Foreword

This article was written by Joe Williams who, in his own words, "... did it so I wouldn't need to rediscover it all six months from now. Perhaps I can send it to a newbie someday to help get him over the mountain that took me two weeks to climb." I'm pleased to make it available here.

Opinions expressed are those of the author and do not necessarily represent the views of www.rogercavanagh.com. Actually, I pretty much agree with everything Joe says - I just couldn't resist putting that in.

For more information on Colour Management, see the links on this page.

Introduction

A Colour Management System (CMS) is essential for serious colour photography. If you ignore it, you may get acceptable results, but understanding CMS is essential for top quality photographic images and prints.

Colour is device dependent, so a colour image data file will produce slightly different colours depending on the device used to capture or display it. If you scan a photo into a computer and then print it, the resulting photo is unlikely to match the original, at least not exactly. Each device works in a different colour space, so an image file will not produce exactly the same colour on every device.

One way to compensate for the device-dependent nature of computer image files is to use ICC profiles. An ICC profile defines the colour space in which a particular device works. Every computer file that represents a colour image exists in some colour space, whether or not it is tagged with an embedded ICC profile. By calibrating a device against known colour standards, a device profile can be produced that describes that device’s colour reproduction.

Colour images are represented in computer files as numbers. In an RGB file, there are three numbers per pixel to represent the values of red, green, and blue in the RGB additive colour model. The more bits used for each number, the more variations of colour can be represented. In an 8-bit JPEG file, for example, there are three bytes per pixel – one for each colour: red, green, blue - so a total of 2^24 colour shades can be represented. That equals 16,777,216 shades of colour. Typically a digital camera will record 12 bits per pixel, so it is theoretically able to represent 2^36, or 68.7 billion colours. Strictly speaking, apart from the Sigma SD9, each pixel in a digicam captures only one colour with a matrix arrangement that allows interpolation of full colour at each site from the surrounding pixels.