It seems like LED’s used specifically for illumination are making their way into the mainstream. I’ve seen lots of advertisements for flashlights where the bulb is actually one or more LED’s, and in many areas incandescent traffic lights are being replaced with LED’s.
So I have a few questions:
[list=1][li]Would LED’s be a good application for household lighting as well?[/li][li]Can an LED emit “natural” looking white light?[/li][li]Are there any LED-based lightbulbs that can be screwed into a lamp in much the same way that I can buy a compact flourescent bulb instead of an incandescent bulb?[/li][li]How would such a lamp be constructed? Would there need to be many outward-facing LED’s to equal the wattage of 60 watt traditional bulb?[/li][li]Would an LED light bulb be more or less energy efficient than a compact flourescent bulb of the same lumens?[/li]
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LED bulbs would be excellent for home use. A typical LED has a lifetime of 100,000 hours, compared to 800-1000 hours for a typical incandescent bulb.
LEDs can emit light as natural as any fluorescent bulb. So-called “white” LEDs are actually UV emitters which stimulate a phosphor layer similar to that used in fluorescent bulbs. Adjusting the phosphor mix can produce various results to suit individial apoplications.
I don’t know, but I’ve not seen any yet. I have seen replacement LED flashlight bulbs designed to replace standard bulbs.
Not rterribly many, I suspect. LEDs are far more efficient at producing light than incandescent bulbs, and better even than fluorescents.
That’s damning with faint praise if I ever heard it. But phosphors have really improved in the last couple decades. A really natural looking light could be just around the corner.
Here’s an outfit that sells screw in (lamp-base) LEDs.
The price of LED lighting is still high though. From the link: “An 18” strip light, which is the equivalent of a 25-watt incandescent lamp, costs $128."
I was doing a bit of googling, mostly because my memory of how a while LED works was slightly different than what QED posted in number 2 above, I thought it was a mixture of different materials, not a phosphor layer… I could be wrong…
Anyway, one of the first links I found happened to be a place that sold (among other things) a white LED residential type light bulb.
For only 89 bucks :eek:
I’ll wait until the price comes down. But, at least they are out there.
The second and third approaches are the most commonly used ones and both use phosphors. To be honest the second one is new to me, I’ve always thought the third method was the most common for single LED lamps.
I read a story a few years back on white LEDs being made via differential doping of the emitter, so that a single unit emits red, green and blue light, but I think QED’s got it right for the mass produced white LEDs that are now so common.
He meant that quite literally, I believe. LEDs and fluorescent bulbs both use phosphors to convert UV into white visible light. So one can have as natural a color as the other, and no more.
Anyway, there are a few problems that make LEDs unsuitable for residential illumination at this point:
[ul]
[li]They are small. The brightest commercial LED is the 5W Luxeon bulb made by Lumileds, but those are only rated for 500 hours of operation. 1W Luxeon LEDs, and most other 1W and smaller LEDs, are rated for 50,000 hours.[/li][li]They are not particularly efficient. Luxeon LEDs are 25-30 Lumen/W while halogen bulbs are 15-25 Lumen/W (both numbers from a Lumileds press release). Fluorescent bulbs can usually achieve 50 Lumen/W or better.[/li][li]They require driver circuits. They can be either efficient or cheap, but not both.[/li][li]They are easily damaged by heat. If you want to make a cluster of white LEDs, you’d probably need to solder all the LEDs to a big heat sink and put a cooling fan on it.[/li][/ul]
For illuminating a room, you can’t beat fluorescent lights for efficiency. The only problem with fluorescent bulbs is that it’s a very large and diffuse source, so you can’t use lenses and mirrors to create a focused beam. For spotlights and car headlights you want a compact light source with good efficiency; that means halogen bulbs or, if you can afford it, high intensity discharge (HID) lamps like xenon bulbs and mercury lamps. HID lights are as efficient as fluorescent lights, and you probalby know, they are already widely used in outdoor stadium illumination and as headlights on luxury cars. For now, white LEDs are only useful for low-power applications like flashlights and bicycle headlights.
Note that I’m only talking about white LEDs here. Colored LEDs are more efficient than white LEDs, and significantly more efficient than using color filters on incandescent bulbs. Which is why we see LEDs widely used as brake lamps on cars and as traffic lights.
