translating watts to lumens/foot candles

korina(9b, Sunset 17)October 24, 2007

Hi. I have four 24" two-tube fluorescent fixtures on my tiny light stands. The tubes are 18 watts; how many foot candles does this translate to, or is there somewhere I can go?

I'm growing African Violets and their cousins, and some small orchids, and it would be useful to know how high up to hang the lights.

Thanks so much for any help you can give.


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shrubs_n_bulbs(z8/9 UK)

It doesn't translate, they are two different things.

Your watts translates, more or less, to lumens. This is the total light that your fluorescents produce. In your case, you'll get around 1200 lumens per tube, maybe a little less depending on the exact specs and age.

Foot-candles are a measure of light intensity. It is how many lumens are falling on each square foot. If you managed to arrange for all the light from your eight tubes (9600 lumens) to fall on four square feet (I can't imagine you get it onto a smaller area than that), then you would have 2400 foot-candles (9600 divided by four).

2400 foot candles is way too much for AVs, you can probably light two to four times that area even allowing for loss of light. Most systems like this lose a lot of light that never gets to the plants, because they don't have good reflectors. Only you can judge how much light is falling somewhere other than the plants and how much area your lights are covering. Raising the tubes will mean the light (probably) falls on a larger area but there is no formula that can help you with this. Start high with AVs and lower the lights slowly until you get satisfactory growth and flowering without any leaf scorch.

    Bookmark   October 25, 2007 at 7:17AM
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korina(9b, Sunset 17)

Thanks, Shrubs.


    Bookmark   October 25, 2007 at 7:13PM
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korina(9b, Sunset 17)

Okay, now that I don't have to run catch my ride...

I wasn't sure how much light my 18 W tubes were putting out. Most growers recommend hanging your standard 4' tubes 10-12" from the tops of the plants, and I figured my 24" tubes were putting out that much less light, I just wasn't sure how much less.

I have two stands, both wire shelving units from Target; one is 24" wide (full of misc. stuff), and the other is 36". The top shelf of the larger stand has one fixture, and the bottom shelf has two overlapping, since no one seems to make 36" light fixtures. The lights on both shelves are about 10" from the tops of the plants. My AVs and other Gesneriads are on the top, with my orchids (two Onc. Twinkles, a Dend. kingmani, a mystery Dend., a Paph., and a Phal.) and some mini Begonias on the edges. It's a little crazy right now, especially with my little Paph suddenly starting to turn into a big one.

Anywho, I just want to make everyone happy, and I was hoping you nice folks could give me some tips on dealing with my odd-size lights.

Thanks again for your time and patience.


    Bookmark   October 28, 2007 at 8:41PM
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watergal(z6/7 Westminster, MD)

You'll be able to tell after a while. The AV leaf rosettes will either stretch up for more light, or curl down if there is too much. I used to shuffle mine around frequently, so that the ones wanting less light were at the ends of the shelf, and the ones wanting more were in the middle.

With the orchids and AV's, ultimately you'll be able to judge how happy they are by how well they flower for you. Don't be afraid to experiment! Good luck.

    Bookmark   October 29, 2007 at 7:49AM
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korina(9b, Sunset 17)

Thanks Watergal; I'll keep my rubber mallet handy, just in case.


    Bookmark   November 2, 2007 at 12:36PM
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