# Light Intensity Variations - some thoughts



## 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!


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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.


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## 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.


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## 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.....


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## hoppycalif

guaiac_boy said:


> 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.


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## hoppycalif

mikeynike said:


> 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


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## hoppycalif

Tex Gal said:


> 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.


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## 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!


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## hectorjohn

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


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## 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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## 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.


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## 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.


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## 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.


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## 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?


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## trag

hoppycalif said:


> 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.


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## trag

hoppycalif said:


> 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.


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## trag

hoppycalif said:


> 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.


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## 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....


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## 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.......


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## 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.


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## hoppycalif

The question about what happens to the light striking the front glass intrigued me. It is a simple first year physics problem, but it has been lots of years since I had that course.

Anyway I found, by googling, http://www.ps.missouri.edu/rickspage/refract/refraction.html, which gives what we need to figure this out, and has a really neat graphical calculator that illustrates the theory. So, using that I came up with:










This tells me that any light that strikes the inner surface of the glass from the water, will be fully reflected if it strikes the glass at an angle less than about 41 degrees. Bigger angles let light escape out the front glass. Since the bottom part of the tank is further from the light, the angle will be the least down there. So, I ran to my aquarium (ok, I climbed the stairs slowly) and used a magazine to block the light escaping various parts of the front glass, by looking at the shadows cast by it on the floor. I found that for my aquarium, virtually all of the escaping light comes from the top third of the front glass, and none I can detect comes from the bottom third. I'm not yet sure how useful it is knowing this - one thing it suggests is that a light near the front of the tank, could reduce the escaping light from the front glass, and near the back could reduce the escaping light from the back glass. But, that back light would lose more light from the front glass and the front light would lose more light out the back glass. Yes, I am just brainstorming now. I may have made a mistake with those angles, but it is easy to get them from that graphical calculator.


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## newbie314

So if one raises the light higher off the surface of the water, the more of the light that get reflected and not escape though the glass.
Of course there is light instensity lost but I guess one could counteract that by a side reflector.

There is some inherent reflection of the glass as we all know when seeing the light bounce from our glass covers.


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## trag

orion2001 said:


> 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.


That's exactly what made me think about this and decide it was worth scribbling some equations. As I mentioned, I took an electromagnetics class about a million years ago, and I vaguely remember those characteristics of point charges vs. line charges.


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## hoppycalif

newbie314 said:


> So if one raises the light higher off the surface of the water, the more of the light that get reflected and not escape though the glass.
> Of course there is light instensity lost but I guess one could counteract that by a side reflector.
> 
> There is some inherent reflection of the glass as we all know when seeing the light bounce from our glass covers.


Yes, the higher the light is above the tank, the smaller the angle of the light "rays" striking the glass, so a lesser amount escapes through the glass. But, of course, the higher the light is above the tank, the more light goes around the tank and is lost anyway, because of our very imperfect reflectors. The compromise would be an enclosure housing the lights, that has extra reflectors on the front and back of the enclosure, with a deep enclosure which moves the lights further from the tank. I stumbled on this design when I made my current fixture: http://www.aquaticplantcentral.com/...cts/49693-diy-cantilevered-light-fixture.html


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## hoppycalif

I have a 10 gallon tank, with two GE 6500K 20 watt powersaver screw-in CFL bulbs, each with a good reflector, but it is obvious that I only have a low light intensity in the tank. This has been a major annoyance to me. But, I think I understand it better now.

Those screw-in CFL bulbs are a somewhat spherical source of light - far from a linear source. So, their light intensity will drop proportional to the inverse square rule. My lights are one inch above the water, and 10 inches above the substrate. So, the light reaching the substrate is reduced by a factor of 100 from what it is at the top of the tank! If I had a 20 watt T5 bulb it would be a linear source, so the intensity would drop only by a factor of 10 at the bottom, giving me 10 times as much light as I now get. (It starts out being twice as good.)

I now think that is the major problem with the screw-in bulbs, much more so than the restrike problem. (I doubt that I lose a lot through restrike because I can look up at the bulb/reflector and see almost every surface of the coiled tube.) I am very tempted to retrofit the Perfecto hood with a linear bulb and reflector. It came originally with a 15 watt linear bulb that looks like a T4 bulb. So, that would give me another data point, an interesting DIY project, and might end the frustration with that tank.


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## BryceM

Before we all get carried away in a wave of point-source disgust and linear source passion, let's setp back for a second. *You must be very careful to understand the assumptions that Trag is making in his post above.* One must remember that modeling a CF or T5 as a true linear light source and a MH or spiral CF as a perfect point light source is a bit of a stretch. I understand the math and physics well enough. The equations are certainly valid. Their applicability to what we do doesn't pass the "real-world" test. Under certain conditions, with certain assumptions, fluorescents might out-perform MH's at a certain depth, due solely to their geometry. Assume, assume, assume ......... we're almost always better off to sticking with emperic data. Use what works for everyone else.

A thought experiment:

It's not much of a stretch to state that a watt of MH and a watt of T5 produce roughly equivalent quantities of useful light. Each is about 25-40% efficient at converting electricity to light. For our purposes, assume identical efficiency. Now, assume for a second that a "linear" CF or T5 fixture is truly 10X as effective as a "point-source" MH fixture for illuminating a remote surface. I know from experience that 3x150W MH HQI's are sufficiently bright to grow HC in a standard 180g tank with a substrate 24" away from the light source.

It's pretty clear that 45W (1/10th) of T5 lighting stretched along my 6' tank wouldn't do it. I doubt 250W of T5 would do it. In fact, I'd guess you'd need pretty close to 450W to get the job done. Where is the error? Didn't we all just accept the "fact" that linear sources are somehow an order of magnitude better? They should be at least 100% better, shouldn't they?

Well, the math for a linear light source assumes an infinite source length and a comparatively short distance to target. A 24" long bulb over a 24"x24"x24" portion of my aquarium still acts a lot like a point source. I'll also argue that my 3 MH fixtures spaced 2 feet apart over a 6' long tank act collectively a lot like a linear source.

Hoppy, your 10g tank is a low light tank for two reasons. First, spiral CF's are lousy designs for aquarium use. Even the best reflector is fighting the complex 3-D geometry of the bulb. There is a ton of restrike. There is tons of re-re-re-re-strike too. Second, the wpg rule doesn't work well for tanks that small. I use 8x23W spiral CF's over my 46g tank. It acts like a rock-solid medium-light tank despite the "apparent" 4 wpg.

