Phaser sweep
Aphaser sweep was a procedure in which aphaser emitter, typically mounted on the wall of a room, emitted aphaser burst that covered the entire area of the room, usually on a setting of three or four.
When this procedure took place, anyChangeling present in the room would convert back to itsgelatinous state, making the Changeling detectable.

A phaser sweep on DS9 in 2372
In early2372, the crew ofDeep Space 9 used phaser sweeps to findOdo, who was helping them to train for a possible infiltration of thestation by theFounders. (DS9: "The Way of the Warrior")
This procedure was used again later that year when aDominion infiltration of Changelings was thought to have occurred onEarth. These phaser sweep devices were implemented in allStarfleet andFederation installations on Earth under orders fromCaptainSisko, who was temporarily promoted to head of Starfleet security. (DS9: "Homefront", "Paradise Lost")


