# Re: Watts/lumens (again?)

```>When we talk about two watts per gallon, are we talking standard
florescents?

Yes, we are talking about standard fluorescents AND standard sized
tanks!  For example, a "tall" tank would have less surface area per
gallon than a "standard" tank, while a wide, short "breeder" tank would
have more surface area per gallon.  Each tank style would need different
amounts of watts per gallon to achieve the same amount of light/surface
area.

I have a reference that may be helpful here:
Moe, Martin.  1992.  The Marine Aquarium Reference:  Systems and
Invertebrates.  Green Turtle Publications, Plantation FL.

He has an equation on page 159 that allows you to calculate how many
watts of light you need for a given amount of lux you desire (remember,
lux is lumens/square meter).  This equation takes into account the
efficacy (i.e., lumens/watt) of the bulbs you want to use, whether they
are NO, VHO, or metal halide, and the surface area of your tank.

The equation is:

watts=[(lux)x(area or tank in square meters)] / [(bulb
efficacy)x(utilization factor)]

You supply the amount of lux you want, the area of your tank in square
meters, the bulb efficacy in lumens/watt, and the utilization factor,
which is the amount of light that actually strikes the water.  Moe sets
this to a constant, 0.5, although I suspect a good reflector can improve
this.  Anyone care to comment?

For example, if you wish to have 10,000 lux over a 75 gallon aquarium
(48 x 18 x 18 inch, or 0.56 square meters), and you intend to use 48"
T-12 NO fluorescents at 79 lumens/watt, then

watts = [(10,000)x(0.56)] / (79)x(0.5) = 141.7 watts.

Of course, since 48" T-12 only come in increments of 40 watts, you would
have to round up to 160 watts, or 4 tubes.  This would put you a little
over your target of 10,000 lux, but pretty close.  It's a simple matter
to rearrange this equation to give you lux for a given wattage and bulb
efficacy.  Note that this is the amount of light at the water's surface,
not the bottom of the tank.  Also note that this barely meets the 2
watts/gallon rule, but nevertheless is a fairly brightly illuminated
tank.

Why are all the best references targeted toward marine aquariums?

Regards,

Mark

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