LED By the Nose

By Phil Plait | June 21, 2005 6:12 pm

There’s a road not far from my house that cuts across a lot of farmland. I drive on it to get to my boss’s house, or when I go to visit the wine country in Sonoma or Napa valley. It’s a beautiful drive, with lots of rolling hills, vineyards, and cows.

At the crest of a hill is a crossroad that leads into a residential neighborhood. To give the folks who live there a chance to get out, the cars on the main road have to stop at the hill top. A stop sign isn’t very useful there, because the speed limit is 60 mph on the big road, and you’d never see it in time. A stop light might help, but they’re expensive, and the small amount of traffic from the neighborhood doesn’t warrant it.

So the engineers put in a flashing red light, which can be seen for hundreds of meters away. On the drive, I can always spot it from quite a distance, blinking on and off about once every second. As I mentioned, the drive is quite nice in that area, which is conducive to long, unbroken chains of thought. A few months ago I was watching the light blinking as I approached it, and I suddenly realized that if the light blinks once per second, it must flash 31 million times per year!

I was amazed. What kind of light bulb could take that kind of stress? A normal bulb would explode after a short time. Ever notice how bulbs always seem to burn out when you flip the switch to turn them on? That’s because the current flowing into them is like a flood, and it stresses the filament. A nice constant flow of electricity doesn’t put much stress on the filament, but a sudden torrent of it does.

So how does the red light at that hilltop last for so long? One day, I happened to mention this story to my officemate, and he looked at me like I was an idiot. “It’s an LED,” he said.

Aha! Light Emitting Diodes answered my question. You’ve seen LEDs: they’re behind the Hulkishly green glowing numerals used in alarm clocks, for example. LEDs are fundamentally different than incandescent bulbs. A regular bulb has a piece of wire which gets hot when you run a current through it. So in a sense it gives off light for the same reason stars do: hot things emit light.

LEDs, though, give off light for a very different reason. Indulge me in an analogy: When you jump down off the last step in a staircase, your feet hit the floor and make a noise. You have just converted gravity to sound! The energy of your descent (which comes from gravity) is used to move air in waves (which makes sound). The same thing happens to electrons, too. When they move in an electric potential (in a sense, the electromagnetic equivalent of gravity) they also convert their energy gained into waves, but in this case those waves are light.

In an LED, electrons move when a current is applied. They respond by giving off light. There are a number of advantages here: for one thing, there is no filament. Electrons are happy to move back and forth zillions of times without worry. So LEDs last a long, long time. It’s not hard to make a lot of light using LEDs, either. I have a pen with an LED in one end, and it’s so bright it’s hard to look at, yet it only uses two little batteries. The color can be controlled as well, so you can have red, green, and blue LEDs.

That’s why the stop light at that hilltop uses LEDs. They’re bright, so I can see the light from a long way off. They can be made red, which is rather useful for a stop light. It can blink on and off literally millions of times a year without malfunction. And they can last for many years, so you don’t have to replace them very often.

And it gets better. You can combine red, green, and blue LEDs to make white light. The light generated this way is bright, cheap (LEDs are tremendously more efficient than not only incandescent bulbs but also compact fluorescents), and last virtually forever. I think they may soon replace light bulbs. Other people do too. This could easily save billions of dollars in energy costs, which is particularly useful these days.

I love stuff like this. It’s so cool! And it’s another reminder to people that understanding science can lead to amazing and unexpected ways of making our lives better.


Comments (36)

Links to this Post

  1. Project Nothing! » Blog Archive » Edison is so 1900 | June 22, 2005
  1. hale_bopp

    That would be cool if LEDs could replace white lights. However, the white LED lights I have seen have a funky whiteness about them…they aren’t tuned to what appears to be a natural white to me.

    I am sure this is an engineering problem that can be fixed, but IMHO, it hasn’t yet.


  2. Jon

    Can they be used to grow plants under them? Effeciently enough to be used for space travel?

  3. Jon Niehof

    Actually, white LED’s aren’t tricolours. They’re violet or UV LED’s with a fluorescent coating, very similar to CF’s. And because LED efficiency decreases rapidly with junction temperature, the brighter white LED’s (3W or so) are actually less efficient than CF’s. LED house lighting is still a ways off.

    I like LED traffic lights, but apparently the very narrow emission width causes problems for people with some forms of colour blindness, so they crank the brightness way up. If I’m the only person on a road without overhead lighting, a green traffic light can seriously ruin my dark adaptation.

