The routine extravehicular activity (EVA) performed from the U.S. Space Station Freedom will require the astronaut to access large amounts of information during the EVA, especially for intensive EVA scenarios such as satellite servicing and emergency or contingent operations. As a result, NASA is presently designing a helmet mounted display (HMD) into the Freedom Station extravehicular mobility unit (EMU) to aid the EVA astronaut. The HMD allows the astronaut to view a virtual image behind a transparent combiner located conveniently above his or her primary field of view (FOV). This HMD system can be voice-driven for "hands-free" operation. NASA is currently exploring four HMD approaches. Two designs utilize cathode ray tubes (CRT's), while the other two use backlit liquid crystal displays (LCD's). Furthermore, two of these designs use purely conventional optics, while the other two employ conventional and holographic optics. A discussion of these designs and some key design issues, such as image source, FOV, exit pupil versus non-pupil-forming systems, monocular versus binocular and biocular viewing, degree of image overlap, and the use of holographic optical elements (HOE's), will be provided in this paper.
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