When this topic arose I wondered what to do with it. The reason I wondered is that this website is designed for beginners and ‘earlier-on’ aquarists (though I notice that experienced aquarists are now visiting, and they’re more than welcome). Lighting is a scientific and technical subject.
Most things can be broken down to basics though, and an attempt has been made to do this. In addition, advice is always given to employ the proper lighting, that is spectrum and power, particularly for reef systems. Perhaps a basic explanation of PAR will demonstrate why this is, and is therefore relevant.
There isn’t a requirement to know any more than the need to employ the proper recommended spectrum and power output over a reef. Fish only systems are not so demanding of course, the fish need to see and be seen, and it is good to see the colours reasonably enhanced.
Having the proper spectrum and power output is important, as said. The power output is measured in watts (W), and with metal halide bulbs this requirement depends on the depth of the aquarium. Fluorescent tubes have power outputs generally relevant to their length, though different tubes exist (eg. normal output (NO), high output (HO), and very high output (VHO)). Spectrum refers to the colour of the light, which is of particular importance to the corals, or more correctly a great many of them. The corals that we have in our aquariums for the most part require the correct spectrum.
Generally, corals contain zooxanthellae, or single-celled symbiotic algae. This algae is very important to the coral (see the text ‘Zooxanthellae’). The algae use photosynthesis and need light to do this.
So PAR then. This stands for Photosynthetically Active Radiation. In other words, light that is potentially useful to photosynthesis. It is measured in nanometers (nm).
In the sea, different colours of light are lost, some before others. The first to be lost is red, within a few feet of depth. Blue penetrates deeply, and is the only light left (except UV) after around 50ft. It is blue that is of interest to aquarists. It is stated that blue light of circa 420nm is of good use to zooxanthellae. Thus there are ‘actinic’ tubes and other tubes that are manufactured to peak at this point. Metal halide bulbs can be obtained with different spectrum outputs.
Blue is not the only colour useful to photosynthesis, but it has been found to be useful in a captive system. There are many corals that use this available light and they are usually lower in depth on the reef. There are corals close to the surface and obviously these receive much stronger light and more than just blue. The corals deal with this naturally, as there is much more light available for photosynthesis than is needed.
I reckon that is all that is needed. It really is a ’skim’ over the surface, as said lighting is a complex subject. As with so many things, one thing could be dependant on another, provided that yet another is present. I apologise to any lighting engineers who might see this text, but it is not intended for you.
Us aquarists need only understand what is needed and basically why on the different aspects of keeping a marine aquarium. In this case, it is the lighting that the zooxanthellae can use to photosynthesize, allowing our corals to prosper.
(Ref: Aquarium Corals. Eric H. Borneham)