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Virtual image distance (VID) is a key parameter in augmented reality (AR) or virtual reality (VR) systems and has a great influence on product performance and user experience. Therefore, it is very necessary to measure the VID accurately and efficiently. In this paper, we develop a VID measurement system which needs no mechanical scanning in the measurement process. The VID can be measured in a short period of time based on one single image generated by the AR/VR system. An image-telecentric lens is designed which is used to capture the virtual image. A diaphragm with two pinholes is placed at the entrance pupil plane of the imaging lens. When the imaging lens is not focused on the virtual image, the captured image contains two copies of the virtual image with a certain interval which is proportional to the VID in units of diopters. By calculating the interval, we can obtain the VID. A simulation is performed in LightTools to verify the effectiveness of the measurement system, and the result indicates that the measurement error is 5 mm when the virtual image is at 3 m.

Compared with the traditional panel display, the head-mounted display (HMD) is a new technology. When using an HMD, it occupies a significant portion of the eyes’ natural field of view (FOV). Thus, it is necessary to ensure the imaging quality of an HMD, which requires accurate measurement and evaluation. Based on the measurement of panel display, this paper proposes a kind of luminance and chromaticity measuring equipment for HMD by combining an optical fiber spectrometer with a translation platform. The equipment measures various parameters such as brightness, chromaticity, contrast, color gamut, eyebox, etc. To prove the validity of the equipment, several HMDs with different luminance are used for multiple measurements. The final results show that the brightness stability of the equipment reaches 2%, and the chromaticity stability reaches 0.002, which fully meets the measurement requirements of HMD. This equipment has certain reference value for HMD production and testing.

We have proposed a design method of a freeform refractive surface for illumination using multiple faceted refractors. The reflection loss on the refractive surface also has been considered during the design process. As an example, a smooth freeform lens with uniform illumination distribution is designed. Compared with the direct method without consideration of the loss of energy, the method achieved a great improvement in the uniformity of illumination distribution with no loss of output efficiency.

We proposed a method to automatically eliminate the obscuration in the design of off-axis mirror systems without plane of symmetry. Off-axis settings and local surface expression are designed to simplify the system representation. Obscuration error function is established based on the positional relation between the proposed mirror boxes and ray boxes, and through function optimization, the obscuration is eliminated. The method is demonstrated by two examples, indicating its ability to eliminate obscuration for totally off-axis mirror system and the searching for such systems from co-axial starting points.

Ultra-throw projector has been widely used in the projection application of large-FOV and short projection distance. However, the projection imaging distortion problem cannot be neglected due to the uncertain factor, such as the unsmooth factor of projection surface. In this paper, we proposed the image predistortion method based on radial basis function (RBF) image warping method to correct the projection distortion due to the unsmooth or wrinkled projection surface. The RBF image predistortion method and chief ray tracing method are used to realize a high-precision image predistortion. In addition, the distortion effect caused by the wrinkled projection surface with different depths are simulated. The distortion correction effects are also verified to less than 1% by the ray-tracing simulations.

Most of the freeform lens optimization methods for LED beam shaping are focused on producing rectangular illumination patterns. Here we demonstrate the generation of non-rectangular irradiance distributions by optimizing the 'uv'-polynomial freeform surface, where (u,v) are stereographic coordinates. An initial freeform surface is designed with an optimal transport ray mapping method that is suitable for unconventional boundaries. Simulated annealing is then used to optimize the polynomial coefficients based on an automated workflow that links Matlab (central platform and optimization engine), Rhinoceros (3D molding) and LightTools (Ray tracing).

It has been a challenge to design an automotive head-up display (HUD) that has large eyebox while maintain a compact structure. In this paper, we present a combiner-type HUD design using freeform mirrors. The HUD is an off-axis fourmirror system consisting of 4 mirrors, which are enclosed in a box to achieve a compact form. The tilt angle of the combiner is adjustable, which can be adapted to different heights of exit pupil. We initially propose an off-axis, fourmirror system which only uses spherical surfaces, then we optimize it by replacing with freeform surfaces, which enables us to balance the residual asymmetrical aberrations. Finally, we obtain an HUD system with good imaging performance. The virtual image is projected at 5 meters away from observer, and the eye box is 106 × 66 mm2 .

