From the Super Mario Wiki
The Portrificationizer Chamber, also called the Ghost Portrificationizer Room, is a location housed in Professor E. Gadd's lab in Luigi's Mansion. This room houses the Ghost Portrificationizer. Luigi only comes to this room for the cutscenes after completing each section of the mansion, and never gets a chance to explore it freely. This room is also where the final cutscene takes place, where Mario is turned from his portrait self back to a human.