How high are your MH's?

[Anonymous]

All of this is assuming we have a real real good reflector, nothing blocks this reflector, and well, it's just a perfect reflector. Agreed under water things happin, I'm trying to avoid that for now, untill I can find more data on it. Lets just BEST case with air to start, or we wont be productive with the discussion. Figuring out reflection is pretty easy with cosine's law, I'm working on it with my reflector, there should be a formula for the bell pendants out there somewhere. Again, the reflector in the above example was helping, but the tested lumens still droped by 50 % in the second 10 centimeters. The reflector avoided a % of loss, but NO where near compairing it to a laser! L8r mega

 

[Anonymous]

I keep a 10k 175 w MH about 3 "" above the water with just computer fans for temp control. No problems thus far...

 

[Anonymous]

Mega:

You consider a shop light a real good reflector? That ought to disappoint the spider light people.

Further, I don't believe that the results in the link that you posted have anything at all to do with reflectors. As pointed out in that link, the light field from a line source (like a very long fluorescent tube) is well known to drop by 1/r, not 1/r2 (inverse relationship, not inverse square). That's the theory, and that's essentially the experimental conclusion in that thread.

Now I'll really provide fodder for the flamers:

The known relationship of the light field coming from an infinite planar source is that it does not drop off at all with distance. Now of course, a bank of VHO tubes in a hood is no more an infinite plane than a single tube is an infinite line, but nevertheless, the light field from a wide bank of fluorescent tubes is likely to drop off significantly SLOWER than 1/r(at least within a few feet of it).

(anyone remembering electric and magnetic field theory from lines and planes will recall these examples).


------------------
Randy Holmes-Farley

 

[Anonymous]

Whoa!
Gone 24 hours and what do I see!
MegaD--
Was not trying to dis you at all. I actually thought you had, or had access to, a spectroradiometer-I was impressed.
You made a reference to using "Sanjays test" and after looking at the article, with the equipment involved, I thought you indicated you were doing the same.
My other comments were just clarifying that you were using the same parmeters and to how you arrived at your figures.
These question were not out of line. Same type of thing I ask for from others when reviewing an analysis of my own.
I think you do a good job of getting some hard data--when we goad you into it.

2)

What happen to my original post--how high do you keep your Mhs and what problems of you have? Some of you folks are going to scare off responces with all this math!

3)

We are throwing around different measurements pretty loosely--lumen, lux, par--different things folks. The equations for all are not interchangable.

4)

The references given are, well--I would look for different ones (see--I am goading MegaD to do the work again

)
Given the measurements we are concerned with, it would probably be more practical to use the secant than cosine, just as a matter of ease of calculation.

5)

There is no doubt this is multi-variable. Hence, infinite posssible solutions. However, some of those variables can be negated as not significant(water clarity, amount of reflective mass within the water,etc) for this application and some can be assumed (distance, lumens, spectral radiation, etc) for ease of calculation and extrapolation. It is possible to calculate, even if rough, and at least for comparison of some of the parameters.

6)

The Planer field theory needs to be elaborated on--this will be interesting.(That goes back to another post, on another bb, with someone else here

)

and finally!!!
7) Exactly where does all that light go when you turn off the "light"!!

b.

 

[Anonymous]

Wouldn't the air itself provide for some reflection/refraction of light? The dust particles and such would actually absorb some of the light. I'd bet this is a significant factor in the inverse square law. Of course, reflection and refraction at the surface of the water is a whole nother issue. There's probably some light lost there, but submerging bulbs isn't really feasible.

 

[Anonymous]

you'd think that the air would be negligable.

