Measuring Florescent light output

[Stray59]

Does anyone know a rule of thumb for measuring light output? Basically, what wattage do I use for cattleya culture, and what should I use for phalaenopsis or paphiopedilum? I know that distance from the bulbs also matter, so how far should they be from the source. Any info or link to a good instruction page would be appreciated!
Steve

[WhiteRabbit]

good luck!

[naoki]

Steve, my phals and paphs grow well with around 800-1000fc for 15 hours. They are in a grow tent, so they don't get any ambient light. At one point, I made a following calculation (I posted it to one of the forum, but I can't remember): the cumulative amount of photosynthetically active radiation (PAR) from this setup is roughly equivalent to that of natural sun light with 1200-1500fc of peak intensity (this kind of "peak intensity" is the type of values used for "recommendation"). I grow only species, so hybrid phals may do ok with less light.

If you are using T8 (32W for 4'), then you want to put them within 2-5 inches. If your grow space is about 1'x4', you probably want to have 4 bulbs (2 stoplights). If you are using T5HO (52W for 4'), then mine is about 7-10 inches from the top of the leaves.

In the summer time, I may need to put them further to prevent heat related problems.

I don't have much experiences with growing Cattleyas under lights, but my 3 rupicolous Laelias are growing well (they are still too small to flower) about 4 inches from T5HO (I think it's getting around 1600-1800 fc). I don't know if they will flower if you are using T8, and T5HO may be better. If you can put them outside during the summer, T8 may be enough for the winter.

I posted some measurement of a T5 fixture, but is it for 2' unit and not 4' unit:
Big Leaf Orchid forum • View topic - 2' 1 or 2 bulb T5 Fc question

[blackvine]

Steve, I struggled with this a few months ago when I first started growing orchids. Focusing on Catts, which need 3,000 - 5,000 footcandles, I found a simple "step down" chart, showing the decrease in footcandles by foot. Here it is:

Trend as we move in one-foot increments from the light on the ceiling to the floor in a normal house with eight-foot ceilings:
1 Ft = 500 Lm
2 Ft = 125 Lm (25% of 500)
3 Ft = 55.5 Lm (11% of 500)
4 Ft = 31.25 Lm (6% of 500)
5 Ft = 20 Lm (4% of 500)
6 Ft = 13.8 Lm
7 Ft = 10.2 Lm
8Ft = 7.8 Lm
By moving just seven feet further from the light source, the intensity of the light has decreased from 500 Lumens to less than eight Lumens.

All you need to know is how many footcandles your light puts out. This should be printed on the box. It may be stated in "lumens", but for all intents and purposes an FC is equal to a lumen. (A footcandle is a unit of measurement describing the intensity of light one foot from a candle).

From that starting point - your footcandles of light - decrease by the percentages I've shown in the above table. So if your light has 20,000 FCs, your Catt will recieve 6% of that if your light source is 4 feet away. I have lots more info, but I just use this chart because the math is mind boggling! If you want it, though, let me know.

[Stray59]

WOW - I can always count on Orchid Board members to come through! Whiterabbit, Naoki and blackvine to the rescue! I can really use all this info. I am currently using a blend of T12 (yes I know, almost useless, but it is what I have now), T8 and will soon be adding some T5's in the near future. In the past, the orchids I have had 'survived' in the lights I had, but now I need to move it up and get some blooms. Currently I have been able to bloom pescatoria under the lights I have, but have many Cattleyas, laelias, brassovola, etc. that I am moving out of the window sills and onto growth shelves. I also am deflasking seedlings for future stock / resale and need to make sure I am providing enough light for them to grow well. I will eventually have 4 distinct growing environments, each adjusted to the varieties that I grow. This is kind of a 'late in life' career attempt and hope all goes well with it. Keep your fingers crossed for me please!
Thanks to all of you for your input. I was basically housebound for 18 months due to a herniated disc and have focused on my orchids a lot. Currently I have about 125, all grown in window sills, of a wide variety of tribes.
Thanks for letting me blab on, but I want you to know you are really appreciated and the info you are sharing is truly assisting me in my new venture.
Have a great day, all of you -
Steve

[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

[Ray]

Originally Posted by blackvine

So if your light has 20,000 FCs, your Catt will recieve 6% of that if your light source is 4 feet away. I have lots more info, but I just use this chart because the math is mind boggling! If you want it, though, let me know.

Actually, that figure would only hold true if the light output was 20,000 footcandles at 1 foot from the bulbs.

The drop off is an inverse square - twice the distance = 1/4th the intensity, 3x the distance, 1/9th, etc. That means that if the output is 20,000fc at 1" from the bulbs, it will be 5000 at 2", etc.

However, don't disregard the effect of reflectors, as they will redirect the "lost" light from the part of the bulb facing away from the plant, down toward it.

I did some measurements once, using a 2', 24W T5 fixture:

[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