Privacy Glass for license plate cover, where to buy privacy glass?

ok, so I’m not looking for window tint. I’m looking for the glass (like in some clubs) that when you close the door, it becomes tinted. somthing about completeing the circuit. Well, i’ve seen it done before, and I want to know how to do it, but i need to know where to buy privacy glass. any help? any ideas?

heres the link where its been done-http://www.jalopnik.com/cars/top/bullrun…