Toward custom VR displays

The first step toward thinking about displays for VR is to consider the distance from the eyes. If it is meant to be viewed from far away, then it is called a naked-eye display. For a person with normal vision (or while wearing prescription glasses), the display should appear sharp without any addition help. If it is close enough so that lenses are needed to bring it into focus, then it is called a near-eye display. This is the common case in current VR headsets because the display needs to be placed very close to the eyes. It remains an active area of research to develop better near-eye display technologies, with a key challenge being whether the solutions are manufacturable on a large scale.

Figure 4.37: An illustration of how a DigiLens waveguide operates, as light is propagated from a small source display to the human eye. (Figure by Christopher Grayson; uploadvr.com/waveguides-smartglasses/)

An important family of near-eye displays is based on a microdisplay and waveguide. The microdisplay is typically based on liquid crystal on silicon (or LCoS), which is a critical component in overhead projectors; microdisplays based on organic LEDs (OLEDs) are also gaining popularity. The size of the microdisplay is typically a few millimeters, and its emitted light is transported to the eyes through the use of reflective structures called a waveguide; see Figure 4.37. The Microsoft Hololens, Google Glass, and Magic Leap One are some well-known devices that were based on waveguides. The current engineering challenges are limited field of view, overall weight, difficult or costly manufacturing, and power loss and picture degradation as the waves travel through the waveguide.

A promising device for future VR display technologies is the virtual retinal display [340]. It works by a scanning beam principle similar to the CRT, but instead draws the image directly onto the human retina; see Figure . A low-power laser can be pointed into a micromirror that can be rapidly rotated so that full images are quickly drawn onto the retina. Current engineering challenges are eye safety (do not shine an ordinary laser into your eyes!), mirror rotation frequency, and expanding the so-called eye box so that the images are drawn onto the retina regardless of where the eye is rotated.

To maximize human comfort, a display should ideally reproduce the conditions that occur from the propagation of light in a natural environment, which would allow the eyes to focus on objects at various distances in the usual way. The previously mentioned displays are known to cause vergence-accommodation mismatch (see Section 5.4), which is knwon to cause discomfort to human viewers. For this reason, researchers are actively prototyping displays that overcome this limitation. Two categories of research are light-field displays [75,163,200] and varifocal displays [4,50,128,189,208].