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Grafik SLM menggambarkan efek waktu respon yang terbatas dan pengalamatan baris-demi-baris pada SLM setelah beralih dari satu hologram ke hologram berikutnya. Grafik lampu latar menunjukkan pengalihan lampu latar yang tertunda dan berturut-turut untuk memberi kompensasi efek ini. Gambar 17. Tampilan atas susunan filter warna dalam tampilan holografik dengan multiplexing warna spasial. Grafik kiri menunjukkan filter warna yang terintegrasi dalam piksel SLM (SLM CF) dan filter warna terpisah yang terpisah (CF). Grafik menunjukkan tiga lensa lenticular positif (L) yang membagi cahaya untuk menghasilkan jendela tampilan kiri (VWL) dan jendela tampilan kanan (VWR). Untuk penyederhanaan, hanya kilau yang menerangi VWR yang diperlihatkan dan cahaya terang yang menerangi VWL dihilangkan. Sumber cahaya dan lensa Transformasi Fourier tidak diperlihatkan. Holografik 3-D Menampilkan - Electro-holografi dalam Pegang Komersialisasi Stephan Reichelt 1. Ralf Hussler 1. Norbert Leister 1. Gerald Ftterer 1. Hagen Stolle 1 dan Armin Schwerdtner 1 1 SeeReal Technologies, Germany 1. Pendahuluan Holografi adalah difraksi- Teknik pencitraan koheren berbasis di mana objek tiga dimensi yang rumit dapat direproduksi dari layar dua dimensi datar dengan transparansi kompleks yang mewakili nilai amplitudo dan fasa. Hal ini umumnya disepakati bahwa holografi real-time adalah seni ultra plus dan sains untuk memvisualisasikan adegan 3-D yang cepat berubah sementara. Integrasi prinsip real-time atau electro-holografik ke dalam teknologi display adalah salah satu perkembangan yang paling menjanjikan namun juga menantang untuk tampilan konsumen dan pasar TV masa depan. Hanya holografi yang memungkinkan rekonstruksi pemandangan 3-D yang tampak alami, dan karena itu memberi pengamat pengalaman menonton yang benar-benar nyaman. Namun hingga saat ini beberapa tantangan telah menghalangi teknologi untuk dikomersilkan. Tapi hambatan itu kini mulai bisa diatasi. Baru-baru ini, kami telah mengembangkan pendekatan baru untuk holografi tampilan real-time dengan menggabungkan teknik sub-hologram yang tumpang tindih dengan teknologi layar pandang yang dilacak (Schwerdtner, Leister amp Hussler, 2007. Schwerdtner, Hussler amp Leister, 2007). Untuk pertama kalinya, ini memungkinkan solusi untuk tampilan holografik layar besar interaktif (Stolle amp Hussler, 2008. Reichelt et al 2008). Bab ini menyajikan solusi baru ini untuk tampilan holografik 3-D real-time yang besar dalam konteks pendekatan holografi sebelumnya dan terkini. Tampilan holografik yang dikembangkan oleh kami menggabungkan skema perekaman holografik yang disesuaikan dengan pelacakan aktif pengamat. Pendekatan unik ini secara dramatis mengurangi permintaan akan produk bandwidth-space dari hologram dan dengan demikian memungkinkan penggunaan modulator cahaya ruang angkasa state-of-the-art dan memungkinkan perhitungan real-time. Dasar-dasar dan tantangan teknologi display holografik dijelaskan, implementasinya dalam prototip ditunjukkan, dan prospek cerah untuk pasar display 3-D dibahas. 2. Teknologi tampilan holografik real-time Saat berbicara tentang teknologi display holografik, catatan kehati-hatian mengenai terminologi yang umum digunakan sangat dibutuhkan. Untuk pemasaran atau alasan lainnya, istilah tampilan holografik sering disalahgunakan untuk memberi nama sistem, yang sebenarnya bukan holografik dalam arti holografi video. Sistem, yang menggunakan layar holografik atau elemen optik hologram untuk gambar proyek hanyalah contohnya. Tetapi bahkan menampilkan volumetrik yang menciptakan titik terang di suatu tempat dalam volume mereka disebut dalam banyak kasus holografik. Di sisi lain, rekaman holografik benar-benar disebut display, padahal sebenarnya ada hologram statis atau hologram dinamis, yang belum real-time yang mampu dengan penulisan ulang-kali pada rentang menit dan dengan setup skala besar (Tay et al 2008). Apa yang kita maksud dengan tampilan holografik real-time adalah sistem yang didasarkan pada difraksi untuk merekonstruksi bidang gelombang dari adegan 3-D di angkasa dengan cahaya koheren. Tampilan semacam itu harus beroperasi pada atau di dekat tingkat video agar sesuai dengan nama holografi video. Selanjutnya, holografi real-time tidak hanya harus menampilkan hologram pada tingkat video tetapi juga menghitung frame hologram secara real time untuk memungkinkan interaksi pengguna. 2.1. Mengapa holografi teknologi 3-D utama Dalam pandangan manusia, persepsi tiga dimensi dipicu oleh sejumlah besar isyarat. Diantaranya isyarat monokuler seperti shading, oklusi, ukuran relatif, fogging, distorsi perspektif, dan gradien tekstur serta teropong seperti vergence (disparitas sudut) dan stereopsis (disparitas horizontal). Dalam situasi melihat alami, informasi mendalam adalah isyarat yang selalu ada dalam persepsi visual. Umumnya, dan selain paralaks, isyarat kedalaman akomodasi dan kewaspadaan fisiologis dianggap paling penting untuk persepsi mendalam. Akomodasi adalah mekanisme dimana mata manusia mengubah daya optiknya untuk menahan benda pada jarak yang berbeda menjadi fokus yang tajam pada retina. Perubahan daya disebabkan oleh otot-otot siliaris, yang meruncing lengkungan lensa kristal untuk benda-benda pada jarak yang lebih dekat. Vergence. Sebaliknya, adalah gerakan simultan kedua mata menuju titik ketertarikan. Sumbu optik kedua mata bertemu pada titik ini untuk menggambarkan objek ke masing-masing daerah fovea. Bila mata tidak sejajar satu sama lain, terjadi strabismus yang dapat mengganggu persepsi 3-D. Tapi yang terpenting, pergerakan dan akomodasi yang hina saling terkait satu sama lain secara otomatis dan tidak sadar. Itulah mengapa citra sebuah benda tajam dan kedua perspektif itu menyatu. Bersama dengan isyarat monokuler dan teropong lainnya, isyarat isyarat fokus dan kabur dalam citra retina berkontribusi pada kemampuan visual kita untuk memahami lingkungan dalam tiga dimensi. Selama dekade terakhir, berbagai teknologi untuk memvisualisasikan adegan tiga dimensi (3-D) pada display telah didemonstrasikan dan disempurnakan secara teknologi, di antaranya jenis stereoskopik, multi-view, integral-imaging, volumetrik, atau holografik. Secara umum diyakini bahwa hal besar berikutnya di industri display sudah dekat, yaitu transisi dari visualisasi 2-D ke 3-D. Hal ini dilihat sebagai tidak kurang dari perubahan zaman ketiga dalam industri perfilman, setelah perubahan dari film silent to sound pada 1920-an dan dari hitam-putih menjadi warna pada tahun 1950an. Sebagian besar pendekatan saat ini menggunakan prinsip stereoskopik konvensional, yang pertama kali dijelaskan oleh Wheatstone (Wheatstone, 1838). Tapi kecuali display super multiview, mereka semua tidak memiliki konflik inheren antara kehangatan dan akomodasi karena kedalaman pemandangan tidak dapat direalisasikan secara fisik namun hanya pura-pura dengan menampilkan dua pandangan tentang perspektif yang berbeda pada layar datar dan mengantarkannya ke mata kiri dan kanan yang sesuai. . Ketidakcocokan ini mengharuskan penampil untuk menimpa proses oculomotor yang digabungkan secara fisiologis dari fokus dan fokus mata, yang dapat menyebabkan ketidaknyamanan dan kelelahan visual. Perbedaan antara tampilan normal dan tampilan stereoskopik dengan display 3-D konvensional diilustrasikan pada Gambar 1. Penampakan alami memberi rangsangan nyata bahwa penampil keduanya terpaku dan fokus pada objek, jarak akomodasi dan jarak huniannya sama-sama cocok. Tapi situasi berubah untuk tampilan stereoskopis 3-D. Meskipun penampil masih terpaku pada objek dengan kehangatan yang sama seperti pada tampilan alami, fokus matanya sekarang ada di layar dan bukan di tempat benda itu berada. Itu karena mata pemirsa selalu fokus pada titik terang atau kontras tertinggi. Dengan display stereoskopis 3-D, kedalaman hanyalah ilusi optik. Oleh karena itu dengan tampilan stereoskopis, korelasi fisiologis normal antara vergensi dan akomodasi terganggu (Hoffman et al., 2008). Saat melihat tampilan stereoskopis untuk sementara, ketidaksesuaian kedalaman yang disebut antara konvergensi dan fokus ini menyebabkan ketegangan mata dan kelelahan. Masalah mendasar pada stereoscopic 3-D adalah fisiologis yang tidak bisa dipecahkan dengan cara teknologi. Satu-satunya solusi untuk menampilkan stereoskopik adalah untuk membatasi kedalaman pemandangan ke urutan yang sangat pendek (penayangan singkat) atau untuk mengurangi secara artifisial kedalaman pemandangan (kedalaman yang terjepit dan tidak proporsional). Yang disebut rentang kedalaman kenyamanan display stereoskopis, yang bisa digunakan untuk ilusi mendalam tanpa ketegangan mata dan kelelahan terbatas pada daerah yang dekat dengan layar yang kira-kira kira-kira 20. 