Emergency beacon

From Memory Alpha, the free Star Trek reference.

Emergency beacons are large hand-held lamps that can be used to illuminate dark surroundings, similar to palm beacons and SIMs beacons.

The beacon can be modified to emit only certain wavelenghts 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")