Digital Holograms

Holograms are physical structures used for diffracting light into images. The term is also applied when referring to the encoded material and to the image that results from the encoding. Holographic images are normally seen when one looks at a holographic print that is illuminated or through the process of directing light into a laser past a hologram and then projecting the image on a screen. The computerized generation of holograms began in 1966 but was limited by the issues of computer technology. Earlier models of holograms were recorded on other photographic media. The digital method of their recording provided new options for holographic image production. Digital approach is advantageous in the sense that it offers real-time response and convenience while processing and storaging holographic images. There has been a great progress achieved in the area of the real-time holographic production such as videos and cameras.

Dennis Gabor, a Hungarian physicist, developed the holographic imaging method. He was awarded the Nobel Prize in 1971 for this invention.  Gabor’s work was based on the x-ray microscopy provided by the scientists, such as Mieczyslaw Wolfke and WL Bragg, in 1920 and 1939 respectively. He discovered holograms when he was conducting the research that would help in improving the electron microscopes in the British Thomson- Houston (BTH) company in England. In Dec, 1947. The technique was known as electron holography until the development of the laser technology that holography advanced.  Due to the introduction of the laser, it became practical to develop the first holograms in 1962. They were made by Yuri Denisyuk, Emmett Leith and Juris Upatnieks in the Soviet Union and the University of Michigan in the United States of America. They could record three-dimensional objects.  The transmission process of the holograms was developed after several scientific experiments that were based on the understanding that electromagnetic radiations have some wave nature. Max Von Laue used copper sulphate to demonstrate the diffraction of the x-rays. In 1913, William Henry Bragg and William Lawrence Bragg formulated the results, which were further consolidated into a rule called the Bragg’s law of diffraction.

 2dsin? = n? 

The formula attempts to equate (d) which is the fringe spacing to the deflection angle (?) for a given wavelength (?)5. Through the use of the relationship, it is possible to make diffraction gratings that would help to control the deflected angle of light separating the various wavelengths of light. The early holograms used only silver halide emulsions as the medium for recording.  The lack of laser illuminators limited the efficiency of the early holograms and could produce less clear images because they absorbed most of the incident light. There have been various methods developed to convert transmission variations and promote the production of holograms using v refractive indexes. Digital holograms are recorded and later kept in a computer to be processed. The processed hologram is then displayed on a light modulator upon which a reference wave is directed to reconstruct the image of the original object. In order to ensure good quality of a holographic image, it requires a digital processing that considers the illuminating wavelength as well as the physical attributes of the device used for display. In order to achieve these results, it is necessary for the technicians to transmit holograms in a format that uses standard interchange while finalizing the process on a display device to take into account its unique features. 

Types of Holograms

The computer-generated holograms, also called digital holograms, use the fringe pattern of every pixel, which is determined and recorded in it. According to Yaroslavsky, there are several ways of producing digital holograms. Some regions of films called voxels are exposed to previously calculated fringe patterns such as the use of electron beam or a spatial light modulator (SLM). In the past, the technology used in holography would produce noticeable systems and distort the hologram surface. There are several types of holograms, such as transmission ones, developed by Upatnieks and Leith. These are viewed by shining the laser light through the holograms and observing the reconstructed image from the opposite side of the source.

There is another type of holograms called the rainbow transmission. It applies a more convenient illumination that uses a white light as opposed to lasers. The holograms are useful for authenticating documents for security purposes, such as in product packaging and on credit cards. The laser viewable transmission holograms ensure the perfect reconstruction of the optic fields. The advantage of this aspect is that the recorded scene appears in the object film. 

When laser replaces the scene, it can be very deep and sharp. Moreover, it is possible to use the holograms for master recording that can be further transferred into a transmission or reflection of the holographic print. Paula Dawson described these laser viewable holograms as concrete holograms since, according to the observations, they create a scenario of physical presence. However, there is another type of holograms known as the Denisyuk or the reflection hologram. It can be viewed using the white lit illumination source when the viewer is standing at the same side as the hologram. Reflection holograms are the ones seen in most holographic displays. These types have the capacity of producing multicolor images.