30 jarak antara penampil dan tampilan. Hanya di wilayah ini mata manusia bisa mentolerir sejumlah ketidakcocokan. Menurut dokter mata, toleransi ini berada di kisaran 14 diopter. Meskipun stereo 3-D dapat bekerja dengan baik untuk beberapa aplikasi, misalnya bioskop dengan jarak pengamatan yang jauh atau ponsel dengan waktu pengamatan singkat, hal ini menyebabkan risiko faktor manusia yang signifikan untuk produk utama seperti monitor PC dan TV. Melihat display stereotip 3-D saat ini dan prototip, juga dapat diamati bahwa bahkan diopter 14 hampir tidak digunakan, sehingga membatasi jangkauan kedalaman yang dapat digunakan lebih jauh. Perbandingan antara tampilan alami atau tampilan holografik 3-D (kiri) dan tampilan stereoskopik dengan layar stereo 3-D (kanan). Oleh karena itu, keterbatasan yang melekat pada semua teknologi layar stereo 3-D dapat dirangkum sebagai berikut: Kedalaman ketidaksesuaian konvergensi dan akomodasi menyebabkan ketegangan mata dan kelelahan, Mengurangi rentang kedalaman kenyamanan memerlukan pendalaman yang tidak proporsional atau hanya memungkinkan waktu singkat. Melihat adegan dengan kedalaman yang besar, Potensi untuk penggunaan yang tidak tepat, dan oleh karena itu merupakan risiko aplikasi konsumen. Penting untuk dicatat bahwa rentang kedalaman kenyamanan dalam tampilan dan konten yang dihasilkan misalnya dalam film atau permainan bersifat independen. Hal ini menyebabkan risiko yang signifikan bahwa bahkan dalam tampilan stereoskopik 3-D yang benar-benar baik, konten yang tidak tepat akan membahayakan kenyamanan pengguna atau kesehatan dan mungkin (tidak adil tapi) mungkin dilakukan terhadap produsen display. Berlawanan dengan stereo 3-D, yang secara inheren menyebabkan kelelahan dan ketegangan mata untuk pemandangan kedalaman 3-D alami (yaitu kedalaman yang benar), holografik 3-D menyediakan semua informasi tentang pemandangan alami termasuk fokus mata dan kedalaman yang tak terbatas. Siapa pun yang bisa melihat 3-D dalam kehidupan nyata bisa melihat layar 3-D pada tampilan hologram tanpa kelelahan atau risiko konsumen lainnya. Tampilan holografik didasarkan pada rekonstruksi objek yang koheren. Mereka memberikan isyarat fokus penuh yang diperlukan untuk memberi pengamat pengalaman menonton 3-D yang benar-benar nyaman (Benton amp Bove, 2008). Kami telah mengembangkan dan berhasil menunjukkan pendekatan baru untuk holografi tampilan real-time berdasarkan teknik penyandian sub-hologram dan teknologi layar pandang yang dilacak. Solusi kami mampu memenuhi harapan pemerhati pada persepsi kedalaman nyata. 2.2. Holografi klasik dan rintangan bersejarah Holografi ditemukan oleh Dennis Gabor pada tahun 1947 (Gabor, 1948), namun hologram berkualitas tinggi hanya dapat dilakukan pada film fotografi, yang untuk alasan teknis menghalangi animasi. By this classic holography, the 3-D scene information is encoded in the entire hologram, i.e. every tiny region or pixel of the hologram contributes to each object point. The specialty of such holograms is well-known: If the hologram is broken into pieces, each piece will reconstruct the original scene, even though with less resolution and in smaller size. When the hologram is illuminated by the reference wave, the combination of all of its cells reproduces the complete scene by multiple interferences. A classic film hologram has a large diffraction angle, which means it creates a large angular spectrum. The viewing zone from which the reconstructed object can be seen is large, both eyes of the viewer fit into this zone and the viewer can even move around and see different perspectives of the scene. The difficulties arise when trying to apply the classic approach of holography to digital or electro-holography. The challenges of this approach are twofold: (a) the spatially sampled representation of the hologram by a light modulator (spatial resolution issue) and (b) the fast computation of the hologram (processing issue). Hence, one of the most serious restrictions of video holography has been the dynamic representation of the hologram by an electrically addressed spatial light modulator (SLM) having a pixelized structure with limited spatial resolution. The complex amplitude distribution that reconstructs the desired object or scene is calculated and represented at regular discrete locations, i.e. at the pixel positions of the spatial light modulator. Since the hologram is sampled, aliasing has to be prevented. Otherwise, improper reconstruction with image artifacts would occur. The amount of information that can be recorded in the hologram is directly related to the spatial resolution and the size of the SLM. This fact is represented by the dimensionless space-bandwidth product S B P v x b x v y b y b x 2 x b y 2 y with being the maximum spatial frequency according to the sampling theorem, b the width and the pixel pitch of the modulator in x and y direction (Lohmann, 1967 Lohmann et al. 1996 ). In general, the space-bandwidth product capability of an optical system is directly related to its quality and performance. For example, a present state-of-the-art LCOS microdisplay with 1920 1080 pixel resolution, a pixel pitch of 8 m and a total size of 0.7 gives a space-bandwidth product of 518,400. The Nyquist limit for the maximum spatial frequency is thus 62.5 Lpmm, which translates into a maximum diffraction angle of 2.27 . Principle of classic holography. In conventional holography, every hologram pixel contributes to each object point of the 3-D scene, that is, holographic information existing at a large viewing zone. Let us recall that in conventional holography the diffraction angle must be large to create a viewing zone that covers at least both eye and that different areas of the hologram encode the wave field originating from another perspective of the object (see Figure 2 ). In order words, the primary objective of conventional holography is to reconstruct the 3-D object in space, which can be seen by any viewer binocularly from different view points at different perspectives. To achieve a sufficient viewing zone, pixel sizes in the range of the one micron or less would required. Moreover, to create large objects and fully exploit the 3-D impression the display should be large. However, this corresponds to a huge amount of information that even if large SLM with tiny pixels would be available must still be handled in data processing and computing. To give an example, extreme-resolution displays with a pixel size of roughly 0.5 microns would be required, which translates to the huge demand for calculation of billions to trillions of complex values for each of the 2 million