During World War II, night fighters were either purpose-built night fighter designs, or more commonly, heavy fighters or light bombers adapted for the mission, often employing radar or other systems for providing some sort of detection capability in low visibility. Many WW II night fighters also included instrument landing systems for landing at night, as turning on the runway lights made runways into an easy target for opposing intruders. Some experiments tested the use of day fighters on night missions, but these tended to work only under very favorable circumstances and were not widely successful.