Circadian Science · Interactive Tool · CIE S 026/E:2018
M/P Ratio Calculator —
Quantify Any Light Source's Circadian Impact
The Melanopic-to-Photopic (M/P) ratio is the single most useful number for screening any light source for night-safe use. It tells you, for every unit of visible light a source produces, how much circadian-clock stimulation it delivers to your retina. This page gives you the interactive calculator, the complete reference table for 20+ common sources, a full methodology walkthrough, and the worked examples you need to apply the number correctly.
M/P Ratio Calculator — Select a Source or Enter Your Own Values
Select a light source from the reference library, or enter a known M/P ratio and photopic lux level to calculate melanopic EDI and get an instant night-safety rating. All reference M/P values are derived from published SPD data using CIE S 026/E:2018 methodology.
Select a source to load its M/P ratio, then set the photopic lux level to calculate melanopic EDI.
If you have a verified M/P ratio from a manufacturer datasheet (calculated via CIE S 026), enter it below along with your installation lux level.
What the M/P Ratio Actually Means — The Most Useful Single Number in Circadian Lighting
The M/P ratio is dimensionless — it has no units. It tells you the ratio of a light source's circadian potency to its visual output. A high M/P means the source stimulates your circadian system strongly per unit of visible light. A low M/P means it barely registers on the circadian axis, regardless of how bright it looks.
Think of the M/P ratio as an efficiency rating — but for the wrong thing. A source with M/P = 0.90 is highly efficient at stimulating your circadian clock for every lumen it produces. At night, when you want zero circadian stimulation, this is exactly what you don't want.
The M/P ratio is fundamentally different from melanopic EDI in one critical way: M/P ratio is a property of the light source alone, independent of brightness. Melanopic EDI depends on both the source and the light level. A 2700K warm LED has an M/P of approximately 0.42 whether it's at 5% dim or full brightness. Its melanopic EDI at 10 lux is 4.2 mel. lux; at 200 lux it's 84 mel. lux. The ratio is the same. The absolute exposure is different.
This makes M/P ratio ideal for fixture screening before installation. You can determine whether a light source is architecturally suitable for night use from its M/P ratio alone — without knowing the actual lux levels in your room. If M/P ≤ 0.02 (the LumeCircadian night target), the source will stay below 2 melanopic EDI lux at any illuminance up to 100 photopic lux. If M/P = 0.45, the source will exceed the clinical melatonin threshold at any illuminance above approximately 22 photopic lux.
M/P ratio is also the tool for comparing sources without equalising their brightness. You cannot fairly compare the circadian impact of a 5-watt LED running at 800 lumens and a 2-watt amber emitter running at 80 lumens by comparing their absolute melanopic EDI — the lux levels are different. But comparing their M/P ratios gives you the apples-to-apples comparison: "per unit of useful visible light, which source is more circadianly dangerous?"
The M/P ratio of D65 standard daylight is approximately 0.95. This makes intuitive sense — daytime natural light should have a high M/P, because it is biologically appropriate at the correct time. When we say a nighttime lamp should have M/P ≤ 0.02, we're saying it should deliver less than 2% of the circadian signal that outdoor daylight delivers per lumen. That is what "biologically dark" means in quantitative terms — not darkness for vision, but darkness for the circadian clock.
How M/P Is Calculated — The CIE S 026 Method in Plain Steps
The M/P ratio is derived directly from the CIE S 026/E:2018 standard. It requires the spectral power distribution (SPD) of the source — not just its CCT. Here is the exact calculation chain, followed by the shortcut method that works when you have verified reference values.
The full CIE S 026 calculation requires four steps:
Step 1: Obtain the spectral power distribution (SPD) of the source — the radiant power at every wavelength from 380–780nm. This comes from a spectroradiometer measurement or manufacturer datasheet.
Step 2: Multiply the SPD by the melanopic sensitivity function sc(λ) (from CIE S 026 Table 2) at each wavelength and integrate. Apply the D65 normalisation constant to convert to melanopic EDI.
Step 3: Multiply the SPD by the standard photopic sensitivity V(λ) at each wavelength and integrate. Multiply by Km = 683 lm/W to convert to photopic lux.
Step 4: Divide melanopic EDI by photopic lux to get M/P ratio.
