Image Transmission

Instrumented C-130 Aircraft used for experimental research in Contrast Transmittance and Visibility.

For many years, the Air Force Geophysics Laboratory supported an extensive program in the measurement of atmospheric optics. The goal was to measure everything which affects seeing conditions through the atmosphere, or more specifically, the contrast transmission through the atmosphere. Data from this period include many vertical profiles through the atmosphere of scattering coefficients (see Johnson 1981a, for example), along with relative humidity and other related parameters such as drop size distribution (Fitch and Cress, 1981).

Measured absolute radiance distributions were acquired of the upper and lower hemisphere (ie both above and below the aircraft) under a variety of conditions (ref Johnson 1981d and Johnson and Hering 1981b). The desire to replicate these radiance distributions simultaneously and at higher resolution provided part of the inspiration for the Whole Sky Imagers. A quite large library of radiance distributions was obtained, and are still used by MPL and provided to others on occasion.

The figures below show typical C-130 data. The first plot shows the altitude profiles of total volume scattering coefficient for several flights in Germany. The second set of plots shows the measured radiance distribution at low altitude over water. The x axis is the zenith angle from -180 (the nadir) through -90 (horizon), 0 (zenith), and 90 (opposite horizon), to 180 (nadir). Data of this type is available in several reports in the Publications section.

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