Compare area coverage of all standard gamuts plus your custom primaries against the CIE spectral locus and sRGB baseline.
Paste HEX values (one per line, max 50) to compute xy, u′v′, L*a*b*, C*h° for each colour.
CIE 15:2018 — Colorimetry (4th Edition)
CIE 15:2018 is the primary CIE technical report on colorimetry. It defines the CIE 1931 Standard Colorimetric Observer (2°) and the CIE 1964 Supplementary Standard Colorimetric Observer (10°), including the colour-matching functions x̅(λ), y̅(λ), z̅(λ) that underpin all chromaticity diagrams.
The report also defines the CIE 1931 (x, y) chromaticity diagram, the CIE 1976 Uniform Chromaticity Scale (u′, v′) diagram, standard illuminants (A, D50, D65, etc.), and colour-difference formulae (ΔEab, ΔE94, ΔE00).
CIE S 014-1:2006 — CIE Standard Colorimetric Observer
This CIE standard defines the colour-matching functions of the CIE 1931 2° and CIE 1964 10° standard observers at 1 nm resolution from 360–830 nm. The 2° observer is based on the Wright–Guild experiments (1931), while the 10° observer uses Stiles–Burch data (1959).
The chromaticity coordinates x = X/(X+Y+Z), y = Y/(X+Y+Z) derived from these functions define the horseshoe-shaped spectral locus that forms the boundary of all possible chromaticities for real stimuli.
CIE 1976 UCS — Uniform Chromaticity Scale
The CIE 1976 Uniform Chromaticity Scale (u′, v′) was introduced to provide a more perceptually uniform chromaticity representation than the CIE 1931 (x, y) diagram. Equal Euclidean distances in u′v′ correspond more closely to equal perceived colour differences.
The transformation is: u′ = 4X/(X+15Y+3Z) = 4x/(−2x+12y+3), v′ = 9Y/(X+15Y+3Z) = 9y/(−2x+12y+3). The earlier CIE 1960 UCS uses u = u′, v = (2/3)v′.
MacAdam (1942) — Chromaticity Discrimination Ellipses
David L. MacAdam (1942) published his seminal paper "Visual Sensitivities to Color Differences in Daylight" measuring just-noticeable difference (JND) ellipses at 25 chromaticity centres in the CIE 1931 diagram. These "MacAdam ellipses" demonstrate that the CIE 1931 (x, y) diagram is highly non-uniform — the ellipses vary enormously in size and orientation.
The n-step ellipses (this lab shows step-2) enclose the region where colour differences are below n JNDs. MacAdam's data motivated the development of more uniform colour spaces (CIELUV, CIELAB) and the CIE 1976 u′v′ diagram.
IEC 61966-2-1:1999 — sRGB
The sRGB standard defines the colour space for consumer displays, web content, and the default RGB interpretation in most imaging workflows. Primaries: R(0.64, 0.33), G(0.30, 0.60), B(0.15, 0.06), white point D65 (0.3127, 0.3290). The gamut triangle covers approximately 35.9% of the CIE 1931 spectral locus area.
ITU-R BT.2020-2 — Rec. 2020
Rec. 2020 defines ultra-high-definition television (UHDTV) colour primaries at R(0.708, 0.292), G(0.170, 0.797), B(0.131, 0.046) with D65 white point, covering approximately 75.8% of the CIE 1931 spectral locus — more than double sRGB.
The primaries are located on the spectral locus at approximately 630 nm (R), 532 nm (G), and 467 nm (B), representing monochromatic light. No current display technology can reproduce the full Rec. 2020 gamut.
ISO 11664-6:2014 — CIEDE2000
ISO 11664-6:2014 defines the CIEDE2000 colour-difference formula (ΔE00), the current state-of-the-art metric for quantifying perceptual colour difference. It incorporates lightness, chroma, and hue weighting functions plus a rotation term for the blue region, correcting systematic errors in ΔE*ab (CIE76).
ICC.1:2022 — ICC Profile Connection Space
ICC profiles define a Profile Connection Space (PCS) using CIE XYZ (D50) or CIELAB (D50). All colour transformations in ICC workflows pass through the PCS, requiring chromatic adaptation to/from D50. The chromaticity diagram shows where D50 sits relative to other illuminants and colour space white points.
