Abstract: Apparatus for analyzing flowing fluid which contains light scattering particles comprises a light source; beam-expansion means to expand a light beam from the source to provide a two-dimensional sheet of light, there being at least a component of the fluid flow in the plane of said sheet; sensing means to sense light scattered by the particles; switch means arranged so that the sensing means senses scattered light only intermittently; and control and calculating means to calculate from the intermittently sensed light a property of the flow.

Abstract: An optical particle analyzer in which the size of a particle in a particle stream is determined from the intensity or the duration of light scattered from it. A laser beam is directed transversely onto the stream and scattering occurs at the beam focus. To ensure that only particles which traverse the center of the beam are considered, a smaller, validating beam concentric with the main beam also produces scattered light. A pulse from the narrow beam coincident with one from the broad beam confirms that the associated particle has passed through the center of the broad beam and warrants consideration and assessment.A further narrow beam spaced from the first provides a time delay, dependent on particle velocity, between interception of the two narrow beams by a particle. Particle velocity is thus measured.