Colour matching functions are the analytic multi-lobe Gaussian fit to the CIE 1931 2° standard observer published by Wyman, Sloan & Shirley (JCGT 2:2, 2013) — not the tabulated data. Spectra are sampled at 5 nm across 380–730 nm and integrated by rectangle rule. Blackbody spectra come from Planck's law with c₂ = 1.4388 × 10⁻² m·K, which is also the definition of CIE Illuminant A at 2856 K. Correlated colour temperature and Duv are read off the Planckian locus by nearest-point search in CIE 1960 uv, not by a polynomial fit.
The spectral locus is drawn over 400–645 nm only. Outside that window the fit's Gaussian tails decay faster than the real observer's, the ratios that make a chromaticity go wrong, and the horseshoe's tips land in the wrong place — so the instrument declines to draw them.
Most of the locus lies outside what your screen can emit. Every out-of-gamut colour here is desaturated toward the achromatic point until it fits, then flown at the brightest luminance that chromaticity allows. The horseshoe is therefore a lie your display tells; the coordinates beside it are not. The swatch shows chromaticity only — intensity is reported as a number instead of faked as brightness.
Rev. A · 17 Jul 2026 · No dependencies · No network
Self-calibration — Illuminant E
True (flat spectrum, by definition)—
This instrument reads—
Error—
A perfectly flat spectrum must land at exactly (⅓, ⅓) — that is what Illuminant E is. The gap above is measured at load, and it is the analytic fit standing in for the tabulated observer.
It is also the instrument's best case, and should not be read as an error bar on everything else: Illuminant E tests the three integrals whole, and local errors cancel inside them. Point by point the fit wanders further — worst along the green flank near 500–520 nm, where it leaves the tabulated locus by roughly 0.015 in xy, some thirty times the figure above. Trust the reds, the ambers and the whites. Treat the green edge as a sketch.
Chroma on this page is computed, never chosen: each of the ~350 colours in the locus, every wavelength tint in the wells, and both swatches are calculated from the observer above. Every other pixel of this instrument is exactly achromatic — R = G = B, without exception — because ISO 3664 asks for a neutral surround, and a page that judges colour has no business having a favourite.