scene points (19201080) to determine an HDTV scene in 3-D. When considering these requirements, the insurmountable obstacles to realize conventional holography by using todays technology become immediately obvious. The reasons why all past attempts of transferring conventional holography to display and TV applications have heretofore failed can be summarized as follows: Insufficient display resolution: In order to achieve a viewing angle of 30. which is necessary to serve several users, a pixel pitch of about one wavelength or less is required. This means that for a 47-inch holographic display, for example, a resolution of 250,000 times that of HDTV is necessary. Inadequate data volume and processing requirements: The computation of each display frame requires significantly more steps for a holographic display compared to a 2-D display. Typical hologram computation involves calculations of Fourier transformations. This factor, coupled with the greatly increased number of pixels required, places a demand for enormous amounts of computational power. Thus, real-time videoquality holograms would typically require processing power up to several hundred Peta-FLOPS, i.e. approximately 10 17 floating-point operations per second. This is far more than the current computation power of super computers. 2.3. Full parallax vs. single parallax holography With full-parallax holograms, the holographic information is delivered in both x and y direction. When looking at a full parallax hologram, the perspective of the scene varies with the viewpoint no matter in which direction the observer is moving. In single-parallax holograms, on the other hand, the parallax information is sacrificed in one dimension. That way both the computational effort and data transfer can be substantially reduced. Because of the eyes are side by side, it is common practice to make horizontal-parallax-only holograms. A well-known example of a horizontal-parallax-only (HPO) hologram is the optically recorded white-light rainbow hologram invented by Benton (Benton, 1969. Benton amp H. S. Mingace, 1970 ). The concept of single parallax holograms was later successfully transferred to computational holography (St-Hilaire et al. 1992 ). Examples of full and parallax-limited holograms. The spherical phase of a simple single-point hologram is shown (kinoform of a point or Fresnel zone lens). HPO horizontal-parallax-only VPO vertical-parallax-only. Both benefits and limitations of full and parallax-limited holograms become obvious from Figure 3. which shows a very simple hologram and its parallax-limited versions. A full parallax hologram reconstructs the object point from a large area with spatial frequencies in all directions, which comes along with a large information content that must be all calculated, transferred by computer and spatially resolved by the light modulator. In comparison, a parallax-limited hologram that is a sliced version of the full type, diffract the light basically in one dimension. Beside the reduced computational effort, such a configuration is beneficial for other reasons as well. For example, the remaining pixel (or saved bandwidth) could be used for realizing hologram interlacing for different colors or just for simplifying hologram representation with a given display architecture. However, there are also tradeoffs with single-parallax holograms. As the diffraction occurs mainly in one direction, the diffracted wave is slightly elliptical and the spatial resolution of the reconstruction in the non-diffracted direction might be marginally reduced. However, when taking into account the resolution capabilities of the human eye and generate the hologram and the display system accordingly, the benefits of the parallax-limited holograms outweigh its constraints. It should be noted that SeeReals sub-hologram approach is inherently applicable to both encoding principles with similar gains in efficiency. 2.4. Brief review of previous and current approaches to electro-holography There have been many practical approaches to electro-holography in the past decades. Several of them are briefly presented in this chapter as examples. A pioneering holographic display was set up at the MIT Media Lab in S. A. Bentons group and continuously improved (St-Hilaire et al. 1992. Lucente et al. 1993. St-Hilaire et al. 1993 ). These systems use an acousto-optic modulator (AOM), scanners and an optical imaging system. High-frequency acoustic waves locally modulate the refractive index of the AOM crystal and thus the phase of transmitted light. The AOM generates a horizontal line of the hologram that is vertically continued by a vertical scanner. Recent progress was made with an improved AOM that allows higher bandwidth and a simplified optical setup (Smalley et al. 2007 ). The system is specified with a cube-like object volume with approximately 80 mm edge length and 24 viewing angle at a frame rate of 30 Hz. Another approach was made by QinetiQ using a so-called Active-Tiling technique (Stanley et al. 2003. Slinger et al. 2004 ). A SLM with 1 million pixels is replicated sequentially 25-fold on an optically addressable SLM (OASLM) using 5 x 5 replication optics. Four of these units are stacked horizontally to yield a SLM with 100 million pixels in total at a pixel pitch of 6.6 m. The modular system design allows stacking of more units to achieve higher numbers of pixels. A replay system with an Active-Tiling SLM with 100 million pixels achieved an object with 140 mm width and a viewing zone width of 85 mm at 930 mm distance. Direct tiling of SLMs is used for another holographic display (Maeno et al. 1996 ). Five SLMs with 3 million pixels each are tiled to yield 15 million pixels in total. The object may be as large as 50 mm, 150 mm high and 50 mm deep and can be viewed with both eyes at a distance of 1 m. Effort was also made to optimize the calculation of holograms. A computing system with dedicated hardware performs hologram calculation much faster than a PC. As an example, the HORN-6 cluster uses a cluster of boards equipped with FPGA chips (Ichihashi et al. 2009 ). The system needs 1 second to calculate a hologram with 1920 x 1080 pixels if the object is composed of 1 million points and 0.1 second if the object is composed of 100,000 points. All these approaches have in common that a large number of pixels is needed to reconstruct an object with small or medium size. These requirements for the SLM and the computing system hinder scaling to larger sizes, e.g. 20 object size with unlimited depth for desktop applications or TV. 3. SeeReals novel solution to real-time holography 3.1. Fundamental idea and overview The fundamental idea of our concept is fairly simple when considering holography even literally from an information point-of-view. All visual acuity is limited by the capabilities of the human eye, i.e. its angular and depth resolution, color and contrast sensitivity, numerical aperture, magnification, etc. where the characteristics of the eye may vary widely from individual from individual. It may additionally be confined by monochromatic and chromatic aberrations. The majority of optical instruments, such as visual microscopes or telescopes utilize the eye as the final element of the optical system. The eyes specific capabilities are thus taken into account in the optical system design. We view holography in the same way. When considering the human vision system regarding to where the image of a natural environment is received by a viewer, it becomes obvious that only a limited angular spectrum of any object reaches the retina. In fact, it is limited by the pupils aperture of some millimeters. If the positions of