Holography

Beyond ordinary photography

An ordinary photograph records light intensity from a scene. It does not directly record the phase of the light wave. Holography records information about both amplitude and phase by using interference between an object beam and a reference beam.

This allows a hologram to reconstruct a three-dimensional wavefront, giving depth and parallax.

Recording a hologram

A coherent laser beam is split into two parts. One part illuminates the object and scatters toward the recording medium. This is the object beam. The other part travels directly to the recording medium as the reference beam.

The two beams interfere, producing a pattern of bright and dark fringes. The recorded intensity is related to

$Ipropto |E_o+E_r|^2$

where $E_o$ is the object field and $E_r$ is the reference field.

Expanding this expression gives

$Ipropto |E_o|^2+|E_r|^2+E_oE_r^*+E_o^*E_r$

The cross terms contain phase information about the object wave relative to the reference wave.

Reconstruction

To view the hologram, light similar to the original reference beam illuminates the recorded pattern. The hologram diffracts the light, reconstructing a wavefront similar to the original object beam.

Your eyes receive light as if it came from the original object, producing a three-dimensional impression.

Coherence requirements

Holography requires coherent light because stable interference fringes must be recorded. The path difference between object and reference beams must remain within the coherence length of the source. Mechanical stability is also crucial because vibrations can blur the interference pattern.

This is why lasers are commonly used for holography.

Volume and surface holograms

Some holograms are thin surface patterns; others are volume holograms with interference structure throughout a material thickness. Volume holograms can be highly wavelength-selective and angle-selective.

Security holograms on cards and packaging often use embossed surface structures that diffract light.

Holography and diffraction

A hologram works like a complex diffraction grating. Its recorded fringe pattern redirects light to recreate the original wavefront. The details are more complicated than a simple grating because the fringe spacing and orientation vary across the hologram.

Holography is therefore an application of interference during recording and diffraction during reconstruction.

Applications

Holography is used in security labels, art, microscopy, data storage, interferometry, optical testing, augmented reality displays, and measurement of small deformations. Holographic interferometry can compare wavefronts before and after an object changes shape, revealing tiny displacements.

Misconceptions

A hologram is not simply a 3D photograph. It is a recorded interference structure that reconstructs a light wavefront. Also, a small piece of a hologram can often reconstruct the whole object from a limited perspective because each region receives light from many parts of the object.

The big idea

Holography records phase information by interfering object and reference beams. When illuminated properly, the recorded pattern diffracts light to reconstruct the original wavefront. Holography depends on coherence, interference, and diffraction, and it enables three-dimensional imaging and precision measurement.