Spectral irradiance and power responsivity calibrations from 200 nm to 400 nm using the new NIST Ultraviolet Spectral Comparator Facility

Abstract Ultraviolet radiation is becoming increasingly popular for applications such as germicidal disinfection and photo-polymer additive manufacturing. However, calibrations of UV detectors have been limited by relative expanded uncertainties typically larger than 2% and a lack of National Measurement Institutes that measure wavelengths from 200 nm to 250 nm because of the difficulty working in that range. The National Institute of Standards and Technology (NIST) has established a new facility that achieves diode-to-diode spectral power responsivity transfers with uncertainties of 0.1% or better across the 200 nm to 400 nm wavelength range and a relative expanded uncertainty ( k =2) of less than 1%. The Ultraviolet Spectral Comparator Facility (UVSCF) provides spectral power and spectral irradiance responsivity calibrations of detectors and filter radiometers from 200 nm to 1800 nm. Its low UV transfer uncertainties derive from a combination of common-mode rejection, nitrogen purging the ozone from the system, a careful selection of mirrors, and strategic usage of broadband optical radiation sources, including a laser-driven light source and a deuterium lamp. The measurement stability over long time periods enables spatial mapping of detectors’ responsivities using calibrated x-y motion stages, which allows the facility to directly calibrate the spectral irradiance responsivity. Spectral irradiance is a critical measurement in many UV applications, and legacy calibration methods for spectral irradiance require multiple facilities and measurements. Our detector-based spectral irradiance responsivity method determines the spectral irradiance responsivity of silicon carbide detectors with 1% relative expanded uncertainty ( k =2). Those results agreed to within 0.70% with spectrally integrated irradiance measurements using a calibrated deuterium lamp with an independent scale traceability. This novel comparison of UVC scales validates the performance and uncertainties of the UVSCF and the efficacy of scanning spectral irradiance calibrations down to 200 nm.