Images courtesy HP Canada
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?
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.
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.
