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Introduction
A century has elapsed since electroluminescence was demonstrated in a laboratory and nearly five decades since the invention of the light-emitting diode (LED) by Nick Holonyak, Jr., then of General Electric.
Holonyak succeeded in a method of synthesizing gallium arsenide phosphide (GaAsP) crystals that, when switched on, glowed with a faint but tunable red light (Massachusetts Institute of Technology, 2004). At first, consumer applications for LEDs consisted solely of power-on indicators in portable calculators, music players and television sets.
Progress to finding other consumer applications was slow. For instance, a full decade would go by before a Holonyak student found a way to boost light output ten-fold. In contemporary times, nonetheless, the energy efficiency, increasing light output, durability and much faster switching time of LEDs have paved the way for applications in room lighting, scrolling text displays, video displays, automobile lights, outdoor lighting, and traffic lights. This paper assesses the foreseeable prospects for LED lighting, principally against the existing alternative, fluorescent lamps.
Why LED Technology is Energy-Efficient
LED lighting contributes to retrofitting the built environment to be more sustainable, principally in point of reducing power requirements. Belonging as they do to the product class of semiconductors that replaced transistors, LEDs inherently operate at very low power settings.
The early LEDs used in the first non-analogue wristwatches and to signal power-on state in desk and handheld calculators, portable radios and cassette players, operated at just 30 to 60 milliwatts. It would be the turn of the century (1999) before die sizes became large enough to accept even one watt. This power upgrade also required the innovation of mounting LED dies on metal slugs.
Brightness
Technology in LED lighting has matured to the point where commercially-available products already exceed the luminosity-to-power ratio of incandescent bulbs and even standard fluorescent lights, presently the most widely-used alternative.
Incandescent bulbs rated at 60 to 100 watts emit in the vicinity of 15 lumens per watt (lm/W) while regular, 40-watt fluorescent lights attain up to 100 lm/W. In 2002, Philips Lumileds brought to market five-watt LEDs rated at 1822 lm/W. From 2003 to 2010, American, Japanese, Korean and Taiwanese manufacturers raised the bar, first achieving 65 lm/W and, in early 2010, attaining 208 lm/W in cooled laboratory environments. For commercially-available products, 100 lm/W was already available in 2009.
One hundred lumens is an important benchmark by itself because the Illuminating Engineering Society considers that illuminance level the very minimum for class C applications: Working spaces where visual tasks are only occasionally performed (Williams, 1999, table 1).
Since 1 lumen/square metre = 1 lux, 1 lumen/square foot = 1 foot-candle, illuminance is measured one metre from the source, and the distance from ceiling lights to the desktops or easy chairs where people read is at least three metres, the IES recommendations rise all the way to 2,000 lumens for class F, reading small print or low-contrast materials. This is why fluorescent lamps are routinely installed in sets of at least two in residential and office areas where occupants need to do some reading.
Going by the declared current requirements of 20 to 350 mA (the latter for high-power LED lighting), one can see that the technology already achieves acceptable lumens at under 5 watts in standard household current of 220 volts but in cold laboratory conditions.
To the extent that R & D can find ways to transfer such significant power efficiency to environments of moderate-to-high room temperature and humidity, LED lighting looks to make an enduring contribution to the search for sustainable solutions in the foreseeable future, particularly as energy costs continually rise.
Other Advantages
First, the spectrum of available colours continues to expand. In the 1990s, for example, gallium arsenide (GaN) LEDs were developed by Metal Oxide Chemical Vapor Deposition of an ultra-thin layer of the alloy on sapphire. The result was a device structure that reflected at least 90% of the light generated by the active layer.
Luminance nearly doubled at the same time that the device finally completed (with green) the primary colour palette available from LED lighting (MarkTech OptoElectronics, 2010). Outdoor applications for daytime use, such as traffic lights, became possible.
Further, LED lighting bears important advantages versus current popular lighting in point of waste disposal and ultraviolet radiation. The product class is so fine-tuned that it does not emit UV rays at all (Toolbase.org, 2001).
On the other hand, both the Health Protection Agency (2008) and the National Electrical Manufacturers Association (NEMA) admit that fluorescent lamps emit UV rays, owing to the way the technology works: an electrical current exciting mercury vapour, thence causing UVR to excite the phosphors coating the interior of the fluorescent tube.
So far, laboratory measurements suggest that highly dangerous UV-C is absorbed by the glass fitting while the UV-A and UV-B radiation that leak are well below the official threshold for experiencing acute side-effects (malignant melanoma being the most serious).
On the other hand, the Support Group for Sun Sensitive People (2006) argues that exposure to bright fluorescent lighting in shopping malls is equivalent to 50% the midday UV radiation of summer sunlight, a peril for those who have sensitive skin and suffer from such conditions as lupus and other sun-related sensitivities. But there is no debate that disposal of short-lived fluorescent lamps poses a mercury contamination issue for landfills.
References
Health Protection Agency (2008) Ultraviolet radiation (UVR) from fluorescent lamps. [Internet]
MarkTech OptoElectronics (2010) History of LEDs and LED technology. [Internet]
Massachusetts Institute of Technology (2004) Nick Holonyak, Jr.: 2004 Lemelson-MIT prize winner. [Internet]
Support Group for Sun Sensitive People (2006) UV radiation from lighting, copiers, computer monitors etc. [Internet]
Toolbase.org (2001). LED lighting. [Internet], NAHB Research Center.
Williams, B. (1999) Footcandles and lux for architectural lighting: An introduction to illuminance (Ed. 2.1). [Internet]
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