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LED FlashFigure 2 shows a typical LED bulb. The small device at the center of the bulb is called a Light Emitting Diode (LED), and emits light in all directions as electric current passes through [1,2].
Light Emitting DiodeA diode [4] is a electronic chip made from an n-type semiconductor and a p-type semiconductor, which are shown in Figure 3. Semiconductors [5] are materials that can conduct electricity fairly well, but not as well as metals can. As shown in Figure 3, an n-type semiconductor has an abundance of electrons (negatively charge particles), and a p-type semiconductor has an abundance of protons (positively charged particles). While both types conduct electricity well by themselves, we can produce a lot of energy by forcing their particles to collide. As soon as we bring the two semiconductors together, however, a weak insulation zone forms and the charges are prevented from colliding. Figure 4 shows the charges bunched up around this zone. Fortunately, the zone can be destroyed with enough power. By connecting the p-type and n-type semiconductors to the positive and negative ends of a battery (like we do in Figure 5), we can break the insulation zone to let charges pass through freely. As pairs of charges collide with each other, they release flashes of light, hence the name light emitting diode. Light extraction
The flashes of light produced by the collisions can only escape if they strike the edges of the LED almost head-on, as Figure 6a shows. Otherwise, they will simply bounce back [1], a situation known as total internal reflection [6]. To overcome this problem, we can place a transparent dome (usually made out of epoxy) around the diode, keeping the light from bending too much. Furthermore, we can place a reflector below the diode to direct more light towards the dome. We can see in Figure 6b that most of the light hits the edge of the dome nearly head on, thereby escaping.
Further Reading
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