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Previous applications of coded aperture imaging (CAI) have been mainly in the energetic parts of the electro-magnetic spectrum, such as gamma ray astronomy, where few viable imaging alternatives exist. In addition, resolution requirements have typically been low (~ mrad). This paper investigates the prospects for and advantages of using CAI at longer wavelengths (visible, infrared) and at higher resolutions, and also considers the benefits of adaptive CAI techniques. The latter enable CAI to achieve reconfigurable modes of imaging, as well as improving system performance in other ways, such as enhanced image quality. It is shown that adaptive CAI has several potential advantages over more traditional optical systems for some applications in these wavebands. The merits include low mass, volume and moments of inertia, potentially lower costs, graceful failure modes, steerable fields of regard with no macroscopic moving parts and inherently encrypted data streams.Among the challenges associated with this new imaging approach are the effects of diffraction, interference, photon absorption at the mask and the low scene contrasts in the infrared wavebands. The paper analyzes some of these and presents the results of some of the tradeoffs in optical performance, using radiometric calculations to illustrate the consequences in a mid-infrared application. A CAI system requires a decoding algorithm in order to form an image and the paper discusses novel approaches, tailored to longer wavelength operation. The paper concludes by presenting initial experimental results.

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The extension of holographic techniques from the visible to the infrared is important. Potentially, holographic diffractive elements have a large range of uses in this wave band. Examples include mirrors, lenses, filters, and beam combiners. All these elements would have similar advantages to those enjoyed by their visible band diffractive analogs. The metal photodissolution effect in chalcogenides shows promise as one of the few techniques for producing low-loss holographic materials for use at any given wavelength from 0.6 to beyond 16 microm. To date, the work has concentrated on the photodissolution of silver into arsenic sulfide glasses. Both bulk and surface relief gratings can be fabricated simply by holographic or mask exposure. In principle, kinoforms (e.g., blazed zone plates) and Fresnel lenses can also be made. The results of material studies show that phase gratings with high modulation and low absorption can be produced. A coupled-wave analysis is used to calculate the likely grating performance, and some initial grating characterization results are presented. The limitations of the medium are discussed and possible solutions are considered.

Holographic techniques offer a route to the generation of 3-D images having all the depth cues used by the human vision system. A new electro-optic modulator system has been developed by the authors to replay dynamic holographic images. This Active Tiling (AT) system offers a route to replay giga-pixel computer generated holographic (CGH) images with video refresh rates. A key component of the AT system is an Optically Addressed Spatial Light Modulator (OASLM), onto which segments of the large pixel count CGH are loaded or written sequentially before the whole CGH frame is read out simultaneously. The OASLM device structure used consists of an amorphous silicon photosensor layer combined with surface stabilised ferroelectric liquid crystal (SSFLC) light modulation layer. A number of experiments have been conducted to determine the performance and suitability of this device for replaying a CGH. These experiments include electro-optic switching to determine the operating window and diffraction efficiency (DE) measurements to determine spatial resolution performance. A detailed description of the experimental apparatus and method used for measuring DE is presented and results show the OASLM to be capable of diffracting light from fringe patterns with spatial periods as low as 3 µm (333 lp/mm). Examples of CGH replay of 3D images from the OASLM when operating within the AT system are also presented.