A dual mode light source unit is configured so that it can be secured into a conventional incandescent bulb socket on the aircraft. The light source has a connector portion that fits in the socket and receives the electrical current supplied thereto by the aircraft electrical system. The unit also comprises electric circuitry connected with the connector portion and a visible light source and an IR light source. The electric circuitry processes the input current from the socket and, based thereon, operates in either a civilian, visible mode or a covert IR mode. Where the current is in one electrical state, such as for example a certain voltage, the electric circuit sends power to the visible light source. When the current is in a different electrical state, e.g., a different voltage level, the circuitry sends power only to the IR source, and no visible light is emitted. Such a system allows for ready upgrade of existing aircraft because all control may be accomplished over a single pair of wires, as are already in existing aircraft wiring systems that do not have IR mode capability. To upgrade, light source units according to the invention can be simply inserted into the existing navigational light sockets.