CIE XYZ
This article is designed to get some of the basics of colour theory across to photographers and to prepress professionals.It is not written for colour scientists. So you won't find the maths here!
For background information on the CIE, who define colour models, please see this article.The basic CIE colourmodel, or 'colour space', is based on a 'Standard Observer' and 'StandardIlluminants' (D50, etc.). Thisis a numerical model of colour sensitivitybased on research, commenced in the 1920s, on a sample of people withnormal colour vision.CIE XYZ is a 'universal colour space' representing the colour spectrumvisible to the 'average human'.
The light-sensitive retina at the back of the eye is composed of light-sensitive cells known as
The CIE XYZ tristimulusvalues are assigned to the 'red, green and blue' 'curves'respectively. These curves approximate to thecones in the eye. The relative response of each is plotted on a diagramagainst thewavelength in nanometres.
There are two axes;
The vertical axis represents Relative Response 0 - 2.0 (shown here) orReflective Intensity 0 - 120% (not shown).
The horizontal axis represents Wavelength innanometres, usually from about 380 to about 720.
Why not simply use RGB or CMYK?
The CIE XYZ colour model is a 'device-independent' or 'fixed' colour space, whereas RGB, for example, varies with every individual device(monitor, scanner, camera, etc.), having its own version of RGB. Even 'standard' RGB colour spaces, such as sRGB are based on actual or theoretical devices, such as old-fashioned CRT displays. So you don't need to know which XYZ colour space is being referred to, as there is only one!
Devices measuring XYZ
XYZ is typically used to reportthe spectral response of a sample measured by a colorimeter or aspectrophotometer. Acolorimeter may contain as few as three sensors, one each for red,green and blue, (or X,Y and Z), and will typically be used forcalibrating and profiling displays. A spectrophotometer will report the entire spectral response at frequent intervals along the spectrum, say every 10 nanometres, andwill typically be used to measure printed sheets, either to control a press, or in conjunction with ICC profiling software, to create a printer profile.
Another common use is astheProfile Connection Space (PCS)within an ICC profile, where it may be used instead of CIE Lab.
While CIE XYZ is used to report colour from measuring instruments,it is not so useful for humans to describe colour. In the real world CIE XYZ isn't as useful to experienced photographers and retouchers as CIE Lab is.
You may notice that theY ('greencurve') covers thewidest wavelength. This corresponds to the overall human visualresponse to all colours, orlightness.It is therefore also used to indicate luminance ('lightness').
TheCIE xy Chromaticity Diagram & Colour Gamut
Thesecoordinatesare used to show the spectrum visible to humans in aChromaticity Diagram or a 'horseshoe-locus'. They are used to map adevice's colour gamut showing therange of coloursthat can be reproduced against the visible spectrum. They can also beused to compare differentdevices, as in the example here, where gamuts from a typical offsetpress and a typical LCD display are shown. The White Point of the Standard Illuminant D50, which is 0.34x, 0.35y, is also shown.
The co-ordinates are calculated fromratios of the XYZ tristimulus values.
The two axes are:x which ishorizontal, andy which is vertical.Thex ("little-x") andy ("little-y") are lower-case.
Unfortunately it cannot show luminance.
In order to show luminance Y (upper-case) is used. Inthis case it becomes known as xyY .
Colours in the chromaticitydiagram are not perceptually uniform.
For background information on the CIE and theirLab andLchColourSpaces please see the previous article .
This article is intended to give photographers, designers, etc. an introduction to colour theory, and is not intended for colour scientists!
The Wikipedia page on CIE colour spaces has a morein-depth explanation of the mathematics involved.
http://en.wikipedia.org/wiki/CIE_1931_color_space
If you like thisarticle, please link to it from your site. However please do not copythe text or hot-link the images.
- Basictheory of the CIE XYZ Tristimulus Colour co-ordinates and the xychromaticity (colour gamut) diagrams.
OurRemote Custom Printer Profiling Service
This page updated February 13, 2024.
