Kress and Sterner introduced the critical optical design challenges for AR NEDs, including providing sufficient resolution, large eyebox and wide FOV ( Kress and Starner, 2013). The basic construction for AR NEDs normally includes: 1) a display unit or image source (e.g., a laser projector, a LCD display panel) 2) magnifying optics or relay optics and 3) the medium to transmit and project the virtual imageries into the eyes of the viewers while allowing the lights from the real environment to pass through (e.g., half mirrors, holographic films) ( Cakmakci and Rolland, 2006 Kress, 2020) ( Figure 1). 2 Overview of Different Types of AR Near-Eye Displays We conclude by outlining emerging technologies and unsolved challenges for future research. Then we will describe each of the leading types of AR NEDs in details with the principles and advancements to counter key challenges including eyebox, FOV and VAC. We will begin our review by giving an overview of the leading types of AR NEDs. Although there are two main groups of AR NEDs ( Rolland et al., 1994), namely video see-through and optical see-through, in this paper we will focus introducing the optical see-through AR NEDs because of their potential to provide an extremely high sense of immersion. In this paper, we present a review on the advancements and challenges towards AR NEDs. The requirements on other metrics, including resolutions, FOV, eyebox, and eye relief push the boundaries of diffraction for visible light wavelengths. The requirement that an AR NED be see-through constrains the form factor and optical materials involved. However, to counter the mentioned challenge with AR NEDs requires significant technological advancements. Therefore, the greatest challenge in AR NEDs is not in optimizing any individual metric, but instead simultaneously providing a wide FOV, variable focus to mitigate the vergence-accommodation-conflict (VAC), high resolution, a wide eyebox, ease of manufacturing, a slim form factor, etc ( Hoffman et al., 2008). Although AR NEDs offer a replacement for smartphones and computer monitors and provide visual experience to viewers, all designs for AR NEDs involve tradeoffs between a number of different metrics, including resolution, eyebox ( Barten, 2004), form factor, correct focus cues ( Zschau et al., 2010), field of view (FOV) ( Wheelwright et al., 2018), eye relief, brightness, and full color. The central component of AR is a near-eye display (NED) which is worn by the viewers and is used to combine real and virtual imageries together so that both can be seen at the same time ( Koulieris et al., 2019). Different from smart glasses which simply superimpose two-dimensional (2D) contents in a head-mounted display (HMD), AR allows the viewers to have more natural interactions with the virtual objects. In AR, information is presented to viewers with virtual objects such as graphics and captions fused with real environments without compromising the viewer’s natural vision ( Olbrich et al., 2013 Choi J. We also introduce a number of emerging technologies that are worthy of close study.Īugmented Reality (AR) is widely recognized as the next-generation computing platform replacing smart phones and computers. In this review, we present a brief overview of leading AR NED technologies and then focus on the state-of-the-art research works to counter the respective key challenges with each of the leading AR NED technologies. Although various AR NED products have been successfully commercialized and even deployed into applications, there are still challenges with present AR NED technologies (e.g., limited eyebox, fixed focus, bulky form factors). Optical see-through near-eye display (NED) technologies for augmented reality (AR) have achieved significant advancements recently with investments from both academia and industry.
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