PAR (Photosynthetically Active Radiation)
PAR, short for photosynthetically active radiation, measures the specific range of light wavelengths, roughly 400 to 700 nanometers, that corals, their symbiotic zooxanthellae, and other photosynthetic organisms actually use to drive photosynthesis. Unlike simply measuring overall light output or brightness, PAR specifically quantifies the portion of light spectrum that’s biologically useful, making it a far more meaningful metric for matching lighting to a coral’s actual needs.
Different coral species have evolved to thrive under different PAR ranges depending on their natural depth and habitat, making PAR measurement an important tool for both initial lighting setup and ongoing placement decisions within a tank.
Why It’s Important
- Measures the specific light wavelengths corals actually use for photosynthesis.
- Provides a far more meaningful metric than simple brightness or wattage.
- Varies considerably between coral species based on their natural habitat depth.
- Guides both lighting equipment selection and in-tank coral placement decisions.
How It Works
A PAR meter uses a specialized sensor to measure the intensity of photosynthetically useful light wavelengths at a specific point, typically expressed in micromoles per square meter per second. Reef keepers take PAR readings at various depths and positions within a tank to understand how light intensity varies throughout, since factors like water depth, glass thickness, and fixture height all significantly affect actual PAR delivered to any given coral.
Because different coral species require different PAR ranges to thrive, from lower-light LPS species tolerating relatively modest PAR to demanding SPS corals requiring considerably higher intensity, matching placement to PAR helps prevent both insufficient lighting and photo-stress from excessive intensity.
PAR and Related Concepts
PAR is closely tied to photosynthesis, the biological process it directly measures light for, and to PAR meters, the specialized equipment used to take these readings. It’s also relevant to coral coloration and coral placement, since appropriate PAR levels significantly influence both.
Reef keepers setting up a new tank often map out PAR readings at various locations before finalizing coral placement, using this data to position light-demanding SPS corals in higher PAR zones while placing more shade-tolerant species lower or further from the light source.
Gradual PAR acclimation is important when introducing a new coral or upgrading lighting equipment, since a sudden jump to much higher PAR than a coral is accustomed to can cause stress or bleaching even if the new PAR level is technically appropriate long-term.
Common Misconception
A common misconception is that brighter lighting always means better PAR and therefore better coral growth. In reality, PAR depends on wavelength and measurement position as much as raw brightness, and excessive PAR for a given coral species can cause just as many problems as insufficient PAR.
The Bottom Line
PAR measures the specific light wavelengths corals use for photosynthesis, offering a far more meaningful metric than simple brightness. Matching PAR to each coral’s specific needs through careful measurement and placement supports healthier growth and coloration.
