The mercury part is a definite plus, and I can't wait too -- a genius in the house (not me) was dragging a CFL up the stairs by the cord and it broke. Nice little panic that day. And the same genius broke a bunch (clumsy) by dropping them in the basement.
It's just that I remember years ago when there was talk about how LEDs were just starting to be developed for regular bulbs. I remember hearing that they converted the majority of electricity (moreso than CFLs) into light rather than heat, and they would use less energy.
A bit disappointing that their main selling point is just getting rid of mercury -- it's an important one, but still. Not really concerned about life considering the luck we've had with CFLs. My backlight is going on 5 years now.
It depends on the fluorescent tube and driver. Some fluorescents can be quite impressively efficient. T8 tubes can
easily deliver over 90 lumens per watt, though you'll then get some efficiency losses in the driver. (The same happens with LED drivers.) Some CFLs are down to less than 1mg of mercury, and the linear tubes are also cutting way back.
LEDs range from around 50 lm/W up to 112 lm/W and maybe a bit beyond. Again, you'll then see efficiency losses in the driver.
Usually the lower cost you go on the driver, the worse the efficiency is. Lower-quality components, and
fewer components as well, can end up leading to higher losses to heat.
Some LED lamps or bulb replacements will also use
really cheap power circuits, and then you can be looking at an overall system efficiency that a good halogen tube can beat.:|
The other thing to watch for is CRI - Color Rendering Index. Something like a cheap fluorescent shoplight might have a CRI of 60-70; colors under that are going to look a bit wacky.
LEDs are also not terribly great at converting electricity to light, though they have certainly improved dramatically, thanks in part to the infusion of money from early adopters of LED lighting. Some of that's due to their use of a semiconductor material, but also because the materials they tend to use carry with them a high index of refraction; it ends up being difficult to effectively extract the light from the die once it's been produced.
But they're immensely better at it than incandescent, which is really just a space heater that also happens to put out a little bit of light that we're able to see, and they
can be better at it than fluorescent lights.
LEDs got attention because they could put out a single color of light, useful in indicator applications, including large indicators like traffic signals. So instead of wasting a lot of extra energy that was being put into first making a filament very hot, generating a lot of useless infrared in the process, and then filtering out most of the small amount of visible light it could produce, an LED source would instead directly generate the color desired. That's a big efficiency gain there.
Then someone got the idea to use them for lighting.
They got a shaky start, because the light we're used to includes
a mess of wavelengths all across our visible spectrum. So the goal was to take something meant to put out a narrow band of radiation, and spread it out so it looked white, and to also do so in a way that would make colors look reasonable. Not so simple now.
And you do indeed still see LED lights that look very blue, and have lousy color rendering. White LEDs work by generating short-wavelength blue light, which passes through a phosphor coating that converts some of it to a rough yellowish color. The combination of blue and yellow looks white. But to pump up their lumen ratings, a lot of them like to let more of the blue go through unconverted. So you get that high lumen value that the sales&marketing groups love (ooooh, big numbers!), but the light looks bluer. Using cheap phosphors is also way to keep costs down, but also makes for poor-quality light.
Some manufacturers, at least of the emitters themselves, do focus on quality. Philips has their
Luxeon S and
Luxeon Rebel lines, and Cree's got their
X-Lamp CXAs. Philips' offerings tend to lag in the efficacy department, particularly in the warm (2700-3000K) color temperatures, which is fairly common, but Cree's X-Lamps maintain >100lm/W even at the difficult 2700K color temperature.
The Luxeon Rebel linked above has a nice CRI of 90-95; the X-Lamps
do have that high-CRI option as well, but I don't know where they can actually be purchased, at least if you don't want to buy 100pcs of them, at around $28 each.
Still, the regularly-available ones give a minimum CRI of 80, which certainly isn't bad. And it'll put out as much light as a 4' fluorescent tube, from an emitter that's under 1 square inch.
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