A hologram is the recorded interference pattern between a point sourced of light of fixed wavelength (reference beam) and a wavefield scattered from the object (object beam). A hologram is recorded in a two- or three-dimensional medium and contains information about the entire three-dimensional wavefield of the recorded object. When the hologram is illuminated by the reference beam, the diffraction pattern recreates the lightfield of the original object. The viewer is then able to see an image that is indistinguishable from the recorded object  .
The holographic plate is a kind of recording medium, in which the 3D virtual image of an object is stored. While in a recording media like a CD, the grooves contain information about sound that can be used to reconstruct a song, a holographic plate contains information about light that is used to reconstruct an object .
The information about light is coded in the form of bright and dark microinterferences. Usually, these are not visible to the human eye due to the high spatial frequencies. Reconstructing the object wave by illuminating the hologram with the reference wave creates a 3D image that exhibits the effects of perspective and depth of focus .
This photographic technique of recording light scattered from an object and presenting it as a 3D image is called Holography. The object representations created with this technique are the most lifelike 3D renditions because it uses the same technique as our eyes to see the world around us  . Therefore, it is an attractive imaging technique since it allows the viewer to see a complete three-dimensional volume of one image .
Throughout the years, several types of holograms have been created. These include transmission holograms, that allow light to be shined through them and the image to be viewed from the side, and rainbow holograms. These are common in credit cards and driver’s licenses (used for security reasons) .
While various holograms have been used in movies like Star Wars and Iron Man, the real world technology has not achieved the same level as presented in those cinematic stories . Currently, holograms are still static, but they can look incredible such as in the case of large-scale holograms that are illuminated with lasers or displayed in a darkened room with carefully directed lighting. Some holograms can even appear to move as the viewer walks past them, looking at them from different angles. Others can change colors or include views of different objects, depending on how the viewer looks at them  .
One of the interesting traits of a hologram is that cutting one in half, each half will contain the pattern to recreate the original object. Even if a small piece is cut out, it will still contain the entire holographic image. Another feature is that making a hologram of a magnifying glass will create a hologram that will magnify the other objects in the hologram .
How does it work?
To create a hologram, holography uses the wave nature of light. In a normal photograph, lenses are used to focus an image on film or an electronic chip, recording where there is light or not. With the holographic technique, the shape a light wave takes after it bounces off an object is recorded. It uses interfering waves of light to capture images that can be 3D. When waves of light meet they interfere with each other, analogous to what happens with waves of water. The pattern created by the interference of waves contains the information used to make the holograms .
True 3D holograms could not be a practical reality without the invention of the laser. A laser creates waves of light that are coherent. It is this coherent light that makes it possible to record the light wave interference patterns of holography . While white light contains all of the different frequencies of light traveling in all directions, laser light produces light that has only one wavelength and one color (Figure 1) .
In it basic form, three elements are necessary to create a hologram: an object or person, a laser beam, and a recording medium. A clear environment is recommended to enable the light beams to intersect .
The laser beam is separated into two beams and redirected using mirrors (Figure 2). One of the beams is directed at the object, while the other - the reference beam - is directed onto the recording medium. Some of the light of the object beam is reflected off the object onto the recording medium. The beams intersect and interfere with each other, creating an interference pattern that is imprinted on the recording medium. This medium can be composed of various materials. A common recording medium is a photographic film with an added amount of light reactive grains, enabling a higher resolution for the two beams, and making the image more realistic than using silver halide material .
A developed film from a regular camera shows the negative view of the original scene, with light and dark areas. Looking at it, it is still possible to more or less understand what the original scene looked like. However, when looking at a revealed holographic tape, there is nothing that resembles the original scene. There can be dark frames of film or a random pattern of lines and swirls, and only with the right illumination is the captured object properly shown .
Using a transmission hologram made with silver halide emulsion as an example, there needs to be the right light source to recreate the original object beam. This beam is recreated due to the diffraction grating and reflective surfaces inside the hologram that were caused by the interference of the two light sources. The recreated beam is identical to the original object beam before it was combined with the reference wave. Furthermore, it also travels in the same direction as the original beam. This means that since the object was on the other side of the holographic plate, the beam travels towards the viewer. The eyes focus the light, and the brain interprets it as a 3D image located behind the recording medium (Figure 3) .
1886 - Gabriel Lippmann, in France, develops a theory of using light wave interference to capture color in photography. He presented his theory in 1891 to the Academy of Sciences, along with some primitive examples of his interference color photographs. In 1983, he presented perfect color photographs to the Academy and won a Nobel Prize in Physics in 1908 due to his work in this area. 1947 - Dennis Gabor develops the theory of holography. He coined the term hologram from the Greek words holos (meaning ‘whole’) and gramma (‘message’).
1960 - N. Bassov, A. Prokhorov, and Charles Towns contributed to the development of the laser. Its pure, intense light was optimal for creating holograms.
1962 - Yuri Denisyuk publishes his work in recording 3D images, inspired by the Lippmann’s description of interference photography. He began his experiments in 1958 using a highly filtered mercury discharge tube as his light source.
1968 - Dr. Stephen A. Benton invents the white-light transmission holography while researching holographic television. The white-light hologram can be viewed in ordinary white light.
Main types of holograms
White-light transmission holograms - This type of holograms are illuminated with incandescent light, producing images that contain the rainbow spectrum of colors. Depending on the point of view of the viewer, the hologram’s colors change. They are also called rainbow holograms.
Reflection holograms - Reflection holograms are usually mass-produced using a stamping method. They can be seen in credit cards or in a driver’s license. Normally, these holograms can be viewed in white light.
Transmission holograms - Typically, a transmission hologram is viewed with laser light. The light is directed from behind the hologram and the image projected to the viewer’s side.
Hybrid hologram - These are holograms that are between the reflection and transmission types of holograms. Examples include embossed holograms, integral holograms, holographic interferometry, multichannel holograms, and computer-generated holograms.   
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