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Re: More Light for Deep Tanks - NOT!
Wright, it is not that I did not trust you, but I had to confirm your
statements for myself.:-) After all, there is literature like Kirk's _Light
and Photosynthesis in Aquatic Ecosystems_ that say " downward irradiance
diminishes in an approximately exponential manner with depth."
Since I have a light meter, I put it in a baggy and then made a few
measurements on my two four bulb setups. The numbers in footcandles confirm
your wisdom.
----------------------------------------------------------
125 gal. tank - 4 6ft bulbs | 70 gallon tank, 4 4ft bulbs
a b c d | a b c d
|
Dimpled relector material
____v_______v________v___ ( ) indicates the bulb
+6"--( )--( )--( )--( )---| triton
600 880 980 800 | reflectors <--fc directly under
bulb
| | _____ _____
| / \ / \
+4" +--( )--( )----( )--( )--
| | 800 800 550 500 <--fc directly under
bulb
|
+3" 600 +
| |
| |
+2" + 680
| Water
0~~~ 400~~~480~~~~480~~~~~~~~~~~~~~~~550 <380>~~~~~~~~~~ surface
| |
| |
| |
-6" 320 {350} + 400 <200>
| |
| |
| |
-12" 300 {350} + 400 <200>
. .
. .
. .
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Notes:
6 ft. Bulbs over 125 4 ft. Bulbs over 70g
a. spectralite 55w a. Triton 40w
b. daylight HO 85w b. Cool white 40w
c. cool while 55w c. Sylv. grow lux WS 40w
d. grow lux WS 85w d. Philips Agro lite 40w
-----------------------------------------------------------------------------
Numbers in <> brackets show intensity without triton reflectors.
Numbers in {} brackets show intensity with triton reflectors added.
Underwater measurements are approximate, probably +/- 30 fc because meter
was hard for me to read.
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Wow, thanks Wright. I am glad we have an expert onboard. I never took the
time to take the measurements and always thought that the light attenuation
was the same in air and water. Consequently, I did not pay attention to the
additional 2 inches above the water surface in my 125 setup. That explains
why the plants do not grow as rapidly in that tank. My numbers also show the
advantage of using a good reflector, such as the one made by triton. Now I
have to work on lowering those bulbs, instead of adding more of them.
Kirk also notes that says that turbidity can further attenuate the light
(which must be true), and yellow color from organics (gilvin) absorbs blue.
Any comments from the expert?
Neil
Neil Frank Editor of "The Aquatic Gardener" Aquatic Gardeners Association
Visit the AGA home page at <http://blake.oit.unc.edu/~fish/aga/>