Engineering of head-mounted projective displays

Design of a compact waveguide eyeglass with high efficiency by joining freeform surfaces and volume holographic gratings

In this paper, a compact waveguide eyeglass integrating freeform surfaces and volume holographic gratings (VHGs) is proposed for full-color display with high energy utilization. The in-coupler with four freeform surfaces collimates the light emitting from the micro image source (MIS) and couples them into the waveguide. The six-layer VHGs as an outcoupler are designed to modulate the light propagating toward the user's eye. The chromatic aberrations and aberrations are well optimized and compensated by the in-coupler. The diffraction angular bandwidth of the gratings matches the angular range of the light propagating in the waveguide. The simulation results show that our proposed eyeglass achieves a diagonal field of view (FOV) of 39.5°, the average diffraction efficiency of the outcoupler achieves 95.22%, and the diffraction uniformity is about 0.95. Because of the integrated designs and compact stable structures, the optimized display system is expected to be flexibly used in various applications.

Integrated holographic waveguide display system with a common optical path for visible and infrared light

We propose an integrated holographic waveguide display system. An infrared volume holographic grating (IVHG) and a visible light grating are recorded on the same waveguide to achieve the purpose of a common light path for system miniaturization. Simulated and experimental results verify the feasibility of this method. The coupling efficiencies of the infrared module for eye tracking and the visible light module for augmented reality (AR) display are 40% and 45%. The holographic waveguide has a weight of only 4.3 grams. It is believed that this technique is a good way to achieve a light and thin eye tracking near-eye display.

Design of a high-performance in-coupling grating using differential evolution algorithm for waveguide display

Illuminance nonuniformity caused by natural vignetting can seriously affect the display quality of large-field-of-view (FOV) waveguide displays. In this paper, an optimization method based on the differential evolution algorithm is proposed for in-coupling grating design to improve coupling efficiency and compensate for natural vignetting. The in-coupling grating parameters are optimized to achieve efficiency distributions in which efficiency increases continuously with incidence angle, realizing uniform illuminance over a large FOV of 45°. The angular uniformity reaches 0.89. Additionally, average diffraction efficiency reaches 89.13% for transverse-electric polarization at 532 nm and 76% in the wavelength region between 450 and 700 nm.

Highly efficient waveguide display with space-variant volume holographic gratings

We propose a highly efficient waveguide display based on space-variant volume holographic gratings (SVVHGs). The local period and slant angle of the SVVHG vary along the tangential direction, enabling variant incident angles to satisfy the Bragg condition of the local gratings. As a result, we enlarge the field of view (FOV) without using the conventional multiplexing scheme, while achieving high efficiency and large FOV at the same time. We experimentally record the SVVHGs on Bayfol HX200 films. We demonstrate that the proposed display can achieve 31.9% system efficiency for a broadband light source and 52.3% for a coherent light source, 20° FOV, and high brightness uniformity, making it a promising candidate for widespread applications in the augmented reality (AR) industry.

Design of a uniform-illumination binocular waveguide display with diffraction gratings and freeform optics

Uniform illuminance over the expanded exit pupil is an important requirement for waveguide display systems with a wide field of view (FOV). To address this issue, we develop a monochromatic binocular waveguide display in this paper. Two surface-relief diffraction gratings are designed as in-couplers and out-couplers. The parameters of the gratings are optimized to achieve uniform diffraction efficiency distributions over a broad angular range. The grating couplers enable the system to realize a diagonal FOV of 40°. A freeform surface prism is designed as the projection optics. The diameters of the two exit pupils are 12 mm in the expanding direction at an eye relief of 19 mm.

Design of a large field-of-view see-through near to eye display with two geometrical waveguides

A novel waveguide near to eye display (WGNED), with new in-coupling and propagation subsystems, is proposed for the first time, to our knowledge, to enlarge the vertical field-of-view (FOV) and the vertical size of the eye box. Two waveguides are stacked-one is for in-coupling and the other for out-coupling. A freeform prism is used to correct the aberrations. These components are combined to form the WGNED. We have simulated such a system; as a result, we show that it achieves a FOV of 30°horizontal (H)×60°vertical (V) and an eye box of about 15  mm (H)×12  mm (V). The modulation transfer function of the system is larger than 0.3 at 33  lp/mm and the distortion is smaller than 5%.

