The Model PFC-5001 Portable Field Calibrator is a highly stable irradiance light source for performing absolute characterization of field-calibrating narrow band spectroradiometers in-situ. While it is optimized for use with the Model RSS-1024 and Model UVRSS-1024 spectroradiometers, it can be readily setup and adapted to a wide variety of optical detectors both in the laboratory and in the field. The PFC-5001 is a computer-controlled stabilized single lamp light source. An internal stabilized current regulation circuit ensures precise current regulation to the rugged 200 watt tungsten filament lamp, providing constant optical power output under all environmental temperatures. Two internal independent fans maintain the internal air that surrounds the lamp envelope at a nearly constant temperature. Current monitoring resistive shunt temperatures as well as housing temperatures can be monitored during operation. An exit aperture provides precise positioning of the spatially uniform light flux to the device under test (DUT). Custom mechanical alignment fixtures are typically built to order to mate the system to a variety of radiometers.