A 4000K label is a projection onto a locus, not a fixed color. Two products can carry the same 4000K name, sit at different points along the isotemperature line, and sit at different distances off it. Put both in one ceiling grid and the mismatch is easy to see. SDCM is the number that describes that gap.
Same CCT, Different Chromaticity
Correlated color temperature is found by drawing the closest line to the blackbody locus and reporting the temperature at the crossing point. Every chromaticity on that line scores the same CCT. Duv measures the perpendicular distance from the locus, positive toward green and negative toward pink. Two 4000K luminaires can differ by 0.006 in Duv, which most observers read as one clearly green and one neutral.
MacAdam’s Experiment and the Step
In 1942 David MacAdam asked observers to match a reference color against nearby colors. The matches scattered in an ellipse. One step on that scale, 1 SDCM, is roughly the smallest shift an average observer notices under controlled viewing. The ellipses are not the same size across the diagram: they stretch in the green region and tighten in the blue and purple regions. A 3-step bin in one area therefore covers more chromaticity than a 3-step bin in another.
| Bin width | Ellipse | Typical use |
|---|---|---|
| 1-step | Tightest sold | Color-critical inspection, museum, medical imaging |
| 2-step | Very tight | Premium retail, high-end architectural |
| 3-step | Tight | Architectural and display work |
| 5-step | Standard | General and industrial lighting |
| 7-step | Loose | Cost-driven work with wide fixture spacing |
Where the Bin Boundaries Come From
ANSI C78.377 sets nominal CCTs and a quadrangle around each one. Industry binning then draws MacAdam ellipses inside that quadrangle and sells parts as 3-step or 5-step. Two suppliers can both claim 3-step and still differ, because the ellipse centers are placed differently and because measurement tolerances differ between labs.
Reading a Datasheet
Four numbers matter together: nominal CCT, CCT tolerance in kelvin, Duv or bin point, and SDCM. A sheet that lists only a kelvin tolerance leaves the off-locus distance unstated. Ask for the chromaticity coordinates and the test condition, typically 25C ambient and 1,000 hours per LM-79. Also ask about drive and thermal behavior, because junction temperature and forward current both move chromaticity. A part can hold its bin at 65mA and drift outside it at 150mA.
Specifying Consistency in a Project
- Adjacent fixtures in one sightline: 3-step or tighter.
- Fixtures separated by walls or floor levels: 5-step is usually enough.
- Mixing two production lots in one run: request the same bin point, not just the same SDCM.
- Dimming: some drivers shift tint at low output, so check dimmed photometry if dimming is normal operation.
- Cold stores and outdoor cabinets: check the shift over the real operating temperature range, not just at 25C.
FAQ
Does a wider bin always cause visible mismatch? No. A 5-step pair can look clean when the sources are far apart or when one source serves one room. Risk rises when two bins share one plane of view.
Is SDCM the same as CCT tolerance? No. CCT tolerance describes spread along the locus in kelvin. SDCM describes spread around the target point in perception steps.
Which values belong on the specification? State the nominal CCT, the SDCM, and the bin point, and add the test condition so receiving inspection uses the same basis as the supplier.
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