The purpose of a display is to generate a stimulus for a targeted sense organ. Vision is our dominant sense, and any display constructed for the eye must cause the desired image to be formed on the retina. Because of this importance, Chapters 4 and 5 will explain optical systems and the human vision system, respectively. For CAVE systems, some combination of digital projectors and mirrors is used. Due to the plummeting costs, an array of large-panel displays may alternatively be employed. For headsets, a smartphone display can be placed close to the eyes and brought into focus using one magnifying lens for each eye. Screen manufacturers are currently making custom displays for VR headsets by leveraging the latest LED display technology from the smartphone industry. Some are targeting one display per eye with frame rates above 90Hz and over two megapixels per eye. Reasons for this are explained in Chapter 5.

New display technologies that are customized for VR are rapidly emerging. Direct retinal stimulation is provided by using pico projector technology, including DLP (Digital Light Processing), LCD (Liquid Crystal Display), and LCoS (Liquid Crystal on Silicon). Products that have used this technology include Google Glass, Microsoft Hololens, and Avegant Glyph. To address comfort issues such as vergence-accommodation mismatch (see Section 5.4), more exotic display technologies have been prototyped but await the ability to rival other display technologies in terms of pixel density, field of view, frame rate, manufacturability, and cost. The two main families are light-field displays [74,161,198] and multi-focal-plane displays [4,127,187].

Figure 2.8: Two examples of haptic feedback devices. (a) The Geomagic Phantom allows the user to feel strong resistance when poking into a virtual object with a real stylus. A robot arm provides the appropriate forces. (b) Some game controllers occasionally vibrate.
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(a) & (b)

Now imagine displays for other sense organs. Sound is displayed to the ears using classic speaker technology. Bone conduction methods may also be used, which vibrate the skull and propagate the waves to the inner ear; this method appeared Google Glass. Chapter 11 covers the auditory part of VR in detail. For the sense of touch, there are haptic displays. Two examples are pictured in Figure 2.8. Haptic feedback can be given in the form of vibration, pressure, or temperature. More details on displays for touch, and even taste and smell, appear in Chapter 13.

Steven M LaValle 2016-12-31