How Holography Works: Recording Light as Wavefronts

Why coherent light is essential

A photograph records |E|² — the time-averaged intensity of the electromagnetic field — and loses all phase information. But phase carries the depth information: two points at different distances scatter waves that travel different path lengths and accumulate different phases. Recording phase requires interference, which in turn requires coherent light that maintains a constant phase relationship over the exposure. Ordinary light sources have a coherence length of micrometres; laser light maintains coherence over metres or kilometres. Dennis Gabor demonstrated holography in 1948 using a filtered mercury arc lamp before lasers existed, but contrast was poor until Leith and Upatnieks adapted the technique to lasers in 1960-62.

Recording: splitting a beam into object and reference

A single laser beam is split into a reference beam (travels directly to the film) and an object beam (illuminates the subject). The object beam scatters off the subject in all directions, with every point becoming a secondary wave source carrying its own phase shift. Object and reference waves meet at the holographic film, creating a submicrometre-scale interference pattern that a silver halide emulsion (100-1000 nm grains) records as varying density after development. The plate must remain still to within a fraction of the laser wavelength (~300 nm) during exposure — typical exposure times are 1-30 seconds on a vibration-isolated optical table.

The interference pattern and reconstruction

With reference beam R and object wave O at the film, the recorded intensity is I = |R+O|² = |R|² + |O|² + R*O + RO*. The last two terms encode the holographic information — R*O is a diffraction grating carrying both amplitude and phase of the object wave, distributed across the entire plate. To reconstruct, illuminate the developed hologram with the same reference beam R; the transmitted wave R·I contains a term |R|²·O that is exactly the original object wave, scaled. It radiates from the plate as if the object were still there — a perfect 3D virtual image with full parallax, so moving your head reveals hidden surfaces just as with the real object.

Recorded intensity:   I = |R|² + |O|² + R*O + RO*
Reconstruction:        R·I = R(|R|²+|O|²) + |R|²·O + R²·O*
                                              ↑ this term reconstructs
                                                the object wave exactly

Gabor's discovery and the off-axis breakthrough

Dennis Gabor invented holography in 1948 while improving electron-microscope resolution, and won the 1971 Nobel Prize in Physics. His original single-beam "in-line" setup produced overlapping real and virtual images. Emmett Leith and Juris Upatnieks (1962) introduced the off-axis reference beam — splitting the beam at an angle so the real image, virtual image and zero-order term separate spatially, the configuration used in all modern display holograms. Yuri Denisyuk (1962, USSR) independently developed reflection holograms viewable in white light, the basis of the embossed security holograms found on credit cards and banknotes.

From security strips to holographic data storage

Embossed holograms — a master hologram pressed onto metallised plastic film — are cheap to mass-produce and hard to forge, appearing on credit cards, passports and banknotes. Holographic storage can theoretically reach 1 TB/cm³ by superimposing multiple holograms in the same volume via angular multiplexing; InPhase Technologies reached 515 Gbit/in² before the company folded in 2010. Polarisation-sensitive optical coherence tomography uses tissue birefringence to distinguish tissue types non-invasively, and Microsoft HoloLens uses diffractive waveguide combiners related to computer-generated holography to overlay images on the real world.

Frequently asked questions

Try it live

Everything above runs in your browser — open Holography and record a hologram as the interference of an object wave and a reference beam, then reconstruct the image by dragging object points, changing wavelength and reference angle. Nothing is installed, nothing is uploaded.