Why this matters for the calculator: The reference library values in the calculator above are derived from this full SPD-based calculation for representative sources in each CCT class. However, as the disclaimer notes, individual products of the same CCT can vary significantly in M/P ratio depending on phosphor formulation. A 2700K LED with a large blue spike might have M/P = 0.50, while a 2700K LED with an optimised phosphor might achieve M/P = 0.38. Both label as "2700K." Neither fact appears on the packaging. For precise results, always calculate from the actual product SPD. See the full methodology at the Melanopic EDI article.
All M/P ratio values derived from CCT alone carry a ±15–30% uncertainty. Two products with identical 2700K ratings can have M/P ratios between 0.35 and 0.55 depending on their phosphor composition and blue pump amplitude. This is not a rounding issue — it is the difference between a source that exceeds the clinical threshold at 30 photopic lux vs. one that exceeds it at 20 lux. For residential screening, library values are useful for initial fixture selection. For verified night-safe specifications — especially in nursery environments or migraine-sensitive households — always calculate M/P from the actual product SPD using the CIE S 026 Toolbox. The toolbox is free at cie.co.at.
Complete M/P Reference Table — 25 Common Sources, Fully Annotated
This is the most comprehensive publicly available M/P ratio reference table for common residential and commercial light sources, calculated from published SPD data using CIE S 026/E:2018. Every row includes melanopic EDI at two reference lux levels and comparison against the clinical threshold.
| Light Source | CCT / Peak λ | M/P Ratio | Mel. EDI @ 50 lux | Mel. EDI @ 100 lux | ×threshold (10 mel.lux) | Night Rating |
|---|---|---|---|---|---|---|
| D65 standard daylight | ~6500K | ~0.95 | ~48 lux | ~95 lux | 9.5× | Day only |
| Overcast sky | ~6000K | ~0.80 | ~40 lux | ~80 lux | 8.0× | Day only |
| 6500K Blue-enriched LED (office) | 6500K | ~1.05 | ~53 lux | ~105 lux | 10.5× | Never night |
| 5000K Cool-white LED | 5000K | ~0.90 | ~45 lux | ~90 lux | 9.0× | Day only |
| 4500K Daylight LED | 4500K | ~0.72 | ~36 lux | ~72 lux | 7.2× | Day only |
| 4000K Neutral-white LED | 4000K | ~0.65 | ~33 lux | ~65 lux | 6.5× | Day only |
| T8 Fluorescent 4100K | 4100K | ~0.88 | ~44 lux | ~88 lux | 8.8× | Day only |
| T8 Fluorescent 3000K | 3000K | ~0.55 | ~28 lux | ~55 lux | 5.5× | Day only |
| 3500K Warm-neutral LED | 3500K | ~0.52 | ~26 lux | ~52 lux | 5.2× | Day only |
| 3000K Warm-white LED | 3000K | ~0.45 | ~23 lux | ~45 lux | 4.5× | Inadequate |
| 2700K "Warm" LED (common smart bulb) | 2700K | ~0.42 | ~21 lux | ~42 lux | 4.2× | Inadequate |
| 2700K High-CRI (>90) warm LED | 2700K Hi-CRI | ~0.38–0.44 | ~19–22 lux | ~38–44 lux | 3.8–4.4× | Inadequate |
| Incandescent A19 (2700K) | ~2700K | ~0.40 | ~20 lux | ~40 lux | 4.0× | Inadequate |
| HPS Street lamp | ~2000K | ~0.68 | ~34 lux | ~68 lux | 6.8× | Day only |
| 2400K Very warm LED | 2400K | ~0.35 | ~18 lux | ~35 lux | 3.5× | Poor |
| 2200K High-CRI filament LED | 2200K | ~0.21 | ~11 lux | ~21 lux | 2.1× | Poor |
| Candle flame | ~1800K | ~0.10 | ~5 lux | ~10 lux | 1.0× (at threshold) | Marginal |
| PC-amber LED ~575nm (verify SPD) | ~575nm | ~0.05 | ~2.5 lux | ~5 lux | 0.5× | Verify SPD req. |
| 590nm InGaAlP amber (direct emission) | 590–595nm | ~0.018 | ~0.9 lux | ~1.8 lux | 0.18× ✓ | ✓ Qualified |
| LumeCircadian amber spec (590nm+) | ≥590nm | ≤0.02 | ≤1 lux | ≤2 lux | ≤0.2× ✓ | ✓ Verified safe |
| 610nm deep amber LED | 608–615nm | ~0.006 | ~0.3 lux | ~0.6 lux | 0.06× ✓ | ✓ Preferred |
| 625nm deep red LED | 620–630nm | ~0.002 | ~0.1 lux | ~0.2 lux | 0.02× ✓ | ✓ Maximum safety |
| 660nm red LED (far-red) | 655–665nm | <0.001 | <0.05 lux | <0.1 lux | <0.01× ✓ | ✓ Near-zero |
| Full moon (natural moonlight) | ~4100K reflected | ~0.88 | N/A (0.001 lux) | N/A | ~0.001 mel.lux total | ✓ Biologically safe |
| Night sky (no moon, dark adapted) | Various | — | N/A | N/A | <0.0001 mel.lux | ✓ True biological dark |
Moonlight has an M/P ratio of approximately 0.88 — nearly as high as a 5000K cool LED. Yet it does not suppress melatonin or disrupt sleep, because its absolute illuminance at Earth's surface is approximately 0.001–0.003 photopic lux (a full moon on a clear night). Melanopic EDI = M/P × photopic lux = 0.88 × 0.001 = 0.00088 melanopic EDI lux. This is roughly 10,000× below the clinical threshold for melatonin suppression. The M/P ratio tells you about spectral quality; it tells you nothing about absolute dose without the lux multiplier. Both numbers are always necessary.
