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New York City Street Lights To Go LED
eldavojohn writes "Wired has a short piece on NYC's new street light project. I don't think we need to belabor the many benefits that LEDs hold over traditional light bulbs, but the finishing touches are being addressed, and they will hopefully be put into place sometime next year. This design won a competition back in 2004, and OVI has been whittling down the prototypes. At $1.175 million, this sounds like a pretty cheap deal considering the DOE forked over $21 million to 13 R&D projects along the same lines."
uh... because they do and you can buy them.
Some drink at the fountain of knowledge. Others just gargle.
The white LEDS are doped to generate three distinct colors of light (R,G,B) whose combination yield a very cold blueshifted white light (>6500 K). If one seeks to use these for video, better check to see if the camera works well with such light.
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There are plenty of LED traffic lights around me and I've never noticed any flicker. I imagine it isn't a problem.
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You can buy a single 5 watt led that is the same brightness as a 50 watt incandesscant.
Some drink at the fountain of knowledge. Others just gargle.
LED light bulbs do exist (they're a bunch of small LEDs, not one jumbo one -- I don't know if that's feasible).
I think you hit on the problem in your post though, power. 220V (or 120V) AC certainly has enough watts, but it's not in a usable form for LEDs. They require direct current (DC) at a much lower voltage. So you need some power conversion electronics to make them work. Then, to make them work efficiently, you need more electronics to regulate the current through them. For a standard electronics project, you just use a resistor, but then you're wasting power (to the tune of P=R*I^2). Off the shelf components that regulate the power more efficiently exist, but it adds expense.
Fluorescent lights need some electronics to work too, but I don't think they're as complicated (and are thus, cheaper). Cost is a big factor here, because old incandescent light bulbs don't cost much to purchase.
FTA, the ~$1million is for building and testing six working prototypes. The design will then be added to a catalog the city uses, and they can then install them as they see appropriate.
I'll never make that mistake again, reading the experts' opinions. - Feynman
The lighting product manufacturers quote efficiency in lumens-per-watt(lpw). What they don't shove in your face in marketing is that the devil is in the details.
CFLs, LEDs, incandescents, HPS and metal halides all have drastically different spectrum outputs. Incandescents have a very broad spectrum but their lpw is astonishingly low.
CFLs have as much as 80 lpw, whereas MH and LEDs are currently at about 100 and HPS can be even higher(around 140 lpw initial, which declines over time). LEDs have the potential to be higher than HPS but across the lifetime of the HPS bulb the LED may end up with a higher average lpw and definitely much longer service life.
There are CFL's with a broader spectrum but they're less efficient. While not completely monochromatic, there is a big spectrum spike in reds and yellows for HPS bulbs. Most people find this light to be soothing. Metal halides have a broader spectrum than HPS but are less efficient than even fluorescents. There are new white LEDs in research that produce as much as 145 lpw, but these are not commercially produced yet. Philips produces a 115 lpw white LED which is available in large quantities. You're right about the blue light hazard though - phosphor based white LEDs have a large spike around 465nm.
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You don't have to have a bridge rectifier. Just run two strings of LEDs with opposing polarity, in parallel, and you have the light of one string containing the same number of LEDs, but at 120 Hz. What the bridge rectifier gains you is a fuller duty cycle, rather than one something less than 50%, and just more light from each LED. Whether you want that or not depends on heat.
Bruce Perens.
Actually, LEDs get dimmer as they get used. If they don't fail due to the semiconductor turning into molten metal, they get dimmer and dimmer and dimmer. The 100,000 hour lifetime figure on LEDs is usually the time until 50% brightness (considered to be the point where one would notice the light being dimmer).
There are many reasons for this - degradation of the junction itself, but the semiconductor itself leads to a large index of refraction - a lot of the light in a LED gets reflected back into the semiconductor. And then there's degradation of the epoxy used to seal the LED. All these conspire to make the LEDs much dimmer, and get dimmer over time.
Well, they have been testing these lights in my home town of Scottsdale, and they have three different types installed along one stretch of road way. They are super bright, and there is no flicker whatsoever.
The fact that they are directional is an advantage in this case since they are meant to throw light in a cone shape. The ones I've seen have no secondary lens. If there is any covering at all it is completely transparent glass.
Personally I like them because the light is white, not the orange of sodium vapor. Reminds me of when I was a kid before the move from mercury vapor to sodium vapor...
I still cannot find the droids I am looking for...
The biggest problem with LED traffic lights is that the greens are REALLY bright. You'll be shocked, especially at night how bright the damned things are. In SoCal, we have LED traffic lights everywhere.
Cars and buses with LED tail lights are not running them at 60Hz. Nor are they being pulsed at any rate. The electrical system in a car is 12V DC. The LED assembly is either designed with enough LEDs in series such that the forward voltage drop over the set is 12V (roughly 10 LEDs) or they are in parallel with a buck-style switching power supply in front of it.
Now, before you go on about how the switching power supply causes flicker, you should research how they work. You will find that for cost and size reasons, it is better to run a buck topology as fast as possible. 250kHz, 500kHz, and 1MHz are common frequencies. Of course, the output from the switching portion is put through a LC filter such that the voltage ripple is reduced to a small percentage of the target output voltage. Besides, LED brightness is controlled by current. Even a 5% voltage ripple on a 2V output would trigger a few lumens of brightness difference.
So, if you are seeing flicker in car and bus tail lights, then you can see a 250kHz "flicker" with an average brightness delta of a few lumens. If you can, I'm sure there are plenty of researchers who would love to talk to you as you are the only person on the planet who can.
Of course, since cost is the driving factor in these types of devices, they probably aren't using the switcher at all and thus there is _no_ flicker due to electrical reasons. You are probably being more affected by the directionality of the LEDs and the lenses used being vibrated by the engine at idle speeds. You get the same effect watching a motorcycle headlamp on a rough road. The light isn't flickering, it is just vibrating enough that the beam is falling in and out of your eye.
kc8apf
First, the GP is right: most high power white LEDs are actually blue or UV LEDs with a yellow phosphor in the plastic packaging.
As for phosphors yielding a spiky mess for a spectrum: how exactly do you imagine the spectrum of an RGB LED looks? The individual primaries in such combinations are VERY narrow band, so rather than a continuous spectrum you get three distinct peaks. Phosphors are actually smoother by comparison.
The flicker is not in his head, it's in the taillights. I've seen the flicker, it's caused by a pulse width modulation circuit to make the taillight mode of a combination taillight/stoplight appear dimmer. A quick google search pulls up this article http://findarticles.com/p/articles/mi_m0EIN/is_2006_Nov_8/ai_n27039046 about an automotive product specifically designed to address this issue and stop the flicker by eliminating the pwm circuit. It works by reducing the DC drive to the LEDs in taillight mode instead of using pulse width modulation to reduce the average current and effective brightness.
Incorrect when talking about LEDs. "White" LEDs are covered with a phosphor that takes a blue LED's light and shifts it down. The output from the phosphor is broad spectrum, even if the original LED was a narrow band blue. Thus, these LEDs are a good wide spectrum light, instead of an approximation made from mixing red, green and blue LEDs. Of course, the problem you described can exist, but is commonly seen only with fluorescent bulbs.