Types of Lights and Light Structures
Primary and secondary lights are so designated because of their
importance as ATONs. Primary seacoast aids are distinctive lights in
the U.S. system of ATONs. They are "fixed" as opposed to "floating"
and are maintained on the mainland, or on offshore islands and shoals to
warn mariners of the nearness of land or dangers. They are usually the
first ATONs that the navigator sees when making a landfall. The
navigator can use these lights to keep farther offshore at night. When
lights are located offshore, they mark a specific hazard or serve as a
marker for ships approaching a major harbor.
Many lights are classified as primary lights because of the importance of
their location, their intensity, and the prominence of their structures.
Other aids are classed as secondary or minor lights because of their
lesser qualities in one or more of these characteristics.
The dividing line
is not clear cut, and the difference is of no significance when applied to
These familiar structures are typical primary lights found along the
coastlines around much of the world. Lighthouses are placed on
prominent headlands and other points such as harbor entrances and
isolated dangers to warn mariners of danger or to guide them. The
principal purpose of the structure is to support a light source and lens at
a considerable height above the water. The same structure may also
house a fog signal, a radiobeacon, RACON, and other equipment.
Lighthouses vary greatly in shape and construction (fig. 4-5), which is
determined in part by their location (whether in the water or on shore),
the importance of their light, the kind of soil on which they are
constructed, and the prevalence of violent storms. Since lighthouses are
nonlateral aids, their paint color schemes are quite different than
traditional lateral marks. Lighthouse structures are painted in various
patterns such as stripes and solids, which help mariners to easily
distinguish them from other such structures in the same vicinity.