The photolitic converter.
A photolitic converter was a device invented by Naroq, a Deputy Investigator for Kesat Security, to penetrate the cloaking fields of the secretive Ba'neth species.
In 2376, Naroq lent the USS Voyager crew his photolitic converter, for their investigation into a Ba'neth attack on the Delta Flyer. The crew was able to tie the converter into the main deflector, allowing them to illuminate Ba'neth starships and space stations. In return for information on a weapon that caused neurological damage to Tuvok, Naroq gave the photolitic converter to the Ba'neth so that they could adjust their cloaking technology to compensate for it. (VOY: "Riddles")