liquid core light guides :: piping light around corners

This is a guest post by Victor Babbitt – thank you Victor for sharing your work with us.

Light is much harder to handle than electricity. How do we transfer large amounts of light over distances and around many corners? Fiber optics is one method of transferring light around corners efficiently, but using solid fiber optics to transfer large amounts of light is very expensive, and losses at the point of light injection are very high. If we could transfer large amounts of light around corners, we could capture the sunlight striking the roofs of office buildings and use it to light the interior offices; natural, free sunlight, better for the eyes and better for the environment.

A very early experiment of a neon sign replacement. Tygon SE-200 tubing, mineral oil and rutile TiO2 scattering agents. Low power LED light sources

What about high power LEDs? Today’s high power LEDs are far more efficient than incandescent bulbs, but their pinpoint brilliant brightness is hard to use. Can we create glowing panels using LEDs emitting smooth, even light across their surfaces, tunable to any color you would like, to match your décor or your mood?

To meet these challenges, I developed Liquid Core Light Guides, basically fiber optic type light transport, but far more efficient and able to transport huge amounts of light. I also developed methods of embedding LED dies directly into these liquid core light guides, avoiding optical insertion losses, and developed methods to spread high power LED light into a smooth, even glow.

In 2003-2007 I spent about 2500 hours and $70,000 developing these technologies, but never found a clear path to a profitable business. I am now publishing some of this work on OpenMaterials.org, in the hopes that someone out there will be able to expand on this work, and find useful applications for it. If anyone knows of a high-value application where this technology might be useful, please let me know.

How to Transport Light Around Corners and Over Distance
If you want to pipe light a long distance, a pipe with an inner mirror surface might seem a reasonable answer. However, mirrors are not very efficient at reflecting light. A mirror will typically absorb or scatter 5% of the light striking it. Almost all the light that enters a long reflecting tube will be absorbed as it travels along any distance, reflecting dozens or hundreds of times, losing 5% with each bounce.

Example of Total Internal Reflection in a Tube or Panel.

However, nature has a better mirror, the phenomenon of Total Internal Reflection (TIR). Total Internal Reflection occurs when a light ray is traveling in a medium of high refractive index, such as a transparent solid like the silica glass used in fiber optics, and strikes an interface with a medium of lower refractive index, such as a fluoropolymer, at a low enough angle. In this situation, the light ray is reflected back into the high-refractive index medium without loss, the interface forming a perfect mirror. In this way, a fiber optic cable can bounce a light ray down its’ core for miles, reflecting millions of times, with little loss.

For a light ray to stay within a fiber optic core, it must reflect at a low enough angle. This angle is determined by Snell’s law: ni * sin Өi = nr * sin Өr, where n is the refractive index of the material. The critical angle that a light ray must meet to be reflected losslessly by TIR is sin ӨC = nr / ni .

Fiber optics are wonderful for sending light-based communication signals for miles, but are far too expensive to use to transport large amounts of light. A huge diameter fiber using Silica glass would be heavy, expensive and prone to instant cracking. Acrylic fiber optics of large diameter have been developed, but have very high attenuation over 10’s of meters, and are also very expensive. How can we use the phenomenon of Total Internal Reflection to transport light less expensively? One answer is to use a liquid filled light guide.

A liquid “fiber optic” cable could be inexpensive, flexible, and of large diameter. In the case of using high-power LEDs, the LEDs could actually be embedded into the liquid, and the surrounding liquid could provide some of the needed LED cooling. However, any such liquid light guide would need to meet Snell’s law for Total Internal Reflection. The liquid core would have to have a higher index of refraction than the solid cladding. In general, liquids have lower indexes of refraction than solids, so finding a pair of materials where the solid has a lower index of refraction then the liquid is challenging.

One reasonable choice for a core fluid is regular transparent mineral oil, which has a refractive index near 1.47. Experiments can be made with mineral oil you can find in any drugstore, however mineral oil meant for consumption has certain additives which can cause yellowing under the influence of UV. Very few solids have low refractive indices. Aside from some expensive epoxies, most of the low-refractive transparent index solids are perfluoropolymers, such as various forms of Teflon.

Therefore, some of my earliest experiments used tubing such as Tygon SE-200, which has a co-extruded FEP Teflon lining, of approximately 1.33 refractive index. A Tygon SE-200 tube filled with mineral oil makes a reasonably good liquid filled light guide. However, at $6/foot for 0.5” ID tubing, it is also an expensive solution for most applications. In addition, the co-extruded FEP lining is stiff and prone to kinking and cracking.

Teflon is a substance that is almost universally heat-processed, at temperatures often in the 350C range at a minimum. Therefore, coating tubing with Teflon, or creating a light-capturing cavity of any kind, require substrates that can accept the high temperatures of Teflon processing (such as your kitchen non-stick frying pans). To achieve TIR, the coating of low-refractive index material need only be a few microns thick. How would we inexpensively coat pipes, plates of glass, or other solid cavities with a thin coating of low-refractive index material?

The answer, which is being published for the first time here, is THV. THV is a rarely used special form of Teflon that can be dissolved in solvents, unlike other forms of Teflon. I have found that a 9% by weight solution of THV in MEK (Methyl Ethyl Ketone) can be simply dip-coated on glass and many plastics, and result in a perfect ~10 micron layer of low-refractive index solids. When I first tracked down this solution, I found one of the few bags of this material then in existence. At that time I was told that my 50 pound bag represented about 1/3 of the world’s supply of THV.

Making Chambers that Glow Evenly
Once I found that I could create liquid light guides, and that I could embed LED dies (without lenses), directly into the core fluid, I saw that a potential application could be a replacement for neon type signs and accent lighting. Some of the advantages of a liquid light guide over a neon light are that a liquid light guide using RGB LEDs can change color instantly and dynamically, making it better at attracting attention, which is the purpose of neon signs.

Liquid filled glowing “windows”, driven by LEDs

However, to do this, I would need to scatter the light evenly through a length of tube or sign volume. The final solution that I came up with was nano-crystalline rutile Titanium DiOxide (TiO2). Crystals of TiO2, approximately ~2 microns in size that were properly processed into the core fluid would remain in suspension essentially permanently. These crystals would scatter light, rather than absorbing or coloring the light, very effectively. The high index of refraction of TiO2 is only challenged by powdered diamond, an obviously more expensive substitute.

Applications We Have Considered
We reviewed many applications for this technology. These applications include:
:: Piping sunlight into the interior of office buildings
:: Replacement for Neon signs
:: Replacement for accent lighting
:: Glowing transparent windows
:: Glowing wall accent panels
:: Replacement for standard 2’ x 4’ fluorescent overhead lighting panels, flat panels powered by LEDs.

Resources
Patent filings were made on some of the early applications, later abandoned. While they don’t cover the more recent developments, they do provide much background and describe various techniques. Copies of these filings can be found here: http://www.freepatentsonline.com/20050084229.pdf, and http://www.freepatentsonline.com/20030147261.pdf.

I would be happy to answer any serious questions or comments at victor_babbitt (at) hotmail (dot) com