Super multi-view display based on near-eye pinholes.

Vergence-accommodation-conflict-free super multi-view (SMV) display based on more than one near-eye pinhole group for each pupil of the viewer is developed in this paper. Two-dimensionally arranged pinholes of a group correspond to different subscreens of the display screen, with perspective views projected by each subscreen through a corresponding pinhole splicing into an image of enlarged field of view (FOV). By sequentially switching on/off different pinhole groups, more than one mosaic image is projected to each pupil of the viewer. Adjacent pinholes of a group are endowed with different timing-polarizing characteristics to generate an effective noise-free region for each pupil. In the experiment, four groups of 3×3 pinholes are configured for a proof-of-concept SMV display on a 240 Hz display screen, with a diagonal FOV of 55 deg and a depth of field reaching 1.2 m.

Super multi-view near-eye virtual reality with directional backlights from wave-guides.

Directional backlights have often been employed for generating multiple view-zones in three-dimensional (3D) display, with each backlight converging into a corresponding view-zone. By designing the view-zone interval for each pupil smaller than the pupil's diameter, super multi-view (SMV) can get implemented for a VAC-free 3D display. However, expanding the backlight from a light-source to cover the corresponding display panel often needs an extra thickness, which results in a thicker structure and is unwanted by a near-eye display. In this paper, two wave-guides are introduced into a near-eye virtual reality (NEVR) system, for sequentially guiding more than one directional backlight to each display panel for SMV display without bringing obvious extra thickness. A prototype SMV NEVR gets demonstrated, with two backlights from each wave-guide converging into two view-zones for a corresponding pupil. Although the additional configured light-sources are positioned far from the corresponding wave-guide in our proof-of-concept prototype, multiple light-sources can be attached to the corresponding wave-guide compactly if necessary. As proof, a 3D scene with defocus-blur effects gets displayed. The design range of the backlights' total reflection angles in the wave-guide is also discussed.

Polarization enlargement of FOV in Super Multi-view display based on near-eye timing-apertures.

With strip-type timing-apertures attached to each eye of a viewer, more than one perspective views can be guided to either eye sequentially through different timing-apertures, thus implementing VAC-free (vergence-accommodation conflict-free) SMV (Super Multi-view) 3D (three-dimensional) display. To overcome the FOV (field of view) limitation problem due to small size of the timing-apertures along their arrangement direction, novel polarization architectures are designed to the timing-apertures in this paper. Correspondingly, the display screen of the proposed SMV display system is divided into M > 1 sub-screens along the arrangement direction of the timing-apertures, with adjacent sub-screens emitting light of mutually orthogonal polarization. At a time-point of each time period, a group of M timing-apertures, which correspond to the M sub-screens in a one-by-one manner along the arrangement direction, are turned on for creating an M-fold FOV, with each polarized timing-aperture of the group allowing light from the corresponding sub-screen passing through and blocking light from sub-screen(s) adjacent to the corresponding sub-screen. At 2T > 1 time-points of each time period, 2T groups of timing-apertures are turned on sequentially for presenting more than one two-dimensional images of the displayed scene to each eye, to implement SMV display based on persistence of vision. M stands for the FOV magnification number and T stands for the two-dimensional image number for each eye. As proof, a 3-fold FOV of 41° gets implemented experimentally with a currently available timing-aperture array of M = 3, accompanied by an effective noise-free region (ENFR) of 8.34 mm. Furthermore, the promising of freeing FOV from timing-aperture constraint fundamentally by larger M is described, out-of-screen blur along strip direction of the timing-apertures and the problem of limited ENFR are discussed.

GaN-based mid-power flip-chip light-emitting diode with high -3 dB bandwidth for visible light communications.

By directly flip-chip soldering three mid-power light-emitting diode (LED) chips with periodic micro-via-holes on ceramic substrates, -3 dB modulation bandwidths of 49.9 MHz, 58.8 MHz, and 25 MHz are obtained at the driving current of 170 mA. To the best of our knowledge, these are the reported highest -3 dB bandwidth values for flip-chip power-type LEDs (FC-LEDs) at the low-bias current levels. Moreover, good radiant powers are also achieved: 180.2 mW, 168.8 mW, and 233.8 mW at 150 mA, respectively. With a gold wire-free feature, the fabricated FC-LEDs could offer an opportunity to miniaturize the package volume of a LED module for illumination and free-space high-speed VLC dual-usage applications.

