It has become a tradition for every topical meeting on digital holography (DH) to be succeeded by a feature issue in Applied Optics (AO). The last feature issue was jointly published by AO and Chinese Optics Letters; this time, it is joint between AO and the Journal of the Optical Society of America B (JOSA B). Last year’s topical meeting, which was part of the Imaging Congress held in Heidelberg, Germany, was a great success, with 92 oral presentations along with 15 invited talks, 2 tutorials, 2 keynote addresses, and 51 posters, adding up to a grand total of 162. The DH Topical Meeting is the world’s premier forum for science, technology, and applications of digital holography, and 3D imaging and display methods. It is therefore not surprising that this feature issue has a record number of excellent contributions, of which 31 are being published in AO and 11 in JOSA B. Four of the papers in AO are invited papers, while there are two invited papers in JOSA B. We would like to thank all the contributors and the reviewers, without whom this feature issue would not have been possible. We would also like to thank Grover Swartzlander for his foresight and encouragement to have a part of this feature issue in JOSA B and Ronald Driggers for enthusiastically supporting the joint issue in AO and JOSA B. Thanks are also due to Ting-Chung Poon, who started the DH meeting 10 years ago with the support of Joseph Mait. A special thanks to Hoonjong Kang, who co-chaired the DH meeting in Heidelberg and who is also chairing the meeting this year. Finally, we would like to recognize the chairs, co-chairs, and organizers of DH 2017 at Jeju Island, South Korea, and wish them the very best of success in the upcoming meeting from 29 May to 1 June.

Based on the classification of topics in DH 2016, we have attempted to organize the papers into the following categories: DH microscopy, advances in DH techniques, 3D imaging and display, computer generated holography, metrology and profilometry, and THz DH. A couple of new categories have been added, viz., phase retrieval, and recording materials and techniques. While authors have been free to submit in either AO or JOSA B depending upon the focus of each journal, it is clear that there is considerable overlap with only a fine boundary.

The papers in AO show a wide spectrum (pun unintended!) of applications of DH and recent advancements in DH techniques. The first two papers are in the area of DH microscopy. The first of these, by Abdelsalam and Yasui [1], show DH microscopy for 3D visualization of an in-vitro sandwiched biological sample using a frequency doubled femtosecond pulsed illumination for coherent noise suppression. The second paper, by Yuan et al. [2], demonstrates the use of sinusoidal structured illumination microscopy as a wide-field imaging technique to achieve resolution enhancement, similar in principle to the work by Hussain et al. in JOSA B (below).

The next ten papers pertain to advances in DH techniques. The invited paper by Vijayakumar et al. [3] provides noise improvements on coded aperture correlation holography for recording incoherent digital holograms of general three-dimensional scenes through modifying the reconstruction method, as well as integrating a quadratic phase function, as used in Fresnel incoherent correlation holography (FINCH), with a random coded phase mask. The second paper by Choi et al. [4] proposes optically suppressing the defocus noise in FINCH by making a pinhole on the linear polarizer. Based on FINCH, the third paper, by Man et al. [5], proposes a self-interference compressive digital holography with improved axial-resolution and signal-to-noise ratio and has the potential of super-resolution imaging. The next two papers pertain to the use of multi-wavelength digital holography (MWDH). The first, by Flasseur et al. [6], proposes a parametric inverse problem approach to achieve the self-calibration of a digital color holographic setup in lensless in-line digital color holography used in biomedical imaging and microfluidics. The second paper, by Tahara and Arai [7], demonstrates single-shot MWDH with an extremely large incident angle (approximately 40 deg) for digital recording of multiple objects at multiple wavelengths to generate interference light. The issue of determining the perfect reconstruction (or focusing) distance during DH reconstruction is the general topic of the next two papers. The first, by Mohammed et al. [8], propose a quality assessment of several focusing criteria for imaging in digital off-axis holography. The second, by Lyu et al. [9], demonstrates a DH autofocusing method that computes focused distance using the first longitudinal difference of the magnitude of the reconstructed image. The next two papers assess the contribution of noise and speckle. The first, by Chen et al. [10], analyzes the defocus noise in the focal plane sweeping based light field reconstruction technique, and proposes a method to reduce this noise. The second, by Zhang et al. [11], addresses the issue of degradation of one-shot DH imaging due to laser speckle, and proposes various shapes of resampling masks in the spatial domain for speckle reduction. The tenth paper, by Zhu et al. [12], discusses the use of sinusoidal phase modulation in DH and DH interferometry by utilizing a generalized lock-in technique for optimizing signal extraction.

The fundamental advantage of holography is that it enables 3D imaging and display. A full-color holographic display with increased viewing angle is implemented by Zeng et al. [13]. In this invited paper , they describe its realization using two tiled phase-only spatial light modulators, a $4f$ concave mirror system and a temporal-spatial multiplexing synchronization control method. The next two papers describe the effect of integral imaging on the display process. Yim et al. [14] propose the pickup system of integral imaging using an offset lens array which is useful for the both pickup and display processes, and resolve the pseudoscopic image problem of integral imaging. Kim et al. [15] describe an improved projection-type integral imaging system using a 3D screen consisting of a lens array and a retroreflector film. Image quality improvement for holographic projections using the characteristics of ringing artifacts which, along with speckle noise, cause deterioration of image quality has been proposed by Nagahama et al. [16]. Özgürün et al. [17] show how depth information of a macroscopic 3D object can be displayed from a single digital hologram using stereo disparity where two perspectives of the scene are obtained by dividing the hologram into two parts (two apertures) before reconstruction. Speaking of large displays, Häussler et al. [18] show how SeeReal’s holographic 3D display employs a spatial light modulator along with holographic optical elements (HOEs) in photopolymer films and laser light sources to achieve 3D reconstruction which enables selective accommodation of the observer’s eye lenses and natural depth perception. Finally, as Shimobaba et al. [19] show, the quality of reconstructed images from holograms, otherwise contaminated by direct light, conjugate light, and speckle noise can be restored by using an autoencoder based on artificial neural networks.

