Both systems consist of a heated silicon carbide block that radiates a large amount of infra-red energy. It is surrounded by a large cylindrical mechanical shutter that modulates the infra-red output, producing a pulsing pattern. Early infrared guided missiles used a rotating reticle. When a target was not on the sensor's centerline, it would produce a pulse as the reticle swept over the target. When the target was on the sensor's centerline, the sensor would produce a constant signal. This constant signal was required by the early missiles to produce a "lock on" that would allow a launch.