Lighting, lumens equation

[cassidy_7]

Does anyone know if there is an equation to determine your lumens at a distance? For example, I know the lumens at the surface of my lamp (it says on the box), after 25 inches though, what has the level of lumens been reduced to?

 

[Anonymous]

Take a look at any optic book. The equation is quite difficult, if you want something that real engineers use. You will come to realize that the lumens number on the box is pretty unless, since it does not tell you the surface area of the "surface" of the lamp. The use of reflector complicates the issue of the "inverse square" relationship, and in a few hours, you will be very discouraged. Let us know if you have different experience.

 

[cassidy_7]

Ah, ok. I was just looking for something to tell me how close to the water I really needed to have my light so that enough would be getting through to the bottom of my tank. The tank is 20" tall (12 deep by 24" wide) and I have 150w HQI with 2x65w actinics, I currently have the light system 9inches off the water surface, maybe someone out there "knows" if that is ok for the tank.

 

[ChrisRD]

Without knowing what reflector/fixture you're using it will be hard for anyone to make a specific recommendation, but 9" off the water sounds like a reasonable starting point. If you're getting a lot of light spilling over the sides of the tank try lowering it a bit. If you're not getting even spread, trying raising it a bit.

 

[cassidy_7]

Not sure about the reflector, its just a Current USA OuterOrbiter 24"