both eyes are known, it therefore would be wasteful to reconstruct a holographic scene or object that has an extended angular spectrum as it is common practice in classical holography. As mentioned above, in every part of a classic hologram the entire object information is encoded, cf. Figure 2. This means that a large viewing zone with parallax information within this zone exists by moving within this zone the viewer can look around the reconstructed object and thus sees different perspectives of the scene. This approach is historically explained by the interference-based exposure technique onto high-resolution holographic films and is useful for static holograms as known from artistic holographic recordings. The key idea of our solution to electro-holography is to reconstruct a limited angular spectrum of the wave field of the 3-D object, which is adapted laterally in size to about the humans eye entrance pupil, cf. Figure 4. That is, the highest priority is to reconstruct the wave field at the observers eyes and not the three-dimensional object itself. The designated area in the viewing plane, i.e. the virtual viewing window from which an observer can perceive the proper holographic reconstruction is located at the Fourier plane of the holographic display. It corresponds to the zero-order extension of the underlying SLM cross grating. The holographic code (i.e. the complex amplitude transmittance) of each scene point is encoded on a designated area on the hologram that is limited in size. This area in the hologram plane is called a sub-hologram. The position and size of the sub-hologram is defined by the position of object point and viewing window geometry. There is one sub-hologram per scene point, but owing to the diffractive nature of holography, sub-holograms of different object points may be overlapping. The complex amplitude transmittances of different sub-holograms can be added without any loss of information. Principle of viewing-window holography. With viewing-window holography the essential and proper holographic information exists at the eye positions only. So far, for the sake of simplicity, we have discussed the matter for a single viewing window, which carries the information for one eye only. But how is then parallax information generated Binocular view can be created by delivering different holographic reconstructions with the proper difference in perspective to left and right eye, respectively. For this, the techniques of spatial or temporal multiplexing can be utilized. For such a binocular-view multiplexed hologram, the reconstructed 3-D object can be seen from a single pair of viewing windows only. Advantageously, dynamic or real-time video holography offers an additional degree of freedom in system design with respect to temporal-multiplex operation. Given that the computational power is sufficient and the spatial light modulator is fast enough, the hologram can be updated quickly. By incorporating a tracking system, which detects the eye positions of one or more viewers very fast and precisely and repositions the viewing window accordingly, a dynamic 3-D holographic display can be realized that circumvents all problems involved with the classic approach to holography. The steering of the viewing window can be done in different ways, either by shifting the light source and thus shifting the image of the light source, or by placing an additional steering element close to the SLM that realizes a variable prism function. Selected implementations of steering principles will be explained in more detail in section 3.5. To summarize, the pillars of our holographic display technology are: Viewing-window holography: By limiting the information of the holographic reconstruction to the viewing windows, the required display resolution is decreased dramatically. Pixel sizes in the range of todays commercially available displays are sufficient. Real-time computation of sub-holograms: By limiting the encoding to sub-holograms, the computing requirements are greatly reduced. Sub-hologram encoding brings computation into graphics card or ASIC range. The principle also enables temporal color multiplexing, speckle reduction, and suppression of higher orders within the viewing window. Tracking of viewing windows: An active and real-time tracking of the viewing window allows a free movement of the observer. 3.2. The viewing-window and sub-hologram concept The optical principle of our holographic approach is schematically depicted in Figure 5. Coherent light coming from a point light source is imaged by a positive lens (L) into the observer plane and creates the spherical reference wave for hologram illumination. Very close to the imaging lens, the spatial light modulator (SLM) is positioned. 3.2.1. What is a viewing window The inherent regular SLM structure generates a diffraction pattern in the far field whose zero-order extension is the viewing window (VW) where the eye of the observer is located. Given small angles, the size of the viewing window is obtained from the grating equation and trigonometry to where the maximum allowable spatial frequency is given by the resolution of the spatial light modulator and must satisfy the relation (x) 1 2 x and (y) 1 2 y. Masing-masing. 3.3.2. Fourier-based modeling Figure 6 illustrates an alternative method for hologram synthesis that is based on fast Fourier transforms (FFT) of object planes. It shows a side view of a three-dimensional object or three-dimensional scene, the spatial light modulator (hologram), and the viewing window. The viewing window is positioned at or close to an observer eye. The 3-D-scene is located within a frustum that is defined by the edges of the viewing window and the SLM, respectively (drawn as red dashed lines in Figure 6 ). This frustum may be approximated by a pyramid if the viewing window is much smaller than the SLM. For calculation, the 3-D-scene is sliced in layers (L 1. . L m ) that are parallel with both the SLM and viewing-window plane. The continuously distributed object points are assigned to the closest layer. The extension of each layer is limited by the frustum and depends on the distance from the viewing window. With the approximation of the viewing window being much smaller than the SLM, the extension of a layer is proportional to its distance from the viewing window. FFT-based hologram synthesis. In each layer, the object points are assigned to the nearest sampling point of the layer. The calculation method uses Fresnel transforms between the object layers L 1. . L m . a reference layer L R in the observer plane and a hologram layer L H in the plane of the SLM. The calculation of a Fresnel transform can be mathematically performed as a Fourier transform and multiplication with quadratic phase factors (Goodman, 1996 ). The discrete Fourier transform can be efficiently executed by using the fast Fourier transform (FFT) algorithms. Hologram synthesis comprises three steps: Firstly, the layers L 1 to L m are transformed subsequently to the reference layer L R by m Fresnel-Transforms. Secondly, the wave fields calculated in the first step are summed up to a superimposed complex-valued wave field in the viewing window. This superimposed wave field represents the frequency-limited wave field that would be generated by a real existing 3-D scene. Thirdly, the superimposed wave field in the viewing window is back-transformed to the hologram layer L H by an inverse Fresnel transform. This yields finally the hologram function (x,y). The information in each layer is not continuous but sampled. It is essential that all object layers, the viewing window and the hologram layer L H contain the same