In
optics, the
exit pupil is a virtual
aperture in an optical system. Only
rays which pass through this virtual aperture can exit the system. The exit pupil is the
image of the
aperture stop in the optics that follow it. In a
telescope or
compound microscope, this image is the image of the
objective element(s) as produced by the
eyepiece. The size and shape of this disc is crucial to the instrument's performance, because the observer's eye can see light only if it passes through this tiny aperture. The term
exit pupil is also sometimes used to refer to the
diameter of the virtual aperture. Older literature on optics sometimes refers to the exit pupil as the
Ramsden disc, named after English instrument-maker
Jesse Ramsden.