# Re: Light intensity

```Shouldn't we be talking about the amount of light that strikes the
bottom of tank per
area per day?  That would lead me to a formula like this:

light factor = (wattage/depth^2) / (area of bottom) * (hours of light
per day) * 100

This is barring other factors like efficency of the reflector, spectra
of the bulbs,
etc. (which matter quite a bit)  It also leads me to ask the
question:  Can't I just
adjust the duration of lighting to get what I need, no matter what my
actual
wattage is?

This formula might be total B.S., so lets do some checks:

A 10g tank: bottom area of ~210 in^2. Depth to bottom is ~10in.  I've
got 26 watts
over that tank, running 12 hours a day.
That gives me a light factor of 1.4857

Two 10g tanks under one 80W shoplight: Light factor of 2.28

A 60 gallon 24" cube tank under 250W halide: Light factor of 0.904

The same tank with 175W instead of 250 has a light factor of 0.6329

A "standard" 50g tank has a bottom area of 36*15 in^2 and a depth of
21".  With 100W
lighting @ 12h, we get a light factor of 0.50.  At 175W, it rises to
0.88.

According to my new "light factor" formula, the smaller tanks have
extremely high
lighting, mostly because of their shallow depth. I'd believe that.  Is
250W too much
light for a 60g tank thats 24" deep?  Hard to tell.  Its not as high a
factor as the
smaller tanks, but still nearly twice what the 50g tank has at 2
watts/gallon.   (the
reason I ask is that I'm currently setting up a tank like that, and
after hearing the
discussions here, I think I have too much light)

I'm not sure that aquarium lighting falls off with the square of the
distance,
because there is quite a bit of internal reflection of the lights off
the sides of
the aquarium.  It may be something like depth^1.5, but I really don't
know.

Thoughts?

Steve

```