Holography is a method that records a wavefront and later recreates it. It is most commonly used to create three-dimensional images and has other uses, such as storing data, examining tiny objects (microscopy), and measuring tiny changes (interferometry). In theory, it is possible to make a hologram for any type of wave.

A hologram is a record of an interference pattern that can recreate a 3D light field using a process called diffraction. Usually, a hologram is a record of any wavefront in the form of an interference pattern. It can be made by capturing light from a real scene or by using a computer to generate the image, which is called a computer-generated hologram. These holograms can show virtual objects or scenes. Optical holography requires a laser to record the light field. The recreated light field can create an image that appears to have depth and parallax, like the original scene. A hologram is hard to understand when viewed in normal room lighting. When lit properly, the interference pattern bends the light to recreate the original light field, making objects appear with realistic depth and perspective that change as the viewer moves.

Traditionally, a hologram is made by combining a second wavefront, called the reference beam, with the wavefront of interest. This creates an interference pattern, which is recorded on a physical surface. When the pattern is later lit with the same reference beam, it bends the light to recreate the original wavefront. A 3D image from a hologram can often be seen with regular light. However, in practice, using non-laser light often reduces image quality.

A computer-generated hologram is made by digitally creating and combining two wavefronts to form an interference pattern. This pattern can be printed on a mask or film and lit with the right light to recreate the wavefront. Alternatively, the pattern can be shown directly on a dynamic holographic display.

Holographic portraits often use non-holographic methods to avoid the dangerous, high-powered lasers needed to capture moving subjects. Early holography required expensive, high-power lasers. Today, low-cost laser diodes, like those in DVD players, are used to make holograms. These have made holography easier for researchers, artists, and hobbyists with limited budgets.

Most holograms are of still objects, but systems to display moving scenes on dynamic holographic displays are now being developed.

The word "holography" comes from Greek words meaning "whole" and "writing" or "drawing."

History

In 1948, the Hungarian-British physicist Dennis Gabor invented holography while working to improve image resolution in electron microscopes. His research built on earlier studies in X-ray microscopy by scientists such as Mieczysław Wolfke in 1920 and William Lawrence Bragg in 1939. Gabor’s discovery of holography happened by chance during his work at the British Thomson-Houston Company (BTH) in Rugby, England. The company applied for a patent for his invention in December 1947 (patent GB685286). The original technique, called electron holography, is still used today in electron microscopy. Gabor received the Nobel Prize in Physics in 1971 for inventing and developing the holographic method.

Optical holography did not progress significantly until the invention of the laser in 1960. The first practical optical holograms that recorded three-dimensional objects were created in 1962 by Yuri Denisyuk in the Soviet Union and by Emmett Leith and Juris Upatnieks at the University of Michigan in the United States.

Early optical holograms used silver halide photographic emulsions to record images. These materials were not very efficient because the diffraction grating they produced absorbed much of the light. Scientists later developed methods to change the material’s refractive index, a process called "bleaching," which improved the efficiency of holograms.

A major breakthrough in holography came when Stephen Benton invented a method to create holograms that can be viewed using natural light instead of lasers. These holograms are called rainbow holograms.

Basics of holography

Holography is a method used to capture and recreate light fields. A light field is created when light from a source, such as a lamp or sunlight, reflects off objects. Holography works in a way similar to recording sound, where sound waves from sources like musical instruments or voices are stored and can later be played back without the original source. It is even more similar to a type of sound recording called Ambisonic, which allows sound to be heard from any direction when played back.

In laser holography, a laser light source is used. Laser light is special because all its waves are in sync. Different setups can be used, but all involve light coming from multiple directions and creating a tiny pattern of overlapping light and dark areas, which is recorded on a material like film or a plate.

One common setup divides the laser beam into two parts: the object beam and the reference beam. The object beam is spread using a lens and used to light the subject. The recording material is placed where the light, after reflecting off the subject, will hit it. The edges of the material act as a frame through which the subject can be seen, so its position is carefully chosen. The reference beam is also spread and directed to hit the recording material directly, where it overlaps with the light from the subject to create the pattern.

Like regular photography, holography needs the right amount of time for the recording material to capture the image. However, unlike photography, during this time, the light source, the recording material, the subject, and the tools used must not move relative to each other by more than a small fraction of the light’s wavelength. If they move, the pattern becomes unclear, and the hologram is ruined. With living subjects or unstable materials, this is only possible using a very bright and short laser pulse, which is dangerous and rarely done outside of labs. Most holograms are made using a weaker laser that runs continuously for several seconds or minutes.

