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Documentation and technical study of analog holograms

Background

Since its invention, analog holography enjoyed a rapid development from the 1960s to the 1980s. At the time when various holographic photography techniques and optical elements emerged, a large number of holograms were produced with explorative visual effects. Many of them are currently housed within museums and galleries. Despite the large body of the collection, the study of its documentation and conservation within the field of cultural heritage has been few. In this study, we aim to 1) document the dynamic and parallax visual effects of holograms, and 2) analyze the material's properties using multiple characterization techniques to enhance the understanding of analog holograms.

Photo documentation and photogrammetry

We image the holograms using a conventional DSLR camera. By either manually positioning the camera to acquire the unstructured light field or using a gantry-based system to control the precise location of the camera, the discrete propagating wavefront of the hologram can be synthesized. It represents the dynamic and parallax visual effects. Furthermore, the 3D model of the object recorded in the hologram can be generated using photogrammetry, offering an additional documentation approach.

Materials Analysis

In this study, we apply non-invasive analysis such as X-ray Fluorescence (XRF), visible hyperspectral imaging (HSI), and optical coherence tomography (OCT) to map the chemical and physical properties of the holographic emulsion.

Specifically, XRF provides the elemental fingerprints to identify 1) the type of emulsion (photopolymer or silver halide/gelatin) and 2) the chemical residuals within the hologram, which can potentially indicate the type of bleaching reagents used during chemical processing. While XRF provides chemical information, both OCT and HIS characterize the thickness of the hologram. For reflection as well as transmission holograms, OCT directly images the layered structure of the hologram (substrate, emulsion, etc.) and measures the thickness of each layer at a micron-scale axial resolution. For reflection holograms, HSI combined with spectra analysis maps the thickness heterogeneity of the emulsion layer across the plane of the hologram.

Link to the hologram collection at the MIT Museum https://mitmuseum.mit.edu/collection/art

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