Table 1 compares the different AR-HMD optical combiners. Each has advantages and disadvantages. Based on the optical effect, compared with micro and nanooptics, macrooptics can achieve a higher optical efficiency and a larger FOV. A larger FOV can provide a more immersive experience for customers. The development of high-precision diamond turning and optical injection processing technologies facilitated a submicron machining precision and guaranteed the mass production of AS and FFS lenses. For AS and FFS lenses with small apertures, the submicron machining accuracy is sufficient to satisfy current AR-HMD requirements. However, the detection of AS and FFS still has some limitations, which are limited detection accuracy and speed. The polishing, coating, and cementing processes of geometrical waveguides also interfere with the quality of the geometrical waveguide, because it requires a parallelism of PRMA less than 10”. In terms of the shape factor, the optical combiners of macrooptics are larger than those of micro and nanooptics. AR-HMDs based on combiners of macrooptics still have the problem of being bulky and heavy.
Type Combiner Efficiency Bandwidth FOV (diagonal) Form factor Fabrication requirements Macrooptics Traditional <50% Large 90° Large CNC grinding; diamond turning and injection
(submicron machining precision)
FFS prism <50% Large 120° Large Geometrical waveguide <20% Large 40° Medium Polishing, coating, cementing. (Parallelism of PRMA<10’’) Microoptics SRG <10% Medium 56° Small RIE and EBL; nanoimprinting. (machining precision <100 nm) VHG <10% Small 40° Small Laser exposure; (submicron machining precision) PVG <10% Medium 50° Small Nanooptics Metalens <10% Medium 76° Small RIE and EBL; nanoimprinting. (machining precision <100 nm) Metasurface reflector — Medium 81° Small
Table 1. Comparisons among AR-HMD Optical Combiners
Compared with macrooptics, because the diffraction effect can result in a larger beam deflection angle, combiners based on microoptics are small in size and weight, making them easy to miniaturize. An acceptable FOV larger than 40° can be realized in some aspects. The bandwidth for SRG and PVG are medium, and small for VHG. However, the current microoptics-based diffraction waveguide combiners still encounter many problems such as low efficiency (<10%), chromatic aberration in full-color displays, illuminance nonuniformity, and lack of mass production processing technology, which hinder their commercial application. Furthermore, owing to the large angle and wavelength selectivity and the higher order diffraction light of the diffractive waveguide, the dispersion and stray light are prominent problems that disturb the user experience. RIE and EBL technologies are used to manufacture SRG, and the nanoimprinting process can realize a high-precision (machining precision < 100 nm) large-size and mass production. In EBL technologies, the spin-coated resist layer is first exposed by the electron beam and then etched to reduce its lateral size to generate the nanostructures. However, the angle of the slanted grating and the ratio of line width to depth must be strictly limited in the production of SRG to avoid the direct collapse of the imprint grating, which restricts and influences the optimal optical design effect. High cost and low efficiency are still problems that limit SRG. In the manufacturing of VHG and PVG, submicron machining precision can be achieved using the laser exposure method. The production of holographic materials directly affects the uniformity of the coating. Furthermore, environmental stability requirements, such as humidity, temperature, and air fluidity, affect the exposure quality. This type of exposure method makes it difficult to realize a mass production process that maintains quality, which hinders the popularization and application developments for the devices. The emergence of roll-to-roll production technology has significantly promoted the mass production of waveguides. However, owing to the limitation of the Bragg condition, the realization of full-color waveguide processing without chromatic aberration is still a significant challenge.
For nanooptical combiners, the combiners based on metalens facilitate a small form factor and large FOV. The nanooptics-based combiners achieve the imaging effect through the modulation of the phase and polarization using the arrangement of nanorods. The bandwidth for the metalens and metasurface reflectors is medium. However, the current nanooptics-based combiners have many problems such as low efficiency (<10%), chromatic aberration in full-color displays, and lack of mass production processing technology, which hinder their commercial application. Furthermore, owing to the large angle and wavelength selectivity, and the roughness of the device surface, dispersion problems are still prominent and disturb user experience. In the manufacturing of metalens or metasurface reflectors, because RIE and EBL technologies and the nanoimprint processes (machining precision <100 nm) are used, the problems encountered are similar to those of the SRG. The precision of the sub-structure nanorod contribute significantly to the final optical effect.
