Reduction of aberrations in underwater holography

Holographic measurements of engineering objects made in air have been extended to underwater measurement. This requires that the hologram of an object recorded in water is replayed in air which results in image degradation, especially at large field angles. The effect is basically that of an observer in air viewing an object in water and is caused by the change in refractive index. A technique is proposed to minimize such aberrations by introducing a wavelength change between recording an replay of the hologram. The ratio of wavelengths is selected to be the inverse of the ratio of refractive indices. Initial experiments on the measurement of astigmatism are encouraging and indicate the validity of the technique