A hologram can be made by splitting the laser beam so that one part shines directly onto the recording material and the other part hits the subject, with some of the reflected light from the subject also reaching the material. A more flexible setup uses a beam splitter to divide the laser into two identical beams:

• One beam (called the "illumination" or "object beam") is spread using lenses and mirrors to light the subject. Some of the light reflected from the subject then hits the recording material.
• The second beam (called the "reference beam") is also spread using lenses but is directed to hit the recording material directly, without touching the subject.

Many materials can be used as the recording medium. One common type is a film similar to photographic film but with much smaller light-sensitive particles (less than 20 nm in size), allowing it to capture the fine details needed for a hologram. This film is attached to a transparent surface, such as glass or plastic.

When the two laser beams reach the recording material, their light waves overlap and create an interference pattern. This pattern is imprinted on the material. It appears random because it represents how the subject’s light interacted with the original light source, but not the original source itself. The interference pattern acts like a hidden message that needs a specific key—the original light source—to be read.

This key is provided later by shining a laser identical to the one used to record the hologram onto the developed film. When this light hits the hologram, it bends (diffracts) off the pattern, creating a light field identical to the one originally reflected from the subject.

Holography differs from regular photography in several ways:

• A hologram captures light from the subject scattered in many directions, allowing the scene to be viewed from different angles, as if it were still there. A photograph only captures light from one direction.
• A photograph can be taken with normal light sources like sunlight or lamps, but a hologram requires a laser.
• A lens is needed in photography to focus the image, but in holography, light from the subject is scattered directly onto the recording material.
• Holography uses a second light beam (the reference beam) to record the image.
• A photograph can be viewed in many lighting conditions, but a hologram requires specific lighting to be seen.
• If a photograph is cut in half, each piece shows only part of the scene. If a hologram is cut in half, each piece still shows the whole scene. This is because each point in a hologram contains information about light from every part of the subject, like seeing a street through a small window and still recognizing everything, even though less is visible at once.
• A photographic stereogram creates a 3D-like image from one viewpoint, while a hologram recreates the full range of depth cues from the original scene, allowing the brain to perceive a 3D image as if the scene were real.
• A photograph clearly shows the light field of the original scene, but a developed hologram has a complex, random pattern that does not look related to the scene it recorded.

Physics of holography

To understand the process, it is important to know about interference and diffraction. Interference happens when two or more waves overlap. Diffraction occurs when a wave bends around an object. The process of creating a holographic image is explained below using these two ideas. This explanation is simplified but accurate enough to show how holograms work.

For those who are not familiar with these ideas, it is helpful to read about them first before continuing.

A diffraction grating is a structure with a repeating pattern. An example is a metal plate with evenly spaced slits. When light hits a grating, it splits into multiple waves. The direction of these waves depends on the spacing of the slits and the color (wavelength) of the light.

A simple hologram can be made by combining two flat waves from the same light source onto a recording material. These waves overlap, creating a pattern of lines with varying brightness. The spacing of these lines depends on the angle between the waves and the wavelength of the light.

The recorded light pattern acts like a diffraction grating. When one of the original waves is used to shine on this pattern, a new wave is produced that travels in the same direction as the second original wave. This means the second wave is recreated, which is how a hologram works.

If the recording material is exposed to light from a single point and a flat wave, the result is a pattern called a sinusoidal zone plate. This pattern behaves like a special type of lens that focuses light at a distance equal to the distance between the light source and the recording material.

When a flat wave hits this lens-like pattern, it spreads out into a wave that seems to come from the lens's focal point. This means that when the recorded pattern is illuminated with the original flat wave, some of the light spreads out like the original light from the point source. This creates a hologram of the point source.

If the flat wave hits the recording material at an angle instead of straight on, the pattern becomes more complex. However, it still acts like a lens when illuminated at the same angle.

To make a hologram of an object, a laser beam is split into two beams. One beam shines on the object, causing light to scatter onto the recording material. According to diffraction theory, every point on the object acts like a light source, so the recording material is exposed to many light sources at different distances.

