Disproving the Inverse Square Law Part II

I started a thread a long while ago about this, but here it is again as I believe I have more concrete "evidence"

first:

http://en.wikipedia.org/wiki/Inverse-square_law

explains the law in detail

This diagram shows how the law works. The lines represent the flux emanating from the source. The total number of flux lines depends on the strength of the source and is constant with increasing distance. A greater density of flux lines (lines per unit area) means a stronger field. The density of flux lines is inversely proportional to the square of the distance from the source because the surface area of a sphere increases with the square of the radius. Thus the strength of the field is inversely proportional to the square of the distance from the source.

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that is from the caption of the picture...


Notice how it says that it is BASED of the formula for surface area of a sphere... as you double your distance from the OMNIDIRECTIONAL source point, it makes sense that the light given off from the point would be reduced by this formula because now it has to cover a larger surface area (based on the sphere surface area formula)


Every time I see a indoor grow on this site, with a hood / reflective material on the wall / and the light source way up high you get people saying "oh man lower the light and you'll double your output"


The above ALONE disproves this completely.

yeah you have your source point S, but the thing is, you have a hood reflecting all those flux lines back towards the plant. then you also have all those flux lines bouncing off your reflective side walls...

A greater density of flux lines (lines per unit area) means a stronger field.

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we are REFLECTING those flux lines with the hood, the reflective walls, etc...



Now, the only thing that is diminishing the strength of the field, is the loss of energy from reflecting, and distance travelled THROUGH the medium (air)

or another way,

the hood and reflective walls in a room of LxW will keep the density of the flux lines CONSTANT given infinite distance (Height)*



* again, energy lost from traveling through the medium (air) and reflecting off of not perfectly reflective walls will diminish the returns. also from the link:

The total number of flux lines depends on the strength of the source and is constant with increasing distance.

so we will still lose energy if they are higher up, but it is NOT based on the inverse square law...


Discussion welcome!

I'd like to hear others thoughts.


EDIT:

shine a laser in your eye at 2 ft, and it's bright as hell, shine a laser in your eye at 20 ft and it is still bright as hell. (not 100x less bright)

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You're comparing apples and oranges here. From the article you posted:

In physics, an inverse-square law is any physical law stating that some physical quantity or strength is inversely proportional to the square of the distance from the source of that physical quantity.

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The article is discussing inverse-square laws in general, not a single example of an inverse-square law. Within the context of flux what you're saying is true, but it has nothing to do with the inverse-square law in terms of light. The article does describe how light works lower down, under the heading "light and other electromagnetic radiation". Here's an excerpt:

The intensity (or illuminance or irradiance) of light or other linear waves radiating from a point source (energy per unit of area perpendicular to the source) is inversely proportional to the square of the distance from the source; so an object (of the same size) twice as far away, receives only ¼ the energy (in the same time period).

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So, when you're talking about light the article states that the inverse-square law is exactly as it is used elsewhere on this board: twice the distance, one quarter the light.

Edited to add:

I forgot the laser thing. Pointing a laser at your eye from varying distances doesn't really prove anything. You'd need to use equipment to measure the drop off in intensity over distance.

that square law you are talking about lower down is still considering a point as an omnidirectional source... which means the particles / waves being emitted from it are going everywhere...

we are still reflecting the light waves (flux lines) back to the plant...

For example, the intensity of radiation from the Sun is 9140 watts per square meter at the distance of Mercury (0.387AU); but only 1370 watts per square meter at the distance of Earth (1AU)—a threefold increase in distance results in a ninefold decrease in intensity of radiation.

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the above quote was from the same example area, and again, the sun is considered an omnidirectional source point.


Example:

say we take a source point and put it in in a tube that has a 1 meter radius...
the inside surface is 100% reflective, AND its in a vacuum, so there are no other particles for the photons to collide with and lose energy.


in this perfect tube, we could extend the length to infinity and we would still get the same intensity, since intensity is based on the amount of flux lines, which is based on the strength of the source point to begin with.

the picture quote said it best:

The total number of flux lines depends on the strength of the source and is constant with increasing distance. A greater density of flux lines (lines per unit area) means a stronger field.

right there it says "flux lines are constant with increasing distance"

but say a 400W bulb, with say 40000 lumens produces 40000 flux lines where 1 flux line is == 1 lumen.

since a light source (our bulb) is a "point" thouse 40,000 flux lines radiate from the source in every direction...

if you take a reading at say 1 ft from this point source, you will get X flux lines, and if you take a reading at 2X, you will get a reading of 1/4 flux lines, not because each line itself reduced its strength by 1/4, but because at double the distance the density of flux lines is 1/4



I'll post some diagrams once I can find / create them\


EDIT:

remember, light is no different than other electromagnetic waves, they are still transmitted via a particle (photon)

light acts as both a wave and a particle.

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I think you're right green_tea. As soon as you get your 100% reflective hood, walls, floors and any other exposed surface grow room built, post some pictures will you?

robbiedublu said:

I think you're right green_tea. As soon as you get your 100% reflective hood, walls, floors and any other exposed surface grow room built, post some pictures will you?

