A light characteristic is a graphic and text description of a navigational light sequence or colour displayed on a nautical chart or in a Light List with the chart symbol for a lighthouse, lightvessel, buoy or sea mark with a light on it. The graphic indicates how the real light may be identified when looking at its actual light output type or sequence. Different lights use different colours, frequencies and light patterns, so mariners can identify which light they are seeing.
While light characteristics can be described in prose, e.g. "Flashing white every three seconds", lists of lights and navigation chart annotations use abbreviations. The abbreviation notation is slightly different from one light list to another, with dots added or removed, but it usually follows a pattern similar to the following.
An abbreviation of the type of light, e.g. "Fl." for flashing, "F." for fixed.
The color of the light, e.g. "W" for white, "G" for green, "R" for red, "Y" for yellow. If no color is given, a white light is generally implied.
An example of a complete light characteristic is "Gp Oc W 10s 15m 10M". This indicates that the light is a group occulting light in which a group of three eclipses repeat every 10 seconds; the light is white; the light is 15 metres above the chart datum and is visible for.
A fixed light, abbreviated "F", is a continuous and steady light.
A flashing light is a rhythmic light in which the total duration of the light in each period is clearly shorter than the total duration of the darkness and in which the flashes of light are all of equal duration. It is most commonly used for a single-flashing light which exhibits only single flashes which are repeated at regularintervals, in which case it is abbreviated simply as "Fl". It can also be used with a group of flashes which are regularly repeated, in which case the abbreviation is "Fl" or "Gr Fl", for a group of two flashes. Another possibility is a composite group, in which successive groups in the period have different numbers of flashes, e.g. "Fl. " indicates a group of two flashes, followed by one flash. A specific case sometimes used is when the flashes are longer than two seconds. Such a light is sometimes denoted "long flashing" with the abbreviation "L.Fl". If the frequency of flashes is large the light is denoted as a "quick light", see below.
An occulting light is a rhythmic light in which the duration of light in each period is longer than the total duration of darkness. In other words, it is the opposite to a flashing light where the total duration of darkness is longer than the duration of light. It has the appearance of flashing off, rather than flashing on. Like a flashing light, it can be used for a single occulting light that exhibits only a single period of darkness or the periods of darkness can be grouped and repeated at regular intervals, a group or a composite group. The term occulting is used because originally the effect was obtained by a mechanism periodically shading the light from view.
An isophase light, abbreviated "Iso", is a light which has dark and light periods of equal length. The prefix derives from the Greek iso- meaning "same".
A quick light, abbreviated "Q", is a special case of a flashing light with a large frequency. If the sequence of flashes is interrupted by regularly repeated eclipses of constant and long duration, the light is denoted "interrupted quick", abbreviated "I.Q". Group notation similar to flashing and occulting lights is also sometimes used, e.g. Q. Another distinction sometimes made is between quick, very quick and ultra quick. This can be combined with notations for interruptions, e.g. I.U.Q for interrupted ultra quick, or grouping, e.g. V.Q for a very quick group of nine flashes. Quick characteristics can also be followed by other characteristics, e.g. VQ LFl for a very quick group of six flashes, followed by a long flash.
A Morse code light is light in which appearances of light of two clearly different durations are grouped to represent a character or characters in the Morse Code. For example, "Mo" is a light in which in each period light is shown for a short period followed by a long period, the Morse Code for "A".
Fixed and flashing
A fixed and flashing light, abbreviated "F. Fl", is a light in which a fixed low intensity light is combined with a flashing high intensity light.
An alternating light, abbreviated "Al", is a light which shows alternating colors. For example, "Al WG" shows white and green lights alternately.