Measuring colour
Why the standard observer and CIE XYZ were needed in 1931, what X/Y/Z and x/y mean, how to read a chromaticity diagram and why Rec.709, P3 and Rec.2020 gamuts are drawn on it.
CIE XYZ is a common measurement language for colour. It lets us describe different RGB systems through one reference coordinate system and compare them independently of a particular display or camera.
Quick reference
Foundation of XYZ and the 2° standard observer.
A device-independent reference system for measurement and transforms.
Σ = X + Y + Z; normalisation removes overall luminance scale.
Do not confuse it with Y′ luma in YCbCr.
Primary coordinates are plotted to scale; the outer spectral-locus silhouette is intentionally schematic and is not a precision measurement plot.
Useful for primaries and white points, but xy alone does not retain absolute luminance.
If every camera and display uses its own primaries, RGB values alone cannot provide universal interchange. A device-independent reference system is needed to describe primaries, white points and measurements. Modern colorimetry uses CIE tristimulus systems for that role.
CIE 1931 is rooted in colour-matching experiments in which observers adjusted amounts of three reference stimuli to visually match a test stimulus. The results were standardised as colour-matching functions, converting a perceptual matching task into a reproducible numerical system.
The CIE 1931 standard observer represents averaged colour-matching properties for a small field of view. The modern standard describes use for visual fields of roughly 1° to 4° under photopic adaptation. A supplementary 10° observer from 1964 represents larger fields. These are standardised functions, not the literal spectral sensitivity of one individual.
XYZ are tristimulus coordinates obtained by integrating a spectral stimulus against the CIE colour-matching functions. The system was designed to represent visible colour stimuli conveniently with positive coordinates. Y is deliberately tied to photopic luminous efficiency, so under standard conditions Y carries luminance information while X and Z complete the chromatic description.
Chromaticity removes the overall intensity scale and keeps relative proportions: x = X / (X + Y + Z), y = Y / (X + Y + Z), z = Z / (X + Y + Z). Because x + y + z = 1, x and y are sufficient for a 2D diagram. But normalisation discards absolute luminance, so an xy diagram cannot fully describe colour by itself.
The curved outer boundary is the spectral locus: chromaticities of monochromatic stimuli across wavelengths. The straight lower boundary connects the extreme red and violet regions and is called the line of purples; those chromaticities do not correspond to one single wavelength. Interior points represent chromaticity mixtures but do not show luminance.
Positive mixtures of three RGB primaries lie inside the triangle whose vertices are the primary coordinates. This makes it convenient to compare Rec.709, P3 and Rec.2020 on one diagram. However, triangle area in xy is not a linear measure of perceived colourfulness because CIE xy is not perceptually uniform.
D65, D50 or the ACES reference white can be plotted as ordinary x,y coordinates on the same diagram. The white point defines which primary mixture is treated as neutral white. Changing white point is therefore not equivalent to rotating Hue; it changes the reference neutral of the whole system.
CIE xy does not show luminance, is not perceptually uniform and visually exaggerates some gamut regions. Other spaces and uniform chromaticity diagrams are used for perceptual distance, but xy remains fundamental for defining video primaries and white points, which is why it appears constantly in specifications.
Resolve exposes xy coordinates in custom colour spaces, chromatic adaptation and technical settings. When you see D65 or an x/y pair, it refers to chromaticity, not gamma, saturation or an RGB code value. CST normally wraps standardised primary/white-point sets so you do not have to enter coordinates manually.
The most useful skill after this article is being able to read any standard's primaries and white point as geometry on CIE xy while treating its transfer function as a separate layer of signal description.
Technical definitions are anchored to standards bodies and primary specifications wherever that distinction matters.