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haha, im not saying that all that is possible

but if your hood is 95% reflective, and your walls are 90% reflective...

and we knew how much intensity is lost when going through air...

you aren't going to have 1/4 the intensity when you double the distance...


for added evidence:

http://www.icmag.com/ic/showthread.php?t=51325

(that's the aircooled hood testing with meter thread)

In that thread you have the same light putting out 2250[units]
at 18" away from the light...

at 21" you have 2076[units]

at 24" you have 1918[units]

some quick math:

2250.........x
------ = ---------
1............1/4

X = 570

that is what the intensity (relative to the #'s above) should be if you go from 18" to 36"

problem is we are going from 18" to 24" so we need to find a ratio that can correctly relate the difference between 18" -> 36" and 18" -> 24"

18/36...........1
-------- = ---------
18/24...........x

X ends up being .6666666


now:

2250 X 2/3 = ~1500

no where close to the 1918 that was measured with the meter

** ignore the ....'s they are used as spacers since there are no

Code:

 tags
[/quote]

EVEN BETTER (for us) is that the ratios i used to get this will only work for the first doubling of distance, as the inverse law is not linear like the above equations, but exponential


[url]http://unificationproject.blogspot.com/search/label/C%29%20Inverse%20Square%20Law[/url]

(second image)  

* first image helps explain first post better...

for the non clickers:

[quote]
Any point source which [B]spreads its influence equally in all directions[/B] without a limit to its range will obey the inverse square law. This comes from strictly geometrical considerations. The intensity of the influence at any given radius r is the source strength divided by the area of the sphere. Being strictly geometric in its origin, the inverse square law applies to diverse phenomena. Point sources of gravitational force, electric field, light, sound or radiation obey the inverse square law. It is a subject of continuing debate with a source such as a skunk on top of a flag pole; will it's smell drop off according to the inverse square law? 
[/quote]

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Hey green_tea, it's late and I'm not firing on all four cylinders, so I'm not going to directly reply to your last post right now, but I do want to say that it's great you're questioning something that most folks take on faith. If everyone just agreed with everything they saw on the boards we'd all be in big trouble. It's good to ask questions.

That said, sometime tomorrow I'm looking forward to finding whatever holes there are in your logic

This is why I use the cone shaped hoods with base up bulbs, they let the light spread out horizontally instead of downward. This is all they sold back in the 70's and they are still the best. Even with these its still not perfect but a much better light distribution. . . 70's rule

DangerP said:

I forgot the laser thing. Pointing a laser at your eye from varying distances doesn't really prove anything. You'd need to use equipment to measure the drop off in intensity over distance.

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The inverse square law applies to a point source of light -- one that radiates in all directions. Laser light is coherent, emitted in a narrow beam, so the inverse square law does not apply.

But for any point source of light (which our bulbs approximate) the inverse square law absolutely applies. It's the law! However, as you point out, there are other factors in a grow room that the law does not take into account. For example, reflections.

So yes, in a typical indoor grow setup things are more complicated than just the inverse square law. In fact, with all the various imperfectly reflecting surfaces it gets very complicated. All setups are different, so there's really no way to know how much light is reaching any given point without actually measuring it. Since very few people can or will do this, it's best to assume that with distance from your bulbs light energy spreads out and decreases. Probably less than you would predict based on the inverse square law alone, but the actual amount isn't that important. The point is, if you train your plants and try to create an even canopy with the bulbs as close as possible, and use reflecting surfaces where appropriate, you will insure your plants are getting the most possible energy from your bulbs.

The square law only applies to a puntual source of light emiting with same intensity on all directions. HID bulbs are a close aproximation to it when they are unreflectorized (often in vertical grows: the only situation where the square law may be applied with some sense).

As green_tea explained, its related to pure geometry. A sphere increases its surface by the square of its radius.

But when a bulb is reflectorized, light isnt distributed along a sphere, but in the worst case, along a semisphere. Meaning the light density with fall on a given unit surface not drop for the square of the distance, but, in worst case, for half it.

And that would be true only in the case the light were distributed evenly along that semisphere. But it not happen never. Reflectors, depending of its geometry, gives diferent intensity in each direction, so the rate at what the light density drops depends of the direction from the bulb.

But, let apart theory. Just check it. Its very easy, a cheap lightmeter (luxometer) measures light density. Tie a wire or a a rope to the light socket and put the lightmeter at the end of it. Moves the luxometer along the space below the bulb, and notice how reading varies strongly depending of the position: the light density drop is different in each point of the space at same distance from the bulb.

Now double the wire lenght and repeat measurements. Check how light density drops mostly between 1/2 and 1/4 of the square of the distance. (but it still varies depending of the point of measurement)

If you dont know if a theory works or not in your situation, just check it. That way there isnt any doubt. You dont need to trust anyone opinion, you can know it for yourself. In this case, very easyly.

Its curious how extended myths as this (the square law) survive to its obvious falseness, when its so easy to check that it dont work for reflectorized lamps. And it dont work never with floro tubes or CFLs, BTW.