1. 3D capture and 3D content generation by digital holography

Three-dimensional (3D) displays are the next generation displays. The claim for 3D imaging is indisputable in mass television, game industry, medical imaging, computer-aided design, automated robotic systems, air traffic control, education and cultural heritage dissemination. The ultimate goal of 3D visual communication is 3D capture of a real-life scene that is followed

Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, © 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons

distribution, and eproduction in any medium, provided the original work is properly cited.

by creating its scaled exact optical duplicate at a remote site instantaneously or at a later time moment [1]. Tracking the development of 3D imaging devices from Wheatstone stereoscope designed in 1830 to modern full HD 3D displays with glasses (Figure 1) reveals some memorable periods of booming public interest to this area, e.g. the 3D theatres boom observed in 1950 as a counterpoint to the increasing popularity and commercialization of the television or the last years reaction to the 'Avatar' movie. Multiple parallax 3D display technology has evolved to full-parallax displays for a naked eye observation as integral imaging displays [2] and super-multi-view displays [3]. However, since invention of holography by Dennis Gabor in 1948 [4] and the first holographic 3D demonstration by Emmett Leith and Juris Upatnieks in 1964 [5], the consumer public has high expectations for a truly holographic display in view that holographic imaging is the best candidate for 3D displays.

Holography is the only imaging technique which can provide all depth cues. High quality of 3D imaging from analogue white-light viewable holograms is well known [6]. They provide a wide viewing angle due to the very fine grain size of the holographic emulsions. Realistic images can be viewed by an arbitrary number of viewers with unique perspectives. Motion parallax both vertically and horizontally and tilting of the head are possible. The viewer is capable of focusing at different depths. There are no convergence-accommodation conflict and discontinuities between different views as for multiple parallax displays. Holographic imaging allows for building autostereoscopic displays. To realize a holographic display, the 3D scene description should be encoded as two-dimensional (2D) holographic data. The 3D content generation is carried out by means of digital holography [7] based on the classical holographic principle. According to it, holography is a two-step process enabling storing and reconstruction of a wavefront diffracted by 3D objects. The hologram records as a 2D intensity pattern the

Figure 1. History of 3D displays with some of the major events depicted as a diagram with years along the horizontal axis and intensity of the public interest along the vertical axis.

interference of this wavefront with a mutually coherent reference wave as depicted schematically in Figure 2. The wave field from the object is characterized by a complex amplitude, Oðx, yÞ ¼ aOðx, yÞ exp ½jϕOðx, yÞ�, where aOðx, yÞ and ϕOðx, yÞ are the amplitude and phase of the object beam, respectively. Interference of Oðx, yÞ and the mutually coherent reference beam Rðx, yÞ ¼ aRðx, yÞ exp ½jϕRðx, yÞ� results in four terms superimposed in the hologram plane (x, y):

by creating its scaled exact optical duplicate at a remote site instantaneously or at a later time moment [1]. Tracking the development of 3D imaging devices from Wheatstone stereoscope designed in 1830 to modern full HD 3D displays with glasses (Figure 1) reveals some memorable periods of booming public interest to this area, e.g. the 3D theatres boom observed in 1950 as a counterpoint to the increasing popularity and commercialization of the television or the last years reaction to the 'Avatar' movie. Multiple parallax 3D display technology has evolved to full-parallax displays for a naked eye observation as integral imaging displays [2] and super-multi-view displays [3]. However, since invention of holography by Dennis Gabor in 1948 [4] and the first holographic 3D demonstration by Emmett Leith and Juris Upatnieks in 1964 [5], the consumer public has high expectations for a truly holographic display in view that

Holography is the only imaging technique which can provide all depth cues. High quality of 3D imaging from analogue white-light viewable holograms is well known [6]. They provide a wide viewing angle due to the very fine grain size of the holographic emulsions. Realistic images can be viewed by an arbitrary number of viewers with unique perspectives. Motion parallax both vertically and horizontally and tilting of the head are possible. The viewer is capable of focusing at different depths. There are no convergence-accommodation conflict and discontinuities between different views as for multiple parallax displays. Holographic imaging allows for building autostereoscopic displays. To realize a holographic display, the 3D scene description should be encoded as two-dimensional (2D) holographic data. The 3D content generation is carried out by means of digital holography [7] based on the classical holographic principle. According to it, holography is a two-step process enabling storing and reconstruction of a wavefront diffracted by 3D objects. The hologram records as a 2D intensity pattern the

Figure 1. History of 3D displays with some of the major events depicted as a diagram with years along the horizontal axis

and intensity of the public interest along the vertical axis.

holographic imaging is the best candidate for 3D displays.

