Spectral irradiance and radiance responsivity calibrations using uniform sources (SIRCUS) facility at NIST

Detectors have historically been calibrated for spectral power responsivity at the National Institute of Standards and Technology (NIST) using a lamp-monochromator system to tune the wavelength of the excitation source. Silicon detectors can be calibrated in the visible spectral region with uncertainties at the 0.1% level. However, uncertainties increase dramatically when measuring an instrument's spectral irradiance or radiance responsivity. In addition, the uncertainties are even larger in the UV and IR ranges. We will discuss a new laser-based facility for Spectral Irradiance and Radiance responsivity Calibrations using Uniform Sources (SIRCUS) that was developed to calibrate instruments directly in irradiance or radiance mode with uncertainties approaching those available for spectral power responsivity calibrations. In this facility, high-power, tunable lasers are introduced into an integrating sphere using optical fibers, producing uniform, quasi-Lambertian, high radiant flux sources. Reference standard irradiance detectors, calibrated directly against national primary standards for spectral power responsivity, are used to determine the irradiance at a reference plane. Knowing the measurement geometry, the source radiance can be readily determined as well. The radiometric properties of the SIRCUS source coupled with state-of-the-art transfer standard radiometers whose responsivities are directly traceable to primary national radiometric scales, result in typical combined standard uncertainties in irradiance and radiance responsivity calibrations less than 0.1% in the visible (larger in the UV and IR). Details of the facility are presented and examples of unique calibrations possible in the facility are given, including system-level responsivity calibrations in support of the National Aeronautics and Space Administration's (NASA's) remote sensing activities.