Near-eye light field display

Near-eye displays (NEDs) project images into a viewer’s eyes, creating a virtual image in their field of view. The image appears at a distance, and larger than the small display panel and optics used to create it. However, according to Lanman and Luebke (2013), these kind of displays have a fundamental problem: the unaided human eye cannot accommodate (focus) on objects placed in close proximity .

NEDs are also known as head-mounted displays (HMDs) and encompass electronic viewfinders. Bhakta et al. (2014) referred that “near-eye displays are the headphones of the display world, creating small, portable, personal viewing experiences.” They have several advantages over traditional displays, such has a compact size, being lightweight, demanding low power, and can be see-through, being able to produce a virtual image that looks like a big screen TV from a small form factor. Furthermore, NEDs can be placed in two general categories: immersive and see-through. Immersive NEDs block the user’s view of the real world, creating a large field of view image (e.g., VR headset). See-through NEDs allow for the user to see the real world, generating a transparent image or a very small opaque image that blocks a small portion of the user’s peripheral vision. Examples of see-through NEDs are augmented reality headsets or smart glasses like the Google Glass .

Near-eye light field displays introduce a light-field-based approach to NEDs. This allows for thinner and lighter HMDs capable of depicting accurate accommodation, convergence, and binocular-disparity depth cues. The two human eyes perceive the world slightly differently. In the same way, light rays that enter the pupil at different location will encode a slightly different picture of the world being observed . A light field is composed of all the light rays at every point in space travelling in every direction. It is a 4D data, since every point in three-dimensional space is attributed a direction. This concept came about in the 1990s as a solution to problems in computer graphics and vision . Near-eye light field displays must independently render light rays that are coming from every direction through every point in space in order to trigger accommodation. Sharp images from out-of-focus display elements are depicted by synthesizing these light fields that correspond to virtual scenes located within the viewer’s natural accommodation range. Lanman and Luebke (2013) mention that “conventional displays are intended to emit light isotropically. In contrast, a light field display supports the control of tightly-clustered bundles of light rays, modulating radiance as a function of position and direction across its surface.”

Traditional HMDs only provide a single display plane; without a proper focus cue, the display decouples accommodations from the vergence of the eyes. Since there is a mismatch, the observer has to rely only on the binocular vision to perceive a 3D space. This can lead to visual discomfort, fatique, eye strain, and headaches .

There have been demonstrations that light field displays allow for small form factors of NEDs. This was made by placing a microlens array on a small screen close to the eye. In near-eye light field displays the image created appears to be floating outside the physical device enclose, and the observer can accommodate with a narrow range. However, the lens used in the studies have a tradeoff between achieved spatial resolution and the supported depth range . Another technique used to implement light field displays is to stack liquid crystal displays (LCDs). In this case, the image formation is multiplicative, allowing for correct or nearly-correct focus cues to be supported over larger depth ranges. Alternatively, it reduces the number of required display planes .

With the Oculus Rift, the commercial interest in HMDs increased. Indeed, over the last few years interest in VR has been increasing, both by researchers as well as consumers. NED technology has a vast range of applications besides gaming and entertainment. It can be applied in education, teleconferencing, scientific visualization, training and simulation, phobia treatment, and surgical training, for example. Immersive VR has also been demonstrated to be effective in the treatment of post-traumatic stress disorder. For the continuing development of VR. It is essential to have a visually comfortable experience, such as diminishing the vergence-accommodation conflict that occurs in most HMDs. The improvement of light field displays is a path to the creation of better and more visually comfortable headsets . Huang et al. (2015) wrote that “correct or nearly correct focus cues significantly improve stereoscopic correspondence matching, 3D shape perception becomes more veridical, and people can discriminate different depths better. Vergence and accommodation cues are neurally coupled in the human brain; it seems intuitive that displays supporting all depth cues improve visual comfort and performance in long-term experiences.”