# Light pipes - is there a limit

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1. Mar 24, 2015

### Skeptik22

I have seen light pipes which "conduct" sunlight from an outdoor receptor to the dark recesses of buildings ... but they all seem to be awkward bulky metallised tubes.

Is it not possible to achieve the same effect with fibre optics?

Could one not focus the light at the receptor into a fibre.. and spread it out again at the appropriate point.

I have seen posts about the capacity of fibres to carry data .. but not about the capacity to carry simple white light..

What would limit the amount of light (lumens?) which could be transported?

2. Mar 24, 2015

### DaveC426913

The capacity to carry data is determined by the range of wavelengths of light and the rate of bit transmission. The amount of brute energy required can be arbitrarily small.

If you are attempting to transport light for the sake of illumination, you would want the energy to be arbitrarily large. I imagine you would quite quickly run into overheating problems and melt the fibres.

Calculate the wattage of a given area of solar impingement, then apply it to the cross section of an optic fibre.

3. Mar 24, 2015

### blue_leaf77

It's obvious that if one puts too intense light into a fiber, it will barbecue the fiber. So material damage threshold limits the amount of lumens. The disadvantage of using fiber optics is that first it's difficult to couple light into its core because of the micrometer size of it, while the position of focal spot from a fixed lens is moving according to the relative movement between earth surface and the sun. Secondly, for a given fiber dimension, not all frequency can propagate down the fiber, some of them will interfere destructively during the propagation. This is the concept of fiber modes.

Ah someone has been one step ahead of me.

4. Mar 24, 2015

### Skeptik22

5. Mar 24, 2015

### Skeptik22

Is there a number for "too intense" ?

My model in my mind has no heat only light .. only light ... if such a thing were possible ..

lumens rather than watts

6. Mar 24, 2015

### blue_leaf77

The number would depend on the type of material of which the fiber core is made. I guess the measure would be either in fluence or intensity. You can easily find related topics on the net.
There's no way one can address material damage without considering heating phenomena given the frequency of visible light is well below the ionization potential. Heating is caused by energy transfer from electrons after undergoing excitation to the crystal lattice.

7. Mar 24, 2015

### CWatters

Normally the diameter of the solar tube is quite large to maximise the amount of light collected. I think you would need a similarly large diameter lens. The focal length would need to be quite short otherwise the overall system will still have a large diameter over a significant length. Then I think you may also need more than one lens each end. The second one being a diverging lens to match the acceptance angle of the fibre? All sounds a lot more expensive that a polished metal tube.

8. Mar 24, 2015

### CWatters

http://www.limitless.uk.com/parans-solar-collector/

9. Mar 24, 2015

### DaveC426913

But polished metal tubes have severe physical limitations on their use. If the OP could solve the physics problem, he would have a truly innovative product.

10. Mar 24, 2015

### Skeptik22

My mental image has an optical fibre being the same as a glass window .. with rather more thickness and rather less area..

My "gut feel" is that a glass window will allow almost every lumen which falls on the window to pass through .. with perhaps a very small loss (assuming it is clean).

One wonders whether there is a limit to this ... is there a intensity of light at which the transmission characteristics change?

The fibre has the additional capability to reflect the light from the edges of the "window".. so again ..... is there an intensity at which this reflection is compromised?

11. Mar 24, 2015

### blue_leaf77

I'm not sure how to describe what you said there, but I'm assuming you are imagining the fiber core to be made of the same material as the glass window.
Not all frequencies can successfully propagate through the fiber. Fields that can propagate is called propagation modes and their behavior depends on the dispersion relation of the system, which in turn also on the shape of the medium. Glass windows have large propagation volume, so that we can assume the propagation inside windows to be similar to that in free space, and almost all frequencies can go through. However if you cut a small volume out of the window slab, the dispersion relation would depart significantly from the free space-like one.

yes there is, if the intensity is high enough, nonlinear propagation may come to happen. This will lead to the so called solitons. Increase the intensity further, and you will damage the fiber.

12. Mar 25, 2015

### Andy Resnick

Sunlight can only be focused to a certain limit due to the non-infinitesimal size of the source.

http://en.wikipedia.org/wiki/Nonimaging_optics

Typical fibers have a numerical aperture of about 0.1 (f/5), and this sets a limit on how much light can be coupled into a fiber bundle. (conservation of etendue). And then, since fiber is is highly dispersive in the visible, a lot of light is going to leak out along the length.

