<title>Field Testing Of A Long-Path Laser Transmissometer Designed For Atmospheric Visibility Measurements</title>

Abstract

The measurement of atmospheric properties from which visual range and thus "visibility" can be inferred reliably is a difficult task. One approach to instrumental measurement of atmospheric properties that can be interpreted in terms of visual range, with appropriate assumptions, is to use an optical transmissometer. Such an instrument provides a measure of the atmospheric extinction coefficient averaged over the propagation path and bandwidth of the instrument. When visual ranges are to be measured, the instrument baseline must necessarily be a significant fraction of the maximum visual range to be resolved and both micro-scale and macroscale beam effects are evident in the measurements. To realize the desired measurement of transmittance due to extinction by aerosol, it is necessary that the errors in measurement of atmospheric transmittance due to gaseous absorption and the macroscale components of atmospheric transmittance due to stochastic and deterministic refraction be minimized. This paper describes the field testing on an 18 km folded baseline of a long-path laser transmissometer parametrically designed for visual range measurements under good visibility conditions. Design parameters, instrument performance, measurements and data interpretations are presented. It is concluded that with large collecting optics, aperture averaging and adjustment of beam divergence, stochastic and refractive effects can be sufficiently minimized to make the transmissometer a useful instrument for visual range measurements.