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CHAPTER 1 INTRODUCTION

Three-dimensional video displays that can generate ghost like optical duplicates of 3D objects and scenes have been depicted in science-fiction movies as futuristic means of visual media tools; such display devices always attracted public interest . One immediate question is whether such a display is possible; and a quick answer is “Yes, it is”.Noting that “seeing” is a purely optical interaction, and what we (or any other observer, including living organisms and machines) see is only due to the light that enters through our pupils, the design target for such a display is simple to state: if we can record the volume filling timevarying light field in a 3-D scene, with all its needed physical properties, and then regenerate the same light field somehow at another place, maybe at another time, the observer will not be able to distinguish the original scene from its duplicate since the received light will be the same, and therefore, any visual perception will also be the same. Then the natural question is whether we can record the light with all its relevant physical properties, and then regenerate it. The classical video camera is also a light recorder. However, not all necessary physical properties of light for the purpose outlined above can be recorded by a video camera; indeed, what is recorded by a video camera is just the focused intensity patterns (one for each basic color) over a planar sensing device. What is needed to be recorded instead is indeed much more complicated: we also need the directional decomposition of incoming light as well. Briefly, and in an idealized sense, we can say that we need to record the light field distribution. The term light field distribution is usually associated with ray optics concepts, and therefore, can be a valid optics model only in limited cases. If it can be recorded, we then need physical devices that can also regenerate (replay) the recorded light field. Prototypes for light field recording and rendering devices are reported in the literature . Integral imaging gets close to a light field imaging device in the limit under some mathematical idealizations; however such limiting cases are not physically possible. A better optical model than the ray optics is the wave optics. The propagation of light in a volume is modeled as a scalar wave field; the optical information due to a Dept. of ECE,CEMP

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3-D scene is carried by this wave field. Therefore, if such a wave field can be recorded and replayed, we achieve visual duplication of 3-D scenes; this is holography . Scalar wave model is usually satisfactory, and more accurate models of light are rarely needed, if any, for 3-D imaging and display purposes. Therefore, the term holography refers to recording and replaying optical wave fields. In a more restrictive usage, holography refers only to a specific form of such recording where interference of the desired wave field with a reference wave (sometimes self-referencing is employed, as in in-line holography) is formed and recorded; we prefer the broader usage as stated above. Indeed, the usage of the term may even be further broadened to include all kinds of physical duplication of light, and therefore, may also cover integral imaging, in a sense . Here in this paper, our focus is on the display of holograms. We focus only on dynamic displays for video. Still holographic display technology has been well developed since 1960s, whereas dynamic display technology is still in its infancy, and therefore, a current research topic. We further restrict our focus to pixelated display devices that can be driven digitally. Such displays are usually called digital electroholographic displays since they are driven electronically.

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CHAPTER 2 THREE DIMENSIONAL DISPLAYS

A 3D display is any display device capable of conveying a stereoscopic perception of 3-D depth to the viewer. The basic requirement is to present offset images that are displayed separately to the left and right eye. Both of these 2-D offset images are then combined in the brain to give the perception of 3-D depth. Although the term "3D" is ubiquitously used, it is important to note that the presentation of dual 2-D images is distinctly different from displaying an image in three full dimensions. The most notable difference is that the observer is lacking any freedom of head movement to increase information about the 3-dimensional objects being displayed. Holographic displays do not have this limitation, so the term "3D display" fits accurately for such technology. Similar to how in sound reproduction it is not possible to recreate a full 3dimensional sound field merely with two stereophonic speakers, it is likewise an overstatement of capability to refer to dual 2-D images as being "3D". The accurate term "stereoscopic" is more cumbersome than the common misnomer "3D", which has been entrenched after many decades of unquestioned misuse. The optical principles of multiview auto-stereoscopy have been known for over 60 years.Practical displays with a high resolution have recently become available commercially.

