What is Holography?

What is holography? What are the different types of holography?

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- GoPhotonics

Dec 5, 2022

Holography is a technique used to generate a 3D image of an object. These 3D images are called holograms. This technique was invented by Dennis Gabor in 1947. A hologram is created using three different types of waves such as object wave, reference wave, reconstruction wave, and a photographic plate. Three of the waves mentioned are coming from the same coherent light source.

  • Object wave: Lightwave from a coherent light source that illuminates the object.
  • Reference wave: Lightwave which falls on the photographic plate from the same light source during the recording of the hologram. 
  • Reconstruction wave: Lightwave from the same coherent light source falling on the photographic plate used for the reconstruction of the image. 
  • Photographic plate: A flat metal or glass sheet on which the photographic image is recorded.

To record a hologram, the object wave interferes with the reference wave (plane wave) at the photographic plate. This interference pattern formed on the photographic plate contains information about both the amplitude and phase of the object wave. To see the image, the photographic plate is illuminated with the reconstruction wave and this process is called reconstruction. Red lasers or He-Ne lasers are commonly used to record a hologram. The laser acts as a coherent light source which provides a stable and good holographic image.

There are two types of holography: Analog holography and Digital holography

Analog Holography

The reconstruction of an image physically is called analog holography. In analog holography, the hologram is recorded on a piece of film or a photographic plate. The methods of recording and reconstructing analog holograms are explained below.

In-line and Off-axis holography:

In the initial phases, Gabor's technique involved causing a coherent light beam to be dispersed from an object and then allowing it to overlap another unobstructed coherent beam as shown in figure 1. A photographic plate put in front of the object would produce interference fringes as a result of the two sets of waves combining. This interference fringe pattern is called Gabor zone plate. 

Figure 1: Gabor’s in-line holography

In this process, planar coherent waves illuminate the plate. The hologram's points will all result in diffracted light. It creates a virtual image that may be viewed from the right of the hologram. This holographic technique is called in-line holography. But this method had some limitations. The formation of twin images (real and virtual images) caused difficulties. This problem was solved by Leith and Upatneiks through the development of off-axis hologram in 1962. In off-axis holography, an angle was introduced between the reference and the scattered beam. This separated the real from the virtual image. Figure 2 depicts the recording and reconstruction process in off-axis holography.

Figure 2: Recording and Reconstruction of hologram in off-axis holography

There are two phases in the working of holography: Recording and Reconstruction.

Figure 3: Recording of hologram

Recording of hologram:

Figure 3 depicts the process of recording a hologram. The monochromatic coherent laser beam is divided into two parts by a beam splitter. One of the laser beams from the beam splitter is sent to the photographic plate by reflection from the mirror and this beam is called the reference beam. The other beam hits the object by reflection from another mirror. This beam is called the object beam. The light beam scattered from the object also reaches the photographic plate. This reflected light carries information about the object. Both the reference beam and object beam gets superimposed on the photographic plate forming an interference pattern. This interference pattern contains a multimode of fringes generated by constructive and destructive interference of the two light beams. This pattern also contains information about the object.

Reconstruction of image:

The reference beam is used to reilluminate the hologram that is developed. When the hologram is illuminated with this beam, two images of the object are produced by light diffracted by the hologram. The observer will see an undistorted 3D view of the object and this is a virtual image as it requires a lens which is the observer’s eye to form. The second image formed is a real image formed by light diffracted in a different direction from the virtual image. This is projected directly onto a screen and does not require a lens to form it. Some of the light beams are lost by transmitting through the hologram in the same direction as the reference or reconstruction beam without any diffraction. Figure 4 depicts the image reconstruction in holography.

Figure 4: Reconstruction of image

Types of holograms

Reflection, Transmission, Rainbow, and Hybrid holograms:

In a reflection hologram, the object and reference beams are incidents on the holographic plate from opposite sides of the plate. The object reconstructed is viewed from the side where the reconstructing beam is incident. Since they produce high-quality images, they are very expensive. These types of holograms are mostly found in galleries. They are created with laser light but their structure allows them to be visible only using white light. Directional sources like direct sunlight, spotlights, flashlights, etc are used to view as they are the best light sources that give bright and clear images. In reflection holograms, the light source illuminates the hologram from the front side and the reconstruction of the image "reflection" back of the film's surface. 

