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Photonics researchers at the University of Twente and Philips Lighting have come up with a concept of making white LEDs even more efficient and powerful. The team has found a detailed way for describing the light that stays inside the led by absorption and scattering.
From relatively weak light sources to strong lights at home and in cars, since the blue and white LEDs were invented, there has been a rapid development in possible applications. Low energy consumption and long lifetime are major advantages over existing lighting solutions. White LEDs consist of a semiconductor emitting blue light, with on top of that phosphor plates that turn the blue light into yellow. What one sees then, is white light. The light will be scattered by the phosphor particles, but part of it is absorbed as well. What part of the light will exit the LED is not easy to predict.
What makes good prediction particularly difficult: some of the light is absorbed, but re-emitted in another color. One way is trying to define all possible light rays, and use a lot of computing time to get a result. This doesn’t give much insight in what is actually happening. A theory that is often used for light propagation in a LED is diffusion theory. In strongly absorbing media, however, this approach isn’t valid anymore.
Researchers therefore have built a setup to collect all the light around the phosphor plates, in the whole visual spectrum. Based on this, absorption and scattering can be deduced using the radiative transfer equation, well known in astronomy. This results in a full description of light propagation inside and outside the phosphor plates. Compared to a description using diffusion theory, the absorption level is up to 30 percent higher. At the same time, the method is about 17 times faster than the numerical approach. These new insights, and their level of detail, can lead to powerful and predictive tools for LED designers. They help in further improving the efficiency.
The research has been done in the Complex Photonic Systems group of UT’s MESA+ Institute for Nanotechnology, together with Philips Lighting in Eindhoven.