Better view of scattering light for more efficient white LEDs
Researchers from the University of Twente and Philips have jointly developed a model that they can use to predict the behaviour of white LED light far better than in the past. Thanks to the new insight based on nanophotonics it might be easier in the future to design more efficient and less environmentally harmful white LED lighting. The researchers will shortly publish their findings in the leading American journal Applied Optics.
Wandering photons
White LED light is made by transmitting blue light from a semiconducting diode through a thin polymer layer that contains phosphorous and other particles. Not all of the photons from the light pass straight through the polymer layer. Many photons encounter a particle and are then scattered. They then proceed in a different direction. A number of photons scattered by a phosphorous particle lose part of their energy and therefore acquire a longer wavelength. Most of the photons are scattered more than once and therefore all of the light particles make a so-called random walk, a sort of drunken stagger through the polymer layer.
A vital aspect of this behaviour is the so-called mean free path of the photons: the average distance they travel between two successive encounters with a scattering particle. After all of these wanderings of the photons through the polymer layer the original blue light emerges as a collection of all colours (blue, green, yellow and red): the mixture that we see as white light. Furthermore, the random walk sends the photons in all different directions making the light diffuse. The result is therefore an even-coloured white shining LED lamp, which is exactly what the user wants.
Understanding the scatter better
Up until now, it had only been possible to produce a poor model of the scattering using a very time-consuming simulation method. "Therefore at present we cannot predict the contribution of the different wavelengths to the LED light, or we have to influence the optical parameters of the system," says FOM workgroup leader professor Willem Vos. This strongly hinders the design and development of efficient white LEDs.
The researchers have now chosen a different approach to understand the scattering. They carried out measurements using a standard light-scattering polymer that contained various concentrations of commercially available titanium dioxide (TiO2) particles. These particles vary strongly in size and that is the best guarantee for an even scattering of the light. The researchers illuminated the polymer with a clear white light source and measured characteristics such as transmission, reflection, wavelength distribution and direction of the emerging light.
Calculating and modelling
Using a theory from nanophotonics (the photonic diffusion theory) in which the light scattering is described solely on the basis of fundamental physics principles, they calculated the mean free path from the measurements. The researchers also developed a mathematical model, which contained the measured dimensions of the TiO2 particles, to predict the mean free path. When the measurements were compared with the predictions, the scattering was found mainly to depend on the size of the TiO2 particles, which are made up of clusters of smaller particles. If these clusters were properly included in the model then the measurements and the predictions strongly agreed with each other. This is a surprising new insight that will help in the design of LEDs for white light that are more energy efficient and less harmful to the environment, say the researchers.
The research was partly funded by the Foundation for Fundamental Research on Matter (FOM), the Netherlands Organisation for Scientific Research (NWO), and the European Research Council (ERC).
Further information
Willem Vos, Teus Tukker, Allard Mosk, Ad Lagendijk and Wilbert IJzerman. Broadband mean free path of diffuse light in polydisperse ensembles of scatterers for white LED lighting, Applied Optics (2013).
Source: press release Technology Foundation STW