Melanopic EDI (melanopic Equivalent Daylight Illuminance) is a metric, defined in CIE S 026:2018, that describes how strongly light stimulates the melanopsin-containing cells in the eye that drive the circadian system. It is expressed in lux and answers one question: how much standard daylight (D65) would produce the same melanopic effect as the light actually reaching the eye. Two lamps giving the same 500 lux on a light meter can have very different melanopic EDI, because the metric depends on the spectrum of the light, not just its brightness.
Melanopic EDI is the quantity used by the 2022 international consensus on indoor light exposure and by the WELL Building Standard v2, which makes it the number that ends up in circadian lighting specifications. This page covers the definition, the formula, the design targets, and typical values for common light sources. For the measurement chain — spectroradiometers, wearables, commissioning — see how melanopic EDI is measured; for the full design method, see the circadian lighting guide.
Why Photopic Lux Is Not Enough
Ordinary lux is weighted by the photopic sensitivity curve V(λ), which peaks at 555 nm in the yellow-green and describes how bright light looks. The circadian system does not use that curve. The intrinsically photosensitive retinal ganglion cells (ipRGCs) that signal day and night to the brain contain the photopigment melanopsin, whose sensitivity peaks near 490 nm in the blue-cyan region. A warm 2700K lamp and a cool 6500K lamp set to the same photopic lux therefore deliver very different amounts of light in the band melanopsin responds to. Lux tells you what the lamp does for vision; melanopic EDI tells you what it does for the clock.
The Formula: mEDI = Photopic Illuminance × Melanopic DER
In practice, melanopic EDI is calculated from two quantities:
melanopic EDI (lx) = photopic illuminance at the eye (lx) × melanopic DER
The melanopic Daylight Efficacy Ratio (m-DER) is a property of the spectrum alone. It is the melanopic irradiance a source produces per lux, divided by the same ratio for standard daylight D65 — so D65 has an m-DER of exactly 1.0 by definition, a warm-white LED sits well below 1, and a blue-rich source can exceed 1. Multiply the m-DER by the photopic illuminance measured vertically at the eye, and you have the melanopic EDI. A 4000K LED with m-DER 0.65 delivering 400 lx at the eye gives 400 × 0.65 = 260 lx melanopic EDI.
Strictly, CIE S 026 defines melanopic EDI as the melanopic irradiance of the light divided by the melanopic irradiance of D65 at 1 lux (1.3262 mW/m²); the m-DER shortcut gives the same answer.
Melanopic ratio, m-DER and EML: three names, two conventions
Before CIE S 026 was published, the Lucas et al. 2014 toolbox defined a melanopic ratio (M/P ratio) normalised to an equal-energy spectrum rather than to D65, and the WELL Building Standard v1 built its metric on it: Equivalent Melanopic Lux (EML) = photopic lux × melanopic ratio. The two systems measure the same thing on slightly different scales. D65 has a melanopic ratio of about 1.10 in the Lucas convention, so:
- EML ≈ 1.10 × melanopic EDI (WELL v2 uses 1.104; the CIE/Lucas conversion is melanopic EDI = 0.9058 × EML).
- m-DER ≈ 0.906 × M/P ratio — an older datasheet quoting "M/P 0.55" corresponds to an m-DER of about 0.50.
Melanopic EDI is the current international standard and the metric of the 2022 consensus; EML is still quoted by WELL v2 alongside it. Use whichever the project framework names, and do not mix the two in one calculation.
Design Targets: Brown et al. 2022 and WELL v2 L03
The most widely cited reference values come from the 2022 expert consensus published in PLOS Biology (Brown, Brainard, Cajochen, Czeisler, Hanifin, Lockley and colleagues). They are stated for healthy adults aged 18–55 with regular daytime schedules and measured in the vertical plane at the eye, approximately 1.2 m above the floor for a seated person:
- Daytime: at least 250 lx melanopic EDI throughout the day, ideally from daylight, with electric light enriched in short wavelengths where daylight is insufficient.
- Evening: no more than 10 lx melanopic EDI, starting at least 3 hours before habitual bedtime, from sources depleted in short wavelengths.
- Night: the sleep environment as dark as possible — a maximum of 1 lx melanopic EDI, and no more than 10 lx when light is needed for a night-time activity.
These are science-based recommendations, not building-code requirements, and the authors note that children, older adults, shift workers and people with atypical light sensitivity may need different values. The useful design principle is the contrast: high melanopic exposure by day, very low in the hours before sleep.