Tristimulus Values & Chromaticity Coordinates
Y = ∫ S(λ) · y̅(λ) dλ
Z = ∫ S(λ) · z̅(λ) dλ
x = X / (X + Y + Z)
y = Y / (X + Y + Z)
z = 1 − x − y
S(λ) is the spectral power distribution, x̅, y̅, z̅ are the CIE colour-matching functions. The chromaticity coordinates (x, y) project the 3D tristimulus space onto a 2D plane, discarding luminance information (Y).
Uniform Chromaticity Scale Transforms
u′ = 4x / (−2x + 12y + 3)
v′ = 9y / (−2x + 12y + 3)
Inverse:
x = 9u′ / (6u′ − 16v′ + 12)
y = 4v′ / (6u′ − 16v′ + 12)
CIE 1960 (u, v):
u = u′
v = (2/3) v′
sRGB Gamma & XYZ Matrix
Linear → sRGB: c ≤ 0.0031308 ? 12.92c : 1.055c1/2.4 − 0.055
MsRGB→XYZ =
[0.4124564 0.3575761 0.1804375]
[0.2126729 0.7151522 0.0721750]
[0.0193339 0.1191920 0.9503041]
CIELAB (L*a*b*)
a* = 500 [f(X/Xn) − f(Y/Yn)]
b* = 200 [f(Y/Yn) − f(Z/Zn)]
f(t) = t1/3 if t > 216/24389
f(t) = (24389/27 · t + 16) / 116 otherwise
ΔE Colour Difference Formulas
ΔE = √[(ΔL*)² + (Δa*)² + (Δb*)²]
ΔE*94:
ΔE = √[(ΔL*/SL)² + (ΔC*/SC)² + (ΔH*/SH)²]
SL=1, SC=1+0.045C*₁, SH=1+0.015C*₁
ΔE00 (CIEDE2000):
Includes L′, C′, H′ corrections, parametric weighting SL, SC, SH, and rotation term RT for the blue region.
Gamut Primary Coordinates & Matrix Derivation
[Xw] [Xr/yr Xg/yg Xb/yb] [Sr]
[Yw] = [1 1 1] · [Sg]
[Zw] [Zr/yr Zg/yg Zb/yb] [Sb]
M = [Sr·Xr/yr Sg·Xg/yg Sb·Xb/yb]
[Sr Sg Sb]
[Sr·Zr/yr Sg·Zg/yg Sb·Zb/yb]
This lab's custom gamut panel computes both M (RGB→XYZ) and M⁻¹ (XYZ→RGB) live from your custom primary coordinates.
Planck Locus & Correlated Colour Temperature
T ≤ 4000 K: x = −0.2661e9/T³ − 0.2344e6/T² + 0.8777e3/T + 0.1799
T > 4000 K: x = −3.0258e9/T³ + 2.1070e6/T² + 0.2226e3/T + 0.2404
The Planck locus traces the chromaticity of a blackbody radiator from ~1000 K (deep red) through ~6500 K (daylight white) to ~∞ K (blue-white). The isotemperature lines are perpendicular to the locus, marking constant CCT.
MacAdam Ellipses
y(t) = y₀ + a·cos(t)·sin(θ) + b·sin(t)·cos(θ)
t ∈ [0, 2π]
This lab renders 24 MacAdam ellipses at step-2 (2× the original JND values). In the CIE 1931 diagram, ellipses in the green region are much larger than those in the blue — demonstrating the diagram's perceptual non-uniformity. In the CIE 1976 u′v′ diagram, the ellipses become more nearly circular.
CIE Colorimetry & Chromaticity
Smith, T. & Guild, J. (1931). The C.I.E. Colorimetric Standards and Their Use. Trans. Optical Society 33(3):73–134.
CIE (1964). Proceedings of the CIE 15th Session, Vienna. Definition of the CIE 1964 10° Supplementary Standard Observer.
CIE 15:2018. Colorimetry, 4th Edition. Standard illuminants, observers, CIELAB, colour-difference formulae.
CIE S 014-1:2006. Colorimetry — Part 1: CIE Standard Colorimetric Observers. High-resolution CMF tabulation.
Perceptual Uniformity & Discrimination
CIE (1976). Supplement No. 2 to CIE Publication 15. Recommendation of CIE 1976 UCS (u', v') and CIELAB.