How to Interpret Your Results — The Four Zones and What to Do in Each One
The calculator outputs a melanopic EDI value and a night rating. Here is exactly what each rating zone means, which populations are affected, and what action it implies for your installation.
| Rating Zone | Melanopic EDI Range | Who Is Affected | Physiological Effect | Recommended Action |
|---|---|---|---|---|
| ✓ LumeCircadian Verified | ≤ 2 mel. lux | All populations including infants, elderly, migraine sufferers | No measurable melatonin suppression. Biologically equivalent to a dark room. | No action required. Proceed with 590nm+ amber specification at any residential lux level. |
| ✓ Below Clinical Threshold | 2–10 mel. lux | General healthy adult population. Sensitive individuals may still show response. | Below the onset threshold for the general population. Sensitive subgroups may experience marginal suppression. | Acceptable for most adults. Not recommended for infants, young children, or migraine-sensitive households. Verify infant exposure separately with 3× lens factor applied. |
| ⚠ Above Onset Threshold | 10–30 mel. lux | General healthy adult population shows measurable melatonin suppression above 10 mel. lux. | Measurable melatonin onset delay and partial suppression in the general population. Sleep onset disruption possible. | Replace source or significantly reduce lux level. A 2700K LED requires ≤22 lux to stay below 10 mel. lux threshold — typically too dim for care tasks. |
| ⚠ Significant Suppression | 30–100 mel. lux | All adults. Gooley 2011 documents ~50% melatonin suppression and 90-minute phase delay in this range. | Substantial melatonin suppression. Meaningful circadian phase delay. Elevated cortisol during sleep period. Standard residential white lighting at normal dim levels operates here. | Architectural intervention required. White LED sources at normal household lux cannot achieve safe levels in this range — spectral correction is mandatory. |
| ✕ High Circadian Load | >100 mel. lux | All populations including healthy adults. | Near-maximal melatonin suppression. Circadian resetting possible with extended exposure. Equivalent to significant daylight exposure in terms of clock signal. | Completely unacceptable for night environments. Immediate replacement required. Standard cool-white and daylight LEDs at normal office lux levels operate here. |
If your calculator result is above 10 melanopic EDI lux, the corrective actions in priority order are: 1. Change the light source — switch to a 590nm+ InGaAlP amber emitter with M/P ≤ 0.02. This is the only complete solution. 2. Reduce lux level — dimming reduces melanopic EDI proportionally but does not change M/P ratio. A 2700K source needs to be below 24 photopic lux to achieve 10 melanopic EDI lux — barely functional for most care tasks. 3. Reduce duration — shorter exposure reduces cumulative dose but does not eliminate the hazard for any given exposure event. Actions 2 and 3 are harm reduction measures. Only action 1 solves the problem completely.
M/P Ratio vs. Melanopic EDI — When to Use Which Metric
These two metrics are related but serve different purposes. Using the wrong one for a given task produces either unnecessary conservatism or false confidence. Here is the decision rule for which to apply.
Use M/P Ratio when: You are comparing or screening light sources for their spectral suitability before knowing the installation lux level. M/P ratio is a property of the source — it doesn't require you to specify a location, room, or fixture position. It answers the question: "Is this source architecturally capable of meeting the night-safe specification, regardless of how bright it runs?"
