LEDs vs. Lightbulbs

zymano writes "www.technologyreview.com has this nice article on LEDs vs lightbulbs" Follows the exploits of one Shuji Nakamura, the same man who brought you the practical violet laser.

Comments

efficacy versus efficiency

[Michael.Forman]

This article, like most articles that tout LEDs as a replacement for light bulbs, confuses efficacy and efficiency.

Luminous efficacy is defined as the power of radiated visible light (visible luminous flux) divided by the dissipated electrical power. Given in lumens per watt, efficacy provides a useful means of comparing which source provides the most useful illumination for a given power.

Efficiency on the other hand is defined as the power of radiated visible light (visible luminous flux) divided by the power of all radiated light (total luminous flux). Ideally given as a unit-less ratio, efficiency provides the ratio of useful light to wasted light and does not necessarily correlate with efficacy. It is possible to have simultaneously high efficiency and low efficacy.

The question of why LEDs haven't already replaced all lights can be answered quickly by comparing the efficacy of different sources of light. All numbers below are approximate.

Filament light: 10 lm/W
White LED: 20 lm/W
Halogen light: 25 lm/W
Red LED: 40 lm/W
Florescent light: 100 lm/W
Sodium light: 150 lm/W
It can be seen why red LEDs have replaced halogen bulbs with red filters in stop lights -- red LEDs have a much higher efficacy. Note that the efficacy of LEDs are still well below that of florescent lights. If you feel frustration in seeing how far LED technology still has to go to compete with the boring lighting technologies of yesteryear, assuage your sorrows in the knowledge that you can save billions in energy costs right now, simply by switching to florescent lighting.

Michael.

Re:A long way to go...

[theedge318]

While you are correct that fluorescent bulbs are much more efficient, they serve a considerably different purpose. Fluorescent bulbs provide great omni-directional lighting. Unfortunately they produce some unwanted spectrums, namely don't store wine under anything but an incandescent bulb. Anyways LED provide a very tight cone of light. This means that they are perfectly suited for flashlights, traffic lights, and headlights, where we are looking for a cone of light that only represents 1/100th of the sphere. A fluorescent bulb may be 3 time more efficient, but it does so in directions that need to be reflected into the correct direction, a process that is only 10-20% efficient.

What does this mean ... you won't be replacing your household lamps with LEDs anytime soon (if ever) ... but flashlights, traffic lights, headlights, and other similar lights will all begin to make use of LED technology.