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Although the methods used to measure the velocity and size of moving particles are very different and hence make use of separate apparatus, it is often possible to obtain both information simultaneously, The basic equipment requested for this technique is a standard laser doppler anemometer to which two slight modifications are brought; the photomultiplier is set so as to view the measuring volume at right angles and its's pin-hole is replaced by a slit, In these conditions, the electrical output of the photomultiplier can be best interpreted. The statistical correlation between the velocity fluctuation of the particle and that of its diameter is obtained by computing the joint probability density function of these variables from the probability density of the velocity and the conditional probability density of the peaks of the envelope, This method does not require much investment on the optics which is standard; the effort is however transfered to the electronics, data acquisition and digital processing. Although it, is at present less accurate than those base on Fraunhoffer diffraction and gives relative values since it must be calibrated, it has the advantage to yield the velocity and size-velocity correlation. Moreover the electronics can still be improved and the processing speed increased by the use of a fast compiled language,
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