Morphology evolution and pure red upconversion mechanism of ß-NaLuF4 crystals.

A series of ß-NaLuF4 crystals were synthesized via a hydrothermal method. Hexagonal phase microdisks, microprisms, and microtubes were achieved by simply changing the amount of citric acid in the initial reaction solution. Pure red upconversion (UC) luminescence can be observed in ß-NaLuF4:Yb(3+), Tm(3+), Er(3+) and Li(+) doped ß-NaLuF4:20% Yb(3+), 1% Tm(3+), 20% Er(3+). Based on the rate equations, we report the theoretical model about the pure red UC mechanism in Yb(3+)/Tm(3+)/Er(3+) doped system. It is proposed that the pure red UC luminescence is mainly ascribed to the energy transfer UC from Tm(3+):(3)F4 → (3)H6 to Er(3+):(4)I11/2 → (4)F9/2 and the cross-relaxation (CR) effect [Er(3+):(4)S3/2 + (4)I15/2 → (4)I9/2 + (4)I13/2] rather than the long-accepted mechanism [CR process among Er(3+):(4)F7/2 + (4)I11/2 → (4)F9/2 + (4)F9/2]. In addition, compared to the Li(+)-free counterpart, the pure red UC luminescence in ß-NaLuF4:20% Yb(3+), 1% Tm(3+), 20% Er(3+) with 15 mol% Li(+) doping is enhanced by 13.7 times. This study provides a general and effective approach to obtain intense pure red UC luminescence, which can be applied to other synthetic strategies.

Super multi-view three-dimensional display technique for portable devices.

Portable display devices, such as intelligent telephones and panel PCs, have become parts of modern people's daily life. Their mainstream display interfaces are based on two-dimensional (2D) images. Although some three-dimensional (3D) technologies have been proposed for portable devices, comfortable visual effects are untouched until now. A super multi-view (SMV) system with comfortable 3D effects, constructed by a group of OLED microdisplay/projecting lens pairs, is proposed in this paper. Through gating different segments of each projecting lens sequentially and refreshing the virtual image of the corresponding microdisplay synchronously, the proposed SMV system greatly decreases the demand on the number of employed microdisplays and at the same time takes a thin optical structure, endowing great potential for portable devices.

Robust Eye Center Localization through Face Alignment and Invariant Isocentric Patterns.

The localization of eye centers is a very useful cue for numerous applications like face recognition, facial expression recognition, and the early screening of neurological pathologies. Several methods relying on available light for accurate eye-center localization have been exploited. However, despite the considerable improvements that eye-center localization systems have undergone in recent years, only few of these developments deal with the challenges posed by the profile (non-frontal face). In this paper, we first use the explicit shape regression method to obtain the rough location of the eye centers. Because this method extracts global information from the human face, it is robust against any changes in the eye region. We exploit this robustness and utilize it as a constraint. To locate the eye centers accurately, we employ isophote curvature features, the accuracy of which has been demonstrated in a previous study. By applying these features, we obtain a series of eye-center locations which are candidates for the actual position of the eye-center. Among these locations, the estimated locations which minimize the reconstruction error between the two methods mentioned above are taken as the closest approximation for the eye centers locations. Therefore, we combine explicit shape regression and isophote curvature feature analysis to achieve robustness and accuracy, respectively. In practical experiments, we use BioID and FERET datasets to test our approach to obtaining an accurate eye-center location while retaining robustness against changes in scale and pose. In addition, we apply our method to non-frontal faces to test its robustness and accuracy, which are essential in gaze estimation but have seldom been mentioned in previous works. Through extensive experimentation, we show that the proposed method can achieve a significant improvement in accuracy and robustness over state-of-the-art techniques, with our method ranking second in terms of accuracy. According to our implementation on a PC with a Xeon 2.5Ghz CPU, the frame rate of the eye tracking process can achieve 38 Hz.

