When transmittance of the atmosphere is determined by comparing the irradiancy of a distant light source to the calculated value for the irradiancy in vacuum, the result obtained is dependent on the field of view of the receiver. Measurements of this dependence over water have been made at sea level. For an uncollimated source, the results for any wavelength in the visible and near ultraviolet regions of the spectrum are represented approximately by the relation
Observations have been made in several wavelength regions between 3600A and 6235A for atmospheres in which the transmission for visible light was between 0.35 and 0.85 per sea mile. Two-mile and nine-mile optical paths were used. Observations of Tθ through the nine-mile path were made with a fixed field of view 13.5 degrees in diameter, and those through the two-mile path were made with several values of θ between 5° and 25°.
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