Experimental investigation of Gaussian beam effects on the accuracy of a droplet sizing method

A special sizing technique is applied to measuring the diameter of monosized droplet streams that are used for investigation of fuel droplets in enginelike conditions. For these experiments the droplet diameter must be known precisely. The sizing technique used is based on the evaluation of the fringe spacing of scattered light in the forward direction. This technique is independent of the intensity of the incident light. No absolute intensities need to be measured. The droplets are exposed to a focused laser beam. Therefore the frequently used assumption of plane wave fronts is not fulfilled. Elaborate experiments have been carried out to study the influence of a Gaussian intensity distribution of the laser beam on the accuracy of the sizing technique. It has been shown that the droplet diameter can be measured to an accuracy of better than 2% even if the droplet is illuminated by a Gaussian beam for a droplet diameter that is smaller than the beam diameter.