WELL v2 feature L03 (Circadian Lighting Design) turns the daytime side into certification points. At the time of writing the feature awards points for electric lighting that provides, on the vertical plane at seated eye level (about 0.45 m above the work plane) at the majority of workstations, at least 150 EML (≈136 lx mEDI) for one point and 275 EML (≈250 lx mEDI) for the maximum, for at least four hours beginning no later than noon; lower thresholds of 120 EML (≈109 mEDI) and 180 EML (≈163 mEDI) apply where the project also meets WELL's daylight criteria. Residential spaces have their own daytime and evening limits. WELL publishes quarterly addenda, so verify the thresholds in the current version of the feature before writing them into a specification. UL 24480, the third common framework, uses a different model (Circadian Stimulus, CS) whose values cannot be converted directly to melanopic EDI.
Typical Light Sources: m-DER and Melanopic EDI at 500 lx
The table shows what the formula means in practice. Every row assumes the same 500 lx photopic illuminance measured vertically at the eye; only the spectrum changes. Values for generic sources are approximate mid-points of published ranges — real products at the same CCT vary by ±0.1 or more in m-DER depending on phosphor design, which is exactly why a published spectral report matters more than the Kelvin number.
| Light source | Approx. m-DER | Melanopic EDI at 500 lx | Note |
|---|---|---|---|
| Incandescent / halogen, 2700K | ≈ 0.50 | ≈ 250 lx | Continuous spectrum; slightly higher m-DER than a phosphor LED of the same CCT |
| Warm white LED, 2700K | ≈ 0.45 (0.40–0.50) | ≈ 225 lx | Below the 250 lx daytime reference even at 500 lx |
| Neutral white LED, 4000K | ≈ 0.65 (0.55–0.75) | ≈ 325 lx | Typical office LED; 385 lx at the eye reaches 250 mEDI |
| Cool white LED, 6500K | ≈ 1.0 (0.9–1.2) | ≈ 500 lx | Blue-pump LEDs at 6500K can exceed D65 |
| Daylight D65 | 1.00 | 500 lx | Reference by definition (CIE S 026) |
| Yuji SunWave™ (daylight-oriented, 2700K–6500K) | see product spectral data | see product spectral data | Full-spectrum daylight-oriented design; per-CCT measured spectral reports on the product page |
| Yuji Dim-to-FlameWarm™ at its warmest point (~1250K) | M/P ≈ 0.099 (published) ≈ 0.09 m-DER | ≈ 45 lx | Engineered for the ≤10 lx evening target — reached at roughly 100 lx photopic |
Sources for the generic rows: manufacturer tables for a tunable-white luminaire (Fagerhult: 2700K 0.47, 3000K 0.53, 4000K 0.71, 6500K 1.18), laboratory values for phosphor-converted LEDs in Trinh, Bodrogi and Khanh, Sensors 2023 (2801K 0.46, 3942K 0.67, 4614K 0.75, 5059K 0.83), and the Signify/TU Eindhoven white paper on daytime lighting (≈0.6 at 4000K). The incandescent figure is converted from a published M/P ratio of about 0.58.
Read the table both ways. During the day, a warm-white office at 500 lx on the desk — which is already more than most offices deliver at the eye — does not reach the 250 lx melanopic reference. In the evening, the same warm-white source at 100 lx still delivers about 45 lx melanopic EDI, more than four times the 10 lx recommendation; a flame-spectrum source at the same brightness sits near 10 lx. Spectrum is the lever that lets a space be bright enough to use and still low in melanopic stimulus.
How to Measure It and How to Specify It
Measuring. Melanopic EDI is measured with a spectroradiometer, or a spectrally calibrated meter, held vertically at the eye position and facing the occupant's normal viewing direction. A lux meter cannot measure it. In design, it is calculated from the source's spectral power distribution (SPD) and the predicted vertical illuminance at the eye — lighting-calculation software does this from an SPD file. The full chain, including the accuracy limits of wearable dosimeters, is in how melanopic EDI is measured.
Specifying. A usable specification names the metric (melanopic EDI, not "circadian lighting"), the target value, the time window it applies to, the measurement plane and height, the occupant positions it must be met at, and the verification method at commissioning. It should ask the luminaire or LED supplier for the SPD and m-DER at every relevant CCT or dimming point, not only the endpoints — see LED strips with published spectral data for what to ask for, and the tunable white for circadian lighting page for how a tunable platform fits into a daytime/evening schedule.
Common Mistakes
- Treating CCT as melanopic content. Two 4000K sources can differ in m-DER by 0.2 or more. CCT describes appearance; the SPD decides the melanopic result.
- Measuring on the desk. Horizontal illuminance on the work plane overstates the stimulus. Under ceiling lighting, vertical illuminance at the eye is typically only 50–70 % of the desk value, so 500 lx on the desk may be 300 lx at the eye.
- Ignoring daylight. Even a modest window can dominate daytime melanopic EDI; conversely, an interior space with no daylight needs the electric system to carry the whole 250 lx.
- Mixing metrics. EML, melanopic EDI and CS use different scales. Convert deliberately, or stay inside one framework.