2.1 TYPES OF THREE-DIMENSIONAL DISPLAYS

2.1.1 STERIOSCOPIC Stereoscopy (also called stereoscopic For 3-D imaging) refers to a technique for creating or enhancing the illusion of depth in an image by presenting two offset images separately to the left and right eye of the viewer. Both of these 2-D offset images are then combined in the brain to give the perception of 3-Ddepth. Three strategies have been used to accomplish this: have the viewer wear eyeglasses to combine separate images from two offset sources, have the viewer

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wear eyeglasses to filter offset images from a single source separated to each eye, or have the lightsource split the images directionally into the viewer's eyes (no glasses required). Stereoscopy creates the illusion of three-dimensional depth from images on a twodimensional plane. Human vision uses several cues to determine relative depths in a perceived scene.Some of these cues are: • Stereopsis • Accommodation of the eyeball (eyeball focus) • Occlusion of one object by another • Subtended visual angle of an object of known size • Linear perspective (convergence of parallel edges) • Vertical position (objects higher in the scene generally tend to be perceived as further away) • Haze, desaturation, and a shift to bluishness • Change in size of textured pattern detail All the above cues, with the exception of the first two, are present in traditional two-dimensional images such as paintings, photographs, and television. Stereoscopy is the enhancement of the illusion of depth in a photograph, movie, or other two-dimensional image by presenting a slightly different image to each eye, and thereby adding the first of these cues (stereopsis) as well. It is important to note that the second cue is still not satisfied and therefore the illusion of depth is incomplete. Many 3D displays use this method to convey images. It was first invented by Sir Charles Wheatstone in 1838.Stereoscopy is used in photogrammetry and also for entertainment through the production of stereograms. Stereoscopy is useful in viewing images rendered from large multidimensional data sets such as are produced by experimental data. An early patent for 3D imaging in cinema and television was granted to physicist Theodor V. Ionescu in 1936. Modern industrial three dimensional photography may use 3D scanners to detect and record 3 dimensional information.The three-dimensional depth information can be reconstructed from two images using a computer by corresponding the pixels in the left and right images . Solving the Correspondence problemin the field of Computer Vision aims to create meaningful depth information from two images.

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2.1.2 AUTOSTERIOSCOPIC Autostereoscopy is any method of displaying stereoscopic images (adding perception of 3D depth) without the use of special headgear or glasses on the part of the viewer. Because headgear is not required, it is also called "glasses-free 3D" or "glasses-less 3D". The technology also includes two broad approaches used in some of them to accommodate motion parallax and wider viewing angles: those that use eye-tracking, and those that display multiple views so that the display does not need to sense where the viewers' eyes are located.Examples of autostereoscopic displays include parallax barrier, lenticular, volumetric, electro-holographic, and light field displays. Many organizations have developed autostereoscopic 3D displays, ranging from experimental displays in university departments to commercial products, and using a range of different technologies.The method of creating auto-stereoscopic 3D using lenses was mainly developed by Reinhard Boerner at the Heinrich Hertz Institute (HHI) in Berlin from 1985.The HHI was already presenting prototypes of Single-Viewer displays in the nineties. Nowadays, this technology has been developed further mainly by European companies. One of the most known 3D display developed by HHI was the Free2C, a display with very high-resolution and very good comfort achieved by an Eye tracking system and a seamless mechanical adjustment of the lenses. Eye tracking has been used in a variety of systems in order to limit the number of displayed views to just two, or to enlarge the stereoscopic sweet spot. However, as this limits the display to a single viewer, it is not favored for consumer products. Currently, most flat-panel solutions employ lenticular lenses or parallax barriers that redirect incoming imagery to several viewing regions at a lower resolution. When the viewer's head is in a certain position, a different image is seen with each eye, giving a convincing illusion of 3D. Such displays can have multiple viewing zones allowing multiple users to view the image at the same time, though they may also exhibit dead zones where only a monoscopic, crosseyed, or no image at all can be seen.

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2.1.3 COMPUTER GENERATED HOLOGRAPHIC AND VOLUMRTEIC Computer Generated Holography (CGH) is the method of digitally generating holographic interference patterns. A holographic image can be generated e.g. by digitally computing a holographic interference pattern and printing it onto a mask or film for subsequent illumination by suitable coherent light source.Alternatively, the holographic image can be brought to life by a holographic 3D display (a display which operates on the basis of interference of coherent light), bypassing the need of having to fabricate a "hardcopy" of the holographic interference pattern each time. Consequently, in recent times the term "computer generated holography" is increasingly being used to denote the whole process chain of synthetically preparing holographic light wavefronts suitable for observation. Computer generated holograms have the advantage that the objects which one wants to show do not have to possess any physical reality at all (completely synthetic hologram generation). On the other hand, if holographic data of existing objects is generated optically, but digitally recorded and processed, and brought to display subsequently, this is termed CGH as well. Ultimately, computer generated holography might serve all the roles of current computer generated imagery: holographic computer displays for a wide range of applications from CAD to gaming, holographic video and TV programs, automotive and communication applications (cell phone displays) and many more. A volumetric display device is a graphical display device that forms a visual representation of an object in three physical dimensions, as opposed to the planar image of traditional screens that simulate depth through a number of different visual effects. One definition offered by pioneers in the field is that volumetric displays create 3-D imagery via the emission, scattering, or relaying of illumination from well-defined regions in (x,y,z) space. Though there is no consensus among researchers in the field, it may be reasonable to admit holographic and highly multiview displays to the volumetric display family if they do a reasonable job of projecting a threedimensional light field within a volume. Most, if not all, volumetric 3-D displays are autostereoscopic; that is, they create 3-D imagery visible to the unaided eye. Note that some display technologists reserve the term “autostereoscopic” for flat-panel spatially-multiplexed parallax displays, such as lenticular-sheet displays. However, nearly all 3-D displays other than those requiring headwear, e.g. stereo goggles and stereo headmounted displays, are autostereoscopic. Therefore, a very broad group of display architectures are properly deemed autostereoscopic. Dept. of ECE,CEMP

