Abstract

Forward scattering by particles along a transmissometer path can increase the measured transmittance. The contribution of forward scattering is determined by the scattering phase function of the particles, their concentration and the optical characteristics of the transmissometer. We present simultaneous measurements of visible and infrared transmittance through fog, rain and snow made with transmissometers of different geometry along a 538 m path. The transmissometers operated over nearly identical paths to ensure that the simultaneous measurements represented the same aerosols. The width of the forward scattering peak of hydrometers is on the order of milliradians at visible and IR wavelengths which is of the same order as the field of view (FOV) of typical transmissometer receivers. The receiver can therefore accept a substantial amount of scattered light, increasing the measured transmittance. The result is that the extinction coefficient derived from the transmissometer measurements using the Beer–Lambert law is less than the “true” or single scattering extinction coefficient. Converting the measured transmittances to extinction we find a linear relation between the simultaneously measured extinction values. The linear relationship supports a simple model for the forward scattering effect which is based on the transmissometer characteristics and the phase function of the scatterers.

© 1993 Optical Society of America