Wearable near-eye displays for virtual and augmented reality (VR/AR) have seen enormous growth in recent years. While researchers are exploiting a plethora of techniques to create life-like three-dimensional (3D) objects, there is a lack of awareness of the role of human perception in guiding the hardware development. An ultimate VR/AR headset must integrate the display, sensors, and processors in a compact enclosure that people can comfortably wear for a long time while allowing a superior immersion experience and user-friendly human–computer interaction. Compared with other 3D displays, the holographic display has unique advantages in providing natural depth cues and correcting eye aberrations. Therefore, it holds great promise to be the enabling technology for next-generation VR/AR devices. In this review, we survey the recent progress in holographic near-eye displays from the human-centric perspective.

Toward the next-generation VR/AR optics: a review of holographic near-eye displays from a human-centric perspective.

H J Caulfield (ed) 1979 New York: Academic xiv + 638 pp price $55 This book is for people who want to use holography. It is intended as a reference work but has a textbook approach which makes it enjoyable to read and the 31 authors who have contributed to ten chapters have achieved in one manageable volume a good up to date account of optical holography, its practice and applications.

Handbook of Optical Holography

The noninvasive, fast acquisition of quantitative phase maps using digital holographic microscopy (DHM) allows tracking of rapid cellular motility on transparent substrates. On two-dimensional surfaces in vitro, MDA-MB-231 cancer cells assume several morphologies related to the mode of migration and substrate stiffness, relevant to mechanisms of cancer invasiveness in vivo. The quantitative phase information from DHM may accurately classify adhesive cancer cell subpopulations with clinical relevance. To test this, cells from the invasive breast cancer MDA-MB-231 cell line were cultured on glass, tissue-culture treated polystyrene, and collagen hydrogels, and imaged with DHM followed by epifluorescence microscopy after staining F-actin and nuclei. Trends in cell phase parameters were tracked on the different substrates, during cell division, and during matrix adhesion, relating them to F-actin features. Support vector machine learning algorithms were trained and tested using parameters from holographic phase reconstructions and cell geometric features from conventional phase images, and used to distinguish between elongated and rounded cell morphologies. DHM was able to distinguish between elongated and rounded morphologies of MDA-MB-231 cells with 94% accuracy, compared to 83% accuracy using cell geometric features from conventional brightfield microscopy. This finding indicates the potential of DHM to detect and monitor cancer cell morphologies relevant to cell cycle phase status, substrate adhesion, and motility. © 2017 International Society for Advancement of Cytometry.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/cyto.a.23316

Quantitative assessment of cancer cell morphology and motility using telecentric digital holographic microscopy and machine learning.

This work was supported by Ministerio de Economia, Industria y Competitividad (Spain) under projects FIS2017-82919-R (MINECO/AEI/FEDER, UE) and FIS2015-66570-P (MINECO/FEDER), and by Generalitat Valenciana (Spain) under project PROMETEO II/2015/015.

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Roadmap on holography

Augmenting reality (AR) holds many benefits in how people perceive information and use it in their workflow or leisure activities. A cohesive AR experience has many components; nevertheless, the key is display technologies. The current industry standard for the core solution is still conventional stereoscopy, which has proven to be inadequate for near-work due to the caused vergence–accommodation conflict and the inability to precisely overlay the 3D content on the real world. To overcome this, next-generation technologies have been proposed. While the holographic method holds the highest potential of being the ultimate solution, its current level of maturity is not sufficient to yield a practical product. Consequently, the next solution for near-work-capable AR displays will be of another type. LightSpace Technologies have developed a static multifocal display architecture based on stacked liquid crystal-based optical diffuser elements and a synchronized high-refresh rate image projector. A stream of 2D image depth planes comprising a 3D scene is projected onto respective physically-separated diffuser elements, causing the viewer to perceive a scene as continuous and having all relevant physical as well as psychological depth cues. A system with six image depth planes yielding 6 cpd resolution and 72° horizontal field-of-view has been demonstrated to provide perceptually continuous accommodation over 3.2 Diopter range. A further optimization by using a conventional image combiner resulted in the compact and practical design of the AR display.

