The performance approach groups HMD system and subsystem issues into performance categories. These are: Optical system, visual, helmet, and human factors engineering. The issues under each category will be discussed in terms of how various HMD parameters relate to corresponding human sensory or structural parameters, and how they interact to limit or enhance aviator performance.
In most HMD designs, an image source (e.g., CRT, LCD, etc.) creates on its face a reproduction of the outside scene. This reproduced image then is relayed through a set of optical elements (relay optics) producing a final image which is viewed by the eye. The former image on the image source has certain characteristics. The relay optics have a transfer function which modifies these characteristics in producing the final image. When the aviator dons the HMD, there are both system characteristics (e.g., FOV, magnification, see-through transmittance, etc) and image characteristics (relating to image quality) which define the usefulness of the HMD in helping the aviator perform the mission. The optical performance of an HMD can be evaluated using two approaches. The first addresses the physical characteristics of the HMD and its imagery. The second addresses the perceived performance with regard to the human user.
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