Portable waveguide display system with a large field of view by integrating freeform elements and volume holograms

A compact waveguide display system integrating freeform elements and volume holograms is presented here for the first time. The use of freeform elements can broaden the field of view, which limits the applications of a holographic waveguide. An optimized system can achieve a diagonal field of view of 45° when the thickness of the waveguide planar is 3mm. Freeform-elements in-coupler and the volume holograms out-coupler were designed in detail in our study, and the influence of grating configurations on diffraction efficiency was analyzed thoroughly. The off-axis aberrations were well compensated by the in-coupler and the diffraction efficiency of the optimized waveguide display system could reach 87.57%. With integrated design, stability and reliability of this monochromatic display system were achieved and the alignment of the system was easily controlled by the record of the volume holograms, which makes mass production possible.

Truncated corner cubes with near-perfect retroreflection efficiency

By isolating a finite effective volume from a conventional triangular pyramid corner cube, we obtained truncated corner cube structures with greatly enhanced retroreflection efficiency. We explore an optimal truncated corner cube with near 100% retroreflection efficiency based on the expectation that the traveling paths of the optical rays can be localized in the finite effective volume of the structure, and, as a result, truncated corner cubes with perfect efficiency can be produced. As a case study, the retroreflection efficiency of a commercialized 3M truncated corner cube sample is evaluated. Furthermore, it is shown with numerical verification that a truncated corner cube array sheet with near-perfect retroreflection efficiency can be produced.

Investigation of a 3D head-mounted projection display using retro-reflective screen

We propose a compact head-worn 3D display which provides glasses-free full motion parallax. Two picoprojectors placed on the viewer's head project images on a retro-reflective screen that reflects left and right images to the appropriate eyes of the viewer. The properties of different retro-reflective screen materials have been investigated, and the key parameters of the projection - brightness and cross-talk - have been calculated. A demonstration system comprising two projectors, a screen tracking system and a commercial retro-reflective screen has been developed to test the visual quality of the proposed approach.

Design of an optical see-through head-mounted display with a low f-number and large field of view using a freeform prism

It has been a challenge to design an optical see-through head-mounted display (OST-HMD) that has a wide field of view (FOV) and low f-number (f/#) while maintaining a compact, lightweight, and nonintrusive form factor. In this paper, we present an OST-HMD design using a wedge-shaped freeform prism cemented with a freeform lens. The prism, consisting of three freeform surfaces (FFSs), serves as the near-eye viewing optics that magnifies the image displayed through a microdisplay, and the freeform lens is an auxiliary element attached to the prism in order to maintain a nondistorted see-through view of a real-world scene. Both the freeform prism and the lens utilize plastic materials to achieve light weight. The overall dimension of the optical system per eye is no larger than 25 mm by 22 mm by 12 mm, and the weight is 8 g. Based on a 0.61 in. microdisplay, our system demonstrates a diagonal FOV of 53.5 degrees and an f/# of 1.875, with an 8 mm exit pupil diameter and an 18.25 mm eye relief.

Imaging quality of a retroreflective screen in head-mounted projection displays

A retroreflective screen composed of miniature corner cube reflectors (CCRs) or microbeads redirects an incident ray in the reverse direction. Recently the retroreflective screen was utilized as a key element in head-mounted projection displays (HMPDs). Most prior efforts in developing the HMPD technology have been focused on optimizing the optical design of the projection optics, neglecting the imaging artifacts caused by the screen. Few efforts have been attempted to analyze and evaluate the overall image quality of the HMPD system with the presence of a retroreflective screen. This paper first applies a ray-tracing method to examine the imaging properties of a single CCR. Through the combination of both the geometrical imaging effect and the diffraction effect, the imaging properties of a CCR-based retroreflective screen are analyzed and characterized. Based on these analytical results, the paper further evaluates how the imaging artifacts of a retroreflective screen degrade the spatial resolution of an HMPD system and limit the tolerance range of the distance from an HMPD user to the screen. Finally, a discussion is employed to illustrate potential techniques in minimizing the image quality degradation through the optimization of the corner cube size in a retroreflective screen.

