A transparent optical element, usually of glass or plastic, in the form of an annulus or annular segment, having a prismatic section in sense 2 of 2-2-020. There are both dioptric (2-2-060) and catadioptric (2-2-070) rings, usually forming part of an optical panel or a drum lens.
(Figs. 10 and 16)
Please note that this is the term as it stands in the original IALA Dictionary edition (1970-1989)
International Association of Marine Aids to Navigation and Lighthouse Authorities - AISM