Pioneered by CIE in 1931 from colour-matching experiments, the (x,y) diagram remains the universal map of visible colour. The horseshoe boundary is monochromatic light. The Planckian locus is the path of black-body radiation, where every CCT lives. MacAdam ellipses define how much chromaticity error the eye can detect. LED binning is judged inside those ellipses.
Four ideas to carry into the next specification conversation you have.
The curved boundary plots monochromatic wavelengths from 380 to 780 nm. Inside the horseshoe = every colour the eye can perceive.
A curved line crossing the diagram. Every point on it is a black-body temperature. Where 2700K, 4000K, 6500K live.
A 1-step ellipse is just-noticeable. A 3-step ellipse is what premium LED binning uses (SDCM ≤ 3). 7-step = visibly different lamps.
Positive Duv = greenish. Negative Duv = pinkish. EP11 covered green–magenta shift; this is where it lives on the chart.
Key zones, points, and tolerances every lighting designer should recognise.
| Feature | Where on chart | Meaning |
|---|---|---|
| Spectrum locus | Outer curved boundary | Pure monochromatic light |
| Purple line | Straight bottom edge | Non-spectral magentas |
| Planckian locus | Curve through centre | Black-body radiator CCT path |
| White point D65 | x=0.313, y=0.329 | Daylight 6500K reference |
| White point A | x=0.448, y=0.407 | Incandescent 2856K reference |
| 3-step MacAdam | Small ellipse on locus | Premium binning target |
| 7-step MacAdam | Large ellipse on locus | Cheap fluorescent / poor LED |
Five reasons CIE 1931 dominates LED specification almost a century later.
When a datasheet says 4000K, what it really means is: this lamp's chromaticity falls within a defined ANSI quadrangle around the Planckian locus at 4000K. The chart defines the rules.
An LED 'bin' is a small rectangular zone on the CIE diagram. Premium binning = 3-step MacAdam ellipse. Loose binning = 5- or 7-step. Two lamps from different bins won't match on a wall.
Duv is the perpendicular distance from a point to the Planckian locus on the (u',v') diagram. Positive = green tint, negative = pink. ANSI C78.377 allows ±0.006.
sRGB, DCI-P3, Rec.2020 are all triangles on the CIE diagram. The bigger the triangle, the more colours a display can show. Same physics.
A tunable-white luminaire mixes 2700K and 6500K LEDs. Its operating point traces along the Planckian locus as you dim/tune, a curve you can plot on the chart.
Every premium NLC LED is binned to 3-step MacAdam (SDCM ≤ 3) or tighter. That's the difference between a uniform ceiling and a patchwork of subtly different whites.
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