A calibration method for radiometric imaging systems that relies on an absolute measurement of scene radiance (thereby requiring a baseline measurement of zero radiance), and a shutter assembly for taking the baseline measurement of zero radiance which is operable under cryogenic temperatures as low as 80.5K (-192.65 C.) in vacuums measuring 10^-6 torr (mm Hg). The shutter assembly generally includes an actuator and a shutter mechanism. The actuator is preferably a miniature solenoid assembly capable of operation in extreme environments (e.g. vacuums of 10^-6 torr, temperatures below 90K). The shutter mechanism preferably includes a single shutter blade and is also capable of operation when subjected to extreme environments. The method of using the shutter assembly provides a zero radiance reference measurement for an infrared imaging system, thereby providing a basis upon which absolute scene radiance may be determined.