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Volumetric 3-D displays embody just one family of 3-D displays in general. Other types of 3-D displays are: stereograms / stereoscopes, view-sequential displays, electro-holographic displays, parallax "two view" displays and parallax panoramagrams (which are typically spatiallymultiplexed systems such as lenticular-sheet displays and parallax barrier displays), re-imaging systems, and others. Although first postulated in 1912, and a staple of science fiction, volumetric displays are still under development, and have yet to reach the general population. With a variety of systems proposed and in use in small quantities — mostly in academia and various research labs — volumetric displays remain accessible only to academics, corporations, and the military.

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CHAPTER 3 HOLOGRAPHY

3.1 OVERVIEW OF HOLOGRAPHY Even though we focus on dynamic holographic displays in this paper, we feel that it is appropriate to start with a brief history of holography in general. Gabor (1900–1979) invented the holography to reduce the aberrations in electron microscopy . However, due to low quality of obtained images holography did not become popular until early 1960s. After the developments in laser technologies, Leith and Upatnieks developed the off-axis holography. In the meantime, Denisyuk invented the volume holography by bringing the work of Lippmann to holography . Still holography has been significantly developed since then, and many excellent monochromatic and color holograms have been made. The first computer generated hologram was introduced by Lohmann and Paris in 1967 . In the same year, Goodman and Lawrence brought forward the idea of the digital holography . Then, in 1980, the fundamental theory of digital holography was introduced by Yaroslavskii and Merzlyakov . We use the term digital holography in a broader sense to include all sorts of digital techniques to compute wave propagation, diffraction, and interference, as well as, digital capture and digital display of holograms. Conventional thick holograms on photographic plates can provide high resolution and full parallax. However, dynamic displays for holographic video are still far from providing satisfactory results. In electroholography, the resolution is significantly lower compared to thick holograms. Moreover, pixelated structures bring some additional problems. Pixel period determines the maximum frequency that can be represented when digital-to-analog conversion is conducted in the Shannon sense, and this in turn determines the maximum diffraction angle as outlined. Unfortunately, the pixel periods are not currently small enough to support sufficiently large viewing angles. Problems associated with pixelated electroholographic display are known . Since liquid crystal spatial light modulators (SLMs) are currently the primary choice for digital holographic displays, it is quite relevant to briefly mention current capabilities of such devices. Bauchert et al. reported the desirable features of liquid crystal SLMs. These features can

