An eyepiece was a cybernetic device replacing one of the eyes of some Borg drones.
A removable component, the Borg eyepiece was capable of detecting a wide visual spectrum as well as providing advanced spacial and geometric perspectives. Some versions emitted low-intensity lasers. (TNG: "I Borg"; Star Trek: First Contact)
During the production of Star Trek: First Contact, Borg eyepieces were rigged with complex lighting systems that spelled out, in Morse code, names of several production members, including Michael Westmore and Jonathan Frakes.