Design, fabrication, and validation of an aberration-free integral imaging system for 3D reconstruction

Optical design for reducing aberrations of the micro-lens-array-based integral imaging system is challenging. A design process combining the sequential and non-sequential modes of optical design software is proposed. The process is verified by a system assembled on a coordinate measuring machine with errors of several micrometers. Differences in the modulation transfer function, measured by the slanted-edge method, are less than 0.02 between the paraxial and off-axial regions. Reconstructed images of a U.S. quarter-dollar coin with different refocusing depths show the synthesized defocusing. The estimated depth map and depth-based reconstructed image show the rough shape of the coin.

Self-Organized Liquid Crystal Droplets as Phototunable Softmasks

A single-step self-organized pathway is harnessed to generate large-area and high-density liquid-crystal (LC) microdroplets via rapid spreading of an LC-laden volatile liquid film on an aqueous surfactant bath. The surfactant loading on the water bath and LC loading in the solvent fluid help in tuning the size, periodicity, and ordering of LC microdroplets. Remarkably, the experiments reveal a transition from a spinodal to heterogeneous nucleation pathway of dewetting when the surfactant loading is modulated from below to beyond the critical micellar concentration in the aqueous phase. In the process, a host of unprecedented drop formation modes, such as dewetting and contact-line instability, random ejection, and "fire cracker" toroid splitting, have been uncovered. Subsequently, the LC microdroplets on the air-water interface are employed as photomasks suitable for soft-photolithography applications. Such masks help in the decoration of a host of mesoscale three-dimensional features on the films of photoresists when photons are guided through the LC droplets. In such a scenario, phase transition of LC droplets under solvent vapor annealing is employed to control the movement of photons through drops and subsequently modulate the light exposure on the photoresist surface. Such a simple soft-photolithography setup leads to an array of flattened droplets on a positive resist, while donut features are observed on the negative tone. Remarkably, the orientation of nematogens within 4-cyano-4'-pentylbiphenyl droplets and at the three-phase contact-line provides additional handles in controlling the transmission of photons, which facilitates such a unique pattern formation. A number of low-cost and simple strategies are also discussed to order such soft-photolithography patterns. Importantly, with a minor modification to the same experimental setup, we could also measure the variation in the order parameter of the LC droplet during its phase transitions from the nematic to isotropic state.

Reducing the light scattering impact in liquid-crystal-based imaging systems

We show an experimental method of quantifying the effect of light scattering by liquid crystals (LCs) and then apply rather simple image processing algorithms (Wiener deconvolution and contrast-limited adaptive histogram equalization) to improve the quality of obtained images when using electrically tunable LC lenses (TLCLs). Better contrast and color reproduction have been achieved. We think that this approach will allow the use of thicker LC cells and thus increase the maximum achievable optical power of the TLCL without a noticeable reduction of image quality. This eliminates one of the key limitations for their use in various adaptive imaging applications requiring larger apertures.

Bandwidth-enhanced depth priority integral imaging using a band-limited diffusing illumination technique

The display bandwidth and display mechanism determine the performance of the three-dimensional (3D) display system. In this paper, a bandwidth-enhanced depth priority integral imaging (DPII) technique is proposed. Information transmission efficiency (ITE) defined as the output display bandwidth divided by the input display bandwidth is used to assess the II system. By analyzing the ITE, we find that only a part of the input display bandwidth is used efficiently to present the 3D image in the traditional DPII system. The DPII system sacrifices the ITE for depth enhancement. The low ITE that fundamentally limits the 3D performance of the DPII system is ascribed to the diffusing illumination mechanism of the display system. To enhance the 3D performance, a collimated illumination DPII system as a special case of band-limited diffusing illumination technique has been proposed and demonstrated first. The bandwidth and ITE of such a DPII system are increased. The depth of field (DOF) of the system is doubled. The resolution of the 3D image is increased to the level of the resolution priority II system without sacrificing the viewing angle. A more general case, band-limited illumination DPII system is also demonstrated. By modulating the divergence angle of the illumination system, the 3D image's resolution and DOF can be controlled. The bandwidth and ITE of the DPII system using band-limited illumination are also higher than that of the traditional DPII system. Experiments are presented to prove the bandwidth-enhanced mechanism of the DPII system.

Bi-phase emulsion droplets as dynamic fluid optical systems

Micro-scale optical components play a critical role in many applications, in particular when these components are capable of dynamically responding to different stimuli with a controlled variation of their optical behavior. Here, we discuss the potential of micro-scale bi-phase emulsion droplets as a material platform for dynamic fluid optical components. Such droplets act as liquid compound micro-lenses with dynamically tunable focal lengths. They can be reconfigured to focus or scatter light and form images. In addition, we discuss how these droplets can be used to create iridescent structural color with large angular spectral separation. Experimental demonstrations of the emulsion droplet optics are complemented by theoretical analysis and wave-optical modelling. Finally, we provide evidence of the droplets utility as fluidic optical elements in potential application scenarios.

