Wide-format Printing: Maximizing the potential of colour

by all | 21 February 2018 3:57 pm

By Josh Hope
There are numerous ways to maximize the potential of colour in wide-format printing, including taking advantage of today’s expanded colour ink sets and better understanding the nature and role of input profiles.

Expanded colour ink sets
Adding new ink sets to a print production department is a great way to expand its output gamut and successfully produce challenging colours. The inks themselves, however, are only half the battle. To get the maximum value out of them, the process of extending beyond the standard cyan, magenta, yellow and key/black (CMYK) space also requires planning and attention to how image files are set up.

A standard CMYK workflow, particularly a raster-based one, will not allow the files to request expanded colours from an output device without some additional steps. The default CMYK colour space in Adobe’s Photoshop software, for example, is U.S. Web Coated Specifications for Web Offset Publications (SWOP), which is very limited when compared to the scope of an expanded-gamut inkjet printer.

For the sake of illustration, one could think of the US Web Coated SWOP CMYK colour space as a 473-ml (16-oz) glass of water and a CMYK-plus-orange output device as a 710-ml (24-oz) empty glass. If the water is poured into the larger glass, there will still be one-third of the glass left empty; it cannot be filled. To take advantage of the larger gamut of the output device, there also needs to be a larger colour space at the design stage.

Depending on the specific workflow, there are a few ways to achieve this. One is working in red, green and blue (RGB), if the raster image processor (RIP) software supports it, as then the files can request the larger gamut of the output device. The old-school practice of converting files from RGB to CMYK before using the RIP was sensible in the days of offset presses, but is usually not necessary in an all-digital workflow. Becoming proficient at making edits in RGB can take some getting used to, but the results can be well worth the effort. For vector-based artwork, colour swatches can be defined in RGB, as well, and will reap the same benefits.

If, on the other hand, the raster files must be defined in CMYK, then it may be worth considering standardizing the workflow with the new Expanded CMYK (XCMYK) colour space, developed by the International Digital Enterprise Alliance (IDEAlliance) for four-colour presses and released at the end of 2016. While this data set will still not completely cover the gamut of a printer with orange and/or green inks, it is significantly larger than the default US Web Coated SWOP colour space. It can be downloaded from IDEAlliance’s website (www.idealliance.org[3]) and set as the new default CMYK colour space in Adobe Photoshop or other image editing software.

An even more powerful way for files to request an expanded gamut from a printer is to use colour book swatches defined by L*a*b* values. If the RIP software supports colour books or named colours with L*a*b values, this is probably the best way to take full advantage of the printer’s and the inks’ potential.

By requesting a colour via L*a*b* values, the workflow effectively bypasses any constraints on the design document colour space. When the named colour is correctly referenced in the RIP (which should be checked in the RIP’s colour library), the colour values can be converted directly through the output profile of the device and to a known print condition. Assuming the output profile is accurate, this will ensure the full benefits of the additional ink colours can be exploited.

Yet another way to address additional ink sets is through a direct colour replacement. If a specific design file colour is selected at the RIP stage of the workflow, then it can often be mapped directly to an ink value. By way of example, if a solid magenta square is selected in a design file, then it can be replaced in the RIP software with 100 per cent orange ink or with a mix of orange and other ink colours. Some RIPs only allow this type of replacement to be achieved within vector-based artwork, but others support it in raster-based files, as well.

To make the process simpler, some printer manufacturers provide a colour swatch library that matches up with another predefined colour library within the RIP. In any case, users are advised to read their RIP software manual to see what options are available and then experiment to achieve the best results.

Input profiles
One of the most overlooked—and most often misunderstood—elements in a colour-managed workflow is the input profile or working colour space. It provides a reference to the RGB and CMYK values in a document, which directly affects how much of an output device’s colour gamut can be addressed. One of the core concepts in colour management is the requirement for such a reference to allow the device-dependent RGB and CMYK colour spaces to communicate effectively with each other.

(Editor’s note: Again for the sake of illustration, a document that simply calls for ‘100 per cent cyan’ is roughly equivalent to asking a bartender for a full glass of beer. While the customer may get a glass filled to the brim, there is no guarantee of how large that glass will be or what kind of beer will fill it. A 237-ml (8-oz) glass of pilsner is very different from a 651-ml (22-oz) glass of stout, even though each can legitimately be defined as a full glass of beer!)

To get the desired output, it is important to provide a reference for the requested input colour values. This is where the input profile is a much-needed resource.

Colour standards such as the General Requirements for Applications in Commercial Offset Lithography (GRACoL) or Adobe RGB 1998 are common input colour spaces that define both the ‘volume’ and the ‘contents’ of the ‘glass.’ The GRACoL 2006 data set, for example, defines 100 per cent cyan with the L*a*b* values 55, -37 and -50, while the SWOP 2006 data set defines it as 57, -37 and -45; these are not the same colour. So, taking a document created with the SWOP colour space and then assigning GRACoL as the input profile would change the reference and create a colour shift before any inks ever hit any substrates.

While colour management is mostly a science, it also involves a dash of ‘philosophy.’ There are more than a few competing viewpoints about how best to handle files from outside sources, which can lead to frustration when attempting to rein in a colour workflow and determining which approach works best for
a given sign shop.

One effective mantra is “always embed, always honour.” This means when a file is saved, the profile that was used as the working colour space for that document should be embedded into it, if possible. Then, when that file is processed, the RIP software should look for the embedded profile and ‘honour’ it, rather than assigning a different one.

Indeed, reviewing the RIP software’s colour management settings is one of the best ways to take better control of colour management. Most RIPs provide the options (a) to use an embedded profile if one is available and (b) to assign a default profile if none is found.

While (b) is the trickier proposition, the safest bet is to select a colour profile that matches the default settings used by Adobe’s design software suite for CMYK and RGB colour spaces, i.e. U.S. Coated SWOP for CMYK files and either standard RGB (sRGB) or Adobe RBG 1998 for RGB files. Given these colour spaces are the most commonly used when new files are being created, chances are in the sign shop’s favour the designers have not changed them to anything else along the way.

That said, when incoming jobs place more emphasis on colour saturation than on image fidelity, an alternate colour workflow can then be considered. In these cases, it may make sense to ignore any embedded profiles and assign a larger-gamut input profile at the RIP stage. This approach will tend to use more of the output device’s extended gamut, but at some cost in terms of colour accuracy.

Josh Hope is senior manager of industrial printing business development and marketing for Mimaki, which manufactures digital wide-format inkjet printers and cutters. This article is based on a seminar he presented in October at the 2017 Specialty Graphic Imaging Association (SGIA) Expo in New Orleans, La. For more information, visit www.mimakiusa.com[6] and www.sgia.org[7].

Source URL: https://www.signmedia.ca/35193-2/

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