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Researchers Conquer "LED Droop"
sciencehabit writes "Tiny and efficient, light-emitting diodes (LEDs) are supposed to be the bright future of illumination. But they perform best at only low power, enough for a flashlight or the screen of your cellphone. If you increase the current enough for them to light a room like an old-fashioned incandescent bulb, their vaunted efficiency nosedives. It's called LED droop, and it's a real drag on the industry. Now, researchers have found a way to build more efficient LEDs that get more kick from the same amount of current—especially in the hard-to-manufacture green and blue parts of the spectrum."
The solution is called "LED Viagra"?
I guess that's why their new LED burns-up 26 watts but only created the equivalent of a 100 watt bulb. They are losing efficiency because the LEDs are being driven to high powers. (Lower power 25W or 40W bulbs only use 3 and 6 watts.)
My AC stalker: " I personally agree with your posts most of the time, but that won't keep me from modding you troll"
Why must a single LED provide all the light? Couldn't an array of, say, four LEDs, each equivalent to a 25W incandescent and using mirrors and/or lenses to even out the light distribution, get the same efficiency and substitute for a 100W bulb? Am I missing something obvious?
$_ = "wftedskaebjgdpjgidbsmnjgcdwatb"; tr/a-z/oh, turtleneck Phrase Jar!/; print
Ahite LEDs of various color spectrums are totally different. They usually have a phosphorescent coating that creates the white light. The blue and green LEDs mentioned are single spectrum blue and green lights.
If you increase the current enough for them to light a room like an old-fashioned incandescent bulb, their vaunted efficiency nosedives.
Apparently this droop issue is only a problem for non-blue wavelengths. At least if my subwoofer, PC and external HDD are anything to go by...
My eyes hurt.
"Nine times out of ten, starting a fire is not the best way to solve the problem." - my wife
Typically a "green" produced by GaN is fairly easy to manufacture and fairly efficient, but it is physically a very *hard* material. In contrast, the "blue-green" produced by InGaN (an alloy of a little bit of InN and base of GaN) isn't as efficient as it tends to have lots crystal defects and these defect cause brittle-ness and results in some electron-hole recombinations to be non-radiative (generating heat and not band-gap light emissions).
Regardless of this manufacturability issue, many white LEDs use an InGaN band-gap devices and create the "warmer" parts of the spectrum using phosphors. This makes most of the output light more blue-ish, but only the phosphor re-radiated (stoke's shifted) part in the warmer part of the spectrum where you pay the efficiency cost. For "cool" devices, less of the output is down-converted, so you have less efficiency loss. For "warmer" devices, more of the light is down converted and you pay for more conversion efficiency loss. Some warm devices actually have multiple LEDs (say a red, green, and blue), but color stability is generally hard to maintain over time and temperature, so these devices are generally less efficient and more expensive.
In any case, the effect that was described is that the currently "cheap" way of growing GaN base crystals for LEDs results in a polar orientation which is bad for high-current operation as it tends to generate a back field. This is described in more detail in this other site:
Most of the commercial GaN devices are grown along the [0001] direction, so-called “polar” or “c-plane” structures. However, there is an internal electric field perpendicular to the active regions in the c-plane devices as the c-axis is polar. This will result in band bending and a poor overlap of electron and hole wave-functions (the Quantum confined Stark effect, or QCSE), which reduces the radiative recombination efficiency and affects the device performance. In order to avoid (or reduce the effects of) the QCSE, GaN can be grown in “non-polar”, or “semi-polar”, orientations, in which there is no, or much less, internal polarization fields along the growth direction. In theory, this should increase the efficiency of light emitting structures. The high density of structural defects (such as basal plane stacking faults and partial dislocations) in heteroepitaxially grown non-polar and semi-polar GaN results in low internal quantum efficiency and output power of the devices, as reported in the literature.
Of course the answer is to just grow low-defect GaN in a non-polar or semi-polar orientation, but that's currently hard to do. These UCSB researchers aren't the only group working on this problem, but they apparently have done some cooperation with people doing actual manufacturing (Mitsubishi Chemical).
and yet I just bought a 30 pack of 40 watt Incadescent bulbs for better lighting and environmental efficiency - No Mercury. Do they burn out any faster? Not as far as I've seen based on the quality of the damn CFL bulbs we've been able to get cheaply. Those don't last anylonger then an incadescent bulb and have mercury in them plus they look horid where I really need them.