number of sampling points N. This number corresponds to the pixel number of the spatial light modulator. As mentioned above, the extension of an object layer L m is proportional to its distance d m to the reference layer L R . Hence also the sampling interval p m in a layer is proportional to its distance d m to L R . i.e. p m d m . As a consequence, the periodicity interval w m in the reference layer L R is the same for all object layers L m . that is w m d m p m constant. This ensures that the periodicity interval in the L R has the same extension for all object layers. Therefore, a common viewing window that is located within one periodicity interval can be defined where the wave field within this viewing window is unique. As explained, in the third step the wave field in the viewing window is transformed to the hologram layer. The hologram layer and the reference layer are related by a direct or an inverse Fresnel transform. The number of sampling points N in each object layer is the same as in the layer L H on the SLM. As the viewing window is within one periodicity interval of an object layer, it is also within one periodicity interval of the L H or SLM. Hence, as the hologram reconstructs the wave field in the viewing window, this wave field will be unique therein. Periodic repetitions of this wave field that are inherent for sampled holograms are outside the viewing window. The hologram reconstructs the wave field that would be generated by a real existing 3-D scene in the viewing window. Disregarding reconstruction imperfections, an observer whose eyes are in one or two viewing windows will have the same perception as if the wave field emanates from a real existing object. Periodic repetitions of the reconstructed object are thus not visible. 3.4. Hologram encoding methods Hologram encoding refers to the representation of the complex wavefield at the hologram plane (x,y), i.e. to the process of converting the complex wavefield into a format, which can be displayed at the SLM by addressing its pixel. Hologram encoding is therefore directly related to the hardware implementation of the SLM. In synthetic or digital holography, a fully complex representation would be most qualified. But the major challenge is in finding a method and device to record a complex-valued hologram transmission function. Generally speaking, there are various possibilities for a spatially sampled representation of complex wavefields by spatial light modulators: Complex representation: A spatial light modulator that provides a full complex-valued modulation would be the ideal, whereas independent, non-coupled amplitude and phase addressing is mandatory. Although one can think of such SLM, which may implement the detour-phase principle for example, but thus far such devices are non-existent. Another possibility would be a sandwich of two active modulation layers, which are independently controlled for amplitude and phase modulation (Gregory et al. 1992 ). The challenge is then to put them together as close as possible to avoid cross talk. Both concepts seem difficult to realize even with todays enabling technologies. Decomposition methods: Since the very beginning of computational holography decomposition methods for complex-valued wavefields have been developed 1 - . Famous examples of holograms utilizing the detour-phase concept are those from Brown and Lohmann (Brown amp Lohmann, 1966 ), Lee (Lee, 1970 ), Burckhardt (Burckhardt, 1970 ) and the double-phase holograms from Hsueh and Sawchuck (Hsueh amp Sawchuk, 1978 ). All methods have in common that the hologram is divided into discrete resolution cells having apertures or stops of different size and position or having a certain number of sub-cells. That way both amplitude and phase quantities can be approximated. Originally developed for static holograms or holographic filters, those methods are also suited for implementation with spatial light modulators. However, to modulate both amplitude and phase two or more sub-pixels have to be combined to one macro-pixel. That means part of the light modulators original resolution has to be sacrificed for the sake of full holographic modulation. In the following subsections, two decomposition methods capable for SLM implementation are described in more detail. 3.4.1. Burckhardt amplitude encoding One method to decompose a complex-valued function is the method suggested by Burckhardt (Burckhardt, 1970 ), which is a simplified version of Lees original approach (Lee, 1970 ). One hologram cell is laterally divided into three amplitude-modulating sub-cells. The lateral shift between the sub-cells represents phase angles of 0. 120 and 240 and acts as a phase offset, similar to the detour-phase principle. In holograms of this type, a phasor is decomposed into three vectors that run parallel to exp(i0) 1, exp(i23) 0.5 and exp(i43) 0.5 . Since the phase values are already represented by the lateral displacement of the sub-cells, any complex amplitude transmittance Aexp(i) can be encoded in one macro-pixel. The laterally displaced sub-pixels have positive amplitude transparencies of A 1 ,A 2 ,A 3 . Masing-masing. Hence, H ( x. y ) A 1 ( x. y ) exp i 0 A 2 ( x. y ) exp i 2 3 A 3 ( x. y ) exp i 4 3 As a result, a fast phase-only LC panel can be used as light modulator. A pair of two pixels of a phase-only modulating SLM is then combined into a complex-valued macro pixel (Birch et al. 2000 ). Both pixel act as the intended complex-valued macro pixel only if light modulated by both pixel is superimposed. A physical combination of light modulated by two phase subpixels may be achieved by beam-combining micro-elements. The hologram is encoded by first normalizing the amplitudes of the complex amplitude transmittance in a range from 0 to 1 and then calculating the phase quantities 1 . 2 from the equations above. Because of its phase coding, the diffraction efficiency of dual-phase holograms is compared to Burckhardt-type holograms greatly increased to approx. 10. Again, only a single active layer is required for representing the entire hologram information. Since only two sub-pixels have to be combined to one macro-pixel a better sampling at the hologram plane is being present. 3.5. Tracking methods For a non-tracked viewing-window-type hologram, the reconstructed 3-D object could be seen from a single or pair of viewing windows only. However, SeeReals approach to dynamic holography is directly related to eye tracking. In case of a movement of the observers eyes, the observer window is tracked to the new eye position. Hence it is possible to reduce the size of a viewing window to the size of approximately an eye pupil. Two viewing windows, i.e. one for the left eye and one for the right eye, are always located at the positions of the observer eyes. But then, how to move the viewing window in the observer plane Advantageously, dynamic holography offers the additional freedom of temporal-multiplex operation. By incorporating a tracking system that detects the eye positions of one or more viewers very fast and precisely and repositions the viewing window accordingly, a dynamic 3-D holographic display can be realized. Thus, the viewing angle of the reconstructed object is beneficially enlarged while maintaining the moderate resolution of the spatial light modulator. Tracked viewing-window holography must therefore fulfill the following key functions: Detection of the current eye position in x,y, z and Means for shifting the observer window to this position. To realize the former task, the holographic displays developed by SeeReal are equipped with an eye position detection system composed of a stereo camera and an imaging