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 reeef 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!
How do you measure PAR?
PAR can be measured with a quantum meter with a detachable, and for aquarium use, a submersible probe. Apogee Instruments MQ-200 PAR Meter makes an excellent and affordable meter.
What are acceptable PAR values?
This greatly depends on the species of coral or other photosynthetic animal, but the acceptable range is roughly 100-450 PAR, measured as PPF (which most PAR meters do). Obviously the low end is preferred by low light corals such as mushroom corals and elegance corals, whereas the high end would be acceptable for shallow water 'SPS' species. For those of you with nature or planted freshwater aquariums, the PAR requirements of plants tend to be lower: between 20-200 PAR, depending on the species.
Is PAR the only reef aquarium lighting consideration?
Actually, no. The wavelength of the light must be considered as well, especially in broad-spectrum bulbs (for instance, 'daylight' bulbs) and in LED lighting. PUR (photosynthetically usable radiation) is a measurement that's desirable to have with LED lighting in addition to PAR, and in fact, matters more due to the large amount of photosynthetically unuseable light produced by LEDs. The next installment of the Lighting Lab will discuss why PAR measurments doesn't tell the whole story about LED reef aquarium lighting! In the meantime, feel free to ask any question you may have in the comments section, and don't forget to sign up for our newsletter so that you don't miss next week's installment!