Collimator

Device that narrows or straightens a beam of particles or waves

A collimator is a device which narrows a beam of particles or waves. “To narrow” can mean either to cause the directions of motion to become more aligned in a specific direction (i.e., make collimated light or parallel rays), or to cause the spatial cross section of the beam to become smaller (beam limiting device).

From Wikipedia, under CC BY-SA. More on occurri.