Sectors of colored glass are placed in the lanterns of certain lighthouses
to indicate danger bearings within which a ship will be in danger of
running on rocks, shoals, or some other hazard. The arcs over which a
red light shows are the danger sectors whose bearings usually appear on
the chart. Although the light is red within the danger arc, its
characteristics remain the same. It should be noted, however, that the
red light within the red sector may not be as visible as the white light
outside that sector.
Sectors may be only a few degrees in width, marking an isolated
obstruction, or they may be so wide that they extend from the direction
of deep water to the beach.
In most instances, red sectors indicate water areas to be avoided. A
narrow green sector may signify a turning point or the best water across
a shoal. Exact significance of each sector may be obtained from the
Exercise caution so that the danger sectors are not mistaken for the
sectors of good water or that incorrect bearings are taken from the chart.
All sector bearings are true bearings in degrees running clockwise
around the light as a center and are expressed as BEARINGS
OBSERVED FROM THE SHIP TOWARDS THE LIGHT.
Take a look at the example presented in figure 4-9. The Light List
Remarks column shows Cape Henry Light (LLNR 365) as having a red
sector from 154° to 233°. As long as your ship is within this sector, the
light will appear red. In this same example you will also note that the
nominal range of the red light is 15 miles, while the same light in the
white sector has a nominal range of 17 miles. The reason for this
difference is that a white light of a certain intensity is visible for a
longer distance than a red light of the same intensity.