The Dark-Light Device is an item in Luigi's Mansion 2, used for uncovering hidden furnishings, doors, Boos, the Polterpup's footprints, and other various items. It can also be used on paintings to obtain the item in the frame. By pressing and holding "Y" button on the 3DS, Luigi will be able to use the Dark-Light Device, and if a hidden item is found, a transparent, white silhouette of the item will become more and more visible as colorful rings fly around it. After shining the Dark-Light at it for enough time, several Spirit Balls will pop out of the item, usually the bigger the item, the more Spirit Balls. Luigi must then suck up these Spirit Balls to make the item become tangible and useable again. Should Luigi fail to suck up the Spirit Balls within the time they are present, the Spirit Balls will go back into the item they were in, and more Spirit Balls will be created in the place of the ones Luigi caught.
When finding a Boo, Luigi must use the Dark-Light Device to uncover where the Boo has gone, and eventually, uncover the Boo itself. The Boo will continuously make items in the room invisible, so the Dark-Light Device comes in handy in that regard as well.
There are also several missions where Luigi must track down the Polterpup, using the Dark-Light Device to uncover its footprints. He then must follow the trail of footprints, solving puzzles and capturing ghosts along the way, until he reaches the Polterpup.
The Dark-Light Device is found in Mission A-4, in the Gloomy Manor's Lab, however Luigi must access it through the Parlor as the door to the Lab disappears. He must then fight several Greenies to get back the device they stole, and finally be able to use the Dark-Light Device.