Tuesday, May 18, 2010
Before the invention of incandescent lamps, candles and oil lanterns were used to provide light. There were so many different candles, the National Bureau of Standards specified a certain candle in terms of size and chemistry and specified the light radiated as 1 Candlepower. The term Candlepower is presently used to describe the lighting power of all types of lighting products. The term Lumen is also used and 1 Lumen equals 4∏ Candlepower.
At the same time it was necessary to define the candlepower of a new lighting product. In the case of incandescent lamps, the maximum light output occurs when the lamp is aged until the current is stable and is referred to as the initial candlepower/lumens. This is the point of the highest efficiency for the lamp and the candlepower will continually decrease as the lamp is burned. The same is generally true with all lighting products.
In the beginning, incandescent lamps were vacuum lamps and the bulb blackened due to condensed carbon and later tungsten on the bulb wall and the useable radiated light diminished with lamp use. This value is called Lumen Maintenance. The useful life of a lamp occurs before the lamps actually fail. It was decided that a bulb was no longer useful when the light output had dropped to 70% of initial candlepower/lumens. The 70% designation is still used today for most all lighting products.
The life of a lamp is determined by life testing at rated voltage or at a higher voltage for long life lamps. The average lamp life is often calculated as the test hours when 50% of a group of lamps have failed. The life test history indicates that lamps that fail at less than 40% of rated life are defective usually contamination. In theory, the higher the initial candlepower, the shorter the lamp life and if there is not some indication that lamp life varies as the initial light output, the entire lot is suspect.
Standard Incandescent Lamp Data from the IES Handbook WATTS VOLTS RATED LIFE INITIAL LUMENS % INITIAL LUMENS @ 70% LIFE 60 120 1K Hr 860 93%
Standard Fluorescent Lamp Data WATTS VOLTS RATED LIFE INITIAL LUMENS % INITIAL LUMENS @ 70% LIFE 13 95 6K – 7.5K Hr 860 72%
Standard LED Lamp Data from CREE, Inc WATTS VOLTS RATED LIFE INITIAL LUMENS % INITIAL LUMENS @ 70% LIFE 7.5 120 50K Hr 550 95%
There is a difference between Commercial application LED lamps and Military Application lamps. The guaranteed life for the commercial LED lamps is 20,000 hours and the expected average lamp life is 50,000 hours.
The guaranteed life for Military LED lamps is 5,000 – 10,000 hours per MIL-217 handbook.