lumen help...

wishIwereAnExpert · #1 (**permalink**) 08-18-2006, 07:48 AM

Well, I better get the answer I want to this question 'cause I already bout the fixture (d'oh!)...

As most of you know, I'm about to embark on a 120 gallon vert, 2 X 2 X 4 ft tall.

I ordered this 250 watt metal halide, 20000 lumens at the bulb. Now, obviously that's 5000 foot candles at the bulb (over that 4 sq ft area). Now, the part I'm curious about is how fast the drop off is in foot candles. If it's a 6" gap to the top of the tank (suspended light pendant) and then 4' to the bottom, can someone give me estimates of what kind of lighting I'll have at the top, middle, bottom of the tank?

I'm *hoping* that it'll be a gradient of something like 3500-1000 ft-c, but I'm not sure. Seems intuitional that the light would drop off proportionally to the square of the distance, but I need some coeffecients for that to give me any practical information.

-Solly

defaced · #2 (**permalink**) 08-18-2006, 12:24 PM

It's an inverse relationship. of the form c/x, where c is some constant that is dependant on the initial lumen level of the bulb. I don't have the file handy but I used excel to regress some data someone took and came up with a really easy way to determine the constant.

Hope that was of some help

wishIwereAnExpert · #3 (**permalink**) 08-18-2006, 07:14 PM

I was saying I thought it would be inversely proportional to the square of the distance, because the light is spreading out in two dimensions as it moved away from the bulb. If you could pull out that file, I'd appreciate it...

-Solly

defaced · #4 (**permalink**) 08-21-2006, 02:15 AM

I'll have to do a deeper search, I didn't turn up the file just now.

I was suprised it was an inverse relationship too - maybe the reflector is playing a role we're not accounting for - as I was expecting an inverse square realtionship too. Excel gave an R^2 value of .98 or something obscenely accurate so it's a good statistical fit.

defaced · #5 (**permalink**) 08-21-2006, 04:33 PM

Here's the orignal thread that spawned the PM conversation that gave me another data set. I'll look at it tonight and see if I can recreate that Excel file for you.

http://www.dendroboard.com/phpBB2/viewtopic.php?t=13590

And since you're going to read over the thread, I goofed when I said it was a log trend in my first post. I hadn't crunched the numbers when I said that. 8)