The international unit of luminous flux or quantity of light used as a measure of the total amount of
visible light emitted. The higher the lumens, the “brighter” or more “intense” the light looks to the
human eye. You can figure lumens per watt by dividing the lumens your lamp lists by the wattage the
Knowing your lumens per watt is often as or more important than watts per gallon. For example a
T12 light that is rated at 20 watts with a total lumen output of 800 lumens has a lumen per watt
output of 40. While a 13 watt T2 bulb rated at 950 lumens has a lumen output of 73 lumens per
watt. This is a clear example that the watts per gallon rule is severely flawed as the 13 watt T2 (or
two of these) is clearly the better choice for a 15 gallon planted aquarium (or reef) and this does not
even take into consideration the PAR rating which is also important for plants/corals or lumens per
length of bulb (space).
This lumen comparison also applies to SHO, VHO, and Metal Halide all of which far out produce most
T12 lamps in lumens per watt.
Watts equal one joule of energy per second. For us, it’s a measurement of how much energy our light
fixture is using NOT of light output! This why the 2-3 watts per gallon for FW plants (3-5) for reef can
be deceiving, and this rule is only a starting point similar to the 1 inch of fish per gallon “rule”. This
archaic rule was more accurate when all that was used were T12 lamps which is what this rules is
Keeping this in mind the average T12 has a lumens per watt rating of 40, which means you would
need half as many watts of a bulb that produces 80 lumens per watt (assuming PAR, Kelvin and other
aspects are equal)
The term “watts per gallon” is getting more archaic as newer T-2, T5, compact Fluorescents, and
especially the SHO and some of the new reef compatible LED lights have more watts spread over less
distance. Keeping this in mind; ‘watts’, when applied to a standard fluorescent tube are spread over
longer bulbs as the wattage increases. For instance a standard 30 watt T 8 bulb is 36″ while a
standard 20 watt T-8 bulb is 24″.
For high light requirements such as plants or reefs, at least 1 inch per watt is required when
comparing tube style fluorescents bulbs.
Many high output light such as the Metal Halide or the more economical SHO PC bulbs use a lot of
watts in a small amount of space. The 110 watt SHO bulb uses 110 watts in 10″ or even less if
mounted in a pendant.
Another aspect of watts is the output of lumens per watts actually used. The output of a 400 watt
incandescent bulb is about 25 watts of light, a 400 watt metal halide bulb emits about 140 watts of
light. If PAR is considered to correspond more or less to the visible region, then a 400 watt metal
halide lamp provides about 140 watts of PAR. A 400 watt HPS lamps has less PAR, typically 120 to 128
watts, but because the light is yellow it is rated at higher lumens (for the human eye).
(article by Carl Strohmeyer：Aquarium Lighting）
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