The mercury vapor ones I heard of were made by that one company that hung on to mechanical tv well after electronic was already for sale to the public. The name escapes me at the moment, but they had a ~400 line mechanical tv. I believe it used a spinning drum and projected onto a screen. A large screen... like 24" or so.
EDIT:
Scophony
Although... looking into it, I don't think the light would switch at all... which certainly explains how you'd get any kind of arc lamp to switch so fast... you wouldn't.