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be summarized as higher number of pixels, smaller pixel period, better optical efficiency, and faster operation. There are various SLMs such as liquid-crystal-based devices (liquid crystal devices and liquid crystal on silicon devices), mirror based devices (digital micromirror devices) and solid crystal devices (acousto–optical devices). The acousto–optical modulators (AOMs) are mostly used in 1-D applications. The digital micromirror devices are usually for binary modulation and they may result in additional noise due to vibration of micromirrors. The liquid crystal-based light modulators are more commonly used in electroholographic applications. Michalkiewicz et al. Pre sented the progress in liquid crystal on silicon (LCoS) SLMs and their applications ]. Ohmura et al. proposed a method to increase the viewing angle using such SLMs ].In their proposed system, they used a single SLM that wasdriven by a mirror module. As a consequence of this method the resolution along the horizontal direction increases. Therefore, the horizontal diffraction angle also increases; and thus the viewing angle is improved. Liquid-crystal-based SLMs are classified into various types such as complex amplitude, amplitude-only, phase only, transmissive- and reflective-type SLMs, and so on. The discussions corresponding to the bandwidth restriction and the pixel period given in Section III are valid for all such types of pixelated SLMs. Among them, the fully complex amplitude-type SLM may be the ultimate solution for the accurate reproduction from the hologram corresponding to a 3D object. Ability to support complex functions at the display is highly desirable since diffraction fields are represented as complex valued fields where both the amplitude and the phase are needed. An ideal SLM pixel should modulate both the amplitude and the phase of the incident light. However, it is difficult to manufacture the complex amplitude-type SLMs based on current technology. Phase-only SLMs may be the next best solutions for electroholography because they have several advantages over amplitude-only SLMs such as suppressed zeroth-order and highdiffraction efficiency, which can theoretically reach 100%. Amplitude-only SLMs can also be used for electroholography. However, problems associated with strong undesired diffraction orders are more severe compared to the phase-only case. A research group from Barcelona University, Barcelona, Spain, combined two SLMs to display full complex Fresnel holograms They used one SLM for the amplitude and the other one for the phase. They also investigated the quality of the reconstructions using real-only, imaginary-only, amplitude-only, and phase-only holograms . Schwerdtner et al. reported a novel hologram technology, which they called tracked viewing window (TVW) . By this approach they only calculate a small portion of a hologram, which then reconstructs a narrow angle light that falls onto the tracked pupils of the observer. They Dept. of ECE,CEMP

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demonstrated that thin film transistor (TFT) monitors can then be used as SLMs to build holographic displays. Another electroholographic display technique was presented by Hahn et al. [2. In their research, they used curved array of SLMs to increase the field of view. Spatial Imaging Group at the Massachusetts Institute of Technology (MIT, Cambridge, MA) developed a series of holographic display systems named Mark-I, Mark-II, and Mark-III . Mark-I and Mark-II use acoustooptical modulators, whereas Mark-III uses guided-wave optical scanners. All three can render 3-D objects at video rates. A company developed another holographic display system . The system uses active tiling where an electrically addressed SLM (EASLM) projects tiles of a big hologram onto an optically addressed SLM (OASLM). With the help of the setup, more than 100 megapixels holograms can be displayed. Another system, so-called Horn (HOlographic ReconstructioN), was presented by a group in Chiba University, Chiba, Japan Field programmable gate arrays (FPGAs) were used in the developed holographic display system to achieve video frame rates. Another group from Japan also demonstrated a holographic display system ; the system at the National Institute of Information and Communications Technologies (NICT, Tokyo, Japan) captures the 3-D scene by an integral imaging camera. The digital holograms of the captured scene is calculated and displayed in real time. For further details, the reader is referred to a broad survey on dynamic holographic displays, which was recently published .

3.2 HOLOGRAPHIC TECHNIQUE Holography (from the Greek

hólos, "whole" + grafē, "writing, drawing") is a

technique that allows the light scattered from an object to be recorded and later reconstructed so that when an imaging system (a camera or an eye) is placed in the reconstructed beam, an image of the object will be seen even when the object is no longer present. The image changes as the position and orientation of the viewing system changes in exactly the same way as if the object were still present, thus making the image appear three-dimensional. The holographic recording itself is not an image – it consists of an apparently random structure of either varying intensity, density or profile – an example can be seen below. The technique of holography can also be used to store, retrieve, and process information optically. While it has been possible to create a 3-D holographic picture of a static object since the 1960s, it is only in the last few years that arbitrary scenes or videos can be shown on a holographicvolumetric display. Dept. of ECE,CEMP

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HOW HOLOGRAPHY WORKS

Holography is a technique which enables a light field to be recorded, and reconstructed later when the original light field is no longer present. It is analogous to sound recording where the sound field is encoded in such a way that it can later be reproduced. Though holography is often referred to as 3D photography, this is a misconception. A photograph represents a single fixedimage of a scene, whereas a hologram, when illuminated appropriately, re-creates the light which came from the original scene; this can be viewed from different distances and at different orientations just as if the original scene were present. The hologram itself consists of a very fine random pattern, which appears to bear no relationship to the scene which it has recorded. To record a hologram, some of the light scattered from an object or a set of objects falls on the recording medium. A second light beam, known as the reference beam, also illuminates the recording medium, so that interference occurs between the two beams. The resulting light field generates a seemingly random pattern of varying intensity, which is recorded in the hologram. The figure below is a photograph of part of a hologram – the object was a toy van. The photograph was taken by backlighting the hologram with diffuse light, and focusing on the surface of the plate. It is important to note that the holographic recording is contained in the random intensity structure (which is a speckle pattern), and not in the more regular structure, which is due to interference arising from multiple reflections in the glass plate on which the photographic emulsion is mounted. It is no more possible to discern the subject of the hologram from this random pattern than it is to identify what music has been recorded by looking at the hills and valleys on a gramophone record surface or the pits on a CD. When the original reference beam illuminates the hologram, it is diffracted by the recorded hologram to produce a light field which is identical to the light field which was originally scattered by the object or objects onto the hologram. When the object is removed, an observer who looks into the hologram "sees" the same image on his retina as he would have seen when looking at the original scene. This image is avirtual image as the rays forming the image are all divergent. The figure shown at the top of this article is an image produced by a camera which is located in front of the developed hologram which is being illuminated with the original reference beam. The camera is focused as if on the original scene, not on the hologram itself.