processing means. Images from the observer having a different perspective are captured by two cameras as exemplary shown in Figure 9. Multi-threaded software that comprises of image processing, pattern recognition and artificial intelligence is working in a two step process. In a first step, the face of the observer is recognized within the captured image and afterwards the eyes are detected within the region of the face. Once the face is identified, only the eye detection algorithms have to be executed, which makes the entire recognition process much faster. The results obtained from left and right image of the stereo camera are then combined to a 3-D model that defines the position of the eyes in space. Figure 10. Images captured by the tracking cameras (left and right view of a stereo camera). The current system is capable to track simultaneously up to 4 viewers in real-time. We have developed different alternatives for the tracking means, two of them are explained here in more detail. The steering of the viewing window can be done for example by shifting the light source and thus shifting the image of the light source accordingly, or by placing an additional element close to the SLM that realizes a variable deflection. In the following, we discuss implementations of these alternatives. 3.5.1. Light-source shifting The first principle that was developed and implemented in prototypes is based on light source shifting. The optical principle is schematically sketched in Figure 10. By imaging through a lens, a shift of any light source in object space results in a shift of its image. Since the viewing window is located within the zero order of the spatial light modulator, the holographic reconstruction as can be viewed from the viewing window will be always correct. From a holographic way of thinking, this corresponds to an illumination of the hologram with a tilted reference wave. When a single lens is used, this method allows in principle for a tracking in x,y and z direction. However, there is a practical limit for x and y shifting because of the paraxial limit of the lens. A skew ray path introduces aberrations that, if too large, may deteriorate the holographic reconstruction quality. Although aberrations can be compensated by encoding means, a practical limit has been identified at approximately 10 . Figure 11. Schematic principle of the light source tracking method. The position of the light source does not have to be shifted mechanically. One possibility would be an active array with a large number of light sources only one of them switched on at the same time. Tracking would then be performed by switching between several light sources. Each light source position then corresponds to a distinct tracking position in the viewing plane. Another possibility is the use of a secondary light source. A secondary light source might be an activated pixel in an additional liquid crystal display (LCD) that is illuminated by a homogenous backlight. By activating a pixel at the desired position on the LCD, the light source can be shifted electronically without mechanical movement. Tracking by light-source shifting has been successfully implemented in the prototypes by using a homogenous backlight and an LCD-shutter panel, cf. section 4. For the prototypes however, not a single lens and a single (secondary) light source is used, but a matrix of simultaneously emitting light sources and lenses is utilized instead. For a large display a single lens wouldnt be a feasible solution because of its thickness, weight, costs and display compactness. The shutter pixels act as secondary light sources. By switching on different shutter pixel, the light source position can be changed. An LED array (primary light sources) is used for illuminating the shutter. Pitch of the lens array needs to be large compared to pixel pitch of the SLM such as to still have a certain number of pixels with coherent illumination. Light source tracking has been proven to be a reliable solution. On the other hand, it has also certain disadvantages. For example, the use of secondary light sources is not optimal in terms of light efficiency of the system. Also there may be illumination crosstalk by light from secondary sources passing the wrong lens of the lens array. This does not cause any problem in a single-user system but may be disadvantageous for a multi-user display. The most important drawback of light source tracking is the limitation of the tracking angle by aberrations. Large tracking angles put the need for an oblique optical path from the light source through the lens array. Aberrations may not necessarily degrade the reconstruction of single points, but might somewhat corrupt the observer window leading to vignetting effects in the reconstruction. While light source tracking may be well suited for a single user display with a tracking range of about 10. it is less practicable for multi-user displays and large tracking ranges, as needed for example for TV applications. 3.5.2. Steering of the reconstruction Since the capabilities of the previous tracking method are limited in terms of tracking range, alternative solutions that enable larger ranges have been developed. The conceptual design of a holographic display that steers the holographic reconstruction is shown in Figure 11. With a beam steering element placed at the front of the hologram display, the optical path from the light source to the SLM can be kept constant. As an advantage, the hologram is always illuminated by the same planar wavefront, which is ideal in terms of light efficiency and aberrations. The beam-steering element deflects the light after passing the SLM and directs the light toward the observer eyes. In addition to the prism function, it could realize a focusing function. There exist various promising approaches to nonmechanical beam steering, which are currently at different stages of development (McManamon et al. 2009). The challenge of such beam steering devices is that often both a large deflection angle and a large aperture of the deflector are required. Refractive solutions are thus not suitable because of the thickness a prism would have. But when the optical system operates at coherent or narrow-band light, diffractive approaches can be utilized. For a transmission grating with a local period of , the angle of the diffracted light is given by the grating equation sin m m sin i n where m is the diffraction order, the wavelength of light and in the angle of the incident light. Such variable diffractive gratings can be divided into two categories. Either a sawtooth-like grating is adjustable in its period, or in its blaze angle. The variable period grating most often operates at the first diffraction order (m 1) and the maximum steering angle is defined by the gratings smallest permissible period. The minimum period arises from diffraction efficiency requirements at a given angle as well as the addressing resolution of the grating. The steering up to the maximum angle is beneficially continuous. Variable blaze gratings on the other hand have a fixed period and diffract the light into the designated order by matching the blaze angle to the diffraction order m. Since the variable blaze grating type steers light only at discrete angles, an extra variable period grating stage is required for continuous steering between those angles. Figure 12. Schematic side-view of the steering-of-reconstruction principle. is the tracking angle and z is the focal length variation of the nonmechanical steering element. Light could for example be steered and focused by writing a phase function including a prism and focus term into a liquid crystal layer. The