Home lighting used to be so simple: bulb burns out, you unscrew it, shake it to see if the filament rattles and then replace it with another incandescent. Now, lightbulb shopping is like buying a pair of shoes. Do you go flashy or sensible? Pricey and long-lasting or cost effective with a shorter life? You can choose from incandescents, halogens, compact fluorescents and set alight emitting diodes. It’s this latter category that has people so excited. Not only are LEDs hyper efficient they are also speedily reducing in price. They don’t have the association with mercury (which, as we’ve seen, is exaggerated) that compact fluorescents do.
But LEDs are also the least understood lighting equipment. So we went to General Electric lighting physicistGary Allen for a quick history lesson and a view into the future of LEDs. (GE sponsors this magazine.) It gets a little wonky so we threw in an illustrated glossary of terms to define some terms.
Txchnologist: Where did LEDs come from?
Gary Allen: The LED was a equipment first developed in the 1950s that worked only in the infrared, in the invisible spectrum. The visible LED was invented at GE’s corporate labs in 1962. The inventor’s name wasNick Holonyak. That is not to say that GE is responsible for all of what’s going on in white LEDs for general illumination, there were a lot of other developments in the last 50 years that also enabled that.
Nick Holonyak: Invented the visible LED while working for General Electric in 1962. Later predicted in Reader’s Digest that LEDs would replace Edison’s incandescent set alight bulb
Txch: Is it honest to say that Nick Holonyak was the Edison of this equipment?
GA: Nick Holonyak and any of us who have worked on set alight bulbs since Edison are really only working on a fraction of what Edison’s contribution was. He made an infrastructure. Nevertheless, Holonyak is sort of the Edison of visible LEDs [his was red]. The white LED is credited to Dr. Shuji Nakamura, who invented the first blue LED. Reckon about Holonyak inventing red and then at the very far end of the spectrum in 1993, Nakamura invented the blue. The blue LED made possible the white LED.
Txch: Is this a leapfrog equipment?
Leapfrogging: Skipping frankly to a more efficient equipment without taking intermediate steps.
GA: Absolutely. I would call it disruptive. They are now already passing compact fluorescents in efficacy or lumens per watt. LEDs are increasing in efficacy at 10 to 20 percent per year.
Txch: In view of the fact that LEDs are semiconductors, is there an analogue for Moore’s Law?
GA: It’s called Haitz’s Law: the amount of set alight you get out of an LED is doubling every two years and every decade the cost per lumen decreases by a factor of ten. The performance of LEDs is speedily approaching a theoretical maximum.
Theoretical Maximum: The highest efficiency an LED can achieve. A number somewhere around 400 lumens/watt.
GA: The ideal lightbulb has 5-10 equally vital goals:
2) Cost. LEDs are more pricey per lumen than any other equipment. But LED costs will continue diminishing the same way that transistor and semiconductor costs are diminishing even after they become the cheapest set alight source.
3) Color quality. The LED is approaching the ideal color spectrum.
4) Can you change the color?
5) Can you dim the colors?
6) What is the lifetime of the source ?
7) The ideal set alight source should be vanishingly small. If I want to put it out of the way I should be able to.
Txch: How do you drive the efficiency gains right now?
Lumens per watt (lm/W): A measure of a lamp’s efficiency. Incandescent bulbs rate about 15 lm/W while LEDs range from 50 to upwards of 200 lm/W.
GA: One way is the bandgap, which is two different doped semiconductors that make an energy difference linking the p-doped layer and the n-doped layer. Just like any diode. There are losses in electrons moving linking those materials due to impurities and imperfections in the net structure.
Once you’ve made the photon efficiently, you’ve converted the energy of an electron into the energy of a photon, then you have to get the photons out of the structure. There are photonic tricks and material-based tricks that once the photon is made you get it out of the semiconductor and into your lighting system without further losses. You might call that an extraction efficiency.
Finally there is conversion efficiency. Most white set alight sources now are blue LED generators that then become white by a down conversion of the blue photon by a phosphor. That phosphor efficiency wants to be optimized. As those factors deal with 100 percent efficiency, the LED set alight source will have usable lumens per watt something on the order of 300 or more. That’s compared with our best efficiency sources like fluorescent and HID and so on, that matured at around 100 lumens per watt. LED can go up to two and three times more.
LEDs: Already five times more efficient than the incandescent lightbulb, the cost per lumen is decreasing by a factor of ten every decade.
GA: I’ve been in lighting for 26 years. My private interest in lighting equipment is to be working on what becomes a near ideal set alight source, so we can get all of those six or seven key attributes to fully satisfy what the customer wants. An ideal lumens per watt is 300-400. How close to that can an affordable lightbulb get? Probably somewhere around 200 lumens per watt or more. Once you’ve gotten two-thirds of the way to theoretical max, there are diminishing returns in terms of energy savings.
The LED lightbulb that right now is five times more efficient than the incandescent, when it then becomes another five times more efficient and it’s using 1/25th the amount of electricity as the incandescent, do you still save much more money by getting even more efficient? The answer probably is no. So will we get all the way to ideal efficiency? Probably not. It won’t be economically rational to do that. But LED can get us very close to ideal theoretical efficiency.
Txch: What’s your favorite lightbulb?
GA: You really should question an objective observer.
The lightbulb that I have in the kitchen over the counter is our GE Energy Smart par 38, which is one of my private inventions along with other folks on the innovation team. This is very smooth, very soft looking and the beam is perfectly uniform. The color quality is very high and the efficiency is high. It looks gorgeous in the ceiling.
The other favorite that I have, I burn in desk lamps is another of our inventions. That’s the omnidirectional LED bulb, the direct replacement for Edison’s incandescent bulb. We have right now a 40-watt replacement version. Our 60-watt version is coming out next month. Our 75- and 100-watt replacements are scheduled to come out next year. What we like about that is it is the incandescent replacement type of LED that looks and performs and feels most like Edison’s set alight bulb. When you think about what GE lighting would make as an LED replacement to Edison’s set alight bulb to fill the iconic soft white product, it’s this.