The real question about LED efficiency

Accepted Fact #1: Current mass-market white LEDs are only as efficient halogens bulbs.

Accepted Fact #2: Halogen lights put out a *lot* of heat, despite their efficiency.

Accepted Fact #3: Luxeons (or any large cluster of overdriven LEDs) can get really HOT!

Accepted Fact #4: We have only seen/heard of Luxeons or other new-tech LEDs coming up to sizes of 2-5 watts apiece. Halogens in the household range from 10W all the way to 75W.

So, won't equally high-powered LEDs produce the same amount of heat as halogen bulbs?
LEDs are being touted as cool light(ie. little or no heat), but if the facts above are true, won't this 'cool lightsource' thing now be false? Or are we talking about equivalent electrical-to-light conversion efficiencies here, with no regard to the wavelength of the light produced?

For example, lets say a halogen and a white Luxeon LED put out 25 lumens per watt. All else being equal, will the halogen be the hotter light because a large percentage of light produced has its wavelength within the IR range, as opposed to the higher color temps of white LEDs? Or will both bulbs be equally hot due to their inherent pure electrical resistance?

I'd like to hear anyone's views on this as this might affect our expectations of a future where halogens are possibly replaced by LEDs (such as in that Luxeon-in-car-headlamp post).

Good question, but I believe that LED can be more efficient than halogens. The problem is everyone runs them at such a high rate they aren't. If you are running a white LED at 5mA is should be quite high.

As far as the heat goes, I bet they are similar. I had the prototype 19LED Surefire lamp and it got HOT, very similar to the E1 when they are both left on the same amount of time. They both used about the same power so it is understandable.

Most of the power going into either halogen or LED lights is being converted into heat. A perfectly efficient light source would have around 680 lumens/W efficiency, and existing LED and halogen lamps rarely exceed 80 lumens/W. So at (say) 20 lm/W we're only 3% efficient anyway.

Most of the hype around power consumption of LEDs has come from manufacturers of LED lighting units using lower-powered LEDs. They were able to spin a deficiency (low output levels) into an advantage (long battery life).

Existing white LEDs are actually less efficient than good halogens when you get up to the light levels used in house, car, or industrial lighting. They're only comparable to small (3W or so) halogen bulbs, which are substantially less efficient than their bigger cousins for thermal reasons. LEDs will get better, of course, but they have a long ways to go. Halogens aren't likely to get much better.

However, LED advantages include vibration resistance, long life, and easier packaging and heatsinking.

Colored LEDs are much more efficient than filtered incandescents primarily because putting a filter in front of a white light discards most of the light.

So then are the Metal Halide / ARC lights the most efficient?

My logic (if it's worth anything ) tells me that the energy has to reveal itself in one form or another. With a halogen, it's obvious. A couple watts in - lots of heat and some light out. If LEDs offer similar efficiency then they should produce the same results, mostly heat and some light. IMHO the only reason we consider LEDs as cool is because they are very low power devices so there's not much noticeable heat. Get them upto the LS power levels and lo and behold...they get hot.

<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by JollyRoger:
So then are the Metal Halide / ARC lights the most efficient?<HR></BLOCKQUOTE>

No; I think the most efficient white lamps currently available are the electrodeless sulfur lamps , at 96 lumens/W ("Both feature a constant correlated color of 5,700 Kelvin, color rendering index of 79, light output of 135,000 lumens, and operate at 1,425 watts"). T-8 fluorescent lamps are about 80 lumens/W.

So then, I guess the question has to be asked: Could LEDs really aim to replace incandescents as primary lighting? Or will they be destined for use only in low-power devices?

We're all hoping for the 2W nichia or 5W luxeon, but with the facts so far, are we just kidding ourselves with LEDs and really just heading to the point when we have 'unbreakable bulbs' that last ages, but still produce loads of heat? I'm imagining an LS in something the size of a ArcAAA and getting far too hot to hold after a couple of minutes

The efficiency of LEDs, the performance of the light source, how hot the LEDs run, and their total power output are related, but different thing.

The efficiency of a light source tells you what fraction of the electrical input shows up as optical power output. This is different from illumination performance, because illumination performance considers both the efficiency of photon production and the eye's sensitivity to the color of the photons produced. If you had a light source which had 100% efficiency at producing light of 390nm (on the border of violet and ultraviolet), the lumen per watt performance would actually be pretty low. On the other hand, if the light source was 100% efficient at producing the proper green light, you would get a lumen per watt performance of 683 lumen per watt. For the mix of wavelengths called white light, maximum lumen per watt performance is roughly 300 lumen per watt.

Measured in terms of lumens of light output versus watts of electrical input, the performance of a modern white LED is roughly the same as the efficiency of a 3-10 watt halogen lamp, in the 10-20 lumen per watt range. This means that most of the electrical energy fed either to a halogen or an LED array at these scales is wasted, producing heat or non-visible light.

The halogen lamp produces this visible light with a very hot filament, in the 3000C range. Quite a bit of the energy dissipated by the filament is in the form of invisible infrared light, which doesn't count toward the illumination performance. Everything in the halogen lamp gets quite hot. One important factor with halogen lamps is that much of the wasted energy is emitted as invisible infrared light; it doesn't have to be conducted away.

