Actually, you lot might be able to help me out with something here. I have half a memory of a series that proper gave me the willies as a kid and I’m damned if I know what it is. This is what I remember, though I might not be 100% on the details.
It was a sci-fi show with a basic premise of some sort of clandestine alien invasion I think; I’m guessing it aired around the early 80s. People were being replaced by alien dopplegangers, with the transformation occurring off-camera (presumably for SFX budgetary reasons). When someone was attacked, it showed the view from outside the room, with a trademark throbbing green glow visible round the edges of the door.
There was also some sort of device, like a green lantern, which all(?) the aliens had. I can’t remember what it was for, but finding one of these hidden lights was a shibboleth that someone wasn’t who they seemed.
Does that ring any bells with anyone? It fair gave me the shits as a kid, and it’s bugged me for years as an adult. It might even be something already mentioned here, like Tripods – I vaguely remember that being on but only really remember the Tripods themselves; whether it’s the same show or not, I’ve no idea.