 

[Anonymous]

http://www.actwin.com/fish/aquatic-plants/month.9511/msg00044.html

Here somebody was able to document the affects of inverse square law on florescents, a light that's nothing near point source, but is still affected by the same rules. We simply do not reflect in a manner like a laser beam is focused. I think I'll have to order that quanum meter to prove my points, as I have quite a few combinations of bulbs & reflectors to expermient with. L8r mega


------------------
megareef.freeservers.com
Yup, new url, finally thumbnail index html's on the foldlers, and a live reefcam

 

[Anonymous]

Here is a simple example to consider.
Ever take pictures with a flash?
Wow!!! My flash works better when I'm 6 feet from the subject matter than when I'm 20 feet away! Gee!!!
This is the most basic law for photographers.
Any light source does that. You light your subject, the light is not strong enough, so you move it closer. Decrease the distance of your lights by half and you'll gain 2 stops (4 times the light), that's a huge difference!!!
You can take this one to the bank.
It works everytime.

 

[Anonymous]

I'm with Randy on this one.

*IF* you have good reflective meterial, 99.9% (figuratively) of the light will be directed towards the tank and it won't make any difference how far the bulbs are above the water (as long as the entire hood is coated in reflective material (this includes the sides and back of the inside).

*IF* you have no reflectors and paint the inside of your hood matt black, you will get the 1/r^2 theory where the light drops off with the square of the distance.


One more thing:
Someone said that by lowering their hood 1" it made a huge difference in brightness in their tank. Well, when the bulbs are close to the surface you will get a brighter spot of light directly under the bulb (and darker in the corners). When you raise the bulbs this intensity decreases, but it is not *lost*, it just scatters to the darker area to the left and right of the bulb and creates a more even distribution of light.

So it all comes down to how good your reflective material is.

-Nathan

[This message has been edited by Nathan (edited 23 February 2000).]

 

[Anonymous]

I don't think Mega is implying that the light is lost, but he's trying to achieve the most intense lighting possible. Lowering the lights will do that.

If you had a perfect parabolic reflector, and a set of lenses, you wouldn't lose that much light either, but all things being equal, our reflectors are far from perfect, and I doubt anyone has focusing lenses on their setup. Without a lense, you are going to run into inverse square eventually. (A laser is not a well designed reflector) So given these facts, lower = less loss, where loss = light not going into the tank.

A simple test... Turn all the lights off at night except the fish tank lights. Any light not in the tank, is "lost" light. (If you have a canopy, peek inside to see how well lit the woood of the canopy is, etc.) A perfect system would have it dark everywhere except inside the tank. If you play with the height of the bulbs, you'll probably find the optimum point for your particular setup by watching the shadows the light throws outside the tank. A wider tank would let you set up your lights higher before it spills out.

I've got a set of reflectors that are a bit over 12" off the surface of the water, and they throw a fair amount of light all over the room when I pull up the covers. I'm thinking of coating the covers with some mylar to get some of that light back into the tank, but finding a way to mount these lights closer to the surface while still moving them so I can get in there would be the optimal solution.

 

[Anonymous]

My 250 Watt Iwasakis are about 3-4 inches off the water. I only have temp problems in July and August here in New England. I'm very happy with growth rate, etc. 36" off the water is....unbelieveable!

- Greg Hiller

 

[Anonymous]

I was the one who first invoked the inverse-square law on this discussion. As I think about what I've read, I believe Randy is correct as far as the point source and sphere theory. Despite the reflectors, the light still gets radiated in a sphere from a MH. The reflection moves some of the light in a different direction so it will "augment" the light radiated towards the surface but the surface area of the sphere is still expanding, even though the sphere is now distorted.
We also need to remember absorbance by air, dust and water. If anything, this would make the inverse square law look like a light booster!
Bottom line here? It's complicated at best and with all the variables I don't believe it's predictable.
BTW, did I mention my 2 175's are about 4" above my 30 gallon?

------------------
My drug habit cost less, but reef colors are better!

 

[Anonymous]

I have to disagree, it is predictable, but figuring in all the variables is a pain, some assumptions have to be made. L8r mega

------------------
megareef.freeservers.com
Yup, new url, finally thumbnail index html's on the foldlers, and a live reefcam

 

[Anonymous]

Randy,
Sure would like you to elaborate on your last post--the light plane.
b.

 

[Anonymous]

Light intensity falls off as a function of distance from the bulb because the light is going to cover more area. Inverse square sounds about right.