Reconfigurable and responsive droplet-based compound micro-lenses

Micro-scale optical components play a crucial role in imaging and display technology, biosensing, beam shaping, optical switching, wavefront-analysis, and device miniaturization. Herein, we demonstrate liquid compound micro-lenses with dynamically tunable focal lengths. We employ bi-phase emulsion droplets fabricated from immiscible hydrocarbon and fluorocarbon liquids to form responsive micro-lenses that can be reconfigured to focus or scatter light, form real or virtual images, and display variable focal lengths. Experimental demonstrations of dynamic refractive control are complemented by theoretical analysis and wave-optical modelling. Additionally, we provide evidence of the micro-lenses' functionality for two potential applications—integral micro-scale imaging devices and light field display technology—thereby demonstrating both the fundamental characteristics and the promising opportunities for fluid-based dynamic refractive micro-scale compound lenses.

Micro-lenses are critical components in miniaturized optical devices for imaging and sensing, yet it is challenging to design them with on-demand variable optical properties. Here, Nagelberg et al. use bi-phase emulsion droplets to design reconfigurable micro-lenses with variable focal length.

Light-field and holographic three-dimensional displays [Invited]

A perfect three-dimensional (3D) display that satisfies all depth cues in human vision is possible if a light field can be reproduced exactly as it appeared when it emerged from a real object. The light field can be generated based on either light ray or wavefront reconstruction, with the latter known as holography. This paper first provides an overview of the advances of ray-based and wavefront-based 3D display technologies, including integral photography and holography, and the integration of those technologies with digital information systems. Hardcopy displays have already been used in some applications, whereas the electronic display of a light field is under active investigation. Next, a fundamental question in this technology field is addressed: what is the difference between ray-based and wavefront-based methods for light-field 3D displays? In considering this question, it is of particular interest to look at the technology of holographic stereograms. The phase information in holography contributes to the resolution of a reconstructed image, especially for deep 3D images. Moreover, issues facing the electronic display system of light fields are discussed, including the resolution of the spatial light modulator, the computational techniques of holography, and the speckle in holographic images.

Extended depth-of-field 3D endoscopy with synthetic aperture integral imaging using an electrically tunable focal-length liquid-crystal lens

Conventional synthetic-aperture integral imaging uses a lens array to sense the three-dimensional (3D) object or scene that can then be reconstructed digitally or optically. However, integral imaging generally suffers from a fixed and limited range of depth of field (DOF). In this Letter, we experimentally demonstrate a 3D integral-imaging endoscopy with tunable DOF by using a single large-aperture focal-length-tunable liquid crystal (LC) lens. The proposed system can provide high spatial resolution and an extended DOF in synthetic-aperture integral imaging 3D endoscope. In our experiments, the image plane in the integral imaging pickup process can be tuned from 18 to 38 mm continuously using a large-aperture LC lens, and the total DOF is extended from 12 to 51 mm. To the best of our knowledge, this is the first report on synthetic aperture integral imaging 3D endoscopy with a large-aperture LC lens that can provide high spatial resolution 3D imaging with an extend DOF.

Depth plane adaptive integral imaging using a varifocal liquid lens array

This paper proposes an enhanced integral imaging system with an electrically controllable image plane to address the issue of the limited depth problem in integral imaging. For implementation of the variable image plane, a varifocal liquid lens array and driving device are adopted instead of an ordinary solid lens array. The position of the central depth plane is varied by adjusting the focal length of the lens array. The proposed system enables matching between the object position and depth plane electrically, and thus an object moving from 5.15 to 11.72 cm is clearly displayed with this method.

Extended depth-of-focus 3D micro integral imaging display using a bifocal liquid crystal lens

We present a three dimensional (3D) micro integral imaging display system with extended depth of focus by using a polarized bifocal liquid crystal lens. This lens and other optical components are combined as the relay optical element. The focal length of the relay optical element can be controlled to project an elemental image array in multiple positions with various lenslet image planes, by applying different voltages to the liquid crystal lens. The depth of focus of the proposed system can therefore be extended. The feasibility of our proposed system is experimentally demonstrated. In our experiments, the depth of focus of the display system is extended from 3.82 to 109.43 mm.

Analysis of the depth of field of integral imaging displays based on wave optics

In this paper, we analyze the depth of field (DOF) of integral imaging displays based on wave optics. With considering the diffraction effect, we analyze the intensity distribution of light with multiple micro-lenses and derive a DOF calculation formula for integral imaging display system. We study the variations of DOF values with different system parameters. Experimental results are provided to verify the accuracy of the theoretical analysis. The analyses and experimental results presented in this paper could be beneficial for better understanding and designing of integral imaging displays.

