how fast does PAR drop off when the light is raised higher?

[Ram_Puppy]

i am thinking about putting a metal halide over a small tank, and the 70 and 100 watt versions don't come in color temps i like or need for the project. (not to mention they are insanely expensive bulbs)

so I was thinking about just doing a 150 coralife hanging pendant, and having it higher off the water... like, 12 to 18"

[Richard]

There are two answers...

Inverse Square Law (http://en.wikipedia.org/wiki/Inverse-square_law)

or

Pretty Fast

[Ram_Puppy]

hmm.. maybe i will just go w/ a buble slightly cooler in temperature and get the smaller fixture..

thanks richard!

[HecticDialectics]

Dunno what color you're looking for exactly but myreefshop.com has astralux 70W 14k bulbs and illuming.com has 70W 14k and 20k ushio bulbs

[Ram_Puppy]

actually was looking the other direction - 6K i looked all over the internet and can not find DE HQI 70 Watt at a 6K color temp.

[HecticDialectics]

Can't attest for quality or light intensity or anything, but I found one for ya if you really wanted to go that route...

http://www.businesslights.com/venture-70w-6500k-double-ended-metal-halide-lamp-p-1189.html