A hologram is an image that appears to be three dimensional and which can be seen with the naked eye. Holography is the science and practice of making holograms. Typically, a hologram is a photographic recording of a light field, rather than an image formed by a lens. The holographic medium, i.e., the object produced by a holographic process (which itself may be referred to as a hologram) is usually unintelligible when viewed under diffuse ambient light. It is an encoding of the light field as an interferencepattern of variations in the opacity, density, or surface profile of the photographic medium. When suitably lit, the interference pattern diffracts the light into an accurate reproduction of the original light field, and the objects that were in it exhibit visual depth cues such as parallax and perspective that change realistically with the relative position of the observer. That is, the view of the image from different angles represents the subject viewed from similar angles.

In its pure form, holography requires the use of laser light for illuminating the subject and for viewing the finished hologram. A microscopic level of detail throughout the recorded scene can be reproduced. In common practice, however, major image quality compromises are made to eliminate the need for laser illumination to view the hologram, and in some cases, to make it. Holographic portraiture often resorts to a non-holographic intermediate imaging procedure, to avoid the hazardous high-powered pulsed lasers otherwise needed to optically “freeze” moving subjects as perfectly as the extremely motion-intolerant holographic recording process requires. Holograms can now also be entirely computer-generated to show objects or scenes that never existed.

Holography is distinct from lenticular and other earlier autostereoscopic 3D display technologies, which can produce superficially similar results but are based on conventional lens imaging. Images requiring the aid of special glasses or other intermediate optics, stage illusions such as Pepper’s Ghost and other unusual, baffling, or seemingly magical images are often incorrectly called holograms.