An important area of applications of DH is in metrology and profilometry. In an invited paper, Falldorf et al. [20] argue that recording the mutual intensity instead of the complex wavefront may enable interferometric measurements with multiple independent light sources at the same time. This has potential important applications in metrology where the light field to be recorded may be sparse in phase space. The remaining five papers pertain to specific applications. For instance, Narayanamurthy et al. [21] apply DH to analyze stressed photoelastic materials by examining the photoelastic isochromatic and isopachic fringes. Where precision is important, such as in quantitative deflectometry, different regions of a deflected object being recorded may be unequally projected on the detector. Li et al. [22] provide an improved Southwell zonal integration method to mitigate this problem. Application of time averaged DH to inspect large scale sandwich and composite structures, such as those in the modern aerospace industry, using square wave rather than sinusoidal excitation to enhance the speed and accuracy of inspection is proposed by Thomas et al. [23]. Tavera et al. [24] use DH to determine surface structural damage in cortical bone due to medical drilling. In order to observe the bone’s surface behavior caused by the drilling effects, a DH interferometer is used to analyze the displacement surface’s variations in non-fractured post mortem porcine femoral bones. Finally, Kemppinen et al. [25] perform quasi-3D microparticle imaging by stacking a collection of silhouette-like images of a particle reconstructed from a single in-line hologram, thus enabling estimation of the particle size in the longitudinal and transverse dimensions.

While DH is the process of digital recording of holograms and their numerical reconstruction, computer generated holography pertains to generating the hologram digitally and illuminating it optically for various applications. One of the many uses of computer generated holography is the design of HOEs. A unique HOE, called a chiral square Fresnel zone plate, is introduced by Vijayakumar et al. [26] in the group’s second contribution to the feature issue (see above for the first) which enables generation of optical beams with orbital angular momentum. Such beams can be used in optical trapping applications. Nishi and Matsushima [27] provide a recipe for creating large-scale computer-generated holograms based on a polygon-based method, but allowing for specular curved surfaces to be reconstructed without increasing the number of polygons. Nobukawa and Nomura [28] discuss a simple and compact holographic data storage system based on a computer-generated hologram enabling the hologram of a data page to be recorded through an imaging system without an additional optical path for a reference beam. Finally, Zhang et al. [29] propose a layer based algorithm with single-viewpoint rendering geometry for calculating computer-generated holograms which can generate high quality 3D scenes with accurate depth information as well as occlusion effect.

The principle of DH, developed for optical applications, can readily be extended to other electromagnetic frequencies as well. One of the promising extensions is to terahertz frequencies. In an application of THz DH, Guo et al. [30], in an invited paper, show how a THz DH imaging system is utilized to investigate natural dehydration processes in biological tissues. Differences between water retention ability of different animal tissues are thereby determined.

The last paper in the feature issue on DH in AO deals with noninterferometric phase retrieval. Reminiscent of transport of intensity, Claus et al. [31] describe how phase retrieval can be achieved by recording diffraction patterns with varying distance between illumination-source and the object. This has the same effect as changing the object-sensor distance, albeit offering the advantage of preserving the resolution.

In the spirit of developing more fundamental physics in the area of DH, the first two papers in JOSA-B relate to recording materials and techniques. The invited paper by Serak et al. [32], discuss arrays of diffractive waveplates which present new opportunities for digital light polarization holography. The second paper, by Gao et al. [33], investigates permanent hologram recording in TMPTA-based photopolymer films with high diffraction efficiency as high as 90%. Two papers in DH microscopy follow, the first an invited paper by Jin et al. [34] on tomographic phase microscopy which uses digital holographic measurements of complex scattered fields to reconstruct the 3D refractive index maps of biological cells, and the second by Pourvais et al. [35], which uses DH microscopy for microstructural surface characterization of stainless and plain carbon steel. The next three papers pertain to advances in DH techniques. The first, by Hussain et al. [36], employs a simple fringe illumination technique for optical super-resolution using spatial light modulators to retrieve the missing information of an object. The second, by Kalenkov et al. [37], introduces the registration of hyperspectral holograms in incoherent light by using Fourier transform spectroscopy. The third, by Montrésor et al. [38], performs rigorous analysis of phase errors generated by noise reduction algorithms during 3D deformation measurement using digital holographic interferometry. Speaking of 3D imaging and display, the paper by Zhang et al. [39] describes a table-top 3D display system based on integral imaging by using a HOE, where the 3D image and the real-world scene are seamlessly combined together to produce augmented reality. The second paper on this topic, by Pang et al. [40], develops a bionic compound eye to achieve a large field of view, high image quality, high 3-D detection accuracy and high speed tracking. Computer generated holography, based on a modified Gerchberg–Saxton algorithm for phase retrieval, is proposed by Chen et al. [41] and demonstrated for a panoramic 3D holographic projection display system. Finally, Su et al. [42] propose and implement a configuration for laser direct printing of computer-generated holograms generated by a ray tracing method.

It is hoped that the joint feature issue on DH in AO and JOSA B excites readers to many more fundamental developments in the area and opens the door to even more exciting applications.