Beamsplitter

Here's a neat trick of the beamsplitter glass of prompters.

In this image, the beamsplitter appears as a mirror. But if you were to look at this beamsplitter on-location, it would appear transparent.

This is from the Benjamin Moore How to Paint industrial shoot in Ridgewood, New Jersey. See how in the reflection of the glass, you can see the floorboards? It's picking up the reflection of the 17-inch monitor mounted underneath, facing the ceiling with reversed text. (Which I fuzzed out in the photo.)

In the room, you're looking through the glass to the camera lens.