Colour Management

Colour management is the definition and conversion of colour in digital imaging systems.

Colour Space

Because human eyes have three types of colour receptors corresponding roughly to the primary colours of light (red, green and blue) one could say that colour is three-dimensional, and it is treated as such by most digital imaging systems. In most computer programs you can choose colours using a three-dimensional colour-picker:

• Red, Green and Blue
• Hue, Saturation and Brightness (also known as Luminance or sometimes just Value)

It doesn’t matter which three parameters are used to describe colour, but the international standard is the CIE (International Commission on Illumination) XYZ Colour Space. For technical reasons this is a strange standard that is quite difficult to understand, but the important thing is that it enables the precise definition of any colour.

RGB Colour Space

An RGB colour space can be shown as a cube with axes corresponding to the levels of red, green and blue. At one corner is black where the levels of red, green and blue are all 0%. At the opposite corner is white where the levels of red, green and blue are all 100%. There are also pure red, green and blue corners (see Through the 6x6x6 Color Cube - An Interactive Voyage). The colour space is precisely defined in terms of the CIE XYZ Colour Space.

For example, a television has three types of phosphor dots corresponding roughly to the three primary colours of light (red, green and blue). The range of colours that the television can display is determined by the exact colour and intensity of the red, green and blue phosphors, defined in terms of the CIE XYZ Colour Space.

White Point

Once the red, green and blue points are defined, the "white point" of the colour space is also defined - it is the colour produced when red, green and blue are all 100%.

This is not necessarily a "true" white at all, but is usually close in colour to the light emitted by a hot black body. The white point of television and the internet is the Europe Standard D65, which corresponds to a hot black body at 6500 Kelvin.

Rendering Intent

Because different devices and systems have different colour spaces it is often necessary to convert an image from one colour space to another. If the new gamut is smaller than the old gamut, then some colours cannot be mapped precisely - they are said to be out-of-gamut. In such cases a compromise must be made, and the method one chooses for the conversion is called the rendering intent. There are four rendering intents in common use: Perceptual, Saturation, Relative Colorimetric and Absolute Colorimetric. For photographic images the most useful rendering intents are Perceptual and Relative Colorimetric:

Perceptual

With perceptual rendering intent the conversion is such that overall the new image looks like the old image - accuracy is sacrificed to produce an attractive image. All colours are adjusted so that the old gamut maps to the new gamut, and out-of-gamut colours are brought into gamut.

Relative Colorimetric

With relative colorimetric rendering intent out-of-gamut colours are mapped to the nearest available colour. The white point of the old colour space is mapped to the white point of the new colour space, and all other colours are adjusted accordingly.

Colour Managed Workflow

The international standard default colour space is called sRGB, which has been chosen to approximate the displays that most people use. So if you publish a digital image file in the sRGB Colour Space on the internet, it will display correctly on most peoples computers, even if they have no knowledge of colour management and have not calibrated their monitor. Most digital cameras produce JPEG (Joint Picture Experts Group) or TIFF (Tagged Image File Format) files in the sRGB Colour Space, so that they are ready for immediate use by most people. Furthermore, most modern printers assume that documents are in the sRGB Colour Space, so that they print correctly without any special adjustment. However, these default systems rarely work correctly, and most people are using uncalibrated equipment so the results are average at best. In practice one must actively use Colour Management to achieve good results.