Read the lettering on the burned-out lamp. You will see either T8 or T12 stamped on one side near the end with the prongs. You also will see the lamp's rated wattage, usually 32 watts for T8 and 40 watts for T12.
Measure the diameter of the lamp. T12 lamps are larger than T8 lamps, with a 1 ½-inch diameter. The diameter of a T8 lamp is 1 inch.
Look at the troffer, which is the metal fixture that holds the ballast and lamps and is shaped like an upside-down trough. Determine whether it is a lensed troffer, a louvered troffer or an open troffer, in which the lamps are completely exposed.
Look for an acrylic lens, a flat sheet that spans the troffer below the lamps, diffuses light and reduces glare. This is a lensed troffer, which customarily uses T12 lamps.
For T8 lamps, look for a parabolic louver troffer. It has several slanted openings that help shield the lamps from direct view, gather the light and redirect it out into the room.
Remove the old T12 magnetic ballast and install a new, lighter-weight T12 electronic ballast to reduce light flicker and hum. Install the T12 tubes. Choose from warm white or cool white, depending on your lighting preference.
To improve efficiency, remove either type of T12 ballast and wire a T8 electronic ballast in the current troffer. Purchase and install T8 tubes.
To reduce glare and distribute a more uniform light, remove the open troffer or lensed troffer and install in its place a parabolic louver troffer with T8 tubes.
Choose T8 fluorescent lamps with parabolic troffers and electronic ballasts for most applications, as they last longer than T12s and contain a triphosphor blend capable of producing a more natural light.
Choose T8 lamps where overheating is a concern, as they produce less heat than T12 lamps
Choose T12 lamps if initial expense is a concern, as they cost less than T8 lamps.
Choose T12 lamps with the older-style magnetic ballasts if you need to install lamps where there will be absolutely no interference to nearby electronic equipment. In the alternative, purchase fixtures with T8 electronic ballasts that have passive filtering.