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To measure drop sizes in a spray without disturbing the spray, one can use an optical technique like Phase-Doppler Anemometry (PDA). In this technique drop size and speed are measured at the cross-over of two equivalent laser beams. Since the beams are coherent, at the cross-over (usually a tiny spot) thin bright and dark layers are created. When a drop falls through, it scatters light every time it passes a bright layer. A series of flashes results; this usually is called a 'burst'. The faster the drop falls, the higher the pulse rate of successive flashes. Thus the pulse rate is a measure of drop speed. this measurement is the 'Doppler' part of the technique.

Intensity of the scattered light is proportional to the square of drop size. Yet because the intensity of the laser beams is not constant within the cross-over region, calculating drop size from intensity is not very reliable. A much better sizing method is based on the measurement of the pulsating flashes by two or three small detectors, adjacent to each other. It turns out that these detectors measure the same series of flashes, but slightly shifted in time. This time shift is related to the 'phase' (there it is!) of the burst. from this the size can be determined very accurately.

Since the cross-over spot is very small, the results of the measurement only present the drop sizes and speeds at that particular point in the spray. Therefore, to obtain an average of the whole spray beneath a nozzle, the measuring spot is slowly moved in a plain below the nozzle, thus scanning the whole spray.