Abstract

An apparatus is described for classifying particles and includes an optical system for transmitting to a focal plane which includes at least one particle, two substantially parallel optical beams, the beams being initially mutually coherent but of different polarizations. The beams are displaced and focused in the focal plane. A further optical system is positioned in the path which the beam takes after depating from the focal plane and combines the beams so that a particle-induced phase shift in one beam is manifest by a change in elliptical polarization of the combined beams. A first detector is responsive to the beam's intensity along a first polarization axis to produce a first output and a second detector is responsive to the beams intensity along a second polarization axis to produce a second output. The first and second outputs are added to provide an extinction signal and, in a separate device, are subtracted to provide to phase shift signal. The extinction signal and phase shift signal are both fed to a processor which classifies a particle in accordance therewith.