A laser crossedhyphen;beam anemometer has been applied to the study of capillary blood flow. Analysis of the various contributions to theS/Nratio of this system is presented, in which the effect of finite detector area is taken into account. Experimental data, takeninvivofrom a rabbit omentum, using a coherent frequency tracker, are included. The frequency tracker also serves as a particularly effective phasehyphen;lock detection system, based upon the rotation of the relative polarization of the two incident beams. The data processing circuits are gated synchronously with the cardiac pulse, thus allowing measurement of flow velocity as a function of phase in the cardiac cycle. In spite of the considerable optical phase distortion incurred upon passage of the laser beams through the tissue covering the blood vessels, the autocorrelation of the fringe field (rsquo;rsquo;fringe visibilityrsquo;rsquo;) remained high enough to permit useful velocity measurements. Rotation of the fringe direction allows measurement, with a onehyphen;dimensional velocimeter, of randomly oriented flows.
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