In the future, the improvement of the optical effect in the current combiners will depend on the advancement in the combination of various solutions, the manufacturing technologies, and the adoption of emerging materials and display devices.
Challenges and opportunities coexist for the design method, and the combination of various solutions will introduce complex methods and systems. However, this is a promising approach. FFS elements frequently exhibit continuous smooth and variable curvature characteristics and have a significant correcting effect on imaging aberration, which is beneficial to increasing image quality. The image quality of the system must be increased significantly by applying it to the projection system of waveguide-type combiners. Application examples include the combination of an FFS element with a geometric waveguide51 and an FFS element with a VHG diffraction waveguide110. In addition, AR-HMD solutions based on the combination of retinal projection (acquiring a large FOV) and SRG diffraction waveguide combiners (acquiring small form factor) have also received significant interest and applications69. In addition, multifocal imaging technology is promising for use in AR-HMDs to produce virtual patterns with multiple depths of virtual imaging or real stereoscopic effects111.
In manufacturing of microoptics and nanooptics in particular, the improvement of the optical effect still relies on sub-structure processing. Sub-structures with complex forms (increased freedom of design and feasibility of larger bandwidth) and high precision (satisfying design requirements) will result in better optical effects. The combiner fabrication of SRG and metalens is accomplished using semiconductor fabrication technology. Some problems remain, such as color inhomogeneity, the rainbow effect, and achieving an excellent fabrication rate. The improvement of the resolution and efficiency of EBL and laser exposure methods are expected to be central in the fabrication of micro and nanooptical combiners. Furthermore, we can expect to achieve low cost, large size, and mass production of micro and nanooptics based on advances in nanoimprint technologies. More time is required for manufacturing technologies to mature further and for the rate of mass production to increase.
The emergence and adoption of materials such as polymer nanomaterials will also assist in achieving a better optical effect, such as a decrease in optical chromatic aberration and better imaging quality. The VHG combiners also depend on exposure and substrate materials. From silver halide and dichromate to today's photoinduced polymer materials, the exposure effect of VHG has been significantly improved88−90. From traditional glass substrates to today's resin substrates, the lightweight and shape of the VHG waveguide system has improved87. These emerging microdisplays112 can be further studied to create specialized image sources, such as a special polarization and curved-screen displays.
Design and manufacture AR head-mounted displays: A review and outlook
- Light: Advanced Manufacturing 2, Article number: 24 (2021)
- Received: 30 December 2020
- Revised: 30 August 2021
- Accepted: 31 August 2021 Published online: 26 September 2021
Abstract: Augmented reality head-mounted displays (AR-HMDs) enable users to see real images of the outside world and visualize virtual information generated by a computer at any time and from any location, making them useful for various applications. The manufacture of AR-HMDs combines the fields of optical engineering, optical materials, optical coating, precision manufacturing, electronic science, computer science, physiology, ergonomics, etc. This paper primarily focuses on the optical engineering of AR-HMDs. Optical combiners and display devices are used to combine real-world and virtual-world objects that are visible to the human eye. In this review, existing AR-HMD optical solutions employed for optical combiners are divided into three categories: optical solutions based on macro-, micro-, and nanooptics. The physical principles, optical structure, performance parameters, and manufacturing process of different types of AR-HMD optical solutions are subsequently analyzed. Moreover, their advantages and disadvantages are investigated and evaluated. In addition, the bottlenecks and future development trends in the case of AR-HMD optical solutions are discussed.
Design and manufacturing of AR head-mounted display
Augmented reality head-mounted displays (AR-HMDs) not only enable users to visualize virtual information generated by a computer anytime and anywhere, but also enable users to see real images of the outside world. In the AR-HMDs, optical combiners and display devices are used to mix objects in the real world and the virtual world, which are perceived by the human eye. The team of Yongtian Wang from Beijing Institute of Technology divided the AR-HMD optical solutions for optical combiners into three categories: optical solutions based on macro optics, micro optics and nano optics solutions. The team analyzes the physical principles, optical structure, performance parameters, and manufacturing process of different types of AR-HMD solutions, and they also evaluate their advantages and disadvantages. In addition, the team discusses the bottlenecks and future development trends of AR-HMD optical solutions.
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