Comparison of filter radiometer spectral responsivity with the NIST spectral-irradiance and illuminance scales

Abstract

Narrowband filter radiometers are being used in the development of a new method of irradiance scale realization at the National Institute of Standards and Technology (NIST). To improve the accuracy of the current spectral-irradiance scale, we propose to eliminate several steps from the calibration process and to use the filter radiometers and a high-temperature black body in realizing the scale. Ultimately, the filter radiometers will be calibrated against a trap detector measured at the NIST High Accuracy Cryogenic Radiometer, which maintains an expanded uncertainty at 633 nm of 0,0266% for a coverage factor of k = 2. In this work, the filter and detector were calibrated as a unit using the NIST Detector Comparator Facility, and the aperture areas were measured by flux comparison with a uniform source. Six filter radiometers covering the spectral range 350 nm to 1100 nm were used to measure a known irradiance source; the measurements agreed with predictions to within 1%. Comparisons were also conducted with three NIST standard photometers, and similar agreement was observed. Since the irradiance source and the photometers were calibrated using independent methods, our results not only verify the calibration of the filter radiometers but also imply that the NIST scales of spectral irradiance and illuminance are consistent. Future plans include characterization of a high-temperature black body and the calibration of quartz-halogen irradiance lamps using this black-body source.