Yes, you multiply the initial lumens per lamp (bulb) by the number of lamps (bulbs) to get the total initial lumens. So if you have four lamps rated at 5000 lumens each you'll have 20,000 initial lumens.

Various factors will influence the amount of light actually getting to your plants. Distance is a biggie, so it's best to have the light as close as you can to the plants without burning the plants. For fluorescents they can practically touch the plants without harming them.

Another important, easily controlled factor is reflectance. I drape my fluorescent luminaires with reflective mylar (similar to aluminum foil) to increase the amount of light getting to the plants.

Well, yes and no. It all depends on your reflectors. Remember that the bulb emits light in all directions, not just where you want it. The better measure to use is foot-candles, which is the total lumen output divided by the square footage of the area lit. I usually take use the following formula, since I don't have a light meter.
Total Lumens / Growing area * 50%

The 50% is for all the light that didn't end up on the growing area, whether absorbed by the reflectors, or simply headed the wrong direction. That factor depends on your reflectors and the total size of your area. I picked 50%, but I suspect that it is wildly optimistic, given my setup. I'll be buying a light meter shortly, and I'll find out for sure.

aes123,

If you really want to be rigorous about it, the zonal cavity equation for the illuminance (foot-candles or lux, whether you use square feet or square metres) is:

[(lumens per lamp)*(lamps per luminaire)*(# luminaires)*(coefficient of utilization)*(light loss factors)] all divided by growing area

The coefficient of utilization takes into acount your luminaire properties, growing area geometry, and surface reflectances.

The light loss factors take into account ballast factor, dirt depreciation, and lamp lumen depreciation.

If you've got a half-way decent fluorescent setup (meaning lights close to the plants and reflective surrounding and you dust the lights once in a while, then your figure of 50% (effectively the coefficient of utilization x light loss factors) doesn't sound wildly optimistic at all. Actually probably understated, since the distances are so much closer in grow lighting than human lighting, and the reflectance of mylar is so much higher than the reflectance on standard painted walls.

If nothing else, the equation helps figure out where to concentrate our efforts in order to increase the amount of light reaching the plants.

Thanks for all the information, ralleia and aes123.

The fixture I'm installing has what sounds like a good (or impressive, even) German-made aluminum reflector, so I'm hopeful.

ralleia I do have a light meter and the foot candles do not increase when measuring the six bulbs across the width of the grow area. It remains a steady 600 fc's. None of the available light is escaping up because the bulbs almost touch.

Brooke

600fc is pretty low for starting most plants. Its startlingly low for up close to any fluorescent tube. Do you have an incident light meter or a reflected light meter? A reflected light meter, such as the one in a camera, requires a suitable 18% grey surface if you want a reading in foot-candles, although you can get a decent estimate by using a white card. Measuring against a darker surface such as a plant or soil will give you a very low reading even though the light intensity is still the same. If you have an incident meter then it should be placed almost touching the tubes and pointed in the right direction.

If one tube produces 5,000 lumens than two bulbs produce 10,000 lumens. Lumens are easy :)

Foot-candles are how many lumens you have landing on a square foot (why aren't they called foot-lumens?) and they vary depending on where you measure them. In an ideal world, you would get all your 10,000 lumens evenly spread onto your growing area and know exactly what the foot-candles were, but you always lose some along the way and the light is never spread evenly. I get about 80% of the theoretical maximum in a completely enclosed white growing area. I would consider 50% to be quite poor.

ralleia, Thanks for the details on that. I figured my little formula had a more formal counterpart; I didn't realize how close I was.

I'm working to increase the light on one of my shelves so that I can grow some orchids that require 2000-2500fc. I'm wondering how close I can get using overdriven Sunpark based shoplights. Does this make sense?

[(lumens per lamp)*(lamps per luminaire)*(# luminaires)*(coefficient of utilization)*(light loss factors)]

lumens per lamp = 2800
lamps per luminaire = 2
luminaires = 5

Easy so far. This is where I start guessing:
(overdriven fixtures+mylar on three sides)
combine light loss and utilization = 70%

shelf dimensions = 2'x4'

yields 2800*2*5*80%/8 = 2450fc

That seems awfully high to me, based on what I've read online. Has anyone been able to get that much light out of overdriven fixtures?

Ten (four foot?) fluorescent tubes in a 2'x4' area? If you really could squeeze them into that space, then 2450fc seems reasonable.

I don't grow orchids but I suspect your light requirements may be set too high. Is the 2,000fc-2,500fc perhaps a requirement for the maximum midday light intensity? That would be typical dappled shade, or perhaps an east exposure. Artificial lights are usually operated at lower levels because the full intensity is maintained for twelve hours or more. 2,500fc for 16 hours a day is the sort of level you might use on a cactus!