What I'm doing now is moving towards the halogen based 12v bulbs in low voltage track light systems. quite a bit of light and can be run straight from a 12v source w/o converting to AC first. Very nice as I'm also moving off the grid and beginning to switch to a PV array due to planned rolling blackouts by Edison International this year. Getting tired of having my power go out every damn summer because no one want's to invest in power line maintenance.
Mod me up/Mod me down: I wont frown as I've no crown
I have a DVR with a blue power LED on front that apparently is just the right wavelength to be screwed with by my glasses. As I move my head around (or just move my glasses around) the LED appears to move around on the front of the thing. The closer to the edge of my glasses, the farther the displacement. I can even get it to overlap the other LEDs if I turn far enough, so it seems to just be that wavelength of light that's distorted, and it has to be a fairly narrow band that is affected since I've never noticed it anywhere else.
Drives me crazy sometimes, I try not to look at it.
If I have been able to see further than others, it is because I bought a pair of binoculars.
CFLs burn out quickly if you cycle them. Once you turn them on, they shouldn't be turned off for 20 minutes. This makes them less than ideal for some locations (like the bathroom, hallway, etc). I currently have CFLs as the main lighting in areas like the living room, but LEDs in other areas. LEDs are expensive, but it's not like I'll starve if I spend a couple of hundred dollars on lights. Prices are dropping fast (at least here in Japan). It wouldn't surprise me if the cost per lumen approaches CFLs soon.
I've never been one to dislike CFLs. Personally, I like the color of "daylight" bulbls *much* better than incandescent. But I must say that I like my LEDs better than the CFLs. The biggest issue is that the lumens don't drop off as quickly through use. They also come to full brightness more quickly (basically instantly). I will probably switch over completely in the next couple of years.
Since the have laws banning incandescent bulbs because they are inneficient, when are they going to do something about the large incandescent light source 92 million miles away? Not only is it inefficient, it is the major cause of global warming.
(PK so there might be some issues of jurisdiction, but the owner of said light source (Oracle) is in this country...
Slew is somewhat correct. The nonpolar/semipolar substrates are currently very expensive and small (1 sq. inch at best, compared to 12sq. inches for sapphire substrates that all commercial LED's, except Soraa's, are currently grown on). It has been prohibitively expensive for any other academic institution to do any meaningful research on nonpolar/semipolar GaN LED's and lasers. That is unlikely to change anytime soon. There are industrial companies working on it though. A few Japanese companies and the startup company owned by the UCSB professors, Soraa. This article does give a lot of hype, the 20-2-1 LED's aren't quite the magic bullet that it implies. But nonpolar/semipolar LED's probably are the future once Soraa, Ammono, or Mitsubishi Chemical figures out how to grow large bulk GaN nonpolar/semipolar crystals by the ammonothermal technique. Soraa is releasing (or already released?) an LED based on nonpolar/semipolar technology this year, and lasers probably later this year or next.
There are some good c-plane LED's for sure. Nichia's best c-plane LED's probably have 95% PEAK internal quantum efficiency, but they still have droop problems and are expensive.
>I just bought a 30 pack of 40 watt Incadescent bulbs for better lighting and environmental efficiency - No Mercury.
Unless coal is used to generate some - or worse most - of the electricity where you live, in which case powering those incandescent bulbs will release far more mercury into the environment than an equivalent number of CFLs would.
Worse, the mercury that comes from burning coal isn't elemental mercury, as you'd find in a CFL. Which means it's far more easily absorbed by living things like us.
It's called chromatic aberration, and it is an unavoidable effect of light passing through lenses. When light passes through a lens, it gets bent, which is the whole point. But the amount of bending is wavelength-dependent, and so most lenses will act a bit like a prism and spread the spectrum of the incident light. The stronger the lens, the greater the bending, the more pronounced the effect. The effect is more or less nonexistent along the optical axis, but becomes more pronounced as you go out towards the edges of the lens, where there is both more material to refract through, and the deflection angle is that much greater. This is one reason why big telescopes don't use lenses except for the objectives: the big lenses you'd need for a big telescope would produce too much aberration. Reflector telescopes are, for the most part, immune to this effect, because a mirror reflects all (visible) wavelengths equally.
For myself, with worse than -6 diopter, polycarbonate lenses, I've just learned to live with the effect. It can actually be kinda fun, because it can be like wearing a spectrograph: by putting a light source off to one side of my field of view, I can see the constituent wavelengths. There is no noticeable effect when looking straight ahead.