[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/forumapc/aquascaping/49082-jazzlvr-s-75-gallon-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!

 

[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]

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.

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]

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.

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]

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.....

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.

 

[houseofcards]

For many this is a very simple hobby. Lights, ferts, action and you have plants growing, but for others it could be a very complex affair. Complex in the fact that "growing" plants isn't good enough, but instead they strive to have the plants grow in a particular fashion. Whether that be color, height, fullness, etc. Based on observation, it's no surprise that the par valve is low compared to the surface, but what does it really mean? What is the minimum par threshold that a plant needs to grow. Probably different with different plants. Are we going to see future plant charts stating, Plant Requirements: co2, NPK, Par level 500+

I think it's very interesting information, but difficult to utilize since one's setup will certainly throw par valves all over the place even if those tanks have the same lighting system.

 

[hoppycalif]

I think a PAR meter is just a research tool, not something one of us, trying to keep an attractive planted tank going, would want. One of the significant things that test showed me is the very wide range of light intensities in a typical tank, and how the plants will grow well under most of those intensities. It also added to my knowledge about growing ground cover plants and the reality of really deep tanks.

Another thing: a newly planted tank, even with dense planting, will have low growing plants until they gain some height from growth. So, the need for nutrients will be much less in the beginning vs in a more mature tank. Intuitively I knew this, but didn't realize the effect that light intensity gradient would have on that.

 

[houseofcards]

Interesting about the nutrient relationship with light.

I realize it's a research tool, but it does shade some scientific "light" on some of the issues we know by observation. Hobby the MH/T5 lighting although greatly reduced is still going to have a higher par value than a CF light hung at the same height, no. This being true the MH/T5 lighting might provide the plant with enough light to grow while the CF light would not. I've always felt tank height is one of the hardest fixed varibles to deal with. It's no coincedence that most tanks in scaping contests, ADA, ADG, etc. are shallow. The plants become more interesting since their not always reaching for the light and they interact more with the features in the tank.

I have a 10g tank with 55watts of cf lighting. A standard 10g is only 12" tall and my Rotala sp. green acts like hairgrass. It spreads across the substrate and new stems grow from the horiziontial stem that acts like a runner. I also have the same plant in my 46g under 192watts of cf lights. So again alot of light by the tank is 18" tall and the plant grows pretty much straight up until it hits the surface.

 

[hoppycalif]

No question - MH and T5 lights get more light into the tank for the same wattage, but that light intensity drops off with the square of the distance from the light. What ever we use for light, the intensity drops off that way. So, for two tanks, both 90 gallons, one 24 inches deep and one 16 inches deep, the deeper one would need as much as twice the wattage of light to get good ground cover growth. This is contrary to my intuition, but it seems to be a fact.

Maybe the real reason deep substrates give better growth is because they raise up the bottom towards the light?

 

[trag]

Now, someone will probably look it up and find that theoretically the light doesn't get through the glass

Great posting Hoppy. Regarding the above, it is largely dependent on the angle of incidence of the light, just as it is at the air/water interface. Since the light is probably striking the interior glass at more or less random angles, having been reflected many times, it shouldn't be too hard to take the equations for reflection vs. refraction and calculate what percentage would penetrate.

 

[trag]

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.

I'm hardly the most experienced aquatic planter, but my limited experience is that algae growth seems to be heavily dependent on photoperiod. Folks who have too much algae growth can usually get great reductions by shortening the number of hours per day their lights are on.

I was having algae problems in a couple of my tanks last year, until I realized that I had sort of drifted into leaving the lights on about 13 or 14 hours per day. The tanks get some sunlight starting around sunup, and then if I leave the lights on for convenient viewing late in the evening before I go to bed the lit period is way too long for good algae control.

After than little incident, I thought back 25 years ago (I was away from the hobby for a while) and back then my experiences were the same. Every time I've had an algae problem I was leaving the lights on too many hours per day.

On the other hand, the time I set up a Growlux spotlight on a banged together wooden tripod and shown it in through the front of the glass a couple hours every afternoon, my plants grew great but there was no increase in algae.

 

[trag]

No question - MH and T5 lights get more light into the tank for the same wattage, but that light intensity drops off with the square of the distance from the light. What ever we use for light, the intensity drops off that way.

Hmmm. reading this posting and thinking about it a bit and going back to my electromagnetics class, I think that you actually made a mistake in your original posting. Of course electromagnetics was wayyyyyy back, so I am happy to find that I am wrong and learn something.

Here's the thing. You're approximating the MH light as a point source. This is fine. It works. And that will be consistent wtih the measurements you took the other night. However, your diagram also treats the fluorescent tube as a point source and that is only true when viewed on one axis. The other axis in the horizontal plane shows the fluorescent tube as a line source, which has a different math for how the light drops off. I think you will find, if you have another meeting and take readings on a tank with fluorescent tube lighting, that the drop off is not so dramatic.

The reason light drops off as the square of distance from a point source is that you start with a certain quantity of light at the source. As you move away from the source, that light spreads into an ever-widening circle. The same amount of light is filling a larger and larger circle. The radius of the circle varies linearly with the distance from the source (R = Y/Tan[theta]) and the area of the circle increases as the square of the radius. So the area the fixed amount of light must fill grows in proportion to the square of the distance from the light source. In other words the equation for the area which must be filled with the fixed amount of light is (pi * (y/tan[theta])^^2) where Y is the distance from the source and theta is the angle at which the light spreads from the vertical.

