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Fading White LEDs
By: Dave Johnson  

There is a “dirty little secret” in the LED lighting industry.  You will not see much being written about this because the industry does not want to talk about it.  “White LEDs fade.”  Some LEDs from China fade very rapidly.  I have seen some devices where the light output drops by 50% in only a few weeks.  I cringe when I see those same devices being used in incandescent lamp replacement fixtures, nightlights and flashlights.

Standard Plastic LED    White LED
Let’s imagine you are a company president and are looking at ways to save your company some money.  One way to save money is to reduce your monthly electricity bill.  You look at one floor of your office building and count 250 40 watt lights, which are turned on for 12 hours each day, 6 days each week.  This works out to about 50,000 kilowatt hours each year. At the going rate of $0.1 per kilowatt-hour, the electric bill for these lights is $5,000 each year for each floor.  The president looks around and decides that he can reduce the electric usage for lighting by shifting from fluorescent lamps to the new LED lamps.  The LED lamp Chinese manufacture claims that each lamp will emit the same amount of light as the fluorescent lamps but draw only 15 watts, instead of 40 watts per lamp, and will last 10 years.  By replacing all 250 of the lamps, the president figures that about $3,000 could be saved each year for each of the 20 floors of his company’s building.
Typical Office Ceiling Lights    Typical Office Ceiling Lights 
OK, he decides to make the switch.  Even at a cost of $40 for each LED lamp, for a total of $10,000 per floor, he figures he would get a return on his investment in about 3 years from the energy savings.  He buys the lamps and has a crew of people replace all of the lamps over one weekend.  On the following Monday, his employees are greeted with new lights.  They love them.  There is no flicker so they have less eye fatigue.  There is none of the annoying hum from the fixtures and the light spectrum seems to be more natural. Everything is great and the president pats himself on his back.  Then, after 6 months, one of the LED lamps fails and is replaced by a new lamp, from the same manufacture.  Only then, does the president sees that the new lamp is visibly brighter than the 6 month old lamp.  He calls in a lighting expert, who makes some light level measurements and tells the president that the new lamp is 30% brighter than the older lamps.  This works out to a “half life” of one year for each lamp.  That means that after a full year, those original lamps will be emitting only half as much light as they did when they were first installed.  After two years, the light will be down to only 25%. After three years, they will be useless by emitting only 12% of the original intensity.  The company president has been stung by the erroneous claims of many of the lamp manufactures selling these things.  This sort to thing will happen over and over until some standards are in place.  The lamps coming in from China are especially over rated.
LED Light Replaced for a 40W Fluorescent lamp   Typical Office Ceiling Lights 
There are two main causes for the fading light.  The most important factor is heat.  High temperatures will cause LEDs, especially white LEDs, to fade.  Many lamp designs do not cool the LEDs well enough.  In each fixture, there should be a large metal heat sink.  The next factor is the use of poor phosphors on the part.  White LEDs are really blue LEDs with a special phosphor coating.  The phosphors glow different colors when illuminated with the blue light from the LED.  In many cases, those phosphors are a closely guarded secret.  In many LED designs, inferior phosphors are used, which are bleached by the constant blue light and over time will no long emit much light.  The combination of these two factors means that nearly all white LEDs made today, will emit less light in a few years of operation.  The manufactures are pretty tight lipped about this degradation curve.  Perhaps in time, there will be a solution but until then, don’t expect white LEDs to be in wide use as long-term replacements for general lighting.  Also, do not believe outrageous claims of lifetimes of 5 to 10 years.  Cheap lamps usually mean short lifetimes.  Finally, don’t buy white LEDs from eBay with ridiculous low prices from obscure manufacturers.
An American company CEO would go to jail if they consistently advertised that their bag of cookies contained 12 ounces of product when in fact the packages only had 9 ounces.  Why do we continue to allow Chinese manufacturers to claim a 5 year lifetime for their LED lights, when they don’t even make it through one year?

March 2010     Issue 7

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