long as it is less than 51 mm (2 in.) thick.

These substrates all have unique ink absorption rates, which are also affected by the choice of solvent-based, durable aqueous ‘latex’ or ultraviolet-curable (UV-curable) inks. The best-case scenario for colour matching is to use coated media that can receive inks evenly, without pooling (beading) or bleeding (spreading).

The need to create customized media profiles cannot be stressed enough. The ability to recalibrate a printer each time a given material is used means output can remain consistent over time, no matter how many other ‘things’ get printed.

When developing a profile for a given type of media, signmakers should make sure enough ink will be put down for good coverage without excessive buildup that could lead to banding or oversaturation in the print. This is where RIP feature sets and performance will come into play.

RIP performance
For commercial wide-format inkjet printing, RIPs should offer a feature set that allows for calibration, media profiling, the use of conventional printing profiles and spot colour libraries and the ability to edit job colours if needed.

Some of today’s wide-format printers use both the primary CMYK inks and lighter,saturated cyan and magenta inks to provide smoother transitions for RIPs when handling CMYK and RGB reproduction. Colour saturation is particularly important for colour management of graphics that will be backlit.

Some also add a white ink channel, which in turn allows more accurate top-surface spot colour reproductions on virtually any substrate. Printing with a white undercoat will give spot colours a greater chance of meeting agreed-upon accuracy expectations when printed on off-white media.

Further efforts to expand the print gamut will include other colours, like orange and violet, in the future.

A media profile can be created using the RIP’s own profiling option or third-party software. In each case, the user must determine the individual limits for each channel of ink, so as to achieve optimal coverage per channel, and then set the total ink limit for four-colour (or more) combinations. The goal is to achieve adequate coverage and matched colours without wasting ink or causing too much saturation. When multiple inks are laid down together to create darker colours, for example, too-saturated coverage can cause uneven banding, undesirable ‘overglossing’ and too thick a film of ink.

Once the total ink limits are satisfactory, the next step is to adjust the media profile for a specific printing resolution and speed. These factors will vary somewhat, since solvent inks are typically printed onto more absorbent materials than UV-curable inks, so a spectrophotometer should be used to read all of the colour patches and determine the Delta E variations between expected and actual printed colours. If these variations are too wide, then adjustments must be made using the RIP’s colour manipulation tools.

Once the media profiles have been created, the easiest way to predict the appearance of spot colours is to print the full Pantone library on a profiled substrate, to be used as a reference for future jobs. Third-party software can also be used to fine-tune specific colours for certain jobs, but some RIPs will allow users to print colour iterations, so they can see how minor changes in CMYK ink percentages will alter spot or brand colours, without having to bring in third-party software.

Media profiles are also beneficial in that they can be used as a working space or preview space for a variety of applications, providing a sort of ‘soft proofing’ that allows operators to preview jobs as they create or preflight files (as long as their computer display monitors have also been calibrated for optimum colour matching).

The aforementioned offset and web printing industry standards have helped balance overall colour by providing a starting point once media profiles have been developed and selected. Signmakers will want to make sure all of their raster-based CMYK output uses the same source profile—e.g. Specifications for Web Offset Publications (SWOP), General Requirements for Applications in Commercial Offset Lithography (GRACol) or Forschungsgesellschaft für Druck- und Reproduktionstechnik (FOGRA)—while vector-based CMYK output should be set to the media profile that achieves the most colour gamut with the given printer/substrate combination.

One exception to this rule is when graphics feature a gradient vector blend, which can result in visual ‘stepping’ in the gradation, depending on the number of colour patches that were read to create the media profile. When there is both a gradient vector blend and a raster image, using the same source profile is preferred.

Some RIPs provide recalibration tools. These should be used if printer operators notice colours drifting or other visual differences over time. And if there is a further need to match offset lithographic printing, the International Digital Enterprise Alliance’s (IDEAlliance’s) G7 calibration methodology can also be applied to wide- and grand-format printing to help neutralize grey output and maintain visual consistency within the print shop.

That said, a certain degree of continuity can certainly be achieved by using offset print standards like SWOP, GRACol or FOGRA.

The colour equation
Whatever printing technologies a shop uses, its goal will be to produce stable, predictable and repeatable full-colour output. By following best practices, not only is consistent colour reproduction within reach, but shops can also save time, materials and money.

David Tung is a colour systems specialist in Agfa Graphics’ global services business. This article is based on a seminar he presented at the 2013 Specialty Graphic Imaging Association (SGIA) Expo. For more information, visit www.agfagraphics.com[5] and www.sgia.org[6].

Source URL: https://www.signmedia.ca/managing-colour-for-success/

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