Understanding Circadian Effective Lighting

Press/Media: Expert Comment


The Magazine Designing Lighting (dl) examines the three circadian methodologies in layman’s terms. Most designers do not specify circadian methodologies primarily for two reasons: it is extremely complicated, and the science is ongoing.
New data are still being generated by many labs throughout the world.

Melanopic Equivalent Daylight Illuminance Explained (MEDI) by Luc Schlangen

EML (WELL) by Nathan Stodola and Shengliang (Daniel) Ron,

The Circadian Stimulus MOdel: What is it? by Mariana Figueiro.

Period19 Dec 2022

Media contributions


Media contributions

  • TitleMelanopic Equivalent Daylight Illuminance Explained (MEDI)
    Degree of recognitionInternational
    Media typePrint
    Country/TerritoryUnited States
    DescriptionIn 2018 the International Commission on Illumination (CIE) standardized spectral sensitivity functions and SI-compliant quantities that describe optical radiation for its ability to stimulate each of the five photoreceptor classes that can contribute to eye-mediated non-visual effects of light in humans. This new metrology is defined within international standard CIE S 026:2018 [1, 2] and uses the shorthand α-opic to represent one of the five photoreceptor classes within the human retina: S-cone opic (α=sc), M-cone-opic (α=mc), L-cone opic (α=lc), rhodopic (α=rh) [rods] and melanopic (α=mel) [intrinsically-photosensitive retinal ganglion cells (ipRGCs)]. Under most practically relevant situations (i.e., long duration exposures to polychromatic light) the spectral sensitivity of circadian and neuroendocrine responses to ocular light exposure and, by extension, other related non-visual responses, can be well described by the melanopsin-based spectral sensitivity of the ipRGCs, see Brown et al. [3]. The next section of this article will therefore only discuss the melanopic quantities of CIE S 026 in more detail. However, it’s worth noting that similar quantities can also be defined for the other four α-opic photoreceptors. As such the α-opic metrology provides a suitable framework to develop multi-photoreceptor models of spectral sensitivity that eventually might yield a more accurate prediction of circadian, sleep, neuroendocrine, and/or cognitive responses to light as compared to the melanopic-only model as currently adopted in for instance the CIE Position Statement “Proper Light at the Proper Time” [4] and other recommendations for healthy indoor light exposures [3, 5].
    PersonsLuc J.M. Schlangen


  • circadian rhythm
  • integrative lighting
  • non-visual effects