Light Emitting Diodes or LEDs convert the energy in an electric field directly into light and are more efficient than either incandescent light bulbs or compact fluorescent lamps. Despite the tremendous advantages in energy efficiency, engineers face challenges in extracting the light generated in the semiconducting crystal to the surroundings where it can be used. This problem stems from the high refractive index of the crystal which results in most of the light being reflected back into then material itself and being lost as heat. To overcome these problems, improve light extraction, and minimize losses, engineers employ several techniques.
The underwater lights placed inside a pool not only provide safety at night, but add to the beauty and splendor of an expensive investment. A thoughtful observer can see that one lamp can almost illuminate the entire bottom of the pool as the light bounces around and is internally reflected under the water’s surface. This phenomenon is entirely based in physics and is due to the differences in the refractive indices of water and air.
The refractive index of a material tells us how much light slows down in a material. Water has a refractive index of 1.33 and light travels 1.33 times faster in vacuum or air than in water. Another way of saying this is light moves slower by almost 25% in water than in air. As light moves, from a medium with one reflective index to a medium with a different refractive index, it changes direction or bends.
This phenomenon is seen not only in swimming pools, but poses a problem for physicists and engineers working with LEDs as well.
The Physics of Light Extraction
The refractive index of LED semiconducting materials is quite large and light bends a great deal as it moves from inside the crystal to the air. This means that a small “light cone” makes it out while a substantial amount – more than half – is reflected back into the semiconductor material itself. Semiconducting materials have high refractive indices. This results in a small “light cone” that escapes into the surroundings where it can be used. Much of it gets trapped in the LED material.
This poses two problems; firstly, how to extract as much light as possible from the small “light cone” and secondly, how to prevent the absorption of what is reflected back and subsequently turned into heat. Too much heat will destroy a semiconductor.
Scientists and engineers must not only devise ways to extract as much light as possible, but also ways to keep LEDs cool.
Light Extraction Methods in LED
One method to minimize reflection back and maximize extraction of light is to use a dome-shaped or half-sphere package with the LED material in the middle. This way outgoing light rays strike the surface directly and minimize reflections. The smaller difference between the refractive indices of the LED semiconductor material and the transparent hemisphere dome, which usually has a refractive index between 1.6 and 1.7, also produces a wider light cone and allows for better light extraction.By adding a dome shaped encapsulant, the light cone is broadened. This is due to the smaller difference between the refractive index of the LED material and the refractive index of the hemisphere dome.
LEDs can also be engineered in other ways to maximize light extraction. Reflective layers can be placed behind LED materials to reflect as much light as possible and increase extraction efficiency.
LED materials can also be designed to absorb and re-emit any reflected light in a process called photon recycling.
Some more recent advances have designers etching the semiconductor surfaces to roughen them and reduce internal reflections to allow more light to get out.
Physicists and engineers are constantly exploring and designing new ways and techniques to extract more light from LEDs to further improve their efficiencies.