Procedures for determining the spectral responsivity of an infrared radiometer

With the increasing number of commercial radiometers employing rather varied and sophisticated optical designs, the determination of the spectral responsivity of these radiometers has accordingly become more involved. A joint National Bureau of Standards-U. S. Army (Missile Electronics Warfare Technical Area, White Sands Missile Range, New Mexico) calibration of an ir radiometer was conducted at the White Sands Missile Range facility. The radiometer chosen for the measurements employed an uncooled PbS detector with a 10-cm Cassegrainian optical system. The selection of this particular type of instrument permitted two methods of measurement (each employing different standards and techniques) to be investigated. In order to simulate infinitely distant point sources, a 30-cm off-axis collimator was used in both methods of calibration. The methods of calibration and the uncertainties associated with each method are described and the application of the responsivity values to field measurements is discussed. The agreement of the spectral responsivity values as determined by each method is within the estimated uncertainty computed for each procedure.