Managing color profiles and gamuts

The gamut of an output device (eg. monitor, printer) is the set of colors it can display. There are some colors 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 color – 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 color 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 color spaces have larger gamuts, roughly equivalent to high quality printers or film. Some have extra wide gamuts, covering most of the colors the eye can see and even colors we cannot see such as infrared. Maintaining consistent color appearance in the translation between different devices and color spaces is not straightforward, but color management provides a reasonably succinct and practical solution. While color management can’t make two devices with different gamuts display exactly the same colors, it can represent colors more accurately than working ‘blind’.

In a color-managed workflow, the color 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 color image data. Profiles can define color in devices or image color spaces. All digital images refer to a color 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 color space to another
• Displaying an image in memory on a monitor
• Outputting an image in memory to a printer

How to convert between color profiles

One way to do this would be to use our free online image converter, which can convert between any color profile and RGB, CMYK or grayscale conveniently. Our full product also supports color profiling as part of the download process.

For more information on color 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 color profiling in action?

It is possible to see color 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 color management, you can see distorted color rendition when you compare an unprofiled image to a profiled image. In a color managed application both images look the same.

Colour profile test image 1

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.

Colour profile test image 2

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).