The light meter we have is a "regular" light meter, not the one on our digital SLR Nikon although that light meter was very close to the same readings.

I know zero about overdrive fluorescents, only the typical, buy at a big box store for $2 a bulb light bulb that runs 3000 lumens, when first lit. They decrease over a matter of time.

I do grow orchids including the high light orchids requiring fc of 2,000-2500. These I grow under HPS lights on a moving track. I grow the lower light orchids under the regular fluorescents and keep them actually touching the lights, putting the higher light requirement plants in the middle and the lower light growers on the end. All of these do grow and bloom on schedule but previously, they have been housed outside in the summer months for maximum growth to endure the winter months of less light. Soon, the orchids will be housed in a greenhouse and I can worry about them but in a different way.

You are correct shrubs, most orchids with a required 2000-2500 fc's, take sun until noon, then shade. Most orchids that do prosper under regular fluorescents are the dappled or shade loving plants.

Brooke

I guess I've been getting conflicting/confusing information. Most sources I've read seem to indicate that the light requirements for orchids are for a 12 hour period.

Currently, I only have two of the five planned fixtures in place. I was just planning for growth. According to my measurements, I should be able to fit three more, but I'm no carpenter... If I can only fit four, that may be for the best, based on what you're saying about light requirements.

For those with experience with orchids, I'll be growing Cattleya alliance hybrids(mostly minis), Miltassia, and if it lives through its current root rotted distress, a Howeara (Oncidium hybrid). Is 2000fc for 12 hours overkill? I was under the impression that I might have to keep the lights on even longer to meet thier needs.

Wow, that's a lot of light! I did read a bit about orchids, since I thought they were the sort of thing you could grow indoors without too much trouble. Seems some of them like almost full sun, at least in England.

Some of these orchids are pretty tall though. That can be hard to accomodate if you want full light levels at the bottom of the plant as well as at the top. Fully enclosed side reflectors help, but a large plant necessarily absorbs a good chunk of the light and you might find you are only getting about half the light intensity at the bottom leaves. A whole row of plants under the fluorescents would shade the bottom leaves even more than that. The simplest solution is to get more light and place it further away. Getting slightly more creative, you can arrange some of your tubes at the sides of the plants instead of just above them, for that even all over tan :) Your calculations of light intensity should then take into account the total lit surface area including the sides, not just the top surface area of the plants, so you'd probably need extra tubes.

Luckily, there are tons of different kinds of orchids, so restricting myself to the shorter ones isn't too much of a limitation. Since the blooms last longer with lower light, I'll just move blooming plants to the edge of the shelf, which will also help for plants with taller flower spikes.

I'm tossing around the idea of moving some of the higher light plants out to my balcony for the summer. They can have 'real' dappled sunlight at the edges of my tomatoes and peppers.

A word of caution... If you have an addictive personality, stay AWAY from orchids. They're dangerous. :)

I'm going to be lighting a small area for some imdoor bonsai--very small: 48" x 15." Only 5 square feet. 20,000 lumens: 5,000 each in 4 luminaires. Allowing for 20% lumen loss will leave me with 3200 fc or 7424 lux, unless I'm mistaken. Which is very possible.

I grow a few mini catts myself... I've got them growing under 2 overdriven T8 tubes (6" from top of plant)in my basement and they bloom just fine. I've also had success growing them under regular 40W T12 tubes, but had trouble getting them to bloom, but they stay nice and healthy until I could bring them outside in the summer. That's not to say that they don't bloom under the T12 lights, they do, just not as often!

Oncidiums and Miltassias I haven't really tried. I did it the one time to the one Onicidium (Sherry Baby) and it was fine until it decided to put out a spike... Since it like to be really close to the light and the flower spike grew to about 30", I caved in and got myself an HID light.

I don't have a light meter, so I've no idea how much light they're getting. Also I run my lights in 16-hour light, 8-hour dark cycles.

You're right, orchids are addictive! I even got some seeds planted and germinating in some aggar right now. Now I just gotta save up to get myself a bay or bow window to make some extra space... =)

ccc1 -- Bay or bow? Why stop there if you're a garden nut?

My new triple-pane krypton-filled Schuco garden window is waiting to be installed, and when that space runs out (and we save up some money) we're going to knock down the south-facing wall of the living room and make that a sun room.

My gardening addiction is part of the reason I'm still driving my 14-year old car. Wouldn't it be great if my 15-year mortgage dies before my car does?

A greenhouse or sunroom would be a dream... unfortunately, budget is limited, and until I get rid of that mortgage (only 2 years into it, so long way off), I can't even think about such things! Just that since none of my windows actually have a ledge, changing a few windows is the most proctical way to increase growing space. At least for the time being. =)