Use Of Active Cavity Radiometers As Absolute Radiometers And Transfer Standards

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Abstract
The reference active cavity radiometers (ACRs) form the heart of the Earth Radiation Budget Experiment (ERBE) calibration system. These instruments are designed to measure an absolute input irradiance level, independent of wavelength. As such, the reference ACRs can be configured to accept various input fields of view and measure sources of irradiance from the ultraviolet through infrared regimes without the problem of any spectral response corrections. The use of active cavity radiometers, with optimized duplex cones, provides a direct transfer between: (1) input energy and (2) that which is provided electrically to maintain the same temperature differential relationship between the two cones. Ray trace techniques, described in this paper, were used to design the cone geometry. An effective emittance of over 0.999 resulted from the geometric enhancement over input angles of 0 to 170 deg. The electronic bridge amplifier, coupled with the geometric design of the input aperture, makes these radiometers accurate to well within 0.5%, as shown by the repeatability of the calibration data and the design analysis referenced. This paper describes: (1) the design of these reference ACRs, (2) similarities to the ERBE flight configurations, (3) improvements in design made possible through the use of ground system electronics and heatsinks, and (4) performance accuracy as an absolute measurement of input radiation.

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