This study is an experimental and theoretical investigation of the application of forward-scatter Fresnel holography to water droplets. The theoretical holograms were constructed on a digital computer by determining the irradiance of the interference pattern formed by the light scattered from a droplet and a reference source, at discrete points on a hologram. The forward-scattered light is evaluated by assuming that the droplet is an opaque disk and utilizing the Rayleigh–Sommerfeld theory of diffraction. The reconstructed wave front is evaluated by performing the Fresnel transform on the hologram matrix. The corresponding experimental holograms were made with a helium–neon continuous-wave laser and using glass beads from 80 to 250 μm in diameter as models of the water droplets. The half-radiance width of the reconstructed wave-front radiance distribution is shown theoretically to increase linearly as the distance of the object from the hologram is decreased. Also, the half-radiance width increases logarithmically with hologram area. The theoretical and experimental radiance distributions in the plane of reconstruction were compared. Proper alignment of the hologram during reconstruction is necessary for the reconstruction of the original radiance distribution. Size can be measured by noting edge enhancement (radiance peak at the edge of the droplet) in the reconstructed image. A relationship between the diameter of the peak and the actual droplet diameter is determined for sphere diameters from 20 to 250 μm and object distances from 10 to 20 cm. Experimental accuracy to within 4% is achieved with this technique.
© 1970 Optical Society of AmericaFull Article | PDF Article
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