effective refractive index and hence the deflection angle is controlled by a voltage applied to electrodes at the cells. Embodiments as variable period grating as well as variable blaze gratings can be realized. Another steering or tracking concept for the holographic reconstruction is based on electrowetting. Electrowetting or exactly electrowetting on dielectrics (EWOD) (Beni et al. 1982. Berge amp Peseux, 2000 ) can be regarded as an electrostatic manipulation of liquids that enables to vary the wettability of a conducting liquid (Mugele amp Baret, 2005 ). The conductive liquid and an electrode are separated by a thin dielectric hydrophobic layer thus forming a parallel plate capacitor. By applying a voltage between the electrode and the conductive liquid, the droplet wets the hydrophobic dielectric. Without a voltage, the droplet returns to the dewetted state, i.e. to its initial contact angle 0 . Since the thin dielectric layer prevents the liquid from electrolysis, the process is highly reliable. Below a critical saturation threshold, the behavior of electrowetting-on-dielectrics can be well-predicted by the so-called electrowetting equation cos v cos r 0 2 t l a V 2 which can be derived from Lippmanns electrocapillary equation and Youngs equation for a three-phase contact line. In this equation, 0 is the initial contact angle at zero voltage, and t are the dielectric constant and the thickness of the dielectric layer, respectively, V is the applied voltage and la is the interfacial surface tension of the liquid-ambient (typically electrolyte-oil) interface. In recent years, electrowetting has been successfully applied to various optical applications such as varifocal lenses, amplitude-modulating displays and fiber coupler and switches. How electrowetting can be applied for realizing a liquid prism is illustrated in Figure 12 (Kuiper et al. 2005. Smith et al. 2006 ). At initial state with no voltage, the liquids form a curved meniscus, depending on the interfacial surface tensions between the liquids and the solid. Since here the sidewall is hydrophobically coated, the water-based electrolyte features a large initial contact angle of 0 gt 150 (Figure 12 left, L R 0 ). If a voltage difference is applied between the insulated sidewall electrode and the electrolyte, the contact angle of a conducting droplet can be decreased. At a certain voltage pair of equal voltages, the contact angle at both left and right electrode reaches 90 resulting in a flat meniscus (Figure 12 middle, L R 90 ). The light passing through such a cell will not be altered in its propagation direction. Prism functionality can be realized if the sum of left and right contact angle equals to 180 (Figure 12 right, L R 180. where L R ). Figure 13. Operation principle of an electrowetting prism. For simplicity and visualization, here only a prism with 1D-deflection capability (2 sidewall electrodes) has been drawn. A prism capable for 2D-deflection comprises of 4 sidewall electrodes. It is advantageous to minimize the size of the prisms to get faster response, because dynamic response scales with the volume density product of the used liquids. Therefore, the intended prism size is adapted to the pixel pitch of the SLM. Since the response time of electrowetting cells of that size are in the range of lt1 ms, time sequential tracking of several users becomes feasible. 3.6. Color holography As holography is based on diffraction and as diffraction is wavelength-dependent, the 3-D scene has to be separated in its color components. Usually, these are red, green and blue. Three holograms are computed (one for each color component) and the 3-D scene is reconstructed using three light sources with the corresponding wavelengths. There are several methods to combine the three holograms and the three light sources, for example: Spatial multiplexing. The red, green and blue holograms are spatially separated. For instance, they may be displayed on three separate SLMs that are illuminated by red, green and blue light sources. An arrangement of dichroic beamsplitters combines the output of the SLMs. The optical setup is bulky, above all for large displays. Temporal multiplexing. The red, green and blue holograms are displayed sequentially on the same SLM. The red, green and blue light sources are switched in synchronization with the SLM. Fast SLMs are required to avoid color flickering. 4. Implementations and prototypes Our holographic approach has been successfully demonstrated by prototypes having 20.1- inch diagonal 3 - . The prototypes are intended to demonstrate the key principles of our solution for large-sized real-time holography, i.e. viewing-window holography with sub-hologram encoding technique, cheap and interactive real-time computing, and the feasibility with common pixel sizes. However, it should be emphasized that the prototypes do not represent commercial solutions with a flat design and are not at all optimized in intensity and tracking performance. Although commercial solutions that fulfill the latter features have already been developed, they are not described in this section. Figure 14. Optical principle of the 20.1-inch holographic display prototype. Sizes and distances are not to scale. 4.1. General description of components The second generation of the direct view holographic display prototype (VISIO 20) comprises a grayscale amplitude-modulating liquid crystal panel (NEC NL256204AM15-01) with a 3 5 megapixel resolution at a pixel pitch of p x 156 m and p y 52 m, an operation frequency of 60 Hz with a relatively slow response time of 30 ms (Figure 14 ). The used 1D hologram encoding (here vertical-parallax only) is a common practice to further reduce bandwidth requirements and is well-suited for the given pixel arrangement and geometry. Figure 15. a) 20.1-inch direct view prototype (VISIO 20) and (b) photograph of a holographic reconstruction. The optical scheme of the prototype is depicted in Figure 13. An LED backlight consists of red, green and blue high-brightness LEDs emitting at wavelengths of 627 nm, 530 nm, and 470 nm, respectively. The spectral linewidth (FWHM) of 30nm provides sufficient temporal coherence. Light coming from the RGB-LED backlight is mostly blocked by a first LC display that acts as shutter or variable secondary light source array. Only those pixels that are switched on transmit the light, and thus a variable (secondary) line light source is realized having a spatial coherence corresponding to the pixel opening. A lenticular comprising approximately 60 horizontal cylindrical lenses is used for hologram illumination and for imaging the light sources into the viewing window. Each cylindrical lens is illuminated by a horizontal line light source. Furthermore, secondary line light sources and arrayed cylindrical lenses are aligned such that all light source images coincide onto the viewing window. In the SLM the sum of all complex amplitudes U n (x,y) is encoded by combining three amplitude-modulating pixels for each complex value according to the Burckhardt-encoding scheme described above (cf. section 3.4). Two viewing windows delivering slightly different holographic perspectives of the scene are generated by a vertically aligned lenticular beam-splitter and an interlaced (horizontally multiplexed) hologram. High-precision user tracking is realized by a stereo camera incorporated in the holographic display and advanced eye recognition algorithms combined with active light source shifting by the shutter panel. 