Abstract: In a forward looking infrared (FLIR) sensor, a scanning mirror views an external scene, and the infrared energy from the scanning mirror is directed along an optical path to a solid state detector. In the optical path there is a three-position mirror which is driven by an actuator to three predetermined positions. The FLIR sensor also includes two thermal reference sources which are disposed on either side of the optical path and viewable by the three-position mirror. When in a neutral position the three-position mirror reflects the infrared energy from the external scene onto the detector. When the three-position mirror is moved away from the neutral position to two different predetermined positions, the three-position mirror views each of the thermal reference sources and reflects the thermal energy of the thermal reference sources onto the detector in order to provide two reference irradiance levels for the FLIR sensor.