1.1 How LED works
A light-emitting diode (LED) is a semiconductor device that emits light on certain wavelength (color). A die (active area of LED) is encased in plastic or ceramic housing. The housing may incorporate one or many dies.
Figure 1.1.1: When LED is switched on, electrons recombine with holes within the device, releasing energy in the form of photons with certain wavelength (color). Photon: Unit of light
The term SSL (Solid State Lighting) is common term for LED technology being used for lighting applications. It refers to technology in which the light is emitted by solid-state electroluminescence as opposed to incandescent bulbs (where the light is emitted via thermal radiation in visible part of spectrum – incandescence).
White LED working principle
The most common method involves coating LEDs of one color (mostly blue LEDs made of InGaN) with phosphor (Figure 1.1.2a) of different colors to form white light; the resultant LEDs are called phosphor-based white LEDs. The “blue” photons emitted by High-brightness LED (HB LED) (Figure 1.1.2b) either passes through the phosphor layer without alteration, or they are converted to the “yellow” photons in the phosphor layer (Figure 1.1.2c). The combination of “blue” and “yellow” photons leads to white light (Figure 1.1.2d).
Figure 1.1.2: a) Cross-section of standard phosphor-based white HB LED. b) Recombination of electrons with holes results to “blue” photons. c) “Blue” photons either passes through the phosphor layer without alteration, or they are converted to the “yellow” photons in the phosphor layer. d) Combined together, they create white light.
Spectrum of a phosphor-based white LED clearly showing blue light directly emitted by the LED die and the more broadened yellow light emitted by the phosphor (Figure 1.1.3).
Figure 1.1.3: White light can be produced by combining blue and yellow light only. Sir Isaac Newton discovered this effect when performing color-matching experiments in early 1700s.