BTW, I have them about 4 inches in one tank and about a foot in the other. The foot away lights a lot more of the tank than the ones just 4 inches off the tank.

 

[Anonymous]

Calculating the function that determines light intensity at a point beneath a halide is much more complicated than most people realize.

The inverse-square law is what all calulations should be derived from--However, the geometry of the reflector is what can really makes things difficult.

Reflectors do not focus light--they reflect it. Focusing would only occur if the surface was parabolic or curved.

The Spiderlight reflects multiple images of the bulb. But when you move away or laterally from the source, you will get a slightly different set of reflections. For this reason, much of the area underneath the light cannot be related using the inverse square law alone, you must relate the geometry of the reflections as well. The math involved here is not difficult but it is extremely complex.

And as for the water clarity issue, unless the water is pi$$ yellow, it's a relatively insignificant variable. Most of our reef tanks are pretty clear. At least mine is...

 

[Anonymous]

BMW:

There are two natural consequences of the fact that light drops off from a single point source according to an inverse square law.

The first is that as you move away from an infinite line source (like a very long fluorescent tube) the light drops off as 1/r.

The second is that as you move away from an infinite plane of light (like a large bank of fluorescent tubes) the light does not drop off at all.

Both of these are simple mathematical relationships that are described in electromagnetism textbooks for electrical charge (which happens to drop off exactly like light does: 1/r2 from a point source).

I'm hesitant to say what effect this has on a tank, but I believe that if one tested the drop off in light from a bank of 4 or more fluorescent lamps, that the drop off in the middle of the bank would be minimal until you got as far away as the width of the bank.

------------------
Randy Holmes-Farley

 

[Anonymous]

Randy, could you give a source, any would be fine, for those relationships?
And maybe one giving the relationship of electicity to light in the manner you describe?
Always looking to be better educated.

Thanks,
b.

 

[Anonymous]

Put qualitatively, the argument runs like this: If you have an infinite plane that carries a uniform surface charge, the electric field will be constant regardless of how far away you are. For each point, the field dies off as 1/r**2, but as you move out further and further away, more points come into view, so to speak, proportional to the area, which scales as r**2, so the two effects cancel. This is one of the toy problems one encounters in elementary physics as an applicatioin of Gauss's Law, and it is important to remember that the crucial assumption is that the plane is infinite and carries a uniform charge. So if you had a really big bank of fluorescent lamps, and they were all narrow enough that you could regard the whole thing as one big infinite plane of uniformly irradiating light, and if that light were passing through a vacuum (no light being scattered or absorbed by the air or water), then this would be true. The electric field of an infinite plane is constant (independent of r), that of a sphere dies off as 1/r**2 and that of a line (say a single narrow infinitely long fluorescent tube) as 1/r, again, in a vacuum. Shyte, I still gotta prepare tomorrow's pchem lab lecture....... Check any decent physics book, look under Gauss's Law. The one on my shelf is Jackson's Electrodynamics but we used to refer to the course he taught at Berkeley as Electromasochism; check an easier book first. It should be in all of them, and as I said, it is a standard textbook problem. Remember, light passing through air and water will be scattered, so in reality it doesn't work quite this well, but on the other hand the reflector might help to focus the light partially. The bottom of my aquarium is almost as light as the top, thanks to my avoidance of metal halides

.

Oh, the other thing: The reason you can use the same arguments for light as for this is that they are both the same thing -- the electric field. Electrodynamics treats all electric and magnetic phenomena the same way, and light is just one aspect of this -- visible electromagnetic radiation. The electrostatic interaction between two point charges, what everyone is familiar with, is mediated by the exchange of electromagnetic radiation (photons), they are all ultimately the same physical phenomena.

[This message has been edited by flounder (edited 27 February 2000).]

[This message has been edited by flounder (edited 27 February 2000).]

 

[Anonymous]

Hi Flounder,
Thanks for that info.
Do I get from that you like the vhos over the mhs? Because of the light distribution?
And, electricty is like light. Never realized that!

b.