3.5 Holography

Holographic Bananas

Holography is the only true 3-D photography. It recreates the 3-D environment of light reflecting from an original scene. Just as the imaged object, holograms are viewable from any perspective. In theory, the original and the image should be indistinguishable. For example, if a hologram was made of a stamp under a magnifying glass, the stamps will change as we change our viewing angles exactly as if we were viewing the real scene. The magnifying glass in the hologram magnifies the stamps as we change our point of view. However, technology has yet to catch up with theory. Researchers are continuing to develop ways in which to improve holograms.

Holography is an imaging technique, which uses wave properties of light and/or other energy to create 3-D images. The use of wave properties to create imagery was first proposed by Dennis Gabor (1948), who won a Nobel prize for his idea in 1971.

Creating holograms - First

Holography makes use of binocular parallax, convergence and accommodation cues (Friedhoff and Benzon, 1991). Holographic images can be viewed from as many different perspectives as the object being imaged. Two steps make up the holographic imaging process. First, the interference pattern containing information corresponding to both the wave amplitude and phase scattered by an object is recorded on a transparency to create the hologram. Second, the developed hologram is reconstructed.

Creating holograms - Second

Creating holograms - First

Since the 1960s, researchers have progressed in overcoming disadvantages in the holographic imaging process. Some of the drawbacks of holography are:

• a darkroom is needed to record an object image;
• the object must be stationary;
• laser light illumination is dangerous to the human eye;
• ordinary holograms allow only for a black and white image to be created;
• recording of large objects is impractical because high powered lasers are not yet available;
• speckle noise and modulation noise are present;
• the reconstruction process is very inefficient;
• high-resolution photographic plate or film is very expensive; and
• a large amount of redundant information results from the recording process.

In short, holography is not yet an economical or easy process. Specialized recording and reconstructive devices are needed and an extremely well controlled environment is necessary. Techniques such as white light recording and reconstruction, colour holography, pulse laser beams, and reduced information holography are promising possible solutions to holographic imaging problems.

Holography has been applied to remotely sensed data (Benton et al., 1985). This group of researchers generated white-light viewable holographic stereograms using a digitally and optically processed Landsat MSS stereo pair. This resulted in a black and white 3-D image of the Earth's surface.