HID (High Intensity Discharge) typically use a trapped gas and an arcing electric spark to ignite it. The gas is usually Xenon so the terms are used interchangeably at times. It's my favorite headlamp technology anyway...maybe just because we put 'em on the 787...but I do like them over LED and Hali.
The projector ball represents the optics...elliptical lens. You probably know plenty about that, Jwest, considering your line of work.