I would absolutely love the chance to play with a PAR meter. On my 180g tank I have a combianation of MH's and T5's. It would only take a few mintues to produce loads of data with various lights on and off. It's hard to argue with data. Sadly, this is precisely what we lack during this sort of discussion.


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## hoppycalif

I am borrowing Tom Barr's PAR meter soon, so I will have some more data in a few days. I certainly agree that these simple equations don't accurately describe the non-simple light sources we use, but they do help to understand some of the characteristics we see. I doubt that any real light source has an inverse square drop off - maybe closer to dropping off at the 1.8 power of the distance. And real tubular lights probably drop off at something like the 1.3 power of the distance. I hope to get some usable data from both my AH Supply 110 watts of light and my 2X20 watt screw-in lights. I expect to be surprised again - I just don't know what the surprise will be. But, theoretical simplified considerations can help understand whatever the surprises are.


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## orion2001

Great points guaiac_boy and Hoppy. I'd love to see your data from your experiments with the PAR meter Hoppy. I agree with both of you'll in that the theory ofcourse is overly simplified but it can definitely help in understanding trends we might observe from experimental data. I'm also very curious to see your PAR data for your 10g tank with the spiral CFLs. Considering I have 2x15W CFLs in my 10g (emersed setup), I'd love to find out if I really need to boost the lighting up for my non co2, excel tank once I submerge the setup.


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## BryceM

Please experiment tons. I'd love to see the data.


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## trag

guaiac_boy said:


> Before we all get carried away in a wave of point-source disgust and linear source passion, let's setp back for a second. *You must be very careful to understand the assumptions that Trag is making in his post above.*


Another assumption/factor that I think is getting lost in the discussion is that for equal light production (roughly wattage) the linear source starts out *less* intense because the same light is spread over a larger area. The T5 intensity doesn't cross over to the MH intensity until a fairly substantial distance is reached.

Also, to emphasize guaiac_boy's point about the errors in treating a finite linear source as an infinite linear source...

Consider the target of a 4 ft. linear source with a 45 degree spread of light from the perfectly vertical. The light tends to act linearly, because each point on the target receives light from multiple point sources. The multiple point sources make up the linear light source.

However, the points of the target which are even with the ends of the 4ft. tube only get 1/2 the light intensity that the equations would predict. This is because they get light from multiple point sources on one side, but no light contribution from the other side. 1/2 of their light sources are missing as compared to an infinite linear source.

This effect is true to a lesser degree as you move inward from the end of the tube toward the middle.

Let's call the distance to the target, L. The point at which the light on the target is the same as the theoretical prediction is L inwards from the end of the tube. So, for a 4' bulb, if L = 2', then only at a theoretical dimensionless point at the very center, is the light as bright as infinite linear source theory would predict. If L = 18", then there is a 1' long area in the middle of the target where the light is as bright as theory would predict for an infinite linear source.

If the angle of light spread is 60 degrees, this effect is much worse. If the angle of spread is 30 degrees it is much better.

So, as G_B says, don't get too carried away. Linear bulbs have some advantage over point sources in light spread--certainly more than some MH proponents would allow. But they are not infinite linear sources, unfortunately.

I too look forward to seeing more PAR meter readings. Especially interesting would be reading from end to middle on fluorescent tubes at varying depths.


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## ingg

guaiac_boy said:


> It's pretty clear that 45W (1/10th) of T5 lighting stretched along my 6' tank wouldn't do it. I doubt 250W of T5 would do it. In fact, I'd guess you'd need pretty close to 450W to get the job done. Where is the error? Didn't we all just accept the "fact" that linear sources are somehow an order of magnitude better? They should be at least 100% better, shouldn't they?


Sorry to throw a wrench, but I run 312 watts of T5's over my 180g, and get results in the same arena with a high light tank. Bright colorations, good growth, from some fairly light demanding species such as Stellata, Hygro "bold", Polygonum Porto Velho....I'd put my growth rates up against a 180g with 3-150w MH, and I think you'd be surprised. I can grow that Stellata from 5" stems to the top of the tank in under a month, for instance.

That is running 8 of the available 12 bulbs. When I introduce a noonburst of 2 hours of all 468 watts in the middle of my normal 9 hour lighting period, virtually every plant in the tank is pearling by the end of the noonburst.


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## BryceM

ingg,

There's no denying that T5s are efficient. I'm perfectly willing to believe that 1.73 wpg (312/180) of T5 over a 180g tank acts as a "high-light" setup. If you want to compare growth rates, keep in mind that most pepople who use MH lights only use them for a few hours per day. On my 180g tank the MH's only run for 4 hrs. During the remainder of the 11 hr photoperiod I use 0.86 wpg of T5 (4x39W). This makes it hard to compare apples to apples.

It is still unlcear however, if it is the shape of the light source that is contributing to your success, or simply the fact that T5s are also an inherently effecient method of lighting.

To really answer this, obtaining actual experimental readings would be "illuminating". There are still many variables to control for. Bulb type (mogul vs HQI, regular T5 vs HO vs overdriven) reflector design (Tek, pendants, parabolics), and bulb manufacturer (GE, ADA, etc) all will play into this.

I haven't kept the other plants you mention, but I've not found P. stellatus to be that light-demanding. You can't keep it in a low-light setup, but I've kept it in medium-light tanks without much trouble.

My biggest curiosity with deep tanks is learning how to provide enough light to allow for a nice, dense foreground carpet (of HC, for example) without using so much light that tall stemmies grow insanely fast. I actually recently got rid of my P. stellatus due to its insane growth rate. Trimming three or four times per month gets old pretty quick.


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## ingg

Heh, I mow it to the line of my rock, then watch it grow back in for a few weeks. The splitting of stems is getting insane, though, last trim out was over 40 stems - I started with like 8 in January.

I need another local 180g, that runs MH! There is a 215g inside our club, but it is deeper, so tough comparisons again - it is 750 MH, insane light levels and growth.


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## hoppycalif

Today I measured some PAR values for a 10 gallon, Perfecto hood light fixture modified to take two 20 watt screw-in CFL bulbs, with good DIY reflectors. See http://www.aquaticplantcentral.com/...ts/45208-diy-mod-perfecto-10-gal-light-3.html for details about how I measured this. Here are some graphs of the results:
First - the distribution from one end of the tank to the other at the center of the fixture: (10.25 inches down is about at the substrate)









Then the distribution from front to back:









And, for reference, the distribution from the bulbs to the substrate:









Clearly, the inverse square "rule" almost describes the vertical distribution, at least once you move a small distance from the bulbs.


