Light hanging over tank - how much wattage makes it into the tank?

**feemia** · 03-07-2012, 1:22 PM

I just installed a light fixture, which hold 2 4 foot long 34 watt t8 bulbs. It hangs 5" above the top of my 3 foot long 30 gallon tank. I intentionally installed it to hang above and over the ends of the tank so that the houseplants on either side of the tank would benefit from the light, as well as the plants in the tank.

Is there a rough formula to estimate how much light actually makes it into the tank? If the fixture was resting directly on top of the tank, it would be over 2 watts per gallon; However the way I have it set up there is a lot of light coming out into the room, rather than into the tank.

**Bushkill** · 03-07-2012, 2:59 PM

I don't think it's something that can be calculated. There are special meters called PAR meters that will measure intensity though. Also consider that all flourescants lose intensity from the moment you flip the switch.

**THE V** · 03-07-2012, 4:30 PM

Actually it can be calculated using the Inverse-square law.

http://en.wikipedia.org/wiki/Inverse-square_law

Or a less trustworthy site

http://imagine.gsfc.nasa.gov/YBA/M31...erR2-more.html

Basically the farther away from the light source the intensity of the light decreases by the square of the distance changed. 5 inches above the tank is about 1/3 of the distance from the top of the tank to the substrate. So the light intensity at the substrate will be around 2-3 times less than if the light was right on top of the tank.

So the bottom of the tank is getting very low light levels right now. You'll need in increase your lighting from around 60W to 120W-180W to get up to a "2 WPG" equivalent.