MH lighting is a myth

[bmw]

Well--I am not sure.
But I thought that would get a few peoples' attention.

I cannot figure why Mh bulbs(250 watt) at 16000 lumens at 8" from the water surface are preferred over vho at 3500(110 watt) lumens at 3" over the water surface.
Given that light from a point deliveres energy at the reciprocal of the distance squared, why would one prefer MH at the distances given above? DO I have wrong data?

This one is not an experience or opinion question folks. Just looking for what I must be missing in the physics department.

And I know I am talking about photometric measurements. Lets keep it at that for now, and worry about the radiometric measurements later.
b.

[billsreef]

It's the difference between a point source (MH) and a non-point (difuse) source (FLO).
The diffuse flo. lighting looses intensity very rapidly with the increase in distance from the source, compared to the fall off in intensity from a concentrated point source of MH.

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Bill

If damsels grew as big as sharks, the sharks would run in fear!
My dive photos
ICQ 56222784

[bmw]

Sorry Bill,
that doesn't fly.
b.

[horge]

Won't comment on the photometric angle yet, I'm Mr. Natural Light, and so may not be qualified. Maybe you're misreading bill's comment?

Photometry aside, positioning your lights ala MH allows for pendant mounting --servicing your tank is a breeze compared to having a durn hood to flip up or lift off.
I'll shut up now

[Psyduck]

Ok I dont know the answer to this question, but a VHO bulb is not a point source is it? I mean MH and VHO are 2 different designs the MH being a near point, and VHO being a length across the entire tank? I mean arent we comparing apples to oranges here? Just a thought, please dont yell at me .

[Larry M]

Hah! That subject line is pretty funny. Don't tell me we have another "I-don't-want to-spend-the-money-so-I-am-going-to-re-invent-the-(lighting)-wheel-again-and-go-a different-route reefer in denial????


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Larry M

See my tanks at Northern Reef

It's called the inverse squared law, double the distance, 1/2 the power, this is why I run my halides several inches from tank, as close as is possible. Vho spread this light out the lenght of the bulb, halides concentrate it in one point, I run a combination of both, vho simply dont cut it for the acro's that I keep (yes have kept many of my corals in different lighting) L8r mega

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[bmw]

Horge-
Getting you to shut up? Who has ever been able to accomplish that?
Psyduck,
I would never dream of yelling at you!
Bill,
I apologize, perhaps I replied too quickly.
Teacher in me, easy to forget I am the student here.
So--
No, we are not comparing apples an organges here. Or ease of one or the other. Just light delivery to the water suface.
I believe(and you can correct me here) what you all are referring to is the fact that MH lighting comes from a smaller surface area outside the enclcosure(bulb) than a tube.
Correct.
However, since light travels in a linear direction----
Intensity at a perpindicular angle for MH would be greater? But at greater and greater angles would, it in fact be less?
There is some serious math here, yes?
b.

[NorthCoast]

BMW,

I'm not a lighting expert...someone will write a text book answer. I see your point. However, your linear assumption with the FO and VHO bulbs is wrong to a degree. A tube light is brighter in the center than on the ends. I don't have a formula in front of me to quote (and it depends and length and manufacture). It is that drop-off that you have to consider in the equation. It is not as easy as dividing the watts (or lumens, or whatever) by the length to get a constant value across the length of the tube.

A non-scientific answer FWIW,
NorthCoast

[bmw]

Whoa,
missed 2 replys while I was conversing with the wife.
No, Larry, money ain't the question here. No more than someone a few years back saying they don't need them wet-dry filters.