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## speakerguy

Excellent work! Do you have any results for T5 bulbs or linear PCs?


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## hoppycalif

speakerguy said:


> Excellent work! Do you have any results for T5 bulbs or linear PCs?


I am posting results as I get them, so no, I only have what I have posted. My 45 gallon tank has two 55 watt AH Supply light kits installed, so I will be taking some measurements with it later.

Today is embarrassing to me. It's like I am walking down the street with egg on my face, my fly unbuttoned, a wet splotch on my pants, toilet paper hanging from my belt, etc. You get the idea I'm sure. For a couple of years or more I have been insisting that the best we can do for reflector material is aluminized mylar, and I have never had any doubts about that. Theoretically it just has to be the best.

So, today I covered my mylar reflector surfaces first with aluminum foil, taking only a little care to avoid wrinkles, then with white paper, which I spray painted with glossy white paint. I repeated yesterdays PAR measurements using these two versions of the reflectors. The embarrassing results are:
















It is obvious that I was wrong about the mylar. Aluminum foil, which also gets panned by others, is clearly the superior reflector covering, followed by the glossy white paper, with mylar being the worst. I'm happy to have these results, and I will certainly not forget them, but I really could have done without the embarrassment!

I think the reason why the white paper reflector gives lower PAR readings close to the bulbs is because it gives diffuse reflection, where the shiny materials don't diffuse the light so much. But, at the bottom of the tank, the diffuse reflection is not a problem, and that is where it counts. I'm not sure that is the cause, but it might be.


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## Bert H

Very interesting stuff Hoppy. 

I have 2 different 50's. One (36 inches wide, AGA tank) is lit via a 96W ahs kit plus a 30W strip, total wattage 126W. The second tank is 30 inches wide (Oceanic), and taller than the first tank. It is lit via 2 ahs 55W retrofits total wattage 110W. It is easy to see that these 110W are brighter than the other's 126W. Whether this is a result of the 30W strip being a cheapie strip or not, I don't know. The second tank's depth is deeper than the first, but I can grow L. aromatica in that second tank, while I can't in the first. I'm sure there is some physics involved in how the 3ft 96W light is distributed vs how the 55W smaller strips distribute it. But that's for others to figure.


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## newbie314

Man, that makes me want to rip out my mylar (but it is shiny on one side and white on the other).

Time to get some Tin foil, yee-ha ..too bad I'm not allowed to put it on my windows during the summer.


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## BryceM

Hoppy, *great* stuff. Any idea what these PAR meters cost? I'd love, love, love to get my hands on one. I'm also dying to see what the effect of various reflectors geometries will be. We all regularly decree with reckless abandon that T5's with TEK reflectors are 9,000 times better than anything else......... Show me the numbers baby!

I'd also like to determine the required PAR for HC maintenance, HC growth, and a lush HC carpet. Once you have those numbers, a particular lighitng design can be easily developed for any size or shape of aquarium.

About being embarrassed...... don't be. Almost all of us here spout off things that we _know_ to be true with no actual data to support our position. Removing personal bias is one the greatest obstacles to true scientific progress.


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## hoppycalif

Here is one PAR meter: http://www.apogee-inst.com/qso_spec.htm. Tom's is an Apogee model QMSW-SS with the sensor shown on that page. I don't know what the whole thing costs, but it is around $200 as I recall. More than I want to spend.

One suggestion is that local aquatic plant groups buy one for use by all members. Our group has no treasury at this point, so we couldn't do that. But, it is an instrument of great value when you want to do this kind of measuring, but of no value at all for daily aquarium keeping.


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## orion2001

Aluminum foil FTW!!  

Very interesting stuff. Like guaiac boy mentions, there are soooo many things to play around with. Good luck with all the experimenting!


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## trag

guaiac_boy said:


> Hoppy, *great* stuff.
> 
> <snip>We all regularly decree with reckless abandon that T5's with TEK reflectors are 9,000 times better than anything else......... Show me the numbers baby!
> 
> About being embarrassed...... don't be. Almost all of us here spout off things that we _know_ to be true with no actual data to support our position. Removing personal bias is one the greatest obstacles to true scientific progress.


Well said and a good point.


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## hoppycalif

Today I set up my 45 gallon tank fixture, using two 55 watt AH Supply Kits, with the GE 9325K bulbs, in a stand similar to how I did the 10 gallon tank fixture:









I took many measurements of light intensity at various locations, including a "map" of the intensity at the substrate level, and the drop off in light at the centerline under the fixture. Here are the results at the centerline, with interpolated results for the ten gallon fixture, with screw-in CFL bulbs plotted on the same chart for comparison.









To better compare the drop off in light intensity with distance, I scaled down the readings for the AHS fixture to match the same PAR reading at 4.75 inches. This shows that the linear PC bulbs do better in maintaining intensity with distance than the screw-ins do. Whether this difference is because the linear bulbs lose intensity with the inverse of distance, while the screw-ins lose intensity with the square of distance, isn't clear to me yet.


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## BryceM

Hoppy, are your measurements being taken in air?


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## Brilliant

hoppycalif said:


> 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.


Did I miss the part where this statement was recanted? Tell Tom to send me the PAR meter...I have Orbit 2x96w and Orbit SLR 2x39w and a deep tank to test it on. 

If I understand correctly power compact light is very inefficient because of lost light reflected off water surface. When you make the statement quoted above are you comparing lets say 400w of power compact vs. 200w of T5HO?


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## hoppycalif

Brilliant said:


> Did I miss the part where this statement was recanted? Tell Tom to send me the PAR meter...I have Orbit 2x96w and Orbit SLR 2x39w and a deep tank to test it on.
> 
> If I understand correctly power compact light is very inefficient because of lost light reflected off water surface. When you make the statement quoted above are you comparing lets say 400w of power compact vs. 200w of T5HO?


Light is the same without regard for what bulb produced it. So, it doesn't matter if the light you have comes from a MH bulb, a T5 bulb, a linear PC bulb or screw in bulbs. What does matter is the intensity of the light, and the geometry of the light/reflector combination. The reflector just collects more of the light from the bulb and directs it towards the water, and the configuration of the reflector helps keep that redirected light at a sufficiently small angle to the vertical so it isn't reflected to any great degree by the water/air interface. So, what is needed to "punch through the deep water in a deep aquarium" is enough intensity at the surface, with a reflector shaped to direct that light as near vertically as is possible into the water. That can be MH bulbs, or T5 bulbs, because of their superior reflector configuration, or from PC linear bulbs with very well designed reflectors.