Retinal projection display (RPD) is a research hotspot in the field of near-eye display (NED), which has a long depth of field and is able to overcome the vergence-accommodation conflict (VAC) because of the principle of Maxwellian view. However, the existing problems of RPD, such as small field of view (FOV) and large volume heavy weight, restrict its application and development. In this paper, an RPD system based on micro-electro-mechanical system (MEMS) and conicoid curved semi-reflective mirror is proposed in order to realize a large FOV with compact structure and light weight. The MEMS used is a kind of biaxial scanning mirror, which works in coordination with RGB laser diodes for the sake of scanning a two-dimensional image in a specific direction with high illuminance and high resolution. The conicoid curved semi-reflective mirror is used to project the image onto the retina and form a large FOV with a suitable eye relief distance (ERF), meanwhile to keep the external object visible. The combination of MEMS, RGB laser diodes and conicoid curved semi-reflective mirror retains compact size factor suitable for wearable function. The performance of the proposed RPD is quantitatively analyzed. And the key performance is set forth in detail as follows. The FOV is 70° horizontal (H) × 40° vertical (V), ERF is 30mm. And the MTFs are discussed as well. The proposed RPD system realizes a large FOV, a long depth cue and high performance with compact size, which can be further applied in the next-generation optical see-through NED.

The compound eye optical system has attracted much recent attention, owing to its large field-of-view, compact structure, and rapid imaging capabilities. We designed a compact image scanner (cross-section 36.5 mm × 17 mm) based on the compound eye system. A single imaging unit is mainly composed of two freeform mirrors and a flat mirror, and its field-of-view in object space reaches 10 mm. The modulation transfer function of the system is greater than 0.6 at 12 cycles/mm corresponding to Nyquist spatial frequency of 600 dots per inch in depth-of-field of 0~4 mm. Furthermore, a larger field-of-view is achieved by splicing multiple imaging units with 1-mm overlap in object space. An object-side telecentric system is used to obtain constant magnification at different object distances and the distortion (

Using freeform optical surface in an imaging optical system is a revolution in the field of optical design. Introducing machine learning into freeform imaging optical design will significantly reduce the human effort and even beginners in optical design will be able to perform difficult design tasks. Machine learning has been successfully applied to the immediate generation of starting points with various system specifications for the design of freeform reflective imaging systems. However, the parameters used in the network training, which are the key points in the whole design framework, are determined without proper guidance, which may significantly affect the actual performance of the networks. In this paper, a comprehensive exploration of the training parameters of the neural network used for starting points generation of freeform reflective systems is conducted. The parameters include the number of layers, the number of nodes in the layers, the type of activation function, the type of loss function, the type of optimization algorithm, and the value of learning rate. A detailed comparison and analysis of different training parameters are demonstrated on the training results and the imaging performance of validation output systems with large amount of random system specifications input. Using the obtained results designers can choose proper parameters accordingly and get desired neural networks with shorter training time and better performance. The results also offer insight in the design of imaging systems with other system configurations and more advanced system specifications.

With the widespread use of AR/VR head-mounted displays, there is an increasing demand for pinhole cameras to test their image quality. However, it has been a challenge to design a pinhole camera with a wide field of view (FOV), high image performance and low distortion while maintaining compact and lightweight. In this paper, a pinhole camera optical system is designed. Based on a 1 in. sensor, the pinhole lens has a FOV of 100° and an entrance pupil diameter of 4mm. In our final design, the distortion is limited within 2.5% and the modulation transfer function values across the entire field are above 0.3 at 161 line pairs/mm (lps/mm). The spherical aberration, color aberration and field curvature are corrected. The design result shows that the optical system of the pinhole camera is compact and meets the requirements of the VR/AR image quality test system.

