Transmission of electromagnetic radiation in fogs in the 053–101-μm wavelength range

Abstract

A problem exists in predicting the effects of fogs (and par­ ticulates) on the performance of detection systems which rely on the transmission of electromagnetic waves in the visible and IR. Most reported experimental contemporary programs on atmospheric propagation 1-3 are largely concerned with relatively long ranges and time-averaged absorption and will yield little information for fogs in which the visual range is <1000 m. We have concentrated on conditions in which the visual range is less than this. We show the wide range of spectral attenuation curves that can occur both from fog to fog and at various stages during a single fog. Few measurements have been published showing the wavelength dependence of attenuation in fogs (referred to later as the attenuation curve). The main source of data is found in Ref. 4 which has formed the basis of several discus­ sions 5-6 which show that the attenuation curves can also yield important information on the size distribution for droplets in a fog. Arnulf et al. 4 used relatively long sampling paths (50-1200 m). They found many of the fogs they studied appeared spatially and temporally inhomogeneous. Their sampling period was not stated but appeared to be several minutes. In this work we have adopted a shorter measuring period to re­ duce the effect of inhomogeneity. Our transmissometer was designed to be operated con­ tinuously at the Field Station of the Royal Military College of Science. The site has already been described. 7 The attenuation coefficient λ (km-1) at each wavelength in the atmospheric windows is where If λ is the transmission in fog, and I 0 λ the transmission in clear air, and l the length of the path (in kilometers). Two transmissometers were used in our measurements operating over 17-m optical paths whose axes were never more than 0.25 m apart. One transmissometer had a quartz halo­ gen lamp as source, collimating and focusing optics, and a lead sulfide detector. Six filters, mounted on a rotating disk (5 Hz), sample spectral regions in the wavelength range from 0.53 to 2.69 μm.