identification purposes. Figure 9-8 illustrates
characteristics of lights on lighthouses and lightships.
VISIBILITY OF LIGHT
Geographic range: The maximum distance a
light can be seen under conditions of perfect visibility,
limited only by the curvature of Earth. It is expressed in
nautical miles for the height of the observer's eye at sea
level.
The visibility of a light is the distance in nautical
miles that a navigator can expect to see the aid to
navigation.
Nominal range: Maximum distance at which a
light can be seen in clear weather. Clear weather is
meteorologically defined as a visibility of 10 nautical
When speaking of a light, the following terms will
apply:
miles. Nominal range is listed for all Coast Guard listed
aids except range and direction lights.
CLASS
ABBREVIATION
GENERAL DESCRIPTION
ILLUSTRATION
Fixed light
F
A continuous and steady light.
Occulting light
Oc, Occ
The total duration of light in a period is
longer than the total duration of
darkness and the intervals of darkness
(eclipses) are usually of equal duration.
Period 12s
Eclipse regularly repeated.
Group - occulting light
Oc (2), Gp Occ (2)
An occulting light in which a group of
eclipses, specified in number, is regular-
ly repeated.
Period 14s
Composite group -
Oc (2 + 1), Gp Occ
A light similar to a group-occulting light
occultinglight
(2 + 1)
except that successive groups in a period
have different numbers of eclipses.
Period 12s
Isophase light
Iso
A light in which all the durations of
light and darkness are clearly equal.
5s
5s
Flashing light
F1
A light in which the total duration of
light in a period is shorter than the total
duration of darkness and the ap-
pearances of light (flashes) are usually
Period
of equal duration (at a rate of less than
10s
50 flashes per minute).
Long - flashing light
L F1
A single-flashing light in which an ap-
pearance of light of not less than 2 sec.
duration (long flash) is regularly
Period
repeated.
10s
Group - flashing light
F1 (3), Gp F1 (3)
A flashing light in which a group of
flashes, specified in number, is regular-
ly repeated.
Period 15s
Composite group -
F1 (2 + 1), Gp F1
A light similar to a group-flashing light
flashing light
(2 + 1)
except that successive groups in a period
have different numbers of flashes.
Period 20s
Quick light
Q, k F1
A light in which a flash is regularly
repeated. Flashes are repeated at a rate
of not less than 50 flashes per minute
but less than 80 flashes per minute.
Group quick light
Q (3), Gp F1 (3)
A light in which a specified group of
flashes is regularly repeated. Flashes
Period 10s
are repeated at a rate of not less than 50
flashes per minute but less than 80
Q (9), Gp F1 (9)
flashes per minute.
Period 15s
Q (6) + LF1, Gp F1 (6)
Period 15s
Figure 9-8.—Characteristics of lights—sheet 1.
9-8
