Melanopic Lighting & m-EDI — The Science of Non-Visual Light
How light affects your circadian clock, alertness, and sleep — and the metrics that quantify it.
Beyond Vision — How Light Affects Biology
For over a century, lighting design focused exclusively on visual performance — enough light to see comfortably. But since the discovery of intrinsically photosensitive retinal ganglion cells (ipRGCs) in 2002, we know that light profoundly affects non-visual biological processes: • Circadian rhythm synchronization (sleep-wake cycle) • Hormone regulation (melatonin suppression during the day, production at night) • Alertness, cognitive performance, and mood • Core body temperature regulation • Seasonal affective responses ipRGCs contain the photopigment melanopsin, which is most sensitive to blue-cyan wavelengths around 480-490nm. This means the spectral composition of light — not just its intensity — determines its biological impact.
What Is m-EDI?
Melanopic Equivalent Daylight Illuminance (m-EDI), standardized by CIE S 026:2018, quantifies the melanopic (circadian) effectiveness of a light source. It's expressed in units equivalent to lux. m-EDI answers the question: 'How effective is this lighting at stimulating the circadian system compared to standard daylight (D65)?' • Daylight at 250 photopic lux has m-EDI ≈ 250 (ratio ~1.0) • A 6500K LED at 250 photopic lux might have m-EDI ≈ 200-225 • A 3000K LED at 250 photopic lux might have m-EDI ≈ 100-125 • A 2700K LED at 250 photopic lux might have m-EDI ≈ 80-100 This illustrates why warm-white lighting at the same lux level provides much less circadian stimulation than cool-white or daylight. It's also why WELL Building Standard L03 requires specific m-EDI thresholds rather than just photopic lux.
Melanopic Ratio by CCT (Typical LED)
| 0 | 1 | 2 |
|---|---|---|
| 2700K | 0.40–0.45 | ~340–375 lux |
| 3000K | 0.45–0.52 | ~290–335 lux |
| 3500K | 0.52–0.58 | ~260–290 lux |
| 4000K | 0.58–0.65 | ~230–260 lux |
| 5000K | 0.70–0.78 | ~195–215 lux |
| 6500K | 0.85–0.95 | ~160–180 lux |
| D65 Daylight | 1.00 | 150 lux |
The 2025 CIE/BIPM Framework
In 2025, the International Bureau of Weights and Measures (BIPM) and CIE agreed to formally incorporate melanopic quantities into the SI metrology framework. This means m-EDI is now an internationally standardized, traceable metric — no longer an advisory or research concept. This formalization accelerates adoption in building standards (WELL, EN standards updates), product specifications, and regulatory requirements. Manufacturers will increasingly report melanopic data alongside photometric data on datasheets.
Practical Design Guidelines
For circadian-effective lighting design: Daytime (6am–6pm): • Target ≥ 150 m-EDI at eye level (vertical illuminance at 1.2m seated, 1.5m standing) • Achieve through: high CCT (4000-5000K) general lighting, or tunable white at morning/midday settings, or maximize daylight contribution through window design • Focus melanopic delivery in morning hours (7am-12pm) when circadian system is most responsive Evening (after sunset): • Reduce to ≤ 50 m-EDI to allow natural melatonin rise • Achieve through: dim, warm lighting (2700K or below), amber/warm-dim fixtures, automated CCT scheduling Night (sleeping environments): • Near-zero m-EDI — if wayfinding light is needed, use amber (<1800K) or red wavelengths • Hospital night lighting, hotel corridors, and senior living should prioritize amber/warm-dim fixtures
Tunable White — The Practical Solution
Tunable white (TW) luminaires that adjust CCT from 2700K to 6500K throughout the day are the most practical way to satisfy both visual comfort and circadian requirements. Morning: 5000K at high intensity. Midday: 4000K. Afternoon: 3500K. Evening: 2700K dimmed. Combined with DALI-2 or Bluetooth controls, this creates a circadian-responsive environment without compromising visual comfort at any time of day.