A fluorescent lamp is a glass tube coated with phosphor powder, and filled with an inert gas and a small amount of mercury. There are electrodes on each end which, when turned on, cause electrons to move through the gas from one end of the tube to the other. This transforms the liquid mercury into a gas, and as electrons collide with the gaseous mercury, ultraviolet light is released. This ultraviolet light is not visible to the human eye, but as it hits the coating on the tube, the phosphor atoms give off white light. A flickering in the tube can be caused by one of three things--a defective lamp, defective ballast or defective starter.
Test Fluorescent Tubes
Check the tubes. The tubes are the least expensive part of a fluorescent fixture, and the easiest to replace. Defective fluorescent tubes will flicker and cause other tubes paired with them to flicker. If allowed to continue flickering, these defective tubes will cause the ballast to overheat and fail prematurely. Tubes need to be replaced if the electrodes begin to fail or if gas escapes.
Use a ladder to gain access to the fixture and open the lens to get to the lamps. This may require a screwdriver, but usually can be accomplished manually. Look at the lamp. A key indicator that the lamp is failing will be a blackened area near each end of the tube.
Test the lamps (if there is no blackening on the ends of the tube) by putting them into another working fixture one at a time. If the lamp continues to flicker, it is defective, but if it works in the new fixture, it is still good and the problem is either the ballast or starter.
Replace Defective Starter
Check to see if the fixture has a starter. Older fixtures may have a starter, but newer fixtures will not. A fluorescent starter is a small silver cylinder that plugs into the fixture's frame. It is visible on the fixture near the lamps and will not be hidden by any compartment doors or covers. Starters work by sending high-voltage electricity through the gas in a fluorescent tube in order to ionize the gas so that it can conduct electricity. This is not an instantaneous process, and the tube will flicker momentarily as it starts up. A defective starter will cause the lamp to continue flickering after start up or fail to ignite altogether, leaving the lamp dark.
Replace the starter. This is the only way to determine if it is defective. Make sure the starter is firmly plugged into the fixture before removing. It may have become loose, which will cause it to fail to work properly.
Remove a firmly attached starter by pushing it in and turning it counterclockwise. Take it to an electrical supply store or lighting store to see if a replacement is available. If not, check the ballast before replacing the fixture with a new one.
Replace Ballast or Fixture
Find the ballast and determine if it is a magnetic or electronic model. Older magnetic ballasts typically last 15 to 20 years and will hum throughout their lifetime. As they age, the humming will grow more pronounced. Newer electronic ballasts also have a long life expectancy, but do not hum or make any noise while operating. They are also smaller than magnetic ballasts.
Decide whether to replace the ballast or replace the fixture. Magnetic ballasts typically will cost as much or more than a new fixture. A new fixture will also be more energy efficient. However, if the ballast is going to be replaced, take the burned out ballast to an electrical supply or hardware store to see if a replacement is available.
Install a new ballast. This requires that the power to the fixture be turned off, preferably by switching the circuit breaker off. Follow the wiring diagram on top of the ballast and, using wire strippers and wire nuts, attach wires of the same color according to the instructions.