  4. Bored Huge Krill

    just an aside:

    if I remember my materials science correctly (and I confess it has been quite a long time) the failure of incandescent light bulb filaments is usually caused by fatigue. The filament emits light because it is very hot (duh), but the temperature also causes the springy filament to be sufficiently ductile that it deforms plastically under its own weight. Repeated cycling of the bulb on and off subjects the filament to plastic deformation fatigue cycles until finally it fails – the final “switch on” causes one fatigue cycle too many and it literally breaks under its own weight.

    I vaguely remember the analysis from college days, predicting how many cycles a given filament could take. What I’ve never figured out is why bulb lifetimes are always quoted in hours, given that a significant component of the durability is the number of on-off cycles… oh well

  5. Edward

    From what I’ve read, one reason for short lamp life is “cold inrush current.” The electrical resistance of the incandescent filament is proportional to its temperature. So, the hotter it is the higher its resistance. When the lamp is switched on the filament, due to its low resistance at room temperature, allows several times its working current to go through for a fraction of a second. This quickly heats up the filament, leading to increased resistance and thus limiting the current. After a good number of switchings, the filament eventually capitulates to the repeated shocks.

    Would love to see LEDs replace incandescents too. They’re so cool (uh, literally), very compact, and have such a long life. The catch is they’re still too expensive, particularly the white ones (which have a funny kind of whiteness to them). Got half a dozen of those lying around here, still figuring where to use them. But what I’d really love to get my hands on and toy around with is a Luxeon Star 5-watt LED.

  6. LEDs are only more efficient than compact fluorescents for specific colors–a red LED will beat a filtered CF–but they are about as efficient as a halogen for other uses.

  7. Brock

    Actually, the current going through the light bulb doesn’t only change when you flip the light switch on or off – They use AC power, remember, which means the current goes from flowing in one direction to nothing to flowing the other direction and back again. In north america, this cycle goes on at about 60 Hz, meaning the bulb is essentially turned on and off 120 times per second. In Europe, I believe the frequency is 50 Hz. The only difference between that change in current and the change when you actually flip the switch is in how hot the filament is – Since the frequency is high enough, the filament doesn’t have enough time to cool down when the current is at zero while the switch is on, so its temperature stays pretty constant. But when you flip the switch on or off, the filament does actually heat up or cool down.

  8. Michelle Rochon

    Well I just learned something today… I didn’t know how LEDs worked…

    I know my dad busted one of mine that I had put on a school project when he decided to overload it with current just to see how I did my stuff. That was weird.

  9. Thomas Siefert

    We (me and the better half) lived in Singapore for a while and left in 1999. About a year latter we where back there for a wedding and we found (to my nerdy exhilaration and my wife’s indifference) that all the light bulbs in the traffic lights had been replaced by LED’s. Not just one but ALL island wide.
    Here in Australia you only see one LED traffic light here and there, but most are still old fashioned light bulbs.
    When things happen in Singapore, they happen fast….

    It also reminds me of back in Denmark many years ago I was holidaying on a island called Fyn (funen) in a town called Odense. All the pedestrian crossings had two red lights on top of each other. Years later I found the reason for this weird setup was that an old man had been killed because the light bulb in the red light had blown and he had walked out in front of a car. The city council then decided that to prevent a similar tragedy to happen again, all pedestrian crossings would be equiped with an extra red light.

  10. wedgebert

    I’ve got a few LED flashlights on my keychain and they are pretty cool. They’re about as big as a quarter and maybe a cm tall. They’re quite bright and come in handy for things like working inside a computer case or in my car.

    I’ve got the standard white light so I can tell colors apart, a red one with an enclosed bulb so I don’t ruin my night vision, a turquoise one because it was the brightest one available and finally a violet/UV one because it’s fun to have a small blacklight where I go. They run off watch batters (either one CR2032 or two CR2016s) and last a long time. The red and violet bulbs will last over 100 hours, longer if I set it to medium or dim mode. Unfortunately, they’ve just released versions that are up to 2x as bright with the same battery life, so it looks like I need to go upgrade.

    For those interested, the website is http://www.photonlight.com

  11. Citizen Of Trantor

    Mmm. I think compact flourescents are still the way to go form home lighting. I have one in my porch light that is 12 years old. That was the first one I ever bought, and the only two that have died are a result of dropping them. They are also very cheap now, with Home Depot having six and twelve packs of the things.

    The LED traffic lights are neat and all, but the cluster of little dots always looks out of focus to me. I find myself starting to try and blink away the fuzziness at a long red light.

    For those of you with a new found urge to play with LEDs, always put a resistor in series. The value of the resistor is left as an exercise for the student (always wanted to say that).

  12. Paul West

    The wilderness sportspeople have been using and following the LED illumination device progression for some time. A good review of the stuff that is in the marketplace (or soon will be) is here:


    I would expect these devices to be migrated rather rapidly into the home market.