Freeform optical surfaces can be characterized as nonsymmetric surfaces and they can offer much more degree of freedom for optical design. This kind of optical surface can be seen as a revolution in the optical design and plays a key role in the next generation of high-performance optical systems. Another trend in imaging optics is to use phase elements (such as diffractive elements and metasurface). In specific, the flat or planar phase element can effectively reduce the weight and volume of the total system. Easier-alignment of the system can also be achieved. In this paper, the point-by-point design method are applied to the design of three kinds of nonsymmetric imaging systems: consisting of only geometric freeform surfaces, only flat phase elements, and both of them (the generalized case). The entire design process begins from an initial system using simple geometric planes. Both the geometric freeform surfaces and the phase profiles or functions are generated point-by-point based on specific design requirements. The design results can be taken as good starting points for further optimization. The dependence on existing starting points is significantly reduced and advanced design skills are not required. In addition, three typical three-mirror folding geometries are employed and designed using the proposed method for all the three kinds of systems under same system specifications. The imaging performance and system volume of the different systems after final optimization are analyzed and compared. The results offer insight on the selection of optimal system folding geometry and types of imaging element for the nonsymmetric system design tasks.

As a kind of radial basis functions, the Gaussian function has local feature and therefore has an excellent surface description ability. In this paper, we proposed the design strategy of freeform unobscured three-mirror system using Gaussian radial basis functions surface type and demonstrate a design example. A novel and high-accuracy surface fitting algorithm of Gaussian radial basis functions is proposed for the freeform surface fitting. Successive optimization strategy is employed for the system after surface fitting. The final example system works at the long wave infrared band and has an 8°×6° field-of-view with an F-number of 1.9.

An accurate vertebrae segmentation in the spine is an essential pre-requisite in many applications of image-based spine assessment, surgical planning, clinical diagnostic treatment, and biomechanical modeling. In this paper, we present the stacked sparse autoencoder (SSAE) model for the segmentation of vertebrae from CT images. After the preprocessing step, we extracted overlapped patches from the vertebrae CT images as the inputs of our proposed model. The SSAE model was trained in an unsupervised way to learn high-level features from the input pixels of the unlabeled images patch. To improve the discriminability of the learned features, we further refined the feature representation in a supervised fashion and fine-tuned the whole model by using the feedforward neural network parameters for classifying the overlapped patches. We then validated our model on a publicly available MICCAI CSI2014 dataset and found that our model outperforms the other state-of-the-art methods.

Vascular anastomosis, connecting two vessel ends together, is the foundation of plastic and reconstructive surgery. Every living tissue transplantation requires vascular anastomosis. While optical coherence tomography (OCT) imaging can provide objective information for intraoperative evaluation, currently it suffers from limited imaging penetration depth for relative large vessels, which will result in information loss at the bottom region of vessel. In this work, we designed a multi-view scanning scheme for the vessel imaging to increase the FOV effectively. To push the clinical translation, we used MEMS mirror to steer the beam and made a miniature handheld probe for testing. Preliminary results on IR sensing card, multilayer scotch tape, and plastic tube imaging showed the performance of our probe.

Optical holography became a powerful tool for arbitrarily manipulating the wavefronts of light. With the recent development of metasurface holography it became possible to tailor all the fundamental properties of light (amplitude, phase, polarization, wave vector and frequency) within a thin slab of material. However, for exploring the full capability of the information storage of metasurface holograms and enhance the encryption security, smart multiplexing techniques together with suitable metasurface designs are required. Here, we demonstrate a novel method for achieving multichannel vectorial holography and show its potential for obtaining dynamic displays and high-security applications. We explore birefringent metasurfaces for the complete control of polarization channels with the freedom of designing both the polarization dependent phase shift and polarization rotation matrix. We show that although the target holographic phase profiles have quantified phase relations they can process very different information within different polarization manipulation channels. For our metasurface holograms, we demonstrate high fidelity, large efficiency, broadband operation, and a total of twelve polarization channels. Such multichannel polarization multiplexing can be used for dynamic vectorial holographic display and provide triple protection to the optical security devices. The concept is appealing for applications of arbitrary spin to angular momentum conversion and various phase modulation/beam shaping elements.

