Introduction to Colour Management

Colour management is a large subject and whole books have been devoted to the subject. The intent of this article is to give an overview of the concepts and ideas involved.

Essentially the idea of colour management is to keep colours consistent all the way through from capture to display.

The Problem

Joe paints his boat. He has three pots of Dulux Paint. He mixes them up with the following ratios

Jim sees Joes boat and wants the same colour Joe gives him the ratios of RGB but Jim uses British paints.

It is immediately apparent that the colours don’t match. The numbers on their own are not sufficient to accurately define a particular colour.

Monitors and printers share this basic problem not all primary colours and inks are the same shade.  A common problem is that the print doesn’t look the same as the monitor

The Solution

The International Color Consortium (ICC) was established in 1993 to create an open, standardized colour management system.

This involves three concepts

• colour models or profiles
• colour spaces
• rendering intents.

Before we can get into the application of colour management we need to cover some basic concepts.

Colour models

Colour is a uniquely human perception.  Computers, as wonderful as they are, only work with numbers so we need a method of describing colours as numbers. These methods are called colour models.

There are three common colour models used:

RGB

The RGB (Red Green Blue) model uses three values to describe the primary colours. This is an additive colour model. We start out with no light (black) and add all the primary colours to arrive at white. Eg 255,255,255 represents white 0,0,0 represents black Monitors and projectors are devices that use this method.

LAB:  Luminance & Chroma A&B Model

Lightness, the A component (green-red axis) and B component (blue-yellow axis)
is based on the human perception of color. The numeric values in LAB describe all the colors that a person with normal vision sees.

CYMK Model

Cyan Magenta Yellow Key (Black) is a subtractive colour model.  The colours are subtracted from the white light illuminating the paper.  Printers are an example of the CYMK model, but the print driver is RGB.

Colourspace

A colour model with no reference to an absolute colour space is a more or less arbitrary colour system.  In other words we have not defined the absolute colours that will display or print.

So we need a Colour space/Profile to describe the range of colours a device can reproduce or print, or describe a particular gamut of colours we can see.

There are two kinds of colour space

• device dependant
• device-independent

A device dependant colour space is one that describes the gamut of a physical device such as a printer or monitor. Normally device-dependant colour spaces are usually called profiles.

A device-independent colour space or working colour space are synthetic colour spaces designed to provide a consistent range of colours with useful editing qualities. One of these qualities is that equal values of RGB will result in a neutral colour white through grey to black.  This is referred to as a well behaved colour space.

Common Working Colour-spaces

sRGB IEC61966-2.1.

• a RGB color space proposed by HP and Microsoft to describe the color gamut of the most common computer display devices. It is the standard color space for displaying images on the internet.

Adobe RGB 1998

• designed (by Adobe Systems, Inc.) to encompass most of the colors achievable on
  CMYK printers, but by using only RGB primary colors on a device such as a computer monitor

Prophoto RGB

• Defined by Kodak (the name says it all).

We can see that different colour spaces have varying gamuts.  Indeed, different devices have different gamuts from each other so we need to translate the colour information between devices and colour spaces.  This is done by the colour management module.

The primary role of the CMM is provide a translation service to convert the colours in one space /device to another so they display or print correctly using ICC profiles.
An ICC profile is a set of data that characterizes colour attributes of a particular device or
Colour space. 

As we can see from the colour space diagrams the gamuts can vary considerably so it is possible that a colour existing in one colour space/profile might not exist in a smaller. For example as AdobeRGB is a larger colour space than sRGB.  When we convert an image to sRGB for web use, the colour may not be reproducible and will need to be modified by the CMM to a colour that exists in sRGB.  This modification is called rendering and how this out of gamut colour is changed is defined by the rendering intent.

Rendering Intents

Some standard colour rendering intents are described as follows.

Perceptual (sometimes call photographic)
If the gamut of the source space is wider than that of the destination space, all colours are compressed to fit the destination space. If the colours of the source space are smaller than the destination no change is made.

Relative Colourimetric
If the gamut of the destination space is smaller than the source, identical colours are mapped 1:1.  All colours that are not present in the destination space are mapped to the closest colour, usually at the edge of the colour space.  The white point is mapped to the white point of the destination if they differ. This may result in different colours in the source space being mapped to the same colour. This is the rendering intent used with monitors.

Absolute Colourimetric
This is essentially the same as relative colourmetric, except the white point is not mapped.

Summary

Colour management is about defining absolute colours in order to correctly display or print the images as close to the original as possible. Sometimes the gamut of our medium may limit our ability to reproduce a particular colour, so we need to alter colours as defined by the rendering intent.

I have deliberately avoided discussing the practical application of this theory as it applies to Photoshop as that is the topic for another article.