Ambient humidity control for maximising replay intensity and resolution in aberration-compensated off-axis holograms of underwater objects

Abstract

In hologrammetry it is usually more desirable to reconstruct the real image than the virtual image, since the latter must be viewed at a distance through the window of the holographic plate itself. In applications where the recorded scene was in water but with replay into air it is necessary to correct for the refractive index difference. This can be done by reconstructing the image with shorter wavelength illumination combined with a change in beam angle to satisfy the grating equation, but these changes mean that the Bragg condition may no longer be satisfied during replay, reducing the diffraction efficiency and making the reconstructed images difficult to see. Changing the replay beam angle to better satisfy the Bragg condition makes the images brighter, but also renders them unrecognizable by introducing severe optical aberrations. A possible solution is to alter the Bragg properties of the hologram. In particular, the emulsion thickness can be conveniently controlled by altering the humidity of the atmosphere surrounding the hologram without causing any long-term changes or damage to the holographic plate. The validity of using humidity change to tune the Bragg properties of emulsions during replay has been demonstrated by measuring the brightness and perceived resolution of a reconstructed real image from a hologram over a wide range of humidities. The results have been compared with a simple model based on the Flory-Huggins theory of polymer swelling.