Submarine operations are performed with as much frequency at night as during the day.  Therefore it is important to rig the control room and other spaces "for red."  The process of "rigging for red" is to turn off all lights except red lamps.  This keeps one's eyes accustomed to the dark for time periods when it will be neccesary to operate with the lights off.  The need for a flashlight to carry out tasks is coupled with the need for maintaining "rig for red" integrity.  The submarine penlight has a rotating black barrel at the tip with contains a red filter.  Turning the barrel changes the beam from white to red to enable operation during "rig for red" operations.  The light itself is constructed out of a brass tube with a black bakelite barrel.  The light operates on two AA batteries.  The nylon pouch is open ended with no closure to facilitate constant resheathing of the light.