From International Dictionary of Marine Aids to Navigation
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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)

Ch2 Fig10.jpg

Ch2 Fig16a.jpg

Ch2 Fig16b.jpg

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