That is what I meant by the statement you quoted, and I still believe that to be true.


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## hoppycalif

guaiac_boy said:


> Hoppy, are your measurements being taken in air?


Yes, I posted photos of the set up to make that clear. Adding water at this point doesn't help. Eventually it will be interesting to compare the intensity vs distance in air with the same in water. It would also make this a lot easier if I were to use a light detector with a spherical, integrating sensor, instead of the somewhat direction sensitive detector I'm using now.


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## BryceM

Oh. Hmmm.

There are some factors in an aquarium that might act to mitigate the spread of light as you move away from the source. The sides of the tank will reflect a certain portion of the light back inside. How much depends on the material (glass vs. acrylic) and the specific geometry of the setup.

The air-water interface also comes into play. There is reflection, refraction, and scatter that won't be accounted for during "in-air" tests.

I'm sure Hoppy is aware of all of this, but it's important for everyone to understand exactly how these measurements are being done.

I don't want to sound too skeptical, but I think the data would be quite different in actual aquarium conditions. Is this light meter not submersible?


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## hoppycalif

guaiac_boy said:


> Oh. Hmmm.
> 
> There are some factors in an aquarium that might act to mitigate the spread of light as you move away from the source. The sides of the tank will reflect a certain portion of the light back inside. How much depends on the material (glass vs. acrylic) and the specific geometry of the setup.
> 
> The air-water interface also comes into play. There is reflection, refraction, and scatter that won't be accounted for during "in-air" tests.
> 
> I'm sure Hoppy is aware of all of this, but it's important for everyone to understand exactly how these measurements are being done.
> 
> I don't want to sound too skeptical, but I think the data would be quite different in actual aquarium conditions. Is this light meter not submersible?


Yes, the sensor is submersible, but the effort required to do this in the tank, which is badly overgrown right at the moment, would be excessive. Once I get the aquascape under control, thinned out a great deal, then I can take a few readings to "calibrate" the air readings. I still have no good idea for accurately locating the sensor in the water in three dimensions, which is easy in the air. Then, all of that has to be done reaching under the light fixture and without blocking the light, while reading the meter. If I had three hands and smaller arms it might be easier, but it will be very difficult in any case.


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## Brilliant

hoppycalif said:


> Light is the same without regard for what bulb produced it. So, it doesn't matter if the light you have comes from a MH bulb, a T5 bulb, a linear PC bulb or screw in bulbs. What does matter is the intensity of the light, and the geometry of the light/reflector combination. The reflector just collects more of the light from the bulb and directs it towards the water, and the configuration of the reflector helps keep that redirected light at a sufficiently small angle to the vertical so it isn't reflected to any great degree by the water/air interface. So, what is needed to "punch through the deep water in a deep aquarium" is enough intensity at the surface, with a reflector shaped to direct that light as near vertically as is possible into the water. That can be MH bulbs, or T5 bulbs, because of their superior reflector configuration, or from PC linear bulbs with very well designed reflectors.
> 
> That is what I meant by the statement you quoted, and I still believe that to be true.


I understand your point about light. The optic and reflector is what makes LED and T5HO work so well but their output is not intense. I think MH is just shear intensity. I have what I would consider decent reflectors and see lots of reflection on my ceiling. Ive seen a 24" deep tank grow extemely well with MH+poor reflector raised high above the tank. Metal halide's intensity seems to be overpowering the physics behind it all regardless of reflector. I think PC mimic idea behind MH and try to do the job with shear intensity. I posted because my PC's are much more intense then my T5HO's which are also proving to be over 100% more efficient. My point is regardless of light and its properties theres the whole wattage and efficiency issue which make any other contenders not worthy in deep water applications.


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## orion2001

OK, now I am thoroughly confused. After reading up on Mylar on Wikipedia (yes, I know it isn't the most reliable place for info) :
PET film (biaxially oriented - Wikipedia, the free encyclopedia)

It seems that Mylar as we know it is basically a metallized version of this PET polyester film. The mylar that we use is actually aluminum metal in PET!! Now this makes it extremely hard for me to figure out how Mylar isn't giving better/equivalent results to aluminum foil. Given the processes used to metallize Mylar, I'd assume you have very low surface roughness (which explains why it looks so much like a mirror). Also considering

The wiki on Aluminum states:

"Aluminium is one of the few metals that retain full silvery reflectance in finely powdered form, making it an important component of silver paints. Aluminium mirror finish has the highest reflectance of any metal in the 200-400 nm (UV) and the 3000-10000 nm (far IR) regions, while in the 400-700 nm visible range it is slightly outdone by tin and silver and in the 700-3000 (near IR) by silver, gold, and copper."

Hoppy, this is my theory. Let me know what you think of it. It could be, that in the case of your Spiral CFLs, your reflector geometry isn't optimal for something as highly reflective (next to zero diffuse reflections) as mylar leading to significantly higher light losses due to restrike. On the other hand, Aluminum foil, though not as reflective as Mylar, causes more diffuse reflections which basically lower this restrike as some of the light that would be reflected straight back into the bulb is now being scattered at different angles, bouncing around and in general providing better illumination. This would also explain why something like the white paint on paper behaved quite well since it is entirely diffuse in its reflection of light...again lowering restrike effects.

Something I had thought about a while back but never fleshed out in my head is the following idea:

For non optimal reflector geometries, there is a good chance that a completely diffuse reflector such as white paint or a partially diffuse reflector such as aluminum foil will provide better illumination than a perfect reflector like Mylar since the non optimal reflector design coupled with the perfectly reflecting Mylar surface will cause a significantly larger amount of restrike and thus, losses. On the other hand, diffuse reflectors in such a situation would cause some of this light that is being forced to "Restrike" due to the non-optimal reflector design in the case of Mylar to be scattered in other directions which wouldn't experience "re-strike". This effect would be even more pronounced in the case of Spiral CFLs since they have a geometry which lends itself to a lot of restrike. In addition it is next to impossible to design an "Ideal reflector" for Spiral CFLs. In this case, diffuse reflectors are probably the best materials to use.

I don't know if this is clear in words. I could draw a diagram to explain my idea.

I would postulate that for a T5 tube (restrike being less of an issue) and an optimally shaped reflector (see pic below), Mylar should still be more effective than aluminum foil or white paint (As Tony Gomez in the site linked below seems to believe).










