Especially during the night, when most of the coastline features are not visible,
lights are a great help to the navigator as reference and bearing points.
In order to distinguish one light from another and to allow a unique
identification during the night, each light has distinctive optical characteristics.
This includes a distinctive color and a distinctive sequence of
light and dark intervals. This characteristic sequence is called phase
The colors used for the identification of lighted beacons or marks are
Some special beacons carry a
No other color is used for lights.
Besides the color of the beacon light, which is only visible
between sunset and sunrise, beacons are painted with distinct
daylight colors. Beacons may be painted with the following
The towers of lighthouses are often painted with conspicuous colors or black and white patterns
(e.g. checkerboard patterns).
A complete and detailed description of lighthouse characteristics is found in the "List of Lights"
published by the national lighthouse authorities.
red with an horizontal
green band (lateral marks)
green with an horizontal
red band (lateral marks)
a combination of horizontal
black bands (caridinal marks)
black with an horizontal
red band (isolated danger marks)
white stripes (safe-water marks)
One of the principal characteristics of a light is the duration
of the characteristic sequence. The sequence duration or period is expressed
in seconds and is the first identification characteristic. The period of
a light should be measured with a stop watch to get a reliable identification.
The characteristic sequence of a light consists of an alternating pattern
of light and dark intervals or phases. Depending on the ratio of light
to dark phases, lights are distinguished in:
- fixed lights (F) :
they are always on and have no dark phase;
- occulting lights (Oc):
the duration of light phases is greater than the duration of the
- flashing lights (Fl):
the duration of the light phases is less than the duration of dark
- isophase lights (Iso):
the duration of light phase is equal to the duration of the dark
Depending on the structure of the characteristic light sequence, occulting
and flashing lights may show single or group patterns. Single-occulting
and single-flashing lights have a simple sequence of one light and one
dark phase. Group-occulting and group-flashing lights have more complex
sequences of multiple light and dark phases.
Even more complex sequences are shown in composite group-occulting
or composite group-flashing lights.
Quick lights are special flashing lights. The flashing rate is higher
than 60 flashes per minute.
These lights may have a continuous (Q)
interrupted flashing (IQ) pattern.
- Very quick flashing lights have a rate of more that 120 flashes per minute.
These lights also may have a continuous
(VQ) or interrupted flashing
- A light showing different colors alternatively is described as alternating
light (AL). The alternating color may be combined with an additional
light phase characteristic.
From the interactive lights page examples
of lights with different phase and color characteristics can be selected
and examined. It can be used as training aid for the determination of lights
Almost all lights today are operated automatically. They are extinguished
automatically during daylight hours and switched on during the night by
light sensitive switches. Due to the variations of the sensitivity of the
light switches, all lights are not switched on or off at the same time.
Mariners should account for this when identifying navigation aids during
twilight periods when some lights are on while other lights are extinguished.
Sectors of colored glass or plastic are sometimes placed in the lanterns
of certain lights to indicate dangerous waters. These lights show different
colors when observed from different bearings. A sector changes the color
of a light, but not its characteristic, when viewed from certain directions.
Sectors may sometimes be only a few degrees in width or extend in a
wide arc from deep water toward shore. Bearings referring to sectors are
expressed in degrees true as observed from an approaching vessel.
In most cases, areas covered by red sectors should be avoided. The nature
of the danger can be determined from the chart. In some cases a narrow
sector may mark the best water across a shoal, or a turning point in a
The colors of different sectors change through an arc of uncertainty
of 2° or greater, depending on the optical design of the light. Therefore
bearings to lights should not be determined by observing the color change
(sector boundary) but conventionally by using a hand-bearing compass.
Light Ranges and Weather Conditions affecting Light Characteristics
The condition of the atmosphere has considerable effect upon a light's
range. Sometimes lights are obscured by fog, haze, dust, smoke or precipitation.
On the other hand, refraction may cause a light to be seen farther than
under ordinary circumstances.
Unfavorable weather conditions, may even change some of the lights characteristics.
Haze and distance may reduce the apparent duration of a light's flash.
In some conditions of the atmosphere, white lights may have a reddish hue.
In clear weather green lights may have a more whitish hue. In regions where
ice conditions prevail, an unattended light's lantern panes may become
covered with snow or ice. This may reduce the light's luminous range and
change the light's observed color.
The distance from a light cannot be estimated by its apparent brightness.
There are many factors which can change the perceived intensity. Also,
a powerful, distant light may sometimes be confused with a smaller, closer
one with similar characteristics. Every light sighted should be carefully
evaluated to determine if it is the one expected.
The presence of bright shore lights may make it difficult to distinguish
navigational lights fro the background lighting. Lights may also be obscured
by various shore obstructions.
A light's loom is seen through haze or the reflection from low-lying
clouds when the light is beyond its geographic range. Only the most powerful
lights can generate a loom. The loom may sometimes be sufficiently defined
to obtain a bearing.
At short distance, some of the brighter flashing lights may show a faint
continuous light, or faint flashes, between regular flashes. This is due
to reflections of a rotating lens on panes of glass in the lighthouse.
The apparent characteristic of a complex light may change with the
distance of the observer. This is because for a given candlepower, white
is the most visible color, green less so and red least of the three. This
fact also accounts for the different ranges given in the "Light List" for
dome multi-color sector lights. The same lamp has different ranges according
to the color imparted by the sector glass.