Fluorescence polarimeter for flow cytometry

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Abstract
We have added the ability to measure the fluorescence polarization of single cells to a flow fluorimeter cell sorter system. The fluorescence, which is excited by a vertically polarized laser beam, is split by a Glan–Thompson prism into vertical and horizontal components. These pass through beam diffusers and are measured by two photomultipliers. The signals from the photomultipliers are processed electronically on line by a PDP‐11 computer based system. The ratio of the polarization components can be measured to 1% over a dynamic range of 10:1 in intensity, providing the fluorescence is bright enough. A Fresnel rhomb is used to rotate polarization of the exciting laser beam to the horizontal for calibration. Our system can display in real time the distributions of fluorescence polarization, fluorescence intensity, and cell volume either singly or in two‐parameter correlations, and it can sort cells on the basis of preset limits on these distributions.
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