Absorption line filter window and method for velocity measurements by

Abstract

Velocity is measured by observing velocity-related frequency shifts in light scattered from moving air molecules or particles suspended in moving air, by passing the scattered light through an absorption line filter window gas cell with a notch type attenuation profile as a function of frequency. The scattering region is illuminated with a narrow linewidth light source coincident in frequency with a strong absorption line of an absorption line filter, whereby light scattered from stationary air molecules or particles is passed into the filter and a portion of that light falling within the strongly attenuated region is absorbed. As the velocity of the molecules or particles in the scattering region increases, the scattering frequency is shifted due to the Doppler effect, and the portion of the scattered light falling beyond the filter cutoff increases, causing the intensity of the light transmitted through the filter increase. The intensity of the transmitted light is directly related to the shifted frequency, thereby providing a measurement of the velocity of the gas. The cell transmission can be quantified by comparing the intensity of the light transmitted through the cell with the intensity seen in the absence of the cell, thereby leading to a quantitative measure of the relative velocity between the light source and scattering volume.