Light-emitting diodes, better known as LEDs, are all around us--those little red or green indicator lights that blink at us from our stereo equipment, most new traffic signals, and virtually all new exit signs in commercial buildings. And if you've been to Times Square recently, you've seen way too many LEDs being used for advertising! We've all seen colored LEDs; what's new is the use of white LEDs for indoor lighting.

Three recent columns provided a brief history of lighting, an overview of fluorescent technology, and a look at the challenges of improving streetlights. Following a side trip into the issue of "passive survivability," I'm returning this week to illumination with an overview of high-intensity discharge (HID) lighting.

The ice storm a week-and-a-half ago illustrated, all too clearly, the vulnerability of our homes. Hundreds of thousands of homes in New England lost power in the storm, which deposited up to an inch of ice on trees the night of December 11th, and tens of thousands were still without power a full week later, despite heroic efforts by utility crews.

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Last week we took a look at fluorescent lighting, which is dramatically reducing our energy use for illuminating indoor spaces. This week we'll cover mercury vapor lighting, which is the most common outdoor lighting in many of our towns.

Last week, after an overview of lighting history, we examined incandescent lighting--the lamp technology invented by Thomas Edison. Until the mid-1900s incandescent lighting dominated both commercial and residential lighting applications, indoors and outdoors.

I'm not usually all that comfortable in front of a camera, but I had fun walking the Greenbuild 2008 Expo floor with a video crew from CNNMoney.com and Fortune magazine. We focused on four or five technologies in our tour, only two of which made it into the final two minute video (after a nice lead-in by Scot Horst of 7group). The CNN crew were looking for photogenic presentations, while I was looking for products I believe in to talk about.