Light
Melanopic Equivalent Daylight Illuminance
mEDI
Light weighted by the response of the retinal cells that drive circadian signalling, expressed in lux. The measurable quantity underneath most claims about circadian lighting, standardized in CIE S 026.
In more detail
Melanopic equivalent daylight illuminance expresses how strongly a light source stimulates the retinal cells that drive circadian signalling, in lux. It is one of the alpha-opic quantities standardized in CIE S 026, which weights measured spectral irradiance by the sensitivity of melanopsin-containing retinal ganglion cells rather than by the photopic curve ordinary lux uses.
Two things follow. It is spectrum-dependent, so two sources at the same photopic lux can differ substantially — a difference expressed as their melanopic-to-photopic (M/P) ratio. And it is a quantity at the eye, in the direction of gaze, on a vertical plane: a ceiling fixture producing high illuminance on a desk may deliver far less to an eye looking horizontally, and a bright window in the field of view may deliver far more.
That distinction is what makes the metric necessary and also what makes it hard to verify. Correct measurement takes a spectrometer positioned at eye level facing the direction of view. A lux meter on a countertop is measuring something else.
Why it matters
Melanopic EDI is the only quantity that makes claims about light and circadian rhythm comparable across products and spaces. It is also the quantity almost no residential product publishes, which is itself informative: a lighting specification quoting CCT and wattage has not addressed the question. Where a target is being discussed, it belongs with its source, its measurement plane and its time of day attached — which is guide territory rather than a definition.
The common misunderstanding
That melanopic EDI is a property of a fixture. It is a property of the light arriving at a particular eye, in a particular position, facing a particular direction. Moving a chair, repainting a wall or opening a shade changes it. A published fixture figure is a measurement at a stated distance and geometry, not a value that follows an occupant around the room.
Related terms
On Compound
Where this term does real work — and where the decisions get made.
Sources
Last reviewed July 2026 by Compound Editorial. Definitions are educational and not project-specific engineering or medical advice.