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Fig 3.1Holographic recording

fig 3.2 Holographic reconstruction

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3.3 DIGITAL HOLOGRAPHY

Digital holography is the technology of acquiring and processing holographic measurement data, typically via a CCD camera or a similar device. In particular, this includes the numerical reconstruction of object data from the recorded measurement data, in distinction to an optical reconstruction which reproduces an aspect of the object. Digital holography typically delivers three-dimensional surface or optical thickness data. There are different techniques available in practice, depending on the intended purpose. 3.3.1 DIGITAL ANALYSIS OF HOLOGRAMS Phase-shifting holograms The

phase-shifting

digital

holography

process

entails

capturing

multiple

interferograms that each indicate the optical phase relationships between light returned from all sampled points on the illuminated surface and a controlled reference beam of light that is collinear to the object beam (in-line geometry). From a set of these interferograms, holograms are computed that contain information defining the shape of the surface. Multiple holograms gathered at multiple laser light wavelengths are then combined to compile the full shape of the illuminated object over its full dimensional extent. Off-axis configuration At the off-axis configuration where a small angle between the reference and the object beams is used. In this configuration, a single recorded digital hologram is sufficient to reconstruct the information defining the shape of the surface, allowing real-time imaging. Multiplexing of holograms Digital holograms can be numerically multiplexed and demultiplexed for efficient storage and transmission. Amplitude and phase can be correctly recovered.The numerical access to the optical wave characteristics (amplitude, phase, polarization) made digital holography a very powerful method. Numerical optics can be applied to increase the depth of focus (numerical focalization) and compensate for aberration.

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Wavelength multiplexing of holograms is also possible in digital holography as in classical holography. It is possible to record on the same digital hologram interferograms obtained for different wavelengths.) or different polarizations Super-resolution in Digital Holography Superresolution is possible by means of a dynamic phase diffraction grating for increasing synthetically the aperture of the CCD array Optical Sectioning in Digital Holography Optical sectioning, also known as sectional image reconstruction, is the process of recovering a planar image at a particular axial depth from a three-dimensional digital hologram. Various mathematical techniques have been used to solve this problem, with inverse imaging among the most versatile. Extending Depth-of-Focus by Digital Holography in Microscopy By using the 3D imaging capability of Digital Holography in Amplitude an Phase it is possible to extend the depth of focus in Microscopy. Combining of holograms and interferometric microscopy The digital analysis of a set of holograms recorded from different directions or with different direction of the reference wave allows the numerical emulation of an objective with large numerical aperture, leading to corresponding enhancement of the resolution.This technique is called interferometric microscopy.

