Spray Characteristics 10: Drop Size Distribution: Laser diffraction, a spatial sampling method, relies on the principle of Fraunhofer diffraction, which is caused by the light interacting with the drops in the spray. The scattering angle of the diffraction pattern is inversely related to the size of the drop. This nonintrusive method utilizes a long cylindrical optical probe volume. The scattered light passes through a special transforming lens system and is collected on a number of concentric photodiode rings. The signal from the photodiodes is used to back-calculate a drop size distribution. A number of lenses allow measurements from 1. 2 to 1800 μ. The optical imaging method uses a pulsed light, laser or strobe, to generate the shadow graphic image used to determine the size of the drop in the measurement volume. This spatial measurement method has a range from 5 µ to 10,000 μ with lens and optical configuration changes. Image analysis software processes the raw images to determine a circular equivalent drop diameter. This method is best suited to quantify larger diameter drops in medium to low density sprays, opaque liquids (slurries), and ligaments (partially formed drops)