mEDI

Melanopic equivalent daylight illuminance

mEDI is the light measure weighted to the eye's circadian receptor, capturing the blue-wavelength signal your body clock reads that plain lux misses.

Your eye carries a set of cells that talk straight to the brain's master clock, tuned to the blue-cyan wavelengths near 480 nanometers that tell the body whether it is day or night. Melanopic equivalent daylight illuminance, mEDI, is the light metric weighted to those cells, so it measures the circadian signal you are actually getting. A standard lux meter reads visual brightness instead, and the two come apart. A warm lamp can look bright while delivering almost no circadian signal, and a cooler daylight source at the same brightness can deliver several times more. That gap is why a room can feel well lit yet still leave your clock under-signaled by day, and it is why the consensus daytime, evening, and nighttime light targets are set in mEDI rather than lux. Standardized by the CIE in 2018, mEDI is the unit the research uses to say how much of the right light is reaching the system that sets your sleep and your energy.