MH lighting is a myth

[MegaDeTH]

"While they may be useful for comparing one distance from a light source to another distance" That's all I'm doing, the only numbers that have any usefullness in documenting coral requiremnts are par, I wouldnt and wont use lux, as par is MUCH better. ALL I am doing is showing how much difference a increase in distance make, I have par for my bulbs, which is a much more usefull number as I said above 5 times. L8r mega

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members.xoom.com/megareefs
Yup, new url, finally thumbnail index html's on the folders, and a streaming webcam!

 

[markrb]

This post made my head hurt!

I never had any idea that there was so much to this. I never even considered such things. I guess in my naive way I always assumed that watts=watts give or take. If I hadn't already purchased my VHO ballasts I would have reconsidered.

Thanks for the entirely confusing and way above my head info. Anyone want to know what how many cpu's can go into a Sun 450?

Mark

 

[jameso]

Just FYI guys, the inverse square law does not apply if you use a parabolic (or anything BUT a flat) reflector.

The inverse square function is derived from the surface of a spherical shell (integral of a circle over 2pi).

In any case, the inverse square law is based on the fact that light emmitted from a point source "spreads out" in ever increasing spheres from the point.

Put a reflector next to the lamp, and the rule does not apply.

In fact, if 100% of the light (ideal of course) is directed into the tank, then 1/R^2 goes to 1/1.

Cheers
James Wiseman www.reefs.org

 

[ATJ]

Mega,

I guess one of the points I was trying to make was this: your calculations are OK for comparing the different light initensities for one light source at different distances, however, I'm not sure if the comparisons from VHO to MH are accurate. That is, what you are saying is perfect to get the point across that the closer the lights are to the water surface the better. I don't think you can use it to compare one light source to a different light source.


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ATJ
http://atj777.tripod.com/

 

[ATJ]

James,

The inverse square rule applies whenever the light rays are not parallel. Only when they are parallel will there be no dispersion. If the rays are near parallel, the intensity may be brighter and any point away from the light source but it will still be one quarter as bright at twice the distance.

Even if you had a perfectly parabolic reflector, with the light source at its focus, the light emitted from the near side of the light source would disperse.

With any other reflector all the light disperses. It doesn't matter that the light does not disperse evenly.

It is important to note that the distance from the light source to the reflector will make some differences to the calculations, but if the reflector is fairly close, the differences quickly become insignificant.

In your ideal case, unless you have 100% of the light directed to a spot that is exactly the same size as the light source (i.e. a point), there will be dispersion and so drop of in intensity as per the inverse square rule.


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ATJ
http://atj777.tripod.com/

 

[jameso]

A reflector can and is designed to take 1/2 the photons emmitted from a pt source, and send them all out on parallel paths. This is how reflecting telescopes work, only backwards. I'm pretty sure the function is parabolic.

I agree w/ you that half the photons will still disperse in a hemisphere away from the pt source.

Now that I think about it, I think that you may be right about the dispersion relation for light waves, just because of the way we model them. We assume that a wavefront is an infinite line of emmitters...Ok, I don't want to try to explain it here, but the inverse square law may still apply, but w/ modifications due to the reflector. Interesting.

Cheers
James


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James Wiseman
www.reefs.org

 

[Gannet]

The inverse-square law still applies, even with a reflector. Nominally, all a reflector does is maximize the amount of light available by redirecting light that would otherwise be lost off the "back" of the light. Yes, you could have a focused reflector that does a lot more than that, but is it still a "reflector" at that point, or a lens? If I could focus the output of an EYE 400 to an infinitely small point would it be infinitely bright?

In any case, the reflector has no effect on inverse square. Imagine the emitter, instead of being an assumed point or line, was instead a plane parallel to the plane of measurement. It doesn't matter how the photons got to the backside of the plane...once they emit from the front, inverse-square applies.

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fwiw, imo, ime, ymmv, etc.
when in doubt, change water

 

[bmw]

The function is hyperbolic.
The so called inverse square law is a model for the dispersion of light in a three dimensional space. It assumes linear direction at infinite angles. It works well for short(say under a few light years) distances. Einstein developed some models, though the mathematical ground work had preceded him by 40 years, that showed a different geometry--where parallel lines do in fact meet.

It (inverse square law)incidentally coincides with the geometry of a sphere if you consider a point source. Doesn't have to.

I am not aware of anyone actually demonstrating parallel lines of light from an artificial source. A laser is a closer approximation that had previously been achieved--but even in the best applications it will show dispersion over enough distance, indicating non parallel lines. The more finite the angles, the less the rate of dispersion,

b.

 

[ATJ]

<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by bmw:
It (inverse square law)incidentally coincides with the geometry of a sphere if you consider a point source. Doesn't have to.
<HR></BLOCKQUOTE>

bmw,

This is a very good point. The inverse square rule is more corelated to the geometry of isosceles triangles and areas being a function of the square of the sides or radius.

You could just as easily represent it with a cone or even a pyramid.

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ATJ
http://atj777.tripod.com/

 

[Martyn]

WOW
Only just found this how come these sorts of discussion I have not come across.
just had to refresh printed it all
Martyn