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. The room was specifically designed to house the large Ghost Portrificationizer.
Luigi only visits this room after defeating an area boss in mansion. Like The Lab, Luigi cannot freely roam around the location. The final cutscene takes place in this room, which shows Mario being transformed from a painting and into his normal self.