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.

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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.

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.

You find the darndest things on YouTube sometimes. Southwest Windpower, the maker of the Skystream 3.7 small-scale wind turbine, brought this video (and others like it) to my attention.

Light is one of our most important energy needs. Historically--before the advent of electric lighting--the need for illumination governed architecture. Buildings were designed to facilitate natural daylighting. My office is in one of the old Estey Organ buildings on Birge Street, built in the late 1800s; it's just 28 feet wide, allowing natural light from the windows to reach fully into the building.

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