DOI: 10.17633/rd.brunel.8204339
title: Optimizing replay intensity and resolution in aberration-compensated off-axis holograms by ambient humidity control

description: This is the data behind the plots in "Optimizing replay intensity and resolution in aberration-compensated off-axis holograms by ambient humidity control" by J.J. Nebrensky <em>et al.</em>, Imaging Science Journal (2003) [1]. <br>Abstract<br>In hologrammetry it is desirable to reconstruct the real image rather than the virtual image as the latter must be viewed at a distance through the window of the holographic plate itself. When a scene is located in water but the image is replayed into air it is necessary to correct for the refractive index difference by reconstructing the image with shorter wavelength illumination and changing the beam angle to satisfy the grating equation. However this means that the Bragg condition may no longer be satisfied during replay, reducing the diffraction efficiency and decreasing the SNR of the reconstructed images. Changing the replay beam angle to better satisfy the Bragg condition makes the images brighter, but also renders them unusable by increasing the optical aberrations. Our solution is to alter the Bragg properties of the hologram by altering the humidity of the surrounding atmosphere. This approach has been experimentally demonstrated for Agfa 8E56HD emulsions by measuring the brightness and resolution of a reconstructed real image from an off-axis hologram over a humidity range of 6 to 93%. The emulsion swelling and its effect on the Bragg properties of the hologram were modelled using the Flory-Huggins theory of polymer swelling.<br><br>BackgroundThe Holomar project built an in-situ undersea holographic camera, associated replay system, and developed image-processing and image-classification software.One of the major difficulties is that the off-axis holograms need to be reconstructed at a much shorter wavelength than was used for recording, to correct for the refractive index change between sea water and air. Unfortunately, at the replay angle giving the best image resolution the image brightness is very bad, as the Bragg condition is no longer fulfilled within the grating volume.The strong effect of humidity on holographic materials is a well-known phenomenon, and we therefore investigated the possibility of manipulating the the ambient humidity around the hologram during replay so as to cause the appropriate swelling of the emulsion to satisfy the Bragg condition.This work was first presented at "<em>Optical Holography and its Applications</em>" [2], but the resulting Proceedings volume was never published.Apart from a smattering of papers on using the induced wavelength shift to measure the humidity, the vast majority of work on holographic materials and humidity has been concerned with modifying the material to give dimensional stability by resisting swelling or shrinkage, especially for colour holograms. I'm only aware of Wuest and Lakes [3] and the present work as attempting to control the environment to achieve desired replay conditions.<br>Material<em>bragg6.xls</em> (22/04/2003): replay properties of Bragg holograms: dependence on ambient humidity, based on measurements and Flory-Huggins model predictions (Fig.7('Fig7'))<em>floryhg6.xls</em> (9/10/2002): gelatin swelling - data and Flory-Huggins model predictions (Fig. 2 ('Fig2'))<em>humid36.xls</em> (22/04/2003): response of holographic plates to humidity changes at replay (Figures 4 ('Plt1Res'), 5('Plt1Brt'), 6('CmpBrt') and 8('PRespGrf'))<em>w_lakes.xls</em> (21/05/2002): spreadsheet reproducing Figs. 3 and 6 of Wuest and Lakes[3], with data from GIFs of original figures supplied by Dr. Roderic Lakes extracted using WinDIG version 2.5.<em>w_lakes.xls</em> will be omitted from the public distribution.<br>AcknowledgementsHenry Nebrensky assembled the replay system and made the measurements.<br>Gary Craig, Helge Nareid and John Watson provided the holograms.<br>Peter Hobson provided advice and discussion.<br><br>DisclaimerThis data is provided in the form of spreadsheets as saved to disk over a decade ago (datestamps listed above).The 'Response' tab of <em>humid36.xls</em> refers to "O'Neill <em>et al.</em>", this is: F.T. O'Neill, J.R. Lawrence and J.T. Sheridan: "Improvement of Holographic Recording Material Using Aerosol Sealant" <em>Journal of Optics A : Pure and Applied Optics</em> <strong>3</strong> pp. 20-25 (2001).<br>References1. J.J. Nebrensky, G. Craig, P.R. Hobson, H. Nareid and J. Watson: "Optimizing replay intensity and resolution in aberration-compensated off-axis holograms by ambient humidity control" <em>Imaging Science Journal</em>, <strong>51</strong>(2) pp.111-124 DOI: 10.1080/13682199.2003.11784418 (2003)2. J.J. Nebrensky, G. Craig, P.R. Hobson, H. Nareid and J. Watson: "Ambient humidity control for maximising replay intensity and resolution in aberration-compensated off-axis holograms of underwater objects" presented at <em>Optical Holography and its Applications</em>, House of Sciences, Kiev, Ukraine; 28<sup>th</sup> September – 2<sup>nd</sup> October 20003. D.R. Wuest and R.S. Lakes: "Color Control in Reflection Holograms by Humidity" <em>Applied Optics</em> <strong>30</strong>(17) pp.2363-2367 DOI: 10.1364/AO.30.002363 (1991) <br>