This picture is from Aquarium Plants - Info Pages , interestingly the author claims that white paint is much better than aluminum, and hints that mylar would be the best.

I'd love to hear what people think about this idea.

Edit - I tried to put this reasoning into some sort of math. This is what I came up with:

X is the fraction of light emitted by bulb that restrikes the bulb for a perfect mirror material(mylar) used for a suboptimal reflector geometry.

Fraction of light emerging out of fixture for Mylar = 1-X

Now consider a diffuse reflector.

If light is incident on a diffuse surface (lets assume perfectly diffuse...meaning equal probability of reflection in all directions in a hemisphere from the surface) then a fraction of the total light will actually restrike and be reflected back into the bulb. IF we call this fraction Y, Y can be expressed as the ratio of the angle subtended by the bulb at the point on the reflector to the total angles it could be reflected at (ie 180 degrees for a hemisphere). Now this ratio Y would change depending on which point of the reflector you are at. Right above the bulb, at the point on the reflector closest to the bulb, this ratio would be the greatest (it is clear in the picture above), whereas at a point on the left or right portions of the reflector, this fraction Y, is lower. A good guess at what the mean value of Y should be, if you integrate across the whole surface might be Y=0.2-0.3. Lets assume Y=0.25.

This implies that in the case of a perfectly diffuse material you would expect 1-Y=0.75 of the total light being output by the bulb (in the upwards direction) to be available. So roughly you would get around 175% of the light as compared to if you had a perfectly black reflector.

Now if you have a perfect reflector geometry, X can be extremely small for the perfect mirror material. However the efficiency for a perfectly diffusive surface will not improve much at all since the diffuse nature will cause light to scatter in all directions. An interesting corollary is that perfectly diffuse reflectors will be very insensitive to the geometry of the reflector in terms of how efficient they are. The variability in performance of diffuse reflectors for a whole host of geometries should be pretty low.

It is very easy to see that in the case of a perfect mirror surface like Mylar, for a spiral CFL and non optimal reflector geometry, the fraction X could easily exceed this value of 0.25, and in this case a diffuse reflector would be a better option as compared to a perfectly reflecting material.


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## hoppycalif

(Cross posted at the Barr Report)
Orion: First a bit of nit picking: fluorescent tubes, whether T5, T8 or T12, all emit light from the inside surface of the tube, not its centerline. The centerline of the bulb contains the mercury arc which emits UV, which strikes phosphors on the inner surface of the bulb, which absorb the UV and re-radiate the energy as visible light. So, Tony's chart you showed isn't at all accurate, and there is no optimal reflector for a fluorescent bulb.

Yes, I understand your theory about diffuse or imperfect reflection vs near perfect reflection. You may be correct, but I don't know any way that I have available to verify that. One could do an experiment with a light source that approaches being a point source, with a simple flat reflector, but the sensor would have to be very small for the experiment to work. (I think.)

The mylar I am using is, I think, aluminized on both the front and back surfaces. Neither surface is 100% metalized, but the combination comes close. (Some light goes through the metalizing through the gaps.) That is one source of inefficiency. Another source is the anti-oxidation coating on the aluminum coating, and I don't know what that is or how thick it is.

Mylar looks so "perfect" only because it gives a very nearly true reflection, so it works reasonably well as a mirror for shaving, for example. But, our eyes are extremely capable of using a wide variation in light intensity and ignoring the variations. So, if only 80% of the incident light is reflected the mirror effect will still seem to be perfect.

Aluminum foil, not having nearly as perfect a surface, looks like a poor reflector - it gives a distorted image. But, our eyes concentrate on that aspect and ignore the higher percentage of incident light being reflected. That is my opinion.

White paint gives pure diffuse reflection, but appears blindingly white only because such a high percentage of incident light is reflected. That reflected light is reflected at small angles off of perfect reflection, making it useless as a mirror, but the small angles are not great enough to greatly reduce the total amount of light being usefully reflected. Again, this is my opinion.

Others have been posting that mylar isn't really very good as a reflector, and that white paint is very good. I couldn't accept that since it was so counter intuitive. Those "others" also have said that aluminum foil isn't a good reflector. I intuitively accepted that. What I think we have here is a failure of intuition.


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## BryceM

hoppycalif said:


> If I had three hands and smaller arms it might be easier, but it will be very difficult in any case.


For some reason that sentence make me think of your avatar.  In any case Hoppy, this is one of the more interesting and valuable threads to come along in quite a while. Keep up the good work.


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## hoppycalif

Below is a plot of my data yesterday for my 45 gallon tank fixture with AHS light kits, measured along the length of the fixture at the depth of the top of the substrate. I added the data for the 10 gallon fixture with the screw-in CFL bulbs for comparison.









I think the drop off in intensity results because as you move towards the ends of the fixture the light contribution from the opposite end drops off rapidly, and there is less contribution from the shorter end in the opposite direction.

It should be clear that there is no one number that characterizes the light from a light fixture, no matter what bulbs or reflectors are used. So, we might as well just use wattage and bulb/reflector type as our descriptive "number" for such fixtures. i.e. 110 watts of AHS Bright Light kits with GE 9325K bulbs.

I hope someone with a MH pendant, or even a MH mounted in a fixture, will be able to do similar testing with that type of light. I'm very curious about how nearly the MH light drops off with the inverse square of the distance, and how the intensity varies along the length of a tank with a MH light.

I don't think this type of testing will ever show that one type of lighting is the "best". It does give some information that can help in understanding the light intensity in a typical aquarium, which might help in understanding what we see in the growth patterns of our various plants.


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## orion2001

Hey Hoppy,

To be nit picky about your nit picking , I do understand your point about T5 lighting not being a radial source either but what I meant is that a T5 has a much smaller diameter than a CFL spiral bulb. So the geometry above is still "better" for a T5 than it is for a CFL spiral bulb. Try shrinking the circle in the picture above to 1/4th its diameter. You can then get an idea as to how that might work much better for the same reflector design as in the picture. With the additional spacing between the T5 and the reflector, the angle the total diameter of the tube will subtend at any point will be even smaller making the radial approximation a better one. Ofcourse it cannot be simplified completely to this approximation, but the truth is that this geometry, especially if the tube is separated from the reflector by a bit of space will be much more effective for a T5 than it can be for a CFL Spiral. In such a case, a highly reflective material like Mylar might perform better. I'm curious as to why reflectors for T5 tubes aren't made to be a little bigger and spaced out further from the tube axis. Maybe I'm missing something, but it does seem to me that it could help. 

