Revolutions in light

It started with fire, then light bulbs and now LEDs but how did this come about?
24 January 2017

Interview with 

Colin Humphreys


We started with fire, then Edison's light bulb and today we have the LED but how did these revolutions come about? Colin Humphreys explained to Chris Smith...

Colin - I guess our love affair with light started about 400 thousand years ago when Homo erectus, who preceded Homo sapiens who is us, they started lighting these campfires for heat, and also for cooking, and to scare off wild animals, and also for light.

And then people thought it would be really nice to have a portable source of light but we have to wait until 1750 BC for the Babylonians and the Egyptians to have oil lamps. They had sesame seed oil lamps and it took 50 hours of the average worker to get enough money to buy the oil for 1 hour of light so it was really valuable stuff.

Then if you come up to about 1800 AD when we had candles and you had to work about 6 hours there to get 1 hour of light so that you could buy the candles. Then if you come up nearer to this time you had kerosene lamps and then incandescent light bulbs. And with all of these you had heated sources and light was a byproduct of the heat.

Now we’ve got LEDs and they’re really important because it’s the first light source which doesn’t depend on heat to get the light and in fact today, you have to 0.6 seconds to get an hour of light. So it’s just a huge increase in efficiency and I haven’t worked it out but is about 64 thousand times to go from 1880 to today. But, basically. LEDs they’re very, very efficient because they give out very little heat and light is what they’re really giving out.

Chris - So just to clarify the point you’re making which is that if you look at say and Edisonian light bulb, this thing we plug into the ceiling with a piece of tungsten filament that glows. We’re putting electricity through that, we’re making it get white hot, glow a few thousand degrees and it’s that glowing that’s giving us some light. But only about 15 percent of the electricity we put in turns into anything you can see, the majority of it is just light you can’t see - it’s infrared heat?

Colin - Yeah. It’s actually about 5 percent - it’s even worse…

Chris - … even worse than I said.

Colin - 5 percent comes out of light and 95 percent is heat. The LEDs we have today are about 35 percent efficient. But in the lab, we have LEDs which are 75 percent efficient for light and those LEDs will be working their way into the marketplace in the next few years.

Chris - I was going to say if you’ve got these lamps and they’re 75 percent efficient, that’s 100 percent better than what’s on the market at the moment. Why are they not on the shelves?

Colin - Well, it takes a bit of time for the researcher to get it out into the marketplace - we’re running hard!

Chris - So how have you got a doubling in the efficiency?

Colin - It’s really a lot of research on the details and some major breakthroughs. And the other thing which is happening is the reduction in costs, which is really important. One thing that we’ve done is to grow LEDs on silicon. Other people are growing LEDs on sapphire wafers, the ones you buy in a shop, and that’s very expensive sapphire. We’ve developed ours growing on silicon and, as you say, we’ve set up a couple of companies in Cambridge. They got acquired by Plessey, a UK Company, and Plessey is making the first LEDs on silicon in the world and the first LEDs in the UK at all.

Chris - We’ve got a little demo here to demonstrate and put into reality the points you’re making about the efficiency, we’ve got here a hand crank which has got a drill motor from a DC electric drill in it. When you turn the crank you're obviously turning the motor which then generates some electricity and wired up to it are some LED lights and some old fashioned tungsten lights. So I’m going to turn the handle, and you can switch me on and we’ll do the LEDs first. I’m going to demonstrate really how hard it is or not to do this. So let’s start. I’ll start turning if you could load me up with some LEDs first.

Colin - Let’s start the LEDs first, right…

Chris - Now if I’m honest, it was really easy and I got these two LED bulbs, the kind of thing you’d see in the ceiling of your house and they are bright lights and I’ve now got white spots all over my eyes from doing that. It was really easy to do that.

Colin - Right, well now I’ll go onto the tungsten halogen bulbs and Chris is going to try again…

Chris - Okay, so we’ll turn this on...Oh, God!... Okay, you’ve made your point. I’m actually out of breath from doing that. It’s very, very difficult to turn the crank. Is that because they’re using so much more energy in order to make the same...because they were equivalently bright to the LEDs? I had to wind it a lot harder, though.

Colin - That is right. So these take so much more energy to light up than the LED bulb, so the LEDs are a really very efficient source of light.

Chris - If we were to translate that into carbon equivalents, in other words, carbon dioxide that we’re not emitting because we’re turning far more of the electricity into visible light than heat, how much carbon are we saving by using LEDs in our homes?

Colin - Lighting takes about 20 percent of all electricity. It’s much more than people think. With the LED lighting that comes down to 10 percent and with the future will come down to 5 percent. At the moment we’re saving 10 percent of all that electricity and carbon emissions from power stations. In the future, we’ll be saving 15 percent with LEDs in carbon emissions from power stations. That’s a lot of carbon dioxide.

Chris - It’s a lot of wasted energy at the moment, isn’t it?

Colin - It is. Absolutely, yes. 15 percent is what you’re going to get probably from solar and wind power combined, so it’s really important.


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