High performance radiometers with selected optical detectors are being designed, characterized, and calibrated in the Optical Sensor Group for the 200 nm to 20 µm wavelength range. These radiometers are used as standards to realize new scales based on spectral responsivity. New calibration facilities have been developed to realize, maintain, and disseminate these new detector-based scales.
Improved detector technology in the past two decades has opened a new era in detector metrology of optical radiation measurements. Lower calibration and measurement uncertainties can be achieved with modern detector/radiometer standards than traditionally used source standards (blackbodies and lamps). The achievable lower uncertainties provided the motivation to decrease the gap between the 0.02% (k=2) relative expanded uncertainty of cryogenic radiometer measurements and the two to three orders of magnitude higher uncertainties of field-level optical radiation measurements. Thus, the role of modern detector/radiometer standards is increasing to lower calibration and measurement uncertainties compared to traditionally used source standards.
Standard detector/radiometer developments from the ultraviolet to the infrared
To decrease the large measurement-uncertainty gap between cryogenic radiometer measurements (such as those made by the NIST Primary Optical Watt Radiometer) and field measurements, a large variety of transfer and working standard radiometers are being developed. These radiometers are designed to work in different radiometric and photometric measurement modes and satisfy diverse requirements in different scale realizations, scale propagations, and field applications. The radiometers are optically and electronically characterized and verified so that the scale uncertainty is dominated by the scale realization procedure and not by the performance of the radiometers.
For examples, see Transfer and working standard radiometers and photometers and NIST Technical Note 1438: Optical radiation measurement with selected detectors and matched electronic circuits between 200 nm and 20 μm (14 MB pdf).
New radiometric, radiance temperature, photometric, and color scales have been realized based on the spectral responsivity of standard detectors and radiometers. These reference responsivity scales have been transferred to other NIST calibration facilities to realize and maintain more fundamental detector-based scales and decrease calibration and measurement uncertainties.
For more details, see Spectral responsivity-based calibration of photometer and colorimeter standards and NIST Technical Note 1621: Optical radiation measurements based on detector standards. The latter is an anthology of representative papers, published by the Optical Technology Division from 1996 to present. This collection of papers describe the conversion of the high-performance optical radiometers into standards and the realization the new improved spectral-responsivity based scales.
NIST cooperates with the three services (Air Force, Navy, and Army) to realize a uniform night vision goggle (NVG) calibration system with low measurement uncertainty. These efforts include transferring the NIST detector-based radiance responsivity scale to the primary standards laboratories, developing radiometric models to estimate uncertainty components in NVG calibrations and field measurements, and standardizing the spectral distribution of the light sources of the test sets used in calibrating NVGs. In response to these efforts, NIST has developed a night vision radiometer calibration facility and night vision radiometer transfer standards (NVTS). NIST also participates in the work of the Night-vision Sub-committee of the US Department of Defense’s Calibration Coordination Group (CCG) to establish a new detector-based NVG gain definition.
For more details, see Uniform calibration of night vision goggles and test sets.
Realization of the candela and Kelvin - Based on newly developed high performance radiometers, photometers, and colorimeters, realization of the two SI units, the candela and Kelvin, has been significantly improved.
Spectral irradiance responsivity reference scale - The spectral power responsivity scale was extended to a spectral irradiance reference scale at the SIRCUS facility. Based on this scale, the uncertainty of the SI unit, the candela, was improved. The NIST candela is now directly traceable to the NIST Primary Optical Watt Radiometer (POWR).
Detector-based color scale - Until recently calibrations of tristimulus colorimeters were performed against lamp standards. However, the uncertainty of these source-based calibrations increased with the burning hours of the lamps. These calibrations can now be performed against detector standards, lowering the uncertainty significantly. For more information, see Detector-based color scale.
Detector-based illuminance responsivity scale - The uncertainty of the 1996 NIST illuminance responsivity scale has been improved by a factor of two using newly developed transfer standard photometers.
Detector-based radiation temperature scale - Reference radiance temperature responsivity scales were developed at SIRCUS, based on new transfer standard radiometers for the calibration of radiation thermometers to perform direct thermodynamic temperature measurements over a wide temperature range (430 K to 2700 K) with low uncertainty. This work was done in cooperation with Howard Yoon, project leader of Radiance Temperature.
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