saturation, reflectors, lamps

> From: krombhol at felix_teclink.net (Paul Krombholz)

[we talked about bubbles]

I think we've gone about as far with the bubble thing as we can.  There 
is an underlying assumption in the discussion that I'd like to talk about 
briefly, and end with a question.

We've been talking about the plants as though they are not biological 
systems.  One of the hallmark features of biologically catalyzed 
reactions is that they display saturation phenomena.  At some point, you 
will be adding so much light that the plant can't use any additional 
irradiance.  At some point the pCO2 or [HCO3-] will be so high that the 
plant can't make use of additional carbon.

There are interesting implications in terms of our previous discussion. 
One would expect the plant to hit a different phenomenological saturation
point in stagnant and moderate flow conditions (should hit saturation at
lower pCO2 when water is flowing over the plant, for the reasons you
discussed before.) That's because the important "things" that get
saturated are inside the plant. 

No, for my question:  where are the aquatic plants folks operating their 
aquaria?  Are any of you seeing saturation phenomena in your tanks, due 
to either pCO2 or light?


About reflectors vs. white paint.  I've done both, you can get good 
results either way.  I wish that it were as simple as mylar vs. specular 
aluminum vs. powder coated stee vs. epoxy paint vs. acrylic paint. 

It isn't.  

There are a number of pendent metal halide fixtures around.  They are all 
white on the inside, and my guess is that the reflectivity of the 
surfaces on the inside is all pretty much the same.  Yet some of them 
allow a lot more light to reach the aquarium than others.  The shape of 
the pendent is important, with some, there is excessive trapping of 
radiation inside the bell.  Tall skinny bells (Coralife) don't seem to 
work nearly as well as more open bells (Iwasaki.)

So you can do better or worse even with white surfaces.  When you get 
into shaped specular reflectors, it gets hard really really fast.  You can 
focus light with them.  That can be good, it can also be very bad.

For a bank of fluorescent lights as close to the surface of the water as 
you can get them, I don't think white vs. specular makes much 
difference.  George has shown that before.  Part of the issue is that you 
pack the lights so tightly that there is a lot of trapping of radiation 
on the back sides of the lamps, and it is immaterial what reflector is 
behind the lamps when most of the photons emitted "up" just eventually 
are absorbed by reflectors or lamps.  

I'd like to see George repeat his experiment with four T-8 lamps spread 
out over, oh, say 16" or so.

For metal halides that are 0.2-1.0 meters above the surface of the water,
it makes a lot of difference. 

If anyone is truly hard-core about this, you can start running raytracing 
simulations on your workstation.  There are some interesting public 
domain programs out there, and it turns out that even people who are 
doing light fixture design professionally use this type of code to 
simulate how well a given fixture/array of fixtures will work in a given 

A friend of mine did some measurements with a spectroradiograph recently 
on a number of metal halide lamps and halide fixtures.  The hottest 
ticket in the 150-175 watt class was the fixture being marketed by Two 
Little Fishies.  It has a professionally designed reflector, and does an 
extremely good job of getting the light into the tank.  It is also very 
pretty.  It is also expensive.  ;-)

Another thing that people in the planted tank world might want to 
consider is the spectrum of the various light sources they are using.  
Not just from a "who drives photosynthesis best" perspective, but also 
from a purely asethetic perspective.  There are two main types of metal 
halide lamps sold in the US.  One, championed by Venture, is the Sc-Na 
system.  It produces very spikey spectra.  The other is the so called 
"dense line emitter" system.  This produces a much better simulation of 
sunlight.  I've been amazed at how much better fish look under the 
dense-line emitter type of lamp.  This is much more of a CRI issue than 
"color temperature" and on some level, I think the CRI numbers that I've 
seen are somewhat less objective than the color temp numbers.  Dense-line 
emitter lamps are extremely nice.  I'm in the process of redesigning my 
light hood to let me use them.  I've seen reef fish under both, and there 
is no contest.  Sc-Na system lamps just don't look as good as DLE lamps.