A phaser sweep was a procedure in which a phaser emitter emitted a phaser burst that covers an area of a room. The procedure was introduced across the entire Starfleet in 2372 as a method to detect and stop Changeling infiltrators. On a setting of three or four, the Changeling reverts to it's gelatinous state, making it detectable. (DS9 episodes: "The Adversary", "The Way of the Warrior", "Homefront", "Paradise Lost") Aboard Federation starship's, all cargo and baggage brought aboard were to be subjected to phaser sweeps. (TNG eBook: A Sea of Troubles)