Acousto-optic laser-scanning cytometer

An instrument has been developed that uses a computer-controlled rapidly scanning laser beam to make cytometric measurements on cells or particles and which can measure low levels of fluorescence when using low-power lasers (Gershman, Hoffman, and O'Connell, "Methods and Apparatus for Analysis of Particles and Cells.") The method used is based upon acousto-optic principles of light diffraction. A vertically polarized 5-mW He-Ne laser is directed into an acousto-optic Bragg cell in which a portion of the incident light undergoes a small angular variation or deflection. Suitable optics focus the beam to a 25 microns diameter spot, at the 1/e2 point, in a sample cuvette while translating the angular variation into a linear scan. The cuvette enclosing the sample is slowly moved (approximately 1 micron/ms) via a stepper drive into the scanning beam while the forward angle light scatter sensor is monitored for the presence of valid signal events. When an event occurs, appropriate software optimizes the position of the focused laser beam onto the cell. Subsequently, scanning is stopped to allow for cell interrogation times that last for milliseconds or longer.