Effect of spherical aberration on real-image fidelity from replayed in-line holograms of underwater objects.

The real image of a line object, located in water of refractive index n(w) and recorded on an in-line Fraunhofer hologram, is calculated by use of the Huygens-Fresnel principle. The presence of the water-glass and glass-air interfaces or the change in effective wavelength between recording and replay introduce wave-front aberrations. Spherical aberration dominates for a perfectly aligned finite-aperture hologram, and its effect on the replayed image of a finite-width line object is evaluated. Numerical results are compared with experimental data of a 10-mum wire located in water 50.0 mm from a 10-mm-thick glass window, and good agreement is demonstrated. It is shown that the error on the linewidth is less than 1.5%, and the shift in focal plane from the Gaussian plane is less than 16 microm, for a replay-to-recording wavelength ratio mu in the range 0.98 < mun(w) < 1.02.