Circadian lighting is leaving the era of marketing claims. Peer-reviewed work now evaluates wearable light dosimeters for how accurately they capture melanopic exposure in the field — which means the numbers on a lighting spec are becoming checkable after installation. The number that matters is melanopic EDI. Here is what it measures, how it is derived, and how to put it in a specification that survives commissioning.
What Melanopic EDI Measures
Melanopic EDI (Equivalent Daylight Illuminance, in lux) answers one question: how strongly does this light stimulate the eye's circadian photoreceptors — the melanopsin-containing ipRGCs — compared to daylight? Two light sources with identical brightness on a lux meter can differ substantially in melanopic EDI, because the circadian system's peak sensitivity sits at ~490 nm (cyan-blue), not where the eye's brightness perception peaks.
That is why photopic lux alone cannot describe a circadian design: the metric is standardized in CIE S 026:2018, which defines the melanopic action spectrum and the EDI calculation. (WELL v2 uses the closely related EML; melanopic EDI ≈ 0.906 × EML.)
How It Is Calculated
Melanopic EDI is derived from the spectral power distribution (SPD) of the light reaching the eye:
The SPD is weighted by the melanopic action spectrum — how strongly each wavelength drives the ipRGCs.
The result is expressed as the illuminance of standard daylight (D65) that would produce the same melanopic stimulus — hence "equivalent daylight illuminance."
The ratio between a source's melanopic EDI and its photopic lux is its melanopic DER (efficacy ratio) — a property of the spectrum itself. A high-DER spectrum delivers circadian effect at comfortable brightness; a low-DER spectrum needs uncomfortably high output to hit the same target.
The practical consequence: melanopic performance is decided by the spectrum, which means it is decided at the moment you choose the LED — not at the dimmer.
How It Is Measured in the Field
Design-stage calculations use the supplier's SPD data and vertical illuminance simulation at eye level — 1.2 m seated, 1.5–1.7 m standing. After installation, a spectrometer measurement at the same points closes the loop; research-grade wearable dosimeters now track personal melanopic exposure over full days, and their accuracy is under active scientific validation. Expect commissioning agents and WELL assessors to measure — surfaces, reflectances, and furniture layouts routinely shift the installed numbers from the render.
What to Put in the Specification
Melanopic EDI targets at defined points and times: e.g. ≥ 250 lx melanopic EDI at eye level through the working day (the published consensus recommendation), ≤ 10 lx in pre-sleep spaces.
Supplier data: SPD and melanopic DER at every CCT point of a tunable system — not just the endpoints, where cheap tunable products hide mid-range spectral dips.
Verification method: vertical-plane measurement at eye height at commissioning.
Color quality floor: CRI 95+ maintained across the tuning range — circadian performance must not cost rendering.
Why We Publish the Data
Yuji Lux publishes SPD and melanopic ratios for every product variant, at every CCT point — because engineered spectra are our core product, and defensible numbers are what a maturing, measured market will demand. SunWave™ tunable white was built exactly for this specification style: a high-DER daytime spectrum and verified color quality across the range.
For design targets, WELL v2 thresholds, and the full framework, see our complete circadian lighting guide.