4.2. Color implementation In the prototypes, holographic reconstruction is performed either in monochrome (optionally R, G, or B) or in full-color. Two types of full-color holographic displays have been realized that are based on either a temporal or a spatial multiplexing of colors (Hussler et al. 2009 ). At the system for which temporal color multiplexing is implemented, the colors are displayed sequentially. The SLM displays the holograms of the red, green and blue 3-D scene components one after the other where the backlight is switched between red, green and blue LEDs. Both processes are synchronized. However, two obstacles have to be taken into account to achieve good reconstruction quality: The pixels of the SLM have a finite response time. For a LCD, this is the time the liquid crystals need to align to the electric field applied to the pixel cell. The LCD panel that we use as SLM stems from a medical LCD and has a long response time t on t off of typically 30 ms. The pixels do not switch simultaneously across the SLM as the pixels are addressed in columns and rows. The rows of the SLM are addressed sequentially, with one frame period needed from the first to the last row. As a consequence, there is a time lag of up to one frame period across the SLM. Both effects have to be taken account as each part of the hologram has to be illuminated with the corresponding wavelength. For instance, if SLM and backlight were switched from red to blue simultaneously, the last rows of the SLM would still display the red hologram when the backlight is already switched to blue. Therefore, we used a scanning backlight and a time lag between switching the SLM and the backlight. Figure 15 illustrates this process. Figure 16. State of SLM rows (top) and backlight rows (bottom) versus time. The SLM graph illustrates the effect of finite response time and row-by-row addressing of the SLM after switching from one hologram to the next hologram. The backlight graph shows delayed and row-by-row switching of the backlight to compensate these effects. The top graph shows the states of the SLM rows versus time and the bottom graph the states of the backlight rows versus time. The gradual color transition along the time axis of the SLM graph illustrates the finite response time after switching from a hologram of one color component to the hologram of the next color component. There is no sharp transition from one hologram to the next hologram but an intermediate interval in which the pixels of the SLM transit to the next state. The color transition along the row axis of the SLM graph illustrates that the SLM is addressed row-by-row. At a point in time at which the last row has just received the data of the current frame, the first row will already receive the data of the next frame. As an example, at the second dotted vertical line, the last row has just settled to the red hologram, whereas the first row already starts to transit to the green hologram. The intermediate states are indicated by the slanted gradual color transition. At these points in time, the state of the respective SLM pixel is undefined, and illumination by the backlight has to be avoided. Therefore, we built a scanning backlight in which the rows of LEDs are grouped in 16 groups. Switching of these groups is illustrated in the backlight graph of Figure 15. These groups are switched on and off sequentially such that the corresponding parts of the hologram are only illuminated if its pixels are in a settled state of the associated color. A complete cycle comprises three frames with colors red, green and blue and three intermediate transition frames. As the frame rate of the SLM is 60 Hz, the full-color frame rate is 10 Hz. The human vision perceives a full-color holographic reconstruction, albeit with color flickering. Color flickering will disappear and a steady reconstruction will be visible with availability of faster SLMs. In contrast, the display with spatial color multiplexing shows the three backlight colors and the three holograms for red, green and blue color components simultaneously. The three holograms are interlaced on the same SLM. A color filter is used to achieve that each hologram is illuminated with its associated wavelength only. Six holograms are interlaced on the SLM: three red, green and blue holograms that generate the viewing window for the left eye VW L and also three holograms to generate that for the right eye VW R . A beam-splitting lenticular is used to separate the light for left and right viewing window, VW L and VW R . Color filters are used to separate the wavelengths. Figure 16 illustrates top views of two possible arrangements of color filters. The left arrangement uses color filters that are integrated in the SLM pixels. One lens of the beam-splitting lenticular is assigned to two pixels of the SLM. The light of all left pixels at the lenses coincides in the observer plane and generates VW R . Vice versa the right pixels generate VW L . The color filters are arranged in columns such that each column of the filter extends over two columns of the SLM, as illustrated in the left graph of Figure 16. Such an arrangement of color filters integrated in the SLM pixels and two neighboring pixels having the same color is not commercially available. Standard LCD panels have color filters with color changing from pixel to pixel. An external color filter laminated on the cover glass of the panel would have a disturbing separation between pixel and color filter. Therefore, in our prototype we used the arrangement illustrated in the right graph of Figure 16. The color filters are attached directly to the structured surface of the beam-splitting lenticular. This arrangement avoids a disturbing separation between lenticular and color filter and facilitates tracked viewing windows in the same way as with a monochrome display. The functional principle is analog to that of the arrangement in the left graph of Figure 16 . Figure 17. Top view of an arrangement of color filters in a holographic display with spatial color multiplexing. The left graph shows color filters integrated in the SLM pixels (SLM CF) and the right graph separate color filters (CF). The graphs show three lenses of the positive lenticular (L) that splits the light to generate left viewing window (VW L ) and right viewing window (VW R ). For simplification, only the light illuminating VW R is shown and the light illuminating VW L is omitted. The light sources and the Fourier-transforming lenses are not shown. 5. Conclusions In conclusion, a novel approach for real-time holography that has a strong market potential for desktop, TV, as well as mobile displays has been presented. To date, it is the only practical solution known to the authors that is capable of holographic reconstruction of large 3-D scenes of natural size and depth made with commercially available component technologies. The essential idea of the proprietary and patented approach is that for a holographic display the highest priority is to reconstruct the wavefront at the eye position that would be generated by a real existing object and not to reconstruct the object itself. The tracked viewing-window holographic technology limits pixel size to levels already known for commercially available displays. Sub-hologram encoding brings computation into graphics card or ASIC range. The new concept is applicable to desktop, TV, and mobile imaging. While there have been impressive developments in 3-D display technology in the past decade, the remaining visual conflicts between natural viewing and 3-D stereo visualization have prevented 3-D displays from becoming a universal consumer product. In principle, the only 3-D display capable of completely matching natural viewing is an electro-holographic display. 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