Accurate three-dimensional metrology of underwater objects using replayed real images from in-line and off-axis holograms

In many fields of science and engineering it is important to obtain accurate high-resolution images of underwater objects and scenes to allow precision dimensional measurement and inspection. Optical holography enables true three-dimensional images, with a simultaneous high resolution and large depth of field, to be recorded. The technical challenges of recording such holograms, and their subsequent replay in the laboratory, is considerable as there are increased optical aberrations arising from the change in refractive index when an object recorded in water is replayed in air. Our detailed evaluation of these aberrations, and techniques for their compensation, are described. We have demonstrated a resolution of better than 10 µm using in-line holography and better than 100 µm using off-axis holography in laboratory simulations of underwater scenes. Volumes of water of 2.5 litres (in-line) and up to 100 litres (off-axis) have been recorded. An absolute coordinate accuracy of better than 1% for both in-line and off-axis holograms has been achieved. Results are presented on resolution and coordinate accuracy versus distance, replay wavelength, off-axis angle, and holographic plate misalignment. We discuss the concepts with reference to the design of an underwater holographic camera for the recording of marine organisms and the associated replay of the holograms.