I think with Spiral CFLs you're pretty much screwed whichever way you look at things. There really can't be any optimal reflector design for them and from your experiments, it seems like aluminum foil/white paint will work much better than a reflective material which seems to promote larger restrike.

Keep up the great work Hoppy! This is some really interesting stuff and it is great to exchange ideas and discus this. 

Cheers

PS- I'm not saying that my theory has to be right...I could be totally wrong. But, so far it seems to make some sense to me when I try to justify the results you are seeing to myself. Btw, Hoppy, regarding the 45G AHsupply fixture, are these T5 tubes with polished aluminum reflectors? Is there any chance you could cover this reflector with aluminum foil, and also spray painted (white) paper? I would be extremely curious to see the results for this fixture. Sorry for troubling you so much =)


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## hoppycalif

orion2001 said:


> PS- I'm not saying that my theory has to be right...I could be totally wrong. But, so far it seems to make some sense to me when I try to justify the results you are seeing to myself. Btw, Hoppy, regarding the 45G AHsupply fixture, are these T5 tubes with polished aluminum reflectors? Is there any chance you could cover this reflector with aluminum foil, and also spray painted (white) paper? I would be extremely curious to see the results for this fixture. Sorry for troubling you so much =)


Oh my aching back! Yes, it would be relatively easy to cover those reflectors with either or both aluminum foil and painted white paper. At least it would be a lot easier than taking measurements in the tank full of water. Eventually Tom will want his meter back, so I am trying to get as much as I can done before then. In a few minutes I plan to do a major pruning and rescaping of the 45 gallon tank to make it possible to take some measurements in the tank. Maybe I can do that and the reflector surface change testing this week end. I agree that it would be very interesting to see just how good the AHS reflectors are.


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## hoppycalif

The light from a fluorescent tube is very diffuse, with light going in almost all directions from any spot on the tube. This should explain why this is so:









Therefore, light from the fluorescent tube is striking the reflector at many different angles other than as if the light came from a line source. As the distance to the reflector surface gets bigger, the fluorescent tube looks more like a line sourse, so a small diameter (T5) bulb can have a better reflector than a T12 bulb can have, unless the T12 reflector is about 2.5 times as far from the bulb as the T5 reflector is.

(Just some idle thoughts as I seek excuses to put off working on my pruning.)


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## orion2001

hoppycalif said:


> The light from a fluorescent tube is very diffuse, with light going in almost all directions from any spot on the tube. This should explain why this is so:
> 
> 
> 
> 
> 
> 
> 
> 
> 
> Therefore, light from the fluorescent tube is striking the reflector at many different angles other than as if the light came from a line source. As the distance to the reflector surface gets bigger, the fluorescent tube looks more like a line sourse, so a small diameter (T5) bulb can have a better reflector than a T12 bulb can have, unless the T12 reflector is about 2.5 times as far from the bulb as the T5 reflector is.
> 
> (Just some idle thoughts as I seek excuses to put off working on my pruning.)


Thanks for the pic. Yup that is exactly what I was trying to say and the picture helps explain a lot. I guess the reason reflectors aren't really designed that way (kept further away from the tube) is due to the impracitcality of making them like that. You'd need a fairly large hood and that would probably look pretty ugly.

Sorry for all the trouble Hoppy. I promise not to come up with any more suggestions for experiments to perform  . I can't wait to see what the results of using foil/white paint on your AH supply reflector! I wish it were easier to get my hands on a PAR meter. I'd love to experiment with it too.


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## hoppycalif

Today I did a major thinning of the plants in my 45 gallon tank, so the light can now reach the substrate in a few areas. I managed to get a reading in one location, at the substrate, about 8 inches to the left of the centerline, and near the front-back centerline. The reading was 50 - 80, depending on exactly where I held the sensor, with movement of less than an inch making that big a difference. If I moved it a bit more the light coming from the right half of the fixture was shaded and the reading dropped into the 20's. This is clearly not going to be a precise measurement, nor is a distribution of intensity possible to obtain. I still have no way to accurately position the sensor, and the remaining plants, mostly Limnphila aromatica, do a very good job shading the substrate.

I think one would need a tank without plants, and a grid of fine wire or thread in the tank, before it would be possible to get a distribution of light intensity. It just isn't worth that much effort for me.

Tom bought this PAR meter with the intent of monitoring the light for specific leaves, as the plant grows. I can see doing that, but even that is terribly difficult to do with any accuracy. I suspect that's why he hasn't done that yet.

I'm still thinking about the task of covering the reflectors with aluminum foil and/or white painted paper.


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## orion2001

Well, I think the fact that you measured similar intensities in water as compared to those in air (in places which weren't shaded) is a good sign. Then it might be reasonable to estimate that we might see a similar trend as we did in air. 

The only cause of concern would be the total internal reflections at the glass surfaces, especially the front and back glass which aren't accounted for in your air setup. The wooden planks on the left and right side might approximate this effect to some extent for the left and right glass walls of the tank. 

All-in-all I think it might be safe to assume that there isn't a significant drop in intensity once you fill the tank in water. If anything, due to multiple internal reflections it might boost it some.


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## BryceM

orion2001 said:


> All-in-all I think it might be safe to assume that there isn't a significant drop in intensity once you fill the tank in water. If anything, due to multiple internal reflections it might boost it some.


Um, I hate to be the skeptic here, but this statement is not supported by anything except perhaps a single measurment. You might well be right. Actually, I suspect you are, but until we have good data for comparison with a variety of fixtures, depths, glass vs. acrylic, I don't think it's wise to jump to conclusions just yet.


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## hoppycalif

I am a retired engineer, not a scientist, so I look at things like this a bit differently perhaps. I know that the light fixture on my 45 gallon tank is good enough to grow every plant I have tried, including a ground cover plant. So, I assume that whatever intensity of light I get with it must be at least moderately high intensity. So, when I measure the intensity in air, that tells me that any fixture that can approximate those measurements will very likely give moderately high light intensity in a water filled tank.

Unlike some measurements, PAR meters give a number that intuitively is meaningless. The number would mean something to me only if I had a history of measuring light intensities (PAR) and could correlate the intensities with something related to plant growth. That isn't likely to ever happen, nor am I likely to do sufficient research to develop a feel for PAR as related to plant growth. So, I am left with just looking at the numbers as being typical for moderately high aquarium lighting.

Now I can better understand why my little screw-in CFL fixture for my 10 gallon tank gives such marginal plant growth. The PAR numbers for that fixture just aren't even close to what I get with the bigger fixture and AHS lights. 

