From the Super Mario Wiki
The Projection Room (Japanese: 映写室 Projection Room) is one of the many rooms in Luigi's Mansion. It is located in Area Three.
When Luigi entered the Projection Room, he taps a projector on the front wall, causing some Purple Grabbing Ghosts to appear on the screen. Luigi sucks up the ghosts, causing the lights to turn on and Mario's Glove to appear.
The Projection Room has a couple points of interest. First, obviously, there's the projector and the projector screen, flanked by two oddly-shaped speakers resembling violin cases without necks. Secondly, there is a large cabinet on the eastern wall of the room.
If Luigi tries to suck the projection screen with his vacuum, it turns into the picture of a Boo (possibly Boolossus because of the yellow eyes) that says "Get out of here!". This poster appears again in the washroom on the second floor.
 Ghosts in The Projection Room during the blackout