High-resolution three-dimensional holographic display using dense ray sampling from integral imaging

We present a high-resolution three-dimensional (3D) holographic display using a set of elemental images obtained by passive sensing integral imaging (II). Hologram calculations using a high-density ray-sampling plane are achieved from the elemental images captured by II. In II display, ray sampling by lenslet array and light diffraction limits the achievable resolution. Our approach can improve the resolution since target objects are captured in focus and then light-ray information is interpolated and resampled with higher density on ray-sampling plane located near the object to be converted into the wavefront. Numerical experimental results show that the 3D scene, composed of plural objects at different depths from the display, can be reconstructed with order of magnitude higher resolution by the proposed technique.

Optical implementation of micro-zoom arrays for parallel focusing in integral imaging

We report 3D integral imaging with an electronically tunable-focal-length lens for improved depth of field. The micro-zoom arrays are generated and implemented based on the concept of parallel apodization. To the best of our knowledge, this is the first report of parallel dynamic focusing in integral imaging based on the use of micro-zoom arrays.

Geometrical superresolved imaging using nonperiodic spatial masking

The resolution of every imaging system is limited either by the F-number of its optics or by the geometry of its detection array. The geometrical limitation is caused by lack of spatial sampling points as well as by the shape of every sampling pixel that generates spectral low-pass filtering. We present a novel approach to overcome the low-pass filtering that is due to the shape of the sampling pixels. The approach combines special algorithms together with spatial masking placed in the intermediate image plane and eventually allows geometrical superresolved imaging without relation to the actual shape of the pixels.

Digital slicing of 3D scenes by Fourier filtering of integral images

We present a novel technique to extract depth information from 3D scenes recorded using an Integral Imaging system. The technique exploits the periodic structure of the recorded integral image to implement a Fourier-domain filtering algorithm. A proper projection of the filtered integral image permits reconstruction of different planes that constitute the 3D scene. The main feature of our method is that the Fourier-domain filtering allows the reduction of out-of-focus information, providing the InI system with real optical sectioning capacity.

Three-dimensional visualization of objects in scattering medium by use of computational integral imaging

In this paper, we propose a method to three-dimensionally visualize objects in a scattering medium using integral imaging. Our approach is based on a particular use of the interference phenomenon between the ballistic photons getting through the scattering medium and the scattered photons being scattered by the medium. For three-dimensional (3D) sensing of the scattered objects, the synthetic aperture integral imaging system under coherent illumination records the scattered elemental images of the objects. Then, the computational geometrical ray propagation algorithm is applied to the scattered elemental images in order to eliminate the interference patterns between scattered and object beams. The original 3D information of the scattered objects is recovered by multiple imaging channels, each with a unique perspective of the object. We present both simulation and experimental results with virtual and real objects to demonstrate the proposed concepts.

Three dimensional imaging with randomly distributed sensors

As a promising three dimensional passive imaging modality, Integral Imaging (II) has been investigated widely within the research community. In virtually all of such investigations, there is an implicit assumption that the collection of elemental images lie on a simple geometric surface (e.g. flat, concave, etc), also known as pickup surface. In this paper, we present a generalized framework for 3D II with arbitrary pickup surface geometry and randomly distributed sensor configuration. In particular, we will study the case of Synthetic Aperture Integral Imaging (SAII) with random location of cameras in space, while all cameras have parallel optical axes but different distances from the 3D scene. We assume that the sensors are randomly distributed in 3D volume of pick up space. For 3D reconstruction, a finite number of sensors with known coordinates are randomly selected from within this volume. The mathematical framework for 3D scene reconstruction is developed based on an affine transform representation of imaging under geometrical optics regime. We demonstrate the feasibility of the methods proposed here by experimental results. To the best of our knowledge, this is the first report on 3D imaging using randomly distributed sensors.

Extension of depth of field using amplitude and phase modulation of the pupil function

We analyze the extension of depth of field using both amplitude and phase modulation of the pupil function. In particular, we discuss the advantages and disadvantages of each approach and establish the range of applicability of each method based on the range of spatial frequencies of interest in the imaging system. To the best of our knowledge, this is the first such report on the range of applicability of amplitude and phase modulation to extend the depth of field.

Integral imaging with large depth of field using an asymmetric phase mask

We propose to improve the depth of field of Integral Imaging systems by combining an array of phase masks with the traditional lenslet array. We show that obtained elemental images are sharp over a larger range than with a regular lenslet array. We further increase the quality of elemental images by a digital restauration. Computer simulations of pickup and reconstruction are presented.

Optically-corrected elemental images for undistorted Integral image display

Conventional macro objectives are generally used as relay systems in the capture stage in Integral Imaging. This choice leads to microimage overlap and shift, which produce undesirable effects on the reconstructed three-dimensional images, such as loss in resolution and image distortions. In this paper, we propose and demonstrate a new architecture for the capture stage. Our method uses a telecentric relay system to overcome the conventional drawbacks. Experiments conducted with our system show an important improvement in the quality of displayed images.