Absolute Calibration of Field Reflectance Radiometers
1986
Jackson, Ray D. | Slater, Philip N.
A method is described whereby field reflectance radiometers can be calibrated in an absolute sense using equipment available at most agricultural or environmental research locations. A radiometer is positioned directly above a calibrated standard reflectance panel that is horizontal to the Earth's surface. The sun's direct beam is separated from the total by measuring the total, shading the panel with a nontransparent shield held between the sun and the panel, measuring the diffuse component, and subtracting the diffuse from the total. These measurements are repeated periodically from shortly after sunrise to near solar noon. A graph of the logarithm of the radiometer response to the direct beam versus the secant of the solar zenith angle (known as a Langley plot) yields the spectral-extinction optical thickness of the atmosphere as the slope and the logarithm of the exoatmospheric irradiance divided by the calibration factor as the intercept. Calibration factors for two radiometers were within 10 percent of those obtained by other methods, indicating that this technique is a viable method for the absolute calibration of field radiometers.
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