A datasheet that reports a 3000K white LED has told you one number. The color itself is a pair of coordinates on a chromaticity diagram, and two parts with the same nominal CCT can sit at visibly different points. Reading those coordinates is the difference between accepting a label and knowing what will ship.
From Spectrum to Coordinates
A spectroradiometer measures radiant power at each wavelength from 380nm to 780nm. Weighting that spectrum with the CIE color matching functions gives three tristimulus values, X, Y and Z. Chromaticity is the normalized pair x = X/(X+Y+Z) and y = Y/(X+Y+Z). The pair (x, y) fixes a point on the CIE 1931 diagram, while the absolute stimulus stays in Y.
Two spectra with different shapes can land on the same x, y point. This is metamerism, and it is why a chromaticity match is not a spectral match. For visual work the coordinates are enough; for color rendering, dye matching or camera sensors they are not.
Why There Are Three Diagrams
CIE 1931 (x, y) was built for practical measurement, not for perceptual uniformity. Equal distances on that diagram do not correspond to equal perceived steps, and the distortion is worst in the green region where the diagram is stretched.
CIE 1960 introduced u and v to flatten that distortion. CIE 1976 added the UCS coordinates u’ and v’, which are the standard for color difference work today:
u’ = 4x / (-2x + 12y + 3)
v’ = 9y / (-2x + 12y + 3)
The 1960 pair is still used in one place that matters for anyone specifying an LED: the definition of Duv.
| Diagram | Coordinates | Property | Typical use |
|---|---|---|---|
| CIE 1931 | x, y | Legacy, non-uniform | Datasheets, filter records, historic files |
| CIE 1960 | u, v | More uniform | Duv, sign convention |
| CIE 1976 UCS | u’, v’ | Most uniform of the three | Color difference, binning math, tolerance circles |
CCT, Duv and the Coordinates
Correlated color temperature is derived, not measured. A program draws the Planckian locus, finds the closest point to the measured chromaticity, and reports the temperature at that crossing. Duv is the signed perpendicular distance from the locus, positive toward green and negative toward pink.
Because the closest point can be found from any direction, the same CCT value covers a whole line of chromaticities. Two 4000K parts can differ enough in Duv that one reads green and the other neutral in the same fixture run, and neither label is wrong.
Tolerances and Bins
Suppliers express chromaticity tolerance in one of two ways. One is an explicit radius, for example 0.007 in u’v’. The other is a MacAdam ellipse step, commonly 3-step or 5-step. The two are not interchangeable, because an ellipse step covers a different area depending on where it sits on the diagram.
A specification that states only a kelvin tolerance leaves the off-locus distance unstated. Ask for the coordinates, the tolerance in u’v’, and the bin center. Three numbers, one meaning.
Measurement Practice
Chromaticity is measured with a spectroradiometer or a calibrated colorimeter, in most cases in an integrating sphere. Standard practice holds the part at a controlled ambient, usually 25C, and waits for the junction temperature to stabilize before recording. Drive current must be recorded with the result, because forward current moves both CCT and Duv.
Measurement uncertainty in a well-run lab sits around 0.003 in x and y. That is enough that a part specified at a 3-step bin can sit close to the boundary, so receiving inspection should use the same instrument class and the same sphere geometry as the supplier.
FAQ
Is Duv part of the CCT number? No. CCT describes position along the locus. Duv describes distance away from it. A part can hold 4000K and still shift visibly green or pink.
Which coordinates should appear on a specification sheet? State nominal CCT, the tolerance in u’v’ or the ellipse step, and the bin center. Adding the test condition keeps supplier data and incoming inspection on the same basis.
Do LED strips and modules need the same treatment? Yes. Wide runs of strip and large modules both show bin-to-bin variation, and the effect is more visible on strip because segments sit next to each other in a line.
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