In "Fringe" Season 4 Episode 18, as Olivia and Walter use the Machine Room bridge to cross over to the parallel world, they are "Scanned" by a bright light. What is that scan for ???
A similar scan appeared in an earlier episode when Olivia and Nina Sharp were visiting a secure area in Massive Dynamic.
The purpose of such scans is to reveal hidden weaponry & bugging devices, verify identity, and possibly check for contagion.
Presumably, the ones in the bridge would also be scanning for shape shifters (the original kind).
They are the generic Sci-fi security scanners. They aren't explicitly explained in the show. Any answers will be (educated) speculation.