How high are your MH's?

[Anonymous]

Could kyou desrcibe "sanjay's 400w 10kk halide test" ?
b.

 

[Anonymous]

The top of my tank is open, but like I said earlier I have an 1/8 inch piece of tempered glass fitted into the mh canopy. I tried to get starfire glass made that thin but no one would do it.

Bill

 

[Anonymous]

http://www.aquariumfrontiers.com/fish/aqfm/1998/nov/features/1/default.asp

ALl the details on how the bulbs were tested. L8r mega

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

 

[Anonymous]

MegaDeTH ... Your test has really made me think. I thought that air being a lots less dense than water that some air between the lamp and the water would not hurt. My 400 MH is 36" above the water and the * VHO lamps are about 30" from the water.

I guess that it is time to buy the acme screw drive and some cable and make my lights go up and down. Thanks I guess.

 

[Anonymous]

lower them puppies.

 

[Anonymous]

WOW, 36" to a coral that's 6" under water, directly under the halide, thats 42 ppfd, at the coral, with the haldies 4" above the water, thats 743 ppfd at the coral, I think you should lower those haldies!!!

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

 

[Anonymous]

You have a LiCOR LI-1800/12 portable spectroradiometer ?
With the software?

And all these test were done in certain controlled conditions. You duplicated these?

And all done in air. How did you calculate the amount of distance in water to your par?

b.

 

[Anonymous]

Actually a little common sence helps, as you know the law applys to a vacume I assume, water will filter light depending on water clairty and light wavelenght, and many other factors, reflector also helps quite alot, sanjay's tests were without reflector. So that said, inverse square law is inverse square law in air & in water if anything it's worse in water, hell in a vacume it's staggaring.

Knowing the exact par my corals are getting really isnt important to me, but knowing that by placing x coral 2" higher, and 2" to the left will result in 20 % more par at the coral is important. (definitly dont need equipment to figure this out.) I dont think most people realize the difference a inch or two makes.

Now what I've done is use sanjay's numbers as a baseline as it's all there is, build the formulas into excel and started graphing out everything, if you can do better feel free to do so. You seem to know the subject but are not adding anything to the thread. I fail to see how expensive equipment will change the formula used to extrapolate the data. Someday though, I plan on investing in this guy..
http://www.apogee-inst.com/bqmspecs.htm I think that long term it will be a good investment. L8r mega

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members.xoom.com/megareefs


[This message has been edited by MegaDeTH (edited 21 February 2000).]

 

[Anonymous]

I'll interject a few comments on the "inverse square law" that some are using here.

1. In any fixture that prevents the light from spreading out in a spherical or hemispherical form, the light will not drop off that fast. The light from a pendent, for example, certainly does not drop off that fast. Same with many reflectors in hoods.

2. Where do you guys think the light is going? Unless the lights are so high that a significant portion of the light is spilling out into the room (which would be a bad design) the light is ALL still entering the tank. And it is presumably all still hitting corals.

Consequently, while higher lights may reduce local intensity peaks, it will likely increase intensity elsewhere in the tank, farther from directly beneath the lamps.

Thus, while my mh lamps in my main tank are only a few inches above the water, the pendent in my refugium is about 2 feet above the water. Nevertheless, nearly all of the light from the pendent is hitting the creatures in the refugium.

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Randy Holmes-Farley

 

[Anonymous]

>1. In any fixture that prevents the light from spreading out in a spherical or hemispherical form, the light will not drop off that fast. The light from a pendent, for example, certainly does not drop off that fast. Same with many reflectors in hoods.

Sorry it does, please provide the physics to back this comment up, inverse square law is a law of physics, http://www.intl-light.com/handbook/ch06.html well proven and documented, unfortunatly there is no way around it, certainly a reflector will increase the light, but at any distance under the pendant, the tested light will still drop off at the exact same rate. L8r mega

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

[This message has been edited by MegaDeTH (edited 22 February 2000).]

 

[Anonymous]

Megadeth:

The inverse square law does not apply to focussed, or partially focussed, light.

The inverse law applies to light expanding in a sphere because the surface area of a sphere is 4 x pi x r2, where r is the radius. Thus, as the radius increases, the surface area goes up as the square of the radius. Since the light is spread out over this surface area, the intensity decreases as the square.

For other shapes, especially focussed beams, there can be much less than inverse square drop off. If the light pattern is not spreading out as much as a sphere or cone would, it is not an inverse drop off.

A laser beam is an extreme example. Lasers can be focussed to drop almost not at all.

If you read the reference you posted, it says:

"The inverse square law can only be used in cases where the light source approximates a point source. "

Of course, a focussed beam is not a point source. Neither is a mh lamp that is providing light via a focussed, or partially focussed, reflector.

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Randy Holmes-Farley


[This message has been edited by Randy Holmes-Farley (edited 22 February 2000).]

[This message has been edited by Randy Holmes-Farley (edited 22 February 2000).]

 

[Anonymous]

Wow, deep stuff on the science guys!

Anyway, 55g, 2 175w 10k, about 3" from surface. Results are incredible so far. I firmly believe that one should not have a reef tank with out MHs. Salt >spray< is a bit of a problem though. I just wipe them off a night every few days. Only been on about 2 months though.

