When purchasing a new light source or replacement bulb, lamp life is a very important consideration. Unfortunately purchasers often do not understand exactly what is meant by "lamp life". There are two very different and distinctive terms that describe life: "rated life" and "economic life".
Rated average life for metal halide lamps is defined as: A value of lamp life expectancy based on laboratory tests of representative lamps, burning at rated volts, on an approved system, operating with a burning cycle of 10 hours per start. The "average life" is determined when 50% of the lamps initially installed are still operating.
Various operating conditions can affect the average life of lamps. One of the most important factors is burning position. Position-oriented lamps (designed to burn in a certain position) are tested and rated based on their designated position. Burning these lamps in other positions can dramatically shorten life, lumen output, and color. Universal lamps can be burned in any position, but as a result, they sacrifice life expectancy and lumen output in some operating positions. Published "rated life" for universal lamps are based on the lamps being burned in the vertical position. "Rated life" for lamps burned in the horizontal position is 75% of the published rating for the vertical application.
Lamps, like most electromechanical devices, have a shorter life the more they are turned on and off. Other factors can also affect lamp life, such as high or low operating voltages, marginally operating control devices (ballasts, capacitors, etc.), extremely high operating temperatures, among other conditions. Combinations of these factors can multiply the reduction in "rated life".
"Economic life" is a much better description of actual lamp life than rated life. It refers to the hours of operation a lamp is designed to provide in terms of optimum light output, aesthetic quality, and economic energy consumption. The "economic life" of lamps is generally defined as 60% of the lamps rated life.
"Rated life does not account for the lumen depreciation, color shifting, and loss in efficacy that always occur as lamps age. To consistently provide a quality lighting system, you must not only consider the lamps that fail, but the lamps that continue to operate. Lower light output (lumen depreciation) occurs even though the lighting system continues to consume the same (or sometimes slightly more) electricity. See the section on light output for more information.
The color of light ( (CCT) that lamps generate also changes or "shifts" with age. This is rarely noticed during the "economic life" of lamps but often creates problems in the last 40% of "rated life" when color shift may accelerate. During this period more spot replacements are needed for failed lamps. The original lamps often appear to be a different color than the newly installed replacements In almost any case, the cost of a lamp is the smallest fraction (typically 5%) of maintaining a lighting system compared to the energy usage and the labor cost involved in spot relamping. Though "economic life" is the most important factor when considering lamp life, the lamp data tables contain the lamps "rated life because: 1) it is a means of comparison - other lamp manufacturers only discuss "rated life" and 2) "economic life" is a percentage (generally 60%) of "rated life".