If M/P ≤ 0.02, the source meets the LumeCircadian specification at any sensible residential lux level (≤100 photopic lux). If M/P = 0.45, no amount of dimming makes the source spectrally appropriate — you can only reduce the dose, not the hazard per lumen.
Use Melanopic EDI when: You are comparing a measured or estimated exposure against a clinical threshold. Research thresholds — "10 mel. lux for onset of suppression," "30 mel. lux for clear suppression" — are expressed in melanopic EDI lux. To determine whether your specific installation is above or below the clinical threshold, you need the actual melanopic EDI at the measurement point: M/P × photopic lux at that location.
The combined workflow: Screen by M/P ratio first (is the source spectrally qualified?). Then verify by melanopic EDI at installed lux levels (is the actual exposure below the clinical threshold?). For most 590nm+ amber installations at normal residential lux levels, if M/P ≤ 0.02 is verified, melanopic EDI will be well below 2 mel. lux without further calculation. The M/P check is sufficient as a first-pass qualifier.
Limitations — When CCT-Based M/P Estimates Are Not Good Enough
The calculator reference library uses CCT-class representative M/P values. For most residential screening purposes this is adequate. There are specific situations where it is not — and knowing them prevents false confidence in a failing installation.
Situation 1: You're specifying a nursery or infant environment. The infant lens amplification factor (approximately 3× higher retinal blue exposure than adults at equal fixture output) makes every M/P estimate more critical. A 15% error in M/P estimate means a 15% error in infant melanopic EDI — which can be the difference between 8 mel. lux (marginal) and 9.2 mel. lux (above threshold for sensitive individuals). For nursery applications, always calculate M/P from the actual product SPD, not the CCT-class reference value. See the full infant vulnerability framework at the Pediatric Shield lens transmission section.
Situation 2: You're specifying for a migraine-sensitive household. Migraine sufferers have documented lower-than-average thresholds for light-triggered episodes. A melanopic EDI that is "safe" for the general population may still be provocative for a migraine-sensitive individual. In these cases, apply the most conservative possible specification — verified SPD, not CCT-class estimate, and target melanopic EDI ≤ 1 mel. lux rather than ≤ 10. See the migraine-safe lighting protocol for the complete specification.
Situation 3: The source is marketed as "amber" without SPD data. As documented on the phosphor blue spike page, many products marketed as "amber" are PC-amber sources with residual blue pump emission. CCT-class M/P values do not apply — a source marketed as "575nm warm amber" may have M/P anywhere from 0.02 to 0.20 depending on phosphor design. The CCT-class amber reference value of ~0.05 may significantly underestimate the actual M/P. Always require an SPD for any source marketed as a night-safe amber.
Situation 4: You are comparing sources for a formal HCL specification document. For professional HCL installations, formal specifications, or any document that will be used for compliance verification, only SPD-derived M/P ratios from the specific product lot are acceptable. Library values are for educational and screening purposes — not for formal specifications. Use the CIE S 026 Toolbox with manufacturer SPD data for all formal calculations. See the complete methodology at the HCL Retrofit emitter selection guide.
The reference library lists 2700K warm LED at M/P ≈ 0.42. Real 2700K products range from approximately 0.35 to 0.52 depending on phosphor. At 80 photopic lux (a dim bedroom):
Best case (M/P = 0.35): 0.35 × 80 = 28 melanopic EDI lux — 2.8× above clinical threshold
Reference (M/P = 0.42): 0.42 × 80 = 33.6 melanopic EDI lux — 3.4× above clinical threshold
Worst case (M/P = 0.52): 0.52 × 80 = 41.6 melanopic EDI lux — 4.2× above clinical threshold
All three are above the threshold — the CCT estimate correctly identifies the source as inadequate for night use regardless of the specific M/P. But the exact dose varies by 50%, which matters if you're trying to calculate how much you need to dim to reach a marginal threshold. This is why SPD-based M/P is mandatory when exact dose matters.
The CIE S 026 α-opic Toolbox is a free Excel workbook that implements the complete CIE S 026/E:2018 calculation. Import your product's SPD data (available from the manufacturer as a CSV or table), and the toolbox outputs all five α-opic EDI values including melanopic EDI. Divide by photopic lux to get your verified M/P ratio. Free download at cie.co.at. Takes approximately 15 minutes for someone comfortable with Excel — faster than any other path to a verified number. See the CIE Toolbox reference entry for download details.