Strobulb

The Strobulb is a flashlight tool which appears in Luigi's Mansion: Dark Moon. The Strobulb is required to stun some Ghosts as, unlike in the first game, shining light via flashlight is no longer sufficient to stun all ghosts in the game. Players need to charge up the Strobulb and unleash a powerful beam of light. The longer the charge, the greater the range. Some ghosts wear sunglasses to protect them from the light; these sunglasses must be removed with the Poltergust 5000 before the flashlight or Strobulb is used.

Luigi gets it in the first mission of the game when he enters Gloomy Mansion and finds out two Greenies have it.

Trivia

 * From a development and control standpoint, the Strobulb was most likely added to the game due to the player's lessened control over Luigi's flashlight due to the Nintendo 3DS' lack of two control sticks like the Nintendo GameCube controller.