Prismatic Section

From International Dictionary of Marine Aids to Navigation
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1. Cross-section of a prism by a plane normal to the refracting edge. The section is bounded by straight lines (usually three).

Ch2 Fig11.jpg

2. In lighthouse practice, the term is also used for a cross-section of a transparent optical element bounded by three straight or slightly curved sides, approximating to a triangle, but of such shape that incident parallel rays of a particular wavelength are focussed after refraction or reflection (or both) by the section.

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