The LED (Light Emitting Diode) fixtures have taken the aquarium hobby by storm. We have “single bright” systems designed to replace simple T8 or T12 fluorescent fixtures, “double bright” to replace two bulb systems, and “reef capable” to replace multiple bulb T8 or even T5HO systems. Where it gets confusing is the inability to compare oranges to oranges. The ratings used by the different manufacturers obviously highlight what is best about their system, but might not give the hobbyist the answers they need to make the best choice. Another factor is the rapid development of newer generations of LED’s with better output spectrums and higher output per watt. It’s much like buying computer you need only to realize that the fixture will be obsolete within 2 or 3 years.

Many manufacturers will provide the PAR value for their LED light fixtures. This stands for Photosynthetic Available Radiation and is a measure of the light output between 400-700 nanometers (visible light). A light source with a higher PAR value will look brighter to the human because the human eye “sees” in this spectrum. But, the spectrum that is most responsible for plant/coral growth is in the 400-550 and 620-700 nanometer ranges. These are two of the spectrums that are the least sensitive to the human eye. These two ranges are measured as PUR (Photosynthetic Usable Radiation) and, for the hobby’s purpose, would generally be the best way to compare different LED fixtures. However, these values are rarely given.

Manufacturers generally take one of two approaches with LED aquarium fixtures; multiple lower PUR value LED’s to get a cumulatively higher PAR and PUR, or fewer, newest generation high PUR LED’s that may measure lower on a PAR scale. Fixture A may have 60 W’s producing 100 PAR but only 50 PUR units. Fixture B may have 30W’s producing 50 PAR and 50 PUR units. To the human eye, Fixture A would look twice as bright, but in reality, would only provide the same useful output as Fixture B that would run at a much lower energy cost. (There figures are for example purposes only, rarely will a system have equal PAR and PUR ratings). Throw into this mix the need to also provide an esthetically pleasing “look” to the human eye, and you can understand some of the varied approaches taken by different manufacturers.

Will LED fixtures take over the market? At this point, it would appear so. They have lower energy cost and heat production per equal output, much better longevity for the output source (50,000 hours vs. 8000 hours), and are easier to focus to get all the light to the aquarium. Compared to older systems, 25W’s of LED can equal 100 W’s of PC bulbs, 17 W’s equal 100 W’s of T8 or T12 bulbs. They have replaced 250 W MH fixtures and only lack for systems requiring 400 W MH bulbs for 30+” of depth. But the next generation will probably equal that!

About The Author Don Roberts

comments (2)

  • PUR is a term, generally used by less than reputable companies, to further confuse and mislead consumers rather than using an honest and truly scientific means of measurement such as PAR. It is a theory which would be useful if the sweet spot was known for each coral attempting to be housed, and if measuring it were easily accomplished. In all reality, I don't believe there is a single company that has truly measured the output radiation within a specific band of the spectrum coming from their lights and than went so far as to correlate the numbers to what a specific coral needs.

  • Here’s some current information on how PUR can be used related to PAR and RQE. Also understanding how water plays a role and which light is really useful. It’s not species specific, but it is photosynthesis action response specific.

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