Underwater hologrammetry: reduction of aberrations by index compensation

Abstract

Measurement accuracy in underwater hologrammetry is fundamentally limited by the aberrations which are introduced by replay in air. These aberrations may be compensated for by replaying the hologram in air at a shorter wavelength, taking account of the reduction in wavelength which occurs in the transmission of monochromatic light through water. The authors consider the practical implementation of index compensation. Generalized formulae are given for spherical aberration and astigmatism in the reconstructed image. These expressions are used to establish the conditions required for optimum compensation and to determine the residual aberrations which must be tolerated in a practical scheme for recording underwater holograms. The results of experimental trials demonstrate the effectiveness of the technique and also highlight the inherent differences between schemes employing collimated and diverging reference beams.