Principal plane

principal plane
(Science: microscopy) A plane normal to the principal axis of a lens, passing through a principal point.
In geometrical construction of ray diagrams through a perfect lens, the single plane that can be considered (disregarding the refraction of the rays at each optical interface) to convert paraxial rays into rays converging onto the rear focal point. For thin symmetrical lenses, there is one principal plane that bisects the lens. For thick or complex lenses, there is a principal plane for the paraxial ray to the left, and another for the paraxial ray to the right, of the lens.