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God Bless,
Jerry

 

[Anonymous]

Actually I'll dig up the specifics on why it does apply tonight when I get home from work, it does apply, the are cases it wont is with lasers as the Beam is focused to a extreem level. Yes, it will help, but no there's no way you will not have noticable loss, I can use cosine's law's to figure reflector efficency, I assume 70 % to keep the math simple. A perfect reflector that makes a beam of light 2"x3" the size of the halide tube, wouldnt be affected as much, but we dont use them, we use a reflector that spreds that 2"x3" point source, into a 2'x2' area, so the formula still holds true. L8r mega

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

[This message has been edited by MegaDeTH (edited 22 February 2000).]

 

[Anonymous]

Megadeth:

Look up whatever you want, and I'll be happy to discuss it.

FWIW, where do you think the light is going that is lost? It doesn't just evaporate according to some inverse square law. It gets dispersed. If it's all still hitting the tank, you haven't lost anything.

I agree that at very large distances from any normal reflector, the light will look like a point source, and will drop as an inverse square law. Even a laser will act this way (which operate at the same wavelength as your tank lights).

However, if you look closely at the article that you reference, it states one place that the inverse square law doesn't apply: when you are close to the source relative to its size.
They say:

"the distance to a light source should be greater than five times the largest dimension of the source"

If you take the size of the point source coming from a pendant as being the opening of the bottom of the fixture, then you are talking about the whole tank being much closer than this, and the inverse square law won't apply.

The same would be true for a hood that contained mh lamps.

Further, this disregards focussing effects.

Do you not believe that I could construct a reflector that would put all of the light into a 1" spot 15 feet away from the lamp? I'm sure I could.

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Randy Holmes-Farley


[This message has been edited by Randy Holmes-Farley (edited 22 February 2000).]

 

[Anonymous]

Hey, I've heard Mega claim he was a technogeek. I wouldn't be surprised if he had a electron microscope and high powered liquid chromatagraph to analyze his tank and water!

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

 

[Anonymous]

The inverse square law has nothing to do with reflection or refraction. It is simply the expansion of the light to cover a large area. If the area isn't expanding, or isn't expanding at an inverse square rate, it doesn't follow that law.

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Randy Holmes-Farley

 

[Anonymous]

QED.

Thank you, Mega, for posting that data. It is exactly what I was saying: the data does not even come close to folowing an inverse square law.

Take line 1 of the data:

Distance: 10 20 40 80 120
intensity:10,800 5,300 2,400 940 530

Now look at the same data if it were an inverse square law (assuming 10 cm = 10,800):

Distance: 10 20 40 80 120
intensity:10,800 2,700 675 169 75

In case you don't care for my math, here is the conclusion in your own reference:

"Light intensity from a dual 40w shoplight is inversely proportional to the distance from the bulb."

That's not an inverse square law.



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Randy Holmes-Farley

 

[Anonymous]

Weheh serious stuff folks, hold on there till i really confuse matters,

Ill add my dubious 1/2 pennies/cents worth.

Wihout all the frills, in relation to keeping corals/other animals under a light source, the further away the light source, the less intense it is on the subject.

Source for this ground breaking finding.

My experience of seeing light emitted from a single source, lower in intensity on a focused point, the further away that light source is moved from the subject in question.

Heh, think of a torch shining on a particular point, move that torch further away, the ilumination of this item fades as you move further away.

Jaysus, that actually seems to be quite plausible, i must admit when i started i wasnt really that convinced myself. !!!

I think i need a rest.

Ah well what the hell.


Happy Reefin.


Goby 1 Knoby.

 

[Anonymous]

Mega and Randy;

You guys are both kind-of right. Neither of you is properly explaining what happens to light in our tanks, however.

Randy is right about the inverse square law. It has nothing to do with reflection, refraction, and transmission medium. It has everything to do with the dispersal of photons as a lightwave spreads over a surface area. His explanation is accurate. Focused beams have a different intensity loss curve over distance.

However, what goes on under the hood is much too complex for the inverse square law to be of much use. For one, there are lots of reflective surfaces such as the reflector, obviously, but also the insides of the hood, the surface of the water, the inside surface of the glass or acrylic in the tank, the rock, the sand, and many more. The reflector is more efficient, most other surfaces less so. Some of the light is reflected, some lost to heat, some escapes the tank, and so on. Second, the water itself has great filtering/refracting abilities depending upon its color, clarity, etc.

The bottom line is that in any given tank, the PAR at a particular depth is extremely dependant upon your particular tank details. The loss of intensity in most tanks is pretty drastic with increasing depth, but not simply because of the predictions of the inverse square law. It may be better or worse. Lowering the lights will increase intensity, generally, but not in any mathematically predictable way. Salt creep on the buld will drop it hugely, heat may be a problem, and so on.

Mega, go ahead and buy that PAR meter. The results should be real useful to you, but unfortunately, not to anyone else unless you start loaning it out.

-Jim

 

[Anonymous]

I agree with you, Jim.

Reflections inside the tank make lighting a complicated issue. Because of reflection from the front glass, for example, I have significant light underneath overhangs where there is no direct line of sight to the hood.

Going back to the original question of how high a mh lamp should be, I think one needs to address the question of what one wants in the tank.

If one wants some locations to have as bright of light as possible, lower is almost certainly better.

If one wants to light all areas of the tank, we need to recognize (IMO) that there may be situations where higher is better.

My refugium is a perfect example. If I put the single pendant within a few inches of the water, there is simply no light covering much of the refugium. If I raise it 18 inches, it just covers the water area, with no significant loss to the room. If I raise it still further, then there is a large spillage of light onto the floor.

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Randy Holmes-Farley