PAR and Reef Aquarium Lighting: What is PAR?

Photosynthetically Active Radiation, or PAR as it's commonly called, is a term that's widely used in the hobby, and is often not well understood, especially when applied to LED lighting.  When used correctly, PAR information can be very helpful in determining the best way to light a reef aquarium, and where best to place corals, clams, and other photosynthetic invertebrates.When misunderstood, it can lead to unnecessary and costly lighting purchases and have detrimental affects on your reef aquarium!

So, what is PAR?

This can be a sticky subject, so let's keep it simple and practical for us reef aquarists!  For our uses, PAR is the number of light quanta (particles of light, called photons) that fall in a square meter over the course of one second, that are in between the wavelengths of ~400nm-600nm (nm stands for nanometer, or billionths of a meter).  Still confused?  Think of it as a measurement of the total amount of visible light that shines on a square meter when you start a stopwatch and hit stop afer one second.  That's PAR in a nutshell!

Why is PAR important?

PAR is important because it is roughly the measurement of how much usable light energy is available to your corals, anemones, clams - your reef aquarium in general!  It's also one of the easiest ways for you to measure the exact amount of light your reef aquarium is getting, which is useful because too little and too much light is bad for photosynthetic invertebrates and plants!