PRECISION REPLAY OF UNDERWATER HOLOGRAMS

Optical metrology of holographic images, otherwise known as 'hologramm etry', offers the ability to produce in real space a full-scale, three -dimensional image of a recorded structure, which displays sub-millime tre accuracy in both detail and spatial proportions. In an effort to d evelop this potential for underwater applications, we have been studyi ng the optical aspects of underwater holographic imaging, in particula r those aberrations associated with reconstruction of the conjugate re al image in air. A similar problem is encountered in conventional unde rwater optical systems, where a variety of measures have been adopted to limit the effects, particularly on off-axis image fidelity, of refr action and dispersion at the air/water interface. These include use of compensated lens elements in recording or calibration factors in imag e processing, or some combination of both. If the advantages of underw ater holography are to be fully realized these difficulties must be ad dressed and, if possible, resolved in a context appropriate to hologra phy. In this paper, we have employed both geometrical optics and analy tic approaches to assess the extent of the problem and to evaluate a p ossible solution. By employing third-order numerical methods, we obtai n simple analytic expressions for the aberrations encountered when the conjugate real image is replayed in air, and illustrate that a substa ntial restoration of image fidelity can be realized by introducing a p rescribed wavelength change between recording and replay. Finally, we review practical implementation of the technique and outline the condi tions that must be satisfied before residual aberrations can be reduce d to tolerable levels.