13. Mar 26, 2015

### CWatters

Did some of you miss my post #8 which links to an existing product that appears to do exactly what the OP proposes.

14. Mar 26, 2015

### DaveC426913

Never let reality stand in the way of a good discussion.!

15. Mar 26, 2015

### blue_leaf77

I guess that technique will require the light collector to be able to track the sun's movement. I don't know the case with metal tube though, if it also requires a movable mounting.

16. Mar 26, 2015

### Khashishi

The lossiest parts of an optical fiber are the ends, so these are the points that are most likely to overheat. Making some gross order of magnitude guesses:
max temperature of light pipe (made of some kind of plastic) ~ 40 C
ambient temperature ~ 30 C
heat transfer coefficient (very rough) ~ 5 W/m^2/degC
max cooling capacity ~ 100 W/m^2
end losses of light pipe ~ 5%
max light flux ~ 100 W/m^2 / 5% = 2000 W/m^2

This is just a gross estimate, so I wouldn't be surprised if it's off by an order of magnitude. That's not much more than natural solar insolation, so you couldn't concentrate light much before you ran into heat issues, assuming plastic fibers.

17. Mar 27, 2015

### Skeptik22

Thanks for the link .. not sure how I missed that..

My question was more about the theoretical limits of such a system rather than looking for a specific product ..

was most informative.

Perhaps I should have asked 2 questions ....

1 Is there a maximum intensity for visible light? This seems to be covered by Andy Renick's reference

"Imaging optics can concentrate sunlight to, at most, the same flux found at the surface of the sun. Nonimaging optics have been demonstrated to concentrate sunlight to 84,000 times the ambient intensity of sunlight, exceeding the flux found at the surface of the sun, and approaching the theoretical (2nd law of thermodynamics) limit of heating objects up to the temperature of the sun's surface.[12]"

2 What limits the intensity of visible light which can be passed through a single optical fibre?

I am having a problem getting my head around the repeated reference to heat... Does visible light always include a (large) heat component? Or are we saying that the losses in the fibre are such that a large proportion of the visible is converted to heat?

18. Mar 27, 2015

### blue_leaf77

Heat transport from light to a material is a common and everyday phenomena, we all know that a slab of window will get warm during the day. Related to our problem, the optical damage in a fiber or in any transparent material is more into light-matter interaction rather than just a specified topic on fiber. The heat induced in a material upon radiation by the light comes from the absorbed portion of the incomng light by the material's atoms/molecules. Absorption becomes very high when the incoming intensity drastically increases. You could have found more related topics such as these:
http://www.nktphotonics.com/files/files/Application_Note_-_Damage_threshold_of_fiber_facets.pdf [Broken]
if you had done some online research. Those are more technical and probably mostly talk about pulsed radiations. If you want a more advanced read, I suggest this: http://opticalengineering.spiedigitallibrary.org/article.aspx?articleid=1813808.

Last edited by a moderator: May 7, 2017
19. Mar 27, 2015

### Skeptik22

Is this is question of imprecise language ... my original question was (or should have been) about visible light.. ie that part of the spectrum which is necessary for us to be able to see ... is the heating of the window not down to parts of the spectrum which may be classified as heat?

I am not sure about the references you quoted .. the first seemed to be talking about "hot electrons" - which is probably a topic in itself.

I had the impression that the energy of a photon would only be given up (converted to heat) if it were absorbed... and optical fibre specifications always seem to claim very low absorption rates.

20. Mar 27, 2015

### Skeptik22

This was interesting

http://www.researchgate.net/post/Can_a_optical_fiber_coupler_2x1_4x1_8x1_be_used_as_a_power_combiner [Broken]

I think the consensus was that it was possible to have a Y coupler such that if you had 2 inputs of x lumens the output would be >x lumens nut less than 2x ... which would seem to be what one would expect.

The thread seemed to dwell on the fact that there would e losses and that 2x was impossible... But it was far more interesting that the output would be more than x.. say x(1+n)

This would imply that one could build a tree of such couplers, with say Q levels .. and end up with an intensity of x(1+n) to the power Q

so I think my question is still is there a limit to this increase?

Last edited by a moderator: May 7, 2017