Improved spatiotemporal-multiplexing super-multiview display based on planar aligned OLED microdisplays.

Through gating spectrum plane of multiple planar aligned OLED microdisplays by a timely sequential manner, a super-multiview (SMV) three-dimensional (3D) display based on spatiotemporal-multiplexing was developed in our previous paper. But an upper limit of the allowable sub-viewing-zones (SVZs) for an OLED microdisplay did exist in the previous system, even if microdisplays with very high frame rates could be commercially available. In this manuscript, an improved spatiotemporal-multiplexing SMV displays system is developed, which removes the above limitation through controllable fusing of light beams from adjacent OLED microdisplays. The employment of a liquid-crystal panel as the gating-aperture array allows the improved system to accommodate multiple rows of OLED microdisplays for denser SVZs. Experimentally, a prototype system is demonstrated by 24 OLED microdisplays, resulting in 120 SVZs with an interval small to 1.07mm.

Tuning of structure and enhancement of upconversion luminescence in NaLuF4:Yb(3+),Ho(3+) crystals.

A series of NaLuF4:Yb(3+),Ho(3+) nano/micro-crystals with different crystal structures were synthesized via a hydrothermal method using citric acid as a chelating agent. The influences of NaF content, Li(+) doping, reaction temperature and reaction time on the crystal structure and shape of the as-synthesized NaLuF4 crystals were systematically investigated. To the best of our knowledge, it is the first time to report Li(+) doped α-NaLuF4:Yb(3+),Ho(3+) nanocrystals and the phase transformation by introducing Li(+) in NaLuF4 crystals. As for Li(+) doped α-NaLuF4, UC luminescence intensities of green emission (538 nm) and red emission (644 nm) in α-NaLuF4:Yb(3+),Ho(3+) nanocrystals with 20 mol% Li(+) doping are enhanced by 20 and 3.5 times compared to their Li(+)-free counterpart. As for Li(+) doped α/ß-mixed NaLuF4, with the increase of Li(+) content, the phase transforms from the α/ß-mixed phase to hexagonal then to cubic. UC emissions of 538 nm and 644 nm in NaLuF4:Yb(3+),Ho(3+) crystals doped with 5 mol% Li(+) are enhanced by 26.5 and 23 times, respectively. Besides, it is found that with the higher temperature and prolonged time, the morphology of NaLuF4 changes from nanoparticles to microtubes, resulting in the dramatic increase of UC emission intensity. The effects of Li(+) doping, reaction temperature and reaction time on the enhancement of UC emission intensity are discussed in detail. This study provides an effective and facile approach to obtain nano/micro-crystals with controllable structures and excellent optical properties.

Generation of 360° three-dimensional display using circular-aligned OLED microdisplays.

A 360° all-around multiview three-dimensional (3D) display system is proposed by using coarse-pitch circular-aligned OLED microdisplays. The magnified virtual color images projected from microdisplays serve as stereo images, which can create separate eyeboxes for the viewer. Through inserting baffles, a transitional stereo image assembled by two spatially complementary segments from adjacent stereo images is presented to a complementary fusing zone (CFZ) which locates between adjacent eyeboxes. For a moving observation point, the spatial ratio of the two complementary segments evolves gradually, resulting in continuously changing transitional stereo images and thus overcoming the problem of discontinuous moving parallax. Such a controllable light-ray fusing technology, assured by the inherent large divergent angle of OLED pixels, decreases the required number of display panels for 360° multiview 3D display greatly. A prototype display system with only 67 full-color OLED microdisplays is set up to demonstrate the 360° 3D color display. The develop system is freed from the dependence on mechanical moving elements, high-speed components and diffusion screens.

Multiview three-dimensional display with continuous motion parallax through planar aligned OLED microdisplays.

