EXPANDING COLOUR HORIZONS – UNDERSTANDING COLOUR GAMUTS

When colour characterisation of any printing system is carried out, a colour gamut plays a crucial role in terms of understanding the colour capabilities of that system. In order to reproduce the brand-specific colours – also known as spot colours – it is important to review these colours against the colour gamut of the printing system. When buying a new printing device or press, it can be helpful to compare the colour gamuts and imperative to describe and communicate these accurately. 

International Commission on Illumination (CIE 156) has defined colour gamut as a range of colours achievable on a given colour-reproduction medium under a given set of viewing conditions. Every printing system, along with the given printing conditions – such as specific substrate, inks, print variables and associated viewing conditions (for example, ISO 3664) – has its own colour gamut. Other factors affecting colour gamut include the reference white point – such as the perfect white diffuser illuminated by D50 – and the state of adaptation of the observer. 

Typically, a colour gamut of the printing system is obtained by printing and measuring a test chart containing several colour patches. Each of these consists of different ink value combinations, such as CMYK. For example, profiling test charts such as IT8.7/5 (ISO 12642-2) and TC1617. The test chart is normally printed on the desired substrate without colour management. A spectrophotometer is used for measuring the printed chart, to derive the gamut boundary in 3D colour space, such as CIELAB. 

Every printing system, along with the given printing conditions, has its own colour gamut

Alternately, colour gamut can be derived from the colour profile of the printing system or from the mathematical printer characterisation model. Colour gamuts of the publicly available printing conditions can be calculated from the corresponding characterisation data set – such as FOGRA52 or GRACoL_2013 – or from the published colour profile – such as PSOCoated1 or GRACoL2013. These are available on the ICC International Color Consortium website (www.color.org/registry/index.xalter and www.color.org/chardata/drsection1.xalter)

Usually, it is not possible to achieve the full colour gamut without any restrictions. For example, many printing devices cannot produce a print with 400% total ink coverage of CMYK. This is typically addressed by ink limiting and other controls. In addition, the printer driver and the colour profile – such as ICC – can influence the colour gamut. In practice, a subset of the full colour gamut is obtained, also called the usable colour gamut. 

A gamut volume quantifies how much of a colour space is covered by the colour gamut

In the past few years, there are two important documents that have been published related to colour gamut – ISO/TS 18621-11 and CIE 246. The first covers the technical specifications for describing and analysing colour gamuts for printing systems. The latter provides the guidelines for computing and communicating colour gamuts for printers and displays. Both of these are well aligned. 

A gamut volume quantifies how much of a colour space is covered by the colour gamut. In other words, how many colours are contained within the gamut. Usually, the gamut volume is calculated in terms of ‘cubic CIELAB units’ provided that the gamut is calculated in the CIELAB colour space. Figure 1 shows an example of two colour gamuts in the CIELAB colour space. The first gamut based on CMYK inks and the second gamut adds orange, green and violet inks. It also shows a number of spot colours represented by dots. 

There are a number of metrics for comparing and analysing colour gamuts known as gamut metrics. For example, Gamut Comparison Index (GCI) quantifies how closely two colour gamuts match. This is similar to the colour difference (∆E) which quantifies how close the two colour coordinates are in a 3D colour space. There are other metrics such as ‘gamut volume ratio’, ‘gamut coverage’ and ‘out-of-gamut volume proportion’. These gamut metrics can be used in different scenarios for analysing colour gamuts throughout the print workflow. 

Contrary to the typical representation of colour gamut in 2D diagrams, it is a volume in a 3D colour space, such as CIELAB. Many software applications can show a colour gamut using an interactive 3D gamut plot in the CIELAB colour space using 3D rendering tools. This makes it possible to visualise and review the gamut from different angles. Additionally, these tools can show a 2D projection of a gamut in an a*–b* plane of the CIELAB colour space or a slice of the gamut at a constant hue angle (h*) or constant lightness (L*). 

When comparing two colour gamuts, it is difficult to represent the relative volumes of the gamuts and the volume of their intersection in 3D colour space. This can be simplified by representing them in a 2D illustration of a Venn diagram. Figure 2 shows such a Venn diagram of two colour gamuts (right). Although these two gamuts have similar gamut volumes – as seen from their 2D projections in a*–b* plane (left) – they have a slight shift from each other in some regions. As shown, it is easy to visualise the relative volumes of these two gamuts and the volume of intersection in the Venn diagram on the right.

There are a number of metrics for comparing and analysing colour gamuts known as gamut metrics 

Siegwerk can help businesses to calculate, analyse and visualise colour gamuts for printing systems by using ‘Gamut Analysis Service’ provided by Colorwerk which offers a one-stop solution for colour management from Siegwerk. It includes comparison colour gamuts and calculation of the gamut metrics such as the Gamut Comparison Index. Siegwerk can also find if specific brand colours are inside the colour gamut, i.e., achievable on that printing series or not.

The guidelines for communicating colour gamuts are provided by CIE 246

The guidelines for communicating colour gamuts are provided by CIE 246. There is a list of key information for communicating colour gamuts, such as device type, device name, substrate type, colour channels, measurement description, total area coverage and gamut volume. 

Different file formats can be used to encode and communicate the colour gamut. These include Colour Exchange Format (CxF), ICC profile and the tab-delimited text file. These are widely used in the industry worldwide.

These guidelines help vendors, print suppliers – and the industry at large – to communicate and exchange colour gamut data in a consistent and effective manner across the workflow.   

REFERENCES

CIE (2004) CIE 156:2004: Guidelines for the evaluation of gamut mapping algorithms, CIE Central Bureau, Vienna.

CIE (2021) CIE 246:2021: CIE Central Bureau, Vienna.

Deshpande, K, Green, P and Pointer, MR (2015) Metrics for comparing and analysing two colour gamuts. Color Res. Appl. 40(5), 465-471. www.doi.org/10.1002/col.21930.

ISO (2009) ISO 3664:2009. Graphic technology and photography – viewing conditions. Geneva, Switzerland: ISO.

ISO (2006) ISO 12642-2:2006. Graphic technology – Input data for characterization of 4-colour process printing -- Part 2: Expanded dataset. Geneva, Switzerland: ISO.

ISO (2019) ISO/TS 18621-11:2019. Image quality evaluation methods for printed matter – Part 11: Colour gamut analysis. Geneva, Switzerland: ISO.