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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."
It's being worked on. Basically the issue holding them back is cost/brightness. Given the inevitable lowering of costs of all things technological and the toxicity of CF-bulbs I think it's just a matter of a few years before LEDs take on the consumer lightbulb market in a big way.
uh... because they do and you can buy them.
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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.
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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.
Aren't LEDs less efficient for white light, compared to current streetlights with HPS? Wikipedia says 150 lumens/watt for HPS and only 10-90 for white LEDs.
http://en.wikipedia.org/wiki/Luminous_efficacy
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.
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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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The type of power supply used in LED lighting is called a 'buck/boost' converter. It is a switching supply that merely PWM's the filtered line voltage down at high frequency (40~60KHz) to the operating voltage and current of the load The difference between this and a standard switching supply is that no isolated secondary circuit is required and thus the only 'large' components are the rectified line voltage filter caps, load filter caps, choke and heatsink mounted FETs or IGBT's. This also neatly eliminates the surge problem because the operating frequencies of said supply is high enough to keep the caps relatively small.
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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.
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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...
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The company I work for has actually done a good amount of research on the technologies available for high efficiency lighting right now and they do indeed make warmer white LEDs. They look pretty nice and have an adequate CRI, however, their efficacy is poor enough compared to the cool white LEDs that they are in fact only about as efficient as compact florescent.
I think it has to do with the fact that the visible light generating part of white (and blue) LEDs are phosphors pumped by what is actually a ultra-violet LED. Now I might be wrong on this part, but I think that those larger wavelength colors are less and less efficient to make this way. I'm not sure simply putting in a red or amber LED would fill in enough of the spectrum to generate a pleasant light.
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.
...if every home in the US replaced their five most-used bulbs with CFL's, the energy and greenhouse gas savings would equal taking 8-million cars off the road. The numbers from replacing every street light in a city of 9m people could be just overwhelming. So let's do it nationwide. Now!
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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
LEDs only produce light in narrow bands of spectrum, so even if those bands are far apart, so the light looks white, the reflection from various materials may look nothing like the color seen under wide-spectrum source such as sun, incandescent or mercury vapor light.
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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.
Mercury vapour is MUCH worse than led in terms of "spiky spectrum". They nearly have no continuum at all. LEDs do.
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You need to look at the LED taillights on a Cadillac in moving traffic. In the dim mode used for tail/marker lights (not the full brightness mode used when the driver presses on the brake pedal), the taillights are being dimmed by PWM with no filtering. The flicker is extremely annoying and gives a strobe-like appearance where your eyes see multiple images of the lights in moving traffic.
Why Cadillac chose to dim their LED taillights this way is beyond explanation. It makes the cars look cheap, but it can't be a cost saving, because you can dim an LED array with a simple resistor and eliminate the PWM circuit altogether. LED taillights for heavy trucks use a diode and a resistor for the lower light output level and they look great.
The effect is more noticeable when you're traveling at a different speed than the Cadillac, or if you move your head side to side while looking at the taillights. It's really obvious and undeniable.
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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.
Oh really ?
Those tail lights were not flickering to the naked eye, it was not a police vehicle - it only showed up through the video camera. And BTW, I can see flicker too, not the extent revealed by the video, but almost imperceptibly. I know it exists. Whether it's a DC circuit or not is irrelevant, as the flickering is to do with duty cycle not frequency.
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.
Older LED traffic signal conversions probably replaced the old incandescent bulbs and color filters with LED panels made of LEDs using the appropriate color. They also in some cases may have used separate AC to DC converters located either in the control box for the traffic signal which would have been on the ground near the signal. Potentially this also meant rewiring all of the lights on each pole and replacing more equipment when required. On the other hand, an AC-DC converter could have been in each panel, which would require less retrofitting. Recently though, with the wide availability of 1W, 3W, and 5W high power white LEDs, it makes more sense (and easier) to use an LED fixture that is directly compatible with the old incandescent light socket.
The railroad industry, for instance, has begun using high power white LEDs for many of the various signals near tracks. A high power white LED, properly ventilated, can be lit for 10 years continuously and still retain 70% of its brightness. In both car and rail signaling applications not many lights are powered on continuously 24/7/365 and spend some time turned off this does of course increase the lifespan of the fixture. Still though, the parts of the power conversion for the fixture may fail sooner.
The traffic lights using the green LEDs you mention will become less intense as time passes. For me at least, in Minnesota, comparing from memory the LED traffic lights converted roughly five years ago or more do not seem as intensely bright as they once were. Some of this might be attributable to learning not to stare at such a light at night.
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This is incorrect. Modern white LEDs are quite similar to fluorescent bulbs. They use a high frequency (large bandgap) junction to generate UV light, which is "down converted" by a cocktail of phosphor chemicals, to produce a smooth output covering wide swatches of the visual light range. It is customizable, but it is based on what our eye and brain actually consider to be white (our eye and brain have a very nonlinear response to different wavelengths of light). We're not used to it since other lights are not as white as LEDs are.
Look and page 19 and 20 of this PDF to see what I mean: http://www.philipslumileds.com/pdfs/DS51.pdf