Daylighting Metrics I:
Equivalent Melanopic Lux (EML)

The savings and visual comfort that daylight produces are easy to quantify, there are metrics such as kWh, lux, uniformity or CRI, but how can we measure how much it affects our well-being?

The WELL Building Standard was the first to answer this question by proposing a new concept, the Equivalent Melanopic Lux (EML).

WELL Building Standard v1

As ITG defines, partner in Spain of the International WELL Building Institute (IWBI), the WELL certification is a dynamic scoring system for buildings and communities that allows to identify, measure and monitor the characteristics of built spaces that impact health and the well-being of the occupants.

Among the many aspects it considers, Feature # 54 mentions for the first time the need to measure the biological contribution of light, that is, its ability to stimulate our circadian system.

As a requirement it is imposed that, under certain conditions, the lighting system provides at least 200 Equivalent Melanopic Lux (EML), a new metric that intended to measure the stimulation of the photoreceptors involved in the non-visual effects of light (the ipRGCs, with a maximum at 490nm) compared to the traditional vision ones (the cones, maximum at 555nm).


Equivalent Melanopic Lux (EML)

To understand this concept, let’s see the following graphs, which show the visual -or photopic- (green) and melanopic (blue) response for different light sources:

Irradiance LED4000K
Irradiance DAYLIGHT D65
Irradiance DAYLIGHT D50

For the same visual response (Ev = 100 lux), both the total irradiance in the visible spectrum (Ee) and the melanopic response (Emel) are very different:

Emel 100lux

In this table we can see how the melanopic contribution of natural light is twice that of incandescent light, or how natural light at noon (D65) contributes 20% more than at dawn/sunset (D50).

From these observations, the IWBI proposed that for each source the relationship between visual lux and Equivalent Melanopic Lux (EML) could be calculated through a conversion factor, the Melanopic Ratio (R): EML = R · Ev

This constant can therefore be calculated by dividing the melanopic response by the visual response and multiplying it by a normalization factor for both sensitivities (1.218), obtaining the following values:

Melanopic Ratio

In this table we observe that in the case of natural lighting (D65), for every 100 visual lux, 110 Equivalent Melanopic Lux is obtained.

Another interesting conclusion, obtained from the previous table and the second graph, is the need for the new generation of HCL LEDs, to compensate for that valley right in the zone of highest biological sensitivity of 490nm.

Undoubtedly, the new metric created by the IWBI set a very interesting precedent to be able to measure the well-being produced by light, although this standard underwent important modifications in the WELL v2 version, introducing new concepts such as the Melanopic Equivalent Daylight (D65) Illuminance.



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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Daylighting Metrics I: EML

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