https://www.mdpi.com/2076-3417/9/15/3147/pdf

AR Displays: Next-Generation Technologies to Solve the Vergence–Accommodation Conflict

Holographic optical element has a high diffraction efficiency and a narrow-band frequency characteristic, and it has a characteristic that is able to implement several features in a single flat device. It is widely applied in various fields. In this chapter, the principle and characteristics of the holographic optical elements are described in detail, and few typical holographic optical element-based applications, such as head-mounted display, lens array, and solar concentrator, are introduced. Finally, the futuristic research concepts for holographic optical element-based applications and contents are discussed.

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Holographic Optical Elements and Application

We propose a multi-depth three-dimensional (3D) image cryptosystem by employing the phase retrieval algorithm in the Fresnel and fractional Fourier (Fr-FrF) domains. Encryption was realized by applying the phase retrieval algorithm based on the double-random-phase-encoding architecture in which two encryption keys will be incessantly updated in each iteration loop. The phase-only functions (POFs) are generated in two cascaded Fr-FrF transforms (Fr-FrFT), serving as decryption keys to efficiently reduce the speckle noise and crosstalk between encrypted 3D image depths. The use of Fr-FrFT position parameters and fractional order as decryption keys further extended the key space, enhancing the cryptosystem's security level. Numerical simulations demonstrated the feasibility and robustness of our proposed scheme.

Multi-depth three-dimensional image encryption based on the phase retrieval algorithm in the Fresnel and fractional Fourier transform domains

Holographic optical elements can be fabricated to concentrate the solar energy with no sun tracking system. Concentrated diffraction efficiency (CDE) for the holographic solar concentrator is suggested as a new efficiency calculation method. The exposure response of a monochromatic hologram recorded in photopolymer film is presented. The iterative exposure schedules at three different angles are chosen to optimize the uniformity of the CDE using angular multiplexed technique. The experimental results confirm that the angular multiplexing method and newly proposed iterative recording scheduling are appropriate for fabricating the holographic solar concentrator.

Holographic Solar Energy Concentrator Using Angular Multiplexed and Iterative Recording Method

Holographic optical element is widely developed in augmented reality, virtual reality, and three-dimensional display application. Especially, the full-color HOE has been studied for the wearable device in these days. In this chapter, the basic theories and the specific analysis for the full-color holographic optical element are explained. It also explores the Bragg angle shift phenomenon caused by shrinkage of the recording materials. The full-color holographic optical element with enhanced diffraction efficiencies using the optimum recording intensities for each wavelength is presented. The fabricated full-color holographic optical element can be applied in augmented reality application. In addition, we reviewed the waveguide headmounted display system using full-color holographic optical element.

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Full-Color Holographic Optical Elements for Augmented Reality Display

With the development of the holographic printer, printing synthetic hologram requires smaller holographic element (hogel) size to improve spatial resolution of the reconstruction. On the contrary, a larger hogel size affords higher angular resolution, but it leads to a lower lateral resolution and there exists a trade-off problem. In this paper, a hologram synthesis method based on three-dimensional (3D) rendering of computer-generated holographic stereogram (HS) is proposed to limit the spatial-angular trade-off problem. The perspectives of the 3D scene are captured by re-centering the camera method and transformed into parallax-related images by a proposed pixel re-arrangement algorithm for holographic printing. Unlike the conventional approaches, the proposed algorithm not only improves the angular resolution of the reconstruction while maintaining the hogel size fixed, but also keeps the spatial resolution without degradation. The effectiveness of the proposed method is verified by numerical simulation and an optical experiment.

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Hui-Ying Wu

Papers

Holographic Optical Elements and Application

Multi-depth three-dimensional image encryption based on the phase retrieval algorithm in the Fresnel and fractional Fourier transform domains

Holographic Solar Energy Concentrator Using Angular Multiplexed and Iterative Recording Method

Full-Color Holographic Optical Elements for Augmented Reality Display

Efficient Hogel-Based Hologram Synthesis Method for Holographic Stereogram Printing