Foot Candle conversion - Orchid Board

dennis4246 · #1 (**permalink**) 01-05-2009, 02:50 PM

Hello:
I am in the process of buying a T5 48" 4 bulb grow light system for my orchids. If this system is rated at 20000 lumens how do i convent that to foot candles.

Thanks

Dennis in pensacola

Ross · #2 (**permalink**) 01-05-2009, 03:05 PM

Dennis, you can divide Lumens by 12.57 to get candle power. One foot candle is one candle power light measure at 12" from the light source (I think that is correct). So (if I am correct) you would divide 20,000 by 12.57 (which equals 1591.09 foot candles). This is at 12" from the source. You can feel safe rounding that to 1600 FCs. Best bet is to get a lux/foot-candle meter and measure yourself. I find the above a bit wimpy as mine measures closer to 1800 FCs at 12". It's also a function of the quality of the reflector system used in the fixture. I started with a system with lower output and migrated to one with higher output - based on light measurements. If you want details on brands, please PM me as I don't want to get into any brand bashing here on board.

cb977 · #3 (**permalink**) 01-05-2009, 03:06 PM

I'm moving this thread into the Growing Under Lights forum...I think it's a better place for it

Magnus A · #4 (**permalink**) 01-05-2009, 05:46 PM

Ross, I think you are doing things to easy..... But I can also have got everything wrong

1 lumen that falls on 1 sq meter gives 1 lux
and one lux is 1/10.764 footcandle

The T5 bank of 4 tubes gives 20000 lumens in all direction, (with reflectors you can direct most (90% ??) of the light in one direction but not all.)

Dennis, if you have a cylinder with 1 m circumference and the tubes in the centre, this area defined by the cylinder will get 20000 lux and 20000/10.764 footcandle.

/Magnus

Ross · #5 (**permalink**) 01-05-2009, 06:10 PM

agnus, you are totally correct! I just tried to do the North American conversion. Notice, I recommended a Lux/FC meter cause only us North American folks use the foot-candle term. I always try to use the lux term and let folks convert as they will.

Ross · #6 (**permalink**) 01-05-2009, 06:22 PM

Let me add some sources of info:

Indoor Plant Lighting
ACF Indoor Plant Grow Lights & Information Guide
Lumens, Footcandles, Candlepower, Measuring Light Output

Mostly, we need to be talking in terms outside North America. Thus Lux rather than Foot-candles. Metric, rather than US. Get my drift?

Magnus A · #7 (**permalink**) 01-05-2009, 06:40 PM

Ross,
I think everyone in the orchid buisness should be aware of both systems to measure light and distance in. Therefore it is educational to use both. I have some problems with F to C and the other way but maybe it will come .......

Older and new sources uses both so why not learn both?

Ross · #8 (**permalink**) 01-05-2009, 10:50 PM

Quote:

Originally Posted by Magnus A

Ross,
I think everyone in the orchid buisness should be aware of both systems to measure light and distance in. Therefore it is educational to use both. I have some problems with F to C and the other way but maybe it will come .......

Older and new sources uses both so why not learn both?

I totaly agree Magnus. That's why I typically give measurements in Lux, not Foot-candles. Lux is a better measure world-wide. One can simply divide Lux by 10 to get approx footcandles. Lumens is the best measure of all since it is typically the output of the lamp itself.