For testing of specific lights, visit: the Reef PAR Database 

PAR and PUR are both very important terms that have been mystified to the point that all but experts don’t understand what they truly mean. I’m going to keep this as a permanent page and attempt to make some sense of these two terms, so that everyone may educate themselves further in lighting. I am studied in theoretical physics which includes much of this topic as proven ground rules as well as much more complicated unproven theory. This will be 100% SCIENTIFICALLY based with readings from professional equipment, but a lot of it is still theory due to lack of research, so any argument is welcome to help move us further towards science fact. I hope you enjoy!

PAR-

We first need to start with a basic explanation of these terms that everyone can understand. I will begin with PAR. PAR, or photosynthetically active radiation, is a measure of how much light energy is present that may be used in all known instances of photosynthesis. This is measured a measure of how many radiation particles hit a meter square of space in one second. PAR is the best to date way to easily measure actual usable light. It is rarely measured due to the cost of equipment to do so, but if it’s in your budget a meter such as one from Apogee Instruments. The problem with PAR is that it measures light within all usable wavelengths, and most photosynthetic organisms aren’t capable of using all of these wavelengths. This means that a PAR reading may not be truly showing how much energy is being emitted that your corals or plants can use. This is where PUR comes in.

PUR-

This next part is more of a theory than a science based measurement NO MATTER WHAT ANYONE TELLS YOU! PUR, or photosynthetically usable radiation, is a measure of of how much PAR is present in the wavelengths specifically useful to a designated animal or plant. As depth increases light falls off more in some wavelengths than others, so corals that live deeper are more adapt at using specific wavelengths for photosynthesis. There may be a meter capable of measuring PAR in very specific wavelengths only, but the problem with this is finding the exact wavelengths a coral uses. This is why I say that PUR is a theory not an exact science. If it were possible to find out exactly which wavelengths a coral is capable of using and measure for only those, you could calculate an exact number for PUR. Corals are also very adaptive animals and can adapt or adopt new xoozanthellae that may use new wavelengths, so PUR for some corals may be exactly the same as PAR.

This is where the complicated subject of aquarium lighting often gets derailed. Some insist that PAR is useless compared to PUR, but with the adaptive ever changing nature of coral, it’s hard to pin down exactly which wavelengths to test for. I disagree with the idea of PUR as a reliable means of testing how much photosynthetic energy is making it into an aquarium, but it is a term that is here to stay, so everyone should be informed about what it means.

How much PAR is enough?

This is a question that I feel doesn’t come out often enough. Everyone seems to be so preoccupied with which fixtures put out the most PAR and how to get the most PAR over their tank that I think, in many cases, they don’t even know how much they need. For starters, it is most likely not as much as you would think. The saturation point of many hard corals is above 500, but they grow great much lower than that. Even the most light demanding of corals can, and will, grow pretty quickly in the 250 to 300 range, and lower light demanding corals are even more shocking. I’ve seen it said that almost no coral will grow at levels of below 100, but that is completely un-true. Most soft corals, some LPS and all chalice will grow very well at PAR levels all the way down to 50!

More to come…