Duv, TM-30, SSI, and Melanopic Lighting

CCT describes a direction, not the whole spectrum

Correlated color temperature places a near-white light relative to the blackbody locus and communicates a warm-to-cool appearance. Two sources at the same CCT can differ in green-magenta tint, spectral smoothness, color rendition, and biological effect.

That is why a lighting specification based only on kelvin and CRI can produce disappointing paint, skin, food, or textile appearance.

Duv describes distance from the white locus

Duv indicates how far a chromaticity lies from the Planckian or reference locus in the u-prime v-prime diagram. Positive and negative values generally correspond to opposite green-magenta tint directions, though acceptable preference depends on CCT and application.

A small Duv difference can be visible when lamps are installed side by side or wash the same wall. Binning and consistency matter as much as the nominal product value.

TM-30 separates fidelity and gamut behavior

TM-30 evaluates many color samples and reports a fidelity index, Rf, plus a gamut index, Rg. A color vector graphic shows how hue regions are shifted in saturation and hue.

Two sources can share a similar average fidelity while one boosts reds and another suppresses greens. The vector information is often more actionable than a single CRI value.

SSI compares spectra directly

The Spectral Similarity Index compares one illuminant spectrum with a reference spectrum. It is especially useful in film, photography, and multi-source environments where matching the spectrum can matter more than average color rendition.

A source can render ordinary samples acceptably yet mismatch another source on camera because their spectra differ. SSI helps reveal that relationship.

Melanopic metrics describe one biological pathway

Light also influences circadian and neuroendocrine responses through intrinsically photosensitive retinal ganglion cells and related pathways. Melanopic equivalent daylight illuminance and related metrics estimate stimulation relative to a defined daylight reference.

These metrics do not replace visual illuminance, comfort, glare control, sleep timing, or medical guidance. They add another dimension to lighting design and should be interpreted with exposure timing and duration.

Specify light for the actual task

For interiors, record CCT, Duv, color rendition data, dimming behavior, flicker, distribution, and illuminance. For camera work, include spectral similarity and test the actual cameras. For circadian goals, coordinate melanopic targets with visual comfort and schedule.

HexCheck can model broad lighting direction and material relationships. Final selection should use photometric and spectral data from the real source.

A disciplined verification checklist

Record the full SPD, CCT, Duv, TM-30 data, SSI reference, illuminance, dimming state, flicker behavior, distribution, exposure timing, and intended task. Compare installed samples rather than choosing from a headline metric alone.

• Check Duv consistency between adjacent fixtures.
• Read the TM-30 vector graphic, not only Rf.
• Use SSI when matching camera light sources.
• Separate visual and melanopic goals.
• Evaluate materials under the installed light.

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