A lighthouse is not effective as an aid to navigation unless
it can be seen by the mariner, and can be identified as a particular lighthouse
in a specific location.
Most lighthouse lights rhythmically flash or eclipse to
provide their identification signal. The particular pattern of flashes or eclipses
is known as the character of the light, and the interval at which it repeats
itself is called the period. When lights are designed, the designer must choose
light characteristics that are different from other lights in the vicinity. The
number of different characters allowed in the design is restricted by
international agreement through the International Association of Lighthouse
Authorities in Paris. The majority of maritime nations belong to this
association.
Two classes of lights most used in lighthouses are the fixed
flashing light and the group flashing light.
A fixed flashing light is a single flash, regularly
repeated at perhaps 5-, 10-, or 15- second intervals. An example of this class is
the Heceta Head Lighthouse, located near Florence, OR. (Pictured below.) Its characteristic
or signature is a single white light which flashes every 10 seconds.
The group flashing light exhibits groups of two,
three, or four flashes, with a short eclipse between individual flashes and a
long eclipse of several seconds between successive groups. The whole pattern is
repeated at regular intervals of 10 to 20 seconds. An example of this class is
the Umpqua River Lighthouse, located at Winchester Bay, OR. (Pictured below). Its characteristic or
signature is flashes of white and red light. Two white flashes and one red
flash every 15 seconds.
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