Fluorescent lamps are tube lights that use a gas discharge to function. Electrodes at each end power the light. Phosphor coats the tube, and that produces a light. Some fluorescent lights come on immediately (with rapid-start bulbs); others preheat for a moment before lighting. In either case, when the lamp or fixture doesn't function properly, the fluorescent light may appear dimmer than normal. Often, you can fix the problem yourself.
Check the Fixture, Bulbs
Install a new fluorescent light bulb into the light fixture to replace the dim one. Take the old bulb out to make sure the problem is your fixture and not a burnt out bulb. Insert the new bulb in the fixture by placing the prong side into the corresponding holes in the fixture. Push the other end of the bulb into the opposite side of the fixture.
Use indoor fluorescent fixtures in temperatures above 50° Fahrenheit. Otherwise, the lamp won't be able to generate enough heat to stay fully lit.
Insulate bulbs with plastic lamp sleeves and plastic caps in temperatures colder than 50 degrees Fahrenheit. Use only high-output lamps in temperatures as low as 10 degrees Fahrenheit.
Rotate the fluorescent bulb 180 degrees if you see a dim, feather-like shape.
Replace the Ballast
Replace the ballast in the fluorescent light fixture if there are gray or brown rings in a bulb that has been used for fewer than 100 hours. Replace the ballast if two tubes dim at once. Replace the bulbs with new ones before replacing the ballast in case you just have faulty bulbs.
Unplug the light or turn off the circuit breaker before removing the old ballast. Remove the light bulbs and take off the ballast cover with a screwdriver.
Cut the old ballast wiring with wire snips, about 3 inches from the end of the ballast. Unscrew the nuts that hold the ballast in place. Hold the ballast so it doesn't fall out of the fixture.
Compare the new and old ballasts to make sure that the wiring diagrams, voltage and current are the same. Strip one-half inch off the electrical wiring of the new ballast. Connect it to the existing wiring in the fluorescent fixture.
Connect wires to their corresponding wire color. Twist wire connectors onto each connection. .