Geometrical optics analysis of the structural imperfection of retroreflection corner cubes with a nonlinear conjugate gradient method

Geometrical optics analysis of the structural imperfection of retroreflection corner cubes is described. In the analysis, a geometrical optics model of six-beam reflection patterns generated by an imperfect retroreflection corner cube is developed, and its structural error extraction is formulated as a nonlinear optimization problem. The nonlinear conjugate gradient method is employed for solving the nonlinear optimization problem, and its detailed implementation is described. The proposed method of analysis is a mathematical basis for the nondestructive optical inspection of imperfectly fabricated retroreflection corner cubes.

Design of a polarized head-mounted projection display using ferroelectric liquid-crystal-on-silicon microdisplays

It has been a common problem in optical see-through head-mounted displays that the displayed image lacks brightness and contrast compared with the direct view of a real-world scene. This problem is aggravated in head-mounted projection displays in which multiple beam splitting and low retroreflectance of a typical retroreflective projection screen yield low luminous transfer efficiency. To address this problem, we recently proposed a polarized head-mounted projection display (p-HMPD) design where the polarization states of the light are deliberately manipulated to maximize the luminous transfer efficiency. We report the design of a compact p-HMPD prototype system using a pair of high-resolution ferroelectric liquid-crystal-on-silicon (FLCOS) microdisplays. In addition to higher resolution, the FLCOS displays have much higher optical efficiency than a transmissive-type liquid crystal display (LCD) and help to further improve the overall light efficiency and image quality. We detail the design of a compact illumination unit for the FLCOS microdisplay, also commonly referred to as the light engine, and a projection lens, both of which are key parts of the p-HMPD system. The performances of the light engine and projection lens are analyzed in detail. Finally, we present the design of a compact p-HMPD prototype using the custom-designed light engine and projection optics.

Design of a bright polarized head-mounted projection display

In optical see-through head-mounted displays, it has been a common challenge that the displayed image lacks brightness and contrast compared with the direct view of a real-world scene. Consequently, such displays are usually used in dimmed lighting conditions, which limits the feasibility of applying such information displays outdoors or in scenarios where well-lit environments, such as in operation rooms, are required. The lack of image brightness is aggravated in the design of a see-through head-mounted projection display (HMPD). For instance, the overall flux transfer efficiency of existing HMPD designs is less than 10%. The design of a polarized head-mounted projection display (p-HMPD) is presented. The images of a p-HMPD system can potentially be three times brighter than those in existing HMPD designs. It is further demonstrated that the p-HMPD design is able to dramatically improve image brightness, contrast, and color vividness with experimental results. Finally, the design of a compact optical system and helmet prototype is described.

Design and assessment of microlenslet-array relay optics

Recent progress in micro-optics fabrication and optical modeling software opens the opportunity to investigate how microlenslet-array-based compact relay systems can be designed and assessed. We present various optical configurations that include an appropriate baffle computation to eliminate ghost images, followed by an analysis of image quality. The investigation shows the existing trade-off between compactness of the system and a tiling effect observed in the corresponding image, an effect we refer to as lensletization. To yield meaningful optical modeling results, we provide insight into ray-tracing optimization while ensuring a sufficient signal-to-noise ratio. The results show that, given no discernable lensletization, the most compact configuration to image gray-scale images is the 5f-based system. Finally, simulations of the imaging of gray scale and color bitmaps through microlenslet arrays are demonstrated for the first time to our knowledge.

Design of an ultralight and compact projection lens

Driven by the need for lightweight head-mounted displays, we present the design of an ultralight and compact projection lens for a head-mounted projective display (HMPD). An HMPD consists of a pair of miniature projection lenses, beam splitters, and miniature displays mounted on the helmet and retroreflective sheeting materials placed strategically in the environment. The HMPD has been proposed recently as an alternative modality for three-dimensional visualization. After demonstrating the concept, building a first-generation custom-designed prototype, and investigating perception issues and application potentials, we designed an ultralight and compact projective lens with a diffractive optical element (DOE), plastic components, and aspheric surfaces for the next-generation prototype. The key contribution here lies in the conception, optimization, and assessment of the projection optics. Thus a brief review of the HMPD technology and related research is followed by a detail discussion of the conception and optimization of the ultralight and high-performance projection optics. The design of the DOE will be particularly described in detail. Finally, the diffraction efficiency of the DOE will be evaluated, and the overall performance of the optics will be assessed in both object space for the optical designer and visual space for possible end-users of the technology.