Holography by Shadow Casting
”Coded aperture imaging” is a recently agreed-upon name (1) for a two-step imaging process conceived in the early 1960’s by Mertz and Young (2, 3, 4, 5) at Block Engineering. Their basic idea was to let each point in the object encode its three-dimensional location by casting a shadow of a Fresnel zone plate onto a photographic plate. Thus an object consisting of a collection of points would be encoded as a collection of zone plates. Lateral displacement of the object points leads to lateral displacement of the shadow-cast zone plates. Vertical displacement of the object points leads to scale changes in the shadow-cast zone plates. Given this encoded image, it is easy to arrive at an image. Illumination with coherent light converts each zone plate into a point. The lateral position of the decoded point is determined by the lateral position of the corresponding zone plate. The vertical position of each decoded point is determined by the scale of the corresponding zone plate. Thus the three - dimensional image encoded by shadow casting can be decoded by illumination with coherent light.
KeywordsSpatial Frequency Image Encode Central Disc Object Point Fresnel Zone
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