The LEDs produce light at much lower temperature, with a maximum junction temperature of about 125C. Very little of the output shows up as infrared, but instead shows up as heat which acts to raise the junction temperature. This is why LEDs need so much heat sinking; all of this heat needs to be carried away by the heat sink, and it has to be carried away so well that the LED junction temperature won't get higher than 125C. Whereas a halogen lamp is designed to get hot, and is more efficient at higher temperatures. LEDs get more efficient at lower temperatures; with a good heat sink on a very cold day, you might get 50% to 100% more in the way of lumen per watt with LEDs.

In any case, if we were to find a match pair of a single 5W halogen lamp and a 5W LED array with the same lumen output, we would find that the temperature of the halogen lamp would be much higher, the total heat (in calories) conducted by the LED array heat sinking would be higher, but at a lower temperature, the expected life of the LED array would be much longer, the direct IR heating of the optical output would be much higher with the halogen lamp, and several there would be several other trade-offs.

One truly enormous benefit of LED lighting over halogen for portable illumination is dimmability. When you dim a halogen lamp, it gets much less efficient, and the color changes to much redder. LED lamps can be dimmed while maintaining full efficiency, depending upon the level of heat sinking the efficiency might even improve (if the junction is being run hot). Another benefit of LEDs is that they have the efficiency of medium sized halogen lamps in much smaller, lower power packages.

I don't think that we are deluding ourselves to think of LEDs in terms of general lighting stuff. Halogen lamps are pretty much at the peak of their efficiency, but LEDs are getting more efficient all the time. 40 or 50 lumen per watt could be reasonably expected within the decade for white LEDs, possibly more. If you generate white light by mixing primary colors, you might just be able to reach these efficiency levels today, and have the benefit of being able to change your light source color (but at the cost of having a less well mixed 'white' light). I think that we can expect that even if LEDs don't compete _today_, they will compete _tomorrow_.

-Jonathan Edelson

I personally think we won't be seeing LEDs for general lighting for at least 5, maybe 10 years. Even when they do increase the efficiency and power output, they will cost a lot more than your standard incandescent or CFL.

When we talk about how much LEDs will advance, we cannot forget about all the other light sources.

Just recently, manufacturers like Philips and GE have started using an IR reflecting coating in their halogen lamps, increasing efficiency dramatically. They claim an efficiency improvement of almost 40%

Also white arc lamps (like metal halides) have also improved significantly. The CRI has increased into the 80-90s and the colour temperatures are stable throughout the lamp's life. They also have very long burn times and lets not forget that they put out 80-100 lumens/watt.

In 10 years, IMHO, I see people using CFLs for general lighting and arc-lamps for large area lighting. LEDs will most probably serve the specialised markets like coloured lights.

It would be interesting to see a graph of current vs. light output for an led and an incandescent bulb. I wonder how much the efficiency changes with current?

This is the best I've seen so far:
http://www.ResurgentSoftware.com/Perfect_LED_Light.html

LEDs are best where they are at right now -- flashlights and portable operation.

For small lights, LEDs simply cannot be beat. For battery-powered setups, as the battery dies, the light is still white and usable. This is the nice thing about LEDs. Incadesent lights go yellow as the power drops and they break. CCF and EL require hi-voltage inverters (and the accompanying annoying buzzing), and CCF can break. EL does not age well.

I do not see LEDs replacing 60W bulbs in the next decade. For lights under a watt or two, the LED is the only way to go.

Ironically, I may have proven my own doubts in this post wrong. See my other post here where I pitted 2 overdriven LSs against my Tec40 and a 10-watt/12volt Osram halogen reflectored lightbulb.
http://www.candlepowerforums.com/cgi-bin/ultimatebb.cgi?ubb=get_topic&f=3&t=001967

My experiment may be too over simplistic to really be considered indicative of anything, but it seems to be getting ridiculously easy to get current-day LEDs to seemingly outperform incandescents. Anyone concur?

<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by Harrkev:
LEDs are best where they are at right now -- flashlights and portable operation.
<HR></BLOCKQUOTE>

I think thats where LEDs will penetrate the most, although the majority of the public will still prefer cheaper incandescent torches.

Other than that, coloured lighting is where LEDs really excel. Even LedCorp has released a LED indicator globe for cars based on the Luxeon Star.

<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by Someguy:
Even LedCorp has released a LED indicator globe for cars based on the Luxeon Star.<HR></BLOCKQUOTE>

Yes! I saw those on their website - they've got 2 models. Anyone tried those yet? Some earlier multi-LED taillights got a bad rep because they weren't visible enough off-center.

Take a look at this article:

physics today

from Physics Today which was linked off of the lumileds web page. The article gives an optimistic (just because the authors are people who have a big stake in the future of LEDs) view of the future of LED lighting.

I already see see LEDs used in all new traffic lights here (CA bay area), and in most EXIT signs.

Don't forget that the blue/white LED technology is only about ten years old!