Melanopic EDI Calculator
Melanopic equivalent daylight illuminance (mEDI) is the CIE S 026:2018 metric for how much a light source stimulates the eye's circadian (melanopsin) response — not just how bright it looks. Two sources giving the same photopic lux can deliver very different mEDI, because it depends on the spectrum. For the full definition, formula derivation and design targets, see What Is Melanopic EDI?
Pick a light source and enter the illuminance at the eye to get mEDI, m-DER and EML instantly, compare sources side by side, or paste your own measured spectrum in Advanced mode.
Light source
Melanopic EDI
208 lx
m-DER
0.69
mELR
0.922 mW/lm
EML (WELL v1/v2)
230 EML
At 300 lx at the eye this source delivers ~208 mEDI — 17% short of the 250 lx daytime target. To reach 250 mEDI you need 360 lx at the eye, or a spectrum with m-DER ≥ 0.83.
Calculated from a relative spectrum and the illuminance you enter — it does not replace an in-situ spectroradiometer measurement of an installed system.
m-DER of common light sources
The same figures used by the interactive tool above, at a fixed 500 lx at the eye for comparison — so a link to this page and the live calculator never disagree.
| Light source | m-DER | mEDI at 500 lx | Source |
|---|---|---|---|
| Incandescent / halogen, 2700K | 0.50 | 248 lx | CIE Standard Illuminant A (tungsten reference, ~2856K) — continuous spectrum, no phosphor gap. |
| LED 2700K | 0.48 | 241 lx | Measured phosphor-converted LED spectrum (Ra ≈ 96) from the site's spectral library. |
| LED 3000K | 0.52 | 259 lx | Measured phosphor-converted LED spectrum (Ra ≈ 97) from the site's spectral library. |
| LED 4000K | 0.69 | 348 lx | Measured phosphor-converted LED spectrum (Ra ≈ 97) from the site's spectral library. |
| LED 5000K | 0.85 | 425 lx | Measured phosphor-converted LED spectrum (Ra ≈ 97) from the site's spectral library. |
| LED 6500K | 0.98 | 492 lx | Measured phosphor-converted LED spectrum (Ra ≈ 97) from the site's spectral library. |
| D65 (daylight reference) | 1.00 | 500 lx | CIE Standard Illuminant D65 — the reference source; m-DER = 1.00 by definition (CIE S 026:2018). |
Generic presets are representative measured spectra from the site's spectral library (LED rows) or CIE reference illuminants (incandescent, D65) — a specific product at the same CCT can differ by roughly ±0.1 in m-DER depending on phosphor design. See typical published ranges for cross-reference.
How it works
mEDI = illuminance at the eye (lx) × m-DER. m-DER is a property of the spectrum alone: the melanopic irradiance a source produces per lux, divided by the same ratio for CIE Standard Illuminant D65 — so D65 has an m-DER of exactly 1.00 by definition. Because it is a ratio, m-DER does not depend on the absolute brightness of the spectrum you feed in, only its shape.
This page is a calculator, not the full explainer — for the CIE S 026 formula derivation, the Brown et al. 2022 consensus targets, and the WELL v2 L03 feature, see What Is Melanopic EDI?
Limitations
- This tool calculates from a relative spectrum and the illuminance you enter — it does not replace an in-situ spectroradiometer measurement of an installed system.
- mEDI is conventionally measured vertically at eye level. The horizontal-to-vertical 0.7 ratio offered here is a typical office approximation, not a measurement — actual geometry, fixture aiming and room surfaces change the real ratio substantially.
- The 250 / 10 / 1 lx targets are the Brown et al. 2022 expert consensus for healthy adults with regular daytime schedules — not a medical recommendation, and not a building code requirement.
- Generic presets are representative spectra; a specific product's m-DER can differ from the preset by roughly ±10% depending on the actual phosphor and die combination.
FAQ
What does this calculator compute?
It converts photopic illuminance (lux) at the eye into melanopic EDI (mEDI), using the melanopic DER (m-DER) of the selected or pasted light spectrum, per CIE S 026:2018. It also reports m-DER, mELR and the WELL EML equivalent, and checks the result against the Brown et al. 2022 daytime/evening/night targets.
What lux value should I enter?
Photopic illuminance measured vertically at eye level, not the horizontal desk lux most light meters default to. If you only have a horizontal reading, use the "I only know desk (horizontal) lux" toggle — the default 0.7 ratio is a typical office approximation, not a measurement, so adjust it for your actual room geometry when precision matters.
What counts as a good m-DER?
There is no single "good" value — it depends on the target. Daylight (D65) is the reference at 1.00. For a daytime target of 250 lx mEDI, a source needs m-DER ≥ 250 / (illuminance at the eye). For an evening target, a low m-DER (warm, short-wavelength-depleted spectrum) is what you want instead.
How is this different from EML?
Melanopic EDI (CIE S 026:2018) and Equivalent Melanopic Lux (WELL v1) measure the same underlying quantity on two slightly different scales — EML is normalized to an equal-energy spectrum, mEDI to CIE Standard Illuminant D65. This tool computes mEDI directly and shows EML alongside it (EML ≈ 1.104 × mEDI) for WELL v2 reporting. See our full explainer for the exact conversion and history.
Can I use my own measured spectrum?
Yes — switch to Advanced mode and paste wavelength/value pairs (one per line, or CSV) from a spectroradiometer export, or upload a .csv/.txt file. The spectrum must cover at least 380–780 nm; it is interpolated onto a 5 nm grid and the same CIE S 026 calculation is applied.
Embed this tool
Drop the calculator into an article or press page. Please keep the attribution line linking back to this page.
<iframe src="https://yuji-lux.com/embed/melanopic-edi-calculator" width="100%" height="720" style="border:0" loading="lazy" title="Melanopic EDI Calculator — Yuji Lux"></iframe> <p style="font-size:12px">Calculator by <a href="https://yuji-lux.com/melanopic-edi-calculator">Yuji Lux</a></p>
References
- CIE S 026/E:2018, "CIE System for Metrology of Optical Radiation for ipRGC-Influenced Responses to Light."
- Brown, T.M., Brainard, G.C., Cajochen, C. et al. (2022), "Recommendations for daytime, evening, and nighttime indoor light exposure to best support physiology, sleep, and wakefulness in healthy adults," PLOS Biology.
- WELL Building Standard v2, Feature L03 (Circadian Lighting Design), International WELL Building Institute.
- UL 24480, Circadian Lighting Design.
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