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How a light is read
A tower is what you photograph. A character is what you navigate by. These six steps follow the things a published List of Lights entry actually carries, using the five stations on this site as examples.
Colour
Most of the lights here are white. Longships is the exception on this desk: its published character is Fl (2) WR 10s, so the flash is white or red according to the sector. The published character is Fl (2) WR 10s — a group of two flashes every ten seconds, white or red according to the sector. The published range is 15 nautical miles, and the published intensity is 14,400 candela.1
Character
Is the light a single flash, or a group? Eddystone and Bishop Rock are both group-flashing, Fl (2). Fastnet and Hook Head are single flashing, Fl W. The letter in brackets is the number of flashes in the group, not the length of a flash.
Period
The period is the time in which the whole pattern repeats: 10 seconds at Eddystone, 15 at Bishop Rock, 5 at Fastnet, 3 at Hook Head. Two stations can share a grouping and still be told apart by the period, or by colour.
Nominal range
Trinity House publishes Eddystone at 17 nautical miles and Bishop Rock at 20. Irish Lights publishes Fastnet at 18 and Hook Head at 23. Range is not a promise that you will see the light from that distance on a given night; it is the figure the authority prints.
Height of the light
The height that matters to a mariner is the height of the light above the water, not only the height of the tower. Trinity House publishes the height of the tower as 49 m, and the height of the light above mean high water as 41 m.2 The published height of the tower is 49 m, and the height of the light above mean high water is 44 m.3 The published height of the tower is 35 m, and the height of the light above mean high water is also 35 m.1
The identity of the station
Charts and lists name the station and tie the character to a place. Hook Head is not Fastnet, even though both flash white. The pattern, the period and the position together are what tell them apart.
You can watch those same five characters in the simulator, and you can compare the official numbers on the desk.