The ratio of the amounts of scattered and direct radiation received from a 4π source has been measured in the San Francisco Bay Area for source-receiver distances of 1.18, 4.33, 6.79, 9.81, and 14.37 miles. A large flashlamp surrounded by a frosted globe was used as the source. The receiver consisted of any one of five combinations of Wratten fiters and photomultiplier tubes having peak responses at 0.40, 0.50, 0.70, 0.83, and 0.90 μ and feeding into an oscilloscope, the screen of which was photographed. A device for controlling the field of view of the receiver allowed choice of fields of view from 4 degrees to 58.33 degrees half-angle, and an occulter allowed the direct radiation to be blocked out if desired. It has been found that for a given field of view the ratio of scattered to direct radiation received from such a source under no overcast conditions is greater the shorter the wavelength and that for source-receiver distances of up to about 7 miles this ratio is approximately proportional to the optical thickness (path length times attenuation coefficient) of the path. For the source-receiver distances greater than 7 miles and a given attenuation coefficient and field of view, the ratio in question has been found to decrease slowly with increasing distance.
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