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CHAPTER 4 RECENT DEVELOPMENTS TO HOLOGRAPHIC DISPLAYS Holographic displays have been investigated at Bilkent University, Ankara, Turkey .Recently, they used SLMs for such purposes and demonstrated single and multiple SLM holographic displays. They mostly use phase-only SLMs. For example, in a study involving only one phase-only SLM , in-line phase holograms, which were calculated by Gerchberg–Saxton algorithm , were used to show that reconstructions that are larger than the SLM size are feasible. In another system, three SLMs were used to generate color holographic reconstructions . Again the Gerchberg–Saxton algorithm was used to generate the in-line phase holograms that were written on the SLMs. Three phase holograms were calculated separately (for red, green, and blue channel) and loaded to the SLMs. Color light-emitting diodes (LEDs) were used as light sources; all three reconstructions were combined to obtain a color reconstruction. Yet another system generates and displays holograms in real time . The phase-only holograms for the display were computed using a fast, approximation-based algorithm called accurate compensated phase-added stereogram (ACPAS) , which was implemented on graphics processing units (GPUs) to render the holograms at video rates. LEDs were used as light sources for reconstructions that can be observed by naked eye. Fig.4.1 shows the overall setup for the real-time color holographic display system and Fig. 4.2(a)– (c) shows the original color 3-D model, the computer reconstruction from the phase-only hologram, and the optical reconstruction from the same hologram written on the SLM, respectively. They also compared the quality of optical reconstructions obtained by using a laser and a LED as the light source . Even though LEDs have broader spectra than lasers, they conclude that reconstructions using LEDs can be still satisfactory in quality. In a recent prototype, a curved array of six phaseonly SLMs was used to increase the field of view. As a consequence of the achieved large field of view, the observer can look at the optical reconstruction binocularly and see a real 3-D image floating in the space. Reconstructions can also be observed from different angles without any discontinuity and with a larger horizontal parallax. shows the optical reconstructions of a pyramid recorded from different angles. The ghost-like 3-D image (a real optical image) was positioned next to a similar physical object located at the same depth, and the recording camera was focused to that plane; such a setup shows the depth location of the reconstruction, as well as its quality of parallax and sharpness by providing a similar physical object for comparison. The actual size of the base of

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the pyramid is about 1 cm  1 cm, and its height is about 2 cm. The reconstruction (real image) is about 50 cm in front of the SLMs. This brief overview of current state of the art indicates that dynamic holographic displays do have the potential for highly satisfactory futuristic 3DTV displays; however, they do not yet provide such satisfactory results to the consumer who expects the counterpart of crisp clear conventional 2DTV displays. Further research is certainly needed.

fig 4.1 Overall setup: BEVbeam expander; BSVnonpolarized beam splitter.

fig 4.2 Color holographic reconstruction using SLMs. (a) A rigid color 3-D object. (b) Computer reconstruction from the hologram of the 3-D object. (The hologram was calculated by using the ACPAS algorithm.) (c) Optical reconstruction (a single frame of the holographic video)

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CHAPTER 5 WORKING SETUP OF DIGITAL HOLOGRAPHIC 3D DISPLAY SYSTEM

5.1

INTRODUCTION TO SETUP Electro-holography is one of the common method for holographic display

researches . There are several pixelated devices which are suitable for digital holographic display systems. Liquid crystal (LC) displays, liquid crystal on silicon (LcoS) displays, digital micro-mirror devices are only some of them. Since all those pixelated devices modulate the incoming light in the spatial domain, either transmissively or reflectively, they are called spatial light modulators (SLMs). Full complex modulators would be the ultimate solution; however, most of the spatial light modulators usually modulate only the amplitude or the phase of the incident light. In our holographic display prototypes we use phase-only LCoS SLMs. Although those SLMs are easy to use, have high diffraction efficiency and are rich in term of pixel count, they are quite small in size. Typical phase-only SLM size is approximately 1cm × 2cm. As a result of it, reconstructions are also quite small. It is almost impossible to see the reconstructions binocularly and rotate around them when only one such SLM is used. Therefore researchers are trying to come up with ideas to overcome this bottleneck. Maeno et al. proposed a method to increase the size of the holographic display system and also the field of view . In their system they use five transmissive type SLMs and place them side by side. They increase the field of view in horizontal direction, but they discard the vertical parallax by using a lenticular sheet. Yet another holographic display system is reported by Hahn et al. and they use mirror modules and other optical components to increase the size of the display system . Their horizontal parallax only (HPO) curved holographic display system provides approximately 23◦ viewing angle. A company from Germany, called SeeReal Technologies, developed a novel technique . With the help of the technique, only a part of the wavefront that enters to the eye pupil of the observer is reconstructed. Different from the above systems, we propose to build a fullparallax circularly configured multi-SLM holographic display system. We use six phase-only reflective type spatial light modulators to increase the horizontal size of the holographic display. With the help of the proposed system, observers can see ghost-like 3D images floating in the space binocularly with naked eye. Methods and configurations for enlargement of the viewing angle will be presented in Dept. of ECE,CEMP

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the following section. Next Section presents the optical setup and hologram generation algorithm. Experimental results will be also presented .

