I've seen buses, trucks and a few cars using LED tail lights. I know LEDs have long life times..even more with power limiting and intermittent use... I've also read somewhere that LEDs get dimmer with usage. I wonder... Would there be a time when it's so dim it's a hazard? Would someone have to use a light meter someday to figure out if the tail lights are not bright enough and need changing? Perhaps it's been timed to match the lifetime of the vehicle?
The quirky good thing about incandescent is that it burns out. ("Better to burn out than to faint away"..Who said that?) D from BC
AFAIK your basic bright red LED's will last forever.
Some white LED's use phosphors, as they're actually ultraviolet LED's with the phosphor converting the uv to visible light. Now phosphors do degrade with time.
So red stop lights and yellow turn signals are likely to last forever, much longer than most cars last anyway. White LED backup lights, if they ever come into use, may only last 20,000 hours or so. But how much time do yo spend backing up?
You have a valid point there. When a manufacturer specifies an LED life expectancy, if they do, it is in "half-life." at rated current. There is no standard test technique that the whole industry embraces - ambient temperature, lumen output, color temperature etc..
I'm waiting for the new traffic lights with arrows formed from the LEDs in the green light being dimmer than the rest.
I made an LED night light, that burned 24/7 - initially it lighted the whole bathroom and part of the hall way, at five years it was enough light to tell where the wall was and nothing else. Replaced the LEDs and the newer ones appear to be lasting longer but its only been two years.
Lets hope that cars and trucks don't get the time on the LEDs or they are sufficiently de-rated to keep safe - I know the DOT is way behind on laws and standards and the manufacturers are iffy at best.
Barring that, I really like them - I put four Cree LEDs in my standard motorcycle tail light and it is easily twice as bright when the LEDs are loafing along at 60 milliamps and really hard to look at when at
750. The two 1157's were using 1200 ma when low and 4,200 ma on high. That's another 20 watts my alternator doesn't need to produce. Same game with the turn signals.
I put 56 cheap white LEDs below my headlight and it is more noticeable in daylight than the low beam headlight - so much so I rigged it to turn off when signaling to avoid confusion.
My guess is the technology will improve to the point where the question is moot - or do so before "law makers" (that's too kind - really meant scum sucking, lobbyist supported, politicians) wake up to the fact that technology has changed. In the meantime I'm keeping an eye on my brake, turn and white lights.
Ever watch CSPAN when technology is being discussed? Their questions indicate they have no clue as to what is being discussed - yet they nod their heads sagely pretending to be on top of it . . .
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LED's operated conservatively will last decades until they fail, and when they do fail it is often a packaging failure.
LED's operated without adequate heat sinking, or with excessive mounting stresses, can go poof after just a few hours or after years. These are failures related to how the LED was mounted but in some industries none of the LED's are mounted correctly.
LED's operated in harsh conditions (e.g. truck marker lights) will probably fail for some other reason than the LED failing, e.g. bashed in a collision or the PCB they're mounted on will go open circuit somewhere.
But yes, LED's do get dimmer over time as they are operated. So do incadescents.
The lens housing will also lose transmission over time as the plasticizers darken and scratches abrasions take place, this may occur slightly less for a LED due to less IR heat being produced in the lamp, but most of it is environmental degradation.
Is there an industry standard to express LED lifetime?.. Is that 20
000 hours to complete dead or maybe that's 20 000 hours to half-brightness? I wonder if it's best to ignore sell sheets and just look at the datasheet? D from BC
On 12 Jan 2007 12:03:41 -0800, "Tim Shoppa" wrote: [snip]
[snip]
Got some experience with that.. I did some crappy mounting of Luxeon 3 emitters using spec sheet data and it only lasted 2 years.. (Too embarrassed to say how I did the mounting...) It was running continuously 10hrs./day for 2 years and then failures. D from BC
As far as I know, all led's degrade with time, depending roughly on the time integral of applied current. At rated current, half-brightness levels can range from roughly 100K hours to a lot less. Optocouplers have the same problem: their IR led's degrade and CTR falls, so it's best to keep their led current as low as possible.
Years ago, when I worked on the reliability of LEDs, we used half-life to mean that the brightness decreased to half when measured with a photometer and not by the brightness your eye see; there is a difference. In those days, 2000hrs of continuous operation at room temperature at rated current turned out to be the half-life. The failures occured due to what are called "dark-line" defects. These multiply over time and reduce the active area which produces the light. The "dark-line" defects can be caused by mechanical stress to the die and impurities and defects in its manufacture.
Temperature is the bad guy. Hi intensity LEDs are sure to get hot. So the "half-life" will depend on the design of the heat sink, the ambient temperature, the duty-cylce.....
