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How well do we understand white light?

Posted: 04 Mar 2016 ?? ?Print Version ?Bookmark and Share

Keywords:White light? LED? LED spectra?

White light is essential to a host of applications. But just how well is it understood?

A team of researchers from the University of Twente and Philips Lighting recently published findings in the Journal of Applied Physics that provide additional and important information as to how light scatters, is absorbed, and re-emitted in white LEDs. The research paper, "How to distinguish elastically scattered light from Stokes shifted light for solid-state lighting" is authored by Maryna Meretska, Ad Lagendijk, Henri Thyrrestrup, Wilbert IJzerman, Allard Mosk, and Willem Vos.

According to the researchers, there is a very limited grasp of light scattering, absorption, and re-emission, and current LEDs are explained with models where these three occurrences are described by ray-tracing and Monte Carlo techniques. Vos, the leading researcher, explains in the journal that the LED spectra cannot be predicted quantitatively and that optical parameters are manipulated in order to match up with the data measurements. That should be enough to set back at least some progress.

So what the group did was to collect LED diffused light that exits in multiple directions by filtering incoming light and separately filtering outgoing light (see figure below).

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Figure 1: (Source: University of Twente)

There is a 10 per cent difference in the way transmission is measured using the traditional way and the research, whereby researchers used Nanophotonic Theory so that light scattering is described by figuring from measurements a mean free path. This is the average distance after which photons lose their sense of direction. According to the group, the mean free path can be up to 50 per cent different than what is found using the traditional way. The researchers claim that the findings can improve the design and therefore the energy efficiency of LEDs.

- Carolyn Mathas

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