Ex Mensura, Scientia
Two numbers describe the color of white light. The correlated color temperature (CCT) names the blackbody temperature whose color comes closest. Duv tells how far the light sits from the colors of a blackbody, and on which side.
Move the mark on the diagram, or enter CIE 1931 x and y (or CIE 1960 u and v) in the form. The calculator finds CCT and Duv with Ohno's method on a table of the Planckian locus.
The CIE chromaticity diagram with the Planckian locus (the colors of a blackbody from 1000 K to 25 000 K), its isotherms and lines of equal Duv. The teal string joins the mark to the nearest point of the locus; its length is the Duv. The colors are an approximation, since a screen cannot show the most saturated ones.
−0.00404500 KThe point lies below the Planckian locus: a pinkish or magenta tint. It is within ±0.006, the Duv tolerance of the ANSI C78.377 standard for LED lighting.
0.3592, y = 0.35430.2199, v = 6y / (−2x + 12y + 3) = 0.32540.2199, v′ = 1.5v = 0.4881The display precision set is .4. To change that, edit the variable &digits=x in the URL where 0 <= x <= 9.
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A blackbody glows in a color set by its temperature alone; the filament of an incandescent lamp comes close. Its colors trace the Planckian locus across the chromaticity diagram, from deep orange at 1000 K through white to pale blue. The CCT of a light source is the temperature of the blackbody whose chromaticity lies nearest to it in the CIE 1960 (u, v) diagram.
But two lamps with the same CCT can still look different. Duv is the distance from the Planckian locus in that same (u, v) diagram, with a sign. Positive values lie above the locus, where white light looks greenish or yellowish. Negative values lie below it, where it looks pinkish or magenta. ANSI C78.377, the standard for the chromaticity of LED lighting, allows a Duv tolerance of ±0.006.
The CIE advises against using a correlated color temperature at all when a chromaticity lies more than 0.05 from the Planckian locus.
Drag the red mark, or tap the diagram to move it there. With the mark focused, the arrow keys move it in small steps; hold Shift for larger ones. The buttons switch between the CIE 1976 u′v′ diagram and the CIE 1931 xy diagram, and between a close-up of the Planckian locus and the full diagram. Every move updates the form and the results. And since the address follows along, a bookmark or a shared link brings back exactly this point and view.
x, yu, vx and y are missingcalc=yesdigitsview=xy, zoom=fullformatjson, xml or csv for the API; leave it out for this page