Low-temperature light emission, also known as luminescence, has always surrounded us in the natural world. You may have seen it in the form of an eerie glow emanating from bacterial life in a marsh or recall it in the blinking radiance of lightning bugs you chased as a child. But controllable luminescence is a much more recent invention.
One of the first examples of artificially produced luminescence occurred in Paris in 1936, when George Destriau, working in Madam Curie’s Laboratory, discovered that zinc sulfide compound emitted light in the presence of an electric field. Essentially, he discovered the first synthetic electroluminescent (EL) phosphor.
Later, during World War II, a considerable amount of research was carried out on phosphors of various types, mainly destined for radar screens, the precursors to modern television tubes. And during the late ‘40s and early ‘50s, transparent conductive coatings were created for military applications, such as de-icing systems for aircraft windshields. These coating developments played a key part in another “display” technology liquid-crystal displays (LCDs).
How do EL lamps work? An EL lamp is a loose parallel-plate capacitor. In other words, it’s an electronic device that is electrically charged, then loses its energy in the form of light. To do this, an EL lamp requires an AC current, which is applied to both plates. The current allows energy to build up within a phosphor layer between the plates, and the energy is released as light during each half cycle of the AC current (when the electrical field or charges applied to the plates alternate). The number of times that the field changes within a given time period (the frequency of the current) influences the brightness of the light emitted by the phosphor.
How much light can you expect an EL lamp to generate? To put this question into perspective, consider a typical watch face with an EL light source. When illuminated, such watch faces typically produce around 1 footlambert (3.5 candela/sq m) of light, which is more than adequate to read the information on an LCD or analog watch.
The color of an EL lamp can be influenced by the frequency of the driving voltage. As a result it is extremely important that the driving voltage and frequency be specified during any discussion of color.
An inverter is a device which turns DC electricity into AC electricity and also is used to increase the frequency on AC electricity.
“DC” means Direct Current, and is used to describe electricity which is essentially constant with time (at least over intervals or interest); the voltage is basically constant for a (short) time interval.
“AC” means Alternating Current and is used to describe electricity which varies periodically (fluctuates with a (regular) rhythm) at a given frequency. Power coming out of a wall plug is AC. In the US it has a frequency of 60 cycles per second (Hertz); overseas, it has a frequency of 50 Hz. Appliances are responsive to this difference.