Wyszecki, G. & Stiles, W. S. (2000). Color Science: Concepts and Methods, Quantitative Data and Formulae. Wiley. Comprehensive reference for colour science fundamentals.
Colour Difference Metrics
ISO 11664-6:2014. CIEDE2000 colour-difference formula. Official specification.
CIE 142:2001. Improvement to Industrial Colour-Difference Evaluation. Precursor documentation for CIEDE2000.
Display Colour Spaces & Standards
ITU-R BT.2020-2 (2015). Parameter values for ultra-high definition television. Rec. 2020 primaries.
SMPTE ST 2065-1:2021. Academy Color Encoding Specification (ACES). ACEScg primaries.
ICC.1:2022-05. Image technology colour management. Profile Connection Space definition.
Kang, B., Moon, O., Hong, C. & Lee, H. (2002). Design of advanced color-temperature control system for HDTV applications. J. Korean Physical Society 41(6):865–871. CCT to xy conversion.
About this tool
This tool implements the CIE 1931 (x, y) and CIE 1976 (u′, v′) chromaticity diagrams with support for 2° and 10° standard observers, multiple reference whites and illuminant sets, gamut overlays (sRGB, Display P3, Rec. 2020, Adobe RGB, ACEScg, custom), MacAdam ellipses, spectral locus, Planck/black-body locus, isotemperature lines, CIEDE2000 colour difference playground, and batch chromaticity analysis — entirely client-side (zero network). Not a substitute for calibrated measurement or ICC profile software.
Enable MacAdam ellipses and switch between CIE 1931 xy and CIE 1976 u′v′ modes to visualise the dramatic improvement in perceptual uniformity. In the xy diagram, ellipses in the green region are up to 20× larger than those near blue (480 nm), demonstrating severe non-uniformity. In u′v′, the variation reduces to approximately 4:1.
CIE 1931 xy: ≈ 20:1
CIE 1976 u′v′: ≈ 4:1
CIELAB (a*b*): ≈ 2:1 (not a chromaticity diagram, but included for comparison)
Use the Actions tab gamut comparison to quantify the area coverage of standard colour spaces. Key findings:
- sRGB: ~35.9% of CIE 1931 spectral locus — the web/consumer baseline.
- Display P3: ~45.5% — widely adopted in Apple displays and HDR web content.
- Adobe RGB: ~52.1% — extended green gamut for photography/printing.
- Rec. 2020: ~75.8% — UHDTV target; spectral-locus primaries.
- ACEScg: ~89.4% — ACES rendering space; very wide, scene-referred.
Note: Area in xy² is an imperfect measure of colour gamut because the CIE 1931 diagram is perceptually non-uniform. A more meaningful comparison uses volume in CIELAB or similar uniform spaces. However, the xy area ratio remains the most common industry metric.
The spectral locus boundary consists of all chromaticities realisable by monochromatic light (single wavelengths). The interior plus the "line of purples" (connecting 380 nm and 780 nm) encloses all physically realisable chromaticities for standard observers.
Key wavelength markers:
- 380–420 nm: Violet (very low luminous efficiency)
- 450–480 nm: Blue (peak S-cone sensitivity near 445 nm)
- 500–520 nm: Cyan–Green (high curvature region)
- 520–560 nm: Green (peak luminous efficiency at 555 nm)
- 560–590 nm: Yellow (rapid hue change from green to orange)
- 600–650 nm: Orange–Red
- 650–780 nm: Deep Red (near-straight locus, low photopic sensitivity)
The Planck locus shows the chromaticity of ideal blackbody radiators. Real-world sources approximate blackbodies to varying degrees:
- Incandescent (A, 2856 K): Very close to Planck locus — nearly ideal blackbody.
- D-series daylight: Offset from Planck locus toward green — real sunlight includes atmospheric scattering effects not present in blackbody emission.
- Fluorescent (F-series): Large departures from Planck locus — emission is dominated by discrete spectral lines from phosphor coatings.
The correlated colour temperature (CCT) of a non-blackbody source is the temperature of the Planck radiator whose chromaticity is closest to the source on the isotemperature line. Enable isotemperature lines in the diagram to visualise these perpendiculars.