Megadeth,
Thanks, knew what it was called.
In fact, halides do not concentrate light on one point, and even given that the source is a single point, the envelope (glass housing) negates the "no diffusion" angle. You are trying to compare a single point placing all of its energy at a perpindicular angle to a source with infinite points radiating at perpindicular angles. MH does not radiate all of its energy at a perpindicular angle.
And thanks for the "experience" but not looking for that here.
Now if you are putting your MH 3" from the water surface--well that negates the whole question. There is no question of the fact that MH put out more light energy.
My question was--why do you think that putting MH 8" from the surface was more intense than putting VHO 3" from the surface.
b.

well, your question is why would one prefer metal halide 8" away vs vho 3" away, simple, halides output MUCH MUCH more light. Back to the point source, halides are a 2" x 1" tube that produces light, encased in a UV shielding outer envlope. Vho (what vho produces 3500 lumens, 48"?)and put out that light over 48"x3". THe light radiates from the source in all directions in both cases. I dont fully understand the impacts of the inverse square law, however, your trying to compare apples vs oranges right now, halide at 4" vs halide at 8" would be a much better question. Seriously, that's 3500 lumens / 48" = 72 lumens per inch, vs 8000 lumens per inch of halide. Really are 2 different topics in your question that need to be discussed separatly.

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Ok, using inversed square law, E=I/D^2, and not touching on Lamberts Cosine law (yet, I can do this, once I setup some senarios)

16,000 lumen halide 8" over water, with coral 12" under water will recieve 40 lumens

vho 3500 lumen 3" over water, with said coral 12" under water will recieve 15.55556 lumens. This is why we use halides for intense light.

I was bored and threw some bulb lumens into excel and played with some numbers, fwiw here's some I was able to find.

250w iwasaki 18200 lumens after 100 hours
96w pc 8250 lumens (per custom sea life) (update, freind just tested his and gets a MUCH lower number, 5220 lumens)
If you have the 96w pc & 96w blue pc 3" over the water, the lumins at a coral 12" under the water is 42.38. If you put the 250w iwasaki 8.75" above the water you will get the same, 42.27 lumens at the coral. is this the data you were after? Fwiw I'm trying to find measurements for vho and vho actinic lumens, and for some of the other halide bulbs. L8r mega

[This message has been edited by MegaDeTH (edited 02-19-2000).]

[This message has been edited by MegaDeTH (edited 02-19-2000).]

[This message has been edited by MegaDeTH (edited 02-19-2000).]

[Wolverine]

I always thought that another of the big bonuses of point lighting is the shimmering effect. Now, this is all trying to remember from things I read a couple of years ago, but I think I have it right. When MH light goes through the waves at the surface, it gets focused into bands of more intense and less intense light which move across the corals (and everything else). I know that some feel this alternation of intensity is very important. And I know that the more intense bands end up delivering much more light to a given spot during that time than without them (I can't even begin to remember how much of an increase).

I also know, and can understand, why this happens with MH but not with tube lighting, but there's no way I could even begin to explain, especially mathematically, and especially of the top of my head. And visually this is far more striking (makes me wish we could have a MH with our current system). So, at least as far as this effect is concerned, it would be better to have the MH 8" away rather than the VHO 3" away from the water.

Buy anyway, I guess this is just a different angle that I wanted to throw into the conversation.

Dave

ok, I've been looking at lamberts cosine law, trying to figure out what role it plays in our tanks, with reflectors it shouldnt factor too terrible much, at most 60 degree's is a 50 % loss (that was the easy one hehe) But considering it's a point source, but not a flashlight directed beam, directed in all directions, it doesnt factor in too much, or else my logic is wrong (trying to picture this in my head) However, the inverse squared law is different for a coral 12" down, and also 12" to the left of a halide, a senario that wont happin with a vho, as they emit light along a 48" source. Or am I way off on what your looking for? L8r mega

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[horge]

Sigh. You were right Bruce. Can't shut up
Can I try to pick your mind?

With the math for your initial figures quickly put to sleep, it's easy to see your puzzlement.

On the nanolevel, apples and oranges are the same, as even 'non-pointsource' lighting is composed of conglomerations of pointsources. On the nanolevel, all radiated light (whether rays, waves or packets, all you Planckian activists..) is shooting out of the source at a 'perpendicular angle' anyway.
(Whether halide element or fluorescent phosphor, at least 50% percent of your lightsource is firing in the wrong direction.) Again on that 'nanolevel' you've pretty much erased any other answer than:
"It ISN'T better to have the specified MH at 8 than the specified Fluo at 3". Light bending would seem to be less an issue with fluorescents as the phosphor is right smack up against the glass, whereas an MH element can be less than centred within the bulb, and at the levels of output we're discussing, it's an insignificant factor.

