How a fluorescent light works

You may think that a fluorescent light works the same way an incandescent, or ordinary light bulb, works, but they do not. They work on a completely different principle called Gas Discharge. One of the first fluorescent lights was shown at the world's fair in the 1930's

Electricity can jump across a gap. This is called sparking or arcing. Inside a "standard" fluorescent light tube there are two heating coils, one at each end. When you turn on a fluorescent light, a spark jumps from one of these coils to the other one at the other end. While the light is on, this spark continues to run from one to the other. In normal air this would take a huge amount of electricity and quickly burn the two heating coils, but there is no air inside a fluorescent light tube.

If you have ever seen an electric spark, you know it is an ugly blue or yellow color, but the light from a fluorescent light tube is a "nice" white. The light is white because the fluorescent light tube is coated with phosphor. This phosphor glows white when light or other radiation excites it. The spark jumping between the heating coils makes ultraviolet light. This makes the phosphor glow white.

You may have been wondering why the spark jumps between two heating coils. In order for the spark to start, you can either use really high voltage electricity or you can heat the places where the spark is jumping between. With modern electronics it is easy to make high voltage easily and safely, but when fluorescent lights started being made there were no modern electronics. So the heater system was used. Most new fluorescent lights have "instant start" which uses high voltage to start the arc. But most smaller or older ones use the heaters and relatively low voltage.

The part of the fluorescent light fixture that starts the arc and keeps it going is called the ballast. In the first kind of fluorescent light, ballasts called "pre-heat" or "warm-start" ballasts, the ballast consisted of a simple coil. When you turned on the light, a "starter" would turn on the heaters and the ballast coil would give the electricity a boost to start the spark. When the spark started the starter would turn off and the spark would continue until the light was turned off.

Later systems like "rapid-start" and "trigger-start" eliminated the starter but still used the heaters in the ends of the light to help start the arc.