From International Dictionary of Marine Aids to Navigation
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A circular lens (in sense 1 of 2-2-010), or part thereof, usually plano-convex or double convex in section, the optical axis (2-2-110) of which coincides with the optical axis of the prismatic lens of which it forms a part.

Ch2 Fig10.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