hoppycalif

Last night our local plant club met at jazzlvr123's house, where we got to watch a demonstration of pruning in a 75 gallon tank. The tank has ADA aquasoil, MH lighting, full CO2 mist system, and is heavily planted - it is a tank with a thread about it in the aquascaping forum, http://www.aquaticplantcentral.com/f...planted-5.html. Tom Barr was there and he brought his PAR meter with him - the first time I have seen one.

We played around with it in the 75 gallon tank, and the results were far from what I expected. At the water surface, right below one of the MH lights, it gave a 1500 PAR reading - full sunlight is around 2000. That is high light! But, just a few inches down in the tank the reading was below 500, and near the bottom, it was about 150. That is a much greater range of PAR than I expected to see.

Water does not absorb much light, so this reduction had to be geometric, due to the inverse square rule. My belief had been that the inverse square rule only holds true for lights with no reflectors or bad reflectors. Clearly I was wrong.

Today I spent much of the day thinking this over, and came up with the chart below to explain what I think is reality with our lights:


There are no lights available that violate the inverse square law.

Some things come to mind after seeing this. First, our plants are extremely adaptable to widely varying light intensity. In a single tank the intensity can and does vary by a factor of 50, but the plants usually manage to grow anyway. Second, I have been puzzled about why my Limnophila aromatica was such a mild mannered plant, growing at a sedate pace for a few months, but lately it requires heavy pruning every week. That is because it grew until it is much closer to the light and gets to grow at the fast rate the higher light drives it to.

Another thing: people say that MH or T5 light is needed to "punch thru to the bottom" of a tank. That clearly isn't true. Any light we can use will drop in intensity by the inverse square law, so whatever wattage gives us high light at the water surface, the light will be reduced greatly by the time it reaches the bottom of the tank. It is all in the intensity we get from the light, whatever type it is.

And, pendant lights don't have to be raised much to greatly reduce the light intensity in the tank. If you start with the light 4 inches above the water line, raising it another 4 inches drops the intensity by a factor of 4 - effectively dropping a 4 watt per gallon "intensity" to 1 watt per gallon.

Last night was very educational!

 

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BryceM

YES!!! That's what I've been trying to say. We don't use lasers after all. There's a lot of scatter and re-direction of light at the air-water interface too. Distance from the light source is by far the most important factor when thinking about light intensity at a certain location. If you need a visual, grab an anubias leaf, put it up by the lights, and look at the enormous shadow it casts down at the substrate level. Leaves up high get all the glory. That's why stupid stemmies grow upward so quickly, then become densely branched near the surface.

To be fair, a large quantity of light gets bounced around a few times. Much of it eventually makes its way even into the dark corners of the tank where there is no direct line-of-sight to the light source.

mikeynike

Wow thats alot of light loss. I always believed that some light was regained from reflecting off the interior glass of the aquarium. Thanks for the great info.

Tex Gal

Boy what amazing results! All this talk about too much light.... makes you rethink. We have seen that you can get too much light and have algae all over the place. However with figures like Hoppy has expressed it makes you wonder how this is possible. Maybe our plants don't need as much light as we thought, that high light only exists at the top of the tank...... interesting.....

hoppycalif

The PAR meter never did drop below about 50 even inside the "bushes", so it is true that a lot of light scatters and provides what is apparently a minimum required to keep plants going. I have no idea what percent of light is lost by reflection off the water surface, but it can't be much, since you can easily see down through the water until the angle gets to be too great.

hoppycalif

I told a lot of people that until one day I decided how to prove it to everyone. Just look at the floor around your tank that has no light spilling over the top surface. It will be very dark - so no light comes through the glass and must therefore be reflected internally. Unfortunately, when I actually looked it was obvious that most of the light goes right through the glass and to the floor. This is easily predicted by applying the theory on reflection at a surface between two different index of refraction substances. But, I don't recall the equations. Now, someone will probably look it up and find that theoretically the light doesn't get through the glass

hoppycalif

Apparently algae grow very well at a very wide range of light intensities too. But, that isn't consistent with our usual advice to reduce lighting by half to avoid algae problems. Even if we did, the top parts of the tank would still get far more light than the substrate gets under the brighter light. Frankly, my head is still spinning trying to understand what I saw and what seems to be reality.

jazzlvr123

very interesting write up vaughn im glad tom brought that light meter so we could see how light spread throughout my tank. even in the shaded areas which I considered low light tom showed us that most high light plants could survive fine according to the PAR readings he took. Last night was great fun, im glad you were able to take something from it!

hectorjohn

i was just thinking. what if we submerge the light source..??

BryceM

Hoppy,

I've never been a huge believer in the theory that cutting back light levels is always useful in sorting out algae issues. I do think, however, that there needs to be balance between light intensity and nutrient availability (including carbon). For most people, on average, for most algae situations, the imbalance is probably a lack of carbon or another nutrient. Cutting light in that situation would bring things to a happier place, but I doubt it's a universally true principle. Meet the needs of the plants and the algae magically goes away..... yadda, yadda.

When restricting light does work, it's probably got less to do with the actual intensity of light at a particular algae-prone location and more to do with limiting the total amount of "energy" available to the system. Complex, complex stuff no doubt.

A recent article in TAG looked at crypts in the wild growing in locations only a few feet apart, but with light intensities that varied by a factor of 1000. They might be the masters of adaptability, but plants really are amazing at making use of what they've got. Light intensities are wildly variable in nature too - maybe even to a greater extent than in our little glass boxes.

Oh, and about the % lost at the air-water interface.... I suppose scattered, or redirected would be a better term for what I'm trying to say. Maybe a perfectly flat, calm surface wouldn't do much, but many of us create a few surface ripples to help with scum or to make pretty effects with the MH lights. These ripples have constantly varying angles of tangency which must act to throw light all over the place.

 

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