If PAR meters were real cheap and a lot of us had one to play with, we could accumulate enough data to arrive at some good conclusions, or if one of us wanted to spend the time and money to do a lot of careful testing, we could also arrive at some good conclusions. For now I think about all we can hope for is some general ideas about the subject.

One such general idea is that plants react to PAR, not to the log of PAR - PAR is not like pH, which is a log function. So a light giving 1/2 of the PAR as another light is half as effective. I didn't even have a feel for that before playing around with this. Another general idea is that plants will grow over a very wide range of PAR - within one aquarium the PAR will be many times higher up near the water surface than at the substrate, but most plants will grow both places, just a lot faster and better at near the surface. This supports the idea that growing carpet plants is the crucial test of lighting, something I was not convinced of before.

The next generation of aquatic plant growers will still have some interesting research they can do.


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## hoppycalif

Today I measured the light intensity from my 55 watt AHS Bright light kits in my 45 gallon tank light fixture, for different reflector materials, as suggested by Orion2001. To make this easier, I did the measurements with only one bulb of the two installed. First I checked the reflector with no modifications, using the same technique as I used for this fixture before. Next, I covered the AHS reflector with aluminum foil, trying to minimize the wrinkles in it. And, finally, I covered the reflector with white paper, spray painted with glossy white paint. All of the PAR measurements were taken at the fixture centerline, so the light was affected by the reflector, since that bulb was off center above the sensor.

I plotted all of the data, plus the data taken with both bulbs installed, on one graph:









Two obvious conclusions: I get about 50% more light at the center of the tank by using two light kits instead of one. Intuitively I would expect to get about twice as much light. And, aluminum foil and white paint are effective reflector materials, although not as good as the AHS reflectors. I think I may have misread the meter for the aluminum foil test at the closest to the bulb point. (I suspect all three readings for that location should be about the same, since almost all of the light is directly from the bulb with little from reflection.) It looks like aluminum foil is amazingly good as a reflector when you are considering only light at some distance from the reflector, and white paint is a good reflector although, in my opinion, this doesn't support the idea that it is as good as aluminum foil. I suspect a DIY reflector using aluminum foil would be a difficult project to do successfully because the aluminum surface would deteriorate too fast, but I am just speculating on that.

I'm out of ideas now for using the PAR meter to answer questions I have. But, if I think of something else or if anyone has a reasonable suggestion, I will try it.


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## newbie314

So is the AH reflector with no modifications just shiney aluminium or aluminized mylar?

I have a hunch one doesn't get twice the amount of light from two bulbs is the light of one bulb shiney onto the other.
I my setup (all-glass two bulb hood) it wouldn't surprise me 25% of the light from each bulb is lost to the neighbor. I would think possibly even higher.

The hood would need a good triangular reflect right down the middle of the two so it looks like two single hoods together.
Don't know if this is happening on the AHS or any hoods.


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## orion2001

Great work Hoppy. Thanks for all the time and effort. Those are interesting results. It looks like aluminum foil is quite a good material for a reflector. I think you hit the nail on its head by talking about comparing your "Air" intensity results with a hood which you know for a fact works well in an aquarium. Comparing light levels in air is then a good idea. Btw, why exactly do you feel that aluminum foil isn't a good long term solution for DIY projects? Just because it is flimsy? Wouldn't it be easy enough to put on a new layer of foil every 5-6 months?


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## hoppycalif

newbie314 said:


> So is the AH reflector with no modifications just shiney aluminium or aluminized mylar?
> 
> I have a hunch one doesn't get twice the amount of light from two bulbs is the light of one bulb shiney onto the other.
> I my setup (all-glass two bulb hood) it wouldn't surprise me 25% of the light from each bulb is lost to the neighbor. I would think possibly even higher.
> 
> The hood would need a good triangular reflect right down the middle of the two so it looks like two single hoods together.
> Don't know if this is happening on the AHS or any hoods.


AH Supply reflectors are made from "Miro 4 enhanced aluminum" from Germany. They are visibly a better quality reflector than anything else I have seen. My fixture has two AHS kits mounted side by side, with the sides of the fixture sloped and covered with mylar for additional reflection. AHS keeps the bulbs quite a long distance from the reflective surface too, so they get maximum effect from the reflectors.



orion2001 said:


> It looks like aluminum foil is quite a good material for a reflector. I think you hit the nail on its head by talking about comparing your "Air" intensity results with a hood which you know for a fact works well in an aquarium. Comparing light levels in air is then a good idea. Btw, why exactly do you feel that aluminum foil isn't a good long term solution for DIY projects? Just because it is flimsy? Wouldn't it be easy enough to put on a new layer of foil every 5-6 months?


The reason I don't think the aluminum foil will last long is, first, because I have vague memories of being told that by others, and second because shiny aluminum tends to get a hazy layer of aluminum oxide on it with time. It is certainly cheap, so replacing the surface periodically couldn't be expensive.

I haven't tried to glue aluminum foil to anything yet, so I'm not sure how that will work either. I know of no reason why it wouldn't work, but I just haven't tried it. I'm still recovering from the shock of learning my beautiful mylar isn't as good as I thought it was! And, the white paint I have laughed at for so long turns out to be far better than I ever would have predicted.


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## hoppycalif

Since I last posted on this subject I have been thinking about the results of my "play time" with a PAR meter. One of the things I wanted to check was whether the intensity drop off with distance for a linear source of light differed from that with a "blob" source. Today I downloaded a piece of log log graph paper to further look at this. I plotted my PAR measurements at the centerline of the tank from substrate level up towards the bulbs against distance for both light fixtures I checked, and got this:









What I see in this graph is that once you get a certain distance from the plane of the bulbs, the intensity does drop almost linearly for a linear bulb or set of bulbs, and it drops somewhat close to being an inverse square drop off for "blob" bulbs. But, as you get nearer to the plane of the bulbs, the relationship breaks down, because the light is no longer coming from either a linear source or a discrete blob source, but from multiple directions.

I see this as interesting, but nowhere near conclusive. It just suggests that part of the effectiveness of T5 bulbs is that they are somewhat linear sources. Food for thought only.


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## zer0zax

Good stuff Hoppy! rayer:
IMHO AH reflectors really are worth their salt. Don't feel bad about the low percentages shown in your test (even though they do blow my mind!), take into consideration that you are using the same AH shape for all materials tested

You already know that you are only comparing MIRO4 vs. foil vs. white paint, and AH Supply does claim 162% better reflection, but after looking on the AH website they say 41% comes from the shape and distance of bulbs from the center of the reflector, and 21% comes from MIRO4 vs. Mylar.

