Classic Project: Visible spectrum LEDs
Image credit: Pixabay
The story of the Light Emitting Diode.
A little more than half a century ago, in 1962, the American engineer Nick Holonyak Jr wrote his name in the history books of consumer electronics by announcing to his employer General Electric that he had invented a light-emitting diode (LED) that produced visible light.
The background physics had been known since 1907, when Marconi Labs engineer H J Round first discovered electroluminescence in a solid-state diode. Yet since the light was not visible, there was no obvious application for his discovery.
Even after Holonyak came along, the LED in its infancy was a technology looking for a use. The first application was to replace incandescent and neon light bulb warning lights on instrument panels. Early LEDs were useful for this because their output was red and only red, due to the use of gallium arsenide phosphide in the doping process.
It’s unlikely that Holonyak foresaw a future in which his indicator lights would light up the world, especially as early red LEDs cost US$200 per unit (around $1,500 in today’s money). But by the late 1960s, companies such as Monsanto and Hewlett-Packard, realising their potential, started to mass-manufacture LEDs, and by the time Fairchild Optoelectronics entered the fray in the 1970s, unit price had come down to a practical five cents, opening the door for their use in calculator and wristwatch displays. At this point in the LED evolution, the light intensity was nowhere near enough to use the technology for illumination, and so was confined to indicators.
Although development of the blue LED, which became commercially available in 1989, was heralded as a breakthrough in itself, its main influence was to pave the way for the white LED, which would in turn lead to the technology finding a practical application in illumination.
While early white LEDs suffered from being expensive and inefficient, in a phenomenon similar to Moore’s Law, there was a doubling of potential, linked to a 36-month cycle, that reflected trends in parallel semiconductor technology at the time. The result was that, despite its humble beginnings, the LED was on track to out-perform the incandescent light bulb in terms of energy efficiency and unit cost.
LEDs have several advantages over competing technologies, of which the principal one is efficiency, as they emit more lumens per watt than incandescent light sources. They can also emit an intended colour without having to use filters, which increases efficiency while potentially reducing cost. They can be as small as 2mm and have no discernible warm-up time (a red LED is typically at full brightness in under a microsecond). The manufacturers say LED bulbs last up to 50 times longer than incandescent ones and, because they have no moving parts or fragile components, they are more robust. LED streetlights create less light pollution than the sodium lights they replace.
On the other hand, the capital cost of LED lighting is higher than that of other technologies in price per lumen. LEDs are also sensitive to temperature, and because they emit almost no heat in the beam they can be obscured by snow in traffic and airport applications.
Yet the LED is here to stay. By 2020, half of the domestic lighting in the US will be LED, with companies such as Philips and GE (where it all started) pioneering developments in this market. Our TVs are now LED, while back in 2010 car manufacturer Audi produced the model A8 L in which there were no incandescent bulbs at all. The LEDs that replaced them will probably outlast the car itself.
Facts and figures: visible spectrum LED
Originator: Nick Holonyak Jr, General Electric
Unit cost: Bought in bulk, expect to pay as little as £0.03
At least 95 per cent of an LED is recyclable.
The first application for LEDs was as indicator lights on electronic instruments.
LED bulbs can last up to 50 times longer than conventional incandescent bulbs.
In 2012, MIT researchers demonstrated that an LED could emit more optical power than the electrical power it consumed: 30pW in and 69pW out.
The Times Square Ball in New York is illuminated with 32,256 individual LEDs.
By 2020, half of all US domestic lighting will be LED-based.
The background phenomenon of electroluminescence was discovered in 1907.
LEDs don’t attract as many insects as other light sources because their output has very little UV content.
If the United States replaced half of its incandescent Christmas lights, the projected energy cost savings start at around $17.2bn.
General arrangement of the Light Emitting Diode
Source: Edison Tech Center