If the object beam and the reference beam are incident on the holographic plate from the same side, it is called a transmission hologram. Reconstruction of this type of hologram is possible only using a laser beam or a quasi-monochromatic source. They are laser-viewable holograms. The same laser light that was used to create the original recording can be used to view transmission holograms. During reconstruction, laser light comes from the back of the hologram, passes through it and the image is viewed from the front side. The orientation of the interference fringes obtained on the hologram is perpendicular to the hologram surface. These fringes are good at reconstructing diffracted images but bad at filtering different wavelengths, so reconstructing these holograms with white light causes chromatic blur. In the case of reflection holograms, the orientation of the recorded fringes is parallel to the film's surface. Hence, the fringes look like multiple layers on the film which can filter out different wavelengths, and only the colors required for the image are replayed which allows the non-diffracted colors to simply pass through the film undisturbed. Thus, a white light source is used for the reconstruction of those holograms. If produced massively, transmission holograms are less expensive. A transmission hologram produces a sharp and deep virtual image. Transmission holograms are having higher field depth than reflection holograms. If a hologram of this type is divided into pieces, each piece will still be able to recreate the entire hologram. 

If the object is illuminated by a white light rather than a laser, the generated hologram is called a rainbow hologram. The rainbow hologram is a type of transmission hologram. Dr. Stephen Benton created this type of hologram; hence they are also called Benton holograms. When white light is used to illuminate the transmission holograms, since they have a broad spectrum, they diffract all the wavelengths into the image. Red wavelength is diffracted more than blue wavelength since they are longer and hence the color spreads and a rainbow image is obtained. The image might not appear to change even when it has if viewed from above or below. These types of holograms are commonly used for security purposes. Figure 5 shows the creation of fringes in both transmission and reflection holograms.

Figure 5: Top view of fringe creation in transmission and reflection holograms

Hybrid versions of the combination of transmission and reflection holograms are called hybrid holograms. Examples of Hybrid holograms are Embossed holograms, Integral holograms, Holographic interferometry, Multichannel holograms, Computer-generated holograms, etc. Embossed holograms are produced massively and are used for authentication purposes. They produce transparent type of images and are very complex and difficult to recreate. Holograms seen on credit cards, currencies, passports, and other security applications are embossed holograms. These holograms recorded contain grooves on them, which are then filled with nickel and peeled off later causing a metallic shim on them. This shim is placed on a roller under an atmosphere of high temperature and pressure that makes the shim emboss to the roll. Integral holograms are produced from a series of photographs. Different views of an object are recorded by a camera and are displayed on an LCD which is illuminated with a laser beam. This light beam act as an object beam for recording the hologram on a narrow vertical strip. After all the views are recorded in a similar fashion, the final hologram obtained is viewed which gives a stereoscopic image. In holographic interferometry, the changes that occur to an object are identified. The difference in viewing an image is detected in multichannel holograms. By changing the angle of viewing light, different images are observed. These holograms are used in computer memory applications. Computer-generated holograms are holograms that are analyzed and synthesized using computer software. These holograms can be used to create holographic optical elements. 

Amplitude and phase holograms: 

Holograms carry both amplitude and phase information. Depending on the diffraction efficiency, holograms are categorized into phase and amplitude holograms. The efficiency of a holographic grating is termed diffraction efficiency. The holograms with lower diffraction efficiency are called amplitude holograms and holograms with higher diffraction efficiency are called phase holograms. If the amplitude of light diffracted by the hologram is proportional to the intensity of the recorded light, this is an amplitude hologram. Like how exposure intensity varies spatially, amplitude transmittance also varies. Amplitude transmittance is the percentage of the input amplitude of the light beam that retains after passing through the grating. If the thickness or the refractive index of the material is varied in proportion to the intensity of the holographic interference pattern, a phase hologram is generated. In phase holograms, the magnitude of amplitude transmittance equals one but the phase varies. An amplitude hologram can be converted into a phase hologram by bleaching. Since phase holograms are more efficient than amplitude holograms, it is more preferred in experiments.

Thick and Thin Holograms:

Figure 6: a) Thin hologram and b) Thick hologram

If the thickness of the recording medium is greater than the spacing between interference fringes, the hologram generated is called a thick hologram. In thick holograms, the angle between the object and the reference wave will be greater than 90º. Thick holograms can be viewed with a white light source. No pseudoscopic image is observed from a thick hologram, instead, a very sharp image is reconstructed, and the reconstruction is based on Bragg diffraction. Thick holograms are also called volume holograms or Bragg holograms. A thick hologram can store a large number of images. If the thickness of the recording medium is lesser than the spacing between interference fringes, it is a thin hologram. Since there is no Bragg diffraction, the reproduction of the true color is not possible in thin holograms. The angle between the object wave and the reference wave will be lesser than 90º. Thin holograms are having dispersion. So, when white light is used to illuminate them, a mixture of colors of the spectrum is seen. Similarly, when it is illuminated with a mercury lamp, separate images are seen at different colors of the mercury lamp. Figure 6 depicts the image of thin and thick holograms.

