CCT alone doesn't fully describe a white. A 4000 K fixture can sit above the Planckian locus (greenish) or below (pinkish). The Duv value pins this down, and the binning tolerance decides whether your installation looks unified or measled.
Four ideas to carry into the next specification conversation you have.
Positive Duv = above the curve, slight green cast. Negative Duv = below, slight pink. Premium spec: |Duv| < 0.003.
A 3-step MacAdam ellipse defines a chromaticity zone. Two chips inside it can sit on opposite ends, and look different side-by-side.
Most consumer tunable strips push positive Duv at high CCT. Premium fixtures (Lumileds, Samsung) maintain Duv along a defined locus.
Write: '4000 K, Duv ≤ ±0.003, 3-step MacAdam'. Three numbers, not one.
Per ANSI C78.377 binning and IES TM-30 white-point characterisation.
| Duv value | Visual cast | Acceptability |
|---|---|---|
| Duv = 0.000 | On the locus, neutral white | Reference |
| Duv = +0.003 | Faint green tint | Premium spec limit |
| Duv = +0.006 | Visible green | Reject for premium |
| Duv = +0.010 | Strong green | Reject all |
| Duv = −0.003 | Faint pink, often preferred | Premium spec limit |
| Duv = −0.006 | Visible pink | Acceptable interiors only |
| Duv = −0.010 | Strong pink | Reject |
Five ideas that explain the most-missed spec line in lighting.
Imagine an ellipse-shaped zone of "acceptable 4000 K." The long axis runs along the Planckian locus (CCT shift). The short axis runs perpendicular (Duv shift). CCT measures position along the long axis; Duv measures position along the short.
White LEDs are blue chips with yellow/red phosphor. If the phosphor formula leans green, Duv goes positive. If it leans red, Duv goes negative. Reputable chip-makers (Nichia, Lumileds, Samsung, Osram) hold Duv tight. Generic chips drift.
Pink (negative Duv) often looks subjectively more inviting, closer to candlelight. Some premium hospitality fixtures intentionally spec Duv ≈ −0.002 at 3000 K. Green-shift (positive Duv) almost always reads as "sickly" or "fluorescent."
On phone cameras and social media, slight tint shifts read as full-on green or pink. Retail photographers reject fixtures that look fine to the eye. For Instagram-grade hospitality, spec |Duv| ≤ 0.002 even where the eye would forgive 0.005.
A cheap lux meter cannot measure Duv. A handheld spectroradiometer (Sekonic C-800, Asensetek Lighting Passport) gives CCT, Duv, CRI, TM-30 in one reading. Budget one into the project; you only need it once on site.
Our premium spec ships at |Duv| ≤ 0.003 and 3-step MacAdam. Project lots use single-lot binning so 200 fixtures match.
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