Lamp life is an important consideration when purchasing a new, retrofit or replacement lamp. Two very different and distinct terms describe lamp life: “rated life” and “economic life.”
Rated, or average (median), life for metal halide lamps is a value of lamp life expectancy based on laboratory and field tests of representative lamps, operating on approved ballasts, with a burn cycle of at least 10 hours per start. The average life is determined when 50% of traditional metal halide lamps initially installed are still operating. For 320 – 875 watt Uni-Form Pulse Start lamps, life is determined when 70% of the initial lamps installed are still operating.
Various operating conditions affect lamp life. One key factor is operating position. Position-oriented lamps (designed to operate in one specific position) are tested and rated based on that designated position. Operating these lamps in any other positions can dramatically shorten life, reduce lumen output and cause colour shift. Lamps designated universal can be operated in any position. However, life expectancy and lumen output are sacrificed in certain positions. Published “rated life” for universal lamps is based on operation in the vertical position. “Rated life” for universal lamps operated horizontally is 75% of the published rating.
Economic life refers to the hours of operation during which a lamp is designed to provide optimum light output and colour quality as well as lowest replacement cost. Economic life describes actual lamp life better than rated life because rated life does not account for the lumen depreciation and colour shift that occur as lamps age. The economic life of lamps is generally 60% – 75% of the lamp rated life. Though economic life is important when considering a lighting system, lamp data tables show rated life because they provide a comparison with other lamp manufacturers’ ratings.
Pulse Start metal halide ballasts provide the proper combination of open circuit voltage and high voltage pulses to start the lamp. The pulse is provided by a specially designed ignitor, or starter, that is used in conjunction with the ballast. As soon as the ignitor senses that the lamp has started, it discontinues the pulsing operation. At this point, the ballast sustaining voltage must be sufficient to maintain lamp operation. A positive feature of this system is that the lamp will hot restart in 3 – 4 minutes following a power interruption. Traditional probe start metal halide ballasts can take as long as 10 – 15 minutes to restart the lamp.
The ballast open circuit voltage starts traditional probe start metal halide and mercury vapour lamps. Auxiliary electrodes, or probes, aid these lamps in starting. The probe electrode is disconnected after lamp has warmed up.