Fiber-optic probe

The Fiber Optic Probe is a tool of stealth used by UNSC military personnel who are wearing some form of armor. Fiber Optic Probes are small, wire-shaped devices that are plugged into an outlet on the helmet of the armor on one end, so that the wearer can see through the other end via their HUD.

Fiber Optic Probes allow the wearer to see around corners or through small spaces without having to reveal their positions by looking up and exposing themselves to enemy fire. Some of these probes measure at least three meters long.

Apparently, these are used by both ODSTs and Spartans through their BDUs and MJOLNIR armor.