Existing multiview three-dimensional (3D) display technologies encounter discontinuous motion parallax problem, due to a limited number of stereo-images which are presented to corresponding sub-viewing zones (SVZs). This paper proposes a novel multiview 3D display system to obtain continuous motion parallax by using a group of planar aligned OLED microdisplays. Through blocking partial light-rays by baffles inserted between adjacent OLED microdisplays, transitional stereo-image assembled by two spatially complementary segments from adjacent stereo-images is presented to a complementary fusing zone (CFZ) which locates between two adjacent SVZs. For a moving observation point, the spatial ratio of the two complementary segments evolves gradually, resulting in continuously changing transitional stereo-images and thus overcoming the problem of discontinuous motion parallax. The proposed display system employs projection-type architecture, taking the merit of full display resolution, but at the same time having a thin optical structure, offering great potentials for portable or mobile 3D display applications. Experimentally, a prototype display system is demonstrated by 9 OLED microdisplays.

Tunable multicolor and white-light upconversion luminescence in Yb3+/Tm3+/Ho3+ tri-doped NaYF4 micro-crystals.

NaYF4 micro-crystals with various concentrations of Yb(3+) /Tm(3+) /Ho(3+) were prepared successfully via a simple and reproducible hydrothermal route using EDTA as the chelating agent. Their phase structure and surface morphology were studied using powder X-ray diffraction (XRD) and scanning electron microscopy (SEM). The XRD patterns revealed that all the samples were pure hexagonal phase NaYF4. SEM images showed that Yb(3+)/Tm(3+)/Ho(3+) tri-doped NaYF4 were hexagonal micro-prisms. Upconversion photoluminescence spectra of Yb(3+)/Tm(3+)/Ho(3+) tri-doped NaYF4 micro-crystals with various dopant concentrations under 980 nm excitation with a 665 mW pump power were studied. Tunable multicolor (purple, purplish blue, yellowish green, green) and white light were achieved by simply adjusting the Ho(3+) concentration in 20%Yb(3+)/1%Tm(3+)/xHo(3+) tri-doped NaYF4 micro-crystals. Furthermore, white-light emissions could be obtained using different pump powers in 20%Yb(3+)/1%Tm(3+)/1%Ho(3+) tri-doped NaYF4 micro-crystals at 980 nm excitation. The pump power-dependent intensity relationship was studied and relevant energy transfer processes were discussed in detail. The results suggest that Yb(3+)/Tm(3+) Ho(3+) tri-doped NaYF4 micro-crystals have potential applications in optoelectronic devices such as photovoltaic, plasma display panel and white-light-emitting diodes.

Spatiotemporal multiplexing for holographic display with multiple planar aligned spatial-light-modulators.

A holographic display system combining the spatial- and time-multiplexing together in one system is proposed. The system is constructed by multiple planar aligned spatial-light-modulators (SLMs). A shiftable cylindrical lens is introduced in to build up an "equivalent SLM" by seamlessly linked horizontal images of the SLMs, which are tiled in a time-sequential manner. The proposed system can realize wide horizontal-viewing-angle holographic three-dimensional (3D) display through the "equivalent SLM", but bear with low requirements on the number and frame rate of SLMs, and the numerical aperture of the optical system. In the proposed system, only one parallel incident beam is needed, leading to a simplified optical structure. Using two 60Hz phase SLMs, a 3D display with a horizontal viewing angle (VA) of 27.5° is implemented experimentally.

Super multi-view three-dimensional display through spatial-spectrum time-multiplexing of planar aligned OLED microdisplays.

Existing super multi-view (SMV) technologies depend on ultra-high resolution two-dimensional (2D) display panel or large number of 2D display panels to obtain dense sub-viewing-zones for constructing more natural three-dimensional (3D) display by pure spatial-multiplexing. Through gating the spatial-spectrum of each OLED microdisplay, the present work proposes a new SMV technology combining time- and spatial-multiplexing based on planar-aligned OLED microdisplays. The inherent light emission characteristics of OLED, i.e. large divergence angle, guarantees a homogeneous light intensity distribution on the spectrum plane, which is a necessary condition for successful time multiplexing. The developed system bears with low requirements on the number of 2D display panels. The factors influencing the lateral display resolution limit are discussed and the optimum value is deduced. Experimentally, a prototype system with 60 sub-viewing-zones is demonstrated by 12 OLED microdisplays. The horizontal interval between adjacent sub-viewing-zones is 1.6mm.