5.2 ENLARGEMENT OF THE VIEWING ANGLE For the pixelated devices, local maximum diffraction angle for Shannon recovery case can be calculated as: θmax = sin−1 (λfmax ),

(1)

where; (2)

Here ∆p denotes the pixel period of the device, λ is the wavelength of the incident light. Within the limit of the local maximum frequency, we can reconstruct 3D objects larger than the SLM size . Those reconstructions can be seen on a diffuser or a screen. However, in order to see the full reconstruction by naked eye, 3D object should be in the viewable area (See Fig.5. 1). Otherwise observer can only see a part of it. One way to solve this problem is to increase the hologram size (See Fig. 5.2). Since viewable area is increased, observer can see large 3D reconstructions.

fig 5.1 Illustration of larger object reconstruction with single SLM

fig 5.2 llustration of larger object reconstruction with multiple SLM Dept. of ECE,CEMP

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However, since SLMs have hard mount around it, when we place them side by side a large gap occurs between two SLMs (See Fig.5. 3). There are methods to overcome this problem, such as using mirror modules as in . In our system, we propose to use multiple beam splitters (halfmirrors). As seen in Fig. 5.4, with the help of the beamsplitters, the gap between two SLMs are filled by the image of the third SLM. The same configuration is applied to other three SLMs and finally, these two sets are combined with a large beam-splitter (See Fig. 5.5). Therefore the resultant hologram size increases six times in horizontal direction. This gives a freedom to the observer to see the reconstructions binocularly and to rotate within the limit of the viewing angle.

fig 5.3 Gap between two SLM when they are aligned side by side

fig 5.4 Alignment of three SLMs to have continuous array without any gap

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fig 5.5 Schematic of the setup

5.3 OPTICAL SETUP AND HOLOGRAM GENERATION

Schematic of the setup can be seen in Fig. 5.5 The method to obtain larger hologram size in horizontal direction described in the previous section. After aligning the six SLMs side by side, we rotate the SLMs with the help of high precision stages to configure them cirlularly. Each SLM is rotated by 1◦ with respect to the next SLM (This can be increased up to 3◦ ). Fine adjustments are also needed after the rotations. Fig.5. 6 shows the front view of the holographic display system. Continuous array of SLMs can be seen in the figure. In our experiments, we have used computer generated 3D model of a cube. It is a wire-frame object. The model consists of a point cloud. Each point in the point cloud has 3D coordinate and intensity information. Holograms are calculated by using Fresnel light propagation algorithm . Each point of the 3D model propagated to the hologram plane. Then, contributions of all the points in the point cloud are superposed to obtain complex field at the hologram plane. Since our SLMs are phase-only, we discard the amplitude and set to unity (plane wave illumination). Although discarding the amplitude lowers the quality, degradations in reconstructions are acceptable. By applying the same procedure, six Dept. of ECE,CEMP

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holograms are calculated one by one, by simply rotating the 3D object by the corresponding angle. The rotation angles are −2.5◦ , −1.5◦ , −0.5◦ , +0.5◦ , +1.5◦ and +2.5◦ . Top view of the setup can be seen in Fig. 5.7. There are two sets of Holoeye SLM modules . Each SLM module has three phase only SLMs with 1920 × 1080 pixels each. SLMs in one module are aligned side by side by using non-polarized cubic beam-splitters as described in the previous section. The resultant wide displays (total of 5760 × 1080 pixels) are also combined with a plate beamsplitter (half-mirror) to obtain a grand total of 11520 × 1080 pixels wide holographic display system. All alignments are done with high precision stages.

Fig 5.6 Front view of the display

Fig 5.7 Top view of the display

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fig 5.8 Optical reconstruction of 3D wire-frame model of a cube. (a) Left, (b) center, and (c) right views.

5.4 EXPERIMENTAL RESULTS Fig. 5.8 shows the optical reconstruction of 3D wire-frame model of a cube. Pictures are taken from different viewing angles (left, center and right). Reconstructions are captured when laser is used as a light source. In order to avoid eye-hazard [20], LED illumination is also used and such reconstructions are observed with naked eye. Reconstruction distance is approximately 400mm. We observed that the total field of view increased significantly. With the help of the same configuration, we can increase the field of view and thus viewing angle by using larger number of SLMs and larger beam-splitters.

5.5 CONCLUSION OF SET UP The proposed system has six phase-only spatial light modulators that are aligned side by side to obtain a 11520 × 1080 pixel wide circular holographic display system. All alignments are accomplshed by using high precision stages. System works both with laser and LED illumination. Optical reconstructions show that increase in the viewing angle is significant compared to single SLM case. We conclude that circular configuration is superior than the previous planar configurations. We also notice the parallax with the help of the optical reconstructions. Experimental results are satisfactory and they show that circularly holographic display system works successfully.

