PAR stands for Photosynthetically Active Radiation.
This is commonly our starting point for determining whether or not a light fixture is adequate for our needs since it can be relatively objectively measured.
Before we go into depth about the meat of this article, we can use
the basics of PAR combined with input wattage to get some useful
information about the efficiency of an aquarium LED light (or really any
The reason this is important is that many if not most LED fixtures can keep high light planted or reef aquariums, but many if not most use a lot more energy and last much lower time due to inefficiencies than need be if built with efficiency and durability in mind (which bring up initial costs, but pays for itself long term).
Here are three examples using PAR reading directly under the lights:
Obviously this is but a starting point as this article will clearly
show in PUR & more, as we have to consider what we are using our
lights for from planted freshwater to acropora reef lighting. But these
efficiency readings speak volumes about who PUR, PWM, and wasted energy
running fans affect PAR efficiency.
As per the LED themselves, different optics will also affect readings further out from the the center, which is why all are readings directly under the light.
However this certainly is an eye opening starting point as per the old term; "you get what you pay for" which when long term costs including short lives of popular "value" lights and 4-5 time operating costs in energy often these savings are evaporated after a few years.
PUR stands for Photosynthetically Useable Radiation. It is also sometimes simply known as "useful light energy".
Another description could be: "Quality of light per application" compared to PAR (Photosynthetically Active Radiation) being the "quantity of light energy" used by photosynthetic life.
I think many in the hobby get "hung up" on this term, as it is a "fuzzy term (which I would partly agree since each plant, coral, etc, can be unique), but there are many aspects of science such as we have moved through the discovery of subatomic particles that are based on subatomic behavior, but not as easily measurable such as PAR is.
As well, we also know based on aquarium lighting history that we simply cannot dismiss the evidence supporting PUR as a fact either. It is unfortunate that many in this hobby will dismiss this term which has been around long before many even were born. The FACTS are, it was and is a useful term in that we knew decades ago that there was a pronounced difference in many fluorescent lights when everything else was equal; from input watts, lumens, length, type, etc and the only difference was the spectral output (PUR).