From the Super Mario Wiki
The Dark-Light Device is an add-on to the Poltergust 5000 and the Poltergust G-00 that appears in Luigi's Mansion: Dark Moon and Luigi's Mansion 3. When in use, Luigi's flashlight emits a rainbow-colored light in the direction he is pointing it.
Luigi's Mansion: Dark Moon
Using this attachment, Luigi can see objects that are hidden in a manner similar to an ultraviolet lamp. After shining the Dark-Light Device on these objects, Luigi needs to vacuum the Spirit Balls that are revealed to make the object visible again. The Dark-Light Device is also used to reveal Polterpup tracks so Luigi can follow them, Boo trails to follow Boo, and Toads, coins, or gems trapped in paintings.
Luigi is also able to use the Dark-Light Device on ghosts such as Greenies and Sneakers. Sneakers will be revealed when hit by the light, while Greenies will start to taunt Luigi.
The Dark-Light Device has an energy gauge that is a rainbow-colored meter in the top-right corner. This gauge can be upgraded by E. Gadd by collecting treasure and leveling up. In the ScareScraper multiplayer mode, if one Luigi has been cursed, other Luigis can use their Dark-Light Device on him to remove the curse.
Luigi's Mansion 3
The Dark-Light Device has the same use in Luigi's Mansion 3. It is used by pressing , and can be held indefinitely. It can also reveal purple Spirit Balls that are possessing furniture once they are stunned.
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