Maybe MIRO4 is not really 21% better than Mylar, but IMO it would not be worth the cost to waste Mylar by covering the reflectors in it for testing (or would it? :idea: :idea. At least MIRO4 appears to be 33% better than white paint!!! (at the substrate)

The aluminum foil really :boom: my mind though.............

I wholeheartedly agree with you guys that selecting lights should be done based on the substrate plants that will be used. These plants usually require the most light and get the least.

PS: (long rant, I know...:deadhorse) IMHO using foil as a reflector would not be worth it, if you touch it even a little the shape will get skewed. But if you are using a cheap shoplight with white paint.... than why not?!


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## hoppycalif

The only way I can see using foil is to cement it to something rigid to keep the shape you want. If it isn't perfect that shouldn't be a problem, since the only "perfection" we want is in the percent of the incident light being reflected towards the water. It doesn't matter if the reflected light is not reflected accurately in direction.

I'm intrigued by the white paint. I only used a spray can of bright white paint I had laying around, and didn't take care to get a really good coat of it on the paper I used. I feel sure there are better white paints available, and I know a good job applying it would help some.


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## treefish

I obviously don't browse these forums often enough because I just discovered this thread today.

Very interesting stuff hoppy.

I admittedly haven't read it all yet. Hopefully I will have some time tonight to thoroughly investigate the data here.

I must say, I'm quite disappointed with your results from the mylar tests; especially considering I am 99.9% certain the mylar you used is from my stock. If you want, I can find the manufacturer information for it. Perhaps conclusions will be different for other mylar types, brands, thickness, etc.

Aluminum foil might be a better choice for your solar cooker as well.

I am also now wondering if the mylar is the reason behind my problems being discussed on another thread. I'm using mylar only as a reflector on 4x24 T5HO. My actual wattage is about 3.3wpg. I assumed I could call it 3wpg safely. Maybe I should have gone with plants that are less light demanding than what I currently have. Certain plants are now dying. This might just be the reason why.


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## zer0zax

Geese Hoppy, you might have just invented the next trend! As you guys have stated, the only problem with aluminum foil is the oxidization that can occur. I am wondering how long it would last with all of the humidity from an aquarium... probably not long at all if you run open-top.

Oh well, I am already trendy because I wear a tinfoil hat.


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## hoppycalif

treefish said:


> I obviously don't browse these forums often enough because I just discovered this thread today.
> 
> Very interesting stuff hoppy.
> 
> I admittedly haven't read it all yet. Hopefully I will have some time tonight to thoroughly investigate the data here.
> 
> I must say, I'm quite disappointed with your results from the mylar tests; especially considering I am 99.9% certain the mylar you used is from my stock. If you want, I can find the manufacturer information for it. Perhaps conclusions will be different for other mylar types, brands, thickness, etc.
> 
> Aluminum foil might be a better choice for your solar cooker as well.
> 
> I am also now wondering if the mylar is the reason behind my problems being discussed on another thread. I'm using mylar only as a reflector on 4x24 T5HO. My actual wattage is about 3.3wpg. I assumed I could call it 3wpg safely. Maybe I should have gone with plants that are less light demanding than what I currently have. Certain plants are now dying. This might just be the reason why.


Yes, my mylar is the piece you sent me. It works great for my solar cooker, but now I have to believe that ordinary aluminum foil would work better there too. (tonight we had excellent split pea soup cooked with that solar cooker!)

In my case I was working with a light fixture, the one for the 10 gallon tank, that is very marginal at best, so the loss from the reflector was significant. I don't know if you are that close to having inadequate light, but I doubt it. 3 watts per gallon allows you to lose some of it without a problem.

I'm almost sure the mylar is inferior at doing what we need, which is to redirect the highest percentage of the light from the back side of the bulbs towards the water. It is great at letting us see a good image of the back of the bulbs, but not so great at capturing all of the light energy involved. The aluminum foil is bad at showing us a good image of the backs of the bulbs, but who cares? It does reflect more energy, if somewhat chaotically, which is what we really need. The white paint does the same, but totally chaotically.


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## zer0zax

Hoppy, do you still have the PAR meter? I am really curious about dual bulb reflectors and the amount of light lost. If you have some T8s laying around it would be interesting to see 2 bulbs running in one reflector versus one bulb running in the same reflector (other bulb completely removed).

If anyone has any ideas or charts on this that would be awesome! (Sorry for hijacking thread!)


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## hoppycalif

zer0zax said:


> Hoppy, do you still have the PAR meter? I am really curious about dual bulb reflectors and the amount of light lost. If you have some T8s laying around it would be interesting to see 2 bulbs running in one reflector versus one bulb running in the same reflector (other bulb completely removed).
> 
> If anyone has any ideas or charts on this that would be awesome! (Sorry for hijacking thread!)


I don't have the PAR meter now. It was borrowed from Tom Barr. I'm sure I could borrow it again if I needed to. But, I don't have any T8 fixtures laying around. As long as you are willing to just test a fixture in the air, and not measure the light in the water, you can use almost any incident light meter for this. You will get comparative numbers for different reflector/bulb configurations, but no numbers that can be used to compare to readings other people get. I think the comparative readings are just about as good anyway.


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## newbie314

So you've done the whole solar cooker thing too.
What did you make out of it?
Funny I was thinking the same thing about cookers that aluminum foil should work better than mylar.

I wonder how my mylar with the white backing would work?
Can buy it locally in sac. here.



hoppycalif said:


> Yes, my mylar is the piece you sent me. It works great for my solar cooker, but now I have to believe that ordinary aluminum foil would work better there too. (tonight we had excellent split pea soup cooked with that solar cooker!)
> 
> In my case I was working with a light fixture, the one for the 10 gallon tank, that is very marginal at best, so the loss from the reflector was significant. I don't know if you are that close to having inadequate light, but I doubt it. 3 watts per gallon allows you to lose some of it without a problem.
> 
> I'm almost sure the mylar is inferior at doing what we need, which is to redirect the highest percentage of the light from the back side of the bulbs towards the water. It is great at letting us see a good image of the back of the bulbs, but not so great at capturing all of the light energy involved. The aluminum foil is bad at showing us a good image of the backs of the bulbs, but who cares? It does reflect more energy, if somewhat chaotically, which is what we really need. The white paint does the same, but totally chaotically.


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