Digital Holography

Due to the difficulties of laser illumination, optical hologram recording is only possible for medium-sized objects and is not possible for distant objects. One option for this might be computer-generated holograms (CGH). Therefore, holograms can also be recorded digitally which is called digital holography. In digital holography, photosensitive materials were replaced with electronic devices such as a charge-coupled device (CCD). The optical signal inside a CCD gets digitized into a two-dimensional digital signal and then processed using digital image processing to reconstruct a hologram. Computers are used to analyze and synthesize coherent optical waves. Computer-generated holograms are synthesized here. An object, a source, and a reference wave are defined mathematically at first. Then the computer calculates the amplitude and phase of the wavefront scattered by the object and also calculates the resulting intensity pattern on the holographic plate due to interference of the object and reference beam. Thus, we could generate a hologram of an imagined object. 

Types of digital holography

There are two types of digital holography: Fresnel holography and Fourier holography. Holograms generated by numerical simulation of diffracted light using Fresnel diffraction theory are called Fresnel holograms and using Fourier transform (FT) are Fourier holograms. With the help of a reference point source, Fourier Transform Holography (FTH) records object information as the Fourier spectrum in an interference pattern. So, from a digitally reconstructed FTH, a Fourier transform operation can give information about an entire object.

The object wave on the hologram plane can be described by either Fresnel or the Fourier diffraction of the object. If the generated hologram plane is described with Fresnel diffraction, then we have a Fresnel hologram. Fresnel diffraction happens when the distance between the light source and object or object and image plane is finite or comparable to the size of the object which causes a diffractive behavior that is unique. When the recording photographic plate is in the near field, a fresnel hologram is formed. The fresnel hologram system projects spherical rings called Fresnel zone plates onto the plane of the image. The Fresnel holography system is shown in figure 7. The real and virtual images of the Fresnel hologram are on both sides as shown in the figure below.

Figure 7: Fresnel holography system

If the object wave on the hologram plane requires the Fourier transform of the object, the Fourier hologram is obtained. The Fourier transform is a tool used to process images by separating the image into its sine and cosine components. The input image will be in the spatial domain and the output image obtained is in the frequency domain. The setup shown in Figure 8 suggests that the object is set at the front focal plane of lens 2, resulting in the Fourier transform of the optical field at the back focal plane of lens 2. Also, a focused light spot beside the object through lens 1 acts as a reference light, making a tilted plane wave on the holographic film.

Figure 8: Recording and Reconstruction geometry of Fourier hologram

The reconstruction geometry shown in Figure 8 suggests that the hologram placed at the front focal plane of a lens is illuminated with a plane wave with normal incidence and unit amplitude.

Other types of holograms

Holograms can be classified based on their optical geometry and the recording medium. Some of those holograms are Denisyuk Reflection Hologram, Pseudo-color Reflection Hologram, Laser viewable transmission holograms, Pulse laser holography, Stenciling and Multiplex holography, and Dot-matrix holograms

If more than one coherent source is available, the reflection holograms can be viewed in color and this type of hologram is called denisyuk hologram. Usually, red, green, and blue laser colors are combined to get a true color reflection hologram. In this hologram, the laser beam passes through the photographic film and then reflected from the object. In pseudo-color reflection holograms, by controlling the emulsion thickness, the final image color in the reflection hologram can be controlled. A single white light source can be used to reconstruct all color components. The red line from a helium-neon laser is mainly used to create pseudo-color reflection holograms, which are then reconstructed using white light. In laser-viewable transmission holograms, the image is recorded behind the film and when illuminated with a laser source, a very deep and sharp image is obtained. Perfect reconstruction of the optical field is possible using laser-viewable transmission holograms. 

Holograms of live subjects can be produced by pausing their motion using pulse laser holography. This is done with the help of a pulsed laser producing an ultra-short pulse and they require less exposure time compared to that of a continuous laser. Recording multiple holograms on a single holographic surface is called multiplexing. A short-animated image can be recorded using a photographic approach. This is done by a process of hand animation with image slides and stencils over the hologram. This type of holography is termed stenciling and multiplex holography. Dot-matrix holograms are made of tiny dots or diffractive pixels. An object with a specific geometry will diffract light at certain angles and is recorded with a particular geometry. The light illuminated on these holograms will give a varied color spectrum and the images obtained are very bright. But these holograms do not contain 3d depth information.


  • Holographic data storage: A large amount of data can be stored in a small space. By changing the angle at which the reference wave is incident, different object waves can be recorded on the same hologram.
  • Holography in the medical field: The 3D image of internal organs and body parts is visible using the holographic technique. This helps the doctors to conduct an accurate diagnosis.
  • Holographic Interferometry: Holography is used to trace changes in objects. The deformations in a body due to stress conditions or temperature changes etc can be precisely analyzed using this technique. The reconstructed wave from the hologram of an object will create a true copy of the object at that instant when the hologram was recorded. So, if we compare this wave with another wave reconstructed at a different time, any change in the object that occurred during this time gap can be found.

Some of the other applications are:

  • Holographic microscopy
  • Acoustic holography
  • Security applications
  • Hologram as an optical element
  • Holographic memories
  • Quantum holography
  • Holographic scanners
  • Holographic displays