Freeing the Photons

Technocrat

Freeing the Photons

zogger

Sun May 11 17:35:45 -0700 2008

Electronics

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A relatively simple in theory and harder in practice technique promises to make LED lighting even better. Although they are already very energy efficient, they still don't give up more than one fifth of the light they actually produce, the rest is reflected back internally. By introducing thousands of tiny holes at the surface, more light is emitted, for the same energy cost.

.."As a nanoelectronics researcher in the University of Glasgow, Rahman knows that LEDs are very energy-efficient. But there's a catch. Although 70% of the energy is converted to light, just 20% of that light escapes. Why? A high refractive index at the LED-air interface means the light is reflected straight back inside. Some LEDs are made from gallium nitride, perhaps the most optically-awkward semiconductor material of all."

ed.z.: ..can't wait for good normal plug in home lighting with LED, in particular something you can screw into a normal lamp socket and have it work just like..a normal light bulb, and not cost like 50 bucks or something like that.. For now I am sticking with incandescents, because CFLs give my eyes the willies. I have a few adjustable spotlights and area lights (like camping lanterns, they "sorta" work OK) with LEDs but they are all battery operated devices, used for when the power goes out.

Freeing the Photons

David Leppik

Mon May 12 10:05:06 -0700 2008
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The problem with LEDs isn't their brightness or efficiency, which are good enough, but the quality of the light. White LEDs are just blue LEDs with a yellow fluorescent coating. So you get a pure blue frequency, and a yellow light with a broad enough frequency range to let you see a little red. (I know this for sure from looking at the light with a prism.) Fluorescent lights produce UV light, which hits a fluorescent coating that converts it into the complete spectrum.

I used to have a problem with fluorescents, but now that they have good color and a high enough frequency (120hz), it's not an issue. The main issue is that their base is too big to fit in the antique chandelier over my dining room table. LEDs would be ideal for that, as long as the power converter is small.

Which brings me to another point: LEDs are DC only. So they're perfect for flashlights and other battery-operated conditions. With AC power, they either flicker at 60hz or need a DC converter. And those are inefficient.

You're missing some details...

Bruce Perens

Mon May 12 11:36:18 -0700 2008
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AC to DC converters are not inefficient. Maybe you are thinking of some other sort of converter. If you rectify the AC, you get a 120 Hz folded-sine (it looks as if you just folded the negative part of the wave upward). Silicon diodes have a band-gap of 0.7 volts and low internal resistance, so with a bridge rectifier (two diodes at a time) you lose about 14% at 12 Volts, and only 1.4% if you are feeding a series string of diodes at 120 volts. If you put a capacitor on that, it gets smoothed out to be something close to DC, and you get a little more light. Modern capacitors are efficient enough. I don't see where the problem would be.

If you want more red, add some red diodes. Given more than one diode, you are not constrained to using the various schemes used to make white in a single chip.

Bruce

Wall warts are inefficient, mainly due to their usage

David Leppik

Tue May 13 07:50:26 -0700 2008
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My understanding is that typical "wall warts" are pretty inefficient. Much of that may be simply that they don't shut off when they are no longer needed.
Here we go... Energy star requirements for power adapters. Looks like for a 15-watt light (equivalent to a 40-80 watt incandescent), it would be (0.09 * ln(15 watts))+0.49, or 0.73 watts (thanks Google!) when in use, and 0.75 watts at rest. Elsewhere they say that Energy Star ones are 30% more efficient than standard converters-- and that they consume 11% of the national electric bill. So you're right, they can be quite efficient, they're just not used that way.

I like your idea of adding a capacitor to smooth the electricity; LEDs switch on and off so quickly as to appear harsh. I can always tell when the car in front of me has LED turn signals.

As for adding a red LED, that would work in a pinch, but it wouldn't produce the same natural color as a fluorescent or incandescent. It's one thing to produce an image on a computer screen, where you can get away with just red, green, and blue. It's another thing to be illuminating the wide range of dyes used in clothing and wallpaper, while competing with other lights in the same room. Even the harshest "natural sunlight" fluorescents look fine when they are the only source of light; it's when paired with an incandescent or a setting sun that they look bad.

The fundamental problem with LEDs is that they emit a pure frequency, so you would need a lot of different LEDs to be able to compete with fluorescents. (Paint matching scanners use something like 30 frequencies, though that's an even more demanding task.) Or perhaps they'll work something out using UV LEDs (which I can buy at the local Walgreens as a dollar bill tester) and fluorescent dyes. Either way, I do hope they figure it out. Even I can tell that white LEDs look unnatural, and I'm red/green color blind.

Freeing the Photons

Harry the Bastard

Tue May 13 03:25:47 -0700 2008
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Firstly, I had to say it. These LEDS must be the Holy grail of lighting. (Enough PUNishment for now.)

The fact is that "old fashioned" flourescent lamps were supposed to consist of a ballast, a capacitor, and a starter. The ballast serves the purpose of limiting the current. the capacitor tunes the ballast / lamp combination to approximatelly 250/450 Hz. when the mercury in the tube stikes, the voltage drops to below the sustaining voltage, the voltage then rises again due to back emf, etc, etc. So correctly installed flourescents actually flicker in the 500 / 900 Hz range! Take away the capacitor though ... it drops to 100 Hz (or 120 Hz in the USA) There is an another effect where the old fashioned flouro's flicker. If you have seen a tube doing this you will note that the tube seems to have bright streaks running around the tube, and the rate is nowhere near 60 Hz. It is some other weird effect entirely!

The more modern compact flourescent tubes actaually have a high frequency inverter in their base. This lifts the frequency much higher again. By doing this, there is an improvement in efficiency, as the UV light produced by the mercury gas only emits in the relaxation state. (that is, when the gas stops conducting electricity.

Incidentally ... kettering ignition systems also rely on their capacitor in much the same way, as it forms a series RLC circuit for when the points open, which increases the voltage on the primary to around 150 ... 300 volts. Also, this increases the length of time there is an arc at the plug, as the cap and inductor ring for a significant period after the points open. Amazing how many people think the capacitor (condensor) is only their to prevent arcing at the points.

Refractive index

wowbagger

Mon May 12 12:23:59 -0700 2008
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If the issue is the refractive index of the semiconductor material, it seems to me that a far simpler method would be to deposit a lower refractive index material onto the surface of the diode, so that instead of one big impedance mismatch, you bring the light through 2 lower mismatches (and of course, you can adjust the thickness of the matching layer to cause the reflected wave from the matching layer to be 180 degrees out of phase with the reflected wave in the diode. BAM! phase cancellation of the reflected wave and nearly 100% transmission from semiconductor to matching layer.
Lather rinse and repeat until you get to a close enough match to the encapsulating plastic.
OK, so why WOULDN'T that work?