This document contains a brief outline of a colour management system and definitions of the more common terms associated with it.

Those looking for more authoritative documentation can find it at the International Color Consortium's website, currently at http://www.color.org.

Useful factsheets on setting up colour management in Photoshop CS and CS2, as well as more general information, can be found at Native Digital's website.

ICC Colour Management

ICC Colour Management provides a way to calibrate and maintain a computer system's colour output relatively easily and allows the transfer and the correct viewing of images between systems thus calibrated.

The conversions outlined are carried out by a Colour Management Module (CMM). This is a set of routines made available to an image processing application.

Examples of CMMs are the Windows ICM, Adobe's ACE and LittleCMS, an open source CMS.

The PCS

The CMM maintains an image within the Profile Connection Space (PCS). This is an internal device-independent space which can represent all colours. The CMM also handles the movement of an image into the PCS by using input profiles to convert an image's colours to the chosen colour space on introduction, display profiles to convert the image correctly for rendering to the screen, and output profiles to convert the image from the chosen colour space into the gamut of the output device, normally a printer.

Colour Models.

Colours are often described in a relatively abstract mathematical way. Normally this involves three (in the case of RGB and LAB) or four (in the case of CMYK) values or colour components.

In the RGB Colour Model one can describe a 3-D shape with (say) Red on the x-axis, Green on the y-axis and Blue on the z-axis. The 3-D shape so defined contains all theoretical colours.

Examples of colour models are RGB, CMYK and LAB.

Colour Spaces.

Although all the colours in such a 3-D shape are theoretically possible, in practice this is not so. Hence it is necessary to apply a mapping function to the colour model to define a subset of colours which are reproducible. This subset of colours is called a colour space and it occupies a volume, and is completely contained, within our original 3-D shape.

Examples of colour spaces within the RGB colour model are sRGB, AdobeRGB and ProPhotoRGB.

In the CMYK colour model, a colour space may refer to any of a large number of printer's ink sets, examples are Euroscale, US Sheetfed and Japan Coloured.

Color space is thus a term for a certain combination of a color model plus a mapping function. The term color space is often incorrectly used to also identify color models since identifying a color space automatically identifies the associated color model. Thus although several color spaces are based on the RGB colour model, it would be wrong to refer to the RGB color space.

Gamut

The range of colours occupied by a colour space within a colour model is called the space's Gamut. Any colour which does not fall within this volume is referred to as Out-of-Gamut.

Colour Profiles

A Colour Profile is a piece of computer code which contains a mapping function. It can stand alone or be contained within an image file. There are a number of different types of profile but the most common are:

Document Profiles
This is often a small generic piece of code which defines the colour space in which the colours in an image reside. Several such profiles are normally available on a computer which is capable of colour management normally recognisable by having their colour space name attached to them (such as 'sRGB Colour Space Profile.icm' or 'AdobeRGB1998.icc'). When there is need to embed a profile into an image, it is a document profile which should be embedded. Only images with document profiles applied to them should be edited in a colour managed workflow.

Input Profiles
An input profile is associated with an input device such as a scanner or digital camera. If the computer-based software which operates the input device is colour management-aware then it should assign the input profile to the image prior to its introduction into the desired editing colour space by conversion with the associated document profile.

As many input devices do not have input profiles it is necessary for an image to be converted by a document profile manually if it is to be a part of a colour managed workflow.

Many digital cameras will associate a colour space with an image; normally this will be sRGB although more advanced cameras may give the choice of AdobeRGB as well. This process does not assign a profile to the image (except in a few older cases) but the information gives a suitably capable image manipulation application the information needed to assign the correct profile on loading, however this may require some user input.

Display Profiles
A Display Profile is required to allow the monitor to show the colours correctly managed.

A monitor will always require some degree of calibration to allow it to display colours correctly even in the presence of a monitor profile. This can be achieved moderately well using software such as Adobe's Gamma utility but the best results are achieved using a colorimeter to analyse the monitor's output and create a custom display profile.

Output Profiles
An output device, such as a printer, must have an output device profile to allow the mapping of the image colours to the gamut of the device. Such profiles are normally provided by the device manufacturer.

Rendering Intents

When a colour managing image processing application sends an image to the monitor it does so using the monitor display profile which relates the colours in the colour space assigned to the image to the capabilities of the monitor. Normally this works well because the common colour space gamuts fit well with the monitor's gamut. When the same image is sent to a printer this is not often the case. The inks used in the printer work in a way which reduces the saturation available as the image darkens and a printer has problems rendering shades as they approach black and bright colours will not be as bright as is possible on-screen. In other words a printer's gamut is narrow.

In consequence the printer's output profile must convert any of the image's colours which the printer cannot render into colours which it can. This conversion can be handled in a number of ways according to the Rendering Intent.

Relative Colorimetric
One such method might be to simply clip all the out-of-gamut colours to the nearest available colour while leaving all the in-gamut colours alone. This is the Relative Colorimetric rendering intent. The image's white point is also remapped to coincide with the white point in the new gamut. This may be a good choice when converting an image from a small-gamut space to one with a larger gamut.

Perceptual
Conversely the Perceptual rendering intent converts an image by maintaining the relationship between all the colours in an image whilst pulling the out-of-gamut colours into gamut. Thus all the colours in the image are altered but the relationship between them is preserved. This will work best when converting from a larger-gamut space to a smaller one.

Absolute Colorimetric
The Absolute Colorimetric rendering intent is very much like relative colorimetric except for the handling of the white point. The relative method remaps the white point from its position in the the original gamut to the white point of the new gamut whereas the absolute method does not. The absolute method can thus introduce colour shifts.

Saturation
The Saturation rendering intent aims to maintain saturation during conversions. It is more useful when moving from a more restricted gamut to a wider one. Accurate colour relationships are not maintained during conversion.

The latter two intents are of little interest to image manipulators and photographers. Which of the former intents is chosen is largely down to the individual. Many photographers would always recommend the use of relative colorimetric but maybe the choice is better made on consideration of the individual image requirements.

Choosing a Colour Space

The format of the image may determine what colour space is appropriate. If an image is for publication it is likely to be CMYK. Thus the colour space and the profile will be dictated by the requirements of the printing method, notably the inkset used.

When using RGB images the choice of colour space is largely personal as the differences between many RGB colour spaces is not great. A colour space encompassing the colours present in the gamuts of the devices used would be perfect but is often not possible so it is necessary to choose that which gives the nearest fit.

This is difficult to test in practice and many photographers have concluded, through experience, that AdobeRGB is a better choice than sRGB as the loss of saturation is more than compensated for by the increased range of colours available.

sRGB
The most common space is sRGB which was primarily designed to show well on a CRT monitor from the early 1990s. It is the default space for viewing images on the web and, if that is the intended target for an image, then the choice of colour space should be sRGB.

AdobeRGB
The gamut of the sRGB space is quite limited and many devices can capture or display a wider range of colours than sRGB allows. AdobeRGB was defined to give a wider gamut but does suffer a drop in saturation level. It is more suited to colour printing and is currently the normal choice for almost all photographers who are in a position to choose.

ProPhotoRGB
A newer colour space, called ProPhotoRGB, is gaining in popularity in some circles. It has a much wider gamut than even AdobeRGB but suffers few of the saturation problems normally associated with wide gamut spaces. ProPhotoRGB encompasses better the wide range of colours detectable by modern high-end digital cameras but no output device is currently able to match this. Development in this area is rapid and it is likely that this will change but it may be a space to avoid until this occurs.