Lamp Life Expectancy
Life expectancy is defined as the number of hours of operation for a lamp until 50% of them fail. This means that it is possible for some lamps to fail after a short amount of time and for some to last significantly longer than the rated lamp life. This is an average (median) life expectancy. Production tolerances as low as 1% can create a variance of 25% in lamp life. For LEDs, lamp life is when 50% of lamps have lumen output drop to 70% or less.
Lamps are also sensitive to switching cycles. The rapid heating of a lamp filament or electrodes when a lamp is turned on is the most stressful event on the lamp. Most test cycles have the lamps on for 3 hours and then off for 20 minutes. (Some standard had to be used since it is unknown how the lamp will be used by consumers.) This switching cycle repeats until the lamps fail and the data is recorded. If switching is increased to only 1 hour on, the lamp life is usually reduced because the number of times the lamp has been turned on has increased. Rooms with frequent switching (bathroom, bedrooms, etc.) can expect much shorter lamp life than what is printed on the box.
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—Henry David Thoreau (1817–1862)
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—William Shakespeare (1564–1616)