The Strobulb is an item in Luigi's Mansion 2 that is essential for

The Strobulb being held by Luigi

defeating ghosts. Unlike the ghosts in Luigi's Mansion, the ghosts of Evershade Valley require a bright flash of light to stun them before they can be captured. That's where Professor Elvin Gadd's new invention comes in. As an accessory to the Poltergust 5000, the Strobulb allows for Luigi's flashlight to create a strong blast of light after charging up, by pressing, holding, and releasing "A". This will temporarily stun ghosts and allow Luigi to capture them. The longer Luigi charges the Strobulb, the bigger and stronger the flash of light.

The Strobulb, along with the Poltergust 5000, is found in mission "A-1: Poltergust 5000". The first time it's seen, Luigi can see two Greenies steal in from the Poltergust 5000. Luigi chases these two Greenies to the Guard Hall, where they keep giving it to each other, and then leave. Finally Luigi chases them to the Foyer, where he sees that they have placed it atop a chandelier. He must lower the chandelier by going up to it, spinning a wheel-like contraption to lower it, and run down and get it before the chandelier raises back up. Once Luigi does have the Strobulb, he equips it on his new Poltergust 5000, and accidentally flashes himself in the face with his flashlight's new ability.