So let's get to the real question I could think you're asking: "What structural and personal factors go into preferring MH over Fluo. despite --lookit!!-- my figures? "

If you suppress the macrolevel factors that differentiate apples from oranges, you're left with energy efficiency/spectrum rendition (one issue, really), footprint (in terms of installation space and energy broadcast), and cost-efficiency/lifespan as the bones of contention. And, oh yes, the facilitation of easy tank maintenance

The importances of the first three vary for issues of relative personal wealth, a prevailing lack of knowledge about lighting requirements, and the variables inherent to different tanks and locations. The fourth is only slightly less so. There are bound to be others.


[This message has been edited by horge (edited 02-19-2000).]

[ASH]

NorthCoast,
One correction. One of the characteristics of a fluorescent lamp driven by an IceCap VHO ballast is that the light is more evenly distributed across the lamp. Conventional ballasts are up to 1/3 brighter in the center. That a major reason why some of our ballasts find themselves in indusrial application providing light for inspection machines.
Andy

[bmw]

Hi Megadeth,
thanks for all that--yes, that was the direction I was headed. couple of things--
You are treating water as air so I am not sure the calculations can apply(I know, we are treating air like a vacumn in the first place)
Second, yes, the characteristical differences between a 48" tube and and a 2" one does complicate comparison.
Third, calculate lumens hitting the water surface and the advantage seems to go to vho.Which I did comming home a couple of days ago --which prompted this whole thread.
Fourth, an accounting for the absorbsion of the longer wavelenghts in water would have to be accounted for(and I admit the original post said photometric measurements--this was why-to advoid this point for a while)
Fifth, I'm sick as a dog and going back to bed
Thanks again for putting in the time.I would like to carry this on awhile if you got any interest--but I am signing out for awhile.

And Horge, looking at your "plankian" comment--you know some quantum mechanics that might define things a bit?
b.

[Gannet]

The reason this looks confusing is because of the numbers chosen.

For one thing, we don't care what the illumination at the surface is. We care what the illumination at the coral is.

Let's make some assumptions to simplify the math: 1) assume the MH is a point source, 2) assume the FL is a linear source with even distribution, 3) assume our reflectors are 100% efficient, 4) assume the area we are illuminating occupies the equivalent of a 4x4" square, 5) assume the area of interest is 8" below the surface and centered under the light, 6) assume 2 VHO tubes to make it a little fairer for the VHO, 7) assume other numbers you mentioned, although why you have the MH so high is beyond me. I run mine 2 3/4" off the water.

So:

If the MH is 8" above the water and coral is 8" down, we have the light going over a sphere with a 16" radius. Using the formula A = 4 * Pi * R^2, this will have a surface area of 3217 in^2. But since we assume the reflector is 100% efficient, the area is half that, or 1609. With 16000 lumens we have 9.94 lumens/in^2, or 159 lumens on our area of interest.

If the 2 VHOs are 3" over the water and coral is 8" down, we have a cylinder with an 11" radius that is 48" long. Using the formula A = L * Pi * R^2 this will have a surface area of 18246 in^2. Assume the 100% reflector again and the area is halved to 9123 in^2. With 7000 lumens we have 0.77 lumens/in^2, and our area of interest gets 12 lumens. Less than 10% as much as the MH.

Pretty big difference, no?

[This message has been edited by Gannet (edited 02-19-2000).]

[npaden]

Okay, the math went right by me but one thing missing is # of MH bulbs for the length of the tank. There is no doubt that a coral smack dab under a MH is going to get way more light that one right under anywhere on a VHO. The key is length of tank and a combination of VHO & MH. I am not sure but don't think that 1 250W MH in the middle of a 4' tank will get near the light to the sides of the tank as 2 - 110W VHO's will. I think the old standard 1 MH for every 2' of tank With VHO supplementation is the way to go for almost everything. The math just shows how important placement of the high light loving corals is. (centered under the MH's) Oh well, thought I would throw in my thoughts on the subject. Nathan

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[TheDonger]

My two cents.