Metallic nanorod metamaterials, arrays of vertically aligned nanorods embedded in an alumina matrix (diameter ~80 nm, length 100-250 nm, period ~113 nm), have recently emerged as a flexible platform for applications in photonics, opto-electronics and sensing. The optical constants for these nanostructured materials are directly associated with their crystallinity. Controlling the crystallinity of these metamaterials in a fast manner has presented a new challenge. Here we show a laser annealing with a pulsed Nd:YAG laser (λ = 532 nm, FWHM 15 ns) to rapidly change the crystallinity of the metallic nanorods. The small column X-Ray diffraction characterization shows that not only the crystallinity of the metallic nanorods is changed, but also that evaporation of the metal occurs with laser annealing.

Vision-tangible mixed reality (VTMR) is a further development of the traditional mixed reality. It provides an experience of directly manipulating virtual objects at the perceptual level of vision. In this paper, we propose a mixed reality system called “VTouch”. VTouch is composed of an optical see-through head-mounted display (OST-HMD) and a depth camera, supporting a direct 6 degree-of-freedom transformation and a detailed manipulation of 6 sides of the Rubik’s cube. All operations can be performed based on the spatial physical detection between virtual and real objects. We have not only implemented a qualitative analysis of the effectiveness of the system by a functional test, but also performed quantitative experiments to test the effects of depth occlusion. In this way, we put forward basic design principles and give suggestions for future development of similar systems. This kind of mixed reality system is significant for promoting the development of the intelligent environment with state-of-the-art interaction techniques.

Ray mapping method can greatly simplify the freeform illumination lens design. However, the result obtained by the ray mapping method sometimes deviates much from the design target. Therefore, we present a construction of double freeform surfaces based on nonlinear least squares and non-uniform rational B-spline surfaces (NURBS) to minimize the deviations. By this way, we can get a smooth double-freeform-surface lens with good optical performance.

Compared with light emitting diodes (LEDs), laser diodes (LDs) are attractive for applications in which size and radiance matter. The elliptical, highly divergent, and astigmatic beam characteristics make LD beams difficult to handle. Compare with conventional techniques, freeform optics can precisely control the LD beam. However, freeform optics design for LD beam shaping is a very difficult inverse problem. A simple and efficient method is provided to iteratively construct freeform surfaces for achieving difficult LD beam shaping tasks.

Computer-generated hologram (CGH) is a key technology in electro-holography systems. However, the huge computational complexity is a challenge for practical demands. In this paper, we propose an improved algorithm to accelerate CGH computation based on symmetric compressed LUT (SC-LUT) algorithm. In LUT calculation, the horizontal and vertical modulation factors is reduced to one-dimension arrays instead of dual one-dimension arrays. In hologram calculation, we introduce a one-time generation of color holograms method which is accelerated by matrix convolution operation. Numerical simulation results also show at least 13 times faster than existing algorithms without sacrificing the computation precision. The proposed algorithm accelerated by the graphic processing unit (GPU) is an effective method for fast calculation of CGH in color holographic display.

Hybrid nanostructures, comprising of a metal core and a semiconductor shell layer, shows great potential for a new generation of low-cost solar cells, due to their unique electronic and optical properties. However, experimental results have fallen far short of unltra-high efficiency (i.e. beyond Shockley-Queisser limit) predicted by theoretical simulation, limiting commercial application. Here, we experimentally design a non-transparent organic solar cell containing an array of Ag/ZnO nanowires, increasing internal quantum efficiency (IQE) by a factor of 2.5 compared with the planar counterpart, indicating a great enhancement of charge collection efficiency, due to the ultrafast Ag nanowire channels. Furthermore, we exploit this hybrid nanostructure as a perfect back reflector for semi-transparent solar cells, resulting in enhanced light absorption by a factor of 1.8 compared with the planar counterparts. The ability to enhance the charge collection and light absorption makes these Ag/ZnO nanostructures a flexible platform for the development of modern optoelectronic devices.

The strive for the small volume and light weight in head-up displays has made the conventional configuration combining relay optics and combiner unpractical any more. In this paper, a compact and light-weight head-up display with a large eyebox is designed, with two orthogonal geometrical waveguides adopted to extend the exit pupil in both directions. This two-dimensional waveguide acts as a pupil expander, which can also save the whole system from the trouble of severe aberrations. A triangular prism serves as the input-coupling and the partially reflective mirrors array is used as the output-coupling. Parameters to shape the combined waveguide are derived. The spherical surface based coaxial projection system to match the combined waveguide are designed to project and magnify the virtual image onto a surface at infinity. The compact head-up display integrating projection optics and the combined waveguide has a horizontal field of 20°, with an eyebox of 80 mmx80 mm.

