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.