Daylighting Metrics II:
Melanopic Equivalent Daylight (D65) Illuminance

The CIE S 026 standard, adopted by the WELL Building Standard v2, defined a new concept in which it compared the effect of a light source on each photoreceptor with that produced by the D65 illuminant, α-EDI (D65).

CIE S 026

The standard proposed by the CIE, based on the research of R.J. Lucas (“Measuring and using light in the melanopsin age“, Trends in Neuroscience, 2014), offers a method to calculate the response of each of the 5 photoreceptors (l-cone, m-cone, s-cone, rod and ipRGC) to a given light source based on its SPD.

Let’s see it with the following example, an LED with a CCT of 4000K for an illuminance of 100lux:


The first graph shows the SPD of the light source, the second the sensitivity of the photoreceptors (normalized to max = 1) and the third the multiplication of the previous two, that is, the response (irradiance) of each photoreceptor.

Melanopic Equivalent Daylight Illuminance [M-EDI(D65)]

Following this same example, the standard obtains the information given in the following table, where:

. Ee: total irradiance of the source for that illuminance (100 lux)
. Eα: response of each photoreceptor
. ELR: efficacy of each photoreceptor against visual response
. EDI(D65): illuminance produced by the D65 source that would achieve the same response in each photoreceptor


Focusing on the object of this article, the melanopic response and the stimulus of the circadian system, we see how the standard proposes the Melanopic Equivalent Daylight (D65) Illuminance concept, which defines how many lux of natural light are needed to obtain the same melanopic response as a given source.

In the example we are dealing with, it is obtained that 100 lux of this LED produce the same melanopic response as 63.2 lux of the D65 illuminant.

In fact, there is a relationship between Equivalent Melanopic Lux and this new concept easy to calculate: M-EDI (D65) = 0.9058 · EML, with which we can obtain the values from the following table:


WELL Building Standard v2

The M-EDI (D65) concept has recently been adopted by the WELL Building Standard v2, specifically in Feature # L03 up to 3 points are provided under these conditions:

1. Melanopic response: at least 240 EML [218 M-EDI (D65)]
2. Duration: at least 4h (maximum from 12:00)
3. Measurement: in the vertical plane at 46cm from the work surface (simulating the entrance to the eyes)

We will talk about the importance of the exposure time to which the second section refers in our next article, in which we will take a step further in the attempt to measure the well-being produced by the influence of light on our circadian system.



David Rodríguez

Director of Operations at Lledó Energía.

Physicist and Electronic Engineer. WELL AP.

Professionally and personally involved in energy efficiency and sustainable building, with more than 15 years of experience in Photovoltaics and Daylighting.

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