Colour Management - Part Twenty Three

Kiran Prayagi, print technologist and chairman, Graphic Art Technology & Education demystifies colour management in a series of articles. In this twenty third article, he discusses Colour Gamut Mapping.

21 Oct 2013 | By Kiran Prayagi

Further details on colour management profiles and its creation will be discussed in future articles. Once the ICC colour profiles are calculated for the outputting systems (incorporating rendering intents then the pre-press operations are performed using the appropriate ICC profiles in the application softwares such as Adobe CS suite, CorelDraw, Quark Xpress or any other application systems, including ripping softwares. Following are the examples.

Consisten workflow for consistent colour quality
Workflow means how the information or content or produce flows from one step to the next in the production process. In digital systems the starting point is an analogue picture or a design scanned on a colour scanner to obtain digital conversion, alternatively a digital camera for live pictures. Digital originals can be in variety of file formats for further colour processing in the reproduction systems.

In modern digital workflows from prepress to press there are no standardized ways of communicating all aspects of colours in the whole value chain. This is why Colour Exchange Format (CxF) was developed. CxF supports, automates and simplifies the color communication of colours within a digital workflow. CxF is an XML based data format which can be seamlessly integrated into an internet-based workflow and can be used independently of platform and programming language. XML (Extensible Markup Language), which is oriented to SGML, defines how a data set should be described. XML is sometimes referred to as metadata.
SGML (Standard Generalized Markup Language) is a metalanguage because it is a language used to describe other languages. SGML defines the rules of how the logical structures (titles, paragraphs, contents etc.) of a document should be described. Metalanguage is the definition or description of a language. A metalanguage describes the rules for the generation of a language.
The CxF file contains all the important information that has an influence on the conversion or reproduction of a colour. In this way each colour in the CxF file can be specified differently, whether or not the colour has been measured, typed manually or defined as a spot colour. The user or the application calling the CxF file decides on which attributes to use. The following data is saved in the CxF file :
1. Spectral data
2. Lab
3. Scene light
4. Observer angle (2 and 10 degree)
5. Physical filters (Pol, D65, UV Cut)
6. Recipes
Using CxF files requires a CxF composer which writes and reads the CxF data. With a CxF viewer one can load colours in the CxF file and display and print them in true colour. The future goal is to synchronize all involved input / output devices with ICC based colour management systems and to translate device dependent colour definitions into device independent colour definitions with ICC profiles. This will give a hard copy of the colour patches in true colour as well as a printout of the embedded text and images - platform and software independent.

With the free Software Development Kit (SDK) from X-Rite, implementation in other colour management applications, RIPs, proofers, measuring devices, printing processes and colour applications can be achieved.

Special colour problems
It is also important to understand that ICC profiles is not the panacea for all colour problems and should not be overhyped before discussing more. The measurement and management of special colours need special attention and right kind of instruments. The light interaction with such colorants is more complex.

Fluorescent materials, such as optical brightening agents are used in textile, paper manufacturing, etc. These materials absorb light in the ultra violet region below 380 nm and emits at longer wavelength in the blue part of the spectrum. This gives the appearance of extra whiteness as in some cases blue light reflected from the sample is more than blue light incident on it due to the conversion of UV radiations into blue light emission. As a result the fluorescent material looks brighter than the non-fluorescent material. The measurement of fluorescent colours and its blue light emission can be compared by measuring without filter and then with UV cut filter that prevents UV light falling on the sample, see figure 3.

Figure 3 - fluorescent sample measurement, red line without filter, broken line UV cut filter

Metallic and pearlescent colours are used in many applications like packaging printing and in the products itself like cosmetics, automobiles, etc. Metallic flakes reflect specular light when its size is bigger and diffuse light when its size is smaller. Measuring reflection with normal 45 / 0 or 0 / 45 optical instrument gives different values depending on direction of the measurement instruments while measuring. Therefore, metallic colours are measured with multi-angle instruments. Instrument is called goniospectrophotometers when sample is illuminated at various continuous angles and multi-angle spectrophotometers when illuminated only at a few angles.

Figure 4 - (a) Metallic flakes (b) Metallic surface reflection (c) Perlescent material

Reasons and examples of colour appearances
In daily life it is often the colloquial and poetic expression that supersedes the scientific expression of colour. These expressions are often used by the artists. With scientific measurement of colours using instruments there is no guarantee that colour appears to human eye as expected. Discounting instrument limitations and metamerism there is a much larger issue called ‘colour appearances’ that is affected by surroundings, viewing light, human colour vision, etc.
As explained in article 12 a famous visual communication artist in India, professor Ranjan R. 
Joshi appropriately describes what is ART, his explanation is :

A absorption of the light rays by the object being perceived
R reflection or refraction of light rays
T transmission of light rays