Light is not set by eye — the PAR meter in practice

It is easy to move a reef light’s brightness slider up or down based on how the tank looks. It feels natural — the human eye has evolved to assess adequate light. On a reef, it is not enough.

The problem is two-fold. First, LED lighting’s blue-dominant spectrum distorts perception: in blue light, a tank looks bright and full of light even when the wavelengths and intensity most relevant to coral photosynthesis are underrepresented or wrong. Second, the human eye cannot compare absolute light levels — it adapts quickly to prevailing brightness and produces no comparative data that would tell us anything about coral needs.

A PAR meter does what the eye cannot: it measures the photosynthetically relevant portion of light as a numerical value comparable to coral-specific reference ranges and natural reef illumination levels.

What PAR measures — and what it does not

PAR (Photosynthetically Active Radiation) is the established way to express the portion of light that plants and animals can use for photosynthesis. The measurement range is 400–700 nanometres, and the unit is µmol/m²/s — micromoles of photons per square metre per second. This is also called PPFD (Photosynthetic Photon Flux Density).

In corals, photosynthesis takes place in the zooxanthellae (Symbiodiniaceae) living inside their cells. These symbiotic algae primarily use blue (430–490 nm) and red (620–680 nm) light through the absorption peaks of their chlorophylls.

A PAR reading gives the total photon count across the entire 400–700 nm range, weighted equally. It does not distinguish whether the light is spectrally useful to corals or not — a high PAR value can also come from wavelengths that zooxanthellae do not efficiently use.

ITC Reefculture PARwise — what it measures

Riuttareef’s view is that a plain PAR value is an insufficient tool for modern LED-based reef keeping. We recommend the ITC Reefculture PARwise meter because it produces, in addition to a PAR reading, several parameters that together give a true picture of coral lighting conditions:

cPUR (Coral Photosynthetically Usable Radiation) is PARwise’s key added value. It restricts the measurement to the wavelengths and intensities that corals can actually use for photosynthesis. Two lights’ PAR values may be identical, but their cPUR values very different based on spectral quality.

Spectral distribution shows how light is distributed across wavelength bands. This is critical information in LED lighting where the spectrum is adjustable by the hobbyist but its consequences are invisible to the eye.

DLI (Daily Light Integral) describes the total daily light dose corals receive: intensity multiplied by lighting hours. Two tanks with the same instantaneous PAR can deliver completely different DLI due to different light schedules.

PARwise works both underwater and in air. It connects via USB to a computer or Android device. iOS devices are currently not compatible — measurements require Chrome or Edge browser on a Mac, Windows computer or Android device.

The special problem with LED lighting: uneven field distribution

Older metal halide and T5 fixtures produce an even, diffuse light field. LED fixtures are inherently directional: light concentrates most intensely directly below the fixture and falls off rapidly to the sides and with depth. This is called the hot spot effect.

The practical consequence is that PAR at the top of the tank can be considerably — and surprisingly — higher than a hobbyist would expect, while at the tank edges and deeper zones values may be a fraction of the upper-zone level.

A single measurement directly below the fixture is a misleading starting point. A complete mapping includes measurements at multiple depth levels, from both ends, the centre and corners of the tank.

Species-specific PAR reference values

On natural reefs at shallow depths (0–5 m), PAR in clear water can exceed 800–1,000 µmol/m²/s during the day. Most aquarium corals originate from deeper zones or have adapted to lower intensities in aquarium conditions.

Rule of thumb: too-low PAR rarely kills a coral acutely — too-high can. Always start from a conservative level and raise carefully.

Light acclimatisation — a critical phase

Zooxanthellae adapt to prevailing light levels by changing their chlorophyll density and pigment composition. This process takes days or weeks. An abrupt change — a new fixture, a new position or unexpectedly clarified water — can cause light damage, resulting in bleaching or tissue loss.

New corals should first be placed in the shadiest part of the tank or near the bottom. Move them gradually towards the intended final position over several days or weeks according to their response.

A new fixture or significant power change should be introduced by starting at low intensity and raising gradually over weeks.

Water clarification is a less common but important case: starting GAC filtration, ozone treatment or the end of heavy algae growth can clear the water quickly and significantly increase the amount of light reaching corals without any change to fixture settings. PAR mapping should be redone in such situations.

A meter is a basic tool, not a luxury

Riuttareef’s view is that a PAR meter — or preferably a spectral meter like PARwise — belongs among the basic tools of reef keeping alongside a refractometer or a high-quality alkalinity test.

The meter does not need to be in continuous use — it is an installation tool, not a monitoring tool. Mapping is done during new tank setup, when installing a new fixture, after significant changes, and whenever coral behaviour gives cause for concern.

Owning the device is not essential — we primarily recommend enquiring about rental options from your local aquarium shop or hobbyist community. A single measurement day is sufficient to map the tank.

References

1. Peer-reviewed studies

• Riddle, D. (2004). Feature Article: Light requirements of photosynthetic invertebrates: studies of selected species. Advanced Aquarist.
• Kirk, J. T. O. (2000). Light and Photosynthesis in Aquatic Ecosystems (3rd ed.). Cambridge University Press.
• Levy, O. et al. (2003). Absorption spectra of various photoreactive biochemicals found in the symbionts of Dipsastraea favus. The FEBS Journal.

2. Hobbyist literature and brand documentation

• Aslett, C. G. (2024). Teach a Person to Fish… SPS Academy Part XII: Lighting. Reef Ranch.
• Community Reef Store (2025). Optimal placement of corals in the marine aquarium.
• ITC Reefculture (2024). PARwise — product documentation. itcreefculture.com / par-wise.com.

3. Books and textbooks

• Borneman, E. H. (2001). Aquarium Corals: Selection, Husbandry, and Natural History. T.F.H. Publications.
• Delbeek, J. C. & Sprung, J. (2005). The Reef Aquarium, Volume Three. Ricordea Publishing.