The system response is calibrated by a tungsten lamp with known radiance.
Calibration against tungsten lamp-based uniform sources does not represent scenes remotely sensed.
British Dictionary definitions for tungsten lamp
a lamp in which light is produced by a tungsten filament heated to incandescence by an electric current. The glass bulb enclosing the filament contains a low pressure of inert gas, usually argon. Sometimes small amounts of a halogen, such as iodine, are added to improve the intensity (tungsten-halogen lamp)