The second beam, called the reference beam, shines directly onto the recording material. Each light source from the object overlaps with the reference beam, creating a pattern of lines in the material. These patterns combine to form a random, grainy image called a speckle pattern.

When the hologram is illuminated with the original reference beam, each pattern in the hologram recreates the light that came from the object. These recreated light waves combine to form the complete image of the object. A viewer sees a wave that looks exactly like the light that was originally scattered from the object, making it seem as though the object is still present even if it has been removed.

Applications

Artists recognized the potential of holography as an art form early on and worked in science labs to create their art. Holographic art often results from teamwork between scientists and artists, though some creators consider themselves both artists and scientists.

Salvador Dalí claimed to be the first to use holography artistically. He was the first and most famous surrealist to do so, but earlier exhibitions took place. In 1968, the Cranbrook Academy of Art in Michigan held a holographic art show. In 1970, a show at the Finch College gallery in New York received national media attention. In Great Britain, Margaret Benyon began using holography as an art form in the late 1960s. She had a solo exhibition at the University of Nottingham art gallery in 1969 and another at the Lisson Gallery in London in 1970, which was called the "first London expo of holograms and stereoscopic paintings."

During the 1970s, several art studios and schools were created, each with its own approach to holography. These included the San Francisco School of Holography, founded by Lloyd Cross; the Museum of Holography in New York, started by Rosemary (Posy) H. Jackson; the Royal College of Art in London; and the Lake Forest College Symposiums, organized by Tung Jeong. None of these studios exist today, but the Center for the Holographic Arts in New York and the HOLOcenter in Seoul now provide spaces for artists to create and display holographic work.

In the 1980s, many artists helped spread the use of holography as a new art form. These included Harriet Casdin-Silver from the United States, Dieter Jung from Germany, and Moysés Baumstein from Brazil. Each artist worked to find a unique way to use holography for three-dimensional art, avoiding simple copies of objects. In Brazil, some concrete poets, such as Augusto de Campos, Décio Pignatari, Julio Plaza, and José Wagner Garcia, who worked with Moysés Baumstein, used holography to express ideas and renew their poetry.

A small group of artists still use holographic elements in their work. Some use new techniques, like artist Matt Brand, who used computational mirror design to reduce image distortion in specular holography.

The MIT Museum and Jonathan Ross have large collections of holographic art and online catalogs of holograms.

Holographic data storage is a method that stores information in crystals or photopolymers at a very high density. Storing large amounts of data in a medium is important because many electronic devices use storage. Current methods, like Blu-ray Discs, are limited by the size of the writing beams. Holographic storage could become the next generation of storage because it uses the volume of the medium, not just the surface. Some SLMs can produce about 1,000 images per second at 1024×1024-bit resolution, achieving a writing speed of about one gigabit per second.

In 2005, companies like Optware and Maxell created a 120 mm disc using a holographic layer to store up to 3.9 TB of data, called the Holographic Versatile Disc. However, no commercial product was released by September 2014.

Another company, InPhase Technologies, developed a competing format but went bankrupt in 2011. Its assets were sold to Akonia Holographics, LLC.

Many holographic data storage models use "page-based" storage, where each hologram holds large amounts of data. Recent research into submicrometre-sized "microholograms" has led to new 3D optical storage solutions. While this method does not reach the high data rates of page-based storage, it is less expensive and easier to produce.

In static holography, recording, developing, and reconstructing happen one after another, creating a permanent hologram.

Some holographic materials do not need a developing process and can record a hologram quickly. This allows holography to perform simple operations using only light. Examples include phase-conjugate mirrors, optical cache memories, image processing, and optical computing.

The amount of information processed can be very high (terabits per second) because operations happen across an entire image at once. This balances the longer recording time, which is about a microsecond, compared to the faster processing of electronic computers. However, optical processing with dynamic holograms is less flexible than electronic processing. Operations must be done on the whole image, and the hologram can only perform multiplication or phase conjugation. In optics, addition and Fourier transforms are already easy to perform using linear materials, like a lens. This enables applications such as devices that compare images using light.

Research into new nonlinear optical materials for dynamic holography is ongoing. Common materials include photorefractive crystals, but holograms have also been created in semiconductors, atomic vapors, gases, plasmas, and liquids.

A promising application is optical phase conjugation, which corrects wavefront distortions caused by an aberrating medium. This is useful in free-space optical communications to reduce atmospheric turbulence, which causes stars to twinkle.