Road lighting can benefit strongly from LEDs and freeform lenses. However, freeform lenses design for road lighting applications is complicated and difficult when balancing energy efficiency and lighting quality. In addition, the asymmetric properties of the required light distributions and non-neglectable sizes of LEDs make the design more challenging. Currently, we prefer an efficient design strategy with two steps: light distribution optimization and freeform lens construction. In the first step, we employ a modified polynomial representation of the illuminance distribution and perform constrained optimization. In the second step, we construct the freeform lens with double freeform surfaces to realize the optimized light distribution using the ray mapping method associated with over compensation.

Metasurfaces provide great feasibilities for tailoring both propagation waves and surface plasmon polaritons (SPPs). Manipulation of SPPs with arbitrary complex field distribution is an important issue in integrated nanophotonics due to their capability of guiding waves with subwavelength footprint. Here, with metasurface composed of nano aperture arrays, a novel approach is proposed and experimentally demonstrated which can effectively manipulate complex amplitude of SPPs in the near-field regime. Positioning the azimuthal angles of nano aperture arrays and simultaneously tuning their geometric parameters, the phase and amplitude are controlled based on Pancharatnam-Berry phases and their individual transmission coefficients. For the verification of the proposed design, Airy plasmons and axisymmetric Airy beams are generated. The results of numerical simulations and near-field imaging are well consistent with each other. Besides, 2D dipole analysis is also applied for efficient simulations. This strategy of complex amplitude manipulation with metasurface can be used for potential applications in plasmonic beam shaping, integrated optoelectronic systems and surface wave holography.

The contradiction between large field of view, big exit pupil and large eye relief in immersive virtual reality (VR) head- mounted displays limits the probability that such bulky device would be accepted by the most consumers. Typical of the wide field of view optical systems, Oculus CV1 adopts the Fresnel lens as the eyepiece to display the magnified virtual image. However, complexity in fabricating the Fresnel lens makes the commercial VR products with it cost-inefficient, especially when both surfaces employ the Fresnel surface. A wide field of view coaxial catadioptric system is proposed to serve as the magnified optics in this paper, which consists of a wire grid polarizer, two quarter-wave plates and a powered lens, making the optics thinner and fabrication easier. Note that coated on the surface of one quarter-wave plate is a partially reflecting coating. The powered lens takes the role of imaging, while the wire grid polarizer and the quarter- wave plates work together to adjust the polarization orientation so as to suppress direct transmission. The resulting monocular optical system yields the field of view as large as 110°, with virtual image exhibited 5 meters away from the observer. Exit pupil diameter and eye relief of the designed system are 9 mm and 15 mm, respectively. Considerable distortion will appear inevitably when field of view approaches 100°, which could be tackled via electronic correction. With evaluation in the CODEV, weight of the whole monocular optics is about 35 grams and the distance from the element near the observer to the image source is as thin as 30 mm, showing that the suggested design is superior to the ones in the form of singlet, doublet and the singlet using aspheric surface or Fresnel, in terms of the weight, compactness and fabricating cost. With this monocular optical system, an HMD binocular optical system can be achieved in the form of partially overlapping field of view. The present design offers a solution to acquiring the light-weight and small- volume VR optics and making cost of the commercial VR products acceptable to the public at the same time.

A shape-changing table-top display with haptic feedback allows its users to perceive 3D visual and texture displays interactively. Since few existing devices are developed as accurate displays with regulatory haptic feedback, a novel attentive and immersive shape changing mechanical interface (SCMI) consisting of image processing unit and transformation unit was proposed in this paper. In order to support a precise 3D table-top display with an offset of less than 2 mm, a custommade mechanism was developed to form precise surface and regulate the feedback force. The proposed image processing unit was capable of extracting texture data from 2D picture for rendering shape-changing surface and realizing 3D modeling. The preliminary evaluation result proved the feasibility of the proposed system.