The halo is the glowing ring around the light. Such a light could be built with halogen or HID lights.

A normal bulb lights up by running electricity through a wire till it glows. A HID light is a different critter altogether - runs an electrical arc through a gas capsule that creates the light. The two require different reflector designs to work correctly.

Here's a web page with some interesting comments on HID lights from a guy who works in the lighting industry and put up this page on the side:

http://www.danielsternlighting.com/t...s/Hid/HID.html