What Is a Hologram? Recording a Wave Instead of a Picture

Light from an object reaches your eyes as a complicated wavefront carrying two pieces of information at every point: amplitude, which is brightness, and phase, which encodes the distance the light has travelled. Depth perception comes from phase, because the difference in path length from different parts of an object is what makes the wavefront curved rather than flat. A camera sensor responds only to intensity and averages away phase completely, which is why a photograph is flat and looks the same from any viewing angle. A hologram gets around this by interference: the light reflected from the object is combined on the recording medium with a second beam from the same laser that has not touched the object, called the reference beam. Where the two waves are in step they add, and where they are out of step they cancel, so the phase information is converted into a pattern of light and dark fringes that a film can record as intensity.

The practical requirements follow directly from the physics and are demanding:

• •A coherent light source, meaning a laser, since the interference requires a wave with a consistent phase relationship across the whole beam and over the depth of the scene
• •A beam splitter to divide the laser into object and reference beams, and mirrors to direct them
• •A recording medium with extremely fine resolution, since the fringes are spaced at around the wavelength of light, requiring thousands of lines per millimetre where photographic film for pictures manages a few hundred
• •Mechanical stability better than a quarter of a wavelength during exposure, which means a heavy isolated table, no air currents and no footsteps, since a movement of a fraction of a micrometre smears the fringes away
• •Reconstruction by illuminating the developed plate with light similar to the reference beam, which diffracts off the fringe pattern and regenerates the original object wavefront
• •White light reflection holograms, developed by Yuri Denisyuk, use fringes recorded through the thickness of the emulsion to select one wavelength from ordinary light, which is why some holograms can be viewed under a lamp

Two behaviours follow from the recording being a diffraction pattern rather than an image. The first is that cutting a hologram in half does not cut the image in half: each piece still reconstructs the whole scene, because light from every point of the object reached every part of the plate, though a smaller piece gives a dimmer image and a narrower range of viewing angles, as though looking through a smaller window. The second is that the recording looks like nothing: an undeveloped or unilluminated holographic plate appears to be a uniform grey smudge, since the information is in a fringe pattern far too fine to see. Holograms are also extremely sensitive to the reconstruction geometry, so illuminating with a different wavelength or angle shifts and distorts the image, which is why the rainbow holograms on credit cards, which trade vertical parallax for viewability in white light, change colour as they tilt.

Almost every popular use of the word describes something else. The stage effects that appear to resurrect deceased performers use a technique from the 1860s called Pepper's ghost, in which a bright image on a hidden screen below the stage is reflected off a large transparent film angled across it, producing a two-dimensional image that appears to stand among the performers and looks correct from one direction only. Projections onto water mist or onto glass panels are equally flat. Head-up displays and augmented reality glasses use waveguides and beam combiners, and although some genuinely incorporate holographic optical elements as components, the image itself is not a hologram. Volumetric displays, which do produce real three-dimensional images, work by scanning a laser to excite points in a medium or by trapping and moving a single particle rapidly enough to trace a shape, which is a different technology entirely. The word has broadened so far in ordinary use that correcting it is close to futile, and the physical distinction remains worth knowing.

The applications are mostly unglamorous and substantial. Security holograms on banknotes, passports and credit cards are difficult to reproduce without the original master and the right equipment, which is the point. Holographic optical elements replace lenses and mirrors in compact instruments, and holographic gratings are standard in spectrometers. Holographic interferometry compares two exposures of an object taken before and after stress and reveals deformations of a fraction of a wavelength as fringe patterns, which is used in non-destructive testing of aircraft components and in art conservation to find voids behind paint. Holographic data storage, which writes pages of bits through the volume of a medium rather than on its surface, has repeatedly promised enormous capacity and has not displaced other storage. Dennis Gabor invented the technique in 1947 while trying to improve electron microscopy, two decades before lasers made it practical, and received the Nobel Prize in Physics in 1971.

A hologram records the phase of a light wave as well as its brightness, by interfering light from the object with a reference beam from the same laser and capturing the resulting fringe pattern, which is spaced at the wavelength of light and requires stability better than a fraction of a micrometre. Illuminating the pattern regenerates the original wavefront, giving genuine depth and parallax. Any piece reconstructs the whole scene, and nearly everything called a hologram in public, including the stage projections of dead musicians, is a flat reflection trick from the 1860s.