  13. Timothy Gaede

    You can buy LED Christmas lights. Traditional lights use filaments and are are not too efficient, even less so than standard filament bulbs. Also, most of the visible light produced by traditional lights is absorbed by the colored bulbs.

  14. VisionEngineer

    I have noticed that LEDs are starting to be used for tail lights and brake lights on trucks. I can’t recall seing them on any cars yet, but I’m sure they will come. It seems like it could be a real safety issue since several LEDs could burn out before the light is unusable. I use LED lighting a lot in my profession and over the years I’ve seen LEDs getting cheaper, bigger, and brighter. I think they will be used for household lighting eventually, but it may take a while.

  15. Bored Huge Krill

    I think I agree – but as far as I know the mechanism by which the filament fails is fatigue due to the constant thermal cycling causing plastic deformation of the hot filament each cycle

  16. Bilateralrope

    My dad bought an LED torch (can’t remember if it was 1 LED or 3). After going camping with it a few time he wanted t know how much charge was left in the battries (2 AA battries IIRC). It remained on constantly for 3 days before we even noticed it dimming at all.

    Sure, the light that it gave off seemed odd, but with that much battery life, its worth it.

  17. Thomas Siefert

    To Timothy,

    You said:
    “You can buy LED Christmas lights. Traditional lights use filaments”

    NOT TRUE… traditional Christmas lights use candles…. When I was young we used to… etc. etc. etc……

  18. Videodrone

    Brock is correct in that the light turns on and off 120 times a sec in the US but it is the inrush of current when the filiment is cold that causes the damage
    C Crane is now selling LED “light bulbs” but at 35 bucks a pop I’ll wait a while

  19. ByTheWay

    The first time I noticed LEDs as traffic lights, I also noticed that only the green and red bulbs were being replaced…not the yellow bulbs. I figured that they replaced the old bulbs with the new ones when they burned out, but the old yellow bulbs lasted longer. Well, the yellow light is rarely on…so why not?

    Inexpensive, efficient, –bright– white LEDs are still a ways off — don’t hold your breath. Yeah, the light has a different quality to it. But then again, look carefully at household incandescent lighting — that orangeish color really gets to me sometimes. Or the buish color of others. The fact is, it’s hard to get light that is “normal”…what’s normal, for that matter??

    RGB LEDs offer lots or promise in this regard. Most types of bulbs can be balanced to match, say, sunlight. But 3-color LED’s can be balanced rather easily (even on the fly, if need be). For those that need the right color balance for different situations (photographers, for instance), this could be invaluable.

  20. Thomas Siefert

    To ByTheWay,

    Sunlight is normal…. Halogen lights comes closest in a normal household.
    Big hollywood arclights are closer but they are not practical in a normal home.

  21. LarrySDonald

    I’ve looked into LEDs for household lighting a few times, but every time they’ve been a bit better but not yet beating CFs. I’m continually amazed at how popular filament bulbs are. CFs have been around for ages, and while the initial investment is a little higher for CFs, even there they are usually cheaper per time unit of operation then filaments and that’s not even counting the energy savings. Even so, seeing a person (as opposed to a company or institution) using all CFs is more of an exception then a rule. I do use a few filament bulbs in areas where they are very prone to being smashed (and thus not likely to last more then a month anyhow) but the rest is CF. Given how few people have even given up filaments, I doubt LEDs are poised to take over anytime soon.

    I suppose in one sense I’m less picky; I’m somewhat colorblind and mostly non-artistic so as long as something is bright I could really care less about the spectrum. It could be skewed all over the place and I probably wouldn’t notice.

  22. Glen Thompson

    While newer traffic lights are LEDs, older ones are still incandescent. How do they stand up to that many on/off cycles? Simple – they are never completely off. Enough current is maintained during the off cycle to keep the filament hot but not hot enough to emit noticable light.

  23. I was wondering something about LED lights when considering buying LED Christmas lights last December. Maybe someone can tell me why they are only available in certain colours?

  24. Taks

    on one hand, it is amazing to me that the concept of LEDs is not more widely known, on the other, i suppose my career has spoiled my objectivity in assessing the reasons. as an electrical engineer, i’ve been working with “blinkies” (marketese for LEDs that customers like) for 10 years. everyone has seen LEDs, however, in such notable products as the “graphic equalizer” on our cool 80s car stereos.

    in general, older LEDs were rather large and not very bright, but worked well enough on a faceplate that you could differentiate on/off. newer LEDs, however, have become increasingly brighter. they also come in more than just the standard red and green, with blue, yellow, white et al becoming more and more popular (just looking at a short list at digikey they have “super intensity red”, “super ultra green” and many bi/tri-colored as well). heck, newer LEDs are bright enough for infiniti to use in their taillights… definitely nifty technology.


