Why Is a Hologram Not a 3D Photograph?
Holographic and 3-D photographs, though they're both types of three-dimensional still shots, the similarity ends there. Holograms are regarded as true three-dimensional images, whereas 3-D photographs merely give the illusion of a three-dimensional image.
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Background
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A hologram is a recording of a wave interference pattern. When properly lit (reconstructed), this captured wave interference pattern produces a true three-dimensional image of the object photographed. Three-dimensional photographs, in contrast, use stereoscopic effects to produce the illusion of three-dimensional images.
Production
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Holograms are created using the reflection of coherent light (such as the light from a laser). Three-dimensional photographs are created by taking two pictures of the same object or scene from two different angles, mimicking the perspective of your eyes.
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Viewing
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Regular photographs can only be viewed from a perspective that shows direction. When objects are viewed from two different directions, as with photographs produced using stereographic effects, they appear in a manner that shows both direction and distance--giving the illusion of a 3-D image when viewed using special spectacles. Holographic images, by contrast, can be viewed with the naked eye from the perspective of height, length and depth.
Presentation
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Holographic images pop out, appearing suspended in space. You can walk around a hologram and see all angles of the image in true form. Three-dimensional photographs appear on two-dimensional surfaces and they only show a 3-D image from a single vantage point.
Other Distinctions
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Every part of a piece of holographic film contains the information of the whole. Not even cutting the holographic film into pieces is enough to destroy the hologram it captured. Properly light any piece of the diced up holographic film and it will still reproduce a whole image of the hologram. This cannot be done with a 3-D photograph.
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