Every supplier now claims human-centric lighting. The M/P ratio is the single parameter that separates a calibrated circadian system from a rebrand. A specifier's guide to what it measures and how to require it.
Walk any lighting trade show floor and you will find the phrase on half the booths. Human-centric lighting has become a sales adjective — which is a problem, because the underlying science is precise enough to be measured, and most products on the market cannot pass that measurement.
The M/P ratio — melanopic to photopic illuminance ratio — is the cleanest single-number proxy for how a light source affects your circadian biology. It is not the only parameter that matters, but it is the hardest one to fake.
Photopic illuminance (lux) measures what the eye’s cone cells perceive as brightness. Melanopic illuminance measures the signal reaching the ipRGCs — the intrinsically photosensitive retinal ganglion cells discovered in 2002 — which route directly to the suprachiasmatic nucleus (SCN), the brain’s circadian master clock.
The SCN uses this melanopic signal to suppress or release melatonin, regulate cortisol and synchronise every peripheral clock in the body. Photopic brightness and melanopic stimulus are not the same thing. A warm-white luminaire at 500 lux may carry an M/P ratio of 0.3, while a full-spectrum source at identical lux may reach 0.9 — same perceived brightness, radically different biological effect.
This is why colour rendering (CRI) alone is an insufficient specification criterion, and why watt-per-lumen efficiency curves tell you nothing about circadian performance.
CIE S 026:2018 — the current international standard for circadian-effective light — defines the following reference points:
WELL v2 Feature L07 mandates that occupied spaces achieve ≥ 150 EML at a 1.2 m eye level for at least 4 hours per day. The M/P ratio of the installed luminaire is the primary determinant of whether a compliant system is achievable within a reasonable fixture count and budget.
The circadian effect operates in both directions. The same biology that governs alertness and recovery also shapes the quality of a visitor’s experience in a commercial space.
The behavioural mechanism is documented in circadian and environmental psychology research: lower melanopic stimulus during evening operating hours reduces low-level vigilance arousal, enabling slower, more immersive visual processing. In a retail or gallery context, this effect supports longer dwell and more deliberate product engagement. The precise uplift varies by space type, visitor demographic and programme execution — no large-scale peer-reviewed study applying CIE S 026 metrics to retail dwell behaviour has been independently published at the time of writing. SUMLINK treats this as an applied-research area where outcomes are tracked per project rather than a generalised figure.
Most factory data sheets do not include M/P ratio. This is not necessarily evasion — many manufacturers have not commissioned SPD (spectral power distribution) measurement. The following request exposes the gap immediately:
“Please provide the full SPD curve and the calculated M/P ratio per CIE S 026:2018 at your nominal CCT. We require third-party measurement or SUMLINK Verified data. Factory self-report is insufficient for this specification.”
Suppliers with a genuine full-spectrum chip and calibrated phosphor formulation will have this data. Those relying on standard phosphor-converted LED will not — because their M/P ratios typically fall between 0.25 and 0.40 across the full CCT range, insufficient for morning activation performance.
For projects requiring tunable white (2700–6500 K range), also request the Δu’v’ value across the tuning range. A system that drifts more than 0.006 u’v’ units while dimming is colour-shifting. This is the single most common point of failure in cheap tunable-white systems and is entirely invisible in a showroom demo under a single fixed CCT.
For any HCL-credible luminaire procurement, the following parameters are minimum requirements:
CRI ≥ 97 is a prerequisite for retail and gallery applications but is now achievable by a wide range of full-spectrum products. It should be treated as a threshold qualifier, not a differentiator.
SUMLINK maintains a portable spectroradiometer for on-site verification. Products that pass factory-reported parameters against independent third-party measurement carry the SUMLINK Verified designation in the Catalog. This is not a quality grade — it is a transparency signal. The M/P ratio either measures as specified, or it does not.
Every light. One link.