Focal Plane (of a light)
The horizontal plane through the focus of the optical apparatus of the light.
Note: In lighthouse practice in some countries the term "focal plane" is used with the above meaning, although this is quite a different meaning from that of 2-2-165.
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