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This paper concerns the use of pulsed light sources of extreme luminance and short duration in measuring the transmissivity of the air along a slant path; the glide approach path to an airport runway is of particular interest. The fundamental pulsed-light-transmissometer technique is described for an idealized atmosphere and an idealized single-ended transmissometer (confocal transmitter and receiver). Complications arising for real conditions are avoided by the following modified technique: A transmitter on the ground projects a series of light pulses up the glide path, during which time a receiving system separated to the side collects light scattered from the transmitter beam at each of a sequence of predetermined ranges and plots the scattered light flux received against time. These return plots are graphically superimposed on a single time base and an envelope of the return peaks is drawn. Equations representing individual returns indicate that their peak envelope can be used to determine the transmissivity at any given range and the average transmissivity over a given path.
Signal variations due to clouds and absorbing media are discussed, and the use of the technique in a ceilometer is mentioned. Path length limitations due to equipment features and to secondary scatter under low-transmissivity conditions are discussed. Measurements obtained with an experimental pulsed-light transmissometer are given and compared with measurements by two other systems.
© 1961 Optical Society of America
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