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Old 11-15-2005, 04:29 AM   #1 (permalink)
 
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Does anyone know of a formula for figuring out the variation in intensity of light based upon distance? Here is why I am asking...I have a diy hood that has the lights (2x55w) 4 inches from the surface of the water. Based upon the green leaves on my red ludwigia - I think I need more light. So, how much more intensity would I get by lowering the light by 2 or 3 inches? Is there a way to compute that 110w at 1 inch away from the water surface is the same as X wattage at 4 inches from the water surface? Thanks for helping.
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Old 11-15-2005, 06:09 AM   #2 (permalink)
 
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Okay - I should have checked the forum for previous posts. Relying upon ekim's post of 3/7/2003 - I created an excel spreadsheet to calculate how much light you need. Just input the length, width and height of tank (the height measurement should be the distance between the lights and the top of the substrate) and the photo period and voila...thanks to ekim's formula - you should see wattage you require depending upon the type of plant (light req. - (low, med, high)) you want to grow.
Uh Oh - no matter how I tried, I couldn't attach the spreadsheet or word document...if anyone wants the spreadsheet, send me an email...

[email protected]

Let me know what y'all think...
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Old 11-15-2005, 11:40 AM   #3 (permalink)
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I think I can use it. [email protected]
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Old 11-16-2005, 09:08 AM   #4 (permalink)
 
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Here are some values from my spreadsheet. For a 55gal tank (48inX12in) with a 10 hour photoperiod. You will need the following wattage of lighting. The first column is inches the substrate is from the light. The second through fourth columns are for low, medium and high intensity light plants.

8 36.86 55.30 124.42
9 41.47 62.21 139.97
10 46.08 69.12 155.52
11 50.69 76.03 171.07
12 55.30 82.94 186.62
13 59.90 89.86 202.18
14 64.51 96.77 217.73
15 69.12 103.68 233.28
16 73.73 110.59 248.83
17 78.34 117.50 264.38
18 82.94 124.42 279.94
19 87.55 131.33 295.49

Give me feedback!!
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Old 12-31-2005, 04:32 PM   #5 (permalink)
 
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To calculate the Lux at any given depth use the Beer-Lambert equation: I1 = I0 * exp(-alpha * depth). Where I1 is the Lux at a depth, I0 is the Lux at the surface, exp is the Excel command to raise e (natural logarithm) to a power, alpha is the light extinction coefficient (.02 for very very clear water up to 4.0 for muddy water), depth is in meters, and Lux is lumens per square meter. If you have an aquarium that has .2262 square meters of surface area and you are using a lamp that puts out 4800 lumen then you have 21.22 KLux at the surface; K is thousand, and 4800/.2262. Your irradiance at .43 meters would be: 21.22 * exp(-.5 * .43) = 17.11 KLux. I use .5 for my 109 liter (29g) aquarium as a conservative alpha.

I hope this helps.

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