Definition:
Melanopic Equivalent Daylight Illuminance (EDI), sometimes abbreviated as MEDI, expresses melanopic light in terms of its biological equivalence to standard daylight (D65). It enables direct comparison of the circadian effect of different light sources by translating measurements into a familiar daylight-referenced unit—lux. This metric is defined in CIE S 026:2018.
Why it matters:
By converting melanopic values into a daylight equivalent, Melanopic EDI helps lighting designers, researchers, and smart-building professionals assess how effectively a light source supports circadian health—without needing complex spectral data.
How it is calculated:
First, melanopic irradiance is determined by weighting the spectral power distribution (SPD) of the light source with the melanopic sensitivity curve. This is then scaled against D65 daylight to derive the EDI value in lux, as standardised by CIE S 026.
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