184 Holographic Materials and Optical Systems

$$\begin{split} I\_H(\mathbf{x}, y) &= \left| \mathbf{O}(\mathbf{x}, y) + \mathbf{R}(\mathbf{x}, y) \right|^2 \\ &= \mathbf{O}(\mathbf{x}, y) \mathbf{O}^\*(\mathbf{x}, y) + \mathbf{R}(\mathbf{x}, y) \mathbf{R}^\*(\mathbf{x}, y) + \mathbf{O}(\mathbf{x}, y) \mathbf{R}^\*(\mathbf{x}, y) + \mathbf{R}(\mathbf{x}, y) \mathbf{O}^\*(\mathbf{x}, y) \end{split} \tag{1}$$

where the asterisk denotes a complex conjugation. The sum of intensities of the object and reference beams gives the zero-order term. The last two terms are the +1 and –1 diffraction orders and contain the object wavefront information. The object field OR� R ¼ O or O� RR� ¼ O� brings into focus the virtual or the real image when IHðx, yÞ is multiplied by Rðx, yÞ or its conjugate. These twin images are separated for the off-axis geometry where the object and reference beams subtend an angle θ, and overlap in inline geometry at θ ¼ 0.

Digital holography grew from a purely academic idea into a powerful tool after the recent progress in computers, digital photo-sensors (CCDs or CMOS sensors) and spatial light modulators (SLMs). The capability of digital holography for digital analysis and synthesis of a light field forms two mutually related branches. The branch dedicated to analysis comprises methods for optical recording of holograms using digital photo-sensors. The holograms are sampled and digitized by the photo-sensor, stored in the computer and numerically reconstructed using different approaches to describe diffraction of light from the hologram and free space propagation to the plane of the reconstructed image [8]. Thus, capture of both amplitude and phase becomes possible enabling numerical focusing at a variable depth and observation of transparent micro-objects without labelling [9]. The holographic data are in the

Figure 2. Schematic representation of holographic recording.

form of a real-valued 2D matrix of recorded intensity according to Eq. (1). In this branch, different techniques have emerged for the two decades of existence of digital holography as (i) digital holographic microscopy with a plane wave or a point-source illumination [10, 11]; (ii) optical diffraction tomography with multi-directional phase-shifting holographic capture [12]; (iii) infrared holography in the long wavelength region for capture of large objects [13]; (iv) determination of sizes and locations and tracking of particles in a 3D volume [14]. Feature recognition based on digital holography has been proposed [15]. A lot of efforts were dedicated to instrumental or software solutions of the twin image problem [16].

The branch dedicated to synthesis of a light field comprises methods for computer generation of holograms [17] which are fed to some kind of SLMs for optical reconstruction of the images they encode. Computer-generated holograms (CGHs) are used for holographic displays [18], holographic projection [19] and diffractive optical elements [20]. In principle, the CGHs provide the only means to generate light fields for virtual 3D objects. A CGH is a real-valued 2D matrix of amplitude or phase data; it may have also binary representation.

Both branches are closely related to the task of direct transfer of optically captured digital holograms to a holographic display. To realize the chain 3D capture—data transfer—holographic display digital holography requires coherent light and 2D optical sensors and SLMs with high resolution and big apertures. The 4D imaging, when the time coordinate, is added to the data further aggravates the task of building a holographic display because the latter needs much higher resolution and much more information capacity than other types of 3D displays. Generally speaking, there are two ways for 3D content generation for holographic displays: (i) conversion of optically captured holographic data; (ii) computer generation of holograms. Furthermore, we discuss these two main tendencies—direct feeding of optically recorded digital holograms to a holographic display and computer generation of interference fringes from directional, depth and colour information about the 3D objects—on the basis of our experience in forming the holographic content.
