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December 13, 2018 | By Tom Sarac

by Tom Sarac

AR stands for Photosynthetically Active Radiation.

PAR is, by far, one of the most widely discussed measurements when it comes to marine reef lighting. It is also unfortunately one of the most often misinterpreted, since it covers only one area within the electromagnetic energy radiated by the sun.

PAR can be defined as the total number of photons within the visible light spectrum falling over a square meter in 1 second.

The unit measure of these photons is presented in micromoles, with 1 mole equalling Avogadro’s number, which is 6.02 X 1023.

The quantum sensor that is actually detecting this energy is doing so across the whole visible light spectrum. This means there is no differentiation between photons of light emitted at longer versus shorter wavelengths and, therefore, no differentiation between the actual amount of light energy.

PAR is an unweighted measure, meaning it is derived from the total number of photons across the whole visible light spectrum. Photosynthetic organisms do respond more to certain specific wavelengths – a limitation to understand when applying PAR values. In other words, if a lighting system were to have a higher PAR value that comes from green wavelengths of light and beyond with very little in the blue range, despite the higher PAR it would not be a more desirable light source versus one with majority of its output in the blue range at a lesser PAR.

In an aquatic environment, PAR ratings can be tricky. Remember that PAR is typically measured in air, not water. Water itself is a filter that affects light transmission. Add to this water surface turbulence, floating plants, surface films, staining from organic compounds, light blockage from unclean glass tops, etc. and you will quickly understand how a PAR rating from a light unit might be skewed.


Choose a unit that comes with a quantum sensor, which exhibits a nice full-square looking response curve. When interpreting a PAR chart, make sure to look at where PAR values are highest and lowest, along with the distances involved, and correlate that to your aquarium dimensions. This will help you decide where you should position plants or corals. Remember that although a specific coral species falls in a certain PAR requiring range, it does not mean that all the specimens will do best at the same value. It may be necessary to shift some specimens higher or lower in your tank. Also remember that, after long shipping periods, corals are stressed and should not be exposed to full light levels. Gradually increase intensity on new individuals by moving them up in the water column slowly.

So what specification is the most important?

Light spectrum.

If a light-emitting source is not giving you the wavelengths of light your corals and plants respond to best, you are simply not going to yield the greatest results. Once you do find an appropriate spectrum though, it’s then a question of building enough lighting power to meet your PAR requirements.


Fluval aquatic expert Tom Sarac explains PAR, a widely discussed yet often misunderstood measurement of lighting in marine reef tanks.


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