Before they were commonplace, I had installed in my Jeep a security system. After about 7 years, the blinking red LED had become dim so you could see it blinking only at night. I replaced it and new one still seems bright enough to me. Then again, technology of the LEDs has improved considerably. Oh, and my Jeep is now 16 years old.
They don't last forever but some of the half-life's are in the thousands of hours and driving for thousands of hours is unlikely unless you're a long haul trucker - and they do use LED lights.
Last time I even looked at a specification that gave the half- life it was something like 2,500 hours. That is how long it takes to reach half its lumen output, at 20 degrees Celsius, at 20 milliamps - or was according to that manufacturer. Raise the temperature, and all bets are off - or there would have to be a de rating curve.
Red yellow and green traffic lights do stay on, so those should be the ones to go first - before auto tail lamps - assuming the same quality of manufacture.
I notice some of the traffic lights here have LEDs that don't light at all. They don't all appear in any quadrant but are scattered four at a place, diagonally, in the whole 7" or so a traffic light uses - probably a solder joint or resistor out - smart not wiring a series in only one place.
Presumably some person smarter than you and I (unlikely) takes into consideration the time spent braking, backing, and other factors like if you have a really small light emitting area, a bit of dust, minerals, or bug splat, counts for a lot more than if you have hundreds of square inches, and writes the specs so we don't have to worry. That may be too optimistic, politicians, influenced by lobbyists, write the laws or govern the agencies that write the rules.
On the brighter side - even if all the bean counters, politicians, and corporate CEO's get it wrong - there will still be some lawyer that will profit to the extent that, eventually, it will be made so safe as to be useless.
gospel according to bob
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At current, I can't help but suspect a "Bill Gates" strategy might be in play with power LEDs.. Release it first, make millions...fix it later.. In the meantime, new LEDs give designers something to work with now to prepare for future applications.... D from BC
Was it Douglas MacArthur? "... but I still remember the refrain of one of the most popular barracks ballads of that day which proclaimed most proudly that old soldiers never die; they just fade away."
Jack London? " I would rather that my spark should burn out in a brilliant blaze than it should be stifled by dry-rot."
Saw the same thing in my night light - the technology is improving, but why isn't there some agreed upon testing by all manufacturers to specify half-life with some authority - and then give de-rating curves so one can apply them intelligently.
And do those thingees (tap-light, turtle lite, key fob light, et al) really last 100,000 hours? Or, does it take a really hot photo multiplier tube to see any light at all if it lasts 100K?
I gotta figure you'd be broke buying batteries before they'd honor that warranty.
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Now that's something I have to read up on.. Light energy vs perceived brightness... My memory is foggy in that area... Something like ..eye brightness perception is logarithmic.. Also time and colour dependant. Also I've heard older eyes sense less brightness. When I see a 20 000 life spec....Hopefully I can convert that to..call it .."useless perceived brightness lifetime".. :) D from BC
\\ They never die - they just fade away. Like you wrote in your original post.
Ah yes, the manufacturer's sheets are changing faster than you can keep track of (assuming if they post the half-life at all).
This is one of the things you should treat as religious, either LEDs are good or bad. Take it on faith or personal bias until someone mightier than you and I make them follow some standard - which the market will do, eventually, when they find sales follow performance AND longevity.
If you "grew up" with old LEDs you're bound to be more skeptical than the newbies. Unfortunately some of the newer "high brightness" LEDs die faster than early 1969 parts . . . that's life and manufacture hype.
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All these factors and more were considered in early Hewlett-Packard application notes for automotive LEDs. The take-home message was: LEDs last longer if used gently, but hard drive and faster wear can make sense too--no one needs 100,000 hours of full-intensity brake lighting.
Same applies in traffic lights. Application Brief AB I-004 "The Reliability of Precision Optical Performance AlInGaP LED Lamps in Traffic Signals and Variable Message Signs"
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describes a 55% RED duty cycle in traffic lights. Dimmed at night to comparatively insignificant drive levels, that overall factor is nearly halved, so the Agilent (Avago) LEDs in question OUGHT to last 100,000 operating hours / ~30% duty = 333,000 calendar hours, or about 38 years, I figure.
"Application Brief - AB I021 Long term reliability for A1lnGap T1 3/4 LED" recommends 20-30mA drive for traffic light applications, and no more than 50mA for automotive.
The majority of the green LED traffic lights in San Francisco have failed clusters of led's, some up to maybe half of the total area. These are mostly strange-shaped regions of blackness, with the occasional zone of weak intermittent blinking.
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