How Melanopic EDI Is Measured — and Why It Belongs in Your Spec

Circadian lighting is moving beyond marketing claims toward measurable performance.

A major part of that shift is melanopic EDI — a standardized metric that allows designers to quantify the melanopic stimulus produced by light reaching the eye.

And measurement is moving beyond the laboratory. A 2026 peer-reviewed study evaluating wearable light dosimeters compared personal monitoring devices against a calibrated spectroradiometer and specifically evaluated their ability to measure melanopic EDI under different lighting conditions.

For lighting designers and specifiers, this matters because circadian performance can increasingly be specified, calculated, measured, and verified after installation.

If you need the definition, formula, design targets and typical values first, start with What is melanopic EDI? — this article picks up from there and covers how the number is measured and specified.

What Melanopic EDI Measures

Melanopic EDI — melanopic Equivalent Daylight Illuminance, expressed in lux — quantifies melanopic stimulation in terms of the amount of standard daylight that would produce an equivalent melanopic response.

The metric is part of the standardized CIE S 026:2018 system for characterizing light with respect to the five α-opic photoreceptor responses.

This is fundamentally different from conventional photopic lux.

Two sources producing the same photopic illuminance can produce substantially different melanopic EDI because their spectral power distributions may be different.

That is one reason why CCT alone is insufficient for circadian lighting.

The current CIE Position Statement on Integrative Lighting emphasizes that light exposure is determined by the combination of light level and spectrum, and warns that using CCT alone to characterize biologically appropriate light can be misleading.

How Melanopic EDI Is Calculated

The calculation begins with the spectral power distribution (SPD) of the light reaching the eye.

In simplified terms:

• The SPD is evaluated using the melanopic spectral sensitivity defined by CIE S 026.

• The resulting melanopic stimulus is referenced to standard daylight D65.

• The result is expressed as the illuminance of D65 that would produce an equivalent melanopic stimulus: melanopic EDI.

Another useful metric is melanopic DER (m-DER).

m-DER describes the melanopic effectiveness of a spectrum relative to its photopic illuminance. In practical terms, it helps compare the spectral characteristics of different light sources independently of their absolute light level.

This distinction is important:

m-DER describes the spectrum.
m-EDI describes the melanopic stimulus produced by that spectrum at a given light level.

Dimming therefore reduces melanopic EDI along with photopic illuminance, while changing the spectrum can change the relationship between the two.

This is why spectral engineering provides an additional design variable beyond simply increasing or decreasing brightness.

What the formula means at a fixed 500 lx photopic illuminance at the eye (generic m-DER values are approximate mid-points of published ranges; see the melanopic EDI explainer for sources):

How It Is Measured in a Real Space

Circadian lighting should be evaluated where the light actually matters: at the eye.

During design, melanopic EDI can be estimated using spectral data for the source together with predicted vertical illuminance at the occupant's eye position.

After installation, the same quantity can be verified using a calibrated spectroradiometric measurement at the relevant viewing position and orientation.

This matters because the installed environment changes the light that actually reaches the occupant.

Reflectances, furniture, glazing, daylight, fixture placement, distribution, and viewing direction can all affect the final exposure.

Personal light exposure is more complicated still.

The 2026 study of wearable light dosimeters found meaningful differences in accuracy between devices and emphasized that body position and direction of incident light can cause measurements taken by a wearable sensor to differ from the light actually reaching the eye.

The technology is improving rapidly, but sensor location and measurement method still matter.

What Should Go Into a Lighting Specification?

A useful circadian specification should go beyond simply calling for “Human-Centric Lighting” or a particular CCT range.

Consider specifying:

Melanopic target
Define the required melanopic EDI for the relevant time period and occupant position.

Measurement location
Specify where the requirement applies — for example, a defined vertical plane at the expected eye position — rather than relying only on horizontal workplane illuminance.

Spectral data
Request SPD and melanopic metrics for the actual LED or luminaire configuration being specified.

For tunable systems, evaluate performance throughout the usable tuning range rather than assuming the endpoints describe every intermediate condition.

Verification method
Define how melanopic EDI will be measured or calculated during commissioning.

