The effects of silver halide processing schemes on image fidelity in hologrammetry
An investigation is conducted into the suitability of thirteen different silver halide hologram development schemes with a view towards their use in underwater hologrammetry. To this end the image fidelity indicators of resolution, reconstructed irradiance, noise, signal-to-noise ratio and contrast are extracted from a representative, diffusely reflecting object. Both amplitude and phase holograms are recorded, utilising a range of post-development treatments, on Agfa-Gevaert 8E56 emulsion. In addition, silver halide sensitised gelatin holograms are examined and evaluated. The major conclusion drawn is that suitably chosen phase development schemes are able to outperform amplitude development schemes in each fidelity indicator. A smaller number of development schemes are selected and characterised in terms of repeatability, beam ratio variations, Hurter-Driffield curves, 'amplitude transmittance versus exposure' curves and hologrammetric suitability. Pulsed holograms are recorded of the representative object both in-air and underwater. These holograms are evaluated with a view to determining their comparability and the conclusion is drawn that underwater holograms, in ideal conditions, are equivalent to 'in-air' holograms. It is concluded that, of those tested, the most suitable development scheme for underwater hologrammetry is Tetenal Neofin Blue bleached (without fixation) with ferric EDTA rehalogenating bleach. The optimum beam ratio is when K=5 and the optimum exposure for maximum signal-to-noise ratio, accurate contrast reproduction and high resolution is H = 25 muJ cm-2. Should maximum resolution be required then exposures as low as H = 5 muJ cm-2 should be employed.