An infrared scope for the Semi-automatic Rifle, which when combined creates the Rifle (semi-auto) w/ Infrared Scope. Equipping this to the weapon allows you to target your enemies through the infrared spectrum.
This add-on is particularly useful, and almost necessary, when you encounter the Regenerators and Iron Maidens. These creatures have several Plaga inside their bodies that must be destroyed in order to kill it. Using the Infrared Scope allows you to target these Plaga.