You are trying to emulate the sun. Now it is hard to compare a light bulb to the sun, but Halides do it much better.

First, the fact of the matter is, halides are point source lighting. The filament produces the light inside the charged glass container. Light is coming from the mainly light filament which is a very small source. Flourescents spread light across the entire tube by the gas that is filled in the tube, spread out greatly. The sun is a point source light. Placement of halides over corals is not as an important concern as some may lead you to beleive. The fact is, the angle of the sun as the Earth rotates constantly changes, so corals are always experiencing different point sources of lighting and reflection, just like in your tank, sort of anyway.

Furthermore, the ripple effect, just like the sun creates in the ocean, is emulated much more accurately with halides than flourescents due to this point source lighting and intensity.



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Visit Keith's Reef

[bmw]

Thank you all for your comments.
I do not think using quadratic equations are going to be a good model for this. I suspect a calculus equation involving some trigometric functions is probably the right direction.
I was kinda hoping someone that deals in this area, say Andy!, might give a reply.
Andy--how do your measure light output from various types of sources when you engineer your ballasts? What are the implications for aquarium use?
And does anyone know of readings being done underwater at the depths we are interested in? I know somewhere way back I saw some on depths in the ocean with sunlight. But how about using the artificial sources?
And finally, the lumen figures in the original post were just off the top of my head as a place to start. But the distance over the water figures are from what I have observed. Now I have seen two people say they are putting MHs much closer to the surface.
I had thought that putting MHs this close was asking for problems with water and fish comming into contact with a very hot piece of thin glass. I would like to here more from you on this.
Thanks alot folks.
b.

[Canadian]

Someone is going to have to find out about the diffusion properties of water as well. If we want an accurate calculation we're going to have to apply the properties of water and not assume that the water behaves in a similar manner to air.

In addition, the turbidity of the water is key as well. FWIW, In the Baensch Marine Atlas Vol-1 Baensch has a half-assed calculation for "required wattage" and there is a table for "pollution factor". At 30cm 100% Clear water has a "polution factor" of 86 (Obviously not a particularily profound number).

Just trying to see that this question does get answered to bmw's standards, as I am quite interested in the physics involved here as well. I just want to see the raw calculations not "Metal halide is good."

All in all, great question bmw!

Andrew

[ASH]

I'll add some less technical notes to this number crunching thread. Comparing a MH light source to the sun is interesting but not perfect. In the ocean if two objects are 6 inches apart they'll both be equally lit. In a tank where the point source is a foot away one object may be fully lit, the other only 1/2 lit. Point source in a tank = shadows, linear source = relatively no shadows.

Small correction TheDonger, when you say the "filament produces the light". Before you turn your MH on check it out. The arc exists without a filament. I also agree, the ripple effect is great and closest to what you observe snorkeling.

As far as measurements I'd look to someone like Richard Harker. He describes his methods in a linked article on IceCap's MH page. When you add water penetration to the mix you appreciate what started the 10K+ movement, as this is the spectral portion of light that goes deepest when full spectrum light is observed.

Andy

[PhishHead]

canadian...my thoughts too
does the inverse square law apply at the same ratios for air/water
i still like my halide. law or no law, corals like it 10x more than the no/pc's i had before.

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"Keep It Clean"

[BristolWorm]

I have to agree with Megadeaths analysis.

Water/Air ok fine the numbers might be different but both the MH and the VHO's light is going through the same medium (it has to for this to be a valid discussion).

Second I think that a Luxmeter is what is needed to end this debate.

Ok... to add to this mess, is lumens what should be measured to determine the most benifit light has on coral. And which types of coral? Does UV play a factor? What about alklinity in coloring and water that has more undissolved or dissolved organics than other water. All this will have an effect one way or another.

Thanks Megadeath for the math. I agree with your analysis. And I too run MH with supplimental lighting. IN MY EXPERIENCE it is the way to go.



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BW

'Semper Ubi Sub Ubi'