Solid-state lighting in the form of Light Emitting Diodes (LEDs) converts electrical energy into visible light with greater efficiency and less heat loss when compared to incandescent or fluorescent lights.
Above a certain current, however, LEDs become less efficient and convert less of their electrical energy into light. This is a condition known as “droop,” which makes the use of LEDs in high-power applications difficult.
The next generation of solid-state lighting may use laser diodes, which overcome some of the LEDs limitations by operating at higher power and optical intensities, but are about the same size.
As lasers are brighter than LEDs, they require fewer diodes for an equivalent light source, which makes the device cheaper and more compact.
“White” Light Sources
LEDs and laser diodes share one characteristic – they are monochromatic, or emit a single color of light. To create white light from either source, we use one of two methods.
- The first method combines the primary colors – red, green and blue – to create white light.
- The second method of creating white light uses either a blue or ultra-violet diode along with phosphors to “down convert” the higher frequency light into a range of lower frequencies. By blending phosphors in the right combination, we can get something that looks like “white” light.
Phosphor Driven White Solid-State Lighting
There are several complications affecting our ability to blend phosphors to get white light.
Firstly, as a diode’s color output can degrade with age or drift with temperature, it’s necessary to use complex optics and electronics to actively monitor and control color output.
LEDs can be coated with phosphors to generate white light, but the heat they generate can degrade phosphors over time.
As phosphor-coated LEDs generate visible light sources, we need optics to diffuse the light beam.
If we separate the phosphor from the LED, however, it overcomes these issues. Philips currently uses this “remote phosphor” design, which won the L Prize in 2011, a competition run by the US Department of Energy to encourage lighting manufacturers to develop high-quality, high-efficiency solid-state lighting products.