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Subject: TWL: Light separation
LRZeitlin@XXX.XXX
Date: Sat Feb 01 2003 - 00:33:14 EST
<<"NOTE to paragraph (c): Two unscreened all-round lights that are 1.28
meters apart or less will appear as one light to the naked eye at a distance
of one nautical mile.">>
Mike,
That's the 2 minutes of arc criterion. A nautical mile, 6080 feet, multiplied
by the tangent of 2 minutes of arc is 42". The simplist way to find the
prescribed separation is to divide the distance by 1720. Incidentally, it is
the same resolution criterion used by Eastman Kodak for professional camera
lenses. It is based on research performed at the Naval Research Lab in San
Diego in the 1950s. Eyeballs havn't changed much since then. The best book on
vision in maritime conditions is "Vision Through the Atmosphere" by W. E. K.
Middleton, Univ. of Toronto Press, 1952, Oxford Univ. Press, 1958. It is
certainly out of print by now but most university libraries should have a
copy. My point was that it is the minimum separation for two point
discrimination in the field. Any degradation due to atmospherics or impaired
vision may require a larger distance. This is easily possible on large ships
but may pose difficulties with smaller trawlers.
Larry Z
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