A Laser-Fog Disdrometer

Abstract

An instrument is described for determining the size distribution of fog droplets 4 microns in diameter and larger. A film record of the fog droplets is obtained as they pass relatively undisturbed through the collimated beam of light from a Q-switched pulsed ruby laser. The recording plane is located in the far field of the individual droplets but not in the far field of the diameter of the whole sample volume. Diffraction patterns associated with the individual droplets are then observed and recorded. Measurement of the characteristic dimensions in the diffraction patterns allows the droplet diameters to be accurately calculated from well-established diffraction relationships. The laser disdrometer is capable of sampling volumes up to five cubic centimeters on each frame of 35 mm film at a rate of 10 frames per minute. The short pulse length of the laser, as short as 1.0 microsecond, enables measurements to be made in moderately high winds without loss of accuracy. The measured distribution is relatively unaffected by the measuring technique, since no sample collection or dilution is involved. The method is applicable to sizing both opaque and transparent particles of all geometries and has, therefore, a wider application than discussed in this paper. In addition, availability of higher repetition rate lasers makes measurements from aircraft feasible.