This joint feature issue on Digital Holography and 3D Imaging is dedicated to two renowned scientists in the area of holography, Byoungho Lee and John Sheridan. Both were acclaimed researchers in the field and, most importantly, were regarded by all for their wonderful personalities. Their untimely passing (both were only 58) a short time apart from each other in November 2022 came as a shock to the optics community.

Byoungho Lee’s research interests included optical information processing, fiber sensors, digital holography, 3D displays, metamaterials, and augmented reality, to name a few. Lee was awarded fellowships in Optica (2005), SPIE (2002) and IEEE (2014). Also, in 2014, Lee was inducted into the elite group of Holoknights, formed in 1988 to bring together as friends the top researchers in the field of holography. He received his bachelor’s (1987) and master’s (1989) degrees from Seoul National University and his PhD (1993) from the University of California, Berkeley, before joining his alma mater in South Korea as faculty, where he remained until his passing. Lee was co-organizer of the first digital holography (DH) conference, was very active in DH committees, and remained a member of its advisory committee till his passing. In the area of holography and 3D displays, Lee’s research focused on the holographic projection of light fields that can be perceived as a natural source of light by the naked eye. Lee and his team demonstrated a curved array of spatial light modulators for displaying dynamic holographic stereograms with a wide viewing angle. Later, he found that thin metamaterials can effectively help to broaden the viewing angles of augmented reality devices. By utilizing a transparent metasurface, he designed a compact near-eye display system for augmented reality with a wide field of view.

Much like Byoungho Lee, John Sheridan’s research interests included optical signal processing, electromagnetic theory of diffraction, and applied optoelectronics. He received his bachelor’s degree from University College, Galway, Ireland, in 1985, his MS from Georgia Tech in 1986, and his PhD from Oxford in 1991. It was at Oxford he worked under the mentorship of Laszlo Solymar and Collin Shepherd, specializing in holography and confocal microscopy. Later he was a postdoctoral fellow at Erlangen, working under Adolf Lohmann, before returning to Ireland, where he settled in University College, Dublin, where he remained till his passing. Sheridan was awarded fellowships in SPIE (2014), Optica (2017), and FIMA (2018). He served on many international conferences such as Optica’s Digital Holography and 3D Imaging, was a member of the editorial board of Applied Optics, and most recently served as a guest editor for the Applied Optics (AO)/Journal of the Optical Society of America A (JOSA A) feature issue on DH and 3D Imaging (DH${+}$3D). Sheridan is probably best known for his work on optical encryption techniques. Whereas typical methods are based on the fractional Fourier transform and require random phase screen keys at the detector for decrypting the data, Sheridan’s group demonstrated a new technique based on a random shifting, or jigsaw, algorithm which does not require the use of phase keys, where the image is encrypted by juxtaposition of sections of the image in fractional Fourier domains.

The joint feature issue in AO and JOSA A is a continuation of a tradition to follow the conclusion of the Optica Topical Meeting on DH${+}$3D. In 2022, this conference was held August 1–4 in Cambridge, England. In this meeting, there were about 180 papers in 21 sessions, with only 4 sessions being all-virtual. This included 2 keynote speakers, about 100 oral presentations (about 90 in-person, 10 virtual), and about 80 posters (about 30 in-person, 50 virtual).

This feature issue comprises 23 contributed papers in AO, including a review paper and an engineering lab note, and 7 contributed papers in JOSA A, including a discussion paper. The review paper by Valentino et al. (AO) discusses at length the application of digital holography to detection of microplastics in water. Automatic analysis based on smart DH processing and its application in field-portable holographic flow cytometers is discussed. The discussion by Rabosh et al. (JOSA A) highlights the potential of display holograms in the storage of information about the object morphology, discussing existing and emerging technologies for converting information into a digital format, and addressing one of the most serious challenges to the widespread use of display holography. The follow-up paper by Rabosh et al. (JOSA A) demonstrates the application of photogrammetry in digitizing the information about objects by acquiring a set of photographic images captured from three-dimensional scenes, which are reconstructed from volume reflection holograms.