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CHAPTER 6 APPLICATIONS

➔ Geo-seismic and related 3D data visualization Spatially manipulate complex data to extract key insights; communicate discoveries across disciplines using an easily comprehensible visual representation. ➔ Medical training: Analyze and manipulate complex physiological data sets from multiple dimensions easily understood by experts as well as trainees. Navigate, enlarge and manipulate the imagery to a degree limited only by data resolution. ➔ Military simulation and situational awareness: Enable rapid understanding of regional landscapes and urbanscapes, quickly giving viewers a feeling of familiarity that is essential for successful missions. ➔ 3D gaming: Convey all aspects of the virtual 3D world during planning, production and postproduction; help facilitate the creation of 3D content with immediate access to all perspectives. Your viewers can then escape into lifelike, three-dimensional entertainment and games that explode with full-color, true-3D realism. ➔ In visual communication Scientists are working to improve Video chat to become holography chat - or "3D Presence." The technique uses light beams scattered from objects and reconstructs a picture of that object, a similar technique to the one human eyes use to visualize our surroundings.

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In holographic 3DTV applications The holographic display technology will made revolutionary changes in the

television market.future televisions will show true 3d videos without the help of any 3d glasses

➔

In Cinema entertainment field It will help in the true 3d projection of 3D films with a 360 ° viewing experience

➔

In cartography It will help to create 360° viewable maps.

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CHAPTER 7 LIMITATIONS

➢ The display techniques are bit expensive ➢ The display panels for the system are not developed upto a full extent ➢ In order to create a faithful reproduction of the 3-D image of a physical object, the holographic recording material is required to have a high sensitivity and a high resolution. ➢ The design and implementation of electronically controllable dynamic displays to support holographic video are the key issues for the success of such true 3-D displays. ➢ Currently available devices have quite limited capabilities, and thus, do not yield satisfactory performance, yet. We expect that such products will be significantly improved in the future. ➢ The system will be bulky.

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CHAPTER 8 FUTURE SCOPES

This three dimensional holographic display technology will be a revolutionary invension to many fields.In future you can chat with your friend in abroad by seeing his True 3D video hologram in a 360° angle.We can able to see the live cricket or world cup football match at your home as you where in the gallery with the 360° viewing angle .You will experince 3d video call in your phone,can view the geographical features of an area through 3d hologram,can view internal organs of our body in 3D.In future technologies like interactive skinputs will bring real time communicatin through our gestures.

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CHAPTER 9 CONCLUSION

Digital holographic video displays are strong candidates for rendering ghostlike “true 3-D” motion images. Interest in this technology is increasing among the research community. Many laboratories have already reported different designs with promising results. Most of these designs are based on SLMs; SLMs with different capabilities and specifications have been used. Therefore, it is important to understand the limitations of such devices, and their effects on the resultant 3-D images. Reasonable sizes and resolutions seem to be sufficient for a stationary observer with no lateral or rotational motion. However, the needed SLM size and pixel density quickly increase beyond the capabilities of today’s electronic technology when such motion is allowed as in a natural viewing environment. An alternative is to arrange planar SLMs on a curved mount to relieve the requirement of small and high-density pixels. Since the holograms are quite robust to quantization errors, and since frame refresh rates are satisfactory for continuous perception, the focus of research is rather on designing digital holographic display sets, which can effectively support more freedom in lateral and rotational motion of the observer while providing satisfactory quality 3-D images.

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REFERENCE

1. Digital Holographic Three-Dimensional Video Displays Onural, L.; Yaraş , F.; Hoonjong Kang;Dept. of Electr. & Electron. Eng., Bilkent Univ., Ankara, Turkey, Proceedings of the IEEE April 2011 2. Reliability of 3D Imaging by Digital Holography atLong IR Wavelength Anna Pelagotti, Massimiliano Locatelli, Andrea Giovanni Geltrude, Pasquale Poggi, Riccardo Meucci,Melania Paturzo, Member, IEEE, Lisa Miccio, Student Member, IEEE, and Pietro Ferraro, Senior Member, IEEE,oct 2010 3. Circularly configured multi-slm holographic display system Fahri Yaras, Hoonjong Kang, Levent Onural ,Bilkent University Department of Electrical and Electronics Engineering,TR-06800 Ankara, Turkey [email protected], [email protected], [email protected] 4. Research Trends in Holographic 3DTV Displays Levent Onural Dept. of Electrical and Electronics Eng. Bilkent University TR-06800 Ankara, [email protected]

5. www.ieee.org 6. www.digital holography.eu

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