Blackbody cavities are commonly used in calibration of optical sensors; that is, they provoke an instrument response to a known incident flux. The so-called cavity effect produces an effective emissivity near unity so that, if a uniform temperature distribution is maintained across the cavity wall, the flux exiting the cavity approaches that of a blackbody at that temperature. The effort described here employs Monte Carlo ray-trace (MCRT) models to simulate radiation within a blackbody cavity in order to define the spatial and angular distributions of exiting flux. These models may be used to evaluate the suitability of various cavity geometries for the calibration of radiometers. The sensitivity of exiting flux properties to variations of interior surface properties and temperature may also be evaluated. Described is an effort to develop a tool that can be used to enhance the design and utilization of blackbody cavities as a calibration source.
KEYWORDS: Domes, Sensors, Solar radiation models, Thermal modeling, Atmospheric modeling, Instrument modeling, Finite element methods, Monte Carlo methods, Convection, Shortwaves
The Eppley pyranometer is widely used to measure broadband shortwave irradiances on the earth's surface. Measurements obtained using these instruments are known to be influenced by infrared radiation that produces an offset from the signal that would result solely from the incident shortwave radiation. Described is an effort to model the energy exchanges within the instrument to describe the measurement error as a function of external conditions. A finite element method (FEM) analysis simulates heat diffusion in the instrument, and a Monte Carlo ray-trace (MCRT) code models radiative exchange in the instrument. An FEM analysis models heat diffusion in the outer dome of the instrument and produces the dome temperature distribution resulting from specified external boundary conditions. An MCRT code is used to model the influence of radiative exchanges between the domes and the sensor surface on the sensor signal. The code confirms offsets expected from a variety of ambient conditions. The goal of the effort is to create a working MCRT model of the pyranometer that will be combined with the existing FEM model. The completed tool will allow an accurate study of the signal sensitivity to various external conditions.
KEYWORDS: Domes, Sensors, Glasses, Solar radiation models, Optical filters, Convection, Temperature metrology, Shortwaves, Solar radiation, Data modeling
The Eppley pyranometer is widely used to measure shortwave irradiances. This instrument consists of a blackened surface in intimate thermal contact with the hot junction of a thermopile. The cold junction of the thermopile is in thermal contact with a heat sink. Shortwave radiation transmitted through two concentric hemispherical domes is absorbed by the blackened surface. The voltage developed by the thermopile is then interpreted in terms of the shortwave irradiance. Measurements obtained using these instruments are known to be influenced by thermal radiation that produces an offset from the signal that would result solely from the incident shortwave radiation. The thermal radiation emitted and reflected by the filters modifies the net radiation at the detector surface. The ongoing efforts to model these exchanges and to use experimental results to verify the model are described. The parallel experimental effort consists of determining the sensitivity of instrument response to thermal radiation effects. In this effort, thermistors are used to characterize the thermal gradients responsible for the instrument offset. The ultimate goal of the work described is to provide reliable protocols, based on an appropriate instrument model, for correcting measured SW irradiance for variable thermal radiation effects.
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