Time of exposure
A melanopic value without a time context is incomplete. Daytime and evening objectives are fundamentally different.

Color quality
Circadian performance should not require sacrificing visual quality. CRI, R9, TM-30, chromaticity consistency, and SPD should remain part of the evaluation for demanding architectural applications.

What Is a Good Melanopic EDI Target?

There is no single melanopic EDI value that is appropriate for every space, occupant, and time of day.

The CIE emphasizes high melanopic exposure during daytime and low exposure during evening and night, while also recognizing that biological responses depend on timing, duration, previous light exposure, and individual characteristics.

This is an important distinction for specification.

A target such as 250 lx melanopic EDI can be useful when it comes from a particular guideline or project framework, but it should not be treated as a universal threshold separating “healthy” from “unhealthy” light.

Circadian lighting is a time-dependent exposure strategy, not a single number.

Why Spectrum Matters

Consider two light sources delivering the same photopic illuminance at the eye.

If their spectra are different, their melanopic EDI can also be different.

That creates an important opportunity for lighting design.

During the day, the objective may be to achieve useful melanopic exposure without simply increasing conventional illuminance indefinitely.

During evening and nighttime periods, the opposite problem appears: we may want enough visible illumination to comfortably use the space while minimizing unnecessary melanopic exposure.

Spectrum gives us another degree of freedom.

Instead of controlling only how much light is produced, we can also engineer what wavelengths are present in that light.

Melanopic EDI and WELL v2

WELL is where melanopic metrics most often enter a project brief. WELL v1 used Equivalent Melanopic Lux (EML), based on the Lucas et al. 2014 toolbox; WELL v2 feature L03 (Circadian Lighting Design) keeps EML but now lists the equivalent melanopic EDI next to it, using the conversion EML ≈ 1.1 × melanopic EDI.

At the time of writing, L03 awards points for electric lighting that delivers a minimum melanopic level on the vertical plane at seated eye level (about 0.45 m above the work plane) at the majority of regularly occupied workstations, for at least four hours a day beginning no later than noon: 150 EML (≈136 lx melanopic EDI) for one point and 275 EML (≈250 lx melanopic EDI) for the maximum, with lower thresholds of 120 EML (≈109 mEDI) and 180 EML (≈163 mEDI) where the project also meets WELL's daylight criteria. Dwelling units and guest rooms carry their own daytime minimum and evening maximum.

Two practical consequences follow. WELL is satisfied by calculation or measurement at the eye, not by a fixture's CCT, so the supplier must provide the SPD or m-DER for the actual configuration. And WELL publishes quarterly addenda, so the numbers above should be checked against the current version of the feature before they enter a specification. The thresholds and the Brown et al. 2022 consensus values are compared on the melanopic EDI explainer.

The Yuji Lux Approach

At Yuji Lux, this is why we begin with the spectrum.

Yuji's background in LED and phosphor development allows us to engineer spectral power distributions for specific lighting objectives rather than treating CCT as the primary measure of light quality.

For daytime-oriented applications, SunWave™ provides a broad, continuous, daylight-like spectrum designed for high color quality and daylight simulation.

For evening applications, FlameWarm™ approaches the problem from the opposite direction. Its spectrum is engineered to substantially reduce short-wavelength content while maintaining useful visible illumination.

The objective is not simply:

cool light during the day → warm light at night.

It is to understand and control:

spectrum + intensity + timing + exposure at the eye.

That is what turns circadian lighting from a marketing description into a measurable lighting strategy.

For the broader design framework — including daytime and evening strategies, controls, color quality, and application examples — see our Professional Guide to Circadian Lighting & Human-Centric Lighting.

Planning a Circadian or Human-Centric Lighting Project?

Yuji Lux works with lighting designers, architects, integrators, and manufacturers to develop linear and custom lighting solutions around specific spectral and project requirements.

Talk to our team about spectrum selection, SunWave™, FlameWarm™, controls, and custom circadian lighting solutions.

This article is part of the Human-Centric & Circadian Lighting methodology hub — the framework, melanopic metrics, tools, and spectral platforms in one place.