Atmospheric Attenuation Coefficients in the Visible and Infrared Regions.

Abstract

Abstract : Atmospheric spectral attenuation coefficients were measured in ten narrow wavelength bands between 0.4 and 2.3 microns for a variety of weather conditions using two overwater, sea-level paths of 5.5 and 16.3 km. The wavelengths bands were chosen so as to avoid molecular absorption and were isolated by interference filters. A 60-in. diam, high-intensity source and a 24-in. diam, narrow-field receiver were combined to yield relative scattering attenuation coefficients (Greek sigma) as a function of wavelength (Greek lambda). These were then scaled using values obtained at one wavelength with a visual telephotometer. Log Greek sigma vs log Greek lambda curves show a wide range of slopes and shapes, with a tendency toward less slope in the infrared (indicating that Greek sigma is becoming independent of Greek lambda in the infrared). Some correlation with relative humidity was found for relative humidities greater than 70%. The anomalous slope reversal between 1.68 and 2.27 microns is discussed, and a possible explanation for the reversal is given as selective scattering by the aerosol at these wavelengths. (Author)