A Photoelectric Method of Measuring the Atmospheric Attenuation of Brightness Contrast Along a Horizontal Path for the Visible Region of the Spectrum*

Abstract

A satisfactory method has been developed for measuring the attenuation of brightness contrast by the atmosphere along horizontal paths for the visible region of the spectrum. The method consists of the measurement of the apparent brightness of “black” and “white” test objects (referred to hereafter as targets) located at different distances from a photoelectric telephotometer. The method is being used in an ONR sponsored research program at the University of Texas. In this program seven pairs of “black” and “white” targets are being used. These are rectangular in shape and subtend practically the same angle with respect to the telephotometer. The targets range in size from 0.65×1.30 ft. to 33×66 ft. These are located approximately along a horizontal line running nearly due East starting at a point located about eight miles east of Austin, Texas, and cover a range of nearly 9 miles in the vicinity of the Colorado River. This location of the target range provides a means of making atmospheric attenuation measurements for different azimuth angles between the path of observation and the position of the sun. In addition, the location is such that fogs and dust haze occur fairly frequently as well as a highly clear atmosphere. The data obtained during the first year of operation indicate that, for “white light,” the brightness contrast is exponentially attenuated and that the attenuation coefficient is surprisingly insensitive to the naturally occurring local variations in illumination along the observation path.