In traditional holography an interference pattern is created by splitting a coherent beam of light projecting one part onto an object and the other part, called a reference beam, on to a sensitive film in a location such that it will absorb at least some of the light scattered from the object. Later the film is developed and when lighted with a similar reference beam it reconstructs the object in free space as a hologram. The set-up to create this hologram is quite complex, and usually requires several attempts and after film development it will support the reconstruction of a static image. With digital holography the interference pattern can be computed numerically by simulating the light propagation toward an object’s geometry including how the light will be scattered from the object. Then the interference pattern can be applied on a film, creating a static hologram, or on a “digital film” such as an addressable Liquid Crystal (LC) or Digital Micro-mirror Device (DMD) and then the hologram can be digitally updated thus dynamic. The addressable element is in fact a Spatial Light Modulator (SLM) and is used to digitally modify the phase and/or amplitude of an incident light, which diffract accordingly. This diffraction is designed by the above mentioned numerical method to match the diffraction of the same light as if it diffused/scattered from or otherwise created by a real object and appears as an image of that real object to a viewer. For this reason, holographic 3D images posses all the depth cues and perceive to the human eye as real as possible. This is a unique capability for holography and distinguishes it from other 3D technologies.
The magic of holography – creating a point of light in free air or a cloud of points using an Spatial Light Modulator (SLM) that digitally shapes light to converge and focus in specific locations in space.