The word “holography” aptly means “entire recording” and originates from the Greek words “holos” (“whole”) and “gramma” (“message”). “Entire” refers to the recording of both the intensity and the phase of the object, as opposed to conventional photography, where only the intensity profile of the object is recorded.1 It is the phase (which contains the information of depth or the information about how an object may have changed over time) that ordinary imaging lacks. Holography provides a way to ascertain the phase through comparison with the “known” phase of a reference beam.
Holography is capable of forming what may be termed a “3D” image from a 2D recording because it contains the optical phase information needed to stimulate the human visual system’s depth cues. Other techniques, such as stereoscopic, autostereoscopic, and volumetric imaging, are deficient in some depth cues or may even produce conflicting cues.2 For example, in a stereoscopic display system, convergence (i.e., the angular convergence between each eye’s line of sight when directed at the same point) and accommodation (focusing) depth cues may conflict, which may result in the viewer experiencing discomfort, especially after long-term use.2 However, holography does not actually record the complete 3D information content of an object; only the “3D” information that can be recorded in a given field of view (i.e., the holographic recording would not contain information about the back of an object if recorded only from the front). Complete 3D information can be obtained with tomographic techniques, including holographic tomography.
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