A portable, sterilizable light source unit for being coupled to an instrument is provided. The unit includes a sealed housing having an anodized coating and a port for receiving an end tip of a light guide of a separate instrument, a light source mounted within the sealed housing for directing light through the port, a battery mounted within the housing for powering the light source, a switch assembly mounted on the housing for controlling light output of the light source, and an O-ring in a compressed condition between the switch assembly and the housing providing a fluid-tight seal therebetween that prevents liquid intrusion into the housing during an immersion sterilization process.