Most lighting designers still specify by lux and CCT. Here is why the melanopic-to-photopic ratio is the single number that determines whether a space supports or disrupts circadian health.
Illuminance levels and colour temperature are the two parameters that dominate most lighting specifications. Yet neither captures the biological impact of light on the human circadian system. That job belongs to the M/P Ratio — the ratio of melanopic to photopic illuminance.
The melanopic channel is driven by ipRGCs (intrinsically photosensitive retinal ganglion cells), discovered in 2002. These cells are most sensitive to short-wavelength light around 480nm — the blue-cyan range abundant in cool white LEDs and daylight. They send signals directly to the suprachiasmatic nucleus (SCN), the brain’s master clock, regulating cortisol, melatonin and core body temperature.
An M/P ratio above 0.9 at 5000K signals the circadian system to suppress melatonin and elevate alertness — appropriate for morning activation in offices, schools and retail openings. An M/P ratio below 0.4 at 2700K tells the system to begin melatonin production — appropriate for evening hospitality, residential pre-sleep environments and gallery night modes.
A space operating at the wrong M/P ratio for its time of day is not just “inefficient’ — it is actively working against the biological state of its occupants.
SUMLINK requires M/P Ratio data for every product in the Catalog. Where factory data is unavailable, core SKUs are independently measured using a portable spectroradiometer. Products carrying the SUMLINK Verified badge have confirmed M/P ratios tested against CIE S 026:2018 reference conditions.