Cause of LED Efficiency Droop Finally Revealed

An anonymous reader writes "Researchers from the University of California, Santa Barbara, in collaboration with colleagues at the École Polytechnique in France, have been able to prove the theory behind LED 'droop.' LED droop is the term for how LEDs emit less light when the amount of current being pushed through them goes above a certain level. 'The cost per lumen of LEDs has held the technology back as a viable replacement for incandescent bulbs for all-purpose commercial and residential lighting.' Now that we understand what causes this, we should start to see research go into technology to circumvent LED Droop. 'LEDs have enormous potential for providing long-lived high quality efficient sources of lighting for residential and commercial applications. The U.S. Department of Energy recently estimated that the widespread replacement of incandescent and fluorescent lights by LEDs in the U.S. could save electricity equal to the total output of fifty 1 GW power plants.'" A pre-print of the team's paper is available at the arXiv.

Re:multiply

[Anonymous Coward]

50 1GW plants do not equal 50GW as the output is not continuously at their maximum. It also doesn't equal one 50GW plant in the fact that one central plant would need a lot of infrastructure to distribute the power around the whole country. Since the economy would be very spread and not localized, it means 50 1GW plants distributed across the network.

Answer not in summary

[Anonymous Coward]

It's because of Auger recombination. Basically, you stick in too many electrons, and they all mill around talking with each other instead of getting any work done. This is also known as the 'Water Cooler Effect'.

Re:Looking forward to replacing a bulb... never

[ShanghaiBill]

This is why LEDs are already used in traffic lights. If you look at the cost of sending out a crew, putting up cones, flagging traffic around the workers, etc., the cost of replacing a bulb can run into kilobucks. Even if the bulb itself is more expensive, it is far more cost effective to use LED traffic lights to avoid the traffic problems, labor costs, and safety problems of burnt out incandescents.

Re:Looking forward to replacing a bulb... never

[Jah-Wren Ryel]

You don't even need a resistor, just a smarter hood. [thedenverchannel.com]

Re:The Color...Ugh

[Arkh89]

The white led are in fact blue LEDs with a phosphor layer which "shifts" the emission spectrum towards green and red. Thus, white LEDs looks bluer or "colder" (associated with ice). The bulbs can be considered as black bodies radiators and thus have their spectrum "coming from" the red part of the spectrum (in fact most of the energy is wasted in the IR as heat dissipation). Their color are more yellowish (centered on green, 550nm) just like the Sun and seems warmer (like a camp fire). Now you can combine few color LEDs to reproduce the the D65 illuminant (Black body at 6500K, like our Sun) by balancing the amount of current in them. Other trick : you reprogram your mind to follow "correctly" Wien's displacement law : blue color is for warmer black bodies compared to yellowish and reddish black bodies (thermal emission). To make sure of that : think of a metallic part you heat up, it will start as black (as in not-emitting) then go to red, then yellowish and then bluish (but you will see it white-blue at this point). So, white LEDs should appear "warmer" when considering true physics...

Re:but won't somebody think of the Mercury?

[ThePeices]

but won't somebody think of the Mercury? Or toxins, or radio waves, or autism that LEDs cause?

What mercury? Ain't none in LED's.
LED's don't emit radio waves. The power supply might ( but utterly harmless levels ), but not LED's
LED's don't cause autism.

LED's don't do anything, except emit light and get a bit warm.

OP: Education. Get one.

Re:Cost Per Lumen? BS!

[FireFury03]

The only things I see holding back LED bulbs are misinformation and lack of availability (Home Depot is the only major brick and mortar store I've found that carries them).

I agree, except for replacing "misinformation" with "confusing information", and the manufacturers are responsible for this. Take for example the following photo:
https://fbcdn-sphotos-a-a.akamaihd.net/hphotos-ak-prn1/11607_10151447644678611_203176319_n.jpg [akamaihd.net]

We can see here, 2 GU10 bulbs. The one on the left is a 28W halogen, the one on the right is a 4.5W LED bulb (they both have a similar beam angle - 36 degrees for the halogen, 35 degrees for the LED). Both claim to be "equivalent" to a 35W "conventional" (by which I assume they mean tungsten) bulb. However, look at the light output - the halogen claims to output 600 lumen whilst the LED bulb says 200 lumen. So clearly different manufacturers use different criteria for what "equivalent" means - the halogen appears to be saying that its total light output is equivalent to a 35W tungsten, whilst the LED bulb appears to be saying that its brightness is equivalent to a (presumably unshaded) 35W tungsten. By the criteria used for the LED bulb, you could manufacture a tungsten bulb that is labelled as being "more efficient" than a tungsten bulb, simply by narrowing the beam angle with a reflector!

Some of the bigger brands put even less information on their packaging - on the same shelves were Phillips 5W LED GU10 bulbs that simply gave an "equivalent to" figure - no information about how many lumens or candela they output, no information about beam angle.

Also, people shopping for bulbs are almost certainly going to be doing like-for-like replacement: if I'm buying a GU10 bulb then the chances are I'm replacing an existing GU10 bulb, which is almost certainly going to be a halogen (since traditionally GU10s are halogen), not an unshaded tungsten bulb with an almost isotropic radiant flux. So telling me what "conventional" bulb it is equivalent to (whether thats done by comparing lumens or candela) is pretty much useless. Instead, I'm most likely to want to know what wattage of halogen its going to replace - if I've got a 50W halogen GU10 already and I'm buying an LED bulb, I want to know which LED bulb will give me the same results as the bulb I'm replacing.

How is anyone supposed to make a decision when the information provided is either nonexistent or unstandardised and misleading?

What I needed is for a standardisation of the information provided:
1. The actual wattage of the bulb - i.e. how much power it is going to draw.
2. The total light output in lumens.
3. The brightness in candela. Especially important for bulbs that are traditionally used unshaded, such as GU10s.
4. The beam angle. Again, important for bulbs that are usually unshaded.
5. Colour temperature.
6. What standard bulb this is equivalent to for a like-for-like replacement (i.e. if you're replacing a conventional ~isotropic tungsten bulb then it should be compared against that, if you're replacing a halogen GU10 then that bulb should be the comparison instead). Obviously this becomes problematic where the beam angles are different (e.g. I just bought a LED GU10 with a 120 degree beam - far wider than you'll get from a standard halogen GU10).
7. The life expectancy of the bulb.

And this information should be printed on *all* bulbs, even the conventional ones, so that someone in a shop can pick up any 2 bulbs and compare the information between them.

I was under the impression that the EU had, several years ago, made some of this information (such as the lumen output) mandatory, but there are still a lot of bulbs on the shelves that don't include any of this data.

Re:but won't somebody think of the Mercury?

[mcgrew]

Horses and dogs are mammals, but dogs aren't horses. Light and radio waves are EMF, but light isn't radio waves.