What are Wattage, Lumens, Lux and PAR?
How much Light Do You Need and Which Light Should You Purchase?
Choosing an aquarium light is a journey down a path littered with confusing terminology. In this article we'll explain this terminology, but we'll also explain why you might be better off just using the MarineAndReef.com Reef Lighting Chart and the MarineAndReef.com Freshwater Planted Tank and Fish-Only Tank Lighting Chart to determine which light you need.
These Charts make the choice simpler because all you will need to know is what types of organisms you have in your tank and the dimensions of your tank. These Charts are based on years of experience and research and are a much easier way to choose a light that trying to figure how many watts per gallon or lumens or lux you will need.
The following explanation of the terms watts, lumens, lux and PAR will demonstrate why these measures of light are troublesome.
Watts – Wattage is a measure of how much power a light fixture uses. It is NOT a measurement of light. It is only a measurement of power consumption.
In years past wattage was used as a general indicator of how bright a light should be in order to support different kinds of corals. The old rule was a reef tank needed a minimum of around 3 watts of light per gallon of aquarium water to support hardy beginner corals.
Because most people are using LED lights and LEDs are far more efficient than traditional florescent and metal halide lighting in terms of the amount of power it takes to produce a similar level of light, the old wattage rule no longer applies.
With LEDs, as little as 1 watt per gallon is sufficient to support low light demand corals. However, even that measurement is troublesome because there is a wide gap in efficiency between the most and least efficient LED lights.
Lumens – The total lumens produced by a light is a measurement of the total amount of light photons produced by that light source. A fixture’s lumen rating does NOT measure the amount of light at a given space at a given time.
While there may be a lot of lumens produced by a fixture the actual amount of light photons that are projected into the aquarium is different at each individual point in space and is not described by the lumen value attached to a specific light fixture. Some lights project light better than others. For example, Kessil lights are very good at projecting light deep into an aquarium.
Additionally, when lumens are measured all colors of light wavelengths are counted equally. This is an issue because corals do not use all light wavelengths equally. Some wavelengths they do not use at all. The result is that a light that produces more lumens may actually produce less useable light for corals than another light that produces far fewer lumens.
Lux – Lux is a measurement of light that records the amount of lumens at a particular location at a particular time. This gives a more useful measurement of a light’s performance because it lets you know how much light is actually getting to your particular corals.
However, as with lumens, lux does not take the color of the particular light wavelengths into account. If the color temperatures of the light are not useful for a coral, lux will not be a good measure of useable light. Many reef lights have insufficient near UV lighting for corals.
PAR – PAR stands for photosyntheticly active radiation. PAR is a measurement of the amount of photons hitting a particular location at a particular time within the range of the light spectrum that is used for photosynthesis. PAR is a much more practical measurement of the amount of light that your corals are actually receiving in comparison to any of the above terms.
With many high end lights the manufacturer will publish a complex PAR chart showing the amount of PAR at various distances from the light.
Yet, PAR has its pitfalls. For one, most PAR meters are skewed toward measuring light for photosynthesis. This is great for freshwater planted tanks. But your typical PAR meter does not measure for the light used by corals. Deepwater coral live in areas where much of the red and yellow light spectrum has been filtered out by water. In this case the coral may have a strong preference for the bluer end of the light spectrum.
As we stated at the beginning, there are no easy answer when it comes to choosing an aquarium light. Your best bet is to refer to the MarineAndReef.com Reef Lighting Chart and the MarineAndReef.com Freshwater Planted Tank and Fish-Only Tank Lighting Chart. We’ve incorporated many different factors into these charts—from the data provided by the manufacturers, to personal experience with our own tanks and feedback from hundreds of stores, service professionals, researchers and customers.
Getting the Most out of Your Light
Aquarists spend a lot of money and and expend great effort choosing the perfect light for their aquarium. On larger aquariums the lights are usually the most expensive piece of equipment. It only makes sense that you do what you can to make sure that your high-tech lights are functioning as they should so your corals or plants can remain healthy, and all the money and effort you put in to your lights does not go to waste. Here are a few ways to keep your light functioning at its best.
Replace Your Light Bulbs - Fluorescent and metal halide bulbs should be replaced at least once a year to ensure they're producing the correct spectrum and quantity of light. Old bulbs may still light up but they can decline in output by 30% or more and shift color, resulting in more algae and less light for your corals.
Clean Your Light Fixture - When the lenses on your light fixture get covered in dust or salt creep it can significantly reduce the amount of light getting into the aquarium. Rinse and scrub the lenses to ensure that they're clean. Use white vinegar to clean hard water stains if needed. If your tank has a glass top, it's basically another light lens. So clean it as well. Make sure your light fixtures are clean of dust so the fans cooling the light run properly. Use a moist rag or compressed air to clean dust from around and inside your lights. Clean as often as you can and make it part of your regular maintenance. It's a good idea to do it monthly with your water changes or set a reminder every 3 months. You can’t clean too often.
Keep Your Water Clear - When your water is dirty or tinted it can significantly cut down on the amount of light reaching your plants or corals. Use a protein skimmer, ozone, carbon, or other chemical media to ensure that the light is passing through the water freely. You will get the added benefit of having a cleaner, better looking aquarium. The simplest way to improve water quality is with a good high quality carbon like the Red Sea Reef Spec Carbon. Another option is to use products like Blue Life Clear FX Pro, or Chemi-Pure. If your aquarium doesn’t have a surface skimming filter, add a surface skimmer like the Ista Multi-Surface Skimmer.
Place Your Light at the Proper Distance from the Aquarium - If your light is one that needs to be suspended, make sure the light is not so far away that you get light spillage. Light spillage occurs when light fills the room instead of the aquarium. Follow the manufacturers recommendations for light height. If no recommendations are given, then visually make sure that the light is getting in the aquarium and not spilling over the edges. Keep in mind that a light too close to the water is going to get splashed and damaged.
Put Your Lights on a Timer - Manually turning your light on and off every day at the correct time is very difficult. Having a timer ensures that the light goes on and off when it is supposed to. For a simple solution use the Hamilton Technology Dual Digital Aquarium Timer. For advanced lights look into using a Kessil Spectral Controller or a Neptune Apex. Consistency is the theme of all great tanks, and a light timer is a simple and easy way to add more consistency to your aquarium.
Experiment with Color and Intensity - Whether your light is controllable or not, experiment with the color and intensity of your light. With LEDs, like Kessils, the spectrum can be easily adjusted. Try different settings and see what your corals or plants like the best. Unless the results are obviously negative, give it at least a couple weeks before passing judgment. If you have a florescent or metal halide fixture, then try different bulbs to see if you can get better color and growth. In general, bluer actinic or 20,000k bulbs will give better color, but whiter 6,700k or 10,000k bulbs will give better growth.