Electroluminescent Lamps
Electroluminescent lamps (abbreviated EL) are a flat and flexible type of cold illumination (they do not produce their light from heat). They are often used as backlights for LCDs in watches ("Indiglo®" is just a name used by Timex® for EL backlights), laptops, cell phones, etc., as well as for night-lights, lighting for membrane keypads, safety stripping, or even for lighting wire. They are basically just light emitting capacitors, where at least one of the plates is transparent, and the dielectric is impregnated with phosphor. When an alternating current is applied, the electrons in the phosphor absorb energy as the field intensity rises and move up from their resting valance band to a higher energy band. When the field intensity drops, the electrons fall back to their lower state and release the energy that was absorbed in the form of light. Depending on the type of phosphor used, different colors can be produced. The most common colors are green, blue/green, amber and white.
In order to drive an electroluminescent lamp, some kind of an AC power supply is required. The higher the voltage, and the higher the frequency, the brighter the lamp will glow, although it will also reduce its life. A voltage of at least 20 volts is needed, but they are usually supplied with 100 volts or more. The frequency is usually in the hundreds of hertz to the kilohertz, although 50/60 hertz will drive ELs with a reduced output (such as in night-lights using EL). In small battery powered devices such as watches, an inverter is needed to step up the voltage and produce an AC supply for the lamp.
Unlike most light sources, electroluminescent lamps do not die, rather they fade. The half-life is generally between 3,000 and 10,000 hours, depending on the voltage and frequency they are powered with, as well as the type of phosphor used.
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