Hologram Development

Two geometries include transmission and reflection. The light in transmission geometry is shown through a hologram. In reflection technology, the hologram is the one that reflects the light. The development of the two methodologies started from different fields and has different optical aesthetics. The reflection hologram that was developed by Yuri used one beam to illuminate both the reference and the object. The process follows the spatial and color photographic recording procedures and practices of the Lippmann photography and the Daguerreotypes created on the metal surfaces that are polished to enhance quality. 

In both Daguerreotype, Lippmann and reflection hologram, the image color is selective and is only formed by wavelengths resonating as the fringes move relative to the spaces between them. With multicolor lasers, it is possible to record a hologram that is indistinguishable with the original object. It is important for one to check the authenticity of the image by ensuring that there is an object behind the plate. The alternative technology of producing three-dimensional images is the use of specular holography. The technology functions by controlling the movement of specularities on a two-dimensional surface. The methodology applied in this case is manipulation of light ray bundles in a reflective or refractive manner. However, the Gabor holography uses the diffraction of reconstruction wave fronts. Most of the holograms are of static objects, but the advancement of technology has led to the introduction of systems that can display changing images on a volumetric holographic display.

 producing a hologram

Figure 1:  The stages of producing a hologram 

Holograms can be used to store, process and retrieve information optically. In the past, the production of holographic images required the use of high-power lasers that were making the production more expensive than the ones used today.  According to Picart & Jun-chang (2013), the advancement in technology has led to the production of low-cost diodes or semiconductors lasers that have reduced the cost of producing holographic images. These diode lasers are used in appliances such as the DVD recorders, etc. Consequently, holograms are more accessible to the researchers who operate on low budgets. Artists and hobbyists who use this technology have also benefited. Radiation sources and detectors such as CCDS generate holograms used in x-rays. These are produced using X-ray lasers that are electron-free or synchrotrons. The shorter wavelength of these X-rays allows the production of images objects that have the high spatial resolution. Use of holography has been important for capturing ultrafast processes.

Holograms work on interference. It captures the pattern of interference between two or even more beams of light known as laser light. One of the beams is shown on the acting medium and is used as a reference to the light that is scattered from the light scene. Holography enables a field of light that is generated from a light source to be recorded and reconstructed after the original light has been eliminated due to the absence of the objects that were originally placed there. According to Kim, the technology is similar while recording sounds where their fields are created by the use of vibrating objects such as vocal cords or musical instruments. Their sound is encoded in a manner that allows its reproduction later, when the originally produced matter will no longer be present. While making digital holograms, the physicists intended  to display images in three dimensions. The holographic records include the random structure of different intensities, densities, and surface profile. After the hologram is placed in a suitable manner, the source of light is recreated. Making the objects change the orientation and the position of the viewer, making the objects appear as if they were still in their original position.  

In order to provide perfect performance of the holograms, it is required to use the laser light that illuminates the subject and displays the completed hologram. This method relies on the phenomenon of diffraction and interference. Under ideal conditions, a side-by-side view of a holographic image portrays a visually distinguishable image that is different from the original object if both were lit the same at the time they were recorded. In practice, however, there have been major compromises made on the quality of the image in order to eliminate the requirement for laser illumination while creating and viewing the hologram. However, several intermediate procedures do not use holographic imaging procedures. These techniques are being used as alternatives to the holographic technology that consumes a lot of power and hazardous lasers required for the production of holographic images. Due to the advancement of technology, holograms can be today generated by the use of computers to show the scenes and objects that did not exist.

Computing Concepts Applied In Holograms

Digital holograms technology focuses on recording the phase and amplitude of the light from the original object. According to Picart & Jun-chang, the process of producing holograms is achieved by combining the wavelength of illumination produced by the original object and the reference beam that is produced from the source. This process will lead to the production of patterns of interference. The interference pattern is called a hologram. A reconstruction of the original wave front occurs when the hologram is lit with a beam, which is identical to the first one. The reconstruction creates the view as if the original object still exists. According to the work of Gabor, both the reference beam and the original object followed the same route that provided the possibility for the production of a two-dimensional image. The technology would only be applicable to the small objects. 

