Wide-format Printing: Working with colour profiles

by all | 5 February 2013 8:30 am

By Josh Hope
Understanding the roles of International Color Consortium (ICC) profiles, working colour spaces and raster image processor (RIP) input and output profiles is critically important to a well-managed production workflow in wide-format digital printing. Yet, more often than not, these settings are either overlooked or implemented incorrectly. To reach a sign shop’s output goals, it is crucial to learn how profiles can affect the colour management process.

One common misconception about ICC profiles is they actively do something. In fact, an ICC profile is nothing more than a description of a range of colours, also called a gamut. It indicates which colours are achievable with a particular device or colour space, given a set of conditions the profile was created with, such as print mode and substrate.

ICC profiles can be thought of like a box of crayons. Each crayon represents a single colour within the box, but two crayons can also be used together to create another colour for which no single crayon exists. And other colours exist that cannot be reproduced, no matter which crayons in the box are used together.

In much the same way, ICC profiles define the colour gamut available either as a document colour space at the front end of a workflow or as an output device colour space at the back end. For offset printing, these two gamuts are usually the same or at least very similar. In the wide-format inkjet printing sector, press manufacturers are constantly trying to increase their devices’ printable colour gamut, without aligning to any single colour standard. To this end, there is a need for customized ICC profiles to more accurately define the achievable colours under each printing condition.

This trend leads to a dilemma: if the ‘box of crayons’ used to design a graphic is different from the ‘box of crayons’ used to print it, how will there be any consistency?

Relative and absolute colour
To work efficiently with colour on wide-format imaging devices, it is necessary first to understand how cyan, magenta, yellow and key/black (CMYK) and red, green and blue (RGB) are device-dependent colour spaces. This means any defined colours are relative to the devices that print them.

An ICC profile’s basic function is to accurately describe both the overall gamut of a device or colour space and the specific colours within that gamut. To accomplish this without needing an external reference, the L*a*b* colour space can be used, which is larger than both CMYK and RGB colour spaces and is device-independent. Specifically, L*a*b* colour space is used as a profile connection space (PCS) inside RIP software to convert files from one colour gamut to another.

Working spaces
When an ICC profile is used to describe the colour gamut of a document or its elements, it is referred to as that document’s ‘working space.’ This provides a reference point for the colour values in a document.

A CMYK document may indicate a logo coloured with 100 per cent cyan, for example, but this only provides a portion of the information needed to reproduce it accurately. It is some kind of cyan, but one cannot know specifically what it looks like if there is no reference point.

The colour working space provides the rest of the information needed to solve the equation. If the document was created with a standard default application colour working space, it would use a U.S. Web Coated Specifications for Web Offset Publications (SWOP) ICC profile defining cyan with the L*a*b* values -62, -44 and -50.

In other words, 100 per cent cyan is relative to these values. Once the colour has thus been defined accurately, signmakers can attempt to reproduce it.

This ICC profile is generally used for CMYK offset printing and has a fairly conservative colour gamut. It can cause some issues with wide-format printing, as the colour gamut of the working space may be smaller than that of the output device, in which case the colours requested within the document will not take full advantage of the printer’s potential gamut.

Assign or convert
With design software, signmakers can change the working space by either assigning or converting to a new ICC profile. In essence, this allows the size of the document’s colour gamut to be changed. There is a difference between the two options, however, which is important to understand.

Assigning a new profile takes the current CMYK or RGB values and changes the colour gamut to which they are relative. If a colour is defined as 100 per cent cyan relative to a small gamut, for example, and then a larger colour space is assigned, the defined colour will still be 100 per cent cyan, but relative to the larger space, which will cause a colour shift.

Converting the colour space, on the other hand, scales the gamut but keeps the colour values the same. The 100 per cent cyan relative to a small gamut, for example, would become a 50 per cent cyan when converted to a colour space that is twice as large. The colour remains the same visually, but relative to a larger space.

Assigning an ICC profile, then, should generally be avoided, as it can cause an unwanted colour shift. In some cases, however, there is no other choice. If a file comes into a colour-managed workflow without an embedded ICC profile, there is no reference for its colour values, so a profile must be assigned. This should only be a last option, however, and if any embedded ICC profile is available, it should always be honoured.

The only time an ICC profile does not need to be embedded is when the file has already been converted to a standard colour working space—a process referred to as ‘normalizing’—and the RIP is set to use that space.

RIP colour conversion
Colour is managed the same way in most wide-format RIP software platforms. When a file is brought into the workflow, the RIP should be configured to check first if the file has an embedded ICC profile (which, as mentioned, should then be honoured). If no ICC profile is found, the RIP should assign a default profile.

Once the RIP has a reference colour space defined by the profile, the RGB or CMYK values in the file can be converted through the PCS into L*a*b* values. These values are then converted again to CMYK, relative to the colour space of the output device.

Pantone colours set up as ‘spot colours’ in the file should be defined by L*a*b* values. These values will bypass the input ICC profile, which is typically smaller than the output gamut, and will be mapped from L*a*b* directly to CMYK, relative to the output device. This process ensures the closes match possible to the original Pantone colour.

Standardization misconceptions
One fairly common misconception about the wide-format printing workflow is that by assigning a common ICC profile—and thus a common colour working space—to all incoming work, the printed output will become more consistent. This is incorrect.

In fact, the opposite is true. Ignoring an embedded ICC profile and assigning a different colour working space may create a different reference from that of the file’s creation, which in turn can cause a colour shift in the file before any ink is ever printed on a substrate.

If the desired goal is to standardize colour spaces before files reach the RIP, this standardization has to be undertaken through conversion, rather than assignment. Today, there are systems available that act as a ‘colour server’ to automate the process, based on defined rules, to convert any incoming files to a standard colour space, while at the same time maintaining the original colour intent of each file.

Josh Hope is a colour management professional for HP, working in the large-format Scitex division. This article is based on a seminar he presented at the Specialty Graphic Imaging Association’s (SGIA’s) 2012 Expo. For more information, visit www.hp.ca[5] and www.sgia.org[6].

Source URL: https://www.signmedia.ca/working-with-colour-profiles/

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