Optical Radiation Measurements Based on Detector Standards

A near-IR radiometer standard with similar performance to silicon trap-detectors has been developed to calibrate detectors and radiometers for absolute spectral power, irradiance, and radiance responsivities between 950 nm and 1650 nm. The new radiometer standard is utilized at the Spectral Irradiance and Radiance Responsivity Calibrations using Uniform Sources (SIRCUS) which is the reference calibration facility of NIST for absolute responsivity. The radiometer is a sphere-detector with a unique geometrical arrangement and it can convert the radiant-power responsivity scale of the primarystandard cryogenic-radiometer into a reference irradiance responsivity scale. The 0.05 % (k=2) scale conversion uncertainty is dominated by the two largest uncertainty components of the radiometer: The spatial non-uniformity of responsivity of less than 0.05 % in power mode and the 0.03 % angular responsivity deviation from the cosine function in a five-degree angular range in irradiance mode. These small uncertainty components are the results of a tilted input aperture (relative to the sphere-axis) and four symmetrically positioned InGaAs detectors around the incident beam-spot in the sphere. With the new radiometer standard, it is expected that a thermodynamic temperature uncertainty of 10 mK (k=2) can be achieved at 157 C the freeze temperature of the In fixed point blackbody.