The term biocular refers to an optical system in which the optical components are shared by both of the viewer's eyes. The unusual aspect of a biocular design is that, while it must have a large exit pupil in order to accommodate both eyes, the instantaneous image quality will be determined by the relatively small pupil of the eye.
Critical to good performance in a biocular design is the condition that both eyes perceive the common point being viewed as originating at the same object point in space. Equally important is the correction of aberrations - such as spherical and coma - over the entire lens pupil which, if present, would result in apparent motion of the object (swimming) as the head moves about the exit pupil. This chapter will deal with the basic optical design considerations involved in generating and evaluating optical designs for two biocular devices.
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