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.

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.

BryceM

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.

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.

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 =)

hoppycalif

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.

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.)

orion2001

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.

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.

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.