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Although the life of any one individual fluorescent lamp cannot be predicted, the average life expectancy of large groups of lamps is fairly well known. Because of variations in manufacture, some lamps will achieve a longer or shorter life than others. 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.
The average life span of a very large number of lamps is also known as the average rated life. Half (50%) of the lamps in a sample are likely to fail before the rated lamp life, and half (50%) are likely to survive beyond the rated lamp life.
Published rated life is based on standard test conditions. The average life of a real given individual lamp or group of lamps operated under variable conditions may not equal the published rated lamp life. For discharge light sources, such as fluorescent and HID lamps, lamp life depends on temperature, hours per start, duration of the operating cycle, operating circuit design, ballast design, etc.). The standard operating cycle for fluorescent lamps is 3 hours on, 20 minutes off. They can also be rated at 12 hour starts and for continuous burning.
It should be noted that lamp life will be reduced when lamps are operated for fewer than 3 hours per start, but the lamps may last longer (have a longer service life) because they may be on for fewer hours per day than lamps that are operated for 3 hours per start or longer. Lamp life will be increased when lamps are operated for more than 3 hours per start, but the service life may be reduced because the lamps may be on for more hours per day than lamps that are operated for 3 hours per start or less. The way a lamp fails varies by the fixture system and source technology. Incandescent, fluorescent, HID, and LED lamps all fail differently.
There is a second mode of lamp failure. Even lamps that are still operating have an end of useful life. This second measure of lamp life is called lumen depreciation. This is defined as the time when the output of a large sample of lamps under controlled laboratory conditions reaches a specified fraction of the initial output. End of life is generally specified when the luminous flux (light output) has dropped below 70% (a reduction of 30%) of the original output value.
Definition of “Median”: Given a list of numbers arranged in order from lowest to highest. The number in the middle or the average of the two middle numbers is the median.
Normal End of life and premature lamp failure: In normal operation, the lamp is working but has a light to medium gray, black, or brown ring or rings on the inside of the glass near the ends of the lamp which grows and gradually causes the end of the lamps to become partly opaque, developing the smoky or burnt look. This is normal lamp aging and should be ignored until the lamp eventually fails to start. When the lamp does finally wear out, the discoloration on the end of the lamps can be as dark as coal.
The only case where this would be considered a problem would be if new lamps develop these marks within the first 100 hours (the first month of operation) of use and have trouble starting. That may indicate a issue with the lamp or ballast. Best to first replace the lamps as it is most likely they are defective and it is less costly than needlessly replacing the ballast.
The normal end of life failure mode for fluorescent lamps varies depending on how they are used and their control gear type. Often the light will turn pink from loss of mercury with black burns on the ends of the lamp due to sputtering of emission mix. The lamp may also flicker at a noticeable rate. This is the normal tube failure mode.
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