A wide spectral range transmissometor has been developed at the Phillips Laboratory for long path atmospheric measurements. It was designed to provide narrow band spectral coverage in three bands between 0.4 and 14 micrometers by programmed scanning with circular variable filters. The system uses three detectors so that three spectral regions can be sampled simultaneously. Two sources, a black body and a ribbon filament lamp are combined in a single beam. A two way path is provided by use of cat's-eye retro-reflectors. Transmitter and receiver are combined in the same unit and by using multiple retroreflectors located at different positions the path length can be readily changed by realignment of the unit. This increases the dynamic range of the system and makes possible measurement of rapidly changing atmospheric conditions. The transmissometer was recently used to provide transmission measurements as part of the FLAPIR (Forward looking infrared and Lidar Atmospheric Propagation in the InfraRed) field measurement program at Brunswick Naval Air Station, which had the purpose of determining the feasibility ofusing lidars to predict the effectiveness ofinfrared imaging devices. Measurements were made primarily in fog and rain conditions.
A transmissometer that operates in the visible /near IR region of the spectrum has been modified to measure beam modulation caused by turbulence along a 300 meter path. The system measures both total transmission and the rms value of the intensity fluctuations and has been designed to allow variation of aperture size and optical wavelength. From these measurements, aI2, QX2 and Cn2 are calculated and compared with measurements made with a log-amplitude variance scintillometer.All measurements have been made over a near -ground horizontal path and are compared with the NOAA model that predicts intensity fluctuations based on Cn2, wavelength, and aperture size.
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