High fidelity real-images from replayed holograms of underwater scenes

摘要

Holographic recording and replay has many advantagesrnover conventional imaging techniques for underwater visual inspection and mensuration. The high resolution and three-dimensional nature of holography, allows optical recreation of the underwater scene in the laboratory. Recording the hologram in water and replaying in air increases the magnitude of the optical aberrations (mainly astigmatism and coma for off-axis holograms and spherical aberration for-in line holograms) and limits the attainable resolution. Aberrations can be minimised by replaying the hologram in air at a wavelength that is equivalent to the effective wavelength of the beam in water (index compensation). Under specific conditions, the aberrations can be reduced to a level that allows precision measurement. To control the influence of the aberrations, reconstruction takes place in a micrometer-controlled plate holder to allow precise positioning about all six axes of freedom. Index compensation is effective, for both back-lit and front-lit off-axis holograms, and over a wide range of field angles. For diffraction limited resolution of in-line holograms the replay wavelength should be within 2% of the ideal. Aberrations are sufficiently well-controlled to allow monitoring of size, orientation and distribution of marine and freshwater organisms at resolutions down to 20 μm.