Recently a new instrument termed the laser fog disdrometer was introduced by Silverman, Thompson and Ward. As implied by the name, the function of the instrument is the determination of the size distribution of fog droplets. In design, operation and analysis this instrument represents a significant departure from the customary approaches to the problem. The basic principle of the instrument may be summarized as follows: by suitably storing the diffraction pattern associated with a droplet, both the precise size and location of the droplet may be determined. This principle can be utilized to obtain size distributions without disturbing the statistics of the sample, i.e., finite volumes may be sampled without dilution.
Originally, the data were read directly from the diffraction pattern. This type of readout is subject to two fundamental difficulties: 1) the geometry of droplets is difficult to ascertain except for simple structures; 2) if several droplets are relatively near each other in the sample volume, the resultant diffraction pattern is difficult to interpret. This first consideration does not represent a severe limitation for this application; however, it would be a serious limitation in other applications where non-spherical droplets exist. Both of these restrictions, however, are removed by the present readout technique. Physically, the new readout is based on the realization that the diffraction patterns stored by the instrument are, in fact, a new kind of hologram. Hence, the stored diffraction pattern can be used to create a real three-dimensional image of the sample volume. Since the image is fixed in time, the volume may be explored at will and the size and shape of each particle as well as its position relative to the other particles in the sample may be determined. In the present paper the concept and design of the disdrometer is reviewed and the new readout technique is discussed from both a theoretical and experimental point of view. Typical experimental results are also illustrated.