Professional Colour Management

Managing Colour Profiles and Gamuts

The gamut of an output device (eg. monitor, printer) is the set of colours it can display. There are some colours which can't be made up of a mixture of red, green and blue phosphor emissions and so can't be displayed by any monitor, but could be printed on paper with appropriate inks and paper stock. Conversely, a bright luminous green colour - readily displayed on a monitor - can't be printed on a matte card stock with any conventional ink. How do we prepare images for different output gamuts reliably considering these differences?

To accommodate the wide range of gamuts in different devices - digital cameras, film, scanners, monitors, and printers - a variety of colour spaces have been developed. The standard for the Internet and desktop computers, sRGB, has a relatively small gamut corresponding to a typical CRT monitor. Other colour spaces have larger gamuts, roughly equivalent to high quality printers or film. Some have extra wide gamuts, covering most of the colours the eye can see and even colours we cannot see such as infrared. Maintaining consistent colour appearance in the translation between different devices and colour spaces is not straightforward, but colour management provides a reasonably succinct and practical solution. While colour management can't make two devices with different gamuts display exactly the same colours, it can represent colours more accurately than working 'blind'.

In a colour-managed workflow, the colour response of each device, each image file, and each image in the computer's active memory is characterised by a file called an ICC profile.

ICC profiles are files that define the meaning of numeric colour image data. Profiles can define colour in devices or image colour spaces. All digital images refer to a colour space - if none is supplied, the default is usually sRGB. Gamut mapping is the transformation or conversion that takes place when an image is transferred between devices, for example:

• Changing an image from one colour space to another
• Displaying an image in memory on a monitor
• Outputting an image in memory to a printer

For more information on colour profiling, you may wish to visit The International Color Consortium (ICC) or, if your main graphics package is Adobe PhotoShop, go to the ICC support pages at Adobe.com.

How can I see colour profiling in action?

It is possible to see colour profiling 'go wrong' on the web. Since a typical web browser (with certain exceptions under the Apple Mac OS X operating system) does not perform any colour management, you can see distorted colour rendition when you compare an unprofiled image to a profiled image. In a colour managed application both images look the same.

The colors in this chart should look undersaturated in web browsers, which are not ICC aware, but they should look correct - nearly identical to the sRGB image below - in a properly set up ICC-aware application.

If your monitor is well calibrated, this chart should appear nearly identical to the physical chart viewed under appropriate lighting (except perhaps for the cyan patch, third row, right, which is outside the sRGB gamut).