Re-engineering the Wheatstone stereoscope
Joel Kollin · SPIE Newsroom · 2007
Therapeutic applications using virtual reality (VR) can deliver excellent outcomes in treating phobias1, 2 and show great promise in cases of post-traumatic stress disorder.3 VR analgesia has reduced subjective pain ratings of burn victims undergoing excruciating wound-care procedures by 30–90%.4 And functional magnetic resonance imaging scans have shown that pain-related brain activity levels in healthy individuals drop by 50–90%.5 In both these modalities, immersion is identified as a key factor.6, 7 However, head-mounted displays (HMDs) that even marginally match the capabilities of the human visual system remain extremely expensive. Stereoscopes may provide an excellent compromise at a fraction of the cost of wide field-ofview (FOV) HMDs. Progress in developing the requisite technologies—primarily high-resolution microdisplays—has been slow, and they have proved difficult to interface. Various near-eye optical systems work well for low FOV but rise rapidly in cost, complexity, and size with higher applications. This contrasts markedly with direct-view LCD displays, now commonly available in resolutions beyond high-definition television while costing less than $600. The major limitation with LCD is the 60Hz refresh rate such that two displays are required for stereoscopic use. The issue then becomes how to deploy them so that each display is viewed by only one eye. This can be accomplished with a polarizing beamsplitter,8 but also by optimizing a 150-year-old invention known as theWheatstone stereoscope. Work has been done with these scopes using cathode ray tubes,9–11 but large LCD panels offer a unique opportunity to optimize performance.12 The Wheatstone stereoscope employs two mirrors whose reflections form an overlapped image at an angle, as shown in Figure 1. Optionally, the images can be viewed through a magnifier (such as reading glasses) for greater comfort. Glasses, which also Figure 1. The Wheatstone stereoscope.