The rated average life in burning hours of fluorescent lamps is based upon the average life of large representative groups of lamps tested in a laboratory under controlled conditions. Ordinarily, with suitable auxiliaries and line voltage, you'll see this stated average life in normal service.
The emission material on the cathode erodes during the normal starting and burning cycle of the lamp. Normal end-of-life is reached when the emission material on either electrode is depleted. Failed Preheat lamps flash on and off or extinguish. Rapid Start or Instant Start lamps extinguish, flicker, or operate at reduced brightness.
Longest performance can be achieved by using approved ballasts from a Certified Ballast Manufacturer or ballasts otherwise known to meet the specifications of the lamp manufacturer. Be sure the line voltage to the fixture is as specified on the ballast. While some combinations of lamps and ballasts will give satisfactory performance at voltages above or below rating, in most cases low voltage as well as high voltage will reduce lamp life and increase discoloration.
Although the life of each individual fluorescent lamp cannot be predicted, the average life expectancy of large groups of lamps is fairly well known. Some individual lamps are destined to live longer than the average life of the group while others are destined to burn out before the average life point is reached.
This mortality curve shows the average trend obtained on representative lamps on life tests. The lamps were started once every three burning hours under controlled conditions on approved auxiliary equipment. Rated Average Life is the point at which approximately 50 percent of the lamps in a large group have burned out.
The average life of fluorescent lamps is affected by the number of lamp starts. Frequent starting shortens life while life is lengthened as the number of burning hours per start is increased. Longest lamp life and best lumen maintenance will be obtained with 1) good voltage, 2) proper auxiliaries, and 3) favorable operating conditions.
The life of a fluorescent lamp isn't affected by the ambient temperature in which it operates. However, its life can be affected by starting; and starting is influenced by temperature. For example, the voltage required for Rapid Start lamps at -20°F will "Instant Start" lamps at normal temperatures. The "Instant Starting" will adversely affect lamp life how much depends on the temperature and the frequency of starts.