PAR is the buzzword, even though most people don't know what it is. Coral keepers and plant buffs have an inkling, but it's a mystery for the rest of us.
PAR stands for Photosynthetic Active Radiation and refers to a large portion of the light spectrum, including light between the 400 and 700 nanometres wavelength. Photosynthesis takes place within this range.
Go any further into the red spectrum and light carries too little energy to be of use. Go into the blue and it’s too powerful — generally damaging to life.
Some keepers assume that PAR readings, if high, will be good for all plants and/or coral. This isn’t necessarily the case.
In order to photosynthesise, plants and corals concern themselves with PUR rather than PAR — PUR meaning Photosynthetic Usable Radiation. The two are not necessarily the same.
For example, a light unit may deliver admirable readings of PAR, but it may be delivering 600-700nm wavelength and nothing else. If an organism hypothetically only uses light of 450-500nm, then they’ll not be getting enough to photosynthesise.
Alas, measuring PUR is a bit of a non-starter. Unless you know the exact range of light spectrum a particular organism requires, it’s all a tad superfluous. Plus, measuring a particular PUR reading can be notoriously difficult and expensive. PAR is the more realistic and affordable option.
Most chlorophyll activity of plants and zooxanthellae sits between the 400-500nm and the 600-700nm ranges respectively. To get the most from your light, you want not only something that gives good PAR output, but specifically output in these parts of the spectrum.
How is PAR measured?
The typical way PAR is quantified is with the formula µmoles/m2/s and usually what a PAR meter will read. This basically means we measure how many ‘bits’ of light hit a surface of 1m x 1m over a period of time.
So, the further away from a light we get, the less PAR we will have — and that’s one of the reasons that a PAR reading of a light that fails to mention the distance from its source is immediately rendered useless.
Based on this, and to give a comparison, a bright sunny day will give a PAR reading of between 1,000 and 2,000, moderate sunshine 250 to 1,000 and shade below 250.
Indoors, in daytime with blinds closed, you’ll be lucky to get past 4 or 5.
A 75w bulb at 60cm/24” distance will produce about the same. A 55w compact fluorescent bulb (6,700K) at 60cm gives around 20.
We can suppose that, at the depth the plants live, 120-300 is a high reading, 80-120 is moderate, and 40-80 is low light.
Many reefkeepers like to map out the PAR hotspots and shady areas in their aquaria. This way they know optimal sites for their corals, many of which have a hugely varied PAR requirement.
Why not take out a subscription to Practical Fishkeeping magazine? See our latest subscription offer.