Well, this thread sucked me into taking another look at the LED museum to see what was new in white LEDs. They’re starting to look pretty good. I also see that someone has come up with filter caps that fit on blue LEDs, and can give a wide variety of different whites.
Boeing is adapting LED’s to use as lighting on the inside of aircraft. Newer 747’s and 777’s use LED’s for night and accent lighting applications. It is coming to the 737 line next year.
I love LED penlights! Batteries last forever and they give a nice focused white light. In house applications, beyond all the other reasons mentioned, think what would happen if you knock a lamp over and crack out a $60 - $80 LED light bulb. For a family with pets, growing kids, not too careful adults, WWE wannabes, actor/ singers waiting for callbacks, toad lickers, boxing kangaroos, human flys, poltergeists or ax wielding bunnymen, this could happen several times a year. Incandescent lights are cheap enough to be sacrificial.
What driver circuits? For LEDs all you need is a voltage source with a current-limiting resistor. With the right resistor, it’s possible to run LEDs right off the mains with no power supply circuitry at all. What’s cheaper or more efficient than that?
I meant energy efficiency. A resistor is a cheap and inefficient driver circuit because the resistor uses up power. For example, the CatEye EL-100 bicycle headlight uses 4 AA batteries and a current limiting resistor to drive three parallel LEDs. I took some measurements and found out that over 30% of the power is consumed by the current-limiting resistor. I think 70% efficiency is pretty bad for a driver circuit.
The optimal way of driving an LED is to use an active current regulator. There are some PWM current regulators that have >90% efficiency and I think some high-end products like the SureFire use those, but they’re not very common.
This may surprise some people, but white fluorescent bulbs are more efficient than white LED’s. I believe LED’s only come out ahead when you desire a specific color (red, green, yellow, etc.)
Powering an array of LED’s from 120 VAC requires the use of a pulse modulated driver circuit if you want high efficiency. In fact, there are chips specifically designed for this purpose. IIRC some of these chips can drive up to 200 LED’s. They chips include an efficient PWM circuit, a fairly sophisticated closed-loop power control circuit, and they do not require a line transformer.
If colored LEDs are more efficient than the phosphor white ones, then why not use the RGB (or other color mixing) scheme to produce white LED light? A mixture of colored LEDs can’t be any less efficient overall than the least efficient color.
As for running an LED straight off the mains (or with nothing but a resistor in between), that’s going to be awfully ineffecient, and probably not too good for the LED, either. Remember, the D stands for diode, and house current is AC. Voltage across an LED in the wrong direction won’t light it up, and might damage it.
Arrays of LED’s are being used in traffic control lights. Intial cost high but replacement costs for incandescents and long time lower power consumption make the LED a viable option.
I had some sample white LED’s, one in a two cell mini-mag and it was wonderful. The led was laying out on the desk while the original bulb was used in the mini-mag because it allows for focusing. The LED cannot be manually focused as that is a built in feature. The LED got vacuumed away, sad loss.
Have a prefocused pocket light using 9v. battery, switch to turn on high and lo-power is on all the time, easy to find in dark, and suprisingly bright in the dark. Battery life 2years or so.
IIRC, if you make a white LED out of RGB elements it’s difficult to stabilize the color. Degradation over time and environmental changes (esp. temperature) affect each element differently, causing color shift. Also you need some type of diffuser to mix the color, and there’s a tradeoff between purity of color (how thoroughly the colors are mixed, i.e. lack of color fringing) and efficiency.
Ususally when this is done a regular (non LED type) diode is paralleled across the LED in the opposite direction, which limits the peak inverse voltage to a safe level.
Our local police department just converted their emergency lightbars to the LED type. They provide a much brighter and solid light than the previous strobe lightbars. The use of LED’s also allow them to add more lights to their vehicles since LED’s require little power.
In the Minneapolis area, they are slowly but surely changing over traffic lights from incandescent bulbs to arrays of LED’s. Reason given is cost savings: major saving in not needing city workers to replace them nearly as often, with a minor saving from reduced electricity used.
Oddly enough, this seems to be happening only with the red & yellow signals. I can clearly see when close up that those are now a bunch of individual LED’s. But the green one still seems to be a green lens, with a light source behind it. That could possibly be LED’s, but it sure looks like a single point source, like an incandescent bulb. Possibly it’s still hard to get a good green LED that is bright enough for a traffic signal? I do see green LED’s used in left-turn arrow signals, and they do seem less bright to me.