What Happens when Fluorescent Lamps Wear Out?
(From: Charles R. Sullivan (charless@crissy.EECS.Berkeley.EDU).) The usual failure mode is depletion of the emission mix on the filaments. Then they do not emit electrons, and the arc can’t be sustained. Unless the ballast supplies a high enough voltage that very high field can be set up near the electrode. Then the ions bombarding the electrode have a high enough energy to knock electrons out of the metal even with no emission mix, or to heat the metal to the point it emits electrons. The high field is also sufficient to ionize the argon fill gas—normally only mercury is ionized. The argon radiation is of a more purple color. That is probably what you see. Blackening at Ends of Fluorescent Tubes This is a common phenomenon with most common fluorescent tubes as they age. However, frequent or repeated starting can accelerate the process. The black areas in themselves don’t affect operation except to slightly reduce the amount of light available since the phosphor in that area is dead.
