RESUMEN
A short-wave infrared (700-2500-nm) radiometer has been designed and built to calibrate and cross calibrate spherical-integrating sources used in the calibration of satellite sensors residing on NASA's Earth Observing System platforms. We describe the design, predicted and measured performance, and calibration of the transfer radiometer.
RESUMEN
Over a period of 3 years a precision Sun photometer (SPM) operating between 300 and 1025 nm was calibrated four times at three different high-mountain sites in Switzerland, Germany, and the United States by means of the Langley-plot technique. We found that for atmospheric window wavelengths the total error (2varsigma-statistical plus systematic errors) of the calibration constants V(0) (lambda), the SPM voltage in the absence of any attenuating atmosphere, can be kept below 1.6% in the UV-A and blue, 0.9% in the mid-visible, and 0.6% in the near-infrared spectral region. For SPM channels within strong water-vapor or ozone absorption bands a modified Langley-plot technique was used to determine V(0) (lambda) with a lower accuracy. Within the same period of time, we calibrated the SPM five times using irradiance standard lamps in the optical labs of the Physikalisch-Meteorologisches Observatorium Davos and World Radiation Center, Switzerland, and of the Remote Sensing Group of the Optical Sciences Center, University of Arizona, Tucson, Arizona. The lab calibration method requires knowledge of the extraterrestrial spectral irradiance. When we refer the standard lamp results to the World Radiation Center extraterrestrial solar irradiance spectrum, they agree with the Langley results within 2% at 6 of 13 SPM wavelengths. The largest disagreement (4.4%) is found for the channel centered at 610 nm. The results of these intercomparisons change significantly when the lamp results are referred to two different extraterrestrial solar irradiance spectra that have become recently available.
RESUMEN
Errors can occur in laboratory measurements when the response of a bandpass-filtered radiometer extends into an atmospheric absorption region. Atmospheric models, such as modtran3, can be valuable tools that permit the optical measurements in these regions to be accurately analyzed, provided the models themselves are accurate. Comparisons of modtran3-predicted and laboratory-measured atmospheric transmittance have been made to help establish the validity of modtran3 for use in modeling short-path-length (a few meters or less), low-resolution optical effects over a wavelength range of 700-5700 nm. Comparisons include percentage differences, range of differences, and band-averaged differences. Good agreement is shown for all absorption bands except for the CO(2) band near 4300 nm. In general, the band-average differences are less than ~0.75% for all water bands. For the CO(2) band the modtran3 default CO(2) level is found to be inappropriate, and 610 parts per million in volume is found to give much better agreement with measurement.
RESUMEN
The standard ultraviolet through short-wave infrared (200-2500-nm) diffuse-reflectance material, Halon PTFE, type G-80, is no longer available. Therefore an equivalent diffuse-reflectance standard material must be found. Algoflon F6 is shown here to be an appropriate replacement through the presentation of measurements of various spectral-reflectance properties of Halon and Algoflon F6. The measurements include spectral hemispherical reflectance, spectral bidirectional reflectance factor (BRF), sample BRF repeatability, and sample lifetime.