FASCAL 2: a new NIST facility for the calibration of the spectral irradiance of sources

Abstract

The facility for automated spectroradiometric calibrations (FASCAL) is the primary facility for calibration of spectral irradiance and spectral radiance at NIST and has been in continuous use since the early 1970s. Due to the increasing demands for spectroradiometric calibration, especially for supporting the monitoring of global environmental changes, a new facility, FASCAL 2, dedicated to calibrating spectral irradiance has been built. This facility will enable faster responses to calibration requests and, ultimately, result in lower uncertainties in the disseminated spectral irradiances. The FASCAL 2 facility is designed with the objective of achieving a signal-to-noise ratio exceeding 1000 : 1 from 250 nm to 2500 nm in a bandwidth of 4 nm to 8 nm when measuring a 1000 W FEL lamp at a distance of 50 cm with a receiving aperture of 1 cm 2. The facility will also be capable of calibrating deuterium lamps from 200 nm to 400 nm. The facility has six independent source stations, with four of the stations dedicated to measurements with spectral irradiance lamps and two stations reserved for the realization of spectral irradiance scales and checking the accuracy of automated wavelength. After verifying that the calibrations of spectral irradiance performed in FASCAL and FASCAL 2 agree within their combined uncertainties, FASCAL 2 will become the primary NIST facility for calibration of spectral irradiance.

Calibration of a radiance standard for the NPP/OMPS instrument

“…For the RCDL calibration, the references were FEL523 and FEL592, irradiance standards calibrated by NIST at the Facility for Automated Spectroradiometric Calibrations 2 (FASCAL2) 3,4 . The lamps were calibrated in May 2006.…”

“…The reference standard for the 746/ISIC is a NIST standard lamp, FEL 512, calibrated at FASCAL2 3,4 . Working standard lamps, which were cross-calibrated with the NIST standard, were used for the calibration of the sphere.…”

Results from Verification of Reference Irradiance and Radiance Sources Laboratory Calibration Experiment Campaign

“…With a smaller source, less energy is regained and this can reduce the measured signal. This is known as the "size-of-source effect" [20,21]. Thus, the smaller size of the source leads to a reduced measured signal because of the size-of-source effect and a 500 mm lamp-diffuser distance leads to a smaller signal because of the reduced uniformity of the source across the FOV (the source is less bright away from the centre of the filament).…”

Uncertainty Analysis of Spectral Irradiance Reference Standards Used for NREL Calibrations

“…The expanded uncertainty of the reference irradiance ranged from +1% to +2% for specific wavelengths ranging from 250 nm to 2,400 nm. Because of the FEL lamp instability and low signal (Yoon, Proctor, and Gibson 2003), the uncertainty is higher on the two ends of the spectrum. Further, we validated our method using the Monte Carlo simulation, and the results of the uncertainty were lower than the final estimated uncertainty.…”