Short answer
CIE XYZ is the 1931 international standard for turning a light's spectrum into three numbers that predict whether two lights will match for an average observer. Every other colorimetric space — sRGB, CIELAB, OKLab — is defined from it. Its Y value is luminance, and dividing X and Y by the total gives the chromaticity coordinates x and y, which place every colour on the familiar horseshoe diagram regardless of brightness. XYZ predicts matches, not how different two colours look.
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Colour matching experiments in the late 1920s had observers match spectral lights with mixtures of three primaries. The CIE standardised the averaged results as the 1931 standard colorimetric observer: three colour-matching functions, x̄, ȳ and z̄, tabulated across wavelength. Multiplying a light's spectral power distribution (or a surface's reflectance times an illuminant) by each function and summing gives X, Y and Z. Two stimuli with equal XYZ should match for the standard observer even if their spectra differ — which is metamerism. The functions were chosen so that all values are positive and so that ȳ equals the eye's luminous efficiency, making Y the luminance.
Dividing X and Y by X + Y + Z gives x and y, a two-dimensional description of colour regardless of intensity. Plotted, the pure spectral colours trace a horseshoe-shaped curve, the spectral locus, closed at the bottom by the line of purples; every visible colour lies inside. An RGB space's primaries are three points, and its gamut is the triangle between them; a white point is one point near the centre. The diagram is excellent for comparing gamuts and white points and misleading for anything perceptual: equal distances do not look equal, and the green region is hugely stretched. The CIE 1976 u′v′ diagram reduces that distortion.
A chromaticity diagram printed on paper or shown on screen cannot show its own outer colours; those lie outside the gamut of any printer or display.
The 1931 observer describes vision in a 2° field — roughly a thumbnail at arm's length — and averages a small number of people. The CIE added a 10° observer in 1964 for larger fields, and has since published cone-fundamental-based functions. Individual observers differ from any standard, which is why two people can disagree about a metameric match that the maths says is perfect, and why industries that assess large samples often compute with the 10° functions. CIE 015:2018 is the current reference for both observers and the calculation methods.
Because XYZ is device-independent and additive — the XYZ of two lights added together is the sum of their XYZ — it is the natural connection between devices. An RGB space is defined by a 3×3 matrix to XYZ (derived from its primaries and white point) plus a transfer function; ICC profiles connect devices through a profile connection space based on XYZ or CIELAB relative to D50. Converting between two RGB spaces means going through XYZ, with chromatic adaptation if their whites differ. The table on this page shows XYZ and xy for sRGB's primaries, white and grey: note that grey and white share the same chromaticity, differing only in Y.
| Colour | X / Y / Z | x, y |
|---|---|---|
| sRGB red primary (#ff0000) | 41.2 / 21.3 / 1.9 | 0.6400, 0.3300 |
| sRGB green primary (#00ff00) | 35.8 / 71.5 / 11.9 | 0.3000, 0.6000 |
| sRGB blue primary (#0000ff) | 18.0 / 7.2 / 95.0 | 0.1500, 0.0600 |
| White (#ffffff) | 95.0 / 100.0 / 108.9 | 0.3127, 0.3290 |
| Mid grey (#808080) | 20.5 / 21.6 / 23.5 | 0.3127, 0.3290 |
| Bright mid blue (#1e90ff) | 28.6 / 27.4 / 98.4 | 0.1850, 0.1777 |
Each statement is labelled by kind — established fact, a standard’s requirement, observed market data, a convention, or Colourwise’s own interpretation or analysis — with the strength of the evidence behind it.
StandardStrong evidence
CIE 015:2018 (Colorimetry, 4th edition) is the CIE's reference for the 1931 2° and 1964 10° standard observers and for calculating tristimulus values, chromaticity coordinates and colour differences.
FactStrong evidence
The CIE openly publishes its colour-matching functions and standard illuminant data as downloadable tables.
Source: CIE datasets (colour-matching functions, illuminants)
Colourwise analysisStrong evidence
Computed from sRGB's definition, sRGB red, green and blue have chromaticities (0.64, 0.33), (0.30, 0.60) and (0.15, 0.06), and white and every neutral grey share D65's chromaticity (0.3127, 0.3290).
Based on: Computed by Colourwise from its sRGB-to-XYZ conversion; see the table on this page.
Source: International Electrotechnical Commission (IEC) webstore and catalogue; CIE 015:2018 Colorimetry, 4th edition
FactStrong evidence
Equal distances on the CIE 1931 xy diagram do not correspond to equal perceived colour differences; the 1976 u′v′ diagram was introduced to reduce this non-uniformity.
Source: CIE 015:2018 Colorimetry, 4th edition; CIE (International Commission on Illumination) publications
Reviewed 29 September 2026. Colourwise summarises its sources in its own words and does not reproduce standards text or proprietary colour data. Spotted an error? Tell us.