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.
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 color 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.
life refers to the hours of operation during which a lamp is designed
to provide optimum light output and color 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 color shift that occur as lamps age. The economic
life of lamps is generally 60% to 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.