The digital construction and readout of full-color volume reflection holograms, which have been traditionally recorded in bulk photopolymers, is shown by Qissi and Banerjee (AO), starting from multispectral digital transmission holograms, and supported by numerical and experimental results, with potential applications in image encryption. The physical recording and reconstruction of a PQ:PMMA photopolymer-based volume holographic optical beam shaper for an abrupt autofocusing beam with high angular selectivity, broadband operation, and intrinsically compact size have been demonstrated by Vyas and Luo (AO). On a related note, in-line and off-axis polarization-selective holographic lenses are recorded in azopolymer thin films via polarization holography and polarization multiplexing using a new method to suppress the surface relief grating formation and to improve the polarization properties of the lenses, as reported by Nedelchev et al. (AO). A full-color holographic system featuring a 3D salient object detection model built at the acquisition step is proposed by Bu et al. (JOSA A) along with a deep network architecture U2-reverse attention and residual learning (RAS) algorithm for salient object detection to obtain more efficient and accurate point cloud information. The point cloud gridding method is used to improve the hologram generation speed.

Many papers in this feature issue in AO are, obviously, on applications of DH. Detection of tire defects using double-exposure digital holographic interferometry, implemented with a portable digital holographic camera, is introduced by Chakraborty and Kumar (AO), where the defects are identified from discontinuities in the fringes, and quantified from the fringe displacement. Incidentally, the fractional topological charge of an optical vortex beam can be determined through interference fringe dislocation, even in moderate atmospheric turbulence, using a spiral interferometer and fork-shaped interference patterns, as shown by Shikder and Nischal (AO). Accurate estimation of the topological charge is important due to its applications in micro-particle manipulation, optical communication, quantum information processing, optical encryption, etc. Gröger et al. (AO) apply holographic imaging through fog and its advantages over conventional coherent imaging for road traffic applications, where autonomous driving vehicles require reliable environmental perception in all weather conditions. Multiple wavelength digital holography is applied to freeform shape measurement of diffuse surfaces and optical alignment, optimized for maximal precision by Kaván et al. (AO). Khorshad and Devaney (AO) show the use of GRIN-lens-based in-line digital holographic microscopy with improved resolution to holographically image dilute polystyrene micro-particles with diameters of 3.0 and 2.0 µm. In contrast, an ingenious adaptation of an off-the-shelf Blu-ray optical pickup unit (OPU) into a highly versatile point source for digital lensless holographic microscopy is presented by Tobón-Maya et al. (AO) as an engineering laboratory note. The functionality of the OPU-based point source is validated through the observation of micron-sized biological specimens. 3D complex field reconstruction with a single defocused off-axis digital hologram is investigated by Rajora et al. (AO). Realizing that for phase objects, the amplitude contrast of the backpropagated object field at the focus plane is minimum, the authors use information available on the recovered object field in the hologram plane to device depth-dependent weights that are proportional to the inverse of amplitude contrast, showing applications to healthy and malaria-infected red blood cells. A technique to improve 3D shape measurement quality for digital fringe projector techniques is provided by Zhang et al. (AO). They use an optimized dithering approach which utilizes a genetic algorithm and chaos maps to optimize the bidirectional error-diffusion coefficients. Mutation factors generated by chaotic maps, compared with the mutation rate, determine whether the individual position will mutate, resulting in improved phase reconstruction at different defocus levels.

On a related note, depth map retrieval from computer generated holograms (CGHs) using a depth-from-focus (DFF) approach is shown by Madali et al. (AO). They show that DFF can be used for depth estimation from the hologram if the set of hyperparameters, required for the application of the method, is suitably chosen. In another CGH related paper, Brkić et al. (AO) propose a low-cost and fast hologram manufacturing process for possible applications as security elements from CGHs using a computer to film process, which includes computer-to-plate, offset printing, and surface engraving utilizing the same CGH calculations and prepress. Higashida et al. (JOSA A) evaluate 3D image quality from CGH reconstruction by mimicking how the eye’s lens works, thus allowing for viewing position and eye focus adjustments. The angular resolution of the eye was used to obtain reconstructed images with the requisite resolution, and compare with the original image with incoherent illumination. An innovative optimization of a phase-only CGH for a 3D target that requires limited memory is achieved by Sha et al. (JOSA A) using the Broyden–Fletcher–Goldfarb–Shanno nonlinear optimization technique with sequential slicing for partial evaluation of the hologram during optimization that only computes loss for a single slice of the reconstruction at every iteration.

