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Lighting the way

Consensus is that there is one part of the semiconductor industry that is positively glowing. Light emitting diodes (LEDs), despite present climes, are forecast to achieve double-digit growth this year. From mobile phones to TVs, cars to traffic lights, garden path lighting to street lighting, LEDs are fast replacing traditional light sources as the market evolves towards higher powered sources, as well as creating new applications of their own.

With lower energy consumption, longer lifetimes, higher durability, smaller size and faster switching, LEDs have much to recommend them. According to research analyst Susie Inouye at Databeans: “New products can emit up to 140 lumens per watt, while a traditional incandescent bulb loses over 98 percent of its total energy through the heat it produces. Also, a standard 40 W incandescent bulb has an expected lifespan of 1,000 hours, while an LED can continue to operate with reduced efficiency for more than 50,000 hours.”

The downside of LEDs is that they are relatively expensive due to complex manufacturing processes, particularly for white LEDs. They also require more precise current and heat management than traditional light sources. Notably, as an LED's temperature increases, so its lumens per amp and overall power efficiency decreases. There has also been criticism of the low colour rendering capability of current LEDs – the measure of how accurately a light source renders objects in true colour.

Analog and mixed signal vendors have been swift to respond to these challenges, with suppliers such as TI and Maxim regularly releasing sophisticated new led driver technology, with support for multiple strings of LEDs, on-chip pwm dimming, over-voltage protection, supercapacitor support and ambient light sensing now par for the course.

Commented Inouye recently in Databeans monthly briefing note for June 2009: “The future of LEDs as a mainstream lighting solution is in many ways just beginning. Already, advances such as organic LEDs (OLEDs) have been used to produce visual displays for portable electronic devices such as mobile phones, digital cameras, and MP3 players. Yet even these devices have not yet met their full potential, as they currently operate at lower efficiencies than standard LEDs and their life expectancy is still far too short (under 1,000 hours) to be fully practical.”

Returning home the other night before the streetlights had yet been switched off, I spied a glowworm in the hedgerow and was reminded of just how far we have to go to emulate nature's efficiency. The warmth of its yellowy green glow and the sheer intensity of its output had me transfixed.

The chemical reaction that gives common European glow worms (Lampyris noctiluca) their bioluminescence is the oxidation of luciferin to produce oxyluciferin, catalysed by the enzyme luciferase. Apparently, fireflies have better control over their oxygen supply than glow worms, giving them the power to switch their luminescence on and off in an instant, compared to several minutes for a glow worm.

The efficiency of many examples of bioluminescence in nature is astounding; with more than 90% of energy input turned into light. By contrast, figures quoted for a 150lm/W LED suggest that around 20% of the total energy used is converted to visible light – significantly better than traditional lighting technologies, but way out of nature's range.

A forthcoming upsurge in LED usage may yet astonish us, thanks to catalysts such as government legislation and environmental targets. However, if you want to see something truly amazing, scan the hedgerows between June and August – prime glowing time.

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