Spray Characteristics 11: Drop Size Distribution: Phase Doppler, a flux-based method, measures particle size and velocity simultaneously. This method, also known as PDPA, is unique because the drop size and velocity information is in the phase angle between the detector signals and the signal frequency shift. Because this method is not sensitive to intensity, it is used in more dense sprays. The range of drop sizes is 1 to 8000 µ. At the heart of the method are crossed laser beams that create interference patterns (regular spaced pattern of light and dark lines) and illuminate drops as they pass through the small measurement zone. A series of three off axis detectors collects the optical signal that is used to determine the phase angle and frequency shift caused by the drops. Optical imaging and phase Doppler methods measure the size of individual drops. A sufficient number of drops (order of magnitude 10,000 drops) must be quantified to produce a representative distribution and to minimize the effect of random fluctuations. Often several measurement locations in a spray are necessary because the drop size varies over the spray cross-section