NIST evaluation and calibration of several key parameters for the SORCE Total Irradiance Monitor (TIM): aperture area, diffraction effects, and cavity reflectance

Details NIST evaluation and calibration of several key parameters for the SORCE Total Irradiance Monitor (TIM): aperture area, diffraction effects, and cavity reflectance NIST evaluation and calibration of several key parameters for the SORCE Total Irradiance Monitor (TIM): aperture area, diffraction effects, and cavity reflectance

Dec 2003

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003agufmsh32a1098s&link_type=abstract

American Geophysical Union, Fall Meeting 2003, abstract #SH32A-1098

Physics

1650 Solar Variability, 7536 Solar Activity Cycle (2162), 7537 Solar And Stellar Variability, 7538 Solar Irradiance

Scientific paper

TIM measures total solar irradiance E, through the ratio of total power Φ , recorded by an active-cavity radiometer, to entrance aperture area A. The as-measured power and area have uncertainties and corrections because of many complicating effects. Here we consider uncertainties in aperture-area measurements, diffraction, and cavity reflectance, all of which were analyzed at NIST. TIM has an entrance aperture that limits radiation reaching the radiometer. Geometrically, all radiation passing through should land on the cavity opening. Thus, an entrance aperture area is a critical parameter. Areas were measured by two methods: a non-contact (optical) technique that located points on an aperture perimeter, fitted the points to a circle, and calculated the area, and a relative technique that equated the ratio, of the area to the area of a standard aperture, to the ratio of measured flux transmitted through the two apertures. We will discuss aspects of uncertainty in these measurements. Knowing the area of the entrance aperture is insufficient to know the instrument throughput, however, partly because of diffraction effects. Diffraction at the entrance aperture causes some light that geometrically would enter the cavity to miss it. Also, three non-limiting baffles between the entrance aperture and cavity can cause radiation diffracted at the entrance aperture to be re-scattered onto the cavity opening. We considered all of these diffraction effects in detail, and will discuss the diffraction correction on measured power and its uncertainty. Radiation reaching the cavity can be absorbed or reflected by the radiometer cavity cone. NIST performed directional hemispherical spectral reflectance measurements on several auxiliary TIM radiometer cones from 1 μ m to 18 μ m. The NIST system combines a custom infrared integrating sphere, unique absolute reflectance measurement method, and Fourier-transform spectrophotometer. These data augment the primary reflectance measurements done at seven discrete wavelengths in the visible, near- and mid-infrared, verifying discrete wavelength measurements in the infrared and spectral continuity across the infrared.

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