Thursday, January 17, 2013

R&D News: Firefly-inspired LEDs

Personal Interest Background

 As of late, I have been fascinated by LED light technology because this technology has the potential to drastically reduce energy consumption for lighting solutions. In this quest for more efficient lighting, I found 40W replacement LED lights that use only 6 Watts while producing 400 lumens equivalent to a 66.6 lumens/W. That's not bad considering that an equivalent CFL consumes at least 9 Watts which equals a 44.4 lumens/W. The cutting edge of technology commercially available can be seen in the Phillips L-Prize LED 60 W replacement LED light which produces 940 lumens with only 10W (94 lumens/W!!!). For comparison, CFLs at this range have an efficiency of roughly 61.3 lumens/W. It appears that LED technology still has room to improve which it must to produce lighting solutions at the 100W replacement range. Companies like Osram, GE or Phillips have all announced their first entrance into these market segments. However, the consumption ranges from 20-27 W and the efficiency is at most 80 lumens/W (1600 lumens for 20 Watts). This is only 10% better than current CFLs that have an output of roughly 70 lumens/W (1600 lumens/23Watts). CFLs seem to get more efficient with increasing light output requirements whereas the challenges to keep LEDs cool seems to decrease efficiency at the high end.



Firefly-inspired LEDs

So these were the thoughts that went on in my mind, when I accidentally came across an editorial referencing two scientific articles in Optics Express this month leading us to today's recent discoveries that were both reported in the same issue. It's not what I typically read and write about but I would classify these papers to fall into the bio-inspired design ak biomimmickry category which fits well enough into the bio-based ideas bracket and makes for some nice diversity in the things I cover.

Diagram of Firefly Structures
Diagram of Firefly Microstructures from [2]
Both articles came from the same lab. In the first article, Annick Bay from the Vigneron lab was analyzing various microscopic nano-structures in the firefly reasoning that some of these structures may have light enhancing properties, given that fireflies also have to deal with the challenge of generating light in an efficient way. This paper essentially found that the misfits between the scales (structure 3) of the light-producing organ lower the refractive index to increase light output.

In the second article, the same group applied the insights obtained from analyzing the firefly nano-structures to LED lights by fabricating an overlay structure ontop of GaN LEDs leading to an increase of 55% in light extraction (at its peak an increase from ~9 to ~18 radiance defined as watts per steradian per square metre (W·sr−1·m−2)).

I wonder that this would equal to in lumens?

Why do these discoveries matter?

At any rate, it appears that one major limitation in current LED technology is the ability of produced photons to escape the diodes due to the lower refractive index of air. Enhancing light extraction is one key step to produce LEDs that have even more lumens/W. The reduced electricity consumption will also mean less heat production which is currently a major challenge when going to 1600 lumens as is needed for 100W replacement bulbs.

If you want to read another really well-written blog article that also talks about this go here:
http://www.technologyreview.com/news/509806/fireflies-inspire-brighter-leds/
As usual I encourage, the readers to go directly to the source which I have linked up below.

Literature Cited:

[1] A. Stark, "Scientists Mimmick Fireflies to Make Brighter LEDs," http://www.osa.org/about_osa/newsroom/newsreleases/2013/scientists_mimic_fireflies_to_make_brighter_leds/
[2] A. Bay, P. Cloetens, H. Suhonen, and J. Vigneron, "Improved light extraction in the bioluminescent lantern of a Photuris firefly (Lampyridae)," Opt. Express  21, 764-780 (2013).
[3] A. Bay, N. AndrĂ©, M. Sarrazin, A. Belarouci, V. Aimez, L. Francis, and J. Vigneron, "Optimal overlayer inspired by Photuris firefly improves light-extraction efficiency of existing light-emitting diodes," Opt. Express  21, A179-A189 (2013).

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