- Specifying only the daytime. A system that delivers 250 lx mEDI at 15:00 and 60 lx mEDI at 22:00 has solved half the problem. The evening limit needs a warm, dim, low-melanopic scene and a schedule to reach it.
Frequently Asked Questions
What does melanopic EDI stand for?
Melanopic Equivalent Daylight Illuminance. It is the illuminance of standard daylight (D65) that would produce the same melanopic (ipRGC) stimulation as the light being evaluated, expressed in lux and defined in CIE S 026:2018.
How is melanopic EDI calculated?
Multiply the photopic illuminance measured vertically at the eye by the source's melanopic daylight efficacy ratio (m-DER), which is computed from the spectral power distribution. For example, 400 lx at the eye from a source with m-DER 0.65 gives 260 lx melanopic EDI.
What is a good melanopic EDI during the day?
The Brown et al. 2022 consensus recommends at least 250 lx melanopic EDI at the eye throughout the daytime for healthy adults with regular schedules. WELL v2 L03 awards its maximum points at approximately the same level (275 EML), with a lower tier at 150 EML (about 136 lx mEDI).
What melanopic EDI is recommended in the evening and at night?
No more than 10 lx melanopic EDI from about three hours before bedtime, and no more than 1 lx in the sleep environment, according to the same consensus. Reaching 10 lx with a warm-white LED means dimming to roughly 20 lx photopic; a flame-spectrum source allows several times more visible light for the same melanopic level.
What is the difference between melanopic EDI and EML?
Both quantify melanopsin-weighted light, but EML (WELL v1, Lucas 2014 toolbox) is normalised to an equal-energy spectrum and melanopic EDI (CIE S 026) to daylight D65. EML is approximately 1.1 times melanopic EDI; WELL v2 lists both values side by side.
What is melanopic DER?
The melanopic Daylight Efficacy Ratio is the melanopic effectiveness of a spectrum per lux relative to D65, which scores 1.0. Warm-white LEDs are typically around 0.4–0.5, neutral 4000K LEDs around 0.6–0.7, and cool 6500K LEDs around 1.0. It describes the spectrum only; the actual exposure also depends on how much light reaches the eye.
Can a lux meter measure melanopic EDI?
No. A lux meter is filtered to the photopic curve. Melanopic EDI requires a spectroradiometer or a meter with a calibrated melanopic channel, or a calculation from the source's measured spectrum and the vertical illuminance at the eye.
Does higher CCT always mean higher melanopic EDI?
Not reliably. m-DER tends to rise with CCT, but sources at the same CCT can differ by 0.2 or more depending on their spectrum. Compare published spectral data or m-DER values rather than Kelvin ratings.
Method note: Melanopic quantities follow CIE S 026:2018. Generic m-DER values above are approximate mid-points of published ranges and are labeled as such; product-specific melanopic EDI must be calculated from the manufacturer's measured spectral data and the actual vertical illuminance at the eye. Exposure recommendations are quoted from the cited sources and are not medical advice.
References
- CIE S 026/E:2018. CIE System for Metrology of Optical Radiation for ipRGC-Influenced Responses to Light. International Commission on Illumination. cie.co.at
- Brown TM, Brainard GC, Cajochen C, Czeisler CA, Hanifin JP, Lockley SW, et al. Recommendations for daytime, evening, and nighttime indoor light exposure to best support physiology, sleep, and wakefulness in healthy adults. PLOS Biology. 2022;20(3):e3001571. doi:10.1371/journal.pbio.3001571
- International WELL Building Institute. WELL v2, Feature L03: Circadian Lighting Design. Refer to the current version and addenda for thresholds. v2.wellcertified.com
- UL 24480. Design Guideline for Promoting Circadian Entrainment with Light for Day-Active People. Underwriters Laboratories, 2019.
- Lucas RJ, Peirson SN, Berson DM, et al. Measuring and using light in the melanopsin age. Trends in Neurosciences. 2014;37(1):1–9. Toolbox note: melanopic EDI = 0.9058 × melanopic illuminance (EML).
- Ticleanu C, Flores-Villa L, Littlefair P, Howlett G. Assessing melanopic equivalent daylight illuminance in office spaces using a simplified approach for predominantly cloudy climates. Lighting Research & Technology. 2025.
- Trinh VQ, Bodrogi P, Khanh TQ. Determination and Measurement of Melanopic Equivalent Daylight (D65) Illuminance (mEDI) in the Context of Smart and Integrative Lighting. Sensors. 2023;23(11):5000. doi:10.3390/s23115000
- Schlangen LJM, van der Zande BMI. The power of healthy daytime lighting in indoor settings: melanopic lighting advances and office applications. Signify / Eindhoven University of Technology white paper, 2023.
Related: Human-Centric & Circadian Lighting hub · Winter Light Protocol · What is CCT?