Works on speckle and noise also feature in this issue. Aarav and Fleischer (AO) show that the presence of a point source behind the scatterer enables single-shot reconstruction of multiple objects at multiple depths. The speckle correlation method relies on speckle scaling from the axial and transverse memory effect. Levchenko et al. (AO) analyze the efficiency of the dynamic speckle method in the presence of environmental noise as a tool for estimation of speed of processes. The speed distribution is encoded in a map built by statistical pointwise processing of time-correlated speckle patterns. The effect of phase flicker on sharpness deterioration on holographic displays with digitally driven phase-only liquid crystal on silicon (LCoS) devices is investigated by Wu et al. (AO). Their work reveals that the increment in the magnitude of phase flicker causes an equal sharpness deterioration with the reduction of the numbers of hologram phase modulation levels. The same group also examines the steering angle of LCoS wavelength selective switches (WSSs) and show that the steering angle of LCoS devices is fundamentally determined by the pixel pitch. Accordingly, Chang et al. (AO) offer an approach to increase the steering angle of LCoS devices through the integration with dielectric Huygens-type metasurfaces. Finally, Meteyer et al. (JOSA A) present theoretical modeling of speckle noise decorrelation in digital Fresnel holographic interferometry in the presence of out-of-focus conditions in the reconstructed image by deriving the complex coherence factor which incorporates the focus mismatch. The phenomenon of anti-correlation in phase data from holographic interferometry is discussed.

In other work related to holography in the feature issue, Zhang et al. (AO) introduce Hough transform-based multi-object autofocusing compressive holography. The proposed method effectively obtains 3D information of multiple objects and achieves noise elimination by reconstructing from only one hologram by computing the sharpness of each reconstructed image using a focus metric such as entropy or variance. As a variation to conventional DH recording, Hammond et al. (AO) employ a time of flight (TOF) camera to perform range selective temporal-heterodyne frequency-modulated continuous wave DH (TH FMCW DH). This allows for on-axis recording of both Fresnel and image holograms, and range selective TH FMCW DH imaging with 6.3 cm resolution is shown using a 2.39 GHz chirp bandwidth.

There are four papers in this feature issue related to 3D imaging elements and visualization techniques, and image mapping. The first is work by Li et al. (AO) on the development of an artificial compound eye which naturally has a large field of view and fast motion detection. This is achieved by assembling microlens arrays with different focal lengths that can distinguish objects at different distances without additional components. By adjusting the spacing of the microlens array, secondary microlenses are incorporated between intervals of the primary microlens. The paper by Kalinina et al. (AO) proposes an architecture for holographic augmented reality glasses to provide a wide field of view (60 degrees) and a large eyebox (10 mm). Their solution is based on the combination of an axial holographic optical element (HOE) with a directional holographic diffuser (DHD) illuminated by a projector. The main feature of their system is the coincidence of the DHD position with the planar intermediate image of the axial HOE. Goloborodko (JOSA A) investigates the accuracy of wavefront curvature restoration based on pit displacement measurements in a Talbot wavefront sensor under Gaussian illumination conditions. The effect of wavefront curvature on the measurement error of the Talbot sensor is discussed using a model based on the Fresnel regime to determine the angular spectrum and hence the intensity distribution in the near field. The fourth paper, by Jiang et al. (AO), pertains to precise organ segmentation of the mantis shrimp, which plays a vital role in marine ecosystems. This paper develops a framework from multiview stereo (MVS) point clouds. Transformer-based MVS architecture is applied to generate dense point clouds from a set of calibrated phone images, followed by an improved segmentation (ShrimpSeg) that exploits both local and global features based on contextual information.

We hope that this feature issue provides a valuable source of information on the latest research in the area of DH and 3D imaging and encourages researchers to continue working and contribute. This feature issue serves as our humble tribute to Byoungho Lee and John Sheridan, who dedicated their lives to exploring and enriching this attractive area.

The next DH${+}$3D meeting will be held as part of the Imaging Congress, 14–17 August 2023. Along with the Chairs and committee members of this meeting, we enthusiastically encourage you to participate. We look forward to seeing you all in Boston.