Reef tank lighting: PAR, PUR, and the Apogee correction

PAR vs PUR vs DLI

PAR is what you measure. Photosynthetically Active Radiation is total photon flux from 400 to 700 nm — the visible window. The unit is micromoles of photons per square meter per second (μmol/m²/s, sometimes called "μE"). It treats every wavelength equally: a blue photon and a green photon at the same flux count the same.

That equality is biologically wrong. Coral zooxanthellae (the photosynthetic algae living inside coral tissue) use chlorophyll a, c2, and the carotenoid peridinin as primary pigments. These absorb most efficiently in blue (around 450 nm) and to a lesser degree red (around 660 nm). Yellow and green photons in the 530–570 nm band largely pass through. So a PAR number can mislead — 200 PAR of pure 530 nm green is almost useless to coral; 200 PAR of 450 nm royal blue grows acropora.

PUR — Photosynthetically Usable Radiation — is the conceptual fix. It's the fraction of PAR weighted by the absorption spectrum of the photosynthetic pigments involved. Useful framing. Not measurable. There is no PUR meter — modeling PUR requires the source spectrum and an assumption about which pigments dominate, both of which vary by coral and by lamp.

DLI — Daily Light Integral — is PAR over time. Multiply PAR (μmol/m²/s) by photoperiod in seconds and divide by 1,000,000 to get mol/m²/d. A coral at 200 PAR for 10 hours receives 200 × 36000 / 1,000,000 = 7.2 mol/m²/d. DLI is the right metric for comparing two schedules that differ in both intensity and length. Plant horticulture has used it for decades; reef hobbyists are slowly adopting it.

The Apogee 1.25 correction

Apogee Instruments builds the MQ-510 quantum sensor that most reef hobbyists rent or own. The sensor is calibrated against the sun — that is, it reads correctly in air. Submerged in water, the sensor reads low, because some photons that would have entered the sensor get refracted past it at the air-water interface, and the calibration also assumes the standardized sun spectrum.

The published immersion correction for the MQ-510 is 1.25×. If the sensor displays "300 PAR" in your tank, the true underwater PAR at that point is 300 × 1.25 = 375 μmol/m²/s. The number 1.32 circulates widely on Reef2Reef and in old YouTube videos; it's incorrect for the MQ-510. The 1.32 value applies to an older Apogee model with a different spectral response and was never the right correction for the current full-spectrum meter.

Practically, whenever you read PAR specs from a manufacturer's coverage map, check whether they've already applied the underwater correction. Ecotech's Radion PAR maps are underwater-corrected. AI's Hydra PAR data is underwater-corrected. Older Kessil published in-air numbers — multiply those by 1.25.

Coral PAR targets

These are the consensus ranges, measured underwater with an Apogee MQ-510 (or equivalent) at the coral, not at the surface. Cross-checked against Sanjay Joshi's reefkeeping.com archives and Dana Riddle's Advanced Aquarist column.

• Softies (zoanthids, mushrooms, leathers): 50–100 PAR. They grow at 150 too, but the colour stays better at the lower end.
• LPS (euphyllia, acanthastrea, chalice, hammers, frogspawn): 50–150 PAR. Acanthastrea and chalices want the low end; euphyllia happily takes the high end.
• Mixed reef (a bit of everything): 100–250 PAR at coral, with the most demanding pieces near the top.
• SPS (acropora, montipora, stylophora, pocillopora): 200–400 PAR. Acros at the high end; encrusting Montipora at the low end. Above 400, you're into known-bleaching territory unless slowly acclimated.
• Clams (Tridacna): 250–400 PAR for crocea/maxima up high; 150–250 for derasa/squamosa on the sandbed.

Tank depth matters because PAR falls off roughly with the inverse square of distance from the source. A measurement at sandbed in a 24-inch tall tank is typically 30–50% of the PAR at 6 inches from the lamp. Account for the actual placement of the coral, not the maximum any pendant can produce.

Daily Light Integral (DLI)

DLI matters because coral cares about the daily dose, not just the peak. The published PUR/PAR research from Dana Riddle and the long-tail data from public reefs (Smithsonian, NOAA) suggest that wild Caribbean shallow reefs see DLI in the 12–25 mol/m²/d range. Indo-Pacific lagoons sit at 8–18. Mesophotic reefs at 30 m sit at 1–4.

For an SPS-dominant tank running 250 PAR average for an 8-hour high-noon period plus 2 hours of ramp at 100 PAR average, DLI works out to 250 × 28800/1e6 + 100 × 7200/1e6 ≈ 8.0 mol/m²/d. That's the bottom edge of healthy Indo-Pacific lagoon — defensible.

Pushing photoperiod beyond 12 hours doesn't grow more coral. Coral photosynthesis saturates above a certain intensity; what extra time on the clock buys you is extra algae and cyanobacteria growth on the rocks. A 10-hour total photoperiod with 8 hours at full intensity is the modern best practice.

Spectrum and PUR

The reason modern reef lights are blue-dominant isn't cosmetics — it's PUR. Blue 450 nm sits squarely in the absorption peak of chlorophyll a and the chlorophyll-c2/peridinin complex. Pure white LEDs (3500 K) deliver photons across the visible band but only a small slice lands in the high-absorption windows. A blue-dominant LED at the same PAR has higher PUR.

Practical implication: a "20,000 K" or "Triton" white-blue blend at 200 PAR grows coral as well as a "10,000 K" mostly-white blend at 280 PAR. The blue mix delivers similar PUR with lower total PAR — and lower heat load, lower algae growth, and better coloration fluorescence.

Don't go pure royal blue, though. Some red (620–660 nm) is needed for the secondary chlorophyll absorption peak, and a small slice of green-cyan (490–530 nm) tunes fluorescent pigment expression. The factory-default channels on Radion, Hydra, Kessil A-series, and similar are all reasonable starting points — manufacturers have spent years tuning these.

Acclimating to new light

Coral takes weeks to adapt zooxanthellae density to a new light level. Drop a Bali acropora that lived under 250 PAR for two years into a tank with 450 PAR and you'll get bleaching inside 48 hours — the algae either eject or photoinhibit and the coral pales. Same coral, brought up at 25 PAR per week, ends up at 450 PAR fine.

The default acclimation protocol: start any new coral at 60% of its eventual target PAR, then increase by 10–15% per week for 6–8 weeks. Or — easier — just place new arrivals on the sandbed and physically move them up the rocks over 6–8 weeks. Same effect.

Common mistakes

• Reading PAR off the manufacturer's map alone. The map shows what the light can deliver under ideal conditions. Your tank has glass tops, surface agitation, and a different mounting height. Measure.
• Lux or phone apps. Useless for reef. The blue-heavy spectrum is exactly what consumer lux meters underweight.
• One spot reading. PAR varies 4× across a tank's footprint. Take readings at 6 inches below the surface every 6 inches across the tank in a grid.
• The 1.32 correction. Out of date for the MQ-510 — use 1.25.
• Treating PUR as a number. It's a concept. The "PUR percentage" you sometimes see in lamp marketing has no agreed-on measurement protocol.
• 14-hour photoperiods. You'll grow algae faster than coral. 10 hours is plenty.

Calculators related to this guide: aquarium volume (for sizing fixtures) and temperature reference (heat from over-driven LEDs). For the lighting-influences-everything reef setup overview, see Starting a reef tank.