That 75% rule would imply a linear decrease which is not correct. A point source of light loses intensity according to the inverse square law, which means it decreases exponentially, not linearly. You do in fact lose 75% when going from 1 foot to 2 feet, but then you lose only 55% going from 2 feet to 3 feet, and 44% going from 3 feet to 4 feet. This is the nature of an exponential relationship. You lose intensity very quickly when first moving away from the bulb, then more and more slowly the further you get from it.

Fortunately, this only applies to a point source of light (e.g. a bare metal halide bulb). Putting a reflector around your bulb will not only increase light, but will compensate for light loss due to distance by focusing the reflected light rather than scattering it. The more accurately a reflector focuses light downwards (as opposed to outwards), the less it will drop off with increasing distance. In fact, a perfect parabolic reflector could theoretically reflect a circular pattern of light infinitely with no loss. Of course this is impossible in the real world, but it does show the importance of a good reflector.

As for specific plant/wattage combinations, there are too many variables (plant, bulb, reflector, etc) to make hard rules. Some people might give you some good starting points, but you'll have to test and adjust as necessary. Some good points to keep in mind when battling light drop-off are using good reflectors as I mentioned above, and the idea of using multiple smaller light sources instead of one large one (e.g. two 400w vs one 1000w). When positioned next to each other they will help eliminate shadows and make better use of scattered light.

For plants that require full sun, the growing zone is about 3 to 7 feet for a 1,000 watt bulb with reflector. Closer is too hot; farther is too dim. Put another way, you can illuminate a 5 foot square area to a height of about 2 feet. Here's a chart of growing area by bulb wattage, and an introduction.

If intensity is 100 at 1 foot, it's 25 at 2 feet, 11 at 3 feet, 6 at 4 feet ... going down by a factor of 4 every time you double your distance from the bulb.

I just wanted to thank you both for such great information!

That article and the resources you provided, both from your knowledge and articles, really opened my eyes in a huge way.

So it is not just the kind of light we use that matters, but the distance we keep them can significantly make a huge impact on growth too..

Much appreciated!

Mike

I have a few general questions about this topic as well. I (like meyermike) am growing containerized citrus trees that I just brought in for the winter and am at a bit of a loss over what to do about lighting. I have the trees (about 7 or so all ranging from 1-3 feet tall) in front of an east facing sliding glass door (about a 3 foot by 3 foot area). They get an hour or so of direct sunlight in the early morning but it soon gets blocked out by my balconys overhang and I get indirect light for the rest of the day. I would like to provide the trees with supplemental lighting without costing a fortune. The trees are arranged in roughly three rows and tiers of height (3 that are about a foot tall closest to the window, a few that are 2 feet in the middle and then 3 bigger guys in the back row that are all about 3 feet tall) and I am concerned about light penetration. So far I have found three or so options and would really appreciate any input!

My first option is just to get two or three 2 foot florescent fixtures and try to keep them as close to the tops of the plants as possible. It would be kind of a pain to hang them all and look rather unattractive (gotta make the lady happy also).

The next option I was considering getting a few 65 watt CFLs that I found on the Lowes website for $10 put inside a few basic clap/dome shop light fixtrues.
http://www.lowes.com/lowes/lkn?action=productDetail&productId=57889-57299-9166B&lpage=none
I figure I could get 3 lights and 3 shop fixtures at loews for about $65. I would still only be looking about a foot worth of light penetration though wouldn't I?

The other option I was considering was just getting a 150 watt HPS compact fixture system for about $80.
http://www.shop.cheapgrowlights.com/product.sc?productId=274&categoryId=26
Also, would a 150 watt HPS light be powerful enough to get small veggies to flower? I've also got a jalepeno plant by the citrus trees and would like to add a sugar snap pea plant if at all possible though I know that is asking alot from 150 watts...

I know that the 3 CFLs would be a bit cheaper, more wattage and could be positioned in a few different places over the plants but would the 150W HPS setup give better overal light penetration? Any other ideas? What would you do if you were me?

Thanks folks!
Joe

Wow Joe...Do we love our trees or what ha...?

Not only would I post this question here, but I would also post the same thread under the forums...." Growing under lights"...I always seem to get immediat help there with very knowledgable people. They are so kind, and very helpful..

Try it..ok..I am have learned alot there, but not enough to help on this one, since I don't use these types of lights..:-). I use metal and sodium halide, and a 600 flourescent compact growing one I spent a whopping 300 dollars for. But boy do they work. And by the way, even at these strengths, they only work well if my plants are no further away from them by more than 2-3 feet..

Goodluck friend!

Here is a link that might be useful: Growing under lights

Brancto..Don't laugh..

I think it's time for me to hit the sack..loololol

Would you believe that I thought I was reading your post about this on the citrus forums, since I am so use to chatting with you over there, so in a haste to help, I directed you by a link, to this thread!!!!!
This thread where you posted this question...DIR!!

God...lololololol

Goodnight!

brancato,

Check out e-conolight. I bought a 150 watt HPS fixture and bulb for about $45.

Mike

Mike, no worries! I always appreciate your helpfulness! Also, Mike (wordwiz) thanks for the website!

Joe