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More ligth measurements (long again...)

This is a continuation of my previous reports on ligth level measurements.
I am gathering this information in order to design a ligth canopy for
a planted 120 g aquarium. I will try to report results here as they become 

In http://www.actwin.com/fish/aquatic-plants/current/msg00312.html 
I reported measurements that show that in a completely bare tank with
just water inside, internal reflection effects seem to be very 
important. To get an idea of how much these effects can be disrupted
in a planted aquarium, I measured the light intensity at the substrate
level in my planted aquarium using the same saucer/camera technique.

This tank is a bow front 36" long, 18" deep and 16" wide at the center.
It was illuminated by a double 55 Watt PC strip with flat-topped aluminum
reflector. The target sat at a depth of 14", with the light bulbs sitting 
1"3/4 above the water level, with an (almost) clean glass cover in between. 
The aquarium is very dense with tall plants at the back and both sides,
only the front center is free os tall plants. 

The correct lux values at the target are not as straightforward to get in 
this situation as it is in the clear water case, since the aquarium water
has some visible organics (slight brown tinge, otherwise crystal clear). 
I had to assume a sensible absorption factor and correct for the light 
that gets absorbed *after* being scattered by the target and travels in 
between the target and the front glass pane on its way to the camera. 
Granted, this is a very uncertain factor, but it turns out to be 
relatively small, about 15-20%, and I'm looking for order-of-magnitude 
effects anyway.

I also took a measurement of the same target sitting under the same
strip light, separated by 15", in air. The lux reading in this situation
was about 3,600. The lux readings under water varied from 1,200 to 2,000
(after correcting for the after-target absorption), depending on the 
position of the target in relation to tall plants and the front glass. 
The highest reading was gotten with the saucer sitting close to the front 
glass, where shading by tall plants is less important, and some internal 
reflection still appears to take place at the front glass.

First thing we can do with these results is check for consistency of in-air
measurements. We can make a "theoretical" prediction of what the average
lux level should be close to the strip light, from the data at 
http://www.aquabotanic.com/lightcompare.htm (look for example there). We
get something in the range of 10,000-11,000 lux, assuming a luminaire 
efficiency of 50% and an even spread of the ligth over the entire aquarium 
bottom area. So the measured value, 3,600 lux at 15", implies a decrease 
factor of about 3 due to the distance between source and target. This is 
consistent with the decrease factor observed in the measurements at 
where the factor between the 16" and 29" measuring points (corresponding to 
13" distance) is roughly 2.3 - 2.5 for the cases with a reflector in place.
This of course assumes that ligth intensity from a strip ligth falls of
linearly with the inverse of the distance, which is more or less what I
observe in my in-air measurements.

So there is at least approximate agreement in between my in-air measures and
theoretical calculations, of course within the quite large error margins I am 
working with. This is encouraging, since it suggests that my crude photometry 
"technique" at least does not give purely random results :-)

Now, the readings from inside the planted aquarium are well *below* what 
I got from the in-air measurement. This is the opposite trend I found from
the measurements using the "clean" tank and water. In that case, the 
intensities are *larger* in water than in air, supposedly due to internal 
reflection effects. 

This leads naturally to the conclusion that in a well planted aquarium, the 
lack of internal reflection due to plant shading, and perhaps the presence of 
organics in the water, can cause a relatively large drop in light intensity 
with depth. My test setup includes a glass cover, but it clearly cannot 
account in itself for such a large factor. This drop factor with depth will 
be undoubtedly highly variable from setup to setup and even within the 
same aquarium as it evolves, plants grow and get pruned, and so on. 

Thus the often heard statement that aquariums act as ligth pipes, and there 
is no strong light decrease with depth, apparently only applies to bare 
tanks. Anyway, my aim here wasn't just to get to this perhaps obvious 
conclusion, but to get a numerical factor, albeit approximate, that can be 
used to quantify how much more ligth I have to provide at the surface of
a real planted aquarium in order to a achieve a given illumination level 
at the substrate. In my particular setup, and with the curent conditions 
in the aquarium, that factor is about 5 - 5.5. In other words, only about
20% of the light makes its way to the substrate at a 14" depth.

Simple linear extrapolation (good enough considering the errors involved)
tells that at 20" depth (the 120 g tank with a thin substrate in the front) 
the ligth level from the same strip ligth would drop to about 1,500 lux, a 
factor 7 drop. Thus my estimate is that the new canopy will have to provide 
at least about 14,000 - 15,000 lux just below it to achieve a similar ligth 
level at the substrate.

-Ivo Busko
 Baltimore, MD