Notes I keep while reading about dioptric apparatus. Mostly definitions I
kept forgetting, written down so I stop looking them up.
Order
The size class of a lighthouse lens, set by its focal distance. First
order is 920 mm and stands taller than a person; sixth order is
150 mm and fits on a table. Larger orders throw further, so
landfall lights are first or second order and harbour lights are
fifth or sixth.
Dioptric / catadioptric
A dioptric element only refracts. A catadioptric one refracts and then
reflects internally, which is how the rings far above and below the
focal plane still contribute to the beam instead of spilling light at
the sky.
Focal plane
The height of the light above mean sea level — not above the ground.
It sets geographic range: roughly 2.08 × √height in nautical miles,
before you account for the observer's own eye height.
Range
Three different numbers get called "range". Geographic range is limited
by the horizon, luminous range by the intensity and the weather, and
nominal range is luminous range quoted at a fixed 10 nmi
meteorological visibility so charts can compare lights fairly.