Since the beginning of holography, many artists have explored its uses and shared them with the public.

In 1971, Lloyd Cross opened the San Francisco School of Holography and taught amateurs how to make holograms using a small helium-neon laser and simple homemade equipment. Holography was thought to require expensive setups to prevent vibrations that could ruin the image. Cross’s method used a sandbox made of cinder blocks on a plywood base, supported by old tires to isolate it from ground vibrations. The laser was mounted on the cinder blocks, and mirrors and lenses were attached to PVC pipes in the sand. The subject and photographic plate were placed in the sandbox. The holographer turned off the lights, blocked the laser with a shutter, loaded a plate in the dark, waited for everything to settle, and then remotely activated the laser to take the exposure.

In 1979, Jason Sapan opened the Holographic Studios in New York City. Since then, they have been involved in the production of holographic art and technology.

Holography using other types of waves

Holograms can be made using any type of wave.

Electron holography uses holography methods with electron waves instead of light waves. This technique was created by Dennis Gabor to make images clearer and reduce errors in a type of microscope called a transmission electron microscope. Today, it is often used to study electric and magnetic fields in thin materials because these fields can change the wave pattern of electrons passing through the material. The same principle used in electron holography can also be applied to a method called interference lithography.

Acoustic holography creates sound maps of objects. Scientists measure sound waves at many points near an object. These measurements are processed by computers to create visual representations of the object.

Atomic holography developed from advances in atom optics. Using special lenses and mirrors designed for atoms, atomic holography is a natural step in studying and using beams of atoms. New tools like ridged mirrors have made it possible to create atomic holograms, though these are not yet widely used in commercial products.

Neutron beam holography has been used to examine the inside of solid objects.

X-ray holograms are made using large machines called synchrotrons or x-ray free-electron lasers, along with special cameras called CCDs to record the images. The final image is created using computer calculations. Because x-rays have shorter wavelengths than visible light, this method allows for more detailed images. Free-electron lasers can produce very short and powerful x-ray pulses, which help scientists study fast-moving processes that happen in trillionths of a second.

False holograms

Many images that appear to be three-dimensional, float in space, or look like something else are often mistakenly called holograms. These effects can be created using methods like lenticular printing, iridescent foil printing, bubblegrams, the Pepper's ghost illusion (or modern versions like the Musion Eyeliner), tomography, and volumetric displays. These types of illusions are sometimes called "fauxlography."

The Pepper's ghost technique is often used in 3D displays that claim to be holographic. Originally, this method used real objects and people hidden offstage in theaters. Today, digital screens replace physical objects, showing images made with 3D computer graphics to create the illusion of depth. Flat images reflected in this way may not look as realistic as real 3D objects. Another method uses semi-transparent screens to project realistic images from behind, creating the same illusion. Examples of this technique include virtual performances by the Gorillaz (at the 2005 MTV Europe Music Awards and the 48th Grammy Awards), Tupac Shakur at the 2012 Coachella Valley Music and Arts Festival, the Swedish group ABBA, the American band Kiss, and the Vocaloid singing synthesizer Hatsune Miku.

Holography is different from specular holography, a method that creates 3D images by controlling how light reflects or bends on a flat surface. This technique uses light rays directly, not by using interference or diffraction.

In fiction

Holography has been often mentioned in movies, novels, and TV shows, especially in science fiction stories since the late 1970s. These stories often include ideas about holography that were spread by scientists and business people who were very excited about the technology. This led the public to expect too much from holography because many fictional stories showed it as fully three-dimensional computer images that could sometimes be touched using invisible energy. Examples of these portrayals include the hologram of Princess Leia in Star Wars, Arnold Rimmer from Red Dwarf, who was later changed to "hard light" to make him solid, and the Holodeck and Emergency Medical Hologram from Star Trek.

Holography has also influenced many video games with science fiction themes. In these games, fictional holographic technology is often used to show incorrect ideas about how holograms might be used in real life for military purposes. For example, in Command & Conquer: Red Alert 2, there are "mirage tanks" that can look like trees. Players in games like Halo: Reach and Crysis 2 can use holographic decoys to trick enemies.

Although fictional portrayals of holograms have sometimes led to misunderstandings, they have also inspired real-world technological progress in areas like augmented reality. This technology has the potential to achieve the kinds of holographic effects shown in fiction through different methods.