Compactness and light weight, large exit pupil diameter and distance, small distortion for virtual image, and see-through light paths are pivotal factors to achieve a better, wearable experience of optical see-through head-mounted displays (OST-HMDs). In addition, light efficiency of the virtual image light path is an important factor for heat dissipation in HMD devices. This paper presents a new type of OST-HMD optical system that includes three wedge-shaped freeform prisms and two symmetric lenses. Based on a 0.71 in. microdisplay, an OST-HMD prototype with a diagonal field of view (FOV) of 45.3°, an F-number (F/#) of 1.8, an exit pupil size of 12 mm × 8 mm , and an eye relief of 18 mm is demonstrated. The maximum value of distortion of the final system is 0.6% and 0.4% for virtual image and see-through light path, respectively. The overall dimension of the optical system per eye is no larger than 30 mm ( width ) × 40 mm ( height ) × 14 mm (thickness), and the weight of the optical module including lenses, holder, and microdisplay is 12.8 g. The light efficiency of the virtual image light path is up to 50% higher than those of other OST-HMD optical solutions.

Free-form surfaces (FFSs) provide more freedom to design an optical system with fewer elements and hence to reduce the size and weight of the overall system than rotationally symmetric optical surfaces. In this paper, an optical see-through (OST), head-mounted display (HMD) consisting of a free-form, wedge-shaped prism and a free-form lens is designed and fabricated through the injection molding method. The free-form prism for the projection system is designed with a field-of-view (FOV) of 36°; the free-form lens is cemented to the prism for the see-through system to achieve a FOV of 50°. The free-form prism and lens are expanded at the edge area during the design stage in order to reduce the effects of surface deformation in the working area in molding fabrication process and to improve ergonomic fit with the head of a user. The tolerance analyzes considering the mold design for the free-form optical systems are carried out using the Monte Carlo method. The FFS optical elements are successfully fabricated and the system performance is carefully examined. The results indicate that the performance of the OST-HMD is sufficient for both entertainment and scientific applications.

Augmented reality head-mounted displays (AR-HMDs) based on diffractive waveguides have been a challenging and rewarding research topic focusing on near-eye displays. The size of the exit pupil and uniformity of the image illuminance are two important factors that affect the display performance of the diffractive waveguide. In this paper, a novel method for optimizing high uniformity of two-dimensional (2D) diffractive waveguide is proposed. A straight-line 2D surface relief grating (SRG) waveguide with divided grating regions is designed. An illuminance uniformity evaluation model of the energy propagation process is established, and non-sequential ray tracing is utilized to optimize the diffraction efficiency of multi-regions grating to achieve illuminance uniformity distribution. Then, the uniformity distribution of the diffraction efficiency in different fields of view (FOVs) is realized by combining the particle swarm optimization (PSO) algorithm and rigorous couple wave analysis (RCWA) to optimize the grating structural parameters, which further ensures the uniformity of the exit pupil illuminance and angular illuminance. The waveguide with exit pupil expansion (EPE) has exit pupil size of 16 mm × 14 mm at an eye relief (ERF) of 20 mm, exit pupil illuminance uniformity of 91%, and angular uniformity illuminance of 64%.

In this paper, we propose a framework of starting points generation for freeform reflective triplet using back-propagation neural network based deep-learning. The network is trained using various system specifications and the corresponding surface data obtained by system evolution as the data set. Good starting points of specific system specifications for further optimization can be generated immediately using the obtained network in general. The feasibility of this design process is validated by designing the Wetherell-configuration freeform off-axis reflective triplet. The amount of time and human effort as well as the dependence on advanced design skills are significantly reduced. These results highlight the powerful ability of deep learning in the field of freeform imaging optical design.

It is difficult to design and fabricate a head-up display (HUD) with a large asymmetric field of view (FOV) while maintaining a good image quality. In this paper, we design and develop such a holographic HUD system. To improve luminance and enhance the environmental adaptability of the HUD, we use a liquid crystal display with high brightness as well as a holographic reflection element. The vertical FOV is tilted to an angle of $-7°$, which is consistent with the actual view of the pilot. The decentration and tilt of the optical relay system effectively compensate for the distortion and off-axis aberration due to the asymmetric FOV and tilted, curved combiner. The transition plates used in the mechanical structure significantly decrease the difficulty in fabrication and alignment, and virtual prototyping greatly reduces the risk and shortens the development cycle. Experimental results demonstrate that this prototype achieves a total FOV of 30° (horizontal) $\times 24°$ (vertical), a resolution of $1280\times 1024$, and a 5% distortion.

In this paper, we propose a novel method to construct an optical see-through light-field near-eye display (OST LF-NED) by using a discrete lenslet array (DLA). The DLA is used as a spatial light modulator (SLM) to generate dense light field of three-dimensional (3-D) scenes inside the user’s eyebox of the system and provide correct focus cues to the user. A corresponding light-field image rendering method is also proposed and demonstrated. The light emitted from the real objects passes through the transparent region of the display panel and the planar area of the DLA successively without redirection, so the user can have a clear view of the real scene as well as the virtual information. The stray light that may degrade the image quality has been analyzed in detail. The experimental result shows that the proposed method is capable of obtaining a corrected depth perception of the virtual information in augmented reality (AR) applications.