However, light from a line source does not work this way. Here's why. Pretend that your line is made up of a whole bunch of point sources shoulder to shoulder. Now, you would think that all your (smaller) point sources would behave in the same way as single point source. But here's the catch. LIght which spreads out into that ever widening circle for one point source is joined by light from other point sources.

Light from a line source will fill an ever growing rectangle. Let's see, the width of the rectangle will be 2Y/tan[theta] where Y is the distance from the source. The length of the rectangle will be L + 2 * (Y/tan[theta]), where L is the length of the fluorescent tube. But, the light is not spreading evenly into that expanding length. Mostly just our imaginary point sources near the ends are contributing light to the area added by the growing length.

So the area filled by the linear light source is approximately 2YL/tan[theta], which shows that the area to be filled only grows linearly with distance rather than as the square of distance.

If the fluorescent light were an infinite length the above would be perfectly true. But there is some spread at the ends. This also means that the longer the fluorescent tube, the slower the drop off in intensity with distance.

Of course all of that is based on certain ideal assumptions and real life isn't going to work exactly that way, but in general, the intensity of a long fluorescent tube will drop off linearly with distance rather than as the square of distance.

Keep in mind that for a given wattage of light, the starting intensity will be less with the fluorescent, than with the MH. All the light from teh MH starts out squeezed into a theoretical point. All the light from the fluorescent light is spread across a line. Or if you don't like geometry outside of three dimensions, just imagine a tiny circle compared to a long rectangle with a width equal to the diamter of the tiny circle. If you fill both areas with the same amount of light, the tiny circle will be more crowded.

But as they expand, at some point the rectangle (remember, only its width is growing, not its length) will be smaller than the circle, even though they're both the same distance from their source, and at that point the rectangle will be more crowded and therefore brighter.

 

[trag]

Okay, I couldn't resist doing a little work on the cross-over point.

If we call the amount of light produced 'Q', and we assume that we have a MH and a fluorescent fixture which are producing about the same amount of total light, then the equation for intensity (light per unit area) for the MH light is:

Imh = Q/[pi * (Y/tan[theta])^2]

And the intensity for the fluorescent light is:

Ifl = Q/[2 * L * Y/tan(theta)]

If we put Imh over Ifl: Imh / Ifl and call it R for ratio, then the MH light is more intense when R is greater than 1 and the fluorescent light is more intense when R is less than 1.

Reducing the resulting equation and not laboriously typing it in here results in:

R = 2 * L * tan(theta) / (pi * Y)

The interpretation of the above is that when 2 * L * tan(theta) > pi * Y then the MH is more intense. And when the reverse is true, the fluorescent is more intense.

So Y > 2 * L * tan(theta) / pi indicates when the fluorescent will become brighter than the MH.

For a 4' fluorescent tube and assuming theta is 45 degrees (tan[45] = 1, conveniently) then the fluorescent becomes brighter than the MH at 8/pi or about 2.5 feet. Interesting. Very few aquariums are that deep. But how high are pendants mounted? Y is the distance from the light whether than distance is above the water or in it.

If theta is 60 degrees, then fluorescent becomes brighter than MH at 4.3 feet.

For theta = 30 degrees the cross-over occurs at 1.5' or just a smidgen less than 18 inches.

A few things to note...

First, while the equation above would seem to indicate that a shorter L is advantageous to the fluorescent light, that is a deception brought on by the assumptions I made earlier. True, shorter lengths of tube will yield shorter distances to equivalent brightness, but at some point the assumption of a line source is no longer valid and the calculation becomes meaningless. Taken to its extreme of L = 0, you would simply be back at a point source and should be using an inverse square law instead of a linear drop off.

Second, would one ordinarily light a 4' long tank with a single MH light? Using the assumptions above of 45 degree spread and a 4' long fluorescent light (presumably atop a 4' long aquarium), the MH light would need to be a minimum of 2' above the aquarium for the light to hit the entire surface. Six inches below the water the light from the fluorescent light would be more intense.

Third, all of this ignores light absorption and scatter by the water and the air water interface. Assuming that all geometries and water conditions are the same for the MH and the fluorescent, these details shouldn't affect the results, I think. Not positive about that.

Fourth, if the fluorescent light is mounted at a different distance from the water than the MH, then the above equations need to be adjusted. Generally, this will work in fluorescent's favor, assuming that MH tends to be mounted higher than fluorescent.

Finally, fluorescent tubes are not usually used one at a time. The calculations are even more fun for multiple tubes and/or multiple MH lights.

Anyway, I think this shows (especially when one take pendant height into account) that there is a good chance that the intensity of equivalent fluorescent lighting overtakes MH at some depth.

Of course, if our light came out as lasers....

 

[hoppycalif]

Trag, you are right! I did drop the third dimension from consideration. And, while I didn't check all of your calculations, I think you are correct about the linear drop off with depth, as long as you are not near the ends of the tube. That makes the charts shown in http://web.mac.com/jgoal55/iWeb/Site/Reef Tank.html more understandable.

Back to work! First, get brain up to speed. Second.......

 

[orion2001]

Great work Trag! You could also compare this situation to the electric field for point and line charges. Using Gauss's law you can easily prove that for point charges you have a 1/R^2 dependence, but for line charges (or cylinders...under some conditions) have a 1/R dependence.