Measuring Florescent light output

[Stray59]

blackvine:
I printed out the scale that you printed, then realized you never told me what lumens the light source is. Does the scale apply to one bulb or a multiple of bulbs under one hood? All of my plants can be within one foot of each hood, having 2 - 4 tubes over the shelving.
Thanks for the info and any additional input!
Steve

[blackvine]

Ray, I'm so glad you picked this thread up - you're one of the experts!

You and I are expressing the same thing, except I'm doing it in percentages, only because it's faster to do on the calculator!

True, the inverse squared rule applies . . . but you'll notice that your statement that twice the distance is 2X2 = 4 = 1/4 FCs is the same as saying 25% of the footcandles the light offers. Same with 3 times the distance: 3X3 = 9 = 1/9 FC is the same as 11%. And so on.

You're right about reflectors, though. I did not factor them in. Thanks for the chart - it will help!!

Steve, for your question: You would have to know what lumens/FC your light source is. It should be printed on the box. Then, just take that and multiply it by the percentages shown at every foot step down to get your FCs. So, if your light source is rated 10,000 lumens, the math would be 10,000 X .25 for 2 feet away. The answer is 2,500 lumens. At 3 feet away, the plant would get 3X3 = 1/9 the lumens: 11% which is 10000 X .11 = 1,100 lumens.

But, yes - the reflectors change this. The math would be incredibly complicated depending on quality of reflectors, size, etc., which is probably why Ray did the measurements himself instead of using a formula!

[blackvine]

P.S. Steve, wattage has nothing to do with light intensity. Don't even think about wattage or it will be confusing.

[naoki]

blackvine, lumen doesn't quite tell you the complete story because it is the total "amount" of light emitted from the source. More relevant values for plants are lux or foot-candle. These are more like "density", so you have to consider how much area is illuminated by the source. So if you have 2' light with 3000lumen gives more footcandle than 4' light with the same lumen. lux = lumen / m^2

[Stray59]

- I had no idea that I would start such a involved thread, but I really am learning a lot! I want to thank all of you for your discussion and input. I am sooooo glad that there are people who are good at and enjoy mathematical problems, because I am horrible with them. I am pretty good with orchids though! I will apply this info to my growing room.
Cant thank you enough!
Steve

[samarak]

Remember also that the inverse square law applies to point sources radiating equally in all directions. Fluorescent tubes are not an infinite line source (which would fall off linearly) but from the standpoint of a plant a few inches away they're not point sources either, even before you add a reflector. When you add several tubes side by side (approximating a planar source to a plant in the center), it's even more complex at those short distances. I still do theoretical calculations from time to time, but my recommendation if you really want to know what intensity you have in your setup is to buy or borrow a meter and measure it.

Re lumens, I find it a valuable number to have simply because it's one of the few that's almost always available. But as it's a measure of luminous flux (i.e., adjusted for the way human eyes perceive light, rather than how plants do, or of total radiant energy) it's only one of the things I consider.

Steve

[Ray]

Blackvine - wattage has everything to do with intensity.

T5 and T8 HO bulbs put out about 90 lumens per watt. A 4' T5 bulb is 54 watts, so puts out about 4860 initial lumens. Two 2' T5s, together, total 48 watts, so put out 540 fewer lumens than does the single 4' bulb.

However, I agree that Steve's initial question about wattage misses the point. A single 2', 24W T5 will allow you to bloom a high-light plant like a vanda - if you put it right up against the plant, but that's all you'll be able to illuminate. If you want to cover a large area, you'll need more light output so the "density" (good term, Naoki) is sufficient over that entire area, so you'll want more total wattage of bulbs.

[Stray59]

I am going to bow out of the conversation for a while and let you guys debate such things as wattage, density, etc. But I will be monitoring the conversation and will (hopefully) be able integrate your knowledge. Please continue to converse as I am learning more and more!!!

[blackvine]

Naoki - thanks! As with everything in life, I continue to see that the more I learn, the more I realize what I do not know!

[blackvine]

Ray, ditto what I just posted to Naoki. But, it seems we did not answer Steve's question about light. Most newbie orchid hobbyists (like me) don't have a deep interest in the science of light - they just want to insure they are growing their orchids in adequate - if not ideal - light. One of these days, I'll buy a light meter, but until then, I'll use my chart just because it seems to be working with my orchids. Cheers, all!