The distinguishing qualities of a navigational light, whether from a lighthouse, lightship, light float, or lighted buoy, by which the navigator of a ship can easily identify it. In addition to their colours, white, red, yellow, or green, individual lights can be recognized by whether they are alternating, fixed, flashing, fixed and flashing, or occulting, and by the number, or group, of exposures in each cycle.
An alternating light is one in which two colours are used, exposed alternately in each cycle of the light and always in the same order. The colours are sometimes separated by periods of darkness, and sometimes alternate continuously. A fixed light shows a steady beam with no period of darkness, and thus has no cycle. A fixed and flashing light is one which shows a steady beam varied at regular intervals with a flash of brighter intensity. A flashing light is one in which the total duration of light in each cycle is less than the period of darkness. An occulting light is one in which the period of darkness is less than the period of light in each cycle.
These five main types are further varied for recognition purposes by what is known as grouping, in which a series of flashes or a series of eclipses (occulting) is separated by intervals of darkness or light. Thus in a group flashing light two or more flashes are visible at regular intervals in each cycle, while in a group occulting light there are two or more short intervals of darkness in each cycle. As an example, a light indicated on a chart as a group flashing light with four flashes every 15 seconds (Fl. (4) 15 sec.) would show to the navigator four bright flashes with an interval of a second or so between them followed by a period of darkness occupying the rest of the 15 seconds. The 15 seconds is the period, or cycle, of the light, and is measured from the first flash in one group to the first flash in the next succeeding group. Composite group flashing is where the number of flashes alters and is used to identify modified lateral marks in the IALA maritime buoyage system. A light group flashing 2 + 1 every 15 seconds (Fl. (2 + 1) 15 sec.), for example, is where there are two flashes followed by the period of darkness, then one flash followed by the period of darkness, then two flashes again.
Other differentiations of characteristics are the quick, very quick, and ultra quick, flashing lights which are a modification of the flashing and group flashing systems. The quick flashing light is one which normally flashes at either 50 or 60 flashes per minute, the very quick flashing light normally flashes at a rate of either 100 or 120 flashes per minute, and the ultra quick flashing light normally flashes at between 240 to 300 flashes per minute.
Both very quick (VQ) and quick (Q) flashes are used to identify cardinal marks in the IALA maritime buoyage system. A north cardinal mark has continuous very quick or quick flashes; east has three very quick or quick flashes, then darkness, in a period of five or ten seconds respectively; south has six very quick or quick flashes followed by a long flash, then darkness, in a period of ten or fifteen seconds respectively; and west has nine very quick or quick flashes then darkness, in a period of ten or fifteen seconds respectively. The long flash (not less than two seconds long) which is a characteristic of the south cardinal mark is merely a device for making certain the three or nine flashes cannot be mistaken for six.
Subjects: Maritime History.