Abstract: A continuous wave Doppler LIDAR with an enhanced signal-to-noise ratio (SNR) that greatly enhances its ability to determine relative fluid velocity. A laser source (12, 50, 102) produces coherent light that is split between a reference beam and a test beam by a beam splitter (16, 106). A quarter-wavelength plate (26, 108) changes the linear polarization of the test beam to a circular polarization. The circularly polarized test beam is focused on a target cell (30) having a very small volume. Any particle in a fluid moving relative to the CW Doppler LIDAR system that passes through the target cell causes a Doppler shift in the frequency of the coherent light reflected from the particle and reverses the rotational direction of circular polarization of the reflected beam. The light reflected from the particle is combined with the reference beam, creating an interference pattern (or difference signal) incident on a photodetector (44, 118).

Abstract: An improved integrating nephelometer having superior sensitivity and long-term stability to previous designs includes a container defining a measurement chamber. A continuously energized light source, such as a tungsten filament incandescent bulb, illuminates the measurement chamber through an opal glass diffuser. The intensity of the light source is regulated in one embodiment by a simple optical-electric feedback system, and in another embodiment by a voltage regulated supply. A cone of observation of the chamber is defined by a plurality of spaced, apertured plates located in the chamber, at right angles to the light source. A measuring photomultiplier tube views the cone of observation and provides an output signal including components resulting from detected photoelectrons, noise, and thermally emitted "dark current" electrons. A photon-detecting apparatus removes the noise component and provides an output pulse for each photoelectron and "dark current" electron.