I recently visited Dallas and dined at its beautiful new Ritz Carlton restaurant. One thing I noticed was its innovative restroom design.
The doors are made of frosted glass and when a door is locked, a large red stop sign is displayed in the glass. The sign is visible from the inside and the outside of the restroom, and didn't seem to be projected onto the door. I'm curious how this is done.
Here's a picture I took from the outside of the restroom using my iPhone: