Why Is Light Measured By The Gallon?

[chrisf]

Hi,

I am just wondering why light input into a tank is measured as watts per gallon?

As light hits the surface and travels down shouldnt light input be measured as function of total light divided by the surface area of the tank.

I undersatnd that the effect of light is reduced as it travels down deeper depths but most off the shelf tanks are not that different in height, surely light can travel down 12 inches just as well as 24.

Just wondered if anyone can educate me on this one..

chris

 

[BigIan]

its a VERY rough guide,

there are other more exact formulas out there that do take surface area, volume and depth into account, but as a rough guide i find the wpg rule plus a little comon sence works fine....
For instance my substrate isalmost 20 inches from my tubes so i know i need more light intensity than some one with a 12" deep tank