An emergency beacon
Emergency beacons were large hand-held lamps that could be used to illuminate dark surroundings, similar to palm beacons and SIMs beacons.
The beacon could be modified to emit only certain wavelengths of light. In 2367, Data modified a emergency beacon to work on ultraviolet light. In order to do so, he had to replace the regular emitter module. (TNG: "Identity Crisis")
An emergency beacon was also a device aboard a starship. In August 2151, Matthew Ryan told Captain Jonathan Archer that a younger crewmember hit the emergency beacon aboard the ECS Fortunate causing the automated distress signal. Due to the malfunction of the com system, the Fortunate could not tell Starfleet that everything was alright aboard the Fortunate. (ENT: "Fortunate Son")