Since the introduction of white LED’s, not only the energy consumption of light went down substantially, the LED is also a very versatile light source: from powerful headlights of cars, to house lighting or small solar-powered light sources in areas without a power grid. Although the advantages are clear, designing a white LED with the right specifications still is a tedious ‘trial-and-error’ process. Researchers of the University of Twente, Eindhoven University of Technology and the lighting company Signify now demonstrate a design principle that drastically reduces the time needed to design an white LED: from hours to less than a second, thus improving performance and reducing costs. The researchers present their method in ACS Photonics, of the American Chemical Society.
The invention of the Nobel Prize winning blue LED made the production of a white LED possible, using blue, red and green light. This new light source rapidly became popular for its energy efficiency, mechanical stability, and long lifetime. For an application-specific design, the light intensity and efficiency count, but also the specific combination of colours that form the white light, the so-called colour point. Reaching the desired colour point is still done by matching measurements and simulations until they fit. This is a slow process using so-called ‘ray tracing’: in this, the experience of the designer is more important than a systematic design. The new method changes this, as it is an analytical approach.
Scattering phosphor particles
The researchers therefore look at the conditions leading to the specific colour point. In practice, a white LED is a blue LED with a layer of phosphor on it. The blue light is partly transmitted through the layer, and is partly converted into red, yellow and green light. The resulting white and diffuse light falling on an object, is a combination of the transmitted and scattered light. The colour point is determined by the thickness of the phosphor layer as well as the concentration of phosphor particles, the type of blue LED and optical parameters like the type of lens that is mounted on the LED.
Step forward for industry
“Using our analytic approach, we are able to find the colour point based on the relevant parameters. And in reverse as well: with a colour point as a starting point, we can find the relevant parameters” says Prof Willem Vos of the University of Twente. “Until now, the lighting industry does not have models that adequately describe the scattering of light,” Wilbert IJzerman adds. He is a Professor at the TU Eindhoven and he also works for the Signify lighting company, previously known as Philips Lighting. “A model based on the physical parameters is a true step forward. We can easily produce a lookup table based on the conditions.” The new design tool is based on the so called radiative transfer equation and is very fast: a typical LED design takes less than a second, while the current approach over one hour.
Few solar cells
It is not merely a matter of economics, according to Prof Ad Lagendijk. It is now possible to design very energy efficient, tailor-made light sources, even for use in areas without a good power grid. “If the stored energy of a few solar cells is sufficient for a cheap and small light for reading in the evening, the improved design method will be meaningful in global literacy.”
Earlier on, the research team was successful in improving white LED’s efficiency. Maryna Meretska, author of both papers, is currently working at Harvard University thanks to a Rubicon grant of Dutch Research Council NWO.
The research has been done within the Complex Photonics Group, part of UT’s MESA+ Institute, in collaboration with Prof. Wilbert IJzerman and Dr Gilles Vissenberg, illumination optics experts at the Signify company (former Philips Lighting) and working at Eindhoven University of Technology as well.
The paper ‘Systematic Design of the color point of a white LED’, by Maryna Meretska, Gilles Vissenberg, Ad Lagendijk, Wilbert IJzerman en Willem Vos, appears in ACS Photonics, and is now online as an ‘ASAP publication’.