Techniques and applications of in-line holography

The analysis of aerosols was one of the first direct applications of holography and illustrates the advantage of in-line holography over photographic or direct observation methods. The diffracted light waves from an illuminated sample volume are made to interfere with a phase related reference wave and the resulting hologram can be used to produce a real, magnified, three-dimensional image of the original sample volume. It therefore provides a unique approach to the in-situ analysis of dynamic particle fields. However, although the technique has many important applications, its usefulness has been limited by the quality of the reconstructed image. The choice of recording materials and processing methods and their contribution to the image quality, has been investigated. This has resulted in improved images from both phase and absorption holograms. By direct analysis of the reconstructed image, the size, shape, orientation and velocity of individual droplets or particles is obtained. The technique has been used to determine the droplet size distribution from several agricultural spray devices and in this context has been compared to some of the major existing sizing systems. In addition, in-line holography has been applied to the study of droplet behaviour near cotton leaf surfaces. The effect of parameters such as droplet size and wind speed on the impaction efficiency was investigated to determine the optimum droplet size for specific spraying conditions. Finally, the technique has been applied to problems of concentration studies, the measurement of natural spore velocities and the conditions governing ligament formation from rotary atomisers.