Method of calibrating a radiation thermometer

A method of calibrating a radiation thermometer including a radiation sensor that is characterized by a sensitivity (S) and a temperature-responsive resistor that serves as an ambient temperature sensor, the temperature-responsive resistor having at a specified reference temperature a reference resistance (R.sub.0). The method includes the steps of using a first radiation standard having a known temperature T.sub.S (1); while the ambient temperature sensor is at a first ambient temperature T.sub.U (1), using the radiation thermometer to read the temperature of the first radiation standard; while using the radiation thermometer to read the temperature of the first radiation standard, measuring a first output signal U(1) of the radiation sensor; using a second radiation standard having a known temperature T.sub.S (2); while the ambient temperature sensor is at said first ambient temperature T.sub.U (1), using the radiation thermometer to read the temperature of the second radiation standard; while using the radiation thermometer to read the temperature of the second radiation standard, measuring a second output signal U(2) of the radiation sensor; and calibrating the radiation sensor and the ambient temperature sensor by using T.sub.S (1), T.sub.S (2) and the values obtained for U(1) and U(2) and without using any value for T.sub.U (1) that is derived from measuring an output of the ambient sensor, wherein the step of calibrating involves calculating at least one of the reference resistance (R.sub.0) of the ambient temperature sensor, the ambient temperature (T.sub.u), and the sensitivity (S) of the radiation sensor.