It is worth noting that due to the invention of the off-axis holographic technology by Leith it became possible to produce holograms with large objects that did not overlap. The digital holograms are computer generated and use the computers to calculate the pattern that would be produced by a virtual object. The digital holograms have replaced the analog photo films and plates that were used to record holograms in the past. The computer-generated digitalized detectors are arrayed as recorders for the holograms. Digital off-axis holograms use CCD arrays for recording holograms in cases where the object beam interferes with the reference one. The data produced is further transmitted to the computer for analysis.

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Application of digital holograms in microscopy

The use of digital holography has made great contributions to the field of microscopy, as anticipated by Gabor. He invented digital holograms as a way that would bring improvement in the electron microscopes. The technology used in digital holography is ideal to be applied to the observation of living cells since these can be difficult objects to study using the conventional microscopes. The difficulties are attributable to the soft nature of the living cells as well as the lack of contrast in their natural color. Kim has argued that, due to the digital holograms, it has become possible to record the differences in the refractive indexes. Moreover, laboratory technicians are able to differentiate cellular components due to the image recording. The key advantage of using digital holography in microscopy is that it can study the living cells without labeling, staining, or affecting them in any way. Due to the digital holography, the technicians are able to measure the properties of living cells such as the refractive index, volume as well as cell thickness. The study was previously difficult due to the limitations of the early holography. The use of digital holograms to record information electronically ensures the less time spent for processing photographic data. The technology also allows recording the time-series images that enables one to study dynamic effects. 

The technology is advantageous in the production of timely, complete and accurate optical fields that would otherwise be difficult in the real-space holograms. Additionally, digital holography provides options for processing images such as focusing on the objects from various distances, phase-shift quantitative measurements as well as digital manipulations. Digital holograms can easily be confused with lenticular and autostereoscopic 3D technologies that produce almost similar results. The earlier technologies were using conventional imaging. Some stage illusions, such as pepper’s Ghost and other magical images, are often confused with the holograms.

A hologram is comprised of a photosensitive film that has a fine structure made of grain. The materials used for the hologram are dichromate gelatins, emulsions made of silver-halide, and photopolymers. All these materials have a set of characteristics provided from different processes. In addition, holograms are used with the aim to enhance security, because they are useful in making embosser seals that are present in documents such as credit cards, passports, tickets, and packaging materials. Due to the use of these embossers, it is difficult to copy the respective documents. Digital holograms provided possibility for the reconstruction of dependent images that are spatially dependent. The digital holograms are based on the material interference instead of programs and sensors. 

The information contained in the holograms is enfolded in the surface. The difference is felt by moving around and by returning to find the present image as it was originally placed. Holographic images have their presence that the users can move. There are several ways of making holograms each with its aesthetic attributes, but sharing the same holographic imaging principles. The technology uses interference pattern that help in encoding and recording the image. The reconstruction of the holographic image is achieved through the optical image due to which it appears different from the surface material. The light that flows through the hologram is responsible for illuminating the image thereby making it clear. The hologram is perceived as a space to hold the light that passes through it. The space depends on the corresponding perspective upon which it is being viewed. Therefore, it allows a hologram to produce a dynamic and spatial scene. 

Limitations of Digital Holograms

Digital holography as many other technologies is faced with some shortcomings. The digital cameras used in microscopy have a relatively low spatial response to frequency, and this limits the resolution of the reconstructed holograms. The limitation occurs as a direct consequence of spacing detector elements and the fact that they are larger than the grains used in is photographic media.

Conclusion

The use of digital technology has led to the great improvements in holographic imaging. There is continued progress in the digital imaging that has continued to improve human life due to the production of accurate, timely, and clear images. These images form the basis upon which the informed decisions can be made particularly in the area of patients’ diagnosis and subsequent observations as well as follow-up procedures. The technology is eco-friendly demonstrating considerable improvement in the sense that it safeguards the cells under observation. The technology has been useful in the world of entertainment. It has supported televisions and videos production and thus contributed to the improvement of the living standards of the human population.

 

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Oct 8, 2019 in Informative
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