  25. Terra

    Here is a great resource on LEDs, what you can do with them, what we can expect in the future, and how much they cost.

    Don Klipstein’s LED Main Page

    High efficiency LEDs page

  26. russell

    Domestic lighting may go straight to huge flat screen that cover every wall and display moving scenery like countryside etc and at the same time provide the equivalent light of a 100w light bulb. If cheap enough the whole house could be walled in tv screens(which I suppose are also LED based).

  27. DennyMo

    I just wish the engineers who design those LED traffic lights would put day/night sensors on them, so you don’t go blind when the light turns green at night. Ouch!! Other than that, I love them. It’s taken me a while to get used to seeing LEDs as the primary tail lights in cars. Incandescent bulbs have a “soft on” quality, the LEDs have a very abrupt “ON” quality. Nothing wrong, just different.

    Another advantage: if one LED fails, the rest of them still work. It gives you some cushion before you “have” to replace the light.

  28. Ed Darrell

    LEDs are used in tail and brakelights in the more expensive Cadillacs, BMWs, and Infinitis. You can buy an LED array to replace your taillights, online, or at places like AutoZone and PepBoys.

    It’s a question of engineering that they have not spread farther already.

  29. Marek

    LEDs might be great but the longest burning lightbulb in history is still a traditional one:


  30. um3k

    Marek, you must take into consideration that LEDs have only been around since the 60’s.

  31. LarrySDonald

    Really, it’s entirely possible to make better incandescent bulbs in terms of durabillity. Usually, they are simply drawn thread of wolfram (tungsten if you’re all US) housed in an atmosphere of Argon to keep it from being oxidized. A theoretically perfect filament would be burning long after you’d dirtnap and couldn’t verify. Even very well made filaments would last much longer then the 750-1000 hours most are rated at (which is taking into account the high restistance and damage with the “cold starts” before reaching normal operating temperature). Optimizing this is mostly economics; as filaments get more refined the price difference is by no means linear. Making a filament that will burn 10% longer doesn’t just cost 10% more but increases faster as percitions go up. Hence, you will hit a limit where it’s cheaper to just make a fresh bulb to replace it rather then go through the trouble of making the first one last. This is further complicated by the market, plenty of people would cheerfully buy an 80 cent bulb that would burn out in less then half the time of a $1.60 or even $1 bulb. This is one of the zillions of paradoxes of humans in general and one of the banes of LEDs and CFs. You can, right now at your local walmart (or other convenience store), buy a CF rated at 20k+ hours for $6 or so, way less then 1/20th of a 1k hour bulb (a typical 1khour bulb should at that rate cost $6/20=30 cent. They’re more then that). Yet, people mostly don’t. If one were to go ahead and install CF tubes instead of regular lightbulb sockets, this would plunge the price/lux/hour even further even including the new equipment and considering the energy cost, well, you guessed it, even further still (hint: It’s not a conincience that most corporations have all CFs in their buildings). It’s really quite insane, most Cfs will break even vs bulbs in less than four months counting investment, lasting power and energy and that’s even without counting the more human “The ammount your wife will bug you to change it will be reduced by 95%”. Do it man, get CFs and be a happier human.

  32. I use many CFs in the house. My only complaint is that the ones we bought give off a somewhat harsh white light. We’re used to softer tones, I suppose, but if a CF is out there with bettercolor, I’d like to know.

  33. Jon Niehof

    Phil, you might want to try some full-spectrum CF’s:

    They’re a bit more expensive but pretty nice. They’ll also still look a bit blue to you; incandescent lighting is very low-temperature compared to daylight.

  34. The other massive advantage of LED lighting is that their light spans a very narrow arc and is directional. This would help reduce light pollution immensely, in that streetlights and floodlights built with modern white LED tech would focus much more of their light downward, where it is needed, and very little (only whatever gets reflected back from the ground) would be “leaked” back into space. Which would make a lot of backyard astronomers very happy, since light pollution is the bane of their existence. Moths and baby sea turtles would also be less likely to be confounded by it. More light, better spectra, less energy, and better control over where the light goes. And I’m not even an LED salesman …

  35. David Read

    I stumbled across this thread after a Google search for ‘problems with LED lighting’. I see that these postings are over a year old but I’m wondering if you or anyone reading this might be able to help me.
    I bought a couple of those outrageously expensive LED indoor lights (PAR 30 floodlight). When I installed them, they immediately began to glow – a bit of a surprise since the switch was off. I thought I might have a faulty switch, so I tried another circuit – same thing. I started checking and it seems that, even though the switches are off, the circuits still carry a small amount of voltage (1v – 1.5v). My house is relatively new (8 years old). Was I the recipient of a bad batch of switches? Or is there something else going on here?


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