Abstract
NIST calibrates spectral irradiance standards at discrete wavelengths and at a fixed distance. Users who require spectral irradiances at finer wavelength intervals need to use interpolation functions, and users who use these standards at distances other than 50 cm need to determine the distance dependences of the lamps. In this talk, the different methods for the interpolation of FEL spectral irradiance standards in a spectral region from 250 nm to 2400 nm are compared, both numerically and experimentally, and uncertainties in the determination of the optical center for distance corrections are found from irradiance measurements at various distances. By comparing the interpolated spectral irradiances of the FEL lamp with the measurements of continuous spectra from a tungsten-strip lamp and a blackbody, the interpolations of the FEL lamp are validated. The advantages and the disadvantages of the different interpolation methods are discussed. We find that to obtain the interpolations with the lowest uncertainties, spectral irradiances determined using a polynomial cubic-spline fit and the least-squares fit using an emissivity-corrected Planck function should be compared. We also find that the FEL lamps can be modeled using a point-source geometry at distances greater than 50 cm.
Spectral Interpolations and Distance Dependences of NIST Spectral Irradiance Standards
NIST calibrates spectral irradiance standards at discrete wavelengths and at a fixed distance. Users who require spectral irradiances at finer wavelength intervals need to use interpolation functions, and users who use these standards at distances other than 50 cm need to determine the distance dependences of the lamps. In this talk, the different methods for the interpolation of FEL spectral irradiance standards in a spectral region from 250 nm to 2400 nm are compared, both numerically and experimentally, and uncertainties in the determination of the optical center for distance corrections are found from irradiance measurements at various distances. By comparing the interpolated spectral irradiances of the FEL lamp with the measurements of continuous spectra from a tungsten-strip lamp and a blackbody, the interpolations of the FEL lamp are validated. The advantages and the disadvantages of the different interpolation methods are discussed. We find that to obtain the interpolations with the lowest uncertainties, spectral irradiances determined using a polynomial cubic-spline fit and the least-squares fit using an emissivity-corrected Planck function should be compared. We also find that the FEL lamps can be modeled using a point-source geometry at distances greater than 50 cm.