Wavefront reconstruction · object + reference beams interfere · fringe recording & replay
A hologram does not store a picture — it stores a wavefront. During recording, light from the object (the object beam) and a clean tilted reference beam overlap on a photographic plate. Because both come from the same coherent laser they interfere, and the plate records the intensity I = |E_obj + E_ref|² = I_obj + I_ref + 2√(I_obj·I_ref)·cos(Δφ). That final cross term is the magic: it encodes the phase difference Δφ between the two beams as a microscopic fringe pattern, so both amplitude and phase of the object wave are frozen into the plate.
The fringe spacing depends only on the angle between the beams: Λ = λ / (2·sin(θ/2)). A larger reference angle packs the fringes more tightly (higher spatial frequency), which is why holographic film must resolve thousands of line-pairs per millimetre.
During replay the plate is illuminated with the reference beam alone. The fringes act as a diffraction grating and split the light into three orders: the 0th order (undiffracted reference), the +1 order which reconstructs the original diverging object wavefront — seen as a virtual image behind the plate — and the −1 order, the phase-conjugate that converges to a real image in front of the plate. Reconstructing the wavefront, not a flat image, is what gives holograms their full three-dimensional parallax.
The holography concept encompasses the fundamental ideas that distinguish holographic recording from conventional imaging. At its core, holography encodes the complete three-dimensional information of a light field — both amplitude (brightness) and phase (wave timing) — into a two-dimensional interference pattern. This information allows the original wavefront to be reconstructed perfectly, producing images with true depth and parallax.
Conceptually, every small patch of a hologram contains information about the entire scene, just from a slightly different perspective. This redundancy means a hologram can be cut into pieces, and each piece still reconstructs the full image, albeit with reduced resolution and a more restricted viewing angle. The information is distributed holistically across the recording medium, unlike a photograph where each region corresponds to one part of the subject.
The concept of holography extends beyond optics. Acoustic holography reconstructs 3D sound fields. Seismic holography images underground structures. In theoretical physics, the holographic principle (inspired by black hole thermodynamics) proposes that information in a 3D volume can be encoded on its 2D boundary — a deep connection between information theory, gravity, and quantum mechanics.
Because the interference pattern at any point on the hologram is formed by light arriving from all visible points of the object simultaneously. Each region records a superposition of contributions from the entire scene, making the encoding globally distributed rather than locally partitioned.
The holographic principle, stemming from work by Bekenstein, Hawking, and 't Hooft, proposes that the information content of a region of space is bounded by the area of its boundary (in Planck units), not its volume. This suggests reality may be fundamentally lower-dimensional than it appears.
Acoustic holography uses arrays of ultrasonic transducers to create precise 3D pressure fields by controlling the phase and amplitude of each transducer. This can levitate small objects, manipulate particles contactlessly, and create tactile sensations without touching the skin.
A lenticular print uses a lens array to switch between a few pre-rendered viewpoints as you tilt it, giving a limited 3D or animation effect. A hologram reconstructs a continuous wavefront, providing true parallax in all directions within the viewing angle with no discrete viewpoint switching.
Yes. X-ray holography uses coherent X-ray sources (synchrotrons or free-electron lasers) to image nanoscale structures. Electron holography uses coherent electron beams in a transmission electron microscope to map electric and magnetic fields at atomic resolution.